Urea Complexes Research Articles

Using the MMFF94 model to predict structures and energies for hydrogen-bonded urea–anion complexes

The performance of the MMFF94 model has been compared with density functional theory (B3LYP/DZVP2) in calculation of hydrogen-bonded complexes of urea with three differently shaped Cl −, NO 3 − , and ClO 4 − anions. After modification of selected van der Waals parameters, good agreement between the two methods was obtained for geometric parameters and relative conformational energies. Absolute values of MMFF94 binding energies are underestimated, but application of a systematic correction yields binding energies that are within ±1 kcal/mol of B3LYP/DZVP2 values.

Thermolysis of Urea Complexes of Uranyl Nitrate

Quantitative parameters of thermolysis of uranyl nitrate urea complexes, [UO2(NO3)2{(NH2)2·CO}2], [UO2(H2O){(NH2)2CO}4](NO3)2, and [UO2(H2O){(NH2)2CO}5](NO3)2 at 175, 200, and 225°C were measured. Thermolysis of [UO2(NO3)2{(NH2)2CO}2] at 200°C affords the biuret complex of uranyl nitrate in a 90% yield. The urea ligands in the hydrated complexes completely transform into biuret at 175°C. Thermolysis of [UO2(H2O){(NH2)2CO}5](NO3)2 yields the biuret-cyanurate complexes of uranyl nitrate. The features of thermolysis of the uranyl nitrate complexes originate from the chemical transformations of urea at elevated temperatures.

Toward Understanding Mobile Proton Behavior from First Principles Calculation: The Short Hydrogen Bond in Crystalline Urea−Phosphoric Acid

The dynamics of the intermolecular short hydrogen bond in the molecular complex of urea and phosphoric acid are investigated using plane-wave density functional theory. Results indicate migration of the proton toward the center of the hydrogen bond as temperature is increased, in line with recent experimental measurements. Computed vibrational frequencies show favorable agreement with experimental measurement. An analysis of existing neutron diffraction data leads us to conclude that the effective potential well experienced by the proton is temperature-dependent. Inspired by our computations and theoretical analysis, we offer a possible explanation for the proton migration phenomenon.

Structural Design Criteria for Anion Hosts: Strategies for Achieving Anion Shape Recognition through the Complementary Placement of Urea Donor Groups

The arrangement of urea ligands about different shaped anions has been evaluated with electronic structure calculations. Geometries and binding energies are reported for urea complexes with Cl-, NO(3)-, and ClO(4)-. The results yield new insight into the nature of urea-anion interactions and provide structural criteria for the deliberate design of anion selective receptors containing two or more urea donor groups.

A di-palladium urea complex as a molecular receptor for anions

A new di-nuclear palladium complex containing thiol-urea ligands has been prepared, structurally characterized and its interaction with anionic species studied in solution.

A simple single-source precursor route to the nanostructures of AlN, GaN and InN

In an effort to find a simple and common single-source precursor route for the group 13 metal nitride semiconductor nanostructures, the complexes formed by the trichlorides of Al, Ga and In with urea have been investigated. The complexes, characterized by X-ray crystallography and other techniques, yield the nitrides on thermal decomposition. Single crystalline nanowires of AlN, GaN and InN have been deposited on Si substrates covered with Au islands by using the complexes as precursors. The urea complexes yield single crystalline nanocrystals under solvothermal conditions. The successful synthesis of the nanowires and nanocrystals of these three important nitrides by a simple single-precursor route is noteworthy and the method may indeed be useful in practice.

N-methylmonothiocarbamatopentamminecobalt(III): restricted C-N bond rotation and the acid-catalyzed O- to S-bonded rearrangement.

High-resolution (1)H and (13)C NMR studies on the linkage isomers [(NH(3))(5)CoOC(S)NHCH(3)](2+) and [(NH(3))(5)CoSC(O)NHCH(3)](2+) reveal that the O-bonded form exists as a 5:1 mixture of Z and E isomers arising from restricted rotation about the C-N bond. Similarly, restricted rotation is observed (at 20 degrees C) for the S-bonded isomer (Z/E ca. 18:1), but not for the isoelectronic carbamate ion [(NH(3))(5)CoOC(O)NHCH(3)](2+), nor for the unsubstituted carbamato complex [(NH(3))(5)CoOC(O)NH(2)](2+). An analysis of the variable-temperature NMR data for the O-bonded carbamato and urea complexes has provided quantitative data on the rotational barriers, and these ions involve much faster C-N bond rotations than the thiocarbamato complexes. The acid-catalyzed reaction of [(NH(3))(5)CoOC(S)NHCH(3)](2+) is confirmed, but there is much less parallel hydrolysis (ca. 2%) than previously reported (40 +/- 10%) for 0.1 M HClO(4). In 1 M HClO(4), [(NH(3))(5)CoSC(O)NHCH(3)](2+) and [(NH(3))(5)CoOH(2)](3+) are formed in parallel as an 83:17 mixture. The kinetic data suggest that the protonated form is at least 20-fold more reactive than the free ion and that the linkage isomerization and hydrolysis pathways are both acid-catalyzed, the latter clearly more so than the rearrangement.

Metal‐Urea Complex—A Precursor to Metal Nitrides

A novel and general route to synthesize various metal nitrides (AlN, CrN, and ζ‐Fe2N) from metal–urea complexes is presented. These complexes, especially metal–urea chloride, have proved to be useful precursors to metal nitrides, because urea molecules construct a coordination sphere around the metal atom and form a stable structure, compared with the air‐sensitive halide. Different anions in the second coordination sphere determine the reaction mechanism. The transformation from metal–urea chloride to nitride is thought to follow a nucleation–growth mechanism, while that from metal–urea nitrate is thought to follow a nitridation mechanism. We anticipate that this metal–urea complex will find applications in the fabrication of other, more complex, nitrides.

Hydrophobic interaction chromatography coupled with atomic fluorescence spectrometric detection: Effect of the denaturation on the determination of thiolic proteins

Hydrophobic interaction chromatography coupled online with chemical vapour atomic fluorescence spectrometry (HIC–CVGAFS) has been optimized for the analysis of thiolic proteins in denaturing conditions. Proteins are pre-column simultaneously denatured and derivatized in phosphate buffer solution containing 8.0moldm−3 urea and p-hydroxymercurybenzoate (PHMB) and the derivatized denatured proteins are separated on a silica HIC Eichrom Propyl column in the presence of 8.0M urea in the mobile phase. Post-column online reaction of derivatized denatured proteins with bromine, generated in situ by KBr/KBrO3 in HCl medium, allowed the fast conversion of the uncomplexed PHMB and of the PHMB bound to proteins to inorganic mercury also in presence of urea. Hg2+, present in solution as Hg2+–urea complex, is selectively detected by AFS in a Ar/H2 miniaturized flame after sodium borohydride reduction to Hg. Under optimized conditions, online bromine treatment gives a 100±2% recovery of both free and protein-complexed PHMB. Denatured glyceraldehyde-3-phosphate dehydrogenase, aldolase, lactate dehydrogenase, trioso phosphate isomerase and β-lactoglobulin have been examined. As the sensitivity and limit of detection of proteins in the HIC–CVGAFS apparatus depends on number of SH groups reacting with PHMB, the denaturation process, which increases the number of PHMB-reactive thiolic groups in proteins, improves the analytical performances of the described system in protein analysis. The detection limit for the denatured proteins examined was found in the range of 10−10–10−12moldm−3, depending on the considered protein, with linear calibration curves spanning over four decades of concentration.

Intercalation of magnesium–urea complex into swelling clay

N fertilization is considered as a main source of N 2O and NO emission from agricultural soils. Especially, it is a great challenge to enhance urea efficiency without any deleterious effects on the environment because of massive application of urea. In this study, we attempted to stabilize urea in the interlayer space of montmorillonite (MMT) in order to reduce urea loss from soils. Urea was successfully intercalated in the form of urea–magnesium complexes. The stabilization of urea–magnesium complexes within the interlayer space was confirmed by a significant expansion of the interlayer space and the presence of urea–magnesium complexes. Urea degradation in soils was significantly delayed by application of the urea–magnesium complex intercalated into MMT. It is highly feasible that stabilization of urea within nanosized interlayer space would lead to considerable improvement of nitrogen efficiency in soils.

Gas Phase Reactivity of Ni+ with Urea. Mass Spectrometry and Theoretical Studies

The gas-phase reactivity of Ni+(urea) has been investigated by means of mass spectrometry techniques and density functional calculations. The major fragmentations observed in the MIKE spectrum of [Ni−urea]+ correspond to the loss of CO, NH3, and HNCO. The electrospray MS/MS spectrum shows also these fragmentations; however, an additional intense peak is detected matching to the elimination of water. To explain the differences observed in the reactivity under FAB or ESI conditions, several pathways leading to the experimental fragmentations have been considered theoretically. The exploration of the potential energy surfaces has shown that, although the elimination of NH3 mainly arises from the noninsertion mechanisms, the elimination of CO and HNCO arises exclusively from the insertion mechanisms. Elimination of water shows larger barriers, but under electrospray conditions, the presence of the solvent can reduce these energy barriers leading to isomerizations of the [Ni−urea]+ complex in the source region. The results obtained have been also compared to those previously reported for Cu+. Calculations show that the most stable Ni+−urea complex has the Ni+ cation interacting with the oxygen, with the computed binding energy being 66.3 kcal/mol.

Novel Synthesis of Nanocrystalline Gallium Nitride Powder from Gallium(III)-Urea Complex

Abstract A novel method is reported for the preparation of wurtzite gallium nitride (GaN) powder with a controlled particle size from nanometer to micron scale, using gallium(III)-urea complex as the precursor. The GaN powders as synthesized at various temperatures showed strong and distinct photoluminescence characteristics. The method developed here may provide a simple, potentially economical way to form other nitride materials with desired performances.

Natural gas and the fertilizer industry

Natural gas and the fertilizer industry

A Study of the Equilibrium and Kinetics of Urea Binding by a Biomimetic Dinickel(II) Complex

AbstractThe equilibrium and kinetics of urea binding by model dinickel complexes have been studied, which is relevant to the activity of the urease enzyme. The pyrazolate‐based, O2H3‐bridged dinuclear nickel(II) complex [LNi2(OH)(H2O)]2+ (1) binds urea reversibly in organic solvents with the formation of an N,O‐bridged urea anion complex [LNi2[OC(NH2)NH)]2+ (2) and two water molecules. The equilibrium constant has been measured as 4.3(4) in acetone, 2.7(5) in acetonitrile, and 3(1) in methanol at 25 °C. Upon dissolving 1 in anhydrous methanol, the O2H3 bridge is substituted for an O2Me2H bridge to give [LNi2(OMe)(MeOH)]2+ (4),which has been crystallized as 4·(ClO4)2 and characterized by X‐ray diffraction. Both NiII ions in 4 are five‐coordinate with geometries intermediate between square‐pyramidal and trigonal‐bipyramidal. The H atom in the Me2O2H bridging unit is located in an asymmetric position. The exchange of water and methanol ligands in complexes 1 and 4 is very fast in solution at 25 °C (kobsd. > 103 s−1). Binding of urea by complexes 1 and 4 are slower reactions (kobsd. ≈︁ 10−1 to 101 s−1 under the concentration conditions used) and can be monitored by stopped‐flow techniques. Detailed kinetic studies indicate that binding of urea is a multi‐step process. Steady‐state intermediates of the tentative formula [LNi2(OR)(urea)n]2+ (n = 1, 2) are formed in fast preequilibrium with the starting complexes 1 (R = H) and 4 (H = Me), respectively. The bidentate N,O‐coordination and deprotonation of an O‐bound urea ligand constitute the overall rate‐limiting step. The kinetic data suggest that both mono‐ and bis(urea) complexes participate in the formation of the chelate 2, and that the latter are substantially more reactive. The bis(urea) pathway was unexpected because only one urea molecule is incorporated into the final product 2. Reactive intermediates [LNi2(OH)(urea)n]2+ are close analogs of the reactive intermediate of urease, which also has the hydroxide and urea ligands bound at a dinickel core. However, the reactivities of the intermediates are different. The hydroxide ligand in our model complex acts as a base towards urea, and the urea anion complex 2 is formed. In urease, the hydroxide ligand attacks urea as a nucleophile leading to the hydrolysis of urea. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

An analysis of bifurcated H‐bonds: crystal and molecular structures of O,O‐diphenyl 1‐(3‐phenylthioureido)­pentanephosphonate and O,O‐diphenyl 1‐(3‐phenylthioureido)butanephosphonate

AbstractIn the crystal structures of O,O‐diphenyl 1‐(3‐phenylthioureido)pentanephosphonate (1) and O,O‐diphenyl 1‐(3‐phenylthioureido)butanephosphonate (2) analysed here, bifurcated H‐bonds within R21(6) motifs are formed. It seems that such interactions play a crucial role in the crystal architecture. This is supported by ab initio MP2/6–311 ++ G** calculations on simple, modelled complexes of urea, thiourea and their derivatives with water, where the oxygen atom of water molecule is the bifurcated proton‐accepting centre. The calculations show that single H‐bonds within bifurcated systems are of medium strength (2.7–3.6 kcal mol−1). The topological parameters obtained from the Bader theory are applied for the analysis of these bifurcated H‐bonds. Copyright © 2003 John Wiley & Sons, Ltd.

Bis(pentamethylene)urea complexes of the lanthanide nitrates: synthesis, characterization, properties

Lanthanide nitrate complexes of bis(pentamethylene)urea (BPMU) with general formula Ln(NO3)33BPMU, where Ln: La, Nd, Sm, Eu, Ho and Er have been prepared and characterized based on CHN elemental analyses, lanthanide titration with EDTA, molar conductivity, spectroscopic data and thermal studies. The infrared spectra show that ligands (BPMU) are bonded through the carbonyl oxygen, nitrate counter-ions are bidentate linked to the central ions.The structure of the neodymium complex was determined. The crystal is monoclinic, P21/c,Z=4, with the following parameters: a=10.148(1)Å, b=21.879(2), c=19.154(2)Å, β=104.11(1)°, V=4124.3(7)Å3. The polyhedron is a distorted tricapped trigonal prism, coordination number nine.

Synthesis, Structures and Characterization of the Dinuclear Nickel(II) Complexes Containing N,N,N′,N′-Tetrakis[(1-ethyl-2-benzimidzolyl)- methyl]-2-hydroxy-1,3-diaminopropane and their Urea Complexes Relevant to the Urease Active Site

Abstract New dinuclear nickel(II) complexes ([Ni2(L-Et)(CH2ClCOO)(H2O)2](ClO4)2·H2O (3), [Ni2(L-Et)(CCl3COO)(H2O)2](ClO4)2·H2O (4), [Ni2(L-Et)(CF3COO)(H2O)2](ClO4)2·3H2O (5)) and their urea complexes ([Ni2(L-Et)(RCOO)(ur)(H2O)](ClO4)2 (R = CH3 (6), C2H5 (7), CH2Cl (8), CCl3 (9), CF3 (10)) (HL-Et = N,N,N′,N′-tetrakis[(1-ethyl-2-benzimidazolyl)methyl]-2-hydroxy-1,3-diaminopropane and ur = urea) have been prepared in order to model the active site urease. These complexes were characterized by IR and UV-vis spectra, and by magnetic susceptibilities. The crystal structures of the complexes [Ni2(L-Et)(CH2ClCOO)(CH3OH)2](ClO4)2 (3′) and [Ni2(L-Et)(OAc)(ur)(H2O)](ClO4)2· 2(CH3)2CO (OAc = acetate) (6′) were determined by X-ray crystallography. In the cation of 6′, the two nickel(II) ions are bridged by an alkoxo of L-Et− and an acetato, and a monodentate urea molecule is attached to a nickel(II) through its oxygen atom. Variable temperature magnetic susceptibility measurements of the urea complexes 6, 8, and 10 show an antiferromagnetic exchange coupling (2J = −15.8, −8.0, and −13.3 cm−1 for 6, 8, and 10, respectively, Hamiltonian H = −2JS1·S2 with S1 = S2 = 1) between two nickel(II) ions.

Oxidation of urazoles via in situ generation of Cl+ by using N,N,2,3,4,5,6-heptachloroaniline or a UHP/MCln system under mild conditions

A combination of inorganic hydrolysable chloride salts and UHP (hydrogen peroxide–urea complex) in the presence of wet SiO2 was used as an effective oxidising agent for the oxidation of urazoles and bis-urazoles to their corresponding triazolinediones under mild and heterogeneous conditions in good to excellent yields. Oxidation of urazoles and bis-urazoles also occurred with N,N,2,3,4,5,6-heptachloroaniline. The in situ generation of Cl+ appears to be required for the oxidation of urazoles using these reagents.

A urea complex of copper(II) hypophosphite at 293, 100 and 15 K.

The crystal structure of poly[copper(II)-di-mu-hypophosphito-mu-urea], [Cu(H2PO2)2(CH4N2O)]n, has been determined at 293, 100 and 15 K. The geometry of the hypophosphite anion is very close to ideal, with point symmetry mm2. Each Cu atom lies on an inversion centre and is coordinated to six O atoms from four hypophosphite anions and two urea molecules, forming a tetragonal bipyramid. The unique urea molecule lies on a twofold axis. Each hypophosphite anion in the structure is coordinated to two Cu atoms. The hypophosphite anions, urea molecules and Cu(II) cations form polymeric ribbons. The Cu(II) cations in the ribbon are linked together by two hypophosphite anions and a urea molecule, which is coordinated to Cu via an O atom. The ribbons are linked to each other by N-H...O hydrogen bonds and form polymeric layers.

TRIANGULAR PHASE DIAGRAMS TO PREDICT THE FRACTIONATION OF FREE FATTY ACID MIXTURES VIA UREA COMPLEX FORMATION

The authors previously developed a rapid and inexpensive procedure to fractionate free fatty acid (FFA) mixtures via urea complex UC formation. The process yields two phases in equilibrium, a solvent-rich extract phase containing unsaturated FFA and a solid raffinate phase consisting of UCs of saturated and monounsaturated FFA. It effectively removed saturated FFA, that is, palmitic and stearic acid, from an FFA mixture derived from low erucic acid rapeseed (LEAR) oil. In the present study, triangular phase diagrams were determined for the urea/ethanol/water/LEAR-FFA system at various temperatures, which delineated the phase boundary between the 1-phase and 2-phase regions. Calculations of the amounts and distribution of urea and LEAR-FFA between the two phases based on tie-lines from the phase diagram strongly agreed with experimental data. The diagram accurately predicted the temperature required to cosolubilize urea and LEAR-FFA in solvent. A plot of the percent extraction of an individual FFA species versus percent extraction of total FFA for all major FFA species yielded a universal curve describing results related to various combinations of urea, water, ethanol, and LEAR-FFA. Equations derived from the phase diagram, distribution data for the FFA species, and mass balances were combined to create a mathematical model, which accurately predicted the FFA composition of both phases. The methodology used and results obtained should aid critical evaluation of large scale UC-based FFA fractionation procedures.