Oxidation of C–H and O–H bonds by a copper complex inspired by the Cu(ii)–tyrosyl species formed in LPMOs

Abstract

Cupric tyrosyl intermediates have been invoked as active oxidants in oxidase and oxygenase Cu-dependent metalloenzymes. Inspired by these natural oxidants, we report the proton-coupled electron transfer (PCET) reactivity of Cu complexes bound by a tridentate redox-active ONO pincer ligand and an ancillary amine ligand, [LCu(A)]ⁿ⁺ (L = bis(3,5-di-tert-butyl-2-hydroxyphenyl)amine; A = triethylamine (NEt₃) or N,N,N′,N′-tetramethylpropane-1,3-diamine (tmpda); n = 0, 1). Analysis of the stoichiometry of the reactions indicated that the iminosemiquinone complex [^sqLCu(NEt₃)] acts as 1H⁺/1e⁻ PCET acceptor, while the benzoquinone analogue [^bqLCu(NEt₃)]⁺ reacts in a 2H⁺/2e⁻ fashion. Thermochemical analysis of the PCET reactivity of [^sqLCu(NEt₃)] and [^bqLCu(NEt₃)]⁺ revealed that [^bqLCu(NEt₃)]⁺ is a stronger H-atom acceptor, which led to faster PCET reactions. [^bqLCu(NEt₃)]⁺ reacted with substrates containing weak O–H bonds and, to our surprise, also abstracted H-atoms from C–H substrates. The reactivity of [^bqLCu(NEt₃)]⁺ was compared with other Cu complexes developed in our laboratory that are stronger H-atom acceptors but do not oxidize C–H substrates, suggesting that non-thermodynamic factors contribute to the enhanced reactivity of [^bqLCu(NEt₃)]⁺ towards C–H bonds. This work describes the first example of Cu complex bound by a redox-active ligand able to oxidize C–H bonds, and provides evidence of the involvement of similar species in the oxidation of organic substrates catalyzed by Cu-dependent metalloenzymes such as lytic polysaccharide monooxygenases.