THE BENZO[B]THIOPHENE (BT) COMPLEXES CP'(CO)2RE(BT) - MODELS FOR BT ADSORPTION ON HYDRODESULFURIZATION CATALYSTS

Benzo[b]thiophene (BT) coordinates in Cp(CO)2Re(BT) as an equilibrium mixture of eta1(S) and 2,3-eta2 isomers. When the electron density on the Re is increased by replacing Cp (eta5-C5H5) with Cp* (eta5-C5Me5), the amount of eta2 isomer increases. This shift in equilibrium can be readily explained by assuming that the sulfur in BT is an electron donor toward Re, while bonding of the C(2)-C(3) olefinic bond to Re involves an important component of pi back-bonding. The intramolecular nature of the isomerization between 1a and 1b was established by the observation that neither 2-MeBT nor PPh3 was incorporated into the [GRAPHICS] complexes during the time of isomerization. The observed isomerization and the novel (2,3-eta2)-BT coordination mode in 1a and 2a provide for the first time a basis for understanding the hydrogenation and initial steps in the hydrodesulfurization of BT on HDS catalysts. 2- and 3-Methylbenzo[b]thiophenes, 2-MeBT and 3-MeBT, are stronger sulfur-donor ligands than BT and therefore form only the eta1(S) isomer of the complexes Cp*(CO)2Re(MeBT). The reactions of Cp*(CO)2Re(BT) (1) with Me3O+ and W(CO)5(THF) give the sulfur adducts Cp*(CO)2Re((2,3-eta2)-BT.CH3)+ (5) and Cp*(CO)2Re(eta2:eta(S)-mu2-BT)W(CO)5 (6). An X-ray structural characterization of the eta1(S)-bound 3-MeBT complex CP*(CO)2Re(eta1(S)-3-MeBT) (4) is also reported.