Relating structural and thermodynamic effects of the Pb(II) lone pair: a new picolinate ligand designed to accommodate the Pb(II) lone pair leads to high stability and selectivity

The crystal and molecular structure and the stability of lead and calcium complexes of two chelates containing picolinate chelating groups in different geometries have been investigated in order to relate the ligand affinity and selectivity for lead over calcium with the ability of the ligand to accommodate a stereochemically active lone pair. The crystal structures of the lead complexes of the diprotonated and monoprotonated tripodal ligand tpaa2- show that the three picolinate arms of the tripodal ligand coordinate the lead in an asymmetric way leaving a gap in the coordination sphere to accommodate the lead lone pair. As a consequence of this binding mode, one picolinate arm is very weakly bound and therefore can be expected to contribute very little to the complex stability. Conversely, the geometry of the dipodal ligand H2dpaea is designed to accommodate the lead lone pair; in the structure of the [Pb(dpaea)] complex the donor atoms of the ligand occupy only a quarter of the coordination sphere, reducing the sterical interaction between the lead lone pair with respect to the H3tpaa complexes. As a result, in the lead structures of H2dpaea all the ligand donor atoms are strongly bound to the metal ion leading to increased stability. The high value of the formation constant measured for the lead complex of the dipodal dpaea2- (log beta11(Pb)=12.1(3)) compared to the lower value found for the one of the tripodal tpaa3- (log beta11(Pb)=10.0(1)) provides direct evidence of the influence of the stereochemically active lead lone pair on complex stability. As a result, since the ligand geometry has little effect on the stability of the calcium complex, a remarkable increase in the Pb/Ca selectivity is observed for dpaea-(10(6.6)) compared to tpaa3- (10(1.5)), making the dipodal ligand a good candidate for application as extracting agent for the lead removal from contaminated water.     

Coordination polyhedra PbX<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> (X = F,Cl, Br,I) in crystal structures

The Voronoi-Dirichlet polyhedra (VDP) and the intersecting sphere method were used to analyze the coordination of Pb(II) and Pb(IV) atoms by halogen atoms in the crystal structures of 158 compounds. A decrease in the steric effect of the Pb(II) lone electron pair with a decrease in the electronegativity of the surrounding atoms was established. The influence of the nature of the central atom on the steric effect of the lone pair in the structure of the AX _n ^z? complexes, where X is halogen or oxygen, A = Tl(I), Sn(II), Pb(II), As(III),Sb(III), Bi(III), S(IV), Se(IV), Te(IV), or Cl(V) was considered.     

Accommodation of the irregular coordination geometry of lead(II) by a square planar N2S2 ligand and its preference for zinc(II)

The N2S2 ligand, bis-mercaptoethanediazacyclooctane, coordinates to Pb(II) largely through sulfur donors, enlisting a second unit to fulfill an irregular, hemispherical N2S3 coordination environment in which a void suggests the location of a stereochemically active lone pair on Pb(II). That the highly exposed lead is vulnerable to metal ion displacement is demonstrated on reaction with zinc which results in a regular square pyramidal coordination about zinc within a [N2S2Zn]2 dimer. Analysis of the two dimeric structures finds different connectivities of the monomeric subunits account for the stability of the zinc structure over that of the lead.     

Synthesis of the new ligand bis[3-(2-pyrazinyl-pyrazol-1-yl) dihydroborate,and the crystal structures of its complexes with thallium(I) and lead(II)

Reaction of 3-(2-pyrazinyl)pyrazole with KBH₄ in a 21:1 ratio afforded the new ligand bis3, 2, 1dihydroborate [L]⁻a bis(pyrazolyl)borate in which each pyrazolyl ring is functionalised with a pyrazin-2-yl group at the C³ position[L]⁻ is therefore a potentially chelating tetradentate ligand with two externally-directed N atoms (the pyrazinyl N⁴ atoms) which are available for additional metal–ion bindingleading to eg coordination polymers The crystal structure of [TlL] shows it to be a simple mononuclear complex with the Tl(I) ion coordinated in the N₄ binding pocket of the ligandand the externally-directed N atoms involved only in intermolecular N H–C hydrogen-bonding interactions The two Tl–N bonds to the pyrazolyl N² atoms (average length 270 Å) are much shorter than the bonds to the pyrazinyl N¹ atoms (average length 305 Å) also there is an obvious gap in the apical position of the metal–ion coordination sphere characteristic of a stereochemically active lone pair The crystal structure of [PbL₂] Et₂O shows that the Pb(II) centre is nine-coordinate with two tetradentate chelating ligands and the ninth donor being a pyrazinyl N⁴ atom from an adjacent complex unit The molecules therefore form infinite one-dimensional chains in the crystal via bridging pyrazinyl groups The coordination geometry about the Pb(II) ions is approximately capped square antiprismatic with no obvious gap in the coordination sphere suggesting that the lone pair is stereochemically inactive.     

Enhanced metal ion selectivity of 2,9-di-(pyrid-2-yl)-1,10-phenanthroline and its use as a fluorescent sensor for cadmium(II)

The metal ion complexing properties of the ligand DPP (2,9-di-(pyrid-2-yl)-1,10-phenanthroline) were studied by crystallography, fluorimetry, and UV-visible spectroscopy. Because DPP forms five-membered chelate rings, it will favor complexation with metal ions of an ionic radius close to 1.0 A. Metal ion complexation and accompanying selectivity of DPP is enhanced by the rigidity of the aromatic backbone of the ligand. Cd2+, with an ionic radius of 0.96 A, exhibits a strong CHEF (chelation enhanced fluorescence) effect with 10(-8) M DPP, and Cd2+ concentrations down to 10(-9) M can be detected. Other metal ions that cause a significant CHEF effect with DPP are Ca2+ (10(-3) M) and Na+ (1.0 M), whereas metal ions such as Zn2+, Pb2+, and Hg2+ cause no CHEF effect with DPP. The lack of a CHEF effect for Zn2+ relates to the inability of this small ion to contact all four donor atoms of DPP. The structures of [Cd(DPP)2](ClO4)2 (1), [Pb(DPP)(ClO4)2H2O] (2), and [Hg(DPP)(ClO4)2] (3) are reported. The Cd(II) in 1 is 8-coordinate with the Cd-N bonds to the outer pyridyl groups stretched by steric clashes between the o-hydrogens on these outer pyridyl groups and the central aromatic ring of the second DPP ligand. The 8-coordinate Pb(II) in 2 has two short Pb-N bonds to the two central nitrogens of DPP, with longer bonds to the outer N-donors. The coordination sphere around the Pb(II) is completed by a coordinated water molecule, and two coordinated ClO4(-) ions, with long Pb-O bonds to ClO4(-) oxygens, typical of a sterically active lone pair on Pb(II). The Hg(II) in 3 shows an 8-coordinate structure with the Hg(II) forming short Hg-N bonds to the outer pyridyl groups of DPP, whereas the other Hg-N and Hg-O bonds are rather long. The structures are discussed in terms of the fit of large metal ions to DPP with minimal steric strain. The UV-visible studies of the equilibria involving DPP and metal ions gave formation constants that show that DPP has a higher affinity for metal ions with an ionic radius close to 1.0 A, particularly Cd(II), Gd(III), and Bi(III), and low affinity for small metal ions such as Ni(II) and Zn(II). The complexes of several metal ions, such as Cd(II), Gd(III), and Pb(II), showed an equilibrium involving deprotonation of the complex at remarkably low pH values, which was attributed to deprotonation of coordinated water molecules according to: [M(DPP)(H2O)]n+ <==> [M(DPP)(OH)](n-1)+ + H+. The tendency to deprotonation of these DPP complexes at low pH is discussed in terms of the large hydrophobic surface of the coordinated DPP ligand destabilizing the hydration of coordinated water molecules and the build-up of charge on the metal ion in its DPP complex because of the inability of the coordinated DPP ligand to hydrogen bond with the solvent.     

Stereochemistry of Pb(II) Complexes with Aminopolycarboxylic Ligands. The Role of a Lone Electron Pair

The structures of complex anions of Pb(II) coordination compounds (complexonates) with monoamino-, diamino-, and polyaminopolycarboxylic acids as ligands were interpreted in terms of a model of the valence shell electron pair repulsion. A lone electron pair (E) in all structurally studied Pb(II) complexonates with carboxyl-containing complexones was shown to be stereochemically active, and the structure of Pb coordination polyhedron was found to depend on both the ligand dentate number and on its degree of protonation. As the ligand dentate number increased, the coordination number (C.N.) of a central atom changed from 4 + E for monoamine complexonates to 6 + E for diamine and triamine complexonates. With allowance for secondary bonds in the structure, the C.N. of the Pb atom increased to 7–9. The Pb(II) complexonates with nitrilotriacetic acid exhibit the formation of a new type of coordination polyhedron for post-transition elements in a low-valent state with five electron pairs in a valence shell (including one lone electron pair), i.e., the ψ-trigonal bipyramid with E in the axial position.__________Translated from Koordinatsionnaya Khimiya, Vol. 31, No. 7, 2005, pp. 483–494.Original Russian Text Copyright © 2005 by Davidovich.     

The origin of the stereochemically active Pb(II) lone pair: DFT calculations on PbO and PbS

The concept of a chemically inert but stereochemically active 6s² lone pair is commonly associated with Pb(II). We have performed density functional theory calculations on PbO and PbS in both the rocksalt and litharge structures which show anion dependence of the stereochemically active lone pair. PbO is more stable in litharge while PbS is not, and adopts the symmetric rocksalt structure showing no lone pair activity. Analysis of the electron density, density of states and crystal orbital overlap populations shows that the asymmetric electron density formed by Pb(II) is a direct result of anion-cation interactions. The formation has a strong dependence on the electronic states of the anion and while oxygen has the states required for interaction with Pb 6s, sulphur does not. This explains for the first time why PbO forms distorted structures and possesses an asymmetric density and PbS forms symmetric structures with no lone pair activity. This analysis shows that distorted Pb(II) structures are not the result of chemically inert, sterically active lone pairs, but instead result from asymmetric electron densities that rely on direct electronic interaction with the coordinated anions.     

Metal ion complexing properties of the highly preorganized ligand 2,9-bis(hydroxymethyl)-1,10-phenanthroline: a crystallographic and thermodynamic study

Metal ion complexing properties of the ligand 2,9-bis(hydroxymethyl)-1,10-phenanthroline (PDALC) are reported. For PDALC, the rigid 1,10-phenanthroline backbone leads to high levels of preorganization and enhanced selectivity for larger metal ions with an ionic radius of about 1.0 A that can fit well into the cleft of the ligand. Structures of PDALC complexes with two larger metal ions, Ca(II) and Pb(II), are reported. [Ca(PDALC) 2](ClO 4) 2 ( 1) is triclinic, Pi, a = 7.646(3), b = 13.927(4), c = 14.859(5) (A), alpha = 72.976(6), beta = 89.731(6), mu = 78.895(6) degrees , V = 1482.5(8) A (3), Z = 2, R = 0.0818. [Pb(PDALC)(ClO 4) 2] ( 2) is triclinic, Pi, a = 8.84380(10), b = 9.0751(15), c = 12.178(2) (A), alpha = 74.427(3), beta = 78.403(13), mu = 80.053(11) degrees , V = 915.0(2) A (3), Z = 2, R = 0.0665. In 1, the Ca(II) is eight-coordinate, with an average Ca-N of 2.501 A and Ca-O of 2.422 A. The structure of 1 suggests that Ca(II) is coordinated in a very low-strain manner in the two PDALC ligands. In 2, Pb(II) appears to be eight-coordinate, with coordination of PDALC and four O donors from perchlorates bridging between neighboring Pb atoms. The Pb has very short Pb-N bonds averaging 2.486 A and Pb-O bonds to the alcoholic groups of PDALC of 2.617 A. It is suggested that the Pb(II) has a stereochemically active lone pair situated on the Pb(II) opposite the two N donors of the PDALC, and in line with this, the Pb-L bonds become longer as one moves around the Pb from the sites of the two N donors to the proposed position of the lone pair. There are two oxygen donors from two perchlorates, nearer the N donors, with shorter Pb-O lengths averaging 2.623 A. Two oxygens from perchlorates nearer the proposed site of the lone pair form very long Pb-O bond lengths averaging 3.01 A. The Pb(II) also appears to coordinate in the cleft of PDALC in a low-strain manner. Formation constants are reported for PDALC in 0.1 M NaClO 4 at 25.0 degrees C. These show that, relative to 1,10-phenanthroline, the hydroxymethyl groups of PDALC produce a significant stabilization for large metal ions such as Cd(II) or Pb(II) that are able to fit in the cleft of PDALC but destabilize the complexes of metal ions such as Ni(II) or Cu(II) that are too small for the cleft.     

Structural insights into the coordination and extraction of Pb(II) by disulfonamide ligands derived from o-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. (1)H 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(CH(3))(2)SO)], 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 nu(S)(=)(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.     

Synthesis and characterization of some new Pb(II), Mn(II) and Ag(I) complexes with a pentaaza macrocyclic ligand containing a piperazine moiety

A pentaaza (N₅) 17-membered macrocyclic ligand (L) has been synthesized and its coordination capability toward perchlorate or nitrate salts of Mn(II), Pb(II) and Ag(I) has been investigated. The complexes were characterized by elemental analysis, IR, FAB mass spectrometry, magnetic studies, conductivity measurements, ¹H and ¹³C NMR spectroscopy. The crystal structure of [PbL](ClO₄)₂ has been determined and it shows the presence of a mononuclear complex, with the Pb(II) ion coordinated to the five N donor atoms of the ligand in a hemidirected structure with the presence of a stereochemically active lone pair of electrons on the Pb(II) ion.     

Pb(II) coordination and synergistic ion-exchange extraction by combinations of sulfonamide chelates and 2,2'-bipyridine

The disulfonamide ligands 1,2-C(6)H(4)(NH(2)SO(2)C(6)H(5))(2) (1) and 1,2-C(6)H(4)(NH(2)SO(2)C(6)H(4)-p-Bu(t))(2) (2), which are readily available in good yields from o-phenylenediamine and the corresponding sulfonyl chlorides, efficiently extract Pb(II) from water into 1,2-dichloroethane when used in synergistic combinations with 2,2'-bipyridine via an ion-exchange mechanism. The extraction was shown to proceed via the formation of a ternary Pb-sulfamido-2,2'-bipyridine complex. The X-ray crystal structure of the binary Pb-sulfamido complex 3 shows a coordination polymer with a stereochemically active lone pair on Pb formed by S=O-Pb axial coordination.     

Amide Group Coordination to the Pb(2+) Ion

The binary and ternary (2,2'-bipyridine) complexes of dipositive lead formed by N-carbonyl and N-sulfonyl amino acids, which are ligands containing the peptide and the sulfonamide group, respectively, were investigated in aqueous solution by NMR and differential pulse polarography, and some were also characterized crystallographically. N-Tosylglycine, N-tosyl-beta-alanine, and N-benzoylglycine behave as simple carboxylate ligands at acid pH, while around neutrality they switch to dianionic N,O-bidentate chelating ligands due to the involvement of the deprotonated amide nitrogen as an additional donor site. The same coordination behavior is maintained in the presence of 2,2'-bipyridine. The binary and ternary species formed in solution, and their stability constants were determined and compared with those of the homologous complexes of Pd(2+), Cu(2+), Cd(2+), and Zn(2+). The Pb(2+) ion is the only dipositive metal which is effective in promoting peptide nitrogen deprotonation in benzoylglycine. The molecular structures of [Pb(N-tosylglycinato-N,O)(H(2)O)] (1), [Pb(N-benzoylglycinato-O)(2)(H(2)O)(2)].2H(2)O (2), and [Pb(N-tosylglycinato-O)(2)(bpy)] (3) were determined by X-ray crystallography (O and N,O refer to the ligands binding as carboxylates and as N,O-chelating dianions, respectively). These compounds are all polymeric with six- to eight-coordinate metals showing distorted coordination geometries indicative of a stereochemically active metal lone pair. Polymerization is invariably determined by a bidentate chelate carboxylate group with one oxygen bridging between two metals, and in 2 and 3 it occurs through the formation of chains of Pb(2)O(2) square-planar rings. The binding set in 1, involving a deprotonated amide nitrogen and a sulfonic oxygen, is unprecedented for the Pb(2+) ion. This work provides new information on the solution and solid state chemistry of dipositive lead with ligands of biological interest, a research area that has received little attention in the past, although it is of great relevance for understanding the mechanisms of metal toxicity.     

Macrocyclic receptor showing extremely high Sr(II)/Ca(II) and Pb(II)/Ca(II) selectivities with potential application in chelation treatment of metal intoxication

Herein we report a detailed investigation of the complexation properties of the macrocyclic decadentate receptor N,N'-Bis[(6-carboxy-2-pyridil)methyl]-4,13-diaza-18-crown-6 (H(2)bp18c6) toward different divalent metal ions [Zn(II), Cd(II), Pb(II), Sr(II), and Ca(II)] in aqueous solution. We have found that this ligand is especially suited for the complexation of large metal ions such as Sr(II) and Pb(II), which results in very high Pb(II)/Ca(II) and Pb(II)/Zn(II) selectivities (in fact, higher than those found for ligands widely used for the treatment of lead poisoning such as ethylenediaminetetraacetic acid (edta)), as well as in the highest Sr(II)/Ca(II) selectivity reported so far. These results have been rationalized on the basis of the structure of the complexes. X-ray crystal diffraction, (1)H and (13)C NMR spectroscopy, as well as theoretical calculations at the density functional theory (B3LYP) level have been performed. Our results indicate that for large metal ions such as Pb(II) and Sr(II) the most stable conformation is Δ(δλδ)(δλδ), while for Ca(II) our calculations predict the Δ(λδλ)(λδλ) form being the most stable one. The selectivity that bp18c6(2-) shows for Sr(II) over Ca(II) can be attributed to a better fit between the large Sr(II) ions and the relatively large crown fragment of the ligand. The X-ray crystal structure of the Pb(II) complex shows that the Δ(δλδ)(δλδ) conformation observed in solution is also maintained in the solid state. The Pb(II) ion is endocyclically coordinated, being directly bound to the 10 donor atoms of the ligand. The bond distances to the donor atoms of the pendant arms (2.55-2.60 ?) are substantially shorter than those between the metal ion and the donor atoms of the crown moiety (2.92-3.04 ?). This is a typical situation observed for the so-called hemidirected compounds, in which the Pb(II) lone pair is stereochemically active. The X-ray structures of the Zn(II) and Cd(II) complexes show that these metal ions are exocyclically coordinated by the ligand, which explains the high Pb(II)/Cd(II) and Pb(II)/Zn(II) selectivities. Our receptor bp18c6(2-) shows promise for application in chelation treatment of metal intoxication by Pb(II) and (90)Sr(II).     

Structural and spectroscopic impact of tuning the stereochemical activity of the lone pair in lead(II) cyanoaurate coordination polymers via ancillary ligands

The reaction of Pb(ClO4)2 x xH2O, an ancillary ligand L, and two equivalents of Au(CN)2(-) gave a series of crystalline coordination polymers, which were structurally characterized. The ligands were chosen to represent a range of increasing basicity, to influence the stereochemical activity (i.e., p-orbital character) of the Pb(II) lone pair. The Pb(II) center in [Pb(1,10-phenanthroline)2][Au(CN)2]2 (1) is 8-coordinate, with a stereochemically inactive lone pair; all 8 Pb-N bonds are similar. The Au(CN)2(-) units propagate a 2-D brick-wall structure. In [Pb(2,2'-bipyridine)2][Au(CN)2]2 (2), the 8-coordinate Pb(II) center has asymmetric Pb-N bond lengths, indicating moderate lone pair stereochemical activity; the supramolecular structure forms a 1-D chain/ribbon motif. For [Pb(ethylenediamine)][Au(CN)2]2 (3), the Pb(II) is only 5-coordinate and extremely asymmetric, with Pb-N bond lengths from 2.123(7) to 3.035(9) A; a rare Pb-Au contact of 3.5494(5) A is also observed. The Au(CN)2(-) units connect the Pb(ethylenediamine) centers to form 1-D zigzag chains which stack via Au-Au interactions of 3.3221(5) A to yield a 2-D sheet. (207)Pb MAS NMR of the polymers indicates an increase in both the chemical shielding span and isotropic chemical shift with increasing Pb(II) coordination sphere anisotropy (from delta iso = -2970 and Omega = 740 for 1 to delta iso = -448 and Omega = 3980 for 3). The shielding anisotropy is positively correlated with Pb(II) p-character, and reflects a direct connection between the NMR parameters and lone-pair activity. Solid-state variable-temperature luminescence measurements indicate that the emission bands at 520 and 494 nm, for 1 and 2, respectively, can be attributed to Pb --> L transitions, by comparison with simple [Pb(L)2](ClO4)2 salts. In contrast, two emission bands for 3 at 408 and 440 nm are assignable to Au-Au and Pb-Au-based transitions, respectively, as supported by single-point density-functional theory calculations on models of 3.     

Holo‐ and Hemidirected Coordination Spheres in a Novel Three‐Dimensional Polymeric KIPbII Heteropolynuclear Complex: X‐Ray Crystal Structure of [KPb(AcO)2(SCN)]n

A novel three‐dimensional polymeric K^IPb^II heteropolynuclear complex, [KPb(AcO)₂(SCN)]_n, with mixed acetate and thiocyanate ligands, has been synthesized and characterized. Its single‐crystal X‐ray structure (Fig. 1) shows three types of K⁺ ions with coordination numbers of seven, and three types of Pb²⁺ ions with coordination numbers of eight, eight, and nine, respectively. The Pb centers (Pb(1) and Pb(3); Fig. 1) with coordination numbers of nine and eight, respectively, possess stereochemically ‘inactive’ electron lone pairs, and the coordination sphere is holodirected. However, the arrangement of O‐, N‐, and S‐atoms for the eight‐coordinated Pb(2) suggests a gap or hole in the coordination geometry around this atom. This ‘hole’ is possibly occupied by a stereochemically ‘active’ electron lone pair of Pb(2), and its coordination sphere is, thus, hemidirected.     

Theoretical study on the interaction of sulfur‐ and aminopyridine‐containing chelating resins with Hg(II) and Pb(II)

The geometries and energetics of complexes of Hg(II) and Pb(II) with sulfur‐ and aminopyridine‐containing chelating resin including crosslinked polystyrene immobilizing 2‐aminopyridine via sulfur‐containing (PVBS‐AP), sulfoxide‐containing (PVBSO‐AP), and sulfone‐containing (PVBSO₂‐AP) spacer arms have been investigated theoretically, and thus interactions of the metal ions with chelating resins were evaluated. The results indicate that PVBS‐AP behaves as a tridentate ligand to coordinate with the metal ions by S and two N atoms to form chelating compounds with S atom playing a dominant role in the coordination, whereas PVBSO‐AP and PVBSO₂‐AP interact with metal cations, respectively, in a tricoordinate manner by O and two N atoms forming chelating complexes. Furthermore, it is revealed that O and N2 atoms of PVBSO‐AP are the main contributor of coordination to Hg(II), whereas N2 atom of PVBSO₂‐AP is mainly responsible for the coordination to Hg(II). For PVBSO‐AP‐Pb²⁺ and PVBSO₂‐AP‐Pb²⁺ complex, the coordination is dominated by the synergetic effect of N1, N2, and O atoms. Natural bond orbital and second‐order perturbation analyses suggest that the charge transfer from the chelating resins to metal ions is mainly dominated by the interactions of lone pair of electrons of the donor atoms with the unoccupied orbitals of metal ions. Hg(II) complexes exhibit larger binding energies than the corresponding Pb(II) complexes, implying the chelating resins exhibit higher affinity toward Hg(II), which is consistent with the experimental results. Combined the theoretical and experimental results, further understanding of the structural information of the complexes and the coordination mechanism was achieved. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Supramolecular lead(II) azide complex of 2,6-diacetylpyridine dihydrazone: synthesis,molecular structure,and biological activity

The complex of 2,6-diacetylpyridinedihydrazone (L) with lead(II) and azide has been characterized by elemental analyses, FTIR, and single-crystal X-ray analysis. The Pb(C₉H₁₃N₁₁) (1) crystallized in the monoclinic space group C2/c. The coordination of 1 exhibits a gap around the lead(II), possibly occupied by a stereochemically active electron lone pair on lead(II) resulting in a hemidirected complex. Antimicrobial activity of the complex is higher than the free ligand.     

Stereochemistry of post-transition metal oxides: revision of the classical lone pair model

The chemistry of post transition metals is dominated by the group oxidation state N and a lower N-2 oxidation state, which is associated with occupation of a metal s(2) lone pair, as found in compounds of Tl(I), Pb(II) and Bi(III). The preference of these cations for non-centrosymmetric coordination environments has previously been rationalised in terms of direct hybridisation of metal s and p valence orbitals, thus lowering the internal electronic energy of the N-2 ion. This explanation in terms of an on-site second-order Jahn-Teller effect remains the contemporary textbook explanation. In this tutorial review, we review recent progress in this area, based on quantum chemical calculations and X-ray spectroscopic measurements. This recent work has led to a revised model, which highlights the important role of covalent interaction with oxygen in mediating lone pair formation for metal oxides. The role of the anion p atomic orbital in chemical bonding is key to explaining why chalcogenides display a weaker preference for structural distortions in comparison to oxides and halides. The underlying chemical interactions are responsible for the unique physicochemical properties of oxides containing lone pairs and, in particular, to their application as photocatalysts (BiVO(4)), ferroelectrics (PbTiO(3)), multi-ferroics (BiFeO(3)) and p-type semiconductors (SnO). The exploration of lone pair systems remains a viable a venue for the design of functional multi-component oxide compounds.     

Model peptides based on the binding loop of the copper metallochaperone Atx1: selectivity of the consensus sequence MxCxxC for metal ions Hg(II), Cu(I), Cd(II), Pb(II), and Zn(II)

The amino acid sequence MxCxxC is conserved in many soft-metal transporters that are involved in the control of the intracellular concentration of ions such as Cu(I), Hg(II), Zn(II), Cd(II), and Pb(II). A relevant task is thus the selectivity of the motif MxCxxC for these different metal ions. To analyze the coordination properties and the selectivity of this consensus sequence, we have designed two model peptides that mimic the binding loop of the copper chaperone Atx1: the cyclic peptide P(C) c(GMTCSGCSRP) and its linear analogue P(L) (Ac-MTCSGCSRPG-NH2). By using complementary analytical and spectroscopic methods, we have demonstrated that 1:1 complexes are obtained with Cu(I) and Hg(II), whereas 1:1 and 1:2 (M:P) species are successively formed with Zn(II), Cd(II), and Pb(II). The complexation properties of the cyclic and linear peptides are very close, but the cyclic compound provides systematically higher affinity constants than its unstructured analogue. The introduction of a xPGx motif that forms a type II beta turn in P(C) induces a preorganization of the binding loop of the peptide that enhances the stabilities of the complexes (up to 2 orders of magnitude difference for the Hg complexes). The affinity constants were measured in the absence of any reducing agent that would compete with the peptides and range in the order Hg(II) > Cu(I) > Cd(II) > Pb(II) > Zn(II). This sequence is thus highly selective for Cu(I) compared to the essential ion Zn(II) that could compete in vivo or compared to the toxic ions Cd(II) and Pb(II). Only Hg(II) may be an efficient competitor of Cu(I) for binding to the MxCxxC motif in metalloproteins.     

Poly(N‐acetyl‐α‐acrylic acid): Synthesis,Characterization and Chelation Properties through Liquid Phase Polymer‐Based Retention Technique

The synthesis and characterization of the water‐soluble poly(N‐acetyl‐α‐acrylic acid) by radical polymerization were carried out. The polymer was characterized by Fourier Transform Infrared (FT‐IR), ¹H NMR and ¹³C NMR spectroscopies, and thermogravimetric analysis (TGA). The metal ion binding properties for the metals Cu(II), Co(II), Ni(II), Cd(II), Zn(II), Pb(II), Hg(II), Cr(III) in the aqueous phase were studied using the liquid‐phase polymer‐based retention technique. The metal ion interactions with the hydrophilic polymer were determined as a function of pH and of the filtration factor. The polychelatogen showed a high affinity for metal ions and higher selectivity for Cr(III) at pH = 3.