Impact of Intramolecular Hydrogen Bonding on the Reactivity of Cupric Superoxide Complexes with O-H and C-H Substrates

Abstract

The dioxygen reactivity of a series of TMPA-based copper(I) complexes (TMPA=tris(2-pyridylmethyl)amine), with and without secondary-coordination-sphere hydrogen-bonding moieties, was studied at -135 °C in 2-methyltetrahydrofuran (MeTHF). Kinetic stabilization of the H-bonded [( ${\left(X{1}_{}\right)\left(X{2}_{}\right)}^{}$ TMPA)Cu^II (O₂^.- )]⁺ cupric superoxide species was achieved, and they were characterized by resonance Raman (rR) spectroscopy. The structures and physical properties of [( ${\left(X{1}_{}\right)\left(X{2}_{}\right)}^{}$ TMPA)Cu^II (N₃^- )]⁺ azido analogues were compared, and the O₂^.- reactivity of ligand-Cu^I complexes when an H-bonding moiety is replaced by a methyl group was contrasted. A drastic enhancement in the reactivity of the cupric superoxide towards phenolic substrates as well as oxidation of substrates possessing moderate C-H bond-dissociation energies is observed, correlating with the number and strength of the H-bonding groups.

Keywords: H-atom transfer; copper monooxygenases; cupric superoxide; dioxygen reduction; hydrogen bonding.