Transmetalation of a Primary Amino-Functionalized N-Heterocyclic Carbene Ligand from an Axially Chiral Square-Planar Nickel(II) Complex to a Ruthenium(II) Precatalyst for the Transfer Hydrogenation of Ketones

Abstract

The first homoleptic nickel(II) complex with primary amino-functionalized N-heterocyclic carbene (C−NH2) ligands ([Ni(m-CH2NH2)2](PF6)2, 2) was prepared under mild conditions by the reduction of a nitrile-functionalized imidazolium salt. This axially chiral, square-planar nickel(II) complex was characterized by NMR spectroscopy and an X-ray diffraction study. Enantiopure Δ-TRISPHAT was used as an NMR chiral shift reagent to observe the diastereotopic ion pairs by 1H NMR in acetonitrile-d3. A novel transmetalation reaction moved the primary amino-functionalized N-heterocyclic carbene ligand from the nickel(II) complex 2 to the [Ru(p-cymene)Cl]2 dimer, yielding the complex [Ru(p-cymene)(m-CH2NH2)Cl]PF6, (3), the first ruthenium(II) complex with such a chelating C−NH2 ligand. Complex 3 is a precatalyst for the transfer hydrogenation of acetophenone to 1-phenylethanol in basic 2-propanol at 75 °C with a turnover frequency of up to 880 h−1 and conversion of 96%.