Metal clusters in enzymes carry out the life-sustaining reactions by accumulating multiple redox equivalents in a narrow potential range. This redox potential leveling effect commonly observed in Nature has yet to be reproduced with synthetic metal clusters. Herein, we employ a fully encapsulated synthetic tricopper complex to model the three-electron twoproton reductive regeneration of fully reduced trinuclear copper clusterin multicopper oxidases (MCOs). The tricopper cluster can access four oxidation states (I,I,I to II,II,II) and four protonation states ([Cu 3 (μ 3 -O)] LH, [Cu 3 (μ 3 -OH)]L, [Cu 3 (μ 3 -OH)]LH, and [Cu 3 (μ 3 -OH 2 )]L, where LH denotes the protonated ligand), allowing mechanistic investigation of proton-coupled electron transfer (PCET) relevant to MCOs. Seven tricopper complexes with discrete oxidation and protonation states were characterized with spectroscopy or X-ray single-crystal diffraction. A stepwise electron transfer−proton transfer (ET−PT) mechanism is established for the reduction of Cu II Cu II Cu II (μ 3 -O)LH to Cu II Cu II Cu I (μ 3 -OH)L, while a stepwise PT−ET mechanism is determined for the reduction of Cu II Cu I Cu I (μ 3 -OH)LH to Cu I Cu I Cu I (μ 2 -OH 2 )L. The switch-over from ET−PT to PT−ET mechanism showcases that the tricopper complex can adopt different PCET mechanisms to circumvent high-barrier proton transfer steps. Overall, three-electron two-proton reduction occurs within a narrow potential range of 170 mV, exemplifying the redox potential leveling effect of secondary proton relays in delivering multiple redox equivalents at metal clusters.

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A formal copper(iii) cyanide complex and its C–H cyanation reactivity are reported. The redox potentials of substrates, instead of C–H bond dissociation energies, were found to be the key determinant of the rates of PCET.

While copper nitrosyl complexes are implicated in numerous biological systems, isolable examples remain limited. In this report, we show that [Cl3CuNO]−, with a {CuNO}10 electron configuration, can be generated by nitrite reduction at a copper(I) dichloride anion or by nitric oxide addition to a copper(II) trichloride precursor. The bromide analogue, [Br3CuNO]− was synthesized analogously, and both copper halonitrosyl complexes were characterized by X‐ray diffraction and a variety of spectroscopic methods. Experimental data and multireference (CASSCF/NEVPT2) calculations provide strong evidence for a CuII–NO. ground state. Both [Cl3CuNO]− and [Br3CuNO]− release and recapture NO. reversibly, and exhibit nitrosative reactivities toward a wide range of biological nucleophiles, such as amines, alcohols, and thiols.

Iron and copper enzymes are known to promote reversible S-nitrosation/denitrosation in biology. However, it is unclear how the direction of S–N bond formation/scission is controlled. Herein, we demonstrate the interconversion of metal-S-nitrosothiol adduct M­(RSNO) and metal nitrosyl thiolate complex M­(NO)­(SR), which may regulate the direction of reversible S-(de)­nitrosation. Treatment of a dicopper­(I,I) complex with RSNO leads to a mixture of two structural isomers: dicopper­(I,I) S-nitrosothiol [CuICuI(RSNO)]2+ and dicopper­(II,II) nitrosyl thiolate [CuIICuII(NO)­(SR)]2+. The K eq between these two structural isomers is sensitive to temperature, the solvent coordination ability, and counterions. Our study illustrates how copper centers can modulate the direction of RS–NO bond formation and cleavage through a minor perturbation of the local environment.