Late Transition Metal Complexes Bearing Functionalized N-Heterocyclic Carbenes and the Catalytic Hydrogenation of Polar Double Bonds.

摘要

Late transition metal complexes of silver(I), rhodium(I), ruthenium(II), palladium(II) and platinum(II) containing a nitrile-functionalized N-heterocyclic carbene ligand (C--CN) were prepared. The nitrile group on the C--CN ligand was shown to undergo hydrolysis under basic conditions, leading to a silver(I) carbene complex with a primary-amido functional group, and a trimetallic complex of palladium(II) with a partially hydrolyzed C--N--N--C donor ligand.;The reduction of a nitrile-functionalized imidazolium salt in the presence of nickel(II) chloride under mild conditions yielded an axially chiral square-planar nickel(II) complex containing a unique primary-amino functionalized N-heterocyclic carbene ligand (C--NH2). A transmetalation reaction moved this chelating C--NH2 ligand from nickel(II) to ruthenium(II), osmium(II), and iridium(III), yielding important catalysts for the hydrogenation of polar double bonds.;The ruthenium(II) complex, [Ru(p-cymene)(C--NH 2)Cl]PF6 catalyzed the transfer and H2-hydrogenation of ketones. The bifunctional hydride complex, [Ru(p-cymene)(C--NH 2)H]PF6, which contains a Ru--H/N--H couple showed no activity under catalytic conditions unless when activated by a base. The outer-sphere mechanism involving bifunctional catalysis of ketone reduction is disfavored according to experimental and theoretical studies and an inner-sphere mechanism is proposed involving the decoordination of the amine donor from the C--NH2 ligand.;The ruthenium(II) complex [RuCp*(C--NH2)py]PF 6 showed higher activity than the iridium(III) complex [IrCp *(C--NH2)Cl]PF6 in the hydrogenation of ketones. This ruthenium(II) complex also catalyzes the hydrogenation of an aromatic ester, a ketimine, and the hydrogenolysis of styrene oxide. We proposed an alcohol-assisted outer sphere bifunctional mechanism for both systems based on experimental findings and theoretical calculations. The cationic iridium(III) hydride complex, [IrCp*(C--NH2)H]PF6 , was prepared and this failed to react with a ketone in the absence of base. The crucial role of the alkoxide base was demonstrated in the activation of this hydride complex in catalysis. Calculations support the proposal that the base deprotonates the amine group of this hydride complex and triggers the migration of the hydride to the eta5-Cp* ring producing a neutral iridium(I) amido complex. This system contains an active Ir--H/N--H couple required for the outer sphere hydrogenation of ketones in the bifunctional mechanism.