Cu4S Cluster in "0-Hole" and "1-Hole" States: Geometric and Electronic Structure Variations for the Active CuZ* Site of N2O Reductase

The active site ofnitrous oxide reductase (N2OR), akey enzyme in denitrification, features a unique & mu;(4)-sulfido-bridged tetranuclear Cu cluster (the so-called Cu-Z or Cu-Z* site). Details of the catalytic mechanism haveremained under debate and, to date, synthetic model complexes of theCu(Z)*/Cu-Z sites are extremely rare due to thedifficulty in building the unique {Cu-4(& mu;(4)-S)} core structure. Herein, we report the synthesis and characterizationof [Cu-4(& mu;(4)-S)](n+) (n = 2, 2; n = 3, 3) clusters,supported by a macrocyclic {py(2)NHC(4)} ligand(py = pyridine, NHC = N-heterocyclic carbene), inboth their 0-hole (2) and 1-hole (3) states,thus mimicking the two active states of the Cu-Z* site duringenzymatic N2O reduction. Structural and electronic propertiesof these {Cu-4(& mu;(4)-S)} clusters are elucidatedby employing multiple methods, including X-ray diffraction (XRD),nuclear magnetic resonance (NMR), UV/vis, electron paramagnetic resonance(EPR), Cu/S K-edge X-ray emission spectroscopy (XES), and Cu K-edgeX-ray absorption spectroscopy (XAS) in combination with time-dependentdensity functional theory (TD-DFT) calculations. A significant geometrychange of the {Cu-4(& mu;(4)-S)} core occursupon oxidation from 2 (& tau;(4)(S) = 0.46,seesaw) to 3 (& tau;(4)(S) = 0.03, square planar),which has not been observed so far for the biological Cu-Z(*) site and is unprecedented for known model complexes. The singleelectron of the 1-hole species 3 is predominantly delocalizedover two opposite Cu ions via the central S atom, mediated by a & pi;/& pi;superexchange pathway. Cu K-edge XAS and Cu/S K-edge XES corroboratea mixed Cu/S-based oxidation event in which the lowest unoccupiedmolecular orbital (LUMO) has a significant S-character. Furthermore,preliminary reactivity studies evidence a nucleophilic character ofthe central & mu;(4)-S in the fully reduced 0-hole state.