The Synthesis and Study of Redox-Rich, Amido-Bridged Cu₂N₂ Dicopper Complexes

Citation

Harkins, Seth Beebe (2006) The Synthesis and Study of Redox-Rich, Amido-Bridged Cu₂N₂ Dicopper Complexes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/1X2Q-3P72. https://resolver.caltech.edu/CaltechETD:etd-08132005-093856

Abstract

A Cu₂N₂ diamond core structure supported by an [SNS]^- ligand exhibits a fully reversible one-electron redox process between a reduced Cu^ICu^I, {(SNS)Cu}₂, and a class III delocalized Cu^1.5Cu^1.5 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 Cu_A site found in cytochrome c oxidase. The rate of electron self-exchange k_s) between the Cu^ICu^I and the Cu^1.5Cu^1.5 complexes was estimated to be ≥ 10⁷ M^-1s^-1 by ¹H NMR line-broadening analysis. The unusually large magnitude of k_s reflects the minimal structural reorganization that accompanies Cu^ICu^I ↔ Cu^1.5Cu^1.5 interchange.

A second generation of {(PNP)Cu^I}₂ dimer supported by a [PNP]^- ligand also has been investigated ([PNP]^- = bis(2-(diisobutylphosphino)phenyl)amide). The highly emissive {(PNP)Cu^I}₂ is characterized by a long-lived excited state (τ > 10 μs) with an unusually high quantum yield (φ > 0.65) at ambient temperature. Removal of an electron from the {(PNP)Cu^I}₂ dimer yields a nearly isostructural, Cu^1.5Cu^1.5 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)Cu^1.5}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 Cu^I and one paramagnetic Cu^III 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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