The question of the Cr oxidation state in the {Cr(SNS)} catalyst for selective ethylene trimerization: An unanticipated re-oxidation pathway

Article abstract of DOI:10.1002/anie.200602240

(Chemical Equation Presented) An unprecedented oxidation of chromium(II) by trimethylaluminum which, despite the reducing environment, forms a cationic organochromium(III) complex (see picture; SNS = RSCH₂CH ₂N(H)-CH₂CH₂SR, Cy = cyclohexyl, X = [Me ₆Al₂Cl]), provides a possible catalyst re-activation pathway.

Full text of DOI:10.1002/anie.200602240

Communications
Catalyst Reactivation
RSCH2CH2N(H)CH2CH2SR)[8] initially indicated that even
this system follows the same trend.[9] The main supporting
evidence for this was the isolation of a divalent, cationic
species resulting from the reaction of [(SNS)CrCl3] (1) with
isobutylaluminoxane (iBAO). On the other hand, the iden-
tical reaction of 1 with either methylaluminoxane (MAO) or
AlMe3 did not lead to CrII, but instead gave a thermally robust
cationic organochromium(III) species. This result indicates
that the SNS ligand system has an unanticipated ability to
stabilize some organochromium(III) functions. This observa-
tion led us to question whether the CrII oxidation state is
effective in the SNS system. In other words, could the
DOI: 10.1002/anie.200602240
The Question of the Cr Oxidation State in the
{Cr(SNS)} Catalyst for Selective Ethylene
Trimerization: An Unanticipated Re-Oxidation
Pathway**
Claire Temple, Amir Jabri, Patrick Crewdson,
Sandro Gambarotta,* Ilia Korobkov, and
Robbert Duchateau*
exceptional stability of the cationic CrIII R function be
À
related to the selectivity? This idea was initially discounted
because the CrII precursors of this ligand system display the
same selectivity as the CrIII precursors and that oxidation of
CrII to the trivalent state cannot normally be foreseen in the
presence of reducing alkyl aluminum activators. On the other
hand, a cationic CrIII species has been shown to perform the
trimerization reaction even in the absence of activator.[10]
Herein we propose an intriguing alternative mechanistic
possibility based on further results obtained while studying
the effects of aluminum-based activators on the {Cr(SNS)}
system.
The steady growth of interest in selective ethylene oligome-
rization[1] is easily understood in view of the importance of
linear a-olefins for a wide range of applications. In particular,
clarifying the reaction mechanism and identifying the redox
couple utilized by the catalytically active species is clearly
central to the rational design of new and better catalysts.
Among all the metals that are known to catalyze the process,
chromium is currently the most preferred since it produces
the best activity as well as selectivity.[2] The metallacyclic
mechanism[1,3] has become the most favored explanation for
the selectivity of the process, yet important questions,
including the metal oxidation state,[4] remain unanswered.
Previous work from our group has attempted to address
this issue by examining one of the most versatile catalytic
systems reported to date, one which is able to selectively
tetramerize ethylene. For example, we found that when the
trivalent catalyst precursor of the remarkable PNP system
(PNP = Ph2PN(R)PPh2)[5] was exposed to AlMe3, reduction
to CrII and formation of a cationic complex ensued.[6] Along
the same lines, a comparative testing of two s,p-donating
tripyrrolide CrIII and CrII complexes indicated that the CrII
species was more active while producing an identical product
distribution to the CrIII complex.[7] These results led us to
believe that CrIII is reduced by the alkyl aluminum activator in
the early stages of catalyst formation.
When complex 1 was treated with 10 equivalents of
AlEt2Cl in toluene, the dicationic dimer [{(SNS)Cr(m-
Cl)Et}2][AlEtCl3]2 (2) was obtained with an 83% yield of
crystalline material (Scheme 1). Surprisingly, the crystal
Scheme 1.
Amore recent investigation of the highly selective
structure (Figure 1) clearly indicated that the trivalent state
is preserved in this ethylchromium species.[11] The lack of
metal reduction in this case and the isolation of a cationic
ethylchromium(III) species in good yield was quite unex-
pected given the inclination of 1) trivalent organochromium
towards reduction and 2) the M–Et function towards b-
hydride elimination.[12]
Sasol
SNS
trimerization
system
(SNS =
[*] Dr. C. Temple, A. Jabri, P. Crewdson, Prof. Dr. S. Gambarotta,
I. Korobkov
Department of Chemistry
University of Ottawa
The organochromium complex 2 displays a remarkable
thermal stability in both the solid state and boiling toluene,
where it slowly starts to degrade after one hour under reflux.
On the other hand, if 2 is left in an excess of alkylating agent
10 Marie Curie, Ottawa ON K1N6N5 (Canada)
Fax: (+1)613-562-5170
E-mail: sgambaro@science.uottawa.ca
Prof. Dr. R. Duchateau
Department of Chemistry
Eindhoven University of Technology
P.O. Box 513, 5600 MB (The Netherlands)
Fax: (+31)40-247-4918
(e.g. MAO, AlMe , AlEt3), intractable degradation products
3
form rapidly. Since no well-defined/isolatable CrII complexes
were obtained from any of the above reactions, we deliber-
ately attempted the preparation of divalent species by
treating [CrCl2(thf)2] with either AlEtCl2 or AlMe3 in the
presence of SNS (reaction with AlEt2Cl or NEt3 led only to
intractable materials). In the first case, the reaction did not
directly ethylate the Cr center, but rather produced the
cationic, monomeric, square-planar complex 3 (Figure 2).
E-mail: R.Duchateau@tue.nl
[**] Thiswork wassupported by the Natural Science and Engineering
Council of Canada (NSERC) and by the Eindhoven University of
Technology. SNS=CySCH2CH2N(H)CH2CH2SCy, Cy=cyclohexyl.
Supporting Information for thisarticle isavailable on the WWW
under http://www.angewandte.org or from the author.
7050
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 7050 –7053

Angewandte
Chemie
Scheme 2. X=[Me6Al2Cl].
nation. However, we found no evidence for these species,
although we did observe that a substantial amount of metallic
chromium was invariably present in these reaction mixtures
as a black, pyrophoric, and insoluble material. The X-ray
fluorescence spectrum of this material confirmed the pres-
ence of Cr as the only heavy element while both the IR
spectrum and combustion analysis showed the absence of
organic functions. Furthermore, the EPR spectrum of the
crude reaction mixture was found to be identical to that of an
analytically pure sample of 4,[9] thereby conclusively proving
that 4 is the only EPR-active species formed during this
unusual reaction.
Figure 1. ORTEP view of 2 (thermal ellipsoids set at 30%); Al-contain-
ing counterionshave been omitted for clarity. Relevant bond
À
À
À
lengths[Š]: Cr1 S1 2.4654(14), Cr1 S2 2.4632(14), Cr1 N1 2.090(4),
À
À
À
Cr1 Cl1 2.5529(13), Cr1 Cl1A 2.3574(13), Cr1 C17 2.108(5).
Results of the catalytic testing for ethylene trimerization
(30 mmol of catalyst at 508C, 35 bar of ethylene, 1 h of
reaction time, and 1000 equiv of MAO in 150 mL of toluene)
are summarized in Table 1. Upon activation with MAO, the
Table 1: Oligomerization results.
Catalyst
PE [g] Activity
[g(gCr)À1 hÀ1
Product [mol%]
]
C6
C8
C10
CrIII
[(SNS)CrCl3] (1)
[{(SNS)CrMeCl}2]2+ (4)
[{(SNS)CrEtCl}2]2+ (2)
0.08 5824
0.80 6903
0.80 9383
>98
>98
>98
trace trace
trace trace
trace trace
CrII
[(SNS)Cr(ClAlEtCl2)]+ (3) 0.64 2265
96.39 1.48 0.63
Figure 2. ORTEP view of complex 3 (thermal ellipsoids set at 30%);
Al-containing counterionshave been omitted for clarity. Relevant bond
À
À
À
lengths[Š]: Cr1 S1 2.4828(18), Cr1 S2 2.4844(19), Cr1 N1 2.092(4),
Cr1 Cl2 2.3974(17).
CrIII precursors afford high selectivity. The increase in activity
with no change in selectivity observed for complexes 1, 2, and
4 follows the order Cl < Me < Et. It is tempting to suggest that
this sequence might be related to the different ability of the
dinuclear structure of these complexes to dissociate and form
coordinatively unsaturated catalytically active species.
These observations clearly indicate that, contrary to what
we previously thought, the CrIII oxidation state delivers a
more active and selective catalyst in the case of cationic SNS
complexes. On the other hand, our previous work[9] has also
shown that this particular system is not immune from
reduction to the divalent state. However, we have now
discovered that the alkyl aluminum activator may indirectly
induce re-oxidation of CrII towards the more active and
selective CrIII catalyst precursor. This oxidation by an alkyl
aluminum species is unprecedented and, to our knowledge,
has never even been considered. It is therefore tempting to
conclude that, in the case of the SNS ligand, the selective
trimerization might be performed by a trivalent catalyst.
When reduction occurs, the activity of the catalyst decreases
À
This CrII complex contains a chlorine atom bridging an
{AlEtCl2} unit and is counter-balanced by an [AlEtCl3]À ion.
Thus, cationization also occurs in this case. When the same
reaction was carried out with AlMe3, the cationic CrIII
complex [{(SNS)Cr(m-Cl)Me}2][Al2ClMe6]2 (4)[9] was formed
and subsequently isolated with a 32% yield of crystalline
material. The oxidation of the metal center during the
formation of 4 has no straightforward explanation given
that an alkylating agent, such as AlMe3, can hardly be
regarded as an oxidizing agent. The only possibility of
explaining this behavior is to propose that a disproportiona-
tion reaction may account for the re-oxidation of the metal
center (Scheme 2). This proposal, in turn, implies that a
species with a valence lower than two must be generated as a
result of the formation of the trivalent 4. The well-established
stability of zero- and monovalent chromium–arene com-
plexes[13] could well be a driving force for the disproportio-
Angew. Chem. Int. Ed. 2006, 45, 7050 –7053
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7051

Communications
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
and it loses selectivity. Nonetheless, a disproportionation
pathway promoted by the aluminum activator may restore the
highly active and selective trivalent state, clearly at the
expense of the overall concentration of chromium. Ulti-
mately, the chromium complex is irreversibly decomposed to
metallic chromium, which, incidentally, offers a reasonable
explanation for why these thermally robust trivalent organo-
chromium complexes decompose in the presence of an excess
of activator. The possibility that a lower valent chromium
species may be responsible for the high catalytic activity and
selectivity cannot, in principle, be excluded at this stage.
However, in the absence of evidence for stable mono- or zero-
valent Cr complexes of this ligand system, this possibility
remains, as yet, unsubstantiated. Finally, these results indicate
Received: June 5, 2006
Revised: July 21, 2006
Published online: September 28, 2006
Keywords: alkylation · chromium · oligomerization · oxidation ·
.
trimerization
[1] J. T. Dixon, M. J. Green, F. M. Hess, D. H. Morgan, J. Organo-
met. Chem. 2004, 689, 3641 – 3668.
[2] See for example: A. K. Tomov, J. J. Chirinos, R. J. Long, V. C.
Gibson, M. R. J. Elsegood, J. Am. Chem. Soc. 2006, 128, 7704
and references therein.
À
that the N H group of the ligand backbone is unaffected by
[3] a) P. J. W. Deckers, B. Hessen, J. H. Teuben, Organometallics
2002, 21, 5122 – 5135; b) P. J. W. Deckers, B. Hessen, J. H.
Teuben, Angew. Chem. 2001, 113, 2584 – 2587; Angew. Chem.
Int. Ed. 2001, 40, 2516 – 2519; c) C. Andes, S. B. Harkins, S.
Murtuza, K. Oyler, A. Sen, J. Am. Chem. Soc. 2001, 123, 7423 –
7424; d) E. Y. Chen, W. J. Kruper, G. Roof, D. R. Wilson, J. Am.
Chem. Soc. 2001, 123, 745 – 746; e) W. J. Van Rensburg, C.
Grove, J. P. Steynberg, K. B. Stark, J. J. Huyser, P. J. Steynberg,
Organometallics 2004, 23, 1207 – 1222.
À
the alkylating agents or the Cr R function under the
conditions studied so far for this system.
Experimental Section
2: Amixture of [{CySCH 2CH2N(H)CH2CH2SCy}CrCl3] (0.503 g,
1.1 mmol) and AlEt2Cl (1.32 g, 11 mmol) in toluene (15 mL) was
stirred at 228C to give a dark green solution. After centrifugation and
decantation, the resulting green solution was stored at À358C for one
day. Green crystals of analytically pure 2 (0.570 g, 0.91 mmol, 83%)
[4] a) (CrIII–CrV) N. Meijboom, C. J. Schaverien, A. G. Orpen,
Organometallics 1990, 9, 774 – 782; b) (CrII–CrIV) K. H. Theo-
pold, Eur. J. Inorg. Chem. 1998, 15 – 24; c) D. H. Morgan, S. L.
Schwikkard, J. T. Dixon, J. J. Nair, R. Hunter, Adv. Synth. Catal.
2003, 345, 939 – 942; d) W. J. Van Rensburg, C. Grove, J. P.
Steynberg, K. B. Stark, J. J. Huyser, P. J. Steynberg, Organo-
metallics 2004, 23, 1207 – 1222; e) (CrI–CrIII) R. D. Kohn, M.
Haufe, S. Mihan, D. Lilge, Chem. Commun. 2000, 1927 – 1928;
f) Y. Fang, Y. Liu, Y. Ke, C. Guo, N. Zhu, X. Mi, Z. Ma, Y. Hu,
Appl. Catal. A 2002, 235, 33 – 38.
[5] a) M. J. Overett, K. Blann, A. Bollmann, J. T. Dixon, D.
Haasbroek, E. Killian, H. Maumela, D. S. McGuinness, D. H.
Morgan, J. Am. Chem. Soc. 2005, 127, 10723 – 10730; b) K.
Blann, A. Bollmann, J. T. Dixon, F. M. Hess, E. Killian, H.
Maumela, D. H. Morgan, A. Neveling, S. Otto, M. J. Overett,
Chem. Commun. 2005, 620 – 621; c) A. Carter, S. A. Cohen,
N. A. Cooley, A. Murphy, J. Scutt, D. F. Wass, Chem. Commun.
2002, 858 – 859; d) D. F. Wass (BP Chemicals Ltd), WO 02/04119,
2002.
[6] A. Jabri, P. Crewdson, S. Gambarotta, I. Korobkov, R. Ducha-
teau, Organometallics 2006, 25, 715 – 718.
[7] P. Crewdson, S. Gambarotta, M.-C. Djoman, I. Korobkov, R.
Duchateau, Organometallics 2005, 24, 5214 – 5216.
[8] a) D. S. McGuinness, P. Wasserscheid, D. H. Morgan, J. T. Dixon,
Organometallics 2005, 24, 552 – 556; b) D. S. McGuinness, P.
Wasserscheid, W. Keim, D. Morgan, J. T. Dixon, A. Bollmann,
H. Maumela, F. Hess, U. Englert, J. Am. Chem. Soc. 2003, 125,
5272 – 5273.
were isolated from the solution. IR (nujol): nNÀH = 3185 cmÀ1
.
Elemental analysis (%) calcd for C23.5H45AlCl4CrNS2: C 45.05, H
7.24, N 2.24; found: C 45.41, H 7.63, N 2.01. meff = 3.92mB.
3: Amixture of [CrCl 2(thf)2] (0.250 g, 0.93 mmol),
CySCH2CH2N(H)CH2CH2SCy (0.285 g, 0.95 mmol), and [AlEtCl2]
(1.2 g, 9.5 mmol) in toluene (15 mL) was stirred at 228C to give a blue
solution. Crystals of 3 (0.521 g, 7.7 mmol, 82%) formed over the
course of three days at À358C. IR (nujol): nNÀH = 3174 cmÀ1
.
Elemental analysis (%) calcd for C20H41Al2Cl6CrNS2: C 35.41, H
6.09, N 2.06; found: C 35.61, H 6.33, N 2.01. meff = 4.83mB.
CrII oxidation: Preparation of 4: Amixture of [CrCl 2(thf)2]
(0.122 g, 0.46 mmol) and CySCH2CH2N(H)CH2CH2SCy (0.138 g,
0.46 mmol) in toluene (5 mL) was stirred at 228C to give a blue
suspension. AlMe3 (0.329 g, 4.57 mmol) was added and green crystals
precipitated immediately. After filtration of the green suspension the
solution was stored at À358C for 1 h. Complex 4 (0.100 g, 0.15 mmol,
32%) was isolated as green crystals by removing the mother liquor,
washing with hexanes (2 ” 5 mL), and drying in vacuo. IR (nujol):
nNÀH = 3181 cmÀ1
C30H60Al2Cl2CrNS2: C 53.32, H 8.95, N 2.07; found: C 53.41, H 8.73,
2.01. meff = 3.94mB. Pyrophoric, black, and insoluble metallic
.
Elemental
analysis
(%)
calcd
for
N
chromium separated from the mother liquor when it was left to
stand at room temperature for a few hours.
Crystal data for 2: C23.50H45AlCl4CrNS2, Mr = 626.51, monoclinic,
C2/c, a = 27.329(4), b = 13.296(2), c = 28.167(6) Š, b = 124.302(2)8,
V= 6462.2(17) Š3, Z = 8, 1calcd = 1.288 MgmÀ3, T= 202 K, absorption
coefficient 0.855 mmÀ1, F(000) = 2640, 23795 reflections collected,
5486 independent reflections, GoF = 1.034, R = 0.0605 [I > 2s(I)],
wR2 = 0.1443 [I > 2s(I)].
[9] A. Jabri, C. Temple, P. Crewdson, S. Gambarotta, I. Korobkov,
R. Duchateau, J. Am. Chem. Soc. 2006, 128, 9238.
[10] a) T. Agapie, M. W. Day, L. M. Henling, J. A. Labringer, J. E.
Bercaw, Organometallics 2006, 25, 2733 – 2742; b) S. J. Schofer,
M. W. Day, L. M. Henling, J. A. Labinger, J. E. Bercaw, Organo-
metallics 2006, 25, 2743 – 2749.
[11] Complexes containing the trivalent Cr–Et function are relatively
rare: a) V. C. Gibson, C. Newton, C. Redshaw, G. A. Solan,
A. J. P. White, D. J. Williams, J. Chem. Soc. Dalton Trans. 2002,
4017 – 4023; b) V. C. Gibson, S. Mastroianni, C. Newton, C.
Redshaw, G. A. Solan, A. J. P. White, D. J. Williams, J. Chem.
Soc. Dalton Trans. 2000, 1969 – 1971; c) O. Heinemann, P. W.
Jolly, C. Kruger, G. P. J. Verhovnik, J. Organomet. Chem. 1998,
553, 477 – 479; d) B. J. Thomas, S. K. Noh, G. K. Schulte, S. C.
¯
Crystal data for 3: C20H41Al2Cl6CrNS2, M = 678.32, triclinic, P1,
a = 11.836(3), b = 12.263(5), c = 12.733(3) Š, a = 75.302(7)8, b =
74.194(5)8, g = 76.374(4)8, V= 1692.4(9) Š3, Z = 2, 1calcd
=
1.331 MgmÀ3, T= 200.2 K, absorption coefficient 0.998 mmÀ1, F-
(000) = 704, 12890 reflections collected, 4792 independent reflections,
GoF = 1.025, R = 0.0621 [I > 2s(I)], wR2 = 0.1594 [I > 2s(I)].
CCDC-608869 (2) and CCDC-608870 (3) contain the supple-
mentary crystallographic data for this paper. These data can be
7052
www.angewandte.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 7050 –7053

Angewandte
Chemie
Sendlinger, K. H. Theopold, J. Am. Chem. Soc. 1991, 113, 893 –
902; e) B. J. Thomas, K. H. Theopold, J. Am. Chem. Soc. 1988,
110, 5902 – 5903.
[12] a) H. Sugiyama, G. Aharonian, S. Gambarotta, G. P. A. Yap,
P. H. M. Budzelaar, J. Am. Chem. Soc. 2002, 124, 12268 – 12274;
b) D. Reardon, G. Aharonian, S. Gambarotta, G. P. A. Yap, O
rganometallics 2002, 21, 786 – 788; c) M. Kooistra, Q. Knijnen-
burg, J. M. M. Smits, A. D. Horton, P. H. M. Budzelaar, A. W.
Gal, Angew. Chem. 2001, 113, 4855 – 4858; Angew. Chem. Int.
Ed. 2001, 40, 4719 – 4722; d) V. C. Gibson, M. J. Humphries, K. P.
Tellmann, D. F. Wass, A. J. P. White, D. J. Williams, Chem.
Commun. 2001, 2252 – 2253.
[13] a) F. Calderazzo, U. Englert, G. Pampaloni, M. Volpe, J.
Organomet. Chem. 2005, 690, 3321 – 3332; b) R. Kohn, D.
Smith, M. F. Mahon, M. Prinz, S. Mihan, G. Kociok-Kohn, J.
Organomet. Chem. 2003, 683, 200 – 208.
Angew. Chem. Int. Ed. 2006, 45, 7050 –7053
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7053