Utilizing hydrogen bonds and halogen–halogen interactions in the design of uranyl hybrid materials

Systematic Study of Solid-State U(VI) Photoreactivity: Long-Lived Radicalization and Electron Transfer in Uranyl Tetrachloride

Reported are the syntheses, structural characterizations, and luminescence properties of three novel [UO2Cl4]2- bearing compounds containing substituted 1,1'-dialkyl-4,4'-bipyridinum dications (i.e., viologens). These compounds undergo photoinduced luminescence quenching upon exposure to UV radiation. This reactivity is concurrent with two phenomena: radicalization of the uranyl tetrachloride anion and photoelectron transfer to the viologen which constitutes the formal transfer of one electron from [UO2Cl4]2- to the viologen species. This behavior is elucidated using electron paramagnetic resonance (EPR) spectroscopy and further probed through a series of characterization and computational techniques including Rehm-Weller analysis, time-dependent density functional theory (TD-DFT), and density of states (DOS). This work provides a systematic study of the photoreactivity of the uranyl unit in the solid state, an under-described aspect of fundamental uranyl chemistry.

Structural, Spectroscopic, and Computational Analysis of Halogen- and Hydrogen-Bonding Effects within a Series of Uranyl Fluorides with 4-Halopyridinium

Reported are the syntheses and characterization of five compounds containing one-dimensional uranyl fluoride chains charge balanced by 4-X-pyridinium (X = H, F, Cl, Br, I) cations. Structural analysis reveals molecular assembly via noncovalent interactions in the second coordination sphere with the X···Oyl interaction distances ranging from 2.987(7) to 3.142(3) Å, all of which are less than or close to the sum of the van der Waals radii. These interactions were probed via luminescence and Raman spectroscopy, where the latter indicates slight differences in the U═O symmetric stretches as a consequence of U═O in-phase and out-of-phase Raman-active stretches. The decrease in the X···Oyl sum of the van der Waals overlap between comparable compounds within the series manifests as a red-shifting trend among the Raman symmetric stretches. Computational density functional theory (DFT)-based frequency, electrostatic potential surfaces (ESPs), and natural bonding orbital (NBO) methods support the observed Raman spectroscopic features and provide a comprehensive rationale for assembly.

(NH4)2[UO2Cl4]·2H2O, a new uranyl tetra-chloride with ammonium charge-balancing cations

A new uranyl tetra-chloride salt with chemical formula, (NH4)2[UO2Cl4]·2H2O, namely, di-ammonium uranyl tetra-chloride dihydrate, 1, was prepared and crystallized via slow evaporation from a solution of 2 M hydro-chloric acid. As confirmed by powder X-ray diffraction, the title compound crystallizes with an ammonium chloride impurity that formed as a result of the breakdown of a triazine precursor. The (UO2Cl4)2- dianion is charge balanced by ammonium cations, while an extensive hydrogen-bond network donated from structural water mol-ecules stabilize the overall assembly. Compound 1 adds to the extensive collection of actinyl tetra-chloride salts, but it represents the first without an alkali cation for purely inorganic compounds. Diffuse reflectance and luminescence spectra show typical absorption and emission behavior, respectively, of uranyl materials.

Surprises in the Solvent-Induced Self-Ionization in the Uranium Tetrahalide UX4 (X = Cl, Br, I)/Ethyl Acetate System

The reaction of the uranium(IV) halides UCl₄, UBr₄, or UI₄ with ethyl acetate (EtOAc) leads to the formation of the complexes [UX₃(EtOAc)₄][UX₅(EtOAc)] (X = Cl, Br) or [UI₄(EtOAc)₃]. Thus, both UCl₄ and UBr₄ show self-ionization in ethyl acetate to a distorted pentagonal bipyramidal [UX₃(EtOAc)₄]⁺ cation and a distorted octahedral [UX₅(EtOAc)]^- anion. Surprisingly, the chloride and bromide compounds are not isotypic. While [UCl₃(EtOAc)₄][UCl₅(EtOAc)] crystallizes in the orthorhombic crystal system, space group P2₁2₁2₁ at 250 K, the bromide compound crystallizes in the monoclinic crystal system, P12₁/n1 at 100 K. Unexpectedly, UI₄ does not show self-ionization but forms [UI₄(EtOAc)₃] molecules, which crystallize in the monoclinic crystal system, P2₁/c, at 100 K. The compounds were characterized by single-crystal X-ray diffraction, IR, Raman, and NMR spectroscopy, as well as molecular quantum chemical calculations using solvent models.

Synthesis, Crystal Structure, and Hirshfeld Surface Analysis of Hexachloroplatinate and Tetraclorouranylate of 3-Carboxypyridinium—Halogen Bonds and π-Interactions vs. Hydrogen Bonds

This work aimed to synthesize new platinum and uranium compounds with nicotinic acid. In this article we describe the synthesis of two new anionic complexes (HNic)2[PtCl6] and (HNic)2[UO2Cl4] using wet chemistry methods. The structure of the obtained single crystals was established by single-crystal X-ray diffraction. The Hirshfeld surface analysis of the obtained complexes and their analogue (HNic)2[SiF6] was carried out for the analysis of intermolecular interactions. Hydrogen bonds (H···Hal/Hal···H and O···H/H···O) make the main contribution to intermolecular interactions in all compounds. Other important contacts in cations in all compounds are H···H, C···H/H···C and C···Hal/Hal···C; in anions H···Hal/Hal···H. The Pt-containing complex has a halogen-π interaction and halogen bonds, but Si-containing complex has a π–π staking interaction; these types of interactions are not observed in the U-containing compound.

Non-covalent interactions of uranyl complexes: a theoretical study

Ab initio and DFT data quantify the ability of model uranyl complexes to engage in hydrogen- and halogen-bonding, quantifying the weakness of U–O_yl as an acceptor but the strength of equatorial OH₂ as a donor.

Dicyanoaurate-based heterobimetallic uranyl coordination polymers

The first series of uranyl ([UO₂]²⁺)-dicyanoaurate coordination polymers and molecular complexes has been synthesized. Reactions of [A][Au(CN)₂] (A = [ⁿBu₄N]⁺ or [(Ph₃P)₂N]⁺ ([PPN])) and uranyl nitrate in alcoholic solvents in ambient light led to [A]₂[(UO₂)₂(μ-η²:η²-O₂)(NO₃)₂(μ-Au(CN)₂)₂], which incorporates peroxo ligands into a one-dimensional ladder topology with alternating aurophilic and peroxo rungs. Conducting the reaction with non-alcoholic solvents formed two polymorphs of a one-dimensional chain, [PPN][UO₂(NO₃)₂Au(CN)₂], from acetone, and a molecular analogue, [PPN]₂[UO₂(NO₃)₂(Au(CN)₂)₂], from acetonitrile, none of which exhibited aurophilic interactions. The addition of 2,2'-bipyridine to the initial reaction resulted in [UO₂(bipy)(MeO)(MeOH)]₂[(μ-Au(CN)₂)(Au(CN)₂)], a one-dimensional structure which propagates via a series of linear aurophilic bonds with pendant uranyl complexes; methanol and methoxy ligands provide additional connections through hydrogen bonding. The addition of 5,5'-dimethyl-2,2'-bipyridine using solvothermal conditions resulted in the one-dimensional ladder [UO₂(Me₂bipy)Au(CN)₂]₂[(μ-OH)₂], generated through aurophilic bonds and hydroxide ligands. The incorporation of 2,2':6',2''-terpyridine (terpy) using solvothermal conditions resulted in [[UO₂(terpy)]₂(μ-NO₃)(μ-O)][Au(CN)₂], a molecular salt with no aurophilic interactions. Emission spectra attributable to aurophilic interactions are observed in [ⁿBu₄N]₂[(UO₂)₂(μ-η²:η²-O₂)(NO₃)₂(μ-Au(CN)₂)₂], while all others only show emission typical of the uranyl cation.

Utilizing bifurcated halogen-bonding interactions with the uranyl oxo group in the assembly of a UO2–3-bromo-5-iodobenzoic acid coordination polymer

The synthesis and crystal structure of a new uranyl coordination polymer featuring 3-bromo-5-iodobenzoic acid is described and the luminescent and vibrational properties of the material have been explored. Compound (1), [UO2(C7H3BrIO2)2]n, features dimeric uranyl units chelated and then linked by 3-bromo-5-iodobenzoic acid ligands to form a one-dimensional coordination polymer that is subsequently assembledviabifurcated halogen-bonding interactions with uranyl oxo atoms to form a supramolecular three-dimensional network. The asymmetric, bifurcated halogen-bonding interaction in (1) is notable as it represents the first observation of this synthon in a uranyl hybrid material. Raman and IR spectroscopy showed that halogen-bonding interactions with the uranyl oxo atoms result in small shifts in υ1and υ3frequencies, whereas luminescence spectra collected at an excitation wavelength of 420 nm reveal partially resolved uranyl emission.

Harnessing uranyl oxo atoms via halogen bonding interactions in molecular uranyl materials featuring 2,5-diiodobenzoic acid and N-donor capping ligands

The supramolecular assembly of molecular uranyl species via halogen-oxo interactions and spectroscopic manifestations thereof are probed in the solid state.

A new Pu(iii) coordination geometry in (C5H5NBr)2[PuCl3(H2O)5]·2Cl·2H2O as obtained via supramolecular assembly in aqueous, high chloride media

A novel hydrated Pu(iii) chloride, (C₅H₅NBr)₂[PuCl₃(H₂O)₅]·Cl·2H₂O, is prepared from aqueous media and the non-covalent interaction pairings are rationalized using electrostatic potentials.

The Halogen Bond

The halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. In this fairly extensive review, after a brief history of the interaction, we will provide the reader with a snapshot of where the research on the halogen bond is now, and, perhaps, where it is going. The specific advantages brought up by a design based on the use of the halogen bond will be demonstrated in quite different fields spanning from material sciences to biomolecular recognition and drug design.

Intriguing I2 Reduction in the Iodide for Chloride Ligand Substitution at a Ru(II) Complex: Role of Mixed Trihalides in the Redox Mechanism

The compound [Ru(CN(t)Bu)4(Cl)2], 1, reacts with I2, yielding the halogen-bonded (XB) 1D species {[Ru(CN(t)Bu)4(I)2]·I2}n, (2·I2)n, whose building block contains I(-) ligands in place of Cl(-) ligands, even though no suitable redox agent is present in solution. Some isolated solid-state intermediates, such as {[Ru(CN(t)Bu)4(Cl)2]·2I2}n, (1·2I2)n, and {[Ru(CN(t)Bu)4(Cl)(I)]·3I2}n, (3·3I2)n, indicate the stepwise substitution of the two trans-halide ligands in 1, showing that end-on-coordinated trihalides play a key role in the process. In particular, the formation of ClI2(-) triggers electron transfer, possibly followed by an inverted coordination of the triatomic species through the external iodine atom. This allows I-Cl separation, as corroborated by Raman spectra. The process through XB intermediates corresponds to reduction of one iodine atom combined with the oxidation of one coordinated chloride ligand to give the corresponding zerovalent atom of I-Cl. This redox process, explored by density functional theory calculations (B97D/6-31+G(d,p)/SDD (for I and Ru atoms)), is apparently counterintuitive with respect to the known behavior of the corresponding free halogen systems, which favor iodide oxidation by Cl2. On the other hand, similar energy barriers are found for the metal-assisted process and require a supply of energy to be passed. In this respect, the control of the temperature is fundamental in combination with the favorable crystallizations of the various solid-state products. As an important conclusion, trihalogens, as XB adducts, are not static in nature but are able to undergo dynamic inner electron transfers consistently with implicit redox chemistry.

The role of intermolecular interactions involving halogens in the supramolecular architecture of a series of Mn(ii) coordination compounds

This study reveals the role of halogen-involving interactions in structural changes of supramolecular assemblies of manganese(ii) complexes including N-(4-halo)phenyl picolinamide ligands.

Combining coordination and supramolecular chemistry to explore uranyl assembly in the solid state

Supramolecular assembly of uranyl species via halogen–oxo and halogen–halogen interactions is explored in the solid state.

The role of weak hydrogen and halogen bonding interactions in the assembly of a series of Hg(ii) coordination polymers

A series of eight new Hg(ii) complexes based on the L^4-X ligands, where L is (E)-4-halo-N-(pyridin-4-ylmethylene)aniline, were synthesized and characterized and their supramolecular crystal structures were studied by different geometrical and theoretical methods.

Halogen–halogen interactions and halogen bonding in thiacalixarene systems

5,11,17,23-tetraiodo-25,26,27,28-tetrapropoxythiacalix[4]arene exhibited I⋯I interactions which are ca. 2% shorter than the sum of their respective van der Waals atomic radii. 5,11,17,23-tetraiodo-25,26,27,28-tetrabutoxythiacalix[4]arene were identified to have S⋯I interactions that are ca. 4.5% shorter than the sum of their respective van der Waals atomic radii. We have elucidated both interactions by computational approaches.

Supramolecular interactions in PuO2Cl4(2-) and PuCl6(2-) complexes with protonated pyridines: synthesis, crystal structures, and Raman spectroscopy

The synthesis, crystal structures, and Raman spectra of seven plutonium chloride compounds are presented. The materials are based upon Pu(VI)O2Cl4(2-) and Pu(IV)Cl6(2-) anions that are charge balanced by protonated pyridinium cations. The single crystal X-ray structures show a variety of donor-acceptor interactions between the plutonium perhalo anions and the cationic pyridine groups. Complementary Raman spectra show that these interactions can be probed through the symmetric vibrational mode of the plutonyl moiety. Unlike previously reported studies in similar uranyl(VI) systems, the facile redox chemistry of plutonium in aqueous solution has demonstrated the feasibility of using not only the An(VI)O2Cl4(2-) anion with approximate D4h symmetry but also the approximately Oh An(IV)Cl6(2-) anion in order to manipulate both the structure and dimensionality of such hybrid materials.

Supramolecular architectures and structural diversity in a series of lead (II) Chelates involving 5-Chloro/Bromo thiophene-2-carboxylate and N,N'-donor ligands

Background

Lead is a heavy toxic metal element in biological systems and is one of the major pollutants as a result of its widespread use in industries. In spite of its negative roles the coordination chemistry of Pb(II) complexes is a matter of interest. The N,N’-bidentate aromatic bases such as BPY,4-BPY and PHEN (BPY = 2,2′bipyridine, 4-BPY = 4,4′-dimethyl-2,2′-bipyridine, PHEN = 1,10-Phenanthroline) are widely used to build supramolecular architectures because of their excellent coordinating ability and large conjugated system that can easily form π-π interactions among their aromatic moieties. A series of novel Pb(II) complexes in concert with 5-CTPC, 5-BTPC (5-CTPC = 5-chlorothiophen-2-carboxylate, 5-BTPC = 5-bromothiophen-2-carboxylate) and corresponding bidentate chelating N.N′ ligands have been synthesized and characterized.

Results

Five new Pb (II) complexes [Pb(BPY)(5-CTPC)₂] (1), [Pb(4-BPY)(5-CTPC)₂] (2), [Pb₂(PHEN)₂(5-CTPC)₄] (3), [Pb(4-BPY)(5-BTPC)₂] (4) and [Pb₂(PHEN)₂(5-BTPC)₂(ACE)₂] (5) have been synthesized. Even though in all these complexes the molar ratio of Pb, carboxylate, N,N-chelating ligand are the same (1:2:1), there is a significant structural diversity. These complexes have been characterised and investigated by elemental analysis, IR, ¹H-NMR,¹³C-NMR, TGA, and photoluminescence studies. Single crystal X-ray diffraction studies reveal that complexes (1, 2) and (4) are mononuclear while (3 and 5) are dinuclear in nature which may result from the chelating nature of the ligands, various coordination modes of the carboxylates, and the coordination geometry of the Pb(II) ions.

Conclusions

The observation of structures 2,4 and 3,5 show the structural changes made just chloro/bromo substituent of the thiophene ring. A detailed packing analysis has been undertaken to delineate the role of valuable non covalent interactions like X…π, H…X, (X = Cl/Br). A quadruple hydrogen bond linking the monomeric units and generating a supramolecular architecture is observed in (1). The metal bite unit comprised of PbN₂C₂ (i.e. Pb-N-C-C-N-Pb) is the repeating unit in all the five complexes and they have almost same geometrical parameters. This metal bite has been identified as the self assembly unit in complexes.

Supramolecular assembly in metal complexes: Two structural cases

With the current advances in our understanding of molecular components in the solid-state world, the relation between energy and geometry remains controversial. In this study, we deliver a concise account for supramolecular chemistry, and in order to illustrate some of its concepts we describe some structural and theoretical analyses for two unique cases of our work. We elaborate on a supramolecular model of controlled “one-pot” host–guest metal-mediated self-assembly reaction inside iron coordination polymer grid architecture, and we review halogen bonding by specifically observing M–Cl···Cl–M intermolecular interactions using Cambridge Structural Database (CSD) hit analyses with theoretical calculations.

An exploration of homo- and heterometallic UO22+ hybrid materials containing chelidamic acid: synthesis, structure, and luminescence studies

Eight U(vi)–chelidamate compounds highlight the structural significance of modified ligands as sources for direct metal–ligand coordination or halogen-based interaction platforms.