A di-copper(II) bis-tren cage with thiophene spacers as receptor foranions in aqueous solution

Valeria Amendola a, Luigi Fabbrizzi a,*, Carlo Mangano a, Piersandro Pallavicini a,Michele Zema b

a Dipartimento di Chimica Generale, Universita di Pavia, Via Taramelli 12, 27100 Pavia, Italyb Centro Grandi Strumenti, Universita di Pavia, Via Bassi 6, 27100 Pavia, Italy

Received 22 February 2002; accepted 30 April 2002

Dedicated to Professor Karl Wieghardt

Abstract

The system made of the bis-tren octaamino cage ligand with thiophene spacers (3) and CuII (1:2 molar ratio) has been studied in

aqueous solution by means of potentiometric titrations. Both protonation and formation constants of the metal-containing species

were determined, so that the system can be fully described at any pH value. 6.9 was chosen as the best pH value for the system to

work as receptor for a series of bidentate anionic species: this is the lowest value at which only dimetallic complexes exist. In

particular, [Cu2(3)(OH)]3� is the species prevailing by far at this pH, so that a displacement equilibria takes places on binding

anions, [Cu2(3)(OH)]3��/A��/[Cu2(3)(A)]3��/OH�. The binding process was followed by means of spectrophotometric titrations

and, for the anions bound by the system, a Kobs was determined, related to the constant of the displacement equilibrium by K�/

Kobs[OH�]. A log Kobs value of 6.75 (9/0.09) was determined for N3�, 4.79 (9/0.07) for NCO�, and 2.72 (9/0.08) for NCS�.

SO42�, NO3

�, HCO3�, CH3COO�, Cl�, Br� and I�, were instead not bound by the system or bound with log Kobs values �/2.

The crystal and molecular structure of the complex [Cu2(3)(N3)]3� was also determined, which evidenced the intrinsically increased

length of the cage and of its CuII2 complexes, related to the large atomic radius of the S atom of the thiophene spacers, which

prevents the binding of monoatomic anions (halides).

# 2002 Elsevier Science B.V. All rights reserved.

Keywords: Crystal structure; Kinetics and mechanism; Copper complexes; Amino cage ligand complexes; Dinuclear complexes

1. Introduction

Bis-tren octaamino cage ligands (tren�/N ,N ?,Nƒ-tris-

(2-aminoethyl)amine) can host two metal cations, one

for each tren subunit. These complexes can further work

as anion receptors, provided that the anion has two

donor atoms or two non-bonding electron couples on

the same atom: the envisaged anion can act as a bridge

between the two metal cations (Scheme 1) if their

coordination geometry is not completely fulfilled by

the tetraamine donor set of the tren ligand. To this aim,

Cu2� appears as the best suited cation, as it prefers

trigonal-bipyramidal geometry and, when bound by

tren, one of its coordination sites remains free, posi-

tioned along the tertiary amine-copper axis. Although

some bis-tren dicopper(II) complexes containing brid-

ging anionic ligands have been described and their

crystal structures determined [1], a few works tried to

determine the stability constants in solution for the

inclusion process of a particular anion or, even better, to

observe if any selectivity can be found along a series of

anionic species [2]. Indeed, only two complete thermo-

dynamic studies have been published so far, in which the

behaviour of the dicopper(II) complexes of ligands 1

and 2 as receptors was examined in aqueous solution for

a variety of anions [1d,3]. With the rigid ligand 1, a

selectivity based on the bite of the bidentate anion (i.e.

on the distance between its donor atoms) was found,

* Corresponding author. Tel.: �/39-0382-507 325; fax: �/39-0382-

528 544

E-mail address: fabbrizz@unipv.it (L. Fabbrizzi).

Inorganica Chimica Acta 337 (2002) 70�/74

www.elsevier.com/locate/ica

0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 1 0 2 9 - 0

and only triatomic or larger anions (e.g. N3� or

HCO3�) were bound [3]. With the more flexible ligand

2 also monoatomic anions (halides) were bound at

selected pH, with fairly large binding constants, but

the dicopper complex appeared suitable also for a large

number of different anions, independently on their bite

or shape [1d]. In this work we describe the behaviour of

ligand 3 in aqueous solution, as regards its coordinating

tendencies towards Cu2� and we report about theability of its dimetallic copper complexes to further

bind a series of anions.

2. Experimental

Ligand 3 was prepared as described [4]. Setup and

procedures for spectrophotometric and potentiometrictitration experiments have already been reported [5].

Equilibrium constants were determined by non-linear

least-squares method, through the HYPERQUAD pro-

gram package [6]. Crystal data and details on the

crystallographic study are reported in Table 1.

A small crystal (0.10�/0.18�/0.28 mm), showing very

low diffraction effects, was submitted to X-ray single-

crystal diffraction analysis. Intensity data were obtainedon an Enraf-Nonius CAD4 diffractometer, using gra-

phite monochromated Mo Ka radiation. Unit cell

parameters were obtained by least-squares fitting of 25

centred reflections monitored in the range 3.83B/uB/

11.918. Calculations were performed with the WinGX-

97 software [7]. Corrections for Lp and empirical

absorption were applied [8]. The structure was solved

by direct methods (SIR-92) [9] and refined by full-matrixleast-squares using SHELXL-97 [10] with anisotropic

displacement parameters for all non-hydrogen atoms.

Hydrogen atoms were inserted in calculated positions

with isotropic displacement parameters proportional to

those of their neighbouring atoms and not refined.

Rotational disorder for one perchlorate group was

detected and two alternative positions for the oxygen

atoms were refined. Atomic scattering factors weretaken from International Tables for X-ray Crystallogra-

phy [11]. Diagrams of the molecular structures were

produced by the ORTEP program [12].

3. Results and discussion

The binding tendencies of the thiophene-containing

ligand 3 towards protons and Cu2� cations (ligand/

metal 1:2 molar ratio) were investigated in aqueous

solution by means of potentiometric titration experi-

ments. Treatment of the titration data with non-linear

regression methods [6] allowed us to determine the

formation constants for the protonated and metal-

containing species [13], and, from these data, we were

able to draw a distribution diagram, in which the

percent of each species is plotted as a function of pH

(Fig. 1).

Three di-copper species are present in the 5�/12 pH

range, [Cu2(3)]4�, [Cu2(3)(OH)]3� and [Cu2(3)-

(OH)2]2�. The formation of [Cu2(3)(OH)]3� takes place

with a pKa of 6.06 for the water molecule subject to

deprotonation [14], while the second hydroxy group

forms with a pKa of 7.75, this reflecting the diminished

overall positive charge of the system. Slow evaporation

of a solution buffered at pH 6.9 (where [Cu2(3)(OH)]3�

is prevalent) yielded a green microcrystalline powder

which analyzed as [Cu2(3)(OH)](ClO4)3 �/3H2O and

Scheme 1.

Table 1

Crystal and refinement data

Empirical formula C30H48Cl3Cu2N11O12S3

Formula weight 1084.4

T (K) 298(3)

Crystal system hexagonal

Space group /P62c/

a (A) 9.382(3)

c (A) 27.947(11)

V (A3) 2130.4(13)

Z 2

Dcalc (g cm�3) 1.690

m (mm�1) 1.405

Scan type v �/2u

u Range (8) 2�/30

Absorption correc-

tion method

c -scan

Index ranges �65h 56, 05k 513, 05 l 539

Reflections measured/

unique

6380/2123 (Rint�0.2528)

Refinement type F2

Refined parameters 102

R1a 0.0912

Rall 0.2202

wR2 0.2800

Weighting scheme w�1/[s2(Fo2)�(0.1190P )2�0.00P ] where

P� (Fo2�2Fc

2)/3

(shift/e.s.d.)max 0.001

Goodness-of-fit b 0.937

Max./min. Dr(e A�3)

0.397 and �0.550

a R1�a ½½Fo½�½Fc½½/a ½Fo½ (calculated on 721 reflections with

I �2s1).b Goodness-of-fit�S� [a [w(Fo

2�Fc2)2]/(n�p )]0.5 where n is the

number of reflections and p is the total number of parameters refined.cFor inverted structure 0.993(20) and R1�0.0639.

V. Amendola et al. / Inorganica Chimica Acta 337 (2002) 70�/74 71

showed a sharp IR absorption at 3571 cm�1. Both pKa

and IR data suggest that the coordinated hydroxy anion

is not bridging the two copper atoms. On the basis of the

obtained distribution diagram, it can be chosen the best

pH value to make the system ligand 3�/2Cu2� to work

as anion receptor. [Cu2(3)]4�, i.e. the ‘void’ dimetallic

species, appears as the best suited to bind anions, as it

gives rise to equilibrium (1)

[Cu2(3)]4��A�� [Cu2(3)(A)]3� (1)

where A� is the chosen anion, while in the case of the

hydroxy-containing species diplacement equilibria (2)

and (3) hold, in which the bulk concentration of OH�

disfavours anion binding.

[Cu2(3)(OH)]3��A�� [Cu2(3)(A)]3��OH� (2)

[Cu2(3)(OH)2]2��A�� [Cu2(3)(A)]3��2OH� (3)

Unfortunately, [Cu2(3)]4� reaches a maximum of only

�/45% at pH 5.9, a pH at which (see Fig. 1) large

amounts of monometallic species exist ([Cu(3)H]3�,[Cu(3)H2]4� and [Cu(3)H3]5� represent the 10, 20 and

5%, respectively). Thus, to study the behavior of the

system made of ligand 3 plus two Cu2� as a receptor for

anions, we chose pH 6.9: this is the lowest pH value at

which no more monometallic species exist and, interest-

ingly, it is also the value at which [Cu2(3)(OH)]3�

reaches its maximum abundance (�/80%), thus repre-

senting the species prevailing by far in solution. Anionbinding according to equilibrium (2) has thus been

studied on solutions of ligand 3�/2Cu2� buffered at

pH 6.9, which are pale green and display in the visible

region a shoulder at 366 nm (o�/1490 M�1 cm�1) and

two d�/d bands at 694 nm (o�/195 M�1 cm�1) and 820

nm (o�/230 M�1 cm�1). Displacement of OH� and

binding of the chosen anion were followed by means ofspectrophotometric titrations, by additions of substoi-

chiometric quantities of A�. The examined anions were

N3�, NCO�, NCS�, Cl�, Br�, I�, SO4

2�, NO3�,

HCO3�, and CH3COO�. In some titrations the growth

of new bands was observed, which are consistent with

the well known MLCT bands typical of the Cu2�-anion

interaction with the chosen species. In particular, a band

centred at 440 nm (o�/2020 M�1 cm�1) and at 358 nm(shoulder, o�/1759 M�1 cm�1), were observed in the

case of N3� and NCS�, respectively. In all the other

cases, the binding process was instead followed thanks

to the shift and change of intensity of the bands of the

starting [Cu2(3)(OH)]3� species.

By choosing a wavelength at which significant varia-

tions were observed and by plotting the absorbance

versus quantity of added anion, titration profiles wereobtained for N3

�, NCS� and NCO� which sharply

indicates that at [Cu2(3)(OH)]3�/anion 1:1 molar ratio

the process is complete. Complexation constants were

calculated from the spectrophotometric data [1d,6]:

what is determined is a conditional constant, Kobs,

which is a function of the OH� concentration (main-

tained constant by the buffer) and related to the

authentic constant for equilibrium (2), K , by K�/

Kobs[OH�]. The calculated log Kobs values are 6.75 (9/

0.09) for N3�, 4.79 (9/0.07) for NCO�, 2.72 (9/0.08) for

NCS�. In addition, the absorbance of the MLCT band

centred at 440 nm was examined for the system ligand 3/

2Cu2� plus a five-fold excess of N3� as a function of

pH. What was found (Fig. 1, black triangles) is a curve

which displays its maximum at pH 6.9, confirming that

this pH value is the best suited for binding anions withthis system (decrease of absorbance on increasing pH is

due to the unfavourable role played by OH� in

equilibria (2) and (3)). On the other hand, no variation

or almost negligible linear variations (Abs vs. added

anion) were observed in the solution spectra of the

dicopper complex (pH 6.9) on addition of SO42�,

NO3�, HCO3

�, CH3COO�, Cl�, Br� and I�, up to

a complex/anion molar ratio of 1/3. This indicates thatthese anions are not coordinated by the dicopper

receptor at pH 6.9, or that are bound with very low

constants (log KobsB/2). This result is significantly

different with what found with the analogous system

2/2Cu2� [1d], for which fairly high complexation

constants were found also for monoatomic halide

anions, which formed 1:1 complexes characterized by

strong MLCT bands. To check if at any pH valuehalides may be bound by the dicopper complex of ligand

3, spectra of the system 3/2Cu2�/10X� (X�/Cl, Br)

were taken in the 2�/12 pH range. However, no new

bands were found at any pH and, in addition, the

Fig. 1. Distribution diagram: % of species (left axis) vs. pH for the

system 3�/2Cu2� (ligand concentration�/10�3 M). The curves

relative to dicopper(II) species are explicitly labelled on the diagram.

For the other species: [(3)H6]6� curve a; [Cu(3)H3]5� curve b;

[Cu(3)H2]4� curve c; [Cu(3)H]3� curve d. Black triangles: molar

absorbance at 440 nm (right axis) vs. pH for a solution of 3�/2Cu2��/

5N3� (ligand concentration�/10�3 M).

V. Amendola et al. / Inorganica Chimica Acta 337 (2002) 70�/7472

obtained spectra were superimposable to those found at

the same pH in the absence of halide, this indicating that

halide binding does not take place at all.

Crystals of [Cu2(3)N3�](ClO4)3 were obtained by slow

evaporation of a solution of 3�/2Cu2��/N3� buffered

at pH 6.9, and the crystal and molecular structure was

solved by diffraction methods (Fig. 2). As expected,

N3� is bridging the two copper cations with an almost

linear Cu�/NNN�/Cu geometry, since all the atoms lie

on the threefold axis. While Cu�/N distances are

comparable to what found e.g. in the case of dicop-

per(II)/N3� complexes of ligand 1 [1b], it is interesting

to note that the Cu�/Cu distance is quite long (6.150(3)

A), if compared[1d] with 6.10 A found for [Cu2(1)N3]3�

or even better with 3.87 A found for [Cu2(2)Br]3�.

Moreover, the distance between the carbon atoms

adiacent to the thiophene ring (C3 and C3i in Fig. 2)

is 5.129(37) A. Most probably due to the increased

atomic radius of the S atom, this value is significantly

longer than the distance between the two carbon atoms

of the methylene groups adiacent to the cyclic spacer

which can be calculated (on the basis of the published

crystal structures of the free or protonated ligands [15]

and of their metal complexes [1,16]) in the case of 1 and

2 (5.001(38) and 4.863(24) A, respectively). Thus,

although presumably as elastic as those of ligand 2,

due to the intrinsically enhanced length of the cage

ligand the dicopper(II) complexes of 3 cannot contract

enough to bind monoatomic anions in an advantageous

bridging mode, this suggesting an explanation for the

striking difference of the system 3/2Cu2� with respect to

2/2Cu2�. Moreover, the increased length of the cage,

coupled with a lack of a fixed Cu�/Cu distance (as with

1) results in the satisfactory binding of only those

bidentate anions which display an intrinsically high

affinity towards the Cu2� cation coordinated by the

tren unit [3].

4. Supplementary material

Listings of final atomic coordinates, anisotropicthermal parameters, all bond lengths and angles, inter-

molecular contacts and unit cell and packing diagrams

for the crystal and molecular structure are available as

CIF file. Crystallographic data for the structural analy-

sis have been deposited with the Cambridge Crystal-

lographic Data Centre, CCDC No. 179656 for

compound [Cu2(3)N3�](ClO4)3. Copies of this informa-

tion may be obtained free of charge from The Director,CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK

(fax: �/44-1223-336-033; e-mail: deposit@ccdc.cam.a-

c.uk or www: http://www.ccdc.cam.ac.uk).

Acknowledgements

Thanks are due for the financial support to MURST

(Ministero italiano dell’Istruzione, dell’Universita e

della Ricerca) and to the Univerisita di Pavia (FAR,Fondo di Ateneo per la Ricerca).

References

[1] (a) R. Menif, J. Reibenspies, A.E. Martell, Inorg. Chem. 30 (1991)

3446;

(b) C.J. Harding, V. McKee, J. Nelson, Q. Lu, J. Chem. Soc.,

Chem. Commun. (1993) 1768;

(c) C.J. Harding, F.E. Mabbs, E.J.L. MacInnes, V. McKee, J.

Nelson, J. Chem. Soc., Dalton Trans. (1996) 3227;

(d) V. Amendola, E. Bastianello, L. Fabbrizzi, C. Mangano, P.

Pallavicini, A. Perotti, A. Manotti Lanfredi, F. Ugozzoli, Angew.

Chem., Int. Ed. Engl. 39 (2000) 2917.

[2] (a) V. Amendola, L. Fabbrizzi, C. Mangano, P. Pallavicini, A.

Poggi, A. Taglietti, Coord. Chem. Rev. 219�/221 (2001) 821;

(b) R.J. Motekaitis, A.E. Martell, I. Murase, J.-M. Lehn, M.W.

Hosseini, Inorg. Chem. 27 (1988) 3630;

(c) R.J. Motekaitis, A.E. Martell, J.-M. Lehn, E. Watanabe,

Inorg. Chem. 21 (1982) 4253.

[3] L. Fabbrizzi, P. Pallavicini, L. Parodi, A. Taglietti, Inorg. Chim.

Acta 238 (1995) 5.

[4] M.G.B. Drew, C.J. Harding, O.W. Howarth, Q. Lu, D.J. Marrs,

G. Morgan, V. McKee, J. Nelson, J. Chem. Soc., Dalton Trans.

(1996) 3021.

[5] V. Amendola, L. Fabbrizzi, C. Mangano, P. Pallavicini, A.

Perotti, A. Taglietti, J. Chem. Soc., Dalton Trans. (2000) 185.

[6] P. Gans, A. Sabatini, A. Vacca, Talanta 43 (1996) 1739.

[7] L.J. Farrugia, WINGX-97, An integrated system of publicly

available windows programs for the solution, refinement and

analysis of single-crystal X-ray diffraction, Glasgow, UK, 1997.

[8] A.C.T. North, D.C. Philips, F.S. Mathews, Acta Crystallogr., A

24 (1968) 351.

[9] A. Altomare, G. Cascarano, C. Giacovazzo, A. Gualardi, J. Appl.

Cryst. 26 (1993) 343.

Fig. 2. Molecular structure of [Cu2(3)N3�](ClO4)3. Perchlorate

anions have been omitted for clarity. Selected bond distances and

angles: Cu1�/N1 2.018(12); Cu1�/N2 2.160(11); Cu1�/N3 1.937(15);

N3�/N4 1.138(15) A; N1�/Cu1�/N2 84.9(3)8; N2�/Cu1�/N2ii 119.2(1)8;N2�/Cu1�/N3 95.1(3)8. Symmetry codes: (i) x , y , �/z�/1/2; (ii) �/y , x�/

y , z ; (iii) �/x�/y , �/x , �/z�/1/2; (iv) �/y , x�/y , �/z�/1/2; (v) �/x�/y ,

�/x , z .

V. Amendola et al. / Inorganica Chimica Acta 337 (2002) 70�/74 73

[10] G.M. Sheldrick, SHELXL-97, Programs for Crystal Structure

Analysis, University of Gottingen, Gottingen, Germany, 1998.

[11] International Tables for X-ray Crystallography, vol. 4, Kynoch,

Birmingham, England, 1974, pp. 99�/101 and 149�/150.

[12] L.J. Farrugia, J. Appl. Cryst. 30 (1997) 565.

[13] [3H]� log K�/8.96 (9/0.01); [3H2]2� log K�/17.50 (9/0.01);

[3H3]3� log K�/25.25 (9/0.01); [3H4]4� log K�/32.47 (9/0.01);

[3H5]5� log K�/39.15 (9/0.01); [3H6]6� log K�/45.63 (9/0.01);

[Cu(3)H3]5� log K�/34.53 (9/0.02); [Cu(3)H2]2� log K�/29.07

(9/0.02); [Cu(3)H]3� log K�/22.62 (9/0.02); [Cu2(3)]4� log K�/

21.04 (9/0.02); [Cu2(3)(OH)]3� log K�/14.98 (9/0.02);

[Cu2(3)(OH)2]2� log K�/7.23 (9/0.02).

[14] In this kind of complexes, the [Cu2(ligand)]4� species is thought

to be not ‘empty’ but with water molecules coordinating the

copper centres.

[15] (a) F. Arnaud-Neu, S. Fuangswasdi, B. Maubert, J. Nelson, V.

McKee, Inorg. Chem. 39 (2000) 573;

(b) G. Morgan, V. McKee, J. Nelson, J. Chem. Soc., Chem.

Commun. (1995) 1649;

(c) M.G.B. Drew, V. Felix, V. McKee, G. Morgan, J. Nelson,

Supramolecular Chem. 5 (1995) 281;

(d) S. Mason, J.M. Llinares, M. Morton, T. Clifford, K. Bow-

man-James, J. Am. Chem. Soc. 122 (2000) 1814;

(e) M.J. Hynes, B. Maubert, V. McKee, R.M. Town, J. Nelson, J.

Chem. Soc., Dalton Trans. (2000) 2853;

(f) S. Mason, T. Clifford, L. Seib, K. Kuczera, K. Bowman-

James, J. Am. Chem. Soc. 120 (1998) 8899.

[16] (a) J.-L. Pierre, P. Chautemps, S. Refaif, C. Beguin, A.E.

Marzouki, G. Sterratrice, E. Saint-Aman, P. Rey, J. Am. Chem.

Soc. 117 (1995) 1965;

(b) L. Fabbrizzi, P. Pallavicini, L. Parodi, A. Perotti, N. Sardone,

A. Taglietti, Inorg. Chim. Acta 244 (1996) 7;

(c) O.W. Howarth, J. Nelson, V. McKee, J. Chem. Soc., Chem.

Commun. (2000) 21.

V. Amendola et al. / Inorganica Chimica Acta 337 (2002) 70�/7474