Thiourea complexes

Tellurium pseudohaUdes, such as the dicyanide [14453-24-8] Te(CN)2, the dithiocyanate [83543-04-8] Te(SCN)2, and thiourea complexes with Te(SCN)2, have been prepared. These are similar to the haUdes in properties. 

THE CONSTANTS OF REPLACEMENT AND THE CONSTANT OF STABILITE MIXED BROMID-THIOUREA COMPLEXES OF GOLD (HI)... 

Bases on the data of curve of the potentiometric titrations of [AuBr ] ions by thiourea (Thio), it consistently replaces bromide ions in [AuBr ] ion. They are formed mixed bromide-thiourea complexes of Au(III) AuBr Thio, AuBiyr/iio AuBrThio, AuBrThioJ. ... 

A number of thiourea complexes of silver have shown the tendency to bind up to four ligands, in contrast to gold. Thus Agtu2X (X = Cl, NCS) have essentially 3-coordinate silver (one distant fourth atom) Agtu3C104 is a 4-coordinate dimer (Figure 4.12) [58]. 

Essentially linear coordination is found in thiourea complexes AuBr(S=C(NR2)2) (R = H, Me) and... 

Abstract After an overview of chiral urea and thiourea synthetic methods, this review describes the main applications of urea and thiourea complexes in asymmetric catalysis. Some recent examples of thioureas as catalysts are also presented. Coordination chemistry of ureas and thioureas is briefly discussed. 

Platinum-thiourea complexes have been extensively studied because of their biological activity [54], but few have been used in catalysis. Neutral thioureas are able to coordinate to metal centres through their sulfur atom (Scheme 9) [55,56] monomeric (I) and oligomeric (II) species are known for Rh [57], and an X-ray structure has also been determined for the chiral complex III [58]. In many complexes hydrogen bonding has been observed... 

Thiourea Ugands can be bounded to the metal centre through one nitrogen atom, the sulfur atom, or the C = S double bond. These coordination modes were studied by density functional theory calculations for Rh-thiourea complexes (Scheme 13). No stable structure was attained by optimisation of the nitrogen coordination mode I but optimised geometries as trigonal-bipyramidal complexes were obtained for modes II and III. An coordination is determined for the latter complex through both S and C atoms. As this... 

Disubstituted benzo[fo]furans were also prepared by intramolecular cyclisation in the presence of a [Pd(thiourea)4]l2 catalyst (thiourea = H2NCS NH2). No Pd precipitation occurred with this very stable thiourea complex (Scheme 31) [120]. 

The aqueous decomposition of thiourea to sulfide and cyanamide has been found to be catalyzed by metal hydroxide species and colloidal metal hydroxide precipitates. Kitaev suggested that Cd(OH)2 is actually required for CdS film formation to occur by adsorption of thiourea on the metal hydroxide particles, followed by decomposition of the Cd(OH)2-thiourea complex to CdS. Kaur et al. [241] found... 

Recently Sergeev et al. 90 91> have developed a low temperature condensation method for the formation of inclusion compounds of thiourea with reactive and volatile guests, avoiding the use of solvents. The two guests in the joint inclusion compound of thiourea with 1,3-cyclopentadiene and maleic anhydride underwent Diels-Alder addition at 170 K. These two substances do not react at this low temperature unless they are present in the thiourea complex the usual endo isomer of the product is formed. Apart from copolymerisation reactions this appears to be the first use of the thiourea canal to study reactions between different materials. 

Crystals of [Tc(tu)6]Cl3 or [TcCl(tu)5]Cl2 are often employed for the synthesis of technetium(III) complexes. However, since the direct reduction of pertechnetate with excess thiourea in a hydrochloric acid solution yields [Tc(tu)6]3+ in high yield [37], direct use of the aqueous solution of the thiourea complex would be preferable for the synthesis of the technetium(III) complex without isolation of the crystals of the thiourea complex. In fact, technetium could be extracted from the aqueous solution of the Tc-thiourea complex with acetylacetone-benzene solution in two steps [38]. More than 95% extraction of technetium was attained using the following procedure [39] First a pertechnetate solution was added to a 0.5 M thiourea solution in 1 M hydrochloric acid. The solution turned red-orange as the Tc(III)-thiourea complex formed. Next, a benzene solution containing a suitable concentration of acetylacetone was added. After the mixture was shaken for a sufficient time (preliminary extraction), the pH of the aqueous phase was adjusted to 4.3 and the aqueous solution was shaken with a freshly prepared acetylacetonebenzene solution (main extraction). The extraction behavior of the technetium complex is shown in Fig. 6. The chemical species extracted into the organic phase seemed to differ from tris(acetylacetonato)technetium(III). Kinetic analysis of the two step extraction mechanism showed that the formation of 4,6-dimethylpyrimidine-... 

Remove unreacted N-acetyl homocysteine thiolactone and reaction by-products by gel filtration or dialysis against lOmM sodium phosphate, 0.15M NaCl, lOmM EDTA, pH 7.2. Other buffers suitable for individual protein stability may be used as desired. For the silver nitrate-containing reaction, removal of the silver-thiourea complex may be done by adsorption onto Dowex 50, and the protein subsequently eluted from the resin by 1M thiourea. Removal of the thiourea then may be done by gel filtration or dialysis. 

Electroless deposition should not be confused with metal displacement reactions, which are often known as cementation or immersion plating processes. In the latter, the less noble metal dissolves and eventually becomes coated with a more noble metal, and the deposition process ceases. Coating thicknesses are usually < 1 pm, and tend to be less continuous than coatings obtained by other methods. A well-known example of an immersion plating process that has technological applications is the deposition of Sn on Cu [17] here a strong complexant for Cu(I), such as thiourea, forces the Cu(I)/Cu couple cathodic with respect to the Sn(II)/Sn couple, thereby increasing the thermodynamic stability in solution of thiourea-complexed Cu(I) relative to Sn(II). 

Thiourea complexes of technetium(III) have been shown to be excellent precursors for the synthesis of a large number of Tc and Tc complexes. The [Tc(tu)6] cation is readily formed in a mixture of pertechnetate, tu and 12 M HCl and precipitates as chloride. Due to the different redox potentials of the homologous elements technetium and rhenium, the reducing capacity of HCl is not sufficient to apply the same method for rhenium and tin chloride is added as reducing... 

Scheme 6.53 Proposed mechanism for the 53-catalyzed asymmetric Pictet-Spengler-type cyclization of P-indolyl ethyl hydroxylactams Hydroxylactam (1) forms chlorolactam (2) followed by chiral N-acyliminium chloride-thiourea complex (3) and the observed product generated by intramolecular cyclization catalysis and enantioinduction result from chloride abstraction and anion binding.

Fig. 32a and b. The conformation of the ligand 5c in its thiourea complex and the disorder of the chain ends. Side view of the 5e-thiourea complex showing hydrogen bond contacts (parts of the aliphatic chain have been omitted for clarity)421... 

This technique is not so easily extended to nonoxides. Sulphur, for example, does not form the cross-linking bonds needed to form the sol-gel as readily as does oxygen. However, a related method has been used, albeit to a very small extent, to form CdS films. It is based on the thermal decomposition (at ca. 300°C) of a Cd-thiourea complex, which is formed as a film by slowly withdrawing the substrate from a methanolic solution of a Cd salt and thiourea [196]. 

Betenekov et al. [39] used an isotopic tracer technique to show that, for then-range of solution compositions, the initial deposition involved adsorption of Cd(OH)2 on the glass substrate. At the beginning of the reaction, only Cd was observed to form on the substrate and this was interpreted to be due to Cd(OH)2, since any other insoluble Cd compounds that might be formed from the deposition solution (containing CdCl2, NaOH, NH4OH, and thiourea dissolved in water) were expected to contain either S or C. However, they concluded that the deposition proceeded, not by reaction between Cd(OH)2 and sulphide formed by decomposition of thiourea, but rather by decomposition of a Cd(OH)2-thiourea complex (see Sec. 3.3.3.1). 

Consider the complexation of free Cd by thiourea to give a Cd-thiourea complex ion ... 

The initial nucleation stage of the complex-decomposition mechanism is probably similar to the simple free-anion mechanism. Either ionic or molecular metal species (ion-by-ion) or Cd(OH)2 (cluster) adsorbs on the substrate. However, instead of conversion of the hydroxide to sulphide by topotactic reaction with sulphide ions, the chalcogenide precursor (in almost all studies of this mechanism, that is thiourea) adsorbs on the Cd(OH)2 surface to form a hydroxide-thiourea complex, which then decomposes to CdS. 

This, together with the known tendency of metal ions to form mixed hydroxy-am-mine complexes, snggested to them that two ammonia molecnles were involved in the first step and that the adsorbed species in Reaction (3.58) was a hy-droxy-ammine species, viz. Cd(OH)2(NH3)2. Decomposition of the hydroxide-ammine-thiourea complex was then assumed to occur by nucleophilic attack of an ammonia species on the S=C bond of the thiourea. 

There does not seem to be a clear consensus as to the mechanism of PbS formation in these and similar studies. It is often stated that the formation of PbS occurs via decomposition of a Pb-thiourea complex species [4,21,22]. This was often based on the absence of any measurable concentration of sulphide on alkaline hydrolysis of thiourea. However, as discussed in Section 3.3.3.1, this is not a valid criterion for the absence of a sulphide-mediated reaction. Even today, it cannot be stated categorically which mechanism is operative or even dominant. 

Reference 85 presents the thermodynamic side of the previous paper. It is pointed out that although both ammonia and thiourea are present in the solution, because of the much higher stability constant of the Ag-thiourea complexes compared to the Ag-ammines, essentially all the Ag will be present as a thiourea complex. In this case, it can be assumed that the role of ammonia is only to control pH. 

Thiourea complexes of the type MX(thiourea)4 have been found if the lattice energy of MX is less than 670 kJ mol-1. This leads to complexes of K+, Rb+ and Cs+, especially with Br and I- as anions. The polarizable thiourea separates the anions from the cations and each cation is probably surrounded by eight sulfur atoms at the corner of an approximately regular cube.65... 

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