Rapid Electron Transfer Self-Exchange in Conformationally Dynamic Copper Coordination Complexes

We report the electron transfer (ET) self-exchange rate constants (k₁₁) for a pair of Cu^II/I complexes utilizing dpa^R (dpa = dipicolylaniline, R = OMe, SMe) ligands assessed by NMR line broadening experiments. These ligands afford copper complexes that are conformationally dynamic in one oxidation state. With R = OMe, the Cu^I complex is dynamic, while with R = SMe, the Cu^II complex is dynamic. Both complexes exhibit unexpectedly large k₁₁ values of 2.48(6) × 10⁵ and 2.21(9) × 10⁶ M^-1 s^-1 for [CuCl(dpa^OMe)]^+/0 and [CuCl(dpa^SMe)]^+/0, respectively. Among the fastest reported molecular copper coordination complexes to date, that of [CuCl(dpa^SMe)]^+/0 exceeds all others by an order of magnitude and compares only with those observed in type 1 blue copper proteins. The dynamicity of these complexes establishes pre-steady-state conformational equilibria that minimize the inner-sphere reorganization energies to 0.71 and 0.62 eV for R = OMe and SMe, respectively. In contrast to the emphasis on rigidity in the formulation of entatic states applied to blue copper proteins, the success of these two systems highlights the relevance of conformational dynamicity in mediating rapid ET.