Energy barriers, ethylene insertion

The results of the MD simulations clearly demonstrate that the insertion starting from the higher energy isomers of the ethylene-chelate complexes in which the chelating bond has been broken have much smaller activation barriers, that are comparable to those observed in ethylene homopolymerization. This, however, does not explain the differences in the copolymerization activity of Pd and Ni-diimine complexes, as the barriers for the ethylene insertion into Ni-alkyl bond are smaller (14.2 kcal/mol) than those for Pd-alkyl bond (16.8 kcal/mol). Thus, it may be concluded that the ethylene insertion following the insertion of the polar monomer is not a crucial factor for the diimine catalyst copolymerization activity. It is the initial poisoning of the catalyst by formation of the... 

The alternating copolymerization of CO and ethylene has been studied in much detail over the past two decades. Although a number of Ni-based systems are known to catalyze this reaction,comparative studies have shown that Pd-based systems are by far the most active catalysts for this reaction. The lower activity of the Ni-based systems is somewhat surprising because the requisite CO and ethylene insertion steps are expected to be energetically more facile for the Ni-based systems. A recent study has shown that the energy barrier for CO insertion into the Ni-Me bond is less than 10 kcal mol" (Scheme 25), significantly lower... 

The potential energy profile is smooth without excessive barriers and too stable intermediates which would break the sequence of steps. The rate-determining step is found to be olefin insertion followed by isomerization, supporting the Halpern mechanism. Isomerization of the ethyl hydride complex is an important part of the rate-determining step. These two reactions, exothermic overall, has an overall barrier height of about 20 kcal/mol. The trans ethyl hydride complex, the product of ethylene insertion, may not be a local minimum (per MP2 calculation) and these two steps may well be a combined single step. 

We have studied the energetics of the ethylene insertion reactions in a previous paper [3]. The energy change from the 7t-complex 2a to the direct product of insertion 2b was found to be 10 7 kcal/mol, indicating the chain growing reactions are thermodynamically favorable. However, the kinetic feature of the insertion, i.e. the transition state structure and the reaction energy barrier have not been discussed. 

Compared to the other reaction pathways. Scheme 6 illustrates the most plausible mechanism for chain termination. The reaction barrier of 8.3 kcal/mol is higher than the insertion barrier of 5.7 kcal/mol. Keim and co-workers have successfully trapped the nickel hydride as evidence to support their catalytic mechanism in which the nickel hydride is considered as the active catalyst [15]. We have found in the present study that the nickel hydride actually exists as an intermediate of the chain termination process. The premise for Scheme 6 to be practically competitive to the ethylene insertion reactions is the formation of the 7t-complex 4c. Based on our calculations, 4c is only a shallow minimum with the stabilization energy of 0.2 kcal/mol. Higher ethylene concentration is thus expected to facilitate the formation of the jt-complex and hence to increase the possibility of chain termination in order to generate dimers and trimers. 

An investigation of the oxidative addition of ChUand CD4[57a], as well as ethane [57b], to a bare palladium atom has demonstrated that quantum tunneling plays a very important role in the process. The barrier of insertion of different transition metal atoms into a C-C bond has been found to be 14-20 kcal mol higher than the barrier for insertion into a C-H bond [57c], Calculations for the activation of the C-H bond in ethylene by second row transition metal atoms showed that the oxidative addition barrier is lowest for the atoms to the right (for rhodium there is no barrier and for palladium the barrier is almost zero) [57d], The activation energy for B2 insertion into methane has been predicted to be 4.1 kcal mol while this value increases to 16.2 kcal mol for insertion of B [58], Two mechanisms have been considered by the SCF CNDO/S method for the oxidative addition of methane to the palladium cluster Pd2 [59a], In the first possible reaction, the C-H bond oxidatively adds to different palladium atoms ... 

Shevlin examined the reaction of carbon atoms with ethylene oxide by the MINDO/3 formalism and found no energy barrier for direct removal of the oxygen atom (76). It was also concluded that if insertion of carbon into the C—O had occurred, other products would have been formed. 

The subsequent insertion of ethylene into the Ni-C bond transforms Ilia into the pentyl complex IVa via the insertion transition state TS[IIIa-Va] of Fig. 6. However it is interesting to note that the barrier of insertion has been reduced from 17.5 kcal/mol for the generic system (TS[3a-4a]) to 13.2 kcal/mol for the real system [13b] (TS[IIIa-IVa]). The reduction is not so much due to a change in the relative energies of the transition states since both the monomer and the a-carbon of the growing chain are sitnated in the N-Ni-N coordination plane (Fig. 6) where they are relatively unencumbered by the bulky phenyl groups. Instead it is the steric destabilization of the Ilia resting-state... 

Comparing the systems with isomeric alkyls, the olefin insertion barriers increase from the systems with primary alkyl to those with tertiary alkyl [13c]. As a result, the insertion from the tertiary n-complexes practically does not happen. In the case of the real Pd-diimine systems the TS for the secondary ethylene insertion has slightly higher energy than the primary TS. The computed value of 0.5 kcal/mol is shghtly lower then the experimental result (1 kcal/mol. Fig. 9). 

The Ni(II) based CO/ethylene co-polymerization has been studied computationally by Morokuma et al. [45b] with the bidentate nitrogen phen ligand of Scheme 4. The calculated enthalpies for complexation of ethylene and CO to methyl and acyl complexes of Ni(ll) and Pd(ll) are shown in Table 2. The same table displays in addition energy barrier for the insertion of ethylene and CO into the metal-methyl and metal-acyl bonds. 

Figure 17 presents the energy profiles for the methyl acrylate insertion into the Pd-alkyl and Ni-alkyl bond in the generic diimine systems [31,32]. The results clearly indicate that it is not the insertion barrier which makes the Ni-system inactive in polar copolymerization. The acrylate insertion barrier is substantially lower for the Ni-catalyst then for the Pd-complex. This, in fact, should not be surprising, as the ethylene insertion barriers are also lower for the Ni-system. 

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