The Benzo[b]thlophene (BT) Complexes Cp′(CO)₂Re(BT): Models for BT Adsorption on Hydrodesulfurlization Catalysts

Benzo[b]thiophene (BT) coordinates in Cp(CO)₂Re(BT) as an equilibrium mixture of η¹(S) and 2,3-η² isomers. When the electron density on the Re is increased by replacing Cp (η⁵-C₅H₅) with Cp* (η⁵-C₅Me₅), the amount of η² 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 π 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 [formula omitted]complexes during the time of isomerization. The observed isomerization and the novel (2,3-η²)-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 η¹(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-η²)-BT•CH₃)+ (5) and Cp*(CO)₂Re(η²:η(S)-μ₂-BT)W(CO)₅ (6). An X-ray structural characterization of the η¹(S)-bound 3-MeBT complex Cp^*(CO)₂Re(η¹(S)-3-MeBT) (4) is also reported.