Ruthenium Azobis(benzothiazole): Electronic Structure and Impact of Substituents on the Electrocatalytic Single-Site Water Oxidation Process.

The present article deals with the structurally and spectroelectrochemically characterized newer class of ruthenium-azoheteroarenes [Ru(Ph-trpy)(Cl)(L)]ClO, []ClO-[]ClO (Ph-trpy: 4'-phenyl-2,2':6',2″-terpyridine; L1: 2,2-azobis(benzothiazole) ([]ClO); L2: 2,2'-azobis(6-methylbenzothiazole) ([]ClO); L3: 2,2'-azobis(6-chlorobenzothiazole) ([]ClO)). A collective consideration of experimental (i.e., structural and spectroelectrochemical) and theoretical (DFT calculations) results of []ClO-[]ClO established selective stabilization of (i) the unperturbed azo (N═N) function of L, (ii) the exclusive presence of the isomeric form involving the N(azo) donor of L to Cl, and (iii) the presence of extended, hydrogen-bonded trimeric units in the asymmetric unit of []ClO (CH---O) via the involvement of ClO anions. The detailed electrochemical studies revealed metal-based oxidation of [Ru(Ph-trpy)(Cl)(L)] (-) to [Ru(Ph-trpy)(Cl)(L)] (-); however, the electronic form of the first reduced state (-) could be better represented by its mixed Ru(Ph-trpy)(Cl)(L)/Ru(Ph-trpy)(Cl)(L) state. Both native (-) and reduced (-) states exhibited weak lower energy transitions within the range of 1000-1200 nm. Further, []ClO-[]ClO delivered an electrochemical OER (oxygen evolution reaction) process in alkaline medium on immobilizing them to a carbon cloth support, which divulged an amplified water oxidation feature for []ClO due to the presence of electron-donating methyl groups in the L2 backbone. The faster OER kinetics and high catalytic stability of []ClO could also be rationalized by its lowest Tafel slope (85 mV dec) and choronoamperometric experiment (stable up to 12 h), respectively, along with high Faradic efficiency (∼97%). A comparison of []ClO with the reported analogous ruthenium complexes furnished its excellent intrinsic water oxidation activity.