CO₂ Fixation and Transformation by a Dinuclear Copper Cryptate under Acidic Conditions

CO2 fixation and transformation by metal complexes continuously receive attention from the viewpoint of carbon resources and environmental concerns. We found that the dinuclear copper(II) cryptate [Cu2L1](ClO4)4 (1; L1 = N[(CH2)2NHCH2(m-C6H4)CH2NH-(CH2)2]3N) can easily take up atmospheric CO2 even under weakly acidic conditions at room temperature and convert it from bicarbonate into carbonate monoesters in alcohol solution. The compounds [Cu2L1(mu-O2COH)](ClO4)3 (2), [Cu2L1(mu-O2COR)](ClO4)3 (3: R = CH3; 4: R = C2H5; 5: R = C3H7; 6: R = C4H9; 7: R = C5H11; 8: R = CH2CH2OH), [Cu2L1(mu-O2CCH3)](ClO4)3 (9), and [Cu2L1(OH2)(NO3)](NO3)3 (10) were characterized by IR spectroscopy and ESI-MS. The crystal structures of 2-6 and 10 were studied by single-crystal X-ray diffraction analysis. On the basis of the crystal structures, solution studies, and DFT calculations, a possible mechanism for CO2 fixation and transformation is given.