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

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 [Fe^X(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 [Fe^I(qpy)]⁺, the catalyst can be recovered as CO in [Fe^I(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.