Thiophenes metal -sulfur-bonded complexes

Thiophenes can bind to metals via four bonding modes (i) fj -thiophene complexes (ii) rf-thiophene complexes (iii) f/ -thiophene complexes and (iv) j -S-bonded complexes. In addition to these, the metal might insert into the C—S bond of thiophene to form a six-membered ring containing the metal and sulfur. All these are closely interconnected and have been fully characterized.. 

The group of Jones has described the desulfurization of thiophene and benzothiophene using [Ir2(Cp )2(H)2(p-H)] with excess TBE (t-butylethylene) or [lr(Cl)(Cp )(H)]2 in the presence of H2 [53, 106]. The reactions evenmally afford diiridium complexes with sulfide and q q -butadiene bridges and seem to proceed via two consecutive carbon-sulfur bond cleavages that require more than one metal center and the ability to form bridging thiolate intermediates (Scheme 21) [107]. 

Additionally, in some cases, special metal complexes lead to degradation of the thiophene skeleton by activation of the carbon-sulfur bond [3-5]. 

Carbon-sulfur bond breaking was observed while treating thiophene with Cp"Co(C2H4)2 at 70 °C in benzene. A cobalt metal center had inserted into the C-S bond and a second Cp Co unit had complexed to the cobalt, sulfur, and C-C double bond 330. Further reaction of the resulting compound with H2S produced a new butadiene dithiolate compound, in which a sulfur atom has been inserted into the metal-carbon bond 331. Reaction with dibenzothiophene produced an analogous dinuclear C-S insertion-reaction adduct (Equation (54)). ... 

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>. 

The picture that emerges is that the bonding within the majority of thiophene molecules adsorbed on the catalyst surfaces is hardly perturbed, and this contrasts sharply with the situation in the thiophene complexes. The thiophene molecule parallel to the surface does not correspond to a metal f/ -bound thiophene. Rather, it is suggestive of a weakly chemisorbed precursor state of thiophene that lies parallel to the surface. In this state, the molecule interacts indiscriminately with the alumina, the basal or edge planes, or both. Moreover, the weakness of this binding enhances the surface mobility of thiophene and allows molecules to move across the surface to the catalytic site for reaction with hydrogen atoms. The few sulfur-bound thiophene molecules, no more than 5-10%, would then correspond to thiophene at the coordinatively unsaturated Mo (or Co) atoms. 

Figure 5.11 illustrates the known coordination geometries of thiophene in organometallic complexes, which indicate likely adsorption configurations of thio-phenic compounds on the surface of adsorbents.118 Both thiophenic compounds and nonsulfur aromatic compounds can interact with metal species by -electrons. However, in Figure 5.11, only two types of interaction of thiophene with metal involve sulfur atom in thiophene the rj -S bonding interaction between the sulfur... 

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