Effect of space linkers in dinuclear copper cryptates on the efficiency of atmospheric CO2 uptake: a DFT study

Abstract

In recent years, dinuclear copper cryptates have been investigated for their ability as catalysts for the fixation of atmospheric CO₂. In this context, the catalyst normally hydrates CO₂ to its bicarbonate form via nucleophilic attack on the relatively inactive carbon atom in CO₂. The present study attempts to develop this model of catalyst by changing the space linker via replacement of the head unit, meta-benzene, with other aromatic or heterocyclic units. The efficiency of these novel catalysts is examined by Density Functional Theory (DFT) applied to the associated cascade-reaction mechanism. The Gibbs free energy profiles are computed to examine the energetics and geometries of all stationary points along the pathway. The most significant result is that cryptates linked to the dinuclear cryptate involving thiophene rings are predicted to be the most efficient catalyst in the uptake of CO₂. Consequently, testing of this cryptate as a receptor for CO₂ fixation is to be recommended. In this present study, we initially tested four DFT functions i.e. M06L, B97D MPW1K and B3LYP on experimentally found furano-linked cryptate intermediates. Among these functions, M06L proves to yield geometrical parameters closest to those obtained from experimental crystallographic studies. We also examine the intramolecular H₂O addition to [Cu₂-m-ben]⁴⁺ and the proton elimination steps are explained based on computed activation energy barriers and pK_a calculations. Two different (m-xylyl and thio) dinuclear cryptate-linked spacers for CO₂ fixation reaction pathways are examined at the M06L level of theory.