Conformation-resolved UV spectra of Pb(II) complexes: A gas phase study of the sandwich structures [Pb(toluene)2]2+ and [Pb(benzene)2]2+

Abstract

Toxic heavy metals, such as Pb2+, have become important targets for the development of efficient receptors that are capable of recognizing their presence as environmental and biological pollutants, and an important part of that receptor–metal characterization process is the provision of spectral evidence that identifies the presence of a metal ion. From results reported here on a combined experimental and theoretical study it is shown that, when complexed with aromatic ligands, Pb2+ is capable of yielding structured UV spectra, which: (i) exhibit discrete electronic transitions that include significant contributions from the metal ion; (ii) are very sensitive to the electronic properties of coordinating ligands; and (iii) are sensitive to subtle changes in coordination geometry. Two aromatic sandwich complexes, [Pb(benzene)2]2+ and [Pb(toluene)2]2+ have been prepared in the gas phase and their UV action spectra recorded from ions held and cooled in an ion trap. Whilst [Pb(benzene)2]2+ exhibits a spectrum with very little detail, that recorded for [Pb(toluene)2]2+ reveals a rich structure in the wavelength range 220–280 nm. Theory in the form of density functional theory (DFT) shows that both types of complex take the form of hemidirected structures, and that [Pb(toluene)2]2+ can adopt three distinct conformers depending upon the relative positions of the two methyl groups. Further calculations, using adiabatic time-dependent DFT to assign electronic transitions, provide evidence of individual [Pb(toluene)2]2+ conformers having been resolved in the experimental spectrum. Of particular significance for the development of methods for identifying Pb2+ as an environmental or biological pollutant, is the observation that there are distinct ligand-to-metal charge transfer transitions in the UV that are sensitive to both the geometry and the electronic characteristics of molecules that accommodate the metal ion