Ethylene trimerization catalysts based on chromium complexes with a nitrogen-bridged diphosphine ligand having ortho-methoxyaryl or ortho-thiomethoxy substituents: Well-defined catalyst precursors and investigations of the mechanism

Article abstract of DOI:10.1021/om0506062

Chromium-based ethylene trimerization catalyst precursors ((PNP ^OMe-d₁₂)CrPh₃ (4) and (PNP^OMe-d ₁₂)CrPh₂Cl (7)) having a bis(diphenylphosphino)amine ligand (o-CD₃OC₆H₄)₂PN(CH ₃)P(o-CD₃OC₆H₄)₂ ((PNP^OMe-d₁₂) = 1) have been prepared and characterized. A thioether analogue (o-CD₃SC₆H₄) ₂PN-(CH₃)P(o-CD₃SC₆H ₄)₂ ((PNP^SMe-d₁₂) = 2) and its triphenylchromium complex (PNP^SMe)CrPh₃ (5) have also been synthesized. The solid-state structures of 4 and 7 display octahedral geometries with a κ³-(P,P,O) coordination of pNP^OMe ligands having chromium-oxygen bond lengths of 2.29-2.44 A. Compound 5 differs, exhibiting (S,P,S)-κ³ coordination of the PNP ^SMe ligand. The deuteromethyl groups allow for ²H NMR characterization of these paramagnetic complexes in solution. Dynamic exchange processes occur in solution at room temperature to render all four of the methoxy or thioether groups equivalent on the ²H NMR time scale; two distinct coalescence processes are observed by variable-temperature ²H NMR spectroscopy for all compounds. The neutral species 4 and 7 react with ethylene (1 atm) by insertion into chromium-phenyl bonds with the release of styrene and ethylbenzene, but 1-hexene is not observed under these conditions. Activation of 4 by protonation and activation of 7 by halide abstraction in the presence of ethylene provide active trimerization catalysts that give turnover numbers for 1-hexene as high as 3000 mol 1- hexene·mol^-1 Cr. These catalysts display comparable activity and selectivity for 1-hexene compared to the original BP system, where the catalyst is generated in situ from CrCl₃(THF)₃, 1, and MAO. Both the well-defined systems and the CrCl₃(THF) ₃/PNP^OMe/MAO system provide catalysts that undergo an initiation period followed by an apparent first-order decomposition process. Activated complexes 4 and 7 initiate trimerization primarily through ethylene insertion into the chromium-phenyl bond, followed by β-hydrogen elimination and reductive elimination to give the active species, rather than via reductive elimination of biphenyl.

Full text of DOI:10.1021/om0506062

Organometallics 2006, 25, 2743-2749
2743
Ethylene Trimerization Catalysts Based on Chromium Complexes
with a Nitrogen-Bridged Diphosphine Ligand Having
ortho-Methoxyaryl or ortho-Thiomethoxy Substituents:
Well-Defined Catalyst Precursors and Investigations of the
Mechanism
Susan J. Schofer, Michael W. Day, Lawrence M. Henling, Jay A. Labinger,* and
John E. Bercaw*
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology,
Pasadena, California 91125
ReceiVed July 19, 2005
Chromium-based ethylene trimerization catalyst precursors ((PNPOMe-d12)CrPh3 (4) and (PNPOMe
-
d12)CrPh2Cl (7)) having a bis(diphenylphosphino)amine ligand (o-CD3OC6H4)2PN(CH3)P(o-CD3OC6H4)2
((PNPOMe-d12) ) 1) have been prepared and characterized. A thioether analogue (o-CD3SC6H4)2PN-
(CH3)P(o-CD3SC6H4)2 ((PNPSMe-d12) ) 2) and its triphenylchromium complex (PNPSMe)CrPh3 (5) have
also been synthesized. The solid-state structures of 4 and 7 display octahedral geometries with a κ3-
(P,P,O) coordination of PNPOMe ligands having chromium-oxygen bond lengths of 2.29-2.44 Å.
Compound 5 differs, exhibiting (S,P,S)-κ3 coordination of the PNPSMe ligand. The deuteromethyl groups
2
allow for H NMR characterization of these paramagnetic complexes in solution. Dynamic exchange
processes occur in solution at room temperature to render all four of the methoxy or thioether groups
2
equivalent on the H NMR time scale; two distinct coalescence processes are observed by variable-
2
temperature H NMR spectroscopy for all compounds. The neutral species 4 and 7 react with ethylene
(1 atm) by insertion into chromium-phenyl bonds with the release of styrene and ethylbenzene, but
1-hexene is not observed under these conditions. Activation of 4 by protonation and activation of 7 by
halide abstraction in the presence of ethylene provide active trimerization catalysts that give turnover
numbers for 1-hexene as high as 3000 mol 1-hexene‚mol-1 Cr. These catalysts display comparable activity
and selectivity for 1-hexene compared to the original BP system, where the catalyst is generated in situ
from CrCl3(THF)3, 1, and MAO. Both the well-defined systems and the CrCl3(THF)3/PNPOMe/MAO system
provide catalysts that undergo an initiation period followed by an apparent first-order decomposition
process. Activated complexes 4 and 7 initiate trimerization primarily through ethylene insertion into the
chromium-phenyl bond, followed by â-hydrogen elimination and reductive elimination to give the active
species, rather than via reductive elimination of biphenyl.
(III) precursor,8 a bis(diphosphino)amine ligand ((o-MeO-
C6H4)2PN(CH3)P(o-MeO-C6H4)2, (PNPOMe-d12) ) 1), and
activation by methylaluminoxane (MAO) (eq 1).9
Introduction
Transition metal-catalyzed ethylene oligomerization typically
produces a broad range of R-olefins.1 Linear R-olefins, especially
1-hexene and 1-octene, are desirable comonomers for the
production of linear low-density polyethylene.2 In 1989 Briggs
reported the chromium-catalyzed selective trimerization of
ethylene to produce 1-hexene.3 Several recent reports describe
other chromium-based catalysts that can carry out this process
with high selectivity.4 Catalysts based on titanium,5 tantalum,6
and vanadium7 complexes have also been found to promote
ethylene trimerization.
CrCl3(THF)3 + 1
300 equiv MAO, toluene8 1-hexene
ethylene
(1-20 bar)
(1)
In the preceding article we describe the synthesis and
characterization of chromium(0) carbonyl, chromium(III) halide,
(4) (a) Kohn, R. D.; Haufe, M.; Kociok-Kohn, G.; Grimm, S.; Wasser-
scheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 4337. (b) Reagen,
W. K.; Pettijohn, T. M.; Freeman, J. W. (Philips Petroleum Co.) Patent US
5,523,507, 1996. (c) McGuinness, D. S.; Wasserscheid, P.; Keim, W.; Hu,
C.; Englert, U.; Dixon, J. T.; Grove, C. Chem. Commun. 2003, 334. (d)
McGuinness, D. S.; Wasserscheid, P.; Keim, W.; Morgan, D.; Dixon, J.
T.; Bollmann, A.; Maumela, H.; Hess, F.; Englert, U. J. Am. Chem. Soc.
2003, 125, 5272. (e) Wu, F.-J. (Amoco Corp.) Patent US 5,811,618, 1998.
(5) (a) Deckers, P. J. W.; Hessen, B.; Teuben, J. H. Angew Chem., Int.
Ed. 2001, 40, 2516. (b) Deckers, P. J. W.; Hessen, B.; Teuben, J. H.
Organometallics 2002, 21, 5122. (c) Pellechia, C.; Pappalardo, D.; Oliva,
L.; Mazzeo, M.; Gruter, G.-J. Macromolecules 2000, 33, 2807.
(6) Andes, C.; Harkins, S. B.; Murtuza, S.; Oyler, K.; Sen, A. J. Am.
Chem. Soc. 2001, 123, 7423.
BP has described a particularly active and selective chromium-
based ethylene trimerization catalyst that is comprised of a Cr-
(1) (a) Skupinska, J. Chem. ReV. 1991, 91, 613. (b) Keim, W.; Kowaldt,
F. H.; Goddard, R.; Kru¨ger, C. Angew. Chem., Int. Ed. Engl. 1978, 17,
466. (c) Svejda, S. A.; Brookhart, M. Organometallics 1999, 18, 65. (d)
Killian, C. M.; Johnson, L. K.; Brookhart, M. Organometallics 1997, 16,
2005. (e) Mecking, S. Coord. Chem. ReV. 2000, 203, 325. (f) Ruther, T.;
Braussaud, N.; Cavell, K. J. Organometallics 2001, 20, 1247. (g) Britovsek,
G.; J. P.; Mastroianni, S.; Solan, G. A.; Baugh, S. P. D.; Redshaw, C.;
Gibson, V. C.; White, A. J. P.; Williams, D. J.; Elsegood, M. R. J. Chem.
Eur. J. 2000, 6, 2221.
(2) Vogt, D. In Applied Homogeneous Catalysis with Organometallic
Compounds; Cornils, B., Herrmann, W. A., Eds.; VCH: Weinheim,
Germany, 1996.
(7) Santi, R.; Romano, A. M.; Grande, M.; Sommazzi, A.; Masi, F.; Proto,
A. (ENICHEM S.P.A.) WO 0168572, 2001.
(3) Briggs, J. R. J. Chem. Soc., Chem. Commun. 1989, 674.
(8) Cr(II) has also been shown to be active. See ref 9.
10.1021/om0506062 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 04/28/2006

2744 Organometallics, Vol. 25, No. 11, 2006
Schofer et al.
Scheme 1
halo-alkyl, and dibenzochromacylopentane complexes having
PNPOMe-type ligands, along with some postulates for the general
mechanism.10 This catalytic cycle involves (i) initial coordination
of 2 equiv of ethylene to a ligated Crn species, followed by (ii)
oxidative coupling to form a chromacyclopentane of oxidation
state Crn+2, (iii) ring expansion to seven by ethylene insertion,
and (iv) rapid â-hydrogen elimination and reductive elimination
of 1-hexene to regenerate Crn. More recently it has been
suggested, and supported by DFT studies, that the release of
1-hexene from the chromacyloheptane intermediate is a con-
certed 3,7-hydrogen shift with formal two-electron reduction
of the metal.11,12
In an effort to learn more about the PNP-chromium-based
catalytic system, the synthesis, characterization, and reactivity
of well-defined [(PNPOMe-d12)CrIII] phenyl complexes have been
investigated, along with the corresponding (PNPSMe-d12)CrPh3
(PNPSMe ) (o-CD3S-C6H4)2PN(CH3)P(o-CD3S-C6H4)2). Part of
this work was communicated previously.13 Complexes of the
general form (PNPOMe-d12)CrPh3 and (PNPOMe-d12)CrPh2X (X
) halide) have been prepared and characterized in solution and
in the solid state. These two complexes can be activated
stoichiometrically to catalyze ethylene trimerization, and the
selectivities and activities of these catalysts have been compared
to the CrCl3(THF)3/PNPOMe/MAO system. These well-defined
catalyst precursors have allowed for study of the trimerization
mechanism, as well as an examination of their initiation
pathways.
Figure 1. Molecular structure of 4 with 50% probability ellipsoids.
Hydrogen atoms have been omitted. Selected bond distances (Å)
and angles (deg) are as follows: Cr2-P3 2.6164(12); Cr2-P4
2.5044(12); Cr2-O6 2.279(3); Cr2-C50 2.060(4); Cr2-C51 2.092-
(4); Cr2-C61 2.081(4); P3-N2-P4 105.92(17).
of 1 and 2 show characteristic resonances at δ ) 54 ppm and
2
δ ) 50 ppm in THF and characteristic H resonances at δ )
3.60 ppm and δ ) 2.26 ppm in dichloromethane, respectively.
Synthesis of Chromium Triphenyl Complexes. CrPh3-
(THF)3 (3) is one of few known tris(hydrocarbyl) chromium-
(III) complexes and thus provides an almost unique direct entry
to tris(hydrocarbyl)Cr(III) complexes with these bis(diphen-
ylphosphino)amine ligands. Its synthesis from CrCl3 and Ph-
MgBr in THF was originally reported in 1959,16 and its crystal
structure was reported in 1983.17 Reactions of the deuterated
diphosphine ligands 1 and 2 with 3 in dichloromethane
(reversibly) afford the desired (PNP)CrPh3 complexes in high
yields. Both (PNP)-ligated chromium triphenyl complexes are
significantly more stable than 3 toward reductive elimination
of biphenyl in solution. Ligand 1 reacts with 3 to form (PNPOMe
-
Results and Discussion
d12)CrPh3 (4) as a red, microcrystalline solid in 68% yield (eq
2). Slow diffusion of petroleum ether into a dichloromethane
solution of 4 at -35 °C provides crystals suitable for structural
determination by X-ray diffraction. The solid-state structure of
4 shows the presence of two crystallographically independent
molecules of 4 in the asymmetric unit with very similar
structures; one of them is shown in Figure 1. The chromium
center for 4 is hexacoordinate with the PNPOMe ligand coordi-
nated in a κ3-(P,P,O) mode. The chromium-oxygen bond
lengths of 2.279(3) and 2.293(2) Å are similar to the chromium-
oxygen bond lengths in 3. The structure also indicates that
coordination of methoxy to chromium shortens the chromium-
phosphorus bond distance of the chelating phosphorus by 0.11
Å to 0.17 Å, from 2.6164(12) Å to 2.5044(12) Å, and from
2.6597(12) Å to 2.4897(11) Å. The nitrogen is slightly
pyramidal, with a sum of all three angles around it of 351.5°.
Although the solid-state structure of 4 indicates the coordina-
tion of one of the four methoxy groups, the room-temperature
2H NMR spectrum in dichloromethane displays only a single
peak at δ ) 8.47 ppm, suggesting that a dynamic process occurs
in solution to render all four of the methoxy groups equivalent
on the NMR time scale. Variable-temperature 2H NMR spectra
of 4 show two distinct coalescence points, as shown in Figure
Synthesis of Deuterated PNP Ligands. Because our target
chromium complexes are paramagnetic Cr(III) species, PNP
ligands were designed to have deuterated methyl groups on the
methoxy or thiomethoxy position for the purpose of examining
the complexes by 2H NMR spectroscopy.14 Synthesis of 1 may
be accomplished by either a stepwise or a convergent route.10
The thioether analogue 2, (o-CD3S-C6H4)2PN(CH3)P(o-CD3S-
C6H4)2 (PNPSMe-d12), has been synthesized via the convergent
route (Scheme 1).15 The room-temperature 31P NMR spectra
(9) (a) Carter, A.; Cohen, S. A.; Cooley, N. A.; Murphy, A.; Scutt, J.;
Wass, D. F. Chem. Commun. 2002, 858. (b) Wass, D. F. (British Petroleum).
Patent WO 2002004119, 2002.
(10) Agapie, T.; Day, M. W.; Henling, L. M.; Labinger, J. A.; Bercaw,
J. E. Organometallics 2006, 25, 2733.
(11) (a) Yu, Z.-X.; Houk, K. N. Angew. Chem., Int. Ed. 2003, 42, 808.
(b) Blok, A. N. J.; Budzelaar, P. H. M.; Gal, A. W. Organometallics 2003,
22, 2564. (c) de Bruin, T. J. M.; Magna, L.; Raybaud, P.; Toulhoat, H.
Organometallics 2003, 22, 3404.
(12) Janse van Rensburg, W.; Grove´, C.; Steynberg, J. P. Stark, K. B.;
Huyser, J. J.; Steynberg, P. J. Organometallics 2004, 23, 1207.
(13) Agapie, T.; Schofer, S. J.; Labinger, J. A.; Bercaw, J. E. J. Am.
Chem. Soc. 2004, 126, 1304.
(14) La Mar, G. N.; Horrocks, W. D., Jr.; Holm, R. H. NMR of
Paramagnetic Molecules; Academic Press: New York, 1973.
(15) (a) Dossett, S. J.; Gillon, A.; Orpen, A. G.; Fleming, J. S.; Pringle,
P. G.; Wass, D. F.; Jones, M. D. Chem. Commun. 2001, 699. (b) Balakrishna,
M. S.; Reddy, V. S.; Krishnamurthy, S. S.; Nixon, J. F.; Burkett St. Laurent,
J. C. T. R. Coord. Chem. ReV. 1994, 129, 1.
(16) Herwig, W.; Zeiss, H. J. Am. Chem. Soc. 1959, 81, 4798.
(17) Khan, S. I.; Bau, R. Organometallics 1983, 2, 1896.

Ethylene Trimerization Catalysts Based on Chromium
Organometallics, Vol. 25, No. 11, 2006 2745
structure of 5 shows that the ligand binds to chromium through
two sulfur atoms and a phosphorus (see Supporting Information).
As compared with 4 this complex shows a quite different
(S,P,S)-κ3 binding mode for the PNP ligand with a dangling
[-P(o-CD3S-C6H4)2]. Unfortunately, these crystals were suitable
only for determination of connectivity in this structure, and
reliable bond lengths have not been obtained for this complex.
2. At -95 °C a very broad peak centered at δ ) 50 ppm is
observed, as well as a single peak in the diamagnetic region (δ
≈ 4 ppm). As the sample is warmed, coalescence is observed
at -75 °C, and at -50 °C two broad peaks are observed in a
1:1 ratio. As the solution is warmed further, the spectrum
displays another coalescence at -41 °C; above this temperature,
only one peak is observed for all four of the exchanging methoxy
groups.
2
The room-temperature H NMR spectrum for 5 in dichlo-
The spectrum at -95 °C is consistent with the static solid-
state structure of 4: the broad peak at δ ) 50 ppm is assigned
to the Cr-coordinated methoxy group. The sharp peak in the
diamagnetic region at δ ≈ 4 ppm is assigned to the three
(inequivalent, but unresolved) uncoordinated methoxy groups.
The occurrence of two separate exchange processes would
romethane displays a single peak at δ ) -3.8 ppm, indicating
that like 4 it undergoes a dynamic exchange process in solution.
An examination of the H NMR spectra at -100 °C shows a
2
very broad peak centered at δ ) -38 ppm, as well as a peak
upfield at δ ) 2.52 ppm. Variable-temperature 2H NMR spectra
show two distinct coalescence points, one at -85 °C and another
at 5 °C. Although comparisons of the relative intensities of very
broad and less broad signals does not allow for a definitive
conclusion, in the -100 °C spectrum the apparent relative
integrations of the very broad downfield peak, as compared to
the peak in the diamagnetic region, suggest that the static
structure of 5 in solution involves coordination of a single
thioether group to chromium, possibly in a fashion analogous
to the structure of 4. Although the solid-state structure of 5
involves one phosphine coordinated to chromium and the other
dangling phosphine much farther away from the paramagnetic
chromium center, a 31P NMR signal for this complex is not
2
account for the observed temperature dependence of the H
spectra. The faster exchange process that is evident from the
spectrum at -75 °C involves the dissociation of the methoxy
connected to one phosphorus (OMea), followed by recoordina-
tion of a methoxy group from the other phosphorus on the same
hemisphere of the complex (OMeb), as is discussed in the
preceding article.10
Ligand 2 reacts with 3 to form (PNPSMe)CrPh3 (5) as a brown
powder in 56% yield (eq 3). Slow diffusion of petroleum ether
into a dichloromethane solution of 5 at -35 °C provides crystals
suitable for analysis by X-ray diffraction. The solid-state
Figure 2. Variable-temperature 2H NMR spectra of 4 in dichloromethane-d2. The arrow indicates the broad peak assigned to the methoxy
group coordinated to chromium in the static structure.

2746 Organometallics, Vol. 25, No. 11, 2006
Schofer et al.
Scheme 2
2
As with 4 and 5, the room-temperature H NMR spectrum
of 7 in dichloromethane shows a single, very broad peak at δ
) 6 ppm. Cooling a dichloromethane solution of 7 displays
two distinct coalescence points, both of which are between 0
and 25 °C. These higher temperature coalescence points indicate
that the barriers for exchange processes for 7 are higher than
those for 4. The reasons for this difference are not immediately
obvious, although the lower symmetry of 7 provides for five-
coordinate intermediates of different energies, leading to a more
complex free energy profile, possibly having some higher
barriers.
Figure 3. Molecular structure of 7 with 50% probability ellipsoids.
Selected bond distances (Å) and angles (deg) are as follows: Cr-
P1 2.4414(6); Cr-P2 2.6096(6); Cr-O1 2.4371(14); Cr-Cl 2.3078-
(6); Cr-C10 2.039(2); Cr-C14 2.077(2); P1-N-P2 106.35(9).
observed, even at -95 °C, again suggesting that a structure
analogous to that for 4 is accessible for 5 in solution.
The spin states for chromium complexes 4 and 5 have been
determined. Their EPR spectra at 20 K as toluene glasses are
consistent with Cr(III) species with three unpaired electrons and
S ) 3/2 (see the Supporting Information).18 Evans method
determination of the solution magnetic susceptibility19 for 4
indicates an effective magnetic moment (µeff) of 3.8 µB in
dichloromethane, consistent with a quartet ground state with
three unpaired electrons.
By following the comproportionation reaction of 6 and 4 by
2H NMR spectroscopy, intermediate formation of (PNPOMe-d12)-
CrPhCl2 (8) may be observed. Complex 8 may be independently
prepared by a comproportionation reaction between 2 equiv of
6 and 1 equiv of 4 in dichloromethane. 8 displays two separate
2H NMR signals at room temperature at δ ) 4.4 ppm and δ )
11.8 ppm in an approximate 1:1 ratio. An exchange process
analogous to that proposed for 4 may again be invoked to
Synthesis of Chromium Phenyl Halide Complexes. The
comproportionation reaction between 1 equiv of the previously
reported complex (PNPOMe-d12)CrCl3 (6)14 and 2 equiv of 4 in
dichloromethane provides (PNPOMe-d12)CrPh2Cl (7) as a dark
olive-green powder in 71% isolated yield (eq 4). Slow diffusion
of petroleum ether into a chlorobenzene solution of 7 at -35
°C provides X-ray quality crystals of this complex. The solid-
state structure of 7 (Figure 3) reveals κ3-(P,P,O) coordination
of the PNPOMe ligand with the chloride trans to the (P,O)-κ2
chelating phosphorus. The chromium-oxygen bond length is
2.4371(14) Å, the chromium-phosphorus bond length for the
(P,O)-κ2 chelating phosphorus is 2.4414(6) Å, and the other
chromium-phosphorus bond length is 2.6096(6) Å. The chro-
mium-chloride bond length in 7 is 2.3078(6) Å, which is similar
to the chromium-chloride bond length of 2.3210(7) Å observed
for the chloride trans to the chelated phosphorus in 6.14
2
explain the dynamic H NMR behavior observed for 8. The
downfield peak observed at room temperature for 8 is assigned
to the two methoxy groups that are exchanging for coordination
to chromium, while the upfield peak is assigned to the two
methoxy groups that exchange but are never coordinated to
chromium. A coalescence point is observed at -65 °C; below
this temperature the spectrum is unchanged, in accord with a
static structure with one methoxy group coordinated to chro-
mium, and exchange processes slow at these low temperatures.
Reactivity of 4 and 7: Trimerizations with (PNP)CrPh3
and (PNP)CrPh2Cl as Precursors. Both 4 and 7 react with
ethylene in toluene solution at room temperature over the course
of hours to insert ethylene into their chromium-phenyl bonds
and form styrene, ethylbenzene, 4-phenyl-1-butene, and higher
ethylene insertion products, along with benzene. Formation of
1-hexene is not observed under these conditions.
Protonation of 4 and halide abstraction from 7 in toluene have
been explored as possible routes to an ethylene trimerization
catalyst system. Reactions of 4 with [H(OEt2)2]+{B[C6H3-
(CF3)2]4}- or 7 with Na+{B[C6H3(CF3)2]4}- in toluene at room
temperature result in rapid formation of green solutions with
an oily precipitate. Presumably, these reactions generate [(PNPOMe
-
d12)CrPh2]+B[C6H3(CF3)2]4- in both cases, either via protonation
to liberate an equivalent of benzene or via halide abstraction to
form NaCl, respectively (Scheme 2). After activation, formation
of biphenyl by these systems proceeds, but does so very slowly;
after 22 h less than 0.5 equiv of biphenyl is observed by GC-
MS.
Protonation of 4 with [H(OEt2)2]+{B[C6H3(CF3)2]4}- in
toluene or abstraction of chloride from 7 with Na+B[C6H3-
(18) Bonomo, R. P.; Di Bilio, A. J.; Riggi, F. Chem. Phys. 1991, 151,
323.
(19) (a) Sur, S. K. J. Magn. Reson. 1989, 82, 169. (b) Evans, D. F. J.
Chem. Soc. 1959, 2003. (c) Schubert, E. M. J. Chem. Educ. 1992, 69, 62.
-
(CF3)2]4 in chlorobenzene, followed by exposure of the
resulting mixture to an atmosphere of ethylene, yields active
catalyst(s) for the trimerization of ethylene to 1-hexene (eq 5).

Ethylene Trimerization Catalysts Based on Chromium
Organometallics, Vol. 25, No. 11, 2006 2747
Figure 4. Ethylene consumption over time for trimerization
catalyzed by 4/H+(OEt2)2B[C6H3(CF3)2]4-: 20 µmol of 4, 20 µmol
Figure 5. Ethylene consumption over time for the catalyst
generated from 1 and CrCl3(THF)3 and activated by MAO after
exposure to ethylene: 21 µmol of 1, 21 µmol of CrCl3(THF)3, 6
mmol of MAO in 50 mL of toluene at room temperature; 1 atm of
ethylene.
-
of H+(OEt2)2B[C6H3(CF3)2]4 in a mixture of 2 mL of Et2O and
48 mL of toluene at room temperature; 1 atm of ethylene.
The structure in Scheme 2 with a κ4-coordination of PNP is
suggested on the basis of the normal preference for six
coordination for chromium(III). Activation of complexes 4 and
7 provides a catalyst system that displays selectivity and activity
similar to the PNPOMe/CrCl3(THF)3/MAO system.9 The differ-
ence in reactivity with ethylene observed for this activated,
presumably cationic species versus the neutral precursors
described above suggests that a cationic species with a weakly
coordinating anion is necessary for catalytic activity. Preliminary
experiments with PNPSMe/CrCl3(THF)3/MAO indicate that 2
does not form a catalyst under the same conditions.
rate of ethylene consumption versus time during the decomposi-
tion period are linear, suggesting that catalyst decomposition is
first-order in chromium (see the Supporting Information).
Treatment of 4 with either [Me2PhNH]+{B[C6F5]4}- or MAO
also generates an active ethylene trimerization catalyst. Reaction
of 4 with [Me2PhNH]+{B[C6F5]4}- (1 equiv) in a diethyl ether/
toluene mixture provides a catalyst that achieves a final turnover
number of 3050 mol 1-hexene‚mol-1 chromium at 1 atm C2H4,
which is the highest turnover number observed for the (PNP)-
CrPh3 system to date. By comparison, MAO activation (300
equiv) of 4 produces an active trimerization catalyst that gives
an integrated 1500 mol 1-hexene‚mol-1 chromium over the
period of 1 h, after which point it is inactive for further ethylene
consumption. Under these conditions the catalyst does not show
an initiation period. Table 1 shows representative turnover
numbers and catalyst lifetimes for a variety of catalytic runs.
Comparison to Trimerizations with CrCl3(THF)3/PNP/
MAO. Trimerizations using CrCl3(THF)3/PNP/MAO (1:1:300)
at 1 atm C2H4 give results similar to the better defined systems
generated according to eq 5. MAO-generated catalysts display
variable activities and undergo an initiation period followed by
first-order decomposition. 1-Hexene turnovers range from 1700
to 4600. A representative plot of ethylene consumption over
time for these catalysts is shown in Figure 5.
Although these catalysts are relatively well defined, there are
some indications that the number of active chromium centers
varies and that the catalyst is undergoing slow decomposition
as it trimerizes ethylene. Activation of 4 with [H(OEt2)2]+-
{B[C6H3(CF3)2]4}- generates an ethylene trimerization catalyst
with highly variable activity. Turnover numbers range from
fewer than 10 mol to 2780 mol 1-hexene‚mol-1 chromium. An
examination of ethylene consumption over time for catalytic
runs shows that the catalyst typically undergoes an initiation
period, during which time ethylene consumption increases,
followed by decomposition of the catalyst, as evidenced by a
decrease in ethylene consumption over time. A representative
plot of ethylene consumption over time for a typical trimeriza-
tion run is shown in Figure 4. Plots of the natural log of the
Initiation Mechanisms for 4 and 7. Possible initiation
pathways for this family of catalysts were investigated by
analyzing and quantifying the phenyl-containing organic prod-
ucts generated in trimerization reactions catalyzed by 4/
[H(OEt2)2]+{B[C6H3(CF3)2]4}- or 7/Na+{B[C6H3(CF3)2]4}-.
Activations of 4 and 7 proceed rapidly to generate [(PNPOMe
d12)CrPh2]+{B[C6H3(CF3)2]4}- (vide supra). Once exposed to
-
Table 1. Representative Turnover Numbers and Approximate Catalyst Lifetimes for a Series of Ethylene Trimerization Runs
with 4, 7, or CrCl3(THF)3/PNP as Catalyst Precursor
catalyst precursor
activatora
solvent
total turnoverb
catalyst lifetimec
-
-
-
-
4
4
4
4
4
4
7
[H(OEt2)2]+[BArF
[H(OEt2)2]+[BArF
[H(OEt2)2]+[BArF
[H(OEt2)2]+[BArF
]
]
]
]
toluene
7
2780
1790
735
3050
1490
450
10
>200
180
90
120
65
4
4
4
4
toluene/Et2O
toluene/Et2O
chlorobenzene
toluene/Et2O
toluene
chlorobenzene
toluene
toluene
[Me2PhNH]+[B(C6F5)4]-
MAO
-
Na+[BArF
MAO
]
4
CrCl3(THF)3/PNPOMe
CrCl3(THF)3/PNPOMe
2320
4630
60
60
MAO
-
a [BArF
]
4
) {B[3,5-C6H3(CF3)2]4}-. b Mol 1-hexene/mol Cr. c Minutes.

2748 Organometallics, Vol. 25, No. 11, 2006
Schofer et al.
Scheme 3
to give phenyl-terminated ethylene oligomers and the active
catalytic species.
Experimental Section
General Considerations. All air- and moisture-sensitive com-
pounds were manipulated using standard vacuum line, Schlenk, or
cannula techniques or in a glovebox under a nitrogen atmosphere,
as described previously.20 All gases were purified by passage over
MnO on vermiculite and activated molecular sieves. Solvents were
dried by the method of Grubbs.21 Ethereal solvents were stored
over sodium benzophenone ketyl, hydrocarbon solvents were stored
over titanocene,22 and halogenated solvents were dried over calcium
hydride. Dichloromethane-d2 was purchased from Cambridge
Isotopes and distilled from calcium hydride. Methylaluminumoxane
was purchased from Aldrich as a 10 wt % solution in toluene.
(CH3)N(PCl2)2,23 PNPOMe-d12 (1),10 and (PNPOMe-d12)CrCl3 (6)10
were prepared as previously described.
ethylene, this species may, among other possibilities, undergo
reductive elimination of biphenyl or insertion of ethylene into
a chromium-phenyl bond. In fact, only trace amounts of
biphenyl are observed under trimerization conditions. Signifi-
cantly, styrene and other phenyl-containing hydrocarbons are
observed under trimerization conditions. Thus it appears that
the chromium-phenyl bonds are cleaved primarily by ethylene
insertion(s), which is eventually followed by â-hydrogen
elimination and reductive elimination. The products are similar
to those observed in the reaction of ethylene with the neutral
species 4 and 7. Some likely reaction pathways for the activated
species are shown in Scheme 3. Although much more direct
evidence remains to be gathered, the implications at this stage
of the study are that the active catalyst is a cationic (PNP)-
chromium species that shuttles between CrIII and CrI with the
PNP ligand utilizing the two phosphine donors and one or more
methoxy ligands to stabilize the various intermediates along the
catalytic cycle.
1
Instrumentation. All H and 31P NMR spectra were recorded
on a Varian Mercury 300 spectrometer at 299.868 and 121.389
MHz, respectively, at room temperature unless indicated otherwise.
All 2H NMR spectra were recorded on a Varian INOVA 500
spectrometer at 76.848 MHz at room temperature unless indicated
1
otherwise. All H NMR chemical shifts are reported relative to
1
TMS, and H (residual) chemical shifts of the solvent are used as
a secondary standard. 31P NMR shifts are reported relative to an
external H3PO4 standard. 2H NMR chemical shifts are reported with
respect to CHDCl2 (natural abundance, 5.32 ppm) from the CH2-
Cl2 solvent or to an external D2O reference (4.8 ppm). Temperatures
of the NMR probe were calibrated using a methanol standard. EPR
spectra were recorded on a Bruker EMX spectrometer. Elemental
analysis was performed by Midwest MicroLab, LLC. GC measure-
ments were taken on an Agilent 6890 Series GC using an Agilent
HP-5 column. X-ray crystallography data were collected on a Bruker
SMART 1000 ccd diffractometer.
PNPSMe-d12 (2). A Schlenk flask was charged in the glovebox
with magnesium turnings (1.474 g, 61 mmol, 1.24 equiv). On the
vacuum line dry THF (100 mL) was added to the flask via vacuum
transfer. Neat 2-bromothioanisole-d3 (10.000 g, 49 mmol, 1 equiv)
was added via syringe. The reaction mixture became warm upon
addition and turned a pale brown color, and the reaction was left
to stir at room temperature for 24 h. The Grignard solution was
cannula transferred to a THF solution of (CH3)N(PCl2)2 (2.626 g,
11 mmol, 0.22 equiv), which caused the solution to get warm, and
the resulting yellow solution was stirred at room temperature for 1
week. THF solvent was removed in vacuo, and the product was
isolated by recrystallization from a dichloromethane/methanol
solvent mixture to obtain 1.598 g of 2 (22% yield). 1H NMR (RT,
300 MHz, CDCl3): δ 2.51 (t, 3H, Ar2PN(CH3)PAr2), 7.06 (m, 8H,
ArH), 7.32 (m, 8H, ArH). 31P NMR (RT, 300 MHz, CDCl3): δ 50
Conclusions
Phenyl chromium(III) complexes containing PNP ligands
have been prepared and characterized both in the solid state
and in solution. The triphenylchromium(III) complexes of two
PNP ligands, ((PNPOMe-d12)CrPh3 (4) and ((PNPSMe-d12)CrPh3
(5), display octahedral geometries around chromium in the solid
state; the PNPOMe ligand coordinates to chromium with two
phosphorus atoms and one oxygen, while the PNPSMe coordi-
nates through two sulfur atoms and one phosphorus. The X-ray
2
and variable-temperature H NMR data for 4 and 7 indicate a
labile interaction between chromium and oxygen, which may
be linked to catalyst activity and selectivity. We have proposed
two distinct exchange processes in solution to explain the
observed variable-temperature behavior.
2
ppm (s, Ar2PN(CH3)PAr2). H NMR (RT, 500 MHz, CH2Cl2): δ
Conversion of 4 or 7 to a [(PNP)CrIIIPh2]+ cation by treatment
with 1 equiv of acid or by precipitation of NaCl produces
ethylene trimerization catalyst systems with selectivity and
activity similar to the CrCl3(THF)3/PNPOMe/MAO system, but
that are much better defined in terms of the precatalyst structure.
The behavior of this activated, cationic complex differs from
that for the neutral triphenyl precursor with ethylene, in which
1-hexene is not formed, strongly suggesting that cationic
complexes with noncoordinating anions are necessary for
catalytic activity. Activities for these well-defined catalysts are
variable, and in almost all cases a catalyst initiation period is
followed by apparent first-order catalyst decomposition. Similar
results have been obtained in attempts to reproduce the work
reported by BP. Under trimerization conditions, chromium-
phenyl bonds are cleaved by insertion(s) of ethylene, followed
by â-hydrogen elimination and reductive elimination processes,
2.26 ppm (s, OCD3).
(PNPOMe-d12)CrPh3 (4). In the glovebox, compound 1 (1.069
g, 2.01 mmol, 1.03 equiv) was dissolved in 75 mL of dichlo-
romethane. Portions of CrPh3(THF)3 (0.974 g, 1.95 mmol, total, 1
equiv) were added as a slurry in tetrahydrofuran (approximately 2
mL) to the stirring solution of 1 over 5 min. The color of the
reaction mixture turned deep red upon addition. Volatile materials
were removed in vacuo, and approximately 50 mL of dichlo-
(20) Burger, B. J.; Bercaw, J. E. In Experimental Organometallic
Chemistry; Wayda, A. L., Darensbourg, M. Y., Eds.; ACS Symposium
Series No. 357; American Chemical Society: Washington, D.C., 1987;
Chapter 4.
(21) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.
(22) Marvich, R. H.; Brintzinger, H. H. J. Am. Chem. Soc. 1971, 93,
203.
(23) King, R. B.; Gimeno, J. Inorg. Chem. 1978, 17, 2390.

Ethylene Trimerization Catalysts Based on Chromium
Organometallics, Vol. 25, No. 11, 2006 2749
romethane were added, followed by solvent removal in vacuo. The
red solid residue was dissolved in approximately 25 mL of
dichloromethane, and approximately 30 mL of petroleum ether were
added to precipitate a red solid. This mixture was stored at -35
°C overnight. The solid material collected by filtration of the cold
solution was recrystallized from a dichloromethane/petroleum ether
mixture, collected on a sintered glass frit, and dried under vacuum
to leave 1.050 g of 4 as a red, microcrystalline solid (66% yield).
2H NMR (RT, 500 MHz, CH2Cl2): δ 8.47 ppm (s, OCD3). Anal.
Calcd for C47H34D12NO4P2Cr: C, 69.26; H, 5.70; N, 1.72. Found:
C, 69.03; H, 5.69; N, 1.86. µeff ) 3.8 µB. X-ray-quality crystals of
4 were obtained from slow diffusion of petroleum ether into a
concentrated CH2Cl2 solution of the complex at -35 °C.
toluene were added to give a pale green solution. The flask was
equipped with a 180° needle valve, degassed on the vacuum line
at -78 °C, warmed to room temperature, and backfilled with 1
atm of ethylene. Ethylene consumption was monitored using a
mercury manometer. After 1 h the reaction was quenched with H2O.
The organic fraction was separated and passed through a plug of
activated alumina to remove any chromium compounds, and this
mixture was analyzed by GC and GC-MS. 1-Hexene was quantified
by comparison to a mesitylene standard, which was added to the
reaction mixture. The reaction produces 1-hexene with a range of
10-3000 turnovers in greater than 85% overall selectivity. A similar
procedure was followed in the case of activation of 4 with
[Me2PhNH]+{B[C6F5]4}- or MAO.
General Procedure for Trimerization of C2H4 with 7/Na+-
{B[C6H3(CF3)2]4}-. In the glovebox a 250 mL round-bottom flask
was charged with 7 (0.017 g, 0.022 mmol, 1 equiv) and
Na+{B[C6H3(CF3)2]4}- (0.021 g, 0.024 mmol, 1.1 equiv) and
equipped with a 180° needle valve. The flask was attached to the
vacuum line, and approximately 50 mL of chlorobenzene was added
via vacuum transfer. The flask was backfilled with 1 atm of
ethylene, and the mixture was allowed to warm to room temperature
and stir while open to an ethylene atmosphere. A pale green solution
was formed as the mixture melted, and the reaction was stirred for
20 min to 1.5 h, at which point it was quenched with H2O. The
organic fraction was separated and filtered through a plug of
activated alumina to remove any chromium, and this mixture was
analyzed by GC and GC-MS. The reaction produces 1-hexene with
a range of 300-1000 turnovers in greater than 85% overall
selectivity.
General Procedure for Trimerization of C2H4 with CrCl3-
(THF)3/PNPOMe/ MAO. In the glovebox a 250 mL round-bottom
flask was charged with 1 (0.011 g, 0.021 mmol, 1 equiv) and CrCl3-
(THF)3 (0.008 g, 0.021 mmol, 1 equiv), 50 mL of toluene was
added, and the flask was equipped with a 180° needle valve. The
apparatus was attached to the high-vacuum line, degassed at -78
°C, and backfilled with 1 atm of ethylene. After the solution had
warmed to room temperature, MAO solution (3.481 g, 10 wt % in
toluene, 6 mmol, 300 equiv) was added via syringe under a positive
ethylene flow. Ethylene consumption was monitored using a
mercury manometer. After 1 h the reaction was quenched with H2O.
The organic fraction was separated and passed through a plug of
activated alumina to remove any chromium compounds, and this
mixture was analyzed by GC and GC-MS. The reaction produces
1-hexene with a range of 1000-4700 turnovers in greater than 85%
overall selectivity.
(PNPSMe-d12)CrPh3 (5). Compound 5 was prepared from 2
(1.252 g, 2.10 mmol, 1 equiv) and 3 (1.050 g, 2.10 mmol, 1 equiv)
using a procedure analogous to that used for compound 4. An
orange-brown solid was obtained, which was dissolved in ap-
proximately 25 mL of dichoromethane. Approximately 25 mL of
petroleum ether were added to precipitate a brown powder. This
mixture was stored at -35 °C overnight. The solid material
collected by filtration of the cold solution was recrystallized from
a dichloromethane/petroleum ether mixture, collected on a sintered
glass frit, and dried under vacuum to leave 1.039 g of 5 as a brown
solid (56% yield). 2H NMR (RT, 500 MHz, CH2Cl2): δ -3.8 ppm
(s, SCD3). Anal. Calcd for C47H34D12NS4P2Cr: C, 65.09; H, 5.36;
N, 1.62. Found: C, 64.83; H, 5.35; N, 1.72. X-ray quality crystals
of 5 were obtained from slow diffusion of petroleum ether into a
concentrated CH2Cl2 solution of the complex at -35 °C.
(PNPOMe-d12)CrPh2Cl (7). In the glovebox, solid 4 (0.517 g,
0.63 mmol, 2 equiv) and solid 6 (0.219 g, 0.32 mmol, 1 equiv)
were placed in a 20 mL vial. Approximately 10 mL of CH2Cl2
was added to form a brown solution. The vial was wrapped in
aluminum foil to protect it from light, and the reaction mixture
was allowed to stir at room temperature for 2 h. The mixture turned
a dark green color. Solvent was removed in vacuo to leave a green-
brown powder, which was dissolved in approximately 10 mL of
dichloromethane. Approximately 7 mL of petroleum ether were
added to precipitate a green-brown solid. This mixture was stored
at -35 °C overnight. The solid material was collected by filtration
of the cold solution and then recrystallized from a dichloromethane/
petroleum ether mixture, collected on a sintered glass frit, and dried
under vacuum to leave 0.523 g of 7 as a green-brown powder (71%
yield). 2H NMR (RT, 500 MHz, CH2Cl2): δ 6 ppm (br s, OCD3).
Anal. Calcd for C41H29D12ClNO4P2Cr: C, 64.69; H, 5.44; N, 1.84.
Found: C, 64.87; H, 6.06; N, 1.65. X-ray quality crystals of 7 were
obtained from slow diffusion of petroleum ether into a concentrated
chlorobenzene solution of the complex at -35 °C.
Acknowledgment. We thank Dr. Steven A. Cohen for
helpful discussions. We are grateful to BP Chemicals (now
Innovene) for financial support.
(PNPOMe-d12)CrPhCl2 (8). Solid 4 (0.014 g, 0.017 mmol, 1
equiv) and solid 6 (0.024 g, 0.035 mmol, 2 equiv) were placed in
a 20 mL vial, and approximately 2 mL of dichloromethane was
added. The vial was wrapped in aluminum foil to protect it from
light and left to stir at room temperature for 12 h. The solution
Supporting Information Available: Tables of bond lengths,
angles, and anisotropic displacement parameters for 7 (correspond-
ing tables for 4 were reported previously).13 Drawing of solid-state
structure of 5. EPR spectra of 4 and 5. Plots of the natural log of
ethylene consumption versus time with various catalytic systems.
X-ray crystallographic data (CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
2
turned a dark green color. H NMR (RT, 500 MHz, CH2Cl2): δ
4.4 ppm (s, 6D, OCD3), 11.8 ppm (s, 6D, OCD3).
General Procedure for Trimerization of C2H4 with 4/
[H(Et2O)2]+{B[C6H3(CF3)2]4}-. In the glovebox a 250 mL round-
bottom flask was charged with 4 (0.016 g, 0.020 mmol, 1 equiv)
and [H(Et2O)2]+{B[C6H3(CF3)2]4}- (0.020 g, 0.020 mmol, 1 equiv),
and approximately 2 mL of diethyl ether followed by 48 mL of
OM0506062