Synthesis, spectroscopic, and lead(II) binding behavior of three novel dibenzo-18-crown-6 hydrazones

Three novel hydrazonic derivatives of dibenzo-18-crown-6 were synthesized and their vibrational and electronic absorption spectroscopic properties thoroughly investigated. Their lead binding behaviors were also characterized and the observations rationalized by density functional theoretical modeling. All three cis-dihydrazonic compounds are of low overall molecular symmetry with their ether cavity adopting an open configuration at the global energy minima where the hydrazonic side-arms are electronically and vibrationally nonequivalent. Their optical absorption features are dominated by p?p* transitions where one side of the molecule exhibits Lewis acid behavior whereas the other behaves like a Lewis base. In addition to their sensitivity to pH, all three derivatives form stable 2:1 metal-ligand complexes with lead, due to coordination at the ether cavity, in addition to the hydrazone "backbone" and possibly the heterocyclic ring. The most stable lead complex is formed with the thiophene carboxylic acid hydrazide derivative due to favorable soft-soft interactions between lead and the ring-based sulfur atom.