E. Fujita et al., CARBON-DIOXIDE ACTIVATION BY COBALT MACROCYCLES - EVIDENCE OF HYDROGEN-BONDING BETWEEN BOUND CO2 AND THE MACROCYCLE IN SOLUTION, Inorganic chemistry, 32(12), 1993, pp. 2657-2662
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-coor
dinate 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 F
T-IR. The equilibration is rapid on the NMR time scale, but the spectr
a of the limiting forms have been derived from the temperature depende
nce 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 existenc
e 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) = 162
6 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-b
onded form (nu(C=O) = 1544 cm-1, nu(NH) = 3145 cm-1). The binding of C
O2 via both its electrophilic center (carbon atom) and its nucleophili
c center (oxygen atom) is thus implicated in the stabilization of the
CO2 complex at low temperature.