A One-Hole Cu₄S Cluster with N₂O Reductase Activity: A Structural and Functional Model for Cu_Z*

Abstract

During bacterial denitrification, two-electron reduction of N2O occurs at a [Cu4(μ4-S)] catalytic site (CuZ*) embedded within the nitrous oxide reductase (N2OR) enzyme. In this communication, an amidinate-supported [Cu4(μ4-S)] model cluster in its 1-hole (S = ½) redox state is thoroughly characterized. Along with its 2-hole redox partner and fully reduced clusters reported previously, the new species completes the two-electron redox series of [Cu4(μ4-S)] model complexes with catalytically relevant oxidation states for the first time. More importantly, N2O is reduced by the 1-hole cluster to produce N2 and the 2-hole cluster, thereby completing a closed cycle for N2O reduction. The title complex is thus not only the best structural model for CuZ* to date, but it also serves as a functional CuZ* mimic.

Probing the electronic and mechanistic roles of the μ4-sulfur atom in a synthetic CuZ model system

“…In our previous report of N 2 22 we were able to use NMR spectroscopy, isotopic labeling experiments, and post situ electrophilic trapping to establish the presence of three products: neutral [Cu 4 (m 4 -S)(m 2 -NCN) 4 ] (the 2-hole cluster, referred to here as [4Cu:1S] 0 ), N 2 , and O 2À . However, we were unable to denitively establish the reaction stoichiometry at that time.…”

“…Finally, a formamidinate-supported [Cu 4 S] cluster in its formally 3Cu I :1Cu II ([4Cu:1S] 1À ) state was found to reduce 15 N 2 O to 15 N 2 (Scheme 1b). 22,23 Here the m 4 -sulde bridge remained intact during a formal oxidation to the 2Cu I :2Cu II ([4Cu:1S] 0 ) redox state of the cluster, allowing us to establish a closed cycle for N 2 O reduction. Based on these results, the potential role (or lack thereof) of the bridging sulde ligand in copper-sulfur clusters merits further investigation.…”

N2O Reductase Activity of a [Cu4S] Cluster in the 4CuI Redox State Modulated by Hydrogen Bond Donors and Proton Relays in the Secondary Coordination Sphere

“…This 4Cu I complex lacks any well‐defined redox chemistry or N 2 O reactivity but, separately, has shown intriguing photochemical properties . Our group recently accessed the 2Cu I :2Cu II and 3Cu I :1Cu II redox states of a [Cu 4 (μ 4 ‐S)] cluster using the bridging formamidinate ligands [(2,4,6‐Me 3 C 6 H 2 N) 2 CH] − , but the 4Cu I state that would model the active form of Cu Z was unstable and could not be accessed synthetically.…”

Progress toward the Isolation of Late Metal Terminal Sulfides

“…[117] Most copper sulfide chemistry has focused on the synthesis of models of the Cu z site in N 2 OR, [75,[118][119][120][121][122][123][124] and not necessarily on the synthesis of terminal copper sulfides. In this regard, several groups have successfully generated copper sulfide clusters that closely resemble the [Cu 4 (μ 4 -S)] core of the Cu z site, including [K(18-crown-6)][ NCN LCu 4 (μ 4 -S)] (67; NCN L = (2,4,6-Me 3 C 6 H 2 N) 2 CH), [17,125] which can reduce N 2 O in a stoichiometric fashion (Scheme 17). Although other copper sulfide or thiolate clusters have been shown to reduce N 2 O, [126] such as [{(Me 3 tacn)Cu} 3 (μ-η 2 :η 1 :η 1 -S 2 )][OTf ] 2 , [121] this finding is notable because 67 more closely approximates the structure of the Cu z site in N 2 OR.…”