Co(II) and Cd(II) complexation with two dipyridine-containing macrocyclic polyamines in water and dimethyl sulfoxide.

Solvent effects in the formation of Co( II) and Cd( II) complexes with the two macrocyclic ligands 2,5,8,11, 14-pentaaza[ 15]-[15]( 2,20)[ 1,15]- bipyridylophane (L1) and trimethyl-5,8,11-2,5,8,11,14- pentaaza[ 15]-[ 15]( 2,20) [ 1,15]- bipyridylophane (L2), both containing dipyridine units, were analysed by determining the thermodynamic parameters (logK, Delta H degrees, T Delta S degrees) for the complexation reactions in water and dimethyl sulfoxide ( DMSO) by means of potentiometric, spectrophotometric and calorimetric techniques. N-Methylation leads to different solvation properties of the ligands and to different abilities in stabilizing metal complexes via the formation of M - NH center dot center dot center dot S hydrogen bonds to solvent molecules ( S) which enhances the sigma-donating properties of the donor atom. In contrast to expectations based on the stronger solvation of Cd( II) and Co( II) in DMSO than in water, complexes with L1 display higher stability in DMSO than in water, indicating that ligand solvation, instead of metal ion solvation, plays a major role in determining the stability of L1 complexes in the two solvents. On the other hand, for ligand L2, in which tertiary amino groups are present, an almost opposite trend of stability is observed. Because L2 is less solvated than L1 in water and tertiary nitrogens are weaker bases in DMSO than in water, the stronger solvation of metal ions in DMSO prevails in determining the stability of L2 complexes. Semi-empirical calculations were also performed to obtain some structural information in the gas phase.