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

Abstract

¹H NMR studies of reactions of titanocene [Cp*₂Ti] (Cp* = η⁵-C₅Me₅) and its derivatives [Cp*(η⁵:η¹-C₅Me₄CH₂)TiMe] and [Cp*₂Ti(η²-CH₂[double bond, length as m-dash]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*₂Ti] 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*₂Ti] in a 2 : 1 ratio. Hydrogenation of alkyne complexes [Cp*₂Ti(η²-R¹C[triple bond, length as m-dash]CR²)] (R¹ = R² = Me or Et) performed at room temperature afforded alkanes R¹CH₂CH₂R², and after removing hydrogen, 2 was formed in quantitative yields. For alkyne complexes containing bulkier substituent(s) R¹ = Me or Ph, R² = SiMe₃, and R¹ = R² = Ph or SiMe₃, 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 η⁵-C₅Me₄^tBu and η⁵-C₅Me₄SiMe₃ did not hinder the addition of dihydrogen to the corresponding titanocenes [(η⁵-C₅Me₄^tBu)₂Ti] and [(η⁵-C₅Me₄SiMe₃)₂Ti] yielding [(η⁵-C₅Me₄^tBu)₂TiH₂] (3) and [(η⁵-C₅Me₄SiMe₃)₂TiH₂] (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 [(η⁵-C₅Me₄^tBu)₂TiH] (5) and [(η⁵-C₅Me₄SiMe₃)₂TiH] (6) were obtained by insertion of dihydrogen into the intramolecular titanium-methylene σ-bond in compounds [(η⁵-C₅Me₄^tBu)(η⁵:η¹-C₅Me₄CMe₂CH₂)Ti] and [(η⁵-C₅Me₄SiMe₃)(η⁵:η¹-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 σ-bonded η¹-but-2-enyl titanocenes (7) and (8), instead of the common π-bonded derivatives formed for the sterically less congested titanocenes, including [Cp*₂Ti(η³-(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*₂Ti(η²-H₂)]. 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.