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 II (Ph-trpy)(Cl)(L)]ClO 4 , [ 1 ]ClO 4 -[ 3 ]ClO 4 (Ph-trpy: 4'-phenyl-2,2':6',2″-terpyridine; L1: 2,2 ' -azobis(benzothiazole) ([ 1 ]ClO 4 ); L2: 2,2'-azobis(6-methylbenzothiazole) ([ 2 ]ClO 4 ); L3: 2,2'-azobis(6-chlorobenzothiazole) ([ 3 ]ClO 4 )). A collective consideration of experimental (i.e., structural and spectroelectrochemical) and theoretical (DFT calculations) results of [ 1 ]ClO 4 -[ 3 ]ClO 4 established selective stabilization of (i) the unperturbed azo (N═N) 0 function of L, (ii) the exclusive presence of the isomeric form involving the N(azo) donor of L trans to Cl, and (iii) the presence of extended, hydrogen-bonded trimeric units in the asymmetric unit of [ 2 ]ClO 4 (CH---O) via the involvement of ClO 4 - anions. The detailed electrochemical studies revealed metal-based oxidation of [Ru II (Ph-trpy)(Cl)(L)] + ( 1 + - 3 + ) to [Ru III (Ph-trpy)(Cl)(L)] 2+ ( 1 2+ - 3 2+ ); however, the electronic form of the first reduced state ( 1 - 3 ) could be better represented by its mixed Ru II (Ph-trpy)(Cl)(L •- )/Ru III (Ph-trpy)(Cl)(L 2- ) state. Both native ( 1 + - 3 + ) and reduced ( 1 - 3 ) states exhibited weak lower energy transitions within the range of 1000-1200 nm. Further, [ 1 ]ClO 4 -[ 3 ]ClO 4 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 [ 2 ]ClO 4 due to the presence of electron-donating methyl groups in the L2 backbone. The faster OER kinetics and high catalytic stability of [ 2 ]ClO 4 could also be rationalized by its lowest Tafel slope (85 mV dec -1 ) and choronoamperometric experiment (stable up to 12 h), respectively, along with high Faradic efficiency (∼97%). A comparison of [ 2 ]ClO 4 with the reported analogous ruthenium complexes furnished its excellent intrinsic water oxidation activity.