1. 1 M(XCHX) 2 R + and M(XCHCHCHX) 2 R + (M=Ti,Zr ; X= NH, O, S) as olefin polymerization catalysts and the role of ligand conjugation: A Density Functional Theory(DFT) study Timothy K. Firman and Tom Ziegler University of Calgary

2. 2 Introduction Many of the best olefin polymerization catalysts include  -conjugated ligands. These ligands can change the extent of their bonding to transition metals by changing the bonding along the conjugated ligand. For example: This variable bond order compensates for other metal-ligand bonding changes, such as the net loss of a metal-olefin bond during olefin insertion. A series of compounds with  -conjugated ligands bound to group IV metals is modeled using DFT to examine bonding and catalytic properties. The metal binds to an NH, an O, and an S, quite different chemically but can be considered to be isolobally analogous, with similar  -conjugation and variable bond order. By varying these heteroatoms, a range of different properties was expected.

3. 3 Computational Details All structures and energetics were calculated with the Density Functional Theory (DFT) program ADF 1. All atoms were modeled using a frozen core approximation. Ti was modeled with a triple-  basis of Slater type orbitals (STO) representing the 3s, 3p, 3d, and 4s orbitals with a single 4p polarization function added. Zr was modeled similarly with a triple-  STO representation of the 4s, 4p, 4d, 5s, and a single 5p polarization function. Main group elements were described by a double-  set of STO orbitals with one polarization function (3d for C, N, and O; 4d for S; and 2p for H.) 2 In each case, the local exchange-correlation potential 3 was augmented with electron exchange functionals 4 and correlation corrections 5 in the method known as BP86. First-order scalar relativistic corrections 6 were added to the total energy of all systems. In most cases, transition states were located by optimizing all internal coordinates except for a chosen fixed bond length, iterating until the local maximum was found, with a force along the fixed coordinate less than.001 a.u. For  -hydride transfer, transition states were found using a standard stationary point search to a Hessian with a single negative eigenvalue. All calculations were spin restricted and did not use symmetry. All energies are in kcal/mol unless otherwise stated.

4. 4 M(XCHX) 2  With only one carbon between them, the bite angle of each ligand is only about 70 ˚.  Ligands are not especially bulky, but sterics will be a factor.  In most cases, the two chelating ligands are canted, making the environment asymmetric  Some experimentally known analogues are known. 7  Some alkyls bind with an  -agostic rather than a  -agostic bond.

5. 5 Uptake Enthalpy XCHX Systems  The metal starts pseudo-trigonal planar then becomes very roughly tetrahedral.  The metal-ethylene bond energy would be about 16 kcal/mol in each case, but moving the alkyl out of the plane incurs a significant energetic penalty, which is labeled  E reorganization  E reorganization is the energy required to distort the alkyl minimum to the shape of the adduct (minus the ethylene)

6. 6 Entropy and Uptake Energy  Previous comparisons of computed and actual d 0 systems correlate better activity for systems with larger uptake energy, with improvement through at least -10kal/mol. 8  Binding an olefin will be significantly entropically unfavorable.  Entropy is calculated for this one example. It is not expected to differ substantially between these systems. DS:115 cal/molK +55 cal/molK -121 cal/molK DDS= 50 cal/molK At 300K, this reaction is entropically unfavorable by 15 kcal/mol. At 400K, this will be equal to 20 kcal/mol. This is larger than the enthalpic contribution and repulsive.

7. 7 Catalytic Properties of XCHX system   -hydride transfer is the dominant termination mechanism  While all three insertion barriers are quite low, the termination barrier is far too low for O and NH.  In the O and NH cases, the ligands become non-planar during the  -hydride transfer, while the S ligands do not.  The ligands bend out of plane because they are no longer  -bound to the metal; the transfer transition state has more bonds to C and H than the others, and these bonds displace the metal-ligand  bonding.

8. 8 Zr compounds  Many Zr catalysts are known  Zr was used instead of Ti in a series of otherwise identical computations  In comparison with Ti, l Ti and Zr are chemically similar l Zr is larger, reducing steric interactions l Zr tends to form stronger bonds, which should improve E uptake

9. 9 Results with Zirconium Center  The uptake energy is significantly improved  The termination barrier is about equal to the insertion barrier in all three cases, indicating that none of these would catalyze polymerization  These Ti and Zr compounds gave similar results overall

10. 10 Six Member Metal Ring Systems  Somewhat similar to the earlier systems, but an extra two doubly bonded carbons are added.  Like the smaller ring, the metal-ligand bond order is flexable; some resonance structures are shown above  This longer linker results in a wider bite angle l Steric effects may be more important with wider ligands  -hydride transfer is more sterically demanding than insertion  Some experimental analogues are great catalysts

11. 11 Uptake Energies  All uptake energies for these systems are poor.  Increased steric hinderance may repel incoming ethylene  Reorganization energies are high; the alkyl requires a lot of energy to be bent away from the plane  As before, the Zirconium energies are somewhat better due to stronger bonds to ethylene, but not by much.

12. 12 Transition States  While the insertions are quite facile, so is  -hydride transfer.  The alkyl and ethylene are in a long, narrow space between the two rings; the  -hydride transfer transition state is just such a shape. Steric effects may actually encourage termination in these cases. (Blank spaces are transition states which have not been found to date)

13. 13 Known, Analogous Catalysts  All three of these have substantial catalytic activity, yet similar models show poor catalyic characteristics.  The other characteristics of these systems may be important, such as sterics or the electronic effects of phenyl rings. (7) (9)(10)

14. 14 Larger model of Matsui 10 Catalyst  On the right are two minima, one with and one without olefin  Uptake energy calculated to be: -4.2 kcal/mol  This is not what we would expect for such a good catalyst

15. 15 Conclusions and Future Work  With the possible exception of the SCHS ligand, the systems examined appear to be poor candidates for catalysts  Insufficient uptake energy is a constant problem  There exist real, active catalysts very similar to these systems, so there may be a problem with our model or with our criteria for identifying promising catalysts.  The most likely flaw in our model is the lack of a counterion l Catalytic activity can vary widely with counterion, particularly in d 0 cases, and this model does nothing to simulate a counterion. l A coordinated counterion would bend the alkyl out of the plane, which might lower reorganization costs to uptake. l The departure of the counterion would be entropically favorable, offsetting the enthalpic penalty.  Models which include a counterion will be studied.

16. 16 Acknowledgements This research was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and Novacor Research and Technology Corporation. References: (1) a) ADF 2.3.3, Theoretical Chemistry, Vrije Universiteit, Amsterdam b) Baerends, E. J.; Ellis, D. E.; Ros, P. Chem.Phys. 1973, 2, 41. c) te Velde, G; Baerends, E. J. J. Comp. Phys. 1992, 99, 84. (2) Snijders, J. G.; Baerends, E. J.; Vernoijs, P. At. Nuc. Data Tables 1982, 26, 483. (3) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. (4) Becke, A. Phys. Rev. A 1988, 38, 3098. (5) a) Perdew, J. P. Phys. Rev. B 1986, 34, 7406. b) Perdew, J. P. Phys. Rev. B 1986, 34, 8822. (6) a) Snijders, J. G.; Baerends, E. J. Mol. Phys. 1978, 36, 1789. b) Snijders, J. G.; Baerends, E. J.; Ros, P. Mol. Phys. 1979, 38, 1909. (7) Littke, A.; Sleiman, N.; Bensimon, C.; Richeson, D. S.; Yap, G. P. A.; Brown, S. J. Organometallics 1998, 17, 446. (8) Margl, P.; Deng, L.; Ziegler, T. Organometallics 1998, 17, 933. (9) Matilainen, L.; Klinga, M.; Leskelä, M. J. Chem. Soc. Dalton Trans. 1996, 219. (10) Matsui, S.; Mitani, M.; Saito, J.; Tohi, Y.; Makio, H.; Tanaka, H.; Fujita, T. Chem. Lett. 1999, 1263.