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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 polymerizationSecond row analogies (M=Mo, Ru, Pd)Linking nitrogen ligands with ethyl bridges: effects on bonding mode and catalytic properties
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Computational DetailsAll 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- 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- STO representation of the 4s, 4p, 4d, 5s, and a single 5p polarization function. Main group elements were described by a double- 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 -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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d1V, d2Cr , d3Mn: 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 -bonding metal orbital, and can also share a -bond.
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Shared Orbitals: trans-NH2
H2N-Cr-NH2 orbitals in -hydride transfer TSTwo of four phase-combinations of four Boys localized orbitals
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NH2 orbitals
H2N-Cr-NH2 orbitals in -hydride transfer TSThese two ligands only bind with a total of two metal orbitals
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Metal Alkyl Structures d1V is nearly tetrahedral
NH2 are flat, with bonds not in the same plane
a-agostic hydride
d2Cr is nearly tetrahedral NH2 are flat, aligned(shared) no-agostic hydride
d3Mn includes a 146˚angle NH2 are bent out of plane due to
weakened -interactions no-agostic hydride
V
NH3
NH2NH2
H2C
H2CH
+
Cr
NH3
NH2NH2
CH3CH2
+
Mn
NH3
NH2NH2
CH3CH2
+N
C
C
Mn
N
N
1 4 5 °
1 1 1 °
9 5 °
N
N
Cr
C
N
C
1 2 2 °
1 1 5 °
N
N
C
V
C
N
1 0 6 °
1 0 7 °
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Olefin Adduct d1V is trigonal bipyramidal
NH2 are flat, bonds unshared -agostic hydride
d2Cr is trigonal bipyramidal NH2 are flat, bonds aligned -agostic hydride
d3Mn is trigonal bipyramidal NH2 are bent out of plane
NH2 is apical instead of ethene -agostic hydride
N
N
C
V
C
C
C
N
1 1 1 °
9 5 °
8 3 °
8 0 °
N
C
N
Cr
C
CC
N
1 4 2 °
8 3 °
8 8 °
N
C
N
C
Mn
N
C
C
9 0 °
9 0 °
9 6 °
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Energies of NH2 Rotation
H-N-V-N 0° 45° 90°0° +1.3 +3.5 +0.0445° +8.4 +4.190° +10.1
H-N-Cr-N 0° 45° 90°0° +5.6 +7.5 +8.445° +3.4 +3.890° +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/molRelative to minimum
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Insertion Insertion barriers are
similar Geometries are quite
different from one another
Each has a ligand trans to a forming or breaking bond
E (Barrier)
+16.3kcal/mol
+12.5 kcal/mol
+13.6 kcal/mol
N
N
C
C
V
CC
N
1 3 3 °
2 . 0 1 5 Å
N
N
C
Cr
C
CC
N
1 4 6 °
2 . 1 5 0 Å
9 2 °
C
C
N
MnC
N
N
C
2 . 3 2 4 Å
9 3 °
8 5 °
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Localized Orbitals: insertion
Olefin insertion of d2Cr
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Termination (-hydride transfer)
Each termination barrier is higher than insertion
No -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
E
+16.4 kcal/mol
+19.6 kcal/mol
+20.0 kcal/mol
C
N
C
V
N
N
C
C
8 3 °
8 2 °
N
C
C
CrN
C
C
N
9 0 °
8 9 °
N
C
C
Mn
N
C
C
N
9 2 °
8 6 °
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Localized Orbitals:-hydride transfer
Chain TerminationTransition State
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Enthalpies Summary
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
Energy with respect to reactants Barrier HeightsModel Catalyst EOC EINS ‡ EBHT ‡ EINS ‡ EBHT ‡
V(NH2)2(NH3)C2H5+ -3.0 +13.3 +13.4 +16.3 +16.4
Cr(NH2)2(NH3)C2H5+ -5.2 +7.3 +14.4 +12.5 +19.6
Mn(NH2)2(NH3)C2H5+ -5.4 +8.2 +14.6 +13.6 +20.0
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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
System Euptake Einsertion Etermination
d2 Mo(NH2)2 +25 +19.6
d4 Ru(NH2)2 -38.9 +30 +31.0
d6 Pd(NH2)2 -14.3 +19.3 +23.3
d2 Mo(NH2)2NH3 -18.7 +23.8 +25.2
d4 Ru(NH2)2NH3 -38.9 +25
d6 Pd(NH2)2NH3 -14.3 +19.3 +21.0
While the uptake energies are substantially improved, thesecombinations of ligand and metal do not result in good catalysts.
H2N
M
H2N
H2N
M
H2N
H3N
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Tethered Nitrogen Ligands
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
NH
M
NH
HN
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Uptake Enthalpy of Linked System
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 uptake energy is actually worse with the tether than without.
Ereorganization is the energy required to distort the alkyl minimum to the shape of the adduct (minus the ethylene)
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Catalytic Properties with Tether
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.
Energy with respect to reactants Barrier HeightsModel System EOC EINS ‡ EBHT ‡ EINS ‡ EBHT ‡
LV CH2CH3+ -9.9 -4.2 -1.2 +5.7 +8.7
LCrCH2CH3+ -15.3 -3.2 +10.2 +12.1 +25.6
LMnCH2CH3+ -3.2 +6.4 +15.5 +9.6 +18.8
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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
AcknowledgementsThis research was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and Novacor Research and Technology Corporation.