Fe-quaterpyridine complex: a comprehensive DFT study on the mechanism of CO2-to-CO conversion

Abstract

Catalytic conversion of carbon dioxide can solve many of the environmental problems caused by it. Iron-quaterpyridine is one of the most promising class of CO2-to-CO conversion molecular catalysts. In the present investigation, the geometry, electronic structure and catalytic property of iron-quaterpyridine complex [FeX(qpy)]X+ (X = 0–2) were explored theoretically with density functional theory (DFT). It is found that both monovalent and neutral iron-quaterpyridine complexes can bind and activate carbon dioxide in weak acid solution effectively, but Fe0(qpy) can generate inactive metal-carbonyl species Fe0(qpy)CO after activating carbon dioxide, resulting in catalyst deactivation. With respect to [FeI(qpy)]+, the catalyst can be recovered as CO in [FeI(qpy)]+CO is easy to release. Otherwise, the extra electrons will make it deeply reduced to the inactive species Fe0(qpy)CO. Furthermore, the photo-physical properties of some species involved in the reaction were investigated theoretically with time-dependent density functional theory (TD-DFT). The current investigation provides further insight into the adsorption and catalytic properties of iron-quaterpyridine complex toward CO2 activation, which plays a crucial role in the activation of CO2. In Fe0(qpy)CO2, the two oxygen atoms have negative charges of − 0.780 and − 0.754 e, respectively, and there is a very negative electrostatic potential (MESP) near the two oxygen atoms, which means the oxygen atom in neutral Fe0(qpy)CO2 adduct should be more easily to bind with H+ ion in acid solution.