5.03, Inorganic Chemistry Prof. Daniel G. Nocera Lecture 4 Apr 11: Bent Metallocenes and Ziegler Natta Catalysis

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1 5.03, Inorganic hemistry Prof. Daniel G. Nocera Lecture 4 Apr 11: Bent Metallocenes and Ziegler Natta atalysis The electronic structure of organometallic complexes follows directly from the sandwich compounds of the last lecture. Focusing on the OMO (a 1g ) and OMO+1 (e 2g ) of the sandwich compound, as the ps (cyclopentadienyls) are bent back, the following energy correlation diagram results: 2a 1 a 1g b 2 The antibonding component (arising from a 1g ) is considerably destabilized e 2g 1a 1 A convenient view of the orbitals from the top-down, i.e. rotating to view the orbitals on the yz plane: The d orbital contribution to the bent metallocene orbitals will of course depend on the metal. But to give you a rough idea of the metal d-orbital contribution, for p 2 Ti, θ = 136, 1

2 % omposition Orbital p 2 Ligands Metal 2a z 2, 27 x 2 y 2, 11 s, 8 z b yz, 2 y 1a x 2 y 2, 30 z 2, 1 s What about protonation of p 2 M? ere we are mixing a (1s) orbital to the frontier molecular orbitals of the bent p 2 M fragment: For clarity, the p 2 M fragment will be rotated so that the line of site is along the z-axis This diagram with orbital occupancy corresponds to p 2 Re or p 2 Fe + (both are known compounds). 2

3 More than one ligand can fit into the wedge. The p 2 MX 2 (M = Ti,, f) compounds are convenient starting materials for a number of organometallic compounds. Again, rotating the p 2 M fragment for greater clarity: The dihydride complexes have a similar electronic structure. The may be removed photolytically, and in the presence of bonds such as those of benzene, oxidative-addition may occur, 3

4 Linked p systems are called ansa metallocenes. The linker can stabilize the system to reductive elimination of substituents within the wedge, For the methyl hydride compound, /D scrambling is faster than methane loss. The scrambling occurs via the following sequence, Finally, molecules can reside in the the yz plane ( the wedge ). onsider ethylene: The MOs of the ethylene molecule are: ombining the MOs of ethylene with the wedge orbitals of bent p 2 M yields the following MO diagram: 4

5 b 2 3a 1 b 2 2a 1 b 2 though of proper symmetry to interact with the a1 MO of ethylene, very little overlap, so little perturbed energetically 1a 1 2a 1 1a1 + * b 2 a 1 p 2 Ti p 2 Ti a Ziegler-Natta Polymerization Bent metallocenes are Ziegler Natta catalysts. Ziegler and Natta (ZN) shattered the dogma concerning the difficulty of polymerizing ethylene, permitting olefin polymerization to be performed at low pressure. Olefins polymerize by effectively severing the carbon-carbon π -bond in α-olefins and remaking a carbon-carbon σ- bonds, linking the olefins together in long hydrocarbon chains. These polymers are the ones that you are familiar with in everyday life: high density polyethylene (DPE, R = ), linear low density polyethylene (LLDPE, R = mostly with some Et, Bu or x), polypropylene (R = Me) and ethylene-propylenediene-modified rubber (EPDM, R =, Me and alkenyl). 5

6 atalyst Types The early catalysts were based on Karl Ziegler and Guilio Natta s original formulation of high valent transition metal halide, oxide or oxo-halide with an alkylating co-catalyst such as an alkyl aluminum. urrent industrial catalysts include Til 4 /Mgl 2 /AlEt 3 (for LLDPE, Dow), ro 3 /Al 2 O 3 /AlEt 3 (for DPE, Phillips) and VOl 3 /AlEt 3 (for EPDM, Shell). These catalysts have low tolerance for functional groups because of their Lewis acidic nature and they are not that selective. Moreover, the active site of these catalysts is ill-defined. Little is known about the nature of the actual catalytic species in these systems. Olefin polymerization was revolutionized with the advent of molecular ZN catalysts. Most of these are bent metallocenes. p 2 l 2 in combination with methylalumoxane ([MeAlO] n, MAO, a hydrolysis product of AlMe 3 ) polymerizes ethylene to high density polyethylene. Despite a quarter century of intense academic and industrial research, the nature of MAO is still largely unknown. Various analytical methods suggest that MAO exists as clusters with ~10-15 aluminum atoms and acts as both a methyl transfer agent and a Lewis acid to form anions which are weakly coordinating to the cationic metal methyl complexes which are the active catalysts. The ligand tunes the catalyst properties. For instance, isotactic polypropylene is obtained by replacing p with the 2 symmetric ansa ligand. The language of stereoregular polymers also includes: ationic bent p 2 M catalysts with weakly coordinating anions are especially active catalysts. For instance, reaction of early transition metal alkyl complexes, such as p 2Me 2 with oxidizing tetraphenylborate salts including AgBPh 4 and (p 2 Fe)BPh 4. 6

7 d 0, 16e species weakly coordinating anion 1e oxidant O Fe 3 B 3 p 2 Fe 3 O B + labile solvent molecule large weakly coordinating anion takes the place of MAO The identification of the active site as an alkyl metal cations have led to the rapid development of new catalysts with desired tacticity, molecular weight, and even variable tacticity (blocks of atactic/isotactic polypropylene elastomer). New anions and activators give catalyst systems with activities approaching 50,000,000 grams of polymer/gram of metal. Mechanism The polymerization reaction is exothermic by ~20 kcal/mol, which is effectively the energy difference between the carbon-carbon π- and σ-bonds. For typical early d 0 transition metal, catalysts, the activation energy is ~10 Kcal/mole, making this a very facile reaction. Orbitals for a ZN polymerization are needed for: the metal-alkyl bond an agostic interaction olefin binding An agostic interaction commonly refers to a bond on a ligand that undergoes an interaction with the metal complex. This interaction closely resembles the transition state of an oxidative addition or reductive elimination reaction. Two examples of agostic interactions are: 7

8 Agostic interactions can be detected by various spectroscopic measurements: 1 NMR. The agnostic is shifted away from the usual chemical shift range for alkyl and aryl and appears between +25 to 60 ppm The IR stretch is reduced The M- - - distance of 1.85 to 2.4 Å as measured by neutron diffraction (rather than X-ray) The commonly accepted mechanism for the ZN olefin polymerization is: 16e resting state + complex complex 3 transition state 3 polyethylene + 3 complex complex 3 A severely electron deficient metal center coordinates the π-bond of an olefin to form a weakly bound olefin complex; the bond is weak because there are no d- electrons to form a π-bond. The bound olefin then inserts into a metal-carbon bond within the complex via a four center transition state, forming new metal-carbon and carbon-carbon bonds. This process then repeats, extending the chain indefinitely. 8