Protonation of trans-[Mo(C2H4)2(Ph2PCH2CH2PPh2)2]: sites of protonation and factors influencing the formation of ethane and ethylene

Abstract

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 °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 β-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).