Download No Slide Title - Cobalt

DFT studies on the Co-monomer Binding
in the Copolymerization of a-Olefins
with Polar Monomers
Catalyzed by Late Transition Metal Complexes
Artur Michalaka,b and Tom Zieglera
aDepartment
of Chemistry,
University of Calgary,
Calgary, Alberta, Canada
bDepartment
of Theoretical Chemistry
Jagiellonian University
Cracow, Poland
Introduction
A new emerging frontier in olefin polymerization is the controlled
copolymerization of a-olefins with monomers bearing a polar functional group.
Of particular interest are the copolymers of monomers with oxygen-containing
polar groups. In order to incorporate polar monomers into a polymer chain in
random copolymerization process, it is required that its insertion follows the
same reaction mechanism, as that of a-olefin Ziegler-Natta polymerization.
Thus, the polar monomer must also be initially bound to the metal center by its
double C=C bond rather than by the oxygen atom of the polar group. Therefore,
it seems to be important, that the stabilization energy of the p-complex is larger
than that of complexes in which the monomer is bound by polar group.
Scheme 1. Initial steps in the copolymeryzation of a-olefins with methyl acrylate.
2
3
In the present study we have computationally investigated the binding mode of
the oxygen-containing monomers (methyl acrylate, vinyl acetate, and their
fluorinated analogues) in the complexes involving cationic and neutral Ni- and Pdbased catalysts with the Brookhart and Grubbs ligands.
The static, gradient corrected DFT calculations have been performed for two
binding modes: the p-complexes in which a polar monomer is bound by its olefinic
fnctionality, and the O-complexes with a monomer bound by its carbonyl oxygen.
The role of the electronic and the steric effects has been investigated, by
considering the simplified (generic) models and the examples of the real catalysts. An
energy decomposition of the contributions to the binding energies has been performed in
order to understand the origin of the differences between Brookhart Pd- and Ni-based
system (active and inactive copolymerization catalyst).
Also discussed are the effects of the reduced basicity of the carbonyl oxygen on
the monomer (in the fluorinated compounds) as well as of the reduced oxophilicity of
the catalyst (in the neutral Grubbs system).
Further, the ab initio Molecular Dynamics simulations have been performed to
explore the free enegy surface of the acrylate complexes with Pd- and Ni-based diimine
catalysts. The stability of both types of complexes at finite temperatures has been
studied by free (unconstrained) MD simulation. Also, the slow-growth MD simulations
for the inter-conversion between the two acrylate binding modes (p- and Ocomplexes)have been performed.
4
PART I. Static DFT calculations
for the p-complexes and O-complexes
DFT calculations (ADF program) with Becke-Perdew XC functional;
triple-zeta STO basis set for Pd, double-zeta with polarization function for C, N, O, F,H;
frozen core: 1s for C,N,O, F, 1s-2p for Ni, 1s-3d for Pd;
energies with the first-order scalar relativistic correction.
5
Catalyst Models:
3b
1b
6a
2b
4a
5a
Table 1. The monomer binding energies for the generic models
for the Ni- and Pd-based Brookhart and Grubbs catalysts.
Catalyst
Monomer
DE (C=C)1 DE (O)2
6
O-
E(C=C) - E(O)2
1a. Brookhart/Ni
1b. Brokhart/Pd
MA
MA
-17.10
-20.70
-21.10
-17.30
+4.00
-3.40
1a. Brookhart/Ni
1b. Brokhart/Pd
VA
VA
-17.07
-20.12
-17.75
-14.96
+0.68
-5.16
1a. Brookhart/Ni
1b. Brokhart/Pd
FMA
FMA
-13.93
-17.95
-16.25
-12.92
+2.32
-5.03
1a. Brookhart/Ni
1b. Brokhart/Pd
FVA
FVA
-11.41
-14.76
-9.99
-8.10
-1.42
-6.66
3a. Grubbs/Ni
3b. Grubbs/Pd
MA
MA
-17.74
-24.34
-10.18
-10.17
-7.56
-14.17
3a. Grubbs/Ni
3b. Grubbs/Pd
VA
VA
-16.09
-21.72
-9.72
-9.56
-7.18
-12.16
p-complex stabilization energy, in kcal/mol;
2 stabilization energy of the O-complex, in kcal/mol;
3 the difference in the energies of the p-complex and O-complex;
p-
1a (Ni)
p-
O-
1b (Pd)
1
Fig 1. MA p- and O-complexes
with diimine catalysts
The results of Table 1 show:
• A preference of the MA and VA O-complexes in the case of the Ni-diimine catalyst
(inactive in copolymerization).
• A preference of the MA aqnd VA p-complexes for Pd-diimine system (active
copolymerization catalyst).
• A decrease in the preference of the O-complex or/and increase in the preference of the pcomplexes for the fluorinated monomers (with reduced basicity of their carbonyl oxygen).
This is accompanied by the substantial decrease of the absolute binding energies for both,
O- and p-complexes, that would result in substantially lower incorporation of the polar
monomer in a prospective copolymerization.
• A strong increase in the preference of the p-binding mode for the complexes involving the
neutral Grubbs catalysts based on both, Ni and Pd. Here, unlike for the fluorinated
monomers, the absolute stabilization energies of the p-complexes are not decreased,
compared to the cationic diimine catalysts.
A decomposition of the binding energies (Fig. 3) indicates that the difference between the
Ni- and Pd- diimine catalyst has mainly {electrostatic + Pauli repulsion} origin. Concerning
a comparison between the two binding modes, there is practically no difference in the
orbital interaction terms. This confirms, that use of the nutral catalysts with the reduced
electrophilicity of the metal can be promising.
7
8
p-
Table 2. Key catalyst-monomer interatomic
distances in the p- and O-complexes with
the generic catalyst models.
Catalyst
Monomer
1a. Brookhart/Ni
1b. Brokhart/Pd
1a. Brookhart/Ni
1b. Brokhart/Pd
1a. Brookhart/Ni
1b. Brokhart/Pd
1a. Brookhart/Ni
1b. Brokhart/Pd
3a. Grubbs/Ni
3b. Grubbs/Pd
3a. Grubbs/Ni
3b. Grubbs/Pd
MA
MA
VA
VA
FMA
FMA
FVA
FVA
MA
MA
VA
VA
M-C1
M-O2
2.06, 2.10
2.21, 2.25
2.03, 2.16
2.19, 2.30
2.05, 2.09
2.21, 2.25
2.03, 2.10
2.20, 2.26
2.02, 2.04
2.17, 2.20
2.03, 2.06
2.19, 2.24
1.91
2.14
1.92
2.15
1.91
2.16
1.95
2.20
1.95
2.18
1.97
2.19
metal-carbon distances in the p-complex; in A;
2 metal-oxygen distance in the O-complex; in A.
1
p-
3a (Ni)
O-
O-
Fig 2. The alternative, cis- (left) and trans- (right)
MA p- (top) and O-complexes (bottom) with the
generic model for the Grubbs catalysts. In the case
of the p-complexes the cis-isomer is preferred, while
the trans-O-complex is more stable.
9
Binding Energy Decomposition
DE = DE geom.+ DEtot.
= DE geom.+ [DEsteric + DE orb.int.]
= DE geom.+ [ (DEel. + DEPauli ) + DE orb.int. ]
where
DE geom. is a geometry distortion term,
DE tot. is a total interaction energy between
the distorted reactants.
The latter can be further decomposed into
the orbital interaction term (stabilizing,
two-orbital, two-electron interactions),
DEorb.int., and the steric contribution
consisting of the electrostatic interaction
(stabilizing or destabilizing) and the Pauli
repulsion (destabilizing, two orbital, fourelectrons interaction), DEsteric = DE el. +
DEPauli.
Fig. 3. A schematic representation of the differences in the contributions to the bonding energies between the O- and pcomplexes of the methyl acrylate and vinyl acetate with the Ni- and Pd- based Brookhart catalysts. Panels a-e display the
differences in the orbital interaction, steric, electrostatic, Pauli repulsion and geometry distortion contributions,
respectively. The arrows point the preferred complex of the respective pair. The numbers correspond to the differences in
respective contributions (for vinyl acatate in parantheses).
Table 3. The binding energies for MA complexes
with the real Brookhart and Grubbs catalysts.
Catalyst
DE (C=C)1 DE (O)2
E(C=C) - E(O)3
2a. Brookhart/Ni -10.10
2b. Brokhart/Pd -13.65
-13.09
-10.64
+2.99
-3.01
4a. Grubbs/Ni
5a. Grubbs/Ni
6a. Grubbs/Ni
-6.49
-7.51
-7.31
-6.33
-4.99
-5.84
-12.82
-12.50
-13.15
O-
2a (Ni)
p-
p-complex stabilization energy, in kcal/mol;
stabilization energy of the O-complex, in kcal/mol;
3 the difference in the energies of the p-complex and O-complex, in kcal/mol;
1
2
2b (Pd)
The results of Table 3 show that:
• The steric bulk of the real catalysts results in decrease of
the absolute binding energies for both, O- and p-binding
modes (compare with Table 1).
• There is practically no steric effect on the preference of
the binding mode. In the case of the real Ni-diimine
catalyst the O-complex stays preferred, while for the Pddiimine and Grubbs Ni-based catalysts the p-complex is
substantially more stable.
p-
6a (Ni)
Fig 4. The most stable MA complexes
with the real catalysts.
10
PART II. MD studies on the stability of the p- and
O-complexes and the interconversion pathways
Pd-p
Pd-O
Ni-p
Ni-O
1) For the p- and O-complexes with the Ni- and Pd-diimine catalysts the free
(unconstrained) molecular dynamics simulations at 300 K were performed.
In addition, for the local minima (Pd/O- and Ni-p) similar simulations at 700 K
were performed.
2) For all four possible inter-conversion reactions (Pd: Op; Pd: p O; Ni:
Op; Ni: p O) the slow-growth MD simulation (300K) with the substitution
constraint, R(Me-O)-R(Me-C)=const., changing between the values characterizing
the respective initial (O/p)and the final (p/O) complexes.
The constrained MD were followed by the relaxation simulations (300 K free
dynamics), starting from the final stage of the slow-growth MD.
11
Pd: p
R [A]
R [A]
Pd: O
RPd-C (300K)
RPd-C (700K)
RPd-O (300K)
RPd-O (700K)
RPd-C (300K)
RPd-O (300K)
timestep
timestep
Ni: p
R [A]
R [A]
Ni: O
RNi-C (300K)
RNi-C (700K)
RNi-O (300K)
RNi-O (700K)
timestep
RNi-C (300K)
RNi-O (300K)
timestep
Fig 5. The two M-C(p) and the M-O distances from the unconstrained MD simulations for the MA O- and pcomplexes with the Ni- and Pd-diimine catalysts.
12
13
Stability of the O- and p-complexes
on the free-energy surfaces
The free MD simulations (Fig.5) indicate that both, O- and pcomplexes for both, Ni- and Pd-based catalysts form stable minima on the
free-energy surfaces and are are separated by non-negligible barriers.
All the complexes at 300K stay in a relatively rigid geometries and
they do not inter-convert spontaniously.
The higher energy complexes (local minima; Pd/O, and Ni/p) stay
in their geometries at 700K, and do not evolve toward the global minima
(Pd/p and Ni/O).
Inter-conversion: Pd catalyst; p-complex toward O-complex
Fig. 6.
R[A]
constrained
dynamics
relaxation
RPd-C
RPd-O
RPd-C-RPd-O [A]
timestep
The MA p-complex with Pd-diimine
catalyst does not directly inter-convert
toward the O-complex. The MD simulation
reveals a dissociative pathway; it leads to
the b-agostic alkyl complex (see also Fig.
10) with the acrylate molecule attached to
the NH group of the catalyst with a
hydrogen bond.
14
Inter-conversion: Pd catalyst; O-complex toward p-complex
15
Fig. 7.
R[A]
constrained
dynamics
relaxation
RPd-C
RPd-O
RPd-C-RPd-O [A]
timestep
During the constrained MD simulation,
the MA O-complex with Pd-diimine
catalyst directly converts into one of the
possible p-complexes, that can be easily
transformed into the global minimum
complex by a rotation of MA and/or alkyl.
Figs. 6-9. Top: the geometries from the constrained
MD simulation at s=0.0, 0.25, 0.5, 0.75, 1.0 (sreaction progress variable). Bottom: theM-C(p) and
M-O distances from the constrained MD and the
relaxation (unconstrained) simulations.
Inter-conversion: Ni catalyst; p-complex toward O-complex
Fig. 8.
R[A]
constrained
dynamics
relaxation
RNi-C
RNi-O
RNi-C-RNi-O [A]
timestep
Similarly to the Pd-case, the p-complex
with the Ni-based catalyst dissociates
before going toward the O-complex.
However, the relaxation run leads to the
structure with a-agostic (not the bagostic, as in the Pd-case; see also Fig.
10). It may be expected that this aagostic complex can easily lead to the
O-complex.
16
Inter-conversion: Ni-catalyst O-complex toward p-complex
17
Fig. 9.
R[A]
constrained
dynamics
relaxation
RNi-C
RNi-O
RNi-C-RNi-O [A]
timestep
Unlike for the Pd-catalyst, in the Ni-case
the O-complex does not inter-convert
into the p-complex. The structure formed
here is the ‘mixed’ O-p-complex (1,4arrangement). Although it is a higherenergy intermediate, it forms stable
minimum at the potential energy- and the
free energy surfaces. In the Pd-case such
intermediate either does not exist, or
forms an extremely shallow minimum.
18
Fig. 10. Final products of the four inter-conversion simulations (after a relaxation simulations).
Top-left: Pd-catalyst; an alkyl b-agostic complex with hydrogen-bonded MA molecule (from p  O
simulation). Top-right: Ni-catalyst; an a-agostic complex (from p  O simulation). Bottom-left: Pdcatalyst; the p-complex (from Op simulation). Bottom-right: Ni-catalyst; the 1,4-MA complex (from
Op simulation).
Energetics of the inter-conversion reactions
constrained
dynamics
relaxation
Ni: p  O
Ni: O  p
Pd: p  O
Pd: O  p
timestep
timestep
Fig. 11. The average potential energy (running average with a window of 200 timsteps) from the
constrained and unconstrained MD simulations. An zero value for the energy corresponds to the
preferred complexes (p-complex in the Pd-, and to the O-complex in the Ni-case)
19
Conclusions
• A comparison of the binding mode of polar monomers for the Ni- and Pd-based diimine
complexes (inactive and active co-polymerization catalysts) shows that the preference of the Obound complex in Ni case is reversed in Pd-based system. Further, the difference between the two
catalysts has mainly {electrostatic + Pauli repulson} origin.
•Thus, use of the neutral catalysts in co-polymerization processes seems to be promising. Indeed,
in the case of Grubbs ligand, the p-complex is strongly preferred already in the Ni-system; this
preference is enhanced for Pd catalyst. The absolute p-complexation energies for the Grubbs
catalyst are comparable with those of the diimine systems.
• In the complexes with fluorinated monomers, the preference of the O-binding mode is decreased,
but with a price of decreased absolute complexation energies. This would lead to a low
incorporation of the fluorinated monomer in the co-polymerization.
• The presence of the steric bulk in the real catalysts does not affect the preference of the binding
mode.
• The MD simulations show that all the complexes are stable on the free-energy surfaces and do
not exhibit ant tendency toward a spontaneous inter-conversion. Further, the complexes are
separated by relatively large barriers. In the Ni-case the O p inter-conversion reaction is difficult
and leads first to another ‘inactive’ intermediate, the 1,4-MA complex.
• Thus, an analysis of the polar monomer binding mode can be used as a screening test in a
search for the active co-polymnerization catalyst: the systems with a strong preference of the
O-binding mode can be excluded from further studies.
Acknowledgements. This work was supported by the National Sciences and Engineering Research Council of Canada (NSERC), Nova
Chemical Research and Technology Corporation as well as donors of the Petroleum Research Fund, administered by the American
Chemical Society (ACS-PRF No. 36543-AC3). A.M. acknowledges a University of Calgary Postdoctoral Fellowship. Important parts of the
calculations was performed using the UofC MACI cluster.
20