Thiophenes bonding modes

In the design of a homogeneous catalyst for the plain hydrogenation of thiophenes it is necessary to take into account that, unlike simple alkenes, (102) and (103) are polyfunctional ligands which can bind metal centers in a variety of bonding modes, often in a rapid equilibrium with each other.166-172,192 Among the possible coordination modes, the /-(S) and the 772-(C,C)... 

One of the modes of coordination of thiophene to catalyst surfaces most frequently suggested is the S or 771, in which the bond is formed directly between the sulphur and a transition metal atom. In examples of the S-bonding mode which have been well-characterized by X-ray diffraction, a pyramidal geometry has been found [75], which is also present in the absorption of thiophene on metallic surfaces such as copper [76]. 

Fig. 1.6. Possible bonding modes for thiophenes on a surface metal site.

Fig. 2./. Bonding modes of thiophenes known in metal complexes...

Some early proposals for the modes of adsorption of thiophenes on metal sulfides have been probed by comparisons with the structures of well-characterized metal complexes this has allowed the identification of the most reasonable alternatives and of new possibilities not previously considered. Tlieoretical studies on such complexes at increasing levels of sophistication have also contributed in an Important manner to provide a clear and consistent picture of the different possible bonding modes of thiophenes to metal centers. When these theoretical and experimental results from molecular chemistry are combined with the information available from surface techniques and heterogeneous catalysis, the chemisorption of this type of organosulfur compounds on metal sulfides arises as a very well understood phenomenon. This is no doubt one of the most important achievements of the organometallic modeling approach to HDS chemistry. 

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.. 

Figure 2 Bonding modes of thiophenes in metai compiexes.

The most favorable coordination sites in thiophenes are the C2C3 and C4C5 double bonds ( -coordination, 38). This type of coordination greatly enhances the nucleophilic power of the sulfur atom, which then gives rise to two new modes of binding the metal atoms, as in the V, S-p.2-, 42, and V, S-p.3-species, 43. 

The photocyclodehydrogenation of thienyl ethylenes is well-defined when both thiophene rings are bound via a C(2) atom to the ethylenic bond as in (70). In other cases, however, more cyclization products are possible. To predict the photocyclization mode for heterohelicenes the F s rule fails in many cases, because correction factors for the hetero atoms in the Huckel MO calculation have to be introduced and the systems are not well comparable with carbocyclic diaryl ethylenes. A better reaction parameter in these cases is the Mulliken overlap population (nrs)51), introduced by Muszkat52) for these cases. The overlap populations of the atoms r and s in ground and excited state (nIiS and n s), are calculated using the extended Huckel method. Cyclizations should not occur when nr>s and An s (= nr>s — n s) have negative values. (This method can also be used for diaryl olefins, but in these cases calculation of F s is more simple.). 

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