Thermodynamic and Kinetic Hydricity of Ruthenium(II) Hydride Complexes

Despite the fundamental importance of the hydricity of a transition metal hydride (Δ<i>G</i>_H^–^°(MH) for the reaction M–H → M⁺ + H^–) in a range of reactions important in catalysis and solar energy storage, ours (<i>J. Am. Chem. Soc.</i> <b>2009</b>, <i>131</i>, 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(η⁶-C₆Me₆)­(bpy)­H]⁺ (bpy = 2,2′-bipyridine), we used a thermodynamic cycle based on evaluation of the acidity of [Ru­(η⁶-C₆Me₆)­(bpy)­H]⁺ p<i>K</i>_a = 22.5 ± 0.1 and the [Ru­(η⁶-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 <i>K</i> 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 <i>E</i>°(H⁺/H^–) is taken as 79.6 kcal/mol vs Fc⁺/Fc as a reference, the hydricities of [Ru­(η⁶-C₆Me₆)­(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 p<i>K</i>_a estimated for [Ru­(tpy)­(bpy)­H]⁺ in acetonitrile is 32 ± 3. UV–vis spectroscopic studies of [Ru­(η⁶-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

Formation of η²‑Coordinated Dihydropyridine–Ruthenium(II) Complexes by Hydride Transfer from Ruthenium(II) to Pyridinium Cations

Reactions between various pyridinium cations with and without a −CF₃ substituent at the 3-position and [Ru­(tpy)­(bpy)­H]⁺ (tpy = 2,2′:6′,2″-terpyridine and bpy = 2,2′-bipyridine) were investigated in detail. The corresponding 1,4-dihydropyridines coordinating to a Ru­(II) complex in η² mode through a CC bond were quantitatively formed at the initial stage. The only exception observed was in the case of the 1-benzylpyridinium cation, where a mixture of two adducts with 1,4-dihydropyridine and 1,2-dihydropyridine was formed in the ratio 96:4. Cleavage of the Ru–(CC) bond proceeded at a slower rate in all reactions, giving the corresponding dihydropyridine and [Ru­(tpy)­(bpy)­(NCCH₃)]²⁺ when acetonitrile was used as a solvent. Kinetic activation parameters for the adduct formation indicated that the 1,4-regioselectivities were induced by formation of sterically constrained structures