Brookhart-type catalysts

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. 

Again, calculations by Ziegler [40] on barriers for the 1,2- and 2,1-insertion of methyl acrylate into Brookhart-type diimine complexes show that for sterically unhindered catalysts 2,1-insertion is strongly favored, with the transition state for... 

Diastereoisomeric transition states calculated for propene insertion in a model for a Brookhart-type Ni(II) catalyst, based on diacetylbis(2,6-diisopro-pylphenylimine)nickel derivative,143,144 are shown in Figure 1.21. Diastereomeric transition states for si (Figure 1.21a) and re (Figures 1.21b,c) monomer insertions into a si chain correspond to like (isotactic) and unlike (syndiotactic) propagations, respectively.144,143... 

Figure 1.21 Minimum-energy transition states for primary propene insertion on Brookhart-type Ni(II) catalyst for the model including si growing chain (whose last monomeric unit was generated by insertion of -coordinated monomer), (a) There is only minimum-energy transition state for insertion of, v(-propene (like insertion) while (b,c) there are two nearly energetically equivalent minimum-energy transition states for insertion of re-propene (unlike insertion).

Finally, Hinderling and Chen used ESMS to screen the activity of a small library of eight Brookhart-type palladium(II) complexes in the solution-phase polymerization of ethylene [62]. The crude reaction mixture was quenched with DMSO, diluted, and electrosprayed in order to analyze the growing polymers chains. Upon CID in the gas phase, the polymer chain was fragmented from the catalyst by /J-hydride elimination, thus facilitating the identification of the most active catalysts in an otherwise dauntingly complex mass spectrum of a polymer mixture. Since this analysis can be performed simultaneously for a whole catalyst library, ESMS was hereby proven the method of choice for an assay of multiple, competitive and simultaneously occurring catalytic reactions. 

Acyclic internal olefins such as cis- and /raar-2-butene can be polymerized using Brookhart-type Ni(ll) and Pd(ll) complexes430 but in the absence of isomerization do not homopolymerize using Ziegler-Natta catalysts, although examples of co-polymerizations of ethylene with an internal olefin are known.313... 

In 1999, an attempt was made to synthesize a polyolefin in clay gallery. Specifically, clay was ion-exchanged using tetradecylammonium ions, and this ion-exchanged clay and a paUadium-based complex of the Brookhart-type were mixed and conditioned in toluene to polymerize ethylene. The interlayer distance was initially 1.99 nm. This increased to 2.76 nm after a palladium catalyst was added. It was confirmed that the X-ray peak disappeared 24 hours after ethylene was introduced [40]. 

Brookhart and coworkers [1] have recently developed Ni(II) and Pd(II) bis-imine based catalysts of the type (ArN=C(R)-C(R)=NAr)M-CH3+ (la of Figure 1) that are promising alternatives to both Ziegler-Natta systems and metallocene catalysts for olefin polymerization. Traditionally, such late metal catalysts are found to produce dimers or extremely low molecular weight oligomers due to the favorability of the P-elimination chain termination process [2],... 

In 1998, Grubbs and co-workers [83, 84] reported on a new type of neutral Ni(II) complexes with salicylaldimin ligands (32). With these catalysts low-branched polyethylenes were obtained with a narrow molecular weight distribution. The copolymerization of ethylene and norbomene is possible. Fe and Co catalysts were used for the linear polymerization of ethylene by Gibson [85] and Brookhart [86] independently (33). Activities of 10 TONs were reported. The polyethylenes obtained are highly crystalline with a broad molecular weight distribution. 

The unique structure of the ethylene homopolymers obtained results from a propensity of the metal centers to run along the growing polymer chain between insertions (a similar behavior had been observed previously for 1-olefm polymerization by a neutral nickel catalyst by Fink et al.) [58, 59]. An extensive patent on fhese polymerizations was filed by Brookhart and DuPont, and McLain demonstrated in several examples that fhese polymerizations can also be carried out in water [60]. Detailed investigations by Mecking et al. revealed that in fhis suspension-type aqueous polymerization fhe catalyst is remarkably stable, efhylene being polymerized at a steady rate for several days [61, 65]. At slightly elevated ethylene pressures of 20 bar, with 5 a activities of 10 TO h , similar to... 

Scheme 3.12 Brookhart/Gibson-type BIP polymerization catalyst...

Building off of seminal work on the development of iridium complexes for dehydrogenation - hydrogenation catalysts, an improved, homogeneous, dual-catalytic system for alkane metathesis has recently been discovered [101, 102]. Goldman and Brookhart found that a combination of an Ir-pincer-based catalyst with Schrock-type, Mo- or W-alkylidene complexes (as the olefin metathesis catalyst) transformed alkanes into lower and higher new alkanes (Figure 2.12). 

Iron- and cobalt-based catalysts " " have been reported to be active for the polymerization of norbornene, especially when activated with MAO. Similar to the case of many nickel and palladium catalysts, PNBs generated with simple salts such as cobalt neodecanoate are similar to PNBs prepared with ligand-bearing catalysts (e.g., Brookhart-Gibson type pyridyldiimine cobalt catalysts). 

The next logical step was the employment of another electron donating atom in the ligand structure in order to obtain a trigonal-bipyramidal coordination sphere. Gibson and Brookhart both succeeded with a catalyst system based on an iron-bisiminopyridyl complex. The structures (28)-(30) illustrate the three types of catalysts [169,170]. 

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