Spectroscopic and electronic structure studies of the μ₄-sulfide bridged tetranuclear Cu_z cluster in N₂O reductase: molecular insight into the catalytic mechanism

Spectroscopic methods combined with density functional calculations are used to develop a detailed bonding description of the μ₄-sulfide bridged tetranuclear Cu_z cluster in N₂O reductase. The ground state of Cu_z has the 1Cu^II/3Cu^I configuration. The single electron hole dominantly resides on one Cu atom (Cu_I) and partially delocalizes onto a second Cu atom (Cu_II) via a Cu_I-S-Cu_II σ/σ superexchange pathway which is manifested by a Cu_II → Cu_I intervalence transfer transition in absorption. The observed excitedstate spectral features of Cu_z are dominated by the S → Cu_I charge-transfer transitions and Cu_I based d-d transitions. The intensity pattern of individual S → Cu_I charge-transfer transitions reflects different bonding interactions of the sulfur valence orbitals with the four Cu's in the Cu_z cluster, which are consistent with the individual Cu-S force constants obtained from a normal coordinate analysis of the Cu_z resonance Raman frequencies and profiles. The Cu_I d orbital splitting pattern correlates with its distorted T-shaped ligand field geometry and accounts for the observed low g_II value of Cu_z in EPR. The dominantly localized electronic structure description of the Cu_z site results from interactions of Cu_II with the two additional Cu's of the cluster (Cu_III/Cu_IV), where the Cu-Cu electrostatic interactions lead to hole localization with no metal-metal bonding. The substrate binding edge of Cu_z has a dominantly oxidized Cu_I and a dominantly reduced Cu_IV. The electronic structure description of Cu_z provides a strategy to overcome the reaction barrier of N₂O reduction at this Cu_I/Cu_IV edge by simultaneous two-electron transfer to N₂O in a bridged binding mode. One electron can be donated directly from Cu_IV and the other from Cu_II through the Cu_II-S-Cu_I σ/σ superexchange pathway. A frontier orbital scheme provides molecular insight into the catalytic mechanism of N₂O reduction by the Cu_z cluster.