A calorimetric study of the protonation of vitamin D 3 and of three other ligands and their formation of binary and ternary complexes with Cd(II)

Abstract

The heats of protonation of vitamin D 3 (VD 3) and of glycine (gly), 2,2'-bipyridyl (bipy) and ethylenediamine (en), and their coordination with Cd(II) to form binary and ternary complexes have been determined at 37°C and I = 0.1 in (KNO 3) in 70% (v/v) ethanol, using a model RD-1 automatic conduction calorimeter. The entropies of formation of these systems were calculated. The results show that all the ternary systems have large positive entropy values. It seems that the stacking effect between the alkyl residue, which is far removed from the coordination atom (O) in the VD 3 molecule, and the aromatic ring of bipy or the carbon linkage of gly (or en) plays a very important role in the entropy increase of the ternary complex compounds. Furthermore, the magnitude of the stacking effect may be in the order aryl-aryl > aryl-alkyl ring > alkyl ring-alkyl ring > alkyl ring-carbon linkage.