CO and ethylene migratory insertion reactions and copolymerization involving palladium complexes of a NiN2S2 metallodithiolate ligand

摘要

Neutral ((Ni-1) Pd(CH3)(Cl)) and cationic ([(Ni-1) Pd(CH3)(OEt2)][BAr'(4)]) complexes containing the strongly electron-donating metallodithiolate ligand Ni-1 ([(N,N'-bis(2-mercaptoethyl)-N,N'-diazacyclooctane]nickel(II), (S(CH2)(2)N((CH2)(3))(2)N(CH2)(2)S)Ni) have been prepared. Carbonylation reactions with these Pd-alkyl complexes were investigated by IR and high-pressure NMR spectroscopy. Carbon-13 NMR spectroscopic studies showed that the rates of CO insertion to form the derivatives (Ni-1) Pd(C(O)CH3)(Cl) and [(Ni-1)Pd(C(O)CH3)(CO)](+) were immeasurably fast at -80 degrees C in CD2Cl2, in contrast to slower rates for Pd metal based diphosphine and diimine analogues. It was further shown that displacement of the terminally bound CO by ethylene in the acetylated derivative [(Ni-1) Pd(C(O)CH3)(CO)](+) was slow, attributable to the high binding affinity of CO toward Pd2+ in the presence of the electron-rich nickel dithiolate ligand. Bulk copolymerization studies of CO/ethylene in the presence of the cationic catalyst precursor [(Ni-1)Pd(CH3)(OEt2)][BAr'(4)] find alternating polyketones. The catalytic efficiency is solvent-dependent: CH2Cl2 > CH3CN > THF. From low-pressure and bulk copolymerization studies the resting state of the catalyst was determined to be the open-chain intermediate [(Ni-1)Pd(C(O)R)(CO)](+) rather than the beta- and gamma-keto chelate complexes [(L boolean AND L)Pd(CH2CH2(O)R-3)](+) and [(L boolean AND L)Pd(C(O)CH2CH2(O)R-3)](+) that were found for diimines and diphosphines.