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Single and double tucked-in titanocenes react with excess CO2 by insertion into their Ti–CH2 bonds to form carboxylate-tethered species and subsequently new dimeric carbodiolates.

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.

1H NMR studies of reactions of titanocene [Cp*2Ti] (Cp* = η5-C5Me5) and its derivatives [Cp*(η5:η1-C5Me4CH2)TiMe] and [Cp*2Ti(η2-CH2CH2)] 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-R1CCR2)] (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-C5Me4tBu and η5-C5Me4SiMe3 did not hinder the addition of dihydrogen to the corresponding titanocenes [(η5-C5Me4tBu)2Ti] and [(η5-C5Me4SiMe3)2Ti] yielding [(η5-C5Me4tBu)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-C5Me4tBu)2TiH] (5) and [(η5-C5Me4SiMe3)2TiH] (6) were obtained by insertion of dihydrogen into the intramolecular titanium-methylene σ-bond in compounds [(η5-C5Me4tBu)(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.

A thermally robust triple-decker complex [bis(η5-pentamethylcyclopentadienyltitanium)-μ-(η4:η4-1,2,4,5-tetrakis(trimethylsilyl)cyclohexa-1-4-diene-3,6-diyl)] (3) was obtained in 4% yield by thermolysing Cp*TiMe3 in the presence bis(trimethylsilyl)acetylene (BTMSA). The solid-state structure of centrosymmetric 3 features rather long all C–C bonds in the nearly planar bridging ligand (1.4720(14)–1.4896(15) A) and a short distance of its least-square plane to the titanium atoms (1.7381(5) A). Computational results revealed the bonding of the central ligand to be accomplished through back-bonding of its two CC bonds and through the simultaneous generation of two σ-Ti–C(H) bonds. Based on CASSCF and CASPT2 results, the molecule acquires several electronic configurations simultaneously, which hinders its representation by one single Lewis structure. Apart from being coordinated to the central ligand, the metal atoms are involved in a direct Ti–Ti bonding by the formation of one σ- and two δ-bonds between them. The bond order of this Ti–Ti overlap shows only a slight decrease upon electronic excitation. The presence of ionic contribution to the bonding of the central ligand is manifested by the charge −1.4e summed on the carbon atoms of the bridging ring. Based on computational results, the spin multiplicity of the ground state is singlet, while the first low lying excitation state is triplet. This is in agreement with the absence of EPR signal in either toluene solution and glass, and with slight downfield shifts of broadened 1H NMR signals of SiMe3 and Cp* methyl groups observed with increasing temperature.

The trinuclear hydride complex [{(C5Me5)2Ti(µ-H2)2}2Mg]1 was obtained from the [(C5Me5Ticl2]–PrImgcl–Et2O system; the X-ray crystal structure of 1shows two mutually perpendicular permethyltitanocene moieties coordinated through four bridging hydride bonds to the central pseudotetrahedrally coordinated magnesium atom.

Electron spin resonance spectra of [Ti(C5H5 –nMen)2X](n= 0,1,3,4 or 5; X = Cl, Br or I) have been recorded in toluene and 2-methyltetrahydrofuran (mthf) solution at 77 and 295 K. Anisotopic spectra in frozen glasses were satisfactorily simulated and the parameters interpreted in terms of a generalised molecular orbital energy diagram. The monomeric species were characterised by a highly asymmetric g tensor, the lowest component being strongly dependent on the number of methyl groups (n) and the nature of the halide ligand; it moved to a lower value on going from Cl to I and with an increase in n. The ESR spectra revealed that compounds with n= 3–5 were monomeric in toluene whereas those with n= 0 or 1 were dimers. The latter compounds dissociated in mthf with the co-ordination of the solvent molecule. Mixtures of [Ti(C5H2Me3)2Cl] or [Ti(C5H2Me3)2Br] with their co-ordinated solvates were found in mthf at room temperature while [Ti(C5H2Me3)2I] and all the C5HMe4 and C5Me5 compounds remained unsolvated. At 77 K the monomer–solvate equilibria were shifted so that all the C5H2Me3 compounds formed only solvates, the C5HMe4 compounds were only partly solvated and the C5Me5 compounds remained unsolvated.

The catalyst system TiCl4–Et2AlCl induces [π6s+π2s] cycloadditions of cycloheptatriene to buta-1,3-diene, norbornadiene, and acetylenes.

Some information concerning the mechanism of the radiolysis of perfluorinated carboxylic acids is presented from an examination of the e.s.r. spectra of radicals trapped in polycrystalline samples of the acids between 77 and 300 K. Three types of radicals have been identified in γ-irradiated samples of RCF2COOH (R = F, CF3, CF3CF2, C6H13, CF2COOH, CF2CF2COOH). These are (1) parent radical anions RCF2ĊOOH–, (2) primary fluoralkyl radicals RĊF2, and (3) secondary radicals RĊFCOOH. The radical anions lose F– on thermal annealing to yield additional RĊFCOOH radicals, but when samples containing RCF2ĊOOH– are u.v.-irradiated at 77 K these species disappear without the appearance of any identifiable radical product, as do the corresponding species in non-fluorinated carboxylic acids. The RĊF2 radicals are believed to originate from the cationic species formed in the initial ionization following proton transfer and elimination of CO2.