Synthesis and Characterization of Rare-Earth Metal Complexes

Article pubs.acs.org/Organometallics

Synthesis and Characterization of Rare-Earth Metal Complexes Supported by 2‑Imino or Amino Appended Indolyl Ligands with Diverse Hapticities: Tunable Selective Catalysis for Isoprene Polymerization Guangchao Zhang,† Shaowu Wang,*,†,‡ Xiancui Zhu,† Shuangliu Zhou,† Yun Wei,† Zeming Huang,† and Xiaolong Mu† †

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

ABSTRACT: The reaction of 2-(2,6-DippNHCH2)C8H5NH (Dipp = 2,6-iPrC6H3, C8H5NH = indolyl) with 1 equiv of (Me3SiCH2)3Yb(THF)2 at room temperature generated mononuclear ytterbium complex 1 having the indolyl ligands in η1:η1 mode with reduction of Yb3+ to Yb2+ and oxidation of the amino to imino group. In the case of Er and Y, the reactions produced dinuclear complexes 2 and 3 having the indolyl ligands in μ-η2:η2:η1 modes with the central metals. When the rare-earth metal is dysprosium, the reaction afforded mixed ligated dinuclear complex 4a having indolyl ligands in μη5:η1:η1 and μ-η6:η1:η1 modes with Dy, and its isomer 4b having the indolyl ligands only in μ-η5:η1:η1 modes with Dy. However, when the rare-earth metal is Gd, the reaction only produced the mixed ligated dinuclear gadolinium complex [(μ-η5:η1:η1)-2-(2,6-DippNCH2)Ind(μ-η6:η1:η1)-2-(2,6-DippNCH2)Ind][Gd(CH2SiMe3)(thf)]2 (5), having indolyl ligands in μ-η5:η1:η1 and μ-η6:η1:η1 modes with Gd. In addition, treatment of 2(2,6-DippNHCH2)C8H5NH with 1.25 equiv of (Me3SiCH2)3Gd(THF)2 produced the alkoxido-bridged trinuclear gadolinium complex [(μ-η3:η2:η1:η1)-2-(2,6-DippNCH2)Ind(μ-η2:η1:η1)-2-(2,6-DippNCH2)Ind-(η1:η1)-2-(2,6-DippNCH2)Ind]-Gd3[(μ3-O(CH2)5SiMe3)(μ2-O(CH2)5SiMe3)(thf)3] (6) having indolyl ligands in η1:η1, μ-η2:η1:η1, and μ-η3:η2:η1:η 1 modes with metals, respectively. In complex 6, sp2 C−H activation is observed at the 7-indolyl position producing unique 2-amido substituted indolyl-1,7-dianions having a μ-η3:η2:η1:η1 bonding modes with three metals. The O(CH2)5SiMe3 arises from the ring-opening of THF by attack of CH2SiMe3. Moreover, when 2-(2,6-DippNHCH2)C8H5NH was treated with 1 equiv of (Me3SiCH2)3Sm(THF)2, a dinuclear samarium complex [μ-η3:η1:η1-2-(2,6-DippNCH2)Ind]3Sm2(thf)3 (7) having a bridged indolyl ligand in μη3:η1:η1 hapticities was isolated. All structures of the complexes have been determined by X-ray crystallographic analyses. Dinuclear alkyl complexes 2−5 have been tested as isoprene polymerization initiators in the presence of AliBu3 and [Ph3C][B(C6F5)4]. The regioselectivity for isoprene polymerization is tunable from 1,4-cis (up to 93.5%) to 3,4- (up to 86.2%) selectivity by these catalysts simply by adjusting the addition order of AliBu3 and [Ph3C][B(C6F5)4].

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INTRODUCTION

Yb(thf)4 and {[2-(Ph)Ind]2Yb(DME)}2 were found to bind with the ytterbium in η1 and μ2-η1:η1 modes (coordination types a and b in Chart 1).10a The η3 bonding mode via the nitrogen atom and the fused carbons of the indolyl ring of the 2,3-dimethylindolyl ligand with rare-earth metals was found in complexes [(2,3−Me2)Ind]LnCp*2 (Cp* = η5-C5Me5, Ln = Y and Sm) (type c).10b The simple indolyl ligand was found to bond with europium(II) ion in μ−η1:η5 modes in [(Ind)2Eu-

Indole and its derivatives are widely distributed in biological systems as an important structural motif of biomolecules and natural products.1 The chemistry of indolyl compounds with transition metal have been developed with findings of versatile bonding modes and plenty of chemistry for their electron-rich property.2−10 In contrast, the chemistry of indolyl rare-earth metal complexes is yet to be developed. Early works on indolyl rare-earth metal complexes focused on coordination modes of the indolyl ligands with rare-earth metals. For example, the 2phenylindolyl ligands in the ytterbium complexes [2-(Ph)Ind]© XXXX American Chemical Society

Received: July 28, 2017

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DOI: 10.1021/acs.organomet.7b00575 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

complex 1 (Scheme 1, please also see Scheme S1 for the proposed formation pathyway in SI) having indolyl ligands in

Chart 1. Coordination Modes of the Indolyl Ligands with Rare-Earth Metals

Scheme 1. Synthesis of Complex 1

η1:η1 mode with central metal with reduction of Yb3+ to Yb2+ and oxidation of the amino group to the imino group. This chemistry is similar to our previous findings of the reaction of 2-(2,6-DippNHCH2)C8H5NH with 1 equiv of [(Me3Si)2N]3Yb(μ-Cl)Li(THF)3 producing the corresponding ytterbium(II) complex at elevated temperature.11i However, the homolytic δYb−C bond, which is consistent with the result that we found in samarium chemistry,11j could result in the reduction of Yb(III) to Yb(II). When the rare-earth metals are Er and Y, the reactions produced dinuclear complexes 2 and 3 (Scheme 2) having

(II)(NH3)3]2 (type d) via the nitrogen atom and the fivemembered heterocyclic indolyl moiety.3a Recently, our group has studied the reactivity of functionalized indoles with rareearth metal amides or alkyls with findings of new bonding modes of the indolyl ligands with rare-earth metals (types e−j) and new reactivity patterns and catalyst systems.11a−i In this work, new bonding modes were found (types k−n). In contrast, a variety of rare-earth metal catalytic systems have been reported for the polymerization of conjugated dienes, such as butadiene and isoprene, with high regio- and stereoselectively specificity.11d,e,12−16 However, the catalytic systems that can conveniently tune the selectivity for the polymerization of 1,3-conjugated dienes via changing the catalytic precursors and cocatalysts are limited.12 It is reported that 1,4-cis, 1,4-trans, and 3,4-selective polymerizations of isoprene can be regulated by simply modifying the auxiliary ligands, metal centers, and initiation groups of rare-earth metal complexes and cocatalysts.12a The metal-dependent control of the 1,4-cis and 1,4-trans selectivity for the polymerization of butadiene was found with the Cp*2Ln[(μ-Me)AlMe2(μMe)]2LnCp*2 and cocatalyst systems.12b It is also found that replacements of cocatalysts MAO by AlEt3 or Al(iBu)3 have influence on not only the activity of the catalysts but also the selectivity (1,4-cis or 1,4-trans-) for the isoprene polymerization.12d A controllable 1,4-cis and 3,4- selective polymerization of isoprene by addition of AlMe3 was reported with the yttrium amidinate catalyst and cocatalyst systems.12e These results have been attributed to different active centers which initiated the polymerization in different ways. Very recently, metal and counteranion effects on microstructure of polymers were found in scandium complexes catalyzed isoprene polymerization and copolymerization.12f Herein, we wish to report the studies of the reactions of 2(2,6-DippNHCH2)C8H5NH with (Me3SiCH2)3RE(THF)2 generated a series of new mono-, di-, or trinuclear rare-earth metal complexes incorporating 2-imino or amino appended indolyl ligands with findings of diverse hapticities of the ligands with central metals depending on the metals, and tunable selectivity for isoprene polymerization.

Scheme 2. Synthesis of Complexes 2 and 3

indolyl ligands in μ-η2:η2:η1 modes with the central metals. The chemistry is different from not only the above ytterbium chemistry but also the results of reactions of 2-(2,6DippNHCH2)C8H5NH with 1 equiv of [(Me3Si)2N]3Y(μCl)Li(THF)3 at elevated temperature.11i These differences can be attributed to larger ionic radii of Y and Er, the larger redox potentials for RE3+/RE2+ (RE = Er and Y),17 and the ligands’ different steric demand (Me3SiCH2 vs (Me3Si)2N). When the rare-earth metal ion was changed to Dy, with a bit larger ionic radius than that of Y or Er, the reaction afforded new mixed ligated dinuclear complex 4a having indolyl ligands in μ-η5:η1:η1 and μ-η6:η1:η1 modes with metal and its isomer 4b having the indolyl ligands in μ-η5:η1:η1 modes with central metal (Scheme 3). When the rare-earth metal is Gd, the reaction only produced mixed ligated dinuclear complex 5 having indolyl ligands in μ-η5:η1:η1 and μ-η6:η1:η1 modes with metal (Scheme 3). However, treatment of 2-(2,6-DippNHCH2)C8H5NH with 1.25 equiv of (Me3SiCH2)3Gd(THF)2 produced alkoxidobridged trinuclear complex 6 having indolyl ligands in η1:η1, μη 2 :η 1 :η 1 , and μ-η 3 :η 2 :η 1 :η 1 modes with central metals respectively (Scheme 4). In complex 6, sp2 C−H activation is observed at the 7-indolyl position producing a 2-amido substituted indolyl-1,7-dianions having a new μ-η3:η2:η1:η1 bonding modes with three metals (highlighted with blue in the scheme). The alkoxido group O(CH2)5SiMe3 is obviously

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RESULTS AND DISCUSSION Synthesis and Characterization of Rare-Earth Metal Complexes 1−7. The reaction of 2-(2,6-DippNHCH2)C8H5NH with 1 equiv of (Me3SiCH2)3Yb(THF)2 in THF or toluene at room temperature generated mononuclear ytterbium B


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Organometallics 1

H NMR spectra of complex 3 showed two double peak signals at high field at −0.84 and −0.99 ppm exhibiting an AB spin with JH−H = 11.4 Hz (see Figure S8), clearly indicating the presence of the alkyl ligand and the two methylene protons are diastereotopic. Moreover, the methylene protons of the Y− CH2SiMe3 gave doublet resonances at −0.84 ppm, due to coupling with the yttrium ion (2JY−H = 3 Hz). In addition, the signals centered at 31.2 ppm in the 13C NMR spectra (see Figure S9) are assigned to the resonances of the methylene carbon of the Y−CH2SiMe3 coupled to the yttrium nucleus with 1JY−C = 42.5 Hz. Furthermore, the signals of the proton of the amido group of the indolyl ligand (−H2C−N) were found at 4.33 and 5.15 ppm, suggesting that the methylene amido group CH2N are inactive under this condition, which is different to our previous findings.11b X-ray analyses revealed that 2−5 are dinuclear complexes (Figures 1−3), that two central metals were bridged by two

Scheme 3. Synthesis of Complexes 4a, 4b, and 5

Scheme 4. Synthesis of Complex 6

arising from the ring-opening of THF by attack of Me3SiCH2.11k−m When the rare-earth metal is Sm, having a larger ionic radius than Gd, the reaction of 2-(2,6-DippNHCH2)C8H5NH with 1 equiv of (Me3SiCH2)3Sm(THF)2 at room temperature afforded new dinuclear samarium complex 7 (Scheme 5), in which the Scheme 5. Synthesis of Complex 7

Figure 1. Representative molecular structure of complexes 2 and 3. Thermal ellipsoids are set at 30% probability. Hydrogen atoms and 2,6-iPr2C6H3- groups on N2 and N4 atoms are omitted for clarity.

functionalized indolyl ligands, and that each metal center was surrounded by one alkyl Me3SiCH2 group and one THF. In complexes 2 and 3, each metal ion has a coordination number of 5, while in complexes 4a, 4b, and 5, each metal ion has a coordination number of 7. These results indicated that larger ionic radius of central metal ion favors larger coordination number complexes. In the formation of complexes 2 and 3, the 2-[(N-2,6-diisopropylphenyl)aminomethyl)]indole was deprotonated by the metal alkyls of (Me3 SiCH 2 ) 3 RE(THF) 2 affording the new dianionic species that bonded with central metals forming the dinuclear rare-earth metal alkyl complexes (Scheme 2). The RE−CH2SiMe3 bond lengths in 2 (2.336(1) Å) and 3 (2.351(5) Å) are in good agreement with their corresponding ionic radii sequence. X-ray analyses revealed that the bond length of C(30)−N(4), 1.458(1) Å in 2 and 1.472(6) Å in 3 (see Table S1), is close to the bond length of C(5)− N(2) (1.467 (9) Å) found in pyrrolyl-functionalized secondary amines ligated complex {2-[(2,6-Dipp)NCH2]C4H3N]ErN(SiMe3)2}2.18a The RE(1)−N(1), RE(1)−C(8), RE(2)−N(1), RE(2)−C(1), RE(1)−N(3), RE(1)−C(22), RE(2)−N(3), and RE(2)−C(29) bond lengths of 2.422(6), 2.908(9), 2.427(8), 3.048(1), 2.422(7), 3.054 (6), 2.442(6), and 2.867(9) Å in 2, respectively, and 2.428(4), 3.021(5), 2.474(4), 3.052(2), 2.419(4), 3.043(4), 2.430(4), and 2.964(5) Å in 3, respectively, suggested that the indolyl moiety intended to bond with the metal in new bridged μ-η2:η2 modes. To the best of our

indolyl ligands took the bridged ligation forms. One of the samarium ions was ligated by the ligands in the new μ-η3 hapticity of the fused ring atoms and three oxygen atoms of THF have coordination numbers of 9, while the other one was ligated by six nitrogen atoms with coordination numbers of 6. The above results indicated that the central metal not only impacts on the reactivity pathways but also affects the bonding modes of the ligands with central metal ions. X-ray diffraction revealed that the central metal of complex 1 adopts a six-coordinate octahedral geometry with the central metal ions bonding with the nitrogen atoms of indolyl moiety and the nitrogen atoms of imino groups and the oxygen atoms of THF, and structural parameters are similar to our previous work (Figure S1),11i but the synthesis route is different. The bond length of N2−C9 of 1.372(12) Å in 1 is a normal CN double bond length, indicating that the amino group was oxidized to the imino group, as further proved by the NMR results. In 1H NMR spectrum, the resonance of the proton of the imino group (NCH) of indolyl ligand in 1 is found at 8.06 ppm. In 13C NMR spectrum, the resonance of the carbon of the imino group (NCH) is observed at 162.3 ppm. These data are also in agreement with our previous report.11i C


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Organometallics

η6:η1:η1 hapticities to generate a distorted trigonal bipyramidal geometry of each central metal by setting the centroids of the five-membered heterocylic ring (or the centroids of the sixmembered ring) of the indolyl groups, the amino nitrogen N4, and alkyl carbon C51 occupying the equatorial positions, while indolyl nitrogen N3 and oxygen of THF are at axial positons. It represents the first example of an organometallic complex having the indolyl ligands bonding with metal in the mixed ligated fashions. The bond distances between the central metal ion and the five-membered heterocyclic ring range from 2.779(6) to 2.838(5) Å, with an average Dy(1)−Ind length of 2.818(5) Å in 4a and that from 2.807(4) to 2.867(6) Å with an average of Gd(1)−Ind of 2.835(6) Å in 5 (Table 2). These distances are comparable with the average Sm−Ind distance of 2.875(10) Å and Nd−Ind distance of 2.882(3) Å found in the corresponding amido complexes.11i These values are comparable to the corresponding values in the dinuclear indolyl complex of [(Ind)2Eu(II)(NH3)3]210c and the dinuclear c o m p l e x o f [ 2 -( 2 -Ph 2 PC 6 H 3 NC ( H ) (C H 2 S iM e 3 )) C4H3N]2Y2(CH2SiMe3)2(THF);19b therefore, the bonding of the five-membered heterocyclic ring of indolyl ligand with the metal ion in 4a and 5 is best described as the η5 mode on the basis of these structural parameters, when the difference of the ionic radii is considered.18c The RE(2)−C(C6 ring, [C(24)− C(29)]) bond distances range from 2.843(5) to 2.981(5) Å (with an average Dy(2)−C distance of 2.921(5) Å) in 4a and from 2.863(5) to 2.997(5) Å (with an average Gd(2)−C distance of 2.938(5) Å) in 5 (Table 1). These values are considered to represent a π-arene−RE interactions that are consistent with the corresponding values in complexes {[2-(2,6DippNCH2)Ind]RE[2-(2,6-DippNCH)Ind]}2 (RE = Y (with an average Y−C distance of 2.863(5) Å) and Er (with an average Y−C distance of 2.888(5) Å),11f complex {[ 2-(2,6DippNCH)C 8 H 5 N]Eu II [2-(2,6-DippNCH)C 8 H 5 N]} 2

knowledge, these complexes represent the first example of the rare-earth metal alkyl complexes bearing indolyl ligand in this bonding mode. The N(3)−RE(1)−N(4) angles of 76.9(2)° in 2 and 77.0(1)° in 3 are larger than the corresponding N(1A)− Y−N(2A) angle of 66.55(8)° found in [(2-(Me2NCH2)C4H3N)Y(CH2SiMe3)2]218b and the N(1)−Y−N(2) angle of 72.6(6)° found in [(2,6-Dipp)NC(CH2)C(CH2)N(C6H32,6-Dipp)]Y(μ2-Cl)2Li-(THF)2,19a owing to electronic and steric effects from different ligands. The representative molecular structure of 4a and 5 is displayed in Figure 2, and the selected bond lengths and angles

Figure 2. Representative molecular structure of complexes 4a and 5. Thermal ellipsoids are set at 30% probability. Hydrogen atoms and 2,6-iPr2C6H3-groups on N2 and N4 atoms are omitted for clarity.

are given in Table S1. X-ray analyses revealed that complexes 4a and 5 are dinuclear complexes having two indolyl ligands bonding with metals in bridged ways in novel μ-η5:η1:η1 and μTable 1. Representative Data of Polymerization of Isoprenea

entry

[Cat.]

[IP]/[Cat.]

[AlR3]

[B]/[Cat.]

t (h)

conv. (%)

1,4-cis (%)e

3,4 (%)e

Mn (× 10−4)d

PDId

1 2 3b 4 5c 6 7 8 9 10 11 12 13 14 15 16 17

3(Y) 3(Y) 3(Y) 3(Y) 3(Y) 3(Y) 3(Y) 3(Y) 2(Er) 2(Er) 2(Er) 4(Dy) 4(Dy) 4(Dy) 5(Gd) 5(Gd) 5(Gd)

1000:1 1000:1 1000:1 1000:1 1000:1 2000:1 3000:1 4000:1 1000:1 3000:1 5000:1 1000:1 2000:1 3000:1 1000:1 3000:1 5000:1

AlMe3 AlEt3 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

5 5 1 1 1 2 3 5 0.5 1.5 4 1 2 3 0.5 1 2

0 trace 97 99 97 99 99 98 99 99 98 99 99 98 99 99 97

90.1 93.5 89.3 92.6 91.7 91.7 79.6 81.3 80.7 92.6 90.1 88.5 92.6 91.7 90.1

9.9 6.5 10.7 7.4 8.3 8.3 20.4 18.7 19.3 7.4 9.9 11.5 7.4 8.3 9.9

6.6 7.2 9.3 7.6 7.8 9.6 9.1 10.7 12.2 5.5 8.0 9.5 6.2 6.8 9.0

4.18 3.15 4.71 3.22 3.12 3.36 2.79 2.72 2.72 2.63 2.92 2.70 2.01 2.98 2.68

Conditions: IP/C6H5Cl (solvent) = 1:10 (v/v) ; Cat. = 10 μmol; [AlR3]/[Cat.] = 10:1; T = 25 °C ; [B] = [Ph3C][B(C6F5)4. b[AlR3]/[Cat.] = 7.5:1. c[AlR3]/[Cat.] = 12.5:1. dDetermined by means of GPC against polystyrene standards in THF at 30 °C. eDetermined by 1H NMR spectroscopy in CDCl3. a

D


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Organometallics (with an average EuII−C distance of 3.038(3) Å),11b and complex (η6-C6Me6)Dy[(μ-Cl)2AlCl2)]3 (with an average Dy− C distance of 2.846(3) Å)20a when the ionic radii difference is taken into account.19c Therefore, the bonding of the sixmembered ring of indolyl ligand with the metal ion in 4a and 5 is best described as a μ-η6:η1 mode . These results indicated the indolyl ligands bonded with the metal ions in complexes 4a and 5 in an unusual mixed ligated fashion. The average bond distances of RE(2)−C (C6 ring, [C(24)−C(29)]) (2.921(5) Å in 4a and 2.938(5) Å in 5) are longer than these of RE(1)−fivemembered heterocyclic ring (2.818(5) Å in 4a, 2.835(6) Å in 5) in the same complexes. As identified by X-ray analysis (Figure 3), complex 4b is an isomer of complex 4a with a centrosymmetric in the solid state.

It was documented that indenyl ligand can bond with transition metals with different hapticities including the expected η5 bonding mode, and the η5 → η3 → η1 modes through “ring slippage”, the η9 bonding mode by “ring folding”,20c,d and the η6 bonding modes found in groups IV20e and VI20f indenyl metal complexes although it is less electron-rich than the indolyl ligand, except with transition metals. However, only the η5, η3, μ−η4:η2, and μ−η4:η5 bonding modes were found in indenyl rare-earth metal chemistry.20g−j The η6 bonding indenyl ytterbium(II) complex was proposed as an intermediate for the reaction of bisindenyl ytterbium(II) complex with excess trimethylaluminum AlMe320k and in a theoretical version.20i Different from the indenyl ligand, the indolyl ligand is an electron-rich heterocyclic ligand with donor nitrogen atom in the five-membered ring, displays rich coordination chemistry, and prefers to form a bridged bonding mode with the Lewis acidic metal center as shown in Chart 1. Thus, in case of indolyl ligand, the μ-η6:η1 and the bridged μη2:η2 bonding modes of rare-earth metal complexes can be stabilized and isolated. X-ray analysis revealed that trinuclear rare-earth metal complex 6 (Figure 4) contains two different bridged (μ3- and

Figure 3. Molecular structure of complex 4b. Thermal ellipsoids are set at 30% probability. Hydrogen atoms and 2,6-iPr2C6H3- groups on N2 and N2i atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Dy(1)−C(1) 2.753(6), Dy(1)−C(2) 2.914(7), Dy(1)−C(3) 2.939(7), Dy(1)−C(8) 2.761(6), Dy(1)−C(9) 2.401(7), Dy(1)−N(1) 2.619(5), Dy(1)−N(2) 2.235(5), Dy(1)− N(1i) 2.452(5), Dy(1)−O(1) 2.389(4), C(29)−N(2) 1.470(8), N(2)−Dy(1)−O(1) 87.69(1), N(2)−Dy(1)−C(9) 124.8(3), O(1)− Dy(1)−C(9) 88.4(2), N(2)−Dy(1)−N(1i) 71.8(1), N(1)−Dy(1)− C(1) 29.7(1), N(2)−Dy(1)−C(8) 92.8(1), O(1)−Dy(1)−C(8) 96.7(1). Figure 4. Molecular structure of complex 6. Thermal ellipsoids are set at 30% probability. Hydrogen atoms, the carbon atoms of the THF molecules and 2,6-iPr2-C6H3- groups on N2, N4, and N6 atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Gd(1)−C(50) 3.029(6), Gd(1)−O(1) 2.271(4), Gd(1)−O(2) 2.466(4), Gd(1)−N(1) 2.488(5), Gd(1)−N(2) 2.221(5), Gd(1)− N(5) 2.546(5), Gd(2)−O(1) 2.261(4), Gd(2)−O(2) 2.415(4), Gd(2)−N(3) 2.351(5), Gd(2)−N(4) 2.237(5), Gd(2)−C(7) 2.521(6), Gd(2)−C(8) 2.829(6), Gd(3)−N(1) 2.672(5), Gd(3)− N(5) 2.514(5), Gd(3)−N(6) 2.255(6), Gd(3)−O(2) 2.488(4), Gd(3)−C(7) 2.570(6), Gd(3)−C(8) 2.881(6), Gd(2)−C(8)− Gd(3) 87.71(1), N(2)−Gd(1)−N(1) 76.6(1), N(4)−Gd(2)−N(3) 77.0(1), N(6)−Gd(3)−N(5) 75.1(1), Gd(2)−O(1)−Gd(1) 106.4(1), Gd(2)−O(2)−Gd(1) 96.0 (1), Gd(2)−O(2)−Gd(3) 107.6(1).

The amido-appended indolyl ligands coordinated to the dysprosium ions in μ-η5:η1:η1 modes to generate a distorted trigonal bipyramidal geometry with the centroids of fivemembered heterocyclic ring of indolyl group, the amino nitrogen N(2), and alkyl carbon C(9) occupying the equatorial positions, while indolyl nitrogen N(1i) and oxygen O(1) of THF are at axial positions. The μ-η5:η1:η1 bonding modes of the amido-appended indolyl ligand are the same as those of samarium and neodymium amido complexes.11i However, the rare-earth metal alkyl complexes with indolyl ligands in this kind of bonding mode has not been documented. Interestingly, the two planes of the indolyl ligands are parallel and the orientations of the six-membered rings of the indolyl ligands are opposite. The bond distances between dysprosium and the fivemembered heterocyclic ring range from 2.753(6) to 2.939(7) Å, with an average of 2.797(5) Å, are comparable to the Sm-η5C(pyrrolyl ring) (2.850(4)−2.928(4) Å) in a pyrrolyl samarium complex.20b The distances are also comparable with those of Nd to corresponding five-membered ring of the indolyl ligand and those of Sm to the corresponding five-membered ring of the indolyl ligand,11i when the difference of the ionic radii is taken into account.19c

μ2-) alkoxido groups Me3SiCH2(CH2)4O arising from the alkyl group Me3SiCH2 attacking the THF molecules11e,21,22 and three different ligated functionalized indolyl ligands. One trianionic indolyl ligand is via unexpected 7-indolyl sp2 C−H activation and deprotonation of two protons on nitrogen atoms with N(1) bridged Gd(1) and Gd(3). C(7) and C(8) bridged with Gd(2) and Gd(3), thus forming unprecedented μ−η3:η2:η1 bonding modes. One dianionic amido-appended E


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Organometallics indolyl ligand coordinated with Gd(1) and Gd(3) in bridged form via N atom of indolyl ring in μ−η2:η1 fashion; one amidoappended indolyl ligand only bonded with Gd(2) through the nitrogen atoms of N(3) and N(4) of the indolyl ring and the appended arm. Therefore, the central metal ions Gd(1), Gd(2), and Gd(3) have coordination numbers 6, 5, and 7, respectively, for different ligation forms resulting in different steric surrounding around the central ions. The bond distances of Gd(1)−O(1) (2.271(4) Å) and Gd(2)−O(1) (2.261(4) Å) are consistent with the corresponding values in SmIII2SmII[nBuN-{CH2-(2-OC6H2-3,5-tBu2)}2]4 (average 2.363(5) Å)23a and [3,5-tBu2-2-OC6H2CH2N-8C9H6NYbCl(THF)]2 (2.230(4) and 2.251(4) Å),23b when the difference in ionic radii is considered, indicating that O(1) atom bridges two gadolinium ions Gd(1) and Gd(2). The bond distances of Gd(1)−O(2) (2.466(4) Å), Gd(2)−O(2) (2.415(4) Å), and Gd(3)−O(2) (2.488(4) Å) suggested that O(2) atom bridges three gadolinium atoms Gd(1), Gd(2), and Gd(3). However, the bond distances of Gd(1,2,3)-μ3-O(2) are much longer than those of Dy-μ3-O (Dy(1)−O(1) (2.123(3) Å), Dy(2)−O(1) (2.281(3) Å), and Dy(1)−O(1) (2.097(3) Å)) found in complex [(3-(2-C4H3N−CHNCH2CH2)C8H5N]Dy3(μ3-O)(μ2-Cl)2(THF)2[N(SiMe3)2], in which the μ3-O atom and the three dysprosium ions are coplanar.11h The bond distances of Gd(1)−C(50) (3.029(6) Å), Gd(1)−N(5) (2.546(5) Å), and Gd(3)−N(5) (2.514(5) Å) reveal that gadolinium ions Gd(1) and Gd(3) are bridged through the indolyl ligand in μ-η2:η1 modes. The bond distances of Gd(3)− C(7) (2.570(6) Å), Gd(3)−C(8) (2.881(6) Å), and Gd(3)− N(1) (2.672(5) Å) indicated that the indolyl ligand bonded with Gd(3) ion in η3 mode; the bond distances of Gd(2)−C(7) (2.521(6) Å) and Gd(2)−C(8) (2.829(6) Å) suggested that this indolyl ligand bonded with Gd(2) ion in η2 mode. The bond distances of Gd(1)−N(1) (2.488(5) Å) indicated the indolyl ligand bonded with Gd(1) ion in an η1 mode through the indolyl nitrogen atom. These results indicated that gadolinium ions Gd(3), Gd(2), and Gd(1) are bridged through indolyl ligand in an unprecedented μ-η3:η2:η1 fashion. In addition, the bond distances of Gd(2)−C(7) (2.521(6) Å) and Gd(3)−C(7) (2.570(6) Å) in complex 6 are closed to RE− C(1) (ranging from 2.506(12) Å to 2.665(8) Å) found in the tetranuclear complexes {[η1:(μ2-η1:η1):η1-3-(tBuNCH)C8H5N]-REIII2(μ2-Cl)2(THF)[N(SiMe3)2](η1:η1-[μ−η5:η2-3(tBu-NCH−)C8H5N]2Li)}2 (RE = Ho, Y, Er, and Yb) having a unique indolyl-1,2-dianion in η1:(μ2-η1: η1) bonding mode through C−H activation at the 2-indolyl position.11a,g Complex 6 represents the first rare-earth metal complex having a unique indolyl ligand bonding with metal in μ-η3:η2:η1 modes. X-ray analyses revealed that the central Sm ions in complex 7 were coordinated by the ligands in different ways (Figure 5): One samarium ion was ligated by indolyl ring atoms and three THF, and the other samarium atom was coordinated by six nitrogen atoms of the indolyl rings and the appended arms. The bond lengths of C(7)−Sm(1), C(8)−Sm(1), and N(1)−Sm(1) in complex 7 are 3.024(4), 3.025(4), and 2.848(4) Å, respectively, indicating that the indolyl ligands bonded with the Sm(1) in η3 mode. The bond lengths of Sm(2)−N(1), Sm(2)−N(2)i, and Sm(2)−N(2ii) are 2.536(4), 2.423(3), and 2.423(3) Å, in the range of normal σ-bridged bond and comparable with those of Er−N or Y−N bond found in complexes 2 or 3, indicating the indolyl moiety bonded with Sm(2) in μ-η3:η1 hapticities, similar to the Evans’ findings.10b This bonding mode is also similar to the recent reported η3

Figure 5. Molecular structure of complex 7. Thermal ellipsoids are set at 30% probability. Hydrogen atoms and 2,6-iPr2C6H3- groups on N2, N2i, and N2ii atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): C(9)−N(2) 1.448(6), C(7)−Sm(1) 3.024(4), C(8)−Sm(1) 3.025(4), N(1)−Sm(1) 2.848(4), Sm(2)−N(2) i 2.423(3), N(2)−Sm(2) 2.423(3), N(1)−Sm(2) 2.536(4), N(1)ii− Sm(1)−N(1) 71.32(12), Sm(2)−N(1)−Sm(1) 88.56(11).

indenyl ruthenium complex,20l but in the present samarium case, the indolyl ligand has coordination of η3:η1 modes for the electron-rich property of indolyl ring and the presence of donor nitrogen atom coordination with Lewis acidic Sm3+ center. As a result, the Sm(1) in complex 7 has a coordination number of 9, while Sm(2) has a coordination number of 6 for the steric demanding substituent 2,6-diisopropylphenyl group attached to the N2, N2i, and N2ii atoms. Isoprene Polymerization. As evidenced, only limited organo-rare-earth metal monoalkyl complexes or complexes bearing one alkyl group/per metal exhibited activity on controllable polymerization of 1,3-conjugated dienes.11e,f,k Dinuclear rare-earth metal alkyl complexes 2−5 were investigated as initiators for the polymerization of isoprene in the presence of cocatalysts AlR3 and borate ([Ph3C][B(C6F5)4]). Representative data are summarized in Tables 1 and 2. The results indicated that dinuclear rare-earth metal alkyl complexes (monoalkyl/per metal) 2−5 alone did not polymerize isoprene, and neither did binary systems (2−5)/borate nor (2−5)/AlR3. Fortunately, the homogeneous ternary systems of (2−5), borate, and AliBu3 exhibited excellent activities and high selectivity. Furthermore, the regioselectivity of polymerization by these catalyst systems can be switched from 1,4-cis (up to 93.5%) to 3,4- (up to 86.2%) selectivity simply by adjusting the addition order of AliBu3 and [Ph3C][B(C6F5)4] in the different solvents. In homogeneous ternary systems (2−5)/AlR3/borate, the catalytic activity was strongly affected by the type of aluminum alkyls with the trend of AliBu3 ≫ AlEt3 > AlMe3. When AlMe3 or AlEt3 was employed to activate 3, no or trace polymer was found in 5 h (Table 1, entries 1 and 2). Homogeneous ternary systems (2−5)/AliBu3/borate showed excellent activity and high 1,4-cis selectivity. The catalytic activity and 1,4-cisselectivity of the systems and the molecular weight of the polymers were also significantly dependent on the molar ratio of [AliBu3]/[Cat]. Increasing the molar ratio of [AliBu3]/[Cat] from 7.5 to 12.5 significantly led to the increase of the F


Article

Organometallics Table 2. Representative Data of Polymerization of Isoprenea microstructure (%) entry

[Cat.]

[IP]/[Cat.]

[AlR3]

[B]/[Cat.]

t (h)

conv. (%)

3,4e

1 2 3b 4 5c 6 7 8 9 10 11 12 13 14

3(Y) 3(Y) 3(Y) 3(Y) 3(Y) 3(Y) 3(Y) 2(Er) 2(Er) 4(Dy) 4(Dy) 5(Gd) 5(Gd) 5(Gd)

1000:1 1000:1 1000:1 1000:1 1000:1 2000:1 3000:1 1000:1 2000:1 1000:1 2000:1 1000:1 3000:1 5000:1

AlMe3 AlEt3 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

5 5 1 1 1 2 5 2 3 0.25 0.5 0.5 2 4

0 62 97 99 99 99 78 99 96 99 97 99 99 97

34.7 82.0 83.3 81.3 82.0 86.2 73.3 81.2 73.5 70.7 28.6 29.0 19.7

mme

69.5 69.8 69.1 68.9 69.0 76.8 76.1 75.3 75.1

1.4-cis (%)e

Mn (× 10−4)d

PDId

65.3 18.0 16.7 18.7 18.0 13.8 26.7 18.8 26.5 29.3 71.4 71.0 80.3

7.0 9.8 12.2 8.9 15.5 22.5 11.6 13.2 9.4 13.6 5.9 6.5 18.1

2.31 1.96 2.33 1.80 2.71 3.63 3.20 3.05 2.85 2.95 3.04 3.09 3.30

Conditions: IP/CH2Cl2(solvent) = 1:10 (v/v) ; Cat. = 10 μmol; [AlR3]/[Cat.] = 7.5:1; T = 25 °C ; [B] = [Ph3C][B(C6F5)4. b[AlR3]/[Cat.] = 5:1. [AlR3]/[Cat.] = 10:1. dDetermined by means of GPC against polystyrene standards in THF at 30 °C. eDetermined by 1H NMR and 13C NMR spectroscopy in CDCl3. a c

molecular weight of the polymers (Table 1, entries 3−5). The results are not consistent with increasing the ratio of Al/Cat., as this usually leads to decrease of molecular weight due to chain transfer;23c−e however, they are consistent with Okuda’s23f and Evans’s23g works, which show that aluminum does not play the role of chain transfer in the catalytic system. Meanwhile, boron compounds such as B(C6F5)3 and [PhNMe2H][B(C6F5)4] initiated any detectable polymerization with addition of AliBu3. Fortunately, [Ph3C][B(C6F5)4] could play the role of the cationizing agent to activate the catalyst. The solvents effects on activity and selectivity of the polymerization of isoprene were surveyed with complex 3.11e,24 When the polymerizations were run in toluene, dichloromethane (CH2Cl2), or chlorobenzene (C6H5Cl) under other same conditions, a 91% conversion with 92.6% 1,4-cis-selectivity in toluene, a 75% conversion with 92.6% 1,4cis-selectivity in CH2Cl2, and a 99% conversion with 93.5% 1,4cis-selectivity in C6H5Cl (Table 1, entry 3), respectively, were observed in 1 h with NMR spectra identification of the polymers, and no 1,4-trans content of polymers were found. These results indicated that C6H5Cl was the best solvent for the polymerization, resulting in polymers with high 1,4-cis selectivity, probably due to its weak coordination with the resulting cationic species·11m,25 Moreover, the influences of the monomer to catalyst ratio on catalytic performance and the microstructures of polymers were found. Taking the catalyst complex 3 as an example, increasing the monomer to catalyst ratio from 1000 to 4000 led to an increase of molecular weight and a relatively narrow molecular weight distribution (Table 1, entries 4, 6−8). In order to evaluate the role of the central metal type in this system, complexes 2, 4 (4a, 4b), and 5 were evaluated under the same conditions (Table 1, entry 9−17). Complex 2 displayed a relatively higher catalytic activity and relatively lower 1,4-cis selectivity than did complex 3 (comparison of the data in entries 4, 6−8, and 9−11, Table 1) although they have the same coordination modes, indicating that metal centers have a certain influence on the catalytic activity and selectivity of isoprene polymerization. Similar results were found between complexes 4 and 5 (comparison of the data in entries 12−14 and 15−17, Table 1) which displayed different catalytic activity

with similar coordination modes. It is found that complex 5 displayed notable catalytic activity and 1,4-cis-selectivity (Table 1, entries 15−17). It is also found that different coordination modes of the complexes only have a little influence on the catalytic activity and the regio- and stereoselectivity (comparison of the data in entries 4, 6−8, and 12−14, Table 1). To our surprise, the regioselectivity for isoprene polymerization is tunable by these catalysts simply by adjusting the addition of cocatalysts AliBu3 and [Ph3C][B(C6F5)4] in sequence of borate, [Ph3C][B(C6F5)4], then AliBu3, when the catalytic polymerization was performed. Homogeneous ternary systems (2−5)/borate/AliBu3 showed excellent activity and high 3,4- selectivity (up to 86.2% in CH2Cl2 and up to 80.6% in chlorobenzene (C6H5Cl)), indicating that the main cause of the selectivity change from 1,4-cis selectivity to 3,4- selectivity is the addition order of AliBu3 and [Ph3C][B(C6F5)4] rather than solvent effects. Homogeneous ternary systems (2−5)/borate/AlR3 were investigated in detail, and representative data are summarized in Table 2. It is found that the type of aluminum alkyls not only have influence on the catalytic activity of the complexes, but also have influence on regioselectivity of the polymerization. AlMe3 displayed no activity, and AlEt3 exhibited moderate activity producing polymers with controlled 1,4-cis selectivity, and the AliBu3 displayed the highest activity among the aluminum alkyls surveyed producing polymers with controlled 3,4- selectivity (Table 2, entry 3). These results are different from the above results from addition of cocatalysts in sequence of AliBu3, then borate [Ph3C][B(C6F5)4]. This difference is probably due to different intermediates produced in the processes. In the present case, addition of borate [Ph3C][B(C6F5)4] will produce the cationic species first,26 while in the above case, addition of aluminum alkyls will probably lead to generate the heterorare-earth metal/aluminum alkyls complexes.12e,27 Increasing or decreasing the [AliBu3]/[Cat.] ratio only has a little influence on the selectivity (Table 2, entries 3−5). It is found that different complexes also have an influence on the selectivity of the ternary catalyst systems (Table 2, entries 3 and 5−10). Erbium complex 2 showed a similar 3,4- selectivity to that of yttrium complex 3, while dysprosium complex 4, which has a larger ionic radius than Er and Y, showed a lower 3,4G


Article

Organometallics

CDCl3 for polyisoprene. 13C-int D1= 5 s NMR spectra were recorded on a Bruker Model AV-500 NMR spectrometer (500 MHz for 1H; 125.0 MHz for 13C) for analyses of polyisoprene in CDCl3. Chemical shifts are reported in ppm. J values are reported in Hz. IR spectra were recorded on a Shimadzu Model FT-IR-8400s spectrometer (KBr pellet). Gel permeation chromatography (GPC) analyses of the polymer samples were carried out at 30 °C using THF as an eluent on a Waters-2414 instrument and calibrated using monodisperse polystyrene standards at a flow rate of 1.0 mLmin−1. Preparation of [η1:η1-2-(2,6-DippNCH)Ind]2Yb(II)(thf)2 (1). To a THF (15.0 mL) solution of 2-(2,6-DippNHCH2)C8H5NH (0.31 g, 1.0 mmol) was added a THF or toluene (15.0 mL) solution of (Me3SiCH2)3Yb(THF)2 (0.58 g, 1.0 mmol) at room temperature. The reaction mixture was stirred at room temperature for 3 h, and the color of the solution remained red during the reaction. The solvent was removed under reduced pressure. The residue was extracted with a mixture of hexane (10.0 mL) and toluene (3.5 mL). Red crystals were obtained upon standing the solution at 0 °C for several days (0.14 g, 31% yield). Mp: 231 °C under Ar. 1H NMR (500 MHz, THF-d8): δ 8.06 (s, 2H, CHN), 7.42 (d, 2H, Ar-H), 7.23 (m, 4H, Ar-H), 7.09 (m, 4H, Ar-H), 6.94 (m, 2H, Ar-H), 6.72 (s, 2H, Ar-H), 5.47 (s, 2H, 3indole-H), 4.15 (s, 8H, CH2-O), 3.14 (m, 4H, CH(CH3)3), 1.69 (s, 8H, CH2CH2-O), 1.17 (s, 24H, CH(CH3)3). 13C NMR (125 M, THFd8): δ 162 (CHN), 148, 143, 138, 136, 135, 130, 127, 122, 121.1, 119, 117, 115, 109, 99 (indole-C and Ar-C), 48 (CH2−O), 30 (CH2CH2−O), 26 (CH(CH3)3), 24 (CH(CH3)3), 12 (CH(CH3)3). IR (KBr pellets, cm−1): v 2955 (w), 1928 (w), 1881 (w), 1456 (s), 1413 (s), 1370 (s), 1317 (m), 1286 (s), 1195 (w), 1064 (m), 1051 (s), 993 (m), 792 (s), 744 (s). Anal. Calcd for C50H62N4O2Yb: C, 64.99; H, 6.76; N, 6.06. Found: C, 64.76; H, 6.69; N, 6.28. Preparation of {[μ-η 2 :η 2 :η 1 -2-(2,6-DippNCH 2 )Ind]Er(CH2SiMe3)(thf)}2 (2). A similar method for preparation of complex 1 was used by treatment of a THF (15.0 mL) solution of 2-(2,6DippNHCH2)C8H5NH (0.31 g, 1.0 mmol) with a THF or toluene (15.0 mL) solution of (Me3SiCH2)3Er(THF)2 (0.57 g, 1.0 mmol) at room temperature. The reaction mixture was stirred at room temperature for 3 h, and the color of the solution changed from faint yellow to yellow during the reaction. The solvent was removed under reduced pressure. The residue was extracted with a mixture of hexane (10.0 mL) and toluene (2.0 mL). Pink crystals were obtained upon standing the solution at 0 °C for several days (0.27 g, 45% yield). Mp: 148 °C under Ar. The NMR spectra data of the complex were not obtained for lack of locking signals due to paramagnetism. IR (KBr pellets, cm−1): v 2955 (w), 2341 (s), 1456 (s), 1413 (s), 1303 (s), 1286 (s), 1195 (w), 1064 (m), 1051 (s), 993 (m), 792 (s), 744 (s). Anal. Calcd for C58H86 Er2N4O2Si2: C, 55.20; H, 6.87; N, 4.44. Found: C, 55.45; H, 6.81; N 4.42. Preparation of {[μ-η 2 :η 2 :η 1 -2-(2,6-DippNCH 2 )Ind]Y(CH2SiMe3)(thf)}2 (3). A similar method for preparation of complex 1 was used for treatment of a THF (15.0 mL) solution of 2-(2,6DippNHCH2)C8H5NH (0.31 g, 1.0 mmol) with a THF or toluene (15.0 mL) solution of (Me3SiCH2)3Y(THF)2 (0.49 g, 1.0 mmol) at room temperature. The color of the solution changed from pink to yellow during the reaction. Faint yellow crystals were obtained upon standing the solution at 0 °C for several days (0.28 g, 51% yield). Mp: 141 °C under Ar. 1H NMR (300 MHz, C7D8, 25 °C, TMS): δ −0.84 (dd, 2H; 2JY−H = 3.0 Hz; Y−CH2−), −0.99 (dd, 2H; Y−CH2−), −0.84/−0.99 (AB, JH−H = 11.4 Hz, 4H, CH2SiMe3), 0.17(s, 18H, SiMe3), 1.00 (m, 12H, hexane), 1.19(m, 24H, (CH3)2CH−), 1.32 and 3.11(m, 16H, THF), 4.27−4.33 (s, 2H, −CH2CN), 5.10−5.16 (s, 2H, −CH2CN), 6.34(s, 2H; 3-indole-H), 6.93−8.46 (m, 8H, ligand ring). 13 C NMR (125 MHz, C7D8, 25 °C, TMS): δ 160.3 (N-CAr), 153.2 (CH2−C2‑indole), 146.1, 137.5, 129.1, 128.9, 128.7, 127.8, 124.9, 124.0, 123.2, 120.2, 118.2, 97.2 (indole-C and Ar-C), 57.6 (CH2−O), 31.3 (d, 1C, 1JY−C = 42.5 Hz, Y−CH2), 30.9 (CH2CH2−O), 27.7 (CH(CH3)3), 25.5 (CH(CH3)3), 20.3 (CH(CH3)3), 14.3 (CH(CH3)3). IR (KBr pellets, cm−1): v 2954(s), 2868 (s), 1587 (s), 1456 (s), 1413 (s), 1286 (m), 1195 (s), 1051 (m), 792 (s), 744 (s). Anal. Calcd for C58 H86Y2N4O2Si2: C, 63.02; H, 7.84; N, 5.07. Found: C, 63.23; H, 7.56; N 5.19.

selectivity than that with Er and Y. Gadolinium complex 5 showed the lowest 3,4- selectivity but produces the 1,4-cis major content polymers. The result can be attributed to the difference of molecular structure and metal ionic radii, increasing the ionic radii will weaken the ligand’s steric effects on controlling the 3,4- regioselectively polymerization. In order to get the difference between homogeneous ternary systems (2−5)/AliBu3/borate and (2−5)/borate/AliBu3, the reactions of 3 with AliBu3, 3 with borate, 3 with AliBu3 and then borate, and 3 with borate and then AliBu3 were monitored independently by 1H NMR technique in C7D8 at room temperature (see Figures S34−S36). The two methylene protons signals of the alkyl ligand have been kept at −0.75 and −1.22 ppm in 1H NMR spectra of the reaction of 3 with AliBu3, compared with those in the 1H NMR spectra of 3, but with changed chemical shifts, indicating that the heterorareearth metal/aluminum alkyls complexes were generated. However, only one alkyl signal was found at −0.75 ppm in 1 H NMR spectra of the reaction of 3 with [Ph3C][B(C6F5)4], suggesting that the other alkyl was abstracted by [Ph3C][B(C6F5)4] generating the cationic species. The differences between the 1H NMR spectrum of the reaction of 3 with AliBu3 and borate and that of 3 with borate and AliBu3 indicated that different kinds of intermediates were produced due to addition order of AliBu3 and [Ph3C][B(C6F5)4]. Different catalysts led to the difference of polymer in the regio- and stereoselectivity, which are consistent with the literature reports in essence.12 These experimental results provided some clues to the catalytic reaction, however, further experiments needs to be carried out to identify the intermediates involved in the catalytic cycles.

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CONCLUSIONS A series of new mono-, di-, or trinuclear rare-earth metal complexes incorporating 2-imino or amino appended indolyl ligands were synthesized and characterized by studying the reactions of 2-(2,6-DippNHCH2)C8H5NH (Dipp = 2,6diisopropylphenyl) with (Me3SiCH2)3RE(THF)2 with findings of different reactivities and diverse hapticities of the ligands with the central metals. These results suggest that the rare-earth metals ionic radii have an important influence on their bonding modes with functionalized indolyl ligands. Study of the catalytic activity of the dinuclear rare-earth metal alkyl complexes revealed that they were distinguished precatalysts for the polymerization of isoprene upon activation with AliBu3 and [Ph3C][B(C6F5)4], and tunable selectivity for isoprene polymerization was found by adjusting the addition sequence of the cocatalysts.

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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. (Me3SiCH2)3RE(THF)2 was synthesized by following documentaion.28 [Ph3C][B(C6F5)4] and [PhNMe2H][B(C6F5)4] were purchased from STREM. 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 analyses data were obtained on a PerkinElmer Model 2400 Series II elemental analyzer. 1H NMR and 13C NMR spectra for analyses of compounds were recorded on a Bruker Model AV-300 NMR spectrometer (300 MHz for 1H; 75.0 MHz for 13C) in D-toluene for rare-earth metal complexes and in H


Article

Organometallics

1,4-cis-Polymerization of Isoprene. Under an argon atmosphere, a 50 mL flask was charged with complex 3 (0.01 g, 10.0 mmol) in C6H5Cl (5.0 mL) and AliBu3 (0.02 g, 100 mmol) in toluene (0.1 mL). To the mixture was added a solution of [Ph3C][B(C6F5)4] (0.01 g,10.0 mmol) in C6H5Cl (2.0 mL). Then, 3.0 mL of C6H5Cl was added to the flask. Isoprene (1.00 mL, 0.68 g, 10.0 mmol) was then added. The mixture was then stirred vigorously for 1 h. The resultant 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%). 3,4-Polymerization of Isoprene. Under an argon atmosphere, a 50 mL flask was charged with complex 3 (0.01 g, 10.0 mmol) in CH2Cl2 (5.0 mL) and [Ph3C][B(C6F5)4] (0.01 g,10.0 mmol) in CH2Cl2 (2.0 mL). To the mixture was added a solution of AliBu3 (0.02 g, 75 mmol) in toluene (0.1 mL). Then, 3.0 mL of CH2Cl2 was added to the flask. Isoprene (1.00 mL, 0.68 g, 10.0 mmol) was then added. The mixture was then stirred vigorously for 1 h. The resultant 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%).

Preparation of [(μ-η5:η1:η1)-2-(2,6-DippNCH2)Ind(μ-η6:η1:η1)2-(2,6-DippNCH2)Ind][Dy(CH2SiMe3)(thf)]2 (4a) and {[μ-η5:η1:η12-(2,6-DippNCH2)Ind]Dy(CH2SiMe3)(thf)}2 (4b). A similar method for preparation of complex 1 was carried out by treatment of a THF or toluene (15.0 mL) solution of 2-(2,6-DippNHCH2)C8H5NH (0.31 g, 1.0 mmol) with a THF (15.0 mL) solution of (Me3SiCH2)3Dy(THF)2 (0.57 g, 1.0 mmol) at room temperature. The color of the solution changed from faint yellow to yellow during the reaction. Faint yellow crystals were obtained upon allowing the solution to stand at 0 °C for several days (0.24 g, 42% yield for 4a and 0.09 g, 16% yield for 4b). Mp: 146 and 158 °C under Ar. The NMR spectra data of the complexes were not obtained for lack of locking signals due to paramagnetism. IR (KBr pellets, cm−1): v 2954(s), 2868 (s), 1587 (s), 1456 (s), 1413 (s), 1361 (m), 1303 (s), 1286 (m), 1195 (s), 1064 (s), 792 (s), 744 (s). Anal. Calcd for C58 H86Dy2N4O2Si2: C, 55.62; H, 6.92; N, 4.47. Found: C, 55.56; H, 6.95; N 4.41. Preparation of [(μ-η5:η1:η1)-2-(2,6-DippNCH2)Ind(μ-η6:η1:η1)2-(2,6-DippNCH2)Ind][Gd(CH2SiMe3)(thf)]2 (5). A similar method for preparation of complex 1 was used via treatment of a THF (15.0 mL) solution of 2-(2,6-DippNHCH2)C8H5NH (0.31 g, 1.0 mmol) with a THF or toluene (15.0 mL) solution of (Me3SiCH2)3Gd(THF)2 (0.56 g, 1.0 mmol) at room temperature. The color of the solution changed from faint yellow to yellow during the reaction. Faint yellow crystals were obtained upon allowing the solution to stand at 0 °C for several days (0.30 g, 48% yield). Mp: 149 °C under Ar. The NMR spectra data of the complex were not obtained due to lack of locking signals as a result of paramagnetism. IR (KBr pellets, cm−1): v 2954(s), 2868 (s), 1456 (s), 1413 (s), 1286 (m), 1195 (s), 1051 (m), 792 (s), 744 (s). Anal. Calcd for C58 H86Gd2N4O2Si2: C, 56.09; H, 6.98; N, 4.51. Found: C, 56.35; H, 6.77; N 4.36. Preparation of [(μ-η 3 :η 2 :η 1 :η 1 )-2-(2,6-DippNCH 2 )Ind(μη 2:η 1 :η 1 )-2-(2,6-DippNCH 2)Ind-(η 1:η 1 )-2-(2,6-DippNCH 2)Ind]Gd3[(μ3-O(CH2)5SiMe3)(μ2-O(CH2)5SiMe3)(thf)3] (6). To a THF (15.0 mL) solution of 2-(2,6-DippNHCH2)C8H5NH (0.25 g, 0.8 mmol) was added a THF (15.0 mL) solution of (Me3SiCH2)3Gd(THF)2 (0.56 g, 1.0 mmol) at room temperature. The reaction mixture was stirred at room temperature for 3 h, and the color of the solution changed from faint yellow to yellow during the reaction. The solvent was removed under reduced pressure. The residue was extracted with hexane (15.0 mL). Yellow crystals were obtained at 0 °C (0.27 g, 28% isolated crystal yield). Mp: 165 °C under Ar. The NMR spectra data of the complex were not obtained for lack of locking signals due to paramagnetism. IR (KBr pellets, cm−1): v 2956 (w), 2341 (s), 1457 (s), 1413 (s), 1303 (s), 1286 (s), 1195 (w), 1064 (m), 1052 (s), 993 (m), 792 (s), 744 (s). Anal. Calcd for C91H133Gd3N6 O5Si2: C, 56.96; H, 6.99; N, 4.38. Found: C, 56.75; H, 6.69; N 4.51. Preparation of [μ-η3:η1:η1-2-(2,6-DippNCH2)Ind]3Sm2(thf)3 (7). A similar method for preparation of complex 1 was used by treatment of a THF (15.0 mL) solution of 2-(2,6-DippNHCH2)C8H5NH (0.31 g, 1.0 mmol) with a THF or toluene (15.0 mL) solution of (Me3SiCH2)3Sm(THF)2 (0.63 g, 1.0 mmol) at room temperature. The color of the solution changed from yellow to red during the reaction. Red crystals were obtained upon standing the solution at 0 °C for several days (0.11 g, 23% yield). Mp: 216 °C under Ar. 1H NMR (300 MHz, C6D6): δ 7.21 (d, 3H), 7.15 (m, 3H), 6.88 (m, 6H), 6.55 (m, 6H), 6.11 (s, 3H), 5.95 (s, 3H), 4.61 (s, 3H), 4.36 (s, 3H), 3.51 (m, 12H), 2.85 (m, 6H), 1.66 (m, 12H), 1.00 (s, 36H). 13C NMR (75 MHz, THF-d8): δ 151 (CHN), 149, 143, 142, 139, 124, 122, 121, 120, 117, 116, 115, 112, 97 (indole-C and Ar-C), 65 (CH2−O), 56 (CH2−O), 52 (CH2CH2−O), 26 (CH(CH3)3), 25 (CH(CH3)3), 24 (CH(CH3)3), 23 (CH(CH3)3), 22 (CH(CH3)3), 21 (CH(CH3)3). IR (KBr pellets, cm−1): v 2957 (w), 2338 (s), 1460 (s), 1415 (s), 1301 (s), 1285 (s), 1194 (w), 1064 (m), 1053 (s), 993 (m), 792 (s), 744 (s). Anal. Calcd for C75H96N6O3Sm2: C, 62.98; H, 6.77; N, 5.88. Found: C, 63.21; H, 6.53; N, 5.86. Polymerization of Isoprene. All complexes were tested as initiators for the polymerization of isoprene in the presence of cocatalysts AliBu3 and [Ph3C][B(C6F5)4].

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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.7b00575. Characterization data and spectra data, tables of crystallographic data, and structure refinement for complexes 1−7, NMR spectra of complexes 1, 3, and PIP, and plausible synthesis pathway for 1 (PDF) Accession Codes

CCDC 1562654−1562661 contain the supplementary crystallographic data for this paper. These data can be obtained free of 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 Author

*E-mail: [email protected]. ORCID

Shaowu Wang: 0000-0003-1083-1468 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation (21432001, 21372010, 21602004), and the Special and Excellent Research Fund of Anhui Normal University.

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REFERENCES

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