Thiophene, metallation numbering

Organosulfur Adsorbates on Metal and Semiconductor Surfaces. Sulfur compounds (qv) and selenium compounds (qv) have a strong affinity for transition metal surfaces (206—211). The number of reported surface-active organosulfur compounds that form monolayers on gold includes di- -alkyl sulfide (212,213), di- -alkyl disulfides (108), thiophenols (214,215), mercaptopyridines (216), mercaptoanilines (217), thiophenes (217), cysteines (218,219), xanthates (220), thiocarbaminates (220), thiocarbamates (221), thioureas (222), mercaptoimidazoles (223—225), and alkaneselenoles (226) (Fig. 11). However, the most studied, and probably most understood, SAM is that of alkanethiolates on Au(lll) surfaces. 

A limited number of non-transition-metal derivatives of thiophene will be considered in this subsection. There are no short-range contacts between the lithium atoms originating from the (LiO)6 cores and the sulfur atoms in [Li—O—EMc2 (2-C4H3S)]6 (E = C, Si) (97OM5032), and evidence for Tr-interactions can be found in the X-ray crystal structures of these compounds. Theoretical computations show that a- (S ) Li" " interactions are weak, whereas Tr-Li" contributions are considerable, in accord with the general reasoning on the electronic characteristics of uncomplexed thiophene. 

Up to 1994 the majority of publications on five-membered monoheterocycles were on phospholes. Since 1995, thiophene has become the ligand of the year. The organotransition metal chemistry of thiophene has been the subject of a substantial number of reviews on its pure and applied chemical... 

Thus, the reduced form of poly-3-methyl thiophene is an intrinsic semiconductor and the Fermi level lies between the valence band and the conductivity band. The effect of oxidation is to introduce a surface state in the band gap between n and n orbitals. The Fermi level is decreased when the compound loses electrons, and metallic properties appear when an increasing number of electrons build a new but only half-filled band. These situations are shown in Fig 11.7. 

Ab initio MO calculations have been carried out on benzo[. ]thiophene (BT), 2-methylbenzo[/ ]thiophene (2-MeBT), 3-methylbenzo[ ]thiophene (3-MeBT), and a number of organometallic complexes containing these thiophenic moieties in an attempt to understand the usual preference for insertion of metal fragments into the sulfur-carbon(vinyl) bond of BT and the recent observations of insertion into the sulfur-carbon(aryl) bond <19980M3798>. 

As previously mentioned, the stoichiometric desulfurization of thiophenes has been achieved with a relatively large number and variety of metal complexes. In general, polynuclear complexes containing both component (Mo or W) and promoter (Ni, Co, Ir, Ru) metals turn out to be more active than mononuclear complexes containing promoter metals [1, 2]. A paradigmatic case has been reported in which the hydrogenolysis of BT to either 2-vinylthiophenol or 2-ethylthiophe-nol is a facile process for the promoter (Rh), but the desulfurization step to ethylbenzene requires the compulsory assistance of a component metal (W) to take place [33]. 

Many other metals have been shown to be active in HDS catalysis, and a number of papers have been published on the study of periodic trends in activities for transition metal sulfides [15, 37-43]. Both pure metal sulfides and supported metal sulfides have been considered and experimental studies indicate that the HDS activities for the desulfurization of dibenzothiophene [37] or of thiophene [38, 39] are related to the position of the metal in the periodic table, as exemplified in Fig. 1.2 (a), 1.2 (b), and 1.2 (c). Although minor differences can be observed from one study to another, all of them agree in that second and third row metals display a characteristic volcano-type dependence of the activity on the periodic position, and they are considerably more active than their first row counterparts. Maximum activities were invariably found around Ru, Os, Rh, Ir, and this will be important when considering organometallic chemistry related to HDS, since a good proportion of that work has been concerned with Ru, Rh, and Ir complexes, which are therefore reasonable models in this sense however, Pt and Ni complexes have also been recently shown to promote the very mild stoichiometric activation and desulfurization of substituted dibenzothiophenes (See Chapter 4). 

In conclusion, an extensive literature is available on the reaction networks that are thought to operate in HD,S of various types of thiophenic molecules besides the great advances that have been made in direct studies on molybdenum sulfides and related catalysts, this is another area in which organometallic chemistry has made an impressive contribution to HD,S catalysis, as a number of reaction pathways and mechanisms for the hydrogenation and hydrogenolysis of thiophenes on metal complexes in solution has been well established with the aid of a variety of physical techniques. 

---