Aryloxy Alkyl Magnesium versus Dialkyl Magnesium in the

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Aryloxy Alkyl Magnesium versus Dialkyl Magnesium in the Lanthanidocene-Catalyzed Coordinative Chain Transfer Polymerization of Ethylene Mikhail E. Minyaev,† Pavel D. Komarov,† Dmitrii M. Roitershtein,*,†,‡,§ Konstantin A. Lyssenko,⊥,∥ Ilya E. Nifant’ev,*,†,∥ Lada N. Puntus,†,¶ Evgenia A. Varaksina,†,# Roman S. Borisov,†,△ Viktor P. Dyadchenko,∥ and Pavel V. Ivchenko†,∥ Downloaded via BUFFALO STATE on July 19, 2019 at 09:28:50 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

A.V. Topchiev Institute of Petrochemical Synthesis, Leninsky pr. 29, Moscow 119991, Russian Federation N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky pr. 47, Moscow 119991, Russian Federation § National Research University Higher School of Economics, Miasnitskaya Str. 20, Moscow 101000, Russian Federation ⊥ G.V. Plekhanov Russian University of Economics, 36 Stremyanny Per., Moscow 117997, Russian Federation ∥ Chemistry Department, M.V. Lomonosov Moscow State University, 1 Leninskie Gory Str., Building 3, Moscow 119991, Russian Federation ¶ V.A. Kotel’nikov Institute of Radioengineering and Electronics, Russian Academy of Sciences, 11-7 Mokhovaya Str., Moscow 125009, Russian Federation # P.N. Lebedev Physical Institute, Russian Academy of Sciences, 53 Leninsky Prospect, Moscow 119991, Russian Federation △ Peoples’ Friendship University of Russia, 6 Miklukho-Maklaya Str., Moscow 117198, Russian Federation ‡

S Supporting Information *

ABSTRACT: Complexes [(1,2,4-Ph3C5H2)2NdCl2K(THF)2]2 (Nd1), {[1,2-Ph2 -4-(4-MeOC6H 4)C 5 H2 ]2 NdCl2 K(THF) 2 } 2 (Nd2), {[1,2-Ph2-4-(2-MeOC6H4)C5H2]2NdCl2[K(THF)4]}(THF)0.5 (Nd3), and [(1,2,4-Ph3C5H2)2TbCl2K]2 (Tb1) have been synthesized, studied by X-ray diffraction analysis, and used in coordinative chain transfer polymerization (CCTP) of ethylene upon activation by alkyl magnesium derivatives. The complexes Nd1 and Tb1 exhibiting similar molecular structures and the same core type have demonstrated similar catalytic activities. Two types of alkylating/chain transfer agents, namely, di-n-butyl magnesium and heteroleptic complex (BHT)Mg(THF)2nBu Mg1 (BHT = 2,6-di-tert-butyl-4-methylphenoxide), have been studied in this reaction. We have found that (BHT)Mg(PE) products (PE is an oligoethylene chain) are being formed at a relatively high rate while using Mg1 at 40 °C in the solution polymerization of ethylene; the oligomeric products comprise more than 40 ethylene fragments, unlike Mg(PE)2 derivatives, which are obtained from MgnBu2 and contain about 20 ethylene fragments. Luminescence spectroscopy study of the reaction mixtures, while initiating the complex Tb1 by MgnBu2 or Mg1, confirmed the structural proximity and high symmetry of the catalytic complexes for both types of Mg reagents. These experimental results reaffirmed the hypothesis about the CCTP mechanism, suggesting the formation of trinuclear LnMg2 catalytic species. Within this mechanism, we can explain the increase in the polymerization degree (Pn) when Mg1 is used by growing a single oligoethylene chain (PE) per a Mg atom to form (μ-BHT)2Mg2(PE)2 species, whereas application of MgnBu2 provides the growth of two PE chains to form the Mg2(PE)4 product with lower solubility.

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INTRODUCTION

polymer chain exists as Mg(PE)2 (Scheme 1). These highly reactive species can be easily converted into end-functionalized polyethylenes or PE-containing copolymers with polar monomers.2,4−17 Bis(cyclopentadienyl) rare-earth complexes, which have currently found applications in the CCTP of ethylene, are represented by three structural types: pentamethylcyclopentadienyl complexes (type I, Scheme 1),7,8,12,13,15,16 SiMe2-bridged

The development of synthetic strategies to incorporate polar functional groups or polymeric segments into polyolefins is one of the most effective approaches in the development of new polymeric materials.1−4 Coordinative chain transfer polymerization (CCTP) is an effective tool for the synthesis of αfunctionalized oligo- and polyethylenes.2,5,6 Lanthanidocenes, in combination with dialkyl magnesium MgR2, represent effective catalytic systems for the CCTP of ethylene.7,8 MgR2 acts as both an alkylating agent and a chain transfer initiator, and the growing © XXXX American Chemical Society

Received: April 15, 2019

A

DOI: 10.1021/acs.organomet.9b00243 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Scheme 1. Coordinative Chain Transfer Polymerization Catalyzed by Lanthanidocene/MgR2 Catalytic Systems (MgnBu2 Is Drawn as an Example) and Bis(cyclopentadienyl) Rare-Earth Precatalysts Bearing Different Types of Cp′ Ligands

Scheme 2. Synthesis of Complexes Nd1−Nd4 and Tb1

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RESULTS AND DISCUSSION Synthesis of Cp′2-Nd/Tb Complexes. To date, tris(arylcyclopentadienyl) complexes of lanthanides have been studied little. 1 4 , 1 8 − 2 2 Cp′ 2 -Ln precatalysts [(1,2,4Ph 3 C 5 H 2 ) 2 NdCl 2 K(THF) 2 ] 2 (Nd1), {[1,2-Ph 2 -4-(4MeOC6H4)C5H2]2NdCl2K(THF)2}2 (Nd2), {[1,2-Ph2-4-(2MeOC6H4)C5H2]2NdCl2[K(THF)4]}(THF)0.5 (Nd3), and [(1,2,4-Ph3C5H2)2TbCl2K]2 (Tb1) were prepared by the reaction of NdCl3(THF)2.5 or TbCl3(THF)3 with potassium triaryl-substituted cyclopentadienides that were generated in situ from corresponding cyclopentadienes and PhCH2K.14,18 Lanthanidocenes bearing 1,2,4-triphenylcyclopentadienyl and 1,2-diphenyl-4-(4-methoxyphenyl)cyclopentadienyl ligands (Nd1, Nd2, and Tb1) were obtained as dimeric ate complexes (Scheme 2, left), whereas in the case of the 1,2-diphenyl-4-(2methoxyphenyl)cyclopentadienide anion, the monomeric ate complex Nd3 was formed (Scheme 2, right). The ate complex Nd3 became a neutral monomeric complex [1,2-Ph2-4-(2MeOC6H4)C5H2]2NdCl (Nd4) in toluene/THF that was separated after crystallization from n-hexane (Scheme 2, right). X-ray Diffraction Study of Nd Complexes. Structural features of various polyphenyl-substituted cyclopentadienyl Ln(III) complexes have been previously discussed,14,18,23,24 including those of Tb1.18 In the present work, we performed single-crystal X-ray diffraction analysis for Nd1−Nd4 (for details, see section S3 in the Supporting Information). The structures of dinuclear ate complexes Nd1 and Nd2 are similar to previously reported structures of [(η5-1,2,4-Ph3C5H2)2Nd(μ2-Cl)(μ3-Cl)K(THF)0.5(1,4-dioxane)1.5]2(1,4-dioxane) and coordination polymer [{η 5 -1,2-Ph 2 -4-(4-MeO-C 6 H 4 )C 5 H 2 } 2 Nd(μ 2 -Cl)(μ 3 -Cl)K] 2 (μ 2 -1,4-dioxane)(toluene) 4 , which were formed in toluene medium.14 The complexes Nd1 (Figure 1A) and Nd2 (Figure 1B) exhibit the expected binuclear [K2Nd2(μ2-Cl)2(μ3-Cl)2] core located on an inversion center in both structures. The Nd coordination number (CNNd) is 8. The same core type has been found previously for various bis(cyclopentadienyl) rare-earth complexes bearing polyalkyl/ aryl substituents in the cyclopentadienyl ring.14,18 Compared to Tb1, the K+ cations in Nd1 and Nd2 are coordinated by two THF molecules. Moreover, Tb1 demonstrates η6-coordination of two phenyl groups from different cyclopentadienyl ligands with the K+ cation,18 whereas complexes Nd1 and Nd2 exhibit

ansa-metallocenes with bulky substituents (type II),10,11 and 1,2,4-triarylcyclopentadiene derivatives (type III).14 When using MgR2 as an initiator under conditions of solution polymerization, the maximum possible length of the polymer chain in the reaction product Mg(PE)2 is limited by the solubility of the latter. We suppose that the application of a chain transfer agent with only one Mg−alkyl bond should provide the growth of only a single PE chain on each magnesium atom, which should ensure that higher molecular weights of the PE chain are achieved without using higher reaction temperatures that cause chain termination, providing formation of α-olefins. The purpose of this paper is experimental verification of this assumption within the general mechanism of the CCTP of ethylene catalyzed by Ln/Mg systems. This work includes the synthesis, studies of the molecular structures of complexes Nd1, Nd2, and Nd3 (Scheme 1, type III, and Scheme 2, R = Ph, 4-MeOC6H4, and 2-MeOC6H4, respectively), and characterization of their molecular structures, as well as catalytic activity studies of Ln/Mg systems based on these Nd complexes and on the terbium derivative (Tb1), which has a similar crystal structure and the same core as Nd1, and luminescence spectral studies of the reaction mixtures formed by catalysis with Tb1. Di-n-butyl magnesium and the complex (BHT)Mg(THF)2nBu (Mg1) containing the bulky aryloxy ligand (BHT = 2,6-di-tert-butyl-4-methylphenoxy) were used as both an alkylating agent and a chain transfer agent/initiator. The obtained results confirm the general concept of the reaction mechanism and provide an explanation for the effect of (BHT)Mg n Bu application in the synthesis of Mg−PE derivatives with enhanced Pn. B


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Organometallics

Figure 1. Crystal structures of Nd1 (A), Nd2 (B), Nd3 (C), and Nd4 (D). Displacement ellipsoids are drawn at the 50% probability level. For clarity, only Cipso atoms (labeled as Ph) are shown for noncoordinated Ph groups, and hydrogen atoms and disordered components are omitted.

Scheme 3. CCTP of Ethylene Catalyzed by Lanthanidocenes and Dialkyl Magnesium Proposed by Boisson et al.15 (A), BHT−Mg Complex Used in This Work (B), and Stable Heteroleptic Dimer Containing a Mg(μ-O)2Mg Core (C)26−28

Nd3. Other structural details for Nd1−Nd4 are presented in the Supporting Information (section S3). Ethylene Polymerization Catalyzed by Lanthanidocenes and Mg nBu 2 or BHT-Mg- nBu Initiators. We previously demonstrated14 that the Cp′2-Nd lanthanidocenes (III, Scheme 1) activated by MgnBu2 are not inferior in their reactivity to alkylcyclopentadiene-based complexes I. During these studies, we found that the rate of CCTP of ethylene in toluene at 40 °C dramatically decreases after the formation of Mg(PE)2 species with Pn ∼ 16−20.14 Additionally, it has been established experimentally that higher molecular masses of Mg(PE)2 (Pn ∼ 70−150, SEC data) can be achieved for

coordination with only one Ph group (see Figure 1 and Tables S2 and S4 in the Supporting Information). The ate complex Nd3 has a [Nd(μ-Cl)2K] core (Figure 1C); CNNd = 9. Its mononuclearity was likely caused by contact of the Nd atom to one ortho-methoxy group (dNd1−O1 = 2.974(2) Å). The second methoxy group remains noncoordinated (dNd1−O2 = 4.075(3) Å). The neutral complex Nd4 (Figure 1D) lies on a two-fold rotation axis passing through the Nd−Cl bond. The Nd atom (CNNd = 9) is coordinated with both ortho-methoxy groups (dNd1−O1/O1A = 2.653(2) Å). It is noted that the Nd−OMe distance in Nd3 is considerably larger (by 0.32 Å) than that in Nd4, which could be explained by greater steric hindrance in C


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Organometallics

Table 1. Ethylene Oligomerization Catalyzed by Cp′Ln Complexes and MgnBu2 or Mg1 Initiator ([Ln]/[Mg] = 1:20, Toluene) run 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Cp′Ln

Nd1 Nd1 Nd2 Nd2 Nd3 Nd3 Tb1 Tb1 Nd2 Nd2 Nd2 Nd2

[Ln], ×10−4 M

4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 2.0 4.0

p, MPa 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.30

T, °C

alkyl Mg complex

40 40 40 40 40 40 40 40 40 40 80 80 80 40

n

Mg Bu2 Mg1 MgnBu2 Mg1 MgnBu2 Mg1 MgnBu2 Mg1 MgnBu2 Mg1 MgnBu2 Mg1 Mg1 Mg1

TOF, h−1

Mna

Đ Ma

P na

DPRb

mp, °Cc

0 0 870 590 990 660 400 350 835 495 1590 1170 595 1900

715 1155 768 1403 598 845 748 1231 609 1100 1261 1651d

1.005 1.061 1.023 1.083 1.007 1.027 1.011 1.066 1.019 1.260 1.076 1.075

21 37 23 46 17 26 22 39 17 35 41 55

0.57 0.57

107 124 110 131 102 114 111 128 107 127 121 128

0.50 0.65 0.56 0.49

a The data were obtained for corresponding [PE-DMAP]+I− derivatives by the mass spectrometry method.35 bDPR is degree of polymerization ratio; the ratio of the numbers of ethylene fragments incorporated into Mg−PE oligomeric compounds that were obtained in the presence of MgnBu2 (P1) and Mg1 (P2) activators. DPR = P1/P2. cMelting temperature peaks (second heating) from differential scanning calorimetry data for [PE-DMAP]I derivatives. dBimodal MWD according to MS data. See Figure S30 in the Supporting Information.

complexes I and II (Scheme 1) at ∼80 °C;10−12 however, at increased temperatures, the reaction is complicated by βhydride elimination with the formation of terminal vinyl groups.7,14,25 The scheme for Ln/Mg chain transfer polymerization previously proposed by Boisson et al.15 involved the formation of trinuclear (LnMg2) complexes with bridging alkyl fragments; polymer chain growth includes the dissociation of this complex, coordination and insertion of an ethylene molecule, and the subsequent recombination of L−Ln−alkyl and Mg2(alkyl)4 (Scheme 3A). Fast polymer chain transfer via dissociation− recombination of the LnMg2 species ensures the near equal growth of all f ive alkyl fragments bound to the metal atoms of a trinuclear catalytic complex, which is confirmed by the narrow molecular weight distribution (MWD) of α-functionalized PE derivatives formed.15 The value of polymerization degree Pn is limited by solubility of the CCPT product of Mg2(PE)4 composition in accordance with the scheme of Boisson.15 We assumed that higher Pn in the pseudoliving polymerization mode at moderate temperatures can be achieved by using heteroleptic magnesium complexes containing only one active alkyl substituent. When choosing such a complex, we were guided by the criteria of synthetic availability and stability under polymerization conditions. As a result, we have selected the n-butyl magnesium complex (BHT)Mg(THF)2nBu (Mg1, Scheme 3B)26 containing the bulky 2,6-ditert-butyl-4-methylphenoxy (BHT) ligand. In the absence of donors, Mg complexes bearing both alkyl and bulky aryloxy ligands demonstrate a tendency to form dimeric complexes similar to Mg2 (Scheme 3D) due to the high stability of the Mg2O2 core.26−30 Note that BHT−Mg complexes retain their dimeric structures in polymerization processes such as the ringopening polymerization of cyclic esters.26,29,31 We found that, in the absence of rare-earth metal complexes, both MgnBu2 and Mg1 are fully inert in ethylene oligomerization (Table 1, runs 1 and 2, respectively). We assumed that by using Mg1 instead of MgnBu2, three but not f ive (as stated in Scheme 3) oligoethylene fragments should grow, and dimagnesium complex (μ-BHT)2Mg2(PE)2 (Scheme 3C) formed during the reaction by the analogy with Boisson scheme should contain two longer alkyl chains and have a higher solubility in hydrocarbons

compared to that in Mg2(PE)4 (Scheme 3A). To confirm this hypothesis, we conducted a series of polymerization experiments. In the preliminary ethylene oligomerization experiments with Mg1, we used controlled consumption of ethylene to synthesize BHT−Mg−PE derivatives with a short PE chains to study the products by NMR spectroscopy. The spectrum of the reaction mixture obtained in Nd1/Mg1-catalyzed ethylene oligomerization is presented in Figure 2A (see also Figures S18 and S19 in the Supporting Information). The reaction mixture contains (BHT)Mg(PE) as the main product. The minor product (BHT)2Mg apparently forms by metathesis according to the Schlenk mechanism.32 Bearing in mind the full inertness of (BHT)MgnBu in ethylene oligomerization, we found that the formation of (BHT)Mg(PE) directly confirms alkylation of Nd1 with a formation of Nd−Mg complexes without the cleavage of a BHT−Mg bond in the course of oligomerization. We made attempts to detect Ln−Mg alkyl complexes by NMR monitoring of Nd2 mixtures with Mg1. The increase of the Mg/Nd ratio resulted in the complete disappearance of the signals assigned to the Nd complex in the 1H NMR spectra (see the Supporting Information, Figure S20), and this observation can be attributed to paramagnetic relaxation, conformational lability, and increased symmetry in Nd−Mg alkyl complexes.33,34 To compare the molecular weight characteristics of the ethylene oligomerization products (“Mg(PE)2” and “(BHT)Mg(PE)”), we have optimized the method of their derivatization and analysis (Scheme 4). The NMR spectroscopic study of (BHT)Mg(PE) oligomers containing longer PE chains (Pn ∼ 15 and more) is difficult due to their low solubility and thermal instability. The conversion of Mg−PE products into iodine derivatives, followed by quaternization with NEt3 or PPh3, and analysis of the resulting [PE−ER3]I salts by the matrix-assisted laser desorption/ionization (MALDI)-TOF method can be used to establish an average length of the oligoethylene chain.14,35 We have found that N,N-dimethylpyridin-4-amine (DMAP) is a more convenient derivatization reagent because it forms quaternization products in quantitative yield under mild conditions. An example of the 1H NMR spectrum of the [PEDMAP]I salt bearing a short PE chain is shown in Figure 2B; the corresponding 13C{1H} NMR spectra, MALDI spectra, and D


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Organometallics

Figure 2. (A) 1H NMR spectrum (THF-d8, 400 MHz) of the reaction mixture after ethylene oligomerization catalyzed by Nd1/Mg1; Pn ∼ 10. (B) 1H NMR spectrum (CDCl3, 400 MHz) of a [PE-DMAP]I derivative obtained from the BHT−Mg−PE product of the Nd1/Mg1 catalytic system followed by successive reactions with iodine and DMAP (Scheme 4); Pn ∼ 20. The signals of solvent impurities are marked with an asterisk.

calculated based on ethylene consumption during 1 h of the reaction. The experimental results are presented in Table 1. The data for runs 3 and 4 show that a more active catalyst is formed when Nd1 is activated by MgnBu2. The activation by Mg1 leads to a lower reaction rate but provides a PE−Mg product with a significantly longer PE chain length.

differential scanning calorimetry (DSC) curve are given in Figures S21−S24 in the Supporting Information. The next stage of experiments was performed by using the precatalysts Nd1−Nd3 and Tb1 and two types of initiators, traditional MgnBu2 and Mg1. Reactions were carried out for 2 h or until ethylene consumption stopped. The TOF values were E


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Organometallics

demonstrate a functionalization degree in the range of 79−92% (1H NMR data). For these [PE-DMAP]I samples, the DSC method shows the presence of a single narrow peak, the melting temperatures correlate with the Pn determined by MS (Table 1, runs 3−10). According to Scheme 3, Mg2(PE)4 and (BHT)2Mg2(PE)2 products are assumed to be formed when using MgnBu2 and Mg1 as initiators, respectively. Based on the assumption that the total number of ethylene units in the reaction product affects its solubility and determines the progress of the CCTP of ethylene, we can expect that the degree of polymerization ratio (DPR), which is defined as the ratio of the lengths of polyethylene fragments in Mg2(PE)4 and in (BHT)2Mg2(PE)2, should be ∼0.4−0.6. The DPR values, calculated for each pair of experiments, fit into this interval (Table 1). Therefore, we have successfully introduced a new type of activatorthe heteroleptic aryloxy−alkoxy magnesium complex Mg1into the Cp′2−Ln/Mg-catalyzed CCTP. Additionally, we have experimentally established that the presence of a bulky BHT ligand slightly slows the polymerization rate but simultaneously allows an increase in the length of the PE chain by a factor of 2 or more, compared to catalysts based on the traditional chain transfer initiator, Mg(alkyl)2. Taking into account the experimentally proven stability of the BHT−Mg bond under the reaction conditions, the length ratios obtained for polyethylene fragments in Mg−PE products by using MgnBu2 and Mg1 initiators suggest the generality of the CCTP mechanism, proposed by Boisson et al. (Scheme 3A).15 Luminescent Studies of the CCTP of Ethylene. Lanthanide cations, especially Eu3+ and Tb3+, are widely used as luminescence probes, owing to their unique electronic transitions, long-lifetime excited states (ms), and high quantum yields (up to 80%).36−42 Because precatalysts Tb1, Nd1, and Nd2 have the same [Cp′2LnCl2K]2 core and display similar catalytic activity in ethylene polymerization, it was intriguing to use luminescence spectroscopy to identify the structure of the polymerization center. This task was simplified by the fact that complex Tb1 has a sufficiently high quantum yield (25%) and that its crystal structure is known.18 Moreover, the luminescence data obtained for Tb1 before and after dissolution in toluene followed by solvent evaporation indicate the same coordination environment of the Tb3+ ion. The list of Tb/Mg organometallic compounds having relatively short PE chains that have been studied by luminescence spectroscopy is given in Table 2. The luminescence spectra of all Tb compounds, recorded in the range of 470 to 720 nm under excitation at the Cp ligand transitions, exhibit characteristic narrow emission bands that are assigned to the 4f8−4f8 transitions of the Tb3+ ion (Figure 3). The following electronic transitions are observed: 5D4 → 7F6 (480−500 nm), 5D4 → 7F5 (535−555 nm), 5D4 → 7F4 (575−

Scheme 4. Ethylene Oligomerization and Conversion of Mg− PE Derivatives into Ammonium Salts

When ethylene oligomerization is catalyzed by the Nd2, Nd3, and Tb1 complexes and carried out under the same conditions (Table 1, runs 5−10), similar patterns are observed. Upon initiating the reaction by MgnBu2, all catalysts display higher activities, but Mg−PE products with a substantially longer chain are formed upon initiation by Mg1. The maximum Pn value of 46 was obtained using the Nd2 precatalyst (Table 1, run 6). The activity of the studied catalytic systems based on either MgnBu2 or (BHT)MgnBu decreases in the order Nd2 > Nd1 ∼ Tb1 > Nd3 (see Figure S17 in the Supporting Information for ethylene consumption kinetic profiles). A rather low catalytic activity of the catalyst based on complex Nd3 can be explained by the coordination of the ortho-methoxy group of the cyclopentadienyl ligand to the Nd atom (see Figure 1C,D). The next experimental stage involved further investigation of Nd2-based catalysts, which demonstrated the best performance. An increase in temperature to 80 °C (Table 1, runs 11−13) resulted in a decrease of the Pn of Mg−PE species due to occasional temperature-induced chain termination. For example, when using the Nd2/MgnBu2 catalytic system (Table 1, run 11), the resulting product contained a mixture of alkanes, αolefins, and [PE-DMAP]I in a 2:5:3 molar ratio (1H NMR data), which correlated with DSC and mass spectroscopy (MS) data (monomodal distribution), as well as with the higher consumption of ethylene than observed at 40 °C (Table 1, run 5). When Nd2/Mg1 was used (Table 1, runs 12 and 13), the resulting mixtures contained low molecular weight oligomers and small amounts of oligomers with higher Mn (up to 3100, Pn ∼ 110 for run 12; see Figures S28 and S29 in the Supporting Information). At increased pressure (Table 1, run 14), we observed an expected increase in catalyst productivity, but the final reaction product had a bimodal MWD (Figure S30 in the Supporting Information). A similar effect has been recently observed for the I/Mg(n-octyl)nBu catalytic system.15 PE-I and [PE-DMAP]I, which are the derivatization products of Mg−PE samples obtained at 40 °C and 0.15 MPa (Table 1, runs 3−10),

Table 2. List of the Reaction Mixtures Containing Complex Tb1 and Relative Integrated Intensities of Electronic Transitions 5D4 → 7FJ (J = 3−6) at 300 K entry

reagents

Pn

Tb1 Tb1-a Tb1-b Tb1-c Tb1-d Tb1-e

20 equiv of MgnBu2 20 equiv of MgnBu2 + ethylene (middle of oligomerization) 20 equiv of MgnBu2 + ethylene (end of oligomerization) 20 equiv of (BHT)MgnBu 20 equiv of (BHT)MgnBu+ ethylene

0 13a 34a 0 31a

5

D4 → 7F6 0.14 0.11 0.17 0.19 0.18 0.17

5

D4 → 7F5 0.68 0.54 0.56 0.64 0.63 0.66

5

D4 → 7F4 0.10 0.17 0.13 0.10 0.11 0.10

5

D4 → 7F3 0.06 0.11 0.08 0.06 0.06 0.05

a

The average length of the PE chain was calculated from MS data (Mn) or corresponding [PE-DMAP]I derivatives. F


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Organometallics

Figure 3. Luminescence spectra of Tb1, Tb1-a, Tb1-b, Tb1-c, Tb1-d, and Tb1-e (left, λexc= 280 nm) and luminescence excitation spectra of Tb1-a, Tb1-c, Tb1-d, and Tb1-e (right, λreg = 545 nm) at 300 K.

595 nm), 5D4 → 7F3 (610−630 nm), 5D4 → 7F2 (640−660 nm), 5 D4 → 7F1 (660−675 nm), and 5D4 → 7F0 (675−685 nm). The last three transitions have expectedly low intensity. The 5D4 → 7 F5 transition is the most prominent and accounts for ∼50−65% of the total emitted intensity. It is important that the 5D3 → 7FJ transitions are not exhibited in the luminescence spectra of the studied Tb systems (the spectral interval between 400 and 475 nm). In contrast to the Eu3+ ion, the full assignment of Stark components of the electronic transitions in the luminescence spectrum of the Tb3+ ion is impossible due to the large number of closely spaced energy levels. To overcome this issue and to analyze the structural peculiarities of the Tb compounds, the radiative rates can be useful. The radiative rates are important parameters for analysis of the energy transfer process and are associated with geometry of the complexes. It is generally accepted that an increasing asymmetry of the ligand field is reflected as an increase in the branching ratio (β) resulting in an increase in the radiative rate. The branching ratio is defined as the intensity of the electric-dipole transition (5D4 → 7F6, ΔJ = 2) over the magnetic-dipole transition (5D4 → 7F5, ΔJ = 1) and is presented in Table 3. Therefore, several points can be indicated for the luminescence spectra of the considered compounds. Namely, the interaction of Tb1 with MgnBu2 leads to remarkable changes in the nearest surroundings of the Tb3+ ion (Table 2 and Table 3, entries Tb1-a) and, as one can propose from the Stark splittings of the electronic transitions and their integrated intensities, the site symmetry of the luminescence center slightly

Table 3. Lifetime (τ) of the 5D4 Level at 300 K and Branching Ratio (β) entry

τ, ms

β

Tb1 Tb1-a Tb1-b Tb1-c Tb1-d Tb1-e

0.32 ± 0.01 0.34 ± 0.01 0.63 ± 0.03 0.49 ± 0.02 0.41 ± 0.02 0.55 ± 0.03

0.33 0.34 0.31 0.33 0.29 0.29

higher. Indeed, the 5D4 → 7F6 transition is presented by one line only in the comparison with complex 1Tb. The luminescence spectra of Tb1-a and Tb1-b look similar, which points to the preservation of the luminescence center in the middle of the polymerization process. The luminescence spectrum of entry Tb1-c is more broadened than is expected when longer ethylene chains are taken into account. The branching ratio (β) is quite similar for all of these compounds (Table 3), and these results are in line with the luminescence analyses. The interaction of the complex Tb1 and (BHT)MgnBu (entries Tb1-d and Tb1-e) leads to changes in the site symmetry of the terbium ion stronger than those in the previous case. The β value is the same in entries Tb1-d and Tb1-e, whereas it is different in comparison to that with complex Tb1 (0.29 vs 0.33). The broad lines corresponding to the electronic transitions of the Tb3+ ion are probably determined by the bulky structure of the BHT ligand. G


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Organometallics

conjugation degree between phenyl groups and the Cp ring.18 Based on this result and taking into account the position of the excitation bands and their intensity, one can assume that the conjugation between a phenyl ring and the π-system of the Cp ring is lost in the case of the initiator MgnBu2 (entry Tb1-a and Tb1-c). As the excitation luminescence spectra for entries Tb1d and Tb1-e are similar to that for complex Tb1, we can propose a similar structure. Thus, the analysis of the luminescence spectra of the reaction mixtures obtained at different stages of the catalytic process indicates a bis(cyclopentadienyl) structure of Tb intermediates with a more symmetrical Tb3+ environment in comparison to that with Tb1. These results are consistent with the assumption of generality for the ground-state structures of the catalytic complexes formed during initiation by dialkyl magnesium reagents (Scheme 3)15 or by (BHT)MgnBu.

The luminescence decay curves were measured for all Tb compounds, and they were adjusted with a single exponential function that points to the presence of only one type of luminescence center. Moreover, the lifetime values observed (τ) for the 5D4 emitting level are similar (Table 3), indicating small changes in the Tb3+ ion coordination environment compared to those with complex Tb1, which probably indicates the presence of coordinated Cp ligands. The steady-state luminescence excitation spectra of the Tb compounds at room temperature monitored around the peak of the intense 5D4 → 7F5 transition of the Tb3+ ion at 545 nm are given in Figure 3 (right). These spectra exhibit overlapped, intense, and broad bands corresponding to the π−π* transitions of the Cp ligands extending from 250 to 350 nm for Tb1-a and Tb1-c and up to 450 nm for the entry Tb1-e. Moreover, the excitation spectra display weak narrow absorption bands in the spectral range from 470 to 500 nm that are assigned to the 4f8intraconfigurational transition 7F6 → 5D4 (488 nm). These data indicate that the Cp ligands are coordinated by the terbium ion during polymerization and continue to serve as an efficient antenna for sensitization the Tb3+ ion luminescence as in complex Tb1. In the luminescence excitation spectrum of complex Tb1, the low-lying excited state in the region of 410−430 nm is assigned to the intraligand charge transfer (ILCT) state formed by additional η6-interactions between a phenyl group of the Cp ligand and a K+ cation (K+CT state).18 In this complex, the coordinated phenyl group is almost coplanar with the cyclopentadienyl ring. In the excitation spectra of samples Tb1-a and Tb1-c, the band corresponding to this state is absent and can be explained by a disruption of the above-mentioned interaction. This result correlates well with the proposed structure of the LnMg2 catalytic species (Scheme 3).15 To evaluate the possibility of such interactions being typical for bis(triarylcyclopentadienyl) complexes of Tb and Gd,18 we performed DFT modeling of the terbium complex [(η5C5H2Ph3)2Tb(μ-nBu)2Mg(μ-nBu)2MgnBu] and have found no Ph−Mg interaction (minimum interatomic Mg−Ph distance was 3.199 Å, Figure 4), which was in good agreement with the absence of ILCT transitions. Moreover, we showed earlier that in phenylcyclopentadienyl complexes, the energy of S1 states can be regulated by the introduction of different numbers of phenyl rings and by

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CONCLUSIONS In this work, we have synthesized a number of bis(triarylcyclopentadienyl) complexes of Nd and Tb, studied the molecular structure of Nd complexes via single-crystal X-ray diffraction, and explored the CCTP of ethylene catalyzed by these complexes activated by traditional MgnBu2 and novel (BHT)MgnBu initiators. We have found that the BHT−Mg fragment is stable during the polymerization and that the use of (BHT)MgnBu achieves doubled Pn values compared to those using MgnBu2. The results of the polymer tests, DFT modeling, and comparative luminescence studies of reaction mixtures containing the Tb complex [(1,2,4-Ph3C5H2)2TbCl2K]2 (Tb1) allow us to suggest the generality of the reaction mechanisms for CCTP initiated by polyaryl-substituted lanthanidocenes upon activation by MgnBu2 or (BHT)MgnBu, compared with the mechanism previously proposed for the Cp*2NdCl2Li ate complex upon activation by dialkyl magnesium.15 Within this general mechanism, the solubility factor for Mg− PE products limits the potential for the molecular mass of αfunctionalized polyethylenes, which can be synthesized by the pseudoliving chain transfer polymerization of ethylene under conditions of Ln/Mg catalysis. We suppose that the observed effect of using a (BHT)MgnBu chain transfer agent initiator is due to the fundamentally higher solubility of the reaction product (BHT)2Mg2(PE)2 compared to that of Mg2(PE)4 formed after conventional activation by dialkyl magnesiums within the concept of Boisson.15

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.9b00243. Experimental details (general remarks, synthesis of lanthanidocenes, ethylene polymerization, derivatization of Mg(PE), DFT modeling) and additional characterization data (X-ray crystallographic details, MALDI-TOF and DSC data, NMR spectra of cyclopentadienyl ligand precursors and Nd complexes) (PDF) Computed Cartesian coordinates of the molecule reported in this study (XYZ) Accession Codes

Figure 4. Calculated geometry of the complex [(C5H2Ph3)2Tb(μ-nBu)2Mg(μ-nBu)2MgnBu] (PBE0 functional with ECP54MWB basis set for Tb and 6-311+g* basis set for other atoms).

CCDC 1870072−1870075 contain the supplementary crystallographic data for this paper. These data can be obtained free of H


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Organometallics

(12) Chenal, T.; Olonde, X.; Pelletier, J.-F.; Bujadoux, K.; Mortreux, A. Controlled polyethylene chain growth on magnesium catalyzed by lanthanidocene: A living transfer polymerization for the synthesis of higher dialkyl-magnesium. Polymer 2007, 48, 1844−1856. (13) Briquel, R.; Mazzolini, J.; Le Bris, T.; Boyron, O.; Boisson, F.; Delolme, F.; D’Agosto, F.; Boisson, C.; Spitz, R. Polyethylene Building Blocks by Catalyzed Chain Growth and Efficient End Functionalization Strategies, Including Click Chemistry. Angew. Chem., Int. Ed. 2008, 47, 9311−931. (14) Minyaev, M. E.; Vinogradov, A. A.; Roitershtein, D. M.; Borisov, R. S.; Ananyev, I. V.; Churakov, A. V.; Nifant’ev, I. E. Catalytic activity of phenyl substituted cyclopentadienyl neodymium complexes in the ethylene oligomerization process. J. Organomet. Chem. 2016, 818, 128− 136. (15) Ribeiro, R.; Ruivo, R.; Nsiri, H.; Norsic, S.; D’Agosto, F.; Perrin, L.; Boisson, C. Deciphering the Mechanism of Coordinative Chain Transfer Polymerization of Ethylene Using Neodymocene Catalysts and Dialkylmagnesium. ACS Catal. 2016, 6, 851−860. (16) Godoy Lopez, R.; Boisson, C.; D'Agosto, F.; Spitz, R.; Boisson, F.; Gigmes, D.; Bertin, D. New Functional Polyolefins: Towards a Bridge Between Catalytic and RAFT Polymerizations? Macromol. Rapid Commun. 2006, 27, 173−181. (17) Nzahou Ottou, W.; Norsic, S.; Belaid, I.; Boisson, C.; D’Agosto, F. Amino End-Functionalized Polyethylenes and Corresponding Telechelics by Coordinative Chain Transfer Polymerization. Macromolecules 2017, 50, 8372−8377. (18) Roitershtein, D. M.; Puntus, L. N.; Vinogradov, A. A.; Lyssenko, K. A.; Minyaev, M. E.; Dobrokhodov, M. D.; Taidakov, I. V.; Varaksina, E. A.; Churakov, A. V.; Nifant’ev, I. E. Polyphenylcyclopentadienyl Ligands as an Effective Light-Harvesting π-Bonded Antenna for Lanthanide+3 Ions. Inorg. Chem. 2018, 57, 10199−10213. (19) Bonnet, F.; Visseaux, M.; Barbier-Baudry, D. New divalent samarocenes for butadiene polymerisation: influence of the steric effect and the electron density on the catalytic activity. J. Organomet. Chem. 2004, 689, 264−269. (20) Visseaux, M.; Zinck, P.; Terrier, M.; Mortreux, A.; Roussel, P. New ionic half-metallocenes of early lanthanides. J. Alloys Compd. 2008, 451, 352−357. (21) Zinck, P.; Valente, A.; Terrier, M.; Mortreux, A.; Visseaux, M. Half-lanthanidocenes catalysts via the ‘borohydride/alkyl’ route: A simple approach of ligand screening for the controlled polymerization of styrene. C. R. Chim. 2008, 11, 595−602. (22) Barbier-Baudry, D.; Bouyer, F.; Madureira Bruno, A. S.; Visseaux, M. Lanthanide borohydrido complexes for MMA polymerization: syndio- vs iso- stereocontrol. Appl. Organomet. Chem. 2006, 20, 24−31. (23) Roitershtein, D. M.; Minyaev, M. E.; Mikhailyuk, A. A.; Lyssenko, K. A.; Belyakov, P. A.; Antipin, M. Yu Lutetium complexes with the 1,3diphenylcyclopentadienyl ligand. Syntheses and molecular structures of the complexes {(Ph2C5H3)Lu(C2Ph4)(THF)} and {(Ph2C5H3)LuCl2(THF)3}. Russ. Chem. Bull. 2007, 56, 1978−1985. (24) Roitershtein, D. M.; Minyaev, M. E.; Mikhaylyuk, A. A.; Lyssenko, K. A.; Glukhov, I. V.; Belyakov, P. A. Polyphenylcyclopentadienyl complexes of rare earth elements. Russ. Chem. Bull. 2012, 61, 1726−1732. (25) Chenal, T.; Visseaux, M. End-Capped Oligomers of Ethylene, Olefins and Dienes, by means of Coordinative Chain Transfer Polymerization using Rare Earth Catalysts. In Oligomerization of Chemical and Biological Compounds; Lesieur, C., Ed.; InTech, 2014. (26) Nifant’ev, I. E.; Shlyakhtin, A. V.; Tavtorkin, A. N.; Ivchenko, P. V.; Borisov, R. S.; Churakov, A. V. Monomeric and dimeric magnesium mono-BHT complexes as effective ROP catalysts. Catal. Commun. 2016, 87, 106−111. (27) Henderson, K. W.; Honeyman, G. W.; Kennedy, A. R.; Mulvey, R. E.; Parkinson, J. A.; Sherrington, D. C. Magnesium aryloxides: synthesis, structure, solution behavior and magnesiate ion formation. Dalton Trans 2003, 1365−1372. (28) Flörke, U.; Henkel, G.; Kuhn, A.; Kuhn, N.; Laufer, S.; MaichleMößmer, C. {nBuMg(OR)}2 und {Mg(OR)2}2 (R = 2,4,6-tBu3C6H2) −

charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: + 44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Mikhail E. Minyaev: 0000-0002-4089-3697 Dmitrii M. Roitershtein: 0000-0003-0320-1775 Ilya E. Nifant’ev: 0000-0001-9151-1890 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Russian Science Foundation (Grant 17-13-01357, for part of the synthesis, X-ray diffraction analysis, catalytic and luminescence study of Ln complexes) and carried out within the State Program of TIPS RAS (for part of the PE functionalization and analysis). Authors are grateful to Kirill P. Birin for acquiring NMR spectra and to Georgiy A. Shandryuk for DSC studies.

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ABBREVIATIONS Bu, butyl; DMAP, N,N-dimethylaminopyridine; PE, polyethylene REFERENCES

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Aryloxy Alkyl Magnesium versus Dialkyl Magnesium in the

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