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

AbstractA flexible macrocyclic ligand with two tridentate {CNC} compartments can host two Cu ions in reversibly interconvertible states, CuICuI (1) and mixed‐valent Cu1.5Cu1.5 (2). They were characterized by XRD and multiple spectroscopic methods, including EPR, UV/Vis absorption and MCD, in combination with TD‐DFT and CASSCF calculations. 2 features a short Cu⋅⋅⋅Cu distance (≈2.5 Å; compared to ≈4.0 Å in 1) and a very high delocalization energy of 13 000 cm−1, comparable to the mixed‐valent state of the biological CuA site. Electron self‐exchange between 1 and 2 is rapid despite large structural reorganization, and is proposed to proceed via a sequential mechanism involving an active conformer of 1, viz. 1′; the latter has been characterized by XRD. Such electron transfer (ET) process is reminiscent of the conformationally gated ET proposed for biological systems. This redox couple is a unique pair of flexible dicopper complexes, achieving fast electron self‐exchange closely related to the function of the CuA site.