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

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(η2-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.