Tris(pyrazolyl)methanide Complexes of Trivalent Rare-Earth Metals

Article pubs.acs.org/Organometallics

Tris(pyrazolyl)methanide Complexes of Trivalent Rare-Earth Metals Tengfei Li,† Guangchao Zhang,‡ Jingjing Guo,† Shaowu Wang,*,‡ Xuebing Leng,† and Yaofeng Chen*,† †

State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, People’s Republic of China ‡ 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 S Supporting Information *

ABSTRACT: Three types of trivalent rare-earth-metal complexes supported by a monoanionic tris(pyrazolyl)methanide ligand were synthesized and structurally characterized, and the catalytic activity of the dialkyl derivatives for isoprene polymerization was investigated. Reactions of the lithium salt of tris(3,5-dimethylpyrazolyl)methanide LLi(THF) with 1 equiv of ScCl3(THF)3, YCl3, or LuCl3 in THF provided the ion-pair complexes [LLnCl3][Li(THF)4] (Ln = Sc (1), Y (2), Lu (3)). Dialkyl complexes LLn(CH2SiMe3)2(THF) (Ln = Y (4), Lu (5)) were prepared by salt metathesis of LLi(THF) with 1 equiv of [Y(CH2SiMe3)2(THF)3][BPh4] or [Lu(CH2SiMe3)2(THF)3][BPh4] in toluene. Reaction of 5 with PhSiH3 provided the unexpected alkylidene-bridged dinuclear complex L2Lu2(μ-η1:η1-3,5-(CH3)C3HN2)2(μ-CHSiMe3) (6). Complexes 1−6 were structurally characterized by single-crystal X-ray diffraction, showing that the tris(pyrazolyl)methanide ligand acts as a κ3coordinating six-electron donor in all complexes. The dialkyl complexes catalyzed 1,4-cis polymerization of isoprene with high selectivity upon activation with borate and alkylaluminum.

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replacing the apical [BH]− moiety with the [C]− moiety. Recently, tris(pyrazolyl)methanide has received growing attention due to its feature of bearing an anionic κ3coordinating six-electron-donor pocket, and an increasing number of tris(pyrazolyl)methanide metal complexes have been reported.6 Although several rare-earth-metal complexes bearing neutral tris(pyrazolyl)methane have been reported,7 to the best of our knowledge, only one trivalent rare-earth-metal complex, LScCl2(THF) (L = tris(3,5-dimethylpyrazolyl)methanide), and one divalent rare-earth-metal complex, L′YbI(THF) (L′ = tris(3-(1-adamantyl)-5-methylpyrazolyl)methanide), have been synthesized by the groups of Mountford and Jones.8 Considering the rich and interesting coordinating properties and reactivity shown by the tris(pyrazolyl)borate rare-earth-metal complexes,9 the chemistry of tris(pyrazolyl)methanide rare-earth-metal complexes is worthy of further exploration. Herein, we report the synthesis and structural characterization of ion-pair complexes [LLnCl3][Li(THF)4], dialkyl complexes LLn(CH2SiMe3)2(THF), and the alkylidenebridged dinuclear complex LLu2(μ-η1:η1-3,5-(CH3)C3HN2)2(μCHSiMe3) supported by a tris(pyrazolyl)methanide ligand. The catalytic activity of the dialkyl complexes for isoprene polymerization is also presented.

INTRODUCTION Organometallic complexes of rare-earth metals have rich and diversified coordinating properties and reactivity1 and have been widely utilized in organic2 and polymer synthesis.3 The most widely investigated organometallic complexes of the rareearth metals are those bearing Cp-type ligands. To further explore the chemistry of the rare-earth-metal complexes, ancillary ligands other than Cp and its derivatives recently have been introduced.4 In this connection, ligands with nitrogen donor atoms have received much attention, as they form strong N−Ln bonds with the acidic and hard Ln3+ ions and are expected to stabilize the highly electrophilic rare-earthmetal complexes. Tris(pyrazolyl)methanide (Chart 1a) is as a cousin of the widely used tris(pyrazolyl)borate (Tp) (Chart 1b),5 by Chart 1. Tris(pyrazolyl)methanide (a) and Tris(pyrazolyl)borate (b)

Special Issue: Organometallics in Asia Received: February 27, 2016

© XXXX American Chemical Society

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DOI: 10.1021/acs.organomet.6b00166 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

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[LLuCl3]− (2.380(5) and 2.532(2) Å) are almost same as those in [LYCl3]− if the difference in Ln3+ radii is considered. The Ln3+ radii affect the N1−Ln−N1a and Cl1−Ln−Cl1a angles. When the Ln3+ size increases, the N1−Ln−N1a angle decreases (79.1(1), 76.9(2), and 75.8(2)° for 1, 3, and 2, respectively) and the Cl1−Ln−Cl1a angle increases (97.85(5), 99.26(6), and 99.94(6)° for 1, 3, and 2, respectively). Complexes 1−3 have similar structures in the solid state, but their 1H NMR spectra in C6D6 at room temperature are significantly different (Figure 2). The 1H NMR spectrum of 1

RESULTS AND DISCUSSION Synthesis and Characterization of [LLnCl3][Li(THF)4]. Reactions of the lithium salt of tris(3,5-dimethylpyrazolyl)methanide LLi(THF) with 1 equiv of ScCl3(THF)3, YCl3, or LuCl3 in THF provided the ion-pair complexes [LLnCl3][Li(THF)4] (Ln = Sc (1), Y (2), Lu (3)) in 65−71% yields, as shown in Scheme 1. The reaction rate is influenced by rareScheme 1. Synthesis of Complexes 1−3

earth-metal ions and is as follows: Y > Lu ≫ Sc. Therefore, the reactions with YCl3 and LuCl3 were carried out at room temperature, while that with ScCl3(THF)3 was carried out at 75 °C. Complexes 1−3 were characterized by NMR spectroscopy, elemental analysis, and single crystal X-ray diffraction. Complex 1 is soluble in toluene, benzene, and THF; complexes 2 and 3 are soluble in THF and slightly soluble in toluene and benzene. These complexes slowly lose THF molecules when the samples are kept at room temperature. Single-crystal X-ray diffraction analysis showed that complexes 1−3 have very similar structural features. The molecular structure of the anion [LYCl3]− of 2 is shown in Figure 1, while

Figure 2. 1H NMR spectra of complexes 1−3 (400 MHz, C6D6, 25 °C): (a) 1H NMR spectrum of 1; (b) 1H NMR spectrum of 2; (c) 1H NMR spectrum of 3.

in C6D6 is apparently inconsistent with what is expected for a C3-symmetric structure: the resonances for the pyrazolyl rings are split into two sets in a 2:1 intensity ratio, the resonances for the THF molecules are split into two sets in a 3:1 intensity ratio (Figure 2a). The resonances for the THF molecules with an integration value of 12 appear at 3.58 and 1.42 ppm, which are almost the same as those reported for the noncoordinated THF molecules in C6D6 (3.57 and 1.40 ppm).10 Resonances with the integration value of 4 appear at 3.91 and 1.14 ppm; these two chemical shifts are very close to those for the THF molecules in (TpMe2)ScCl2(THF) (3.93 and 1.16 ppm).11 It was also observed that a white precipitate (LiCl) was formed during dissolution of 1 in C6D6, and the dissolution is slow. Therefore, complex 1 loses LiCl and three THF molecules to form LScCl2(THF) in C6D6. The six-coordinate LScCl2(THF) has a Cs symmetry, which is responsible for the split of the pyrazolyl resonances into a 2:1 ratio. Variable-temperature 1H NMR spectra of 1 in toluene-d8 showed that increasing the temperature from 25 to 105 °C results in broadening and coalescence of two sets of the pyrazolyl resonances (Figure S5 in the Supporting Information), and the coalescence temperature is about 105 °C. The resonances of THF molecules also broaden and coalesce when the temperature is raised. The behavior of 1 in THF-d8 was also investigated. Variabletemperature 1H NMR spectra of 1 in THF-d8 showed three sets of pyrazolyl resonances when the solution temperature was below 0 °C (Figure 3). Two sets of pyrazolyl resonances (1H NMR (0 °C): δ 5.76 and 5.71 for the pyrazolyl 4-H, respectively) which are in a 2:1 intensity ratio can be ascribed to the Cs-symmetric LScCl2(THF), the remaining resonance (1H NMR (0 °C): δ 5.57 for the pyrazolyl 4-H) is due to the C 3 -symmetric [LScCl 3 ] − . Two sets of resonances for LScCl2(THF) broaden and coalesce when the temperature

Figure 1. Molecular structure of the anion [LYCl3]− of 2 (ball and stick representation). Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Y−N1 2.425(5), Y−Cl1 2.571(2), C1−N2 1.441(5), N1−N2 1.390(7); N2−C1−N2a 110.2(4), N1−Y−N1a 75.8(2), Cl1−Y−Cl1a 99.94(6).

those of the anions [LScCl3]− and [LLuCl3]− of 1 and 3 are given in Figures S1 and S2 in the Supporting Information. All these anions have C3 symmetry; each rare-earth-metal ion is coordinated by three nitrogen atoms of the L− ligand and three chlorides in a distorted-octahedral geometry. Altough the complex contains the tris(3,5-dimethyl-1-pyrazolyl)methanide anion, the Y−N and Y−Cl bond lengths in [LYCl3]− (2.425(5) and 2.571(2) Å) are similar to those in the tris(3,5-dimethyl-1pyrazolyl)methane complex [(LH)YCl 3] (2.459(2) and 2.555(2) Å).7c The C1−N2 and N1−N2 bond lengths in [LYCl3]− (1.441(5) and 1.390(7) Å) are also similar to those in [(LH)YCl3] (1.483(5) and 1.374(4) Å). The Ln−N and Ln− Cl bond lengths in [LScCl3]− (2.284(3) and 2.431(1) Å) and B


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Organometallics

rare-earth-metal trialkyl complexes (LH)Ln(CH2SiMe3)3 (Ln = Sc, Y); the alkyl elimination did not occur.7b For the purpose of synthesizing rare-earth-metal dialkyl complexes with tris(pyrazolyl)methanide, the salt metathesis strategy was used, which has a strong driving force. Okuda’s reagents, [Lu(CH2SiMe3)2(THF)3][BPh4] and [Y(CH2SiMe3)2(THF)3][BPh4],12 were chosen as the starting materials. Reactions of LLi(THF) with one equiv of [Y(CH2SiMe3)2(THF)3][BPh4] or [Lu(CH2SiMe3)2(THF)3][BPh4] in toluene provided the desired dialkyl complexes LLn(CH2SiMe3)2(THF) (Ln = Y (4), Lu (5)) in good yields (Scheme 2). Both complexes were Scheme 2. Synthesis of Complexes 4 and 5

Figure 3. Variable-temperature 1H NMR spectra of 1 in THF-d8 (600 MHz).

rises from 0 to 65 °C. The coalescence temperature for LScCl2(THF) in the THF-d8 solution is much lower than that in the toluene-d8 solution, indicating that the fluxional process is possibly due to the association−disassociation of THF rather than the in-place rotation of the L− ligand. The 1H NMR spectra of 2 and 3 at room temperature are simple, as shown in Figure 2b,c. However, with a decrease in the solution temperature, the pyrazolyl resonances are split into two sets, and the two sets of the pyrazolyl resonances are not in a 2:1 intensity ratio (Figure 4). Therefore, the solution

characterized by NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction. Different from the case for their chlorides, complexes 4 and 5 are highly soluble in toluene and benzene. The room-temperature 1H NMR spectra of 4 and 5 in C6D6 were similar and exhibited two sets of pyrazolyl resonances in a 2:1 intensity ratio, consistent with a Cssymmetric structure. The molecular structure of 4 is shown in Figure 5, while that of 5 is presented in Figure S3 in the Supporting Information.

Figure 4. Variable-temperature 1H NMR spectra of 2 in toluene-d8 (600 MHz). Figure 5. Molecular structure of 4 (ball and stick representation). Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Y−N1 2.407(3), Y−N3 2.464(3), Y−N5 2.465(3), Y−C17 2.402(4), Y−C21 2.409(4), Y−O 2.344(3), C1−N2 1.446(5), C1−N4 1.448(4), C1−N6 1.438(5); N2−C1−N4 109.8(3), N2−C1− N6 109.4(3), N4−C1−N6 110.4(3), N1−Y−N3 78.34(10), N1−Y− N5 75.58(11), N3−Y−N5 73.07(11), C17−Y−C21 101.38(14).

behaviors of 2 and 3 are not consistent with the C3-symmetric [LLnCl3]− or the Cs-symmetric LLnCl2(THF). In toluene-d8, 2 and 3 possibly exist as ate-type complexes, in which the Li+ ion has a close contact with Cl− ions bound to the Ln3+ ion. It should be noted that no white precipitate (LiCl) was formed during dissolution of 2 or 3 in C6D6 or toluene-d8, and the solubility of 2 and 3 in C6D6 or toluene-d8 is much poorer than that of 1. Synthesis and Characterization of LLn(CH2SiMe3)2(THF). To the best of our knowledge, no rareearth-metal dialkyl complex supported by anionic tris(pyrazolyl)methanide has been reported. Mountford and coworkers found that reactions of Ln(CH2SiMe3)3(THF) (Ln = Sc, Y) with LH gave tris(3,5-dimethyl-1-pyrazolyl)methane

The structural features of 4 and 5 are very similar, and 4 was taken as an example to analyze the structural features. The yttrium ion is coordinated by three nitrogen atoms of L−, two alkyls, and one oxygen atom of THF in a distorted-octahedral geometry. Two Y−N bond lengths, 2.464(3) and 2.465(3) Å, are equa;, the remaining bond, 2.407(3) Å, is shorter. These Y− N bond lengths are remarkably shorter than those in the tris(3,5-dimethyl-1-pyrazolyl)methane yttrium trialkyl complex C


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Organometallics (LH)Y(CH2SiMe3)3, where the Y−N bond lengths are in the range of 2.563(2)−2.601(2) Å.7b The Y−N bond lengths in 4 are also somewhat shorter than those in its tris(pyrazolyl)borate analogue (TpMe2)Y(CH2SiMe3)2(THF) (2.4911(19), 2.4918(17), and 2.4288(17) Å, respectively). The Y−C bond lengths in 4 (2.402(4) and 2.409(4) Å) are slightly shorter than those in (LH)Y(CH2SiMe3)3 (2.434(3) Å in the average). Complex 4 shows local Cs symmetry in the solid state; two pyrazolyl rings of the L− ligand are nearly equivalent, while two SiMe3 groups of the alkyl ligands are oriented “up” or “down” toward the L− ligand. The C1−N bond lengths and N−C1−N angles of L− indicate a localized carbanionic center at C1; therefore, 4 is a zwitterionic complex. The zwitterionic structure was commonly observed for the reported tris(pyrazolyl)methanide metal complexes.6a,b,c,d Reactions of LLn(CH2SiMe3)2(THF) with PhSiH3. The σbond metathesis reaction of rare-earth-metal alkyl complexes with PhSiH3 is a convenient method to synthesize rare-earthmetal hydrides. For the purpose of synthesizing the rare-earth metal hydrides containing tris(pyrazolyl)methanide, reactions of LLn(CH2SiMe3)2(THF) (Ln = Y (4), Lu (5)) with PhSiH3 were carried out. 1H NMR spectral monitoring showed that the reaction of 4 with PhSiH3 in C6D6 gave a complicated mixture at room temperature or elevated temperature. The reaction of 5 with 2 equiv of PhSiH3 in C6D6 was very slow at room temperature. When the reaction temperature was raised to 50 °C, the reaction was significantly accelerated and the complex 5 was nearly completely converted into the new complex 6 in 18 h (Figure S22 in the Supporting Information). When the reaction temperature was raised to 75 °C, the reaction was complete in 3 h. The reaction was scaled up in toluene at 75 °C; complex 6 was isolated and characterized by NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction. Complex 6 is an unexpected alkylidene-bridged dinuclear complex which contains two L− ligands and two pyrazolyl ligands, as shown in Scheme 3. Although recent

Figure 6. Molecular structure of 6 (ball and stick representation). Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Lu1−N1 2.345(4), Lu1−N3 2.398(8), Lu1−N5 2.326(5), Lu−C18 2.300(6), C1−N2 1.416(5), C1−N4 1.451(8); N2−C1−N4 108.6(3), N2−C1−N2a 112.7(5), Lu1−C18−Lu1a 97.5(4), C18−Lu1−N5 90.1(2), N5−Lu1−N5a 96.2(4).

on average. The Lu−C bond length (2.300(6) Å) is shorter than those in the lutetium dialkyl complex 5 (2.367 Å on average) but similar to that in the reported lutetium-bridged alkylidene (ArNC(Me)CHCOCHC(Me)NAr)Lu2(CH2SiMe3)2(μ-CHSiMe3)(THF)2 (2.309(6) Å). In addition to 6, PhSiH2CH2SiMe3 was also generated in the reaction of 5 with PhSiH3. PhSiH2CH2SiMe3 is a common byproduct of the σ-bond metathesis reaction of rare-earth-metal alkyl complexes with PhSiH3 producing rare-earth-metal hydrides. The formation of PhSiH2CH2SiMe3 implies that the reaction might proceed through a rare-earth-metal alkyl hydride intermediate. The formation of the anionic pyrazolyl ligand in 6 is associated with the separation of pyrazole; the separation of pyrazole is well-known in Tp chemistry.9,17 The detailed reaction mechanism for the formation of 6 is unclear at present. Polymerization of Isoprene. 1,4-cis-Polyisoprene is one of the most important rubbers. Recent reports showed that rareearth-metal dialkyl complexes are precatalysts for isoprene 1,4cis polymerization to produce 1,4-cis-polyisoprene, and rareearth-metal dialkyl complex/borate/AlR3 ternary systems are usually the most efficient catalytic systems.18 Complexes 4 and 5 were tested for isoprene polymerization upon activation with borate and alkylaluminum; representative polymerization data are summarized in Table 1. Complexes 4 and 5 alone and the binary systems (4 or 5)/borate and (4 or 5)/AlR3 are inactive for isoprene polymerization in toluene at 10 °C. For the ternary systems 5/AlR3/borate, the ratio [complex]/[borate]/[AlR3] = 1/1/10 is inactive for the polymerization, while the ratio [complex]/[borate]/[AlR3] = 1/2/10 displayed activity. The requirement of 2 equiv of borate is probably due to the coordination of 1 equiv of Ph3C+ of the borate to the anionic methanide site of the L− ligand. The catalytic activity of the ternary systems 5/AlR3/borate (1/2/10) is influenced by the type of AlR3. When AlMe3 or AlEt3 was used, no polymer or trace amounts of polymer were obtained (Table 1, entries 1 and 2). When AliBu3 was used, a 35.2% conversion of monomer to produce the polymer with 81.9% 1,4-cis selectivity was observed (Table 1, entry 3), indicating a steric effect of the aluminum alkyl group on the polymerization. The polymerization is also influenced by the type of solvent. Using chlorobenzene as the solvent, the conversion increased to 74%

Scheme 3. Synthesis of Complex 6

reports have described an increasing number of rare-earth-metal alkylidene complexes,13,14 only one structurally authenticated rare-earth-metal complex bearing the alkylidene ligand of [CHSiMe3]2− has been reported.15,16 Complex 6 did not form when 5 was heated at 75 °C in the absence of PhSiH3; the reaction gave complicated mixtures. In 6, each lutetium ion adopts a distorted-octahedral geometry, being coordinated by a L− ligand, a μ-alkylidene ligand, and two μ-η1:η1-pyrazolyl ligands (Figure 6). The distances from the lutetium ion to the nitrogen atoms of pyrazolyl ligands, 2.326(5) Å, are slightly shorter than those to the nitrogen atoms of L− ligand, 2.362 Å D


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Organometallics Table 1. Isoprene Polymerization with 4 or 5 as the Precatalysta

1,4 (%)g entry

complex

AlR3 (amt (equiv))

solvent

time (h)

conversion (%)

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

Lu (5) Lu (5) Lu (5) Lu (5) Lu (5) Lu (5) Lu (5) Lu (5) Lu (5) Y (4) Y (4) Y (4) Y (4)

AlMe3 (10) AlEt3 (10) AliBu3 (10) AliBu3 (10) AliBu3 (10) AliBu3 (7.5) AliBu3 (5) AliBu3 (2.5) AliBu3 (2.5) AliBu3 (5) AliBu3 (5) AliBu3 (5) AliBu3 (5)

toluene toluene toluene dichloromethane chlorobenzene chlorobenzene chlorobenzene chlorobenzene chlorobenzene chlorobenzene chlorobenzene chlorobenzene chlorobenzene

8 8 8 8 8 3 2 2 8 2 5 8 1

0 trace 35.2 0 74 94 97 99 32 99 88 74 99

10

−4

Mnf

PDI

f

cis

trans

3,4 (%)g

6.8

1.7

81.9

0

18.1

7.8 7.3 6.7 8.7 17.6 7.8 9.1 16.3 13.3

1.7 2.0 1.9 2.2 2.0 2.2 2.0 1.9 2.0

84.0 84.0 90.1 88.5 88.1 90.1 94.3 90.1 88.5

0 0 0 0 0 0 0 1.8 0.9

16.0 16.0 9.9 11.5 11.9 9.9 5.7 8.1 10.6

General conditions: 10 μmol of complex, [IP]/[complex] = 500, [borate]/[complex] = 2, temperature 10 °C, 5 mL of solvent. bTemperature −10 °C. c7 mL of solvent. dTemperature 0 °C. eTemperature 30 °C. fDetermined by means of GPC against polystyrene standards in THF at 30 °C. g Determined by 1H NMR and 13C NMR. a

[BPh4] in toluene. Reaction of 5 with PhSiH3 did not provide the hydride but the alkylidene-bridged dinuclear complex L2Lu2(μ-η1:η1-3,5-(CH3)C3HN2)2(μ-CHSiMe3) (6) in high yield. The dialkyl complexes 4 and 5 are precatalysts for isoprene 1,4-cis polymerization to produce 1,4-cis-polyisoprene.

and the 1,4-cis selectivity increased to 84.0% (Table 1, entry 5) due to its weak coordination to the resulting cation;19 no polymer was obtained with dichloromethane as the solvent (Table 1, entry 4). Chlorobenzene was subsequently used as solvent for further study. The amount of AliBu3 used affects the polymerization; decreasing the AliBu3/catalyst ratio from 10 to 5 led to higher conversion and higher 1,4-cis selectivity (Table 1, entries 5−7). Further lowering the AliBu3/catalyst ratio led to a decrease in 1,4-cis selectivity (Table 1, entry 7 vs 8). Decreasing the polymerization temperature from +10 to −10 °C resulted in a polymer with higher molecular weight (Mn = 17.6 × 104 vs Mn = 8.7 × 104; Table 1, entry 8 vs 9), but the catalytic activity became much lower. The ternary system 4/ AlR3/borate ([complex]/[borate]/[AlR3] = 1/2/5) also showed good catalytic activity and 1,4-cis selectivity for the isoprene polymerization (Table 1, entry 10). With an increas in the amount of solvent, the catalytic activity decreased but the 1,4-cis selectivity increased (Table 1, entry 10 vs 11). A polymer with a molecular weight of 9.1 × 104 (PDI = 2.0) and 1,4-cis selectivity of 94.3% was obtained at 10 °C when 7 mL of solvent was used. When the polymerization temperature was increased from 0 to 30 °C, the catalytic activity significantly increased; the 1,4-cis selectivity slightly decreased.

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EXPERIMENTAL SECTION

General Considerations. All operations related to air- or moisture-sensitive rare-earth-metal complexes were carried out under an atmosphere of argon using Schlenk techniques or in a nitrogenfilled glovebox. Tris(3,5-dimethyl-1-pyrazolyl)methane (LH),20 LLi(THF),6b [Lu(CH2SiMe3)2(THF)3][BPh4], and [Y(CH2SiMe3)2(THF)3][BPh4]12 were prepared according to literature procedures. Phenylsilane was dried over activated 4 Å molecular sieves, distilled under vacuum, and stored in the glovebox. [Ph3C][B(C6F5)4] was purchased from Strem and used as received. AlMe3, AlEt3, and AliBu3 were purchased from Sigma-Aldrich and used as received. Isoprene was purchased from TCI, dried over CaH2, and distilled prior to use. Toluene, THF, hexane, benzene, C6D6, and THF-d8 were dried over Na/K alloy, transferred under vacuum, and stored in the glovebox. Toluene used for isoprene polymerization was distilled over sodium benzophenone ketyl under argon prior to use. PhCl and CH2Cl2 were dried over CaH2 and distilled prior to use. 1H and 13C NMR spectra were recorded on a Varian 400 MHz, an Agilent 400 MHz, or an Agilent 600 MHz spectrometer. All chemical shifts were reported as δ units with reference to the residual solvent resonance of the deuterated solvents for proton and carbon chemical shifts. Elemental analyses (C, H, N) were performed by the Analytical Laboratory of the Shanghai Institute of Organic Chemistry. 1H NMR and 13C NMR spectra of polyisoprene in CDCl3 were recorded on a Bruker Model AV-300 NMR spectrometer. 13C-int D1 = 5 s NMR spectra of polyisoprene in CDCl3 were recorded on a Bruker Model AV-500 NMR spectrometer. 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 mL min−1. Synthesis of Complex 1. LLi(THF) (151 mg, 0.40 mmol) and ScCl3(THF)3 (147 mg, 0.40 mmol) were mixed in 6 mL of THF at room temperature. After it was stirred for 3 h at 75 °C, the reaction mixture was filtered. The volume of the filtrate was concentrated to approximately 1.5 mL under vacuum and cooled to −35 °C to give 1 as a brown crystalline solid. Crystallization of the concentrated mother

CONCLUSIONS

Trivalent rare-earth-metal chloride, dialkyl, and alkylidene complexes supported by a tris(pyrazolyl)methanide ligand were successfully prepared. Reactions of the lithium salt of tris(3,5-dimethylpyrazolyl)methanide LLi(THF) with ScCl3(THF)3, YCl3, or LuCl3 in THF provided the ion-pair complexes [LLnCl3][Li(THF)4] (Ln = Sc (1), Y (2), Lu(3)) rather than the zwitterionic complexes LLnCl2(THF). Complex 1, which has the smallest Ln3+ radius, loses LiCl and three THF molecules to form the zwitterionic complex LScCl2(THF) in nonpolar solvents (C6D6 and toluene-d8). The dialkyl complexes LLn(CH2SiMe3)2(THF) (Ln = Y (4), Lu (5)) were prepared by the salt metathesis of LLi(THF) with [Y(CH2SiMe3)2(THF)3][BPh4] or [Lu(CH2SiMe3)2(THF)3]E


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Organometallics liquor at −35 °C afforded a second crop of the product. A 210 mg portion of 1 was obtained in total (71% yield). 1H NMR (400 MHz, C6D6, 25 °C): 5.42 (s, 2 H, pyrazolyl 4-H), 5.22 (s, 1 H, pyrazolyl 4H), 3.91 (m, 4 H, OCH2CH2 of THF), 3.58 (m, 12 H, OCH2CH2 of THF), 2.86 (s, 3 H, pyrazolyl Me), 2.38 (s, 6 H, pyrazolyl Me), 2.33 (s, 6 H, pyrazolyl Me), 2.28 (s, 3 H, pyrazolyl Me), 1.42 (m, 12 H, OCH2CH2 of THF), 1.14 (m, 4 H, OCH2CH2 of THF). 13C NMR (100 MHz, C6D6, 25 °C): 150.9, 149.3, 146.2, 145.7 (pyrazolyl 3- and 5-C), 104.4, 104.3 (pyrazolyl 4-C), 72.0 (Canionic), 73.0, 67.8 (OCH2CH2 of THF), 25.8, 25.4 (OCH2CH2 of THF), 15.2, 15.1 (pyrazolyl Me), 12.5, 12.3 (pyrazolyl Me). Anal. Calcd for C32H53Cl3LiN6O4Sc: C, 51.65; H, 7.18; N, 11.29. Found: C, 50.92; H, 7.20; N, 11.46. The lower C content found is due to the loss of THF. Synthesis of Complex 2. Following the procedure described for 1, reaction of LLi(THF) (112 mg, 0.30 mmol) and YCl3 (59 mg, 0.30 mmol) for 12 h at room temperature gave 2 as a brick red crystalline solid (162 mg, 68% yield). 1H NMR (400 MHz, C6D6, 25 °C): 5.36 (s, 3 H, pyrazolyl 4-H), 3.64 (m, 16 H, OCH2CH2 of THF), 2.59 (br, 9 H, pyrazolyl Me), 2.34 (s, 9 H, pyrazolyl Me), 1.39 (m, 16 H, OCH2CH2 of THF). 1H NMR (400 MHz, THF-d8, 25 °C): 5.59 (s, 3 H, pyrazolyl 4-H), 3.62 (m, 16 H, OCH2CH2 of THF), 2.47 (s, 18 H, pyrazolyl Me), 1.78 (m, 16 H, OCH2CH2 of THF). The solubility of 2 in C6D6 is very low; therefore its 13C NMR spectrum was recorded in THF-d8. 13C NMR (100 MHz, THF-d8, 25 °C): 149.7, 145.9 (pyrazolyl 3- and 5-C), 103.5 (pyrazolyl 4-C), 68.0 (OCH2CH2 of THF), 26.2 (OCH2CH2 of THF), 14.9, 12.5 (pyrazolyl Me). The signal for C a n i o n i c was not observed. Anal. Calcd for C32H53Cl3LiN6O4Y: C, 48.77; H, 6.78; N, 10.66. Found: C, 48.50; H, 6.53; N, 10.63. Synthesis of Complex 3. Following the procedure described for 1, reaction of LLi(THF) (151 mg, 0.40 mmol) and LuCl3 (113 mg, 0.40 mmol) for 24 h at room temperature gave 3 as a brick red crystalline solid (228 mg, 65% yield). 1H NMR (400 MHz, C6D6, 25 °C): 5.37 (s, 3 H, pyrazolyl 4-H), 3.70 (m, 16 H, OCH2CH2 of THF), 2.77 (br, 6 H, pyrazolyl Me), 2.50 (br, 3 H, pyrazolyl Me), 2.34 (s, 9 H, pyrazolyl Me), 1.39 (m, 16 H, OCH2CH2 of THF). 1H NMR (400 MHz, THF-d8, 25 °C): 5.60 (s, 3 H, pyrazolyl 4-H), 3.62 (m, 16 H, OCH2CH2 of THF), 2.49 (s, 9 H, pyrazolyl Me), 2.47 (s, 9 H, pyrazolyl Me), 1.77 (m, 16 H, OCH2CH2 of THF). The solubility of 3 in C6D6 is very low; therefore, its 13C NMR spectrum was recorded in THF-d8. 13C NMR (100 MHz, THF-d8, 25 °C): 150.1, 145.7 (pyrazolyl 3- and 5-C), 103.7 (pyrazolyl 4-C), 68.0 (OCH2CH2 of THF), 26.2 (OCH2CH2 of THF), 15.1, 12.5 (pyrazolyl Me). The signal for C a n i o n i c was not observed. Anal. Calcd for C32H53Cl3LiN6O4Lu: C, 43.97; H, 6.11; N, 9.61. Found: C, 43.30; H, 6.14; N, 9.83. Synthesis of Complex 4. LLi(THF) (113 mg, 0.30 mmol) and [Y(CH2SiMe3)2(THF)3][BPh4] (240 mg, 0.30 mmol) were mixed in 6 mL of toluene at room temperature. After the reaction mixture was stirred for 4 h at room temperature, the volatiles were removed under vacuum to give a pale yellow solid, which was extracted with hexane (3 × 4 mL). The extract was concentrated to approximately 2 mL under vacuum and cooled to −35 °C to provide 4 as a pale yellow crystalline solid. A 129 mg amount of 4 was obtained in total (68% yield). 1H NMR (400 MHz, C6D6, 25 °C): 5.55 (s, 2 H, pyrazolyl 4-H), 5.46 (s, 1 H, pyrazolyl 4-H), 3.69 (m, 4 H, OCH2CH2 of THF), 2.66 (br, 3 H, pyrazolyl Me), 2.36 (s, 9 H, pyrazolyl Me), 2.19 (br, 6 H, pyrazolyl Me), 1.16 (m, 4 H, OCH2CH2 of THF), 0.34 (s, 18 H, SiMe3), −0.07 (br, 2 H, YCH2), −0.28 (br, 2 H, YCH2). 13C NMR (100 MHz, C6D6, 25 °C): 147.9, 146.9 (pyrazolyl 3- and 5-C), 103.9 (pyrazolyl 4-C), 72.0 (Canionic), 71.0 (OCH2CH2 of THF), 32.8 (d, 1JYC = 36.4 Hz, YCH2), 25.1 (OCH2CH2 of THF), 14.9, 13.0 (pyrazolyl Me), 4.9 (SiMe3). Anal. Calcd for C28H51N6OSi2Y: C, 53.14; H, 8.12; N, 13.28. Found: C, 53.17; H, 7.76; N, 13.13. Synthesis of Complex 5. Following the procedure described for 4, reaction of LLi(THF) (113 mg, 0.30 mmol) and [Lu(CH2SiMe3)2(THF)3][BPh4] (266 mg, 0.30 mmol) for 0.5 h at room temperature gave 5 as a pale yellow crystalline solid (158 mg, 73% yield). 1H NMR (400 MHz, C6D6, 25 °C): 5.56 (s, 2 H, pyrazolyl

4-H), 5.48 (s, 1 H, pyrazolyl 4-H), 3.68 (m, 4 H, OCH2CH2 of THF), 2.66 (s, 3 H, pyrazolyl Me), 2.36 (s, 6 H, pyrazolyl Me), 2.34 (s, 3 H, pyrazolyl Me), 2.19 (s, 6 H, pyrazolyl Me), 1.16 (m, 4 H, OCH2CH2 of THF), 0.31 (s, 18 H, SiMe3), −0.31 (d, 2JHH = 11.6 Hz, 2 H, LuCH2), −0.45 (d, 2JHH = 11.6 Hz, 2 H, LuCH2). 13C NMR (100 MHz, C6D6, 25 °C): 149.2, 148.2, 146.9, 146.7 (pyrazolyl 3- and 5-C), 104.1 (pyrazolyl 4-C), 72.4 (Canionic), 71.2 (OCH2CH2 of THF), 36.0 (LuCH2), 25.2 (OCH2CH2 of THF), 15.7, 15.0, 12.8 (pyrazolyl Me), 5.1 (SiMe3). Anal. Calcd for C28H51N6OSi2Lu: C, 46.78; H, 7.15; N, 11.69. Found: C, 46.70; H, 7.11; N, 11.42. Synthesis of Complex 6. LLu(CH2SiMe3)2(THF) (216 mg, 0.30 mmol) and PhSiH3 (65 mg, 0.60 mmol) were mixed in 6 mL of toluene at room temperature. After the reaction mixture was stirred for 4 h at 75 °C, the volatiles were removed under vacuum to give an orange oil. The oil was washed with hexane (3 × 2 mL) and dried under vacuum to give 6 as an orange solid (90 mg, 73% yield). 1H NMR (400 MHz, C6D6, 25 °C): 5.78 (s, 2 H, pyrazolyl 4-H), 5.65 (s, 4 H, pyrazolyl 4-H), 5.52 (s, 2 H, pyrazolyl 4-H), 2.60 (s, 1 H, CHSiMe3), 2.52 (s, 12 H, pyrazolyl Me), 2.37 (s, 6 H, pyrazolyl Me),2.36 (s, 12 H, pyrazolyl Me), 1.69 (s, 6 H, pyrazolyl Me), 1.58 (s, 12 H, pyrazolyl Me), 0.23 (s, 9 H, SiMe3). 13C NMR (150 MHz, C6D6, 25 °C): 150.4, 149.9, 149.8, 147.5, 146.8 (pyrazolyl 3 and 5-C), 126.3 (CHSiMe3), 105.7 104.0, 103.8 (pyrazolyl 4-C), 72.9 (Canionic), 15.2, 13.0, 12.9, 12.8, 12.6 (pyrazolyl Me), 9.3 (SiMe3), 137.9, 129.3, 128.6, 125.7, 21.4 (toluene), 31.9, 23.0, 14.3 (hexane). A satisfactory elemental analysis result for 6 could not be obtained. The NMR (1H, 13C) spectra of the complex are provided in the Supporting Information. X-ray Crystallography. Single crystals of 1−3 suitable for singlecrystal X-ray diffraction were grown from THF solutions, those of 4 and 5 were grown from hexane solutions, and those of 6 were grown from benzene solution. The single crystals of 1−6 were mounted under a nitrogen atmosphere on a glass fiber, and data collection was performed on a Bruker APEX2 diffractometer with graphitemonochromated Mo Kα radiation (λ = 0.71073 Å). The SMART program package was used to determine the unit cell parameters. The absorption correction was applied using SADABS. The structures were solved by direct methods and refined on F2 by full-matrix least-squares techniques with anisotropic thermal parameters for non-hydrogen atoms. Hydrogen atoms were placed at calculated positions and were included in the structure calculations. Due to the disorder of the SiMe3 group, the position of the hydrogen atom on C18 in 6 could not be solved. Calculations were carried out using SHELXL-97, SHELXL2014, and Olex2.21 Crystallographic data and refinement for 1−6 are given in Table S1 in the Supporting Information. Polymerization of Isoprene. The procedures for isoprene polymerization catalyzed by 4 and 5 were similar; a typical polymerization procedure is given below. Complex 5 (7.2 mg, 10.0 μmol) in PhCl (5.0 mL) and [Ph3C][B(C6F5)4] (18.4 mg, 20.0 μmol) and AliBu3 (0.01 g, 50 μmol) in toluene (0.2 mL) were added sequentially to a 50 mL flask in the glovebox. Isoprene (0.5 mL, 0.34 g, 5.0 mmol) was then added, and the mixture was stirred vigorously for 2 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.

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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.6b00166. Crystallographic data and refinement parameters for complexes 1−6, molecular structures of the anion [LScCl3]− in 1, the anion [LLuCl3]− in 3, and 5, 1H and 13C NMR spectra of complexes 1−6, and 1H and 13 C NMR spectra of polyisoprene synthesized according to Table 1 (PDF) X-ray crystallographic data for complexes 1−6 (CIF) F


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Organometallics

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for S.W.: [email protected]. *E-mail for Y.C.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the State Key Basic Research & Development Program (Grant No. 2012CB821600), the National Natural Science Foundation of China (Grant Nos. 21325210, 21132002, 21272256, 21432001 and 21421091), and the CAS/SAFEA International Partnership Program for Creative Research Teams.

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Organometallics WI, 2002. (f) SAINT+ Version 6.22a; Bruker AXS Inc., Madison,WI, 2002. (g) SAINT+ Version v7.68A; Bruker AXS Inc., Madison, WI, 2009. (h) SHELXTL NT/2000, Version 6.1; Bruker AXS Inc., Madison, WI, 2002.

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Tris(pyrazolyl)methanide Complexes of Trivalent Rare-Earth Metals

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