Synthesis and Reactivity of Tervalent Paramagnetic Titanium Compounds (η5-C5Me5)2TiR: Molecular Structure of (η5-C5Me5)2TiCH2CMe3

Paramagnetic, tervalent titanium compounds Cp*2TiR with R = Me (2), Et (3), n-Pr (4), CH2CMe3 (5), CH2Ph (6), η3-C3H5 (7), η3-C4H7 (8), CH=CH2 (9), C≡CMe (10), Ph (11) have been prepared by salt metathesis from Cp*2TiCl (1). The 1H NMR spectra show characteristic broad resonances (width at half-maximum between 0.88 and 5.5 kHz) in the range δ 14.5-20.9 ppm, which are due to the 1H nuclei of the Cp* ligands. Only part of the 1H resonances of the carbyl ligand R are observed. 2H NMR spectra of Cp*2TiR with deuterated ligands R allow assignment of all 2H nuclei in R. These are observed between δ -79 and 125 ppm. Solution EPR spectra show broad singlet signals in the range g = 1.941-1.992, without hyperfine interaction with 1H nuclei of the ligands or titanium isotope splitting. The crystal structure of 5 was determined by a single-crystal X-ray diffraction study. The compound crystallizes in the monoclinic space group P21/n with four molecules in the unit cell. The cell dimensions are a = 10.221 (2) Å, b = 15.609 (3) Å, c = 14.107 (3) Å, and β = 94.21 (2)°. The refinements converged at R(F) = 0.059 for 2801 observed reflections and 359 parameters. Thermolysis of the compounds leads to quantitative formation of RH and a (pentamethylcyclopentadienyl)(fulvene)titanium compound, Cp*FvTi (Fv = η6-C5Me4CH2). The 15-electron compounds Cp*2TiR do not form stable adducts Cp*2TiR·L, although intermediate adduct formation is indicated in reactions with substrates such as CO and isonitriles. With CO a complicated reaction occurs in which disproportionation to Ti(II) and Ti(IV), acyl formation, and nucleophilic attack on the Cp* ligand with ring expansion takes place. With isonitriles RN≡C insertion in the Ti-R bond leads to formation of η2-iminoacyls. Carbon dioxide reacts to give carboxylates Cp*2Ti(η2-O2CR). For alkyls Cp*2TiR, where R bears a β-hydrogen, the dominant process is β-H transfer to an incoming substrate molecule and extrusion of the olefin R(-H). This has been observed for 3 and 4 with carbon dioxide, olefins, and but-2-yne.

IR spectroscopic investigations into reactions between unsaturated titanium complexes and CO or N2 in liquid xenon or heptane

The reactions of Cp*2TiR compounds with CO in liquid xenon (LXe) or heptane were studied by IR spectroscopy in special high-pressure cells. All complexes incorporate a single CO molecule to fill up their coordination sphere, forming Cp*2TiR(CO) intermediates. These species undergo secondary reactions at elevated temperatures, such as disproportionation to Ti(II) and Ti(IV) complexes. The formation of the 18 VE species Cp*2Ti(CO)2 seems to be the driving force of the observed reactions. A similar behavior was found in the reaction of Cp*2TiH with dinitrogen, where the adduct Cp*2TiH(N2) and its 14/15N2 isotopomer was identified together with Cp*2Ti(N2)2. In addition to these orientating experiments, the stable 15 VE Cp*FvTi (Fv = η6-C5Me4CH2) and 16 VE species Cp*AdTi (Ad = η7-C5Me3(CH2)2) show the addition of carbon monoxide to form new, reactive titanium carbonyls, which undergo ligand exchange reactions at higher temperatures and form Cp*2Ti(CO)2 as one of the final products.