Isolating an Inner-Sphere Adduct of [Ru(═O)(terpy)(bpy)] and [Ce(OH)(NO)] with the Oxo Bonded to the Ce Center.

Water oxidation is a key to achieving sustainable energy cycles, for which higher-valent metal-oxo species often play a key role to accelerate the rate-limiting O-O bond formation. The present study undertook efforts to clarify one of the steps postulated for the water oxidation (WO) catalyzed by [Ru(terpy)(bpy)(OH)] (terpy = 2,2':6',6″-terpyridine, bpy = 2,2'-bipyridine). This study focuses on inner-sphere electron transfer for the Ce-driven oxidation of the Ru═O species into the Ru═O species. The approach to this step became possible by inventing a feasible method to isolate an air-stable Ru═O powder sample in this work. Importantly, by mixing the thus-obtained Ru═O sample with CAN (cerium ammonium nitrate), the inner-sphere adduct [Ru(═O)(terpy)(bpy)][Ce(NO)(OH)] was successfully isolated. The IR spectrum of the isolated adduct exhibits a strong band at 774 cm attributable to the Ru═O-Ce stretching vibration, proving covalent bonding of the oxo to the Ce center. Furthermore, the absorption spectrum of this greenish black powder shows a broad absorption band at 600 nm, suggesting a charge transfer transition from the π* orbital of Ru═O to the 4f orbital of Ce, as supported by TD-DFT calculations. The addition of one equivalent of CAN to the Ru═O solution induces the spectral change due to formation of the 1:1 adduct identical to the isolated adduct. Our study provides a clue to the formation of an inner-sphere adduct having a Ru═O-Ce core in the Ce-driven WO catalysis.