CuS Cluster in "0-Hole" and "1-Hole" States: Geometric and Electronic Structure Variations for the Active Cu* Site of NO Reductase.

The active site of nitrous oxide reductase (NOR), a key enzyme in denitrification, features a unique μ-sulfido-bridged tetranuclear Cu cluster (the so-called Cu or Cu* site). Details of the catalytic mechanism have remained under debate and, to date, synthetic model complexes of the Cu*/Cu sites are extremely rare due to the difficulty in building the unique {Cu(μ-S)} core structure. Herein, we report the synthesis and characterization of [Cu(μ-S)] ( = 2, ; = 3, ) clusters, supported by a macrocyclic {pyNHC} ligand (py = pyridine, NHC = -heterocyclic carbene), in both their 0-hole () and 1-hole () states, thus mimicking the two active states of the Cu* site during enzymatic NO reduction. Structural and electronic properties of these {Cu(μ-S)} clusters are elucidated by 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-edge X-ray absorption spectroscopy (XAS) in combination with time-dependent density functional theory (TD-DFT) calculations. A significant geometry change of the {Cu(μ-S)} core occurs upon oxidation from (τ(S) = 0.46, seesaw) to (τ(S) = 0.03, square planar), which has not been observed so far for the biological Cu(*) site and is unprecedented for known model complexes. The single electron of the 1-hole species is predominantly delocalized over two opposite Cu ions via the central S atom, mediated by a π/π superexchange pathway. Cu K-edge XAS and Cu/S K-edge XES corroborate a mixed Cu/S-based oxidation event in which the lowest unoccupied molecular orbital (LUMO) has a significant S-character. Furthermore, preliminary reactivity studies evidence a nucleophilic character of the central μ-S in the fully reduced 0-hole state.