Structural Insights into the Coordination and Extraction of Pb(II) by Disulfonamide Ligands Derived from o-Phenylenediamine

Spectroscopic and Theoretical Insights into Surprisingly Effective Sm(III) Extraction from Alkaline Aqueous Media by o-Phenylenediamine-Derived Sulfonamides

Alkaline high-level waste (HLW) generated as a result of years of nuclear weapons production has complicated composition and requires comprehensive treatment methods, which would allow concentrating its most radiotoxic components in a small volume for geological disposal. We have investigated six alkyl-substituted o-phenylenediamine-derived sulfonamides for extraction and consecutive stripping of Sm(III) from alkaline aqueous media. Up to 81% of Sm(III) recovery at pH 13.0-13.5 was achieved by disulfonamide (dsa) or dsa/Et₃N in CH₂Cl₂, measured after contact with organic phases and subsequent stripping with 0.1 M HNO₃. The use of Et₃N dramatically enhances Sm(III) extraction at lower pH ranges (10.5-11.5) but decreases extraction at pH 13.0-13.5, while control experiments with Et₃N and no dsa showed no extraction. Analysis of the extraction equilibria gave a 1:1 sulfonamide-Sm(III) complexation ratio, with the extracted species also presumed to contain coordinated H₂O or OH^-, as also shown by DFT calculations. Titration experiments of sulfonamides with Sm(III) in CH₃CN were consistent with a 1:1 complexation ratio of dsa-6 to Sm(III) with a K₁₁ = 6.6 × 10⁷ M^-1 derived from nonlinear regression analysis of the 1:1 binding isotherm. Theoretical DFT calculations determined the structures of possible species formed during extraction and the thermodynamics of extraction processes based on several initial [Sm(OH)_y(NO₃)_z(H₂O)_x]^3-y-z species and 1:1 Sm(III)/dsa-3^2- complexes formed in the organic phase, in which dsa complexes to Sm(III) in its bis-deprotonated form (denoted below as dsa-3^2-). Organization of close ion pairs of type {Na[Sm(dsa-3^2-)(OH)₂]·2H₂O} was shown to be thermodynamically favorable for extraction from alkaline aqueous media with pH = 13.0-13.5. Theoretical calculations also demonstrated thermodynamically favorable coordination to Am(III).

Tuning PtII -Based Donor-Acceptor Systems through Ligand Design: Effects on Frontier Orbitals, Redox Potentials, UV/Vis/NIR Absorptions, Electrochromism, and Photocatalysis

Asymmetric platinum donor-acceptor complexes [(pimp)Pt(Q2- )] are presented in this work, in which pimp=[(2,4,6-trimethylphenylimino)methyl]pyridine and Q2- =catecholate-type donor ligands. The properties of the complexes are evaluated as a function of the donor ligands, and correlations are drawn among electrochemical, optical, and theoretical data. Special focus has been put on the spectroelectrochemical investigation of the complexes featuring sulfonyl-substituted phenylendiamide ligands, which show redox-induced linkage isomerism upon oxidation. Time-dependent density functional theory (TD-DFT) as well as electron flux density analysis have been employed to rationalize the optical spectra of the complexes and their reactivity. Compound 1 ([(pimp)Pt(Q2- )] with Q2- =3,5-di-tert-butylcatecholate) was shown to be an efficient photosensitizer for molecular oxygen and was subsequently employed in photochemical cross-dehydrogenative coupling (CDC) reactions. The results thus display new avenues for donor-acceptor systems, including their role as photocatalysts for organic transformations, and the possibility to introduce redox-induced linkage isomerism in these compounds through the use of sulfonamide substituents on the donor ligands.

Remote site-selective C-H activation directed by a catalytic bifunctional template

In chemical syntheses, the activation of carbon-hydrogen (C-H) bonds converts them directly into carbon-carbon or carbon-heteroatom bonds without requiring any prior functionalization. C-H activation can thus substantially reduce the number of steps involved in a synthesis. A single specific C-H bond in a substrate can be activated by using a 'directing' (usually a functional) group to obtain the desired product selectively. The applicability of such a C-H activation reaction can be severely curtailed by the distance of the C-H bond in question from the directing group, and by the shape of the substrate, but several approaches have been developed to overcome these limitations. In one such approach, an understanding of the distal and geometric relationships between the functional groups and C-H bonds of a substrate has been exploited to achieve meta-selective C-H activation by using a covalently attached, U-shaped template. However, stoichiometric installation of this template has not been feasible in the absence of an appropriate functional group on which to attach it. Here we report the design of a catalytic, bifunctional nitrile template that binds a heterocyclic substrate via a reversible coordination instead of a covalent linkage. The two metal centres coordinated to this template have different roles: one reversibly anchors substrates near the catalyst, and the other cleaves remote C-H bonds. Using this strategy, we demonstrate remote, site-selective C-H olefination of heterocyclic substrates that do not have the necessary functional groups for covalently attaching templates.

Aryl Bis-Sulfonamide Inhibitors of IspF from Arabidopsis thaliana and Plasmodium falciparum

2-Methylerythritol 2,4-cyclodiphosphate synthase (IspF) is an essential enzyme for the biosynthesis of isoprenoid precursors in plants and many human pathogens. The protein is an attractive target for the development of anti-infectives and herbicides. Using a photometric assay, a screen of 40 000 compounds on IspF from Arabidopsis thaliana afforded symmetrical aryl bis-sulfonamides that inhibit IspF from A. thaliana (AtIspF) and Plasmodium falciparum (PfIspF) with IC50 values in the micromolar range. The ortho-bis-sulfonamide structural motif is essential for inhibitory activity. The best derivatives obtained by parallel synthesis showed IC50 values of 1.4 μm against PfIspF and 240 nm against AtIspF. Substantial herbicidal activity was observed at a dose of 2 kg ha(-1) . Molecular modeling studies served as the basis for an in silico search targeted at the discovery of novel, non-symmetrical sulfonamide IspF inhibitors. The designed compounds were found to exhibit inhibitory activities in the double-digit micromolar IC50 range.

The expanding world of hybrid perovskites: materials properties and emerging applications

Hybrid inorganic–organic perovskites have emerged over the last 5 years as a promising class of materials for optoelectronic applications. Most notably, their solar cells have achieved power conversion efficiencies above 20% in an unprecedented timeframe; however, many fundamental questions still remain about these materials. This Prospective Article reviews the procedures used to deposit hybrid perovskites and describes the resulting crystallographic and morphological structures. It further details the electrical and optical properties of perovskites and then concludes by highlighting a number of potential applications and the materials challenges that must be overcome before they can be realized.

catena-Poly[[(benzoato-κO,O')(2,2'-bipyridine-κN,N')lead(II)]-μ(3)-nitrato-κO:O,O':O'']

In the title coordination polymer, [Pb(C₇H₅O₂)(NO₃)(C₁₀H₈N₂)]_n, the Pb^II ion is eight-coordinated by two N atoms from one 2,2′-bipyridine ligand, two O atoms from one benzoate anion and four O atoms from three nitrate groups (one chelating, two bridging) in a distorted dodecahedral geometry. Adjacent Pb^II ions are linked by bridging nitrate O atoms through the central Pb₂O₂ and Pb₂O₄N₂ cores, resulting in an infinite chain structure along the b axis. The crystal structure is stabilized by π–π stacking inter­actions between 2,2′-bipyridine and benzoate ligands belonging to neighboring chains, with shortest centroid–centroid distances of 3.685 (8) and 3.564 (8) Å.

Bis(benzoato-κO,O')(1,10-phenanthroline-κN,N')lead(II) benzoic acid mono-solvate

The reaction of lead acetate, benzoic acid and 1,10-phenanthroline (phen) in aqueous solution yielded the title complex, [Pb(C₇H₅O₂)₂(C₁₂H₈N₂)]·C₇H₆O₂. In the crystal, the Pb^II ion is hexa­coordinated by two N atoms from one 1,10-phenanthroline ligand and four O atoms from two chelate benzoate anions. If the second benzoate ligand is treated as one coordination site, the overall coordination may be represented as a distorted pseudo-square pyramid. An inter­molecular O—H⋯O hydrogen bond links the solvent benzoic acid mol­ecule with a metal-coordinated benzoate ligand. The shortest Pb⋯Pb distance is 3.864 (4) Å, indicating a weak metal–metal inter­action. Two complex mol­ecules related by an inversion centre form dimeric units via Pb⋯O inter­actions of 3.206 (4) Å.

Bis(benzoato-κO,O')(2,2'-bipyridine-κN,N')lead(II) benzoic acid monosolvate

The reaction of lead acetate, benzoic acid and 2,2'-bipyridine (bipy) in aqueous solution yielded the title complex, [Pb(C(7)H(5)O(2))(2)(C(10)H(8)N(2))]·C(7)H(6)O(2). The asymmetric unit contains two independent complex mol-ecules as well as two independent benzoic acid solvent mol-ecules, one of which is disordered over two positions with almost equal occupancies [0.504 (5) and 0.496 (5)]. The two complex mol-ecules have similar configurations with the hexa-coordinated environment of the Pb(II) atom formed by four carboxyl-ate O atoms of two chelate benzoate ligands and two N atoms of the bipy ligand. The Pb-O bonds involving one of the benzoate ligands are almost coplanar with Pb-N bonds to the bipy ligand [dihedral angles of 12.67 (11) and 14.73 (11)°] ; if the second benzoate ligand is treated as one coordination site, the overall coordination may be represented as a distorted pseudo-square pyramid. Weak inter-molecular Pb⋯O inter-actions [3.046 (3) and 3.359 (3) Å] link each of the complex mol-ecules into two symmetry-independent centrosymmetric dimers. Hydrogen bonds involving the carboxyl H atoms of solvent benzoic acid mol-ecules and metal-coordinated carboxyl-ate O atoms link complex mol-ecules and benzoic acid solvent mol-ecules into insular aggregates.

Pb···π Aryl Interactions as Supramolecular Synthons

A survey of lead (Pb) structures containing Pb···π aryl interactions has been conducted. Such contacts usually lead to zero- or one-dimensional aggregates with rare examples of two- and three-dimensional architectures. The Pb···π aryl interactions are found only in crystal structures containing lead(ii) centres and arise as a result of electron donation of the lead-bound lone pair of electrons to the lowest unoccupied molecular orbital of the accepting aryl ring. The prevalence of Pb···π interactions as a supramolecular synthon is relatively low, occurring in ~3% of all structures containing lead and at least one aryl ring, but these are more likely to form compared with Sn···π interactions.

Lead-poisoned zinc fingers: quantum mechanical exploration of structure, coordination, and electronic excitations

Density functional theory (DFT) structure calculations and time-dependent DFT electronic excitation calculations on simple mononuclear lead structures confirm recent reports on the stabilization of tricoordinated structural domains in poisoned proteins. However, the possibility of the formation of tetracoordinated lead complexes should not be disregarded in studies on mechanisms of lead toxicity because structures with both coordination modes are plausible and might contribute to observed UV spectra. Reported calculations along with detailed molecular orbital analysis confirm that the intense UV signal at around 260 nm is an indicator of the ligand-to-metal charge transfer (LMCT) band where the electrons are transferred from the sulfur 3p orbital to the lead 6p orbital. The composition of the LMCT band reveals significant excitations not only from the Pb-S bonding orbitals but also from sulfur lone-pair orbitals to the Pb-S antibonding orbitals for which the electron density is largely localized on the Pb "6p-like" molecular orbitals. There is a solid indication that the stereochemically active pair orbital of lead is not strongly hybridized and remains largely of the 6s character in tricoordinated lead structures and is minimally hybridized in tetracoordinated lead structures. Computed UV spectra of lead model complexes are compared to experimental UV spectra of model lead peptides. The comparison shows a good agreement with the major spectral trends and changes observed in these experiments.

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

An electrospray ionization mass spectrometry (ESI-MS) method, in both positive and negative ion modes, was developed for characterization of disulfonamide ligands derived from o-phenylenediamine and their Pb(II) complexes. For the ligands, negative ion mode ESI-MS in methanolic solutions gave simple and easily interpretable mass spectra. However, the spectra of Pb complexes were not readily interpretable under the same conditions. Protonated ligands and their Pb(II) complexes were observed in methanolic solutions by ESI-MS in positive ion mode. The formation of Na(+), K(+), or NH(4) (+) adducts was also observed, complicating the mass spectra and decreasing the signal intensity. In order to optimize the detection of the ligands and the Pb complexes, a method was developed by adding NaOAc in the solutions. The presence of 0.2 mM NaOAc simplified the mass spectra of the ligands and the Pb complexes, and significantly increased sensitivity in both negative and positive ion modes. This modification makes ESI-MS in both modes suitable for characterization of sulfonamide ligands and their Pb complexes, thus providing a potentially powerful tool for evaluating formation of metal complexes and screening combinatorial ligand libraries.