Abstract
Reaction of Mo(N[R]Ar) 3 (R = t Bu or C(CD 3 ) 2 CH 3 ) with N 2 O gives rise exclusively 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 dinitrogen. Solution calorimetry measurements were used to determine the heat of reaction of Mo(N[R]Ar) 3 with N 2 O 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 enthalpy for the terminal nitrido complex, NMo(N[R]Ar) 3 . Comparison of the new calorimetry data with those obtained previously for oxo transfer to Mo(N[R]Ar) 3 shows that the nitrous oxide N-N bond cleavage reaction is under kinetic control. Stopped-flow kinetic measurements revealed the reaction to be first order in both Mo(N[R]Ar) 3 and N 2 O, consistent with a mechanism featuring post-rate-determining dinuclear N-N bond scission, but also consistent with cleavage of the N-N bond at a single metal center in a mechanism requiring the intermediacy of nitric oxide. The new 2-adamantyl-substituted molybdenum complex Mo(N[2-Ad]Ar) 3 was synthesized and found also to split N 2 O, resulting in a 1:1 mixture of nitrosyl and nitride products; the reaction exhibited first-order kinetics and was found to be ca. 6 times slower than that for the tert-butylsubstituted 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 N 2 O, activation reactivity. Bradley's chromium complex Cr(N i Pr 2 ) 3 was found to be competitive with Mo(N[R]Ar) 3 for NO binding, while on its own exhibiting no reaction with N 2 O. Competition experiments permitted determination of ratios of second-order rate constants for NO binding by the two molybdenum complexes and the chromium complex. Analysis of the product mixtures resulting from carrying out the N 2 O cleavage reactions with Cr(N i pr 2 ) 3 present as an in situ NO scavenger rules out as dominant any mechanism involving the intermediacy of NO. Simplest and consistent with all the available data is a post-rate-determining bimetallic N-N scission process. Kinetic funneling of the reaction as indicated is taken to be governed by the properties of nitrous oxide as a ligand, coupled with the azophilic nature of three-coordinate molybdenum(III) complexes.