Electrochemical properties and C-H bond oxidation activity of [Ru(tpy)(pyalk)Cl]+ and [Ru(tpy)(pyalk)(OH)]. Dalton Trans 2018;47:9701-9708. [PMID: 29978176 DOI: 10.1039/c8dt02260g]

Electrocatalytic properties of a novel ruthenium(II) terpyridine-based complex towards CO2 reduction

The electrocatalytic properties of Ru complexes are of great technological interest given their potential application in reactions such water splitting and CO₂ reduction. In this work, a novel terpyridine-based Ru(II) complex, [RuCl(trpy)(acpy)], trpy = 2,2':6',2''-terpyridine, acpy^- = 2-pyridylacetate was synthesized and its spectroscopic, electrochemical and catalytic properties were explored in detail. In dry acetonitrile, the complex exhibits two reduction peaks at -1.95 V and -2.20 V vs. Fc/Fc⁺, attributed to consecutive 1 e^- reduction. Under CO₂ atmosphere, a catalytic wave is observed (E_onset = 2.1 V vs. Fc/Fc⁺), with CO as the main reduction product. Bulk electrolysis reveals a turnover number (TON) of 12 (k_obs = 1.5 s^-1). In the presence of 1% water, an improvement in the catalytic activity is observed (TON_CO = 21 and k_obs = 2.0 s^-1) and, additionally, formate was also detected (TON_HCOO = 7). Spectroelectrochemical experiments allowed the identification of a metallocarboxylate (Ru-COO^-) intermediate under anhydrous conditions, while in water, the partial labilization of the acpy^- ligand was observed in the course of the catalytic cycle. The experimental data was combined with DFT calculations, allowing the proposal of a catalytic cycle. The results establish important relationships between selectivity, ligand structure and reaction conditions.

Catalytic water oxidation by an in situ generated ruthenium nitrosyl complex bearing a bipyridine-bis(alkoxide) ligand

Oxidative degradation and transformation of catalysts are commonly observed in water oxidation by molecular catalysts, especially when a highly oxidizing reagent such as (NH₄)₂[Ce(NO₃)₆] [Ce(IV)] is used. We report herein the synthesis of a ruthenium(III) complex bearing an oxidative resistant bipyridine-bis(alkoxide) ligand, [Ru(bdalk)(pic)₂]⁺ (1, H₂bdalk = 2,2'-([2,2'-bipyridine]-6,6'-diyl)bis(propan-2-ol), pic = 4-picoline) as a water oxidation catalyst (WOC). A ruthenium(II) nitrosyl complex [Ru(Hbdalk)(NO)(pic)₂]²⁺ (3) was also formed during the water oxidation process by 1/Ce(IV), and was isolated and structurally characterized. Complex 3 was found to be an active WOC, with the nitrosyl group remaining intact during water oxidation.