Amido-Bridged Cu_2N_2 Diamond Cores that Minimize Structural Reorganization and Facilitate Reversible Redox Behavior between a Cu^1Cu^1 and a Class III Delocalized Cu^(1.5)Cu^(1.5) Species

Abstract

A novel Cu_(2)N_(2) diamond core structure supported by an [SNS]^(-) ligand (1) ([SNS]^(-) = bis(2-tert-butylsulfanylphenyl)amido) has been prepared. This dicopper system exhibits a fully reversible one-electron redox process between a reduced Cu1Cu1 complex, {[SNS][Cu]}_(2) (2), and a class III delocalized Cu^(1.5)Cu^(1.5) state, [{[SNS][Cu]}_(2_][B(3,5-(CF_(3))_(2)C_(6)H_(3))_(4)] (3). Structural snapshots of both redox forms have been obtained to reveal remarkably little overall structural reorganization. The Cu···Cu bond distance nonetheless undergoes an appreciable compression (0.13 Å) upon oxidation, providing a Cu···Cu distance of 2.4724(4) Å in the mixed-valence state that is virtually identical to the Cu···Cu distance observed in the reduced form of the Cu_(A) site of thiolate-bridged cytochrome c oxidase. Despite the low structural reorganization evident between 2 and 3, the [SNS]^(-) ligand is quite flexible. For example, square-planar geometries can prevail for divalent copper ions supported by [SNS]^(-) as evident from the crystal structure of [SNS]CuCl (4). Physical characterization for the mixed valence complex 3 includes electrochemical, magnetic (SQUID), EPR, and optical data. The complex has also been examined by density functional methods. An attempt was made to measure the rate of electron self-exchange ks between the Cu^(1)Cu^(1) and the Cu^(1.5)Cu^(1.5) complexes 2 and 3 by NMR line-broadening analysis in dichloromethane solution. While the system is certainly in the fast-exchange regime, the exchange process is too fast to be accurately measured by this technique. The value for ks can be bracketed with a conservative lower boundary of ≥107 M^(-1) s^(-1), a value that appears to be larger than other low molecular weight copper model complexes for which similar data is available. The unusually large magnitude of ks likely reflects the minimal structural reorganization that accompanies Cu^(1)Cu^(1) ↔ Cu^(1.5)Cu^(1.5) interchange.

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