Mechanism of Atmospheric CO2 Fixation in the Cavities of a Dinuclear Cryptate

Using density functional theory (DFT) methods, we have investigated two possible mechanisms for atmospheric CO2 fixation in the cavity of the dinuclear zinc (II) octaazacryptate, and the subsequent reaction with methanol whereby this latter reaction transforms the (essentially) chemically inert CO2 into useful products. The first mechanism (I) was proposed by Chen et al. [Chem. Asian J. 2007, 2, 710], and involves the attachment of one CO2 molecule onto the hydroxyl-cryptate form, resulting in the formation of a bicarbonate-cryptate species and subsequent reaction with one methanol molecule. In addition, we suggest another mechanism that is initiated via the attachment of a methanol molecule onto one of the Zn-centres, yielding a methoxy-cryptate species. The product is used to activate a CO2 molecule and generate a methoxycarbonate-cryptate. The energy profiles of both mechanisms were determined and we conclude that, while both mechanisms are energetically feasible, free energy profiles suggest that the scheme proposed by Chen et al. is most likely.