Chemistry Reference
In-Depth Information
Lohrenz et al. reported [
296
] quantummechanical calculations on model systems [
297
] support the
Cossee-Arleman mechanism. Insertion of ethylene into Cp
2
Zr
+
-Me is preceded by an initial olefin
complexation with the vacant coordination site and formation of
p
-complex 1 as shown below:
complex 1
complex 2
complex 3
H
H
H
H
H
H
H
H
H
C
C
C
H
H
Zr
Zr
C
Zr
H
H
C
H
C
C
C
C
H
H
-128 kJ/mol
H
H
H
-93 kJ/mol
-96 kJ/mol
complex 3a
complex 3b
H
H
H
H
H
H
C
C
H
C
Zr
H
C
Zr
H
H
C
C
H
H
H
H
0.0 kJ/mol
-22 kJ/mol
After the formation of complex 1, the insertion follows a low activation barrier (
E
act
ΒΌ
3 kL/mol),
yielding the
-agostic product 3a. These calculations also led to the conclusion about the importance
of agostic interactions present in the product [
296
]. The most stable conformation is the
g
-agostic
structure (3b) at 22 kJ/mole that is more stable than the primary insertion product (3a). It is assumed
that this conformation serves as a model for the resting state of the growing chain attached to the
cationic group-4 metallocenes between insertions [
297
].
To shed additional light on complex 3b, shown above, calculations were carried out [
296
] on the
reaction of ethylene with a model complex, Cl
2
Zr + Et. Two reaction paths (A) and (B) are possible
as illustrated below:
b
CH
2
CH
2
HC
CH
3
insetion
and inversion
A
Zr
front side
attack
H
CH
2
A
CH
2
CH
2
H-exchange
and chain
Zr
Zr
CH
2
H
termination
H
2
C
B
H
Zr
CH
2
inserion
no inversion
CH
2
back side attack
B
CH
2
CH
2
