Carboxylate pentapyridines: Pathway to surface modification and tuneable catalytic proton reduction

The ability to synthetically tune catalytic performance is a key advantage in the use of molecular catalysts. A series of cobalt pentapyridine carboxylate esters [Co(Py5Me₂COOMe)(CH₃CN)]²⁺ (Co-Me), [Co(Py5Me₂COOn-Pr)(CH₃CN)]²⁺ (Co-Pr) and [Co(Py5Me₂COOPh)(CH₃CN)]²⁺ (Co-Ph) were successfully synthesised and characterised through HD-MS, FT-IR and cyclic voltammetry. Electrochemical studies reveal the complexes are active for electrocatalytic proton reduction in acetonitrile with acetic acid as the proton source. At an acid concentration of 2 mM, the required overpotential for proton reduction was calculated to be 465 mV for Co-Me, 485 mV for Co-Pr and 360 mV for Co-Ph. The rate constant is calculated to be 9.41 s⁻¹, 4.09 s⁻¹ and 7.25 s⁻¹ for complexes Co-Me, Co-Pr and Co-Ph respectively. The cobalt carboxylate complex [Co(Py5Me₂COO⁻)(OH⁻)]¹⁺ (Co-COOH) was used to prepare chemically modified electrodes from fluorine doped tin oxide (FTO) coated glass and glassy carbon with a calculated surface coverage of 6.38 × 10⁻¹¹ mol/cm² and of 6.18 × 10⁻¹¹ mol/cm² respectively. The successful reduction of catalytic overpotential through esterification offers an alternative pathway for the design of molecular catalysts.