Probing the electronic and mechanistic roles of the μ 4 -sulfur atom in a synthetic Cu Z model system

• Published in: Chemical science (Cambridge) 2020-04, Vol.11 (13), p.3441-3447
• Main Authors: Rathnayaka, Suresh C, Islam, Shahidul M, DiMucci, Ida M, MacMillan, Samantha N, Lancaster, Kyle M, Mankad, Neal P
• Format: Article
• Language: English
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/c9sc06251c

Nitrous oxide (N O) contributes significantly to ozone layer depletion and is a potent greenhouse agent, motivating interest in the chemical details of biological N O fixation by nitrous oxide reductase (N OR) during bacterial denitrification. In this study, we report a combined experimental/computational study of a synthetic [4Cu:1S] cluster supported by N-donor ligands that can be considered the closest structural and functional mimic of the Cu catalytic site in N OR reported to date. Quantitative N measurements during synthetic N O reduction were used to determine reaction stoichiometry, which in turn was used as the basis for density functional theory (DFT) modeling of hypothetical reaction intermediates. The mechanism for N O reduction emerging from this computational modeling involves cooperative activation of N O across a Cu/S cluster edge. Direct interaction of the μ -S ligand with the N O substrate during coordination and N-O bond cleavage represents an unconventional mechanistic paradigm to be considered for the chemistry of Cu and related metal-sulfur clusters. Consistent with hypothetical participation of the μ -S unit in two-electron reduction of N O, Cu K-edge and S K-edge X-ray absorption spectroscopy (XAS) reveal a high degree of participation by the μ -S in redox changes, with approximately 21% S 3p contribution to the redox-active molecular orbital in the highly covalent [4Cu:1S] core, compared to approximately 14% Cu 3d contribution per copper. The XAS data included in this study represent the first spectroscopic interrogation of multiple redox levels of a [4Cu:1S] cluster and show high fidelity to the biological Cu site.