Physico-Chemical Properties of bis-2-(2-Hydroxyphenyl)-thiazole-carboxamide (BHPTC) Chelators

In this study, we investigate three members of the HPTC family─the monomeric ligand L¹ and the bipodal ligands L^2E and L^2A, 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. L¹ forms mono- and bis-chelates with Fe(III) and Ni(II), while L^2E and L^2A yielded stable monochelates with Fe(III) and other divalent metal ions. Hydrolysis of ester functionalities in L^2A markedly enhances Cu(II) complex stability relative to L^2E. Electrochemical measurements on L¹ 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.