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)]+ reacted 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 but, to our surprise, it also abstracted H-atoms from C–H substrates. The reactivity of [bqLCu(NEt3)]+ was compared with other Cu complexes developed by our lab that are stronger H-atom acceptors but do not oxidize C–H substrates, concluding 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 it provides evidence on the involvement of similar species in the oxidation of organic substrates catalyzed by Cu-dependent metalloenzymes such as lytic polysaccharide monooxygenase.