Palladium diimine complexes

Highly branched ethene-methyl acrylate polymers. The cationic palladium diimine complexes are remarkably tolerant towards functional groups, although the rates decrease somewhat when polar molecules are added. In ETM catalysis addition of polar molecules or monomers kills the catalyst and therefore it was very interesting to see what the new palladium catalysts would do in the presence of polar monomers. Indeed, using methyl acrylate a copolymerisation... 

Single site catalysts, such as metallocene compounds, CGCs, and nickel or palladium diimine complexes, used in combination with MAO or borate cocatalysts, are highly active for the homopolymerization of norbornene and its copolymerization with ethylene. The structure of the norbornene homo- and copolymers can be widely influenced by the symmetry and structure of the ligands on the transition metal complexes. 

McLain et polymerized cyclopentene by late transition metal catalysts using MAO and borate-activated nickel and palladium diimine complexes. The nickel diimine complexes produce crystalline materials showing a ds-1,3 enchainment with a melting point of 240-330 °C. The hydroligomers were mainly atactic. Palladium catalysts gave pure atactic polymers. It is also possible to polymerize substituted cyclopentenes such as 3-methyl- or 3-ethyl-cyclopentene. 

Several combinatorial approaches to the discovery of transition metal based catalysts for olefin polymerization have been described. In one study Brookhart-type polymer-bound Ni- and Pd-(l,2-diimine) complexes were prepared and used in ethylene polymerization (Scheme 3).60,61 A resin-bound diketone was condensed with 48 commercially available aminoarenes having different steric properties. The library was then split into 48 nickel and 48 palladium complexes by reaction with [NiBr2(dme)] and [PdClMe(COD)], respectively, all 96 pre-catalysts being spatially addressable. 

Figure 4-26. Unconstrained MD simulations for methyl acrylate bound to the palladium/nickel diimine complex through the oxygen (top-right/bottom-right) and the C=C (top-left/bottom-left) functionalities. The three panels in each of the four graphs represent variations in the metal-carbon (top two panels) and metal-oxygen (bottom panel) distances. The simulations were carried out at 300 K for all the systems, and 700 K for the local minima, as indicated...

Shultz LH, Brookhart M, Measurement of the Barrier to -Hydride Elimination in a -Agostic Palladium-Ethyl Complex A Model for the Energetics of Chain-Walking in (-Diimine)PdR+ Olefin Polymerization Catalysts, Organometallics, 20, 3975-3982 (2001)... 

Burmeister and colleagues have described the related pseudohalogen derivatives MfterpyjXj (X = SCN or SeCN) (90-92). The platinum compound exhibits the two thiocyanate stretching frequencies expected for a square-planar complex, and is formulated [Pt(terpy)(NCS)][NCS], However, the palladium complexes are less easily formulated, exhibiting absorptions due to coordinated ECN (E = S or Se) only. These observations were interpreted in terms of a square-planar structure, with a bidentate terpy ligand in view of the known ability for palladium and platinum diimine complexes to form five-coordinate species, this formulation must also be considered. In the absence of definitive structural evidence, the formulation as five-coordinate species must be regarded as speculative. 

Yet another era of organometallic polymer chemistry appears to arise from new cationic nickel(II)- and palladium(II) complexes 41 of Al,A-chelate (diimine) ligands (cf. eq. (19)). According to Brookhardt et al., both the homo- and copolymerization of a-olefins proceed with activities that compare in the case of... 

Following the pioneering studies of Keim [19] and Fink [20] in the 1970s and 1980s, mainly Brookhart and coworkers have reported on the development of palladium(II) and nickel(ll) diimine complexes (Fig. 2.4, F, G) that polymerize ethene to high molecular weight polymers that have a branched microstructure [21]. 

The palladium diimine catalysts can be used to prepare polymers of a-olefins as well as cyclic internal olefins. For a-olefins, the rate of polymerization is slower, compared to ethene, but high molecular weigh polymers are formed. Chain walking takes place with a-olefins, but the a-olefin does not insert into secondary methylene carbons. Copolymers of a-olefins and ethene give polymers with complex microstructures since ethene will insert into secondary... 

Brookhart and co-workers recently reported tantalizing results that were close to constituting true copolymerizations of ethylene and methyl acrylate. ° ° The catalyst employed was the palladium version of the diimine complexes that were previously reported for ethylene and a-olefin homopolymerizations (complexes IV). °° The close qualification... 

Syntheses of discrete cationic a-diimine complexes of palladium and nickel are well developed. DAB complexes containing halo ligands can be reacted with a variety of salts of noncoordinating anions to yield cationic organometallic species. For instance, the reaction in eq 2 illustrates chloride abstraction with NaBAF. " The resulting complexes are... 

Most mechanistic and theoretical work published to date has been directed at the a-diimine complexes of nickel and palladium. For that reason, mechanistic work is discussed at this point in the review. While details will differ for the particular metal and ligand set, all of the pertinent details are covered with these two systems. The details then can be drawn upon where appropriate for other systems. 

The cationic palladium a-diimine complexes are remarkably functional-group tolerant. Ethylene polymerizations can be carried out in the presence of ethers, organic esters, and acids, but nitriles tend to inhibit polymerizations. In addition, polymerizations have been carried out in the presence of air and in the presence of an aqueous phase.Aqueous emulsion and suspension polymerizations using these catalysts have been developed as a route to microspheres of polymer for adhesives as well as for other applications.2 ° 2 Preparation of elastomers is often complicated by difficult solvent removal, so polymerizations in supercritical CO2 have been investigated. It is also possible to combine the activity of the palladium catalysts with other polymerization techniques such as living-free-radical polymerizations. One interesting observation is that the... 

The active species for the PBI complexes is not as well characterized as in the nickel and palladium systems. It is assumed to be a cationic alkyl complex formed by reaction of the dihalo precatalyst with a cocatalyst such as methylaluminoxane (MAO). The resulting active species polymerizes ethylene at unusually high rates to form linear high-density polyethylene. Even at ethylene pressures as low as 1 atm, the polymerization is extremely exothermic and the crystalline polymer product rapidly precipitates from solution. Computational chemistry is proving to be of utility in understanding the mechanistic aspects of this chemistry. - Lower barriers to insertion, relative to the nickel a-diimine complexes, support the higher activity. 

Unlike the platinum analogs, direct observation of palladium(IV) alkyls has been limited to those with nitrogen-based ligands such as 2,2 -bipyridine and a-diimines [49]. However, reductive elimination from palladium(IV) complexes has... 

In addition to nickel (71a-c), Suzuki et al. have explored a-diimine complexes of palladium (71d,e, Figure 9.9) for the polymerization of 1-hexene at very high pressures (up to 750MPa) (Suzuki et al, 2003). While nickel catalysts displayed non-living behavior, the palladium catalysts 71d,e were living for 1-hexene polymerization and polydispersities decreased at higher pressures (MJMn= 1.27-1.29 at 0.1 MPa vs. 1.11-1.17 at 500MPa with 71e). 

Merna, J., Cihlar, J., Kucera, M. etal. (2005) Polymerization ofhex-l-ene initiated by diimine complexes of nickel and palladium. European Polymer Journal, 41,303—312. 

Schmid, M., Eberhardt, R., Klinga, M et aL (2001) New C y- and chiral C2-symmetric olefin polymerization catalysts based on nickel(II) and palladium(II) diimine complexes bearing 2,6-diphenyl aniline, moieties Synthesis, structural characterization, and first insight into polymerization poperties. Organometallics, 20,2321 2330. 

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