Three- urea

Urea has no promoting effect on the A AM polymerization initiated with Ce(lV) ion. Recently, Qiu et al. [24] have studied the effect of N-acryloyl-N -4-tolylurea (ATU), N-methacryloyl-N -4-tolylurea (MTU), and N-acetyl-N -4-tolylurea (AcTU) on A AM polymerization initiated with Ce(IV) ion and found that these three urea compounds have a high promoting effect on the polymerization of AAM. The data are cited in Table 4. 

Amino resins are condensation thermosetting polymers of formaldehyde with either urea or melamine. Melamine is a condensation product of three urea molecules. It is also prepared from cyanamide at high pressures and temperatures ... 

Iron(II) complex of tris(N -tert-butylurea-ylato)-N-ethylene]aminato activates dioxygen at room temperature to afford an iron(III) complex containing a single terminal oxo ligand. X-ray structures show that the three urea molecules act as a tridentate N,N,N-hgand [52]. The tripodal ligand was also used to synthesise complexes of cobalt, iron or zinc with terminal hydroxo ligands (Scheme 8) [53]. 

The structures of three urea complexes of dioxouranium(VI) sulfate have also been reported. In the bis complex, UO2S04-2urea, the structure consists of infinite ribbons of OUO groups... 

Around the same time as this, Reinhoudt developed a calix[4]arene system with only two urea or thiourea functionalities attached on opposite faces 70-72 [167]. These less substituted systems display both inter- and intramolecular hydrogen bonding as a result of the calixarene adopting a pinched cone conformation (demonstrated by the use of NOESY NMR). In some spectra it is impossible for the connectivities to be made within a single molecule, so the only possibility left is that dimerisation occurs. As with the initial experiments of Rebek and Bohmer, the extent of hydrogen bonding was observed to be solvent dependent. Concentration dependant FTIR was also used, to observe the effects on the NH stretching vibrations, but no concentration dependence was observed. Of the three urea derivatives used, only 72 showed no evidence of dimerisation... 

Three urea plants with Stamicarbon s new Granulation technology are under construction. 

The carboxylate/RE ratio of the complex is 3, but the structure of [ErLsL ] is very much the same as [LaLe] " (HE = acetic acid). The three acetates coordinate to the Er(III) ion in a chelating mode (p ), while the three urea molecules act as another three unidentate acetates [36], So far, this type of monomeric complex has only been found in the formatate and acetate complexes. 

Prelab Exercise Predict what the melting points of the three urea-cinnamic acid mixtures will be. 

S), have been prepared. The first set of compounds binds anions in the order Cl > Br > CN, has only a small affinity for 1 and SCN, and has no affinity for F and H2P04. . The second set of compounds, possessing C3 symmetry and containing three urea or thiourea groups, shows selectivity for Br over Cl and a high affinity for tricarboxylate anions such as benzene-1,3,5-tricarboxylate — 87,000 when X = O 190,000 M" when X = S),... 

Reinhoudt, Ungaro, and coworkers synthesized ca-lix[6]arenes 38 and 39 that are substituted at the lower rim with three urea or thiourea groups linked via alkyl chains. "" The threefold symmetry of the receptor imparts a selectivity for threefold symmetric tricarboxylates such as 1,3,5-benzenetricarboxylate. This anion was bound by receptor 39 with an association constant of290,000 M " in chloroformwhile benzoate was bound with an association constant of 1400 ... 

An alternative way to construct an anion-controlled, self-assembled polymer is with 57, which has a shortened ethylene chain instead of the pentamethylene spacer used in 56. In this case, the molecules are assembled through trigonal Ag(I) nodes to form a hexanuclear ring with a diameter of about 31.5 A. As shown in Figure 32, three urea groups are directed toward the cavity and involved in coordinating two nitrates via a total of 12 H-bonds (N- O = 2.960 to 2.944 A) to form a Borromean weave. Each nitrate ion is surrounded by three urea groups from three different molecules with six H-bonds, whereas the other nitrate (interanion distance of 3.21 A) is linked with... 

Bohmer and co-workers have synthesized a class of macrocycles in which three urea subunits were linked by xanthene (52) and/or diphenyl ether (53) spacers. Macrocycle 52 was expected to be very rigid, while macrocycle 53 should exhibit a higher degree of flexibihly. The complex of 53 with tetrabutylammonimn chloride showed the chloride anion was trapped in the center of the macrocycle, held by six NH-Cl H-bonds (Fig. 5.42) [105]. 

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