Metal to metal multiple bonds in ordered assembliesMalcolm H. Chisholm†

Newman–Wolfrom Chemical Laboratories, Ohio State University, Columbus, OH 43210

This contribution is part of the special series of Inaugural Articles by members of the National Academy of Sciences elected on May 3, 2005.

Contributed by Malcolm H. Chisholm, December 5, 2006 (sent for review October 20, 2006)

The synthesis and properties of metal–metal (MM) quadruplybonded compounds of molybdenum and tungsten in liquid crys-talline phases are described. Covalently linked MM quadruplybonded complexes, dimers of dimers, are shown to give rise tomixed valence complexes upon oxidation and the degree of elec-tron delocalization is shown to be correlated with the M2� bridge� interactions. Class III behavior or complete delocalization isobserved to M2 to M2 distances of up to 14 Å. When the M2 unitis attached to oligothiophenes via a carboxylate tether, the pho-tophysical properties of the thienyl units are greatly modified dueto M2�–thienyl � conjugation and spin–orbit coupling. This leadsto long-lived emissive states and electroluminescence in solid-statedevices.

mesogens � mixed valency

The metal–metal (MM) quadruple bond exists in dinucleartransition metal complexes that have fourfold symmetry (or

approximately fourfold) along the MM axis and where the metalions have a d4 configuration. Prime examples include theRe2Cl8

2� and Mo2Cl84� ions and neutral molecules such as

Mo2(O2CMe)4 (1). If the MM axis is defined as the z axis, thenthe metal–ligand bonds can be considered to use metal s, px, pyand dx2–y2 orbitals thus leaving the metal dz2 to form a � bond, thedegenerate dxz and dyz interact to form the � bonds and lastly thedxy to form the � bond which has two nodes intersecting alongthe MM axis. The dinuclear center is redox active and uponoxidation electrons are removed from the � orbital to give MMconfigurations of �2�4� and �2�4 leading, respectively, to MMbond orders of 3.5 and 3. These compounds have challenged ourunderstanding of chemical bonding and their attendant spectralfeatures such as the assignment of �(MM) or the � to �*transition have often been the subject of intense debate (2–5).This article focuses on chemistry derived from M2(O2CR)4compounds, where M � Mo and W, and covers work completedin my laboratory principally over the past decade.

The M2(O2CR)4 compounds can be viewed as coordinativelyunsaturated in as much as the metal ions only attain a share of16 valence electrons as a result of forming four metal–ligand �bonds and the MM quadruple bond. By the use of the metal pzorbital (or an s, pz, dz2 hybrid) each metal may coordinate anadditional ligand along the MM axis and in the solid state thesecomplexes generally adopt a laddered structure wherein oneMo2(O2CR)4 unit is weakly coordinated to its neighbors asdepicted in I. In fact, this type of association is common for mostM2(O2CR)4 compounds which adopt the paddle-wheel or lan-tern-like structure when there is not a donor molecule in theaxial position [M � Cr, Mo, W, Ru, Rh] (1).

MesogensThe laddered structure shown in Scheme 1 led us to investigatethe thermal properties of a series of M2(O2C(CH2)nCH3)4compounds. For n � 3, the n-alkanoate dimetal complexesshowed a mesophase upon heating to �100°C with the excep-tion of M � W, whose complexes simply underwent a crystalto isotropic liquid transition (6, 7). The thermotropic proper-ties correlated with the M2���O distances observed in thesolid-state. The stronger the MM bond the longer and pre-

sumably weaker were the intermolecular M2���O stackingforces. As a consequence complexes containing Ru2

4� andRh2

4� units having MM double and single bonds, respectively,did not show a clearing temperature, before thermal decom-position �250°C. The Mo2

4� containing complexes showed welldefined enantiotropic behavior. The introduction of sidebranching in the alkyl groups lowered the crystal to mesophasetransition temperature, whereas perf luoro-n-alkanoates,O2C(CF2)nCF3 raised the clearing temperatures for a given n(8). The mesophases revealed fan-shaped birefringent texturescharacteristic of discotic hexagonal disordered Dhd liquidcrystal phases. Pictorially, we envisage the mesophase consistsof columns of Mo2

4� units wherein the intermolecular Mo2���Obonds are continuously being broken and reformed, as de-picted by Scheme 2.

Support for this view was also found from x-ray diffractionstudies, which showed that the intercolumnar spacing increasedwith increasing n and that the coherence length within a givencolumn was �35 Å i.e., �8–10 M2 units (7).

The MM quadruple is known to have a large magneticanisotropy and from studies of 13C-labeled n-alkanotes it wasshown that the columns align in a magnetic field such that theMM axis is perpendicular to the applied field (7).

The mesophases of Mo2(O2C(CH2)nCH3)4 compounds alsoshowed interesting nonlinear viscoelastic properties as a functionof temperature and strain sweep (9). In a macroscopic pictorialmodel, these could be compared with ‘‘spaghetti in a blender.’’The resistance decreased with increasing strain sweep (radiansper s) as the strands were ‘‘chopped up’’ to form a mush.However, on decreasing the strain sweep, the resistance in-creased as the M2 units ‘‘repaired their strands’’ by reformingtheir intermolecular Mo2���O units within columns.

Mo2(O2CR)4 compounds are kinetically labile to scram-bling, and so in examining the thermotropic properties ofmixtures of Mo2(O2CR)4 and Mo2(O2CR�)4 compounds, it wasnot surprising to find that properties of closely related R and

Author contributions: M.H.C. designed research and wrote the paper.

The author declares no conflict of interest.

Abbreviation: MM, metal–metal.

†E-mail: chisholm@chemistry.ohio-state.edu.

© 2007 by The National Academy of Sciences of the USA

Scheme 1.

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R� e.g., an octanoate and a nonanoate, showed near idealbehavior and conformed to predictions based on the Shroder–van-Laar equation (10). However, mixtures of n-alkanoatesand perf luoro-n-alkanoates showed markedly different prop-erties (9). We also attempted to introduce paramagnetic M2units within these Mo2

4�-containing mesophases. We hadantipated that the introduction of Mo2(O2CR)4

� with a single� electron or Ru2(O2CR)4 with two unpaired electrons mightallow electron transfer through a column. However, studies ofthese mixtures revealed that phase separation occurred as aresult of the different intermolecular M2���O bonding e.g.,Ru2���Ru2 � Mo2���Mo2.

It was shown that the Mo2-containing mesophase is an opti-cally positive material with the largest component of the indexof refraction coincident with the columnar axis. We had hopedthat the alignment of MM multiple bonds might lead to someinteresting large third-order nonlinear optical responses butmore extensive studies of the optical responses of MM quadruplybonded complexes indicated that this was improbable (11). Thus,mesophases of these MM quadruply bonded complexes, al-though fascinating in their own right, would seem to have noimmediate commercial use.

Dimers of Dimers and Studies of Mixed ValencyAnother way in which we attempted to order these dinuclearcomplexes was by use of ligands that would covalently link thedinuclear units and place them in either a parallel or perpen-dicular manner, as represented in Schemes 3 and 4, where thewavy line represents the bridging ligand.

Polymers of this type proved problematic because the mate-rials were insoluble and hard to characterize. (I shall return tothe matter of preparing soluble oligomers.) It is, however,possible to prepare the dimers of dimers by metathetic reactionsinvolving either carboxylate exchange or a salt metathesis asshown in Eqs. 1 and 2, respectively (12).

2M2�O2CBut�4 � HO2C-X-CO2HO¡

25C

toluene

M2�O2CBut�3�2�O2C-X-CO2� � 2ButCOOH [1]

2Mo2�O2CBut�3�CH3CN�2��BF4

� � 2Q�bridge2�

O¡

25C

CH2Cl2Mo2�O2CBut�3�2�bridge� � 2Q�BF4

�� . [2]

Reaction 1 is an equilibrium and the desired product, the dimerof dimers, is generally precipitated from solution, being lesssoluble in hydrocarbons than the parent M2(O2CBut)4 com-pounds. Reaction 1 works well for both M � Mo and W and isalso applicable to other bridging groups having acidic hydrogens.Reaction 2 is only applicable to M � Mo because the cationicanalog of tungsten is not known.

When the two dinuclear units are linked together electroniccoupling arises as a result of the frontier molecular orbitalinteractions of the M2 units and the bridge. For a dicarboxylatebridge these involve the CO2� system and the M2� orbitals asshown in the orbital representations for the oxalate bridge inSchemes 5 and 6 below.

Of these interactions, the in-phase M2� combination with theoxalate lowest unoccupied molecular orbital, Scheme 6, is muchmore important because of energy and overlap factors. The M2�out-of-phase combination which has a symmetry match with thefilled CO2� bonding MO shown in Scheme 5 is a relatively weakinteraction because the CO2� orbitals lie �6 eV lower in energythan the M2�s. The electronic coupling in related bridgedcompounds is always greater for M � W than for M � Mobecause the W2� orbitals are �0.5 eV higher in energy than theirMo2 counterparts, and furthermore, the W5d orbitals providegreater overlap with the orbitals on the bridge.

The orbital interactions resulting from Schemes 5 and 6 leadto a splitting of the two M2� combinations and the magnitude ofthis indicates the degree of electronic coupling. As shown in Fig.1, this is calculated to be �0.5 eV for the model compound[(HCO2)3W2]2(�-O2CCO2) and is a maximum for the planarbridged structure with D2h symmetry (13). Indeed, for thetwisted D2d structure, where the two M2 units are perpendicular,the M2� orbitals form a degenerate pair and the electroniccoupling of the two M2 centers is lost. Thus, control of CO2-ring-CO2 dihedral angles forms the basis for a molecular switch.For oxalate, which as a free dianion has a D2d structure thatminimizes O���O electrostatic repulsions, the bridged complexesare calculated to favor the planar D2h structure by �5–9 kcal

Scheme 2.

Scheme 3.

Scheme 4.

Scheme 5.

Scheme 6.

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mol�1 and in the solid-state they have been found by x-raydiffraction to have near planar structures (13).

As can be anticipated from the MO energy level diagramshown in Fig. 1, the lowest energy electronic transition involvesmetal to bridge charge transfer. Because of the relatively small

rotational barriers the oxalate bridged compounds show ther-mochromic behavior in solution and the electronic absorptionspectra of the [(ButCO2)3W2]2(O2CCO2) complex recorded in2-methyltetrahydrofuran are shown in Fig. 2 over the tempera-ture range of 298 to 2 K. With decreasing temperature, there isa noticeable bathochromic shift and a sharpening of the onset ofthe (0, 0) electronic absorption band. We attribute this to ‘‘thefreezing out of rotation’’ about the oxalate COC bond as oneapproaches zero K with the adoption of the planar geometry.The vibronic features that are very prominent at low tempera-tures arise from the totally symmetric modes of the oxalatebridge in the excited state of the molecule. Photoexcitationplaces an electron in the oxalate �* orbital that has COCbonding and COO antibonding character.

The oxalate bridged complexes show pronounced resonanceRaman enhancements of the totally symmetric modes �(MM),and �1, �2, and �3 of oxalate with excitation into the intensemetal-bridge charge transfer band as shown for[(ButCO2)3Mo2]2(�-O2CCO2) in Fig. 3 (13).

We have made extensive studies of the oxalate bridged com-pounds because an understanding of their properties is prereq-uisite for understanding extended bridges of the typeO2COXOCO2, where X � a conjugated organic group such asO(CHACH)O, aryl, thienyl, etc. A planar CO2OringOCO2structure allows maximum electronic coupling via the bridgeand, as noted earlier, if this can be controlled by a chemicalmeans, it would represent the basis for molecular signaling andswitching.

The 2,5-dihydroxyterephthalate bridge shown below inScheme 7 not only favors a planar bridge structure by way of M2�to bridge �* back-bonding, but also because of intramolecularO���HOO hydrogen bonding. These bridged complexes show

Fig. 1. Comparative molecular orbital energy level diagram for D2h and D2d

[(HCO2)3W2](�-O2CCO2).

Fig. 2. Electronic absorption spectra for [(ButCO2)3W2]2(�-O2CCO2) in2-methyltetrahydrofuran at various temperatures.

Fig. 3. Raman spectrum of [(ButCO2)3Mo2]2(�-O2CCO2) as a 50% KCl disc at80 K with 100 mW of 647.1 nm excitation and a spectral slit width of 2 cm�1.

Scheme. 7.

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intense absorptions in the visible region due to M2� to bridgecharge transfer (14).

However, the colors are discharged upon treatment withhydroxide ion in THF solutions as a result of deprotonation ofthe phenolic OH protons. This bleaching is reversible, andtreatment with acid restores the intense color. At this time, wedo not have structural verification of this reversible process, butwe envisage that it involves the turning on and off of M2� bridge�-conjugation as depicted in Fig. 4 (14).

Electronic coupling of the M2 centers occurs in the ground andphotoexcited states of these molecules and upon oxidation.Single-electron oxidation with an oxidizing agent such as AgPF6or Cp2FePF6 yields mixed valence compounds that may beclassified according to the scheme of Robin and Day (15) as classI, valence trapped where just one M2 center is oxidized, or classIII, where the oxidation results in complete delocalization overthe two M2 redox active centers. Class II is an intermediatesituation where the two redox centers are coupled but at anygiven time are not equivalent, and much attention is currentlygiven to the study of compounds that are close to the class II/IIIborder.

Many spectroscopic techniques can be used in the study ofmixed valence compounds, and this topic has been the subject ofa recent review for ‘‘dimers of dimers’’ involving MM quadruplebonds of molybdenum and tungsten (16). Consequently, I shallfocus on the use of just two techniques that are particularlypertinent: (i) EPR spectroscopy and (ii) UV-vis-near IR (NIR)spectroscopy.

EPR is a most powerful technique because, in the firstinstance, it can distinguish between a metal-based and bridgeoxidation. The large spin-orbit coupling of the heavy metalsleads to metal based oxidations having g values of �1.9 for M �Mo and 1.8 for M � W. Bridge-based oxidations, which arecommonly seen for [RuII-bridged-RuII] chemistry, have g � 2.0(17, 18). Second, both molybdenum and tungsten have a rangeof naturally occurring isotopes, most of which have I � O, butsome of which have spin: 183W, 14.5% natural abundance, I � 1/2and 95Mo, 97Mo, which have I � 5/2 and essentially the samemagnetic moment, have combined abundances �25%. Thus aM2

5� center with a MM bonding configuration �2�4�1 shows anisotropic spectrum consisting of a central resonance flanked bya satellite spectrum due to the hyperfine coupling with the spinactive nuclei. The relative intensities of the satellites and themagnitude of the hyperfine coupling constants are informativeof the degree of delocalization of the electron.

The EPR spectrum of the MoW(O2CBut)5� cation is shown in

Fig. 5 and is particularly illustrative of the power of EPRspectroscopy (19). There is an intense central resonance thatarises from an electron in a � orbital where both Mo and Wnuclei have I � O and a satellite spectrum arising from MoW5�

centers where either Mo, I � 5/2 or W, I � 1/2. The Ao valuesare particularly interesting: Ao for 183W � 44G and Ao for95/97Mo � 31G. These can be compared with 183W, Ao � 51Gand 95/97Mo, A � 27G in the respective homonuclear W2

5�- andMo2

5�-pivalate-containing cations. This is therefore indicative ofa polar M2�1 orbital, where the electron density is polarizedtoward Mo which is, with respect to W, the more electronegativeelement in the MoW quadruple bond. The � orbital is roughlycomprised of 70% Mo 4dxy and 30% W 5dxy (19).

In the homonuclear M4 oxalate-bridged radical cations, the Aovalues are �14G for 95/97Mo and 28G for 183W, roughly one-halfof the M2

5� Ao values and indicative of electron delocalizationover all four metal centers; i.e., class III behavior on the EPRtime scale, which is �10�8 s (20).

Increasing the M2 to M2 separation reduces the degree ofcoupling, and terephthalate Mo4 containing compounds arevalence trapped on the EPR time scale, whereas their tungstenanalogs are fully delocalized (20). Indeed, even the 2,6-azulenedicarboxylate bridge, which separates the M2 centers by�14 Å and affords a polar bridge as a result of the resonancestructure shown in Scheme 8, affords a fully delocalized [W4]�

cation (21). Interestingly, two sets of Ao values due to couplingto 183W are observed: Ao � 40G and 20G, indicative ofpolarization of the HOMO M2� combination as a result of thepolar nature of the bridge (21).

UV-vis-NIR spectroscopy is also an extremely valuablemethod for studying mixed valence compounds (22, 23). Class IIcompounds show low-energy absorptions in the NIR that arebroad and Gaussian in shape and conform well to Hush theory

Fig. 4. The turning ‘‘on’’ and ‘‘off’’ of the intense MLCT absorption in the2,5-dihydroxy-1,4-terephthalate bridged to Mo4-containing complex by suc-cessive deprotonation and protonation.

Fig. 5. The EPR spectrum of the [MoW(O2CBut)4]� cation recorded in tetra-hydrofuran at 223 K showing hyperfine coupling to 95,97Mo and 183W thatreveals the polar nature of the MoW �1 bond.

Scheme 8.

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(24). These are called intervalence charge-transfer bands. Incontrast, complexes near the class II/III border show markedlyasymmetric absorptions with a very sharp onset at the low energyside (22, 23). Class III compounds show more Gaussian shapedabsorptions in the NIR that are much less broad than thosepredicted by Hush theory for class II compounds. For a class IIIcompound, the low-energy absorptions are best referred to ascharge resonance bands and the energy of these absorptions is arough measure of the electronic coupling: 2HAB. The low-energycharge resonance band can be accompanied by MLCT andLMCT and for d5–d6 metal containing ions this is furthercomplicated by d–d transitions and spin–orbit coupling factors(25). The case for the M2�2–M2�1 is much simpler as predictedby the MO diagram shown in Fig. 1. As noted earlier, the filledCO2� orbitals are too low in energy to contribute significantlyin mixing with the M2� orbitals and so the dicarboxylate bridgedcomplexes do not show LMCT bands. The mechanism of elec-tronic coupling is via electron transfer and not hole transfer inthe superexchange model (26). Even when the HOMO of thebridge is close in energy to the M2� orbital combinations, as inthe azulenedicarboxylate bridge, only a charge resonance bandand a MLCT absorption are observed (see Fig. 6). The CO2 unitsthus serve as a gate in shutting down hole transfer via the bridge.

If oxygen in the carboxylate group is substituted for NR or S,the electronic coupling can be significantly changed. Cotton et al.(27) have shown, for example, that Mo2

4� units, supported byp-anisoleformamidinate ligands and bridged by the dianionformed from the double deprotonation of fluroflavine, shown inScheme 9, show much stronger electronic coupling in the mixedvalence oxidized state. Here, the Mo2 separation is essentiallyidentical to that with an oxalate bridge.

Another interesting bridge that is informative with respect tothe mechanism of electronic coupling is the 2,6-dioxypyradizineligand shown in Scheme 10.

With this bridge, the M2 units are aligned in a plane such thatthe inner metal atoms are separated by only 3.3 Å (28). Again forMo2

4� units (supported by pivalate ligands), oxidation producesa more strongly coupled mixed valence ion than is seen foroxalate.

With the employment of the bridging ligands shown inSchemes 9 and 10, the presence of the higher energy filled Np�orbitals allows for a greater mixing with the filled M2� combi-nation, and consequently, hole transfer may contribute signifi-cantly toward the mechanism of electronic coupling. Ratherinterestingly, however, the use of the 2,6-dioxypyradizine bridgeto link two W2(O2CBut)3 centers is not as effective as oxalatedespite the shorter W2 to W2 separation: 3.3 Å vs. 6 Å. This, too,can be understood on the basis of orbital energetics. The lowestunoccupied molecular orbital of the bridge �-system is lower inthe case of oxalate and the W2� orbital is higher in energy inrelation to its Mo2 counterpart. Thus, for tungsten the dominantmechanism of coupling involves electron transfer via the bridge�* and hole transfer contributes little.

Substitution of S for O also has a pronounced effect as seenin a comparison of the properties of the terephthalate bridgedcomplexes, O2CC6H4CO2, versus OSCC6H4COS. Indeed, theelectronic coupling of W2 centers in the thiocarboxylate tereph-thalate complexes is nearly as great as that for oxalate despite themuch longer M2 to M2 separation: 11 Å vs. 6 Å (29).

The magnitude of the electronic coupling can be gauged fromthe energy of the charge resonance bands, as noted earlier, and

Scheme 10.

Fig. 6. The electronic absorption spectrum of the [(ButCO2)3W2]2(�-2,6-azulenedicarboxylate)� radical cation showing the intense MLCT band at �1,200 nmand the asymmetric low-energy charge resonance band at �3,200 cm�1.

Scheme 9.

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from the magnitude of Kc, the comproportionation constantrelating the position of equilibrium of the singly oxidized mixedvalence species in relation to the neutral and doubly oxidizedspecies as defined in Eq. 3 (30).

M2MM2� � M2MM2�2� º 2M2MM2�

�, Kc . [3]

Kc can be determined by electrochemical studies (cyclic volta-mmetry and differential pulse voltammetry) and the magnitudeof Kc and HAB generally show a correlation for closely relatedcomplexes in a common solvent, with a common counter anionand electrochemical set-up. Examples of these measures ofelectronic coupling are shown in Table 1.

One last point should be mentioned: the electronic couplingof the M2 quadruple bonds always arises as a result of M2� tobridge � bonding because, even when the M2 units are alignedat a short distance of �3 Å, direct through-space coupling isinept. This is readily apparent from a comparison of Kc valuesfor the stereochemically correspondent bridging ligands 1,8-

anthracene dicarboxylate (Scheme 11) and 2,7-dioxynapthyri-dine (Scheme 12), which for Mo4 units yield Kc values of �4 and104, respectively (12).

Incorporating MM Quadruple Bonds into OligothiophenesBecause the carboxylate unit is an effective covalent and elec-tronic link, it serves as an alligator clip to incorporate the M2 unitinto oligothiophenes. To investigate the influence of the M2 uniton the properties of the thiophene we have made two types ofmodel compounds that can be studied at the molecular level (31).The first are ‘‘dimers of dimers’’ where a dicarboxylate is usedto link two M2 units as already described. These compounds,prepared by reaction 1, show intense metal to bridge chargetransfer bands that span the visible region of the electronicspectrum and are red shifted with increasing number of thienylunits within the bridge. The compounds show reversible oxida-tion waves that are metal based, and with increasing number ofthienyl units, electronic coupling of the M2 centers rapidly dropsoff.

A second type of model compound has been prepared ac-cording to the reaction shown in Eq. 4 where TiPB � the bulky2,4,6-triisopropylbenzoate.

M2�TiPB�4 � 2�Th�nCOOHO¡

25

tolueneM2�TiPB�2�O2CThn�2

� 2TiPBH. [4]

The molecular structure of the complex where n � 2 is shown inFig. 7. The trans geometry is favored by the presence of the twobulky TiPB ligands and coplanarity of the O2C-(Th)2 groupmaximizes M2�-ligand � conjugation as noted earlier. Thesecomplexes also show MLCT absorptions that fall to lower energyas n � 1 3 3. They show metal based oxidation waves and

Table 1. Comparison of Kc and HAB parameters for variouslinked MM quadruply bonded complexes of molybdenumand tungsten

Metal Bridge �E1/2/mV Kc HAB/cm�1 Ref.

MoO O

O O

280 5.4 104 2,000 12

WO O

O O

717 1.3 1012 2,980 12

MoO

OO

O

FF

F F

65 13 – 10

WO

OO

O

FF

F F

285 6.6 104 1,610 10

MoO

S

S

O

184 1.3 103 – 22

WO

S

S

O

518 5.7 108 2,160 22

MoN N

O O 427 1.7 107 – 21

WN N

O O 630 4.5 1010 1,890 21

Fig. 7. ORTEP plot of Mo2(O2C-2,4,6-trisopropylphenyl)2(O2C-bithienyl)2 (C,black; Mo, green; O, red; S, yellow) with anisotropic displacement parametersdrawn at the 50% probability level and hydrogen atoms omitted for clarity.Scheme 11.

Scheme 12.

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reduction waves associated with thienyl ligands but the latter arenot fully reversible, indicating that the radical anions are kinet-ically labile.

The third class of materials that we have prepared are solublepolymers/oligomers from the reactions shown in Eq. 5 (n � 1 or2) (31).

M2�TiPB�4 � HO2C-Thn�3,4-hex2-C4S�ThnCOOH

O¡

25

tolueneM2�TiPB�2�O2CThn�3,4-hex2-C4S��ThnCO2��x

� 2TiPBH. [5]

The degree of oligomerization i.e., the value of x in theproducts of Eq. 5, is not known, and by MALDI-TOF MS,some lower mass oligomers can be identified corresponding toloops, triangles, and squares. The THF solutions of theseoligomers form gels which probably ref lect imperfectionswithin the ‘‘polymer’’ where some cis-substitution is possiblealong with tri-substitution at the M2 center. The latter leads tocross linking. The soluble nature of the products arises fromthe introduction of the n-hexyl groups at the 3,4-positions ofthe central thienyl unit and the presence of the isopropylgroups on the benzoate ancillary ligands. That these oligomersare soluble in THF solutions allows them to be cast as thinfilms.

Based on electronic structure calculations on model com-pounds where HCO2 substitutes for TiPB we can reasonablyformulate the polymer/oligomers as n-doped semiconductorswhere the M2� orbitals lie between the filled �-band of thethiophene and its empty �* band. The thin films show I–V curvescharacteristic of semiconductors and, when formed into a sand-wich device involving InSnO and Al or Ca electrodes, the thinfilms show electroluminescence (31).

The electronic absorption spectra of the complexes havingthree linked thiophene units and the polymeric/oligomericmaterials formed in reaction 5 have very similar spectralfeatures and all show long lived photo-excited states (32). Mostinterestingly the emissive properties of the molybdenum andtungsten species differ from one another in one importantregard. For molybdenum phosphorescence from the 3MLCT ismuch more prominent than the f luorescence from the 1MLCT,whereas for tungsten the f luorescence is more prominent (seeFig. 8).

From picosecond and femtosecond transient absorption spec-troscopy, the dynamics of the photoexcited states have beenprobed and a comparison of related molybdenum and tungstencomplexes is shown in Fig. 9 (unpublished data).

These metallated polymers show significantly different elec-trochemical and photophysical properties from the metallatedthienyl oligomers studied by Scott and Wolf (32) and Raithbyand colleagues (33). In the latter study, the metals are latetransition metals, which are effectively more electronegative.Their lowest energy electronic transitions arise from the thienyl� to �* transitions with some admixture of metal d character.Also the photophysical properties of the M2-containing com-plexes are very different from those of Ru(bpy)2

2� and relatedpolypyridyl complexes that only show emission from the 3MLCTstate (34). The lifetimes of the 1MLCT states for the M2containing species described are sufficiently long that they canbe observed directly by transient absorption spectroscopy (35).The lifetime for a series of M2-O2C(Th)n containing compoundsincreases with n � 1 3 3, which suggests that the influence ofspin orbit coupling that facilitates inter system crossing isdecreasing as the charge on the thienyl ligand is removed furtherfrom the metal center. Thin films of the M2-thienyl complexesshow similar photophysical properties, but the lifetimes of the3MLCT state are longer, perhaps because of excimer formation.Also, their emission spectra are blue shifted relative to thesolution phase, probably as a result of unfavorable thienyl–thienyl dihedral angles in the solid state that reduce the extentof �–� conjugation along a chain.

Concluding RemarksThe chemistry of compounds containing MM multiple bondsis one of the most fascinating chapters in modern coordinationchemistry. These compounds have challenged our understand-ing of chemical bonding and reactivity as in the facile revers-ible dimerization of two MM triple bonds to form an inorganicanalogue of cyclobutadiene in the reaction 6 (36). Such a [2 �2] reaction does not occur thermally in organic chemistry andis indeed symmetry forbidden (38), but not so for tungsten.

2W2�OPri�6� º W4�OPri�12. [6]

These matters have been a consuming interest to me for the pastthree decades and, as can be seen from this brief account, muchstill remains to be done. It is, however, a beginning.

I thank the National Science Foundation for their continuing support ofthis inquiry driven research, and the Ohio Supercomputer Center forgenerous allocations of resource units. Of course, the greatest thanksgoes to my coworkers and collaborators, many of whom are cited in thereferences. Without these persons, there would have been no story to tell.

Fig. 9. Jablonski diagram comparing the photophysical properties of relatedthienyldicarboxylate bridged Mo4- and W4-containing compounds in THFsolutions at 298 K. *, Due to substantial overlap of fluorescense andphosphorescence.

Fig. 8. Comparison of the fluorescence and phosphorescence of relatedthienyl carboxylate bridged to Mo4- and W4-containing compounds.

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1. Cotton FA; Murillo CA, Walton RA (2005) in Multiple Bonds Between MetalAtoms (Springer, New York), 3rd Ed.

2. Hopkins MD; Gray HB, Miskowski V (1987) Polyhedron 6:705–714.3. Cotton FA, Nocera DG (2000) Acc Chem Res 33:483–490.4. Clark RJH, Franks ML (1976) J Am Chem Soc 98:2763–2767.5. Clark RJH (1990) Chem Soc Rev 90:107–131.6. Cayton RH, Chisholm MH, Darrington FD (1990) Angew Chem Int Edit

29:1481–1483.7. Baxter DV, Cayton RH, Chisholm MH, Huffman JC, Putilina EF, Tagg SL,

Weismann JL, Zwanzinger JL, Darrington FD (1994) J Am Chem Soc116:4551–4566.

8. Baxter DV, Chisholm MH, Lynn MA, Putilina EF, Trzaska ST, Swager T(1998) Chem Mater 10:1758–1763.

9. Chisholm MH, Mackley MR, Marshall RJ, Putilina EF (1995) Chem Mater7:1938–1941.

10. Schroder I (1893) Phys Chem 11:449.11. Pate BP, Thorne JRG, Click DR, Chisholm MH, Denning RG (2002) Inorg

Chem 41:1975–1978.12. Cayton RH, Chisholm MH, Huffman JC, Labkovsky EB (1991) J Am Chem Soc

113:8709–8724.13. Bursten BE, Chisholm MH, Clark RJH, Firth S, Hadad CM, Macintosh AM,

Wilson PJ, Woodward PM, Zaleski JM (2002) J Am Chem Soc 124:3050–3063.14. Chisholm MH, Fiel F, Hadad CM, Patmore NJ (2005) J Am Chem Soc

127:18150–18158.15. Robin MB, Day P (1967) Adv Inorg Chem Radiochem 10:247–422.16. Chisholm MH, Patmore NJ (2007) Acc Chem Res 40:19–27.17. Meacham AP, Druce KL, Bell ZR, Ward MD, Keister JB, Lever APB (2003)

Inorg Chem 42:7887–7896.18. Maurer J, Winter RF, Sarker B, Schwederski B, Kaim W, Zalis S (2006)

Organometallics 25:3701–3712.

19. Chisholm MH, D’Acchioli JS, Pate BD, Patmore NJ, Dalal NS, Zipse DJ (2005)Inorg Chem 44:1061–1067.

20. Byrnes MJ, Chisholm MH, Pate BD (2002) Chem Commun 1084–1085.21. Babyrin MV, Chisholm MH, Dalal NS, Holovics TH, Patmore NJ, Robinson

RE, Zipse DJ (2005) J Am Chem Soc 127:15182–15192.22. Brunschwig BS, Creutz C, Sutin N (2002) Chem Soc Rev 31:168–184.23. Nelson SF (2000) Chem Eur J 6:581–588.24. Hsuh N (1967) Progr Inorg Chem 8:391–444.25. Demadis KD, Hartshorm CM, Meyer TJ (2001) Coord Chem Rev 101:2655–

2685.26. McConnell HM (1961) J Chem Phys 35:508–515.27. Cotton FA, Li Z, Lin CY, Murillo CA, Villagran D (2006) Inorg Chem 45:767–778.28. Chisholm MH, Clark RJH, Gallucci JC, Hadad CM, Patmore NJ (2006) J Am

Chem Soc 126:8303–8313.29. Chisholm MH, Patmore NJ (2006) Dalton Trans 3164–3169.30. Taube H, Richardson DE (1984) Coord Chem Rev 60:107–129.31. Chisholm MH, Epstein AJ, Gallucci JC, Fiel F, Pirkle W (2005) Angew Chem

Int Edit 44:6537–6540.32. Scott TL, Wolf MO (2003) Coord Chem Rev 246:89–101.33. Khan MS, Al-Mandary MRA, Al-Suti MK, Corcoran TC, Al-Mahroogi Y,

Attfield JP, Feeder N, David WIF, Shankland K, Friend RH, et al. (2003) NewJ Chem 27:140–149.

34. Meyer TJ (1989) Acc Chem Res 22:163–170.35. Byrnes MJ, Chisholm MH, Gallucci JC, Liu Y, Ramnauth R, Turro C (2005)

J Am Chem Soc 127:17343–17352.36. Chisholm MH, Clark DL, Hampden-Smith MJ (1989) J Am Chem Soc

111:574–589.37. Woodward RB, Hoffman R (1969) The Conservation of Orbital Symmetry

(Academic, New York).

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