In Situ EPR Monitoring of Chromium Species Formed during Cr

Organometallics 2010, 29, 2943–2950 DOI: 10.1021/om100215t

2943

In Situ EPR Monitoring of Chromium Species Formed during Cr-Pyrrolyl Ethylene Trimerization Catalyst Formation Igor Y. Skobelev,†,‡ Valentina N. Panchenko,† Oleg Y. Lyakin,†,‡ Konstantin P. Bryliakov,†,‡ Vladimir A. Zakharov,† and Evgenii P. Talsi*,†,‡ †

Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, 630090, Novosibirsk, Russian Federation, and ‡Novosibirsk State University, 630090, Novosibirsk, Russian Federation Received March 19, 2010

The catalyst systems Cr(acac)3/pyrrole/AlEt3/AlEt2Cl and Cr(EH)3/pyrrole/AlEt3/AlEt2Cl in cyclohexane (EH = 2-ethylhexanoate), modeling the Phillips ethylene trimerization catalyst, have been studied by EPR spectroscopy. The effect of various components of these catalyst systems on the concentration of Cr(III) and Cr(I) species and ethylene trimerization activity has been investigated. The relationship between the trimerization data and the nature of the chromium species present in the reaction solution has been studied.

Introduction Linear R-olefins such as 1-hexene and 1-octene are important comonomers for the copolymerization with ethylene to generate linear low-density polyethylene. They are generated industrially predominantly via nonselective oligomerization of ethylene.1 This oligomerization generally produces a broad range of olefins characterized by a Schulz-Flory distribution. Therefore, catalyst systems that are selective for specific desirable alkenes would be of great industrial and academic interest. The first process for the selective production of 1-hexene was commercialized in 2003 by Chevron-Phillips.2 The typical Phillips trimerization catalyst is prepared by combining chromium(III) 2-ethylhexanoate (Cr(EH)3), 2,5-dimethylpyrrole, triethylaluminum (AlEt3), and diethylaluminum chloride (AlEt2Cl) in toluene in a molar ratio of 1:3:7.8:10.3 at room temperature. The trimerization of ethylene is performed at a reaction temperature of 115 C and a reaction pressure of 100 bar in cyclohexane.2,3 Up to now, the Phillips trimerization process is the sole example of commercial selective trimerization of ethylene, despite other chromium-based catalyst systems capable of selectively trimerizing and tetramerizing ethylene having been found.4,5 Due to the ill-defined structure of paramagnetic chromium species present in the Phillips catalyst systems, their mechanistic studies are complicated. Very recently, in studies on the catalyst systems related to the Phillips ethylene trimerization *Corresponding author. Fax: þ7 383 3308056. E-mail: [email protected]. (1) Camara Greiner, E. O.; Gubler, R.; Inoguchi, Y. Chemical Economics Handbook Marketing Research Report: Linear Alpha Olefins; SRI: Menlo Park, CA, 2004. (2) Freeman, J. W.; Buster, J. L.; Knudsen, R. D. (Phillips Petroleum Company) US 5856257, 1999. (3) Dixon, J. T.; Green, M. J.; Hess, F. M.; Morgan, D. H. J. Organomet. Chem. 2004, 689, 3641. (4) Wass, D. F. Dalton Trans. 2007, 816. (5) Overett, M. J.; Blann, K; Bollmann, A.; Dixon, J. T.; Haasbroek, D.; Killian, E.; Maumela, H.; McGuinness, D. S.; Morgan, D. H. J. Am. Chem. Soc. 2005, 127, 10723. r 2010 American Chemical Society

systems, complexes of Cr(II) (A-C) and Cr(I) (D) were isolated (Scheme 1).6,7 It was shown that complexes A and C act as self-activating ethylene polymerization catalysts, and complexes B and D as self-activating ethylene trimerization catalysts. It was proposed that complexes of chromium(I) are responsible for selective trimerization.6,7 To support this assumption, it is important to find the connection between the concentration of Cr(I) species in the reaction solution and trimerization activity. This has not been done so far. Recently, in situ EPR spectroscopy has been used to monitor the structure and valence state of chromium species in the catalyst system for ethylene tetramerization Cr(acac)3/PNP/MMAO (PNP = Ph2PN(i-Pr)PPh2, MMAO = modified methylaluminoxane).8 In this work, we have undertaken the EPR spectroscopic monitoring of chromium species formed in two catalyst systems related to the Phillips catalyst. These systems were prepared by combining Cr(acac)3 or Cr(EH)3 with pyrrole (HPyr), AlEt3, and AlEt2Cl in cyclohexane. The main goal was to study the effect of various components of these catalyst systems on the concentration of mononuclear Cr(III) and Cr(I) species in the reaction solution and to establish the correlation between the concentrations of the particular chromium species and the trimerization activity.

Results and Discussion Effect of Various Components of the Catalyst System Cr(acac)3/HPyr/AlEt3/AlEt2Cl on the Concentration of Cr(III) and Cr(I) Species. The System Cr(acac)3/AlEt3. The EPR spectrum of a frozen solution of Cr(acac)3 (cyclohexane, -196 C) exhibits resonances at effective g-values 3.75 (ΔH = 250 G) and (6) Jabri, A.; Mason, C. B.; Sim, Y.; Gambarotta, S.; Burchell, T. J.; Duchateau, R. Angew. Chem., Int. Ed. 2008, 47, 9717. (7) Vidyaratne, I.; Nikiforov, G. B.; Gorelsky, S. I.; Gambarotta, S.; Duchateau, R.; Korobkov, I. Angew. Chem., Int. Ed. 2009, 48, 6552. (8) Br€ uckner, A.; Jabor, J. K.; McConnell, A. E. C.; Webb, P. B. Organometallics 2008, 27, 3849. Published on Web 06/08/2010

pubs.acs.org/Organometallics

2944

Organometallics, Vol. 29, No. 13, 2010

Skobelev et al.

Scheme 1. Complexes of Cr(II) and Cr(I) Isolated from the Catalyst Systems Related to the Phillips Trimerization Catalyst6,7

1.98 (ΔH = 120 G) (Figure 1a). This spectrum is typical for a Cr(III) (S = 3/2) species in octahedral coordination with moderately large zero field splitting (ZFS) and small rhombicity (D > hν ≈ 0.3 cm-1, E/D , 1)9-14 and can be interpreted on the basis of the spin Hamiltonian:

H ¼ βðgx Hx Sx þ gy Hy Sy þ gz Hz Sz Þ þ D½fSz 2 - ð1=3ÞSðS þ 1Þg þ ðE=DÞðSx 2 - Sy 2 Þ ð1Þ The dotted line in Figure 1a shows the simulated spectrum with parameters presented in Table 1. Our data slightly differ from those reported previously (g = 1.975 vs 1.980, D = 0.413 vs 0.6 cm-1, and E = 0.011 vs 0.0087 cm-1).15 This can be caused by different solvents (n-butanol in ref 15 and cyclohexane in our study). The addition of AlEt3 to the solution of Cr(acac)3 in cyclohexane at 20 C ([AlEt3]:[Cr(acac)3] = 20:1, [Cr(acac)3] = 0.01 M) results in the immediate disappearance of the EPR resonances of Cr(acac)3, while those of three new chromium complexes (1, 10 , and 2) are observed (Figure 1b). Complex 1 displays a broad resonance at g = 1.97 (ΔH = 270 G) (Figure 1b, Table 1). This spectrum is characteristic of Cr(III) species with small ZFS (D < hν).11-13 Complex 10 exhibits a weak and sharp resonance at g = 4.00 (ΔH = 50 G), corresponding to a Cr(III) species with large ZFS (D > hν).11-13 Complex 2 shows a relatively sharp resonance at g ≈ 1.98 (ΔH = 60 G) characteristic of a low-spin Cr(I) (S = 1/2) (9) McGarvey, B. R. J. Chem. Phys. 1964, 41, 3743. (10) Weckhuysen, B. M.; Ramachandra Rao, R.; Pelgrims, J.; Schoonheydt, R. A.; Bodart, P.; Debras, G.; Collart, O.; Van Der Voort, P.; Vansant, E. F. Chem.;Eur. J. 2000, 6, 2960. (11) Shaham, N.; Cohen, H.; Meyerstein, D.; Bill, E. J. Chem. Soc., Dalton Trans. 2000, 3082. (12) Pedersen, E.; Toftlund, H. Inorg. Chem. 1974, 13, 1603. (13) Hempel, J. C.; Morgan, L. O.; Burton Lewis, W. Inorg. Chem. 1970, 9, 2064. (14) Sato, K.; Shiomi, D.; Takui, T.; Itoh, K.; Shimozono, T.; Yoshida, H.; Tajima, K.; Azuma, N. Bull. Magn. Reson. 1996, 18, 171. (15) Doeuff, S.; Henry, M.; Sanchez, C.; Livage, J. J. Non-Cryst. Solids 1987, 89, 84.

Figure 1. EPR spectrum (-196 C) of Cr(acac)3 in C6H12 ([Cr(acac)3] = 10-2 M) (a). EPR spectrum (-196 C) of the sample Cr(acac)3/AlEt3 ([AlEt3]:[Cr(acac)3] = 20, [Cr(acac)3] = 10-2 M, C6H12) after storing at 20 C for 6 min (b). Dotted line shows the simulated spectrum of Cr(acac)3 with parameters presented in Table 1.

species (Figure 1b, Table 2).8,16 The concentration of 2 slowly grows upon storing the sample at room temperature (Figures 2a-c). The rate of this growth increases at an elevated temperature (Figure 2d). It is worth noting that all EPR spectra presented in this work were recorded at -196 C. At room temperature, EPR signals of Cr(III) species present in the catalyst systems studied were not observed, probably due to short relaxation times, and the only sharp signal of the Cr(I) species at g ≈ 1.98 was detected. Note also that in Figure 2 and all the next figures in the text the spectra marked by letters a, b, c, etc., were recorded with the same conditions and the same amounts of precatalysts; therefore the intensities of the particular signals reflect the concentrations of the corresponding complexes. The increase of AlEt3 concentration in the system Cr(acac)3/ AlEt3 favors the formation of Cr(I) species. The EPR spectrum of the sample [AlEt3]:[Cr(acac)3] = 200:1 recorded just after mixing the reagents at 20 C displays the resonance typical for Cr(I) species (Figure 3a). The shape and intensity of this resonance change during the first 30 min of storing at 20 C (Figure 3b,c). Further storing at this temperature does not essentially change the shape of the signal. The EPR spectrum of Figure 3c is identical to that of 2. Hence, the initially observed signal (Figure 3a) belongs to some unidentified Cr(I) species. Storing the sample at 20 C results in the growth of the concentration of 2. The EPR spectrum of 2 displays small axial anisotropy (gx,y = 1.976, gz = 2.001) (Figure 3c, inset). The total concentration of complex 2 evaluated from the intensity of the corresponding EPR signals is at least 5 times lower than the concentration of the initial complex Cr(acac)3. Thus, after the addition of AlEt3 to the solution of Cr(acac)3, predominantly EPR-silent species are formed, and only a minor part of Cr(acac)3 (less than 20%) converts to the EPR-active species 2. Unfortunately, EPR spectra of 1, 10 , and 2 display no additional hyperfine splitting from ligands. Without this splitting, it is difficult to derive the unambiguous structural information from the EPR spectra. Therefore, 1 can only (16) K€ ohler, F. H.; Metz, B.; Strauss, W. Inorg. Chem. 1995, 34, 4402.

Article


2945

Table 1. EPR Spectroscopic Data (-196 C) and Tentative Structures of Cr(III) Species parameters of the simulated spectruma

experimental parameters complex

gef ( 0.01

ΔH ( 10, G b

gx = gy = gz

D, cm-1

E, cm-1

σ, Gc

CrIII(acac)3 CrIII(EH)3 CrIII(acac)xEtyL (1) d CrIII(Pyr)xClyEtzL (3) d CrIII(EH)xClyEtzL (5) d

3.75, 1.98 1.96 1.97 4.17 4.34, 3.30, 1.94

250, 120 290 270 130 110, 160, 100

1.975 1.982

0.413 0.052

0.011 0.008

85 308

0.548

0.036

51

a

d

b

1.981 c

Lorentzian line shape. Peak-to-peak widths of the resonances in the experimental spectra. Line width parameter applied for spectrum simulation. L is unidentified ligand or ligands.

Table 2. EPR Spectroscopic Data (-196 C) and Tentative Structures of Cr(I) Species complex

gz ( 0.001

gx,y ( 0.001

CrI(acac)L (2) a CrI(Pyr)L (2-Pyr) CrI(EH)L (4)

2.001 2.005 2.000

1.976 1.978 1.980

a

L is unidentified ligand or ligands.

Figure 3. EPR spectra (-196 C) of the sample Cr(acac)3/AlEt3 ([AlEt3]:[Cr(acac)3] = 200, [Cr(acac)3] = 10-2 M, C6H12) after various treatments: 1 (a) and 5 min (b) storing at 20 C; 20 min storing at 20 C and then 1 min at 65 C (c). Dotted line shows the simulated spectrum of species 2. Figure 2. EPR spectra (-196 C) of the sample Cr(acac)3/AlEt3 ([AlEt3]:[Cr(acac)3] = 20, [Cr(acac)3] = 10-2 M, C6H12) after various treatments: 1 (a), 6 (b), and 15 min (c) storing at 20 C; 1 min after storing sample in “c” at 65 C (d).

tentatively be assigned to a complex of the type CrIII(acac)xEtyL or some oligomers of these species, and 2 to a complex of the type CrI(acac)L, where L denotes unidentified ligand or ligands. The nature of the minor Cr(III) complex 10 is unclear. The presented data show that just after mixing AlEt3 with Cr(acac)3 at 20 C in cyclohexane Cr(acac)3 converts predominantly to EPR-silent species. Then further transformations lead to the gradual growth of the concentration of Cr(I) complex 2. A similar result was previously reported for the Cr(acac)3/PNP/MMAO system.8 After mixing the reagents, the authors observed a relatively fast drop of the concentration of Cr(III) followed by a slower increase of the concentration of Cr(I). They suggested that complexes of Cr(II) and/or antiferromagnetically coupled Cr(I) 3 3 3 Cr(I) dimers are formed at the initial stage of the reaction.8 In our case, the formation of Cr(II) species seems to be more likely than the formation of Cr(I) 3 3 3 Cr(I) dimers, since rather stable Cr(II) complexes A-C can be isolated from the catalyst systems related to the Phillips ethylene trimerization system (Scheme 1).6,7 The System Cr(acac)3/HPyr/AlEt3. The EPR spectrum (-196 C) of the sample Cr(acac)3:HPyr:AlEt3 = 1:3:20

Figure 4. EPR spectra (-196 C) of the sample Cr(acac)3:HPyr: AlEt3 = 1:3:20 ([Cr(acac)3] = 10-2 M, C6H12) after various treatments: 1 (a), 5 (b), and 28 min (c) storing at 20 C.

([Cr(acac)3] = 0.01 M) recorded immediately after mixing the reagents at 20 C displays the signal typical for Cr(I) species (Figure 4a). The shape and intensity of this signal

2946


Figure 5. EPR spectra (-196 C) of the sample Cr(acac)3: AlEt2Cl = 1:20 ([Cr(acac)3] = 10-2 M, C6H12) 13 min after mixing the reagents at 20 C (a) and 10 min of storing the sample in “a” at 65 C (b).

change with time (Figure 4b,c). The EPR spectrum of Figure 4c is a superposition of the resonances of at least two Cr(I) complexes with different values of gz (Figure 4c, inset). One of these complexes (gz = 2.001) can be assigned to 2, and another complex (gz = 2.005) to the Cr(I) complex incorporating pyrrole as a ligand. The latter complex, which will be further refered to as 2-Pyr, can be tentatively presented as CrI(Pyr)L, where Pyr is a deprotonated pyrrole molecule (Table 2). Comparison of the EPR spectra of the systems Cr(acac)3: AlEt3 = 1:20 (Figure 2a-c) and Cr(acac)3:HPyr:AlEt3 = 1:3:20 (Figure 4a-c) shows that in the presence of pyrrole the concentration of Cr(I) species increases faster than in the system containing no pyrrole. Thus, pyrrole promotes the formation of mononuclear Cr(I) species in the catalyst system Cr(acac)3/HPyr/AlEt3. The Systems Cr(acac)3/AlEt2Cl and Cr(acac)3/HPyr/ AlEt3/AlEt2Cl. The EPR spectrum of the sample Cr(acac)3: AlEt2Cl = 1:20 recorded 13 min after mixing the reagents at 20 C (Figure 5a) displays the resonance at effective g-value of 4.2 (ΔH = 120 G), typical of Cr(III) complexes with large ZFS (D > hν),11-13 and those at g = 2.54 and 2.00 (Figure 5a). Storing the sample in “a” for 10 min at 65 C leads to the decrease of the resonance at g = 4.2, whereas the resonances at g = 2.54 and 2.00 remain unchanged (Figure 5b). We cannot assign the observed resonances to a particular chromium species. Noteworthy, in contrast to the sample Cr(acac)3:AlEt3 = 1:20 (Figure 2a-d), the sample Cr(acac)3:AlEt2Cl = 1:20 displays no peaks of Cr(I) species. This is evident from the absence of the corresponding resonance at g = 1.98 in the spectrum of the sample in Figure 5, recorded at 20 C. The EPR spectrum of the sample Cr(acac)3:HPyr: AlEt2Cl:AlEt3 = 1:3:10:10 ([Cr(acac)3] = 0.01 M) shows the resonance of 2-Pyr and that of the new Cr(III) complex 3 at g = 4.17 (ΔH = 130 G) (Figure 6a-c, Table 1). Tentatively, 3 can be represented as CrIII(Pyr)xClyEtzL. The ratio of the reagents in the sample Cr(acac)3:HPyr:AlEt2Cl:AlEt3 = 1:3:10:10 is close to that used in the commercial catalyst systems. The maximum concentration of Cr(I) complex 2-Pyr in this sample does not exceed 10% of the total concentration of chromium in the reaction solution. To study the effect of ethylene on the concentration of various chromium species in the reaction solution, the sample

Skobelev et al.

Figure 6. EPR spectra (-196 C) of the sample Cr(acac)3:HPyr: AlEt2Cl:AlEt3 = 1:3:10:10 ([Cr(acac)3] = 10-2 M, C6H12) after storing for 1 (a), 5 (b), and 15 min (c) at 20 C.

Figure 7. EPR spectrum (-196 C) of Cr(EH)3 in C6H12 ([Cr(EH)3] = 10-2 M) (a). EPR spectra (-196 C) of the sample Cr(EH)3:AlEt3 = 1:20 ([Cr(EH)3] = 10-2 M, C6H12) after various treatments: after storing for 5 (b) and 15 min (c) at 20 C; after storing for 15 min at 20 C and then 10 min at 65 C (d). Dotted line shows spectrum simulated with parameters presented in Table 1.

Cr(acac)3:HPyr:AlEt2Cl:AlEt3:C2H4 = 1:3:10:10:50 was prepared. In this sample, only complex 2-Pyr was observed, in contrast to the sample Cr(acac)3:HPyr:AlEt2Cl:AlEt3 = 1:3:10:10, where complexes 2-Pyr and 3 are present. The concentration of 2-Pyr in the sample containing ethylene was at least 2 times higher than in the sample without ethylene. The maximum concentration of 2-Pyr does not exceed 20% of the initial concentration of chromium. Effect of Various Components of the Catalyst System Cr(EH)3/HPyr/AlEt2Cl/AlEt3 on the Concentration of Cr(III) and Cr(I) Species. The System Cr(EH)3/AlEt3. The EPR spectrum of a frozen solution of Cr(EH)3 in cyclohexane (-196 C) displays a broad resonance at g = 1.96 (Figure 7a, Table 1), characteristic of Cr(III) species with small ZFS (D < hν).11 The sample Cr(EH)3:AlEt3 = 1:20 ([Cr(EH)3] = 0.01 M) exhibits a

Article

Figure 8. EPR spectra (-196 C) of the sample Cr(EH)3:AlEt3 = 1:200 ([Cr(EH)3] = 10-2 M, C6H12) after various treatments: after 3 (a) and 15 min (b) of storing at 20 C; after 5 min of storing the sample in “b” at 65 C (c).

broad and intense resonance of Cr(III), resembling that of the starting complex Cr(EH)3, and very weak and relatively sharp resonances at g = 4.00 and g = 1.98 (Figure 7b,c). The concentration of Cr(III) in the samples of Figure 7b,c is about 30% of the total chromium concentration. This is evident from the comparison of the intensities of the broad resonances at g = 1.96 in Figure 7a,b. By analogy with the catalyst system based on Cr(acac)3, the sharp resonance at g = 1.98 can be assigned to Cr(I) complex 4 with proposed structure CrI(EH)L, while the resonance at g = 4.00 belongs to unidentified Cr(III) species. The broad resonance at g = 1.96 can be assigned to the new complex of the type CrIII(EH)xEtyL or to the superposition of the resonance of this new complex and that of the remaining Cr(EH)3. Heating the sample to 65 C leads to the decrease of the concentration of 4 (Figure 7d). EPR spectra of the sample Cr(EH)3:AlEt3 = 1:200 ([Cr(EH)3] = 0.01 M) recorded at various times after storing at 20 C (Figure 8a,b) show a more intense resonance of 4 than the sample Cr(EH)3:AlEt3 = 1:20 (Figure 7b,c). Storing the former sample for 10 min at 65 C results in the disappearance of the resonances of Cr(III) species, and only that of Cr(I) complex 4 is observed (Figure 8c, Table 2). The System Cr(EH)3/HPyr/AlEt3. The EPR spectrum of the system Cr(EH)3:HPyr:AlEt3 = 1:3:20 ([Cr(EH)3] = 0.01 M), recorded 1 min after mixing the reagents at 20 C, shows the relatively intense resonances of Cr(EH)3/CrIII(EH)xEtyL and 2-Pyr and a weak resonance at g = 4.00 (Figure 9a). Storing this sample at 20 C leads to a decrease of the concentration of 2-Pyr and to a smaller drop of the concentration of Cr(EH)3/CrIII(EH)xEtyL (Figure 9b,c). In the presence of pyrrole, the concentration of Cr(I) species in the reaction solution increases (compare Figures 7b,c and 9a-c). Hence, as in the case of the system Cr(acac)3/AlEt3, the addition of pyrrole promotes the formation of mononuclear Cr(I) species. However, the positive effect of pyrrole on the concentration of Cr(I) species for the Cr(EH)3-based system is more pronounced than for the Cr(acac)3-based analogue. In the case of Cr(acac)3, the sample Cr(acac)3: AlEt3 = 1:20 displays a rather intense resonance of Cr(I)


2947

Figure 9. EPR spectra (-196 C) of the sample Cr(EH)3:HPyr: AlEt3 = 1:3:20 ([Cr(EH)3] = 10-2 M, C6H12) after 1 (a), 5 (b), and 10 min (c) of storing at 20 C.

Figure 10. EPR spectra (-196 C) of the sample Cr(EH)3: HPyr:AlEt3 = 1:3:200 ([Cr(EH)3] = 10-2 M, C6H12) after 1 (a), 5 (b), and 10 min (c) of storing at 20 C.

complex 2 even in the absence of pyrrole (Figure 2), whereas in the case of Cr(EH)3, the sample Cr(EH)3:AlEt3 = 1:20 shows a very weak resonance of Cr(I) complex 4 (Figure 7), and only in the presence of pyrrole does the intensity of the resonance of Cr(I) species dramatically increase (Figure 9). The sample Cr(EH)3:Pyr:AlEt3 = 1:3:200 ([Cr(EH)3] = 0.01 M, cyclohexane, 20 C) displays only the resonance of 2-Pyr (Figure 10a-c). The maximum concentration of 2-Pyr is about 20% of the total concentration of chromium. The fact that one and the same complex 2-Pyr is observed in the systems Cr(acac)3/HPyr/AlEt3 and Cr(EH)3/HPyr/AlEt3 evidences in favor of the replacement of acac and EH ligands by Pyr to afford complex 2-Pyr. This agrees with its proposed structure CrI(Pyr)L. The System Cr(EH)3/AlEt2Cl. The EPR spectrum of the sample Cr(EH)3:AlEt2Cl = 1:200 recorded 5 min after mixing the reagents at 20 C displays resonances of the new complex 5 at effective g-values 4.34, 3.30, and 1.94

2948


Figure 11. EPR spectrum (-196 C) of the sample Cr(EH)3: AlEt2Cl = 1:200 5 min after storing at 20 C ([Cr(EH)3] = 10-2 M, C6H12). Dotted line shows the simulated spectrum with parameters presented in Table 1.

Skobelev et al.

Figure 13. EPR spectra (-196 C) of the sample Cr(EH)3:HPyr: AlEt2Cl:AlEt3 = 1:3:10:10 ([Cr(EH)3] = 10-2 M, C6H12) after various treatments: 1 (a), 10 (b), and 15 min (c) after storing at 20 C; 10 min after storing the sample in “b” at 65 C (d). Scheme 2. Metallacyclic Trimerization Mechanism4,17

Figure 12. EPR spectrum (-196 C) of the sample Cr(EH)3: AlEt2Cl:AlEt3 = 1:10:200 recorded 6 min after mixing the reagents at 20 C (a); EPR spectrum (-196 C) of the sample Cr(EH)3:HPyr:AlEt2Cl:AlEt3 = 1:3:10:200 recorded 20 min after mixing the reagents at 20 C (b) ([Cr(EH)3] = 10-2 M, C6H12).

(Figure 11). This complex is unstable at 20 C and decays with the half-life time of 15 min. The EPR spectrum of 5 is typical for Cr(III) complexes (S = 3/2) with ZFS comparable with hν and small rhombicity E.11,13 The weak signal at g ≈ 5.6 (Figure 11) belongs to the almost forbidden “Δm = 3” transition within the |(3/2æ Kramer’s doublet.11,14 The tentative structure of 5 can be presented as CrIII(EH)xClyEtzL. The Systems Cr(EH)3/AlEt2Cl/AlEt3 and Cr(EH)3/HPyr/ AlEt2Cl/AlEt3. The EPR spectrum of the sample Cr(EH)3: AlEt2Cl:AlEt3 = 1:10:200 recorded 10 min after mixing the reagents at 20 C exhibits mainly the resonances of complex 5 (Figure 12a), like that of the sample Cr(EH)3:AlEt2Cl = 1:200 (Figure 11). The situation changes dramatically in the presence of pyrrole. The EPR spectrum of the sample Cr(EH)3:HPyr:AlEt2Cl:AlEt3 = 1:3:10:200 displays resonances of 2-Pyr and that of unidentified Cr(III) complex 6 (g = 3.78) (Figure 12b). These data again demonstrate that pyrrole favors the reduction of Cr(III) to Cr(I).

The sample Cr(EH)3:HPyr:AlEt2Cl:AlEt3 = 1:3:10:10 with the ratio of the reagents close to that in commercial systems displays resonances of complexes 2-Pyr and 3 (Figure 13a-c) like the sample Cr(acac)3:HPyr:AlEt2Cl: AlEt3 = 1:3:10:10 (Figure 6a-c). Complex 2-Pyr is stable even at 65 C, whereas the concentration of 3 noticeably decreases (Figure 13d). In the sample containing ethylene Cr(EH)3:HPyr:AlEt2Cl:AlEt3:C2H4 = 1:3:10:10:50, the concentration of 3 is smaller (by a factor of 4) and the concentration of 2-Pyr is higher (by a factor of 3) than in the sample without ethylene. The maximum concentration of 2-Pyr does not exceed 20% of the total concentration of chromium in the reaction solution. Correlation of the Oligomerization and EPR Spectroscopic Data. It is widely accepted that trimerization of ethylene follows the mechanism previously proposed by Briggs (Scheme 2).17 Two ethylene molecules coordinate to the chromium center and oxidatively couple to form a metallacyclopentane. The third ethylene molecule is coordinated and inserts to form a metallacycloheptane. This metallacycle can undergo β-H elimination to form an alkyl-hydride species, which after reductive elimination produces 1-hexene and regenerates the initial chromium species. The above mechanism is based on shuttling the active sites between Crn and Crnþ2 valent states. In support of the mechanism (17) Briggs, J. R. J. Chem. Soc., Chem. Commun. 1989, 674.

Article


2949

Table 3. Ethylene Trimerization with CrX3/HPyr/AlEt3/AlEt2Cl Systems (X = acac or EH)a

entry

complex

HPyr (equiv)

AlEt3 (equiv)

1 2 3 4 5 6 7 8 9

Cr(acac)3 Cr(acac)3 Cr(acac)3 Cr(acac)3 Cr(acac)3 Cr(EH)3 Cr(EH)3 Cr(EH)3 Cr(EH)3

3

30 30 30

3 3 3 3

AlEt2Cl (equiv)

10 10 10

30 30 30

10 10

trimerization productivity (g 1-C6/mol Cr/h)b

polymerization productivity (g PE/mol Cr/h)

1066 562 525 21 21 5496 801 42 1461

94 88 302 2720 432 660 738 2912 260

a Reaction conditions: 60 μmol of CrX3, 100 mL of cyclohexane, 12 bar of ethylene, 80 C, 3 h. b 2-C6, 3-C6, and higher oligomers were also observed, but their total amount does not exceed 5 wt % of the liquid fraction.

involving metallacyclic intermediates, Jolly and co-workers have reported well-characterized η5-cyclopentadienyl-stabilized chromacyclopentane and chromacycloheptane complexes; the latter decomposes more readily and yields 1-hexene.18 The compelling evidence for the metallacycle mechanism of trimerization was provided by Bercaw and co-workers.19,20 However, at this stage, there are no data that reliably distinguish between the Cr(II)-Cr(IV)21-23 and Cr(I)-Cr(III)6,7,19,20,24,25 mechanisms of selective trimerization. Our oligomerization studies show that the catalyst system Cr(acac)3:HPyr:AlEt3 = 1:3:30 is more active toward ethylene trimerization than the catalyst system Cr(acac)3:AlEt3 = 1:30 (Table 3, entries 1 and 2). Hence, the addition of pyrrole promotes the trimerization of ethylene. The comparison of the EPR spectra of the systems Cr(acac)3/AlEt3 and Cr(acac)3/ HPyr/AlEt3 indicates that the more active system displays a larger concentration of the mononuclear Cr(I) species (compare Figures 2a-c and 4a-c). A stronger positive effect of pyrrole on the trimerization activity was observed for the systems Cr(EH)3:HPyr:AlEt3 = 1:3:30 and Cr(EH)3:AlEt3 = 1:30 (Table 3, entries 6 and 7). This result is in good agreement with the aforementioned stronger effect of pyrrole on the concentration of Cr(I) species in the Cr(EH)3-based system than in the Cr(acac)3based analogue (compare Figures 2a-c and 4a-c; Figures 7b-d and 9a-c). Thus, the trimerization activity correlates with the presence of mononuclear Cr(I) species in the reaction solution. It is worth mentioning that the starting concentrations of Cr in the EPR and polymerization studies differ by an order of magnitude. However, this difference is not so dramatic to exclude the reasonable correlation of the results of the EPR and polymerization studies. The sample Cr(acac)3:AlEt2Cl = 1:20 shows only the resonances of Cr(III) species (Figure 5). In contrast, EPR (18) Emrich, R.; Heinemann, O.; Jolly, P. W.; Kr€ uger, C.; Verhovnik, G. P. J. Organometallics 1997, 16, 1511. (19) Agapie, T.; Schofer, S. J.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2004, 126, 1304. (20) Agapie, T.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2007, 129, 14281. (21) Jabri, A.; Crewdson, P.; Gambarotta, S.; Korobkov, I.; Duchateau, R. Organometallics 2006, 25, 715. (22) McGuinness, D. S.; Brown, D. B.; Tooze, R. P.; Hess, F. M.; Dixon, J. T.; Slawin, A. M. Z. Organometallics 2006, 25, 3605. (23) van Rensburg, W. J.; Grove, C.; Steynberg, J. P.; Stark, K. B.; Huyser, J. J.; Steynberg, P. J. Organometallics 2004, 23, 1207. (24) 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. (25) K€ ohn, R. D.; Smith, D.; Mahon, M. F.; Prinz, M.; Mihan, S.; Kociok-K€ ohn, G. J. Organomet. Chem. 2003, 683, 200.

spectra of the sample Cr(acac)3:AlEt3 = 1:20 display resonances of Cr(III) and Cr(I) species (Figure 2a-d). The oligomerization data show that the systems Cr(acac)3: AlEt2Cl = 1:10 and Cr(acac)3:HPyr:AlEt2Cl = 1:3:10 produce predominantly PE (entries 4 and 5), whereas the systems Cr(acac)3:AlEt3 = 1:30 and Cr(acac)3:HPyr:AlEt3 = 1:3:30 produce 1-hexene and PE (entries 1 and 2). Thus, again the trimerization activity correlates with the presence of a mononuclear Cr(I) species in the reaction solution. A similar result was obtained for the Cr(EH)3-based catalysts. The system Cr(EH)3:HPyr:AlEt3 = 1:3:30, exhibiting resonance of the Cr(I) species in the EPR spectra, effectively trimerizes ethylene (Table 3, entry 6), whereas the system Cr(EH)3:AlEt2Cl = 1:10, exhibiting a resonance of the Cr(III) species, is inert in this reaction, and predominantly PE is formed (entry 8). All presented data evidence in favor of the participation of Cr(I) species in the trimerization of ethylene and the realization of the Cr(I)-Cr(III) rather than the Cr(II)-Cr(IV) mechanism of ethylene trimerization by Phillips catalyst. However, the supporters of the Cr(II)-Cr(IV) paradigm can argue that the majority of Cr present in the catalysts systems studied is EPR silent, and it is probable that some complexes of this majority bear catalytic activity. We can not totally exclude such a possibility, but only those catalyst systems that contain mononuclear Cr(I) species show noticeable trimerization activity. This fact strongly supports the Cr(I)-Cr(III) mechanism. The Phillips trimerization catalyst contains both AlEt3 and AlEt2Cl in its composition. Under the conditions of our oligomerization studies, we have found no positive effect of AlEt2Cl on the yield of 1-hexene for the samples Cr(acac)3: HPyr:AlEt3:AlEt2Cl = 1:3:30:10 and Cr(acac)3:HPyr:AlEt3 = 1:3:30 (entries 1 and 3) and for the samples Cr(EH)3:HPyr: AlEt3:AlEt2Cl = 1:3:30:10 and Cr(EH)3:HPyr:AlEt3 = 1:3:30 (entries 6 and 9). Apparently, the positive effect of AlEt2Cl on the yield of 1-hexene is weak under the conditions of our oligomerization studies (reaction temperature 80 C, ethylene pressure 12 bar) and becomes more pronounced under conditions of the real trimerization (reaction temperature 115 C, ethylene pressure 100 bar). The catalyst systems Cr(acac)3:HPyr:AlEt2Cl:AlEt3 = 1:3:10:10 and Cr(EH)3:HPyr:AlEt2Cl:AlEt3 = 1:3:10:10 with the ratio of reagents close to the actual Phillips catalyst display resonances of Cr(III) complex 3 and Cr(I) complex 2-Pyr in the EPR spectra (Figures 6a-c and 13a-d). It is tempting to assume that complexes 3 and 2-Pyr participate in ethylene trimerization. Further studies are needed to verify this assumption.

2950


Conclusions EPR spectroscopic study of the catalyst systems Cr(acac)3/ pyrrole/AlEt3/AlEt2Cl and Cr(EH)3/pyrrole/AlEt3/AlEt2Cl has shown that two types of EPR-active chromium species are present in both systems at the ratio of reagents approaching that of the commercial trimerization. They are Cr(III) and Cr(I) complexes with tentative structures CrIII(Pyr)xClyEtzL and CrI(Pyr)L, where L is unidentified ligand or ligands. The maximum concentration of these species did not exceed 20% of the total concentration of chromium. The major part of chromium, probably, exists in the reaction solution in the form of EPR-silent Cr(II) species. The ethylene trimerization activity of the catalyst systems studied correlates with the concentration of Cr(I) species in the reaction solution. The obtained data support the Cr(I)Cr(III) mechanism of ethylene trimerization.

Experimental Section Materials. Cyclohexane (C6H12) was dried over molecular sieves (4 A˚), purified by refluxing over sodium metal, and distilled under dry argon. All experiments were carried out in sealed high-vacuum systems using break-seal techniques. Complexes Cr(acac)3 and Cr(EH)3 were synthesized as described.26,27 Complexes Cr(acac)3 and Cr(EH)3 were transferred and stored in a glovebox. Commercial samples of triethylaluminum (AlEt3) and diethylaluminum chloride (AlEt2Cl) were used as solutions in C6H12 of various concentrations. Pyrrole (HPyr) was purchased from Alfa Aesar. It was distilled just before sample preparation and used as a solution in C6H12. Preparation of Samples for EPR Measurements. A weighed amount [2.5 mg (7 10-6 mol) of Cr(acac)3 or 0.3 mL of a Cr(EH)3 solution in C6H12 (5 10-6 mol)] was placed into the dried, argon-filled quartz EPR tube (l = 200 mm, d = 5 mm), equipped with fine glass break-seals. The system with complex was evacuated and sealed off from the vacuum line. In the case of Cr(EH)3, the sample was frozen before the evacuation. Then 0.5 mL of the solution containing appropriate amounts of AlEt3, AlEt2Cl, and pyrrole was transferred under vacuum into the EPR tube and immediately cooled to -196 C by immersion in liquid nitrogen. To give more details of the break-seal technique, it is worth noting that for mixing two or more reagents, a quartz EPR tube containing Cr(acac)3 or Cr(EH)3 solution in C6H12 was soldered to the glass tube containing the appropriate number of branches with break-seals. The glass ampules with the other reagents were soldered to these branches. All parts of this construction were evacuated and sealed off from vacuum line. As a result, all reagents were separated by break-seals and can be mixed and transferred into the EPR tube by breaking the seals. The seals were broken by an inner steel bar using an outer magnet. In the case of ethylene, the calibrated volume with defined pressure of ethylene was soldered to the EPR tube through the break-seal. After breaking the seal, ethylene was completely transferred into the sample by immersion of the EPR tube into liquid nitrogen. The amount of ethylene in the sample corresponded to the amount of ethylene in the calibrated volume. When all the reagents were transferred into the EPR tube and frozen there, the tube was sealed off from the system and warmed to the appropriate temperature by immersion in a thermostat, the reagents were mixed by carefully (26) Brauer, G. Handbuch der Pr€ aparativen Anorganischen Chemie; Ferdinand Enke Verlag: Stuttgart, 1975. (27) Steele, R. B.; Katzakian, A., Jr.; Scigliano, J. J.; Hamel, E. E. (Aerojet-General Corporation) US 3962182, 1976.

Skobelev et al. shaking the tube, and then the tube was stored for the appropriate interval of time at the given temperature. To stop the reaction, the tube was immersed in liquid nitrogen and transferred to a quartz finger Dewar, and the EPR spectrum was recorded. All EPR spectra unless particularly noted were recorded at -196 C. EPR Measurements. EPR spectra were measured on a Bruker ER-200D spectrometer at 9.3 GHz, modulation frequency 100 kHz, modulation amplitude 4 G. A periclase crystal (MgO) with impurities of Mn2þ and Cr3þ, which served as a side reference, was placed into the second compartment of the dual cavity. EPR spectra at -196 C were recorded in liquid N2 using the quartz finger Dewar. EPR spectra of Cr(I) were quantified by double integration with a frozen solution of copper(II) acetylacetonate as a standard at -196 C. The relative accuracy of the quantitative EPR measurements was (30%. The simulation of the EPR spectra was performed using the EasySpin software package.28 Ethylene Oligomerization. Ethylene oligomerization was performed in a steel 0.3 L reactor. Complex Cr(acac)3 or Cr(EH)3 (6 10-5 mol) was introduced into the autoclave in an evacuated sealed glass ampule. Cr(acac)3/AlEt3 and Cr(acac)3/HPyr/AlEt3 Systems. The reactor was evacuated for 2 h at 80 C, cooled to 20 C, and filled with argon, and then a glass ampule with the solution containing appropriate amounts of Cr(acac)3 or Cr(acac)3 and pyrrole was placed into the reactor. The reactor was evacuated for 1 h at 50 C, filled with ethylene, cooled to 20 C, and charged with 100 mL of C6H12 and AlEt3. After setting the oligomerization temperature to 80 C and the ethylene pressure to 12 bar, the reaction was started by breaking the ampule. During the oligomerization, ethylene pressure, temperature, and stirring speed were maintained constant. The experimental unit was equipped with an automatic computer-controlled system for the ethylene feed, maintaining the required pressure, recording the ethylene consumption, and providing the kinetic curve output both in the form of a table and as a graph. After 3 h of reaction, the reaction solution was analized by GC to determine the oligomerization products. The amount of solid PE was determined by weighing. Cr(EH)3/AlEt3 and Cr(EH)3/HPyr/AlEt3 Systems. The reactor was evacuated for 2 h at 80 C, cooled to 20 C, filled with argon, and then charged with 80 mL of Cr(EH)3 or Cr(EH)3/ HPyr solution in C6H12. After that, 20 mL of AlEt3 solution in C6H12 was added, and oligomerization temperature (80 C) and ethylene pressure (12 bar) were established. After 3 h of reaction, the reaction solution was subjected to GC analysis. CrX3/AlEt3/AlEt2Cl and CrX3/HPyr/AlEt3/AlEt2Cl Systems (X = acac, EH). The reactor was evacuated for 2 h at 80 C, cooled to 20 C, and filled with argon, and then a glass ampule with AlEt2Cl solution in C6H12 was placed into the reactor. The reactor was evacuated for 1 h at 50 C, cooled to 20 C, and charged with 80 mL of CrX3 or CrX3/HPyr solution in C6H12. The ampule with AlEt2Cl was broken, the reagents were mixed during one minute, and 20 mL of an AlEt3 solution in C6H12 was added. The reactor was heated to 80 C, and 12 bar of ethylene pressure was established. After 3 h of reaction, the reaction solution was analyzed by GC. Product Analysis. Product analyses were performed on a Chromos GC-1000 gas chromatograph (SE-30 column, 25 m) with a flame-ionization detector. Dodecane (n-C12H26) was used as internal standard in each analysis.

Acknowledgment. This work was supported by the Russian Foundation for Basic Research and by Deutsche Forschungsgemeinschaft, grants 09-03-00485 and 10-03-91330. (28) Stoll, S.; Schweiger, A. J. Magn. Reson. 2006, 178, 42.

In Situ EPR Monitoring of Chromium Species Formed during Cr

Recommend Documents