ESR and electrochemical studies of Group VIII transition metal complexes

Abstract

The work in this thesis combines the techniques of electrochemistry and electron spin resonance spectroscopy, ESR, to investigate the one-electron reduction and oxidation products of Group VIII complexes of the following ligands: dithiocarbamate, dithiolate, catecholate, triphenylphosphine, or bidentate phosphine or arsine. The redox properties of these complexes were first characterized by cyclic voltammetry. Suitable complexes were then either reduced or oxidized in a bulk electrolysis experiment in the cavity of the ESR spectrometer, and the resulting species investigated by ESR. The first chapter surveys the current literature of paramagnetic complexes of nickel, palladium, and platinum in the formal +1 oxidation state. Chapter Two outlines the theory of cyclic voltammetry and ESR spectroscopy, whilst Chapter Three details the experimental apparatus and procedures developed, and used in this thesis. Chapter Four presents a study of the reduction of some bisdialkyldithiocarbamato complexes of nickel and palladium. The reduction of NiII(R2NCS2)2 at platinum electrode in acetone solution gives an initial planar nickel(I) species which rapidly interconverts to a new nickel(I) species with "reversed" g-values. In contrast the reduced PdII(R2NCS2)2 complexes were found to be more stable in solution. The redox properties of the nickel (11) complexes [(Ni(PPh3)2(L))]n+, and [Ni(L2)(L)]n+ (L2 = bidentate phosphine or arsine; L = dithiolate or catecholate, n = 0, or dithiocarbamate n = 1) have been investigated in Chapter Five. In the case of the triphenylphosphine, PPh3, complexes the voltammetry shows evidence of a dissociation equilibrium involving loss of PPh3 from the nickel species present after the electron transfer process. These mixed ligand complexes are readily reduced to a Ni(I) species which exhibit hyperfine coupling to two equivalent phosphorus nuclei bound to the metal. The frozen solution31p hyperfine coupling parameters have been analyzed for some representative complexes and the amount of unpaired electron spin density transferred from the metal to the phosphine ligands have been estimated. The catecholate-phosphine mixed ligand complexes of Ni were also found to undergo quasi-reversible one-electron oxidations to give Ni(II) complexes containing coordinated semiquinone radical anions. In Chapter Six the electrochemical reductions of the complexes Ni(L)(X)2 (L = bidentate phosphine; X = halide) have been investigated. The species NiI(L)(X) were shown to rapidly form from the initial reduction product. Nickel complexes with four phosphorus σ-donor ligands were also investigated by cyclic voltammetry and ESR. In the case of the Ni complex of 1,3-bis(diphenylphosphino)propane, dpp, the species [NiI(dpp)2]+ was found to form spontaneously in the presence of an excess of the dpp ligand. The one-electron reduction and oxidation processes of the complex NiII((CN)2C2S2)((CH3)2C2N2Ph2) have been investigated. The one-electron reduction of the complex was shown by ESR to produce a Ni(I) species. A review of the general trends exhibited by the Ni(I) complexes studied in this work is included at the end of Chapter Six. In Chapter Seven the redox processes of the "dithiobenzil" complexes M(Ph2C2S2)2 (M = Ni, Pd, Pt) have been investigated. The one-electron reduction processes of these complexes were found by ESR to be mainly ligand based. The complexes were also found to undergo quasi-reversible oxidations, but ESR studies show that more than one species is formed in this process. The stilbenedithiolate complexes M(dpe)(Ph2C2S2) (dpe = 1,2-bis(diphenyl-phosphino)ethane; M = Ni, Pd, Pt) and M(PPh3)2(Ph2C2S2) (M = Pd, Pt) were each found to undergo a reversible one-electron oxidation to give a long-lived species. The oxidation process was shown by ESR to be mainly ligand based, and to give a complex with a coordinated dithioketyl (Ph2C2S2) ligand. Finally, in Chapter Eight the 1:l and 1:2 stannic chloride adducts of is(dithiooxalato)nickel, palladium and platinum have been investigated by cyclic voltammetry and the one-electron reduction products characterized by ESR spectroscopy. The reductions were shown to take place mainly on the dithiooxalate ligands. The cyclic voltammetry properties of the parent dithiooxalate complexes were also investigated for comparison.