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

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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.