Steric Effects in Reactions of Decamethyltitanocene Hydride with Internal Alkynes, Conjugated Diynes, and Conjugated Dienes

Hydrogenation of titanocene and zirconocene bis(trimethylsilyl)acetylene complexes

Reactions following the addition of dihydrogen under maximum atmospheric pressure to bis(trimethylsilyl)acetylene (BTMSA) complexes of titanocenes, [(η5-C5H5-nMen)2Ti(η2-BTMSA)] (n = 0, 1, 3, and 4) (1A-1D), and zirconocenes, [(η5-C5H5-nMen)2Zr(η2-BTMSA)] (n = 2-5) (4A-4D), proceeded in diverse ways and, depending on the metal, afforded different products. The former complexes lost, in all cases, their BTMSA ligand via its hydrogenation to bis-1,2-(trimethylsilyl)ethane when reacted at 80 °C for a prolonged reaction time. For n = 0, 1, and 3, the titanocene species formed in situ dimerised via the formation of fulvalene ligands and two bridging hydride ligands, giving known green dimeric titanocenes (2A-2C). For n = 4, a titanocene hydride [(η5-C5HMe4)2TiH] (2D) was formed, similarly to the known [(η5-C5Me5)2TiH] (2E) for n = 5; however, in contrast to this example, 2D in the absence of dihydrogen spontaneously dehydrogenated to the known Ti(iii)-Ti(iii) dehydro-dimer [{Ti(η5-C5HMe4)(μ-η1:η5-C5Me4)}2] (3B). This complex has now been fully characterised via spectroscopic methods, and was shown through EPR spectroscopy to attain an intramolecular electronic triplet state. The zirconocene-BTMSA complexes 4A-4D reacted uniformly with one hydrogen molecule to give Zr(iv) zirconocene hydride alkenyls, [(η5-C5H5-nMen)2ZrH{C(SiMe3)[double bond, length as m-dash]CH(SiMe3)}] (n = 2-5) (5A-5D). These were identified through their 1H and 13C NMR spectra, which show features typical of an agostically bonded proton, [double bond, length as m-dash]CH(SiMe3). Compounds 5A-5D formed equilibria with the BTMSA complexes 4A-4D depending on hydrogen pressure and temperature.

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

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.

C(sp3 )-H Oxidative Addition and Transfer Hydrogenation Chemistry of a Titanium(II) Synthon: Mimicry of Late-Metal Type Reactivity

Two-electron reduction of the Ti^IV compound (^ket guan)(Im^Dipp N)Ti(OTf)₂ (3) gives the arene-masked complex (^ket guan)(η⁶ -Im^Dipp N)Ti (1) in excellent yield. Upon standing in solution, 1 converts to a Ti^IV metallacycle (4) through dehydrogenation of a pendant isopropyl group. Spectroscopic evidence shows this transformation initially proceeds via the oxidative addition of a C(sp³ )-H bond and can be reversed upon exposure of 4 to H₂ . Interestingly, treatment of 1 with cyclohexene gives cyclohexane and 4 via a titanium-mediated transfer hydrogenation reaction, a process that can be extended to catalytically hydrogenate other unsaturated hydrocarbons under mild conditions. These results, rare for the early-metals, suggest 1 possesses chemical characteristics reminiscent of noble, late-metals.

Displacement of ethene from the decamethyltitanocene-ethene complex with internal alkynes, substituent-dependent alkyne-to-allene rearrangement, and the electronic transition relevant to the back-bonding interaction

The titanocene-ethene complex [Ti(II)(η(2)-C2H4)(η(5)-C5Me5)2] (1) with simple internal alkynes R(1)C≡CR(2) gives complexes [Ti(II)(η(2)-R(1)C≡CR(2))(η(5)-C5Me5)2] {R(1), R(2): Ph, Ph (3), Ph, Me (4), Me, SiMe3 (5), Ph, SiMe3 (6), t-Bu, SiMe3 (7), and SiMe3, SiMe3 (8). In contrast, alkynes with R(1) = Me and R(2) = t-Bu or i-Pr afford allene complexes [Ti(II)(η(2)-CH2=C=CHR(2))(η(5)-C5Me5)2] (11) and (12), whereas for R(2) = Et a mixture of alkyne complex (13A) and minor allene (13) is obtained. Crystal structures of 4, 6, 7 and 11 have been determined; the latter structure proved the back-bonding interaction of the allene terminal double bond. Only the synthesis of 8 from 1 was inefficient because the equilibrium constant for the reaction [1] + [Me3SiC≡CSiMe3] ⇌ [8] + [C2H4] approached 1. Compound 9 (R(1), R(2): Me), not obtainable from 1, together with compounds 3–6 and 10 (R(1), R(2): Et) were also prepared by alkyne exchange with 8, however this reaction did not take place in attempts to obtain 7. Compounds 1 and 3–9 display the longest-wavelength electronic absorption band in the range 670-940 nm due to the HOMO → LUMO transition. The assignment of the first excitation to be of predominantly a b2 → a1 transition was confirmed by DFT calculations. The calculated first excitation energies for 3–9 followed the order of hypsochromic shifts of the absorption band relative to 8 that were induced by acetylene substituents: Me > Ph ≫ SiMe3. Computational results have also affirmed the back-bonding nature in the alkyne-to-metal coordination.

Probing the coordination environment of Ti3+ ions coordinated to nitrogen-containing Lewis bases

A combined EPR and DFT study on model systems reveals fingerprint ¹⁴N hyperfine and quadrupole data to identify binding of nitrogen-containing Lewis bases to Ti(iii).

Titanocene(iii) pseudohalides: an ESR and structural study

ESR spectroscopy in both solution and the solid state revealed doublet, triplet and quartet ground states of new titanocene(iii) pseudohalide complexes.