Spectral studies of a Cr(PNP)-MAO system for selective ethylene trimerization catalysis: Searching for the active species

Article abstract of DOI:10.1021/cs400778a

Variable temperature spectroscopic, kinetic, and chemical studies were performed on a soluble Cr^IIICl₃(PNP) (PNP = bis(diarylphosphino)alkylamine) ethylene trimerization precatalyst to map out its methylaluminoxane (MAO) activation sequence. These studies indicate that treatment of Cr^IIICl₃(PNP) with MAO leads first to replacement of chlorides with alkyl groups, followed by alkyl abstraction, and then reduction to lower-valent species. Reactivity studies demonstrate that the majority of the chromium species detected are not catalytically active.

Full text of DOI:10.1021/cs400778a

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Letter
Spectral studies of a Cr(PNP)-MAO system for selective
ethylene trimerization catalysis: searching for the active species
Loi H Do, Jay Alan Labinger, and John Edward Bercaw
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/cs400778a • Publication Date (Web): 07 Oct 2013
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Spectral studies of a Cr(PNP)-MAO system for
selective ethylene trimerization catalysis:
searching for the active species
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Loi H. Do, Jay A. Labinger,* John E. Bercaw*
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, Pasadena, California
91125
Keywords: chromium, ethylene trimerization, metallacycle, intermediates, methylaluminoxane, active species
ABSTRACT: Variable temperature spectroscopic, kinetic, and chemical studies were performed on a soluble CrIIICl3(PNP) (PNP =
bis(diarylphosphino)alkylamine) ethylene trimerization precatalyst to map out its methylaluminoxane (MAO) activation sequence.
These studies indicate that treatment of CrIIICl3(PNP) with MAO leads to first replacement of chlorides with alkyl groups, followed
by alkyl abstraction, and then reduction to lower–valent species. Reactivity studies demonstrate that the majority of the chromium
species detected is not catalytically active.
Development of selective oligomerization technologies to
access α-olefins from inexpensive building blocks is an active
area of petrochemical research.1 The global demand for linear
α-olefins alone is more than four million tons per year, pri-
marily for the manufacture of chemical commodities such as
polymers, detergents, and lubricants. Industrially, α-olefins
are typically obtained from metal-catalyzed ethylene oli-
gomerization that occurs via a Cossee-Arlman-type mecha-
nism: sequential insertion of ethylene into metal-hydride or
metal-alkyl species, followed by β-hydride elimination to
afford a statistical distribution of α-olefins. In recent years,
the discovery of metal-catalyzed transformations that can
Chart 1. Structures of the precatalysts 1 and 2.
mental5-8,10-14 and theoretical15,16 studies of these systems, the
convert ethylene selectively to either 1-hexene or 1-octene has
attracted considerable interest in the community.2-4 Although
nature of the active catalyst is still largely under debate, with
unresolved questions regarding the oxidation state, the coor-
a wide assortment of selective ethylene trimerization catalysts
dination geometry, and the ligand composition at the metal
have now been reported, so far only one has been commer-
center.4,17-23
cialized, the Phillips CrIII/2-ethylhexanoate/pyrrole/alkyl alu-
minum system.2
Characterization of these chromium species is particularly
challenging because of their paramagnetism and the need for
a large excess of MAO; application of techniques such as
EPR and X-ray absorption (XAS) spectroscopies under cata-
A mechanism that is generally proposed to account for this
selectivity, involving metallacyclic intermediates formed by
the oxidative coupling of two ethylenes followed by ring ex-
pansion (Scheme 1), has been experimentally confirmed by an
isotopic labeling experiment.5,6 However, there is very little
definitive knowledge about the active catalytic species for any
of these systems.4 Work in our group5-9 has focused on a
class of catalysts generated from chromium(III) complexes of
bis(diarylphosphino)alkylamine (PNP) ligands by activation
with methylaluminoxane (MAO). Despite extensive experi-
lytically relevant conditions have implicated both CrI/CrIII 24
18,25
and CrII/CrIV
redox cycles. A fundamental ambiguity in
such investigations, however, is the possibility that a very
small amount of a highly active catalyst is responsible for
most or all of the activity, while the species observed may
take no part in the catalytic cycle — may not even be catalyst
precursors — and hence are essentially irrelevant to catalysis.
In such a situation, which has been suggested as a possibility
for metal-catalyzed olefin polymerization,26-28 cycloisomeriza-
tion,29 1,4-addition to enones,30 and C–C bond cross-
coupling,31 it will be very difficult to establish any direct link
between structure and activity. We report here observations
that strongly indicate such is indeed the case for these
Cr(PNP) systems.
We initially examined the precatalyst CrIIICl3(PNMeP) (1,
PNMeP = (bis(bis(2-methoxyphenyl)phosphino))methylamine,
Chart 1),7,13 using several spectroscopic techniques to follow
its activation by MAO at low temperature, with the ultimate
Scheme 1. Metallacyle mechanism for ethylene trimerization
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Figure 1. UV-vis absorption spectra obtained from addition of MAO to 2 (A, blue trace) immediately after mixing at –40°C (A, red trace),
reaction at -40°C (B), and reaction at 25°C (C). The concentration of the starting chromium complex is 946 µM. The traces are color-
coded according to the species to which they are assigned: 2 (blue), 3 (red), 4 (green), 6/7 (purple).
hope of identifying the active form of the chromium species
by correlation with catalytic behavior under similar condi-
tions. These studies were hampered by the limited solubility
of 1, so a more soluble form was generated by installing an
octadecyl chain in the amine backbone and tert-butyl groups
on the methoxyphenyl substituents of the PNP ligand. This
ligand variant, PNC18P, was prepared according to Scheme S1
(in the Supporting Information) and metalated with
CrIIICl3(THF)3 to afford CrIIICl3(PNC18P) (2, Chart 1).32
Upon addition of 500 equiv of MAO,33 2 exhibits ethylene
trimerization catalysis very similar to 1 under 50 psi of eth-
ylene in aromatic solvents (Table 1). 1-Hexene is the predom-
inant product, along with small amounts of higher oligomers
(mostly C10)8 and polymer. Activity is favored by more elec-
Scheme 2. Proposed interpretation of spectral observations. 5 is
most likely multiple species.
tron deficient solvents: 1,2-difluorobenzene > chlorobenzene
> toluene, a trend that has been noted previously in related
systems.25,34 No product is obtained in either dichloromethane
in chlorobenzene frozen at –196 °C exhibits a broad rhombic
or diethyl ether, possibly due to solvent coordination and/or
signal, with g values at ~1.99, 3.02, and 4.57 (Figure 2A),
rapid catalyst decomposition. Interestingly, activity with 2 is
consistent with other chromium(III) (S=3/2) complexes.36
approximately seven times higher than with 1 (entries 1 vs. 2
When 2 is treated with MAO at –40 °C and immediately fro-
in Table 1), which may reflect the improved solubility of 2.
zen, a new signal is observed, with g = 1.98, 3.50, and 4.34
(Figure 2B), characteristic for octahedral CrIII with a large
The reaction of MAO with 2 is accompanied by a sequence
zero-field splitting and small rhombicity E.24,37 This spectrum
of color changes that can be followed by UV-visible (UV-vis)
absorption spectroscopy.35 Treatment of 2 in chlorobenzene
closely
resembles
that
reported
for
CrIII/2-
ethylhexanoate/alkyl aluminum,24 suggesting that the chlo-
rides in 2 have been replaced by alkyl groups from MAO to
give CrIIIR3(PNC18P) (3, where R = either methyl or butyl33
groups from the MAO reagent). Double integration, using 2
as a spin standard, indicates that the conversion of 2 to 3 is
nearly quantitative.
with MAO at –40 °C results in an instantaneous color change
from blue (λmax= 500, 660 nm) to red (λmax= 540 nm) (Figure
1A). The color further changes to green (λmax = 448, 630 nm)
over about an hour at –40 °C (Figure 1B); this conversion
exhibits an isosbestic point at 602 nm and is first-order in
chromium from a single wavelength kinetic analysis (Figure
S2). The green color is relatively stable at –40 °C over the
course of several hours. When 2 and MAO are mixed at 25
°C no red color is observed (it would be expected to decay
rapidly at this temperature); instead, the species correspond-
ing to the green color forms directly, and then gradually
changes to a light blue-green color over the course of an hour.
This transformation appears to take place in stages rather than
being a simple conversion of one species to another: the spec-
tra exhibit an isosbestic point only during the first phase (Fig-
ure 1C), and the data could not be fit to any simple kinetic
function (Figure S3), suggesting that the blue-green color
When the solution of 3 is allowed to stand at –40 °C, sever-
al new EPR signals (Figures 2C and S5) grow in, over rough-
ly the same time span as the color change from red to green
(see above). The major signal, which integrates as ~ 98% of
the total amount of chromium (CrTotal), is characteristic of
high-spin CrIII (S=3/2), with g = 3.85, 4.17, and 4.50. We
tentatively assign this signal to the cationic complex
[CrIIIR2(PNC18P)]+ (4),38 obtained from abstraction of an alkyl
group from 3 by MAO.16 Such a transformation would be
consistent with the reaction being first order in chromium.
The minor component initially appears as a rhombic signal
with g = 1.98, 2.00, and 2.03, characteristic of low-spin CrI
(S=1/2, designated as 5), and integrating at only ~2 % of CrTo-
tal. Over time another S = 1/2 signal grows in, similar to but
2
corresponds to at least two distinct species. H NMR spectra
recorded at –40 °C show significantly broadened peaks (Fig-
ure S4), consistent with paramagnetic species (see below).
Additional information on the sequence of formation of
species was obtained from EPR spectroscopy. A solution of 2
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Table 1. Ethylene trimerization data
1
2
3
Activitye
(g trimers/g
Cr/h)
Higher
Trimers
(wt %)
Activation
T
1-Hexene
(wt %)
PEf
(g)
Entry Cat.
Methoda (°C)
4
5
6
7
8
9
10
11
12
13
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15
16
17
18
19
1
2
3
4
5
1
2
2
2
2
Ib
Ib
25 220 ± 120
25 1446 ± 76
99
90
78
95
95
1
10
22
5
1.17
0.82
0.10
0.75
0.30
Ib
– 40
25
75 ± 1
IIc
IIId
512 ± 49
25 672 ± 260
5
aGeneral reaction condition: complex 2 (11.8 µmol) in 25 mL PhCl, MAO
(250 mg, ~500 equiv), ethylene (50 psi). bMethod I: pressurize reactor with
ethylene before adding MAO. cMethod II: add MAO, wait 15 s, then pres-
d
surize reactor with ethylene. Method III: add MAO, wait 15 h, then pres-
surize reactor with ethylene. eDetermined from the average of two inde-
pendent trials. fPolyethylene.
tion activity. The clear implication is that none of the major
species observed by UV-vis and/or EPR — 3, 4 and 7 — is
involved in catalysis.
Ruling out (or in) 5 or 6 as an active catalyst (or direct pre-
cursor thereto) is less clear cut, as they are never present in
more than small amounts. There is evidence that ethylene
reacts with 5: comparison of the EPR spectra obtained from
the reaction of 2 with MAO at –40 °C in the presence and
absence of ethylene reveals that the S=1/2 signal attributed to
5 is suppressed in the former; there is a much weaker, similar
but clearly different signal, which intensifies when the solu-
tion is allowed to warm to room temperature (Figure S7). In
contrast, introduction of ethylene to a reaction mixture pre-
pared at room temperature (blue-green) does not appear to
result in any change in the EPR signal assigned to 6 (or the
UV-vis spectrum that characterizes 7).
To sum up: the preparation of a soluble CrIIICl3(PNC18P)
precatalyst has allowed us, for the first time, to attempt de-
tailed in situ characterization of the species generated during
its activation by MAO under catalytic conditions. We can
identify the sequential major species — 3, 4 and 7 — by their
spectral signatures, and show that i) they account for most or
all of the Cr in solution, and ii) none of them is relevant to
catalysis. At present the CrI species 5 appears to be the most
viable candidate for the ethylene trimerization catalyst or
catalyst precursor — even though it does not appear to accu-
mulate to more than a small percent of the total, implying it
would have to be highly active — as it does react with eth-
ylene. Alternatively, 5 could be yet another red herring, such
that some species at low concentration that we have not been
able to observe at all with our methodology is the active cata-
lyst. Because 7 is the main species present (~94%) when
activity is highest, even though all the indications so far argue
against its participation in catalysis, its identification will be
key to developing a more complete picture of the MAO reac-
tion scheme. It must be noted, however, that MAO can also be
involved in deactivation processes,42 which could produce
dormant states that reduces the number of chromium species
available to react with substrates. Ongoing work is focused on
further characterizing the reaction intermediates observed
with the aim of identifying catalyst compositions and condi-
tions that most favor generation of the active species.
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Figure 2. X-band EPR spectra of compound 2 in chlorobenzene
A) before addition of MAO, B) immediately after mixing with
MAO at –40 °C, C) 1 h after mixing with MAO at –40°C, and D)
15 h after mixing with MAO at 25 °C. All spectra were recorded
as frozen solutions at –196 °C. g values are given and also
summarized in Table S2.
clearly distinct from the first (Figure S5B), representing <1%
of CrTotal
.
Finally, the reaction of 2 with MAO at 25 °C leads, after ~
1h, to yet another S = 1/2 signal (Figures 2D and S6), in this
case axial (g = 1.98, 2.00). Such a signal has been shown to
be characteristic of bis(arene)CrI sandwich complexes, which
in the present case could be either [CrI(η6-C6H5Cl)2]+ or
39
[CrI(bis-η6-PNC18P)]+.40,41 However, this signal accounts for
only ~6% of CrTotal; the remainder of the chromium in solu-
tion is EPR silent. A similar situation has been reported for a
related Cr-PNP (tetramerization) catalyst, where the EPR-
active CrI component comprised only a small percentage of
the total; the EPR-silent majority was suggested to be CrII
based on XAS measurements,18,25 although alternatives such
as di- or polynuclear species are also possible.
The reaction sequence suggested by the combined UV-vis
and EPR studies, along with proposed assignments (where
possible), is summarized in Scheme 2. To assess possible
participation of the observed intermediates in ethylene trimer-
ization, catalytic trials were carried out under different condi-
tions (Table 1). When an ethylene-saturated solution of 2 is
cooled to –40 °C and then treated with MAO, a significant
amount of catalytic trimerization can be observed after 1 h
(entry 3), even at such a low reaction temperature. Under
these conditions the only spectrally observable species by
EPR are 3, 4, and 5. Additional reactions were conducted by
pre-treating 2 with MAO at 25 °C approximately 15 s (entry
4) and 15 h (entry 5) before introduction of ethylene. The
spectral findings above indicate that no substantial amount of
either 4 or 5 would be present in either case, while 3 would be
present (but unstable) only in the first. The main compo-
nent(s) in both of these cases would be the EPR-silent blue-
green species 7, along with a smaller amount of 6; but neither
appears to have formed under the conditions of entry 3.
Nonetheless, all of these experiments exhibit good trimeriza-
ASSOCIATED CONTENT
Supporting Information. Experimental details, ligand syn-
thesis and characterization, spectroscopic data, and catalytic
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(19) McDyre, L. E.; Hamilton, T.; Murphy, D. M.; Cavell, K. J.;
Gabrielli, W. F.; Hanton, M. J.; Smith, D. M. Dalton Trans. 2010, 39,
7792-7799.
results. This material is available free of charge via the Inter-
net at http://pubs.acs.org.
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7
(20) Jabri, A.; Temple, C.; Crewdson, P.; Gambarotta, S.; Korobkov, I.;
Duchateau, R. J. Am. Chem. Soc. 2006, 128, 9238-9247.
(21) Jabri, A.; Mason, C. B.; Sim, Y.; Gambarotta, S.; Burchell, T. J.;
Duchateau, R. Angew. Chem., Int. Ed. 2008, 47, 9717-9721.
(22) Vidyaratne, I.; Nikiforov, G. B.; Gorelsky, S. I.; Gambarotta, S.;
Duchateau, R.; Korobkov, I. Angew. Chem., Int. Ed. 2009, 48, 6552-
6556.
AUTHOR INFORMATION
Corresponding Authors
bercaw@caltech.edu (J.E.B.) and jal@caltech.edu (J.A.L.)
ACKNOWLEDGMENT
8
9
(23) Monillas, W. H.; Young, J. F.; Yap, G. P. A.; Theopold, K. H.
Dalton Trans. 2013, 42, 9198-9210.
This work was supported by BP. L.H.D. acknowledges the
National Institute of General Medical Sciences for a postdoc-
toral fellowship (F32 GM099189-02). The authors thank An-
gelo DiBilio for assistance with EPR measurements and Da-
vid Leitch and Aaron Sattler for helpful discussions.
(24) Skobelev, I. Y.; Panchenko, V. N.; Lyakin, O. Y.; Bryliakov, K. P.;
Zakharov, V. A.; Talsi, E. P. Organometallics 2010, 29, 2943-2950.
(25) Rabeah, J.; Bauer, M.; Baumann, W.; McConnell, A. E. C.;
Gabrielli, W. F.; Webb, P. B.; Selent, D.; Brückner, A. ACS Catal. 2013,
3, 95-102.
(26) Busico, V.; Cipullo, R.; Talarico, G.; Caporaso, L. Macromolecules
1998, 31, 2387-2390.
(27) Bochmann, M. Organometallics 2010, 29, 4711-4740.
(28) Coevoet, D.; Cramail, H.; Deffieux, A.; Mladenov, C.; Pedeutour, J.-
N.; Peruch, F. Polym. Int. 1999, 48, 257-263.
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28
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58
59
60
REFERENCES
(1) Nexant Process Evaluation Research Planning (PERP) Report–Alpha
Olefins 06/07-5, 2008.
(2) Dixon, J. T.; Green, M. J.; Hess, F. M.; Morgan, D. H. J. Organomet.
Chem. 2004, 689, 3641-3668.
(3) McGuinness, D. S. Chem. Rev. 2011, 111, 2321-2341.
(4) Agapie, T. Coord. Chem. Rev. 2011, 255, 861-880.
(5) Agapie, T.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2007,
129, 14281-14295.
(6) Agapie, T.; Schofer, S. J.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem.
Soc. 2004, 126, 1304-1305.
(7) Agapie, T.; Day, M. W.; Henling, L. M.; Labinger, J. A.; Bercaw, J.
E. Organometallics 2006, 25, 2733-2742.
(8) Do, L. H.; Labinger, J. A.; Bercaw, J. E. Organometallics 2012, 31,
5143-5149.
(9) Schofer, S. J.; Day, M. W.; Henling, L. M.; Labinger, J. A.; Bercaw,
J. E. Organometallics 2006, 25, 2743-2749.
(10) Overett, M. J.; Blann, K.; Bollmann, A.; Dixon, J. T.; Haasbroek, D.;
Killian, E.; Maumela, H.; McGuinness, D. S.; Morgan, D. H. J. Am.
Chem. Soc. 2005, 127, 10723-10730.
(11) Blann, K.; Bollmann, A.; de Bod, H.; Dixon, J. T.; Killian, E.;
Nongodlwana, P.; Maumela, M. C.; Maumela, H.; McConnell, A. E.;
Morgan, D. H.; Overett, M. J.; Prétorius, M.; Kuhlmann, S.;
Wasserscheid, P. J. Catal. 2007, 249, 244-249.
(12) Bowen, L. E.; Charernsuk, M.; Hey, T. W.; McMullin, C. L.; Orpen,
A. G.; Wass, D. F. Dalton Trans. 2010, 39, 560-567.
(13) Carter, A.; Cohen, S. A.; Cooley, N. A.; Murphy, A.; Scutt, J.; Wass,
D. F. Chem. Commun. 2002, 858-859.
(14) Stennett, T. E.; Haddow, M. F.; Wass, D. F. Organometallics 2012,
31, 6960-6965.
(15) Janse van Rensburg, W.; Grové, C.; Steynberg, J. P.; Stark, K. B.;
Huyser, J. J.; Steynberg, P. J. Organometallics 2004, 23, 1207-1222.
(16) Janse van Rensburg, W.; van den Berg, J.-A.; Steynberg, P. J.
Organometallics 2007, 26, 1000-1013.
(29) Bray, K. L.; Charmant, J. P. H.; Fairlamb, I. J. S.; Lloyd-Jones, G. C.
Chem. Eur. J. 2001, 19, 4205-4215.
(30) Kina, A.; Iwamura, H.; Hayashi, T. J. Am. Chem. Soc. 2006, 128,
3904-3905.
(31) Schmidt, A. F.; Al-Halaiqa, A.; Smirnov, V. V.; Kurokhtina, A. A.
Kinet. Catal. 2008, 49, 638-643.
(32) Variable temperature 2H NMR spectra (Figure S1) show that the
methoxy groups in 2 exchange readily in solution, simiilar to those in 1
(see ref. 7).
(33) Modified MAO (MMAO, see Supporting Information) was used in
this study, which contains butyl in addition to methyl groups. All
references to MAO hereafter refer to the use of MMAO.
(34) Elowe, P. R.; McCann, C.; Pringle, P. G.; Spitzmesser, S. K.;
Bercaw, J. E. Organometallics 2006, 25, 5255-5260.
(35) The same color changes do not occur upon the reaction of MAO with
CrCl3(THF)3, in which case the magenta colored chlorobenzene solution
turns yellow.
(36) Bonomo, R. P.; Di Bilio, A. J.; Riggi, F. Chem. Phys. 1991, 151,
323-333.
(37) Hempel, J. C.; Morgan, L. O.; Lewis, W. B. Inorg. Chem. 1970, 9,
2064-2072.
(38) Macchioni, A. Chem. Rev. 2005, 105, 2039-2074.
(39) Aspley, C. J.; Boxwell, C.; Buil, M. L.; Higgitt, C. L.; Long, C.;
Perutz, R. N. Chem. Commun. 1999, 0, 1027-1028.
(40) McDyre, L.; Carter, E.; Cavell, K. J.; Murphy, D. M.; Platts, J. A.;
Sampford, K.; Ward, B. D.; Gabrielli, W. F.; Hanton, M. J.; Smith, D. M.
Organometallics 2011, 30, 4505-4508; Carter, E.; Cavell, K. J.; Gabrielli,
W. F.; Hanton, M. J.; Hallett, A. J.; McDyre, L.; Platts, J. A.; Smith, D.
M.; Murphy, D. M. Organometallics 2013, 32, 1924-1931.
(41) Köhn, R. D.; Smith, D.; Mahon, M. F.; Prinz, M.; Mihan, S.;
Kociok-Köhn, G. J. Organometal. Chem. 2003, 683, 200-208.
(42) Chen, E. Y.-X.; Marks, T. J. Chem. Rev. 2000, 100, 1391-1434.
(17) Jabri, A.; Crewdson, P.; Gambarotta, S.; Korobkov, I.; Duchateau, R.
Organometallics 2006, 25, 715-718.
(18) Brückner, A.; Jabor, J. K.; McConnell, A. E. C.; Webb, P. B.
Organometallics 2008, 27, 3849-3856.
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ACS Paragon Plus Environment