Thermodynamic and kinetic hydricity of ruthenium(II) hydride complexes

Despite the fundamental importance of the hydricity of a transition metal hydride (δG^° _H-(MH) for the reaction M-H → M⁺ + H^-) in a range of reactions important in catalysis and solar energy storage, ours (J. Am. Chem. Soc.2009, 131, 2794) are the only values reported for water solvent, and there has been no basis for comparison of these with the wider range already determined for acetonitrile solvent, in particular. Accordingly, we have used a variety of approaches to determine hydricity values in acetonitrile of Ru(II) hydride complexes previously studied in water. For [Ru(ν⁶-C₆Me₆)(bpy)H] ⁺ (bpy = 2,2′-bipyridine), we used a thermodynamic cycle based on evaluation of the acidity of [Ru(ν⁶-C₆Me ₆)(bpy)H]⁺ pK_a = 22.5 ± 0.1 and the [Ru(ν⁶-C₆Me₆)(bpy)(NCCH₃) _1/0]^2+/0 electrochemical potential (-1.22 V vs Fc ⁺/Fc). For [Ru(tpy)(bpy)H]⁺ (tpy = 2,2′:6′, 2″-terpyridine) we utilized organic hydride ion acceptors (A⁺) of characterized hydricity derived from imidazolium cations and pyridinium cations, and determined K for the hydride transfer reaction, S + MH⁺ + A⁺ → M(S)²⁺ + AH (S = CD₃CN, MH ⁺ = [Ru(tpy)(bpy)H]⁺), by ¹H NMR measurements. Equilibration of initially 7 mM solutions was slow-on the time scale of a day or more. When E°(H⁺/H^-) is taken as 79.6 kcal/mol vs Fc⁺/Fc as a reference, the hydricities of [Ru(ν⁶-C ₆Me₆)(bpy)H]⁺ and [Ru(tpy)(bpy)H]⁺ were estimated as 54 ± 2 and 39 ± 3 kcal/mol, respectively, in acetonitrile to be compared with the values 31 and 22 kcal/mol, respectively, found for aqueous media. The pK_a estimated for [Ru(tpy)(bpy)H] ⁺ in acetonitrile is 32 ± 3. UV-vis spectroscopic studies of [Ru(ν⁶-C₆Me₆)(bpy)]⁰ and [Ru(tpy)(bpy)]⁰ indicate that they contain reduced bpy and tpy ligands, respectively. These conclusions are supported by DFT electronic structure results. Comparison of the hydricity values for acetonitrile and water reveals a flattening or compression of the hydricity range upon transferring the hydride complexes to water.