NMR and EPR Spectroscopic Identification of Intermediates Formed

Article pubs.acs.org/Organometallics

NMR and EPR Spectroscopic Identification of Intermediates Formed upon Activation of 8‑Mesitylimino-5,6,7-trihydroquinolylnickel Dichloride with AlR2Cl (R = Me, Et) Igor E. Soshnikov,†,‡ Nina V. Semikolenova,† Konstantin P. Bryliakov,†,‡ Vladimir A. Zakharov,†,‡ Wen-Hua Sun,§ and Evgenii P. Talsi*,†,‡ †

Boreskov Institute of Catalysis, Pr. Lavrentieva 5, Novosibirsk 630090, Russian Federation Novosibirsk State University, Pirogova 2, Novosibirsk 630090, Russian Federation § CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡

S Supporting Information *

ABSTRACT: The intermediates formed upon activation of 8mesitylimino-5,6,7-trihydroquinolylnickel dichloride [LNiCl2] with AlR2Cl (R = Me, Et) have been studied by 1H NMR and EPR spectroscopy. Activation of LNiCl2 with AlEt2Cl has been shown to afford diamagnetic ion pair [LNiIIEt]+[AlEt3Cl]−, whereas the use of AlMe2Cl as activator yields the paramagnetic ion pair with proposed structure [LNiII(μR)2AlMeCl]+[AlMe3Cl]− (R = Cl or Me). With time, both ion pairs convert to bis-ligated Ni(I) complexes with proposed structures [L2NiI]+[A]−, where [A]− = AlEt3Cl− or AlMe3Cl−, respectively. Ethylene reactivity examination reveals that ion pairs [LNiIIEt]+[AlEt3Cl]− and [LNiII(μ-R)2AlMeCl]+[AlMe3Cl]− are the closest precursors of the active species of polymerization.

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INTRODUCTION Highly branched ethylene oligomers have attracted growing interest as components of lubricants and surface modifiers.1,2 Such oligomers can be obtained using cationic or neutral Ni(II) catalysts1−27 capable of relatively rapid (comparable to propagation) β-hydride elimination and 2,1-reinsertion, resulting in extensive chain walking.26,28 In 2010, some groups employed bulky substituents to modify α-diiminonickel precatalysts, which resulted in highly active and thermally stable ethylene polymerization catalysts capable of forming elastomeric polyethylenes.29,30 An alternative approach assumed the design of 8-arylimino-5,6,7-trihydroquinolinylnickel chlorides that demonstrated high activities in ethylene polymerization upon activation with MAO and AlEt2Cl to form highly branched polyethylenes with a narrow molecular weight distribution.31 Further modifications led to catalysts affording branched polyethylene waxes, which are potentially interesting as additives to lubricants and pour-point depressants.32 The nature of the active species operating in these catalyst systems remains unclear. Previously, some groups showed that activation of bis(imino)pyridine nickel(II) dichloride complexes (LNiCl2) and vanadium(III) trichloride complexes (LVCl3) with methylalumoxane (MAO) afforded ion pairs [LNiMe]+[Me-MAO]−33 and [L(R)V(μ-R)2AlMe2]+[Me-MAO]− (R = Me or Cl)34 capable of conducting olefin polymerization. However, until now, there have been no examples of spectroscopic characterization of intermediates formed upon the activation of postmetallocene catalysts with AlR2Cl (R = Et, Me). In this work, we present the 1H NMR and EPR spectroscopic detection and characterization of species formed upon the © XXXX American Chemical Society

activation of 1 (Chart 1) with AlMe2Cl and AlEt2Cl. The nature of the closest precursors of active sites of ethylene polymerization and of catalyst deactivation products is discussed. Chart 1. Structure of Complex 1

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RESULTS AND DISCUSSION Ethylene Polymerization Data. The activation of 1 with AlMe2Cl and AlEt2Cl leads to active ethylene polymerization catalysts (Table 1). The rate profiles of the ethylene polymerization by 1/AlMe2Cl and 1/AlEt2Cl systems (Figure 1) were rather similar, exhibiting high initial polymerization rates followed by gradual decay. The 1/AlMe2Cl catalyst system exhibited higher initial activity, whereas the deactivation rates of both catalyst systems were similar (the rate of polymerization for both catalyst systems decreases almost 3× within a 30 min period). The polymers obtained had low Mw values (∼600 g/ mol) and high number of branches (∼45/1000 C) with Mw/Mn Received: March 27, 2015

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DOI: 10.1021/acs.organomet.5b00263 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 1. Ethylene Polymerization Catalyzed by 1/AlR2Cla

a

curve no.

n (Ni), μmol

cocatalyst

m (PE), g

activity, gPE(mmolNi bar h)−1

Mn, g/mol

Mw, g/mol

Mw/Mn

branches/1000 Cb

1 2

1.8 2.0

AlMe2Cl AlEt2Cl

6.8 5.7

756 570

310 310

620 580

2.0 1.9

44 45

Conditions: heptane (50 mL), 50 °C, PC2H4 = 5 bar, [Al]/[Ni] = 200, 60 min polymerization time. bBy 13C NMR spectroscopy.

Figure 1. Kinetic profile of ethylene polymerization over the catalyst systems 1/AlMe2Cl (curve 1) and 1/AlEt2Cl (curve 2). Curve numbers correspond to the respective numbers in Table 1.

∼ 2, which is evidence of the presence of only one type of active site of ethylene polymerization. Spectroscopic Monitoring of the Activation of 1 with AlMe2Cl. Precatalyst 1 displays broad paramagnetically shifted 1 H NMR resonances spread over the +250 to −45 ppm range (Figure 2a). Most of them can be readily assigned on the basis of integral values and analysis of line widths (Table 2). Mixing 1 with AlMe2Cl ([Al]/[Ni] = 120) in toluene-d8 for 1 min at 0 °C results in the conversion of 1 to new paramagnetic complex 2 (Figure 2b, Table 2). 2 is the major product (>70% yield) of the reaction between 1 and AlMe2Cl. In addition to 1 H NMR resonances of the ligand,35 a new broad resonance from AlMeCl moiety at δ ∼3 (3H, Δν1/2 ∼ 540 Hz) was found, which could reflect the formation of either neutral heterobinuclear complex LNiIIR(μ-R)2AlMeCl (R = Cl or Me) or ion pair [LNiII(μ-R)2AlMeCl]+[AlMe3Cl]−. An attempt to discriminate between these two possibilities using AlMe3/[Ph3C]+[B(C6F5)4]− as an ion-pair-forming activator was not successful; even at −40 °C, the reaction of 1 with AlMe3/[Ph3C]+[B(C6F5)4]− immediately afforded EPR active (S = 1/2) Ni(I) species with uninformative NMR spectra. Further warming the sample of 1/AlMe2Cl ([Al]/[Ni] = 120) to +25 °C leads to the disappearance of 2 and formation of EPR-active complex 3. The EPR spectrum of a frozen solution of 3 is characteristic of Ni(I) species36 (Figure 3a). The EPR spectrum of 3 exhibits almost axial g-tensor anisotropy (g1 = 2.046, g2 = 2.077, g3 = 2.223), which is evidence of nearly axial symmetry of the ligand environment of the Ni(I) species. The g1 and g2 components of the EPR spectrum display partially resolved hyperfine splitting from nitrogen atoms. We have simulated the EPR spectrum of complex 3 (Figure 3b) and shown that the observed HFS structure is caused by the interaction of the unpaired electron with two equivalent nitrogen atoms (AN1 = 9.28 G, AN2 = 10.85 G, AN3 = 8.85 G). This picture corresponds well with the [L2NiI]+[AlMe3Cl]− ion pair, assuming that only one nitrogen atom of each iminoquinoline ligand displays hyperfine splitting. The possibility that

Figure 2. 1H NMR spectrum (0 °C, acetone-d6) of 1 ([1] = 10−3 M) (a); 1H NMR spectra (0 °C, toluene-d8) of catalyst systems 1/ AlMe2Cl ([Al]/[Ni] = 120, [1] = 10−2 M) (b) and 1/AlEt2Cl ([Al]/ [Ni] = 20, [1] = 10−2 M) (c). Asterisks denote signals of diamagnetic complex 5 (see Figure 4). The residual solvent peak is marked by “s”.

3 incorporates one imino-quinoline ligand and the observed splitting is caused by two equivalent nitrogen atoms of this ligand seems to be far less probable because the electronic properties of the quinoline and imine nitrogens are obviously quite different. Thus, complex 3 is an ion pair of the type [L2NiI]+[AlMe3Cl]−, where L = initial ligand.37 Prolonged storage of the 1/AlMe2Cl ([Al]/[Ni] = 120) sample at room temperature leads to a decrease of the EPR resonances of Ni(I) accompanied by visible formation of Ni black on the walls of the NMR tubes. B


Article

Organometallics Table 2. 1H NMR Data (δ, ppm and Δν1/2, Hz) for Complexes 1, 2, and 4 in Toluene-d8 at 0 °C 1 2 4

δ Δν1/2 δ Δν1/2 δ

A

B

C

∼247 (∼1300) ∼208 (∼4300) 220÷205

79.5 (350) 63.4 (350) 67÷62

25.6 (190) 17.1 (80) 17.7÷17.0

D, E, and Ha 19.4 (90) 19.5 (140) 20.8÷20.0

15.2 (160) 14.3 (160) 15.4÷13.9

13.0 (50) 9.7 (100) 10.2÷9.2

F

G

I

AlRCl

−41.7 (330) −32.1 (340) −31÷−34

8.4 (230) 18.6 (410) 20÷17

28.9 (210) 26.4 (190) 28.4÷26.1

∼3 (∼540) NF

a

Resonances D, E and H protons cannot be unambiguously distinguished. NF = not found (signals of Et protons overlap with the intense peak of AlEt2Cl).

Figure 4. 1H NMR spectrum (−20 °C, toluene-d8) of sample 1/ AlEt2Cl recorded after 10 min of mixing the reagents ([Al]/[Ni] = 50, [1] = 10−2 M). Asterisks mark signals of AlEt2Cl. Residual solvent peaks are marked by “s”.

ization of a similar diamagnetic species [L1NiIIEt]+[BAr′4]− (L1 = neutral bidentate α-diimine ligand, Ar′ = 2.6-C6H3Me2) was previously reported by Brookhart and co-workers.26 Prolonged storage of 5 at −20 °C results in a decrease of its concentration (∼3× within 2 h) accompanied by the formation of EPR active Ni(I) species 3′ (with EPR parameters coinciding with those of 3), which can be tentatively assigned to the [L2NiI]+[AlEt3Cl]− structure. Warming the sample containing species 5 accelerates the reduction to Ni(I). Overall, NMR and EPR spectroscopic studies of the catalyst systems 1/AlMe2Cl and 1/AlEt2Cl have shown that nickel complexes 2−5 can be observed in these systems. Diamagnetic ion pair 5 ([LNiIIEt]+[AlEt3Cl]−) can be reliably characterized by NMR. The structures of species 2 and 4, due to their paramagnetism, are less firmly established. According to 1H NMR spectroscopic data, 2 and 4 contain one ligand L at Ni(II) as well as a NiII(μ-R)2AlR1Cl moiety (for complex 2, R = Cl or Me, R1 = Me; for complexes 4, R = Cl or Et, R1 = Et). However, on the basis of the NMR data, one cannot conclude whether complexes 2 and 4 are neutral LNiIIR(μ-R)2AlR1Cl complexes or [LNiII(μ-R)2AlR1Cl]+[Al(R1)3Cl]− ion pairs. However, taking into account the high ethylene polymerization activity of species 2 and 4 (see below), we favor the assignment of 2 and 4 to the ion-pair structure of the type [LNiII(μR)2AlR1Cl]+[Al(R1)3Cl]−. Proposed reactions in systems 1/ Al(Alk)2Cl are shown in Scheme 1. The major distinction between the 1/AlMe2Cl and 1/ AlEt2Cl systems is the formation of temperature-unstable species 5, which was only observed in the latter system. Apparently, in the 1/AlMe2Cl system, corresponding intermediate 5′ is too unstable and either rapidly undergoes reduction (detected by the formation of EPR-active bis-ligated complex 3) or accumulates in the form of the ion pair [LNiII(μMe)2AlMeCl]+[AlMe3Cl]−. In intermediate 5, β-agostic interaction of the Et with the Ni center is possible, which may stabilize intermediate 5 as compared to 5′. Catalyst System 1/AlR2Cl/C2H4 (R = Me, Et). Catalyst systems 1/AlMe2Cl and 1/AlEt2Cl display similar activities in

Figure 3. Experimental (a) and simulated (b) EPR spectra of complex 3. The following parameters were used for the simulation: g1 = 2.046, A1N1 = A1N2 = 9.28 G, g2 = 2.077, A2N1 = A2N2 = 10.85 G, g3 = 2.223, A3N1 = A3N2 = 8.85 G. Relative orientation of the A tensor with respect to the g tensor is given by the Euler angles α = 0°, β = 0°, γ = 1.127°. Line width tensor σ has the same principal directions as the g tensor and principal components σ1 = 4.0 G, σ2 = 4.50 G, σ3 = 6.58 G.

Spectroscopic Monitoring of Activation of 1 with AlEt2Cl. The reaction of 1 with AlEt2Cl in toluene results in the formation of new Ni(II) species: paramagnetic complexes of type 4 and diamagnetic complex 5. The ratio between 4 and 5 strongly depends on the temperature and the [Al]/[Ni] ratio. At [Al]/[Ni] = 20, paramagnetic complexes 4 predominate in the reaction solution at 0 °C, and only small amount of 5 is observed (Figure 2c). Increasing the temperature to 25 °C leads to a complete conversion of 4 to 5. At [Al]/[Ni] > 50, only species 5 was found even at −20 °C (Figure 4). 1H NMR resonances of 4 resemble those of 2. However, several (at least two) resonances are observed for each particular proton of 4 instead of one resonance for 2 (Table 2, Figure 2c). This may be due to the existence of several structurally similar species of type 4. The spectrum of diamagnetic complex 5 (see Experimental Section) corresponds to a Ni(II) complex bearing one bidentate ligand and one Ni-CH2-CH3 moiety (Figure 4). The assignment of Ni-CH2-CH3 was confirmed by the 1H-1H COSY experiment. Most likely, 5 is an outer-sphere ion pair of the type [LNiIIEt]+[AlEt3Cl]−. The diamagnetic nature of 5 is evidence of its square-planar topology. 1H NMR characterC


Article

Organometallics Scheme 1. Proposed Reactions in System 1/Al(Alk)2Cl/(C2H4)

stitution of RAlMeCl with ethylene, followed by ethylene insertion.

ethylene polymerization and produce PEs with the same molecular structures (Table 1). However, according to NMR data, different Ni(II) species predominate in the 1/AlMe2Cl and 1/AlEt2Cl systems at room temperature: paramagnetic ion pair with proposed structure [LNiIIR(μR)2AlMeCl]+[AlMe3Cl]− (R = Me, Cl) (2) and diamagnetic ion pair [LNiIIEt]+[AlEt3Cl]− (5), respectively (Scheme 1). Adding C2H4 (200 equiv) to sample 1/AlEt2Cl ([Al]/[Ni] = 50) at −20 °C leads to immediate disappearance of 5 and rapid ethylene polymerization. Over 4−5 min, ethylene was completely consumed, and the 1H NMR resonances of 5 were restored. A similar result was obtained for the 1/AlMe2Cl ([Al]/[Ni] = 120) system. After the addition of 200 equiv of ethylene to the 1/AlMe2Cl ([Al]/[Ni] = 120) sample at 0 °C, complex 2 immediately disappeared. After ethylene consumption, the NMR pattern of 2 appeared again. Brookhart and co-workers have shown that cationic complex [L1NiIIMe]+[BAr′4]− generated in the model system L1NiIIMe2 + [H(OiPr2)2]+[BAr′4]− is capable of inserting ethylene into the Ni−Me bond to form complex [L1NiIIPr]+[BAr′4]−. Further ethylene insertion into the Ni−Pr bond yields complex [L1NiII(C5H11)]+[BAr′4]−. The cationic [L1NiII(alkyl)]+[BAr′4]− complexes were postulated as the active species of ethylene polymerization.26 On the basis of this analogy, intermediate [LNiIIEt]+[AlEt3Cl]− (5) may be considered as the true direct precursor of the active species of polymerization. In the case of the 1/AlMe2Cl system, the corresponding [LNiIIMe]+[AlEt3Cl]− (5′) was not observed, presumably due to its much lower stability. In this system, the heterobinuclear ion-pair intermediate [LNi I I R(μR)2AlMeCl]+[AlMe3Cl]− (R = Me, Cl) (2), predominating in the reaction solution, is the last detectable precursor of the active sites of ethylene polymerization, which is expected to convert to the [LNiII(polymeryl)]+[A]− species upon sub-

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CONCLUSIONS Using NMR and EPR spectroscopy, the activation of an ethylene polymerization precatalyst based on the [LNiCl2] complex (L = 2,4,6-trimethyl-(N-5,6,7-trihydroquinolin-8ylidene)phenylamine) with dialkyl aluminum chlorides AlR2Cl (R = Me, Et) in toluene was investigated at −20 to 0 °C. At the initial stage of activation (at [Al]/[Ni] ≈ 20), paramagnetic ion pairs with heterodinuclear cationic parts [LNi II (μR)2AlR1Cl]+[Al(R1)3Cl]− (R = Cl, Me, Et; R1 = Me, Et) were formed. In the [LNiCl2]/AlMe2Cl system, this ion pair remains the major Ni(II) species in solution even at high [Al]/ [Ni] (100−120). In the system containing AlEt2Cl, raising the [Al]/[Ni] ratio to 50 or higher results in the formation of a diamagnetic ion pair [LNiIIEt]+[AlEt3Cl]−. Addition of ethylene to either [LNi I I (μ-R) 2 AlMeCl] + [AlMe 3 Cl] − or [LNiIIEt]+[AlEt3Cl]− results in a fast reaction with the formation of PE (which is in agreement with virtually identical catalytic properties of the systems [LNiCl2]/AlMe2Cl/C2H4 and [LNiCl2]/AlEt2Cl/C2H4); after complete ethylene cons u m p t io n , [ L N i I I ( μ - R) 2 A l M e C l ] + [ A lM e 3 C l] − a n d [LNiIIEt]+[AlEt3Cl]− restore their concentrations. In the absence of C2H4, [LNiII(μ-R)2AlMeCl]+[AlMe3Cl]− and [LNiIIEt]+[AlEt3Cl]− species gradually reduce to bis-ligated nickel(I) species [L2NiI]+[AlMe3Cl]− and to Ni(0) black. One could expect that similar reduction processes account for the catalyst deactivation processes under practical polymerization conditions.

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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 and distilled under dry argon. Complex 1 was synthesized D


Article

Organometallics according to a published procedure.31 AlMe3, AlEt3, AlMe2Cl, and AlEt2Cl were purchased from Aldrich. Ethylene polymerization was performed in a 0.3 L steel reactor. Precatalyst 1 (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 AlR2Cl or AlR3 (R = Me, Et) in heptane (50 cm3) with [Al]/[Ni] = 200. After setting up the desired polymerization temperature (50 °C) and ethylene pressure (5 bar), the reaction was started by breaking the ampule with the precatalyst. During the polymerization, ethylene pressure, temperature, and stirring speed were maintained to be 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. 1 H NMR spectra were measured on a Bruker Avance 400 MHz NMR spectrometer at 400.130 using 5 mm o.d. glass NMR tubes. 1H chemical shifts were referenced to the residual CD2HC6D5 peak at δ 2.09. The samples for NMR spectroscopy were prepared as follows: Desired amounts of complex 1 were weighed in the glovebox and transferred into the NMR tube, which was then closed with a septum stopper. The solutions of AlMe2Cl and AlEt2Cl in toluene-d8 were then added via a gastight syringe upon proper cooling (−40 °C). EPR spectra were measured on a Bruker ER-200D spectrometer at 9.3 GHz, modulation frequency of 100 kHz, and modulation amplitude of 4 G. Periclase crystal (MgO) with impurities of Mn2+ and Cr3+, which served as a side reference, was placed into the second compartment of the dual cavity. EPR spectra were quantified by double integration with TEMPO toluene solution as standard. The relative accuracy of the quantitative EPR measurements was ±30%. Species 5: NMR data (toluene-d8, −20 °C), δ 8.49 (br d, 1H, PyH0), 6.98 (m, 1H, Py-Hm), 6.64 (d, JHH = 6.8 Hz, 1H, Py-Hp), 6.59 (s, 2H, Ar-Hm), 2.04 (s, 6H, Ar−CH3 (ortho)) 1.89 (t, JHH = 5.9 Hz, 2H, -NC-CH2-CH2-), 0.71 (q, JHH = 5.8 Hz, 2H, Ni-CH2-CH3), −0.60 (t, JHH = 5.8 Hz, 3H, Ni-CH2-CH3). The resonance of Ar−CH3 (para) overlaps with the intense peak of CD2HC6D5.

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ASSOCIATED CONTENT

S Supporting Information *

NMR spectra with the integral intensities, EPR spectra of complexes 3 and 6. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00263.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Ministry of Science and Education of the Russian Federation and by the Russian Foundation for Basic Research (Grant RFBR 14-03-91153/291 NSFC 21211120163). The authors are grateful to Dr. D. E. Babushkin for fruitful discussions, Dr. M. A. Matsko for analysis of the polymers’ MWD, Mrs. O. K. Akmalova for assistance with the ethylene polymerization experiments, and Dr. A. A. Shubin for the EPR spectrum simulation.

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REFERENCES

(1) Wiedemann, T.; Voit, G.; Tchernook, A.; Roesle, P.; GöttkerSchnetmann, I.; Mecking, S. J. Am. Chem. Soc. 2014, 136, 2078−2085. (2) Dong, Z.; Ye, Z. Polym. Chem. 2012, 3, 286−301. (3) Wang, C.; Friedrich, S.; Younkin, T. R.; Li, R. T.; Grubbs, R. H.; Bansleben, D. A.; Day, M. W. Organometallics 1998, 17, 3149−3151. E


Article

Organometallics (37) We have undertaken the NMR-tube synthesis of the similar compound [L2NiI]+[B(C6F5)4]− (6) according to the following reactions: (1) Ni0(COD)2 + 2L → Ni0L2 + 2COD (COD = cyclooctadiene) and (2) Ni 0 L 2 + [CPh 3 ] + [B(C 6 F 5 ) 4 ] − → [NiIL2]+[B(C6F5)4]− (6) + CPh3. EPR spectra of complexes 3 and 6 are presented in the Supporting Information (Figure S3).

F


NMR and EPR Spectroscopic Identification of Intermediates Formed

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