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 [Ru II (H)(L1)(PPh 3 ) 2 (CO)]ClO 4 ([1a]ClO 4 -[1b]ClO 4 )/[Ru II (H)(L2)(PPh 3 ) 2 (CO)]ClO 4 ([2a]ClO 4 -[2b]ClO 4 ) involving azo coupled L1 [L1: ( E )-1,2-bis(1-methyl-1 H -pyrazol-3-yl)diazene]/L2 [L2: ( E )-1,2-bis(4-iodo-1-methyl-1 H -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]ClO 4 /[1b]ClO 4 or [2a]ClO 4 /[2b]ClO 4 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]ClO 4 /[2b]ClO 4 or [1b]ClO 4 , 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]ClO 4 /[1b]ClO 4 or [2a]ClO 4 /[2b]ClO 4 immobilised on the carbon cloth support underwent various electrochemical acidic HERs (hydrogen evolution reactions) with TOF/10 -1 s -1 : [1a]ClO 4 (0.83) > [1b]ClO 4 (0.68) > [2a]ClO 4 (0.50) > [2b]ClO 4 (0.37).