Stable [Pb(ROH)(N)](2+) complexes in the gas phase: softening the base to match the Lewis acid

Effects of electronic structure on the hydration of PbNO3(+) and SrNO3(+) ion pairs

Hydration of PbNO3(+) and SrNO3(+) with up to 30 water molecules was investigated with infrared photodissociation (IRPD) spectroscopy and with theory. These ions are the same size, yet the IRPD spectra of these ion pairs for n = 2-8 are significantly different. Bands in the bonded O-H region (∼3000-3550 cm(-1)) indicate that the onset of a second hydration shell begins at n = 5 for PbNO3(+) and n = 6 for SrNO3(+). Spectra for [PbNO3](+)(H2O)2-5 and [SrNO3](+)(H2O)3-6 indicate that the structures of clusters with Pb(ii) are hemidirected with a void in the coordinate sphere. A natural bond orbital analysis of [PbNO3](+)(H2O)5 indicates that the anisotropic solvation of the ion is due to a region of asymmetric electron density on Pb(ii) that can be explained by charge transfer from the nitrate and water ligands into unoccupied p-orbitals on Pb(ii). There are differences in the IRPD spectra of PbNO3(+) and SrNO3(+) with up to 25 water molecules attached. IR intensity in the bonded O-H region is blue-shifted by ∼50 cm(-1) in nanodrops containing SrNO3(+) compared to those containing PbNO3(+), indicative of a greater perturbation of the water H-bond network by strontium. The free O-H stretches of surface water molecules in nanodrops containing 10, 15, 20, and 25 water molecules are red-shifted by ∼3-8 cm(-1) for PbNO3(+) compared to those for SrNO3(+), consistent with more charge transfer between water molecules and Pb(ii). These results demonstrate that the different electronic structure of these ions significantly influences how they are solvated.

THE GEOMETRIES AND PROTON TRANSFER OF HYDRATED DIVALENT LEAD ION CLUSTERS [Pb(H2O)n]2+(n = 1–17)

The low-lying candidates of hydrated divalent lead ion clusters [ Pb(H2O) n]2+ with up to n = 17 have been extensively studied by using density functional theory (DFT) at B3LYP level. The optimized structures show that for n = 5–13 the lowest-energy structures prefer tetracoordinate with hemi-directed geometries, while the best candidates with n = 14–17 are hexacoordinate with holo-directed geometries, which is just consistent with the experimental observation. Furthermore, the origin of hemi-directed and holo-directed geometries has been revealed. It is found that in the hemi-directed geometries, the lone pair orbital has p character and fewer electrons are transferred from the water molecules to the Pb2+ ion. Contrarily, in the holo-directed geometries, the lone pair orbital has little or no p character and more electrons are transferred to the Pb2+ ion. On the other hand, the proton transfer reactions of the [ Pb(H2O) n]2+(n = 2, 4, 8) complexes have been examined, from which the predicted products of these complexes are in good agreement with the experimental observation.

Carbon dioxide activation by "non-nucleophilic" lead alkoxides

A series of terminal lead alkoxides have been synthesized utilizing the bulky beta-diketiminate ligand [{N(2,6-(i)Pr(2)C(6)H(3))C(Me)}(2)CH](-) (BDI). The nucleophilicities of these alkoxides have been examined, and unexpected trends were observed. For instance, (BDI)PbOR reacts with methyl iodide only under forcing conditions yet reacts readily, but reversibly, with carbon dioxide. The degree of reversibility is strongly dependent upon minor changes in the R group. For instance, when R = isopropyl, the reversibility is only observed when the resulting alkyl carbonate is treated with other heterocumulenes; however, when R = tert-butyl, the reversibility is apparent upon any application of reduced pressure to the corresponding alkyl carbonate. The differences in the reversibility of carbon dioxide insertion are attributed to the ground-state energy differences of lead alkoxides. The mechanism of carbon dioxide insertion is discussed.

Characterization of the glycosidic linkage of underivatized disaccharides by interaction with Pb(2+) ions

Electrospray ionization in combination with tandem mass spectrometry and lead cationization is used to characterize the linkage position of underivatized disaccharides. Lead(II) ions react mainly with disaccharides by proton abstraction to generate [Pb(disaccharide)(m)-H](+) ions (m = 1-2). At low cone voltages, an intense series of doubly charged ions of general formula [Pb(disaccharide)(n)](2+) are also observed. Our study shows that MS/MS experiments have to be performed to differentiate Pb(2+)-coordinated disaccharides. Upon collision, [Pb(disaccharide)-H](+) species mainly dissociate according to glycosidic bond cleavage and cross-ring cleavages, leading to the elimination of C(n)H(2n)O(n) neutrals (n = 2-4). The various fragmentation processes allow the position of the glycosidic bond to be unambiguously located. Distinction between glc-glc and glc-fru disaccharides also appears straightforward. Furthermore, for homodimers of D-glucose our data demonstrate that the anomericity of the glycosidic bond can be characterized for the 1 --> n linkages (n = 2, 4, 6). Consequently, Pb(2+) cationization combined with tandem mass spectrometry appears particularly useful to identify underivatized disaccharides.

Hydrated metal ions in the gas phase

Studying metal ion solvation, especially hydration, in the gas phase has developed into a field that is dominated by a tight interaction between experiment and theory. Since the studied species carry charge, mass spectrometry is an indispensable tool in all experiments. Whereas gas-phase coordination chemistry and reactions of bare metal ions are reasonably well understood, systems containing a larger number of solvent molecules are still difficult to understand. This review focuses on the rich chemistry of hydrated metal ions in the gas phase, covering coordination chemistry, charge separation in multiply charged systems, as well as intracluster and ion-molecule reactions. Key ideas of metal ion solvation in the gas phase are illustrated with rare-gas solvated metal ions.

The synthesis of monomeric terminal lead aryloxides: dependence on reagents and conditions

The successful synthesis of terminal lead aryloxides is shown to be dependent upon reaction conditions, including choice of solvent and alkali metal aryloxide precursor.

Synthesis and theoretical studies on rare three-coordinate lead complexes

A series of beta-diketiminate lead halide complexes has been synthesised LPbCl (2), LPbBr (3) and LPbI (4) (L = {N(2,6-(i)Pr(2)C(6)H(3))C(Me)}2CH]), which includes a rare example of a three-coordinate lead iodide (4). The chloride and bromide complexes, 2 and 3, are relatively stable in both the solid and solution states, only slowly decomposing to elemental lead over the course of a month in solution, the lead iodide 4 appears to be less stable and decomposes after 3 d in the solid state at ambient temperatures. The lead chloride complex 2 was treated with KN(SiMe3)2 to yield an unusual terminal lead amide complex LPbN(SiMe3)2 (5). Unlike three-coordinate beta-diketiminate transition metal-halide complexes, the ligands are present in a pyramidal arrangement around the lead centre, commonly attributed to the presence of a stereochemically active lone pair. We have investigated the influence of this lone pair on the geometry of the metal halide complexes 2-4, as well as the isostructural germanium and tin complexes (6 and 7, respectively) using DFT calculations. The lone pair in the lead complexes is significantly more diffuse than in the tin and germanium analogues and only a small amount of hybridisation between the 6s and 6p orbitals is observed.

Lone-Pair Activity in Lead(II) Complexes with Unsymmetrical Lariat Ethers

We have carried out a study about the structural effect of the lone-pair activity in lead(II) complexes with the unsymmetrical lariat ethers L(7), L(8), (L(8)-H)-, (L(9)-H)-, and (L(10)-H)-. All these ligands are octadentate and differ by the aromatic unit present in their backbones: pyridine, phenol, phenolate, thiophenolate, and pyrrolate, respectively. In these lead(II) complexes, the receptor may adopt two possible syn conformations, depending on the disposition of the pendant arms over the crown moiety fragment. The conformation where the pendant arm holding the imine group is placed above the macrocyclic chain containing two ether oxygen atoms has been denoted as I, whereas the term II refers to the conformation in which such pendant arm is placed above the macrocyclic chain containing the single oxygen atom. Compounds of formula [Pb(L(7))](ClO4)2 (1) and [Pb(L(8)-H)](ClO4) (2) were isolated and structurally characterized by X-ray diffraction analyses. The crystal structure of 1 adopts conformation I and shows the lead(II) ion bound to the eight available donor atoms of the bibracchial lariat ether in a holodirected geometry, whereas the geometry of 2 is best described as hemidirected, with the receptor adopting conformation II. The five systems [Pb(L(7))]2+, [Pb(L(8))]2+, [Pb(L(8)-H)]+, [Pb(L(9)-H)]+, and [Pb(L(10)-H)]+ were characterized by means of density functional theory calculations (DFT) performed by using the B3LYP model. An analysis of the natural bond orbitals (NBOs) indicates that the Pb(II) lone-pair orbital remains almost entirely s in character in the [Pb(L(7))]2+ complexes, whereas in [Pb(L(8)-H)]+, the Pb(II) lone pair is polarized by a certain 6p contribution. The reasons for the different roles of the Pb(II) lone pair in compounds 1 and 2 as well as in the related model compounds are discussed. Our results point to the presence of a charged donor atom in the ligand (such as a phenolate oxygen atom, pyrrolate nitrogen atom, or even thiophenolate sulfur atom) favoring hemidirected geometries.

[Pb(H2O)]2+ and [Pb(OH)]+: Four-component density functional theory calculations, correlated scalar relativistic constrained-space orbital variation energy decompositions, and topological analysis

Within the scope of studying the molecular implications of the Pb(2+) cation in environmental and polluting processes, this paper reports Hartree-Fock and density functional theory (B3LYP) four-component relativistic calculations using an all-electron basis set applied to [Pb(H(2)O)](2+) and [Pb(OH)](+), two complexes expected to be found in the terrestrial atmosphere. It is shown that full-relativistic calculations validate the use of scalar relativistic approaches within the framework of density functional theory. [Pb(H(2)O)](2+) is found C(2v) at any level of calculations whereas [Pb(OH)](+) can be found bent or linear depending of the computational methodology used. When C(s) is found the barrier to inversion through the C(infinityv) structure is very low, and can be overcome at high enough temperature, making the molecule floppy. In order to get a better understanding of the bonding occurring between the Pb(2+) cation and the H(2)O and OH(-) ligands, natural bond orbital and atoms-in-molecule calculations have been performed. These approaches are supplemented by a topological analysis of the electron localization function. Finally, the description of these complexes is refined using constrained-space orbital variation complexation energy decompositions.

Formation of Abundant [Pb(H2O)]2+ by Ligand-Exchange Reaction between [Pb(N2)n]2+ (n = 1−3) and H2O

Doubly charged lead monohydrate, [Pb(H2O)]2+, was predicted to be unstable in the gas phase, but it has recently been observed to form in low yield via ligand change between [Pb(CH3CN)]2+ and H2O [Shi, T.; Orlova, G.; Guo, J.; Bohme, D. K.; Hopkinson, A. C.; Siu, K. W. M. J. Am. Chem. Soc. 2004, 126, 7975-7980]. Here we report that abundant [Pb(H2O)]2+ is formed in the gas phase by ligand-exchange reaction between [Pb(N2)n]2+ (n = 1-3) and water after collisional activation. Density functional theory has been used to examine the ligand-exchange reaction profile. A comparison of the potential-energy surfaces between [Pb(N2)]2+ and [Pb(CH3CN)]2+ reacting with H2O provides strong evidence that the ligand-exchange reaction of [Pb(N2)]2+ with H2O to form [Pb(H2O)]2+ is more efficient than that of [Pb(CH3CN)]2+ with H2O.

Lead(II) Thiocyanate Complexes with Bibracchial Lariat Ethers: An X-ray and DFT Study

Compounds of formula [Pb(L2)(NCS)2] (1) and [Pb(L4)(SCN)2] (2) (where L2 is the lariat crown ether N,N'-bis(3-aminobenzyl)-4,13-diaza-18-crown-6 and L4 is the Schiff-base lariat crown ether N,N'-bis(3-(salicylaldimino)benzyl)-4,13-diaza-18-crown-6) were isolated and structurally characterized by X-ray diffraction analyses. The X-ray crystal structures of both compounds show the metal ion coordinated to the six donor atoms of the crown moiety, leaving the corresponding pendant arms uncoordinated. The coordination sphere of lead(II) is completed by two thiocyanate groups that coordinate either through their nitrogen (1) or sulfur (2) atoms. The organic receptor adopts a syn conformation in 1, while in 2 it shows an anti conformation. To rationalize these unexpected different conformations of the L2 and L4 receptors in compounds 1 and 2, as well as the different binding modes found for the thiocyanate ligands, we have carried out theoretical calculations at the DFT (B3LYP) level. These calculations predict the syn conformation being the most stable in both 1 and 2 complexes. So, the anti conformation found for 2 in the solid state is tentatively attributed to the presence of intermolecular pi-pi interactions between phenol rings, for which the dihedral angle between the least-squares planes of both rings amounts to 2.6 degrees and the distance between the center of both rings is 3.766 A. On the other hand, the analysis of the electronic structure has revealed that the Pb-ligand bonds present highly ionic character in this family of compounds. They also suggest a greater transfer of electron density from the NCS- ligands when they coordinate through the less electronegative S atom. The Pb-SCN covalent bond formation mainly occurs due to an effective overlap of the occupied 3p z orbitals of the S atoms and the unoccupied 6p z AO of the Pb atom, while the Pb-NCS bonding interaction is primarily due to the overlap of the 6s and 7s AO of Pb with sp(1.10) hybrids of the N donor atoms. Our electronic structure calculations can rationalize the different coordination of the thiocyanate groups in compounds 1 and 2: the simultaneous formation of two Pb-SCN bonds is more favorable for S-Pb-S angles close to 180 degrees , for which the overlap between the occupied 3p z orbitals of the S atoms and the unoccupied 6 pz AO of the Pb atom is maximized.

Gas-Phase Study of the Chemistry and Coordination of Lead(II) in the Presence of Oxygen-, Nitrogen-, Sulfur-, and Phosphorus-Donating Ligands

Using a pickup technique in association with high-energy electron impact ionization, complexes have been formed in the gas phase between Pb(2+) and a wide range of ligands. The coordinating atoms are oxygen, nitrogen, sulfur, and phosphorus, together with complexes consisting of benzene and argon in association with Pb(2+). Certain ligands are unable to stabilze the metal dication, the most obvious group being water and the lower alcohols, but CS(2) is also unable to form [Pb(CS(2))(N)](2+) complexes. Unlike many other metal dication complexes, those associated with lead appear to exhibit very little chemical reactivity following collisional activation. Such reactions are normally promoted via charge transfer and are initiated using the energy difference between M(2+) + e(-) --> M(+) and L --> L(+) + e(-), which is typically approximately 5 eV. In the case of Pb(2+), this energy difference usually leads to the appearance of L(+) and the loss of a significant fraction of the remaining ligands as neutral species. In many instances, Pb(+) appears as a charge-transfer product. The only group of ligands to consistently exhibit chemical reactivity are those containing sulfur, where a typical product might be PbS(+)(L)(M) or PbSCH(3)(+)(L)(M).

Existence of Doubly Charged Lead Monohydrate: Experimental Evidence and Theoretical Examination

Despite reports to the contrary, doubly charged lead monohydrate is a stable species against both proton and charge transfers. [Pb(H(2)O)](2+) has been observed as a minor product in the ligand-exchange reaction of [Pb(CH(3)CN)](2+) with H(2)O after collisional activation. Density functional theory has been used to examine reaction profiles of [Pb(H(2)O)(n)](2+) where n = 1, 2, and 3.

Molecular View of the Anomalous Acidities of Sn2+, Pb2+, and Hg2+

Experimental results taken from both the condensed and gaseous phase show that, when associated with water, the three dications Sn(2+), Pb(2+), and Hg(2+) exhibit a facile proton-transfer reaction. In the gas phase, no stable [M.(H(2)O)(n)](2+) ions are observed; but instead the cations appear to undergo rapid hydrolysis to give ions of the form M(+)OH(H(2)O)(n-1). A series of ab initio calculations have been undertaken on the structures and proton-transfer reaction profiles associated with the complexes [M.(H(2)O)(2,4)](2+), where M is one of Sn, Pb, Hg, and Ca. The latter has been used as a reference point both in terms of comparisons with previous calculations, and the fact that Ca(2+) is a very weak acid. The calculations show that for Sn(2+), Pb(2+), and Hg(2+), the only barriers to proton transfer are those associated with the movement of water molecules. In the gas phase, these barriers could be overcome through energy gained during ion formation, and in the condensed phase the thermal motion of water molecules would be sufficient. In contrast, the calculations show that for Ca(2+) it is the proton-transfer step that provides the most significant reaction barrier. Proton transfer in Sn(2+) and Pb(2+) is further assisted by distortions in the geometries of [M.(H(2)O)(2,4)](2+) complexes due to voids created by the 5s(2) (6s(2)) inert lone pair. For Hg(2+), ease of proton transfer is derived partly from the high degree of covalent bonding found in both the reactants and products.

Production and collision-induced dissociation of gas-phase, water- and alcohol-coordinated uranyl complexes containing halide or perchlorate anions

Electrospray ionization was used to generate mono-positive gas-phase complexes of the general formula [UO2A(S)n]+ where A = OH, Cl, Br, I or ClO4, S = H2O, CH3OH or CH3CH2OH, and n = 1-3. The multiple-stage dissociation pathways of the complexes were then studied using ion-trap mass spectrometry. For H2O-coordinated cations, the dissociation reactions observed included the elimination of H2O ligands and the loss of HA (where A = Cl, Br or I). Only for the Br and ClO4 versions did collision-induced dissociation (CID) of the hydrated species generate the bare, uranyl-anion complexes. CID of the chloride and iodide versions led instead to the production of uranyl hydroxide and hydrated UO2+. Replacement of H2O ligands by alcohol increased the tendency to eliminate HA, consistent with the higher intrinsic acidity of the alcohols compared to water and potentially stronger UO2-O interactions within the alkoxide complexes compared to the hydroxide version.

Elucidation of the collision induced dissociation pathways of water and alcohol coordinated complexes containing the uranyl cation

Multiple-stage tandem mass spectrometry was used to characterize the dissociation pathways for complexes composed of (1) the uranyl ion, (2) nitrate or hydroxide, and (3) water or alcohol. The complex ions were derived from electrospray ionization (ESI) of solutions of uranyl nitrate in H2O or mixtures of H2O and alcohol. In general, collisional induced dissociation (CID) of the uranyl complexes resulted in elimination of coordinating water and alcohol ligands. For undercoordinated complexes containing nitrate and one or two coordinating alcohol molecules, the elimination of nitric acid was observed, leaving an ion pair composed of the uranyl cation and an alkoxide. For complexes with coordinating water molecules, MS(n) led to the generation of either [UO2(2+)OH-] or [UO2(2+)NO3(-)]. Subsequent CID of [UO2(2+)OH-] produced UO2(+). The base peak in the spectrum generated by the dissociation of [UO2(2+)NO3(-)], however, was an H2O adduct to UO2(+). The abundance of the species was greater than expected based on previous experimental measurements of the (slow) hydration rate for UO2(+) when stored in the ion trap. To account for the production of the hydrated product, a reductive elimination reaction involving reactive collisions with water in the ion trap is proposed.