Vanadium halides

Vanadium Halides and Oxyhalides. Known haUdes and oxyhahdes of vanadium, their valences, and their colors are Hsted in Table 3. 

A dicyclohexylphosphide (Cy2P) complex, [Li(DME)][V(Cy2P)4] (DME = 1,2-dimethoxyethane) was isolated by the addition of a THF solution of LiCy2P to a suspension of a vanadium halide THF adduct at — 80 °C, addition of DME and cooling to -30°C.449 The detailed structure was not determined oxidation with [Cp2Fe](BF4) gives the thermally unstable [V(Cy2P)4]. 

The vanadium tetrachloride distils over and is purified from any ferric chloride present either by distillation or by extraction of the product with carbon tetrachloride, in which only the vanadium halide is soluble.2 Sulphuryl chloride, thionyl chloride, sulphur monochloride, and phosgene can all be used in the last reaction instead of chlorine, and the ferrovanadium also can be substituted by vanadium carbide, V4C3,3 nitride, VN,4 subsilicide, V2Si,5 disilicide, VSi2,8 or pentoxide.7... 

Soluble catalysts denved from oiganoaluminum compounds and vanadium halides promote formation of atactic elastomeric propylene polymers and copolymers such as ethylene-propylenediene terpolynier rubbers (EPDM). Amorphous adhesive propylene homopolymers have some commercial use. Syndiotaclic or DLDL propylene polymers have been reported but their structures and properties have not been completely established. 

Vanadium forms halides in the +2, +3, +4, and +5 oxidation states. For the +2 and +3 states, all of the halides are known, but only the fluoride is well characterized for the +5 state. Both VC14 and VBr4 are rather rare compounds. Some of the vanadium halides can be prepared by the direct reaction of the elements as illustrated by the reaction... 

The maximum oxidation state of vanadium is V, but apart from this there is little similarity, other than in some of the stoichiometry, to the chemistry of elements of the P group. The chemistry of Viv is dominated by the formation of oxo species, and a wide range of compounds with V02+ groups is known. There are four well-defined cationic species, [Vn(H20)6]2+, [VU1(H20)6]3+, Viv02+(aq), and Vv02+(aq), and none of these disproportionates because the ions become better oxidants as the oxidation state increases both V11 and Vm ions are oxidized by air. As with Ti, and in common with other transition elements, the vanadium halides and oxohalides... 

The vanadium halides provide an example. The liquid tetrachloride VCl can be synthesized from vanadium and excess elemental chlorine at 500 °C. However, on heating in the absence of chlorine, VCl loses chlorine to form VCI3, which in turn loses chlorine on further heating to give VCl., which is stable at its melting point of 1350 °C (equations 3.7-3.9). 

Fig. 9. Propene polymerization by aluminium triethyl and vanadium halides [30], (a) VCl3/AlEt3 AlEt3 = 0.019 mole 1 , Al/V = 3 propene pressure, 1 atm temp., 60°C. (b) VCls/AlEtjCl AlEtjCl = 0.064 mole Al/V = 4 [M] = 1.785 mole 72-heptane solvent.

Titanium-based TiC /AlRs (R = Me, Et, i-Bu) catalysts produce PI with high trans contents (>85%). However, the most active and stereoselective systems for the synthesis of trans-, A-polyisoprene are those based on vanadium compounds. Vanadium halides and mixtures of vanadium and titanium halides, in combination with aluminum alkyls, give high trans-1,4-polyisoprene ( 90%).i ... 

Scheme 5.3 Synthesis of vanadium-halide derivatives and thdr reactivities.

Natta and Ziegler found early on that vanadium halides could be treated with aluminum alkyls to form catalysts competent for alkene polymerizations. A variety of simple precursors in various oxidation states, for example, VCI3, VCI4, and VOCI3. can be treated with aluminum alkyl halides to produce active catalysts. Unlike the titanium and chromium systems, the vanadium catalysts can be single sited with narrow MWDs and more importantly narrow composition distributions. Also unlike the other systems, vanadium incorporates a-olefms at rates somewhat slower but comparable to ethylene. For this reason, vanadium catalysts are used commercially to make EPMs and EPDMs that can have at least 50 mol.% ethylene with the balance being predominantly other olefins and much smaller amounts of diolefins (Figure 9). 

The most widely used transition metal salts have been the relatively cheap titanium or vanadium halides, reduced with aluminum alkyls. In Ziegler s work, titanium tetrachloride was reduced to brown yS-titanium trichloride, which was able to polymerize ethylene. However, when yS-titanium trichloride catalyst was used in the polymerization of propylene, the product contained a high proportion of the gum like atactic polymer, which was not viable for commercial use. In contrast, Natta, in his work on the polymerization of propylene and other a-olefins, showed that violet a-titanium trichloride could polymerize propylene to the useful, crystalline, isotactic polymer. Nevertheless, a relatively large quantity of atactic polypropylene still had to be separated from the commercial isotactic product. 

VX2-6ROH [T148, T149] X = Br, R = Et X = I, R = Me Dehydration of hydrated vanadium halides by TEOF or TMOF. ROH molecules may be coordinated. 

---