Transition Metal Component

The most widely studied transition metal is titanium. At various times, all oxidation states of titanium (II, III, IV) have been proposed for the active site of titanium-based initiators. Most of the evidence points to titanium (HI) as the most stereoselective oxidation state, although not necessarily the most active nor the only one [Chien et al., 1982]. (Data for vanadium systems indicate that trivalent vanadium sites are the syndioselective sites [Lehr, 1968].) Initiators based on the a-, y-, and 8-titanium trihalides are much more stereoselective (iso-selective) than those based on the tetrahalide or dihalide. By itself, TiCl2 is inactive as an initiator but is activated by ball milling due to disproportionation to TiCl3 and Ti [Werber et al., 1968]. The overall stereoselectivity is usually a-, y-, 8-TiCl , TiCL TiCLj P-TiCl3 [Natta et al., 1957b,c], 

Changes in the ligands of the transition metal component affect stereoselectivity [Natta et al., 1957a,b Rishina et al., 1976], For propene polymerization by titanium compounds in 

Changes in the transition metal itself have a large effect. Titanium compounds are the most isoselective, while vanadium compounds are the most syndioselective. 

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Most Ziegler-Natta components participate in a complex set of reactions involving alkylation and reduction of the transition-metal component by the group I—III component as shown below for TiCl4 + A1R3 ... 

The group I III metal component increases activity and stereoselectivity by alkylation (often together with reduction) (Eqs. 8-28 through 8-32) of the transition metal component to form more active and stereoselective reaction sites. The group I—III metal component may also be involved in stabilizing the active transition metal sites by complexation and may be... 

Methane dehydroaromatization on zeolites Mo/HZSM-5 was also investigated by solid-state MAS NMR spectroscopy 162. Both variation of the state of the transition metal component and products (such as ethane, benzene, and ethylene) adsorbed in zeolite were observed after reaction at high temperature (900-1000 K). Molybdenum carbide species, dispersed on the external surface or in the internal channels of the zeolite catalysts, had formed during the reaction 162. ... 

Both heterogeneous and homogeneous disproportionation catalysts are known. All contain a transition metal component with derivatives of Mo, W, and Re being the most important. Heterogeneous catalysts are generally metal oxides deposited on a support such as silica or alumina (1, 4). Homogeneous catalysts in general require a non-transition metal derivative as cocatalyst (2, 3). 

The transition-metal component of [Pt(bipy)2][TCNQ]3 is also diamagnetic. The structure86 consists of stacks of trimers, [T CNQ], surrounded by non-interacting Pt(bipy)2+ cations. Adjacent trimers are closely spaced with an interplanar separation of only 3.33 A as compared with 3.23 A within the trimer. The magnetic properties of this system have been studied much less extensively than those of [TMPD]2[Ni(mnt)2]. Single crystal EPR spectra do show, however, that spins are interacting along the TCNQ stacks. Structurally, this compound is very different from the 1 2 compound [Pt(bipy)2][TCNQ]258> (Sect. 2) which forms Da stacks with the two TCNQ moieties <7-bonded to each other.. ... 

Most of the compounds listed in this class (Table 7) are tetraalkylammonium (R4N+) salts of metal dithiolene complexes. These salts generally exist in a 2 1 or 1 1 stoichiometry. The 2 1 salts, such as [ra-Bu4N]2[Cu(mnt)2]114) and [Et4N]2-[Cu(mnt)2]115,116) can exhibit interesting properties, but they will not be considered here since the transition-metal components are isolated from each other (d(M-M) > 5 A). 

This treatment of coordination polymerization with Ziegler type catalysts is independent of whether the growing polymer end is attached to a Group I—III metal or to a transition metal. Both types can be obtained depending upon the alkylating ability of the alkyl metal with respect to the transition metal component. In both cases the ionic character of the M-R bond can vary appreciably with the cation electronegativity, i. e., with the metal, its oxidation state and ligands. The... 

In the coordinated anionic polymerizations with Group I—III metal alkyls alone, monomer coordination involves overlap of the olefinic jr-electrons with vacant sp3 hybrid orbitals. Since this interaction is very weak it is most effective with easily polarized monomers. In the coordination polymerizations with Ziegler type catalysts, stronger monomer coordination is obtained by overlap of jr-electrons with vacant -orbitals of the transition metal component. The complexes have structures of the type proposed by Dewar (199b) and by Chatt and Duncanson (200) and applied to Ziegler type catalysts by Cossee (201) (Fig. 6). The olefin yr-electrons overlap with the orbital of... 

Polymerization activity was obtained with a variety of catalyst compositions. The best stereospecific catalyst was the split pretreated type (357) in which one mole of VC14 was reduced by a stoichiometric amount of an alkyl metal (0.34 mole AlEt3) in heptane at room temperature and heated 16 hours at 90° C. to obtain the purple crystalline VC13-1/3 A1C13. This reduced transition metal component was then treated with two moles of (i-Bu)3Al tetrahydrofuran complex for 20 hours at room temperature to obtain a chocolate-brown catalyst consisting predominantly of divalent vanadium with 0.21 Al/V and 1.4 i-Bu/Al. Polymerizations at 30° C. gave crystalline polymers from methyl, ethyl, isopropyl, isobutyl, tert.-butyl, and neopentyl vinyl ethers. 

At the present time, the most likely concept of the mechanism of a heterogeneous polymerization catalyzed by a Ziegler-Natta catalyst involves a complex in which the organometallic component and the transition metal component—i.e., the A1 and Ti atoms—are joined by electron-deficient bonds. Natta, Corradini, and Bassi (13) have reported such a structure for the active catalyst prepared from bis (cyclopentadienyl) titanium dichloride and aluminum triethyl. Natta and Pasquon (14), Patat and Sinn (18), and Furukawa and Tsuruta (2) have proposed mechanisms for the stereospecific polymerization of a-olefins in terms of such electron-deficient complexes. 

With the system TiCl3/AlEt3, the rate of isomerization of cA-2-butene to 1-butene can be increased by the incorporation of a late transition metal component such as NiC. 435 Isomerization polymerization using 2-butene has also been shown to be useful for the synthesis of co-polymers of 1-butene with ct-carbon-branched 1-alkenes having low polymerization activity.436 The difference in reactivity between the branched monomer and 1-butene is compensated for by the low concentration of 1-butene formed by isomerization from 2-butene. 

Unreducible ions of the transition metal component of supported catalysts, which survive a severe reduction, are clear evidence of a strong interaction between the support and the metallic element. Currently there is a discussion in the literature on this subject, specifically 1) Do some Pt-ions survive a severe reduction 2) Are they accessible 3) Do they play a role in the metal-support interaction and reactions of hydrocarbons ... 

The studies of n-heptane and methylcyclopentane conversion provide insight into the advantages of platinum-iridium and platinum-rhenium catalysts over catalysts containing only one of the transition metal components, that is, platinum, iridium, or rhenium. If, for example, we consider an iridium-alumina catalyst for the reforming of a petroleum naphtha fraction, we find that it produces a substantially higher octane number reformate than a platinum on alumina catalyst under normal reforming conditions. The iridium-alumina catalyst will also exhibit a lower rate of formation of carbonaceous residues on the surface, with the result that the maintenance of activity with time will be much superior to that of a platinum-alumina catalyst. 

The magnetic properties were studied in the temperature range of 2-1100 K (Hiebl et al. 1987). The compounds in which the transition-metal component is from the same column of the Periodic Table display a similar behaviour. A temperature-independent exchange-enhanced susceptibility was observed for T = Fe, Ru, and Os. In the case of Co, Rh, and Ir, the molar susceptibility is somewhat lower, but a broad maximum in the temperature dependence of x around 600 K, observed in all three compounds, is reminiscent of spin fluctuators. Antiferromagnetic ordering, indicated for T = Ni, Pd, and Pt by sharp cusps in the x versus T curves, was confirmed by the observation of linear magnetization curves at low temperatures. MCW behaviour is found above the antiferromagnetic transition. 

The alcoholic dehydrogenation reaction requires a transition metal component unless it is possible to operate above 470 K, where moderately basic catalysts also become active for this reaction. An example catalyst would be a MgAlOx hydrotalcite of low A1 content. These catalysts can activate H2/D2 exchange at these temperatures. The alcohol adsorbs dissociatively to surface alkoxide and hydroxy, a step requiring proximity of a basic and a Lewis acid site. There follows a hydride abstraction, mediated by another Lewis acid group, and heterolytic association to give H2. 

Throughout this chapter we have tried to cover the different methods described in the literature to prepare chiral supramolecular assemblies. In the first part of the chapter we described the use of achiral bridging ligands of the right match that bind to the transition metal components of different geometry to give chiral supramolecular architectures where the chirality arises only from the asymmetric disposition of these achiral constituents in three-dimensional space. Many examples were reported and this is attributed to the simple access of the reactants where there is no need to prepare beforehand the optically pure constituents. 

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