Selenium electron donors

Table 3 Geometric parameters for halogen bonded complexes with arsenic and selenium electron donors... 

In addition to halogen bonded complexes or ionic salts, it is also possible for sulfur and selenium electron donors to form complexes in which the electron donor atom inserts into the X2 bond, giving a hypervalent donor atom with a T-shaped geometry. It has been recently reported [147] that for dibromine and selenium, this type of complex is favored over halogen bonded complexes. While no purely halogen bonded complex is reported for dibromine, there is one complex (IRABEI) in which one selenium atom of each of several selenanthrene molecules in the asymmetric unit does insert into a Br2 bond, but for one of the molecules, the other selenium atom forms a halogen bond with a Br2 molecule to form a simple adduct (A). 

The interaction of dihalogens, particularly diiodine, with sulfur and selenium electron donors has been an area of increasing interest over the past decade because of potential biological, pharmaceutical, and electronic materials applications [35,179]. Devillanova and coworkers have recently reviewed the solution behavior of a large number of chalcogenides and I2, particularly thiones, selones, sulfides, and selenides [180]. Correlations between computational methods, thermodynamic parameters, and spectroscopic data (UV/Vis, 13C NMR, Raman, UPS) were discussed. 

Like sulfur, selenium based electron donors are heavily studied due to their potential as antithyroid agents. Of the 28 complexes reported, the majority (20) are diiodine based, and most of these (16) are simple adducts. One diiodine complex is bridged, one contains only extended adducts, one contains only bridged adducts, and one contains both bridged and extended adducts. All of the interhalogen complexes are simple adducts except one with iodine monobromine, which forms an extended adduct. 

When the BETS donor replaces the BEDT-TTF electron donor molecule during the electrocrystallization process, crystals of KL-(BETS)2Ag(CF3)4(TCE) have been prepared [29] and structurally characterized. Replacement of the inner sulfur atoms of BEDT-TTF with selenium results in a slight expansion of the unit cell and prevents the stabilization of a superconducting state above 1.2 K. Disorder in one of the BETS ethylene endgroups has been offered as a possible explanation. 

One-electron oxidation of organoselenium and organotellurium compounds results in initial formation of a radical cation (equations (19) and (20)). The eventual fate of the radical cation depends on several variables, but is typically a Se(lV) or Te(lV) compound. The scope of this section will be the one-electron oxidation of selenides and tellurides that are not contained in a heteroaromatic compound, and ones in which the Se and Te are bonded to two carbons, rather than to other heteroatoms. Tellurium- and selenium-containing electron donor molecules have been reviewed. 

Although in humans only MsrBl is a selenoprotein, the depletion of selenium from the diet of mice led to increases in both R and S stereoisomers. This was not initially explained, yet a subsequent study has shown that small molecule selenols (organic selenocysteine homologues) could act as efficient electron donors in vitro for MsrA enzymes. ° This effect has only been shown in vitro, but the possibility that small molecular selenium reductants, or more likely that some selenoproteins that contain reduced selenols (in redox-active motifs) is quite intriguing. Several small selenoproteins do not have real roles and reside in nearly all subcompartments of the cell (mitochondria, ER) where electron donors for Msr enzymes are probably critical to maintain protein stability. Low selenium nutritional status would then have a significant impact on all methionine oxidation, as Future studies to address selenium nutrition and methionine oxidation could prove to be... 

The structures of selenaaromatic compounds are closely related to those of analogous sulfur compounds. The best known is selenophene (1). As for thiophene, the idea can be offered in terms of resonance theory (Fig. 3). This means that outer valence shell resonance structures la-d may be hybridized into an aromatic sextet. In these cases, selenium acts as an electron acceptor and negative charge is localized on the selenium atom. In contrast, structures le-h have octet formulation not involving valence shell expansion. Selenium acts as electron donor and in these cases positive charge is localized on the selenium atom (Fig. 3). 

On the contrary, selenourea 296 (X = Se) is a ligand with selenium as electron-donor center and forms complexes of type 297 (X = Se) [112,634]. 

Chemically, thionyl chloride can function as either a Lewis acid (sulfur is the acceptor atom) or a Lewis base (oxygen is the usual electron donor atom). It behaves in the same manner as other oxyhalides of sulfur and selenium with regard to hydrolysis, and the reaction can be shown as... 

Examples for square planar complexes of Se(II) and Te(II) with other sulfur and selenium donor ligands such as thio- and selenocarbamates are given (e.g., in 29,165, 205). When triarylphosphane ligands were introduced as two-electron donors by the reaction of triarylphosphane selenide with Br2, T-shaped Ar3PSeBr2 with the Br ligands in trans positions was prepared (435). Possibly, the molecules are dimerized via bromine bridges as in Se2(tmtu)2Br4 (445). 

The substituted TTF derivatives 81 containing peripheral selenium atoms showed two one-electron reversible waves. They revealed good electron-donor abilities which made these compounds suitable for application in the synthesis of CT complexes and radical cation salts <2004JMC351>. 

Like their sulfur analogues, tetraselenafulvalenes and the mixed sulfur/selenium systems are excellent electron donors which are able to form radical cation salts and charge transfer complexes with appropriate electron acceptors. 

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