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 ≤...