Dihydrogen activation by titanium sulfide complexes

The titanocene sulfide complex Cp*Ti-2(S)py (1, Cp* = pentamethylcyclopenta dienyl; py = pyridine) is synthesized by addition of a suspension of S-8 to a toluene solution of Cp*Ti-2(CH2CH2) (2) and py. The rate of rotation of the pyridine ligand in solution was determined by H-1 NMR spectroscopy, and the structure of 1 was determined by X-ray crystallography. Complex 1 reac ts reversibly with dihydrogen to give Cp*Ti-2(H)SH (6) and py, Reaction of 1 with HD gives an equilibrium mixture of Cp*Ti-2(D)SH and Cp*Ti-2(H)SD; H- 2 and D-2 are not formed in this reaction, ID H-1 NMR magnetization transfe r spectra and 2D EXSY H-1 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 dihydrog en complex intermediate is proposed to explain this observation. The infrar ed spectrum of 6 shows an absorption assigned to the Ti-H stretching mode a t, 1591 cm(-1) that shifts upon deuteration to 1154 cm(-1). Reaction of 1 w ith trimethylsilane, diethylsilane, or dimethylsilane gives Cp*(2)-Ti(H)SSi Me3 (7), Cp*Ti-2(H)SSiHEt2 (8), or Cp*Ti-2(H)SSiHMe2 (9), respectively. The isotope effect for the reaction producing 7 has been measured, and a mecha nism is proposed. Treatment of 1 with an additional equivalent of Ss result s in the formation of the disulfide Cp*Ti-2(S-2) (4). Acetylene inserts int o the Ti-S bond of 4 to produce the vinyl disulfide complex 5. The structur es 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 p resence of H-2 to provide 6 and the corresponding phosphine sulfide. Reacti on 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 b y metal-sulfide hydrodesulfurization catalysts.