Decamethyltitanocene hydride intermediates in the hydrogenation of the corresponding titanocene-(η2-ethene) or (η2-alkyne) complexes and the effects of bulkier auxiliary ligands

H-1 NMR studies of reactions of titanocene [Cp*Ti-2] (Cp*=eta(5)-C5Me5) and its derivatives [Cp*(eta(5):eta(1)-C5Me4CH2)TiMe] and [Cp*Ti-2(eta(2)-CH2=CH2)] with excess dihydrogen at room temperature and pressures lower than 1 bar revealed the formation of dihydride [Cp*2TiH2] (1) and the concurrent liberation of either methane or ethane, depending on the organometallic reactant. The subsequent slow decay of 1 yielding [Cp*2TiH] (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-2] 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-2] in a 2 : 1 ratio. Hydrogenation of alkyne complexes [Cp*Ti-2(eta(2)-(RC)-C-1 CR2)] (R-1 = R-2 = Me or Et) performed at room temperature afforded alkanes (RCH2CH2R2)-C-1, and after removing hydrogen, 2 was formed in quantitative yields. For alkyne complexes containing bulkier substituent(s) R-1 = Me or Ph, R-2 = SiMe3, and R-1 = R-2 = Ph or SiMe3, successful hydrogenation required the application of increased temperatures (70-80 degrees 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 eta(5)-(CMe4Bu)-Me-5-Bu-t and eta(5)-C5Me4SiMe3 did not hinder the addition of dihydrogen to the corresponding titanocenes [(eta(5)-(C5Me4Bu)-Bu-t)(2)Ti] and [(eta(5)-C5Me4SiMe3) Ti-2] yielding [(eta(5)-(C5Me4Bu)-Bu-t)(2)TiH2] (3) and [(eta(5)-C5Me4SiMe3)(2)TiH2] (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 [(eta(5)-(C5Me4Bu)-Bu-t)(2)TiH] (5) and [(eta(5)-C5Me4SiMe3)(2)TiH] (6) were obtained by insertion of dihydrogen into the intramolecular titanium-methylene s-bond in compounds [(eta(5)-C5Me4 Bu-t)(eta(5):eta(1)-C5Me4CMe2CH2)Ti] and [(eta(5)-C5Me4SiMe3)(eta(5):eta(1)-C5Me4SiMe2CH2)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 sigma-bonded eta(1)-but-2-enyl titanocenes (7) and (8), instead of the common pi-bonded derivatives formed for the sterically less congested titanocenes, including [Cp*Ti-2 (eta(3)-(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*(2) Ti(eta(2)-H-2)]. 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(-1)), 3 (-121.11 kJ mol(-1)), and 4 (-112.35 kJ mol(-1)), in accordance with the more facile dihydrogen dissociation.