Carbon dioxide activation by cobalt macrocycles: Evidence of hydrogen bonding between bound CO2 and the macrocycle in solution

The equilibrium between five-coordinate CoL(CO₂)⁺ (L=5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene) and six-coordinate CoL(CO₂)(CD₃CN)⁺ (formed at the expense of the five-coordinate form as the temperature is lowered) was examined by both ¹H NMR and FT-IR. The equilibration is rapid on the NMR time scale, but the spectra of the limiting forms have been derived from the temperature dependence of the spectrum. The FT-IR spectra measured over the range 25 to -75°C in CD₃CN and in a CD₃CN/THF mixture indicate the existence of four CO₂ adducts, two with intramolecular hydrogen bonds between the bound CO₂ and the amine hydrogens of the ligand: a five-coordinate, non-hydrogen-bonded form (ν_C=O = 1710 cm^-1, ν_NH - 3208 cm^-1), a five-coordinate intramolecularly hydrogen-bonded form (ν_C=O = 1626 cm^-1), a six-coordinate non-hydrogen-bonded form (ν_C=O = 1609 cm^-1, ν_NH = 3224 cm^-1), and a six-coordinate intramolecular hydrogen-bonded form (ν_C=O = 1544 cm^-1, ν_NH = 3145 cm^-1). The binding of CO₂ via both its electrophilic center (carbon atom) and its nucleophilic center (oxygen atom) is thus implicated in the stabilization of the CO₂ complex at low temperature.