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12 Although a tricopper active site in pMMO has been disputed, the conversion of methane to methanol was demonstrated with small-molecule tricopper complexes. Synthetic tricopper clusters have been a prominent synthetic target for the (bio)inorganic community over the past few decades, 1–9 since tricopper centers were identified/proposed as essential active sites for biological reduction of O 2 to H 2O in multicopper oxidase (MCO) 10,11 and aerobic hydroxylation of methane in particulate methane monooxygenase (pMMO). Fast electron transfer rates (10 5 to 10 6 M −1 s −1) were observed for both Cu ICu ICu I/Cu IICu ICu I and Cu IICu ICu I/Cu IICu IICu I redox couples, approaching the rapid electron transfer rates of copper sites in MCO. For the first time, a synthetic tricopper cluster was isolated and fully characterized at Cu ICu ICu I (4a), Cu IICu ICu I (4b), and Cu IICu IICu I (4c) states, providing structural and spectroscopic models for many intermediates in MCOs. The geometric constraints provided by the cryptand are reminiscent of Nature's multicopper oxidases (MCOs). Indeed, the fully reduced (PF 6) 2 can reduce O 2 under acidic conditions. The distinct electrochemical behaviors of (PF 6) 3 and its solvent-exposed analog (PF 6) 4 suggest that isolation of tricopper core in a cryptand enables facile electron transfer, allowing potential application of synthetic tricopper complexes as redox catalysts. Contrasting the limited redox capability of synthetic tricopper complexes reported so far, (PF 6) 3 exhibits several reversible single-electron redox events. The fourth TREN caps on top of the tricopper cluster to form a cryptand, imposing conformational constraints and preventing solvent interaction. The macrocyclic azacryptand TREN 4 contains four TREN motifs, three of which provide a bowl-shape binding pocket for the 3+ core. One-pot reaction of tris(2-aminoethyl)amine (TREN), PF 6, and paraformaldehyde affords a mixed-valent (PF 6) 3 complex.
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