7 research outputs found
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
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.