1983 Volume 56 Issue 12 Pages 3642-3646
Heptamethyl cobyrinate perchlorate, [Cob(II)ester]ClO4, was prepared in a good yield from cyanocobalamin. This hydrophobic complex is structurally comparable to the base-off form of vitamin B12r. Thermodynamic parameters (ΔG, ΔH, and ΔS) for axial ligation of [Cob(II)ester]ClO4 with various amines in dichloromethane and toluene were determined by electronic spectroscopy. The isoequilibrium temperatures obtained from plots of ΔH vs. ΔS were 194±20 and 317±20 K in dichloromethane and toluene, respectively. The equilibrium is primarily controlled by entropy at ordinary temperatures and the logK1 value varies sensitively along with the change in pKa of amine in dichloromethane, while the equilibrium values were almost invariant to the choice of amine in toluene because the measurements were carried out near the isoequilibrium temperature. The logK1 value for [Cob(II)ester]ClO4, different from the case of cobalt(II)-porphyrin complexes, depends on the nature of solvent used and undergoes wide variations. The possible functions of equatorial ligands involved in these two different complexes in the axial ligation behavior were discussed.
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