Selenium coordination

Nickel-selenium coordination compounds have received attention in recent years, because a unique Ni-selenocysteine interaction was revealed in the active site of [FeNiSe]-hydrogenases.1083,1084 Of particular interest in this regard are mixed CO/selenolate complexes. Distorted square planar (393) was prepared from [CpNi(CO)]2, PhSeSePh, and [Fe(CO)3(SePh)3] and provides the first example of CO bound to a square planar Ni11 center in thiolate/selenolate environment.1085,1086 Upon addition of RSSR, species of the series [Ni(CO)(SR) (SePh)3 ] are formed. //(CO) ranges from 2,023 cm-1 to 2,043 cm-1 and is regarded as a spectroscopic reference for the CO binding site in [NiFeSe] hydrogenases. 

A silacyclopropene intermediate 56 was trapped by a reaction with 2-propanol during the thermal degradation of pentacoordinate disilanes 55 bearing an 8-thiona- or 8-seleno-l-naphthyl group and a methoxy group on the same silicon center. This reaction generates intramolecular sulfur- or selenium-coordinated silylenes 57 (Scheme 10) <2003OM3299>. 

Another member of this family that has been studied by XAS is the selenate reductase from Thauera selenatisJ Like DMSO reduetase itself, this en me possesses a mono-oxo Mo(vi) site and a des-oxo Mo(iv) site with elose to four Mo-S ligands in both, eonsistentwith the coordination by two molybdopter-ins that is a signature of this family XAS also deteeted a low-valent selenium coordinated to an unknown metal ion (possibly iron) in the enzyme, although at the time of writing the role of this selenium remains unknown. 

Most studies reported on ruthenium chalcogenides containing Se, S, and Te relate, so far, to the acid medium. It is, however, interesting to compare some feamres from the cyclic voltammetry results generated in acid as well as in alkaline media for the metal core as well as the selenium coordinated to the metallic core (Fig. 14.11). 

Ti(OfPr)4, and chiral l,l -binaphthol (BINOL) gave enantiomerically enriched cfs-cyclopropanes (Scheme 22) (110). The proposed reaction pathway was via silicon migration and ring closure with the aid of selenium coordination. 

The second characteristic we have to consider is the coordination of the R elements in their compounds. First of all, we notice that a large proportion of the structures have flat cells (thickness of about 4 A for sulfides), probably because the rare earths have a strong tendency to the prismatic environment, and the thickness of the cells corresponds to the height of the prisms. The study of the environment of the R elements in these flat cells is relatively easy. Two kinds of sulfur (or selenium) coordination polyhedra are formed around the rare earth. One kind is related to the triangular prism, and the other is derived from the octahedron. 

Ligands Other than Oxygen and Sulfur. See Sec. 3.1.7, Coordination Compounds, for acids containing ligands other than oxygen and sulfur (selenium and tellurium). 

Trialkyl- and triarylarsine sulfides have been prepared by several different methods. The reaction of sulfur with a tertiary arsine, with or without a solvent, gives the sulfides in almost quantitative yields. Another method involves the reaction of hydrogen sulfide with a tertiary arsine oxide, hydroxyhahde, or dihaloarsorane. X-ray diffraction studies of triphenylarsine sulfide [3937-40-4], C gH AsS, show the arsenic to be tetrahedral the arsenic—sulfur bond is a tme double bond (137). Triphenylarsine sulfide and trimethylarsine sulfide [38859-90-4], C H AsS, form a number of coordination compounds with salts of transition elements (138,139). Both trialkyl- and triarylarsine selenides have been reported. The trialkyl compounds have been prepared by refluxing trialkylarsines with selenium powder (140). The preparation of triphenylarsine selenide [65374-39-2], C gH AsSe, from dichlorotriphenylarsorane and hydrogen selenide has been reported (141), but other workers could not dupHcate this work (140). 

The unknown selenium-nitrogen anions [ScsN], [ScaNa] and [ScaNaH] have also been stabilized by coordination to platinum.A few examples of complexes involving anionic ligands with two different chalcogens, e.g., [ESNa] and [ESNaH] (E = Se, Te), are known. 

The structure of the isomeric benzo-l,2,3-thiadiazole 11.30 is unknown, but the 1 1 adduct with AsFs (11.31) has been structurally characterized. The AsFs molecule is coordinated to the carbon-bonded nitrogen atom. Cycloocteno-l,2,3-selenadiazole is an effective source of selenium for the production of semi-conductors such as cadmium selenide." ... 

Examples of the C2N3S ring system containing three- or four-coordinate sulfur are also well known. The monohalogenated derivatives 12.2 (E = S, Se), are best prepared by the condensation of imidoyl amidines with SCI2 or SeCU, respectively (Scheme 12.1)." In the case of the selenium derivative, the initial product is heated at 60°C and then at 120°C in order to convert it to 12.2 (E = Se) via (PhC)2(NH)N2SeCl2. In the solid state this intermediate is a weakly associated, centrosymmetric dimer with Se-Cl and Se Cl distances of 2.42 and 3.39 A, respectively. ... 

Intramolecular chalcogen interactions may also stabilize reactive functional groups enabling the isolation of otherwise unstable species or their use as transient intermediates, especially in the case of selenium and tellurium. For example, tellurium(II) compounds of the type ArTeCl are unstable with respect to disproportionation in the absence of such interactions. The diazene derivative 15.23 is stabilized by a Te N interaction. Presumably, intramolecular coordination hinders the disproportionation process. Other derivatives of the type RTeX that are stabilized by a Te N interaction include 8-(dimethylamino)-l-(naphthyl)tellurium bromide, 2-(bromotelluro)-A-(p-tolyl)benzylamine, and 2-[(dimethylammo)methyl]phenyltellunum iodide. Intramolecular donation from a nitrogen donor can also be used to stabilize the Se-I functionality in related compounds." ... 

Selenoaldehydes 104, like thioaldehydes, have also been generated in situ from acetals and then directly trapped with dienes, thus offering a useful one-pot procedure for preparing cyclic seleno-compounds [103,104], The construction of a carbon-selenium double bond was achieved by reacting acetal derivatives with dimethylaluminum selenide (Equation 2.30). Cycloadditions of seleno aldehydes occur even at 0 °C. In these reactions, however, the carbon-selenium bond formed by the nucleophilic attack of the electronegative selenium atom in 105 to the aluminum-coordinated acetal carbon, may require a high reaction temperature [103], The cycloaddition with cyclopentadiene preferentially gave the kinetically favorable endo isomer. 

The first product of dicoordination, 18, was synthesized via treatment of phos-phinoiminophosphanes with selenium. The latter then inserted into the PP-bond and formed finally a bis-coordinated donor-acceptor complex [55]. The overall synthetic route is shown in Scheme 10. 

The two selenium atoms are attached to the low-coordinated phosphorus atom, with two lengthened dative bonds (2.788 and 2.637 A). Interestingly the phosphorus atom is here strongly pyramidalized, the Z SePSe angle was 80.6°. In other words the lone pair orbital at the electrophilic phosphorus atom remains stereo chemically active, as one would expect on the basis of the quantum chemical theory of bonding within this species. 

The first step consists in the attack of a proton on the W-H bond to yield a labile dihydrogen intermediate (Eq. (3)) that rapidly releases H2 to form a coordi-natively unsaturated complex (Eq. (4)). This complex adds water in the next step to form an aqua complex (Eq. (5)) that completes the reaction by substituting the coordinated water by the X anion (Eq. (6)). Steps (3)-(6) are repeated for each W-H bond and the factor of 3 in the rate constants appears as a consequence of the statistical kinetics at the three metal centers. The rate constants for both the initial attack by the acid (ki) and water attack to the coordinatively unsaturated intermediate (k2) are faster in the sulfur complex, whereas the substitution of coordinated water (k3) is faster for the selenium compound. 

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