On the origin of selective nitrous oxide N-N bond cleavage by three-coordinate molybdenum(III) complexes

Reaction of Mo(N[R]Ar)(3) (R = Bu-t or (CD3)(2)CH3) with N2O gives rise exc lusively to a 1: 1 mixture of nitride NMo(N[R]Ar)(3) and nitrosyl ONMo(N[R] Ar)(3), rather than the known oxo complex OMo(N[R]Ar)(3) and dinitro-en. So lution calorimetry measurements were used to determine the heat of reaction of Mo(N[R]Ar)3 with N2O and, independently, the heat of reaction of Mo(N[R ]Ar)3 with NO. Derived from the latter measurements is an estimate (155.3 /- 3.3 kcal . mol(-1)) of the molybdenum-nitrogen bond dissociation enthalp y for the terminal nitrido complex, NMo(N[R]Ar)(3). Comparison of the new c alorimetry data with those obtained previously for oxo transfer to Mo(N[R]A r)(3) shows that the nitrous oxide N-N bond cleavage reaction is under kine tic control. Stopped-flow kinetic measurements revealed the reaction to be first order in both Mo(N[R]Ar)(3) and N2O, consistent with a mechanism feat uring post-rate-determining dinuclear N-N bond scission, but also consisten t with cleavage of the N-N bond at a single metal center in a mechanism req uiring the intermediacy of nitric oxide. The new 2-adamantyl-substituted mo lybdenum complex Mo(N[2-Ad]Ar)(3) was synthesized and found also to split N 2O, resulting in a 1:1 mixture of nitrosyl and nitride products;,the reacti on exhibited first-order kinetics and was found to be ca. 6 times slower th an that for the tert-butyl-substituted derivative. Discussed in conjunction with studies of the 2-adamantyl derivative Mo(N[2-Ad]Ar)(3) is the role of ligand-imposed steric constraints on small-molecule, e.g. N-2 and N2O, act ivation reactivity. Bradley's chromium complex Cr((NPr2)-Pr-i)(3) was found to be competitive with Mo(N[R]Ar)(3) for NO binding, while on its own exhi biting no reaction with N2O. Competition experiments permitted determinatio n of ratios of second-order rate constants for NO binding by the two molybd enum complexes and the chromium complex. Analysis of the product mixtures r esulting from carrying out the N2O cleavage reactions with Cr((NPr2)-Pr-i)( 3) present as an in situ NO scavenger rules out as dominant any mechanism i nvolving the intermediacy of NO. Simplest and consistent with all the avail able data is a post-rate-determining bimetallic N-N scission process. Kinet ic funneling of the reaction as indicated is taken to be governed by the pr operties of nitrous oxide as a ligand, coupled with the azophilic nature of three-coordinate molybdenum(III) complexes.