Uranyl Compounds Research Articles

(UO2)(C10H8N2O2)2][HPW12O40]: The First Case of a Uranyl Coordination Network Containing a Keggin‐Type Polyoxometalate

A uranyl compound [(UO2)(C10H8N2O2)2][HPW12O40] has been hydrothermally synthesized (150 °C for 72 h) in the presence of typical Keggin‐phosphotungstic acid. This compound exhibits a regular layered structure which is composed of [UO2]2+ and bridging ligand 4,4'‐bipyridine‐N‐dioxide (4,4'‐BPDO). The nanosized cavities are occupied by Keggin‐anions [HPW12O40]2– through covalent bonds and hydrogen bonds, in order to hold the charge equilibrium and enhance the stability of the structure. The adsorption properties of this compound have been primarily investigated. The results indicate that the incorporation of Keggin‐anions plays an important role in the fast capture of cationic dye MB+. Compound 1 is the first case of uranyl coordination network decorated by POMs, which evidently provides a feasible strategy for controlling the structural connectivity and functionality of uranyl organic framework.

Auxiliary Ligand-Dependent Adaptive Regulation of Uranyl Coordination in Mixed-Ligand Uranyl Compounds of Flexible Biphenyltetracarboxylic Acid.

The mixed-ligand strategy is one of the important methods for preparing new materials and regulating the properties of materials. In this work, by introducing different auxiliary ligands (ALs), we have obtained a series of mixed-ligand uranyl complexes (1-6) from a flexible biphenyltetracarboxylic acid (H4bptc) with an adjustable orthogonal conformation and studied the influence of different organic base molecules on the coordination and assembly of H4bptc with a uranyl cation. It is found that the coordinated ALs, including 4,4'-bipyridine-1,1'-dioxide and 1,10-phenanthroline, partially occupy the coordination sites of the uranyl center and directly affect the molecular conformations and uranyl coordination of flexible bptc linkers. On the other hand, noncoordinated ALs such as protonated 4,4'-bipyridine ([H2(4,4'-bpy)]2+) or dimethylammonium, which work as counterions in the form of encapsulated guests or hydrogen-bonded templates, also have a nonnegligible impact on the conformation and coordination of bptc linkers. Most interestingly, the AL-mediated evolution of uranyl coordination by the bptc linker and coordination geometry of the uranyl center is clearly observed, which suggests the adaptability of flexible bptc linkers to take suitable molecular configurations and uranyl coordination modes so as to adapt to the external regulator agents and varying environment. The physicochemical characterization of these uranyl compounds, especially photoluminescence, is addressed and discussed, and the results reveal that compound 5 has the potential to serve as a multifunctional radiation detection material for UV light and X-ray radiation.

Ligand-Supported Facile Conversion of Uranyl(VI) into Uranium(IV) in Organic and Aqueous Media.

Reduction of uranyl(VI) to UV and to UIV is important in uranium environmental migration and remediation processes. The anaerobic reduction of a uranyl UVI complex supported by a picolinate ligand in both organic and aqueous media is presented. The [UVI O2 (dpaea)] complex is readily converted into the cis-boroxide UIV species via diborane-mediated reductive functionalization in organic media. Remarkably, in aqueous media the uranyl(VI) complex is rapidly converted, by Na2 S2 O4 , a reductant relevant for chemical remediation processes, into the stable uranyl(V) analogue, which is then slowly reduced to yield a water-insoluble trinuclear UIV oxo-hydroxo cluster. This report provides the first example of direct conversion of a uranyl(VI) compound into a well-defined molecular UIV species in aqueous conditions.

Ligand‐Supported Facile Conversion of Uranyl(VI) into Uranium(IV) in Organic and Aqueous Media

AbstractReduction of uranyl(VI) to UV and to UIV is important in uranium environmental migration and remediation processes. The anaerobic reduction of a uranyl UVI complex supported by a picolinate ligand in both organic and aqueous media is presented. The [UVIO2(dpaea)] complex is readily converted into the cis‐boroxide UIV species via diborane‐mediated reductive functionalization in organic media. Remarkably, in aqueous media the uranyl(VI) complex is rapidly converted, by Na2S2O4, a reductant relevant for chemical remediation processes, into the stable uranyl(V) analogue, which is then slowly reduced to yield a water‐insoluble trinuclear UIV oxo‐hydroxo cluster. This report provides the first example of direct conversion of a uranyl(VI) compound into a well‐defined molecular UIV species in aqueous conditions.

Dimensional evolution in hydrated K+-bearing uranyl sulfates: from 2D-sheets to 3D-frameworks

Six new uranyl compounds were synthesized within the K+-bearing uranyl sulfate system. An unexpected example of dimensional evolution is demonstrated.

Targeted crystal growth of uranium gallophosphates via the systematic exploration of the UF4–GaPO4–ACl (A = Cs, Rb) phase space

The flux synthesis of a uranium gallophosphate and a uranium gallate, Cs4[UO2Ga2(PO4)4] and Cs2UO2Ga2O5, and 4 uranium phosphates, [Rb2Rb3.93Cl0.93][(UO2)5(PO4)5], Rb11[(UO2)8(PO4)9], Rb7.6[(UO2)8O8.6F0.4(PO4)2], and Rb6[(UO2)5O2(PO4)4], is reported.

Synthesis of Anhydrous Acetates for the Components of Nuclear Fuel Recycling in Dialkylimidazolium Acetate Ionic Liquids.

A series of anhydrous acetate salts with uranium {[C2C1im][UO2(OAc)3] (1), [C2C2im][UO2(OAc)3] (2), and [C4C1im][UO2(OAc)3] (3)}, lanthanides {[C2C2im]2[La(OAc)5] (4) and [C2C1im]2[Nd(OAc)5] (5)}, and strontium {[C2C1im]n[Sr(OAc)3]n (6)} (where C2C1im = 1-ethyl-3-methylimidazolium, C2C2im = 1,3-diethylimidazolium, C4C1im = 1-butyl-3-methylimidazolium, and OAc = acetate) have been prepared and structurally characterized. Both lanthanides and strontium are common components of the nuclear fuel waste, and their separation from uranium is an important but still challenging task. A new synthetic approach with dialkylimidazolium acetate ionic liquids (ILs) as the solvent has been developed for the direct synthesis of homoleptic acetates from the corresponding hydrates and, unexpectedly, hardly soluble f-element oxides. Although the group of characterized compounds shows perfect structural variability, all actinide and lanthanide metal ions form monomeric complex anions where the metal cation coordinates to five ligands including two oxygen atoms in the case of uranium, as is commonly observed for uranyl compounds. Crystallographic analyses revealed that the complex [UO2(OAc)3]- anions possess rather standard D3h symmetry featuring a hexagonal-bipyramidal coordination environment, while the lanthanide anions [Ln(OAc)5]2- are fully asymmetric and the Ln3+ cations are 10-coordinated in the form of a distorted bicapped tetragonal antiprism. This is the first report of lanthanide ions coordinated in this fashion. For Sr2+, 9-fold coordination through oxygen atoms in the form of a strongly distorted tricapped trigonal prism is observed. The crystallization of anhydrous, homoleptic, anionic acetate complexes from such a large variety of different metal salts appears to be due to the properties of dialkylimidazolium acetate ILs themselves, including enhanced basicity from the high concentration of free anions and their greater affinity for hydrogen-bonding solutes relative to metal cations.

Noncomplexed Cucurbituril-Mediated Structural Evolution of Layered Uranyl Terephthalate Compounds.

Template synthesis is one of the most feasible ways to explore new uranyl compounds with intriguing structures and properties. Here we demonstrate the preparation of six novel "sandwichlike" uranyl coordination polymers (UCPs) based on two-dimensional uranyl-terephthalate acid (H2TP) networks using CBn (n = 5, 6, 8) as template ligands in the presence of different cations (Na+, K+, Cs+, or H2N(CH3)2+). Compound 1 ([UO2(TP)2][Na2(CB5)(H2O)](H2O)5) is composed of layered uranyl-TP networks with the complex of CB5 and sodium cations as template ligands. In compound 2 ([(UO2)2(TP)3]2(CB6)(H2O)10), CB6 located between uranyl-TP networks contacts them by π-π interactions and hydrogen bonds. Compound 3 ([(UO2)2(TP)3]2[Na2(H2O)10(CB6)]) is the same as compound 2 except for sodium cations bonding with CB6. Similarly in compound 4 ([(UO2)2(TP)3][Cs(H2O)3(CB6)]), CB6 is a capsulelike structure capped with two cesium cations and interacts with uranyl-TP networks through π-π and C-H···π interactions. Compound 5 ([(UO2)2(TP)3(HCOO)2][K(H2O)2(CB5)]2[H2N(CH3)2]2(CB6)(H2O)6) consists of both templates of CB6 and CB5 in which each CB5 is capped with one potassium cation while the H2N(CH3)2+ cation is held at CB6 portals. In compound 6 ([(UO2)2(TP)3]2[UO2(TP)2(H2O)2][Cs(CB8)3(H2O)4](H2O)16), CB8 ligands are connected by cesium cations to form a triangle motif and are further located between the uranyl-TP networks as template agents. All of the 2D layered structures with free CBn or cation-anchored CBn intercalate into the laminates of uranyl-terephthalate and shows a cucurbituril-mediated structural evolution. The regulating role of CBn as structure-directing template agents for the construction of layered UCPs through outer-surface interactions with layers of uranyl terephthalate is demonstrated, especially for the case of CB6 with contractive interlayer spacing.

Synthesis and Crystal Structure of [Benzyl N'-[[2-(hydroxy-kO)phenyl]ethylene]-N-[[2-(hydroxy-kO)phenyl]methylene]-carbamohydrazonothioato-kN,kN']-ethanol-dioxido-uranium

The uranyl compound UO2(L')EtOH (L' = benzyl N-(2-(oxidanyl)benzylidene)-N'-((2-(1-oxidanyl)phenyl)ethylidene) carbamohydrazonothioate) was synthesized and its molecular and crystal structure was determined by X-ray diffraction. The compound crystallizes in the triclinic sp. gr. P$$\bar {1}$$, Z = 2. The coordination geometry about the uranium atom is distorted pentagonal bipyramidal consisting of a UO2O4N2 polyhedron where the oxido ligands occupy axial positions with a symmetric linear structure (O=U=O angle = 179.11(12)°). The pentagonal plane (N2O3) is formed by the dinegative, tetradentate Schiff base ligand (N2O2) and the oxygen atom of the coordinated alcohol ligand. In the complex, the ligand adopts a propeller-like conformation.

Peculiarities of the Supramolecular Assembly of Tetraethylammonium and 3-Bromopropionate Ions in Uranyl, Neptunyl, and Plutonyl Coordination Compounds.

Synthesis and X-ray diffraction studies of {N(C2H5)4}[AnO2(C2H4BrCOO)3] [An = U (I), Np (II), or Pu (III)] and C2H4BrCOOH (IV), where C2H4BrCOO- is an anion of the 3-bromopropionic acid, are reported. The isostructural coordination compounds I-III contain mononuclear anionic complexes [AnO2(C2H4BrCOO)3]- belonging to the crystal chemical group AB013 (A = AnO22+; B01 = C2H4BrCOO-). In the crystal structure of IV, the C2H4BrCOOH molecules are hydrogen bonded into centrosymmetric dimers R22(8). Using the method of molecular Voronoi-Dirichlet (VD) polyhedra, the features of intermolecular interactions in crystals of I-IV are discussed in support of the results of IR and UV spectroscopy experiments. Actinide contraction in I-III manifests itself in a regular reduction of the average length of the axial and equatorial bonds in hexagonal bipyramids AnO8, in an increase in νas(AnO22+) wavenumbers, and in a simultaneous decrease in the volume and sphericity degree of VD polyhedra of An atoms in the U-Np-Pu series. The title compounds represent an interesting architecture, where 3-bromopropionate ions penetrate through the square 44 net of tetraethylammonium ions and thus bind adjacent nets via the "locking effect".

Probe Raman Spectroscopy of Uranyl-Compound Polycrystals

Regularities in the Raman spectra of acetate uranyl compounds CsUO2(CH3COO)3, CsUO2(CD3COO)3, NaUO2(CH3COO)3, NaUO2(CD3COO)3, UO2(CH3COO)2, and RbUO2(CH3COO)3 have been analyzed. The Raman spectra are excited by 785-nm laser radiation and recorded using a miniature fiber-optic spectrometer with a multielement detector. It is established that the spectra of all these compounds contain a strong Raman satellite with a frequency in the range of 850–860 cm–1. This satellite corresponds to symmetric stretching vibrations (type A1) of the uranyl group. The obtained results make it possible to detect and analyze small amounts of uranyl compounds.

A study of non-iterative triples contributions in relativistic equation-of-motion coupled-cluster calculations using an exact two-component Hamiltonian with atomic mean-field spin-orbit integrals: Application to uranyl and other heavy-element compounds.

The implementation of an equation-of-motion coupled-cluster singles and doubles augmented with a noniterative triples correction [EOM-CCSD(T)(a)*] method [D. A. Matthews and J. F. Stanton, J. Chem. Phys. 145, 124102 (2016)] with an exact two-component Hamiltonian using atomic mean-field spin-orbit integrals (X2CAMF) is reported. Benchmark calculations show that both the correction from triple excitations in the cluster operator to the similarity-transformed Hamiltonian and the contribution from triple excitations in excited-state eigenvalue equations make important contributions to computed excitation energies of molecules containing heavy elements. X2CAMF-EOM-CCSD(T)(a)* excitation energies and excited-state properties for low-lying excited states of As2, Sb2, and Bi2 are in significantly better agreement with corresponding experimental values than EOM-CCSD ones. X2CAMF-EOM-CCSD(T)(a)* calculations have also been demonstrated to provide accurate results for excitation energies of low-lying excited states of bare uranyl ion UO2 2+, hereby resolving a long-standing debate on this topic.

Topological analysis of the layered uranyl compounds bearing slabs with UO2:TO4ratio of 2:3

AbstractNine new templated uranyl sulfates and selenates, [(H9O4)2(H2O)][(UO2)2(SO4)3(H2O)2] (H9US), [(C5H7NO)2(H2O)][(UO2)2(SeO4)3(H2O)2](H2O) (OUSe), [C6H6N3][H5O2] [(UO2)2(SeO4)3(H2O)] (BH5USe), [C6H6N3][H7O3][(UO2)2(SO4)3(H2O)](H2O) (BH7US), [C6H16N][H5O2][(UO2)2(SeO4)3(H2O)] (TeH5USe), [C6H18N2][(UO2)2(SO4)3(H2O)] (TmUS), [H5O2]2[(UO2)2(SeO4)3(H2O)](H2O) (H5USe-1), [H5O2]2[(UO2)2(SeO4)3(H2O)2](H2O)9(H5USe-2), and [C4H14N2][(UO2)2(SeO4)3(H2O)](H2O) (DmUSe) have been prepared by isothermal evaporation of aqueous solutions containing extra sulfuric or selenic acid. Their crystal structures can be considered as organo-inorganic hybrids constructed of alternating [(UO2)2(TO4)3(H2O)n]2−slabs (T = Se6+, S6+,n = 1, 2) and layers containing templating organic moieties and/or hydronium ions and water molecules. The organic and inorganic parts of the structures are linked by multiple hydrogen bonds. Besides structure description, we offer topological analysis of the inorganic fragments with UO2:TO4ratio of 2:3 as modular units resulting from self-assembly of fundamental chains formed by [(UO2)2(TO4)2] tetramers andTO4tetrahedra.

Highly conjugated systems with pedal motion in uranyl crotonate compounds with 1,2-bis(4-pyridyl)ethylene as a neutral ligand or a counter cation

Crystal structures of [UO2(crt)2(bpe)2] (I) and (H2bpe)2(OH)2[UO2(crt)3]2[UO2(crt)2(H2O)2] (II), where crt = crotonate ion C3H5COO− and bpe = trans-1,2-bis(4-pyridyl)ethylene C12H10N2, were solved via single crystal X-ray diffraction analysis and characterized via FTIR spectroscopy. Even though I and II are composed of similar species, their FTIR spectra feature certain differences that can be used for rapid identification of the two phases. The bpe species in I and II act either as monodentate ligands coordinated to uranyl ions through nitrogen atoms, or as protonated counter-cations. Both compounds represent π-systems conjugated in all three dimensions due to the prevalence of aromatic pyridine rings, double CC bonds and carboxylate anions with lots of π-stacking, CHπ and CHN interactions. The XRD data shows temperature variable disorder patterns of one bpe specie in II implying its pedal motion at 120 K. Analysis of all available to date uranyl compounds with bpe molecules/cations in the CSD by means of Voronoi–Dirichlet tessellation is provided. It reveals that the arrangement of bpe is topologically equivalent in most of the studied compounds and represents the thinnest covering of 3D space with 14 neighbors as opposed to classical densest sphere packing with 12 neighbors. This is consistent with the earlier proposed mechanism of crystal growth which implies such a packing of bulky species that allows to minimize the overlap of their outer electron shells.

Uranyl Coordination Compounds with Alkaline Earth Metals and Crotonate Ligands

AbstractCrystal structures of three new uranyl compounds were solved using single crystal X‐ray diffraction and studied using FTIR spectroscopy. Although the syntheses conditions were very similar differing only in the alkaline earth metal (Mg, Ca or Sr), the resulting structures are different, containing, respectively, mono‐, tri‐ and hexanuclear metal‐containing building units. This fact proves the profound crystal‐chemical role of secondary metal atoms on the formation of crystal structures. Parameters of alkaline earth metal cations in the three newly synthesized compounds as well as in the earlier reported Ba‐containing analog are discussed in detail using Voronoi–Dirichlet polyhedra and the method of intersecting spheres. Differences in packing of building units in the four mentioned compounds, as well as inter‐ and intramolecular noncovalent interactions in their structures are reported. The results of analysis are consistent with the stereoatomic model of crystal structures.

Photoluminescence Spectra of UO2(NO3)2

We have measured photoluminescence spectra of uranyl nitrate (UO2(NO3)2) crystals and a UO2(NO3)2-filled opal crystal. The spectra have been compared to the corresponding photoluminescence spectrum of photonic glass. The photoluminescence spectra were excited by semiconductor light-emitting diodes, a cw laser, and repetitive laser pulses. The excitation wavelengths were located in the resonance absorption band of the uranyl compound. This has offered the possibility of measuring the photoluminescence spectrum of UO2(NO3)2 at an acquisition time of 1 ms, using the smallest possible amount of the substance (down to 10–9 g). The spectra have been obtained in the range 200–1000 nm using a fiber-optic minispectrometer.

Persistent Superprotonic Conductivity in the Order of 10 −1 S·cm −1 Achieved Through Thermally Induced Structural Transformation of a Uranyl Coordination Polymer

Despite tremendous efforts having been made in the exploration of new high-performance proton-conducting materials, systems with superprotonic conductivity higher than 10−1 S·cm−1 are scarcely reported. We show here the utilization of bridging uranyl oxo atoms, traditionally termed cation–cation interaction (CCI), as the hydrogen bond acceptor to build a dense and ordered hydrogen bond network, affording a unique uranyl-based proton-conducting coordination polymer (H3O)4UO2(PO4)2 (HUP-1). This compound contains a densely connected hydronium network that is substantially stabilized by uranyl oxo atoms and exhibits high proton conductivities over a wide temperature range. At 98 °C, 98% relative humidity, a superprotonic conductivity of 1.02 × 10−1 S·cm−1 is observed for the system, one of the highest values reported for a solid-state proton-conducting material. This property originates from the thermally induced phase transformation from HUP-1 to another uranyl compound also with a CCI bond, (H3O)UO2PO4·(H2O)3 (HUP-2), accompanied by the partial generation of phosphorus acid that is further trapped in the structure of HUP-2, demonstrated by solid-state NMR analysis. The superprotonic conductivity of H3PO4@HUP-2 is persistent under the testing condition.

Bipyridine-Directed Syntheses of Uranyl Compounds Containing Semirigid Dicarboxylate Linkers: Diversity and Consistency in Uranyl Speciation.

Bipyridine organic bases are beneficial to the synthesis of novel uranyl-organic hybrid materials, but the relationship between their molecular structures and specific roles as structure-directing agents, especially for the semirigid dicarboxylate systems, is still unclear. Here we demonstrate how the bipyridine ligands direct the coordination assembly of uranyl-organic compounds with a semirigid dicarboxylate linker, 4,4'-dicarboxybiphenyl sulfone (H2dbsf), by utilizing a series of bipyridine ligands, 1,10-phenanthroline (phen), 2,2'-bipyridine (2,2'-bpy), 5,5'-dimethylbipyridine (5,5'-dmbpy), 4,4'-bipyridine (4,4'-bpy), or 1,3-di(4-pyridyl)propane (bpp). Under hydrothermal conditions, eight uranyl-organic coordination polymers (UCPs), four of which [[UO2(dbsf)(phen)] (1), [UO2(dbsf)(phen)]·H2O (1'), [U4O10(dbsf)3]2[H2bpp]2 (6), and [U4O10(dbsf)3]2[H2bpp] (6')] were reported previously, were synthesized and divided into two types based on the chelate or template effect of these bipyridine ligands. 1, 1', [UO2(dbsf)(2,2'-bpy)] (2), and [(UO2)2(dbsf)2(5,5'-dmbpy)2] (3) are springlike triple helices with bipyridine ligands (phen, 2,2'-bpy, or 5,5'-dmbpy) as chelate ligands, while [U4O10(dbsf)3][H2(4,4'-bpy)] (4), [U4O10(dbsf)3]2[H(4,4'-bpy)]2[Ni(H2O)6] (5), 6, and 6' are tetranuclear uranyl-mediated 2-fold-interpenetrating networks with 4,4'-bpy or bpp as template ligands and charge-balancing agents. The participation or not in uranyl coordination of different bipyridine ligands promotes not only diversity in uranyl speciation and final topological structures among different classes of organic bases but also consistency for the same types of bipyridine ligands, which thus endows the possibility of the rational design of UCPs based on semirigid dicarboxylate ligands with the aid of cautiously selected bipyridine ligands.

Removal of tetracycline hydrochloride from aqueous solution by three 3D uranyl-organic frameworks

Nitroterephthalic acid (H2ntp), 2,5-dibromoterephthalic acid (H2bdc) and N,N′-donor ligands have been employed to synthesize three uranyl compounds: (H2-tmp)[(UO2)(ntp)2]·H2O (1), (2,2′-Hbpy)[(UO2)2(ntp)2.5]·H2O (2) and [(UO2)(bdc)(4,4′-bpy)0.5] (3) [where 2,2′-bpy = 2,2′-dipyridyl, 4,4′-bpy = 4,4′-dipyridyl, tmp = tetramethylpyrazine]. Single-crystal X-ray crystal structures of compounds 1–3 have been determined. Compounds 1 and 2 consist of dinuclear units, which are linked by ntp2− ligands on the equatorial plane to form 2D network, the layers are further bridged by the carboxylate at the angles of 118.69° (1) and 106.18° (2) in the axial position to form the 3D frameworks. In compound 3, each uranyl center is bridged by four bdc2− ligands to form a 3D framework with the carboxylate at the angle of 73.96° on the equatorial plane, and the 4,4′-bpy further consolidates the crystal structure. Compounds 1–3 have been characterized by IR, powder X-ray diffraction, UV-vis absorption spectroscopy, thermogravimetric analysis and photoluminescence spectroscopy. In addition, the photocatalytic degradation of 1, 3 and the performance for tetracycline hydrochloride adsorption of 2 have been investigated.

Novel Heterometallic Uranyl-Transition Metal Materials: Structure, Topology, and Solid State Photoluminescence Properties.

Six new uranyl hybrid materials have been synthesized solvothermally utilizing the ligands 2,2'-bipyridine-3,3'-dicarboxylic acid (H2L) and 2,2':6',2''-terpyridine (TPY). The six compounds are classified as either molecular complexes (I0O0 connectivity), [(UO2)(L)(TPY)]·H2O (1), [Ni(TPY)2][(UO2)(L)2]·3H2O (2), and [Cu(TPY)2][(UO2)(L)2]·3H2O (3), or 3D metal-organic frameworks (MOFs, I0O3 connectivity), [Cu2(UO2)2(OH)(C2H3O2)(L)3(TPY)2]·6H2O (4), [Zn2(UO2)2(OH)(NO3)(C2H3O2)(L)3(TPY)2]·4H2O (5), and Na[Ni(UO2)3(OH)(O)(L)3]·9H2O (6). A discussion of the influence of transition metal incorporation, chelating effects of the ligand, and synthesis conditions on the formation of uranyl materials is presented. The structure of compound 6 is of particular note due to large channel-like voids with a diameter of approximately 19.6 Å. A topological analysis of 6 reveals a new topology with a 9-nodal 3,3,3,3,3,3,3,4,5-connected network, designated geg1 hereafter. Further, solid state photoluminescence experiments show emission and lifetimes values consistent with related uranyl compounds.