Insertion barriers

In studies of vibrationally excited hydrocarbons with transition metal atoms to be carried out in our laboratory, reaction of the unpumped molecules cannot occur at collision energies below the C-H insertion barrier for v = 0. Thus, no background signal from unpumped molecules will be... 

Figure 19. DFT-calculated He- and H2-insertion barriers inside bislactam 53.

The electronics behind the insertion reaction is generally explained in terms of a simple three-orbitals four-electrons scheme. Hoffmann and Lauher early recognized that this is an easy reaction for d° complexes, and the relevant role played by the olefin n orbital in determining the insertion barrier [26], According to them, the empty Jt orbital of the olefin can stabilize high energy occupied d orbitals of the metal in the olefin complex, but this stabilization is lost as the insertion reaction approaches the transition state. The net effect is an energy increase of the metal d orbitals involved in the d-7t back-donation to the olefin n orbital. Since for d° systems this back-donation does not occur, d° systems were predicted to be barrierless, whereas a substantial barrier was predicted for dn (n > 0) systems [26],... 

Regarding the height of the insertion barrier, the situation is much more controversial, since pure density functionals and some MP2 calculations suggest that this a barrierless reaction, or it occurs with a negligible barrier. HF, hybrid density functionals and several post-HF calculations, instead, suggest a barrier in the range of 5-10 kcal/mol, roughly. 

Since the situation about the height of the insertion barrier is not so clear, we performed a systematic comparison of the performances of different computational approaches in determining insertion barriers and geometries, with the aim to offer a further contribution to the discussion. The insertion transition state was located with different pure and hybrid DFT functional, and at the HF and MP2 level of theory. The main geometrical parameters of the transition state for the insertion reaction of ethene into the Zr-C bond of the H2Si(Cp)2ZrCH3+ species are reported in Table 4. 

In order to investigate the basis set effects on the insertion barrier, single point MP2 calculations on the MP2/MIDI-SVP geometries are reported in Table 5. 

Table 5. Post-HF activation barriers for the insertion reaction of ethene into the Zr-CH3 bond of the HjSifCpEZrCH species. All the reported insertion barriers were obtained through single point calculations on the MP2 geometries of Tables 3 and 4 (corresponding to run 3 in this Table). In the valence calculations the Is orbitals on the C atoms, the orbitals up to 2p on the Si atom and up to the 3d on the Zr atom where not included in the active orbitals space. In the full MP2 calculations all occupied orbitals were correlated.

Firstly, inclusion of polarization functions on the C and H atoms of the reactive groups (CH3 and C2H4) reduces considerably the insertion barrier (compare runs 1 and 2 as well as runs 6 and 7 ) and seems to be mandatory. Instead, inclusion of polarization functions on the ancillary H2Si(Cp)2 ligand has a negligible effect on the calculated insertion barrier (compare runs 2 and 3 as well as runs 7 and 8). Extension of the basis set on the reactive groups lowers further the insertion barrier (compare runs 7 and 9). Both the MIDI basis set on Zr, and the SVP basis set on the remaining atoms decrease the insertion barrier (compare runs 3, 5 and 8). Finally, the extension of the active orbitals space to include all the occupied orbitals reduces sensibly the insertion barrier (compare runs 3 and 4). 

In Table 5 the insertion barrier at levels of theory higher than MP2 are also reported (runs 10-13). The MP3 and MP4 insertion barriers are both remarkably higher than the MP2 barrier. The CCSD insertion barrier also is quite larger than the MP2 barrier (5.2 kcal/mol above), but the perturbative inclusion of triple excitations in the couple cluster calculations reduces considerably the CCSD barrier, which is 8.7 kcal/mol (3.1 kcal/mol above the MP2 insertion barrier). The insertion barriers reported in Table 5 can be used to obtain a further approximation of the insertion barrier. In fact, the CCSD(T) barrier of 8.7 kcal/mol should be lowered by roughly 3 kcal/mol if... 

Insertion of ethylene into the Ni-C bond in 3a leads to the alkyl complex 4a via the transition state TS[3a-4a] with a barrier [13a] of 17.5 kcal/mol relative to 3a It is worth to note that in TS[3a-4a both ethylene and the a-carbon of the growing (propyl) chain are situated in the N-Ni-N plane. For the corresponding palladium complex the insertion barrier [13c] is somewhat higher at 19.9 kcal/mol. 

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. 

We calculate only a 0.7 kcal/mol difference between R-DIHY-B and S-DIHY-B, with nearly identical migratory insertion barriers, so we would predict only modest enantioselectivity if a DuPHOS-ligated catalyst reacted along the hydride route. However, we are not aware of any evidence that a solvated Rh-DuPHOS catalyst reacts with hydrogen to form dihydrides. 

Contrary to experimental evidence, the CO insertion step is predicted as the rate-determining step of the catalytic cycle at all reported levels of theory. The difference between of the computed results and the experiment has been attributed [17] to effects of solvation. Oxidative addition is the only step that involves an unsaturated reactant. The solvent is supposed to stabilize all transition states (TS) in the same extent, but further stabilize the unsaturated complex, which would increase the activation barrier. When a single ethene molecule was used to model the solvent, the activation barrier of H2 oxidative addition increased [17], to almost the same size as the CO insertion barrier. At this point, it seems that theory has not yet managed to distinguish which is the faster step. 

The calculated activation barrier for migratory insertion of the substrate into the palladium-hydride bond was determined to be 4.2 kcal/mol for the pathway 8a to 9a. For isomer 8c, a large thermodynamic insertion barrier of AEins = +15.2 kcal/mol exists, so the activation barrier transforming 8c to 9c was not examined further. 

In situ NMR analysis has also been used to determine the kinetic barriers for the migratory insertions of methyl carhonyl complexes [Pd CO) Me)(PPh2 CH2) PPh2)] (n = 2-4) relevant to propagation in ethene/CO copolymerisation. It was found that the steric bulk of the diphosphine has a significant effect on the insertion barriers with the most bulky ligand having the lowest barrier. 

Aluminium is much cheaper than transition metals, and aluminium oxide is non-toxic. Aluminium residues in a polymer would probably not be harmful. Thus, a catalyst based on aluminium could be extremely attractive, even if it were significantly less active than a transition metal catalyst. This has probably contributed to the continued interest in (potential) aluminium polymerization catalysts. However, such studies are difficult, as even traces of transition metal contamination may lead to erroneous conclusions. According to calculations, insertion barriers at aluminium are typically >10 kcal/mol higher than at transition metal catalysts, corresponding to a reactivity difference of 10, so... 

The relatively high tendency of aluminium alkyls to undergo j -hydrogen transfer, coupled with olefin insertion barriers that are much higher than those for transition metals, makes it unlikely that very efficient high-MW olefin polymerization catalysts based on aluminium will be discovered. Even for the reaction of aluminium alkyls with ketones, using ligands to tune towards insertion selectivity will be difficult. 

It is probably safe to assume that the insertion barriers into the Fe-C(alkyl) bonds will decrease with the BDEs (even though the range of k0 s values, which cover two orders of magnitude, suggest a narrower span of actual insertion barriers than obtained for the BDEs, which vary by 11 kcal/mol). Nevertheless, the data in Table I constitute a plausible rationalization of the observed NMR/reactivity correlation. 

Figure 3. a) Predicted correlation between the ethylene-insertion barrier AEa and (51V) for V(=0 X)Me3 complexes (Adapted from ref. 12b). b) The same for V(=Y)Me3 complexes. (Figure 3a is adapted with permission from reference 12b. Copyright 1998 Wiley-VCH.)... 

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