CARBON-DIOXIDE ACTIVATION BY COBALT(I) MACROCYCLES - FACTORS AFFECTING CO2 AND CO BINDING

The cobalt(I) complexes of several 14-membered tetraazamacrocycles were prepared in CH3CN by either electrochemical or sodium amalgam reduction. The electronic absorption spectra and other physical properties of the Co(I), Co(I)-CO2 and CO(I)-CO complexes are reported. The CO2 and CO binding constants were determined by spectroscopic and/or electromechanical methods. The binding constants range from 5 x 10(4) to greater-than-or-equal-to 3 x 10(8) M-1 for CO and from less-than-or-equal-to 0.5 to > 10(6) M-1 for CO2 at 25-degrees-C. Both binding constants increase as the CoL2+/+ reduction potentials (which range from -0.34 to -1.65 V vs SCE in CH3CN) become more negative. Thus charge transfer from Co(1) to CO2 or CO is an important factor in stabilizing these adducts. However, hydrogen-bonding interactions between the bound CO2 and amine macrocycle N-H protons may serve to additionally stabilize the adduct in some cases, while steric repulsion by the macrocycle methyl groups may destabilize the adducts, depending upon the complex. The equilibrium ratios of N-meso and N-rac isomers of (5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene cobalt(I) and -(II) complexes were determined by H-1 NMR; the N-rac isomers of both predominate in CD3CN at room temperature. The crystal and molecular structure of the perchlorate sale of (3,5,7,7,10,12,14,14-octamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene cobalt(I) was determined from single-crystal X-ray diffraction data collected with use of Mo K-alpha-radiation. Crystallographic data: space group Pl with a = 8.433 (2) angstrom, b = 18.333 (4) angstrom, c = 7.257 (2) angstrom, alpha = 100.22 (2)-degrees, beta = 91.29 (2)-degrees, gamma = 87.68 (2)-degrees, V = 1103 (1) angstrom 3, Z = 2 (R = 0.085, R(w) = 0.105). The two square-planar cobalt atoms in the asymmetric unit are situated on crystallographic inversion centers.