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 achieve 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)(OH2)]2+ (terpy = 2,2':6',6''-terpyridine, bpy = 2,2'-bipyridine). This study focuses on the 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)(NO3)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 the 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 the Ru(IV)=O to the 4f orbital of Ce(IV), as supported by TD-DFT calculations. The addition of one equivalent 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 for the first time 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.