Cu<sub>4</sub>S Cluster in “0-Hole” and “1-Hole” States: Geometric and Electronic Structure Variations for the Active Cu<sub>Z</sub>* Site of N<sub>2</sub>O Reductase
Yang Liu, Sayanti Chatterjee, George E. Cutsail, Sergey Peredkov, Sandeep K. Gupta, Sebastian Dechert, Serena DeBeer, Franc Meyer
Abstract
High Resolution Image Download MS PowerPoint Slide The active site of nitrous oxide reductase (N 2 OR), a key enzyme in denitrification, features a unique μ 4 -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 {Cu 4 (μ 4 -S)} core structure. Herein, we report the synthesis and characterization of [Cu 4 (μ 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), in both their 0-hole ( 2 ) and 1-hole ( 3 ) states, thus mimicking the two active states of the Cu Z * site during enzymatic N 2 O reduction. Structural and electronic properties of these {Cu 4 (μ 4 -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 4 (μ 4 -S)} core occurs upon oxidation from 2 (τ 4 (S) = 0.46, seesaw) to 3 (τ 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 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 μ 4 -S in the fully reduced 0-hole state.