Carbon Dioxide Fixation: Bifunctional Complexes Containing Acidic and Basic Sites Working as Reversible Carriers

Article abstract of DOI:10.1021/ja00383a015

Carbon dioxide fixation in its intact form is achieved from bifunctional complexes containing in their structure a nucleophilic cobalt(I) and an alkali cation: Co(R-salen)M .The fixation of CO2 by such systems displays different degrees of reversibility depending on the reaction solvent and the nature of the alkali cation.The X-ray analysis carried out on shows carbon dioxide anchored to the cobalt through a Co-Co ? bond, while the oxygens interact with the alkali cation, in a polymeric structure.IR spectra are diagnostic for the presence of CO2 bonded as a bent molecule in a reduced form displaying three strong bands in the following ranges: 1700-1650, 1300-1250, and 1250-1200 cm^-1.The binding of CO2 is partially prevented in the presence of a crown ether, which can complex the alkali cation.The reaction of and with dicyclohexano-18-crown-6 (DCHC) gave a complex, 2(DCHC), whose structure shows the presence of the intact bifunctional unit where, however, all the coordination sites around the alkali cation are filled by the oxygens from salen and DCHC and are not further available for binding the oxygen atoms of the CO2 molecule.Crystallographic details for n, form A, are as follows: space group C2/c (monoclinic); a = 17.805 (4) Angstroem, b = 21.251 (5) Angstroem, c = 16.192 (3) Angstroem; β = 112.50 (3)<*>; Z = 8; D_calcd = 1.325 g cm^-3.The final R factor was 0.085 for 1096 observed reflections.Crystallographic details for n, form B, are as follows: space group C2/c (monoclinic); a = 22.921 (4) Angstroem, b = 16.388 (3) Angstroem, c = 18.800 (3) Angstroem; β = 121.74 (3)<*>; Z = 8; D_calcd = 1.248 g cm^-3.The final R factor was 0.090 for 2287 observed reflections.Crystallographic details for 2(DCHC) are as follows: space group P2₁/a (monoclinic); a = 14.547 (3) Angstroem; b = 16.597 (4) Angstroem; c = 14.063 (3) Angstroem; β = 116.80 (3)<*>; Z = 2; D_calcd = 1.249 g cm ^-3.The final R factor was 0.076 for 682 observed reflections.