Contrasting copper-dioxygen chemistry arising from alike tridentate alkyltriamine copper(I) complexes

Article abstract of DOI:10.1021/ja0125265

Copper(I)-dioxygen interactions are of great interest due to their role in biological O₂-processing as well as their importance in industrial oxidation processes. We describe here the study of systems which lead to new insights concerning the factors which govern Cu(II)-μ-η²:η² (side-on) peroxo versus Cu(III)-bis-μ-oxo species formation. Drastic differences in O₂-reactivity of Cu(I) complexes which differ only by a single -CH₃ versus -H substituent on the central amine of the tridentate ligands employed are observed. [Cu(MeAN)]B(C₆F₅)₄ (1) (MeAN = N,N,N′,N′,N′-pentamethyl-dipropylenetriamine) reacts with O₂ at -80 °C to form almost exclusively the side-on peroxo complex [{Cu^II(MeAN)}₂(O₂)]²⁺ (3) in CH₂Cl₂, tetrahydrofuran, acetone, and diethyl ether solvents, as characterized by UV-vis and resonance Raman spectroscopies. In sharp contrast, [Cu(AN)]B(C₆F₅)₄ (2) (AN = 3, 3′-iminobis(N,N-dimethyl-propylamine) can support either Cu₂O₂ structures in a strongly solvent-dependent manner. Extreme behavior is observed in CH₂Cl₂ solvent, where 1 reacts with O₂ giving 3, while 2 forms exclusively the bis-μ-oxo species [{Cu^III(AN)}₂(O)₂]²⁺ (4^Oxo). Stopped-flow kinetics measurements also reveal significant variations in the oxygenation reactions of 1 versus 2, including the observations that 4^Oxo forms much faster than does 3; the former decomposes quickly, while the latter is quite stable at 193 K. The solvent-dependence of the bis-μ-oxo versus side-on peroxo preference observed for 2 is opposite to that reported for other known copper(I) complexes; the factors which may be responsible for the unusual behavior of 1/O₂ versus 2/O₂ (possibly N-H hydrogen bonding in the AN chemistry) are suggested. The factors which affect bis-μ-oxo versus side-on peroxo formation continue to be of interest. Copyright