Timothy K. Firman and Tom Ziegler University of Calgary

1. M(XCHX)2R+ and M(XCHCHCHX)2R+ (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. Introduction Many of the best olefin polymerization catalysts include p-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 p-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 p -conjugation and variable bond order. By varying these heteroatoms, a range of different properties was expected.

3. Computational Details All structures and energetics were calculated with the Density Functional Theory (DFT) program ADF1. All atoms were modeled using a frozen core approximation. Ti was modeled with a triple-z 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-z STO representation of the 4s, 4p, 4d, 5s, and a single 5p polarization function. Main group elements were described by a double-z 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 potential3 was augmented with electron exchange functionals4 and correlation corrections5 in the method known as BP86. First-order scalar relativistic corrections6 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 b-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. 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 a-agostic rather than a b-agostic bond.

5. Uptake Enthalpy XCHX Systems DEreorganization is the energy required to distort the alkyl minimum to the shape of the adduct (minus the ethylene) • 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 DEreorganization

6. Entropy and Uptake Energy • Previous comparisons of computed and actual d0 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. Catalytic Properties of XCHX system • b-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 b-hydride transfer, while the S ligands do not. • The ligands bend out of plane because they are no longer p-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 p bonding.

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, • Ti and Zr are chemically similar • Zr is larger, reducing steric interactions • Zr tends to form stronger bonds, which should improve Euptake

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. 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 • Steric effects may be more important with wider ligands • b-hydride transfer is more sterically demanding than insertion • Some experimental analogues are great catalysts

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. Transition States • While the insertions are quite facile, so is b-hydride transfer. • The alkyl and ethylene are in a long, narrow space between the two rings; the b-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. 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. Larger model of Matsui10 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. 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 • Catalytic activity can vary widely with counterion, particularly in d0 cases, and this model does nothing to simulate a counterion. • A coordinated counterion would bend the alkyl out of the plane, which might lower reorganization costs to uptake. • The departure of the counterion would be entropically favorable, offsetting the enthalpic penalty. • Models which include a counterion will be studied.

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 Tables1982, 26, 483. (3) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. (4) Becke, A. Phys. Rev. A1988, 38, 3098. (5) a) Perdew, J. P. Phys. Rev. B1986, 34, 7406. b) Perdew, J. P. Phys. Rev. B1986, 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. Organometallics1998, 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.