Investigation of disulfonamide ligands derived fromo-phenylenediamine and their Pb(II) complexes by electrospray ionization mass spectrometry

Lead is highly toxic and lead poisoning is a threat to human health, especially for children. Development of selective Pb chelates is necessary for its extraction, sensing and detoxification in environmental and biological systems.1 Numerous examples of coordination and extraction of Pb(II) by macrocycles have been reported.2–6 Ionizable chelates, which extract metals via ion exchange, offer potential advantages, such as versatility, synthetic ease, and favorable complexation-decomplexation kinetics. The recently reported disulfonamides, 1,7 2,7 and 3,1 are practical Pb(II) ion-exchange extractants that take advantage of the preference of Pb(II) in lower coordination numbers for irregular ‘hemidirected’ coordination geometries with a stereochemically active lone pair. These sulfonamides are very effective in Pb(II) extraction, even at micromolar levels, from water into 1,2-dichloroethane, and 3 in particular is very selective against Zn(II) and Cu(II).1,7 Electrospray ionization (ESI) is a soft ionization technique, and ESI-MS has been widely used to investigate non-covalent complexation, such as protein-protein interactions, enzyme-inhibitor complexes, nucleotides, drugs, and the formation of metal complexes.8,9 Although a number of cationic macrocycle–Pb complexes have been studied by ESI-MS,3–5,10 a method has not been established to investigate the neutral Pb complexes of ionizable chelators, such as disulfonamides, using MS. Recently, ESI-MS has been employed to measure stability constants for metal–crown ether complexes,11,12 to estimate relative stability of metal complexes,13,14 and to evaluate the binding selectivity of macrocycles.4 Therefore, ESI-MS could be potentially used to determine the stability of Pb–disulfonamide complexes, and a method based on ESI-MS could be eventually established for rapid screening of disulfonamide ligands for Pb complexation. Understanding the ionization mechanisms of these ligands and their Pb complexes involved in the ESI-MS processes is critical in order to achieve our long-term goal of developing MS methods for rapid detection of metal–complex formation. In this paper, we present an ESI-MS method for characterization of the disulfonamide ligands 1–4 and their Pb(II) complexes in both positive and negative modes.