title: 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 Pb(2+), 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, Pb(2+) 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 Pb(2+) 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.
authors: School of Chemistry, University of Nottingham, University Park, Nottingham, United Kingdom.
authors: Lifu
authors: L
authors: Ma
authors: Tsukiko
authors: T
authors: Takashima
authors: Joseph
authors: J
authors: Koka
authors: Helen J
authors: HJ
authors: Kimber
authors: Hazel
authors: H
authors: Cox
authors: Anthony J
authors: AJ
authors: Stace
mesh_terms: D001554:Benzene / Q000737:chemistry*; D007854:Lead / Q000737:chemistry*; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000737:chemistry*; D011789:Quantum Theory; D013056:Spectrophotometry, Ultraviolet; D014050:Toluene / Q000737:chemistry*
chemical_list: D009942:Organometallic Compounds; D007854:Lead; D014050:Toluene; D001554:Benzene