Physico-Chemical Properties of <i>bis</i> -2-(2-Hydroxyphenyl)-thiazole-carboxamide (BHPTC) Chelators

In this study, we investigate three members of the HPTC family─the monomeric ligand <b>L</b>^<b>1</b> and the bipodal ligands <b>L</b>^<b>2E</b> and <b>L</b>^<b>2A</b>, HPTC = 2-(2-hydroxyphenyl)-thiazole-4-carboxamide─to elucidate their acido-basic behavior, photophysical characteristics, and metal-binding properties. Protonation studies revealed that phenolate and carboxylate sites significantly influence fluorescence responses, including tautomeric equilibria and ESIPT processes. <b>L</b>^<b>1</b> forms mono- and bis-chelates with Fe(III) and Ni(II), while <b>L</b>^<b>2E</b> and <b>L</b>^<b>2A</b> yielded stable monochelates with Fe(III) and other divalent metal ions. Hydrolysis of ester functionalities in <b>L</b>^<b>2A</b> markedly enhances Cu(II) complex stability relative to <b>L</b>^<b>2E</b>. Electrochemical measurements on <b>L</b>^<b>1</b> demonstrate strong Fe(III)/Fe(II) selectivity, and kinetic data support a dissociative ligand exchange mechanism limited by metal desolvation. Overall, the presence of carboxamide and carboxylate groups plays a pivotal role in tuning metal affinity and reactivity.