Synthesis and Reactivity of Rare-Earth-Metal Monoalkyl Complexes

Aug 21, 2015 - Preparation of {[μ-η6:η1:η1-2-(2,6-iPr2C6H3NCH2)Ind]Er[η1:η1-2-(2,6-iPr2C6H3N═CH)Ind]}2 (7). PhSiH3 (0.14 g, 1.2 mmol) at room ...
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Synthesis and Reactivity of Rare-Earth-Metal Monoalkyl Complexes Supported by Bidentate Indolyl Ligands and Their High Performance in Isoprene 1,4-cis Polymerization Guangchao Zhang,† Shaowu Wang,*,†,‡ Shuangliu Zhou,† Yun Wei,† Liping Guo,† Xiancui Zhu,† Lijun Zhang,† Xiaoxia Gu,† and Xiaolong Mu† †

Key Laboratory of Functional Molecular Solids, Ministry of Education, Anhui Laboratory of Molecule-Based Materials, College of Chemistry and Materials Science, Anhui Normal University, Wuhu, Anhui 241000, People’s Republic of China ‡ State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, People’s Republic of China S Supporting Information *

ABSTRACT: A series of novel rare-earth-metal monoalkyl complexes incorporating partially rotation restricted [N,N]-bidentate indolyl ligands were synthesized and characterized, and their reactivities and catalytic activities were investigated. Treatment of [RE(CH2SiMe3)3(thf)2] with 1 equiv of 2-[(N-2,6-diisopropylphenyl)iminomethyl)]indole (2-(2,6-iPr2C6H3NCH)C8H5NH) in toluene at room temperature afforded the rare-earth-metal monoalkyl complexes [η1:η1-2-(2,6-iPr2C6H3NCH)Ind]2RE(CH2SiMe3)(thf) (Ind = indolyl; RE = Yb (1), Er (2), Y (3), Dy (4), Gd (5)) and the samarium complex [η1:η1-2-(2,6-iPr2C6H3NCH)Ind]3Sm (6) via alkane elimination in good yields. Treatment of complex 2 or 3 with 1 equiv of PhSiH3 in toluene at 80 °C for 12 h afforded the dinuclear complexes {[μ-η6:η1:η1-2-(2,6-iPr2C6H3NCH2)Ind]RE[2(2,6-iPr2C6H3NCH)Ind]}2 (Ind = indolyl, RE = Er (7), Y (8)) in good isolated yields. Treatment of complex 2 or 3 with 1 equiv of amidine (2,6-iPr2C6H3)NCHNH(2,6-iPr2C6H3) in toluene produced the corresponding complexes [η1:η1-2(2,6-iPr2C6H3NCH)C8H5N]2RE[(2,6-iPr2C6H3)NCHN(2,6-iPr2C6H3)] (RE = Er (9), Y (10)) possessing the amidinate ligand [(2,6-iPr2C6H3N)2CH]−. The molecular structures of all complexes were determined by X-ray crystallography. The monoalkyl complexes 1−5 were tested as isoprene polymerization initiators. Among the complexes investigated, the optimum combination 5/AliBu3/[Ph3C][B(C6F5)4] displayed a high catalytic activity in isoprene polymerization, producing polymers with an extremely high 1,4-cis selectivity (up to 99%), a high number-average molecular weight (Mn = 7.2 × 105), and a narrow molecular weight distribution (PDI = 1.34) at an isoprene to initiator molar ratio of 6000:1.

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INTRODUCTION Organo rare-earth-metal chemistry has experienced spectacular growth in the past 30 years. In this development, rare-earthmetal alkyl complexes have attracted much attention because of their high activities and excellent performances in various organic transformations and polymerizations.1 To date, a large number of rare-earth-metal dialkyl complexes of the type LRE(R)2 (L = supporting ligands, RE = rare-earth metals, R = alkyl) have been reported. Upon treatment with an appropriate boron compound or a combination of appropriate boron compound and aluminum alkyls, dialkyl complexes can be converted into the corresponding cationic monoalkyl species [LRER]+, which can serve as excellent catalysts for the polymerization and copolymerization of a variety of olefins, especially higher olefins and conjugated 1,3-dienes, to yield a series of new polymer materials with novel properties.2−5 Organo rare-earth-metal monoalkyl complexes of the type L2LnR (L = supporting ligands, R = alkyl) can function as single-component catalysts in the polymerization of ethylene or © XXXX American Chemical Society

active polar monomers, such as alkyl acrylates and lactones, without requiring a cocatalyst.6 Unfortunately, these rare-earthmetal monoalkyl complexes have been shown to exhibit no or very low activity in the polymerization of higher olefins, conjugated 1,3-dienes, and cyclic olefins.7−13 In contrast to the relatively well developed organo rare-earth-metal dialkyl complexes, very few examples of organo rare-earth-metal monoalkyl complexes have been reported to exhibit activity toward polymerization of conjugated 1,3-dienes upon activation with appropriate boron compounds and aluminum alkyls. This observation has been attributed to the formation of stable η3allyl species in the initiation step; this species is inactive in the subsequent propagation of polymer chains.1f−h,12b The examples to date of rare-earth-metal monoalkyl complexes as initiators for 1,3-diene polymerization are as follows. Cui and co-workers reported that diphenylphosphine-functionalized Received: May 31, 2015

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DOI: 10.1021/acs.organomet.5b00467 Organometallics XXXX, XXX, XXX−XXX

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Organometallics aminopyridinato-supported rare-earth-metal monoalkyl complexes with cooperation from aluminum alkyls and an appropriate boron compound exhibited good 1,4-cis selectivity (up to 97.7%) in isoprene polymerization.12b Wakatsuki et al. reported that the Cp*2Gd[(μ-Me)AlMe2(μ-Me)]2GdCp*2, generated by treatment of monomethyl [Cp*2Gd(μ-Me)]2 with AlMe3, displayed good 1,4-cis selectivity (97.3%) in 1,3butadiene polymerization.14a,b We found that the amidinatesupported rare-earth-metal monoalkyl complexes were good catalysts for isoprene with a 1,4-selectivity up to 98%; however, the stereoselectivity (cis selectivity) was not very good.14c These results indicate that development of a new ligation system for organo rare-earth-metal monoalkyl complexes displaying high activity with high regio- and stereoselectivity for 1,3-conjugated diene polymerization is greatly needed. Recently, we developed μ-η2:η1:η1-ligated indolyl dinuclear rare-earth-metal alkyl complexes (monoalkyl/per metal) displaying high catalytic activity with high 1,4-cis selectivity (1,4-cis content up to 99%).14d We found that carbon σbonded indolyl-supported rare-earth-metal monoalkyl complexes exhibited high catalytic activity in isoprene polymerization with an extremely high regio- and stereoselectivity (1,4cis selectivity up to 99%).14e These results are different from those of Cui and co-workers, who found that 7-N-(aryl)iminomethyl-indolyl and iminomethyl-pyrrolyl ligand supported rare-earth-metal monoalkyl complexes are inert in the polymerization of isoprene.11b,19d These results led to the hypothesis that electron-rich indolyl ligation systems may have a preference for rare-earth-metal monoalkyl complexes as catalysts for selective isoprene polymerization. Thus, this prompted us to use the [N,N]-bidentate ligation system with rare-earth-metal monoalkyl complexes to determine whether this type of ligand-supported rare-earth-metal monoalkyl complex, in combination with cocatalysts, could polymerize conjugated 1,3-dienes. However, in recent years, there has been a surge in the development of noncarbocyclic ligands that support highly reactive rare-earth-metal complexes.15 The “post-metallocene era” has witnessed a diverse array of new ligand scaffolds that can support these metal ions by providing a wide variety of different steric and electronic environments. Among them, rareearth-metal complexes incorporating nitrogen-containing ligands, including α-diketimines (dianionic or radical anion forms of the diazadienes), amidinates, guadinates, β-diketimines, diamines, amino-functionalized pyridines, quinolines, and pyrroles (see Figure 1S in the Supporting Information), have attracted much attention due to their interesting structural features and applications in molecular catalysis such as polymerization of alkenes and lactides, intramolecular hydroamination of alkenes, and hydrophosphonylation of aldehydes and aldimines.16−21,23 More recently, we also reported a series of rare-earth-metal complexes supported by amino- or imino-functionalized indolyl ligands (see Figure 1S, j−l, in the Supporting Information).22a,b On the basis of the observations described above and our continuous interest in indolyl rare-earth-metal chemistry, we report the design, synthesis, and reactivities of a series of mononuclear rare-earth-metal monoalkyl complexes incorporating partially rotation-restricted [N,N]-bidentate 2-[(N-2,6diisopropylphenyl)iminomethyl)] indolyl ligands (Figure 1) that display high activity and high 1,4-cis selectivity (up to 99%) in isoprene polymerization in the presence of the cocatalysts AliBu3 and [Ph3C][B(C6F5)4]. To the best of our

Figure 1. Ligand design.

knowledge, this is the highest reported performance of a noncyclopentadienyl (non-Cp) mononuclear rare-earth-metal monoalkyl precursor (L2LnR) initiating isoprene 1,4-cis polymerization at an isoprene to initiator molar ratio of 6000:1.10b,12b,c,19f

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RESULTS AND DISCUSSION Synthesis and Characterization of Rare-Earth-Metal Monoalkyl Complexes. Treatment of [RE(CH2SiMe3)3(thf)2] with 1 equiv of 2-(2,6-iPr2C6H3N CH)C8H5NH at room temperature for 3 h afforded the mononuclear rare-earth-metal monoalkyl complexes [η1:η1-2(2,6-iPr2C6H3NCH)Ind]2RE(CH2SiMe3)(thf) (Ind = indolyl, RE = Yb (1), Er (2), Y (3), Dy (4), Gd (5)) (Scheme 1) in Scheme 1. Synthesis of the Complexes

good isolated yields in the reaction; the formation of the final complexes involved a ligand distribution process. These complexes can also be prepared in similar isolated yields by treatment of [RE(CH2SiMe3)3(thf)2] with 2 equiv of 2(2,6-iPr2C6H3NCH)C8H5NH at room temperature. Complexes 1−5 are extremely sensitive to air and moisture. However, attempts to isolate the samarium analogue from the corresponding reaction mixture under the same conditions failed because it produced the complex [η 1 :η 1 -2(2,6-iPr2C6H3NCH)Ind]3Sm (6) as the only isolated product, probably due to the larger ionic radius of samarium ion causing ligand redistribution (Scheme 1). Complexes 1−5 are soluble in common organic solvents such as THF and toluene, and complex 3 gave resolved NMR spectra in C7D8. In the 1H NMR spectra of complex 3, the methylene protons of Y−CH2SiMe3 exhibited an AB spin at −0.50 and −1.00 ppm B

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Organometallics with JH−H = 15.0 Hz, indicating that the two methylene protons are diastereotopic. Moreover, the methylene protons of Y− CH2SiMe3 gave doublet resonances at −0.50 ppm, due to coupling with the yttrium ion (2JY−H = 6 Hz). In addition, the signals centered at 28.6 ppm in the 13C NMR spectra are assigned to the resonances of the methylene carbon of Y− CH2SiMe3 coupled to the yttrium nucleus with 1JY−C = 25.0 Hz. Furthermore, the signal of the proton of the imino group of the indolyl ligand (−HCN) was located at 8.11 ppm, suggesting that the imino CN bonds are inactive in this reaction; this is different from our previous findings that reactions of 3(tBuNCH)C8H5NH with RE(CH2SiMe3)3(thf)2 produced amido-functionalized indolyl complexes through insertion of the alkyl CH2SiMe3 of RE(CH2SiMe3)3(thf)2 into the imino CN group.14d X-ray analyses (Figure 2) revealed that 1−5 are monoalkyl complexes incorporating two functionalized bidentate indolyl

Figure 3. Molecular structure of complex 6. Thermal ellipsoids are set at 30% probability. Hydrogen atoms are omitted for clarity.

the central metal on Y−CH2SiMe3 bonding. The corresponding RE(1)−N(1) and RE(1)−N(3) bond lengths (anionic indolyl nitrogen atoms) (Table 1) are not identical in the same complex, as for the RE(1)−N(2) and RE(1)−N(4) (the imino nitrogen atoms) bond lengths, indicating asymmetric arrangement in the complexes probably due to crystal-packing forces. However, the RE−N bond distances are also consistent with the trend in ionic radius of the corresponding elements. These RE−N bond distances fall within the range of 2.04−2.49 Å observed for the RE−N bonds in reported α-diketiminato rareearth-metal complexes.18e For example, the Y(1)−N(1) and Y(1)−N(3) bond lengths in 3 are longer than the Y(1)−N(2) bond length (2.288(3) Å) in the α-diketiminato ligated complex [(2,6- i Pr 2 C 6 H 3 )NC(Me)C(CH 2 )N(C 6 H 3 2,6-iPr2)]Y(CH2SiMe3)2(THF).19f The most striking features are the N(1)−RE(1)−N(2) angles of 72.0(1)° in 1, 71.6(1)° in 2, 71.8(1)° in 3, 71.2(1)° in 4, and 70.1(9)° in 5, which generally decrease with an increase in the corresponding metal ionic radius (Yb3+ < Er3+ < Y3+ < Dy3+< Gd3+ with coordination number 6). Similar results were obtained for the N(3)− RE(1)−N(4) angles. The N(1)−Y(1)−N(2) angles (71.8(1)°) in 3 are larger than the corresponding N(1)−Y−N(2) angle of 67.6(1)° found in [(2,6-iPr2C6H3)NC(Me)C(CH2)N(C6H3-2,6-iPr2)]Y(CH2SiMe3)2(THF)19f but smaller than the N(1)−Y−N(2) angle of 75.8(1)° found in [2-MeO-C6H4C(Me)CHC(Me)NC6H4-2-OMe]Y(CH2SiMe3)223 of the α-diketiminato chelating complexes. The N(1)−Yb(1)−N(2) angle (72.0(1)°) in 1 is very close to the N(1)−Yb−N(2) angle (72.8(5)°) found in [(2,6-iPr2C6H3NCHCHNC6H3iPr22,6)Yb(C5MeH4)2]23a but smaller than the N(1)−Yb(1)−N(2) angle (74.2(5)°) found in [Yb2(C9H7)3{PhNC(Me)C(Me)NPh}]23b containing the α-diketiminato ligand, indicating supporting ligand steric effects on the bonding angles. Reactivity Study of the Rare-Earth-Metal Monoalkyl Complexes. Rare-earth-metal hydride complexes are receiving intense interest because of their fascinating structures, high

Figure 2. Representative molecular structure of complexes 1−5. Thermal ellipsoids are set at 30% probability. Hydrogen atoms and the isopropyl groups are omitted for clarity.

ligands, one alkyl CH2SiMe3 group, and one THF and that the six-coordinate geometry of the central metal can be described as a distorted-octahedral geometry. The six-coordinate samarium complex 6 also adopts a distorted-octahedral geometry with six nitrogen atoms (Figure 3). The RE−CH2SiMe3 bond lengths in 1 (2.352(5) Å), 2 (2.379(4) Å), 3 (2.399(4) Å), 4 (2.381(5) Å), and 5 (2.427(4) Å) are generally in good agreement with the corresponding sequence of their ionic radii. The Y−C(CH2SiMe3) distance of 2.399(4) Å in 3 is slightly different from those found in [Me 3 C(2,6- i Pr 2 C 6 H 3 N)CHC(Me)Y(NCH 2 CH 2 N(2,6Me2C6H3))] (2.411(5) Å),18f {[3-(tBu-NCH(CH2SiMe3))Ind]Y(THF)(CH2SiMe3)}2 (2.410(5) Å),14d and [HC(N-2,6Me2C6H3)2]2YCH2SiMe3·(THF) (2.371(4) Å)14c and is slightly longer than those of 2.354(5) and 2.339(4) Å found in the corresponding five-coordinate carbon σ-bonded supported indolyl yttrium complexes,14e indicating the effect of the supporting ligands’ steric and coordination geometry of C

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Organometallics Table 1. Selected Bond Distances (Å) and Angles (deg) for 1−5 RE(1)−N(1) RE(1)−N(2) RE(1)−N(3) RE(1)−N(4) RE(1)−C(43) RE(1)−O(1) C(30)−N(4) C(9)−N(2) C(43)−RE(1)−N(1) C(43)−RE(1)−N(2) C(43)−RE(1)−N(3) C(43)-RE(1)-N(4) C(43)−RE(1)−O(1) O(1)−RE(1)−N(1) O(1)−RE(1)−N(2) O(1)−RE(1)−N(3) O(1)−RE(1)−N(4) N(1)−RE(1)−N(2) N(1)−RE(1)−N(3) N(1)−RE(1)−N(4) N(2)−RE(1)−N(3) N(2)−RE(1)−N(4) N(3)−RE(1)−N(4)

1

2

3

4

5

2.303(3) 2.493(3) 2.337(3) 2.534(3) 2.352(5) 2.299(3) 1.285(6) 1.291(5) 109.0(1) 101.8 (2) 154.1 (1) 84.6 (1) 95.2(1) 87.1(1) 156.4(1) 83.6(1) 91.8(1) 72.0(1) 96.8(1) 166.4(1) 88.1(1) 105.9(1) 69.6(1)

2.326(3) 2.500(3) 2.370(3) 2.577(3) 2.379(4) 2.314(3) 1.286(5) 1.295(5) 110.1 (1) 102.5 (1) 152.1 (1) 84.0(1) 95.2(1) 86.6(1) 155.5(1) 83.3 (1) 92.5(1) 71.6(1) 97.6(1) 165.9(1) 88.6(1) 106.0(1) 68.3(1)

2.323(3) 2.517(3) 2.391(3) 2.556(3) 2.399(4) 2.318(3) 1.300(5) 1.296(5) 108.4(1) 100.7(1) 156.1(1) 87.2(1) 95.1(1) 87.2(1) 156.8(1) 83.5(1) 91.0(1) 71.8(1) 95.4(1) 164.4(1) 88.7(1) 106.6(1) 69.0(1)

2.360(4) 2.527(3) 2.379(4) 2.576(4) 2.381(5) 2.346(3) 1.280(6) 1.301(6) 110.4(2) 103.3(2) 151.3(2) 83.7(2) 94.6(2) 86.4(1) 155.0(1) 83.5(1) 92.5(1) 71.2(1) 98.1(1) 165.9 (1) 88.8(1) 106.6(1) 67.9(1)

2.394(3) 2.552(3) 2.412(3) 2.630(3) 2.427(4) 2.372(3) 1.286(5) 1.289(4) 111.9(1) 104.5(1) 148.9(1) 82.2(1) 94.3(1) 86.0(9) 153.7(9) 83.3(1) 93.2(1) 70.1(9) 99.0(1) 166.0(1) 89.6(1) 107.3(9) 67.0(1)

are dinuclear complexes possessing two amido-appended indolyl ligands bonded to the central metals in a μ-η6:η1:η1 bridged manner and two imino-appended indolyl ligands bonded to metals in η1:η1 modes as the terminal ligands. The coordination modes of the bridged amido-appended indolyl ligands are similar to our previous findings in a europium(II) complex incorporating the imino-functionalized indolyl ligand, {[μ-η 6 :η 1 :η 1 -2-(2,6- i Pr 2 C 6 H 3 NCH)C 8 H 5 N]Eu I I [2(2,6-iPr2C6H3NCH)-C8H5N]}2, which revealed the redox chemistry between the europium(III) amide [(Me3Si)2N]3Eu(μ-Cl)Li(THF)3 and the indolyl-functionalized secondary amines that leads to oxidative dehydrogenation of the secondary amines to imino groups and the reduction of europium(III) to europium(II).22b To the best of our knowledge, this represents the first example of trivalent organo rare-earth-metal complexes that have an amido-functionalized indolyl ligand bonded to a metal in a μ-η6:η1:η1 mode. The 1H NMR spectrum of complex 8 shows a single resonance at 4.71 ppm, which can be assigned to the protons in −CH2−N, indicating that one of the CHN double bonds is reduced to a C−N single bond; this result is also supported by the X-ray analysis data (see Figure 4). Furthermore, the resonances of the alkyl and THF moieties are absent in comparison to the 1H NMR spectra of complex 3, suggesting that THF left complex 3 and the alkyl group in complex 3 was transferred in the process. The representative structures of 7 and 8 are shown in Figure 4, and the selected bond lengths and angles are given in Table 2. The Er(2)−C (C6 ring, C(24)− C(29)) distances range from 2.769(5) to 2.942(4) Å (with an average Er−C distance of 2.880(5) Å) in 7, and the Y(1)−C (C6 ring, C(24)−C(29)) bond distances range from 2.779(3) to 2.980(3) Å in 8 (with an average Y(2)−C distance of 2.888(3) Å) (Table 2). These values represent a significant πarene−RE interaction that is consistent with the average EuII− C distance (3.038(3) Å) found in {[μ-η 6 :η 1 :η 1 -2(2,6-iPr2C6H3NCH)C8H5N]EuII[2-(2,6-iPr2C6H3NCH)-

reactivities toward various unsaturated substrates, and activation of small molecules (for example: styrene, 1,3-cyclohexadiene, 1,4-bis(trimethylsilyl)-1,3-butadiyne, carbon (di)oxide, and benzonitrile)1g,h with isolation of the rare-earthmetal complexes from the corresponding reactions.1i,24 Given the advantages of the rare-earth-metal hydrides, reactions of the above monoalkyl complexes with the silane PhSiH3, generally used for transformation of the rare-earth-metal alkyls to rareearth-metal hydrides, were investigated (Scheme 2). To our surprise, when either complex 2 or 3 was treated with 1 equiv of PhSiH3 in toluene at 80 °C for 12 h, the novel dinuclear complexes {[μ-η6:η1:η1-2-(2,6-iPr2C6H3NCH2)Ind]REIII[2(2,6-iPr2C6H3NCH)Ind]}2 (Scheme 2) (Ind = indolyl, REIII = Er (7), Y(8)) were isolated in good yield as deep yellow crystals. X-ray analyses revealed that complexes 7 and 8 Scheme 2. Reactions of the Monoalkyl Complexes with PhSiH3

D

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Organometallics

amido group. A similar transformation of the imino group to an amido group by reduction of a rare-earth-metal hydride has also been proposed in the preparation of a yttrium hydride-silane complex.24g,h Reassembly with changes in the coordination modes of one of the ligands affords the final complexes (Scheme 2). Amidines are useful ligands of transition metals, and they can coordinate to the metal center in η1:η1 and η1:η6 modes and also induce changes in the coordination modes of other supporting ligands of the complexes.22b,25 Considering these characteristics and the above results, the reactions of the monoalkyl complexes 2 and 3 with [(2,6-iPr2C6H3)NCHNH(C6H3iPr2-2,6)] were examined to determine whether these complexes would create variations in the bonding modes of the indolyl or amidinate ligands. As shown in Scheme 3, reactions of 2 or 3 with 1 equiv of [(2,6-iPr2C6H3)NCHNH(C6H3iPr2-2,6)] in toluene at room Figure 4. Representative molecular structure of complexes 7 and 8. Thermal ellipsoids are set at 30% probability. Hydrogen atoms and 2,6-iPr2-C6H3− groups on N2, N4, N6, and N7 atoms are omitted for clarity.

Scheme 3. Reactions of the Monoalkyl Complexes with Amidine

Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) of Complexes 7 and 8 N(6)−C(51) N(7)−C(72) RE(2)−N(5) RE(2)−N(6) RE(2)−N(7) RE(2)−N(8) RE(2)−C(24) RE(2)−C(25) RE(2)−C(26) RE(2)−C(27) RE(2)−C(28) RE(2)−C(29) N(1)−Y(1)−N(2) N(5)−RE(2)−N(6) N(7)−RE(2)−N(8)

7

8

1.297(6) 1.445(6) 2.328(4) 2.468(4) 2.176(4) 2.360(4) 2.919(5) 2.819(5) 2.769(5) 2.811(5) 2.919(5) 2.942(4) 70.3(1) 70.5(2) 75.2(1)

1.299(4) 1.471(4) 2.323(3) 2.457(3) 2.163(3) 2.363(3) 2.905(4) 2.811(3) 2.779(3) 2.880(4 2.980(3) 2.975(3) 70.6(1) 70.4 (1) 75.2 (1)

temperature afforded the corresponding complexes [η1:η1-2(2,6- i Pr 2 C 6 H 3 NCH)-Ind] 2 RE[(2,6- i Pr 2 C 6 H 3 )NCHN(C6H3iPr2-2,6)] (Ind = indolyl, RE = Er (9), Y (10)). Complexes 9 and 10 are extremely sensitive to air and moisture. X-ray analyses (Figure 5) revealed that the amidine [(2,6-iPr2C6H3)NCHNH(C6H3iPr2-2,6)] replaced the alkyl group and THF molecule by deprotonation and coordination. These results are consistent with the 1H NMR spectrum of complex 10, in which no resonances of alkyl and THF were

C8H5 N]}2,22b the average DyIII−C distance (2.846(3) Å) found in the π-arene−Dy complex (η 6 -C 6 Me 6 )Dy[(μCl)2AlCl2)]3,22c and the average Nd−C distance (3.035 Å) found in Nd2(O-2,6-iPr2C6H3)622d when the difference in ionic radii is taken into account. In addition, the C(51)−N(6) bond lengths are 1.297(6) Å in 7 and 1.299(4) Å in 8, which are similar to 1.295(5) Å in 2 and 1.296(5) Å in 3, indicating a double-bond character of these bonds. However, the C(72)− N(7) bond lengths are 1.445(6) Å in 7 and 1.471(4) Å in 8, which are within the range of normal C−N single bonds, indicating that one of the imino groups in complexes 2 and 3 was reduced to an amido group. The N(5)−Y(2)−N(6) angle of 70.4° in 8 is close to the N(1)−Y(1)−N(2) angle of 70.6° in 8 but is much smaller than the N(7)−Y(2)−N(8) angle of 75.2° in 8, suggesting that the interaction of different ligands with the same metal center is different. The formation pathway for complexes 7 and 8 was proposed as follows. Reactions of 2 or 3 with PhSiH3 produce the corresponding hydride (monomer or dimer) as an intermediate. The resulting hydride immediately reacts with one of the imino groups of the ligands and produces the corresponding

Figure 5. Representative molecular structure of complexes 9 and 10. Thermal ellipsoids are set at 30% probability. Hydrogen atoms and 2,6-iPr2− groups are omitted for clarity. E

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Organometallics Table 3. Representative Data for the Polymerization of Isoprenea

entry 1 2 b 3 4 c 5 6 7 8 9 10 11 12 13 14 d 15 d 16 d 17 d 18 d 19 d 20 e 21

cat. 3 3 3 3 3 3 2 2 1 4 4 4 4 5 5 5 5 5 5 5 5

(Y) (Y) (Y) (Y) (Y) (Y) (Er) (Er) (Yb) (Dy) (Dy) (Dy) (Dy) (Gd) (Gd) (Gd) (Gd) (Gd) (Gd) (Gd) (Gd)

[IP]/[cat.]

[AlR3]

[B]/[cat.]

time (h)

conversn (%)

500/1 500/1 500/1 500/1 500/1 1000/1 500/1 1000/1 1000/1 500/1 1000/1 2000/1 3000/1 1000/1 1000/1 2000/1 3000/1 4000/1 5000/1 6000/1 3000/1

AlMe3 AlEt3 AliBu3 AliBu3 AliBu3 AliBu3 AliBu3 AliBu3 AliBu3 AliBu3 AliBu3 AliBu3 AliBu3 AliBu3 AliBu3 AliBu3 AliBu3 AliBu3 AliBu3 AliBu3 AliBu3

1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1 1/1

5 5 2 2 2 2 2.5 2.5 5 1 1.5 2.5 5 0.5 1 2 3 4 5 6 5

0 trace 82 ∼100 85 99 ∼100 97 0 ∼100 ∼100 99 98 ∼100 ∼100 ∼100 ∼100 ∼100 ∼100 97 99

1,4-cisg (%)

3,4g (%)

Tgh (°C)

Mnf (×10−4)

PDIf

97.3 98.0 98.0 98.0 97.1 97.1

2.8 2.0 2.0 2.0 2.9 2.9

−57 −58 −58 −59 −56 −57

15.3 21.5 21.8 41.1 21.1 27.8

2.01 2.11 2.13 1.75 2.02 2.01

96.2 96.2 96.2 96.2 98.0 99.0 99.0 99.0 99.0 99.0 99.0 99.0

3.8 3.8 3.8 3.8 2.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

−57 −58 −58 −57 −60 −60 −60 −61 −61 −60 −61 −61

20.9 32.9 39.1 42.0 33.7 34.2 35.1 36.5 38.1 49.1 71.5 63.9

2.28 1.92 1.80 1.76 2.00 1.83 1.72 1.65 1.54 1.45 1.34 1.59

Conditions unless specified otherwise: IP/C6H5Cl (solvent) = 1/20 (v/v); cat., 10 μmol; [AlR3]/[cat.] = 10/1; T = 10 °C; [B] = [Ph3C][B(C6F5)4]. b[AlR3]/[cat.] = 7.5/1. c[AlR3]/[cat.] = 12.5/1. dT = −10 °C. eIP/C6H5Cl (solvent) = 1/30 (v/v); T = −10 °C. fDetermined by means of GPC against polystyrene standards in THF at 30 °C. gDetermined by 1H and 13C NMR spectroscopy in CDCl3. hMeasured by differential scanning calorimetry (DSC). a

entries 3−5). These results suggest that AliBu3 plays the role of chain-transfer agent during polymerization. Meanwhile, various boron compounds, B(C 6 F 5 ) 3 , [Ph 3 C][B(C 6 F 5 ) 4 ], and [PhNMe2H][B(C6F5)4], were tested in the ternary catalytic system. Neither B(C6F5)3 nor [PhNMe2H][B(C6F5)4] initiated any detectable polymerization with addition of AliBu3. Fortunately, [Ph3C][B(C6F5)4] could act as a cationization agent. The solvent effect was also investigated for the catalytic reaction; when the polymerization was run in toluene or dichloromethane (CH2Cl2) under otherwise the same conditions, a 75% conversion with 91% 1,4-cis selectivity in toluene and a 75% conversion with 86% 1,4-cis selectivity in CH2Cl2 were observed, respectively. These results indicate that chlorobenzene (C6H5Cl) was the best solvent for the polymerization, resulting in polymers with a high 1,4-cis selectivity, probably due to its weak coordination with the resulting cationic species.27 The catalytic polymerization was effective at room temperature; low-temperature polymerization (−10 °C) improved the selectivity with a slight increase in the molecular weight of the polymers and a decrease in the molecular weight distribution (comparison of the data in entries 14 and 15, Table 3). There is a large difference in catalytic activity among the complexes investigated. Complex 1 showed no activity in isoprene polymerization under the same conditions, which can be attributed to the redox nature of Yb3+/Yb2+.22c,26 An increase in the monomer/catalyst ratio from 1000 to 6000 led to an increase in molecular weight and a decrease in the molecular weight distribution; meanwhile, 1,4cis selectivities remained stable at 99%, in which the polymerization time was proportional to the monomer/catalyst

observed, and the proton of the amidinate (N−HCN) resonated at 8.41 ppm in comparison with the 1H NMR spectra of complex 3. The structures of complexes 9 and 10 are similar to that of complex 6, and the central metal ion adopts a distorted-octahedral geometry with six nitrogen atoms on the basis of analyses of the bond lengths and bond angles of complexes 9 and 10 (see the Supporting Information). Isoprene Polymerization. The rare-earth-metal monoalkyl complexes 1−5 were tested as initiators of the polymerization of isoprene in the presence of cocatalysts AlR3 and borate ([Ph3C][B(C6F5)4]). Representative data are summarized in Table 3. The results indicate that the rare-earth-metal monoalkyl complexes 1−5 alone did not polymerize isoprene; neither did the binary systems (1−5)/borate and (1−5)/AlR3. Fortunately, the homogeneous ternary systems (2−5)/borate/ AliBu3 exhibited excellent activities and high 1,4-cis selectivities. The polymerization activities of the systems were significantly dependent on both the type of AlR3 and the [Al]/[Ln] molar ratio. When AlMe3 was used, no polymerization occurred (Table 3, entry 1). Only trace amounts of polymer were detected when AlEt3 was used (Table 3, entry 2). Using the same conditions but AliBu3 instead of AlMe3 or AlEt3 resulted in an 82% conversion of the monomer with 97.3% 1,4-cis selectivity in 2 h (Table 3, entry 3) when the AliBu3 to catalyst ratio was 7.5, indicating a steric effect of the aluminum alkyl group on the conversion of the homogeneous catalytic systems. A molar ratio of 10/1 is optimal for AliBu3 to catalyze the polymerization of isoprene; higher or lower ratios will decrease the conversion, and the molecular weight of the polymers and the molecular weight distribution will also be affected (Table 3, F

DOI: 10.1021/acs.organomet.5b00467 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Scheme 4. Proposed Mechanism of Isoprene 1,4-Cis Polymerization

propagation, the polymerization reaction produces 1,4-cispolyisoprene.

ratio for the gadolinium system, suggesting that the polymerization proceeded as a somewhat living character polymerization (Table 3, entries 15−20). The molecular weight of the polymers increases and the molecular weight distribution became narrow when the monomer/solvent ratio was changed from 1/20 to 1/30 (comparison of the data in entries 17 and 21, Table 3). Unexpectedly, the gadolinium system displayed excellent activity in isoprene polymerization, even with an isoprene/catalyst ratio of 6000/1, with an extremely high 1,4cis selectivity (up to 99%), producing polymers with a high number-average molecular weight (Mn = 7.2 × 105) and a very narrow molecular weight distribution (PDI = 1.34). To the best of our knowledge, this is the first example of mononuclear rareearth-metal monoalkyl catalytic systems displaying catalytic activity at such a low catalyst loading. The result is due to the perfect unity of the steric effect and electronic effect of the electron-rich indolyl ligand and to the effect of gadolinium. The 1H NMR probe reaction of 3 with [Ph3C][B(C6F5)4] indicates the abstraction of the alkyl species CH2SiMe3 in the process, which is similar to our recent findings of abstraction of the alkyl group from the carbon σ-bonded indolyl-supported complexes when the monoalkyl complex was treated with [Ph3C][B(C6F5)4]14e (see the Supporting Information); thus, the following polymerization mechanism is proposed (Scheme 4). THF is abstracted from A upon addition of AliBu3 to generate B and [AliBu3(thf)x] after combination with AliBu3. The cationic species C is generated after reaction of B with the borate [Ph3C][B(C6F5)4] by the alkyl group, rather than the ligands, as evidenced by the NMR probing results (see Supporting Information). Isoprene coordinates with the cationic center (D), and the subsequent 3,4-isoprene insertion into the RE−R (R = iBu) bond leads to the formation of an η3σ-R intermediate (E) as a secondary growing chain. The η3-σ-R species E is converted into intermediate F via rotation of the C3−C2 single bond. F is then converted into G through electronic delocalization. G then coordinates with a second molecule of isoprene at the metal center to form H. After

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CONCLUSIONS In summary, a series of mononuclear rare-earth-metal monoalkyl complexes [η1:η1-2-(2,6-iPr2C6H3NCH)Ind]2RE(CH2SiMe3)(thf) (Ind = indolyl, RE = Yb, Er, Y, Dy, Gd) incorporating a new type of partially rotation restricted electron-rich [N,N]-bidentate indolyl ligands were synthesized and characterized. The reactions of the monoalkyl complexes with PhSiH3 produced the first example of dinuclear rare-earthmetal(III) complexes {[μ-η6:η1:η1-2-(2,6-iPr2C6H3NCH2)Ind]REIII[2-(2,6-iPr2C6H3NCH)Ind]}2 (Ind = indolyl, REIII = Er, Y) possessing amido-appended indolyl ligands in μ-η6:η1:η1 hapticities through transformation of one of the imino functionalities to an amido functionality in the process. Reactions of the monoalkyl complexes with the amidine (2,6-iPr2C6H3)NCH−NH(2,6-iPr2C6H3) produced [η1:η1-2(2,6- i Pr 2 C 6 H 3 NCH)Ind] 2 RE[(2,6-iPr 2 C 6 H 3 )NCHNH(C6H3iPr2-2,6)] (Ind = indolyl, RE = Er, Y). In addition, the monoalkyl complexes 1−5 were tested as initiators in the polymerization of isoprene in the presence of cocatalysts AlR3 and borate ([Ph3C][B(C6F5)4]). Borate [Ph3C][B(C6F5)4] and the sterically bulky AliBu3 are the best cocatalyst reagents for the polymerization. The central metal ions greatly influence the catalytic activities of complexes in the order Gd > Dy > Er > Y ≫ Yb. Among the complexes investigated, the optimum combination, 5(Gd)/AliBu3/[Ph3C][B(C6F5)4], displayed excellent activity for isoprene polymerization with an extremely high 1,4-cis selectivity (up to 99%), producing polymers with a very high number-average molecular weight (7.2 × 105) and a very narrow molecular weight distribution (PDI = 1.34). This represents the first example of mononuclear rare-earth-metal monoalkyl catalytic systems displaying catalytic activity at such a low catalyst loading. This work further implies that the electron-rich indolyl ligands may have prevalence as supporting ligands in rare-earth-metal complexes as catalysts for selective isoprene polymerization. Further work is in progress. G

DOI: 10.1021/acs.organomet.5b00467 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

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(s), 2868 (s), 1632 (s), 1614 (s), 1016 (m), 923 (s), 802 (s), 739 (s), 667 (s). Anal. Calcd for C50H65N4OSiY: C, 70.23; H, 7.66; N, 6.55. Found: C, 69.90; H, 7.56; N, 6.67. Preparation of [η 1 :η 1 -2-(2,6- i Pr 2 C 6 H 3 NCH)Ind] 2 Dy(CH2SiMe3)(thf) (4). A method similar to that for the preparation of complex 1 was used, consisting of the treatment of a toluene (15.0 mL) solution of 2-(2,6-iPr2C6H3NCH)C8H5NH (0.30 g, 1.0 mmol) with a toluene (15.0 mL) solution of Dy(CH2SiMe3)3(THF)2 (0.57 g, 1.0 mmol) at room temperature. Faint yellow crystals were obtained at 0 °C (0.22 g, 48% yield based on the indolyl proligand). Mp: 172 °C under Ar. 1H NMR spectra of the complex were not obtained, due to the lack of locking signals due to paramagnetism. IR (KBr pellets, cm−1): ν 3053 (w), 2961 (s), 2870 (s), 1595 (s), 1578 (s), 1437 (w), 1016 (m), 932 (s), 923 (s), 802 (s), 756 (s), 667 (s). Anal. Calcd for C50H65DyN4OSi: C, 64.67; H, 7.05; N, 6.03. Found: C, 64.81; H, 7.01; N, 5.87. Preparation of [η 1 :η 1 -2-(2,6- i Pr 2 C 6 H 3 NCH)Ind] 2 Gd(CH2SiMe3)(thf) (5). A method similar to that for the preparation of complex 1 was used, consisting of the treatment of a toluene (15.0 mL) solution of 2-(2,6-iPr2C6H3NCH)C8H5NH (0.30 g, 1.0 mmol) with a toluene (15.0 mL) solution of Gd(CH2SiMe3)3(THF)2 (0.56g, 1.0 mmol) at room temperature. Faint yellow crystals were obtained at 0 °C (0.20 g, 43% yield based on the indolyl proligand). Mp: 180 °C under Ar. 1H NMR spectra of the complex were not obtained due to the lack of locking signals due to paramagnetism. IR (KBr pellets, cm−1): ν 2961 (s), 2870 (s), 1632 (s), 1614 (s), 1460 (m), 1016 (m), 923 (s), 862 (m), 816 (s) 802 (s), 739 (s). Anal. Calcd for C50H65GdN4OSi: C, 65.03; H, 7.09; N, 6.07. Found: C, 65.08; H, 7.08; N, 5.91. Complexes 1−5 can also be prepared in almost similar isolated yields by treating the corresponding RE(CH2SiMe3)3·2THF compound with 2 equiv of 2-(2,6-iPr2C6H3NCH)C8H5NH using similar workup procedures. Preparation of [η1:η1-2-(2,6-iPr2C6H3NCH)Ind]3Sm (6). A method similar to that for the preparation of complex 1 was used, consisting of the treatment of a toluene (15.0 mL) solution of 2(2,6-iPr2C6H3NCH)C8H5NH (0.30 g, 1.0 mmol) with a toluene (15.0 mL) solution of Sm(CH2SiMe3)3(THF)2 (0.63 g, 1.0 mmol) at room temperature. Faint yellow crystals were obtained at 0 °C (0.19 g, 54% yield based on the indolyl proligand). Mp: 186 °C under Ar. 1H NMR spectra of the complex were not obtained, due to the lack of locking signals due to paramagnetism. IR (KBr pellets, cm−1): ν 3051 (m), 2961 (s), 2864 (w), 1593 (s), 1576 (s), 1470 (w), 1016 (m), 922 (s), 878 (s), 800 (s), 756 (s), 730 (s). Anal. Calcd for C63H69N6Sm: C, 71.34; H, 6.56; N, 7.92. Found: C, 71.59; H, 6.32; N 7.65. This complex can also be prepared in 60% yield by the reaction of Sm(CH2SiMe3)3·2THF with 3 equiv of 2-(2,6-iPr2C6H3NCH)C8H5NH. Preparation of {[μ-η6:η1:η1-2-(2,6-iPr2C6H3NCH2)Ind]Er[η1:η12-(2,6-iPr2C6H3NCH)Ind]}2 (7). PhSiH3 (0.14 g, 1.2 mmol) at room temperature was added to a 20 mL toluene solution of 2 (0.56 g, 0.6 mmol). The reaction mixture was stirred at 80 °C for 12 h, and the solution turned from yellow to red. The solution gradually turned from muddy to clear at high temperature. The solvent was removed under reduced pressure. The residue was extracted with a mixture of hexane (10.0 mL) and toluene (2.0 mL). Yellow crystals were obtained at 0 °C (0.24 g, 51% yield). Mp: 262 °C under Ar. 1H NMR spectra of the complex were not obtained, due to the lack of locking signals due to paramagnetism. IR (KBr pellets, cm−1): ν 3053 (m), 2961 (s), 2884 (w), 1628 (m), 1617 (m), 1595 (s), 1576 (s), 1456 (m), 924 (s) 800 (s), 733 (s), 667 (s). Anal. Calcd for C84H94Er2N8: C, 65.08; H, 6.11; N, 7.23. Found: C, 65.32; H, 5.87; N, 7.57. Preparation of {[μ-η6:η1:η1-2-(2,6-iPr2C6H3NCH2)Ind]Y[η1:η1-2(2,6-iPr2C6H3NCH)Ind]}2 (8). A method similar to that for the preparation of complex 7 was used, consisting of the treatment of a toluene (25.0 mL) solution of 3 (0.51 g, 0.6 mmol) with PhSiH3 (0.14 g, 1.2 mmol) at 80 °C. Yellow crystals were obtained at 0 °C (0.19 g, 45% yield). Mp: 228 °C under Ar. 1H NMR (300 MHz, C7D8, 25 °C, TMS): δ 0.24−0.27 (m, 6H; (CH3)2CH−), 0.24−2.04 (m, 6H; (CH3)2CH−), 3.02 (m, 1H, −CHCH3)2), 3.19 (m, 1H, −CHCH3)2),

EXPERIMENTAL SECTION

General Methods. All syntheses and manipulations of air- and moisture-sensitive materials were performed under a dry and oxygenfree argon atmosphere using standard Schlenk techniques or in a glovebox. All solvents were distilled over sodium benzophenone ketyl under argon prior to use unless otherwise noted. B(C6F5)3, [Ph3C][B(C6F5)4], and [PhNMe2H][B(C6F5)4] were purchased from STREM in Shanghai, People’s Republic of China. AlMe3, AlEt3, and AliBu3 were purchased from Sigma-Aldrich and used as received. Isoprene was purchased from TCI, dried with CaH2, and distilled before polymerization. Elemental analysis data were obtained on a PerkinElmer Model 2400 Series II elemental analyzer. 1H NMR and 13C NMR spectra for compound analysis were recorded on a Bruker Model AV-300 NMR spectrometer (300 MHz for 1H; 75.0 MHz for 13C) in C6D6 for the rare-earth-metal complexes and in CDCl3 for polyisoprene. 13C-int D1 = 5 s NMR spectra were recorded on a Bruker Model AV-500 NMR spectrometer for analysis of polyisoprene in CDCl3. Chemical shifts are reported in ppm. J values are reported in Hz. IR spectra were recorded on a Shimadzu Model FTIR-8400s spectrometer (KBr pellet). Gel permeation chromatography (GPC) of the polymer samples was carried out at 30 °C using THF as an eluent on a Waters-2414 instrument calibrated using monodisperse polystyrene standards at a flow rate of 1.0 mL min−1. Preparation of [η 1 :η 1 -2-(2,6- i Pr 2 C 6 H 3 NCH)Ind] 2 Yb(CH2SiMe3)(thf) (1). A toluene (15.0 mL) solution of [Yb(CH2SiMe3)3(thf)2] (0.58 g, 1.0 mmol) was added to a toluene (15.0 mL) solution of 2-(2,6-iPr2C6H3NCH)C8H5NH (0.30 g, 1.0 mmol) at room temperature. The reaction mixture was stirred at room temperature for 3−4 h, and the solution turned red. The solvent was removed under reduced pressure. The residue was extracted with a mixture of hexane (20.0 mL) and toluene (20.0 mL). Dark red crystals were obtained at 0 °C (0.20 g, 43% yield based on the indolyl proligand). Mp: 215 °C under Ar. 1H NMR spectra of the complex were not obtained, due to the lack of locking signals due to paramagnetism. IR (KBr pellets, cm−1): ν 3048 (w), 2961 (s), 2868 (s), 1632 (s), 1614 (s), 1585 (s), 1460 (w), 1016 (m), 923 (s), 816 (m), 802 (s), 739 (s). Anal. Calcd for C50H65N4OSiYb: C, 63.94; H, 6.98; N, 5.79. Found: C, 63.75; H, 7.11; N 5.46. Preparation of [η 1 :η 1 -2-(2,6- i Pr 2 C 6 H 3 NCH)Ind] 2 Er(CH2SiMe3)(thf) (2). A method similar to that for the preparation of complex 1 was used, consisting of the treatment of a toluene (15.0 mL) solution of 2-(2,6-iPr2C6H3NCH)C8H5NH (0.30 g, 1.0 mmol) with a toluene (15.0 mL) solution of Er(CH2SiMe3)3(THF)2 (0.57 g, 1.0 mmol) at room temperature. Yellow crystals were obtained at 0 °C (0.24 g, 52% yield based on the indolyl proligand). Mp: 176 °C under Ar. 1H NMR spectra of the complex were not obtained, due to the lack of locking signals due to paramagnetism. IR (KBr pellets, cm−1): ν 3049 (w), 2961(s), 2868 (s), 1632(s), 1614 (s), 1584 (s), 1458 (s), 1016 (m), 923 (s), 800 (m), 737 (s), 667 (s). Anal. Calcd for C50 H65ErN4OSi: C, 64.34; H, 7.02; N, 6.00. Found: C, 63.96; H, 7.19; N 5.79. Preparation of [η 1 :η 1 -2-(2,6- i Pr 2 C 6 H 3 NCH)Ind] 2 Y(CH2SiMe3)(thf) (3). A method similar to that for the preparation of complex 1 was used, consisting of the treatment of a toluene (15.0 mL) solution of 2-(2,6-iPr2C6H3NCH)C8H5NH (0.61 g, 1.0 mmol) with a toluene (15.0 mL) solution of Y(CH2SiMe3)3(THF)2 (0.49 g, 1.0 mmol) at room temperature. The solvent was removed under reduced pressure. The residue was extracted with a mixture of hexane (16.0 mL) and toluene (16.0 mL). Yellow crystals were obtained at 0 °C (0.18 g, 41% yield based on the indolyl proligand). Mp: 195 °C under Ar. 1H NMR (300 MHz, C7D8, 25 °C, TMS): δ −0.50 (d, 1H; 2 JY−H = 6.0 Hz; Y−CH2−), −1.00 (s, 1H; Y−CH2−), −0.50/−1.00 (AB, JH−H = 15.0 Hz, 2H, CH2SiMe3), −0.06 (s, 9H; SiMe3), 0.48− 1.14 (m, 12H; (CH3)2CH−), 0.77−0.81 (m, 4H; hexane), 1.33 and 2.46 (m, 4H; THF), 3.01 (m, 1H; −CHMe2), 3.81 (d, 4H; THF), 6.37(s, 2H; 3-indole), 8.11(s, 2H; HCN), 6.86−7.98 (m, 14H; ligand ring), 1.05−1.33 (m, 6H; hexane). 13C NMR (75 MHz, C7D8, 25 °C, TMS): δ 170.1, 168.1, 150.5, 148.3, 142.2, 141.8, 131.7, 129.7, 128.9, 125.8, 115.2, 32.5, 28.6 (d, 1C; 1JY−C = 25 Hz; Y−CH2−), 25.5, 23.8, 22.4, 21.9, 15.7, 14.4. IR (KBr pellets, cm−1): ν 3049 (w), 2961 H

DOI: 10.1021/acs.organomet.5b00467 Organometallics XXXX, XXX, XXX−XXX

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Organometallics 3.44 (m, 1H, −CHCH3)2), 4.10 (m, 1H, −CHCH3)2), 4.49 (s, 2H; −CH2−N), 4.71 (s, 2H; −CH2−N), 7.94 (s, 1H; HCN), 7.97 (s, 1H; HCN), 6.00−7.86 (m, 8H; ligand ring). 13C NMR (75 MHz, C7D8, 25 °C, TMS): δ 168.9, 166.7, 164.1, 150.3, 147.9, 145.8, 143.1, 133.1, 123.9, 122.4, 117.7, 97.5, 57.3, 31.9, 28.7, 27.6, 26.2, 25.3, 22.9. IR (KBr pellets, cm−1): ν 3053 (m), 1595 (s), 1578 (s), 1437 (m), 924 (s), 802 (s), 756 (s), 667 (s). Anal. Calcd for C84H94N8Y2: C, 72.40; H, 6.80; N, 8.04. Found: C, 72.56; H, 6.71; N, 8.31. Preparation of [η 1 :η 1 -2-(2,6- i Pr 2 C 6 H 3 NCH)Ind] 2 Er[(2,6-iPr2C6H3)NCHN(C6H3iPr2-2,6)] (9). [(2,6-iPr2C6H3)NCHNH(C6H3iPr2-2,6)] (0.22 g, 0.6 mmol) was added to a 30 mL toluene solution of 2 (0.56 g, 0.6 mmol) at room temperature. The reaction mixture was stirred at room temperature for 12 h, and the solution turned yellow. The solvent was removed under reduced pressure. The residue was extracted with a mixture of hexane (15.0 mL) and toluene (3.0 mL). Yellow crystals were obtained at 0 °C (0.307 g, 45% yield). Mp: 196 °C under Ar. 1H NMR spectra of the complex were not obtained, due to the lack of locking signals due to paramagnetism. IR (KBr pellets, cm−1): ν 2961 (s), 2961 (s), 2866 (s), 1663 (s), 1632(s), 1452 (s), 1287 (s), 1179 (m), 924 (s) 800 (s), 739 (s), 660 (s) cm−1; Anal. Calcd for C67H81ErN6: C, 70.73; H, 7.18; N, 7.39. Found: C, 70.52; H, 6.95; N, 7.23. Preparation of [η 1 :η 1 -2-(2,6- i Pr 2 C 6 H 3 NCH)Ind] 2 Y[(2,6-iPr2C6H3)NCHN(C6H3iPr2-2,6)] (10). A method similar to that for the preparation of complex 9 was used, consisting of the treatment of a toluene (25.0 mL) solution of 3 (0.51 g, 0.6 mmol) with [(2,6iPr2C6H3)NCHNH(C6H3iPr2-2,6)] (0.22 g, 0.6 mmol) at room temperature. Yellow crystals were obtained at 0 °C (0.29 g, 46% yield). Mp: 216 °C under Ar. 1H NMR (300 MHz, C7D8, 25 °C, TMS): δ 0.57 (m, 6H; (CH 3 ) 2 CH−), 0.75−0.76 (m, 6H; (CH 3 ) 2 CH−), 1.09 (m, 6H; (CH 3 ) 2 CH−), 1.16 (m, 6H; (CH 3 ) 2 CH−), 1.50 (m, 6H; (CH 3 ) 2 CH−), 2.74 (m, 1H; −CHMe2), 2.95 (m, 1H; −CHMe2), 3.16 (m, 1H; −CHMe2), 3.60 (m, 1H; −CHMe2), 7.75 (s, 1H; HCN), 8.41 (s, 1H; N−HCN), 6.70−7.20 (m, 8H; ligand ring). 13C NMR (75 MHz, C7D8, 25 °C, TMS): δ 176.7, 166.9, 149.1, 146.9, 146.0, 144.6, 141.6, 141.0, 130.2, 129.1, 128.9, 125.4, 116.0, 32.0, 28.9, 25.3, 24.5, 22.4, 22.2, 20.6, 20.4, 20.1. IR (KBr pellets, cm−1): ν 2961 (s), 1595 (s), 1578 (s), 1470 (m), 1425 (m), 1342 (s), 924 (s), 880 (s), 785 (s), 756 (s), 667 (s). Anal. Calcd for C67H81 N6Y: C, 75.97; H, 7.71; N, 7.93. Found: C, 75.86; H, 7.50; N, 7.82. Polymerization of Isoprene. All complexes were tested as initiators of the polymerization of isoprene in the presence of cocatalysts AlR3 and [Ph3C][B(C6F5)4]. Under a nitrogen atmosphere, complex 4 (0.01 g, 10.0 μmmol) in C6H5Cl (3.0 mL) and AliBu3 (0.02 g, 100 μmmol) in toluene (0.1 mL) were placed in a 50 mL flask. A solution of [Ph3C][B(C6F5)4] (0.01 g,10.0 μmmol) in C6H5Cl (2.0 mL) was then added. Then, C6H5Cl (15.0 mL) was added to the flask. Isoprene (1.00 mL, 0.68 g, 10.0 mmol) was then added, and the mixture was stirred vigorously for 2 h. The resulting viscous solution was poured into a large quantity of methanol to afford polyisoprene solids, which were dried under vacuum at 40 °C to a constant weight (0.67 g, 99%).

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21432001, 21372010, 21372009), the National Basic Research Program of China (2012CB821600), and the Special and Excellent Research Fund of Anhui Normal University.

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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.5b00467. Characterization data and spectral data and tables of crystallographic data and structure refinement details for complexes 1−10 (PDF) X-ray crystallographic data for complexes 1−10 (CIF)

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DOI: 10.1021/acs.organomet.5b00467 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.5b00467 Organometallics XXXX, XXX, XXX−XXX