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(η⁶-C₆Me₆)(bpy)(H)⁺ in aqueous media at 25 °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) × 10¹ M^-1 s^-1, but for Ru(η⁶-C₆Me₆)(bpy)(H)⁺, the reaction was very slow, k ≤ 0.1 M^-1 s^-1. Ru(terpy)(bpy)H⁺ and Ru(η⁶-C₆Me ₆)(bpy)(H)⁺ react with CH₂O with rate constants of (6 ± 4) × 10⁶ and 1.1 × 10³ M ^-1 s^-1, respectively. The reaction of Ru(η ⁶-C₆Me₆)(bpy)(H)⁺ with acid exhibits straightforward, second-order kinetics, with the rate proportional to [Ru(η⁶-C₆Me₆)(bpy)(H)⁺] and [H₃O⁺] and k = 2.2 × 10¹ M^-1 s^-1 (μ = 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 HCO ₂^-, 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 CO ₂ reveal a "product-like" transition state with short C-H and long M-H distances. (Reactant) Ru-H stretched 0.68 Å (product) C-H stretched only 0.04 Å. The role of water solvent was explored by including one, two, or three water molecules in the calculation.