Diolefin Coordination Compounds

Polymerization of olefins such as styrene is promoted by acid or base or sodium catalysts, and polyethylene is made with homogeneous peroxides. Condensation polymerization is catalyzed by acid-type catalysts such as metal oxides and sulfonic acids. Addition polymerization is used mainly for olefins, diolefins, and some carbonyl compounds. For these processes, initiators are coordination compounds such as Ziegler-type catalysts, of which halides of transition metals Ti, V, Mo, and W are important examples. 

Addition polymerization is employed primarily with substituted or unsuhstituted olefins and conjugated diolefins. Addition polymerization initiators are free radicals, anions, cations, and coordination compounds. In addition polymerization, a chain grows simply hy adding monomer molecules to a propagating chain. The first step is to add a free radical, a cationic or an anionic initiator (I ) to the monomer. For example, in ethylene polymerization (with a special catalyst), the chain grows hy attaching the ethylene units one after another until the polymer terminates. This type of addition produces a linear polymer ... 

Neutral and anionic organoimidoimido clusters of rhodium)I) with diolefin and carbonyl ligands are accessible and stable. The diolefin neutral compounds [Rh4(/i-N-/7-tolyl)2(diolefin)4] are a convenient entry into this chemistry for which the synthetic route is an apparently simple replacement reaction, but the reproducibility of the results requires a careful technique. These tetranuclear compounds contain a trimetallic triangular cluster face capped on both sides by two pnra-toly-limido ligands and a further rhodium fragment coordinated to a para-to y ring in a 7 -fashion, giving an overall valence electron count of 64e. 

The stability of Ag(I) complexes with cyclic olefins (Table 6.11) decreases according to the series C5>C7>C6>C8, which is caused almost exclusively by enthalpy changes the entropy changes are almost the same for all olefins. The stability of Ag(I) coordination compounds with diolefins of the type CH2CH(CH2) CHCH2 is maximum for = 2. Therefore, the chain containing six carbon atoms is probably optimal for the formation of a chelate with silver. 

The importance of the electrophilic character of the cation in organo-alkali compounds has been discussed by Morton (793,194) for a variety of reactions. Roha (195) reviewed the polymerization of diolefins with emphasis on the electrophilic metal component of the catalyst. In essence, this review willattempt to treat coordination polymerization with a wide variety of organometallic catalysts in a similar manner irrespective of the initiation and propagation mechanisms. The discussion will be restricted to the polymerization of olefins, vinyl monomers and diolefins, although it is evident that coordinated anionic and cationic mechanisms apply equally well to alkyl metal catalyzed polymerizations of polar monomers such as aldehydes and ketones. 

However, the well-known ability of organolithium compounds to form associated species or to form complexes with electron donor compounds (240—242) provides strong support for mechanisms involving cationic attack by the lithium cation on the monomer prior to an anionic addition. With three orbitals available for coordination, a monomeric lithium alkyl should be able to complex both double bonds of a diolefin to provide the orientation for making cis-1,4 polymer and still have an orbital available for forming associated species in hydrocarbon solvents. The lithium orbitals are presumed to be directed tetrahedrally. Looking at the top of a tetrahedron with the fourth lithium oibital above and normal to the plane of the paper, the complex could have structure A below. In the transition state B for the addition step, the structure... 

The chemistry of the zerovalent state in nickel, palladium, and platinum compounds is reviewed. After a historical introduction in which the development of this chemistry is analyzed in terms of the current theory of the stabilization of low valency states, the most interesting classes of zerovalent compounds are described. The stability and properties of these compounds are discussed and related to the nature of the ligands and the coordinated metal. The catalytic properties of these zerovalent derivatives toward olefins, diolefins, and acetylenes are considered in connection with the facility of ligands exchange, the variation of coordination number, and the stereochemistry. A discussion of the % bond is reported. 

For Ziegler-type catalysts based on Ti there are examples of monodentate trans- or bidentate cw-coordination of the diolefin to the Ti which, respectively, gives a trans- or cw-configuration in the polymer ". The halide y-TiCl3, which offers only one coordination site at the active center, promotes formation of frans-1,4-polymers in contrast )8-TiCl3, which provides more sites, favors formation of a mixture of homo-ci5 and homo-trans polymers . Butadiene, isoprene, and 2,3-dimethylbutadiene can be treated in the same manner. Rare earth catalysts, e.g., Nd compounds, are also used. ... 

Also included in Table 1 are known structures that involve diolefins. Complexes 3 and 4 both have coordinated butadiene. Compound 4 is rather poorly defined the butadiene ligand appears to he coordinated the hydride ligand was not located. Another diolefin, 1,1-dimethylallene, is coordinated to a PtHP2 center in compound 5. The H ligand, though not located, is presumed to be trans to the allene, which is coordinated to the Pt center through the C(2)-C(3) bond. 

Under the reaction conditions the starting complexes react with CO with displacement of coordinated diolefin to form iridium carbonyl compounds, which are the active catalytic species for the WGSR. The catalytic activity slightly increases when bidentate ligands are used and appears to be rather sensitive to temperature. 

In all compounds, the Pd atom would be linearly coordinated by two olefinic bonds. At lower charges of Pd, a onedimensional polymer CsoPdi is formed, with each Cso acting as a diolefin toward two different Pd atoms. As Cso Pd ratio is decreased (more Pd), new olefin bonds of Cso are used and the newly incorporated Pd atoms create links between polymer chains. The limit ratio for a three-dimensional polymer is CsoPda, with each Ceo ball surrounded by six Pd atoms in an octahedral arrangement (as in C6o (Pd(PR3)2)6 )- The CeoPd products aggregate and precipitate out... 

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