Isolating an Inner-Sphere Adduct of [RuIV(═O)(terpy)(bpy)]²⁺ and [CeIV(OH)(NO₃)₅]²– with the Oxo Bonded to the CeIV 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 [RuII(terpy)(bpy)(OH2)]2+ (terpy = 2,2′:6′,6″-terpyridine, bpy = 2,2′-bipyridine). This study focuses on inner-sphere electron transfer for the CeIV-driven oxidation of the RuIV═O species into the RuV═O species. The approach to this step became possible by inventing a feasible method to isolate an air-stable RuIV═O powder sample in this work. Importantly, by mixing the thus-obtained RuIV═O sample with CAN (cerium ammonium nitrate), the inner-sphere adduct [RuIV(═O)(terpy)(bpy)][CeIV(NO3)5(OH)] was successfully isolated. The IR spectrum of the isolated adduct exhibits a strong band at 774 cm–1 attributable to the RuIV═O–CeIV stretching vibration, proving covalent bonding of the oxo to the CeIV 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 RuIV═O to the 4f orbital of CeIV, as supported by TD-DFT calculations. The addition of one equivalent of CAN to the RuIV═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 RuIV═O–CeIV core in the CeIV-driven WO catalysis.