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.