β-diketiminate ligand backbone structural effects on Cu(I)/O₂ reactivity: Unique copper-superoxo and Bis(μ-oxo) complexes

Copper(I) and -(II) complexes of β-diketiminate ligands with identical flanking 2,6-diisopropylphenyl groups but divergent backbone substitution patterns were prepared and structurally characterized, and reactions of the Cu(I) species with O₂ at low temperature were explored. Despite being far removed from the coordinated metal ion, the different backbone patterns significantly influence the steric encumbrance exerted by the ligands, as revealed by differences in (a) the structural features of the Cu(I) and Cu(II) complexes and (b) the course of the oxygenation reactions of the Cu(I) compounds. With the less hindered ligand, a rare example of a neutral bis(μ-oxo)dicopper complex was identified on the basis of its diagnostic spectral features (UV-vis, resonance Raman, EPR) and the stoichiometry of O₂ uptake (Cu:O₂ = 2:1). In contrast, oxygenation of the Cu(I) complexes supported by the more hindered ligands yielded novel (superoxo)copper complexes, identified by a Cu:O₂ ratio of 1:1, a lack of an EPR signal, and O-isotope sensitive resonance Raman spectral features (v_O-O = 968 cm^-1, δ¹⁸O₂ = 51 cm^-1). Symmetric coordination of the superoxo ligand is proposed on the basis of Raman data acquired using ¹⁶O¹⁸O (single peak at 943 cm^-1).