The ferrocene effect: enhanced electrocatalytic hydrogen production using meso-tetraferrocenyl porphyrin palladium(ii) and copper(ii) complexes†
Copper(II) and palladium(II) meso-tetraferrocenylporphyrins (CuTFcP and PdTFcP) were employed as catalysts for electrochemical proton reduction in DMF using trifluoroacetic acid (TFA) or triethylamine hydrochloride (TEAHCl) as acids. Gas analysis under electrocatalytic conditions at a glassy carbon working electrode confirmed the product as H2. CuTFcP showed catalytic behavior for both TFA and TEAHCl, whereas only TFA worked for PdTFcP. The performance of the two compounds for electrocatalytic hydrogen generation was compared to the analogous copper(II) and palladium(II) meso-tetraphenylporphyrins (CuTPP and PdTPP) under identical conditions. The presence of the ferrocence groups on the porphyrin favourably shift the overpotential to a less negative value by around 200 mV and increases the catalytic rate of hydrogen production in DMF/TFA by an order of magnitude to 6 × 103 s−1. Moreover, while CuTPP is fully inactive in a DMF/TEAHCl mixture, the ferrocene subunits activate the CuTFcP catalyst. Spectroelectrochemistry experiments and DFT calculations were consistent with a catalytic process proceeding via the phlorin anion.