Bent Metallocenes and Ziegler Natta Catalysis the Electroni

Home , Metallocene, Oxidative addition, Sandwich compound

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

The electronic structure of organometallic complexes follows directly from the sandwich compounds of the last lecture. Focusing on the HOMO (a1g) and HOMO+1 (e2g) of the sandwich compound, as the Cps (cyclopentadienyls) are bent back,

the following energy correlation diagram results:

2a1

The antibonding component (arising from a1g) is a1g b2 considerably destabilized

1a1 e2g

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 Cp2Ti, θ = 136°,

% Composition Orbital Cp2 Ligands Metal

2 2 2 2a1 21 33 z , 27 x –y , 11 s, 8 z

b1 34 64 yz, 2 y

2 2 2 1a1 18 51 x –y , 30 z , 1 s

What about protonation of Cp2M? Here we are mixing a H(1s) orbital to the frontier molecular orbitals of the bent Cp2M fragment:

For clarity, the Cp2M fragment will be rotated so that the line of site is along the z-axis

This diagram with orbital occupancy + corresponds to Cp2ReH or Cp2FeH (both are known compounds).

More than one ligand can fit into the wedge. The Cp2MX2 (M = Ti, Zr, Hf) compounds are convenient starting materials for a number of organometallic compounds. Again, rotating the Cp2M fragment for greater clarity:

The dihydride complexes have a similar electronic structure. The H2 may be removed photolytically, and in the presence of CH bonds such as those of benzene, oxidative-addition may occur,

Linked Cp systems are called “ansa” metallocenes. The linker can stabilize the system to reductive elimination of substituents within the wedge,

For the methyl hydride compound, H/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”). Consider ethylene:

The MOs of the ethylene molecule are:

Combining the MOs of ethylene with the wedge orbitals of bent Cp2M yields the following MO diagram:

3a1 b2

2a1

though of proper symmetry to interact with the a1 MO of ethylene, very little overlap, so little perturbed energetically b2

1a1 2a1

b2 1a1+ *

a1 Cp2Ti C2H4 Cp Ti 2

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 (HDPE, R = H), linear low density polyethylene (LLDPE, R = mostly H with some Et, Bu or Hx), polypropylene (R = Me) and ethylene-propylene- diene-modified rubber (EPDM, R = H, Me and alkenyl).

Catalyst 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. Current industrial catalysts include TiCl4/MgCl2/AlEt3 (for LLDPE, Dow), CrO3/Al2O3/AlEt3 (for HDPE, Phillips) and VOCl3/AlEt3 (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. Cp2ZrCl2 in combination with methylalumoxane ([MeAlO]n, MAO, a hydrolysis product of AlMe3) 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 Cp with the C2 symmetric “ansa” ligand.

The language of stereoregular polymers also includes:

Cationic bent Cp2M catalysts with weakly coordinating anions are especially active catalysts. For instance, reaction of early transition metal alkyl complexes, such as

Cp´2ZrMe2 with oxidizing tetraphenylborate salts including AgBPh4 and (Cp2Fe)BPh4.

weakly coordinating 0 – – d , 16e species anion 1e oxidant

O CH 3 + + Zr B Fe CH3

CH3 Cp2Fe

CH3 + Zr B O

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 d0 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 C–H 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:

Agostic interactions can be detected by various spectroscopic measurements: • 1H NMR. The agnostic H is shifted away from the usual chemical shift range for alkyl and aryl and appears between +25 to –60 ppm • The IR CH stretch is reduced • The M- - -H—C 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:

– -agostic -agostic 16e -agostic -agostic transition state resting state complex complex H2 H2 C C CH + 2 Zr CH2 Zr H Zr CH CH3 Zr CH CH3 H C CH2 H H H2

H2 H2 C C + CH2 polyethylene Zr CH CH2 CH3 Zr CH H H CH3 -agostic -agostic complex complex

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.