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

Abstract

The present article deals with the structurally and spectroelectrochemically characterized newer class of ruthenium-azoheteroarenes [RuII(Ph-trpy)­(Cl)­(L)]­ClO4, [1]­ClO4–[3]­ClO4 (Ph-trpy: 4′-phenyl-2,2′:6′,2″-terpyridine; L1: 2,2′-azobis­(benzothiazole) ([1]­ClO4); L2: 2,2′-azobis­(6-methylbenzothiazole) ([2]­ClO4); L3: 2,2′-azobis­(6-chlorobenzothiazole) ([3]­ClO4)). A collective consideration of experimental (i.e., structural and spectroelectrochemical) and theoretical (DFT calculations) results of [1]­ClO4–[3]­ClO4 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]­ClO4 (CH---O) via the involvement of ClO4 – anions. The detailed electrochemical studies revealed metal-based oxidation of [RuII(Ph-trpy)­(Cl)­(L)]+ (1 +–3 +) to [RuIII(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 RuII(Ph-trpy)­(Cl)­(L•–)/RuIII(Ph-trpy)­(Cl)­(L2–) state. Both native (1 +–3 +) and reduced (1–3) states exhibited weak lower energy transitions within the range of 1000–1200 nm. Further, [1]­ClO4–[3]­ClO4 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]­ClO4 due to the presence of electron-donating methyl groups in the L2 backbone. The faster OER kinetics and high catalytic stability of [2]­ClO4 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]­ClO4 with the reported analogous ruthenium complexes furnished its excellent intrinsic water oxidation activity.

Ruthenium-hydride-mediated stabilisation of the azo anion radical of azobis(benzothiazole) and its reversible electron-reservoir feature

“…Unsymmetric N a (azo) and N r (thiazole) donors of L generated the possibility of two positional isomeric forms, C and D , based on the trans and cis modes of Ru–Cl with respect to the N a of L, respectively (Scheme 4). 11 The exclusive formation of the energetically favoured C form (Table 3) could be rationalised on account of the trans -effect exerted by the azo function of L.…”

“…S7, ESI †). 9,11 The average trans/cis angles of [1]ClO 4 : 173.00(8)1/90.90(9)1, [2]ClO 4 : 171.84(4)1/91.00(6)1, [3]ClO 4 : 172.96(9)1/90.69 (10)1 a DFT optimised.…”

“…The ruthenium ion in each case fitted reasonably well in the equatorial plane involving L, CO, Cl, and the transconfigured two PPh 3 units occupied the axial position. 12 The relatively shorter Ru1-N1(azo) ( 1.249( 7) Å; L2: 1.268(4) Å; L3: 1.266(6) Å), 8,9,11 reflected the impact of dp(Ru II )p*(azo) back-donation. The distances corresponding to Ru-P, Ru-Cl, Ru-CO, and C-O were in good agreement with reported analogous systems.…”

“…ClO 4 ) and their facile interconversion (Scheme 6) indeed delineated its reversible electron-reservoir feature. 8,9,11 The selective participation of the L-dominated orbitals in the oxidation (O1: L À -L 0 ) and reduction (R1: L À -L 2À ) steps of 1-3 without the involvement of a metal-based orbital (Scheme 6) was supported by collective consideration of (i) the DFT-calculated MOs (Table 5 and Tables S5-S13, ESI †), (ii) the shortening/lengthening of the calculated azo (N1-N4) distances of L, respectively (Table 6), as well as (iii) by their EPR silent feature. The absence of otherwise expected metal-based oxidation (Ru II -Ru III ) within the experimental potential range of +1.5 V versus SCE in CH 2 Cl 2 could be a reflection of the impact of p-accepting 2PPh 3 /CO in the coordination sphere.…”

“…The spectral profiles of 1 n -3 n varied to some extent in terms of the wavelength and intensity as a function of R (H/Me/Cl) in the framework of L as well as the redox state of L. In general, the TD-DFT calculations suggested a severe mixing of the metal and ligand-based orbitals in the electronic transitions of 1 n -3 n due to the inherent covalency factor. 11,19 The key features are highlighted below:…”