Three coordinate models for the binuclear CuA electron-transfer site

Abstract

Truly three coordinate Cu_A synthetic models were developed by employing bulky N-heterocyclic carbene ligands along with simple C₃ and C₄ dithiolate linkers to emulate the role of histidine and cysteine ligands at trigonal copper sites. The reduced [Cu^ICu^I] models [NHC]Cu–S(CH₂)_nS–Cu[NHC] (n = 3 (3), 4 (4)) were characterized by NMR and X-ray crystallography. The structure of the reduced model with the C₃ linker closely mimics the low coordinate, trigonal environment at the reduced dinuclear Cu_A site while the C₄ linker resulted in an “open” solid state structure with two coordinate Cu centers. Oxidized species 3⁺+++ and 4⁺++ were observed by UV-vis and EPR spectroscopy upon the addition of [Cp₂Fe]⁺ to 3 and 4, generating spectroscopic features similar to those of the dinuclear [Cu^1.5Cu^1.5]⁺ Cu_A resting state. Key spectroscopic features of 3⁺+++ and 4⁺++ are low energy intervalence charge transfer bands centered near 1150 nm and EPR spectra indicating delocalization of the unpaired e⁻ among two Cu centers. Electrochemistry of the oxidized [Cu₂S₂]⁺ models reveals reduction potentials (E_1/2 = 0.11 V (3⁺+++); 0.36 V (4⁺++) vs. NHE) similar to the Cu_A site in CcO (E_1/2 = 0.22–0.30 V vs. NHE).