Spectroscopic and electrochemical characterization of a terpyridine-based Ru(II) complex with 2-pyridylacetate as ancillary ligand

Abstract

Coordination compounds are capable of catalysing reactions of great technological interest, for example, the watter splitting or the CO2 reduction. In this project, the new Ru(II) complex, [RuCl(trpy)(acpy)], trpy = 2,2’:6’,2”-terpyridine, acpy = 2-pyridylacetate was synthesized and its spectroscopic, electrochemical and catalytic properties explored in detail. Hydrogen nuclear magnetic resonance and vibrational infrared spectra were used to prove the structure of compound and suggest the bidentated coordination of acpy ligand. Spectrophotometric essays, corroborated by theoretical simulations, reveal that lower electronic transitions are dominated by metal-ligand charge transfer (MLCT). Electrochemical studies revealed that in anodic potentials the peak observed at 0.006 V vs Fc/Fc+ correspond to the redox pair RuII/III. In cathodic potentials, two reduction peaks were detected at -1.95 V and -2.20 V vs Fc/Fc+, being attributed to the trpy and metal center reductions respectively. Under anhydrous conditions and a CO2 atmosphere, the complex presented an electrocatalytic activity converting CO2 to CO with a TONCO = 12 and kcat = 1.5 s-1. In the presence of 1% water (v/v), the TONCO = 21 and kcat = 2.0 s-1 were obtained and, additionally, formate was detected with TONHCOO- = 7. The faradaic efficiencies obtained with 1% water were 76% and 20%, respectively for CO and formate. Spectroelectrochemical experiments in the infrared range and under a CO2 atmosphere reveal the possible formation of the metallocarboxylate (Ru-COO-). In the presence of water the possible chelate breakdown was observed.