Brookhart systems

A comparison of Ni- and Pd-based Brookhart systems leads to the conclusion that the analysis of the polar comonomer binding mode can be used as a screening test to select the best prospective catalytic candidates for the copolymerization the complexes with preference of the 0-complexes can be excluded from further studies. Here, the use of theoretical methods can be very useful. Obviously, the monomer binding is only an initial step in the complex mechanism of the copolymerization processes. Therefore, the studies on the polar comonomer binding mode can be used only for a negative selection. To get more definitive answers about the catalyst activities, a full mechanistic study is needed, involving evaluation of the barriers of all the elementary reactions and the relative stabilities of the reaction intermediates. 

The metal-N bonds in the Grubbs system (3a) are much weaker than in the Brookhart system stabihzation energies not larger than -23.2 kcal/mol (methylamine) have been calculated for the N complexes with the Grubbs catalyst. 

However, compared to the early transition metal-based olefin catalysts, the research on late transition metal catalysts has just begun. Moreover, catalysts based on transition metals are rarely used for polymerizations in SCCO2 and certainly not for olefin polymerization. In this study, we have used a homogeneous diimine palladium complex, also known as the Brookhart system (see Fig. 8.4) [14, 15]. The catalyst has been synthesized according to literature procedures [14, 15] for more details see De Vries [9]. 

Further simulations have been performed. In contrast to what was observed for bis-cyclopentadienyl metallocenes, mono-cyclopentadienyl systems did reveal a significant barrier to insertion [lOj. However, for all these systems it turned out that insertion only proceeded after the formation of a relatively stable agostic interaction, an observation that clearly supports the Brookhart-Green mechanism. 

The Brookhart laboratory has contributed much of the knowledge of the polymerization mechanism for the late transition metal a-diimine catalysts. The review by Ittel provides a concise summary of the mechanistic understanding as of the year 2000 [26]. Some of the early findings will be reviewed here and additional insights reported afterward will be presented. In addition to the experimental work, many theoretical and computational studies worthy of discussion have also been carried out. These efforts have been most important in providing insight into the mechanistic details of the highly reactive nickel system, which is often difficult to study experimentally. 

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],... 

It is worth to note that the calculated insertion barrier [13b] of 13.2 kcal/mol recently has been confirmed by Brookhart [16] et al with an experimental estimate of 12.0 kcal/mol. A similar good agreement betweeen theory and experiment has also been obtained for the palladium [13f] system. 

In the case where one of the elements M is a transition metal, then M—H—M systems are known where M can be B, Al, Si or Zn as well as carbon. BH complexes are an extensive class the ligand can bind via one, two or three H bridges (16)-(18). Compounds containing C—H—M bridges are of particular importance and have been reviewed by Brookhart and Green.29 No pure cluster hydride has yet been observed, but there is no reason to think that these should not be stable. 

A nickel-based catalyst system, which produces, in the absence of comonomers, highly short-chain branched polyethene was developed by Brookhart et al. [23]. Independently, the groups of Brookhart [24, 25, 26] and Gibson [27, 28, 29, 30] developed efficient iron- and cobalt-based catalyst systems. Nickel or palladium is typically sandwiched between two a-di-imine ligands, while iron and cobalt are tridentate complexed with imino and pyridyl ligands. 

One example of minor variation in the catalytic cycle above is shown by Brookhart and co-workers, who performed a low temperature NMR study of their catalyst [Cp(CO)(PR.3)Fe+] in the alcoholysis of triethylsilane with ethanol and phenol.15 They observed catalyst deactivation during the reaction and upon evaluation of the system using low temperature NMR, they discovered that the ethanol disrupts the catalytic cycle in two ways (Figure 6). Ethanol... 

We proposed a mechanistic outline (Figure 10) and carried out several reactions in order to get an insight into the mechanism. We have demonstrated that the silane and alcohol are involved in the rate-determining step for the 10 % Pd/C catalytic system. It was documented with Brookhart s iron catalyst that the rate determining step for that system was the dissociation of molecular hydrogen from the metal assisted by the silane.15 If the rate determining step is the same for our manganese catalyst as Brookhart s catalyst, then the concentration and lifetime of... 

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