Stoichiometrically Activated Catalysts for Ethylene Tetramerization using Diphosphinoamine-Ligated Cr Tris(hydrocarbyl) Complexes

Article abstract of DOI:10.1021/acs.organomet.7b00706

A new, stoichiometric activation mode is presented for Cr-PNP (PNP = diphosphinoamine) complexes for ethylene tetramerization catalysis. To access suitable precatalysts, two robust Cr(III) multiaryl compounds were synthesized as THF adducts. These complexes are supported by a facially coordinated bis(aryl) ligand with an additional ether donor. From these precursors, Cr-PNP tris(hydrocarbyl) complexes were synthesized. Using 1 equiv of Br?nsted acid as an activator, an active species for the catalytic tetramerization of ethylene was produced, without the need for excess alkylaluminum reagents.

Full text of DOI:10.1021/acs.organomet.7b00706

Communication
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX-XXX
Stoichiometrically Activated Catalysts for Ethylene Tetramerization
using Diphosphinoamine-Ligated Cr Tris(hydrocarbyl) Complexes
Nathanael A. Hirscher and Theodor Agapie*
Division of Chemistry and Chemical Engineering, California Institute of Technology, 1200 East California Boulevard MC 127-72,
Pasadena, California 91125, United States
S
* Supporting Information
ABSTRACT: A new, stoichiometric activation mode is
presented for Cr-PNP (PNP = diphosphinoamine) complexes
for ethylene tetramerization catalysis. To access suitable
precatalysts, two robust Cr(III) multiaryl compounds were
synthesized as THF adducts. These complexes are supported
by a facially coordinated bis(aryl) ligand with an additional
ether donor. From these precursors, Cr-PNP tris(hydrocarbyl)
complexes were synthesized. Using 1 equiv of Brønsted acid as
an activator, an active species for the catalytic tetramerization of ethylene was produced, without the need for excess
alkylaluminum reagents.
inear α-olefins (LAOs) are comonomers in the production
of linear low-density polyethylene (LLDPE) and tradi-
Toward developing catalysts for olefin upgrading, a variety of
organochromium complexes have been studied recently.7
Several Cr compounds require no exogenous activators for
ethylene trimerization or nonselective oligomerization.7b,8
Ethylene trimerization catalysts have also been generated
from Cr triphenyl and Cr diaryl halide complexes via
stoichiometric treatment with acid or abstraction of halide.7j−l
Herein, we describe the synthesis of a Cr-PNP precursor
suitable for stoichiometric activation and demonstrate, for the
first time, the generation of ethylene tetramerization catalysts
upon protonation. Remarkably, these catalysts perform
comparably to the MMAO system using CrCl3 precursors.
Chromium compounds featuring three alkyl or aryl ligands
are typically thermally unstable,7g−j,9 although some Cr aryls
are stabilized using chelating interactions.10 Recently, a Cr
compound featuring chelating alkyl ligands was used to
generate an active tetramerization catalyst using excess
aluminum activators, albeit with significantly lower activity
than the chlorinated Cr precatalyst.11 Separately, the
protonolysis of a Cr triaryl complex was reported, but no
oligomerization activity was observed.7c
L
tionally have been produced via nonselective ethylene
oligomerization.1 Due to the high demand for pure fractions
of specific LAOs (typically 1-hexene or 1-octene), selective
ethylene oligomerization catalysts have been targeted for use on
an industrial scale.2 For 1-octene synthesis, the best catalyst
with respect to activity and selectivity is Cr-PNP (PNP =
diphosphinoamine, Scheme 1).3 Despite many studies inves-
Scheme 1. (Top) Selective Oligomerization of Ethylene to 1-
Hexene and 1-Octene and (Bottom) Typical Activation of
Cr-PNP Precatalysts Using a Large Excess of MMAO
Initial attempts to use CrPh3(THF)3 as a precursor were not
promising due to its instability;9b the generation of a five-
coordinate species likely contributed to decomposition. To
access a robust Cr-PNP complex, we used a chelating,
tridentate bis(aryl) ether ligand. Treatment of CrCl2(p-
tolyl)(THF)3 or CrCl2Me(THF)3 precursors with a bis(aryl)
ether organomagnesium reagent results in isolation of 1 or 2,
respectively, as a red crystalline solid (Scheme 2; see the
Supporting Information for experimental details). Single
-crystal X-ray diffraction (XRD) studies of 1 and 2 reveal
tigating the role of the PNP ligand and the activator, this
system still suffers from generation of significant amounts of
polymer or undesired oligomers.3a,4 Additionally, ill-defined
activators such as modified methylaluminoxane (MMAO) are
required in hundreds-fold excess relative to the Cr precatalyst.
Rational improvement of catalysis has been hindered by a lack
of mechanistic understanding related to the oxidation state,
coordination environment, and Cr nuclearity of the active
species.5 Using a single well-defined activator in stoichiometric
amounts is important for addressing mechanistic questions as
well as for industrial applications.6
Received: September 14, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00706
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
Scheme 2. (Top) Synthesis of Tris(hydrocarbyl) Precursors 1 and 2 and Cr-PNP Complexes 3 and 4 and (Bottom) from Left to
a
Right, Solid-State Structures of Compounds 1−4
a
Thermal ellipsoids are displayed at the 50% probability level. Solvent molecules and hydrogen atoms are omitted for clarity.
Table 1. Results of Catalysis Using Stoichiometric Activation (Entries 1−6) and Using 5 equiv of MMAO (Entries 7−11), in
a
Comparison to Activation of CrCl3 Precatalysts Using Excess MMAO (Entries 12 and 13)
b
b
b
b
c
entry
Cr source/PNP
1/iPrPNP
2/iPrPNP
1/tolPNP
2/tolPNP
3/−
nHBAr′/nMMAO
activity(g/g of Cr)
PE
C6
1-C8
C10−C14
1-C6 in C6 (%)
1-octene:1-hexene
1
1 H+
1600
500
<1
0
75
77
62
66
46
43
47
46
52
42
48
43
40
22
21
36
34
43
44
37
48
43
40
36
41
50
3
2
97
97
87
88
75
75
92
77
81
78
81
89
0.23
0.22
0.51
0.44
0.93
1.0
2
1 H+
3
1 H+
980
0
2
4
1 H+
260
0
<1
11
13
16
5
5
1 H+
3300
3700
4200
1500
1900
3900
4600
5200
3100
<1
<1
<1
1
6
4/−
1 H+
7
1/iPrPNP
1/tolPNP
2/tolPNP
3/−
1 H+/5 MMAO
1 H+/5 MMAO
1 H+/5 MMAO
1 H+/5 MMAO
1 H+/5 MMAO
300 MMAO
300 MMAO
0.64
1.0
8
9
0
5
0.75
0.92
0.70
0.80
1.3
10
11
12
13
2
16
15
16
10
4/−
1
CrCl3(THF)3/iPrPNP
CrCl3(THF)3/tolPNP
<1
<1
73
a
b
Conditions: [Cr] = 1 mM; solvent, 7.5 mL of PhCl; pressure, 100 psig C2H4; temperature, 25 °C; reaction time, 45 min. Weight percent (total).
c
Molar ratio.
similar geometries about Cr (see the Supporting Information
for bond metrics). The three hydrocarbyl ligands bind to Cr
facially, as in CrPh3(THF)3.9a,c Consequently, the bis(aryl)
ether ligand also binds facially; the coordination sphere is
completed by two THF ligands.
As designed, 1 or 2 display two THF ligands prone to
substitution with bidentate PNP. Indeed, treatment of 1 or 2
with (p-tol)N(PPh2)2 (tolPNP) yields 3 (47% yield) or 4 (49%
i
yield), respectively (Scheme 2). Metalation of PrN(PPh2)2
(iPrPNP) under similar conditions has not provided the
analogous complexes in practical amounts. XRD analysis
(Scheme 2) shows that the three hydrocarbyl ligands remain
in a facial arrangement, with the phosphorus atoms of tolPNP
binding trans to the chelating aryl donors, in the sites
previously occupied by THF ligands in 1 and 2. Compounds
3 and 4 represent the first structurally characterized Cr(III)
tris(hydrocarbyl) complexes that display an ancillary ligand
established to support selective ethylene tetramerization
catalysis. Notably, compounds 3 and 4 are reminiscent of
multiaryl Cr-PNP precatalysts for ethylene trimerization.7a,j,k In
those compounds, the ether moiety is linked to the PNP ligand
(PNP(OMe)4), limiting catalytic selectivity to trimerization. In
compounds 1−4, the stabilizing ether donor is not a part of the
ancillary PNP ligand, ultimately allowing for ethylene
tetramerization activity.
To employ compounds 1 and 2 as practical precursors for
organometallic catalysis, stability is important. Complex 1 was
compared to CrPh3(THF)3. CrPh3(THF)3 completely decom-
poses within seconds upon dissolution in toluene or Et2O, with
the formation of biphenyl as tracked by gas chromatography/
mass spectrometry (GC/MS). In the presence of 100 equiv of
THF, CrPh3(THF)3 completely decomposes within 3 h in
toluene ([Cr] ≈ 4 mM). Complex 1 decomposes in solution,
producing 0.2 equiv of 4,4′-dimethylbiphenyl within 1 h in
toluene or 5 h in Et2O ([Cr] = 4 mM). No other aryl−aryl
bond formation product is observed, suggesting that the
reductive elimination is an intermolecular process. The only
partial decomposition observed over these longer periods
indicate higher stability for 1 relative to CrPh3(THF)3. In the
presence of THF (100 equiv), no decomposition of 1 was
observed in toluene after 2 days. The significant stability of 1 in
comparison to CrPh3(THF)3 is attributed to the chelating
bis(aryl) ether ligand. Increased stability has been reported for
bidentate aryl ligands, albeit with amine donors.10b−d
With complexes 1−4 in hand, catalytic trials were performed
upon stoichometric activation with a Brønsted acid, [H-
(Et2O)2][BAr′4] (Ar′ = 3,5-(CF3)2-C6H3). Precursor 1 or 2
was mixed with PNP ligands to generate the Cr-PNP species in
B
DOI: 10.1021/acs.organomet.7b00706
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
situ, while 3 and 4 were used directly. Upon addition of
ethylene (100 psi) to the activated Cr species, mixtures of 1-
hexene and 1-octene were produced (Table 1). Preligated
complexes 3 and 4 give substantially higher activity in
comparison to 1 and 2 (entries 3−6, Table 1). This may be
due to partial decomposition of precursors 1 and 2 or their
protonation products before substitution of tolPNP for THF
ligands. Additionally, the presence of 2 equiv of THF could
inhibit catalysis (vide infra). Catalytic activities as high as 3700
g/g of Cr (∼6900 equiv of C2H4 consumed) were obtained
using 4, with selectivity for 1-octene being as high as 44 wt %.
Very small amounts of polymer were produced in the
experiments using stoichiometric activation (entries 1−6,
Table 1). In the trials showing higher activity, oligomers
resulting from cotrimerization/tetramerization of ethylene and
1-hexene or 1-octene (C10-C14 branched α-olefins) were
produced in larger amounts.4i Most notably, the catalytic
performance using 3 or 4 and 1 equiv of [H(Et2O)2][BAr′4] is
very close to that obtained using 300 equiv of MMAO to
activate a mixture of CrCl3(THF)3 and tolPNP or iPrPNP
(Table 1, entries 5, 6, 12, and 13). These results demonstrate
for the first time that large excesses of Al activators are not
required for 1-octene generation. Additionally, very low (or
undetectable) levels of polymer are generated under these
conditions. Both of these features are important for potential
large-scale practical applications.
When the catalysts are prepared from 1 or 2 and PNP in situ,
2 equiv of THF is present in the mixture, in contrast to
activation of 3 or 4. Catalytic trials were performed in the
presence of a small amount of MMAO (5 equiv) toward
binding any THF ligands that may compete with ethylene
coordination to Cr. Significantly improved oligomerization
activity was observed in all cases when precatalysts 1 and 2
were used (entries 1, 3, 4, and 7−9, Table 1). The 1-octene:1-
hexene ratio increased by a factor of 2, on average, upon
MMAO addition in these experiments. It is proposed that Al
species in MMAO abstract THF ligands, generating a more
sterically open and electronically deficient Cr center and
causing an increase in activity and 1-octene selectivity. In
agreement with the above proposal, addition of MMAO (5
equiv) following protonation of 3 or 4, which lack THF, results
in smaller effects on oligomerization activity and selectivity
(entries 5, 6, 10, and 11, Table 1). However, the possibility that
the MMAO alkylates the Cr center or otherwise changes the
mechanism of activation following protonation cannot be
excluded.
Scheme 3. Stoichiometric Activation of 3 via Protonation
followed by β-H elimination and C−H reductive elimination,
are observed. These products are similar to those observed for
the stoichiometric activation of well-defined Cr-PNP(OMe)4
precatalysts for ethylene trimerization.7j,l In contrast, aryl−aryl
reductive elimination was not observed in that case. This
difference in initiation could be a consequence of the lower
denticity of the PNP ligands selective for ethylene tetrameriza-
tion.12 Species corresponding to ethylene insertion followed by
reductive elimination with C−C bond formation are not
observed.
In summary, the first example of Cr catalysis for ethylene
tetramerization upon stoichiometric activation was demon-
strated. Access to appropriate Cr tris(hydrocarbyl) precursors is
instrumental for activation via protonation. The use of chelating
bis(aryl) ether ligands affords organometallic Cr precursors that
are suitably robust. The multidentate ligand was designed to
leave two metal coordination sites available for bidentate PNP
binding. Substitution of THF ligands for PNP affords
complexes that are six-coordinate. Treatment with [H(Et2O)2]-
[BAr′4] leads to aryl or methyl ligand protonolysis. Subsequent
addition of ethylene results in the catalytic formation of 1-
octene and 1-hexene with high activities and low polymer
production. Under the conditions investigated, activity and
selectivity were comparable to those observed using CrCl3
precatalysts with large excesses of MMAO. These catalyst
features are important for industrial applications. With well-
defined precursors capable of stoichiometric activation, future
studies will focus on mechanistic elucidation and character-
ization of reaction intermediates.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00706.
■
S
Analysis of organic byproducts of Cr aryl precatalysts has
been used to infer possible activation mechanisms.7c,j,l Upon
1
protonation of 3, 0.3 equiv of toluene was observed by H
NMR (C6D5Cl), suggesting a ratio of 2:1 for the putative
cationic species 5a and 5b (Scheme 3). Quenching with excess
CD3OD shows partial deuteration of dibenzyl ether via GC/
MS, consistent with hydrocarbyl ligands remaining bound to
Cr. GC/MS analysis also shows the gradual formation of p-
tolyl-substituted dibenzyl ether over 1 h at room temperature,
consistent with reductive elimination from 5a to generate a
Cr(I) species. The catalytic activity was found to decrease upon
storing the activated mixture at room temperature in the
absence of ethylene, indicating that the resulting Cr(I) species
is prone to decomposition in the absence of monomer. Under
normal activation conditions, the same aryl−aryl reductive
elimination is also observed following catalysis. Additionally,
species consistent with one, two, and three ethylene insertions,
Experimental details, synthetic procedures, and crystallo-
graphic information (PDF)
Accession Codes
CCDC 1562429−1562432 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail for T.A.: agapie@caltech.edu.
■
C
DOI: 10.1021/acs.organomet.7b00706
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
A. E. C.; Gabrielli, W. F.; Webb, P. B.; Selent, D.; Bruckner, A. ACS
Catal. 2013, 3, 95.
(6) Stennett, T. E.; Haddow, M. F.; Wass, D. F. Organometallics
2012, 31, 6960.
ORCID
̈
Theodor Agapie: 0000-0002-9692-7614
Notes
(7) (a) Monillas, W. H.; Young, J. F.; Yap, G. P. A.; Theopold, K. H.
Dalton Trans. 2013, 42, 9198. (b) McGowan, K. P.; Abboud, K. A.;
Veige, A. S. Organometallics 2011, 30, 4949. (c) Ronellenfitsch, M.;
Wadepohl, H.; Enders, M. Organometallics 2014, 33, 5758.
(d) Rozenel, S. S.; Chomitz, W. A.; Arnold, J. Organometallics 2009,
28, 6243. (e) Jabri, A.; Crewdson, P.; Gambarotta, S.; Korobkov, I.;
Duchateau, R. Organometallics 2006, 25, 715. (f) Albahily, K.;
Fomitcheva, V.; Gambarotta, S.; Korobkov, I.; Murugesu, M.;
Gorelsky, S. I. J. Am. Chem. Soc. 2011, 133, 6380. (g) Conde-
Guadano, S.; Danopoulos, A. A.; Pattacini, R.; Hanton, M.; Tooze, R.
P. Organometallics 2012, 31, 1643. (h) Kreisel, K. A.; Yap, G. P. A.;
Theopold, K. H. Organometallics 2006, 25, 4670. (i) Ai, P.;
Danopoulos, A. A.; Braunstein, P. Organometallics 2015, 34, 4109.
(j) Schofer, S. J.; Day, M. W.; Henling, L. M.; Labinger, J. A.; Bercaw,
J. E. Organometallics 2006, 25, 2743. (k) Agapie, T.; Schofer, S. J.;
Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2004, 126, 1304.
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to The Dow Chemical Company and Caltech
for funding. We thank Sean Ewart, David Laitar, Mari Rosen,
Joshua Buss, and Dinesh Aluthge for insightful discussions and
Larry Henling and Mike Takase for assistance with
crystallography.
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DOI: 10.1021/acs.organomet.7b00706
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