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†

Abstract

The reaction of [Mo(NNC(5)H(10))(dppe)(2)] (B(Mo)) with an excess of acid, HNEt(3)BPh(4), is investigated applying temperature-dependent stopped-flow measurements. The kinetic data indicate a biphasic process with rate constants k(obs(1)) and k(obs(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, B(Mo) and its Mo(IV) analogue [MoBr(NNC(5)H(10))(dppe)(2)]Br (A(Mo)) 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 alpha-protonated intermediate that is inert with respect to N-N cleavage and is generated from its beta-protonated counterpart by a rapid 1,2-proton shift. The implications of these results with respect to N(2) reduction in the Chatt cycle and the enzyme nitrogenase are discussed.