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

Abstract

The article deals with the development of isomeric ruthenium(II)–hydrido complexes [Ru^II(H)(L1)(PPh₃)₂(CO)]ClO₄ ([1a]ClO₄–[1b]ClO₄)/[Ru^II(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 (NN) 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 ([NN]⁰ → [NN]˙⁻ → [NN]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).