phosphonate and sulfonate metal organic frameworks
DESCRIPTION
Recent progress in phosphonate and sulfonate MOFs is reviewed with an emphasis on openframeworks. These two ligating functionalities are paired due to their structural analogy but thereview will show that their differences likely outweigh their similarities when it comes to theirframework structures and properties. Examples that are highlighted focus on new routes to openstructures, demonstrations of porosity and functionality, and examples with dynamic structures.TRANSCRIPT
This article was published as part of the
2009 Metal–organic frameworks issueReviewing the latest developments across the interdisciplinary area of
metal–organic frameworks from an academic and industrial perspective Guest Editors Jeffrey Long and Omar Yaghi
Please take a look at the issue 5 table of contents to access the other reviews.
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Phosphonate and sulfonate metal organic frameworksw
George K. H. Shimizu,* Ramanathan Vaidhyanathan and Jared M. Taylor
Received 26th January 2009
First published as an Advance Article on the web 12th March 2009
DOI: 10.1039/b802423p
Recent progress in phosphonate and sulfonate MOFs is reviewed with an emphasis on open
frameworks. These two ligating functionalities are paired due to their structural analogy but the
review will show that their differences likely outweigh their similarities when it comes to their
framework structures and properties. Examples that are highlighted focus on new routes to open
structures, demonstrations of porosity and functionality, and examples with dynamic structures.
This critical review is geared to researchers interested in designing open framework solids
(134 references).
Introduction
As part of a special issue concerning coordination network
materials, we will not provide general background on MOFs
other than where specifically germane to phosphonate and
sulfonate frameworks. In the broad domain of coordination
polymer/metal organic framework research, networks based
on phosphonate and sulfonate ligation are, by and large, less
studied. However, both families show great potential for new
functional materials. Phosphonates have demonstrated
promise for robust porous solids while sulfonates generally
show more promise for dynamic materials where structural
pliancy is desired.
For phosphonates, from the authors’ perspective, there are
three points that have likely hindered broader proliferation of
their study as MOFs, say relative to carboxylates. The first is
the predisposition of simple metal phosphonates to a dense
layered motif that, while still offering opportunities for
function, makes forming high surface area materials a
challenge.1–3 The second reason is that growth of single
crystals with phosphonates is generally more difficult as they
often precipitate rapidly as less ordered, insoluble phases.
While this does not preclude interesting properties, it does
make structural characterization, a hallmark property of
MOFs, a challenge. The final point is that, again relative to
other more studied ligating groups (e.g. COO–, pyridyl), the
coordination chemistry of phosphonates is less predictable
owing to more possible ligating modes and three possible
states of protonation.
Sulfonate networks have been studied considerably less than
other classes of ligand but for different reasons than listed for
phosphonates. Coordination of sulfonate anions is relatively
weak.4–6 This facilitates formation of crystalline products but,
oftentimes, the networks are not sufficiently robust to sustain
permanent pores. That said, the weaker ligation does enable
(single-crystal to single-crystal) solid state dynamics that
are increasingly gaining interest in coordination polymer
chemistry.7–9 In parallel with phosphonates, the spherical
ligating ability of a sulfonate does again make a priori
Department of Chemistry, University of Calgary, Calgary, Alberta,Canada T2N 1N4. E-mail: [email protected];Fax: 1 403 289 9488; Tel: 1 403 220 5347w Part of the metal–organic frameworks themed issue.
George Shimizu
George Shimizu completed hisBSc and PhD at the Universi-ties of Winnipeg and Windsor(S. Loeb), respectively. Hethen undertook postdoctoralwork at the University ofBirmingham (J. F. Stoddart)and the National ResearchCouncil of Canada (J. Rip-meester and D. Wayner). Pre-sently, he is a Full Professorand his research concernsinorganic–organic materials forenergy applications includinggas storage/separation andproton conduction. A research
theme is network solids that demonstrate dynamic motion(possibly) reconciled with crystalline architectures.
Ramanathan Vaidhyanathan
Ramanathan Vaidhyanathangained his PhD in Chemistryfrom the Jawaharlal NehruCentre for Advanced ScientificResearch (India), under thesupervision of Prof. C. N. R.Rao and Prof. S. Natarajan.His PhD aimed at investigat-ing the synthesis and applica-tions of open-framework metalcarboxylates. He then joinedthe group of Prof. M. J.Rosseinsky at the Universityof Liverpool, where he investi-gated the magnetic propertiesof Co oxide-hydrides, and
chiral metal–organic frameworks (MOFs). Presently he workswith Shimizu investigating the proton conduction and gas cap-ture applications of MOFs.
1430 | Chem. Soc. Rev., 2009, 38, 1430–1449 This journal is �c The Royal Society of Chemistry 2009
CRITICAL REVIEW www.rsc.org/csr | Chemical Society Reviews
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coordination predictions a challenge although sulfonates
appear almost exclusively as monoanions (RSO3�). As we will
discuss, this lack of regular inorganic assemblies is not wholly
detrimental. New inorganic clusters can form in situ and the
structure-directing role of the organic linker can be enhanced.
In comparing phosphonate and sulfonate coordination
chemistry in framework materials, the contrasts likely
outnumber the similarities so, for clarity, the two families will
be discussed in sequence rather than in parallel beginning with
the phosphonate networks. We will highlight some of the
prominent examples of both families but the overall goal of
this paper will be to provide the reader with a sense of the
challenges and potential of these families relative to more
widely studied MOF components. Several of the more
interesting open-framework compounds presented have not
had gas sorption data reported. In most cases, it is not certain
whether this is because the material collapsed or whether the
experiments were not able to be performed. In this review, we
would prefer to merely state the facts given in the cited work
rather than presuppose any additional properties or lack
thereof.
A brief overview of layered metal phosphonates will
be followed by open-framework phosphonates, loosely
categorized by those with homoleptic phosphonate coordina-
tion and those with additional ligands. The sulfonate examples
will be presented first as layered solids, followed by open
frameworks, hydrogen-bonded coordination complexes, and
finally a separate heading for dynamic materials.
Metal phosphonates
Layered metal phosphonates
As layered metal phosphate and phosphonates have already
been reviewed thoroughly,1–3,10 we will only provide brief
background on this large area. This field is highly relevant
to the core concept of a metal–organic framework. Metal
phosphates had been studied as layered inorganic solids but
evolved into the field of ‘‘hybrid inorganic–organic’’ solids by
appending organic groups off the rigid inorganic layers.11 This
made the interlayer, where guest molecules could be
introduced, much more tunable as it could range from hydro-
philic to hydrophobic; clearly there exists a parallel to MOF
chemistry. Pendant groups have included other functionalities
to impart chirality,12 photoactivity13 or even crown ethers.14
Guest intercalation in such systems is accompanied by layer
swelling and it is possible to even exfoliate the layers to
films;15 both phenomena are confirmation of a robust layered
architecture.
Several tactics exist to generate porosity in a metal-
phosphonate system. Within the layered structures, one can
replace some phosphonates with a non-pillaring group
(e.g. phosphate, phosphite or a small monophosphonate) in
order to create some interlayer space.16 This method was
introduced by Dines and co-workers.17 They prepared a
pillared zirconium phosphate/diphosphonate with rough
composition of Zr(O3PO(CH2)6OPO3)0.5(O3P(CH2)8PO3)0.5and hydrolyzed the hexamethylene ester moieties to obtain
porous materials with high surface areas. The problem with
this approach is that substitution is random so, even though a
porous phosphonate can be formed, structural characteriza-
tion and a narrow pore size distribution are challenges.
A second approach could broadly be classified as examples
where the geometry of the organic core in a polyphosphonate
disrupts the layered motif and necessitates an open framework.
Early work by Alberti and co-workers concerned zirconium
phosphite (3,30,5,50-tetramethylbiphenyldiphosphonate),18
where the pillar moiety enforced porosity. The specific surface
area as determined from BET analysis was 375 m2 g�1. A third
approach would be to employ a second functional group on
the ligand to chelate the metal ion and direct the structure
away from simple layers. This can result in an open phosphonate
framework though often these are heteroleptic structures, with
the second ligating group playing a substantial role. These
latter two approaches will be discussed in more detail as they
are germane to MOF chemistry.
Homoleptic open-framework metal phosphonate frameworks
While a layered motif is unquestionably the most common
observation for a simple metal phosphonate, an important
exception is the family of complexes observed with methyl-
phosphonic acid. The first 3D framework metal phosphonate
b-Cu(O3PCH3), shown in Fig. 1, a 1D channel system with the
methyl groups lining the pores, was reported by LeBideau and
co-workers in 1994.19 The distance between the opposite
methyl carbons was 5.97 A, leaving an effective pore size of
about 3 A. In the same year, Maeda and co-workers reported
the synthesis of polymorphs AlMepO-a and AlMepO-b, theframework compositions of which are Al2(O3PCH3)3,
and revealed their microporous nature by using gas
adsorption.20,21 Building blocks for both are composed of
one AlO6 octahedron, three AlO4 tetrahedra, and six methyl-
phosphonate units with different manners of connectivity. The
corner-sharing aluminates and phosphonate units formed 3D
open frameworks. Both Al materials possessed 1D channels
lined with methyl groups and both channels have triangular
cross sections about 7 A on edge. The nitrogen adsorption
isotherms showed that AlMepO-b was of Type I, as is normal
for zeolites and AlPOs, while that of AlMepO-a had a two-
step adsorption in the low-pressure region. This difference was
explained by the different triangular channel shapes of the twoJared Taylor
Jared Taylor gained his BScHons. in chemistry at theUniversity of Calgary. He iscurrently working towards hisPhD, as an Alberta IngenuityNanotechnology and NSERCof Canada Graduate Scholar,under the supervision ofProf. Shimizu. His researchfocuses on proton conducting,phosphonate-based metal–organic frameworks for lowto intermediate-temperaturefuel cell applications.
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polymorphs. Propane, 2-methylpropane and 2,2-dimethyl-
propane gave Type 1 adsorption isotherms for both adsorbents
at 273 K. In contrast to a model where the inorganic layers
are absolute structure determinants with the organic groups
simple pendant, these structures show that inefficient
packing of the organic interlayer can direct the structure
away from the layered motif to that of 1-D channels.
Structural variation by this approach is quite limited as a small
phosphonate is essential.
After these reports, methylenediphosphonic acid was
successfully used to form a 3-D open framework,
Co2(O3PCH2PO3)�H2O, in 1997 by Lohse and Sevov.22 The
structure showed a 1-D inorganic channel lined with
methylene bridges. This spurred related efforts as several
phosphonate frameworks were made using ligands of the type
H2PO3(CH2)nPO3H2 (n = 1–4) and most of them had
inorganic frameworks with channels lined by the organic
groups.23–28 This approach hinged on the notion that a very
short linker between two phosphonate groups would disfavor
the default layered motif. As a representative example, a nickel
phosphonate [Ni4(O3PCH2PO3)2�(H2O)2], VSB-3, reported by
Cheetham and co-workers29 is presented (Fig. 2). The skeleton
of VSB-3 can be visualized as sheets of edge-sharing NiO6
trimers cross-linked in the third dimension by corner-sharing
NiO4 tetrahedra. This framework is then extensively decorated
by the bridging P–CH2–P units. The cross-linking of the layers
by the tetrahedral Ni centers generates narrow methylene-
lined pores (3.64 � 8.39 A). Completely dehydrated VSB-3
(VSB-4) shows excellent thermal stability up to 575 1C and
ferromagnetic interactions between Ni centers owing to its
three dimensional connectivities.
Varying the chain length in metal alkyldiphosphonates
alters the structures considerably. Compounds with –CH2–
chain lengths of n = 2–4 typically form pillared layered
structures23–28 as opposed to the open 3-D framework
above.20–22,29 No reports exist on n = 5–7 but these would
likely also form pillared layered motifs. When the alkyl chain
length of the diphosphonate was increased to n = 8, with
Co2+, a low-dimensional structure resulted composed of
cationic [Co(H2O)4(H4L)]2+ chains with charge-balancing
1,8-octylenediphosphonate clathrated in the structure.30
In this case, the pH of the complexation likely played a role
but the result is still surprising.
Trivalent vanadium has been complexed to members of the
same family of diphosphonate ligands used above,
[HO3P(CH2)nPO3H] (n = 2, 3).31–33 Five such frameworks
were reported which showed both 3-D and pillared layered
structural types. As these structures were not highly open,
their main interest was in the range of different anionic
vanadium clusters that resulted from the framework
formation. Other vanadium phosphonate structures have been
reported that include M2+-chelate subunits (e.g. [Cu(bipy)2]2+)
as templates and/or building units.34–37 These networks often
employ F� in place of O2� to generate frameworks with large
apertures (up to 19 A) where the guest metal complexes are
situated. In these studies it was shown that fluoride was
essential to effect mineralization and to induce crystallization
at the acidic pH of the reactions, although fluoride was not
incorporated into the structures in most cases.37 Appropriate
modification of reaction conditions could allow incorporation
of fluoride to produce oxyfluorinated materials.37
Use of polyfunctional phosphonate, N,N0-piperaziniumbis-
methylenephosphonate, with lanthanum chloride gave
rise to two polar three-dimensional open frameworks.38
Topologically similar inorganic ‘‘lanthanum phosphate’’
chains were linked in two different ways by the organic
ligands, the nature of which appeared to depend on the guest
Cl� ions. The frameworks were made up from one-dimen-
sional lanthanide phosphonate chains connected in three
dimensions by the piperazinium backbone. The piperazinium
cations imparted a macro-cationic nature to the framework.
The three-dimensional framework contained channels, lined
Fig. 1 Three-dimensional open-framework structure of copper
methylphosphonate19 showing the methylene decorated 1-D channels
running along the framework. Color scheme: Cu-cyan; P-purple;
O-red; C-grey.
Fig. 2 3-D open-framework structure of the nickel phosphonate,
Ni4(O3PCH2PO3)2�(H2O)2 (VSB-3).5 The layers (running perpendicu-
lar to the plane) are built up of trimeric edge-sharing NiO6 octahedra
while the cross-linking units (along the plane) are made up of NiO4
tetrahedra. Color scheme: Ni-cyan; P-purple; O-red; C-grey.
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by these piperazinium and phosphonate units, and occupied
by the Cl� ions. The genuine ‘‘polar’’ ordering of the
Cl-anions within the channels gave a polar framework topology.
No data was available on the porosity of the compound.
In a work related to earlier efforts pillaring phosphonate
layers with crown ethers, Clearfield reported the use of a
bis(phosphonate) derivative of a diazocrown ether to form
one-dimensional leaflet structures using both hydrogen-bond
interactions and covalent bonds.39 The partially deprotonated
form of the ligand was crystallized in the presence of ClO4�
ions to form hydrogen bonded 1-D chains consisting of
macrocyclic leaflet motifs (note: Co2+ ions from the
Co(ClO4)2 used were not incorporated in the structure). The
phosphonate groups from the adjacent ligand hydrogen
bonded among themselves to form a 1-D backbone. The
crown ether groups, which were attached to the phosphonate
groups via the N atoms, were arranged like leaflets around
these chains. The crown groups accommodated the solvent
water molecules and were aligned with respect to one another.
Complexing Cd(NO3)2 with this ligand formed 1-D Cd-N-
(phosphonomethyl)-aza-18-crown-6 chains also with a leaflet
like structure. In this case the Cd2+ centers were coordinated
by the crown phosphonate groups and also by nitrate anions
to again form a one-dimensional coordination polymer. The
author suggested that the work offered novel supramolecular
routes for grafting crown ethers into a polymeric matrix and
that such assemblies might exhibit distinct selectivity for guests
as compared to simple crown ethers in solution or the
solid state.
Another example of coupling phosphonate linkers with
crown ethers was reported but which also incorporated chirality.
Single crystals of homochiral lanthanide crown phosphonates
were prepared using an enantiomerically pure form of
2,2 0-pentaethyleneglycol-3,3 0-diphenyl-1,1 0-binaphthalene-
6,60-bis(phosphonic acid).40 The structure in this case was
made up of 1-D lanthanide phosphonate chains linked by
the binaphthyl backbones of the monoprotonated ligands,
thus creating a 2-D coordination network. The chiral crown
ethers decorate the interlayer spaces. Notably, the framework
was thermally and chirally stable to guest loss (including HCl).
An alternative approach to forming pores in metal phos-
phonates is to employ a large, multidirectional ligand that
would completely disfavor the formation of a layered inorganic
motif. The ligand, 1,3,5,7-tetrakis(4-phenylphosphonic acid)-
adamantane, was such a molecule as it possessed four
phosphonic acid moieties spaced by rigid phenyl groups from
an adamantane core. A number of reports of metal complexes
with this ligand have appeared. Neumann et al. reported two
papers41,42 concerning open, but less ordered, frameworks
with Ti4+ and V3+ and this ligand. No crystal structures were
obtained but the Ti sample41 showed broad features at 5.8 and
11.21 in the PXRD. This, with other supporting data, led to a
model being proposed with 22 � 9 A pores. Gas sorption
measurements gave a surface area of 557 m2 g�1. Using the
same ligand, the same group reported catalytic activity of a
mesoporous V framework42 for aerobic oxidation or benzylic
alcohols to aldehydes.
Taylor et al. reported that the crystal structure of the
Cu2+ network of the same tetraphosphonate ligand.43
[Cu3(H3L)(OH)(H2O)3]�H2O�MeOH, consisted of trigonal
trinuclear copper clusters linked by the organic spacer into a
diamondoid net (Fig. 3). The cluster was composed of three
copper centers coordinated by three separate phosphonate
groups and capped by a fourth phosphonate group to create
a pseudo-tetrahedral arrangement of phosphonates. The other
side of the cluster contained a m3-hydroxyl group to
charge balance. The network was a doubly interpenetrated
diamondoid structure but the bulky copper clusters resulted in
voids filled with H2O/MeOH. CO2 sorption revealed a BET
surface area of 200 m2 g�1. Variable-temperature powder
X-ray diffraction showed even at 30 1C there was some loss
in crystallinity, which was pronounced by 280 1C. This result
indicated that there was some collapse of the structure upon
desolvation but the material did still remain porous. This
main outcome of this work was to show that, with metal
phosphonates, that the organic linker could in fact determine
the inorganic aggregate as new metal clusters were formed
as required by the diamondoid topology. Despite the trimeric
Cu core, the material showed efficient antiferromagnetic
coupling.
Another accessible core unit, analogous to trimesic acid,
would be 1,3,5-benzenetriphosphonate where the trigonal
substitution should again disfavor a simple layered motif.
A study on Cu2+ complexes of this ligand, with and without
additional N-donors, showed a range of results.44 The simplest
combination, {Cu6[C6H3(PO3)3]2(H2O)8}, was a highly
hydrated network with the ligand in the 6� state. Tetracopper
clusters were the building units. Porosity was not examined in
this material. The compounds formed with 4,40-bipy and 4,40-
dipyridylpropane incorporated the ligand in 2� and 4� states,
respectively. Of these two complexes, the dipyridylpropane
complex yielded a material with solvent-filled channels
of B9 � 14 A dimension. These were formed by the
pillaring of Cu(1,3,5-benzenetriphosphonate) layers by the
Fig. 3 Crystallographic representation of a diamondoid cage formed
from the 1,3,5,7-tetrakis(4-phenylphosphonic acid)adamantane tetra-
hedron and pseudo-tetrahedral tricopper clusters. The network is
two-fold interpenetrated and the material displays a BET surface area
of 198 m2 g�1. Color code: Cu-cyan; P-purple; O-red; C-grey.
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dipyridylpropane. Gas sorption was not reported but again,
new inorganic clusters were formed.
Using a similar approach but with a larger core,
Vaidhyanathan et al. reported the ligand 1,3,5-tris(4-phosphono-
phenyl)benzene, a triphosphonate with a rigid D3h geometry.45
Upon hydrothermal reaction with Sr(OH)2 in the presence
of MeOH, the triphosphonate ligand formed a two-fold
interpenetrated, 3-D crystalline solid with the formula,
Sr2(H2L)(CH3OH)(H2O)4, shown in Fig. 4. This compound
contained 1-D SrO chains linked together by the trigonal
ligand. Each ligand bonded to three different chains and vice
versa, so a three-dimensional framework with large 1-D
channels along the a-axis resulted. In a single column, the
ligands were separated by 6.92(1) A, a distance suitable for
inclusion of another aromatic system. The 1-D inorganic
chains from one network filled the channels of the other to
form an interpenetrated structure. Though interpenetrated,
clefts remained between the three peripheral aryl rings. These
were the proposed sites of CO2 uptake as sorption isotherms
indicated that this material was porous with a BET surface
area of 146 m2 g�1. When left standing, it was observed that
the complex took up CO2 from the air, as confirmed by IR
spectroscopy. The topology of this network had design
implications as, with longer phenyl spacers, a third inter-
penetrating network would not be possible and increased
surface area would need to result.
A homochiral open-grid, layered solid has been reported for
the Ln complexes of a bisphosphonated binaphthyl ligand.46
In these structures, Ln atoms as vertices, each linked four
different binaphthyl ligands, to form a 2-D open rhombic grid
which then stacked in the third dimension (Fig. 5). Each Ln
vertex was eight-coordinate incorporating four molecules of
water in its coordination sphere. Upon dehydration, the
network transformed to an amorphous phase but notably,
the parent structure could be reformed upon exposure to water
vapor, illustrating that while long-range order was lost, the
local coordination environment was likely maintained. While
the nature of the amorphous material was not commented on
at length in this work, most likely, the structure shifted to
enable increased ligation of the Ln ions by the phosphonate
groups.
The next section of this review will deal with mixed ligand
phosphonates as a route to generate porosity. Our final
example47 on homoleptic systems actually concerns a complex
that was designed with the heteroleptic approach in mind. The
result was a Zn network with homoleptic PO3 coordination
and permanent porosity.47 The ligand, 1,4-dihydroxy-2,5-
benzenediphosphonic acid was prepared with the intent of
forming a chelating ring between the hydroxyl group and the
phosphonate. With Zn2+ in DMF, a phase-pure product,
Zn(H2L)(DMF)2, was formed with a structure of 1-D columns
of phosphonate-bridged Zn centers. From these columns, the
R groups protruded in four directions to form a square grid.
The role of the non-coordinating hydroxyl groups was not to
perturb the structure by chelation but rather simply by steric
hindrance; in comparison 1,4-benzenediphosphonic acid
readily forms layered structures with most metals.48 Through
PXRD and TGAmeasurements, it was determined that the Zn
complex was stable to loss of B80% of the included DMF
molecules, which inferred porosity. CO2 and N2 sorption
analysis gave BET surface areas of 216 and 209 m2 g�1,
respectively. Upon complete loss of DMF, a loss of order
was observed by PXRD, but order was regained upon resolva-
tion. The free hydroxyl group in this compound represented a
potentially reactive site for post-synthetic modification.
Open-framework phosphonates from mixed ligand systems
The study of novel open-framework metal phosphonates was
accelerated by attaching functional groups to the phosphonic
Fig. 4 The two interpenetrating nets supported by 1-D Sr(PO3)
columns in the Sr2+ structure of tris-1,3,5-(4-phosphonophenyl)-
benzene. Clefts exist between the peripheral aryl rings of like nets.
Fig. 5 Homochiral framework of lanthanide phosphonate material
made using the bulky chiral ligand, 2,20-diethoxy-1,10-binaphthalene-
6,60-bisphosphonic acid. The largest channel has a cross section
of B12 A. The material is able to show interesting sorption and
catalytic activities arising from the microporous nature of the pores.
Color code: Gd-orange; P-purple; O-red; C-grey. The space-filled
model has been superimposed.
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acid ligands.49–52 A new functional group on the organo-
phosphonate ligand would perturb the layered structure in
metal diphosphonates with the result hopefully being a new
3-D open framework. The functional groups attached to the
phosphonic acid to create 3-D open frameworks have included
imino,51 hydroxyl53 carboxylic acid54,55 sulfonic acid56–58 and
pyridine.37,59 This is a large field where many structurally
characterized products with infinite metal ligand connectivity
have formed dense solids. Here, we will present a selected
sampling of those with either open-framework structures or
some interesting functionality.
Stucky and co-workers reported an intriguing open-
framework Zn-carboxyphosphonate, Zn3(O3PCH2COO)2-
(O3PCH2COOH)(NH3CH2CH2NH3)�(BTC).60 This had a
complex, interrupted tetrahedral zeolite-like structure
(Fig. 6) which was markedly different from structures
made with other aliphatic diphosphonates or phosphono-
carboxylates.25–28,31–33,38 The framework had large 24-ring
channels lined by the carboxylate end of the ligand,
H2O3PCH2COOH. The pores were occupied by free benzene-
tricarboxylate (BTC) molecules, which were used in the
synthesis. The structure-directing agents, diprotonated ethylene-
diamine molecules in this case, were not located at the center
of the 24-ring channels, but instead were within the wall that
separated the 24-ring channels. To further probe the effect of
BTC and the role of the carboxylates in the synthesis, three
different carboxylic acids were used in the reaction mixture:
1,2,4,5-benzenetetracarboxylic acid, cis-1,2-cyclohexanedi-
carboxylic acid and cis-1,3,5-cyclohexanetricarboxylic acid.
All of these carboxylic acids led to the formation of the
same framework with the 24-ring apertures. Interestingly, this
particular framework has only been reported in the presence of
templating polycarboxylates. Gas sorption data were not
available.
Ferey and co-workers61 reported the first open-framework
lanthanide-carboxyphosphonates, M4(H2O)7[O2CC5H10-
NCH2PO3]4�(H2O)5 MIL-84 (M = Pr, Y). These isostructural
lanthanide compounds had a three-dimensional open-
framework structure and showed significant thermal stability.
The structure possessed a one-dimensional inorganic sub-
network, built up from chains of edge-sharing rare-earth
polyhedra, interconnected via the organic acids to create an
open-framework structure with small water-filled pores.
X-Ray thermodiffractometry was used to show that the
MIL-84(Pr) was stable up to 523 K and possessed a reversible
hydration–dehydration capability. Gas sorption data were not
available.
Alternatively framework compounds have been made using
pyridyl ligands carrying phosphonate functionalities on their
ring.59,62,63 A series of lanthanide–transition metal compounds
were made using the unsymmetrical 2-pyridylphosphonate
ligands.64 Despite varying the lanthanide metal through
almost the entire lanthanide series the compounds formed
only two different 3-D frameworks. One type was a hydrate
with a chiral framework: Ln2Cu3(C5H4NPO3)6�4H2O I
(Ln = La, Ce, Pr, Nd) and the other was the anhydrous form:
Ln2Cu3(C5H4NPO3)6 II (Ln=Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho).
Structures I had a three-dimensional framework with a
3-connected 10-gon (10,3) topology resembling the Si net of
SrSi2, in which the Ln and Cu atoms are alternately linked by
the phosphonate oxygen atoms. Each 2-pyPO32� ion involved
in the framework served as a tetradentate chelating and as a
bridging ligand. The chiral framework consisted of helical
one-dimensional channels occupied by water molecules and
the solvent-accessible-volume per unit cell was calculated to be
8%. No gas sorption data were reported.
Anhydrous forms II had a complex structure,64 wherein the
Ln centers were connected into a chain exclusively by the
phosphonate groups, whilst the Cu centers coordinated to
both pyridyl and phosphonate groups. The entire three-
dimensional framework could be visualized as being built up
from the inter-linking of Ln chains by the Cu(2-pyPO3) units.
The resulting spaces in the framework were occupied by the
heterocyclic ring backbone of the ligand. Theoretical studies
were carried out to estimate the lattice energy of these
structures and their variation with the ionic radii of the
lanthanide. Interestingly, the correlations of the lattice
energies with the radii of metal ions followed the principle of
lanthanide contraction. Accordingly, the larger ions seemed to
prefer the open-structure capable of accommodating water
molecules, while the smaller ones adopted the denser
anhydrous structure. The calculations also indicated that to
some extent the coordinative versatility of the Cu2+ ion had a
role in determining the framework type.
A relatively longer pyridyl-phosphonate linker was used to
form polar, non-centrosymmetric frameworks by Lin and co
workers.49 Three different compounds formed using ethyl-4-
[2-(3-pyridyl)ethenyl]phenylphosphonate) (L-Et), were
isostructural and crystallized in the noncentrosymmetric space
group Fdd2. They formed a complicated 3D framework
structure composed of [M2(L-Et)4(m-H2O)] building units,
Fig. 6 Open-framework of Zn carboxymethylphosphonate, posses-
sing large 24-membered channels and smaller 8- and 12-membered
rings. The largest channels are lined by the carboxylate end (dark grey)
of the ligand. The larger channels are occupied by free benzenetri-
carboxylate moieties (not shown). Also, the structure-directing
ethylenediamine molecules are within the wall that separates the
24-ring channels. Color code: Zn-cyan; P-purple; N-blue; O-red;
C-grey. The space-filled model has been superimposed.
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far from a layered motif. Within an [M2(L-Et)4(m-H2O)]
building unit, each metal center was connected to the other
metal center through two phosphonate groups and one
bridging water molecule. Each [M2(L-Et)4(m-H2O)] unit was
then linked to four adjacent [M2(L-Et)4(m-H2O)] units via
double L-Et bridges to form a 3D solid. Flexible ethoxy
groups on the L-Et ligands effectively filled all the void spaces
generated within the 3D framework, and these compounds did
not have any included solvent molecules. Although they
crystallized in polar space group Fdd2, all the L-Et ligands
adopted a centrosymmetric arrangement with all their dipoles
cancelling each other. Their polarity resulted from a parallel
arrangement of the bridging water molecules as well as
the phosphonate groups. All these polar compounds
exhibited a second harmonic generation higher than that of
potassium dihydrogen phosphate. In the same article, a
centrosymmetric solid, [Cd(HL)2], was also presented. This
Cd compound had a dense pillared-layered type framework
composed of sinusoidal Cd-phosphonate chains linked by the
pyridyl ligands. No adsorption data were reported on any of
these compounds.
Systematic high-throughput analysis was employed to
screen conditions for growth of a Ln sulfo-phosphonate
network.65 Accordingly, two high-throughput experiments
comprising 96 individual hydrothermal reactions were
performed to systematically investigate the influence of pH,
rare-earth ion, molar ratio of Ln3+ : H3L, and the counterion
in the system LnX3–H3L–NaOH–H2O with X = NO3�, Cl�
and CH3COO�. It was observed that under basic conditions
Ln(OH)3 was formed, while acidic reaction conditions lead to
nine isotypic compounds Ln(O3P–C2H4–SO3)(H2O) with Ln)
La (1), Ce (2), Pr (3), Nd (4), Sm (5), Eu (6), Gd (7), Tb (8) and
Dy (9). These dense pillared-layered 3-D structures were built
up from cross-linking of the Ln-sulfonato-phosphonate layers
by the organic groups. Surprisingly, the only result which
varied was the crystal size of the compounds; the crystal size
increased with decreasing pH and increasing ionic radii of the
lanthanide involved. No significant influence of the
counterions of the rare-earth salts was observed.
Two related Zr4+ compounds have been reported
with bis(aminodiphosphonate) ligands. Zr(HPO3CH2)2N–
C4H8–N(CH2PO3H)2�4H2O66 has a layered architecture
formed by the linking of one-dimensional Zr–PO3 subunits
by the organic spacers. The layers were aligned creating
one-dimensional tunnels (10 � 3.5 A) running along the
interlayer axes, which were occupied by guest water
molecules. An analogous compound where the central
linker was changed to a cyclohexyl ring gave
Zr(PO3CH2)2NC6H10N(CH2PO3)2Na2(H2O)�5H2O, a three-
dimensional open-framework compound. This had a
‘brick-wall‘ type framework formed once again by the linking
of inorganic chains by organic linkers (Fig. 7). The framework
consisted of rectangular channels (12 � 5 A), wherein guest
water molecules and charge balancing Na+ ions reside. This
3D compound showed reversible hydration and dehydration
properties. N2 adsorption studies on this and the butyl
derivative showed negligible surface areas which was attri-
buted to the organic linkers being insufficiently rigid to prevent
structural relaxations occurring during loss of guests.66
None of the compounds presented in this section to this
point have reported gas sorption data to confirm porosity.
There was no report of gas adsorption in non-layered metal
phosphonates until recently. In 2006, MIL-91 (Fig. 8)
which has the proposed formula MX(N,N0-piperazine-
bis(methylenephosphonate)�nH2O (n = 4.5 or 3; MX =
TiO, AlOH) was reported by Ferey.67 These 3-D open frame-
works were built from inorganic corner-sharing chains of TiO6
or AlO6 octahedra linked in two directions via the diphos-
phonate moieties. Small channels (3.5 � 4.0 A2), filled with
free water molecules, were formed in each case. Thermogravi-
metric analysis and X-ray thermodiffractometry of the samples
Fig. 7 Three-dimensional framework of Zr(PO3CH2)2NC6H10N-
(CH2PO3)2Na2(H2O)�5H2O, showing the rectangular channels
(12 � 5 A) present. Color code: ZrO6 octahedra-brown; PO3C
tetrahedra-purple; C-grey; N-blue. The charge-balancing Na+ ions
and guest water molecules in the channels have been removed for
clarity.
Fig. 8 Highly porous framework of the aluminum and isostructural
titanium diphosphonates (MIL-91) formed using N,N0-piperazinebis-
methylenephosphonic acid. View of channels along the c-axis. The
zeolitic structure renders high porosity to the material (B500 m2 g�1).
Color code: Al/Ti-olive green; P-purple; N-blue; O-red; C-grey. The
space-filled model has been superimposed.
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revealed that water was lost reversibly below 423 K and that
both structures were stable up to 463 K. Nitrogen sorption at
77 K gave Langmuir surface areas close to 500 m2 g�1. Strong
Ti–O or Al–O bonds and no interpenetration in the open
frameworks contributed to the thermal stability and relatively
high surface area of these two compounds.
In 2008, Wright and Ferey reported three new phases of
Ni-N,N0-piperazine-bis(methylenephosphonate).68 Structures
of octahydrated (NiSTA-12), dihydrated and anhydrous forms
were determined using synchrotron PXRD measurements.
The structure, shown in Fig. 9, could be broadly described
as helical chains of edge-sharing NiO5N octahedra linked into
a honeycomb arrangement by the ligand. Dehydration of the
parent phase resulted in topotactic formation of the
anhydrous phase which possessed elliptical pores 8 � 9 A in
free diameter. The specific pore volume of dehydrated
NiSTA-12 was B0.21 cm3 g�1, which was comparable to
those in some of the large-pore zeolites (zeolite beta =
B0.22 cm3 g�1). Adsorption measurements were performed
on dehydrated NiSTA-12 using various gases including CO2,
H2, CH4 and CO. At 304 K and 1 atm, the network showed a
10-fold higher uptake for CO2 over CH4. IR measurements
coupled with low-temperature adsorption studies using H2 and
CO as probe molecules, indicated that the Ni2+ centers acted
as weak Lewis acid sites enhancing the interaction of the
adsorbed gases and PQO groups projecting into the pores
were also acting as adsorption sites. Finally, porosity was also
demonstrated using molecular absorption experiments, where
toluene, o-xylene and mesitylene were used as probes giving
uptake of 8.9, 6.2 and 5.8 wt%, respectively.
The use of amino acid derived phosphonates to form
porous architectures resulted in the formation a homochiral
metal-phosphonate solid.69 The chirally pure form (S)-proline,
N-(phosphonomethyl)proline was reacted with a series of
lanthanides to form isostructural solids. These compounds
comprised triple-stranded helical chains wherein the lanthanide
centers were connected via phosphonate units and the
carboxylate end running all around the chains and projecting
into the inter-chain spaces. The chains form strong hydrogen
bonds with other adjacent chains through the carboxylate
groups. Such stacking of the chains result in a three-
dimensional structure with 1D tubular channels of 4.32 �3.81 A free diameter. These channels are occupied by helically
arranged water molecules. The water molecules in the channels
could be removed without collapse of the framework. The
surface area of the Dy compound was determined using N2
adsorption to be 86 m2 g�1, and the compound was shown to
take up water and methanol in a reversible fashion. However,
no adsorption data is available for the other phases.
As the use of a wider R group can exclude the formation of
a simple layer, so can filling the coordination sites on the metal
centers with stronger ligands. The use of ancillary chelating
ligands such as 2,20-bipyridyl, terpyridyl or 1,10-phenathroline
with phosphonate have resulted in lower dimensional
structures.34–37 In particular, the group of Mao has been
studying the effect of ancillary ligation,55–57,63 with both
chelating and bridging ligands, on the networks formed by
metal phosphonates. This had led to the observation of new
inorganic clusters within frameworks.55–57,63 The main
impetus of this research is not so much the generation of
porosity but rather new physical properties stemming from
the inorganic aggregates. Magnetism has been a focus
and much of the research on lanthanide phosphonates is
targeting new luminescence properties.70 As an example, use
of the polyfunctional tetraphosphonic acid ligand, (H8L =
(H2O3PCH2)2NCH2CH2CH2CH2N(CH2PO3H2)2), in combi-
nation with oxalic acid gave rise two different types of
lanthanide-based luminescent three-dimensional frameworks.71
The first type was built up from zigzag LaOx chains
which were interconnected by the phosphonate groups in three
dimensions giving rise to a honeycomb-like open framework
structure with tunnels (4 � 6 A) that are occupied by the
carbon backbone of the organophosphonate ligand (Fig. 10).
The other type made using Nd and Eu were isostructural with
Fig. 9 Framework of nickelN,N0-piperazine-bis(methylenephosphonate)
showing the large open spaces (7–10 A) present within the structure.13
The framework is made up of the linking of homoleptic Ni-phospho-
nate chains by the piperazine backbone. The Co and Fe analogues are
isotypical with this. The pores are occupied by water molecules
(not shown for clarity) and the structure collapses to a denser phase
on dehydration. Color code: NiO6 octahedra-cyan; CPO3 tetrahedra-
purple; N-blue; O-red; C-grey.
Fig. 10 A lanthanum phosphonate/oxalate structure where channels
are defined by crosslinking oxalate and PO3 groups and filled by the
organic linker of the phosphonate.
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a much denser pillared–layered architecture. The La com-
pound with honeycomb-like structure exhibited blue lumines-
cence, while the Nd one showed luminescence in the near-IR
region and the Eu compound showed red luminescence. The
author identified that the use of oxalic acid as a second ligand
provided better control over the crystallization of the
lanthanide phosphonates. Water is also included in the
channel but porosity measurements were not reported.
Metal sulfonates
Metal sulfonates have been examined with transition metals as
potential analogues of metal phosphonates. Under anhydrous
conditions, layered solids reminiscent of a-Zr phosphonates
are obtained and IR spectroscopy can provide a useful
diagnostic for the sulfonate coordination mode.72 Under
hydrous conditions, with transition metals and most hard
metal ions, the most frequent observation is the formation
of ion pairs between fully hydrated metal cations and
sulfonate anions, that is, no true network is formed.73–76 A
key point is that the bonding tendencies of any functional
group in solution, either in molecular complexes or in discrete
assemblies, do not transfer linearly to its coordinative tenden-
cies in network solids. A network solid represents the extreme
case of cooperative bonding interactions between components,
so-called matrix effects, and so what may be a weak inter-
action in a discrete system in solution can have its stability
augmented dramatically in an infinite solid. That said, as will
be shown in the examples to follow, with softer metal ions or
in a hydrogen bonding role, sulfonates can play key roles in
the formation of stable and functional coordination polymers.
Layered and/or dense metal sulfonates
The structure of a prototypical hybrid inorganic–organic solid
is that of rigid inorganic layers that provide a scaffolding of
regular anchor points for pendant organic groups. The
structures of silver p-toluenesulfonate77,78 and silver
benzenesulfonate79 both fit this class of solid but, a closer
look showed that not only were the aryl groups oriented
differently, they were anchored to the layer at different points.
That is, the silver coordination and the layer structure were
fundamentally different in the two compounds. In the benzene-
sulfonate salt, the silver(I) ion adopted a six-coordinate
geometry and the SO3 groups each bridged six different silver
centers (i.e. a saturated m6 mode) whereas in the OTs salt
(OTs = 4-toluenesulfonate also known as tosylate), the
aromatic group was much more tilted and the SO3 group
bridged only five Ag ions. A key observation that was
extracted from these two examples, which diverged from the
pattern observed for metal phosphonates, was that the
inorganic backbone did not provide an inflexible skeleton
upon which the organic groups were merely pendant. In silver
(metal) sulfonates, the organic groups play a structure affecting
role, if not necessarily a structure determining one. The differ-
ences in the two layered solids arose simply from the presence
of the methyl group in the 4-position of the phenyl ring in
AgOTs. The larger inference of this small observation was that
packing and aryl–aryl interactions in the interlayer could affect
the coordination modes of both the silver ions and SO3 groups
in the layer itself.
Given the apparent role of the R group, Cote et al.
performed a study where the breadth of the R group on the
sulfonate was systematically increased and the effect on
structure determined crystallographically.80 To determine this
structural tolerance, it was first necessary to define an ideal
framework as a reference. For this purpose, Ag benzene-
sulfonate, with its m6 coordination mode of the SO3 group,
was chosen. The trace of the unit cell for this compound onto
the inorganic layer gave a rhombus with an area of 23.45 A2.
This was viewed as the area in a plane required for a sulfonate
group to coordinate to silver(I) ions in a simple layered motif.
Taking this area, 23.45 A2, and dividing this value by the
width of an aryl ring (3.66 A), gave 6.41 A. This value
represented, for silver sulfonates, the breadth of an aryl group
that could be accommodated while maintaining a lamellar
structure. This prediction was tested with four different
monosulfonate aromatic R groups, of varying breadth, and
silver(I) giving five complexes: [Ag(4-biphenylsulfonate)]N,
[Ag(2-naphthalenesulfonate)]N, [Ag(H2O)0.5(1-naphthalene-
sulfonate)]N, [Ag(1-naphthalenesulfonate)]N and [Ag(1-pyrene-
sulfonate)]N.80
The compounds with biphenyl and 2-naphthyl appendages,
respectively, had lateral breadths below the calculated
threshold and both formed simple layered solids. For the
1-naphthyl appendage, which exceeded the threshold by a
relatively small amount (6.93 vs. 6.41 A), the Ag sulfonate
backbone was displaced to allow the incorporation of a water
molecule or, under anhydrous conditions, shifted to allow the
formation of Ag–p interactions with half of the naphthyl
groups.80 For the 1-pyrene appendage, which significantly
exceeded the estimated critical breadth to allow a continuum
of SO3-bridged Ag ions (8.14 A vs. 6.41 A), the structure
adapted to a greater extent by forming cation–p interactions
between silver(I) centers and all the aromatic moieties. The
formation of p interactions with the appended arene and the
silver ion necessitated the conversion of 2-D AgSO3 layers into
1-D columns. Based on the structural prediction and the
obtained results, classification for silver sulfonates into three
families was proposed, referred to as Type 1, 2 and 3. Type 1
structures were those sustained exclusively by bonding
between Ag ions and sulfonate oxygen atoms. Type 2 struc-
tures involved a continuum of interactions between Ag ions
and sulfonate oxygen atoms but with ancillary ligation by
additional simple Lewis bases. Type 3 networks involve
coordination between Ag ions and sulfonate oxygen atoms
but with additional coordination of Ag by the p system of the
appended arene. Beyond providing a predictive reference for
the design of silver sulfonates, this work affirmed the structural
role of the organic groups in these networks.
A silver sulfonate was reported by Ma et al. in which
5-sulfosalicylic acid was used as a multifunctional ligand in
order to construct the framework.81 Two structures were
reported, in which both the sulfonate group and the
carboxylate group coordinate to the silver centers. In the first
structure, the 5-sulfosalicylic acid was doubly deprotonated
and in a 2 : 1 metal : ligand ratio, and this formed a 3-D
framework in which the sulfonate, carboxylate and hydroxyl
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group coordinated to Ag centers. The carboxylate groups
served to bridge two unique Ag ions, which were connected
through an Ag–Ag bond. The carboxylate bridged Ag ions
were further connected to two other unique Ag ions through
sulfonate coordination. The second reported structure was a
2-D framework where the metal : ligand ratio was now 1 : 1
and there was no coordination from the hydroxyl group. In
this framework, sulfonate groups served to link Ag centers
into a 1-D coordination polymer and these chains were linked
into two dimensions through bridging carboxylate groups.
Solid-state photoluminescent properties were investigated on
both structures.81
Studies have also been published which investigated the
structure directing role of neutral ligands on Ag sulfonate
frameworks. In one report by Li et al., three sulfonates:
1-naphthalene sulfonate, p-toluenesulfonate and 1,3,6,8-
pyrenetetrasulfonate were used in order to construct silver
frameworks, with pyrazine as the neutral linker.82 The
1-naphthalene sulfonate ligand was also used with hexamethyl-
enetetramine and b-picoline to construct silver frameworks.
It was found that for all the structures, the neutral ligand takes
on a structure directing role, for example the pyrazine bridges
silver centers into 1-D chains, where the chains are linked into
ladders through bridging sulfonates. The divergent pyrene
sulfonate linked the 1-D chains into a 2-D layer. The hexa-
methylenetetramine linked Ag centers into 2-D layers, where
the sulfonate only served to fill a single coordination site on
the silver center and the b-picoline forced the silver to
crystallize as 0-D dimeric pairs which were capped by
sulfonate groups.82
In a separate report by Li et al., further neutral ligands were
used to form frameworks with three mixed ligand mono-
sulfonates.83 The neutral ligands used were 4,40-bipyridine,
1,10-(1,4-butanediyl)-bis(imidazole) and b-picoline, and the
sulfonates used were 5-sulfosalicylic acid, p-aminobenzene-
sulfonate and p-hydroxybenzenesulfonate. Again the neutral
ligands took on the structure directing role and frameworks
formed were either 1-D chains with the bridging neutral
ligands or 0-D dimers with the b-picoline. The sulfonates
served as either non-coordinating counter anions or capped
a single coordination site on the Ag. Photoluminescence was
investigated on the solids in both of the above reports.83
The steric effects of the neutral ligands were also investi-
gated in silver sulfonate frameworks through the use of alkyl-
substituted pyrazine derivatives.84 In this case, two sulfonate
ligands were used, p-aminobenzenesulfonate and 6-amino-1-
naphthalenesulfonate, and five mono- or dialkyl-substituted
pyrazine derivatives were used. Eleven structures were
reported, with eight having the expected 1-D chain motif from
bridging pyrazine units; one chain structure was linked into
2-D sheets through a bridging p-aminobenzenesulfonate unit,
and one was linked into 2-D sheets through the 6-amino-1-
naphthalenesulfonate. One interesting structure had the 1-D
chains linked through the 6-amino-1-naphthalenesulfonate
ligand, with the pyrazine acting as a monodentate ligand on
the Ag center. In general it was found that the pyrazine again
played the major structure directing role, although the steric
effects of the alkyl groups and bulky sulfonate ligand, as well
as p-interactions between the neutral and sulfonate ligands
could disrupt the coordination of the pyrazine. Photolumines-
cence studies were performed on these materials.84 More
comment on predictive trends in metal sulfonates will follow
later in this review when discussing alkaline-earth complexes.
For solids that are strongly bonded in two dimensions, such
as metal phosphonates, as mentioned, a fundamental physical
observation is that the layers can be separated and other
molecular species intercalated between the layers. The pliancy
of the layer structure of silver sulfonates would seem to
preclude that but, to certain extents, this is not the case. In
one example,76 after treatment of AgOTs with nonylamine and
heating to 70 1C, the indexed PXRD pattern gave a unit cell
with two axes, closely related to AgOTs, while the third axis
increased markedly. These data were consistent with retention
of the inorganic layers, albeit with minor rearrangement, and
the expected swelling of the structure. A more detailed study
on the intercalation in silver sulfonates was reported by Cote
et al.80 In this case, single-crystal X-ray structural data of an
actual intercalate complex was obtained which showed the
ethanol solvate formed a coordinative interaction. A series of
alcohols of varying chain length were examined and, by
PXRD and thermogravimetric analysis (TGA), it was
confirmed that the entire series was structurally related with
the guest alcohols adopting an identical coordination mode to
the layer as with ethanol, i.e. the intercalation was topotactic.
Intercalation of amines, at lower loadings, was also
observed for the compound Ag(4-pyridylsulfonate).85 This
solid also formed a layered network but not of a prototypical
hybrid inorganic organic solid. Here, linear silver-pyridine
units were crosslinked by 1-D columns of silver sulfonate
aggregates. Unlike hybrid-inorganic–organic solids, these
layers incorporated the R group rather than it being pendant
to the layers, however, like these solids, the layers formed
continuous sheets. At loadings up to a 1 : 1 amine : L ratio,
Ag(4-PSA) showed intercalation confirmed by PXRD experi-
ments. At higher loadings, a network rearrangement occurred
that necessitated cleavage of Ag–pyridine bonds as confirmed
by solid-state 109Ag NMR.86 This type of structural rearran-
gement has been observed with some Cd sulfonate networks
where the guest amines coordinate the metal ion and displace
sulfonate ligands.87,88
The metal sulfonate examples above have their functions
associated with the accessibility of their interlayer regions.
There are examples of functional metal sulfonate networks
where layered structures are observed and the interlayer is
largely densely packed. Monge et al. have reported a family of
lanthanide sulfonates which act as oxidation catalysts.89 Eight
lanthanide arene disulfonate structures were reported of three
structural types, five isostructural frameworks using a 1,5-
naphthalenedisulfonate (1,5-NDS) as linker, and three struc-
tures of two types using a 2,6-naphthalenedisulfonate
(2,6-NDS) linker. The three frameworks (as Nd salts) were
tested as oxidation catalysts for the conversion of linalool into
cyclic hydroxy ethers. Further, the 1,5-NDS framework
was tested as La, Pr and Nd salts for comparison between
lanthanide ions. It should be noted that the pores in these
solids were not of sufficient size to accommodate the molecules
and the authors confirm that the reactions were occurring
on external surfaces. Of the five frameworks tested, the
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La(1,5-NDS) showed the best conversion (100% in 22 h) with
only a minor selectivity for the furan over the pyran rings
(3 : 2 ratio in all cases). The catalytic activity was dependant
on the coordination number around the Ln centers, with the
3-D 2,6-NDS frameworks with nine-coordinate Ln having the
least catalytic activity (17% in 48 h). These authors have also
reported Co2+ and Ni2+ phenanthroline/1,5-naphthalene-
disulfonate coordination polymers which they examined
as oxidation catalysts for the conversion of thioethers into
sulfones and sulfoxides.90
Kennedy et al. have examined the effect of packing on the
properties of sulfonated dyes.91 In these solids, the crystal
structure affects stability to heat, light and solvents, as well as
the crystal morphology. The morphology in turn affects
binding strength, stability, flow and dispersion properties of
the dyes as a crystalline colorant. In one work, they examined
the Na+ and Ca2+ salts of three sulfonated diazobenzene dyes
where the remote phenyl ring was further substituted with
amine or hydroxyl groups. This work was a nice example of
the flexibility of the SO3 group and how coordination was
influenced by small changes in ligand structure. The SO3
adopted Z1, Z2 or Z3 coordination modes and either linked
the structures into 1-D chains, or left the metal ions as discrete
units. Since the metal coordination spheres were pliant alkali
or alkaline-earth ions, the salts crystallized in frameworks that
maximized intermolecular forces between ligands, while still
filling the coordination sphere of the metal to satisfy ionic
interactions. These subtleties were relevant as the lmax for the
salts was affected by small changes in packing.
The same group reported a more encompassing article92
categorizing a range of sulfonated azo dyes with a range of
s-block metals to generalize the observed structures. Li, Na,
Mg, Ca, K, Rb and Ba were crystallized (Fig. 11) with eight
different m- and p-sulfonated azo dyes for the investigation to
form 43 structures. A subset of 12 of these structures was
discussed in the article in order to form general postulates as to
structure directing factors. It was found that all of the
structures formed alternating layers of organic and inorganic
regions. The harder Mg2+ formed solvent separated ion pairs
in all cases. Ca2+ did as well with some of the m-sulfonated
azo dyes. Ca, Ba and Li typically formed 0-D or 1-D
coordination networks where sulfonates bridged metal centers
in the case of the 1-D networks. Na, K and Rb were found to
form a wider range of networks (0-D, 1-D and 2-D) where
both sulfonate and bridging water molecules served to increase
the dimensionality of the networks. The authors rationalized
the trends through electronegativities of the metal ions,
where the more electronegative metals tended to form less
ionic interactions, hence favoring water coordination.
Conversely, the less electronegative metals tended to form
more ionic interactions thereby disfavoring water and favoring
sulfonate coordination. Previous work by Cote et al. related
similar observations to hard-soft acid base theory.93 Although
the metal plays a significant role in the type of structure
formed, the types and positions of substituents on the ligand
also had a key role in the type of structure formed and affected
the coordination of the SO3 group. These authors followed
this study with related work on alkali and alkaline-earth
complexes of the disulfonated azo dye, Orange G.94
The coordinative tendencies of the sulfonate group have
also been investigated for copper(II) and cadmium(II) ions by
Cai et al. In one study Cu(II) was investigated, where 4,40-
biphenyldisulfonate, and 1,5- and 2,6-naphthalenedisulfonate
were used as the anions.74 Bidentate ethylenediamine deriva-
tives as well as cyclam were used in order to coordinate to the
equatorial positions on the octahedral Cu center, and the
sulfonate groups acted as either uncoordinated counter anions
or as bridging ligands coordinated to the axial positions on the
Cu, forming 1-D chains. It was found that increased steric
bulk on either the amine or the sulfonate would preclude the
formation of a Cu–sulfonate bond, and water would fill the
axial position. As well, decreasing steric bulk on the amine or
sulfonate allowed for favorable hydrogen bond interactions in
the network, which allowed for direct coordination of the
sulfonate to the Cu.
In another study by Cai et al., seven cadmium(II) sulfonates
were reported, with the same sulfonate anions as the copper
study above, as well as 4,40-phenyletherdisulfonate.95 For this
study, 2,20-bipyridine, isonicotinamide and cyclam were used
as neutral ligands. In all cases the Cd center was octahedral
and had at least one coordinated sulfonate group. The neutral
ligands again played a major structure directing role, with the
larger 2,20-bipy ligands forcing 0-D molecular units to form,
and the smaller cyclam and isonicotinamide units causing the
formation of 1-D chains where Cd centers were bridged by
sulfonate units. The absence of neutral ligands with 1,5-
naphthalenedisulfonate allowed for the formation of a 2-D
layered solid in which the sulfonate groups bridged Cd centers
into 1-D chains and the chains were bridged through the
naphthyl groups into a layer. The effects of N-methyl- and
N,N0-dimethylethylenediamine on Cd sulfonates was also
investigated in a separate report, with the same sulfonate
anions as above.96 Again, the dominant structure-directing
ligand was the amine, where equatorially coordinated, octa-
hedral [CdN4]2+ centers were bridged into 1-D chains through
axial disulfonate ligands. Significant hydrogen-bonding inter-
actions between sulfonates and amines on adjacent chains
served to link the chains into 2-D networks.
Fig. 11 Classic observation of close packing R groups as well as non-
coordinating H-bonding sulfonate groups with (a) [Mg(H2O)6]+ and
bridging sulfonate groups with (b) Ca2+ in studies of sulfonated azo
dyes.
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Open-framework metal sulfonates
With the observed versatility of the coordination mode of the
sulfonate to layers of metal ions, an easy approach to open
the structure would seem to be to simply employ pillars for the
layers that would disfavor dense packing. To form a layered
sulfonate with an open pillared-layered structure, Cote et al.
examined the tironate anion, 1,3-disulfo-4,6-dihydroxybenzene.97
The SO3 groups on this ligand do not orient at 1801 and so
it was anticipated that some interlayer porosity would be
created. Indeed, the bent conformation of the pillar resulted
in micropores in the interlayer (Fig. 12). The stability of this
structure was augmented by chelation of the hydroxyl groups
to the Ba centers in addition to the SO3 ligation. The pores
were occupied by water molecules, which was to be expected.
Unexpectedly, the hydrogen atoms of the intrachannel water
molecules were readily observed in the single-crystal X-ray
structure (Fig. 12). This observation led to experiments with
the structurally related guest molecule, H2S, that showed that
upon activation, for every water molecule removed,
0.93 equivalents of H2S were taken up. This reversible H2S
uptake constituted the first illustration of functional porosity
in a layered metal sulfonate. More generally, it showed that
permanent pores could exist in a metal sulfonate material. In
this solid, the structural integrity (TGA showed stability
to 4400 1C) was augmented by a chelating catechol moiety
in the tironate ligand. A later example illustrates that a
homoleptic barium sulfonate could be sufficiently robust to
allow for interlayer chemistry.
The pillaring group, 1,3,5-tris(sulfomethyl)benzene could be
envisioned to open channels between the layers of a metal
sulfonate. Similar structures were obtained with this ligand
and both Ag+ and Ba2+�X�. The silver complex98 was formed
as part of a larger study, akin to opening pores in layered
metal phosphonates, where larger organic cores were screened
for their ability to form open channels but in a metal sulfonate.
The silver(I) complex of 1,3,5-tris(sulfonomethyl)benzene did,
in fact, form a pillared layered solid. The overall structure was
that of sheets of SO3-bridged Ag ions separated by mesitylene
units. Each ligand had one sulfonate group coordinating to
one layer and two sulfonate groups ligating to an adjacent
layer. Notably, the pillars were not densely packed which
resulted in the creation of interlayer void space in which water
molecules were included. The channels had dimensions of
8.50(5) � 4.25(5) A and were occupied by two crystallo-
graphically unique water molecules. Gas sorption was not
reported.
In addition to the mesitylene core already mentioned,
silver(I) networks were reported with a,a0,a00,a0 0 0-durenetetra-sulfonate, to form {[Ag4(L)(H2O)2]}N, and 1,3,5,7-tetra-
(4-sulfonophenyl)adamantane, to form {[Ag4(L)(H2O)2]�1.3H2O}N.98 The sulfonated durene complex formed a
three-dimensional structure but not that of a pillared layered
type. With the durene core, the layers were disrupted and 1-D
inorganic columns resulted. Pores defined in this solid were
small as the columns were realistically separated by only a
Ag-coordinated water molecule. Going to the much larger
tetrasulfonate ligand based on the tetraphenyladamantane
core gave much larger pores. The structure of the silver(I)
complex of 1,3,5,7-tetra(4-sulfonophenyl)adamantane
(Fig. 13(a)) clearly showed that the structure was composed
of 1-D columns of Ag-SO3 aggregates crosslinked in two
dimensions by the ligand. Two types of channel were defined
by the adamantane cores of the ligand. The first was primarily
occupied by coordinated water molecules with approximate
dimensions 7.4 � 5.9 A. The second was occupied by dis-
ordered water molecules and had dimensions of 8.3 � 6.0 A.
The distances between adjacent columns were 11.46(1) and
12.49(1) A. In this structure, guest and coordinated water
molecules could be removed and resorbed but the structure did
not persist in the absence of the guest molecules.
To extract some design principles from the structures of the
durene and tetraphenyladamantane analogues,98,99 the
approach of defining an ideal reference structure was again
employed. As with the monosulfonate study, Ag benzene-
sulfonate was employed as a reference and treated as an ideal
2-D silver sulfonate. The most pertinent parameter for struc-
tural comparison was determined to be the distance between
sulfonate S atoms in the structures as they represented the
anchor points to the layers/aggregates and did not vary greatly
with the identity or orientation of the organic moiety to which
they were linked. This distance in Ag benzenesulfonate
wasB5.2 A. It should be noted that this sulfur–sulfur distance
was somewhat flexible. For example, Ag 2-naphthalene-
sulfonate, maintained a layered motif by the Ag sulfonate
layer rearranging to 4.53(1) and 6.01(1) A to accommodate the
broader pendant group. However, these two values were still
centered roughly on the 5.2 A value. The durene analogue was
viewed as having two sets of meta-xylyl spacers constraining
the structure. Only half of each ligand was crystallographically
unique and so there were only two independent sulfonate
groups. The meta sulfur–sulfur distance in this complex was
Fig. 12 Structure of Ba tironate showing the pillared layer motif. The
H atoms of free intra-channel water molecules are clearly visible
(located in the structure) and indicated a pore well-suited for H2S
inclusion.
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5.835(2) A, considerably above the ideal 5.2 A distance. This
resulted in the observed one-dimensional structure rather than
a two-dimensional network. As a comparison, with the
mesitylene analogue only one side of the ligand was
constrained by the meta-orientation of the sulfonate groups
and so the third SO3 group could effectively function as a
‘‘filler’ to permit the formation of continuous layers. With the
tetraphenyladamantane derivative, sulfonate sulfur atoms
were situated distance of 12.65 A apart, well beyond that
required to preclude a simple layered solid. The design
principles which evolved from this work were that for silver
sulfonate networks, any spacer that would rigidly position
sulfonate groups farther than 5.2 A apart should disfavor a
layered solid and likely form 1-D columns. Going a step
further, one could say that a ligand which imposed such the
same constraint in two dimensions could dictate the formation
of 0-D clusters. This potential was illustrated by the Ba2+
complex of 1,3,5,7-tetra(4-sulfonophenyl)adamantane.99 With
each Ba ion carrying double the charge of a silver ion, half the
metal centers were available to crosslink the ligands resulting
in the first observation of 0-D clusters in metal sulfonate
chemistry (Fig. 13(b)).
As mentioned above, the Ba2+ complex100 of 1,3,5-tris-
(sulfonomethyl)benzene (Fig. 14) had a very similar structure
to that of the Ag+ complex. This was a result of the channels
in the network including a chloride ion. This solid demons-
trated the first example of ion exchange in a metal sulfonate.
In {Ba2[(L)(H2O)5]Cl}, the dimensions of the channels were
approximately 9.1 � 7.9 A and the Cl� ions were supported
only by long H-bonds to coordinated water molecules. Given
the ‘‘free’’ nature of the chloride ion, anion exchange was
examined. As a general comment, it is not typical that metal
sulfonate structures form as bulk solids with mixed anions.
Without exception, the few which do possess a very stable
three-dimensional cationic metal sulfonate skeleton which
requires additional charge compensation. Screening the
BaX2 family with 1,3,5-tris(sulfonomethyl)benzene gave inter-
esting results. With BaF2, the only solid isolable was BaF2
itself. With BaBr2, an isomorphous material to the chloride
complex was obtained. Iodide was too large for the pores and
a different crystalline, yet undetermined, phase was obtained.
Interestingly, when the triacid form of the ligand was
complexed with Ba(OH)2 so that no secondary ion was
present, the mixture remained soluble inferring that an
extended network was not forming. Attempts to exchange
Br� for Cl� in {Ba2[(L)(H2O)5]Cl}, were unsuccessful,
however, F� could be efficiently exchanged. This selectivity
is contrary to that typically observed for ion-exchange
materials.101 In the presence of one equivalent of fluoride
ion, a 75–80% exchange of the chloride ions was observed in
3 h and complete exchange was obtainable in 3 days. This was
confirmed by AgCl gravimetric analysis, PXRD, 19F NMR
spectroscopy, and elemental analysis. In this publication, it
was also proposed that exchange occurred via two-way
passage of halide ions providing intrachannel solvents were
mobile, a condition that was confirmed by NMR experiments
in D2O. A final noteworthy comment concerning the F� ion
exchange is that whereas the F� adduct could be prepared by
ion exchange, it could not be prepared directly from BaF2.
This is perhaps the strongest evidence for a heterogeneous
exchange mechanism with retention of framework integrity.
Fig. 14 Structure of the Ba2+ complex of 1,3,5-tris(sulfonomethyl)-
benzene. Exchangeable Cl� ions are included in the channels. If, in this
depiction, the Cl� ions were replaced with free water molecules, the
image would be an accurate representation of the Ag+ complex of the
same ligand.
Fig. 13 Two complexes of 1,3,5,7-tetra(4-sulfonophenyl)adamantane:
(a) the Ag+ structure showing the 1-D columns (into the page) and
(b) the Ba2+ structure, with ligated dioxane) showing 0-D clusters.
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In keeping with the theme of using larger organic cores to
disperse crosslinking sulfonate groups, Chandler et al.
published two works concerning the use of sulfonated metal
complexes as building blocks for MOFs.102,103 A goal of this
research was to link luminescent metal complexes into a
porous, guest-accessible structure. Both papers reported a
series of lanthanide ions complexed to disulfonated bipyridine
N-oxide ligands. One paper dealt with a series of 3:1
ligand : Ln complexes that were then crosslinked by Ba2+
ions102 and the other dealt with a series of 4 : 1 ligand : Ln
complexes that were bridged by [Na4Cl]3+ aggregates.103 For
the 3 : 1 Ba complexes, all networks were isomorphous and
contained open channels which readily absorbed and desorbed
water accompanied by a sponge-like shrinkage and expansion
of the host. CO2 sorption measurements confirmed micro-
porosity giving a DR surface area of 718 m2 g�1 and an
average pore size of 6.4 A. The luminescent properties of the
lanthanide building blocks were retained in the porous
network solid. From the luminescence data, it was possible
to attribute the sponge-like properties of the network to the
Ba2+ coordination sphere rather than the Ln3+ center.
For the NaCl bridged 4 : 1 complexes,103 the cross-linking
by [Na4Cl]3+ clusters, which resembled ideal faces of the halite
structure, gave a three-dimensional network with two-
dimensional channels. Both coordinated and uncoordinated
water molecules, up to B80%, could be reversibly removed
leaving a rigid and porous framework with a DR surface area
of 426 m2 g�1 as determined using CO2 adsorption. The
permanent microporosity of the framework was also
supported by the energy level splitting in the luminescence
spectra which was maintained in both the hydrated and mostly
dehydrated frameworks indicating minimal variation in the Ln
coordination spheres.
Hydrogen-bonded metal sulfonate complexes
Sulfonates have been observed to participate extensively in
regular hydrogen bonding schemes primarily to metal-bound
water and ammonia. While typically, a H-bonded network
would not fall under the umbrella of MOF chemistry, a few
examples will be presented here as: (1) some contain metal ions
which form coordinate bonds to the sulfonates in addition to
H-bonds; (2) some of these networks demonstrate guest
exchange and one example even presents permanent porosity
as confirmed by reversible gas sorption.
Sulfonate anions are highly complementary with ligated
water molecules in a cis orientation on a metal center.104 We
had shown that the mesitylenetrisulfonate ligand could
perfectly cap the triangular face of a metal octahedron
forming six charge-assisted H-bonds.105,106 Excluding inter-
molecular H-bonds, the divalent and trivalent metal
complexes of this ligand did not form extended structures.
The ligand, hexakis(sulfonomethyl)benzene, could be viewed
as two such mesitylene ligands placed back to back.
With divalent metal ions, H-bonded solids where each
mesitylene-like face capped the triangular face of a metal ion
were observed. However, with Al3+, an unusual network was
formed resulting from each sulfonate group forming both
primary and secondary sphere interactions with the Al centers
(Fig. 15).107 Each SO3 group formed two H-bonds to water
molecules coordinated to an Al center, as expected, however,
the third oxygen formed a bond directly to another Al ion. The
result was the formation of a 3-D open structure sustained by
a combination of primary and secondary sulfonate ligation.
Porosity by gas sorption analysis was not confirmed in
this work.
Much of the inspiration for the work on H-bonded
sulfonate inclusion complexes came from the work on guani-
dinium sulfonates.108,109 These H-bonded solids could be
perceived as expanded versions of metal phosphonates where
the metal ion is replaced by a larger guanidinium cation and
M–O bonds are replaced by longer H-bonds. The net result is
that whereas pillars in metal phosphonates are densely packed,
the guanidinium sulfonates are the most extensive host–guest
inclusion family reported. Based on this work, it was
postulated that a metal hexaamine complex could mimic two
guanidinium cations and form layered networks with inter-
layer void space.110 Trivalent hexaamine cations offer more
inert building blocks but not an ideal charge stoichiometry to
mimic two monovalent cations. Despite that, layered solids
with the ability to reversibly uptake guest molecules have been
reported. The p-xylenedisulfonate complex of [Co(NH3)6]3+
formed both as a hydrate and as p-aniline inclusion
complex.111 A series of PXRD studies confirmed that the
aniline inclusion was reversible provided a trace amount of
water (vapor) was present. The inclusion was shape selective as
1,4-diaminobenzene was also sorbed but not broader diamine
guest molecules.
Further design advances from the previous work led to the
first observation of a permanently porous H-bonded solid.112
The first design advance concerned the stabilization of a
divalent ‘‘hexaamine’’, the ideal charge complement, using
the tripodal ligand tris(aminomethyl)ethane (tame) complexed
to Ni2+. The second involved removing steric congestion
around the methylene linkages of the tame ligand by using
Fig. 15 The channels permeating the [Al(H2O)3]3+ complex of
hexakis(sulfonomethyl)benzene. Each Al center forms both primary
and secondary sphere interactions with the sulfonates.
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sulfoethynyl substituted aryl pillars rather than using an aryl
sulfonate. The net result of these modifications was a highly
insoluble product that, when refluxed with a benzene template,
transformed to the desired network. From PXRD data and
the single-crystal structure of the monosulfonated analogue,
the structure of {[Ni(tame)](4,4 0-bis(sulfoethynyl)biphenyl)}
was determined (Fig. 16). N2 sorption isotherms gave surface
areas of B560 m2 g�1 for Langmuir and DR equations. This
complex made two key illustrations. First, the weaker inter-
actions allowed the network solid to optimize its structure in
the presence of the aromatic guests. Secondly, the solid that
resulted was the first example of a permanently porous solid
sustained exclusively by charge-assisted hydrogen bonds.
In separate reports, Sevov et al. have investigated the effects
of both the guest molecules and the charge of the metal center
in similar hydrogen bonded frameworks. In one report, frame-
works were formed from 4,40-biphenyldisulfonate and
[Co(NH3)6]3+ with a variety of guest solvents; all of the
frameworks also include water as a secondary guest.113 Six
structures were reported, where all exhibited a 3-D structure of
alternating H-bonded [Co(NH3)6]-sulfonate layers bridged
through the organic biphenyl unit. In all of the structures,
the metal : ligand ratio was 2 : 3 and the octahedral metal
centers had their C4 axes oriented perpendicular to the plane
of the layers. The sulfonate pillars adjusted position slightly
depending on the guest in order to accommodate the steric
demands of the guest and maximize hydrogen bonding inter-
actions. In the second report, the charge of the metal center
was adjusted by changing the primary coordination sphere of
Co3+ to include an oxalate anion and ethylenediamine, form-
ing a [Co(en)2(ox)]+ complex.114 The purpose of adjusting the
charge was to give a metal : ligand ratio of 2 : 1, thereby
increasing the space allowed for guest molecules as fewer
pillars were required. Six structures were reported, in all
structures the typical bilayer motif was again observed as
above, although in this case there were H-bond interactions
between metal complexes as well, from the presence of the
oxalate anion. These frameworks were able to accommodate
larger aromatic guest molecules due to the increased space
between organic pillars. An example was also given where two
protonated guests were included and the overall charge of the
framework was adjusted to a 4 : 3 metal : ligand ratio.
There have also been reports of numerous hydrogen-bonded
networks formed from p-sulfonatocalix[n]arenes, systems
containing sulfonated phenoxy groups linked into rings
through methylene bridges. The chemistry of both calixarenes
and sulfonated calixarenes was initially driven by molecular
inclusion phenomena but these molecules have shown
interesting network properties as well. Solid-state structures
of sulfonated calix[n]arene units with a variety of metals,
particularly the lanthanides, have been reported. The first
crystal structure report on a metal p-sulfonatocalix[4]arene
appeared in 1988.115 As well this ligand was used in order to
demonstrate the first aromatic–p hydrogen bonds to water
crystallographically.116 A large amount of work on this subject
was performed in the 1990s, and since then two reviews were
published; the first review highlighted ‘‘Russian-doll’’
complexes formed with p-sulfonatocalix[4,5]arenes and
trivalent cations, and the second reviewed work on
p-sulfonatocalix[4,5]arene metal complexes up to 2001.117,118
The present discussion will briefly cover work done beyond
2001 on p-sulfonatocalix[n]arenes, where n = 4, 5, 6 and 8.
The p-sulfonatocalix[n]arenes where n = 4 or 5 are typically
locked into a bowl-like conformation due to the lack of
flexibility in these systems. These smaller ring systems tend
to encapsulate chelated cationic centers into so-called
‘‘Russian-doll’’ complexes, where each side of the capsule is
connected through a second-sphere coordination to a
hydrated metal cation. The p-sulfonatocalix[4]arenes have also
been characterized to form ‘‘Ferris wheel’’ complexes where a
single sulfonate group will coordinate to a crown-chelated
metal cation. For the most part these structures are dense,
zero-dimensional hydrogen bonded networks with alternating
hydrophilic and hydrophobic layers, formed with di-120,122,123
or trivalent119,121 cations. In some cases where the trivalent
metal cation is large enough, direct coordination of sulfonate
groups is possible and 2-D coordination polymers result.119,121
Larger ringed p-sulfonatocalix[n]arenes where n = 6 and 8
have also been used; in the larger ring systems there is larger
conformational flexibility in the calix[n]arene ring and the
bowl shape present in the calix[4,5]arenes is typically
not observed. Again, the structures containing the larger
p-sulfonatocalix[6]arenes tend to be zero-dimensional hydrogen-
bonded networks.124–126 One-dimensional coordination poly-
mers have also been observed with p-sulfonatocalix[6]arene,
4,40-bipyridine N-oxide and Eu3+, where the Eu3+ centers are
bridged by both the N-oxide and calix[6]arene.126 One struc-
ture has also been reported using p-sulfonatocalix[8]arenes,
where Eu3+ centers are linked by 4,40-bipyridine N-oxide into
a 2-D wavy-brick coordination polymer and the coordination
polymer is extended into three dimensions through coordina-
tion by the calix[8]arene.127 This 3-D structure appeared to be
dense. There is one report of an unusual 2-D coordination
Fig. 16 Carbon dioxide sorption isotherm of hydrogen bonded
{Ni(tame)(4,40-bis(sulfoethynyl)biphenyl)} showing permanent porosity.
Top left inset: Representation of hydrogen bonds (dashed lines)
between sulfonate groups and Ni(tame) clusters. Bottom right inset:
Representation of a model from PXRD of the open pore structure of
Ni(tame). Color code: Ni-cyan; S-yellow; O-red; N-blue; C-grey;
H-white.
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polymer formed with p-sulfonatocalix[4]arene, hexamethylene-
tetramine (hmt) and Ag+, shown in Fig. 17. Here, Ag
coordination by bridging hmt units created layers. One of five
unique Ag centers was coordinated to a sulfonated calixarene.
The layers combined through hydrogen-bonding interactions
creating large channels along one axis of the network that were
filled with solvent water and acetonitrile. Porosity was claimed
but gas sorption data were not given.
A recent report was given by Geremia et al. on a supra-
molecular framework formed from a modified sodium
p-sulfonatocalix[4]arene and a tetracationic porphyrin.128 In
this case the hydroxyl groups on the calixarene have been
modified to have additional carboxylic acid groups attached
through an ether linkage, and meso-tetrakis(4-N-methyl-
pyridyl)porphyrin was used as the porphyrin (Fig. 18). In this
case the calix[4]arene : porphyrin ratio is 4 : 3 with sodium ions
satisfying the remaining charge imbalance. The porphyrin
units occur as staggered trimeric stacks with the middle
porphyrin’s pyridinium groups fitting in the bowls of four
calix[4]arenes. The sodium ions serve to bridge two calix[4]-
arene units through the carboxylate groups, forming a 2-D
layered solid in the xy plane which interpenetrates with the
same 2-D layer in the xz plane, leaving large interconnected
voids in the framework filled with solvent molecules. The
uptake of ZnCl2 and NiCl2 was demonstrated for this frame-
work, with resulting single-crystal to single-crystal trans-
formations observed. Interestingly, due to the stacked nature
of the porphyrins, the divalent metals served only to displace
Na+ from the carboxylate groups capping the calix[4]arene
cones, rather than being coordinated by the porphyrins.
Dynamic behavior in metal sulfonates
In comparison to phosphonates (and even carboxylates), the
trend with sulfonate frameworks is that, with a few exceptions,
they are generally less robust. That said, the spherical and
weaker ligating nature of the SO3 group predisposes the
network to certain degrees of flexibility. For sulfonate
frameworks, this dynamic behavior is more the rule than the
exception. A larger discussion of the types of framework
callisthenics possible with coordination frameworks will
undoubtedly be presented in this special issue and so here,
we will highlight a few examples that show the role sulfonate
ligation can play in solid state dynamics. Our experience has
been that solid-state dynamics are enhanced by pairing
sulfonates with equally pliant metal cations such as d10 centers
or alkaline earth ions.
A silver sulfonate has been reported that showed selective
guest uptake through a structural rearrangement.129 The
network [Ag(3-pyridylsulfonate)(MeCN)0.5], formed 24-
membered rings with exclusively pyridine and sulfonate ligated
silver centers. The rings packed in an eclipsed fashion to give
channels containing the MeCN guests. Heating to remove the
MeCN brought about conversion to a denser still crystalline
phase. Treatment of this phase with MeCN liquid or vapor
brought about a reversion to the nascent structure. This work
presented X-ray structures of the dry network and the MeCN
solvate but, most importantly, offered a mechanism of inter-
conversion based upon this data (Fig. 18). The macroscopic
analogy of the mechanism was that of a cardboard box folding
sideways and this was confirmed by site occupancies and
changes in coordination spheres of the Ag ions. Fundamental
to the conversion was the ability of the SO3 group to offer
simultaneous multidirectional ligation and the d10 silver(I) ion
to accommodate the structure shift in its coordination sphere.
Notably, no Ag–py bonds needed to be cleaved, only the
weaker Ag–O (sulfonate) bonds. This type of behavior led to
the analogy of the ligating ability of a sulfonate group with a
‘‘Ball of Velcro.’’ Beyond the reversible MeCN sorption
observed for this system, selectivity with other structurally
similar molecules was examined; propionitrile, MeOH, EtOH
or THF were not taken up.
Fig. 17 Open-framework structure made using hexamethylenetetramine,
p-sulfonatocalix[4]arene and Ag+. Two layers are represented,
with all non-coordinating solvent molecules removed. Color code:
Ag-pink; S-yellow; O-red; N-blue; C-grey.
Fig. 18 Crystal structures of the solvated (left) and dry (right) forms
of Ag(3-pyridylsulfonate); MeCN has been removed from the channels
on the left to show the proposed rearrangement that occurs in the solid
state to covert to the dry phase in a crystal-to-crystal transformation.
All metal ions are Ag+ and the different colors represent crystallo-
graphically unique centers.
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Chandler et al. reported the Ba2+ salt of 1,3,5-trisulfono-
benzene which is a solid that undergoes multiple single-crystal
to single-crystal transformations while trapping and releasing
gas molecules (Fig. 19).130 An initial structure of a highly
hydrated phase (17.5% water) was presented which had 1-D
channels permeating it. Single-crystal structures were also
presented which showed the pores collapsing at 12% hydra-
tion and then a dense phase at 3% hydration. All water
molecules in the 3% structure were Ba-ligated. Further heating
to remove ligated water molecules gave a structure that
revealed that the sulfonate groups had rotated in order to
maintain oxygen atom ligation to the Ba centers; the Ba
centers were eight-coordinate in all four structures. This
rearrangement necessitated shifting of components and the
formation of ‘‘Ba-belted’’ closed pores between ligand
molecules. This anhydrous phase was observed to release
bubbles of gas upon rehydration to reverse the structural
transformation. The bubbling observed from the network
was an exceptionally odd phenomenon and merited closer
examination. Varying the atmosphere to CO2, O2, N2, CH4,
Ar, or NH3 during heating to the anhydrous phase yielded
solids which released gas on rehydration. To confirm that the
bubbles of gas were in fact originating from the external
atmosphere a colorimetric experiment was performed.
Crystals of the hydrated Ba sulfonate were fully dehydrated
in NH3(g) then further heated under vacuum to liberate
externally adsorbed NH3. The dried crystals were then placed
in an aqueous Co(NO3)2 solution. The Co2+ ions readily
adsorbed to the crystal’s surface to give a pink color. This
coincided with release of gas from the crystal as water
rehydrated the Ba ions and opened the closed pores.
In B2 min, the crystals changed color from pink to a deep
green characteristic of hexaammine Co(II). As any externally
sorbed gases would have been removed in the desorption cycle,
the only source for NH3(g) was the interior of the solid,
confirming that the external atmosphere was trapped during
dehydration. With this material, any gas, with the exceptions
of H2 and He, composing the external atmosphere during the
dehydration was captured and stored. The main illustrations
from this particular work were that a sulfonate solid can be
truly adaptable leading to facile transformations between
different structures. The second, more practical illustration
concerns the gas storage. The vast majority of molecular
materials for storing gases rely on either physisorption or
chemisorption of gases as their means of storage. A dichotomy
in the storage problem is that materials which store gases
efficiently typically do not release them rapidly, and conversely,
facile release is often correlated with ineffective storage at
elevated temperatures. This network presented a solid able to
absorb the ambient atmosphere and store it at 150 1C under
vacuum while providing instantaneous release of the gas at
room temperature simply by addition of water to the solid.
Ribas et al. reported a material with a radical sulfonate
ligand that displayed single-crystal to single-crystal
conversion.131 The ligand, monosulfonated polychlorotriphenyl-
methane, was oxidized by treatment with I2 into the radical
species. Two copper complexes were reported. The first
had the ligand H-bonding via the sulfonate group to
[Cu(py)2(H2O)4]2+ units and the second had the sulfonate
ligand directly coordinating to axial sites of a Jahn–Teller
distorted [Cu(cyclam)]2+ unit. Ethanol molecules were
included in both structures that, upon removal, brought about
single-crystal to single-crystal transformations involving
reorientation of the organic radical. The cyclam system, with
direct ligation of the radical, showed magnetic interaction
between spins however, this was still a relatively weak
magnetic coupling. The weak interaction was attributed to
poor magnetic exchange through the Z1-SO3 group as the
carboxylate analogue showed a markedly stronger magnetic
interaction.
Hu et al. reported the synthesis and adsorption properties of
a linear polymer built from [Cu(2-pyridylsulfonate)2] units
linked via 4,40-bipyridine in the axial sites.132 The chains
packed to form layers while the chains in adjacent parallel
layers were rotated by 901. There were p–p and C–H–pinteractions between adjacent layers but there were still 1-D
pores (5.27 � 4.92 A) present filled with water. The framework
could be desolvated and subsequently resolvated with water,
MeOH or iPrOH vapour all via single-crystal to single-crystal
transformations. The desolvated network could also adsorb
small amounts of benzene and toluene, with selectivity for
smaller guests.
Summary and outlook
The examples presented in this article span a broad range of
structure topologies, inorganic building units, and potential
functions. Here, we will attempt to make some generalizations
from the presented works and from our own experience.
Fig. 19 (Top left) Crystallographic representation of the ‘‘Ba-belted’’
closed pore in Ba 1,3,5-trisulfonobenzene with simulated gas mole-
cules. (Top right) Crystallographic representation of the open-pore Ba
salt with water molecules removed from the channels. (Bottom left)
Photograph of dehydrated single crystals of the Ba salt immediately
after submersion in hydrated paratone oil. (Bottom right) Photograph
of dehydrated single crystals of the Ba salt after readsorption of water
in hydrated paratone oil and the subsequent release of trapped gas.
Color code: Ba-light grey; S-yellow; O-red; N-blue; C-dark grey.
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From the synthetic perspective, for both phosphonates and
sulfonates, the spherical ligating ability of the functional group
makes it easier to extend coordination in three dimensions.
For phosphonates, this is a bit of a double-edged sword as,
while this facilitates the formation of network structures, it
also can lead to insoluble precipitates with low degrees of
order. For phosphonates, especially factoring in their variable
states of protonation, kinetic control of self-assembly,
particularly pH, is crucial. For sulfonates, amorphous
materials are not typically observed. In this case, the possible
variations in ligating mode of the sulfonate are more
manifested as lower degrees of predictability and possibly
the observation of multiple polymorphs.133 That said, there
are general trends one can extract regarding the assembly of
metal phosphonates and sulfonates:
- With the exception of the smallest R groups, for simple
mono- or diphosphonate/sulfonates, layered solids are the
default structure.
- Each layered structure has a certain critical threshold
regarding the breadth of the pendant R group. Organic groups
that exceed the threshold will render the layer structure
impossible and so the assembly will default to the next highest
order of structure, 1-D columns.
- With homoleptic phosphonates and sulfonates, it is
difficult to extrapolate the concept of secondary building units,
as isolable discrete clusters are rarely observed and not regular
in their structures when they are. That said, in some examples
there are clearly infinite SBU motifs which would be expected
to persist with simple modification/extension of the organic
linkers. The fact that new inorganic aggregates are not
uncommon is actually of interest from the perspective of
new electronic/magnetic materials.
- Being sustained more typically by inorganic skeletons with
1-D or 2-D structures, interpenetrated structures are much less
common with phosphonates and sulfonates.
- For sulfonates, hard metal ions remain highly hydrated
and typically zero- or one-dimensional structures results.
Softer metal ions are much better bonding partners.
Regarding the potential applications of phosphonate
and sulfonate MOFs, of course new materials with regular
micropores would be of interest for gas storage or separations.
In contrast to other materials, both phosphonate and
sulfonate MOFs would offer a much greater likelihood of
forming polar pores.134 This would translate to different
selectivity than observed for non-polar pores that are much
more typically observed in MOFs. With sulfonates, their
forte appears to be less in the domain of permanently porous
solids, despite the H-bonded example, and more in the
realm of dynamic and switchable materials. Weaker ligation
coupled with a range of coordination modes is an ideal recipe
for structural dynamics so this is an application where one
would expect sulfonate solids to excel. Another potential
application of materials with polar pores would be ion
conductors. Within the pore size domains of MOFs, one could
envision conduction of protons or Li+ ions. This aspect has
not been extensively studied in MOFs but, given that some
structural mobility in the solid state should augment ion
conduction, a material such as a sulfonate MOF would be
an intriguing candidate.
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