Hydride Ion Transfer from Ruthenium(II) Complexes in Water: Kinetics and Mechanism

Reactions of hydride complexes of ruthenium(II) with hydride acceptors have been examined for Ru(terpy)(bpy)H+, Ru(terpy)(dmb)H+, and Ru(eta(6)-C6Me6)(bpy)(H)(+) in aqueous media at 25 degrees C (terpy = 2,2';6',2 ''-terpyridine, bpy = 2,2'-bipyridine, dmb = 4,4'-dimethyl-2,2'-bipyridine). The acceptors include CO2, CO, CH2O, and H3O+.CO reacts with Ru(terpy)(dmb)H+ with a rate constant of 1.2 (0.2) x 10(1) M-1 s(-1), but for Ru(eta(6)-C6Me6)(bpy)(H)(+), the reaction was very slow, k <= 0.1 M-1 s(-1) Ru(terpy)(bpy)H+ and Ru(eta(6)-C6Me6)(bpy)(H)(+) react with CH2O with rate constants of (6 +/- 4) x 10(6) and 1.1 x 10(3) M-1 s(-1), respectively. The reaction of Ru(eta(6)-C6Me6)(bpy)(H)(+) with acid exhibits straightforward, second-order kinetics, with the rate proportional to [Ru(eta(6)-C6Me6)(bpy)(H)(+)] and [H3O] and k = 2.2 x 10(1) M-1 s(-1) (mu = 0.1 M, Na2SO4 medium). However, for the case of Ru(terpy)(bpy)(H)(+), the protonation step is very rapid, and only the formation of the product Ru(terpy)(bpy)(H2O)(2+) (presumably via a dihydrogen or dihydride complex) is observed with a k(obs) of ca 4 s(-1). The hydricities of HCO2-, HCO-, and H3CO- in water are estimated as +1.48, -0.76, and +1.57 eV/molecule (+34, -17.5, +36 kcal/mol), respectively. Theoretical studies of the reactions with CO2 reveal a "product-like" transition state with short C-H and long M-H distances. (Reactant) Ru-H stretched 0.68 angstrom (product) C-H stretched only 0.04 angstrom. The role of water solvent was explored by including one, two, or three water molecules in the calculation.