Transcript Bis-amides and Amine Bis-amides as Ligands for Olefin

M(XCHX)2R+ and M(XCHCHCHX)2R+
(M=Ti,Zr ; X= NH, O, S) as olefin
polymerization catalysts and the role of
ligand conjugation: A Density Functional
Theory(DFT) study
Timothy K. Firman and Tom Ziegler
University of Calgary
1
Introduction
Many of the best olefin polymerization catalysts include p-conjugated ligands.
These ligands can change the extent of their bonding to transition metals by
changing the bonding along the conjugated ligand. For example:
HN
H
C
M
+
-
NH
H
C
HN
M
NH
HN
H
C
NH
M
This variable bond order compensates for other metal-ligand bonding changes,
such as the net loss of a metal-olefin bond during olefin insertion. A series of
compounds with p-conjugated ligands bound to group IV metals is modeled
using DFT to examine bonding and catalytic properties. The metal binds to an
NH, an O, and an S , quite different chemically but can be considered to be
isolobally analogous, with similar p -conjugation and variable bond order. By
varying these heteroatoms, a range of different properties was expected.
2
Computational Details
All structures and energetics were calculated with the Density Functional Theory
(DFT) program ADF1. All atoms were modeled using a frozen core approximation.
Ti was modeled with a triple-z basis of Slater type orbitals (STO) representing the
3s, 3p, 3d, and 4s orbitals with a single 4p polarization function added. Zr was
modeled similarly with a triple-z STO representation of the 4s, 4p, 4d, 5s, and a
single 5p polarization function. Main group elements were described by a double-z
set of STO orbitals with one polarization function (3d for C, N, and O; 4d for S; and
2p for H.)2 In each case, the local exchange-correlation potential3 was augmented
with electron exchange functionals 4 and correlation corrections5 in the method
known as BP86. First-order scalar relativistic corrections6 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
b-hydride transfer, transition states were found using a standard stationary point
search to a Hessian with a single negative eigenvalue. All calculations were spin
restricted and did not use symmetry. All energies are in kcal/mol unless otherwise
stated.
3
M(XCHX)2
With only one carbon between them, the bite
angle of each ligand is only about 70˚.
Ligands are not especially bulky, but sterics will
be a factor.
In most cases, the two chelating ligands are
canted, making the environment asymmetric
Some experimentally known analogues are
known.7
Some alkyls bind with an a-agostic rather than a
b-agostic bond.
4
Uptake Enthalpy XCHX Systems
N
C
N
C
Ti
C
N
N
C
C
Ti
C
C
N
N
N
C
N
C
C
Eu ptake Ere organiz ation
Model System
+
3
(HNCHNH)2TiCH2CH
(OCHO)2TiCH2CH3+
(SCHS)2TiCH2CH3+
-4.4
-9.8
-8.6
+9.7
+7.4
+7.8
Ereorganization is the energy
required to distort the alkyl
minimum to the shape of the
adduct (minus the ethylene)
 The metal starts pseudo-trigonal planar then becomes very roughly tetrahedral.
 The metal-ethylene bond energy would be about 16 kcal/mol in each case, but
moving the alkyl out of the plane incurs a significant energetic penalty, which
is labeled Ereorganization
5
Entropy and Uptake Energy
 Previous comparisons of computed and actual d0 systems correlate
better activity for systems with larger uptake energy, with
improvement through at least -10kal/mol.8
 Binding an olefin will be significantly entropically unfavorable.
 Entropy is calculated for this one example. It is not expected to differ
substantially between these systems.
HN
H
C
+
HN
Ti
Ti
HC
HC
N
H
DS:
+
NH
H
N
NH
H
N
H
C
N
H
CH2 CH3
115 cal/molK
+
55 cal/molK
-
CH2 CH3
121 cal/molK
DDS= 50 cal/molK
At 300K, this reaction is entropically unfavorable by 15 kcal/mol.
At 400K, this will be equal to 20 kcal/mol.
This is larger than the enthalpic contribution and repulsive.
6
Catalytic Properties of XCHX system
C
C
C
N
C
N
N
C
C
N
Ti
C
N
Ti
N
C
N
C
N
C
C
C
Model System
Einsertion Ete rm in ation
(HNCHNH)2TiCH2CH3+ +5.8
(OCHO)2TiCH2CH3+
+4.2
(SCHS)2TiCH2CH3+
+4.2
+5.9
+4.5
+10.2
 b-hydride transfer is the dominant termination mechanism
 While all three insertion barriers are quite low, the termination barrier is far
too low for O and NH.
 In the O and NH cases, the ligands become non-planar during the b-hydride
transfer, while the S ligands do not.
 The ligands bend out of plane because they are no longer p-bound to the
metal; the transfer transition state has more bonds to C and H than the others,
and these bonds displace the metal-ligand p bonding.
7
Zr compounds
Many Zr catalysts are known
Zr was used instead of Ti in a series of
otherwise identical computations
In comparison with Ti,



Ti and Zr are chemically similar
Zr is larger, reducing steric interactions
Zr tends to form stronger bonds, which should
improve Euptake
8
Results with Zirconium Center
Model System
Eu ptake Ere organiz ation Einsertion Ete rm in ation
(HNCHNH)2ZrCH 2CH3+ -11.0
(OCHO)2ZrCH 2CH3+
-14.3
(SCHS)2ZrCH 2CH3+
-12.3
+9.7
+6.6
+4.9
+4.4
+9.7
+7.3
+5.3
+9.3
+7.1
The uptake energy is significantly improved
The termination barrier is about equal to the
insertion barrier in all three cases, indicating that
none of these would catalyze polymerization
These Ti and Zr compounds gave similar results
overall
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Six Member Metal Ring Systems
HC
CH
HC
CH
HN
NH
HN
NH
M
M
H
C
H
C
HC
H
C
H
C
HC
CH
HC
CH HC
CH
HN
NH
HN
NH
HN
NH
M
+
-
M
+
M
-
+
 Somewhat similar to the earlier systems, but an extra two
doubly bonded carbons are added.
 Like the smaller ring, the metal-ligand bond order is flexable;
some resonance structures are shown above
 This longer linker results in a wider bite angle


Steric effects may be more important with wider ligands
b-hydride transfer is more sterically demanding than insertion
 Some experimental analogues are great catalysts
10
Uptake Energies
Model System
Eu ptake Ere organiz ation
(HNCHCHCHNH)2TiCH2CH3+
(OCHCHCHO)2TiCH2CH3+
(SCHCHCHS)2TiCH2CH3+
(HNCHCHCHNH)2ZrCH 2CH3+
(OCHCHCHO)2ZrCH 2CH3+
(SCHCHCHS)2ZrCH 2CH3+
-2.1
+0.9
-3.1
-6.9
-2.3
-6.2
+11.7
+15.7
+11.8
+6.5
+13.1
+13.3
 All uptake energies for these systems are poor.
 Increased steric hinderance may repel incoming ethylene
 Reorganization energies are high; the alkyl requires a lot of
energy to be bent away from the plane
 As before, the Zirconium energies are somewhat better due
to stronger bonds to ethylene, but not by much.
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Transition States
Model System
Ei nsertion Etransfe r
(HNCHCHCHNH)2TiCH2CH3+
(OCHCHCHO)2TiCH2CH3+
+0.9
(SCHCHCHS)2TiCH2CH3+
+3.4
(HNCHCHCHNH)2ZrCH 2CH3+
(OCHCHCHO)2ZrCH 2CH3+
+2.0
(SCHCHCHS)2ZrCH 2CH3+
+5.6
+5.9
+5.8
+5.3
+4.4
(Blank spaces are transition states which have not been found to date)
 While the insertions are quite facile, so is b-hydride
transfer.
 The alkyl and ethylene are in a long, narrow space between
the two rings; the b-hydride transfer transition state is just
such a shape. Steric effects may actually encourage
termination in these cases.
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Known, Analogous Catalysts
'RN
R
C
+
NR'
R'
N
O
RC
N
R'
(7)
N
O
O
Ti
CH2 CH3
+
+
O
O
Zr
Ti
O
(9)
CH2 CH3
N
CH2 CH3
(10)
 All three of these have substantial catalytic activity, yet
similar models show poor catalyic characteristics.
 The other characteristics of these systems may be
important, such as sterics or the electronic effects of
phenyl rings.
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Larger model of Matsui Catalyst
10
C
C
C
C
C
C
C
C
C
C
O
N
C
C
O
Zr
C
C
N
C
C
C
C
C
N
C
O
C
C
Zr
C
C
N
C
C
C
O
C
C
C
C
C
 On the right are two
minima, one with and
one without olefin
 Uptake energy
calculated to be:
-4.2 kcal/mol
 This is not what we
would expect for such a
good catalyst
C
C
14
Conclusions and Future Work
 With the possible exception of the SCHS ligand, the systems
examined appear to be poor candidates for catalysts
 Insufficient uptake energy is a constant problem
 There exist real, active catalysts very similar to these
systems, so there may be a problem with our model or with
our criteria for identifying promising catalysts.
 The most likely flaw in our model is the lack of a counterion



Catalytic activity can vary widely with counterion, particularly in d0
cases, and this model does nothing to simulate a counterion.
A coordinated counterion would bend the alkyl out of the plane,
which might lower reorganization costs to uptake.
The departure of the counterion would be entropically favorable,
offsetting the enthalpic penalty.
 Models which include a counterion will be studied.
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Acknowledgements
This research was supported by the Natural Sciences and Engineering
Research Council of Canada (NSERC) and Novacor Research and
Technology Corporation.
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