2286
Inorg. Chem. 1991, 30, 2286-2290
modes and the latter through strong coupling to carbonyl modes is likely (particularly in the D4clusters)-and only vMZH(asym) to be a "pure" mode. It is to be hoped that the principles we have used in this study may enable the spectra of other clusters with multiple hydride ligands to be interpreted satisfactorily. Experimental Section [H4Ru4(CO)Iz]and [D4Ru4(CO)Iz]were prepared by the literature method.' Samples of [H40S4(C0)1z]and [D40s4(CO)Iz]were kindly provided by Dr. B. F. G. Johnson, University of Cambridge. Infrared
spectra of samples in KBr disks were measured on a Digilab FTS-20 m-IR spectrophotometer; an RIlC low-tempcrature4 1 with liquid Nz as coolant was used for the low-temperature (ca. 100 K) spectra. The
,(exciting " h "line, 64,.1 ~ ~ ~ twith~ ' ~ ~ ~ t rof~ca.~250~ mW. s~~~~~~~~m Acknowledgment. P.L.S. and S.F.A.K. are indebted to NATO for financial support. The Italian Minister0 delle Pubblica Istruzione is thanked for a grant (to R.R.). We are grateful to the SERC for assistance toward the purchase of a replacement laser tube.
Contribution from the Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204
Investigation of tbe Redox States of [ R U ( ~ ~ ~ ) ~ C N ] ~Evidence C N + : for Valence Delocalization of tbe Singly Oxidized Complex J. B. Cooper, T. M. Vess, W. A. Kalsbeck, and D. W. Wertz* Received July 31, 1990 A spectroelectrochcmicalstudy of both the oxidized and reduced forms of [Ru(bpy),CNI2CN+is reported. Th UV-vis spectra of the parent and reduced species indicate that the complex consists of two identical chromophores, which are nearly identical with the Ru(bpy)z(CN)z chromophore, even though one ruthenium is bound to the carbon of the bridging CN while the other is nitrogen bound. Only the shift in the bridging CN stretching frequency with reduction offers any distinction between the two metal centers. The one-electron oxidation product of the bimetallic species is assigned as valence delocalized on the basis of the following observations: ( I ) only two CN stretches are observed in the oxidized form, (2) the shift to higher energy of the terminal CN stretches for the bimetallic species is half of that observed upon oxidation of the monometallic complex, (3) the sharpness and solvent independence of the near-IR band is not consistent with Hush predictions for the IVT band, and (4) the separation of the oxidation waves implies a comproportionation constant for the bimetallic species of 1.38 X 1O1O (assuming no difference in metal centers) comparable in magnitude to other delocalized systems.
Introduction Recently, there has been a great deal of interest in the synthesis of new coordination compounds in which a photosensitizer center is bound to a reactive center that acts as an electron donor, electron acceptor, or energy acceptor.' The presence of both moieties in the same complex alleviates the need for long photosensitizer lifetimes as well as high concentrations of the reactive moiety. These multimetallic coordination compounds are also being suggested as possible multielectron donors or acceptors, a necessary requirement for artificial photosynthesis.'.2 Another application is the design of supramolecular photochemical devices using these coordination compounds as building blocks? Central to the design of all of these schemes is the requirement of efficient electron or energy transfer between the two centers. Since there is usually no direct overlap between two metal centers, the transfer pathway is mediated by the bridging ligand. Several methods for evaluating the degree of communication between the centers exist in the With the synthesis and subsequent investigations ( I ) Peterscn, J. D. Supramoleculor Photochemistry; Balzani, V., Ed.;
Dordrecht, The Netherlands, 1987; p 135 and references therein. Balzani, V.; Indelli, M. T.; Scandola, F. Supramolecular Phorochemisrry; Balzani, V.; Ed.: Reidel: Dordrecht, The Netherlands, 1987; p 1 and references therein. Richardson, D. E.; Taube, H. J. Am. Chem. Soc. 1983, 105.40. Richardson. D. E.;Taube, H. Coord. Chem. Reo. 1984,60, 107. Palaniappan, V.;Singru, R. M.;Agarwala, U.C. Inorg. Chem. 1988, Reidel:
(2) (3) (4) (5) (6) (7) (8)
27, 181.
Tom, G. M.; Taube, H. J. Am. Chem. Soc. 1975,97, 5310. Baumann, J. A.; Meyer. T. J. Inorg. Chem. 1980, 19, 345. Weaver, T. R.; Meyer, T. J.; Adeyemi, S.A.; Brown, G. M.;Eckberg, R. P.; Hatfield, W. E.;Johnson. E. C.; Murray, R. W.; Untereker, D.
J . Am. Chem. Soc. 1975,97, 3039. Krentzien, H.; Taube. H. J. Am. Chem. Soc. 1976, 98, 6379. (IO) Girerd, J. J. J. Chcm. Phys. 1983, 79, 1766. ( 1 1 ) Mayoh, B.; Day, P. Inorg. Chem. 1974, 13, 2273. ( I 2) Mayoh, B.; Day, P.J. Chem. Soc.. Dalton Trans. 1974, 846. (9)
0020-1 66919111330-2286302.50/0
of the Creutz-Taube ion. a nreat deal of controversv has arisen
concerning strongly interacting systems approaching a' delocalized limit in terms of the valence shell.*'' Theoretical treatments 01 these systems have contributed much to the basic understanding of electron-transfer theory.4.'1-'3,'5.'8-M The complex [Ru(bpy)2CNI2CN+,thus, has multifold aspects of interest. It not only meets the requirements of a photosensitizer strongly absorbing in the visible region and possessing a long-lived emission ( T = 90 ns)14 but also possesses additional coordination sites (CN) so that supramolecular species can be made. Indeed, such systems based on a [ R U ( ~ ~ ~ ) ~ C Nbuilding ] ~ C N block + have already been reportede2 In addition, the mixed-valence species offers the o g (13)
Hush, N. S.J. Chem. Phys 1958,28,962; Z . Elekrrochem. 1957.61, 734; Trans. Faraday Soc. 1%1,57,557; frog. Itwrg. Chem. 1%7,8, 391.
(14)
Bignozzi, C. A.; Roffia, S.;Chiorboli. C.; Davila, J.; Indelli, M. T.; Scandola, F. Itwrg. Chem. 1989, 28, 4350. Marcus, R. A. J. Chem. Phys. 1965.45, 679. Meyer, T. J. Acc. Chcm. Res. 1978, 11.94. K o k , E. M.;Goldsby, K. A.; Narayana, D. N. S.;Meyer, T. J. J. Am.
(15) (16) (17)
Chem. Soc. 1983, 105,4303. (18) Demas, J. N.; Turner, T. F.; Crosby, G. A. Inorg. Chem. 1%9,8.674. (19) Calef. D. F.; Wolynes, P.G. J. Phys. Chem. 1983, 87, 3387. (20) Brunschwig, B. S.; Ehrenson. S.;Sutin. N. J. Phys. Chem. 1986. 90. 3657. (21) Blackbourn, R. L.; Hupp, J. T. J. Phys. Chem. 1988, 92, 2817. (22) Sutin, N.; Creutr, C. J. Chem. Educ. 1983, 60, 809. (23) Blackbourn, R. L.;Hupp, J. T. J. Phys. Chcm. 1990, 94, 1788. (24) Hammack. W. S.; Drickamer. H. G.; Lowery, M. D.; Hendrickson. D. N . Chem. Phys. Lcrr. 1986. 132, 231. (25) Lowery, M.D.; Hammack, W. S.;Dirckamer, H.G.; Hendrickson, D. N. J. Am. Chem. Soc. 1987, 109, 8019. (26) Hammack, W.S.;Drickamer, H. G.; Lowery. M. D.; Hendrickson. D. N. Inorg. Chem. 1988, 27, 1307. (27) Lewis, N. A.; Okng, Y.S.1. Am. Chem. Soc. 1988, 110. 2307. (28) Nelson,S.F.; Kim. Y.;Blackstock. S . C. J. Am. Chcm. Soc. 1989.111, _2 0_ 4 5 . (29) De la Rosa, R.; Chang, P. S.;Salaymeh, F.; Curtis, J. C. Inorg. Chcm. 1985, 27, 1294. (30) Lay, P. A. J. Phys. Chem. 1986, 90, 878.
0 1991 American Chemical Society
Redox States of [Ru(bpy),CNI2CN+
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wavenumbers Figure 1. Cyclic voltammogram of I[Ru(~~Y)~(CN)~~(CN)~~PF,) in 0.1 M TBAH/DMF at Pt and at a 200 mV/s scan rate. Figure 2. Electronic spectra of [Ru(bpy),(CN)],(CN)+ (dotted) and Ru(bpy),(CN), (solid) in DMF. Arrows indicate the appropriate exportunity to investigate the extent of interaction for a nonsymtinction coefficient axis for each. metrically bridged system. This study focuses on the assignment of the redox orbitals for this complex and the effect of the non-2.00 V. The separation between the anodic and cathodic peaks symmetrical bridge with respect to optical and redox properties for each couple (-60 mV) as well as the regeneration of the CV of the complex. In addition, the degree of delocalization for the upon bulk electrolysis passed any of the redox couples maintains mixed-valence species will be investigated and interpreted in terms that all four redox processes are completely reversible. Sine the of the unsymmetrical nature of the complex. two ruthenium centers are identical with the expection that one is bound to the nitrogen at the CN bridge while the other is bound Experimental Section to the carbon, it is reasonable that the first redox electron enters Cknicrla Anhydrous, 99+% N,N-dimethylformamide (DMF) and a bpy orbital on the carbon-bound side, since the bridge will be acetonitrile purchased from Aldrich were used as supplied from the acting as a weaker u base and a better ?r acid with respect to the manufacturer. The sample of I[RU(~~~)~CN],CN)(PF,) was used as nitrogen-bound side. However, it should be noted that the small received from Dr. Franco Scandola. cis-Ru(bpy),(CN), was synthesized separation of redox waves for the complex suggests that the energy according to the method of Demas et al.l* Tetrabutylammonium hexafluorophosphate (TBAH) was purchased from Aldrich, recrystallized difference between the redox sites is relatively small and due in twice from ethanol, and dried under vacuum. large part to electrostatic effects. Ekctroebcmistry. A solution of the sample ( 5 mM for FTIR and Electronic Spectra. The near degeneracy of bpy LUMO's resonance Raman; 0.5 mM for UV/vis) and TBAH (100 mM) was suggested by the electrochemistry is also reflected by the UV-vis prepared in a Vacuum Atmospheres Co. drybox under nitrogen atmospectra of [Ru(bpy),(CN)],(CN)+ and Ru(bpy),(CN),. As sphere. The solution was loaded into an electrochemical H-cell with shown in Figure 2, the two spectra exhibit similar features with Pt-mesh working and counter electrodes and a saturated calomel referalmost a perfect overlap throughout the UV-vis region, the only ence electrode (WE). Voltammetry was carried out with a Princeton difference being the approximate 2-fold intensity increase of the Applied Research (PAR) Model 173 potentiostat/l75 universal probinuclear species. On the basis of the assignments of the mogrammer system connected to a Nicolet 3091 digital oscilloscope, while the extent of reduction was monitored with an in-line PAR Model 379 nonuclear complex,32the peak at 35.0 X lo3 cm-l and its highdigital coulometer. The data were downloaded to an AT&T 6300 comenergy shoulder aan be assigned as a ?r **intraligand transition, puter for analysis. while both the 20.0 X lo3 and the 28.5 X lo3 cm-I peaks correResonance Ramen Spectra. The sealed H-cell was placed in the samspond to MLCT's. The relatively small effect of the binding site ple compartment of a Jarrell-Ash double I-m Raman spectrometer at the bridging CN is perhaps best demonstrated by the single equipped with a cooled RCA c31034A photomultiplier tube, and the peak in the visible portion of the spectrum. The peaks are at the reductions were carried out in situ. A Spectra Physics Model 171 Ar+ same energy (A300 cm-') and exhibit the same structure (the ion laser was used either to serve as a stand alone source or to pump a high-energy shoulder has been assigned for the mononuclear Spectra Physics Model 375 dye laser with Coumarin 540 dye (Exciton). species as vibronic) for both complexes, indicating that the two The spectra were collected and stored on computer for analyses. All frequencies were determined relative to the solvent peak at 865 cm-' lowest states are nearly degenerate. Although it could be argued (DMF) and are expected to be accurate to -2 cm-I. that the increase in the d-orbital energies of the N-bound Ru due FITR Spectra. A Mattson Polaris FTIR spectrometer equipped with a decreased interaction with the CN r* is matched by the increase an NEC Powermate 2 computer, KBr bcamsplitter, and MCT detector of the bpy ?F* caused by an increased interaction with the d r with was used to obtain the spectra. The R I R spectra were acquired by using no net change in the MLCT energy, the electrochemical data a specular reflectance cell.3' The spectra were acquired by reflecting suggest that the changes in orbital energy must be small. off of the polished platinum working electrode, which was flush against With each consecutive reduction of binuclear species, there is the cell window (
