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Organometallics 2010, 29, 3676–3678 DOI: 10.1021/om100576u
On the Formation of Odd-Number Olefins in Chromium-Catalyzed Selective Ethylene Oligomerization Reactions: Evidence for Chain Transfer to Aluminum Thomas W Hey and Duncan F. Wass* School of Chemistry, University of Bristol, Cantock’s Close, Bristol, BS8 1TS, U.K. Received June 11, 2010 Summary: The formation of odd-numbered olefins in chromiumcatalyzed ethylene oligomerization in which substoichiometric quantities of diphosphine ligand are used can be attributed to chain transfer between diphosphine-free chromium species and the AlMe3 present in MAO cocatalysts.
Scheme 1. Mechanism for Formation of Odd-Numbered Olefins Proposed by Rosenthal and Co-workers
Introduction In recent years, catalysts capable of the selective trimerization and tetramerization of ethylene to 1-hexene or 1-octene have revolutionized olefin oligomerization.1 In 2002, we reported catalysts based on chromium complexes of ligands of the type Ar2PN(Me)PAr2 (Ar = ortho-methoxy-substituted aryl group) with productivity figures over an order of magnitude better than previous systems.2 This unprecedented performance led to interest both from a mechanistic viewpoint, which confirmed the distinctive metallacyclic mechanism for such catalysts, and in extending the range of substrates used in these reactions.3 However, the most significant subsequent development has been the report from Bollmann and co-workers that demonstrated that relatively minor changes to ligand structure and reaction conditions can lead to ethylene tetramerization rather than trimerization.4 In 2009, Rosenthal and co-workers5 in a detailed study of these systems reported the surprising result that in some circumstances *To whom correspondence should be addressed. E-mail: duncan.wass@ bristol.ac.uk. (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) Carter, A.; Cohen, S. A.; Cooley, N. A.; Murphy, A.; Scutt, J.; Wass, D. F. Chem. Commun. 2002, 858. (3) (a) Agapie, T.; Schofer, S. J.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2004, 126, 1304. (b) Blann, K.; Bollmann, A.; Dixon, J. T.; Hess, F. M.; Killian, E.; Maumela, H; Morgan, D. H.; Neveling, A.; Otto, S.; Overett, M. J. Chem. Commun. 2005, 620. (c) Overett, M. J.; Blann, K.; Bollmann, A.; Dixon, J. T.; Hess, F.; Killian, E.; Maumela, H.; Morgan, D. H.; Neveling, A.; Otto, S. Chem. Commun. 2005, 622. (d) Bowen, L. E.; Wass, D. F. Organometallics 2006, 25, 555. (e) Bowen, L. E.; Charernsuk, M.; Wass, D. F. Chem. Commun. 2007, 2835. (f) 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.; Pretorius, M.; Kuhlmann, S.; Wasserscheid, P. J. Catal. 2007, 249, 244. (g) McGuinness, D. S.; Overett, M.; Tooze, R. P.; Blann, K.; Dixon, J. T.; Slawin, A. M. Z. Organometallics 2007, 26, 1108. (4) Bollmann, A.; Blann, K.; Dixon, J. T.; Hess, F. M.; Killian, E.; Maumela, H; McGuiness, D. S.; Morgan, D. H.; Neveling, A.; Otto, S.; Overett, M.; Slawin, A. M. Z.; Wasserscheid, P.; Kuhlmann, S. J. Am. Chem. Soc. 2004, 126, 14712. Overett, M. J.; Blann, K.; Bollmann, A.; de Villiers, R.; Dixon, J. T.; Killian, E.; Maumela, M. C.; Maumela, H.; McGuinness, D. S.; Morgan, D. H.; Rucklidge, A.; Slawin, A. M. Z. J. Mol. Catal. A: Chem. 2008, 283, 114. (5) W€ ohl, A; M€ uller, A; Peulecke, N; M€ uller, B. H.; Peitz, S; Heller, D.; Rosenthal, U. J. Mol. Catal. A: Chem. 2009, 297, 1. pubs.acs.org/Organometallics
Published on Web 07/27/2010
odd-numbered 1-olefins were the product of such catalytic reactions. No selectivity within the odd-numbered olefin distribution was observed beyond a statistical Schulz-Flory distribution, but, nevertheless, odd-numbered olefins in some cases made up to 7 wt % of the total products. The catalysts that favored such products were obtained when substoichiometric quantities of ligand ([Ph2PN(iPr)PPh2] in most cases) to Cr were used in catalytic runs rather than the usual equimolar quantities of ligand and Cr source. This led Rosenthal and co-workers to speculate that binuclear chromium species with bridging diphosphine ligands may be the active catalysts for such transformations. They further speculated that the odd-numbered olefins are the result of metathesis reactions, catalyzed by hypothesized dichromium alkylidene species (Scheme 1). The precise reaction sequence that leads to cleavage of the alkylidene, an oddnumbered olefin, and regeneration of the starting complex is not discussed. In this note, we demonstrate that in fact chain transfer to the aluminum cocatalyst from diphosphine-free chromium species is the more likely mechanism for the formation of odd-numbered olefins, and this has consequences in other chromium-based oligomerization catalysts.
Results and Discussion During our ongoing ligand screening program for new chromium-based oligomerization catalysts, we have also occasionally observed trace amounts of odd-numbered olefin products.6 Our standard screening method is similar to (6) Odd-numbered olefins are also observed for recently reported chromium heteroscorpionate ethylene oligomerization catalysts, with a similar chain transfer to aluminum mechanism being proposed: Kilpatrick, A. F. R.; Kulangara, S. V.; Cushion, M. G.; Duchateau, R.; Mountford, P. Dalton Trans. 2010, 39, 3653. r 2010 American Chemical Society
Note
Organometallics, Vol. 29, No. 16, 2010 Table 1. Ethylene Oligomerization Results: Effect of Cr:Ligand Ratio
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Table 2. Ethylene Oligomerization Results: Effect of Added AlMe3
oligomers (wt %)b
runa
molar ratio Cr:ligand
productivity (g (gCr h)-1)
C6
C8
total even
total odd
runa
1 2 3 4
1:1 1:0.50 1:0.25 1:0
11 000 7500 5200 4000
18.6 18.5 18.2 19.2
74.5 64.7 30.6 16.0
100.0 98.5 93.3 92.0
0.0 1.5 6.7 8.0
5 6 7 8
a Conditions unless stated otherwise: 2.5 μmol of [Cr(acac)3], ligand Ph2PN(iPr)PPh2, 300 equiv of MAO, cyclohexane diluent, 40 bar of ethylene, 45 °C. b Determined by GC; remainder of mass balance is C>8 oligomers.
that of Rosenthal and co-workers5 and involves in situ catalyst formation by mixing ligand, a chromium source (typically [Cr(acac)3], acac = acetylacetonate), and a large excess of methyl aluminoxane (MAO). It is noteworthy that odd-numbered olefins were usually observed in cases when the catalyst being screened proved otherwise to be very poor in terms of productivity and selectivity. This led us to hypothesize that odd-numbered olefins are in fact being generated by a “ligand-free” chromium species, formed because of incomplete complexation during the activation step. Removal of diphosphine ligands in such systems has been previously suggested to lead to the formation of Schulz-Flory distributions.3g To test this, we completed a series of experiments in which the molar quantity of bis(diphenylphosphino)amine ligand is reduced relative to chromium (Table 1). The results support our hypothesis. With a ligand:Cr ratio of 1:1 good selectivity to 1-octene is observed (run 1). At ligand:Cr ratios of 0.5:1 or 0.25:1, selectivity is reduced and an underlying Schulz-Flory distribution of even olefins (k =0.7 in both cases) becomes apparent (runs 2 and 3). Odd-numbered olefins are also observed at these ligand:Cr ratios, also in a Schulz-Flory distribution with k = 0.7. If no ligand is added, a purely Schulz-Flory distribution is observed for even numbers and, crucially, an even larger percentage of the total olefinic products are odd numbered. These data support there being a mixture of ethylene-oligomerizationactive species when substoichiometric quantities of ligand are used: (1) diphosphine-chromium species, which give good selectivity to 1-hexene and 1-octene via metallocyclic mechanisms, and (2) “ligand-free” chromium species, which give a Schulz-Flory distribution of even olefins and a parallel Schulz-Flory distribution of odd olefins. We speculated that a mechanism for the formation of these odd-numbered olefins would be the addition of a C1 synthon to growing ethylene oligomer chains; such a C1 synthon readily exists in the trimethyl aluminum present in the MAO cocatalyst (or indeed the MAO itself). Analysis of the MAO solution used by 1H NMR spectroscopy reveals that it contains up to 40 wt % “free” trimethyl aluminum. At the cocatalyst loadings typically used (300 equiv to Cr) this is more than enough aluminum reagent to give the observed quantities of odd-numbered olefins: 2.4 mmol of AlMe3 compared to 0.1 mmol of odd-numbered olefins for run 4. Chain transfer to aluminum alkyls is well known for a range of ethylene polymerization catalysts, including those based on chromium.7 To investigate this possibility, we conducted (7) Review: Kempe, R Chem.;Eur. J., 2007, 13, 2764. Chromium catalysts: Rogers, J. S.; Bazan, G. C. Chem. Commun. 2000, 1209. Bazan, G. C.; Rogers, J. S.; Fang, C. C. Organometallics 2001, 20, 2059.
oligomers (wt %)b
equiv of AlMe3
productivity (g (gCr h)-1)
total even
total odd
50 120 500 200c
3200 4000 500 1970d
95.6 92.0 84.6 45.8e
4.4 8.0 15.4 54.2f
a Conditions unless stated otherwise: 2.5 μmol of [Cr(acac)3] (no diphosphine ligand), 300 equiv of dried MAO, cyclohexane diluent, 40 bar of ethylene, 45 °C. b Determined by GC. c AliBu3. d 5.0 μmol of [Cr(acac)3], no ligand. e Percent without iBu end groups. f Percent with i Bu end groups.
Scheme 2. Proposed Mechanism
a series of experiments in which additional AlMe3 was added to (ligand-free) ethylene oligomerization reactions (Table 2). To ensure a low benchmark in terms of AlMe3 concentration, MAO cocatalyst from which volatiles had been removed was used. As predicted, as trimethyl aluminum concentration is increased, the amount of odd-numbered olefins also increases. In each case, both odd and even numbers are obtained as Schulz-Flory distributions with k between 0.65 and 0.70. Further evidence for chain transfer reactions comes from run 8, in which 200 equiv of triisobutyl aluminum was used in place of AlMe3. In this case, isopropyl end groups derived from the aluminum reagent isobutyl substituents are observed in the oligomeric products. A consequence of the operation of chain transfer to aluminum is that longer chain trialkyl aluminum species should be produced, which, given the aqueous acidic workup of these catalytic runs, are expected as a distribution of alkanes. Surprisingly, such alkanes are not observed, alkenes being the only products on the basis of GC analysis. Methylation occurring exclusively on putative chromium hydrides formed via β-elimination is one possibility. However, certain transition metal salts are known to catalyze the elimination of alkenes from aluminum alkyls, presumably by alkyl chain transfer to the transition metal species followed by β-elimination.8 To test whether the chromium species present in these reactions can catalyze this process, tri-n-octyl aluminum (50 equiv) was added to a catalyst prepared as run 5 (in the absence of ethylene); after 1 h at 45 °C, octene was observed by both GC and 1H NMR spectroscopy. These experiments allow us to put together a modified catalytic cycle for the formation of odd-numbered olefins with chromium catalysts (Scheme 2). (8) Britovsek, G. J. P.; Cohen, S. A.; Gibson, V. C.; van Meurs, M. J. Am. Chem. Soc. 2004, 126, 10701. Britovsek, G. J. P.; Cohen, S. A.; Gibson, V. C.; Maddox, P. J.; van Meurs, M. Angew. Chem., Int. Ed. 2002, 41, 489.
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Transfer to trimethyl aluminum produces a [Cr]-Me species, which ultimately leads to odd-numbered olefins, whereas β-hydride elimination yields a [Cr]-H species leading to even numbers. We have assumed a simple CosseeArlman mechanism for chain propagation; a metallacyclic mechanism cannot be completely ruled out at this stage, but we note that the cyclic byproducts often associated with such a mechanism for C8 and longer products are not observed.
Conclusions Our results suggest the mechanism for the formation of odd-numbered olefins in chromium-catalyzed ethylene oligomerization in which substoichiometric quantities of diphosphine ligand are used is chain transfer between “ligand-free” chromium species and the AlMe3 present in MAO cocatalysts. These results shed light on the odd-numbered olefins that are an unexpected feature of some ethylene oligomerization reactions and, more generally, on systems in which activation methods involving alkyl aluminum reagents can influence olefin distribution patterns.9 (9) (a) Rucklidge, A. J.; McGuinness, D. S.; Tooze, R. P.; Slawin, A. M. Z.; Pelletier, J. D. A.; Hanton, M. J.; Webb, P. B. Organometallics 2007, 26, 2782. (b) Bowen, L. E.; Haddow, M.; Orpen, A. G.; Wass, D. F. Dalton Trans. 2007, 1160.
Hey and Wass
Experimental Section General Comments. All procedures were carried out under an inert (N2) atmosphere using standard Schlenk techniques or an inert atmosphere (Ar) glovebox. Chemicals were obtained from Sigma Aldrich and used without further purification unless otherwise stated. The synthesis of bis(diphenylphosphino)isopropylamine was performed according to literature procedures.3b All solvents were purified using an Anhydrous Engineering Grubbs-type solvent system. 1H NMR spectra were recorded on Varian 500 and 400 spectrometers at 500 and 400 MHz, respectively, at room temperature. 1H NMR chemical shifts are referenced relative to the residual solvent resonances in the dueterated solvent, and oligomerisation products were analyzed by GC-FID and GC-MS, using a Varian L3800, with a Varian WC07 fused silica capillary column, 25 m 0.25 mm, i.d. coating CP-Sil 5CB, df = 0.25. Ethene oligermerization method: 40 to 80 °C at 2 °C min-1, then 80 to 250 °C at 10 °C min-1. Catalytic Runs: Ethylene Oligomerization. A suspension of Cr(acac)3 (3.5 mg, 2.5 μmol) and ligand in cyclohexane (1 mL) was formed in a 10 mL autoclave. MAO (2 mL, 10 wt % in toluene) and trimethylaluminum (2.0 M in hexanes) were added, and the autoclave was sealed and put under 40 bar of pressure of ethylene. After stirring for 60 min, the reactor was cooled in an acetone/dry ice bath for 10 min before being vented to the atmosphere. Residual MAO was quenched by slow addition of 10% aqueous HCl (3 mL), mesitylene standard (10 μL, 0.072 mmol) was added, and a sample of the organic fraction was removed for analysis by GC-FID.
