Hydride Transfer Reactions of Transition Metal Hydrides: Kinetic Hydricity of Metal Carbonyl Hydrides

Abstract

Hydride transfer from neutral transition metal hydrides (MH) to Ph3C+BF4- gives M-FBF3 and Ph3CH. The rate law −d[Ph3C+BF4-]/dt = k[Ph3C+BF4-][MH] was established from kinetic measurements using stopped-flow methods. Second-order rate constants determined in CH2Cl2 solution at 25 °C range from kH− = 7.2 × 10-1 M-1 s-1 to kH− = 4.6 × 106 M-1 s-1. The order of increasing kinetic hydricity is (C5H4CO2Me)(CO)3WH < (CO)5MnH < Cp*(CO)3CrH < Cp(CO)3WH < HSiEt3 < cis-(CO)4(PCy3)MnH < cis-(CO)4(PPh3)MnH < (C5H4Me)(CO)3WH < Cp(CO)3MoH < Cp*(CO)3WH < (indenyl)(CO)3WH < (CO)5ReH < Cp*(CO)3MoH < cis-(CO)4(PPh3)ReH < Cp(NO)2WH < trans-Cp(CO)2(PCy3)MoH < trans-Cp(CO)2(PPh3)MoH < trans-Cp(CO)2(PMe3)MoH (Cp = η5-C5H5, Cp* = η5-C5Me5, Cy = cyclohexyl). Ranges of activation parameters for hydride transfer from trans-Cp(CO)2(PMe3)MoH, trans-Cp(CO)2(PCy3)MoH, cis-(CO)4(PPh3)ReH, and Cp*(CO)3MoH are ΔH⧧ = 3.0−5.9 kcal mol-1 and ΔS⧧ = −18 to −24 cal K-1 mol-1. The rate constant for hydride transfer (kH−) from cis-Cp(CO)2(PCy3)MoH at −55 °C is 3 orders of magnitude lower than that for trans-Cp(CO)2(PCy3)MoH. Phosphine substitution for CO generally enhances the kinetic hydricity, with trans-Cp(CO)2(PMe3)MoH being 104 times as reactive as Cp(CO)3MoH. The electronic effect of phosphine substitution is attenuated by steric factors when the phosphine is cis to the metal hydride. The hydride transfer kinetics reported here are interpreted to be single-step hydride transfers, rather than a multiple-step mechanism involving an initial electron transfer followed by hydrogen atom transfer. A distinction is made between hydricity and nucleophilicity of metal hydrides.