Cu(I)/O₂ Chemistry Using a β-Diketiminate Supporting Ligand Derived from <i>N,N</i>-Dimethylhydrazine: A [Cu₃O₂]³⁺ Complex with Novel Reactivity

A Cu­(I) complex, LCu­(CH₃CN), was prepared and characterized, where L^– is a sterically unencumbered β-diketiminate ligand, the deprotonated version of 4-(2,2-dimethylhydrazino)­dimethylhydrazone-3-penten-2-one (LH). Analysis of FTIR spectra of the products of the reaction of LCu­(CH₃CN) with CO indicate that L^– is strongly electron donating, and support an equilibrium in solution between monomeric and dimeric forms with terminal and bridging CO ligands, respectively. Low temperature oxygenation of LCu­(CH₃CN) generated a bis­(μ-oxo)­tricopper complex with a <i>S</i> = 1 [Cu₃O₂]³⁺ core that was identified on the basis of UV–vis (λ_max (ε, M^–1 cm^–1 per Cu) = 328 (10700), 420 (1500), 590 (835) nm) and X-band electron paramagnetic resonance (EPR) spectroscopy (Δ<i>m</i>_<i>s</i> = 2 transition at 1500 G), electrospray ionization (ESI) mass spectrometry, and spectrophotometric titration (0.35(2) equiv of O₂ per copper atom), magnetic susceptibility (μ_eff = 2.8(1) BM), and H₂O₂ detection experiments (no H₂O₂ evolved upon acidification). Unlike other reported variants supported by neutral N-donor ligands, L₃Cu₃O₂ is not reduced by ferrocene, does not abstract H-atoms from phenols or 1,2-dihydroanthracene, oxidizes PPh₃ to Ph₃PO, and generates carbonate species upon exposure to CO₂. This unique reactivity for a [Cu₃O₂]³⁺ complex may be traced to the anionic charge and strong electron donating characteristics of L<sup>–