Synthesis, characterisation and properties towards CO2 reduction into CO of a Cr(III) diphenyl quaterpyridine complex.

Abstract

During the last decade, group VI metals have regained interest in synthesizing inorganic complexes that demonstrate catalytic properties for CO2 reduction. However, these complexes often exhibit high overpotential and poor efficiency compared to those constructed with less-abundant metals. In this work, a polypyridinyl complex consisting of a Cr(III) centre coordinated to the tetradentate 4',4''-diphenyl-2,2':6',2'':6'',2'''-quaterpyridine ligand and two monodentate chloride ligands ([Cr(dpqpy)Cl2]+) has been synthesized and characterized. Cyclic voltammetry performed under argon displays three quasi-reversible one-electron systems in the cathodic part. Zn(II) parent complex ([Zn(dpqpy)Cl2]) was also synthesized and investigated to identify the nature of the reduction events. The three reduced species of [Cr(dpqpy)Cl2]+ were generated by exhaustive electrolysis and characterized by ultraviolet-visible and electron paramagnetic resonance spectroscopies. Density functional theory calculations confirmed that the two last reduction are localized on the quaterpyridine ligand. Under CO2 with [Cr(dpqpy)Cl2]+ dissolved in DMF in the presence of phenol, a catalytic current is observed during the last reduction at an onset potential of -1.72 V vs Ag/AgNO3 (10-2 M). Exhaustive electrolysis of [Cr(dpqpy)Cl2]+ performed at -1.85 V under CO2 in DMF + 0.1 M TBAPF6 in presence of 1.25 M of phenol led to the formation of CO with a faradaic efficiency of 60% after 2hrs, with no H2 being detected during the reaction time. Longer experiments lead to a partial degradation of the complex.