Fully Delocalized Mixed-Valent Cu1.5Cu1.5 Complex: Strong Cu-Cu Interaction and Fast Electron Self-Exchange Rate Despite Large Structural Changes

A flexible NHC/pyridine macrocyclic ligand scaffold that provides two tridentate {CNC} compartments is shown to host two Cu ions in reversibly interconvertible redox states, CuICuI (1) and mixed-valent Cu1.5Cu1.5 (2). They were comprehensively characterized by X-ray diffraction (XRD) and multiple spectroscopic methods, including electron paramagnetic resonance (EPR), UV-Vis absorption and magnetic circular dichroism (MCD), in combination with TD-DFT and CASSCF calculations. 2 features an extremely short Cu⋯Cu distance (~2.5 Å; compared to ~4.0 Å in 1) and is a Robin Day class III mixed-valent system with a very high delocalization energy of 13000 cm-1, which is comparable to the mixed-valent Cu1.5Cu1.5 state of the biological CuA site. Electron self-exchange between 1 and 2 is found to be rapid despite large structural reorganizations, which is proposed to proceed via a sequential mechanism involving an active conformational isomer of 1, viz. 1’; the latter has been isolated and characterized by XRD. This electron transfer (ET) process is reminiscent of the conformationally gated ET pathway proposed for biological systems. This redox couple represents a unique pair of dicopper complexes with flexible ligand system, achieving fast electron self-exchange closely related to the function of the CuA site.