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

1
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

2
Outline

• RM(NH2)2NH3 (MV,Cr,Mn) bonding and ethylene
  polymerization
• Second row analogies (MMo, Ru, Pd)
• Linking nitrogen ligands with ethyl bridges
  effects on bonding mode and catalytic properties

3
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.
4
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 metals 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.

5
Shared Orbitals trans-NH2
H2N-Cr-NH2 p orbitals in b-hydride transfer
TS Two of four phase-combinations of four Boys
localized orbitals
6
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
7
Metal Alkyl Structures

• d1V is nearly tetrahedral
• NH2 are flat, with p bonds not in the same plane
• a b-agostic hydride
• d2Cr is nearly tetrahedral
• NH2 are flat, p aligned(shared)
• no b-agostic hydride
• d3Mn includes a 146angle
• NH2 are bent out of plane due to weakened p
  -interactions
• no b-agostic hydride

8
Olefin Adduct

• d1V is trigonal bipyramidal
• NH2 are flat, p bonds unshared
• b-agostic hydride
• d2Cr is trigonal bipyramidal
• NH2 are flat, p bonds aligned
• No b-agostic hydride
• d3Mn is trigonal bipyramidal
• NH2 are bent out of plane
• NH2 is apical instead of ethene
• No b-agostic hydride

9
Energies of NH2 Rotation

• A 90? torsion directs the p to the plane of the
  other N
• At 90? and 90?, the two p orbitals are in the
  same plane- both N share a single metal orbital.
• V prefers two separate p orbitals
• Cr prefers to share one p orbital between both
  ligands
• The difference is due to the Crs additional
  unpaired electron

Energies are in kcal/mol Relative to minimum
10
Insertion
DE (Barrier)

• Insertion barriers are similar
• Geometries are quite different from one another
• Each has a ligand trans to a forming or breaking
  bond

16.3kcal/mol 12.5 kcal/mol 13.6
kcal/mol
11
Localized Orbitals insertion
Olefin insertion of d2Cr
12
Termination (b-hydride transfer)
DE

• 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

16.4 kcal/mol 19.6 kcal/mol 20.0
kcal/mol
13
Localized Orbitalsb-hydride transfer
Chain Termination Transition State
14
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

15
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

16
Second Row Results
While the uptake energies are substantially
improved, these combinations of ligand and metal
do not result in good catalysts.
17
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

18
Uptake Enthalpy of Linked System
DEreorganization is the energy required to
distort the alkyl minimum to the shape of the
adduct (minus the ethylene)

• 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.

19
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.

20
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 Research and
Technology Corporation.