Dihydrogen Activation by Titanium Sulfide Complexes.

Abstract

The titanocene sulfido complex Cp*(2)Ti(S)py (1, Cp* = pentamethylcyclopentadienyl; py = pyridine) is synthesized by addition of a suspension of S(8) to a toluene solution of Cp*(2)Ti-(CH(2)CH(2)) (2) and py. The rate of rotation of the pyridine ligand in solution was determined by (1)H NMR spectroscopy, and the structure of 1 was determined by X-ray crystallography. Complex 1 reacts reversibly with dihydrogen to give Cp*(2)Ti(H)SH (6) and py. Reaction of 1 with HD gives an equilibrium mixture of Cp*(2)Ti(D)SH and Cp*(2)Ti(H)SD; H(2) and D(2) are not formed in this reaction. 1D (1)H NMR magnetization transfer spectra and 2D EXSY (1)H NMR spectra of 6 in the presence of H(2) show that in solution the H(2), hydride, and hydrosulfido hydrogen atoms exchange. A four-center mechanism for this exchange is proposed. The EXSY studies show that the Ti-H and S-H hydrogens exchange with each other more rapidly than either of those hydrogens exchanges with external H(2). A transient dihydrogen complex intermediate is proposed to explain this observation. The infrared spectrum of 6 shows an absorption assigned to the Ti-H stretching mode at 1591 cm(-1) that shifts upon deuteration to 1154 cm(-1). Reaction of 1 with trimethylsilane, diethylsilane, or dimethylsilane gives Cp*(2)-Ti(H)SSiMe(3) (7), Cp*(2)Ti(H)SSiHEt(2) (8), or Cp*(2)Ti(H)SSiHMe(2) (9), respectively. The isotope effect for the reaction producing 7 has been measured, and a mechanism is proposed. Treatment of 1 with an additional equivalent of S(8) results in the formation of the disulfide Cp*(2)Ti(S(2)) (4). Acetylene inserts into the Ti-S bond of 4 to produce the vinyl disulfide complex 5. The structures of 4 and 5 have been determined by X-ray diffraction. Compound 4 reacts with 2 in the presence of py to produce 1. Phosphines react with 4 in the presence of H(2) to provide 6 and the corresponding phosphine sulfide. Reaction of hydrogen with 4 gives Cp*(2)-Ti(SH)(2) (3). The reactions of 1 and 4 with dihydrogen provide a model for possible mechanisms of H(2) activation by metal-sulfide hydrodesulfurization catalysts.