Decamethyltitanocene hydride intermediates in the hydrogenation of the corresponding titanocene-(η-ethene) or (η-alkyne) complexes and the effects of bulkier auxiliary ligands.

H NMR studies of reactions of titanocene [Cp*Ti] (Cp* = η-CMe) and its derivatives [Cp*(η:η-CMeCH)TiMe] and [Cp*Ti(η-CH[double bond, length as m-dash]CH)] with excess dihydrogen at room temperature and pressures lower than 1 bar revealed the formation of dihydride [Cp*TiH] (1) and the concurrent liberation of either methane or ethane, depending on the organometallic reactant. The subsequent slow decay of 1 yielding [Cp*TiH] (2) was mediated by titanocene formed in situ and controlled by hydrogen pressure. The crystalline products obtained by evaporating a hexane solution of fresh [Cp*Ti] in the presence of hydrogen contained crystals having either two independent molecules of 1 in the asymmetric part of the unit cell or cocrystals consisting of 1 and [Cp*Ti] in a 2 : 1 ratio. Hydrogenation of alkyne complexes [Cp*Ti(η-RC[triple bond, length as m-dash]CR)] (R = R = Me or Et) performed at room temperature afforded alkanes RCHCHR, and after removing hydrogen, 2 was formed in quantitative yields. For alkyne complexes containing bulkier substituent(s) R = Me or Ph, R = SiMe, and R = R = Ph or SiMe, successful hydrogenation required the application of increased temperatures (70-80 °C) and prolonged reaction times, in particular for bis(trimethylsilyl)acetylene. Under these conditions, no transient 1 was detected during the formation of 2. The bulkier auxiliary ligands η-CMeBu and η-CMeSiMe did not hinder the addition of dihydrogen to the corresponding titanocenes [(η-CMeBu)Ti] and [(η-CMeSiMe)Ti] yielding [(η-CMeBu)TiH] (3) and [(η-CMeSiMe)TiH] (4), respectively. In contrast to 1, the dihydride 4 did not decay with the formation of titanocene monohydride, but dissociated to titanocene upon dihydrogen removal. The monohydrides [(η-CMeBu)TiH] (5) and [(η-CMeSiMe)TiH] (6) were obtained by insertion of dihydrogen into the intramolecular titanium-methylene σ-bond in compounds [(η-CMeBu)(η:η-CMeCMeCH)Ti] and [(η-CMeSiMe)(η:η-CMeSiMeCH)Ti], respectively. The steric influence of the auxiliary ligands became clear from the nature of the products obtained by reacting 5 and 6 with butadiene. They appeared to be the exclusively σ-bonded η-but-2-enyl titanocenes (7) and (8), instead of the common π-bonded derivatives formed for the sterically less congested titanocenes, including [Cp*Ti(η-(1-methylallyl))] (9). The molecular structure optimized by DFT for compound 1 acquired a distinctly lower total energy than the analogously optimized complex with a coordinated dihydrogen [Cp*Ti(η-H)]. The stabilization energies of binding the hydride ligands to the bent titanocenes were estimated from counterpoise computations; they showed a decrease in the order 1 (-132.70 kJ mol), 3 (-121.11 kJ mol), and 4 (-112.35 kJ mol), in accordance with the more facile dihydrogen dissociation.