Isolating an Inner-Sphere Adduct of [Ru IV (═O)(terpy)(bpy)] 2+ and [Ce IV (OH)(NO 3 ) 5 ] 2- with the Oxo Bonded to the Ce IV 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 ^II (terpy)(bpy)(OH ₂ )] ²⁺ (terpy = 2,2':6',6″-terpyridine, bpy = 2,2'-bipyridine). This study focuses on inner-sphere electron transfer for the Ce ^IV -driven oxidation of the Ru ^IV ═O species into the Ru ^V ═O species. The approach to this step became possible by inventing a feasible method to isolate an air-stable Ru ^IV ═O powder sample in this work. Importantly, by mixing the thus-obtained Ru ^IV ═O sample with CAN (cerium ammonium nitrate), the inner-sphere adduct [Ru ^IV (═O)(terpy)(bpy)][Ce ^IV (NO ₃ ) ₅ (OH)] was successfully isolated. The IR spectrum of the isolated adduct exhibits a strong band at 774 cm ^-1 attributable to the Ru ^IV ═O-Ce ^IV stretching vibration, proving covalent bonding of the oxo to the Ce ^IV 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 ^IV ═O to the 4f orbital of Ce ^IV , as supported by TD-DFT calculations. The addition of one equivalent of CAN to the Ru ^IV ═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 ^IV ═O-Ce ^IV core in the Ce ^IV -driven WO catalysis.