Electrochemical studies of nickel complexes containing phosphorus(III) ligands and their related Ziegler catalysts

Article abstract of DOI:10.1016/0022-328X(89)87220-1

All of the data obtained by electrochemical methods involving the mercury electrode, spectrophotometry and conductometry have showed the stepwise formations of a neutral ?-donor-acceptor pentacoordinated NiX2L3 and a cationic tetracoordinated NiL4²⁺, associated with a shift in the dissociation-coordination equilibrium owing to an excessive concentration of the ligand in solutions of the binary systems containing Ni^II compounds coordinated with acid ligands (X=Cl^-, NO3^-, acac) and the trivalent phosphorus ligands (L=PBu3, PPh3, P(OEt)3, P(O-i-Pr)3, P(O-s-Bu)3, P(OC6H11), P(OPh)3).The complexes have for the first time been prepared for all the phosphines and phosphites studied.Ni(acac)2 was found to enhance coordination by the phosphines and phosphites in acetonitrile.The value of the semiwave potential shift (ΔE_1/2, V) towards less negative potentials depends on the number of ligands present in the inner coordination sphere of metal, and was found to be 200-300 and 800-1300 mV for NiX2L2 and NiL4²⁺, respectively.The electochemical reduction of NiL4²⁺ complexes into the zero-valent nickel isostructural complexes is reversible with values similar to that of the nickel formal potential Ni²⁺/Ni⁰ (E_1/2=E₀=-0.25 V).For the first time, the homogeneous formations of catalitically active complexes have been studied electrochemically in situ with the tricomponent Ziegler-Natta catalytic systems to give stable Ni^II complexes in toluene solutions of Ni(acac)2--OAC (Ni/P/Al=1/4/4) at T^+-.The hydride complexes of divalent nickel HNiL4⁺ were found to be as catalitically active as the Ni(acac)2-P(OR)3 -AlEt3 (N/P/Al=1/4/4) systems formed in toluene-isopropanol solvent at T=-10 to +25 deg C.The activating effect by the organic phosphorus ligand on the electronic state of Ni has been found for the first time to decrease the energy levels of the boundary orbitals as the coordination sphere becomes saturated with the ligand, thus changing the ligand's environment and configuration of the complex, the degree of alkylation and the degree of stabilization of the catalitically active complexes.In the case of the NiL4²⁺ complexes, the ligand-to-metal charge transfer energies permitted the estimation of the affinity to the nickel electron inherent to the nickel LUMO energy level in the given complex, that energy being similar to HOMO energy in Ni⁰L4 under the conditions of the reversible electronic process.