Mechanisms of reactions of bases with trans-[Mo(NNH2)X(Ph2PCH2CH2PPh2)2]+(X = F, Br, or p-MeC6H4SO3)

Abstract

The mechanisms of the reactions between trans-[Mo(NNH2)X(dppe)2]+(X = F, Br, or p-MeC6H4SO3; dppe = Ph2PCH2CH2PPh2) and the bases NEt3 and MeO– under an atmosphere of dinitrogen, to give trans-[Mo(N2)2(dppe)2] have been investigated in both methanol and tetrahydrofuran. Initial deprotonation of the hydrazido(2–)-ligand generates trans-[Mo(NNH)X(dppe)2] and the subsequent pathway adopted is determined by the relative labilities of X and the acid strength of the hydrazido(3–)-ligand. Rapid deprotonation of the hydrazido(3–)-complex (X = F or Br) yields trans-[Mo(N2)X(dppe)2]– which loses halide in the rate-limiting step to generate [Mo(N2)(dppe)2]. Subsequent attack by dinitrogen in solution yields the product. However, when X =p-MeC6H4SO3, rapid loss of this ligand occurs from the hydrazido(3–)-complex, to generate, after attack by a molecule of solvent (S) the species trans-[Mo(NNH)S(dppe)2]+. Pathways leading to the product involving either rate-limiting deprotonation or exchange of the co-ordinated solvent for dinitrogen prior to deprotonation have been established, and the factors influencing the choice of pathway are discussed.