Presentation on theme: "Bis-amides and Amine Bis-amides as Ligands for Olefin Polymerization Catalysts Based on V(IV), Cr(IV) and Mn(IV). A Density Functional Theory Study Timothy."— Presentation transcript:
Bis-amides and Amine Bis-amides as Ligands for Olefin Polymerization Catalysts Based on V(IV), Cr(IV) and Mn(IV). A Density Functional Theory Study Timothy K. Firman and Tom Ziegler University of Calgary
Introduction Following the discovery of catalytic olefin polymerization activity of group IV metallocene-based systems 1976 1, a great variety of homogeneous transition-metal based olefin polymerization catalysts have been discovered. Many d 0 group III or group IV metal based systems, with a widening variety of ligands were found, from “constrained geometry” catalysts 2 to the diamide system of McConville et al. 3 Computational modeling of d 0 systems has also progressed rapidly. 4 More recently, a number of late transition metal catalysts have been developed. Brookhart et al. discovered d 8 Ni and Pd compounds 5 which are active polymerization catalysts with certain bulky ligand systems. Several systems with a Cr center and a few d electrons have recently been found to be active catalysts. 6,7 With these indications of catalytic potential in compounds with electron counts between zero and eight, we present a computational study of d 1, d 2, and d 3 metal systems with nitrogen-based ligands. A previous study 8 had indicated some potential in bis-amide systems, we explore here systems with an additional amine. The amine was added in the hope of destabilizing the termination transition state. We use an analysis of the localized electron density to explore how the nature and placement of the ligands affect catalytic properties.
Computational Details All structures and energetics were calculated with the Density Functional Theory (DFT) program ADF 9. All atoms were modeled using a frozen core approximation. V, Cr, and Mn were 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. Mo, Ru, and Pd were similarly modeled 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- STO orbitals with one polarization function (3d for C, N and 2p for H.) 10 In each case, the local exchange-correlation potential 11 was augmented with electron exchange functionals 12 and correlation corrections 13 in the method known as BP86. First-order scalar relativistic corrections 14 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 unrestricted and did not use symmetry. The Boys and Foster method was used for orbital localization, and the orbitals were displayed using the adfplt program written by Jochen Autschbach.
d 1 V, d 2 Cr, d 3 Mn: a Comparison All have formal oxidation states of +4, but will each have a net charge of about +1. Each has six valence orbitals (an s and 5 d). l After filling the SOMOs, V has 5 empty orbitals, Cr 4 and Mn 3 Each available orbital has a bonding interaction with the ligands. l These orbitals must be orthogonal l Individual orbitals of V will be more ligand centered, to balance charge Metal bonding orbitals are often shared between ligands, e.g. trans- ligands typically share a single -bonding metal orbital.
Metal Alkyl Structures d 1 V is nearly tetrahedral NH 2 are flat, with bonds not in the same plane a -agostic hydride d 2 Cr is nearly tetrahedral NH 2 are flat, aligned(shared) no -agostic hydride d 3 Mn includes a 146 ˚ angle NH 2 are bent out of plane due to weakened -interactions no -agostic hydride
Olefin Adduct d 1 V is trigonal bipyramidal NH 2 are flat, bonds unshared -agostic hydride d 2 Cr is trigonal bipyramidal NH 2 are flat, bonds aligned -agostic hydride d 3 Mn is trigonal bipyramidal l NH 2 are bent out of plane -agostic hydride l NH 2 is apical instead of ethene
Insertion Insertion barriers are similar and fairly small Geometries are quite different from one another Each has a ligand trans to a forming or breaking bond EE +8.3kcal/mol +12.5 kcal/mol +13.6 kcal/mol
Termination ( -hydride transfer) Each termination barrier is substantially higher than the insertion No -hydride elimination TS found lower in energy The amine is either trans to the hydride, or to one of the reacting M-C bonds EE +16.4 kcal/mol +19.6 kcal/mol +20.0 kcal/mol
Enthalpies Summary Insertion and termination numbers are promising, particularly for a system lacking steric bulk Uptake energy is too low. l Entropy will be unfavorable by about 12-15 kcal/mol l Displacement of counterion will also effect uptake energetic
The Second Row Transition Metals Good second row olefin polymerization catalysts exist, including d 0 Zr and d 8 Pd Olefin uptake energies are expected to increase due to generally stronger bonds These compounds are found to be low spin Compounds with a like number of occupied metal orbitals may be analogous Model systems with d 2 Mo,d 4 Ru, and d 6 Pd were calculated
Second Row Results While the uptake energies are substantially improved, these combinations of ligand and metal do not result in good catalysts.
Tethered Nitrogen Ligands Electronically similar to the previous systems Chelation will keep the ligands bound All three nitrogen will stay on one side; this will leave the other side vacant and may help uptake Limited conformational flexibility Sterically unhindered, as in the untethered case
Uptake Enthalpy of Linked System The metal-ethylene bond energy would be about 20 kcal/mol in each case, but large differences in reorganization energy result in differences in uptake energies. The shapes of the untethered ethylene adducts predict energetics l In the untethered Cr adduct, the two NH 2 groups are near the NH 3 group with a hydrogen pointing toward it. The tethers hold it in just this position. l V has the N ligands close together, but must twist one of the NH 2 groups. l Mn has an NH 2 trans to the NH 3, which cannot occur with a tether, so the uptake energy is actually worse with the tether than without. E reorganization is the energy required to distort the alkyl minimum to the shape of the adduct (minus the ethylene)
Catalytic Properties with Tether The tether again has a large effect on the energetics that is very different for different metals l In the V system, The N ligands are held in a position close to both transition states; the energies of both are decreased. l In Cr, the insertion is close to the tethered case, but the untethered termination prefers trans NH 2 groups, which is impossible for the tethered case. The resulting energy is much higher. In Mn, the tether is different from all three shapes, causing them each a similar energy penalty. The net result is similar Es.
Conclusions The occupation of metal orbitals has a substantial effect on molecular properties The binding changes in transition states substantially in flexible, untethered systems Tethering the ligands alters the energetics differently for different transition states; matching tether types with metal is important
Acknowledgements This research was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and Novacor Research and Technology Corporation. References: (1) Andersen, A. A.; Cordes, H. G.; Herwig, J.; Kaminsky, W.; Merck, A.; Mottweiler, R.; Pein, J.; Sinn, H.; Vollmer, H. J. Angew. Chem., Int. Ed. Engl. 1976, 15, 630. (2) a) Stevens, J. C.; Timmers, F. J.; Wilson, D. R.; Schmidt, G. F.; Nicklas, P. N.; Rosen, R. K.; Knight, G. W.; Lai, S. Y.: Eur. Pat. Appl., EP 416815, 1990. b) Stevens, J. C.; Neithammer, D. R.: Eur. Pat. Appl., EP 418044, 1990. c) Canich, J. A. M.: Eur. Pat. Appl., EP 420436, 1990. (3) Scollard, J. D.; McConville, D. H.; Payne, N. C.; Vittal, J. J. Macromolecules 1996, 29, 5241. (4) Margl, P.; Deng, L.; Ziegler, T. Organometallics 1998, 17, 933. (5) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 6414. (6) White, P. A.; Calabrese, J.; Theopold, K. H. Organometallics 1996, 15, 5473. (7) Emrich, R.; Heinemann, O.; Jolly, P. W.; Krüger, C.; Verhovnik, G. P. J. Organometallics 1997, 16, 1511. (8) Schmid, R. Ziegler, T. Submitted for publication. (9) 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. (10) Snijders, J. G.; Baerends, E. J.; Vernoijs, P. At. Nuc. Data Tables 1982, 26, 483. (11) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. (12) Becke, A. Phys. Rev. A 1988, 38, 3098. (13) a) Perdew, J. P. Phys. Rev. B 1986, 34, 7406. b) Perdew, J. P. Phys. Rev. B 1986, 34, 8822. (14) 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.