Metal coordination complexes of the sterically hindered β-diketonate, 2,6-dimesitylbenzoyl pinacolone (esac), are reported for Co, Ni, Cu, and Zn.

Bulky β-diketones have rarely exceeded dipivaloylmethane (DPM) in steric demand, largely due to synthetic limitations of the Claisen condensation. This work demonstrates hindered acid chlorides to be selective electrophiles in noncoordinating solvents for condensations with enolates. An improved synthesis of DPM is described (90% yield), and crowded β-diketones featuring bulky o-biphenyl or m-terphenyl fragments were prepared in good to excellent yields. These compounds are anticipated to have a steric profile far greater than that of DPM. General reaction conditions and mechanistic considerations are included.

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 CuII/I electrochemical reduction, which is likely due to inhibited ligand rotation. Chemical reduction of CuII forms CuI, which disproportionates to Cu0 and CuII. The CuI 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.