Neutral Salicylaldiminato Ni_II_ Complexes _Grubbs Catalyst_ as

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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                                                                                        2

          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.
                                                        Table 1: Reactivity of the Catalyst

                        i-pr                        R                     X            Catalyst*
            Ph3P        Ph
               O         N                  H                            H                26.7
      R                                     t-Bu                         H                46.7
                               H            Ph                           H                81.3
                                            9-Phenanthrenyl              H                93.3
                                            9-Anthracenyl                H                98.7
               X                            H                            OMe              13.3
                                            H                            NO2             253.3
Figure 1: Skeleton Structure of Catalyst
                                            * kg of PE/mol of Ni
Introduction: reactivity of catalyst                                                                     3

   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.
Objectives of Theoretical Study: mechanism                                                                                                 4

   To investigate the fundamental reactions of activation, propagation and termination in
    polymerization process as shown in Figure 2 for the salicylaldiminato nickel catalyst.
       Generation of the Active Species:
                       P(CH 3 )3                                  R
              O                                      O
                                                                          +   P(CH3)3
                        H                                         H

             O          P                                                               O                                 O
                  Ni                                 O                                      Ni                                Ni       P
                                     C2H4                    Ni       P                     N                                 N
                        H                                    N                                   H                                 H

       Termination via -Hydrogen Transfer:
                             P                               P                                                    P

              O              H                   O                                                   O
                   Ni                                Ni           H                                      Ni       H
                   N                                 N                                                   N
                        H                                    H                                                H
       Termination via -Hydrogen Elemination:
                                 P                        P
                                                                               O        H
                   Ni        H
                                                Ni       H
                                                                                                 +                    P

                   N                            N                                       H
                         H                           H

        Figure 2: Fundamental Reactions of the Polymerization Process
Objectives of Theoretical Study: substitution                                                        5

   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.
                                      (CH3) 3P        P
     (CH3) 3P        P                           Ni
                                                                      (CH3) 3P        P
                                                           H                     Ni
                Ni           H              O         N                                    H
           O            N                                                  O          N

                                            OCH 3

        System (1)                      System (2)                     System (3)
    (CH3) 3P         P                   (CH3) 3P              ipr       (CH3) 3P              ipr
                                                           P                               P
                                                      Ni                              Ni
          O         N
                                                 O         N                      O        P
                                                               ipr                             ipr

          NO2                                    NO2                              NO2

     System (4)                          System (5)                            System (6)
     Figure 3: Structures of Catalyst Systems to be Studies
Computational Details                                                                              6

      The density functional theory calculations were carried out using the Amsterdam
 Density Functional (ADF) program version 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

      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.
Results: generation of the active catalyst                                                                7

     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
Table 2: Phosphine Dissociation Enthalpy Changes                dissociation enthalpy. A comparison
                                               ∆H               of entries 3 and 4 shows that the 2,6-
System     R           X         R*         (kcal/mol)
                                                                diisopropylphenyl group on the
                                                                imino ligand has little effect on the
 1         H           H        N–H            27.3
                                                                dissociation enthalpy. Finally,
 2         H           OMe      N–H            27.5
                                                                systems 4 and 5 shows that the
 3         H           NO2      N–H            28.2
                                                                anthracenyl group on the 3 position
 4         H           NO2      N–Iph          27.9
                                                                decreases the dissociation enthalpy
 5         An          NO2      N–Iph          23.7
                                                                by 4.2 kcal/mol.
 6         An          NO2      P–Iph          21.0

An = 9-anthracenyl
Iph = 2,6-diisopropylphenyl
Results: chain propagation energies                                                                      8

    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.

                Table 3: Olefin Complextion Energy and Insertion Barriers

     Catalyst     Olefin Complexation (kcal/mol)             Insertion Barrier (kcal/mol)
                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
Results: chain propagation mechanism                                                                      9

      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.

                  Ni     +   C2 H4

 2.               Ni
                                                 N                 isomerization

 3.          O
                                            Ni                         Ni
                                        N                          N

Figure 4: Lowest Energy Pathway for the Insertion Process
     Results: termination mechanism                                                                                               10

                                          hydride + propylene                                  The reaction profiles for the -
                                                                                          hydrogen transfer (BHT) as well as
                                                                                          the -hydrogen elimination (BHE)
                                                       38.0 kcal/mol
                        BHE                                                               chain termination mechanism for
                                     BHE                                                  system 4 appears in Figure 5. The
                                   transition                          hydride propene
                                                    state                                 BHE mechanism involves the transfer
                                      state                               complex
                                                                                          of a  hydrogen to the metal. The
                                                                         propene ethene   barrier to form the nickel hydride is
                    12.5 kcal/mol                                           complex
                                                                                          12.5 kcal/mol, but dissociation of
active catalyst                                                                           polymer requires another 38.0
   + ethene
                                                                                          kcal/mol. The BHT mechanism
                                       29.9 kcal/mol                                      involves the transfer of the  hydrogen
                  -16.1 kcal/mol                                                          to a coordinated monomer. Although
                                                                                          the BHT termination barrier is high at
                                                                                          29.9 kcal/mol, the relative energies
                                                                 propene                  required for the various steps show
                           p-complex                          ethyl complex               that BHT mechanism is still the
   Figure 5: Potential Energy Profile for Termination Reactions                           preferred pathway.
 Results: termination barriers                                                                                11

       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
           Table 4: BHT Termination Barriers
                                                                the termination barrier drastically by 6.0
System        R          X         R*          (kcal/mol)       kcal/mol. The effect have been
                                                                rationalized by the destabilization of the
   1          H          H         N–H            25.7          transition state by repulsive steric
   2          H          OMe       N–H            26.2          interactions.11 Entries 5 and 6 show that
   3          H          NO2       N–H            23.9          changing the donor atom from nitrogen
   4          H          NO2       N–Iph          29.9          to phosphorus decreases the termination
   5          An         NO2       N–Iph          29.3          barrier by 3.3 kcal/mol. Closer analysis
   6          An         NO2       P–Iph          26.0          of the geometries show that this may be
                                                                due to steric rather than electronic
An = 9-anthracenyl
Iph = 2,6-diisopropylphenyl
Influence of Substitution on Catalyst Activity                                                      12

   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.
Conclusions                                                                                               13

   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 -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

   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.
 Acknowledgments                                                                                               14

       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.

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