3-Carboxymethylthio-1,5-diphenylformazan: a potential terdentate ligand with unusual properties

Abstract

3-Carboxymethylthio-1,5-diphenylformazan (8), prepared by two synthetic routes as a potential terdentate ligand, is a very weak base (pK₁=–0.43) and an exceptionally weak acid (pK₂= 12.48) which does not yield metal complexes. No carboxylic proton resonance appears in the n.m.r. spectrum of the yellow, monomeric solution of (8) in CDCl₃ until it is diluted progressively with (CD₃)₂CO when equilibrium with a purple isomer occurs. Comparable phenomena occur with 3-[1(or 2)-carboxyethyl]thio-1,5-diphenylformazan and the unusual properties of these formazans are attributed to configurations in which the carboxylic proton is strongly intramolecularly hydrogen-bonded to a quasi-aromatic formazan ring system.