Copper Chemistry of β-Diketiminate Ligands:  Monomer/Dimer Equilibria and a New Class of Bis(μ-oxo)dicopper Compounds

A series of Cu(I) and Cu(II) complexes of a variety of β-diketiminate ligands (L^-) with a range of substitution patterns were prepared and characterized by spectroscopic, electrochemical, and, in several cases, X-ray crystallographic methods. Specifically, complexes of the general formula [LCuCl]₂ were structurally characterized and their magnetic properties assessed through EPR spectroscopy of solutions and, in one instance, by variable-temperature SQUID magnetization measurements on a powder sample. UV−vis spectra indicated reversible dissociation to 3-coordinate monomers LCuCl in solution at temperatures above −55 °C. The Cu(I) complexes LCu(MeCN) exhibited reversible Cu(I)/Cu(II) redox couples with E_1/2 values between +300 and +520 mV versus NHE (cyclic voltammetry, MeCN solutions). These complexes were highly reactive with O₂, yielding intermediates that were identified as rare examples of neutral bis(μ-oxo)dicopper complexes on the basis of their EPR silence, diagnostic UV−vis absorption data, and O-isotope-sensitive resonance Raman spectroscopic features. The structural features of the compounds [LCuCl]₂ and LCu(MeCN) as well as the proclivity to form bis(μ-oxo)dicopper products upon oxygenation of the Cu(I) complexes are compared to data previously reported for complexes of more sterically hindered β-diketiminate ligands (Aboelella, N. W.; Lewis, E. A.; Reynolds, A. M.; Brennessel, W. W.; Cramer, C. J.; Tolman, W. B. J. Am. Chem. Soc. 2002, 124, 10600. Spencer, D. J. E.; Aboelella, N. W.; Reynolds, A. M.; Holland, P. L.; Tolman, W. B. J. Am. Chem. Soc. 2002, 124, 2108. Holland, P. L.; Tolman, W. B. J. Am. Chem. Soc. 1999, 121, 7270). The observed structural and reactivity differences are rationalized by considering the steric influences of both the substituents on the flanking aromatic rings and those present on the β-diketiminate backbone.