Influence of solvent polarity from water to toluene on the co-ordination of pyridines by the cobalt(III) corrinoid sulfitocobyrinic acid heptamethyl ester

Abstract

Equilibrium constants K have been determined by UV/VIS spectrophotometry for co-ordination of 4-cyanopyridine, pyridine and 4-dimethylaminopyridine (pK 1.9, 5.2 and 9.8 respectively) by the uncharged five-co-ordinate sulfitocobyrinic acid heptamethyl ester in six solvents (water, MeOH, PrOH, MeCN, CH₂Cl₂, toluene) at 25 °C in order to establish the solvent dependence of the values of a and b in the linear free-energy relation (l.f.e.r.) log K=a·pK+b, where pK refers to protonation of the free base in aqueous solution. The results reveal a simple pattern in which the decrease in solvent polarity from water to toluene is accompanied by a decrease in a from 0.36 to 0.25 and in b from –0.3 to –1.7. This provides the first direct comparison of the l.f.e.r. for any ligand-binding equilibrium in both protic and aprotic solvents. The UV/VIS spectrum of this ester (in contrast to those of five-co-ordinate alkylcobyrinic acid heptamethyl esters) shows a marked solvent dependence, indicating strong interaction between the solvent and the ^δ+Co–SO₃^δ– dipole, and it is suggested that the main variable which causes the decrease in a and b with solvent polarity is the dipolar solvation term.