PROTONATION OF TRANS-[MO(C2H4)2(PH2PCH2CH2PPH2)2] - SITES OF PROTONATION AND FACTORS INFLUENCING THE FORMATION OF ETHANE AND ETHYLENE

The mechanism of the formation of ethane and ethylene in the reactions of trans-[Mo(C2H4)2(dppe)2] (dppe = Ph2PCH2CH2PPh2) with HX (X = Cl or Br) in tetrahydrofuran at 25.0-degrees-C has been established by a combination of stopped-flow spectrophotometry, detailed product analysis and kinetic investigation of the evolution of hydrocarbons, together with characterisation of the key intermediates by multinuclear NMR spectroscopy. Addition of anhydrous HX to trans-[Mo(C2H4)2(dppe)2] rapidly generates [MoH(C2H4)(dppe)2]+ by two pathways: direct protonation at the metal or protonation at the ethylene ligand to form [Mo(C2H5)(C2H4)(dppe)2]+ followed by migration of a beta-hydrogen atom from the ethyl ligand to the metal. This migration step is reversible and at low concentrations of HCl [Mo(C2H5)(C2H4)(dppe)2]+ slowly loses ethylene. Subsequent binding of chloride ultimately results in the formation of ethane, and trans-[MoCl2(dppe)2]. At higher concentrations of HCl further protonation of [MoH(C2H4)2(dppe)2]+ occurs and [MoH2(C2H4)2(dppe)2]2+ is the dominant solution species which loses both ethylene ligands to form [MoH2Cl2(dppe)2]. Quantitative analysis of the hydrocarbon product distribution at various concentrations of HCl confirms the nature of these pathways under an atmosphere of dinitrogen or argon. In the presence of carbon monoxide or dihydrogen the hydrocarbon product distribution is different. The relevance of these studies to the understanding of the different substrate specificities of the molybdenum- and vanadium-based nitrogenases is discussed, as are the factors influencing the rates of protonation of the metal in [ML2(dppe)2] (M = Mo or W; L = N2, C2H4 or 2 H).