EPR Monitoring of Vanadium(IV) Species Formed upon Activation of

Nov 11, 2009 - Russian Federation, ‡Novosibirsk State University, 630090, Novosibirsk, Russian Federation, and. §School of Chemistry, University of...
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Organometallics 2009, 28, 6714–6720 DOI: 10.1021/om900515h

EPR Monitoring of Vanadium(IV) Species Formed upon Activation of Vanadium(V) Polyphenolate Precatalysts with AlR2Cl and AlR2Cl/Ethyltrichloroacetate (R = Me, Et) Igor E. Soshnikov,†,‡ Nina V. Semikolenova,† Alexander A. Shubin,†,‡ Konstantin P. Bryliakov,†,‡ Vladimir A. Zakharov,† Carl Redshaw,§ and Evgenii P. Talsi*,†,‡ †

Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, 630090, Novosibirsk, Russian Federation, ‡Novosibirsk State University, 630090, Novosibirsk, Russian Federation, and § School of Chemistry, University of East Anglia, Norwich NR4 7TJ, U.K. Received June 17, 2009

Reactions of the vanadyl polyphenolate complexes {VO[p-tert-butylcalix[4]arene(O)3(OMe)]} (1) and [VO(OArCH2Ar0 CH2OAr)]2 (Ar = 4,6-tert-butylphenol; Ar0 = 4-tert-butylphenol) (2) with AlR3 and AlR2Cl, in the absence and in the presence of reactivator (ethyltrichloroacetate, ETA), were monitored by EPR (R = Me, Et). It was shown, that vanadium(IV) complexes with proposed structures L1VIVR(AlR3) and L1VIVR(AlR2Cl) are formed upon interaction of 1 with AlR3 and AlR2Cl, respectively (20 C, L1 is the initial oxo-calix[4]arene ligand). Similarly, vanadium(IV) complexes with proposed structures L2VIVR(AlR3) and L2VIVR(AlR2Cl) are formed upon interaction of 2 with AlR3 and AlR2Cl (L2 is the initial ligand of 2). The growth of the concentration of L1VIVR(AlR2Cl) and L2VIVR(AlR2Cl) correlates with the increase of the ethylene polymerization activity of the catalyst systems 1,2/AlR2Cl/ETA. It is proposed therefore that vanadium(IV) species of the type L1VIVR(AlR2Cl) and L2VIVR(AlR2Cl) can be the active species (or their immediate precursors) of the catalyst system 1,2/AlR2Cl/ETA.

Introduction Vanadium-based olefin polymerization systems have been reported recently by a number of groups (see ref 1-23 and refs therein). The vanadium(V) complexes bearing chelating *Corresponding author. Fax: þ7 383 3308056. E-mail: [email protected]. (1) Henderson, R. A.; Hughes, D. L.; Janas, Z.; Richards, R. L.; Sobota, P.; Szafert, S. J. Organomet. Chem. 1998, 554, 195. (2) Desmangles, N.; Gambrotta, S.; Bensimon, C.; Davis, S.; Zahalka, H. J. Organomet. Chem. 1998, 562, 53. (3) Reardon, D.; Conan, F.; Gambarotta, S.; Yap, G.; Wang, Q. J. Am. Chem. Soc. 1999, 121, 9318. (4) Nomura, K; Sagara, A.; Imanishi, Y. Chem. Lett. 2001, 36. (5) Milione, S.; Cavallo, G.; Tedesco, C.; Grassi, A. J. Chem. Soc., Dalton Trans. 2002, 1839. (6) Hagen, H.; Boersma, J.; van Koten, G. Chem. Soc. Rev. 2002, 31, 357. (7) Nakayama, Y.; Bando, H.; Sonobe, Y.; Suzuki, Y.; Fujita, T. Chem. Lett. 2003, 32, 766. (8) Gambarotta, S. Coord. Chem. Rev. 2003, 237, 229. (9) Wang, W.; Yamada, J.; Fujiki, M.; Nomura, K. Catal. Commun. 2003, 4, 159. (10) Nakayama, Y.; Bando, H.; Sonobe, Y.; Fujita, T. J. Mol. Catal. A 2004, 213, 141. (11) Nakayama, Y.; Bando, H.; Sonobe, Y.; Fujita, T. Bull. Chem. Soc. Jpn. 2004, 77, 617. (12) Redshaw, C.; Warford, L.; Elsegood, M. R. J.; Dale, S. H. Chem. Commun. 2004, 1954. (13) Tomov, A. K.; Gibson, V. C.; Zaher, D.; Elsegood, M. R. J.; Dale, S. H. Chem. Commun. 2004, 1956. (14) Wang, W.; Nomura, K. Macromolecules 2005, 38, 5905. (15) Redshaw, C.; Rowan, M. A.; Homden, D. M.; Dale, S. H.; Elsegood, M. R. J.; Matsui, S.; Matsuura, S. Chem. Commun. 2006, 3329. (16) Cuomo, C.; Milione, S.; Grassi, A. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 3279. pubs.acs.org/Organometallics

Published on Web 11/11/2009

polyphenolate ligands (complexes 1 and 2, Scheme 1) serve as very active, thermally robust vanadium-based precatalysts for ethylene homo- and ethylene/propylene copolymerization.12,17 The best results were obtained using dimethylaluminum chloride (AlMe2Cl) or diethylaluminum chloride (AlEt2Cl) as activator and ethyltrichloroacetate (ETA) as reactivator. The nature of the active species formed upon activation of complexes 1 and 2 with AlMe2Cl or AlEt2Cl is unclear. Recently, we have reported an EPR spectroscopic study of the interaction of vanadium complexes 3-5 (Scheme 1) with AlEt3 (AlEt2Cl) and ETA. It was suggested that vanadium(IV) species of the type LVIVR(AlR2Cl) are the active species (or their immediate precursors) of the systems (3-5)/AlEt2Cl (L is a polydentate ligand).24,25 (17) Redshaw, C.; Rowan, M. A.; Warford, L.; Homden, D. M.; Arbaoui, A.; Elsegood, M. R. J.; Dale, S. H.; Yamato, T.; Casas, C. P.; Matsui, S.; Matsuura, S. Chem.;Eur. J. 2007, 13, 1090. (18) Homden, D.; Redshaw, C.; Hughes, D. L. Inorg. Chem. 2007, 46, 10827. (19) Abbo, H. S.; Mapolie, S. F.; Darkwa, J.; Titinchi, S. J. J. J. Organomet. Chem. 2007, 692, 5327. (20) Jabri, A.; Korobkov, I.; Gambarotta, S.; Duchateau, R. Angew. Chem., Int. Ed. 2007, 46, 1. (21) Onishi, Y.; Katao, S.; Fujiki, M.; Nomura, K. Organometallics 2008, 27, 2590. (22) Blalek, M.; Czaja, K. J Polym. Sci., Part A: Polym. Chem. 2008, 46, 6940. (23) Wu, J. Q.; Pan, L.; Li, Y. G.; Liu, S. R.; Li, Y. S. Organometallics 2009, 28, 1817. (24) Soshnikov, I. E.; Semikolenova, N. V.; Bryliakov, K. P.; Shubin, A. A.; Zakharov, V. A.; Redshaw, C.; Talsi, E. P. Macromol. Chem. Phys. 2009, 210, 542. (25) Soshnikov, I. E.; Semikolenova, N. V.; Bryliakov, K. P.; Zakharov, V. A.; Redshaw, C.; Talsi, E. P. J. Mol. Catal. A 2009, 303, 23. r 2009 American Chemical Society

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Scheme 1. Structures of Vanadium Precatalysts Studieda

a

In the complexes 1 and 3, tert-butyl groups on the phenyl rings were omitted for clarity.

In this work, we have undertaken the EPR spectroscopic study of V(IV) species formed in the systems 1,2/AlR3, 1,2/ AlR2Cl, and 1,2/AlR2Cl/ETA (R = Me, Et). The EPR spectra of the V(IV) species formed in these systems are more intense and better resolved than those in the previously studied catalyst systems. This allowed us to derive more reliable information on the structure of vanadium(IV) species present in the reaction solution. The main goal was to ascertain the nature of the active species (or their precursors) of the catalyst systems (1,2)/AlEt2Cl/ETA.

Results and Discussion Reaction of 1 with AlEt2Cl. The EPR spectrum recorded 10 min after mixing oxo-calix[4]arene complex 1 with AlEt2Cl at -70 C ([AlEt2Cl]:[1] = 10:1, [1] = 10-2 M, toluene) displays intense resonances of complex 1a and relatively weak resonances of complex 1b (Figure 1A). Storing the sample in “A” 20 min at -40 C leads to the EPR spectrum, which shows resonances of complex 1b and complex 1c (Figure 1B). Further warming this sample to room temperature gives rise to the EPR spectrum, exhibiting only resonances of 1c (Figure 1C). The concentration of 1c decreases with a half-decay time of 30 min at 20 C via the reduction of vanadium(IV). EPR parameters of 1a-c are collected in Table 1. The precise parameters from the experimental EPR spectra were derived using theoretical spectra simulation. One example of such simulation is presented in Figure 1D by a dotted line (for other examples see the Supporting Information). The quantitative EPR measurements show that up to (50 ( 15)% of the starting vanadium(V) complex was converted into vanadium(IV) species upon interaction with AlEt2Cl. At high [AlEt2Cl]:[1] ratio (100:1), that is, under conditions more approaching those employed for real polymerization, the EPR spectrum of the sample 1/AlEt2Cl recorded 10 min after storing at -50 C displays resonances of 1b (Figure 2A). Further storing this sample at -10 C leads to the EPR spectrum exhibiting resonances of 1b and 1c (Figure 2B), and only 1c is observed at higher temperatures (Figure 2C). The maximum concentration of vanadium(IV) was (60 ( 20)% of the initial vanadium complex concentration. However, the concentration of vanadium(IV) species

Figure 1. EPR spectra (-196 C) of the 1/AlEt2Cl sample in toluene ([AlEt2Cl]:[1] = 10:1, [1] = 10-2 M) after various treatments: 10 min after mixing of reagents at -70 C (A); 20 min after storing sample in “A” at -40 C (B); 10 min after storing sample in “B” at 20 C (C). Dotted line shows simulated spectrum of 1c with parameters presented in Table 1 and in the Supporting Information (D).

sharply decreases upon warming the sample from -50 to 20 C (Figures 2A-D). One day after mixing the reagents at 20 C, no EPR signals were observed in the sample due to the reduction of vanadium(IV) into EPR-silent vanadium species. The addition of ETA slows the reduction of vanadium(IV). The EPR spectrum of the sample ([AlEt2Cl]:[ETA]: [1] = 100:100:1, [1] = 10-2 M, toluene, -50 C) displays resonances of complexes 1b and 1c (Figure 3A). Warming this sample to 5 C gives rise to the disappearance of 1b, and

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only 1c is observed (Figure 3B). In contrast to the sample containing no reactivator, the decay of 1c in the sample containing ETA at 20 C occurs much slower (compare Figures 2A-D and 3A-C). Moreover, storing the sample containing ETA for 1 day at 20 C results in the conversion of 1c back to 1b (Figure 3D), but not to the reduction of vanadium(IV). Apparently, the calix[4]arene ligand is not released or essentially changed during the course of transformation 1 f 1a f 1b f 1c. For the reverse, it is difficult to explain in what way 1b first transforms to 1c, then after prolong storing, 1c converts back to 1b (Figure 3A-D). If it is assumed that calix[4]arene ligand remains intact upon activation, and AlEt2Cl attacks the VdO bond of 1, the observed transformation 1 f 1a f 1b f 1c can be hypothetically presented by Scheme 2. To get additional evidence in favor of the structures presented in Scheme 2, we have compared EPR spectra of the intermediates formed in the systems 1/ AlEt2Cl, 1/AlMe2Cl, and 1/AlEt3. Reaction of 1 with AlMe2Cl. The EPR spectrum recorded 5 min after mixing 1 with AlMe2Cl at -70 C ([AlMe2Cl]:[1] = 10:1, [1] = 10-2 M) displays resonances of two types of complexes, namely, 1bMe and 1cMe (Figure 4A). Only 1cMe is

observed at -30 C (Figure 4B). The EPR spectrum of 1bMe is partially obscured by the EPR spectrum of 1cMe; therefore, only high-field resonances of 1bMe are observed (Figure 4A). These resonances coincide with those for 1b. The EPR parameters of 1cMe are very similar but not identical to those for 1c (Table 1). This difference is in favor of the incorporation of various alkyls into 1cMe and 1c (L1VIVMe(S) and L1VIVEt(S), respectively (L1 is calix[4]arene ligand, S is donor molecule occupying the sixth coordination site of vanadium)). Both 1bMe and 1b can be assigned to vanadium(IV) species of the type L1VIVCl(S). If this assumption is valid, then the complex related to 1b should be absent in the system 1/AlEt3, which contains no chlorine atoms. Indeed, EPR spectra of the system 1/AlEt3 have confirmed this prediction. Reaction of 1 with AlEt3. The EPR spectrum recorded 10 min after mixing 1 with AlEt3 at -70 C ([AlEt3]:[1] = 10:1, [1] = 10-2 M) is a superposition of the EPR spectra of two vanadium(IV) complexes, 1a0 and 1c0 (Figure 5A). Warming the sample in “A” results in the disappearance of complex 1a0 ,

Figure 2. EPR spectra (-196 C) of the 1/AlEt2Cl sample in toluene ([AlEt2Cl]:[1] = 100:1, [1] = 10-2 M) after various treatments: 10 min after mixing of reagents at -50 C (A); 10 min after storing sample in “A” at -10 C (B); 10 min after storing sample in “B” at 5 C (C); 5 min after storing sample in “C” at 20 C (D).

Figure 3. EPR spectra (-196 C) of the 1/AlEt2Cl/ETA sample in toluene ([AlEt2Cl]:[ETA]:[1] = 100:100:1, [1] = 10-2 M) after various treatments: 10 min after mixing of reagents at -50 C (A); 10 min after storing sample in “A” at 5 C (B); 10 min after storing sample in “B” at 20 C (C); 24 h after storing sample in “C” at 20 C (D).

Table 1. Selected EPR Spectroscopic Data (-196 C) for Vanadium(IV) Species Formed in the 1/AlR3 and 1/AlR2Cl Systems in Toluene (R = Me, Et) complex IV

L1V (OAlEtCl) L1VIVCl(AlEt2Cl) L1VIVEt(AlEt2Cl) L1VIVCl(AlMe2Cl) L1VIVMe(AlMe2Cl) L1VIV(OAlEt2) L1VIVEt(AlEt3)

1a 1b 1c 1bMe 1cMe 1a0 1c0

g1 ( 0.002

A1 ( 1, G

g2 ( 0.002

A2 ( 1, G

g3 ( 0.002

A3 ( 1, G

1.980 1.984 1.978

36 41 34

1.955 1.960 1.949

50 49 47

1.982 1.979 1.974

33 34 25

1.955 1.964 1.949

45 46 45

1.936 1.947 1.969 1.947 1.967 1.932 1.969

153 160 148 160 149 151 145

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Scheme 2. Proposed Transformations of 1 after Interaction with AlR3-xClx (x = 0, 1; R = Me, Et)a

a

In the 1b-type and 1c-type species AlR3-xClx is coordinated to the vanadium center through an R or Cl bridge.

Figure 4. EPR spectra (-196 C) of the 1/AlMe2Cl sample in toluene ([AlMe2Cl]:[1] = 10:1, [1] = 10-2 M) after various treatments: 5 min after mixing of reagents at -70 C (A); 10 min after storing sample in “A” at -30 C.

and only complex 1c0 is observed (Figure 5B and C, Table 1). The EPR parameters of 1a0 are very close but not identical to those of 1a, and the EPR parameters of 1c0 are close but not identical to 1c (Table 1). As predicted, the system 1/AlEt3 contains no complexes resembling 1b, confirming the assignment of 1b to a complex of the type L1VIVCl(S). The structures of complexes 1a and 1a0 can be tentatively presented as L1VIV(OAlEtCl) and L1VIV(OAlEt2), respectively. These species exist in the reaction solution only at the early stage of activation and do not participate in polymerization. More interesting is comparing the structures of complexes 1c0 and 1c. At 20 C, 1c0 is present in the inert system 1/AlEt3, and 1c in the active system 1/AlEt2Cl. The small difference in the EPR parameters of 1c0 and 1c can be caused by the various nature of the ligand S bound to the sixth coordination site of vanadium complexes L1VIVEt(AlEt2Cl) (1c) and L1VIVEt(AlEt3) (1c0 ). Reaction of 2 with AlEt2Cl and AlEt3. The EPR spectrum of the sample ([AlEt2Cl]:[2] = 100:1, [2] = 10-2 M) recorded 10 min after mixing of the reagents at -50 C exhibits weak resonances of complex 2c. Warming this sample from -50 to -15 C leads to the increase of concentration of 2c (Figure 6A, B). Further storing the sample at 20 C leads to the decrease of 2c concentration (Figure 6C-E). The maximum concentration of 2c corresponds to (80 ( 20)% of the starting concentration of 2.

Figure 5. EPR spectra (-196 C) of the 1/AlEt3 sample in toluene ([AlEt3]:[1] = 10:1, [1] = 10-2 M) after various treatments: 10 min after mixing of reagents at -70 C (A); 10 min after storing sample in “A” at -60 C (B); 10 min after storing sample in “B” at -50 C (C).

Addition of ETA prevents the decay of 2c. In contrast to the sample without ETA (Figure 6C, D), the concentration of 2c increases, but does not further decrease upon storing the sample for 10 min at 20 C (Figure 7B, C). It is worth noting that the EPR spectra of vanadium(IV) species formed upon activation of 1 and 2 with AlEt2Cl are very different (Tables 1 and 2). Hence, the initial polyphenolate ligands of 1 and 2 are not released in the course of activation. The EPR spectra of the system 2/AlEt3 display only resonances of complex 2c0 , resembling those of 2c (Figure 8A-C, Table 2). It is seen that the concentration of 2c0 decreases upon warming the sample due to the reduction of vanadium(IV). Prolonged storing of the sample at room temperature leads to the formation of low-valent vanadium species (Figure 8D). A similar spectrum was previously observed in the systems 4(5)/ AlEt3 and assigned to LVIIEt2 species.25 However, more careful analysis of the literature data shows that the vanadium species observed belong to the sandwich-like zerovalent vanadium complex [V0(toluene)2].26 (26) See, for example: (a) Henrici-Olive, G.; Olive, S. Z. Phys. Chem. (Frankfurt) 1967, 56, 223. (b) Andrews, M. P.; Mattar, S. M.; Ozin, G. A. J. Phys. Chem. 1986, 90, 1037. (c) Nozawa, Y.; Takeda, M. Bul. Chem. Soc. Jpn. 1969, 42, 2431.

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Soshnikov et al. Table 2. Selected EPR Spectroscopic Data (-196 C) for Vanadium(IV) Species Formed in the 2/AlR3 and 2/AlR2Cl Systems in Toluene (R = Me, Et)

L2V Et(AlEt2Cl) 2c L2VIVMe(AlMe2Cl) 2cMe 2c0 L2VIVEt(AlEt3) L2VIVMe(AlMe3) (2c0 )Me

Figure 6. EPR spectra (-196 C) of the 2/AlEt2Cl sample in toluene ([AlEt2Cl]:[2] = 100:1, [2] = 10-2 M) after various treatments: 10 min after mixing of the reagents at -50 C (A); 10 min after storing sample in “A” at -15 C (B). Sample in “B” at various moments of time after storing at 20 C: 1 min (C), 10 min (D), 24 h (E).

Figure 7. EPR spectra (-196 C) of the 2/AlEt2Cl/ETA sample in toluene ([AlEt2Cl]:[ETA]:[2] = 100:100:1, [2] = 10-2 M) after various treatments: 10 min after mixing of reagents at -70 C (A); 10 min after storing sample in “A” at -15 C (B). Sample in “B” at various moments of time after storing at 20 C: 10 min (C), 24 h (D).

The EPR parameters of 2c0 differ from those for 2c (Table 2). As for the related calix[4]arene system, 2c0 can be assigned to L2VIVEt(AlEt3) and 2c to L2VIVEt(AlEt2Cl), where L2 is the initial ligand (Scheme 3). Reaction of 2 with AlMe2Cl and AlMe3. The EPR spectrum of the system 2/AlMe2Cl ([AlMe2Cl]:[2] = 100:1, [2] = 10-2 M, 20 C) displays resonances of one type of vanadium(IV) species. This complex, denoted as 2cMe, differs from complex 2c observed in the system 2/AlEt2Cl (Table 2). Therefore, as in the case of the calix[4]arene system, species 2c and 2cMe

2.001 2.004 1.995 1.995

73 71 67 62

)

g^ ( 0.002 A^ ( 1, G g ( 0.002 A ( 1, G )

complex IV

1.928 1.930 1.933 1.933

185 182 174 170

Figure 8. EPR spectra (-196 C) of the 2/AlEt3 sample in toluene ([AlEt3]:[2] = 100:1, [2] = 10-2 M) after various treatments: 10 min after mixing of reagents at -30 C (A); 10 min after storing sample in “A” at 0 C (B). Sample in “B” at various moments of time after storing at 20 C: 10 min (C), 24 h (D).

can be assigned to L2VIVEt(AlEt2Cl) and L2VIVMe(AlMe2Cl), respectively. The EPR parameters of 2cMe formed in the system 2/AlMe2Cl differ from those of complex (2c0 )Me formed in the system 2/AlMe3 (Table 2); therefore (2c0 )Me can be presented as L2VIVMe(AlMe3) (Scheme 3). Possible Active Species (or their precursors) of the Systems 1,2/AlEt2Cl/ETA. As was suggested above on the basis of EPR data (Figures 3A-D), the activator (AlEt2Cl) reacts with 1 via attack on the VdO bond of 1, and the strongly bound tetradentate calix[4]arene ligand remains intact upon activation. This should simplify the identification of possible vanadium(IV) species formed in the reaction solution. For the previously studied systems 3-5/AlEt2Cl, we could not exclude the possibility of the transformation of the initial ligand upon activation. Moreover, the intense and wellresolved resonances here allow us to find differences in the EPR spectra of the related vanadium(IV) species formed in the 1,2/AlMe2Cl and 1,2/AlEt2Cl systems. This supports the incorporation of an alkyl moiety into the composition of vanadium(IV) species 1c and 2c, which are formed in the systems 1,2/AlEt2Cl at 20 C. On the basis of the obtained EPR data (Tables 1 and 2), we can assign 1c and 2c to L1VIVEt(AlEt2Cl) and L2VIVEt(AlEt2Cl), respectively. According to ethylene polymerization studies, the activities of the catalyst systems 1/AlEt2Cl/ETA and 2/AlEt2Cl/ ETA were noticeably higher than that of the systems 1/ AlEt2Cl and 2/AlEt2Cl, containing no reactivator (Table 3 runs 1, 2 and 4, 5). At high polymerization temperature the

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Scheme 3. Proposed Structures of the VIV Species 2c, 2cMe, 2c0 , and (2c0 )Mea

a

In the 2c-type species AlR3-xClx is coordinated to the vanadium center through a Et(Me) or Cl bridge.

Table 3. Ethylene Polymerization Data for Complexes 1 and 2a precatalyst cocatalyst reactivator run (μmol/L) ([AlEt2Cl]:[V]) ([ETA]:[V]) T, C PE yield b activity c 1 2 3d 4 5 6 7d 8 9e

1 (13.4) 1 (13.4) 1 (13.4) 2 (6.7) 2 (13.4) 2 (6.7) 2 (6.7) 3 (13.4)

1000 1000 1000 0 1000 1000 500 500 500 0 500 500 500 500 500 500 2.0 mmol 2.0 mmol

80 80 80 70 70 25 70 80 80

3580 240 0 4950 110 1350 0 1500 0

490 30 0 660 40 110 0 640 0

a Polymerization in toluene (150 cm3), 2 bar of C2H4, for 30 min. b kg PE 3 mol V-1 3 bar-1. c kg PE 3 mol V-1 3 bar-1 3 min-1 (calculated according to the kinetic curve from the PE yield for 2 min). d AlEt3 was used as activator. e Control experiment (without vanadium-based precatalyst).

catalysts 1/AlEt2Cl/ETA and 2/AlEt2Cl/ETA exhibited high initial activities that fall with the increase of polymerization time (Figure 9, curves “A” and “B”). At low temperature, close to EPR measurements, the decrease of the activity of the system 2/AlEt2Cl/ETA is less rapid (Figure 9, curve “C”). The growth of the activity of the catalyst systems studied in the presence of ETA correlates with the increase of concentration of complexes 1c and 2c. The drop of the polymerization activity with time is paralleled by the drop of the concentration of complexes 1c and 2c. Concentration of complex 2c in the system 2/AlEt2Cl/ETA first increases with time and then drops (20 C, Figure 7C, D). A similar trend was observed for the activity of this system at 20 C (Figure 9C). Hence, 1c and 2c with the proposed structures L1VIVEt(AlEt2Cl) and L2VIVEt(AlEt2Cl), correspondingly, are possible active species (or their precursors) of the catalyst systems 1/AlEt2Cl/ETA and 2/AlEt2Cl/ETA. Among the several redox states available to vanadium, the trivalent and tetravalent states are considered as the most probable ones for the active species of polymerization.6-8,28 We cannot exclude the participation of V(III) species in the polymerization of ethylene by the systems 1,2/AlEt2Cl/ETA. However, the key role of V(IV) species seems to be more probable. The activity of the catalyst system 2/AlEt2Cl/ETA is higher than that of the system 3/AlEt2Cl/ETA with similar structure of the precatalyst (Table 3, run 8). According to EPR data, the maximum concentration of V(IV) species is a factor of 2.5 larger for the former system than for the latter. This indirectly supports the assumption that vanadium(IV) species of the type L1VIVEt(AlEt2Cl) are active species of polymerization of the catalyst systems studied herein. The systems 1/AlEt3/ETA and 2/AlEt3/ETA are inert in ethylene polymerization (Table 3 runs 3 and 7). Therefore, complexes 1c0 and 2c0 , with proposed structures L1VIVEt(AlEt3) and L2VIVEt(AlEt3), are poor polymerizing agents. The sharp difference in the reactivity of 1c and 1c0 (2c and 2c0 ) can be caused by various degrees of polarization of the V-Et bond upon interaction with AlEt2Cl and AlEt3 (the former

Figure 9. Time dependence of ethylene polymerization activity of complexes 1 and 2 activated with AlEt2Cl/ETA: [1]:[AlEt2Cl]:[ETA] = 1:1000:1000 at 80 C (A), [2]:[AlEt2Cl]: [ETA] = 1:500:500 at 70 C (B), [2]:[AlEt2Cl]:[ETA] = 1:500:500 at 25 C (runs 1, 3, and 5 in Table 3) (C).

obeys higher Lewis acidity)27. Further studies are needed to verify this assumption.

Conclusions Vanadium(IV) complexes formed in the systems 1,2/AlR3, 1,2/AlR2Cl, and 1,2/AlR2Cl/ETA (R = Me, Et) were monitored by EPR. It was shown that at 20 C complexes with the proposed structures L1VIVR(AlR2Cl) and L2VIVR(AlR2Cl) are predominant vanadium(IV) species in the systems 1/ AlR2Cl and 2/AlR2Cl (L1 and L2 are polydentate ligands of 1 and 2). The concentration of these vanadium(IV) species correlates with ethylene polymerization activity for the 1,2/ AlR2Cl/ETA systems. Hence, complexes of the type L1VIVR(AlR2Cl) and L2VIVR(AlR2Cl) could be the active species (or their immediate precursors) of the catalyst systems 1,2/ AlR2Cl/ETA. The related complexes L1VIVR(AlR3) and L2VIVR(AlR3), formed in the systems 1,2/AlR3/ETA are inert toward ethylene polymerization. The nature of the dramatic effect of the chlorine atom on the polymerization activity of vanadium(IV) alkyl species is still unclear.

Experimental Section Materials. Toluene was dried over molecular sieves (4 A˚), purified by refluxing over sodium metal, and distilled under dry (27) Talsi, E. P.; Semikolenova, N. V.; Panchenko, V. N.; Sobolev, A. P.; Babushkin, D. E.; Shubin, A. A.; Zakharov, V. A. J. Mol. Catal. A 1999, 139, 131. (28) Lorber, C.; Wolff, F.; Choukroun, R.; Vendier, L. Eur. J. Inorg. Chem. 2005, 14, 2850.

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argon. Solids were transferred and stored in a glovebox. All experiments were carried out in sealed high-vacuum systems using break-seal techniques. Complexes 117 and 212 were synthesized as described. Commercial samples of triethylaluminum (AlEt3), trimethylaluminum (AlMe3), diethylaluminum chloride (AlEt2Cl), and dimethylaluminum chloride (AlMe2Cl) were used as solutions in toluene (heptane/toluene mixture (1:1) in the AlMe2Cl case) with various Al concentrations. Ethyltrichloroacetate (ETA) was purchased from Aldrich. Preparation of Samples for EPR Measurements. A weighed amount (∼5  10-6 mol) of 1 (or 2) was placed into a dried, argon-filled EPR tube, equipped with fine glass break-seals. The system with complex was evacuated to 2  10-2 Torr and sealed off from the vacuum line. Then 0.5 cm3 of the required solution with AlR3-xClx was transferred under vacuum into the EPR tube (length 200 mm, d = 5 mm) and immediately cooled to -196 C (liquid nitrogen temperature). For monitoring the reaction of 1 (or 2) with AlR3-xClx, the EPR tube was sealed off from the system and warmed to -70 C by immersion in a thermostat (a Dewar with cooled ethanol and thermometer), the reagents were mixed by carefully shaking the EPR tube in the thermostat, and then the tube was stored for the appropriate interval of time at -70 C or other given temperature. To stop the reaction, the tube was immersed in liquid nitrogen and transferred to the quartz Dewar, and the EPR spectrum was recorded. All the EPR spectra 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. Periclase crystal (MgO) (29) Shubin, A. A.; Zhidomirov, G. M. Zh. Struct. Khim. 1989, 30, 67.

Soshnikov et al. with impurities of Mn2þ and Cr3þ, which served as a side reference, was placed into the second compartment of the dual cavity. EPR spectra were quantified by double integration with Cu(acac)2 toluene/chloroform (3:2) solution as standard. The relative accuracy of the quantitative EPR measurements was (30%. EPR spectra were simulated using the modified version of the program ESR1.29 Ethylene Polymerization Procedure. Ethylene polymerization was performed in a steel 0.3 L reactor. Complex 1 (2) (2  10-6 mol) was introduced into the autoclave in an evacuated sealed glass ampule. The reactor was evacuated at 80 C, cooled to 20 C, and then charged with the freshly prepared solution of AlR2Cl (R = Me, Et) and ETA in toluene (150 cm3). After setting up the polymerization temperature and the ethylene pressure, the reaction was started by breaking the ampule with complex 1 (2). During the polymerization, ethylene pressure, temperature, and stirring speed were all 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.

Acknowledgment. This work was supported by the Russian Fund of Basic Research, grant 09-03-00485. The authors are grateful to T. M. Ivanova for help with the EPR sample preparation. Supporting Information Available: Results of the EPR spectra simulation for 1a, 1b, 1c, 1cMe, and 1c0 . This material is available free of charge via the Internet at http://pubs.acs.org.