Vibrational Study of Organometallic Complexes with Thiophene Ligands:  Models for Adsorbed Thiophene on Hydrodesulfurization Catalysts

Interpretation and comparison of the vibrational spectra of organometallic complexes containing thiophene ligands with IR spectra of adsorbed thiophene on sulfided Mo/Al2O3 catalysts have permitted a definitive assignment of the adsorption mode of the surface-bonded thiophene. Infrared and Raman spectra were acquired for three organometallic complexes: (η5-C5D5)Re(CO)2(η(S)-C4H4S), (η5-C4H4S)Cr(CO)3, and [(η5-C4H4S)Mn(CO)3]BF4. The vibrational properties of η1(S)- and η5-coordinated thiophene were further investigated through normal-mode analyses of the IR spectra of thiophene coordinated in (η5-C4H4S)Cr(CO)3 and (η5-C5D5)Re(CO)2(η1(S)-C4H4S). Perturbations among the force constants of a gas-phase thiophene model, consistent with the structure of η1(S)- and η5-coordinated thiophene ligands, gave rise to respective thiophene force fields that allowed for the accurate determination of the IR spectra of thiophene coordinated in the Cr and Re complexes. Spectral shifts observed for the IR spectrum of the η1(S)-coordinated thiophene ligand, with respect to the IR spectrum of free thiophene, are similar to those observed for analogous bands of thiophene adsorbed at the surface of sulfided Mo/Al2O3 catalysts. Furthermore, perturbations among the force constants of η1(S)-coordinated thiophene, necessary to model such shifts, indicate that the hydrocarbon backbone of thiophene is strengthened upon η1(S) adsorption, whereas the C−S bonds are significantly weakened. These bonding changes, along with the assignment of the adsorption mode of thiophene, suggest an HDS mechanism in which the initial steps are η1(S) adsorption of thiophene followed by subsequent cleavage of one of the C−S bonds.