Redox Thermodynamics of Dinuclear Transition-Metal Complexes: Unusual Entropy and Electronic Coupling Effects in Mixed Solvents

As a guide to the thermodynamics of intramolecular electron-transfer processes, the redox thermodynamics of three dinuclear transition-metal systems have been investigated in mixtures of acetonitrile and dimethyl sulfoxide (DMSO) as solvent. The specific systems are (2,2'-bipyridine)₂ClRu-L-Ru(NH₃)₄(pyridine)^{5+ /4+/ 3+}, where L is pyrazine, 4,4'-bipyridine, or bis(pyridyl)ethane. A special feature is that the tetraammineruthenium redox site in each interacts specifically with hydrogen-bond-accepting (electron-pair-donating) solvents (as demonstrated by various optical and electrochemical measurements (Curtis et al. Inorg. Chem. 1986, 25, 4233; 1987, 26, 2660)) whereas the (polypyridyl)ruthenium site does not. Thus, the formal potential (E_f) for the ammine fragment is shifted to progressively less positive values as the solvent is enriched in DMSO. Measurements of E_ffor the Ru^III/^II-polypyridyl fragment demonstrate that the solvational effects are readily transmitted electronically from the ammine fragment when pyrazine is the bridge. Variable-temperature E_fmeasurements reveal a sharp positive “spike” in plots of the half-reaction entropy for each of the tetraammine couples versus mixed-solvent composition. A statistical calculation shows that the entropy spike is a direct consequence of the unsymmetrical preferential solvation of (2,2'-bipyridine)₂Ru^II-L-Ru^III(NH₃)_4-(pyridine)⁴⁺versus (2,2'-bipyridine)₂Ru^II-L-Ru^II(NH₃)₄(pyridine)³⁺, as found elsewhere for monomeric redox couples and as earlier predicted from optical intervalence studies. Further examination shows that the entropy effects also are transmitted electronically from the tetraammine site to the polypyridyl site and are detectable in both the pyrazine- and 4,4'-bipyridine-bridged cases. An evaluation of the overall intervalence thermodynamics reveals that ΔS^o“leads” ΔG° on a solvent molar composition coordinate. This suggests that unusual mixed-solvent-induced variations in activation parameters might also be observed in related kinetic experiments.