Synthesis and Reactivity of Reduced α-Diimine Nickel Complexes Relevant to Acrylic Acid Synthesis

Abstract

The aryl-substituted α-diimine (DI) nickel vinyl complex (iPrDI)Ni(CH=CH2) (iPrDI = [2,6-(iPr)2C6H3N=C(CH3)]2) was synthesized and structurally characterized. The complex is dimeric in the solid state and has a distorted-square-planar geometry at nickel. A combination of single-crystal X-ray diffraction, EPR, magnetic susceptibility, and NMR analyses was used to elucidate the electronic structure of the compound, and it is best described as a low-spin Ni(II) derivative with a singly reduced α-diimine chelate. Addition of CO2 to the nickel vinyl complex resulted in insertion into the nickel-carbon bond to yield the corresponding nickel acrylate (iPrDI)Ni(κ2-O2CCH=CH2). EPR spectroscopy coupled with DFT calculations established that the S = 1/2 product maintains the nickel(II) oxidation state with an α-diimine-centered radical. Addition of acrylic acid to (iPrDI)Ni(CH=CH2) induced rapid, net bimetallic reductive elimination to release butadiene and produced the metastable olefin-bound acrylic acid complex (iPrDI)Ni(κ2-CH2=CHCO2H). Over the course of 2 h at 23 °C, this complex underwent a net oxidation to produce (iPrDI)Ni(κ2-O2CCH=CH2), with concomitant loss of H2.