Vibrational spectra of cyclopentadienyl chlorides of titanium,zirconium and hafnium. internal rotation and thermodynamic functions

The infrared and Raman spectra of some cyclopentadienyl compounds of the transition metals, namely Ti(C₅H₅)Cl₃ and M(C₅H₅)₂Cl₂ (M = Ti, Zr and Hf), are reported and discussed. The infrared spectra of the gaseous species isolated in argon matrices at 10 K provide structural information about the single molecules. Particular attention has been paid to the low-frequency region in order to achieve more reliable assignments for the internal-rotation modes. The structural data and the fundamental frequencies derived from the spectra are employed in a calculation of the thermodynamic functions for these compounds in the ideal gas state.     

Analysis of FT-IR spectra of dicyclopentadienyl (bis-substituted cyclopentadienyl) dithiocyano of titanium, zirconium and hafnium

The FT-IR spectra of 18 (R-Cp)2M(NCS)2 were measured. The M-Cp, M-NCS (M=Ti, Zr, Hf) and other vibration modes were reasonably assigned. All complexes of (R-Cp)2M(NCS)2 determined in this paper are bonded by N-M, and the absorption of upsilon(s)(M-Cp)(A1) (M=Ti, Zr and Hf) vibration all appear in 365 cm(-1) or so, while upsilon(as)(M-Cp)(B) appear successively around 420, 350 and 320 cm(-1) in order of Ti, Zr and Hf. The influence of the center metal atoms and the substituents on cyclopentadienyl upon the spectra was discussed. It is mainly in far infrared region that center metal atoms influence upon the infrared spectra. The influence of the substituents to cyclopentadienyling upon its vibration is not significant. Only between 1500 and 1480 cm(-1) did a new absorbing peak appear due to the introduction of substituents to activate upsilon(CC) vibration.     

Vibrational spectra and structure of zirconium and hafnium cyclopentadienyl hydrides

Raman and IR spectra (4000-50 cm^–1) of solid cyclopentadienyl zirconium and hafnium hydrides Cp₂MH₂, Cp₂MD₂, Cp₂Zr(H)X, and Cp₂Zr(D)X (Cp = ⁵-C₅H₅; M = Zr, Hf; X = Cl, Br) have been studied. The vibrational modes of MH groups, Cp-rings, and metal-ligand bonds are discussed and the band assignments are proposed. In the solid state, these complexes form polymers with linear hydride bridges of the M-H-M type. The force constants of the M-H and M-Cp bonds increase on going from Zr to Hf.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1604–1609, September, 1994.     

Far IR and Raman vibrational spectra of nitramines

Vibrational spectra of several nitramines in the long-wave region (50–450 cm⁻¹) were studied. The frequencies of intra- and intermolecular vibrations were separated and a tentative assignment of the frequencies of self-associative complexes was performed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2241–2244, November, 1998.     

Dynamic transformations of cyclopentadienyl chelate complexes of zirconium and hafnium

Russian Chemical Bulletin -     

He(I) and He(II) photoelectron spectra of some mixed sandwich compounds of titanium,zirconium and hafnium

The He(I) and He(II) photoelectron spectra are reported for two series of transition metal mixed sandwich complexes of general formula L_aML_b (M = Ti, L_a = η⁵-C₅H₅, η⁵-CH₃C₅H₄, η⁵-C₅(CH₃)₅; M = Zr, Hf, L_a = η⁵-C₅(CH₃)₅; L_b = η⁷-C₇H₇ (series I); M = Ti, L_a = η⁵-CH₃C₅H₄, η⁵-C₅(CH₃)₅; M = Zr, L_a = η⁵-C₅(CH₃)₅; L_b = η⁸-C₈H₈ (series II)). Assignments were made of the metal d, cyclopentadienyl and carbocyclic π orbitals on the basis of He(I)/He(II) intensity ratios and shift effects and by comparison with UP data for related compounds. For series I no influence of the central metal upon the IEs of the highest occupied molecular orbital e₂ was observed. The IE of the non-bonding metal d_z2 orbital of Ti or Zr (5.28 and 4.70 eV, respectively) in the complexes of series II (L_a = η⁵-C₅(CH₃)₅) is very low.     

Low-temperature synthesis of nanodispersed titanium,zirconium, and hafnium carbides

Nanosized refractory titanium, zirconium, and hafnium carbides were manufactured from highly dispersed metal dioxide-carbon starting mixtures at moderate temperatures of 1200°C or lower. The products were characterized by powder X-ray diffraction, elemental analysis, and transmission electron microscopy. The average size of particles (in nanometers) manufactured at 1200°C was as follows: for TiC, 13 ± 4; for ZrC, 17 ± 3; and for HfC, 16 ± 3; the average crystallite size (in nanometers) was as follows: for TiC, 8 ± 2; for ZrC, 5 ± 2; and for HfC, 8 ± 3. Thermodynamic modeling was performed for the synthesis of Group IVB carbides via carbothermal reduction of the corresponding oxides. The results show that in the titanium dioxide-carbon system, for example, titanium monocarbide formation is possible at a temperature as low as 850°C (p = 10⁻⁴ MPa). Highly dispersed metal dioxide-carbon starting mixtures were prepared using solgel technology from metal alkoxyacetylacetonates in the presence of a polymeric carbon source.     

Dinitrogen functionalization with bis(cyclopentadienyl) complexes of zirconium and hafnium

The rich chemistry of substituted bis(cyclopentadienyl)zirconium and hafnium complexes bearing side-on coordinated dinitrogen ligands is highlighted in this Perspective. Our studies in this area were initially motivated by the desire to understand side-on vs. end-on dinitrogen coordination in bimetallic zirconocene and hafnocene N2 compounds. In the cases where eta2,eta2-dinitrogen compounds were isolated, both structural and computational data have established significant imido character in the metal-nitrogen bonds. This additional bonding interaction, which is diminished in end-on complexes bearing both terminal and bridging N2 ligands, facilitates dinitrogen functionalization by non-polar reagents including dihydrogen, carbon-hydrogen bonds and weak Br?nsted acids such as water and ethanol. In hafnocene chemistry, where unwanted side-on, end-on isomerization is suppressed, cycloaddition of phenylisocyanate to coordinated N2 has also been accomplished. For N-H bond forming reactions involving H2, kinetic measurements, in addition to isotopic labelling and computational studies, are consistent with dinitrogen functionalization by 1,2-addition involving a highly ordered, four-centred transition structure.     

Silylmethyl and related complexes II. Preparation,spectra, and thermolysis of the tetraneopentyls of titanium,zirconium, and hafnium

The preparation and characterisation of the Group IVA neopentyls (Me₃CCH₂)₄M (M = Ti, Zr, or Hf) is described. Spectroscopic data (IR, Raman, mass, ¹H NMR, and PE) are provided; IR and Raman bands have been assigned by comparison with results on Group IVB analogues (M = Ge or Sn). MC₄ stretching vibrations fall in the range 540–485 cm^?1, and bending modes at 240–283 cm^?1. Thermal decomposition gives neopentane as the sole detectable product; qualitatively, stability increases in the order M = Ti < Zr < Hf and for R₄M : R = Me ? Me₃CCH₂ ≈ Me₃SiCH₂. (Me₃CCH₂)₄Ti is aerobically oxidised in benzene to give (Me₃CCH₂O)₄Ti     

Catalytic dehydrogenation of cyclooctane with titanium, zirconium and hafnium metallocene complexes

Metallocene complexes in combination with cocatalysts like methylalumoxane (MAO) are not only excellent catalysts for olefin polymerization but also appropriate catalysts for the activation of alkanes in homogeneous (autoclave) and heterogeneous (fixed bed reactor) reactions. The activities of the catalysts depend on the temperature, the cocatalysts, additives, the central metal and the ligand structure. Generally, complexes with low steric demands and MAO as cocatalyst gave the highest activities. The comparison of different π-ligands resulted in the following activity order: cyclopentadienyl > indenyl > fluorenyl. The influence of σ-ligands and n-donor ligands gave the following activity order: -Cl > -PMe₃ > -CH₂Ph > -(CH₂)₄CH₃ > -NPh₃. The activities depended on the nature of the cocatalyst and decreased in the following order: MAO ? AlMe₃ > AlEt₃. The addition of aluminum powder and the Lewis base NPh₃ increased the activity of the Cp₂ZrCl₂/MAO catalyst. The Cp₂ZrCl₂/MAO/NPh₃ catalyst showed the highest activity in homogeneous reactions with 458 turnovers in 16 h at 300 °C. The Cp₂ZrCl₂/MAO/NPh₃/SI1102 catalyst gave the highest activity in heterogeneous catalysis with 206 turnovers in 5 h at 350 °C. None of the catalysts required a hydrogen acceptor like an external olefin.     

Polymer-supported organometallic compounds of titanium,zirconium and hafnium as hydrogenation catalysts

Cyclopentadienyl compounds of titanium, zirconium and hafnium have been prepared attached to polymers by first binding cyclopentadiene to styrene-divinylbenzene (20%) copolymers and then converting them to cyclopentadienyl anions. The resin-bound anions were treated with MCl_n and polymer-attached Mcl_n?1 compounds were formed (where M = Ti, Zr and Hf). The polymer-attached monocyclopentadienyl titanium trichloride on reduction with butyl lithium produces active catalysts whose hydrogenation efficiencies are 10 to 20 times as great as the corresponding non-attached species under the same conditions. Studies of the physical nature of the metal catalyst dispersions on the polymer supports by using an electron-X-ray fluorescence microprobe indicate that the dispersion is uniform throughout the entire section of the bead.