A Cu2N2 diamond core structure supported by an [SNS]¯ ligand exhibits a fully reversible one-electron redox process between a reduced CuICuI, {(SNS)Cu}2, and a class III delocalized Cu1.5Cu1.5 state, [{(SNS)Cu}2][B(C6H3(CF3)2)4] ([SNS] ¯ = bis(2-t?butylsulfanylphenyl)amide). The Cu×××Cu distance compresses appreciably (~0.13 Å) upon oxidation; a metal-metal distance of 2.4724(4) Å is observed in the mixed-valence molecule that is nearly identical to the dicopper CuA site found in cytochrome c oxidase. The rate of electron self-exchange (ks) between the CuICuI and the Cu1.5Cu1.5 complexes was estimated to be ³ 107 M-1s-1 by 1H NMR line-broadening analysis. The unusually large magnitude of ks reflects the minimal structural reorganization that accompanies CuICuI ? Cu1.5Cu1.5 interchange.
A second generation of {(PNP)CuI}2 dimer supported by a [PNP]¯ ligand also has been investigated ([PNP]¯ = bis(2-(diisobutylphosphino)phenyl)amide). The highly emissive {(PNP)CuI}2 is characterized by a long-lived excited state (t > 10 ms) with an unusually high quantum yield (f > 0.65) at ambient temperature. Removal of an electron from the {(PNP)CuI}2 dimer yields a nearly isostructural, Cu1.5Cu1.5 complex [{(PNP)Cu}2][B(C6H3(CF3)2)4]. With a highly reducing excited state reduction potential (~ ?3.2 V vs. Fc+/Fc) as well as the availability of two reversible redox processes, these bimetallic copper systems may be interesting candidates for photochemically driven two-electron redox transformations.
Studies of Cu2N2 diamond core complexes supported by the [tBu2-PNP]¯ ligand revealed that the dicopper complex {(tBu2-PNP)Cu}2 can not only be oxidized by one electron to [{(tBu2-PNP)Cu1.5}2][B(C6H3(CF3)2)4], but also by two-electrons to [{(tBu2-PNP)Cu}2][SbF6]2([tBu2-PNP]¯ = bis(2-diisobutylphoshino-4-tbutylphenyl)amide). These Cu2N2 complexes show remarkably low structural reorganization for all oxidation states as evidenced by the solid-state molecular-structures. Based on these studies of [{(tBu2-PNP)Cu}2][SbF6]2, we propose a formulation of one CuI and one paramagnetic CuIII nuclei in compressed?tetrahedral environments in the Cu2N2 core. Spectroscopic, redox, and magnetic data are consistent with a highly covalent M2N2 core supported by a rigid ligand scaffold. These complexes are excellent mimics of the entatic state found in bimetallic copper proteins.
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