Copper(II) as a Platform for Probing the Steric Demand of Bulky β-Diketonates

The synthesis and characterization of a series of copper bis(beta-diketonate) complexes, functionalized with sterically hindered o-biphenyl and m-terphenyl functional groups, are reported. X-ray structural analysis reveals that the ligands exhibit several modes of flexibility in order to accommodate the steric demand. Increased steric bulk of the ligands influences the Cu-II/I electrochemical reduction, which is likely due to inhibited ligand rotation. Chemical reduction of Cu-II forms Cu-I, which disproportionates to Cu-0 and Cu-II. The Cu-I species could be quantitatively trapped using triphenylphosphine to form Cu(beta-diketonate)(PPh3)(2) (7), which is also characterized. The catalytic ability of these complexes, along with several common precatalysts, was determined for the reaction of bromobenzene and 2-naphthol, an Ullmann-type C-O bond coupling reaction. Control experiments in toluene show no catalytic ability in the absence of beta-diketonates, suggesting involvement of the ligand in catalytic turnover.