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

IntroductionFollowing the discovery of catalytic olefin polymerization activity of group IV metallocene-based systems 19761, a great variety of homogeneous transition-metal based olefin polymerization catalysts have been discovered. Many d0 group III or group IV metal based systems, with a widening variety of ligands were found, from “constrained geometry” catalysts2 to the diamide system of McConville et al.3 Computational modeling of d0 systems has also progressed rapidly.4 More recently, a number of late transition metal catalysts have been developed. Brookhart et al. discovered d8 Ni and Pd compounds5 which are active polymerization catalysts with certain bulky ligand systems. Several systems with a Cr center and a few d electrons have recently been found to be active catalysts.6,7 With these indications of catalytic potential in compounds with electron counts between zero and eight, we present a computational study of d1, d2, and d3 metal systems with nitrogen-based ligands. A previous study8 had indicated some potential in bis-amide systems, we explore here systems with an additional amine. The amine was added in the hope of destabilizing the termination transition state. We use an analysis of the localized electron density to explore how the nature and placement of the ligands affect catalytic properties.

Computational Details

All structures and energetics were calculated with the Density Functional Theory (DFT) program ADF9. 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.)10 In each case, the local exchange-correlation potential11 was augmented with electron exchange functionals12 and correlation corrections13 in the method known as BP86. First-order scalar relativistic corrections14 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.

d1V, d2Cr , d3Mn: a Comparison All have formal oxidation states of +4, but will each have a net

charge of about +1. Each has six valence orbitals (an s and 5 d).

After filling the SOMOs, V has 5 empty orbitals, Cr 4 and Mn 3

Each available orbital has a bonding interaction with the ligands. These orbitals must be orthogonal Individual orbitals of V will be more ligand centered, to balance charge

Metal bonding orbitals are often shared between ligands, e.g. trans- ligands typically share a single -bonding metal orbital.

Metal Alkyl Structures

N

N

Cr

C

C

N

N

Mn

C

C

N

N

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

N

C

N

VC

N

V

NH3

NH2NH2

H2C

H2CH

+

Cr

NH3

NH2NH2

CH3CH2

+

Mn

NH3

NH2NH2

CH3CH2

+

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 -agostic hydride NH2 is apical instead of ethene

CC

N

N

C Mn

C

N

NC

C

N

VC

C

N

N

CC

C

N

Cr

C

N

Insertion Insertion barriers are

similar and fairly small Geometries are quite

different from one another

Each has a ligand trans to a forming or breaking bond

CC N

N

Mn

N

C

C

N

C

C

N

Cr

CC

N

N

C

N

C V

C

C

N

E

+8.3kcal/mol

+12.5 kcal/mol

+13.6 kcal/mol

Termination (-hydride transfer)

Each termination barrier is substantially higher than the insertion

No -hydride elimination TS found lower in energy

The amine is either trans to the hydride, or to one of the reacting M-C bonds

N

C

C

V

N

C

C

N

N

C

C

C

Cr

C

N

N

N

C

C

C

Mn

C

N

N

E

+16.4 kcal/mol

+19.6 kcal/mol

+20.0 kcal/mol

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

Metal Euptake Einsertion Etermination

d1 V -3.0 +8.3 +16.4

d2 Cr -5.2 +12.5 +19.6

d3 Mn -5.4 +13.6 +20.0

LocalizedOrbitals

Olefin insertion of d2Cr

More Localized Orbitals

Chain Termination-d2 Cr(via -hydride transfer)

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

These compounds are found to be low spin Compounds with a like number of occupied

metal orbitals may be analogous Model systems with d2 Mo,d4 Ru, and d6 Pd

were calculated

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

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

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)

Catalytic Properties with TetherModel System Euptake Einsertion Etermination

LVCH 2CH3+ -9.9 +5.7 +8.7

LCrCH 2CH3+ -15.3 +12.1 +25.6

LMnCH2CH3+ -3.2 +9.6 +18.8

The tether again has a large effect on the energetics that is very different for different 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. The resulting energy is much higher.

In Mn, the tether is different from all three shapes, causing them each a similar energy penalty. The net result is similar Es.

Conclusions

The occupation of metal orbitals has a substantial effect on molecular properties

The binding changes in transition states substantially in flexible, untethered systems

Tethering the ligands alters the energetics differently for different transition states; matching tether types with metal is important

AcknowledgementsThis research was supported by the Natural Sciences and EngineeringResearch Council of Canada (NSERC) and Novacor Research andTechnology Corporation.

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