Chromium Aryl Complexes with N-Donor Ligands as Catalyst

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Chromium Aryl Complexes with N‑Donor Ligands as Catalyst Precursors for Selective Ethylene Trimerization Mathias Ronellenfitsch, Hubert Wadepohl, and Markus Enders* Institute of Inorganic Chemistry, University of Heidelberg, Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany S Supporting Information *

ABSTRACT: A series of 8-amino-2-arylquinoline ligands (1− 6) were synthesized and reacted with CH3CrCl2(thf)3. Under these conditions a CH bond of the 2-aryl substituent is metalated, leading to organochromium complexes with monoanionic tridentate ligands (8−13). The presence of a chromium−carbon σ bond in these complexes has been established by X-ray analysis. Furthermore, 8-(piperidin-1yl)quinoline (14) was used as neutral bidentate ligand in addition to an external aryl group, leading to complex 15. Finally, the tris-aryl complex 18 was synthesized, which features a rare five-coordinate chromium(III) metal center. All chromium complexes were tested as catalysts for the selective trimerization of ethylene after activation with methylaluminoxane (MAO). Several of the new catalyst precursors show good behavior for the selective trimerization of ethylene. Although chlorido ligands in the catalyst precursor will be substituted by methyl groups during the activation with MAO, there is a clear difference in the catalytic behavior when the complex contains a methyl (or aryl) group prior to addition of MAO. The mechanism of catalyst activation has been studied in more detail with the tris-aryl complex 18.

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and tridentate SNS, PNP, and NNN ligands.9 The catalyst precursors become active upon addition of aluminum alkyl reagents, often as methylaluminoxane (MAO). Therefore, it is obvious that initially chromium alkyl species are formed which upon reaction with ethylene somehow enter into the catalytic cycle, which itself operates via metallacycles and Cr(III)/Cr(I) or Cr(IV)/Cr(II) redox couples. A small number of chromacycles have been isolated, but they usually do not allow the catalytic formation of 1-hexene.10−12 From these observations a working hypothesis is that chromium-based trimerization catalyst precursors should enable the formation of monocationic chromium dialkyl (or diaryl) species. Practically all of the hitherto reported precatalysts fall into that scheme. MAO as activator is able to alkylate the metal center and to abstract an anionic ligand (i.e., Cl− or CH3−) and is therefore a privileged cocatalyst. Complexes of the type LnCrCl3 activated with MAO should lead to LnCr(CH3)2 cations, which could form the proposed Cr(I) species by reductive elimination. However, Bercaw et al. have shown that, during the initiation process, reductive elimination is observed only after ethylene insertion into the chromium−carbon bonds (in this case into the Cr− phenyl bond).13 Consequently, the nature of the carbanionic ligand in the catalyst precursor is able to influence the initiation process.

INTRODUCTION Linear α-olefins (LAO), which are important industrial products, are predominantly synthesized by the nonselective oligomerization of ethylene. Well-established industrial processes lead to α-olefins with distributions of the chain lengths (e.g., Shell Higher Olefin Process (SHOP) or processes based on the Ziegler Aufbau reaction).1,2 A growing demand for uniform α-olefins such as 1-hexene or 1-octene is due to the increasing production of linear low-density polyethylene (LLDPE), where such α-olefins are used as comonomers.3 The catalytic systems developed so far predominantly contain chromium as the catalytic center.4 The observation that chromium-based ethylene polymerization catalysts may also produce small amounts of 1-hexene5 has culminated in the development of the so-called Phillips trimerization catalyst, which was presented in 1991.6 Even in 1977 researchers from Union Carbide proposed a metallacyclic mechanism for selective ethylene trimerization catalyzed by a combination of a chromium(III) source and partially hydrolyzed triisobutylaluminum.7 Later it was found that addition of the simple bidentate ligand dimethoxyethane improved the selectivity for 1-hexene to 74%.8 From the year 2000 onward there have been many reports about molecular chromium systems which catalyze the trimerization or the tetramerization of ethylene with high activity and selectivity. Most of the reported ligands are neutral or monoanionic chelate ligands with two or three donor atoms.2 Examples are bidentate PNP ligands (P coordination and sometimes additional coordination of a phenoxy moiety) © XXXX American Chemical Society

Special Issue: Catalytic and Organometallic Chemistry of EarthAbundant Metals Received: May 2, 2014

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order to verify under which conditions a NH or a CH activation occurs. All chromium complexes used and prepared in this work are paramagnetic and give relatively badly resolved 1 H NMR spectra. Nevertheless in situ NMR spectroscopy was a valuable tool for monitoring the reaction with 1. In 19F NMR a signal could only be detected if the aryl did not coordinate to the chromium center. First, the reaction of 1 with CH3CrCl2(thf)3 under varying conditions was tested. In toluene as solvent complexation occurs rapidly at room temperature and the NH activation product 7 is obtained (see Scheme 2). In tetrahydrofuran (thf), however, complexation does not take place without heating to about 70 °C. Stirring the mixture in a closed tube for 1 h at 100 °C completes the reaction, giving the CH activation product: i.e., complex 8. Thus, with CH3CrCl2(thf)3 as precursor the complexes 7 and 8 were obtained as dark red and brown solids, respectively, and could be recrystallized and characterized by X-ray analysis. When using p-tolylCrCl2(thf)3 as chromium precursor and thf as solvent, heating of the reaction mixture was necessary as well. However, in this case only the NH activation product 7 was formed, as could be shown by NMR spectroscopy. Analyzing the reaction mixtures by 19F and 1 H NMR spectroscopy revealed that generally under a given condition either complex 7 or 8 is formed, but not a mixture of the two compounds. The presence of an intramolecular carbon−chromium bond in 8 was established from single-crystal X-ray diffraction. A comparison of the solid-state molecular structures of complexes 7 and 8 substantiates that in the case of 7 the amine function was deprotonated, resulting in a monoanionic, bidentate ligand (Figure 1). The two chlorido ligands are located trans to each

Some of the tridentate PNP and SNS ligands provide the opportunity for deprotonation of the secondary amine function by aluminum alkyls during activation to give an anionic ligand. Indeed, the best results with these catalyst systems have been obtained with ligands containing a secondary amine function.9e,g,h,14 However, the products isolated after treatment of a SNSCrCl3 complex with various aluminum alkyls did show that the NH moiety is not deprotonated under these conditions.9f,15 We present here our results on the synthesis and catalytic properties of catalyst precursors where there is already a chromium−carbon bond established prior to addition of MAO as co cat alyst. Th is is achieved via th e use of CH3CrCl2(thf)316,17 and p-tolylCrCl2(thf)318 as well-known chromium complexes already containing a chromium−carbon bond.

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RESULTS Synthesis and Characterization of Organochromium Complexes. The starting point of the work presented here was a patent describing trimerization catalysts using 2-aminomethyl5-arylpyridines as ligand precursors.19 Many of the ligands described there have a NH group which could probably be deprotonated by reaction with metal alkyl precursors. However, upon reaction with CH3CrCl2(thf)3 or with p-tolylCrCl2(thf)3 the NH functionality remains intact while an ortho C−H bond of the ligand aryl group is activated, leading to complexes with a new chromium−carbon bond to the resulting tridentate monoanionic ligands. After activation with MAO these systems produced 1-hexene with up to 90% selectivity and an activity of 5800 g/((g of Cr) h). We chose a related ligand system where aryl groups are attached to the 2-position of 8-aminoquinolines. Some molecules of that type have already been described in the literature, and we added the new derivatives shown in Scheme 1. Scheme 1. Ligand Precursors 1−6 Based on 8Aminoquinolines with an Aryl Substituent in the 2-Position

Figure 1. Molecular structures of 7 and 8. For clarity, hydrogen atoms have been omitted and only one of the two disordered molecules of 8 is shown; a figure showing the disorder is given in the Supporting Information. Selected bond lengths (Å) and angles (deg): 7, Cr−N1 2.1080(15), Cr−N11 1.9575(15), Cr−O41 2.1202(13), Cr−O31 2.0612(12), N11−Cr−N1 81.67(6), N11−Cr−O41 176.78(6); 8, Cr−N1 2.0159(15), Cr−N11a 2.326(3), Cr−C19 2.0416(19), Cr−O1 2.0414(13), N1−Cr−N11a 75.24(8), C19−Cr−N11a 149.96(8).

The ligands were prepared following a synthetic route described for a derivative.20 We wanted to evaluate the reactivity toward organometallic chromium precursors in

Scheme 2. Complexes Obtained by Reaction of 1 with CH3CrCl2(thf)3 and p-tolylCrCl2(thf)3 in Tetrahydrofuran (thf) or Toluene

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temperature. The complexes are formed immediately under these conditions, which can be observed by their precipitation after a few minutes from a formerly clear solution. By this procedure the complexes were obtained in isolated yields ranging from 52 to 85%. Complexes 9, 11, and 13 could be analyzed by X-ray structure analysis (see Figure 2 for 9). Their

other, and the octahedral geometry is completed by two thf molecules. In 8 the octahedral coordination sphere is distorted, since the relatively inflexible ligand is coordinating in a meridionally tridentate fashion. This is reflected by the C− Cr−N(amino) angle, which shows a value of about 150° instead of the optimum 180°. The tridentate coordination is the result of the formation of a chromium−carbon bond connecting the metal center with the aryl remainder of the ligand and the two nitrogen atoms acting as neutral donors. The secondary amine function of the ligand is not deprotonated, so that the ligand is monoanionic as well. Consequently, we could show that ligand 1 provides different reaction pathways during complexation with the chromium precursors leading to 7 or 8. With CH3CrCl2(thf)3 as precursor, both complexes can be obtained, depending on the solvent used. In thf the chromium precursors are stable at room temperature and reaction with the ligands occurs only at elevated temperatures. However, at such temperatures the chromium precursor MeCrCl2 (thf) 3 or a short living decomposition product attacks the aromatic CH bond of the ligand, leaving the NH function unchanged. We performed DFT calculations in order to estimate the relative energies and free energies of complexes 7 and 8. The calculations show that 7 is lower in enthalpy by 70 kJ/mol in comparison to 8, indicating that the formation of the latter is kinetically controlled. The calculated Gibbs energy difference of 13 kJ/mol, still indicates a preference for 7. Heating of a solution of 8 in thf did not lead to a rearrangement with formation of the thermodynamically favored 7. Several chromium-based trimerization catalyst precursors feature a NH moiety at the ligand which could possibly react with the cocatalyst (i.e., with MAO). However, it is not clear if such a ligand reactivity is beneficial for a good trimerization catalyst. Our initial catalytic experiments did not exhibit a clear advantage having a NH moiety present in the ligand, and so we decided to eliminate one of the reaction pathways by synthesizing ligands with a tertiary amine function. Consequently, we should obtain complexes containing a metal− carbon bond between the ligand and chromium with better yields, as the system has fewer opportunities for undesired reactions. Thus, we synthesized the derivatives 2−6, where the anilinyl remainder is replaced by piperidinyl and pyrrolidinyl groups, respectively (see Scheme 1 and the Supporting Information for details). After reaction with CH3CrCl2(thf)3 complexes 9−13 were obtained (Scheme 3). As mentioned before, 2−6 offer no opportunity to form an anionic heteroatomic donor, making it possible to carry out the reaction in toluene and thus at room

Figure 2. Molecular structures of 9 (left) and 15 (right). Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): 9, Cr−N1 2.0060(12), Cr−N2 2.4074 (12), Cr−C12 2.0369(14), Cr−O1 2.0871(11), N2−Cr−N1 77.67(4), N2−Cr−C12 157.67(4); 15, Cr−N1 2.051(2), Cr−N2 2.432(2), Cr−C16 2.083(3), Cr−O1 2.092(2), N1−Cr−N2 77.30(9), N2−Cr−C16 170.97(9).

molecular geometries are all similar to that of the previously discussed complex 8. In every case the chromium atom is coordinated by the two neutral donor atoms of the ligand in addition to a bond to the ortho C atom of the aryl substituent. This shows that the CH activation of the ligand by the chromium alkyl precursor is a reliable reaction. As expected, two chlorido ligands and one THF molecule complete the distorted-octahedral coordination geometry. In order to evaluate the influence of the chelating aryl ligand for the catalytic properties, we replaced it by an external aryl group. Therefore, ligand 14 was synthesized and reacted with the chromium precursors p-tolylCrCl2(thf)3, CH3CrCl2(thf)3, and CrCl3(thf)3 respectively (Scheme 4). The new complexes 15−17 were obtained and could be characterized by elemental analysis and by X-ray structure analysis (see Figure 2 for 15). The green complex 16 is formed only after recrystallization from thf. The initially formed compound is a yellow powder and does not contain coordinated thf, as could be shown by elemental analysis. The catalytic experiments were performed with yellow 16-thf. The geometry of 15 with an external p-tolyl ligand is very similar to those of complexes 9, 11, and 13 (with an internal aryl ligand). The angle N2−Cr−C12 of 157.76(4)° in 9 opens up to 170.97(9)° (N2−Cr−C16) in 15. Not only the aryl ligand in 15 but also the methyl ligand in 16 (see the Supporting Information) is located in a position trans to the tertiary amine group. Therefore, the meridional arrangement of the two N-donor atoms together with the carbanionic ligand is favored irrespective of the covalent attachment of the carbanionic group to the aminoquinoline ligand. Synthesis of the Triphenyl Complex 18. Reaction of 3 equiv of the Grignard reagent p-tolylMgCl with the trichlorido complex 17 led to the tris-aryl complex 18 in a relatively high yield of 81%. The elemental analysis already showed that no thf is present in the solid. Single-crystal X-ray analysis confirmed the composition (Scheme 5). The chromium center is coordinated by only five ligand atoms, resulting in a squarepyramidal coordination geometry. The Cr atom is slightly displaced (0.356(2) Å) from the plane defined by the ligand

Scheme 3. Chromium Complexes 9−13 Synthesized by Conversion of Ligands 2−6 with CH3CrCl2(thf)3 in Toluene at Room Temperaturea

a Molecular structures of 9, 11, and 13 have been determined by X-ray analysis (see the Supporting Information).

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Scheme 4. Complexes 15−17 Obtained by Use of Ligand 14

Scheme 5. Preparation of Complex 18 (Left) and Molecular Structure As Obtained from X-ray Diffraction (Right)a

a

Selected bond lengths (Å) and angles (deg): Cr−N1 2.138(3), Cr−N2 2.253(3), Cr−C16 2.073(4), Cr−C23 2.054(4), Cr−C30 2.034(4); N1− Cr−N2 76.0(1), C23−Cr−C16 89.6(2), C23−Cr−N2 153.3(2), C16−Cr−N1 165.7(2), C30−Cr−C16 101.2(2).

2). With 10 the selectivity for 1-hexene in the LAO fraction is better but the major product is still PE (entry 3). However, the formation of PE can be suppressed efficiently by keeping the temperature at 60 °C or below (see entries 3−6). Under these conditions an overall selectivity toward 1-hexene of over 90% was observed (entry 6). A remarkable increase in activity could be achieved by shortening the activation time, which means the time the chromium complex and MAO are stirred in toluene before the trimerization is started by the insertion of ethylene. Starting the reaction by pressurizing the autoclave immediately after the chromium complex comes in contact with MAO instead of stirring it for 20 min increased the activity by a factor of 1.7, while the selectivity stayed almost the same (compare entries 6 and 7). The replacement of the piperidyl by a pyrrolidinyl remainder (complex 13), which means a relatively small change in the ring size of the cycloamine, results in a catalyst performing with considerably worse selectivity (compare entries 8 and 13). A comparison of the results of 9−11 (see entries 11, 12, 8) shows that the electron-withdrawing trifluoromethyl group seems to be disadvantageous concerning the selectivity. On the other hand, complex 12 gives a comparably low activity, even though it has two electron pushing methyl groups at the aryl remainder of the ligand at positions ortho and para to the chromium− carbon bond. If a more electron rich aryl remainder is positive in general, this effect must be overbalanced by the increased steric hindrance in this case. Ethylene Trimerization with Complexes 15−17 (Bidentate Ligands). The complexes 15 and 16-thf showed relatively high activity, comparable to those complexes with tridentate ligands. This shows that a linkage of the chromium to the ligand via an alkyl or aryl remainder is not beneficial for the catalytic

atoms N1−N2−C16−C23 (RMSD 0.120 Å). A number of molecular structures of tris-aryl or tris-alkyl chromium(III) complexes have been described, and the majority are sixcoordinate, as for example Ph3Cr(thf)3.21 Five-coordinate chromium(III) complexes usually feature a square-pyramidal geometry similar to that of 18. A couple of examples have been published with five-coordinate tris-aryl complexes.22 Catalytic Ethylene Trimerization. Complexes 7−13 and 15−18 were tested as catalysts for the selective trimerization of ethylene to 1-hexene after activation with methylaluminoxane (MAO) under various conditions (see Table 1 and the Supporting Information for experimental details). The denoted α-olefins were identified by their retention times in the gas chromatograms after reference measurements with the pure substances. All other signals in the chromatogram that could not be identified in this way were summarized under “other”. The amount of a single unidentified product was typically not more than 2%. Thus, cyclic or branched isomers of LAOs could not be identified under our conditions. Before we discuss the performance of the complexes, it must also be mentioned that the compounds CH3CrCl2(thf)3, p-tolylCrCl2(thf)3, and CrCl3(thf)3 give oligomerization catalysts under our conditions with high activities (see entries 19−21). Under the given reaction conditions only a small amount of polymer and a broad distribution of LAOs was obtained. In comparison to this result the ligands used in this report lowered the overall activity but shifted the selectivity from a range of LAOs toward 1hexene. Ethylene Trimerization with Complexes 7−13 (Tridentate Ligands). Complexes 7 and 8 gave unsatisfying results. A larger amount of polyethylene (PE) was produced in the reaction, and no selectivity was observed in the LAO fraction (entries 1 and D

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Table 1. Results of the Ethylene Trimerization Experimentsa entry

complex

cat. amt (μmol)

1b 2c 3c 4c 5c 6c 7 8d 9d 10e 11d 12d 13d 14d 15d 16d 17e 18e 19 20 21e 22e 23e 24e 25f 26g 27h

8 7 10 10 10 10 10 10 10 10 9 11 13 12 15 16-thf 16-thf 16-thf p-tolylCrCl2(THF)3 CH3CrCl2(THF)3 CrCl3(THF)3 17 18 16-thf 16-thf 16-thf 16-thf

7.9 10.0 12.7 10.5 11.1 12.5 11.3 8.5 10.9 10.1 8.9 7.6 8.9 9.8 8.4 8.1 11.1 12.3 8.8 10.4 17.1 10.4 10.0 11.4 11.4 11.4 11.4

T (°C) 85−95 85−95 90−95 70−75 60−63 50−53 50−55 49−51 49−51 49−51 49−51 49−51 50−55 49−51 49−51 49−51 45−65 50−52 49−51 49−51 45−55 50−58 48−52 50−54 50−53 50−53 48−53

p (bar)

activity (g/((g of Cr) h))

1-hexene activity (g/((g of Cr) h))

1-hexene (wt %)

1-oct (wt %)

1-dec (wt %)

1-dodec (wt %)

other (wt %)

PE (wt %)

24 24 25 25 25 25 25 25 10 10 25 25 25 25 25 25 25 10 25 10 10 10 10 10 10 10 10

1760 8800 31800 13800 8200 5600 9300 12600 4600 6800 11600 9800 11400 6300 14000 13800 26500 10400 27600 40100 27200 3000 22200 15900 26000 30600 18300

230 1500 10800 6800 7400 5200 8400 10900 4100 5000 10100 7300 6400 4900 11000 10800 22000 9000 8200 13500 10500 1700 18900 13200 19800 25400 15200

13 17 34 49 90 92 90 87 88 74 87 74 56 77 79 78 83 87 30 34 38 54 85 83 76 83 83

9 17 1 3 3 1 4 5 3 3 4 3 14 3 13 13 8 4 24 25 25 6 6 7 5 6 7

8 14 1 3 2 1 2 3 2 2 3 2 11 1 0 0 0 0 18 18 17 1 0 0 1 1 0

7 12 1 2 2 0 2 2 1 1 2 1 6 1 0 0 0 0 13 11 11 0 0 0 0 0 0

10 14 1 4 2 3 1 2 4 6 2 1 6 2 6 6 4 5 11 11 7 6 4 3 3 3 3

53 26 62 39 1 3 1 1 2 14 2 19 7 16 2 3 5 4 4 1 2 33 5 7 15 7 7

a

Conditions: 83 mL of toluene, cocatalyst 2 mL of a 10% solution of MAO in toluene (“MAO 1”) or 1 mL of a 30% solution of MAO in toluene (“MAO 2”)). The autoclave was pressurized with ethylene immediately after addition of the chromium complex to the toluene MAO mixture, reaction time 30 min. bActivation time 30 min. cActivation time 20 min. dReaction time 60 min. e1 mL of MAO 2. f2 mL of MAO 2. g3 mL of MAO 2. h1 mL of MAO 2, 2 mmol of AlMe3. Note that two different activation parameters have been used, differing in amounts and charges of MAO, resulting in different activities (compare entry 9 with 10 and entry 18 with 19).

ligands (9−13), where other LAOs in addition to 1-hexene and 1-octene were also produced. Increasing the amount of MAO by a factor of 3 doubles the catalytic activity, whereas the selectivity remains unchanged (see entries 24−26). Addition of AlMe3 has almost no influence on the catalytic behavior, with only a slight increase in activity and no changes in selectivity (see entry 27 with addition of 1 mmol of AlMe3). Investigation of Catalyst Activation. Bercaw, Labinger, and co-workers have demonstrated that a triphenyl chromium(III) catalyst precursor is active for selective ethylene trimerization only after abstraction of a carbanionic phenyl group. They also showed that ethylene inserts into the chromium−phenyl bonds prior to reductive elimination under formation of the active catalyst.13 We used complex 18 in order to investigate the activation process. In a first experiment we heated a solution of 18 in the absence of the cocatalyst and in the absence of ethylene. At a typical trimerization reaction temperature (i.e., 50 °C), the complex decomposes slowly over hours. At 80 °C the decomposition is complete within 60 min, as has been monitored by 1H NMR (disappearance of paramagnetically shifted signals and considerable increase of signals in the diamagnetic region (i.e., δ 0−10 ppm)). The reaction mixture was filtered through a short silica gel column in order to eliminate the decomposed chromium compounds. Analysis of the solution by GC-MS revealed the formation of ligand 14 and 4,4′-dimethylbiphenyl as the only

ethylene trimerization. Nevertheless, a chromium−carbon bond in the catalyst precursor enhances strongly both activity and selectivity, as a comparison between complexes 16 and 17 shows (compare entry 18 with 22). This is surprising, since during activation with MAO the complexes are already expected to be alkylated. However, in the end all of the chlorido ligands have to be removed from the complex to get to the active species. Thus, many reaction steps are necessary in this process, surely providing many side reactions leading to inactive compounds. A detailed study of the activation process of similar catalysts was reported by Bercaw et al.23 They were able to identify spectroscopically two main products, but these were not the active catalysts. Considering this, it seems reasonable that providing a complex that is one step closer to the active species gives better results. Furthermore, it is remarkable that 15 and 16-thf gave very similar results both in activity and selectivity, indicating that the same active species is formed in both cases. This implies that the initial aryl or alkyl remainder, respectively, is no longer present in the active species. It also shows that the coordinated thf molecule in 15 does not influence the catalytic properties, as it is most probably abstracted by the Lewis acidic cocatalyst. Having a look at the selectivity in the LAO fraction, one can also notice that considerable amounts of 1-octene (between 4 and 13%; see entries 15−18) but no other LAO is produced. This distinguishes these complexes from those with tridentate E

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chromium aryl moiety occurs and after β-H elimination this leads to 22 (a cis or trans isomer is possible). From these results we propose the activation pathways given in Scheme 7, which are similar to those suggested by Bercaw, Labinger, and co-workers for the triphenyl chromium(III) catalyst with a PNP ligand. Cocatalysts Other Than MAO. Several catalytic runs with activators other than MAO were performed (see Table 2), and the reaction solutions were analyzed by GC. No formation of any LAO nor polyethene was observed.

organic products. The nature of the resulting chromium compounds could not be clarified. This experiment was repeated in the presence of ethylene at atmospheric pressure. Again the mixture needed heating to 80 °C in order to be complete within 60 min. The analysis of the organic byproducts now showed a third component, which was identified as 4methylstyrene. This molecule must have formed after insertion of one molecule of ethylene into the chromium−phenyl bond and subsequent β-elimination. In a third experiment MAO was added, ethylene was fed into the solution, and the mixture was stirred for 1 h at 50 °C. The reaction mixture then was quenched with water, filtered over glass wool, and analyzed by GC-MS and by GC-FID (FID denotes a flame ionization detector). The analysis showed that 1-hexene was formed in the amount we would expect under these conditions, showing that the catalytically active species must have been formed. It also revealed that, in contrast to the aforementioned experiments in the absence of MAO, no biphenyl and no methylstyrene had formed but instead the product from multiple insertions of ethylene into the chromium−aryl moiety (23−25, Scheme 6). The GC-FID analysis showed that the amounts of the ligand 18 and the compounds 23−26 are all of the same order of magnitude (see Figure S2 in the Supporting Information).

Table 2. Attempts To Activate the Complexes 10, 16-thf, and 18 without MAO

a b

activation

conditions

10 16-thf 16-thf 18 18 18

596 equiv Me3Al 9 equiv Et3Al 8 equiv Et3Al a b 240 equiv Me3Al

25 bar, 90 °C, 30 min atm, room temp, 1 h atm, room temp, 1 h atm, 50 °C, 1 h atm, 50 °C, 30 min atm, 50 °C, 30 min

1 equiv of N,N-dimethylanilinium tetrakis(pentafluorophenyl)borate. 1 equiv of [H(Et2O)2]+[B(C6H3(CF3)2)4]−.

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Scheme 6. Compounds Detected after Trimerization with 18 at Atmospheric Pressurea

a

complex

CONCLUSION The reaction of CH3CrCl2(thf)3 or p-tolylCrCl2(thf)3 with ligands 1−6 leads to CH-activated ligands in most cases. However, in the presence of a NH functionality also the deprotonation of the amino group was observed, and therefore ligands with secondary amines may lead to different species upon reaction with organometallic chromium precursors. The chromium aryl ligands in complexes 8−13 are part of a tridentate ligand, and in order to simplify the system, we replaced the internal aryl by an external aryl (or methyl respectively) combined with the bidentate ligand 14. The three complexes 15−17 should lead to the same active species after activation with MAO, and indeed the results for the methyl complex (16) in comparison with those for the p-tolyl complex (15) are almost identical. However, the trichlorido complex 17 is much worse than 15 or 16 with respect to catalytic activity and selectivity. This shows that the aryl (or methyl) group in the catalyst precursor is important for the activation pathway leading to a good trimerization catalyst.

For 22 the trans isomer is also possible.

Some of the identified compounds are a result of β-H eliminations (19 and 21). In addition to these, we identified considerable amounts of 1-butyl-4-methylbenzene (20). Quenching of the reaction mixture in the end with D2O gave the monodeuterated compound. This shows that 20 is not formed during the activation but by the hydrolysis of a corresponding chromium alkyl or aluminum alkyl species. In addition, we suggest that insertion of 1-hexene into the

Scheme 7. Proposed Activation Pathways for Complex 18 As Concluded from the Analysis of Organic Byproducts after a Catalytic Trimerization Run under Typical Conditions

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The tris-aryl complex 18 features a rare five-coordinate chromium(III) metal center. This complex was used for the identification of catalyst activation products ,which gave insight into the mechanism of catalyst activation.

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could be analyzed by X-ray crystallography. 19F NMR (thf-d8, 376.27 MHz): no signal. 1H NMR (thf-d8, 399.89 MHz): δ 12.83 (bs), 11.70 (bs), −10.86 (bs). Anal. Calcd (found) for C26H22Cl2CrF3N2O: C, 55.93 (54.70); H, 3.97 (4.10); N, 5.02 (5.04). We attribute the low C value in the elemental analysis to insufficient combustion in the presence of the CF3 moiety.28 Synthesis of Complex 9. A mixture of 350 mg (1.21 mmol, 1 equiv) of 2-phenyl-8-(piperidin-1-yl)quinoline, 429 mg (1.21 mmol, 1 equiv) of CH3CrCl2(thf)3 and 8 mL of toluene was stirred for 24 h at room temperature. Then the solvent was removed under reduced pressure. The obtained solid was first washed with a hexane/thf mixture (1/1) and afterward with pure hexane before it was dried under reduced pressure. The product was obtained with a yield of 496 mg (85%) as a brown solid. Diffusion of hexane into a solution of the complex in thf gave crystals suitable for X-ray analysis. Anal. Calcd (found) for C24H27Cl2CrN2O: C, 59.79 (59.58); H, 5.64 (5.76); N, 5.81 (5.72). Synthesis of Complex 10. A mixture of 200 mg (0.66 mmol, 1 equiv) of 8-(piperidin-1-yl)-2-p-tolylquinoline, 232 mg (0.66 mmol, 1 equiv) of CH3CrCl2(thf)3, and 4 mL of toluene was stirred for 24 h at room temperature. Then the solvent was removed under reduced pressure. The obtained solid was washed three times with thf and dried under reduced pressure. The product was obtained with a yield of 228 mg (69%) as a brown solid. Anal. Calcd (found) for C25H29Cl2CrN2O: C, 60.49 (60.35); H, 5.89 (5.55); N, 5.64 (5.72). Synthesis of Complex 11. A mixture of 200 mg (0.56 mmol, 1 equiv) of 8-(piperidin-1-yl)-2-(4-(trifluoromethyl)phenyl)quinoline, 197 mg (0.56 mmol, 1 equiv) of CH3CrCl2(thf)3, and 4 mLof toluene was stirred for 24 h at room temperature. Then the solvent was removed under reduced pressure. The obtained solid was washed with hexane and recrystallized by diffusion of hexane into a solution of the solid in thf. In this way crystals suitable for X-ray structure analysis were obtained. After it was washed with hexane and dried under reduced pressure, the product was obtained with a yield of 109 mg (31%) as a brown solid. Anal. Calcd (found) for (C25H26Cl2CrF3N2O)(C4H8O): C, 55.96 (54.00); H, 5.51 (5.37); N, 4.50 (4.60). We attribute the low C value in the elemental analysis to insufficient combustion in the presence of the CF3 moiety.28 Synthesis of Complex 12. A mixture of 230 mg (0.73 mmol, 1 equiv) of 2-(3,5-dimethylphenyl)-8-(piperidin-1-yl)quinoline, 257 mg (0.73 mmol, 1 equiv) of CH3CrCl2(thf)3 and 6 mL of toluene was stirred for 24 h at room temperature. Then the solvent was removed under reduced pressure. The obtained solid was washed with thf and afterward with pentane. After drying under reduced pressure the product was obtained with a yield of 245 mg (66%). Further purification by washing with thf at 65 °C gave 192 mg of the very pure product. Anal. Calcd (found) for C26H31Cl2CrN2O: C, 61.18 (61.01); H, 6.12 (6.31); N, 5.49 (5.73). Synthesis of Complex 13. A mixture of 196 mg (0.68 mmol, 1 equiv) of 8-(pyrrolidin-1-yl)-2-p-tolylquinoline, 241 mg (0.68 mmol, 1 equiv) of CH3CrCl2(thf)3, and 5 mL of toluene was stirred for 24 h at room temperature. Then the solvent was removed by filtration. The obtained solid was washed with thf and pentane. After drying under reduced pressure the product was obtained with a yield of 274 mg (84%). Further purification by washing with thf at 65 °C gave 107 mg of the pure product. Crystals suitable for X-ray analysis were obtained by storing a solution of the complex in thf at −30 °C. Anal. Calcd (found) for C24H27Cl2CrN2O: C, 59.76 (59.18); H, 5.64 (5.53); N, 5.81 (5.75). Synthesis of Complex 15. A solution of 342 mg (3.67 mmol, 1 equiv) of 8-(piperidin-1-yl)quinoline and 693 mg (1.61 mmol, 1 equiv) of p-tolylCrCl2(thf)3 in 5 mL of thf was stirred for 2 h at 60 °C. After cooling to room temperature the solvent was removed from the precipitating solid by filtration. The brown product was recrystallized from thf, giving crystals suitable for X-ray analysis. After drying under reduced pressure the product was obtained with a a yield of 176 mg (20%). Anal. Calcd (found) for (C25H31Cl2CrN2O)(C4H8O): C, 61.05 (60.95); H, 6.89 (6.96); N, 4.91 (5.27). Synthesis of Complex 16. A mixture of 778 mg (1.61 mmol, 1 equiv) of 8-(piperidin-1-yl)quinoline, 1.30 g (3.67 mmol, 1 equiv) of

EXPERIMENTAL SECTION

General Synthetic Procedures. All manipulations were performed under an argon atmosphere using standard Schlenk techniques or in an argon-filled drybox. Solvents were purchased anhydrous and stored over molecular sieves or in a solvent purification system after degassing. Methylaluminoxane was purchased from Sigma-Aldrich (MAO 1, 10% in toluene) and from Chemtura Organometallics GmbH (MAO 2, 21−23% MAO, 7−9% aluminum alkyls). 2,8-Dibromoquinoline, 2 4 8-bromo-2-phenylquinoline, 2 5 CH3CrCl2(thf)3,26 and p-tolylCrCl2(thf)327 were synthesized according to the literature. Catalytic Ethylene Trimerization. In all cases (except for 16-thf, where we used the yellow material without coordinated thf) we used the crystalline material from X-ray analysis for catalytic applications. The autoclave was heated to 100 °C under vacuum for 30 min, flushed with argon, and cooled to the desired temperature. The solvent (80 mL of toluene), the MAO solution (MAO 1, 2 mL; MAO 2, 1 mL) and the chromium complexes were transferred into the autoclave as a solution or a suspension in toluene. Immediately after this, the catalysis was started by pressurizing the autoclave with ethene. In some cases the chromium compound, MAO, and toluene were stirred for a certain time at room temperature before they were transferred into the autoclave. After the reaction time, the autoclave was cooled by the internal cooling system and depressurized carefully. A 1−3 mL sample of the reaction mixture was quenched with 5−10 drops of water, filtered, and diluted with diethyl ether before analyzing it by gas chromatography. To calculate the amount of 1-hexene, the toluene signal was used as an internal standard and referenced to measurements with 1-hexene/ toluene mixtures. The 1-octene and 1-dodecene signals were identified by reference measurements. The 1-decene signal was identified by a GC/MS measurement. The amounts of LAOs other than 1-hexene were calculated by referring to the 1-hexene signal under the assumption that the intensity is proportional to the molecular weight of the compound due to their structural similarity (FID detector). The amount of polyethylene was determined by quenching the reaction mixture with a mixture of 50 mL of methanol and 10 mL of concentrated hydrochloric acid. After filtration the solid obtained was dried at 100 °C and weighed afterward. Synthesis of Complex 7. A 102 mg portion (280 μmol, 1 equiv) of N-phenyl-2-(4-(trifluoromethyl)phenyl)quinolin-8-amine and 103 mg (291 μmol, 1.04 equiv) of CH3CrCl2(thf)3 were dissolved in 4 mL of toluene in a Schlenk tube and stirred for 6 h at room temperature. Afterward the mixture was cooled to −30 °C overnight before it was filtered. The obtained red solid was purified by crystallizing it via diffusion of hexane into a solution of the product in thf. Then the product was washed three times with hexane and dried under reduced pressure. It was obtained in a yield of 73 mg (41%). Crystals were grown by diffusion of hexane into a solution of the complex in thf and could be analyzed by X-ray crystallography. 19F NMR (thf-d8, 376.27 MHz): δ −55.73 ppm. 1H NMR (thf-d8, 399.89 MHz): δ 22.56 (bs), 20.80 (bs), 16.64 (bs). Anal. Calcd (found) for C30H30Cl2CrF3N2O2: C, 57.15 (56.09); H, 4.80 (4.89); N, 4.44 (4.65). We attribute the low C value in the elemental analysis to insufficient combustion in the presence of the CF3 moiety.28 Synthesis of Complex 8. A 123 mg portion (338 μmol, 1 equiv) of N-phenyl-2-(4-(trifluoromethyl)phenyl)quinolin-8-amine, 128 mg (361 μmol, 1.1 equiv) of CH3CrCl2(thf)3, and 2 mL of thf were stirred in a closed Schlenk tube with a Teflon valve at 100 °C for 2 h. Afterward the mixture was cooled to −30 °C and the solvent was removed from the precipitating solid by filtration. After it was washed with hexane and dried under reduced pressure, the product was obtained with a yield of 17.5 mg (9%) as a brown solid. Crystals were grown by diffusion of hexane into a solution of the complex in thf and G

dx.doi.org/10.1021/om500459k | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

J. T.; Green, M. J.; Hess, F. M.; Morgan, D. H. J. Organomet. Chem. 2004, 689, 3641. (e) Agapie, T. Coord. Chem. Rev. 2011, 255, 861. (3) Forestière, A.; Olivier-Bourbigou, H.; Saussine, L. Oil Gas Sci. Technol. 2009, 64, 649. (4) For systems based on tantalum or titanium see: (a) Andes, C.; Harkins, S. B.; Murtuza, S.; Oyler, K.; Sen, A. J. Am. Chem. Soc. 2001, 123, 7423. (b) Arteaga-Muller, R.; Tsurugi, H.; Saito, T.; Yanagawa, M.; Oda, S.; Mashima, K. J. Am. Chem. Soc. 2009, 131, 5370. (c) Deckers, P. J. W.; Hessen, B.; Teuben, J. H. Angew. Chem., Int. Ed. 2001, 40, 2516. (d) Deckers, P. J. W.; Hessen, B.; Teuben, J. H. Organometallics 2002, 21, 5122. (e) Hessen, B. J. Mol. Catal. A: Chem. 2004, 213, 129. (f) Hagen, H.; Kretschmer, W. P.; van Buren, F. R.; Hessen, B.; van Oeffelen, D. A. J. Mol. Catal. A: Chem. 2006, 248, 237. (g) Otten, E.; Batinas, A. A.; Meetsma, A.; Hessen, B. J. Am. Chem. Soc. 2009, 131, 5298. (5) Manyik, R. M.; Walker, W. E.; Wilson, T. P. (Union Carbide Corporation) US 3300458, 1967. (6) Reagen, W. K. EP 0 417 477 B1, 1991. (7) Manyik, R. M.; Walker, W. E.; Wilson, T. P. J. Catal. 1977, 47, 197. (8) Briggs, J. R. Chem. Commun. 1989, 674. (9) (a) Carter, A.; Cohen, S. A.; Cooley, N. A.; Murphy, A.; Scutt, J.; Wass, D. F. Chem. Commun. 2002, 858. (b) Bollmann, A.; Blann, K.; Dixon, J. T.; Hess, F. M.; Killian, E.; Maumela, H.; McGuinness, 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. (c) 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. (d) 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. (e) McGuinness, D. S.; Wasserscheid, P.; Keim, W.; Morgan, D.; Dixon, J. T.; Bollmann, A.; Maumela, H.; Hess, F.; Englert, U. J. Am. Chem. Soc. 2003, 125, 5272−5273. (f) Jabri, A.; Temple, C.; Crewdson, P.; Gambarotta, S.; Korobkov, I.; Duchateau, R. J. Am. Chem. Soc. 2006, 128, 9238. (g) McGuinness, D. S.; Wasserscheid, P.; Morgan, D. H.; Dixon, J. T. Organometallics 2005, 24, 552. (h) McGuinness, D. S.; Wasserscheid, P.; Keim, W.; Hu, C.; Englert, U.; Dixon, J. T.; Grove, C. Chem. Commun. 2003, 334. (i) Maas, H.; Mihan, S.; Köhn, R.; Seifert, G.; Tropsch, J. US 6844290 B1, 2005. (j) Mihan, S.; Molnar, F.; Maas, H.; Prinz, M. WO 03/076367 A2, 2003. (k) Mihan, S.; Maas, H. WO 03/ 076368 A1, 2003. (l) Köhn, R. D.; Haufe, M.; Kociok-Köhn, G.; Grimm, S.; Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39 (23), 4337. (m) Köhn, R. D.; Haufe, M.; Mihan, S.; Lilge, D. Chem. Commun. 2000, 1927. (n) Köhn, R. D.; Smith, D.; Mahon, M. F.; Prinz, M.; Mihan, S.; Kociok-Köhn, G. J. Organomet. Chem. 2003, 638, 200. (10) Emrich, R.; Heinemann, O.; Jolly, P. W.; Krueger, C.; Verhovnik, G. P. J. Organometallics 1997, 16, 1511. (11) (a) Agapie, T.; Schofer, S. J.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2004, 126, 1304. (b) Agapie, T.; Day, M. W.; Henling, L. M.; Labinger, J. A.; Bercaw, J. E. Organometallics 2006, 25, 2733. (c) Agapie, T.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2007, 129, 14281. (12) Monillas, W. H.; Young, J. F.; Yap, G. P. A.; Theopold, K. H. Dalton Trans. 2013, 42, 9198. (13) Schofer, S. J.; Day, M. W.; Henling, L. M.; Labinger, J. A.; Bercaw, J. E. Organometallics 2006, 25, 2743. (14) Temple, C. N.; Gambarotta, S.; Korobkov, I.; Duchateau, R. Organometallics 2007, 26, 4598. (15) Temple, C.; Jabri, A.; Crewdson, P.; Gambarotta, S.; Korobkov, I.; Duchateau, R. Angew. Chem., Int. Ed. 2006, 45, 7050. (16) Kurras, E. Monatsber. Dt. Akad. Wiss. 1963, 5, 378. (17) Nishimura, K.; Kuribayashi, H.; Yamamoto, A.; Ikeda, S. J. Organomet. Chem. 1972, 37, 317. (18) Daly, J. J.; Sneeden, R. P. A.; Zeiss, H. H. J. Am. Chem. Soc. 1966, 88 (18), 4287. (19) Ackerman, L. J.; Bei, X.; Boussie, T. R.; Diamond, G. M.; Hall, K. A.; Lapointe, A. M.; Longmire, J. M.; Murphy, V. J.; Sun, P.;

CH3CrCl2(thf)3, and 21 mL of toluene was stirred for 24 h at room temperature. Then the solvent was removed from the precipitated solid by filtration. The yellow product was washed two times with thf and dried under reduced pressure to give a yield of 1.03 g (66%). Anal. Calcd (found) for C15H19Cl2CrN2 (16-C4H8O): C, 51.44 (51.44); H, 5.47 (5.21); N, 8.00 (8.11). Dissolution of the yellow complex (without coordinated thf) in thf leads to a green solution which upon cooling to −30 °C gives crystals suitable for X-ray analysis. The green compound contains one coordinated thf. The catalytic experiments have been performed with the yellow complex 16-thf. Synthesis of Complex 17. A mixture of 850 mg (4.00 mmol, 1 equiv) of 8-(piperidin-1-yl)quinoline and 1.50 mg (4.00 mmol, 1 equiv) of CrCl3(thf)3 in 10 mL of thf was stirred under reflux for 1.5 h. After cooling to room temperature the solvent was removed from the precipitating solid by filtration. The gray product so obtained was washed with thf. After drying under reduced pressure the product was obtained in a yield of 1.37 g (77%). Crystals suitable for X-ray analysis were obtained by diffusion of heptane into a solution of the complex in thf. Anal. Calcd (found) for C19H27Cl2CrN2O: C, 48.83 (48.88); H, 5.46 (5.14); N, 6.33 (7.26). Synthesis of Complex 18. A 3.4 mL portion of a 2 M solution of p-tolylMgCl in thf (6.80 mmol, 3 equiv) was dropped slowly into a suspension of 1.00 g of 17 (2.26 mmol, 1 equiv) in 16 ml of thf at 0 °C. After the addition was complete, the mixture was stirred for 2 h at room temperature. The solvent was removed from the precipitating solid by filtration. The dark green product was dried under reduced pressure, giving a yield of 980 mg (81%). Further purification by recrystallization from toluene gave 465 mg of the very pure product. Crystals suitable for X-ray analysis were obtained by diffusion of heptane into a solution of the complex in toluene. Anal. Calcd (found) for C35H37CrN2: C, 78.18 (78.49); H, 6.94 (7.09); N, 5.21 (5.04).

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, tables, and CIF files giving instrumentation and general experimental procedures, general synthetic procedures, synthesis and analytical data of ligands 1−6 and 14, identification of organic byproducts during initiation of catalytic trimerization, data from X-ray diffraction analysis, the molecular structure of 8 showing the disordered molecules, and details of DFT calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*M.E.: tel, +49-6221-546247; fax, +49-6221-541616247; e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Federal Ministry of Education and Science (BMBF) under the project funding reference number 03X3565B. We thank our cooperation partners Basell Polyolefine GmbH (Germany) and the groups from the Universities of Freiburg and Konstanz (Rolf Mülhaupt and Stefan Mecking, respectively) for fruitful interactions within our joint research project.

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REFERENCES

(1) Skupinska, J. Chem. Rev. 1991, 91, 613. (2) (a) McGuinness, D. S. Chem. Rev. 2011, 111, 2321. (b) Wass, D. F. Dalton Trans. 2007, 816. (c) van Leeuwen, P. W. N. M.; Clément, N. D.; Tschan, M. J. L. Coord. Chem. Rev. 2011, 255, 1499. (d) Dixon, H

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Verdugo, D.; Schofer, S.; Dias, E.; McConville, D. H.; Li, R. T.; Walzer, J.; Rix, F.; Kuchta, M. (Exxon Mobil) WO 2006/096881 Al. (20) Mao, L.; Moriuchi, T.; Sakurai, H.; Fujii, H.; Hirao, T. Tetrahedron Lett. 2005, 46, 8419. (21) (a) Herwig, W.; Zeiss, H. J. Am. Chem. Soc. 1959, 81, 4798. (b) Kahn, A. I.; Bau, R. Organometallics 1983, 2, 1896. (22) (a) Monillas, W. H., Theopold, K. H. Yap, G. P. A. Private Communication in CCDC 667984 and 667985. (b) Kreisel, K. A.; Yap, G. P. A.; Theopold, K. H. Organometallics 2006, 25, 4670. (23) Do, L. H.; Labinger, J. A.; Bercaw, J. E. ACS Catal. 2013, 3, 2582. (24) Lord, A.; Mahon, M. F.; Lloyd, M. D.; Threadgill, M. D. J. Med. Chem. 2009, 52, 868. (25) Mao, L.; Moriuchi, T.; Sakurai, H.; Fujii, H.; Hirao, T. Tetrahedron Lett. 2005, 46, 8419. (26) Nishimura, K.; Kuribayashi, H.; Yamamoto, A.; Ikeda, S. J. Organomet. Chem. 1972, 37, 317. (27) Daly, J. J.; Sneeden, R. P. A.; Zeiss, H. H. J. Am. Chem. Soc. 1966, 88 (18), 4287. (28) Fadeeva, V. P.; Tikhova, V. D.; Nikulicheva, O. N. J. Anal. Chem. 2008, 63, 1094.

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dx.doi.org/10.1021/om500459k | Organometallics XXXX, XXX, XXX−XXX