Electrochemical and magnetic resonance studies of tertiary phosphine complexes of nickel

Abstract

In this thesis is presented the results of a study of di-tertiary phosphine compounds, their quaternary derivatives and various of their nickel(II) complexes using electrochemical and magnetic resonance methods. In chapters one and two are presented relevant literature reviews. An outline of relevant theory and the experimental technique used are given in chapters three and four respectively. The synthesis and characterisation, by 1H nmr and 31P nmr and elemental analysis, of several reported and hitherto unreported di-tertiary phosphines, their quaternary derivatives and their nickel(II) complexes is described in chapter five. The synthesis of cis- and trans-1,2-C2H2(PPh2)2, was found to be non-stereospecific, whereas the contrary had previously been reported. New so-termed unsymmetrical di-tertiary phosphines were synthesised. These compounds and their quaternary and nickel(II) derivatives exhibited AB quartets as their 31P nmr spectra from which chemical shifts and coupling constants were directly obtained. In chapter six the 1H nmr and 31P nmr spectral parameters of the compounds synthesised are quantitatively discussed. The coordination chemical shift and the ring effect for one group of compounds was investigated. The 400 MHz 31P nmr spectrum of one compound was conclusively assigned to two unique AA'BB' spin systems. Another compound exhibited a line pattern suggestive of a one-ended dissociation of a coordinated ligand to produce a three-coordinate complex. The redox processes of the (diphosphino)nickel(II)(dithiolate) complexes prepared, and the esr parameters of their electrogenerated redox products were investigated, and are presented in chapter seven. These complexes exhibited quasi-reversible one-electron reduction processes to form stable nickel(I) complexes. The unique, successive one-electron transfer processes of two new dinuclear nickel(II) complexes were observed and investigated. The factors which produce these effects were discussed. One compound was observed to undergo an oxidation process to produce a species tentatively assignable as a nickel(II)-stabilised coordinated di-phosphorus radical cation, with observed hyperfine coupling to two inequivalent 31P nuclei. It was observed that systematic variations in the chelate ring size, chelate linkage, and the number and type of alkyl substituents on the phosphorus nuclei produced systematic variations in the reduction potentials and esr parameters obtained for these [(diphosphino)nickel(II)(dithiolate)]n complexes and their nickel(I) redox products. The (diphosphino)NiS2C2Ph2 complexes studied exhibited one or two one-electron oxidation processes, and esr spectra typical of nickel(II)-stabilised coordinated dithioketyl complexes. One such complex was assigned to undergo rapid dimer formation upon freezing. The production of [(diphosphino)2Ni(I)]+ complexes from the reaction of [(diphosphino)2Ni(II)]2+ complexes and [Ni(S2C2R2)2]- complexes, R = CN and Ph, was observed and concluded to be due to an electron transfer process more complex than that involving an outer-shell mechanism. Presented in chapter eight is an investigation of the redox properties of the complexes of general form [(L-L)2Ni(II)]2+, [(L-L)2Ni(X)]+, (VPP)NiX2, and [(trans-VPP)NiX2]n, L-L = a diphosphino moiety, and X = halogen or SCN, and the esr parameters of their redox products. The successive one-electron redox processes which some of these complexes exhibited were quantitatively investigated, and comparisons were made with the other nickel(II) complexes studied. Several nickel(III) complexes were produced in-situ, and from the resulting esr spectra, assignments of the molecular structures of the complexes were made.