Cu₄S Cluster in "0-Hole" and "1-Hole" States: Geometric and Electronic Structure Variations for the Active Cu_Z* Site...

The active site of nitrous oxide reductase (N₂OR), a key enzyme in denitrification, features a unique μ₄-sulfido-bridged tetranuclear Cu cluster (the so-called Cu_Z or Cu_Z* site). Details of the catalytic mechanism have remained under debate and, to date, synthetic model complexes of the Cu_Z*/Cu_Z sites are extremely rare due to the difficulty in building the unique {Cu₄(μ₄-S)} core structure. Herein, we report the synthesis and characterization of [Cu₄(μ₄-S)]ⁿ⁺ (n = 2, 2; n = 3, 3) clusters, supported by a macrocyclic {py₂NHC₄} ligand (py = pyridine, NHC = N-heterocyclic carbene), in both their 0-hole (2) and 1-hole (3) states, thus mimicking the two active states of the Cu_Z* site during enzymatic N₂O reduction. Structural and electronic properties of these {Cu₄(μ₄-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 {Cu₄(μ₄-S)} core occurs upon oxidation from 2 (τ₄(S) = 0.46, seesaw) to 3 (τ₄(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 single electron of the 1-hole species 3 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 μ₄-S in the fully reduced 0-hole state.