Inner-Sphere Electron Transfer Induced Reversible Electron Reservoir Feature of Azoheteroarene Bridged Diruthenium Frameworks

Fine Tuning between Radical versus Nonradical States of Azoheteroarenes on Selective Osmium Platforms

The article highlights the cooperative impact of azoheteroarenes [abbt: 2,2'-azobis(benzothiazole), L1-L3; bmpd: (E)-1,2-bis(1-methyl-1H-pyrazole-3-yl) diazene, L4] and coligands [bpy: 2,2'-bipyridine; pap: 2-phenylazopyridine] in tuning radical (N-N•-) versus nonradical (N═N0) states of L on selective OsII-platforms in structurally/spectroscopically characterized monomeric [1]ClO4-[6]ClO4 and [1](ClO4)2-[2](ClO4)2/[7](ClO4)2-[8](ClO4)2, respectively. The preferred syn-configuration of L in the complexes prevented obtaining ligand bridged dimeric species. It revealed that {Os(bpy)2} facilitated the stabilization of both nonradical ([1](ClO4)2-[2](ClO4)2) and radical ([1]ClO4-[2]ClO4) states of L1/L2, while it delivered exclusively the radical form for L3 in [3]ClO4. In contrast, {Os(pap)2} generated radical states of L1-L3 in [4]ClO4-[6]ClO4, respectively, without any alteration of the redox state of OsII and azo (N═N0) function of the pap coligand. The neutral state of L4 was, however, ascertained in [7](ClO4)2 or [8](ClO4)2 irrespective of the nature of the metal fragment {Os(bpy)2} or {Os(pap)2}, respectively. Switching between radical and nonradical forms of L in the complexes as a function L and coligand could be addressed based on their relative FMO (frontier molecular orbital) energies. Multiple close redox steps of the complexes extended a competitive electron transfer scenario between the redox active components including metal/L/bpy/pap, leading to delicate electronic forms in each case.

The isomer-sensitive electrochemical HER of ruthenium(II)-hydrido complexes involving redox-active azoheteroaromatics

The article deals with the development of isomeric ruthenium(II)-hydrido complexes [RuII(H)(L1)(PPh3)2(CO)]ClO4 ([1a]ClO4-[1b]ClO4)/[RuII(H)(L2)(PPh3)2(CO)]ClO4 ([2a]ClO4-[2b]ClO4) involving azo coupled L1 [L1: (E)-1,2-bis(1-methyl-1H-pyrazol-3-yl)diazene]/L2 [L2: (E)-1,2-bis(4-iodo-1-methyl-1H-pyrazol-3-yl)diazene], respectively. Structural evaluation of the complexes affirmed the syn conformation of the coordinated/uncoordinated pyrazole groups of L and its unperturbed neutral azo (NN) state. Isomeric forms in [1a]ClO4/[1b]ClO4 or [2a]ClO4/[2b]ClO4 differed with respect to the cis and trans orientations of the coordinated CO and N(azo) donor of L, respectively. It also demonstrated the formation of intermolecular hydrogen-bonded dimeric or 1D-polymeric chains in [1a]ClO4/[2b]ClO4 or [1b]ClO4, respectively. Successive two-electron reductions of the complexes varied to an appreciable extent as a function of the heterocycles connected to L. The involvement of the azo function of L towards the reductions ([NN]0 → [NN]˙- → [NN]2-) was supported by the DFT calculated MOs and Mulliken spin density at the paramagnetic state, which was further validated by the radical EPR profile of the first reduced (S = 1/2) state. Isomeric [1a]ClO4/[1b]ClO4 or [2a]ClO4/[2b]ClO4 immobilised on the carbon cloth support underwent various electrochemical acidic HERs (hydrogen evolution reactions) with TOF/10-1 s-1: [1a]ClO4 (0.83) > [1b]ClO4 (0.68) > [2a]ClO4 (0.50) > [2b]ClO4 (0.37).

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)]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 [Ru^II(Ph-trpy)(Cl)(L)]⁺ (1⁺-3⁺) to [Ru^III(Ph-trpy)(Cl)(L)]²⁺ (1²⁺-3²⁺); 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]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.