Thioureas coordination chemistry

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. 

Several metal complexes have been described with urea, thiourea or dimethyl derivatives [49,50]. We will focus in this section on the coordination chemistry of substituted ureas and thioureas used as neutral Ugands as well as many ureato and thioureato anions complexed to metal centres. 

Koch, K. R. Sacht, C. Grimmbacher, T. Bourne, S. New Ligands for the platinum-group metals deceptively simple coordination chemistry of N-acyl-N -alkyl and N-acyl-N, N -dialkyl-thioureas. S. Afr. J. Chem. 1995, 48, 71-77. 

In the evolution of anion coordination chemistry, it was discovered that neutral molecules can also operate as effective receptors for anions provided that they contain polarised N-H fragments (e.g. of amides [53], ureas [54], thioureas [55] or pyrroles [56]), which act as H-bond donors for anions. These receptors cannot compete for hydrogen bonding with water and alcohols and must be studied in aprotic solvents of varying polarity, e.g. CHCI3, MeCN, DMSO. In this vein,... 

The coordination chemistry of NS (thionitrosyl), thiazate (NSO ), disulfidothionitrate (SsN"), sulfur monoxide (SO), disulfur monoxide (S2O), SO2, DMSO, thiosulfate, sulfite, and thiourea ligands is extensive. " Scheme 23 illustrates their coordination modes and some of the complexes that can be generated. 

On the coordination chemistry side, ligand substitution on metal complexes in ILs has attracted quite some interest. This is mainly due to the fact that both spectroscopic and catalytic properties are strongly governed by the nature of the ligands and the stability of their bond to a metal center. Begel et al. have studied the role of different ILs on ligand substitution reactions on [Pt(terpy)Cl]+ (terpy = 2,2 6, 2"-terpyridine) with thiourea with stopped-flow techniques. The substitution kinetics show similar trends if compared to conventional solvents with similar polarities. Moreover, much like in conventional solvents, the authors find an associative character of the substitution reaction [205], These results are essentially supporting an earlier study by Weber et al., who found the same behavior [206],... 

Square-planar, four-coordinate tellurium(II) complexes, whose structural chemistry has recently been summarized by Foss 89>, offer illustrations of this highly approximate, constant-volume rule. Fig. 27 is a drawing, approximately to scale, of a schematic, electron-domain representation of a section through the 1 1 complex of benzenetellurenyl chloride with thiourea 90>. Shown are the electron domains of the tellurium kernel (largest, nearly centrally located solid circle) the ligands kernels (two chlorine kernels, a sulfur kernel of thiourea, and a carbon kernel of benzene) and the domains of the tellurium atom s shared valence-shell electrons (open circles) and, most schematically of all, the tellurium atom s unshared valence-shell electrons (shaded region). 

In the presence of millimolar concentrations of stronger nucleophiles such as CN and thiourea for Pt + complexes, or molar concentrations of Cl for Hg+ complexes, nncleic acid base substitution takes place. However, steric constraints can lead to surprising inertness, as demonstrated for N-3-platinated T and U. In contrast to coordination bonds of Hg + to heterocyclic N-atoms of nucleic acid bases, the covalent bond of mercury ion to C-5 of pyrimidines see Mercury Organometallic Chemistry) is stable even in the presence of 1M NaCl. ... 

---