Modulation of peripheral functional groups for the enhancement of electrochemical HER activity in bimetallic Ni-containing triply-fused porphyrins

We report here nickel(II) complexes (Ni-1 and Ni-2) of triply fused (covalently linked through β–β, meso–meso, β′–β′ linkages) porphyrins functionalized with tert-butyl and O-hexyl chains for the investigation of the electrocatalytic hydrogen evolution reaction (HER). The analogous monometallic porphyrins Ni-3 and Ni-4 were also synthesized for comparative analysis. Cyclic voltammetry of the bimetallic complexes functionalized with tert-butyl and O-hexyl chains showed an anodic shift in reduction potentials of Ni-1. The influence of the long alkyl chains was even more pronounced during electrochemical proton reduction with trifluoroacetic acid (TFA), as confirmed by the higher icat/ip and rate constants (kobs) of Ni-2 compared to Ni-1. Additionally, Faradaic efficiencies for the HER is more than 90 % for Ni-2, which is three times larger than Ni-1. Ni-2 also displays turnover numbers that are more than 200 times larger than Ni-1. A mechanistic investigation using UV-vis-NIR and EPR spectroscopy as well theoretical calculations indicate the involvement of the doubly reduced species as an active catalytic intermediate in the electrocatalytic process.