Hydrophobic interactions in ternary zinc(II) and copper(II) complexes containing 1,10-phenanthroline or 2,2′-bipyridyl and an alkane carboxylate or sulphonate

Abstract

The methyl resonances of [Me₃Si(CH₂)₃SO₃]^– and of [Me₃Si(CH₂)₂CO₂]^– are shifted upfield by [Zn(bipy)]²⁺ or by [Zn(phen)]²⁺, whereas Zn²⁺ causes no shift at all; 2,2′-bipyridyl (bipy) or 1,10-phenanthroline (phen) alone cause only much smaller upfield shifts. This effect is attributed to a hydrophobic interaction between the trimethylsilyl group and the heterocyclic aromatic ring system. The stability constants of all four ternary complexes have been determined from the variation of the n.m.r. spectra with [Zn(bipy)]²⁺ or [Zn(phen)]²⁺ concentration. The stability constants of the binary Cu²⁺ and Zn²⁺ and the ternary [Cu(bipy)]²⁺, [Zn(bipy)]²⁺, [Cu(phen)]²⁺, and [Zn(phen)]²⁺ complexes of [Me₃Si(CH₂)₂CO₂]^– have been determined by potentiometric titrations, and indicate small positive values of Δ log K. The methyl resonances of a range of medium-length straight- and branched-chain carboxylic acids are also shifted, showing that a hydrophobic interaction between an aromatic group and a methyl, isopropyl, or t-butyl group can also occur in a ternary complex.