Structural Insights into the Coordination and Extraction of Pb(II) by Disulfonamide Ligands Derived from <i>o-</i>Phenylenediamine

The o-phenylenediamine-derived disulfonamide ligands 1 and 2 complex and efficiently extract Pb(II) from water into 1,2-dichloroethane via ion-exchange, in combination with 2,2‘-bipyridine (97.5% and 95.0%, respectively, for 1:1 ligand-to-Pb ratios). The corresponding Pb(II)−sulfonamido binary complexes of ligands 1 and 2 (3 and 4, respectively), and ternary complexes with 2,2‘-bipyridine (5 and 6, respectively), were isolated and characterized. 1H NMR spectra of the organic phases after extraction show the formation of ternary Pb−sulfonamido−bipy complexes. X-ray characterization of 3, 4, and the ternary complex 5 consistently demonstrates four primary coordination sites and a stereochemically active lone pair on Pb. The X-ray structure of 3 shows a pseudo trigonal bipyramidal configuration on Pb, with the lone pair occupying one of the equatorial sites, and the formation of an unusual “hemidirected” coordination polymer via axial SO−Pb coordination. The same axial SO−Pb coordination pattern with two DMSO molecules is observed in the structure of 4·[2(CH3)2SO)], thus rationalizing the high solubility of the binary complexes in strongly coordinating solvents. In contrast, the X-ray structure of the ternary complex 5 reveals a distorted four-coordinate configuration with only weak SO−Pb coordination leading to dimer formation, thus explaining its higher solubility in weakly coordinating solvents. FT-IR spectroscopy confirms the X-ray data, since the ligand νSO stretching frequencies shift to lower values in the binary Pb(II)−sulfonamido complexes and are again altered upon formation of the ternary Pb(II)−sulfonamido−bipy complexes, as would be expected for 2,2‘-bipy complexation and hindered SO−Pb coordination.