TiS-Bonded metal thiophene complexes

Examples of S-bonded thiophene complexes are known for a wide variety of metals, viz. Mn, Re, Cr, Mo, W, Fe, Ru, Co, Rh and Ir, as shown in Table 2.1 the procedures used to synthesize such compounds are usually straightforward, involving addition of the thiophene to an unsaturated metal precursor or displacement of a labile ligand. This type of binding has generally turned out to be weak, resulting in rather unstable compounds the stability increases along the trend thiophenes benzothiophenes dibenzothiophenes. 

Some early examples of S-bonded Ru-thiophene complexes provided by Rauchfuss were stabilized by attaching the thiophenic moiety to a strongly coordinating group such as a phosphine (a) [30] or acyclopentadienyl(b) [29] but even in these cases dissociation of the thiophene was apparent from the NMR spectra. 

Also interestingly, bases such as OH or EtjN induce the activation of the C-H bond a to S in [Cp (NO)(PPh3)Re(Ti S-T)] (Eq. 2.2) this C-H activation can be related to the mechanism of the a-H/D exchange known to occur in thiophene over solid HDS catalysts [43]. 

NMR measurements of the equilibrium constants for a series of ruthenium complexes [Cp(CO)(L)Ru(Ti S-Th)f (Cp = cyclopentadienyl L = CO, PPh, Th = T, 2-MeT, 3-McT, 2,5-Me2T, MCjT, BT, DBT)) (Eq. 2.3) have been performed by Angelici [44], showing that increased methylation on the thiophene results in stronger binding to the metal center and that steric effects are important for the stability of the complexes. t) S-BT and q S-DBT complexes are also more stable than their T analogues. Tetrahydrothiophene (THT) was found to bind 7.1 x 10 times more strongly than thiophene. 

Furthermore, the unusually high stability of these Ir(in) hydrido-thiophene complexes allowed some interesting studies to be carried out, involving the transformation of the coordinated thiophene through intramolecular migration of a coordinated hydride to the C=C bond (Eq. 2.6). This led to a particularly rare example of the mild homogeneous hydrogenation of thiophene to tetrahydrothiophene described in detail in Chapter 3. 

---