Crystallographic revelation of the role of AIMe3 (in MAO) in Cr [NNN] pyrazolyl catalyzed ethylene trimerization

Article abstract of DOI:10.1021/om9002347

A series of Cr-Al intermetallic complexes, viz. [(Pz₃CH) ₂Cr¹¹₂(μ-Cl)₂]₂ ⁺ 2[(Me₂XAl)(μ-Cl)(Me₂XAl)]^- (X = Me/Cl), Pz₃CHCr¹¹¹

Full text of DOI:10.1021/om9002347

Organometallics 2009, 28, 2935–2937
2935
Crystallographic Revelation of the Role of AlMe3 (in MAO) in Cr
[NNN] Pyrazolyl Catalyzed Ethylene Trimerization
Jun Zhang,† Aifang Li,‡ and T. S. Andy Hor*,†
Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, Kent Ridge, Singapore,
and Department of Chemistry, College of Chemistry and Chemical Engineering, and the MOE Key
Laboratory of Ananlytical Sciences, Xiamen UniVersity, People’s Republic of China
ReceiVed March 28, 2009
Summary: A series of Cr-Al intermetallic complexes, Viz.
[(Pz′3CH)2CrII (µ-Cl)2]2+ · 2[(Me2XAl)(µ-Cl)(Me2XAl)]- (X )
2
Me/Cl), Pz′3CHCrIIIMeCl(µ-Cl)(AlMe2X) (X ) Me/Cl), and
Pz′2CHCH2(µ-N)CH2(µ-Ph)CrIIICl(µ-Cl)AlMe2, haVe been iso-
lated from the reactions of CrCl3(NNN) (NNN ) tridentate
pyrazolyl ligand) with neat AlMe3 or commercial MAO solu-
tions. The products generally show high actiVity and selectiVity
in ethylene trimerization. Crystallographic analysis has proVided
structural eVidence for Cr(III) reduction by residual AlMe3 in
a mixture containing the “real” actiVator (MAO) as well as
other transformations: Viz., adduct formation, methylation,
cationization, halide abstraction, N-H deprotonation, and C-H
cleaVage of an unactiVated arene.
Figure 1. [NNN] pyrazolyl Cr(III) Complexes 1-4.
performance, whereas the fairly acidic N-H bond is expected
to be deprotonated in the process. Hence, the observed lack of
reactivity of N-H toward Al(III) or Cr-R is perplexing.4
We have recently isolated some unusual bi- and intermetallic
intermediates that are catalytically relevant.5 This was followed
by the report of a highly selective trimerization Cr(III) catalytic
system with heteroscorpionate pyrazolyl ligands of the type
Pz′2CHCH2XR (Pz′ ) 3,5-dimethylpyrazol-1-yl, X ) O, S, R
) alkyl, aryl).6 These results prompted us to probe the activity
of the actual activator MAO with precatalyst Cr(III) as a means
to gain some structural insights into the mode of interaction.
We herein choose model Cr(III) systems with [NNN] ligands
of the type Pz′2CHX (1-4; X ) N-heterocycle or CH2NHCH2-
Ph) (Figure 1) and their reaction with commercial MAO reagent
that contains AlMe3. Use of a similar tris(pyrazolyl)methane
ligand with Cr(III) (i.e., 1) as a highly selective trimerization
catalyst has been patented.7 X-ray single-crystal crystallographic
analysis of the isolated materials revealed some intriguing
intermetallic interactions and crystallographic evidence of the
role of AlMe3 (in MAO) in the reduction of the precatalyst.
We also reveal the structural outcome of unexpected amine
N-H deprotonation and arene C-H activation reactions on the
bimetallic core.
The selective trimerization of ethylene has attracted vigorous
attention recently, due largely to the importance of 1-hexene in
the production of linear low-density polyethylene (LLDPE).1
The trimerization mechanism is generally believed to involve
metallacyclic intermediates,2 but despite intensive studies, there
remains considerable debate on the mechanistic details, such
as the dynamics of the chromium oxidation states at different
stages of the catalytic cycle.1a,b It has been proposed that the
cocatalyst reduces the metal at the early stage of activation.1a,b
One of the most commonly used trimerization cocatalysts is
methylaluminoxane (MAO), the role of which is generally
assumed to be metal alkylation followed by alkyl abstraction
to give an ion pair.1a However, as the structural elucidation of
the intermediates formed between the precatalytic Cr(III) and
MAO remains a formidable challenge, the precise role of MAO
in the trimerization pathway is still uncertain.3 In the remarkable
PNP (PNP ) Ph2PN(R)PPh2) and Sasol SNS (SNS ) RSCH2-
CH2N(H)CH2CH2SR) systems, several Al(III) activators (e.g.,
isobutylaluminoxane (iBAO), AlMe3, AlEt2Cl, AlMeCl2, and
AlCl3) are found to be active in the reduction,4a,d alkylation,4a,b
chloride abstraction,4a-d cationization,4b and oxidation (or
disproportionation) of the precatalysts.4b Attempts to probe the
interaction between Cr(III) and the “real” cocatalyst MAO only
resulted in “intractable materials”.4a In the Sasol system, the
N-H function in the SNS ligands is believed to be key to its
The [NNN] pyrazolyl ligands were prepared by modified
literature procedures.8 Their treatment with [CrCl3(THF)3] in
THF afforded the corresponding Cr(III) complexes 1-4 in good
yields (88-92%).
(4) (a) Jabri, A.; Temple, C.; Crewdson, P.; Gambarotta, S.; Korobkov,
I.; Duchateau, R. J. Am. Chem. Soc. 2006, 128, 9238. (b) Temple, C.; Jabri,
A.; Crewdson, P.; Gambarotta, S.; Korobkov, I.; Duchateau, R. Angew.
Chem., Int. Ed. 2006, 45, 7050. (c) Temple, C.; Gambarotta, S.; Korobkov,
I.; Duchateau, R. Organometallics 2007, 26, 4598. (d) Jabri, A.; Gambarotta,
S.; Korobkov, I.; Duchateau, R. Organometallics 2006, 25, 715.
(5) (a) Weng, Z. Q.; Teo, S.; Koh, L. L.; Hor, T. S. A. Angew. Chem.,
Int. Ed. 2005, 44, 7560. (b) Weng, Z. Q.; Teo, S.; Koh, L. L.; Hor, T. S. A.
Chem. Commun. 2006, 1319. (c) Weng, Z. Q.; Teo, S.; Liu, Z. P.; Hor,
T. S. A. Organometallics 2007, 26, 2950.
(6) Zhang, J.; Braunstein, P.; Hor, T. S. A. Organometallics 2008, 27,
4277.
(7) Yoshida, T.; Yamamoto, T.; Okada, H.; Murakita, H. (Tosoh
Corporation) US2002/ 0035029, March 21, 2002.
* To whom correspondence should be addressed. Fax: (+65) 6873 1324.
E-mail: andyhor@nus.edu.sg.
† National University of Singapore.
‡ Xiamen University.
(1) (a) Dixon, J. T.; Green, M. J.; Hess, F. M.; Morgan, D. H. J.
Organomet. Chem. 2004, 689, 3641. (b) Wass, D. F. Dalton Trans. 2007,
816.
(2) (a) Briggs, J. R. J. Chem. Soc., Chem. Commun. 1989, 11, 674. (b)
Agapie, T.; Schofer, S. L.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem.
Soc. 2004, 126, 1304.
(3) (a) van Rensburg, W. J.; van den Berg, J.-A.; Steynberg, P. J.
Organometallics 2007, 26, 1000. (b) Bru¨ckner, A.; Jabor, J. K.; McConnell,
A. E. C.; Webb, P. B. Organometallics 2008, 27, 3849.
(8) (a) Reger, D. L.; Foley, E. A.; Semeniuc, R. F.; Smith, M. D. Inorg.
Chem. 2007, 46, 11345. (b) Maria, L.; Cunha, S.; Videira, M.; Gano, L.;
Paulo, A.; Santos, I. C.; Santos, I. Dalton Trans. 2007, 3010.
10.1021/om9002347 CCC: $40.75
2009 American Chemical Society
Publication on Web 04/23/2009

2936 Organometallics, Vol. 28, No. 10, 2009
Communications
Table 1. Ethylene Trimerization Catalyzed by 1-7a
entry (complex)
amt of Cr (µmol)
PE (wt %)
C6 (wt %)
C8 (wt %)
C10 (wt %)
1-C6 (wt %)
activity (g/((g of Cr) h))
1 (1)
2 (2)
3 (3)
4 (4)
5 (5)
6 (5)b
7 (6)
8 (7)
8.7
8.9
7.9
8.0
5.5
6.1
5.3
8.9
<0.1
<0.1
<0.1
<0.5
<0.1
<0.1
<0.1
<0.1
97.6
97.8
98.5
92.1
98.7
97.8
97.8
93.0
1.3
1.1
1.4
3.5
1.2
2.1
1.1
3.7
1.0
1.0
0
3.9
0
0
1.0
3.2
98.4
99.3
99.1
98.7
99.1
99.1
99.3
98.3
32 400
38 200
53 000
10 500
36 300
630
37 400
13 300
a Conditions: 200 MAO, 30 bar, 60 mL of toluene, 80 °C, 30 min. b 50 · AlMe3.
Scheme 1. Reactions of 1 with AlMe3 or Residual AlMe3 in
MAO
Figure 2. Molecular structure of 5. The two [(Me2AlMe(Cl))2(µ-
Cl)]- counteranions in 5 are omitted for clarity. Selected bond
lengths (Å) and angles (deg) for 5: Cr1-N1 ) 2.091(3), Cr1-N3
) 2.087(3), Cr1-N5 ) 2.290(3), Cr1-Cl1 ) 2.384(1), Cr1-Cl2
) 2.376(1); N3-Cr1-Cl1 ) 173.27(9), N1-Cr1-Cl1 ) 94.29(8),
N(3)-Cr(1)-N(1) ) 83.68(11), N(3)-Cr(1)-N(5) ) 83.01(11),
N(1)-Cr(1)-N(5) ) 85.85(11), Cl2-Cr1-Cl1 ) 85.52(3),
Cr2-Cl1-Cr1 ) 94.48(3).
The catalytic performance of 1-4 toward ethylene trimer-
ization is summarized in Table 1. At 80 °C under 30 bar of
ethylene pressure and in the presence of MAO (Al:Cr ) 200:
1), the precatalysts with a heterocycle group, i.e. a pyridinyl,
imidazolyl, or pyrazolyl group, are highly selective toward
trimerization (total C6 selectivities >97.6%) to 1-hexene
(>96.0%) with good activities of 32 400-53 000 g/((g of Cr)
h) (entries 1-3). The highest activity is found in 3, which has
an imidazolyl group. Complex 4, with an amino group, is less
active and selective than those bearing a heterocyclic group
(compare entry 4 with entries 1-3).
The reaction of 1 with 6 equiv of AlMe3 results in a reduction
to give cationic [(Pz′3CH)2Cr2(µ-Cl)2]2+ · 2[(Me2XAl)(µ-Cl)-
(Me2XAl)]- (5; X ) Me/Cl), isolated as green crystals in 79%
yield (Scheme 1). It consists of a dinuclear chloride-bridged
dication balanced by two dialuminate [(Me2XAl)2(µ-Cl)]-
anions. Each Me2XAl moiety is crystallograhphically repre-
sented as a mix of [Me3Al] and [Me2ClAl].9 The planar [CrII Cl2]
2
rhombic core together with the supporting [NNN] ligands form
an edge-sharing bi-square-pyramidal structure (Figure 2). Use
of a 10 equiv excess of MAO gives the same product (61%)
but also introduces the side product Pz′3CHCrMeCl(µ-
Cl)(AlMe2X) (6; 9%) (Figure 3).9 The latter is a product of
methylation and Lewis adduct formation with AlMe3, giving a
Figure 3. Molecular structure of 6. The toluene solvate in 6 has
been omitted for clarity. Selected bond lengths (Å) and angles (deg)
for 6: Cr1-N1 ) 2.235(2), Cr1-N3 ) 2.085(2), Cr-N5 )
2.094(2), Cr-Cl2 ) 2.3213(7), Cr-Cl1 ) 2.4182(7), Al-Cl1 )
2.346 (1); N(3)-Cr(1)-N(5) ) 84.23(8), N(1)-Cr(1)-N(5) )
84.89(8), Cl2-Cr1-Cl1 ) 89.09(3), Al-Cl(1)-Cr ) 128.55(4).
(9) The crystals were mounted on quartz fibers, and X-ray data were
collected on a Bruker AXS APEX diffractometer, equipped with a CCD
detector at -50 °C, using Mo KR radiation (λ ) 0.710 73 Å). The data
were corrected for Lorentz and polarization effects with the SMART suite
of programs and for absorption effects with SADABS. Structure solution
and refinement were carried out with the SHELXTL suite of programs. In
complex 5, each of the two (Me3Al)2(µ-Cl) counteranions has two of the
six methyl group carbon atoms not properly behaving during the refinement.
By arbitrarily attributing 13% chlorine character to one carbon atom, 22%
chlorine character to another carbon atom, and the remaining as carbon
character, respectively, it was possible to obtain a satisfactory refinement.
Complex 6 has one molecule of toluene per chromium atom in the lattice,
with one of the three methyl group (in AlMe3) carbon atoms not properly
behaving during the refinement. By arbitrarily attributing 33% chlorine
character and the remaining as carbon character, it was possible to obtain
a satisfactory refinement. The structures were solved by direct methods to
locate the heavy atoms, followed by difference maps for the light non-
hydrogen atoms.
single Cr-Cl-Al bridge. These reactions suggest that the
residual AlMe3 in commercial MAO solution (prepared from
partial hydrolysis of AlMe3) is an active ingredient in both metal
reduction and M-Cl activation. When the MAO reagent was
pretreated by removing AlMe3 in vacuo at 50 °C for 5 h, neither
5 nor 6 was isolated in either reaction, suggesting that, in the
early stage of activation, AlMe3 is the active reductant in MAO.
Complete removal of AlMe3 was not successful in our hands.
The isolation of 6 enables a rare glimpse of the possible mode
of interaction between AlMe3 and the Cr(III) species before
reduction or ion formation takes place. The reaction of 6 and 2
equiv of AlMe3 leads to the formation of 5 in 89% yield,
suggesting that 5 is the reductive product of 6 by AlMe3. In the

Communications
Scheme 2. Reaction of 4 with AlMe3
Organometallics, Vol. 28, No. 10, 2009 2937
Cr pyrrolyl ethylene trimerization system, Cr(III) ethyl hex-
anoate, [Cr3+(EH)3], is proposed to be activated through one-
electron inner shell reduction to Cr2+(EH)2 initiated by AlEt3
and accompanied by ligand exchange and formation of ethyl
radical.10 Activated by MAO, both 5 and 6 show activities and
selectivities similar to those of 1 (entries 1, 5, and 7 in Table
1), also suggesting that they could lead to similar catalytic
species. Use of 5 with AlMe3 as cocatalyst shows very low
activity, indicating the indispensable role of MAO, especially
in the late stages of the activation and trimerization process.
AlMe3 is believed to serve as the main reductant in the early
stages of activation (compare entries 5 and 6).
Figure 4. Molecular structure of 7. Selected bond lengths (Å) and
angles (deg) for 7: Cr-C19 ) 2.043(4), Cr-N1 ) 2.058(3), Cr-N5
) 2.096(3), Cr-N3 ) 2.186(3), Cr-Cl1 ) 2.463(1), Al-N5 )
1.947(4), Al-Cl1 ) 2.286(2); Al-N5-Cr ) 100.5(1), Al-Cl1-Cr
) 81.69(5), N5-Al-Cl1 ) 93.1(1), C-Cr-N3 ) 170.06(14),
C-Cr-N5 ) 82.86(14), C-Cr-Cl1 ) 90.35(12).
trimerization process that contains the “real” coactivator: viz.,
MAO. Our study unveils some unexpected intermetallic-assisted
ligand transformation, suggests that we may have underestimated
the importance of adventitious AlMe3 in MAO-activated eth-
ylene oligomerization processes, and offers a possible reductive
process for the Cr(III) precatalyst. The isolation of these
catalytically relevant intermediates point to a complex and
multifaceted role played by AlMe3 (in MAO) in Cr-Al-
catalyzed ethylene oligomerization processes, especially the
reduction of precatalyst. It also offers a possible explanation of
the complex and at times conflicting results of some related
systems. While we are still far from deciphering the mechanistic
details of the trimerization process, it is clear that interaction
with the Al(III) activator with the commonly used tridentate
ligand used in ethylene oligomerization cannot be ignored. Our
next target is to probe the catalytic advantage of using
multidentate ligands that can actively promote such interactions
with the Al(III) activator and the stabilizing effect thus obtained
from intermetal cooperativity in conjunction with C-H and
N-H activations.
C-H activation11 is generally more common among heavy
metals,11b partly because of the M-C and M-H bonds are
stronger than for 3d metals such as Cr.12 In the presence of a
6-fold excess of AlMe3, complex 4 readily undergoes cyclom-
etalation with C-H activation and a concomitant N-H cleavage
to capture the Al moiety to give an unusual heterobimetallic
Al-Cr complex, Pz′2CHCH2(µ-N)CH2(µ-Ph)CrCl(µ-Cl)AlMe2
(7) (Scheme 2 and Figure 4). The four-membered ring
{Cr-N-Al-Cl} is possibly formed through a cyclohetero-
metalation step via CH4 elimination from a Cr-Cl-Al bridged
adduct similar to that witnessed in 6. Acid condensation would
then explain the arene metalation. Similar intermolecular arene
C-H activation has been reported in the reaction of [N(o-
C6H4PiPr2)2]NiMe with benzene in the presence of AlMe3,
giving [N(o-C6H4PiPr2)2]NiPh.11b Complexes 7 and 4 show
similar trimerization activities (Table 1, entries 8 and 4).
It is remarkable that, within a single model system, there is
crystallographic evidence of different functions of AlMe3 as the
catalyst activator: viz., adduct formation, methylation, reduction,
cationization, halide abstraction, N-H deprotonation, and C-H
cleavage of an unactivated arene. Among all the heterometallic
species isolated, there is no spectroscopic or structural evidence
of any MAO-stabilized adduct. Instead, the products conclu-
sively point to an active and key reductive role of AlMe3 in a
Acknowledgment. We acknowledge Professor Y. B. Jiang
(Xiamen University) for helpful discussions and cosupervi-
sion of A.L. We are grateful to the Agency for Science,
Technology & Research (Grant No. R143-000-364-305), the
Ministry of Education (Grant No. R143-000-361-112), and
the National University of Singapore (Grant No. ICF-AH-
48-07) for financial support.
(10) van Rensburg, W. J.; Grove, C.; Steynberg, J. P.; Stark, K. B.;
Huyser, J. J.; Steynberg, P. J. Organometallics 2004, 23, 1207.
(11) (a) Labinger, J. A.; Bercaw, J. E. Nature (London) 2002, 417, 507.
(b) Liang, L.-C.; Chien, P.-S.; Huang, Y.-L. J. Am. Chem. Soc. 2006, 128,
15562.
(12) (a) Coles, M. P.; Gibson, V. C.; Clegg, W.; Elsegood, M. R. J.;
Porrelli, P. A. Chem. Commun. 1996, 1963. (b) Rudler, H.; Parlier, A.;
Lafolle´e-Bezennine, S.; Vaissermann, J. Eur. J. Org. Chem. 1999, 2825.
Supporting Information Available: Text and figures giving
details of the syntheses and characterization data for all new
compounds and CIF files giving X-ray crystallographic data for
5-7. This material is available free of charge via the Internet at
http://pubs.acs.org.
OM9002347