Hydride transfer from (η⁵-C₅Me₅)(CO)₂MH (M = Fe, Ru, Os) to trityl cation: Different products from different metals and the kinetics of hydride transfer

Reactions of the metal hydride complexes Cp*(CO)₂MH (M = Fe, Ru, Os) with Ph₃C⁺BF₄^- in CH₂C1₂ were studied. Hydride transfer from Cp*(CO)₂FeH to Ph₃C⁺BF₄^- gives Cp*(CO)₂FeFBF₃. Hydride transfer from the ruthenium hydride Cp*(CO)₂RuH to Ph₃C⁺ BF₄^- produces the bridging hydride complex {[Cp*(CO)₂Ru]₂(μ-H)}⁺BF₄ ^-, indicating that Cp*(CO)₂RuH exhibits significant nucleophilicity in addition to hydridic reactivity of the Ru-H bond. The osmium hydride Cp*(CO)₂OsH reacts with Ph₃C⁺BF₄^- in CH₂C1₂ to give a mixture of Cp*(CO)₂OsFBF₃ and [Cp*(CO)₂Os(ClCH₂Cl)]⁺BF₄ ^-. The kinetics of these hydride transfer reactions were monitored by stopped-flow methods, leading to the second-order rate law -d[Ph₃C⁺ BF₄^- ]/dt = k[Ph₃C⁺BF₄^-] [MH]. The temperature dependence of the rate constants was determined for the iron hydride, the osmium hydride, and the osmium deuteride Cp*(CO)₂OsD. Activation parameters for hydride transfer from Cp*(CO)₂FeH are ΔH^‡ = 2.6 ± 0.1 kcal mol^-1 and ΔS^‡ = -22.1 ± 0.4 cal K^-1 mol^-1; for Cp*(CO)₂OsH the activation parameters are ΔH^‡ = 4.9 ± 0.1 kcal mol^-1 and ΔS^‡ = -16.8 ± 0.5 cal K^-1 mol^-1. A kinetic isotope effect (k_H/k_D = 1.6 at 0 °C) was found for the reaction of Cp*(CO)₂OsD. The order of kinetic hydricity is HRu > HFe > HOs. Second-order rate constants (extrapolated to 25 °C from data collected at lower temperatures) are k = 3.2 ± 10⁵ M^-1 s^-1 for Cp*(CO)₂OsH and k = 1.1 × 10⁶ M^-1 s^-1 for Cp*(CO)₂FeH; for Cp*(CO)₂RuH, k > 5 × 10⁶ M^-1 s ^-1 is estimated at 25 °C. Rate constants were also determined for hydride transfer to Ph₂(p-MeOC₆H₄)C⁺ at 25 °C: k = 1.5 × 10⁵ M^-1 s^-1 for Cp*(CO)₂RuH, k = 4.1 × 10⁴ M^-1 s^-1 for Cp*(CO)₂FeH, and k = 3.2 × 10³ M^-1 s^-1 for Cp*(CO)₂OsH.