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 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 Cu⁰ and Cu^II. The Cu^I species could be quantitatively trapped using triphenylphosphine to form Cu(β-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 β-diketonates, suggesting involvement of the ligand in catalytic turnover.