NUCLEAR MAGNETIC RESONANCE STUDIES OF THE SOLUTION CHEMISTRY OF METAL COMPLEXES. X. DETERMINATION OF THE FORMATION CONSTANTS OF THE METHYLMERCURY COMPLEXES OF SELECTED AMINES AND AMINOCARBOXYLIC ACIDS

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Abstract
The aqueous solution chemistry of the methylmercury complexes of a series of amines and amino acids has been investigated by proton magnetic resonance spectroscopy. Methylmercury-amine complexes form at intermediate pH values; in acidic solution the complex is dissociated due to protonation of the amine while in basic solution the complex dissociates through formation of methylmercuric hydroxide. Formation constants of the complexes were determined from the pH-dependence of the chemical shift of the methyl group of methylmercury, from the pH-dependence of the mercury-proton spin-spin coupling constant of methylmercury, and from the pH-dependence of the chemical shift of the carbon-bonded ligand protons in solutions containing amine and methylmercury at a molar ratio of either one or two. The nature of the complex formed by the amino acids is pH dependent, with methylmercury binding to the carboxylate group at low pH and to the amino group at higher pH. Formation constants were determined for binding of methylmercury by the carboxylate and amino groups. The methylmercury-phenylalanine complex (amino coordination) is appreciably more stable than the other complexes. The greater stability is discussed in terms of a specific interaction between the methyl group of methylmercury and the phenyl ring; the methyl protons are shifted upfield by 0.5 ppm consistent with the proposal that the methyl group lies close to and above the plane of the phenyl ring.