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- 1,4,8,1 1 -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 H-1 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-degrees-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 (nu(C=O) = 1710 cm-1, nu(NH) = 3208 cm-1), a rive-coordinate intramolecularly hydrogen-bonded form (nu(C=O) = 1626 cm-1), a six-coordinate non-hydrogen-bonded form (nu(C=O) = 1609 cm-1, nu(NH) = 3224 cm-1), and a six-coordinate intramolecular hydrogen-bonded form (nu(C=O) = 1544 cm-1, nu(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.