Structural, Spectroscopic, and Kinetic Investigation of the Molybdenum Dialkylhydrazido Complexes [MoBr(NNC₅H₁₀)(dppe)₂]Br and [Mo(NNC₅H₁₀)(dppe)₂]: Activation Parameters and Revised Mechanism for N−N Cleavage

The reaction of [Mo(NNC5H10)(dppe)2] (BMo) with an excess of acid, HNEt3BPh4, is investigated applying temperature-dependent stopped-flow measurements. The kinetic data indicate a biphasic process with rate constants kobs(1) and kobs(2) which are both slower than the single rate constant reported by Henderson et al. (Henderson, R. A.; Leigh, G. J.; Pickett, C. J., J. Chem. Soc., Dalton Trans. 1989, 425−430). Moreover, both rate constants exhibit a linear dependence on the acid concentration with a large intercept which is attributed to acid-dependent and acid-independent components of each reaction phase, respectively. All four reaction channels exhibit temperature-dependent reaction rates. Furthermore, BMo and its Mo(IV) analogue [MoBr(NNC5H10)(dppe)2]Br (AMo) are characterized structurally and spectroscopically. Density-functional theory calculations are performed to locate possible barriers in the overall reaction scheme and determine their energies, providing additional information for the formulation of a mechanism. The temperature-dependent rate of N−N cleavage is explained by a revised mechanism which involves an α-protonated intermediate that is inert with respect to N−N cleavage and is generated from its β-protonated counterpart by a rapid 1,2-proton shift. The implications of these results with respect to N2 reduction in the Chatt cycle and the enzyme nitrogenase are discussed.