Studies of the kinetics of the reactions of nitriles with dinitrogen complexes of molybdenum(0) and tungsten (0). Factors affecting the rate of dissociative loss of dinitrogen

Abstract

The kinetics of the substitution of nitriles for dinitrogen in trans-[M(N 2) 2-(dppe) 2] (M = Mo, W; dppe = 1,2-bis(diphenylphosphino)ethane) and cis-[M(N 2) 2(PMe 2Ph) 4] (M = Mo, W) complexes have been studied in toluene solution at various temperatures. In the case of trans-[Mo(N 2) 2(dppe) 2], one dinitrogen is replaced by a nitrile in toluene and the kinetic data support an S n1 dissociative mechanism ( k(PhCN) 3.6 X 10 -4 s -1 at 303 K; E a 28 kcal/mol; Δ H ≠ 27 kcal/mol; Δ S ≠ 15 cal/mol-deg). The analogous reaction with trans-[W(N 2) 2(dppe) 2] was found to proceed only photochemically, with loss of one dinitrogen. The thermal activation energy for N 2 dissociation from trans-[W(N 2) 2(dppe) 2] in toluene is estimated to be 38 kcal/mol. In the cis-[M)N 2) 2(PMe 2Ph) 4] (M = Mo, W) reactions in toluene, both dinitrogen ligands are replaced by nitriles in two consecutive dissociative steps ( k(Mo)/ k(W) ≈ 10 2 for each step).