CO and ethylene migratory insertion reactions and copolymerization involving palladium complexes of a NiN2S2 metallodithiolate ligand
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abstract
Neutral ((Ni-1)Pd(CH3)(Cl)) and cationic ([(Ni-1)Pd(CH 3)(OEt2)][BAr4]) complexes containing the strongly electron-donating metallodithiolate ligand Ni-1 ([(N,N-bis(2-mercaptoethyl)-N,N-diazacyclooctane]nickel(II), (S(CH2)2N((CH2)3) 2N(CH2)2S)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)-CH 3)(Cl) and [(Ni-1)Pd(C(O)CH3)(CO)]+ were immeasurably fast at -80C 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)][BAr4] find alternating polyketones. The catalytic efficiency is solvent-dependent: CH 2Cl2 > 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 - and -keto chelate complexes [(L down curve sign L)Pd(CH 2CH2(O)R3)]+ and [(L down curve sign down curve sign L)-Pd(C(O)CH2CH2(O)R3)] + that were found for diimines and diphosphines. 2007 American Chemical Society.
