Assessment of two cobalt(II) complexes with pincer ligands for the electrocatalytic hydrogen evolution reaction. A comparison of the SNS vs ONS coordination

Abstract

Bis-N-(2,5-dimethoxyphenyl)pyridine-2,6-dicarbothioamide (pdcta) and N-(2,5- dimethoxyphenyl)-6-[(2,5-dimethoxyphenyl)carbamothioyl]pyridine-2-carboxamide (pcta) were synthesized from a one pot mixture as pincer ligands. Crystals of pcta, grown from CDCl3 were structurally characterized, and found to crystallize in the space group P21/c. The Co(II) complexes [Co(II)(κ3-SNS-pdcta)(CH3CO2)(H2O)] (1) and [Co(II)(κ3-ONS-pcta)(CH3CO2)(H2O)]·H2O (2) were prepared from the reaction of pdcta and pcta and Co(CH3CO2)2·4H2O in ethanol at reflux under argon atmosphere. The identities of 1 and 2 were confirmed from their elemental analyses, ESI MS and a series of spectroscopic measurements. Both complexes displayed electrocatalytic hydrogen evolution with p-toluene sulfonic acid monohydrate in the presence and absence of bpy or PPh3 co-ligands. The hydrogen evolution occurred at moderate overpotentials (605–780) mV, with 1 giving better Faradaic efficiencies than 2 under all the conditions explored herein. Thermodynamic estimates, based on the reduction potentials, suggest that the homolytic pathway for the production of hydrogen from the CoIII-hydride species generated in the presence of the proton source, is more favourable than the heterolytic pathway in CH3CN.