SYNTHESIS, STRUCTURE, AND REACTIVITY OF THIENYLCARBENE, BENZOTHIENYLCARBENE, AND SELENYLCARBENE COMPLEXES OF RHENIUM - A NEW MECHANISM FOR H/D EXCHANGE DURING HYDRODESULFURIZATION

A series of eta(1)(E)-coordinated (E = S or Se) thiophene, benzo[b]thiophene and selenophene complexes [Cp(NO)(PPh(3))Re(yl(E)-L)](+), Cp = C5H5, L = thiophene (T), 2-methylthiophene (2-MeT), 2,5-dimethylthiophene (2,5-Me(2)T), benzo[b]thiophene (BT), 3-methylbenzo[b]-thiophene (3-MeBT), selenophene (Sell, 2-methylselenophene (2-MeSel), and 2,5-dimethylselenophene (2,5-Me(2)Sel) are prepared by the reaction of [Cp(NO)(PPh(3))Re(ClC6H5)](+) with the appropriate ligand. The T, 2-MeT, BT, 3-MeBT, Sel, 2-MeSel complexes are deprotonated at C(2) by strong, non-nucleophilic bases to give the neutral Cp(NO)(PPh(3))Re(2-L-yl) complexes, where 2-L-yl = 2-thienyl (2-Tyl), 2-(5-methylthienyl) (2-(5-MeTyl)), 2-benzothienyl (2-BTyl), 2-(3-methylbenzothienyl) (2-(3-MeBTyl)), 2-selenyl (2-Selyl), and 2-(5-methylselenyl) (2-(5-MeSelyl)). The pK(a), of the base required to effect this deprotonation increases with the L ligand in the complex in the following order: Sel < T < BT. The 2-Tyl, 2-BTyl and 2-Selyl complexes react with either HBF4.Et(2)O or HO3SCF3 at -42 degrees C to give the corresponding carbene complexes [Cp(NO)(PPh(3))Re(2-L-ylcarbene)](+) re suiting from proto nation at C(3). The molecular structure of [Cp(NO)(PPh(3))Re(2-BTylcarbene)]O3SCF3, as determined by an X-ray diffraction study, exhibits a Re=C bond distance of 1.992(7) Angstrom. The carbene complexes do not react with nucleophiles; however, those nucleophiles that are sufficiently basic deprotonate C(3) to give back the L-yl compound. The pK(a) values of bases that are strong enough to cause deprotonation increase with the L-ylcarbene ligand in the order: Selylcarbene similar to Tylcarbene similar to BTylcarbene. The carbene complexes [Cp(NO)(PPh(3))Re(2-(5-MeTylcarbene)](+) and [Cp(NO)(PPh(3))Re(2-(5-MeSelylcarbene)](+) are unstable and rearrange to their more stable isomers [Cp(NO)(PPh(3))Re(eta(1)(S)-2-MeT)](+) and [Cp(NO)(PPh(3))Re(eta(1)(Se)-2-MeSel)]+. A new mechanism for H/D exchange of thiophene on hydrodesulfurization catalysts is proposed based on deuterium labeling studies of these thiophene complexes.