Hydrophobic and steric effects on the ion-pair formation of tris(1,10-phenanthroline)iron(II) and arenesulfonate ions. Kinetic determination of the formation constants of the ion pairs and a proton NMR study of their structures

Abstract

Ion-pair formation constants (K) for Fe(phen)/sub 3//sup 2 +/ and six kinds of arenesulfonate ions were obtained from kinetic studies of the aquation of the complex ion in aqueous sodium arenesulfonate solutions: K = 5 +/- 1, 13 +/- 2, 28 +/- 5, 19 +/- 3, 8 +/- 2, and 5 +/- 1 mol/sup -1/ dm/sup 3/ for benzene-, 4-methylbenzene-, 4-ethylbenzene-, 2,4-dimethylbenzene-, 1-naphthalene-, and 2-naphthalenesulfonate, respectively. An arenesulfonate of greater hydrophobicity showed a larger formation constant, except that small formation constants were shown by bulky naphthalenesulfonates. The formation constant was smaller for an arenesulfonate than for an alkanesulfonate with the same number of carbon atoms. The /sup 1/H NMR signal of arenesulfonate in the ion pair was found to shift upfield. Comparison of the observed shifts with those calculated on the basis of the current loop model supported a model of the ion pair in which the arenesulfonate ion lies in the hydrophobic cavity between two phenanthroline ligands of the complex ion with the sulfonate group directed outside the cavity.