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

Bis-amides and Amine Bis-amides as
Ligands for Olefin Polymerization
Catalysts Based on V(IV), Cr(IV) and
Mn(IV). A Density Functional Theory
Study
Timothy K. Firman and Tom Ziegler
University of Calgary
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Outline
 RM(NH2)2NH3+ (M=V,Cr,Mn) :
bonding and ethylene polymerization
Second row analogies (M=Mo, Ru,
Pd)
Linking nitrogen ligands with ethyl
bridges: effects on bonding mode and
catalytic properties
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Computational Details
All structures and energetics were calculated with the Density Functional Theory
(DFT) program ADF. All atoms were modeled using a frozen core approximation.
V, Cr, and Mn were 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. Mo, Ru, and Pd were similarly modeled 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 STO orbitals with one polarization function (3d for C, N and
2p for H.) In each case, the local exchange-correlation potential was augmented with
electron exchange functionals and correlation corrections in the method known as
BP86. First-order scalar relativistic corrections 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 unrestricted
and did not use symmetry. The Boys and Foster method was used for orbital
localization, and the orbitals were displayed using the adfplt program written by
Jochen Autschbach.
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1
d V,
2
d Cr
,
3
d Mn:
a Comparison
All three metals were high spin in compounds analyzed
As the number of SOMOs increases, the metal will
have correspondingly fewer available bonding orbitals
Amides can bind with either single or double bonds,
depending on the metal’s available orbitals
Metal bonding orbitals are often shared between
ligands, e.g. trans- ligands share a single s-bonding
metal orbital, and can also share a p-bond.
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Shared Orbitals: trans-NH2
H2N-Cr-NH2 p orbitals in b-hydride transfer TS
Two of four phase-combinations of four Boys localized orbitals
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NH2 s orbitals
H2N-Cr-NH2 s orbitals in b-hydride transfer TS
These two ligands only bind with a total of two metal orbitals
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Metal Alkyl Structures
NH
H2
C
d1V is nearly tetrahedral
+
2
NH
N
V
H2C
H
V
C
NH
N
107 °
2
10 6°
C
3
N
NH 2
NH 2
+

N
N
C

Cr
Cr
NH2 are flat, with p bonds not
in the same plane
a b-agostic hydride
d2Cr is nearly tetrahedral
122°
CH 3 CH 2

C
115°
NH 3

NH2 are flat, p aligned(shared)
no b-agostic hydride
N
1 45 °
NH 2
NH 2
CH 3 CH 2
d3Mn includes a 146˚angle
N
+
N
95 °
Mn

Mn
N
C
NH 3
C
11 1°

NH2 are bent out of plane due
to weakened p -interactions
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no b-agostic hydride
Olefin Adduct
C
C
d1V is trigonal bipyramidal
80°
8 3°
N
C
V

111°
C
N

9 5°
NH2 are flat, p bonds unshared
b-agostic hydride
N
C
C
d2Cr is trigonal bipyramidal
83 °
N
142 °

Cr
C
C
N

8 8°
NH2 are flat, p bonds aligned
No b-agostic hydride
N
d3Mn is trigonal bipyramidal
N
90°
90°
N

C
Mn
C
C
C

96°
N

NH2 are bent out of plane
NH2 is apical instead of ethene
No b-agostic hydride
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Energies of NH2 Rotation
H -N -V-N
0
0
+1.3
45 
45 
+3.5
+0.04
+8.4
+4.1
90 
H -N -C r-N
0
45 
90 
90 
+10.1
0
+5.6
45 
90 
+7.5
+8.4
+3.4
+3.8
+0.9
 A 90 torsion directs the π to the
plane of the other N
 At 90 and 90, the two π orbitals
are in the same plane- both N share
a single metal orbital.
 V prefers two separate π orbitals
 Cr prefers to share one π orbital
between both ligands
 The difference is due to the Cr’s
additional unpaired electron
Energies are in kcal/mol
Relative to minimum
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Insertion
C
DE (Barrier)
2
.0
1
5
Å
C
C
N
V
13 3°
+16.3kcal/mol
N
C
N
C
C
2.15 0Å
C
N
C
Cr
14 6 °
N
+12.5 kcal/mol
92°
N
Insertion barriers are
similar
Geometries are quite
different from one
another
Each has a ligand trans
to a forming or
breaking bond
C
N
85 °
C
+13.6 kcal/mol
2 .3 2 4 Å
C
Mn
N
C
N
93 °
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Localized Orbitals:
insertion
Olefin insertion of d2Cr
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Termination (b-hydride transfer)
DE
N
C
83°
C
+16.4 kcal/mol
V
N
C
82°
C
N
C
C
9 0°
N
+19.6 kcal/mol
Cr
N
N
C
8 9°
C
C
C
92 °
N
+20.0 kcal/mol
Mn
N
Each termination barrier
is higher than insertion
No b-hydride elimination
TS found lower in energy
than this transfer
The amine is either trans
to the hydride, or to one
of the reacting M-C
bonds
N
8 6°
C
C
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Localized Orbitals:
b-hydride transfer
Chain Termination
Transition State
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Enthalpies Summary
Ene rgy w it h re spec t t o reac ta nt s
E OC
E IN S ‡
E BH T ‡
Mo d el Cata lyst
+
V (N H 2 ) 2 (N H 3 )C 2 H 5
+
Cr (N H 2 ) 2 (N H 3 )C 2 H 5
+
M n (N H 2 ) 2 (N H 3 )C 2 H 5
-3.0
-5.2
-5.4
+13 .3
+7 .3
+8 .2
+13 .4
+14 .4
+14 .6
B arrier H eigh ts
D E IN S ‡ D E B H T ‡
+16 .3
+12 .5
+13 .6
+16 .4
+19 .6
+20 .0
Insertion and termination numbers are promising,
particularly for a system lacking steric bulk
Uptake energy is too low.


Entropy will be unfavorable by about 12-15 kcal/mol
Displacement of counterion will also effect uptake energetic
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The Second Row Transition Metals
Good second row olefin polymerization
catalysts exist, including d0 Zr and d8 Pd
Olefin uptake energies are expected to
increase due to generally stronger bonds
Model systems with d2 Mo,d4 Ru, and d6 Pd
were calculated
These compounds are found to be low spin
Compounds with a like number of occupied
metal orbitals may be analogous
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Second Row Results
H 2N
H 2N
M
H 3N
H 2N
Sys tem
M
H 2N
E up ta k e E ins ert ion E ter m in at ion
d 2 M o(N H 2 ) 2
+ 25
+ 19.6
d 4 R u (N H 2 ) 2
-38.9
+ 30
+ 31.0
d 6 P d (N H 2 ) 2
-14.3
+ 19.3
+ 23.3
d 2 M o(N H 2 ) 2 N H 3
-18.7
+ 23.8
+ 25.2
d 4 R u (N H 2 ) 2 N H 3
-38.9
+ 25
d 6 P d (N H 2 ) 2 N H 3
-14.3
+ 19.3
+ 21.0
While the uptake energies are substantially improved, these
combinations of ligand and metal do not result in good catalysts.
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Tethered Nitrogen Ligands
NH
HN
M
NH
Electronically similar to the previous systems
Chelation will keep the ligands bound
All three nitrogen will stay on one side; this will
leave the other side vacant and may help uptake
Limited conformational flexibility
Sterically unhindered, as in the untethered case
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Uptake Enthalpy of Linked System
M o d el Sy stem
D E up ta k e D E reo r gan iza ti on
L VCH 2 CH 3 +
-9.9
+ 11. 8
L CrCH 2 CH 3 +
-15 .3
+ 3.7
L M n C H 2 CH 3 + -3.2
DEreorganization is the energy
required to distort the alkyl
minimum to the shape of the
adduct (minus the ethylene)
+ 16. 4
 The metal-ethylene bond energy would be about 20 kcal/mol in
each case, but large differences in reorganization energy result in
differences in uptake energies.
 The shapes of the untethered ethylene adducts predict energetics



In the untethered Cr adduct, the two NH2 groups are near the NH3 group
with a hydrogen pointing toward it. The tethers hold it in just this position.
V has the N ligands close together, but must twist one of the NH2 groups.
Mn has an NH2 trans to the NH3, which cannot occur with a tether, so the
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uptake energy is actually worse with the tether than without.
Catalytic Properties with Tether
E nergy w it h respec t to re ac tan ts
Mo d el Sy st em
+
LV C H 2 C H 3
+
LC r C H 2 C H 3
+
L M nC H 2 C H 3
EOC
E INS
-9.9
-15.3
-3.2
-4.2
-3.2
+6 .4
‡
E BH T
-1.2
+10 .2
+15 .5
‡
B a rri er He igh ts
D E INS
‡
+5 .7
+12 .1
+9 .6
D E BH T
‡
+8 .7
+25 .6
+18 .8
 The tether has a large effect on the energetics, in a
substantially different way for each metals



In the V system, The N ligands are held in a position close to both
transition states; the energies of both are decreased.
In Cr, the insertion is close to the tethered case, but the untethered
termination prefers trans NH2 groups, which is impossible for the
tethered case. EBHT is much higher as a result.
In Mn, the tether causes each shape a similar energy penalty.
Energies are similar to the untethered case.
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Conclusions
The occupation of metal orbitals by single electrons
has a substantial chemical effect
NR2 can vary its bonding orbitals to compensate for
other bonding changes, such as during insertion
Tethering the ligands alters the energetics
differently for each transition state
Matching tether types with metal is important
Acknowledgements
This research was supported by the Natural Sciences and
Engineering Research Council of Canada (NSERC) and Novacor
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Research and Technology Corporation.