Urea Complexes Research Articles

Differential scanning calorimetry studies on urea-carboxylic acid inclusion complexes

Inclusion complexes of urea with several long chain fatty acids (of up to 30 C atoms) were prepared and their thermal behavior was investigated by differential scanning calorimetry (DSC). The host-guest ratios in these complexes were determined by titration against standard NaOH solution using pH to determine the end point. The DSC thermograms showed that complexes of the higher saturated acids (with 23 or more C atoms) are stable even above the melting point of urea. This is contrary to the currently accepted view that for complexes of urea and fatty acids, the outer urea lattice simply melts when the melting point of urea is reached, causing the complex to decompose.

Mössbauer Study of Some Iron Organo-Metallic Complexes

Abstract Mossbauer absorption spectra were obtained for ferric complexes of urea and thiourea derivatives with dibasic acids. The complexes were prepared in the two ratios 1:1 and 2:1 metal to ligand. The Mossbauer parameters were analysed inconsistent with the structural data obtained from I.R. spectra. It was found that changing the metal -ligand ratio has nearly no effect on both the isomer shift and quadrupole splitting values.

Raman spectral studies of uranyl sulphate and its urea complex structural isomers

Abstract Using Raman spectroscopy, the authors have studied two forms of the dioxouranium(VI) sulphate complexes of the urea molecule, the bis(urea)dioxouranium(VI) sulphates α- and β-UO 2 SO 4 ·2CO(NH 2 ) 2 . Raman spectra were recorded using λ = 488 and 647.1 nm laser excitation lines. For λ = 448 nm, the crystals exhibit strong, structured luminescence, with peaks separated by 857 cm −1 (α-form) and 853 cm −1 (β-form). For λ = 647.1 nm, very intense Raman spectra are obtained which are similar but slightly different and can be used to distinguish each form. A comparison with the urea and uranyl sulphate hydrate (USH) spectra has clarified the Raman band assignment of the two parent α- and β forms. Contributions of the urea donor ligand and the sulphate have been identified. Charge transfer to uranium and the uranium—oxygen force constant in the uranyl ion have been evaluated. The results agree with previous X-ray structural investigations.

Production and partial purification of γ-linolenic acid and some pigments fromSpirulina platensis

The polyunsaturated fatty acid γ-linolenic acid (GLA, 18:3ω6) is of potential pharmaceutical value. The cyanobacteriumSpirulina platensis could become an excellent source for this fatty acid, provided that GLA content could be increased and a GLA concentrate could be obtained at a low cost. Increasing the cell concentration inSpirulina platensis enhanced the fatty acid content and thus the GLA content. This effect was used to further enhance the GLA content of GLA-overproducing strains. Separation of the galactolipids and their purification via urea complexes formation, resulted in a GLA concentrate of over 90% purity.

Binaphthyl metallomacrocycles for complexation of neutral molecules

AbstractBinaphthyl salophen crown ethers 1 and 2, containing an immobilized electrophilic uranyl cation, were synthesized by cyclization of dialdehyde 10 with 1,2‐benzenediamine (14) or cis‐1,2‐cyclohexanediamine (15), respectively, in the presence of Ba2+ as a template ion and subsequent transmetallation with UO22+. Solid complexes of urea and 1 and 2 were isolated. Molecular‐mechanics calculations show that the geometry of 1 and 2 is suitable for the introduction of functional groups close to the cavity of the metallomacrocycles.

Functionalized binaphthyl salophen crown ethers as models for the enzyme urease

AbstractFunctionalized binaphthyl salophen crown ethers 2 and 3 containing a Lewis‐acidic uranyl cation were prepared by the Ba2+‐templated cyclization of dialdehydes 15 and 22, for which two convenient synthetic routes were developed, with diamines 23 and 24. Intramolecular complexation of the amide groups in metallomacrocycles 2 and 3 was demonstrated by the isolation of two different forms of 2a, which were observed to interconvert, by 1H‐NMR and IR spectroscopy and by electrochemical measurements. Use of diamine 24 during the cyclization resulted in the formation of two different stereoisomers of the monofunctionalized metallomacrocycles 3a‐d and in “sidedness” of the bisfunctionalized metallomacrocycles 3e and 3f. Solid complexes of urea and 2c and 2d were isolated, but no catalytic activity was observed for 2a in the hydrolysis of urea in dioxane / water.

Synthesis and reactions of the cyclic silyl peroxide 1,1,4,4-tetramethyl-2,3-dioxa-1,4-disilacyclohexane

The first simple cyclic silyl peroxide, namely 1,1,4,4,-tetramethyl-1,4-disila-2,3-dioxane ( 1), was prepared by classical synthetic methodology from its corresponding cyclic disilazane and the urea complex of hydrogen peroxide.

Thermal behaviour of Cu(II)-urea complex

The urea complex of copper was synthesized and its structure was established by Fourier transform infrared (FTIR), electron spin resonance (ESR) and atomic absorption spectroscopy and elemental analysis to be Cu(urea)4Cl2. The thermal behaviour of this complex has been studied by thermogravimetry and differential thermal analysis and FTIR and ESR. Thermal analysis shows that the decomposition of the complex occurs in four stages of weight loss of different intermediates followed by three endothermal effects. The complex is thermally stable up to 428 K. The ESR and FTIR behaviour of the Cu(II)-urea complex during thermolysis was studied between 428 and 633 K. The experimental results suggest that in this temperature range the complex decomposition occurred forming thermodynamically stable regions of Cu(II) which are ferromagnetically coupled.

Hydrogen‐1 and aluminum‐27 NMR study of Al(ClO4)3 in aqueous mixtures of dimethylformamide and urea

AbstractA competitive solvation study of Al(III) in water–dimethylformamide (DMF) and water–urea mixtures was carried out by 1H and 27Al NMR spectroscopy. At temperatures low enough to slow proton and ligand exchange, separate 1H resonance signals were observed for bulk and coordinated ligand molecules, and for 27Al in different solvation complexes. The 1H NMR data confirmed hexacoordination of Al(III) at all concentrations, monodentate binding at the oxygen atom of both DMF and urea, preferential solvation by both organic ligands, the presence of steric hindrance to complexation, particularly by DMF, and the identification of the complexes responsible for the 27Al spectra. It also showed a non‐equivalence of the amido groups in urea, in both the complex and free states, presumably owing to hindered rotation about the carbon—oxygen bond. In contrast to previous reports, the 27Al spectra for the water–DMF and water–urea solutions showed well resolved signals for the species [Al(H2O)n–6Ln]3+ (n = 0–6; L = DMF or urea), including two isomers for one of the water–urea complexes. The 27Al NMR data reflected the stronger preferential solvation by urea.

Urea and Thiourea Inclusion Complexes of Conjugated Polyenes: Polarized Fluorescence Excitation and Resonance Raman Studies

Abstract Urea and thiourea inclusion complexes have been prepared containing conjugated polyene chromophores. Dialkylpolyenes with n-alkane tails form complexes with urea; carotenoids and diphenylpolyenes form complexes with thiourea. In some cases the chromophoric groups are diluted by incorporation of saturated hydrocarbons into the inclusion complexes. The resulting crystals are highly dichroic with their maximum absorption along the crystallographic hexagonal c-axis. Polarized fluorescence excitation studies (of the urea n-alkylpolyene complexes) and polarized single crystal resonance Raman studies (of the thiourea carotenoid and diphenylpolyene complexes) have been performed and are interpreted in terms of the orientation of the transition dipole of these polyenes with respect to the molecular axis.

Ready formation of stable cationic Rh(I) complexes of MeCN, formamide, urea and related species via replacement of the triflate in trans-(Ph 3P) 2Rh(CO)(OTf)

The weakly-coordinated trifluoromethanesulfonate (triflate) in trans-(Ph 3P) 2Rh(CO)(OTf) (OTfOSO 2CF 3) can be readily replaced by ligand L to form stable ionic complexes [ trans-(Ph 3P) 2RhL(CO)]OTf (LMeCN, formamide pyrone and urea). The replacement of triflate by carbon monoxide forms an unstable species that gradually loses CO and returns the starting material. Sterically hindered ligands such as 1,1,3,3-tetramethylurea and xanthone do not react.

ChemInform Abstract: Synthetic Molecular Receptors for Urea. Macrocyclic Ligands with Intraannular Acidic Groups and the Complexes with Urea.

AbstractA systematic study is carried out on the effect of intraannular acidic groups present in the cavities of the macrocycles (III), (IV), (VIII), (X) on the complexation of urea.

ChemInform Abstract: Reversible Linkage Isomerization of Pentaamminecobalt(III) Complexes of Urea and Its N‐Methyl Derivatives.

AbstractSpecific rates and conditions for the observation of hitherto unrecognized urea‐O to urea‐N isomerization reactions are reported.

ChemInform Abstract: Synthesis of Urea Complexes of Cd(II) and Cu(II) and the Reaction of Bipyridine and Phenanthroline with Cu(MA)Cl2H2O and Ni(MA)2Cl2.

ChemInformVolume 19, Issue 41 Organoelement Compounds ChemInform Abstract: Synthesis of Urea Complexes of Cd(II) and Cu(II) and the Reaction of Bipyridine and Phenanthroline with Cu(MA)Cl2H2O and Ni(MA)2Cl2. N. RETTA, N. RETTA Dep. Chem., Addis Ababa Univ., Addis Ababa, EthiopiaSearch for more papers by this authorO. M. NUR KEKIA, O. M. NUR KEKIA Dep. Chem., Addis Ababa Univ., Addis Ababa, EthiopiaSearch for more papers by this author N. RETTA, N. RETTA Dep. Chem., Addis Ababa Univ., Addis Ababa, EthiopiaSearch for more papers by this authorO. M. NUR KEKIA, O. M. NUR KEKIA Dep. Chem., Addis Ababa Univ., Addis Ababa, EthiopiaSearch for more papers by this author First published: October 11, 1988 https://doi.org/10.1002/chin.198841296AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume19, Issue41October 11, 1988 RelatedInformation

Reversible linkage isomerization of pentaamminecobalt(III) complexes of urea and its N-methyl derivatives

Abstract The (NH3)5CoOC(NH2)23+ ion is consumed in water according to the rate law k(obs.) = k1 + k2[OH−], where k1 = 4.0 × 10−5 s−1 and k2 = 14.2 M−1 s−1 (0–0.1 M [OH−];μ = 1.1 M, NaClO4, 25 °C). A hitherto unrecognized intramolecular O- to N- linkage isomerization reaction has been detected. In strongly acid solution only aquation to (NH3)5CoOH23+ is observed, but in 0.1–1.0 M [OH−], 7% of the directly formed products is the urea-N complex (NH3)5CoNHCONH22+ which has been isolated. In the neutral pH region a much greater proportion (25%) of the products is the urea-N species. These results are interpreted in terms of an urea-O to urea-N linkage isomerization reaction competing with hydrolysis for both spontaneous (k1) and base-catalyzed (k2) pathways; the rearrangement is not observed in strongly acidic solution (pH ⩽ 1) because the protonated N-bonded isomer (pK′a ≈ 3) is unstable with respect to the O-bonded form. The appearance of the isomerization pathway as the pH is raised in the 0–6 region is commensurate with a rate increase which cannot be attributed to a contribution from the base catalysis term k2[OH−]. It is argued that this observation establishes, for the spontaneous pathway, that hydrolysis and linkage isomerization are separate reaction pathways — there is no common intermediate. The product distribution and rate data lead to the complete rate law, k(obs.) = k1 + k2[OH−] = (ks + kON) + (kOH + k′ON) [OH−] for the reactions of the O-bonded isomers, where ks, kOH are the specific rates for hydrolysis, and kON, k′ON are the specific rates for O- to N-linkage isomerization, by spontaneous and base-catalyzed pathways respectively; kON = 1.3 × 10−5 s−1 and k′ON = 1.1 M−1 s−1 (μ = 1.0 M, NaClO4, 25 °C). The O- to N- linkage isomerization has been observed also for complexes of N-methylurea, N,N-dimethylurea and N-phenylurea, but not for the N,N′-dimethylurea species. There is an approximately statistical relationship among the data for −NH2 capture (versus H2O), while −NHR and −NR2 do not compete with water as nucleophiles for Co(III) in either the spontaneous or base-catalyzed hydrolysis processes. For each urea-O complex, O- to N-isomerization is a more significant parallel reaction in the spontaneous as opposed to the base-catalyzed pathway. This is interpreted as being indicative of more associative character in the spontaneous route to products, a conclusion supported by other evidence. Some activation parameter data have been recorded and the effect of the N-substitution on the rates of solvolysis (H2O, Me2SO) is discussed. The urea-N complexes have been isolated as their deprotonated forms, [(NH3)5CoNHCONRR′](ClO4)2·xH2O (R,R′ = H, CH3). They are kinetically inert in neutral to basic solution but in acid they protonate (H2O, pK′a 2–3; μ = 1.0 M, 25 °C) and then isomerize rapidly back to their O-bonded forms. Some solvolysis accompanies this N- to O-rearrangement in H2O and Me2SO. Specific rates and activation parameters are reported. The kinetic data follow a rate law of the form kNO(obs.) = (k + kNO)[H+]/(K′a + [H+]) and the active species in the reaction is the protonated form; k, kNO are the specific rates for hydrolysis and isomerization, respectively. Proton NMR data establish that the site of protonation (in Me2SO) is the cobalt-bound nitrogen atom. For the unsubstituted urea species (NH3)5CoNH2CONH23+, diastereotopic exo-NH2 protons arising from restricted rotation about the CN bond are observed. The relevance to the mechanism of the linkage isomerization process is considered. 13C and 1H NMR and electronic absorption spectral data are presented, and distinctions between linkage isomers and the solution structures (electronic and conformational) are discussed. The urea-N/urea-O complex equilibrium is governed by the relation K′NO(obs.) = K′NO[H+]/[H+](K′a), where K′NO is the equilibrium constant = [(NH35Co(urea-O)3+]/[(NH3)5Co(urea-N)3+]. Values for K′NO(=kNO/kON = 260 and pK′a ≈ 3 for the NH2CONH2 system are consistent with the stability of the N-isomer in feebly acidic to basic solution (e.g. pH 6, K′NO(obs.) = 2.6 × 10−2) and instability in acid solution (e.g. pH 1, K′NO(obs.) = 240). The equilibrium data for this and other urea complexes of (NH3)5Co(III) are contrasted with the result for the analogous Rh(III)NH2CONH2 system K′NO ≈ 1).

ChemInform Abstract: Benzyloxydimethylsilane. A Generator of Dimethyl‐ and Methylsilanone.

AbstractPyrolysis of the benzyloxysilane (III), prepared from the chlorosilane (I) and benzyl alcohol (II) via an urea complex, produces a mixture of the cyclosiloxanes (IV) ‐ (VII), benzene (VIII), toluene (IX), and ethylbenzene (X).

Novel Strategies for the Complexation of Neutral Guest Molecules by Synthetic Macrocyclic Hosts

Abstract Complexes of macrocyclic polyethers with neutral guests have been studied in solution. The first part, a systematic study of complexes of malononitrile with a variety of macrocyclic polyethers, allowed the evaluation of the thermo-dynamic parameters of complexation. The second part deals with the complexation of urea. Complexes with simple 18-membered macrocycles were prepared. Protonation of urea allowed the phase transfer of urea from aqueous to chloroform solution. Finally the complexation of urea by 2-carboxy-1,3-xylyl crown ethers is described.

ChemInform Abstract: Macrocyclic Receptor Molecules with a Pendant Carboxylic Acid Group for the Complexation of Urea.

AbstractThe crown ethers (I)‐(IV) are prepared by common methods and their pKa values are determined.

Preparation and X-ray structures of complexes of 18-membered crown ethers with polyfunctional guests: Urea and (O-alkyliso)uronium salts

The preparation and X-ray structure determinations of six complexes of urea and (O-n-butyliso)uronium salts with crown ethers are presented. Urea forms isostructural 5:1 adducts with 18-crown-6 (1) and aza-18-crown-6 (2), in which two urea molecules are each hydrogen bonded to two neighbouring hetero atoms of the macroring. The remaining urea molecules form two-dimensional layers alternating with crown ether layers. In both complexes the macroring has theg+g+a ag−a ag−a g−g−a ag+a ag+a conformation withCi symmetry. In the solid 1:1 complex of O-n-butylisouronium picrate with 18-crown-6 (3) two types of conformations of the macroring were observed: theg+g+a ag−a ag+a ag−g−ag−a ag+a conformation with approximateCm symmetry and to a lesser extent theg+g+a ag−a ag+a g+g+a ag−a ag+a conformation with approximateC2 symmetry. Both conformations allow the guest to form three hydrogen bonds to the macrocyclic host. Three complexes of 18-crown-6 and uronium salts have been prepared and characterized by X-ray crystallography. The 1:1 complexes with uronium nitrate (4) and uronium picrate (5) both exhibit the sameC2 conformation and the same hydrogen bonding scheme as in the least occupied form of the previous complex. A 1:2 complex with uroniump-toluenesulphonate (6) has a different hydrogen bonding scheme (two hydrogen bonds per cation to neighbouring oxygen atoms of the macroring) and a different conformation of the host molecule (theag+a ag−a ag+a ag−a ag+a ag−a conformation with almostD3d symmetry). An attempt to prepare a solid uronium nitrate complex with diaza-18-crown-6 in the same way as the 18-crown-6·uronium nitrate (1:1) complex did not yield the expected result. Instead X-ray analysis revealed that the uronium ion is dissociated, resulting in the nitrate salt of the diprotonated diaza crown ether (7).

Macrocyclic receptor molecules with a pendant carboxylic acid group for the complexation of urea

AbstractThe synthesis of 5,5′‐di‐tert‐butyl‐3‐(carboxymethyl)biphenyl crown ethers and several lariat ethers with pendant carboxylic acid groups, together with the determinations of the pKa values of these crown ether carboxylic acids, are described. In addition, the complexation of urea by these crown ethers has been investigated. Both the larger‐ring biphenyl crown ethers (≥ 29 ring atoms) and lariat ethers (≥ 27 ring atoms) solubilized urea in chloroform. The data for urea complexation by the lariat ethers show that, for assistance in urea complexation, it is important for the carboxylic acid group to be located close to the macrocyclic ring. When the carboxylic acid group of the lariat ethers is located in an appropriate position, urea is complexed by 21–27% of these macrocyclic ligands, as is also the case with the biphenyl crown‐ether carboxylic acids. These results demonstrate that a pendant carboxylic acid group can assist in the complexation of a neutral molecule.