1H and 2H NMR Spectroscopic Characterization of Heterobinuclear

Soshnikov , I. E.; Semikolenova , N. V.; Bryliakov , K. P.; Zakharov , V. A.; ...... Igor E. Soshnikov , Nina V. Semikolenova , Konstantin P. Bryliako...
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H and 2H NMR Spectroscopic Characterization of Heterobinuclear Ion Pairs Formed upon the Activation of Bis(imino)pyridine Vanadium(III) Precatalysts with AlMe3/[Ph3C]+[B(C6F5)4]− and MAO Igor E. Soshnikov,†,‡ Nina V. Semikolenova,† Artem A. Antonov,†,‡ 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 Novosibirsk State University, 630090, Novosibirsk, Russian Federation

‡

S Supporting Information *

ABSTRACT: Until recently, attempts to characterize vanadium(III) species formed upon the activation of vanadium(III) αolefin polymerization precatalysts with AlMe3/[Ph3C]+[B(C6F5)]4− or MAO were unsuccessful. In this contribution, 1H and 2H NMR spectroscopy was used to study the activation of bis(imino)pyridine vanadium(III) chloride LVIIICl3 {L = [2,6(ArNCMe)2C5H3N], Ar = 2,6-iPr2C6H3; 2,6-Me2C6H3; 2,4,6Me3C6H2; 3,5-F2C6H3} with AlMe3/[Ph3C]+[B(C6F5)]4− or MAO. Formation of heterobinuclear ion pairs of the type [L(R)V III (μ-R) 2 AlMe 2 ] + [A] − ([A] − = [B(C 6 F 5 )] 4 − or [MeMAO]−, R = Me or Cl) was observed, which are the most likely direct precursors of active sites of ethylene polymerization.

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afford the active catalyst for ethylene polymerization.3,5,26 Gambarotta and co-workers found that the reaction of 1 with 1 equiv of MeLi or 2 equiv of MAO in toluene resulted in the formation of X-ray-characterized V(III) complex 1a (Chart 1). The activity of the system 1a/MAO was the same as that of the system 1/MAO, and identical polymers were produced with both systems. Therefore, it was suggested that 1 is the precursor of 1a, which is in turn a precursor of the catalytically active species.3 However, the active species of the catalyst system 1/MAO were not identified. Herewith, a 1H and 2H NMR spectroscopic detection of heterobinuclear vanadium(III) ion pairs formed upon the activation of LVCl3 precatalysts 1−4 (Chart 1) with AlMe3/ [Ph3C]+[B(C6F5)]4− and MAO is presented.

INTRODUCTION In spite of the undiminishing interest in the synthesis of new vanadium polymerization catalysts,1−41 little is known about the polymerization mechanism and the nature of active species. The major technical drawback is the paramagnetism of the vanadium alkyl compounds, precluding a straightforward NMR spectroscopic characterization. Data on the chemistry and X-ray structure of vanadium alkyls are limited with respect to welldeveloped chemistry of group 4 complexes.42−46 For catalyst systems based on VIII precatalysts, complexes of trivalent vanadium are usually implicated as active sites of polymerization.6,8,41 For the catalyst systems based on vanadium(V) precatalysts, trivalent, tetravalent, and pentavalent alkyl vanadium complexes have been considered as potential active species of polymerization.8,37 Recently, EPR spectroscopic investigations of compounds formed upon the reactions of a number of high-valent (IV, V) vanadium aryloxide complexes with AlR3 and AlR2Cl (R = Et, Me) in the presence of reactivator (ethyltrichloroacetate (ETA)) were reported.28−30 EPR spectra consistent with VIV species of the type L′VIV(R)(AlR3) and L′VIV(R)(AlR2Cl) were observed, where L′ is a modified initial ligand. One could not exclude that vanadium(IV) species, observed in these EPR experiments, might precede the catalytically active vanadium(III) species. However, until now there have been no reported attempts of NMR spectroscopic detection of vanadium(III) species formed in the active polymerizing systems. Previously, treatment of bis(imino)pyridine vanadium(III) complex LVCl3 (1) {L = [2,6-(ArNCMe)2C5H3N], Ar = 2,6-iPr2C6H3, Chart 1} with MAO or AlEt2Cl was shown to © 2014 American Chemical Society

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RESULTS AND DISCUSSION The activation of complexes 1−4 with MAO leads to active ethylene polymerization catalysts (Table 1). Complexes 2 and 3 (runs 2 and 3, Table 1) afford much more active catalysts than complexes 1 and 4 (runs 1 and 4). Low molecular weight polymers were obtained for 1−3, and liquid oligomeric products for 4. The activity of 2 is essentially higher for MAO than for AlMe3/[Ph3C]+[B(C6F5)4]− as an activator (runs 2 and 5, Table 1). For both activators polymers with similar Mw, Mn, and MWD were formed. Previously, it was shown that ion pairs formed upon the activation of metallocene and postmetallocene catalysts with MAO and AlMe 3/ Received: March 13, 2014 Published: May 8, 2014 2583

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Chart 1

[Ph3C]+[B(C6F5)4]− feature the same cationic parts.42−46 For the present NMR study, we mostly focused on the systems 1− 4/AlMe3/[Ph3C]+[B(C6F5)4]−, since the 1H NMR resonances of the ion pairs formed in the systems 1−4/MAO were partially obscured by intense peaks of MAO and toluene. The starting paramagnetic complexes 1−4 display 1H NMR resonances in the range +110 to −32 ppm. As an example, Figure 1A shows the 1H NMR spectrum of the most active complex (2) in CDCl3 at −20 °C (Table 2). The 1H NMR spectra of 1, 3, and 4 can be found in the Supporting Information (Figures S1−S4, Tables S1−S4). The 1 H resonances of complexes 1−3 were previously assigned on the basis of integration and comparison of the spectra of different complexes, containing various substituents in the benzene rings.5,26 Using the same approach, we have assigned the resonances of complex 4 (Table S4). Complex 1a prepared as described in ref 3 displayed no observable 1H NMR resonances. The System 2/AlMe3/[Ph3C]+[B(C6F5)4]−. Mixing the components of the sample 2/AlMe3/[Ph3C]+[B(C6F5)4]− ([V]:[Al]:[B] = 1:10:1.2) in toluene-d8 for several minutes at −20 °C results in the conversion of 2 into a new complex, 2b

Figure 1. 1H NMR spectrum (chloroform-d, −20 °C) of a saturated solution of 2 (A). 1H NMR spectrum (toluene-d8, −20 °C) of 2/ AlMe3/[Ph3C]+[B(C6F5)4]− ([V]:[Al]:[B] = 1:10:1.2, [2] = 0.01 M) (B). 2H NMR spectrum (toluene, −20 °C) of 2/Al(CD3)3/ [Ph3C]+[B(C6F5)4]− ([V]:[Al]:[B] = 1:10:1.2, [2] = 0.01 M); asterisks mark peak of CH3C6H4D (C). 1H NMR spectrum (toluene-d8, −20 °C) of 2/MAO ([V]:[Al] = 1:20, [2] = 0.01 M) (D). Spectra B−D were measured 10 min after mixing the reagents.

(Figure 1B), assigned to a heterobinuclear ion pair, [L(R)VIII(μ-R)2AlMe2]+[B(C6F5)4]− (R = Me or Cl). The 1H NMR spectrum of 2b displays the following resonances (toluene-d8, −20 °C): δ 80.2 (6H, Δν1/2 = 1100 Hz, N−CH3); 47.0 (2H,

Table 1. Data on Ethylene Polymerization Catalyzed by 1−4a run

precatalyst

time, min

P(C2H4), bar

m(PE), g

activity, gPE/(mmolV·bar·h)

1 2 3 4 5

1 2 3 4 2c

60 30 30 30 30

2 1 2 2 2

2.2 15.5 13.5 1.0b 3.3

550 15 500 6750 500 1 650

Mw × 10−3

Mn × 10−3

Mw/Mn

4.3

1.6

2.7

3.7

1.8

2.1

a Conditions: toluene (50 mL), 60 °C, 2 μmol of V, activator: MAO ([Al]:[V] = 500:1) for runs 1−4 and AlMe3/[Ph3C]+[B(C6F5)4]− ([Al]:[B]:[V] = 100:1.2:1) for run 5. bLiquid oligomeric products were obtained (ca. 70 mol % of 1-butene and 30 mol % of 1-hexene). cActivator: AlMe3/ [Ph3C]+[B(C6F5)4]− ([Al]:[B]:[V] = 100:1.2:1).

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Table 2. 1H NMR Data (δ, ppm and Δν1/2, Hz) for Complexes 2 and 2b at −20 °C 2a 2bb a

δ Δν1/2 δ Δν1/2

N-CH3

Py-Hm

Py-Hp

Ar-Hm

Ar-Hp

Ar-CH3

AlMe2

72.6 (100) 80.2 (1100)

−8.3 (90) 47.0 (580)

−19.8 (100) 7.7 (140)

6.5 (60) 10.1 (100)

7.1 (70) 6.0 (65)

5.7 (80) ∼6 (∼1000)

−2.9 (290)

In CDCl3. bSample 2/AlMe3/[Ph3C]+[B(C6F5)4]− ([V]:[Al]:[B] = 1:10:1.2) in toluene-d8 at −20 °C.

Scheme 1. Reactions in the Systems 1−4/AlMe3/[Ph3C]+[B(C6F5)4]−

Δν1/2 = 580 Hz, Py-Hm); 10.1 (4H, Δν1/2 = 100 Hz, Ar-Hm); 7.7 (1H, Δν1/2 = 140 Hz, Py-Hp); 6.0 (2H, Δν1/2 = 65 Hz, ArHp); ∼6 (12H, Δν1/2 ∼ 1000 Hz, Ar-CH3);47 −2.9 (6H, Δν1/2 = 290 Hz; Al(CH3)2) (Figure 1B, Table 2). This assignment is in good agreement with the 1H NMR data for the systems 1, 3, 4/AlMe3/[Ph3C]+[B(C6F5)4]− (see below). Apparently, 2b is an ion pair rather than a neutral complex of V(III): it is formed only in the presence of the cationizing reagent [Ph3C]+[B(C6F5)4]− and is not found in the course of reaction of 2 with AlMe3 (Scheme 1). For reliable assignment of the 1H NMR peaks of the paramagnetic complexes, selective introduction of deuterium labels can provide valuable information. The 1H NMR spectrum of the sample 2/Al(CD3)3/[Ph3C]+[B(C6F5)4]− ([V]:[Al]:[B] = 1:10:1.2) displayed the same peaks as 2b, besides that at δ −2.9. At the same time, the resonance at δ −2.9 (Δν1/2 = 25 Hz) from the Al(CD3)2 moiety was found in the 2H NMR spectrum of the sample 2/Al(CD3)3/[Ph3C]+[B(C6F5)4]− ([V]:[Al]:[B] = 1:10:1.2) (Figure 1C), thus confirming its assignment to the AlMe2 moiety of the heterobinuclear ion pair [L(R)VIII(μ-R)2AlMe2]+[B(C6F5)4]− (2b, R = Me or Cl). Using the same approach as described in ref 48, we have partially replaced the N−CH3 protons of 2 with the N−CD3 congeners. Activation of the thus obtained complex 2 with AlMe3/[Ph3C]+[B(C6F5)4]− affords complex 2b, exhibiting the 2H NMR resonance at δ 78 (Δν1/2 = 180 Hz) (toluene, −20 °C). This result confirms the assignment of the 1 H resonance of 2b at δ 80.2 (Figure 1B) to the N−CH3 group. The evaluated concentration of 2b is ca. 2 times lower than that expected based on the known initial amount of precatalyst 2. Apparently, this discrepancy is due to the low solubility of 2b, which precipitates at the bottom of the NMR tube. The Systems 1, 3, 4/AlMe3/[Ph3C]+[B(C6F5)4]−. Similar to 2, the reaction of 1, 3, and 4 with AlMe3/[Ph3C]+[B(C6F5)4]− in toluene-d8 leads to the formation of heterobinuclear ion pairs 1b, 3b, and 4b ([L(R)VIII(μ-R)2AlMe2]+[B(C6F5)4]−; R = Me or Cl). Complexes 1b−4b contain the same pyridinic moiety but different Ar groups. The resonance at δ ∼45 (2H) is present in all complexes 1b−4b (Figures S5−S8). Hence, this resonance belongs to the Py-Hm protons. The assignment of the resonance at δ ∼10 (4H) to the Ar-Hm protons is in

accordance with the absence of this resonance in the 1H NMR spectrum of 4b (Ar = 3,5-F2C6H3). Resonances of ArCH3 and Ar-iPr protons were assigned on the basis of integration and analysis of their half-widths. Thus, the comparison of the 1H NMR spectra of complexes 1b−4b allows the assignment of most of their 1H NMR resonances (Tables S1−S4). Complexes 1b−4b have different stabilities: the half-life time (τ1/2) of 1b is about 10 min at 0 °C, 2b and 3b are more stable (τ1/2 ∼10 min at 20 °C), and 4b is the least stable (τ1/2 ∼10 min at −10 °C). A comparison of the 1H NMR spectra of the samples 1/AlMe 3 /[Ph 3 C] + [B(C 6 F 5 ) 4 ] − ([V]:[Al]:[B] = 1:10:1.2) and 1a/AlMe3/[Ph3C]+[B(C6F5)4]− ([V]:[Al]:[B] = 1:10:1.2) in toluene-d8 at −20 °C shows that the same complex 1b is formed in both samples. The Systems 1−4/MAO. As mentioned, some of the 1H resonances of VIII species formed upon the activation of 1−4 with MAO are obscured by the intense resonances of MAO and toluene. Nevertheless, the key resonances of the ion pairs 1b′− 4b′ formed in the systems 1−4MAO can be detected. These resonances coincide with the corresponding resonances of 1b− 4b. As an example, Figure 1D shows the 1H NMR spectrum of 2b′, which is rather similar to that of 2b (Figure 1B). Hence, 1b′−4b′ can be reasonably assigned to ion pairs [L(R)VIII(μR)2AlMe2]+[MeMAO]− (R = Me or Cl). This is the first reported example of characterization of such species in catalyst systems LVIIICl3/MAO (L = bis(imino)pyridine ligand). Possible Precursors of Active Species of the Catalyst Systems 1−4/AlMe3/[Ph3C]+[B(C6F5)4]− and 1−4/MAO. Ion pairs 1b−4b are the only observed vanadium(III) species in the catalyst systems 1−4/AlMe3/[Ph3C]+[B(C6F5)4]− after mixing the reagents at low temperature. When ethylene was injected into the NMR tubes containing freshly generated 1b− 4b at −10 °C, monomer consumption was observed, accompanied by the precipitation of polyethylene onto the inner surface of the tubes. It is logical to conclude that it is ion pairs [L(R)VIII(μ-R)2AlMe2]+[B(C6F5)4]− (R = Me, Cl) that are the direct precursors of the true active species of polymerization. Similarly, ion pairs [L(R)VIII(μ-R)2AlMe2]+[MeMAO]− (R = Me, Cl) are the likely species responsible for the ethylene polymerization with the catalyst systems 1−4/ MAO. 2585

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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. 1 H and 2H NMR spectra were measured on a Bruker Avance 400 MHz NMR spectrometer at 400.130 and 61.422 MHz, respectively, using 5 mm o.d. glass NMR tubes. 1H chemical shifts were referenced to the residual solvent peaks (δ 2.09 in toluene-d8 or δ 7.26 in chloroform-d). 2H chemical shifts were referenced to the CH2DC6H5 impurities (δ 2.1) in toluene. The t1ir1d pulse program was used for the inversion−recovery NMR experiments. The samples for NMR spectroscopy were prepared as follows. Desired amounts of the vanadium complex and [Ph3C]+[B(C6F5)4]− were weighed in the glovebox and transferred into the NMR tube, which was then closed with a septum stopper. The solution of AlMe3 in toluene-d8 was then added via syringe upon proper cooling (−30 °C). Ion Pair [LAlMe2]+[B(C6F5)4]− {L = [2,6-(ArNCMe)2C5H3N], Ar = 2,6-Me2C6H3} Observed in the Catalyst system 2/AlMe3/ [Ph3C]+[B(C6F5)4]−/C2H4. 1H NMR (400 MHz, toluene-d8, 25 °C): δ 7.88 (br t, 1H, Py-Hp); 7.42 (br d, 2H, Py-Hm); Ar-H resonance overlaps with toluene and Ph3CCH3 aromatic protons; 1.78 (s, 12H, Ar-CH3); 1.72 (s, 6H, N-CH3); −0.80 (s, 6H, AlMe2). NMR Reaction of L with AlMe3 and [Ph3C]+[B(C6F5)4]− {L = [2,6-(ArNCMe)2C5H3N], Ar = 2,6-Me2C6H3}. Ligand L (1.8 mg, 5.0 μmol) and [Ph3C]+[B(C6F5)4]− (5.5 mg, 6.0 μmol) were transferred into the NMR tube in the glovebox, and a solution of AlMe3 in toluene-d8 (0.5 mL, 0.1 M, 50 μmol) was then added via syringe at room temperature. 1H NMR (400 MHz, toluene-d8, 25 °C): δ 7.89 (t, 1H, 3JHH = 7.9 Hz, Py-Hp); 7.42 (d, 2H, 3JHH = 7.9 Hz, PyHm); Ar-H resonance overlaps with toluene and Ph3CCH3 aromatic protons; 1.78 (s, 12H, Ar-CH3); 1.72 (s, 6H, N-CH3); −0.80 (s, 6H, AlMe2).

This picture is analogous to that for bis(imino)pyridine complexes of FeII, where heterobinuclear ion pairs [LFeII(μMe)2AlMe2]+[A]− ([A]− = [MeMAO]− or [B(C6F5)4]−) are proposed to be the precursors of active species of polymerization of catalyst systems LFeCl2/AlMe3/[Ph3C]+[B(C6F5)4]− or LFeCl2/MAO.49−53 As mentioned above, ion pairs [L(R)VIII(μ-R)2AlMe2]+[A]− (R = Me, Cl) are unstable and convert with time into other vanadium species. Three possible routes of this transformation can be proposed: (1) modification of the starting bis(imino)pyridine ligand of the ion pairs [L(R)VIII(μ-R)2AlMe2]+[A]− (R = Me, Cl) upon the reaction with AlMe3, (2) reduction of vanadium(III) to a lower valence state, (3) bis(imino)pyridine ligand transfer (from V to Al). The analysis of the 1H NMR spectra of the sample 2/AlMe3/ [Ph 3 C] + [B(C 6 F 5 ) 4 ] − /C 2 H 4 ([V]:[Al]:[B] = 1:10:1.2; N(C2H4):N(V) = 50:1) has shown that in the presence of ethylene the decay rate of 2b markedly increases (τ1/2 = 15 min at −10 °C in the presence of ethylene, and τ1/2 > 80 min at −10 °C in the absence of ethylene). In the course of the decay of complex 2b, resonances of complex [LAlMe2]+[B(C6F5)4]− {L = [2,6-(ArNCMe)2C5H3N], L = 2,6-Me2C6H3} turn up (Experimental Section). The identity of the latter complex was established by an independent synthesis by reacting L with AlMe3 and [Ph3C]+[B(C6F5)4]− (Figures S9 and S10). The 1H NMR spectrum of [LAlMe2]+[B(C6F5)4]− witnesses that the structure of L does not undergo modifications in the course of this ion pair formation. The evaluated concentration of [LAlMe2]+[B(C6F5)4]− accounts for at least 30% of the concentration of the starting complex 2; thus, the ligand scrambling (to form [LAlMe2]+[B(C6F5)4]−) may be regarded as one of the major deactivation pathways of 2b.

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S Supporting Information *

1 H NMR data of complexes 1−4, 1b−4b, and [LAlMe2]+[B(C6F5)4]−. This material is available free of charge via the Internet at http://pubs.acs.org.

CONCLUSIONS Using 1H and 2H NMR spectroscopy, the activation of bis(imino)pyridine vanadium(III) chloride LVIIICl3 catalysts {L = [2,6-(ArNCMe)2C5H3N], Ar = 2,6-iPr2C6H3; 2,6Me 2 C 6 H 3 ; 2,4,6-Me 3 C 6 H 2 ; 3,5-F 2 C 6 H 3 } with AlMe 3 / [Ph3C]+[B(C6F5)]4− or MAO has been studied. Formation of heterobinuclear ion pairs of the type [L(R)V III (μR)2AlMe2]+[A]− ([A]− = [B(C6F5)]4− or [MeMAO]−, R = Me or Cl) has been observed, which are the most likely direct precursors of active sites of ethylene polymerization. The bis(imino)pyridine ligand transfer to AlMe3 has been identified as one of the major deactivation pathways of the ion pairs [L(R)VIII(μ-R)2AlMe2]+[A]−.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +7 383 3308056. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Education of the Russian Federation and by the Russian Foundation for Basic Research (Grant No. 12-03-00133). The authors are grateful to Dr. D. E. Babushkin for fruitful discussions and for the synthesis of Al(CD3)3 and Dr. M. A. Matsko for the analysis of the polymers’ MWD. Also the authors thank Mrs. O. K. Akmalova for assistance with the ethylene polymerization experiments.

EXPERIMENTAL SECTION

All manipulations with air-sensitive materials were performed in an argon-filled glovebox. All solvents used were dried with 4 Å molecular sieves (toluene and toluene-d8) or with P2O5 (CH2Cl2 and CDCl3) and distilled under dry argon. Complexes 1−4 and 1a were synthesized according to published procedures.3,6,15 AlMe3, MAO, and [Ph3C]+[B(C6F5)4]− were purchased from Aldrich. Al(CD3)3 was synthesized by Dr. Babushkin according to a published procedure.54 Ethylene polymerization was performed in a 0.3 L steel reactor. Precatalyst (2 μmol) was introduced into the reactor 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 MAO or AlMe3/[Ph3C]+[B(C6F5)4]− in toluene (50 cm3). After setting up the desired polymerization temperature (60 °C) and ethylene pressure, the reaction was started by breaking the ampule with the precatalyst. During the polymerization, ethylene pressure, temperature, and stirring speed were maintained constant. The experimental unit was

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REFERENCES

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Organometallics

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dx.doi.org/10.1021/om500267e | Organometallics 2014, 33, 2583−2587