Download 1 5.03, Inorganic Chemistry Prof. Daniel G. Nocera Lecture 4 Apr 11

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
a1g
b2
e2g
1a1
The antibonding component
(arising from a1g) is
considerably destabilized
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°,
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% Composition
Orbital
Cp2 Ligands
Metal
2a1
21
33 z2, 27 x2–y2, 11 s, 8 z
b1
34
64 yz, 2 y
1a1
18
51 x2–y2, 30 z2, 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).
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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,
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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:
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b2
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
a1
Cp2Ti
C2H4
Cp2Ti
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 σbonds, 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-propylenediene-modified rubber (EPDM, R = H, Me and alkenyl).
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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.
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d0, 16e– species
weakly coordinating
anion
1e– oxidant
O
Zr
CH3
CH3
+
B
+
Fe
CH3
Zr
CH3
O
Cp2Fe
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 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:
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Agostic interactions can be detected by various spectroscopic measurements:
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•
H 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 16e–
resting state
H2
C
+
CH2
Zr
H
polyethylene
-agostic
complex
Zr
-agostic
complex
H
CH CH3
Zr
C CH2
H2
-agostic
transition state
H2
C
CH2
H
H
H2
C
H2
C
+
CH CH2 CH3
Zr
CH CH3
Zr
H
-agostic
complex
Zr
CH2
CH
H
CH3
-agostic
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 delectrons 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.
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