Vanadium complexes in relation to dinitrogen fixation.

The aim of this work was to investigate the chemistry of vanadium in relation to dinitrogen fixation and consequently the literature concerning vanadium systems which reduce dinitrogen in protic media and the coordination compounds of vanadium(II) and vanadium(III) has been surveyed. In view of the importance of tertiary phosphines in the stabilisation of dinitrogen complexes of second and third row transition metals, methyldiphenylphosphine complexes of vanadium(III), (VCl3(PPh2Me)2) and (VCl3(MeCN) (PPh2Me)2) have been prepared and their electronic spectra and magnetic moments measured. The structures, determined by single crystal X-ray diffraction methods, are trigonal bipyramidal, (VCl3(PPh2Me)2), and octahedral, (VCl3(MeCN) (PPH2Me)2), with the phosphines occupying axial positions in both complexes. Attempts to reduce these complexes did not give any products reactive towards dinitrogen. Hydrated vanadium(II) salts (V(H2O)6)SO4, trans-(V(H2O)4Cl2), and (V(H2O)6)Br2 , were prepared by electrolytic reduction of the corresponding vanadyl solutions for use in the preparation of more reactive vanadium(II) complexes. (V(MeOH)6)Br2 was prepared from the hydrate and used to prepare (VBr2(thf)2) which was used in turn to prepare (VBr2(dppe)2). The diffuse reflectance spectra and magnetic moments of these complexes are consistent with octahedral stereochemistries throughout. The tetrahydrofuran complex is thought to be a bromide-bridged polymer and the phosphine complex is thought to be trans-(VBr2(dppe)2)