1 / 14

Density Functional Study of Neutral Salicylaldiminato Ni(II) Complexes as Olefin Polymerization Catalysts

Density Functional Study of Neutral Salicylaldiminato Ni(II) Complexes as Olefin Polymerization Catalysts. Mary Chan, Liqun Deng and Tom Ziegler. Department of Chemistry, University of Calgary Calgary, Alberta Canada T2N 1N4. Introduction. Table 1: Reactivity of the Catalyst. Catalyst*

oakley
Download Presentation

Density Functional Study of Neutral Salicylaldiminato Ni(II) Complexes as Olefin Polymerization Catalysts

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Density Functional Study of Neutral Salicylaldiminato Ni(II) Complexes as Olefin Polymerization Catalysts Mary Chan, Liqun Deng and Tom Ziegler Department of Chemistry, University of Calgary Calgary, Alberta Canada T2N 1N4

  2. Introduction Table 1: Reactivity of the Catalyst Catalyst* Activity R X H H 26.7 t-Bu H 46.7 Ph H 81.3 9-Phenanthrenyl H 93.3 9-Anthracenyl H 98.7 H OMe 13.3 H NO2 253.3 * kg of PE/mol of Ni The recent discovery of the ability of salicylaldiminato Ni(II) complexes to promote ethene polymerization creates the potential for the new class of olefin polymerization catalyst.1 The skeleton structure of the catalyst is shown in Figure 1. The major advantage of this type of catalysts is that they produce a neutral active center and thereby avoids the ion-pairing problems encountered with the homogenous single-site catalysts in current use. The influence of the substituents on the catalyst backbone have been studied and the activity of various substituted catalysts are summarized in Table 1. Figure 1: Skeleton Structure of Catalyst

  3. Introduction: reactivity of catalyst • The nickel complexes are inactive as polymerization catalysts without the presence of phosphine scavenger such as Ni(COD)2 and B(C6F5)3, and activity appears to be independent of scavenger type. • One of the two areas of structural modification is at the 3 position of the salicylaldiminato ring (labeled as R in Figure 1). The experimental data show that bulky substituents at this position enhances catalyst activity. • The second area of structural modification is at the 5 position of the salicylaldiminato ring (labeled as X in Figure 1). It was found that electron donating groups at this position decrease the catalyst activity while catalysts with an electron withdrawing group significantly increases the activity. • For compounds where R=H an induction period was observed, ranging 8 min for X=H to 20 min for X=NO2. No induction period was observed when R is substituted with a bulky organic group. It was speculated that this may be due to slow olefin insertion into the Ni–Ph bond or slow abstraction of the phosphine from the nickel.

  4. Objectives of Theoretical Study: mechanism • To investigate the fundamental reactions of activation, propagation and termination in polymerization process as shown in Figure 2 for the salicylaldiminato nickel catalyst. Figure 2: Fundamental Reactions of the Polymerization Process

  5. Objectives of Theoretical Study: substitution System (1) System (2) System (3) System (4) System (5) System (6) • To determine the effects of the substituent X by comparing the energy of the model systems 1, 2 and 3 shown in Figure 3. • To determine the effects of the bulky organic groups by comparing the energy of the model systems 3, 4 and 5 shown in Figure 3. • To determine the effects of changing the donor atom from nitrogen to phosphorus by comparing the energy of the model systems 5 and 6 shown in Figure 3. Figure 3: Structures of Catalyst Systems to be Studies

  6. Computational Details The density functional theory calculations were carried out using the Amsterdam Density Functional (ADF) program version 2.3.3.2 Geometry optimizations were carried out by augmenting the local exchange-correlation potential of Vosko et al.3 with Becke’s nonlocal exchange corrections4 and Perdew’s nonlocal correlation corrections.5 The frozen-core approximation was used to treat the core orbtials for all atoms. A Slater type triple-zeta basis set was used to describe the valence orbitals for the nickel whereas a double-zeta basis set was used for the non-metals. A single-zeta polarization function was also included for all atoms. Catalyst systems 4, 5 and 6 were investigated by a combined DFT and molecular mechanics approach using the QM/MM implementation in the ADF program.6 It incorporates a modified Amber957 force field which includes Rappé’s universal force field van de Waals parameters for nickel.8 The partition scheme developed by Morokuma and Maseras was used to couple the QM and the MM regions.9 The MM regions were defined as the bulky 2,6-diisopropylphenyl or 9-anthracenyl groups when they are present.

  7. Results: generation of the active catalyst The activation of the salicylaldiminato nickel complexes requires the removal the phosphine ligand to produce a coordinately unsaturated site. This process is modeled by the taking the enthalpy change of the dissociation of the phosphine ligand from the square planar precursor. The dissociation enthalpies are reported in Table 2. The first 3 entries show that the electronic nature of the substituent X at the 5 position of the salicylaldiminato ring has a small influence on the enthalpy of dissociation. Entries 5 and 6 show that changing the electronic nature of the donor atom have a larger effect of 2.7 kcal/mol on the dissociation enthalpy. A comparison of entries 3 and 4 shows that the 2,6-diisopropylphenyl group on the imino ligand has little effect on the dissociation enthalpy. Finally, systems 4 and 5 shows that the anthracenyl group on the 3 position decreases the dissociation enthalpy by 4.2 kcal/mol. Table 2: Phosphine Dissociation Enthalpy Changes ∆H (kcal/mol) System R X R* 1 H H N–H 27.3 2 H OMe N–H 27.5 3 H NO2 N–H 28.2 4 H NO2 N–Iph 27.9 5 An NO2 N–Iph 23.7 6 An NO2 P–Iph 21.0 An = 9-anthracenyl Iph = 2,6-diisopropylphenyl

  8. Results: chain propagation energies Table 3: Olefin Complextion Energy and Insertion Barriers Olefin Complexation (kcal/mol) Insertion Barrier (kcal/mol) Catalyst Alkyl trans to N Alkyl trans to O Alkyl trans to N Alkyl trans to O 1 -18.2 -17.1 15.3 25.0 2 -18.3 -17.4 15.5 25.0 3 -17.1 -16.7 14.1 24.0 4 -16.1 -16.1 14.0 24.3 5 -17.1 -11.1 14.0 23.3 6 -16.8 -10.8 15.6 25.2 Chain propagation is assumed to follow the Cossée-Arlman mechanism.10 The insertion is initiated by the coordination of the olefin to the metal center followed by insertion into the carbon-metal bond. Two geometrical isomers are possible for each of the p-complex, the transition state, and the insertion product: one where the alkyl chain is trans to the nitrogen and one where is it trans to the oxygen. The complexation energies and insertion barriers for both isomers are reported in Table 3. The insertion barriers as well as the complexation energy from the trans to N isomers are relatively insensitive to changes in the catalyst structure. The anthracenyl group decrease the olefin complexation energy for the trans to O isomer by 5 kcal/mol.

  9. Results: chain propagation mechanism The insertion barriers in Table 3 indicate that there is a significant difference between the two isomers. The geometrical arrangement around the nickel changes after each insertion and therefore, insertion cannot proceed through the energetically more favorable pathway at all times due to the formation of the undesired isomer. The barrier of cis/trans isomerization from the p-complex was determined to be 11.4 kcal/mol. This suggests that the lowest energy pathway for the insertion process follows the sequence outlined in Figure 4 of 1) complexation with the olefin, 2) cis/trans isomerization, and 3) insertion to extend the polymer chain. complexation isomerization insertion Figure 4: Lowest Energy Pathway for the Insertion Process

  10. Results: termination mechanism The reaction profiles for the b-hydrogen transfer (BHT) as well as the b-hydrogen elimination (BHE) chain termination mechanism for system 4 appears in Figure 5. The BHE mechanism involves the transfer of a b hydrogen to the metal. The barrier to form the nickel hydride is 12.5 kcal/mol, but dissociation of polymer requires another 38.0 kcal/mol. The BHT mechanism involves the transfer of the b hydrogen to a coordinated monomer. Although the BHT termination barrier is high at 29.9 kcal/mol, the relative energies required for the various steps show that BHT mechanism is still the preferred pathway. hydride + propylene BHT transition state BHE transition state hydride propene complex propene ethene complex active catalyst + ethene propene ethylcomplex olefin p-complex Figure 5: Potential Energy Profile for Termination Reactions

  11. Results: termination barriers Table 4: BHT Termination Barriers ∆H (kcal/mol) System R X R* 1 H H N–H 25.7 2 H OMe N–H 26.2 3 H NO2 N–H 23.9 4 H NO2 N–Iph 29.9 5 An NO2 N–Iph 29.3 6 An NO2 P–Iph 26.0 An = 9-anthracenyl Iph = 2,6-diisopropylphenyl The BHT termination barriers do not show the same isomeric dependence as the insertion barriers. The termination barriers form the two isomers of system 3 are 23.9 and 23.7 kcal/mol and therefore, only the barrier form the more stable p-complex was compared. The results are summarized in Table 4. The first 3 entries show that the electronic nature of the substituent X at the 5 position have relatively small effects on the termination barrier. The electron releasing group appears to increase the barrier while the electron withdrawing group shows a decrease. Entries 3 and 4 shows that the bulky 2,6-diisopropylphenyl group increases the termination barrier drastically by 6.0 kcal/mol. The effect have been rationalized by the destabilization of the transition state by repulsive steric interactions.11 Entries 5 and 6 show that changing the donor atom from nitrogen to phosphorus decreases the termination barrier by 3.3 kcal/mol. Closer analysis of the geometries show that this may be due to steric rather than electronic reasons.

  12. Influence of Substitution on Catalyst Activity • Electron withdrawing substituents at the 5 position of the salicylaldiminato ring alters the energies in favor of polymerization by decreasing the phosphine dissociation energy and the insertion barrier. Electron releasing groups at this position would hinder polymerization as they increase the phosphine dissociation energy and insertion barrier. The magnitude of the changes in energy caused by this substituent is relatively small. • The effects of varying the electronic nature of the donor atom from nitrogen to phosphorus is not easily predictable due to the opposing trends observed. The phosphorus analog has a lower phosphine dissociation energy, but at the same time shows a lower termination barrier. • The most significant effect of the 2,6-diisopropylphenyl group on the imino ligand is to increase the termination barrier by 6.0 kcal/mol. Catalyst with this substituent should show marked increase in activity. • The function of the 9-anthracenyl group on the 3 position is to decrease the phosphine dissociation energy by 4.2 kcal/mol. This is due to the destabilization of the precursor by steric repulsion between this substituent and the phosphine ligand. Therefore, bulky substituents at this position should enhance catalyst activity.

  13. Conclusions • The polymerization mechanism followed by neutral salicylaldiminato nickel complex have been determined. They are activated by the dissociation of the phosphine ligand followed by the coordination of a monomer to the metal center. The p-complexes thus form undergoes cis to trans isomerization before insertion. Termination of the polymer occurs via the b-hydrogen transfer mechanism. • The electronic nature of the substituent X on the 5 position of the salicylaldiminato ring have relatively small effects on the energies of all fundamental polymerization reactions and is expected have little influence on catalyst activity. Therefore, the large increase in activity of the nitro substituted catalyst observed experimentally cannot be attributed to electronic factors alone. • The calculated insertion barriers are relatively insensitive to electronic or steric changes to the catalyst backbone. Therefore, the induction period observed is due to slow dissociation of the phosphine from the nickel rather than slow insertion. • The steric effects of adding bulky groups on the 3 position of the salicylaldiminato ring and the imino nitrogen were large in comparison to electronic effects. The calculated barriers and enthalpy changes suggest that the addition of bulky substituents at these positions should significantly enhance catalyst activity.

  14. Acknowledgments This investigation was supported by the Natural Science and Engineering Research Council of Canada (NSERC) and by Novacor Research and Technology (NRTC) of Calgary, Alberta, Canada. We wish to thank Dr. A. Michalak for helpful discussions. References 1. Wang, C.; Friedrich, S.; Younkin, T.R.; Li, R.T.; Grubbs, R.H.; Bansleben, D.A.; Day, M.W. Organometallics1998, 17, 3149. 2. (a) Baerends, E.J.; Ellis, D.E.; Ros, P. Chem. Phys.1973, 2, 41. (b) Baerends, E.J.; Ros, P. Chem. Phys.1973, 2, 52. 3. Vosko, S.H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. 4. Becke, A. Phys. Rev. A1988, 38, 3098. 5. Perdew, J. P. Phys. Rev. B1986, 34, 7406. 6. Woo, T.K.; Cavallo, L.; Ziegler, T. Theor. Chem. Act. 1998, 100, 307. 7. Cornell, W.D,; Cieplk, P.; Bayly, C.I.; Gould, I.R.; Merz, K.M.Jr.; Ferguson, D.M.; Spellmeyer, D.C.; Fox, T.; Caldwell, J.; Koolman, P.A. J. Am. Chem. Soc.1995, 117, 5179. 8. Rappé, A.K.; Casewit, C.J.; Colwell, K.S.; Goddard, W.A. III; Skiff, W.M. J. Am. Chem.Soc.1992, 114, 10024. 9. Maseras, F.; Morokuma, K. J. Comput. Chem.1995, 16, 1170. 10. (a) Cossée, P. J. Catal. 1964, 3, 80. (b) Arlman, E. J. J. Catal.1964, 3, 89. 11. Deng, L.; Woo, T.K.; Cavallo, L.; Margl, P.M. Ziegler, T. J. Am. Chem. Soc. 1997, 119, 6177.

More Related