CHAPTER 1
The Early Transition Metals
BY C. D. GARNER
1 Titanium
Introduction. — A text describing the chemistry of titanium has been publishedand the organometallic chemistry of titanium reported during 1971 has been reviewed. The compound C8H8TiC4Ph4 has been prepared by treating TiCl3 with Pr'MgBr in ether containing cyclo-octatetraene and diphenylacetylene. This green compound is diamagnetic and air-stable in the solid state and its i.r., mass, and 1H n.m.r. spectra are consistent with a structure involving π-bonded cyclo-octatetraene and tetraphenylcyclobutadiene rings. A new type of fluxional process for an organometallic system has been described for bis(cyclo-octatetraene)titanium(II) in which formal oxidation and reduction occur for the planar and bent C8H8 rings, (1), respectively, through reciprocal ring bending and flattening with an activation energy of 70 [+ or -] 1 kJ mol-1.
Cyclopentadienyl(cycloheptatrienyl)titanium has been shown by an X-ray diffraction study to be a sandwich compound, the dihedral angles between the C5H5 and C7H7 rings being 2.2°. Although the distance of the titanium atom from the carbon atoms of the former ring (232 pm) is normal, that from the carbon atoms of the latter (219 pm) is unusually short. Trisfcyclopentadienyl)titanium involves two h5- and one h2-C5H5 groups (2); it is suggested that in the bonding of this latter group to the metal, the cyclopentadienyl radical acts as a three-electron ligand thus giving the titanium a 17-electron configuration. This three-electron h2-C5H5 arrangement may serve as a model for the intermediate state in the 1,2-shift mechanism in fluxional π-C5H5 systems.
The fixation of dinitrogen by organic compounds in the presence of titanocene derivatives has been accomplished, and amines can be obtained directly by treatment of the corresponding aldehyde, α-keto-ester, acid chloride, or anhydride with a mixture of [(π-Cp)2TiCl2] and Mg–Mgl (or EtMgBr) in a current of dinitrogen. The paramagnetic complex [{(7i-Cp)2Ti}N2MgCl] has been isolated at –60 °C in the system [(π-Cp)2TiCl2]- PriMgCl–N2 in ether. This complex affords hydrazine when decomposed by HCl.
A model olefin polymerization catalyst previously characterized as [(C5H5)2TiAlEt2]2 has been studied by X-ray diffraction, 1H n.m.r., and mass spectral techniques. The compound contains (1–5η–C5H5) and µ(l–5η: σ–C5H5) rings and is the first well-characterized dimeric titanium–aluminium hydride: structure (3) has been suggested. Alkyl exchange between a polymeric alkyltitanium compound and alkylaluminium compound, present in excess, is usually assumed to be the main transfer process in Ziegler–Natta olefin polymerization. Such an exchange process has been identified between TiMe4 and Al2Me6. MO calculations have been performed along the reaction coordinate for the insertion of ethylene into a titanium–carbon σ-bond and also for the titanium–aluminium–ethyiene complex (4). These latter results showed that the Ti-olefin bond involves no back-bonding and that the π*-orbital of the olefin acquires little stability on co-ordination to titanium. The Ti–Me bond of (4) is localized almost completely in the highest bonding level of the complex, with the metal contribution being almost pure d in character. The results of these calculations suggest that the R2Al group in the molecular catalyst functions merely as a substrate which maintains a high co-ordination number at the titanium site. Calculations along the reaction co-ordinate indicate that the negatively charged methyl group may readily migrate to the olefin, which carries a net positive charge, consistent with the phenomenon of catalysis.
Binary Compounds and Related Systems. — Halides and Oxyhalides. Thermal decomposition of NH4TiF4 in a H2 or Ar atmosphere at 600 — 650 °C has been shown to afford a convenient route to high-purity TiF3. The thermodynamic relationships between the chlorides of titanium have been investigated and standard heats and entropies of formation of TiCl3(l), TiCl6(l), and TiOCl(s) determined as –525 kJ mol-1 and 328 e.u., –1180 kJ mol-1 and 520 e.u., and –760 kJ mol-1 and 75 e.u., respectively. The preparation of TiCl4 or TiBr4 by the reaction of titanium metal with molten metal halides such as PbX2, AgX (X = Cl or Br), or CuCl has been studied at 450 — 550 °C. The reaction of titanium with an excess of molten PbCl2 at 530 °C affords pure TiCl4 in 9.8% yield after 15 min. The vacuum-u.v. electronic spectrum of TiCl4(g) has been recorded and discussed in comparison with available photoelectron data and theoretical results. The energy required to reorganize TiCl4 from tetrahedral to various other geometries has been evaluated by the self-consistent MO method. The results suggest that the tetrahedral geometry is a consequence of nuclear repulsions rather than bonding interactions.
An equilibrium diagram has been constructed for the TiCl4–TiBr4 system and the crystallization temperatures of the mixed halides TiCl3Br, TiCl2Br2, and TiClBr3 were shown to be –19.4, –5.7, and 13.4 °C, respectively. These liquids have standard heats of formation of –760, –710, and –660 kJ mol-1, respectively. The chemical shift data for the Ti and Ti n.m.r. of TiCl4, TiBr4, [TiF6]2-, and binary mixtures of TiCl4 with both TiBr4 and TiI4 have been determined. The order of increasing shielding of the titanium nucleus by the halogens is I,Br < Cl. The resonance absorptions for TiCl4and TiBr4 are unshifted on dilution in inert solvents, thus suggesting that the neat liquids of these halides involve monomeric molecules. Mixtures of TiCl4and TiBr4 exhibit one resonance signal, the chemical shift of which varies linearly with the mole fraction of the components, consistent with rapid halogen exchange between the TiClxBr4-x (x = 0–4) molecules. A similar exchange process probably takes place between two or more species in TiCl4–TiCl4 mixtures since again only one signal is observed. Raman spectra of TiCl4 solutions in BrCH2OMe show lines typical of the TiCln Br4-n (n = 1 — 4) molecules.
Oxides. The structures and properties of titanium dioxide have been reviewed and the bonding in anatase has been suggested to be more ionic than that in rutile from a study of their K X-ray emission spectra. The structure of the TinO2n-1 oxides has been classified as an infinitely adaptive one in crystallographic shear phases (4 ≤ n ≤ 9 and 16 [less than or equal to] n ≤ 36). The linewidths of the e.s.r. spectra of these oxides has been shown to be a sensitive indicator of their stoicheiometry, for 2 ≤ n ≤ 10. The structural aspects of the metal-insulator transition in Ti4O7 have been investigated by X-ray crystallography. The triclinic structure of this oxide consists of rutile-like layers of TiO6 octahedra extended in the ab-plane and four octahedra thick along the c-axis. At 120 K there is a clear separation into strings of TiIII or TiIV ions running parallel to the c-axis; the TiIII centres are paired to form Ti — Ti bonds, whereas the TiIV atoms are strongly bonded to one oxygen, Ti — O = 178 — 179 pm.
Chalcogenides. The composition of TiS3 has been shown to be 1:3.00 by X-ray diffraction and density measurements and its vibrational spectra have been reported.
The mixed cation disulphides [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] have been prepared by grinding a mixture of TiS2 with the appropriate metal and sulphur, followed by prolonged heating at 950 °C. Ti0.167NbS2 has been shown to have a structure in which the titanium atoms occupy octahedral sites between the NbS6 prisms, the site symmetries of the metal atoms being C3v and D3h, respectively. The compounds NixTiS2 (x = 0.25, 0.33, 0.40, or 0.75) have been characterized in the Ni–TiS2 system by X-ray diffraction studies. These compounds probably involve nickel atoms inserted into octahedral sites of the host lattice. TiS2 reacts with solutions of K+(naphthalene)- to give a metal intercalation derivative KnTiS2. Layer intercalation compounds of TiS2 with NH3, N2H4, NH2NHMe, MeNHNHMe, and py, other nitrogen heterocyclics, and their N-oxides have been prepared, and the layer expansions determined.
The phase diagrams of the Ti–Hg–Se and TiSe–HgSe systems have been constructed using X-ray and thermal analyses and a compound of composition Ti3Hg7Se10 was identified in the latter.
Carbides, Silicides, and Germanides. The standard heat of formation and the dissociation energy of TiC(g) have been determined as –730[+ or -]9 and 160[+ or -]8 kJ mol-1, respectively, using the mass spectrometric Knudsen effusion technique. The standard heat and entropy of formation of Ti5Si3 (s) have been reported as –580 kJ mol-1 and 272 e.u., respectively. A thermodynamic analysis of the co-reduction of TiO2 with SiO2 by carbon at elevated temperatures has shown that the formation of TiSi is more probable than TiSi2. Phase equilibria in the Ti–Nb–Ge ternary system have been investigated.
Titanium(II). — Pulse radiolysis of aqueous solutions of titanium(III) containing formic acid affords TiIIvia reduction of TiIII by • CO2H. Density determinations of molten NaCl solutions (800 — 950 °C) of TiCl2 suggest that substantial amounts of [TiCl3] are formed in such media. The d–d spectrum of TiII ions isolated in NaCl crystals have been obtained, and absorption maxima identified at ca. 8000 and 15000 cm-1 and assigned to [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] transitions, respectively. The samples were prepared from CdCl2 and titanium metal in NaCl at 950 °C.
Titanium(II) complexes, ArTiCl2,Al2Cl6 (Ar = tetra-, penta-, or hexa-methylbenzene), may be prepared by the reduction of TiCl4 with metallic Al in the presence of AlCl3 in benzene containing the polymethylbenzene. These compounds, together with the analogous pentamethylbenzene derivative, may also be prepared by ligand exchange reactions from C6H6TiCl2,Al2Cl6. This study has provided further support for the π-interaction between titanium and the aromatic molecules suggested earlier.
New evidence has been presented indicating the participation of TiII in various dinitrogen-fixing systems, although other studies of these systems (p. 10) indicate that TiIII is also involved. An investigation of the systems [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] has led to the conclusion that the active species for dinitrogen absorption is probably titanocene, with perhaps more than one molecule of dinitrogen bonded to each titanocene dimer. The kinetics and stoicheiometry of dinitrogen fixation by TiCl3–Mg mixtures in THF solution have been reported. These systems react with N2 at 25 °C (1 atm) to form a species believed to be TiNMg2Cl2(THF)n and it is proposed that the mechanism involves the complexing of N2 with a dimeric TiII species, followed by a rate-determining reaction with metallic Mg. The reactions of transition-metal complexes with azo-compounds are also of interest in connection with dinitrogen fixation. [(π-Cp)2Ti(CO)2] reacts during two days at 25 °C with azobenzene to form black-maroon crystals of [(π-Cp)2Ti(Ph — N=N — Ph)] which are thermally stable, soluble in aromatic hydrocarbons but readily hydrolysed: structure (5) has been suggested.
[FORMULA NOT REPRODUCIBLE IN ASCII] (5)
Oxidative additions of alkyl and acyl halides to [(π-Cp)2Ti(CO)2] affording TiIV derivatives have been reported (p. 25). Titanocene has been shown to reduce a variety of organic molecules including alcohols, aldehydes, ketones, and organic halides.
Titanium(III). — Halides and Oxyhalides. Semi-empirical MO calculations on [TiF6]3- using a new parameter-free method have been published, and the calculated and experimental values of the ligand-field splitting, super-hyperfine coupling constants, and spin densities were in excellent agreement.
Density determinations of NaCl solutions (800 — 950°Q containing TiCl3 have led to the suggestion that substantial quantities of [TiCl4]- are formed under these conditions. The phase diagram for the TiCl3–NaCl–AlCl3system has been presented; Na3TiCl6 is the only compound formed. A thermal analysis of the TiCl3–VCl3–KCl system has been performed and the only compounds identified were K3MCl6 (M = Ti or V).
Treatment of TiBr4 with B2(NMe2)4 produces TiBr3,B2Br2(NMe2)2, which has been characterized by i.r. and electronic spectral and magnetic studies as a dinuclear species: structure (6) has been suggested. The compound reacts with HBr or NMe3 to form TiBr3,B2Br4(NMe2H)2 or TiBr3,2NMe3, respectively; pyrolysis affords B2Br2(NMe2)2.
[FORMULA NOT REPRODUCIBLE IN ASCII] (6)
O-Donor Ligands. YTiO3 has been prepared from Ti2O3 and Y2O3 and its X-ray diffraction characteristics have been reported. TiTaO4 has been obtained from the corresponding oxides by ceramic techniques under an inert atmosphere; X-ray and neutron diffraction and magnetic measurements indicate that the metal atoms are distributed statistically over the metal sites of the rutile structure. Treatment of an aqueous HCl solution of titanium(m) chloride with alkali affords a dark-brown precipitate of Ti2O3,nH2O, which is rapidly oxidized to TiO2,nH2O via a blue-black intermediate. The reflectance spectrum of the latter is very similar to that of the corresponding iron system and therefore the intermediate probably involves oxygen bridging between TiIII and TiIV centres.
