Catching an Entatic State—A Pair of Copper Complexes

The structures of two types of guanidine-quinoline copper complexes have been investigated by single-crystal X-ray crystallography, K-edge X-ray absorption spectroscopy (XAS), resonance Raman and UV/Vis spectroscopy, cyclic voltammetry, and density functional theory (DFT). Independ- ent of the oxidation state, the two structures, which are virtually identical for solids and complexes in solution, resemble each other strongly and are connected by a reversible electron transfer at 0.33 V. By resonant excitation of the two entatic copper complexes, the transition state of the electron transfer is accessible through vibrational modes, which are coupled to metal-ligand charge transfer (MLCT) and ligand-metal charge transfer (LMCT) states. Copper is one of the most important redox-active metals and plays a central role in many biological processes. (1) Blue copper electron-transfer proteins are natures workhorses for electron transfer (ET). They use copper as a one-electron relay that shuttles between the cuprous and cupric oxidation states. Their Cu II/I reduction potentials span a large window (E8' = 0.18 to > 1 V versus NHE (normal hydrogen elec- trode)), (2) which tailors these proteins to interact with a wide variety of ET partners. The inner coordination sphere most directly affects the redox properties of metal ions. (3) Hence, tuning the Cu II/I redox couple is central for electron transfer in nature and also important in synthetic complexes for catalytic applications. (4-6) There are multiple factors governing the redox potential, including the effects of the first coordination