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

The synthesis and characterization of a series of copper bis(β-diketonate) complexes, functionalized with sterically hindered -biphenyl and -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 electrochemical reduction, which is likely due to inhibited ligand rotation. Chemical reduction of Cu forms Cu, which disproportionates to Cu and Cu. The Cu species could be quantitatively trapped using triphenylphosphine to form Cu(β-diketonate)(PPh) (), 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 β-diketonates, suggesting involvement of the ligand in catalytic turnover.