Intramolecular electron transfers within the mixed valence states of the ligand bridged hexaruthenium clusters Ru3(μ3-O)(μ-CH3CO2)6(CO)(L)(μ-L‘)Ru3(μ3-O)(μ-CH3CO2)6(CO)(L) (L‘ = 1,4-pyrazine; L = 4-dimethylaminopyridine (1), pyridine (2), 4-cyanopyridine (3), or L‘ = 4,4‘-bipyridine; L = 4-dimethylaminopyridine (4), pyridine (5), 4-cyanopyridine (6)) were examined. Two discrete and reversible single electron reductions are evident by cyclic voltammetry in the redox chemistry of 1−5, and the intercluster charge-transfer complexes are well-defined. The splitting of the reduction waves, ΔE, is related to the electronic coupling H AB between the triruthenium clusters, and varies from 80 mV for 5 to 440 mV for 1. In the case of 6, the splitting of the reduction waves, ΔE, is <50 mV and the intercluster charge-transfer complex is not defined. The mixed valence states of 1−3 also exhibit intervalence charge transfer (ICT) bands in the region 12 100 (1) to 10 800 cm-1 (3) which provide spectroscopic estimates of H AB in the range 2180 (1) to 1310 cm-1 (3). The magnitude of the electronic coupling H AB is found to strongly influence the IR spectra of the singly reduced (−1) mixed valence states of 1−6 in the ν(CO) region. In the case of relatively weak electronic coupling (4−6), two ν(CO) bands are clearly resolved. In the cases of strong electronic coupling (1−3), these bands broaden to a single ν(CO) absorption band. These data allow the rate constants, k e, for electron transfer in the mixed valence states of 1, 2, and 3 to be estimated by simulating dynamical effects (Bloch-type equations) on ν(CO) absorption band shape at 9 × 1011, 5 × 1011, and ca. 1 × 1011 s-1, respectively. The less strongly coupled 4,4‘-bipyridine-bridged complexes 4−6 exhibit IR line shapes in the −1 mixed valence states that are not as strongly affected by electron-transfer dynamics. The rate constant for the −1 mixed valence state of 4 is close to the lower limit that can be estimated by this approach, between 1 × 1010 and 1 × 1011 s-1.

Single-electron reductions of linked triruthenium clusters of the general type Ru,-pyrazine-Ru, produced mixed valence systems showing spectroscopic characteristics of rapid intramolecular electron transfer. Reflectance infrared spectroelectrochemistry was used to characterize the vibrational spectra of mixed valence systems that contained one carbon monoxide ligand on each Ru, cluster. Infrared spectra in the CO stretching region showed two resolved, partially coalesced, and coalesced v(C0) bands for clusters with rate constants for intramolecular electron transfer k, increasing from = 1 x 1 O9 s-' up to 5 x 10" and 9 x 10'' s-', respectively. These data provide a strong correlation between rates of intramolecular electron transfer and infrared spectral bandshape.Single-electron transfers are the simplest chemical reactions and are f~tndamentally important in biology (photosynthesis and respiration) and technology (photography, electrophotography, and solar energy conversion) (1-3). The fastest electron transfers involve the transfer of charge frorn an electron-donor site to an electron-acceptor site within the same molecule. The Creutz-Taube ion, [(NH,) jRu-pyrazine-R~(NH3) j]5+, is a now classical example of an intramolecular chargetransfer complex (4-6). Rather than being viewed as a localized valence (Ru"1Ru"') state, it is generally accepted that charge is delocalized over the complex (5, 6). The semiclassical expressio~l for the rate constant for intramolecular electron transfer ke in a symmetric charge-transfer complex with no net free energy change (lGO = 0) is given by k, = KV, X exp [-(AG,,' " H.q, + HA4,"4AG,*)/RT]where K is the adiabaticity factor (unity for adiabatic reactions), v.. is the nuclear frequency factor, which ir;'cludes both the solvent dielectric response frequency and bond-length/bond-angle reorganizations required by charge transfer between the localized valence states, AG,'%s the reorganizational energy barrier, and HAB is the electronic coupling between the metal centers (6-8). Thus, the theoretical maximum rate show that the rates of intramolecular electron transfer in a series of pyrazine-bridged assemblies of ruthenium clusters can be varied over orders of magnitude by moderating the electronic coupling HAB through ancillary ligand variation. In the rnost rapidly exchanging systems, electron transfer dynamical effects were observed in the infrared (IR) vibrational spectra.The pyraiine-bridged ruthenium clusters R y i P3-O)( P-CH~CO:)~(CO) Qi ~z -p z ) Ru3( p,-0) ( k-CH,CO2),(CO) (L) [pi = pyraiine; L = 4-dirnethylaminopyridine (dmap, I ) , pyridine (py, 2 ) , and 4-cyanopyridine

We previously reported the measurement of rates of electron transfer (ET) by the coalescence of vibrational absorption bands in infrared (IR) spectra. [1,2] To observe the coalescence of vibrational bands in IR spectra, rates of chemical exchange

Seven new oxo-centered triruthenium complexes with 2,6-dimethylphenyl isocyanide (CNXy) as a terminal ligand, [Ru3(O)(CH3CO2)6(CNXy) n (L)3 - n ] {L = CO, n = 2 (1); L = pyridine (py), n = 1 (2); L = 4-(dimethylamino)pyridine (dmap), n = 1 (3); L = py, n = 2 (4); L = dmap, n = 2 (5); n = 3 (6); L = 5-(4-pyridyl)-10,15,20-triphenylporphine, n = 2 (7)}, three new pyrazine (pz) bridged dimers of triruthenium complexes with terminal isocyanide ligands, [{Ru3(O)(CH3CO2)6(CNXy) n (L)2 - n }2(μ-pz)] {L = py, n = 1 (8); L = dmap, n = 1 (9); n = 2 (10)}, and three new 1,4-phenylene diisocyanide bridged dimers, [{Ru3(O)(CH3CO2)6(L)2}2(μ-CNC6H4NC)] {L = py (11); L = dmap (12); L = CNXy (13)}, were prepared. All the Ru3(O) cluster core in these compounds have the formal oxidation state of RuIII 2RuII in the isolated state. In contrast to the related carbonyl derivatives, the isocyanide complexes gave triruthenium complexes with more than one isocyanide ligand, reflecting the lower π acidity of isocyanides. In the mixed carbonyl/isocyanide complex [Ru3(O)(CH3CO2)6(CO)(CNXy)2] (1), the carbonyl and isocyanide ligands compete in π acidity for the Ru3(O) cluster core, and as a result 1 undergoes a facile elimination of the CO ligand followed by substitution. With use of this reaction, many other triruthenium isocyanide complexes were prepared. Coordination of isocyanide ligands to these triruthenium complexes causes the redox potentials to be shifted anodically relative to the corresponding pyridine derivatives. The pz bridged dimers, 8 and 9, gave stable one-electron-reduced species which are in the mixed-valence state with respect to the Ru3 core, as is evidenced by a large redox wave splitting for the RuIII 2RuII−pz−RuIII 2RuII/RuIII 2RuII−pz−RuIIIRuII 2/RuIIIRuII 2−pz−RuIIIRuII 2 process (ΔE 1/2 = 349 mV for 8 and 393 mV for 9). On the other hand, 1,4-phenylene diisocyanide bridged Ru3 dimers 11−13 gave no stable mixed-valence state, indicating that the electronic communication between the Ru3 units through this bridge is negligibly small. The compound [Ru3(O)(CH3CO2)6(CNXy)(py)2] (2) crystallized in the orthorhombic Pbcn space group with a = 18.393(5) Å, b = 14.428(2) Å, c = 17.396(2) Å, and Z = 4. A comparison of Ru−O(acetate) distances reveals that the site coordinated by the isocyanide ligand is formally Ru(II).