Dinuclear Aluminum Poly(phenolate) Complexes as Efficient Catalysts

Article pubs.acs.org/Organometallics

Dinuclear Aluminum Poly(phenolate) Complexes as Efficient Catalysts for Cyclic Carbonate Synthesis Pengfei Gao, Zhiwen Zhao, Lijuan Chen, Dan Yuan,* and Yingming Yao* Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, Dushu Lake Campus, Soochow University, Suzhou 215123, People’s Republic of China S Supporting Information *

ABSTRACT: A series of dinuclear aluminum complexes 1−4 stabilized by amine-bridged poly(phenolato) ligands have been synthesized, which are highly active in catalyzing the cycloaddition of epoxides and CO2. In the presence of 0.3 mol % complex 3 and 0.9 mol % NBu4Br at 1 bar CO2 pressure, terminal epoxides bearing different functional groups were converted to cyclic carbonates in 60−97% yields. Complex 3 is one of the rare examples of Al-based catalysts capable of promoting the cycloaddition at 1 bar pressure of CO2. Moreover, reactions of more challenging disubstituted epoxides also proceeded at an elevated pressure of 10 bar and afforded cyclic carbonates in 52−90% yields.

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INTRODUCTION CO2 is an abundant, renewable, and nontoxic C1 feedstock, and transformation of CO2 to value-added chemicals is attracting increasing interest.1 For instance, coupling reactions of CO2 and epoxides generate cyclic carbonates in a 100% atomeconomic manner (Scheme 1), which find application as

1 bar CO2 at room temperature using 2.5 mol % catalyst. Cooperation of both metal centers is reported to contribute to the high activity.9a−c Inspired by these reports, we worked on the design and synthesis of dinuclear aluminum complexes aiming at developing catalysts active under mild conditions (i.e., low temperature or pressure). On the basis of our previous study on lanthanide complexes bearing ethylenediaminebridged tetraphenolato ligands, which efficiently catalyzed the cycloaddition of CO2 and epoxides,13 multidentate poly(phenolato) ligands are employed to stabilize aluminum centers. Four dinuclear aluminum complexes were prepared and applied in cyclic carbonate synthesis.

Scheme 1. Synthesis of Cyclic Carbonates from Epoxides and CO2

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RESULTS AND DISCUSSION Synthesis of Complexes 1−4. Ligand precursors oxyethylamine-bridged bis(phenol) (LnH3)(n = 1, R = tBu; n = 2, R = CH3; and n = 3, R = Cl) and amine-bridged tri(phenol) (L4H3) were easily prepared according to literature methods.14 Alkane elimination reactions of LnH3 with AlMe3 in THF at 25 °C proceeded straightforwardly and gave the corresponding aluminum complexes 1−4 in good yields of 70−82% (Scheme 2). 1 H NMR spectra of 1−4 show the absence of signals for phenolic and alcoholic protons (for 1−3), corroborating successful complexation. Moreover, no resonances of coordinating methyl groups were observed, suggesting a 1:1 ratio of ligand to Al in complexes. Methylene protons are found to resonate in the range of 4.20−2.90 ppm. Solid state structures of complexes 3 and 4 reveal dinuclear centrosymmetric structures (Figures 1 and 2). Each central metal ion is five-

pharmaceutical intermediates and aprotic solvents.2 A variety of catalysts have been developed to catalyze this transformation,3 including transition metal4 (e.g., Zn, Cr, Co, and Fe) and main group metal (e.g., Al5−12) complexes. Since aluminum is earth-abundant and nontoxic, application of Al-based catalysts in CO2 transformation is highly desirable. Aluminum complexes stabilized by various ligands, including porphyrin,5 phthalocyanine,6 triphenolate,7 salen,8,9 salalen,10 pyrazole,11 and aminoethanol-derived12 ligands, have been reported to catalyze this cycloaddition reaction. However, high temperatures and pressures are in general required. Highly active Al-based catalytic systems include an aluminum triphenolate complex reported by Kleij et al., which, in combination with ammonium iodide, efficiently catalyzed reactions of mono- and disubstituted epoxides with 10 bar CO2 at 70−90 °C in the presence of low catalyst loading of 0.05−0.5 mol %.7b Dinuclear aluminum(salen) complexes,9 mainly reported by North and co-workers, are exceptionally effective in catalyzing the cycloaddition of terminal epoxides at © XXXX American Chemical Society

Received: February 24, 2016

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DOI: 10.1021/acs.organomet.6b00153 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 2. Synthesis of Complexes 1−4

Figure 2. Solid state structure of one independent molecule of complex 2×4·9THF. Thermal ellipsoids are drawn at the 50% probability level, and hydrogen atoms and solvent molecules are omitted for clarity. Selected bond lengths [Å] and bond angles [deg]: Al1−O1 1.745(2), Al1−O2 1.734(2), Al1−O3 1.871(2), Al1−O6 1.900(2), Al1−N1 2.053(3), Al2−O4 1.728(2), Al2−O5 1.743(2), Al2−O6 1.868(2), Al2−O3 1.894(2), Al2−N2 2.065(2), O2−Al1−O1 116.87(11), O2−Al1−O3 114.63(11), O1−Al1−O3 128.46(11), O2− Al1−O6 104.56(10), O1−Al1−O6 91.25(10), O3−Al1−O6 73.96(9), O2−Al1−N1 96.35(10), O1−Al1−N1 94.11(10), O3−Al1−N1 82.23(9), O6−Al1−N1 153.20(10), O4−Al2−O5 117.52(12), O4− Al2−O6 115.72(11), O5−Al2−O6 126.69(10), O4−Al2−O3 103.18(10), O5−Al2−O3 91.42(10), O6−Al2−O3 74.16(9), O4− Al2−N2 96.07(10), O5−Al2−N2 94.45(10), O6−Al2−N2 82.45(9), and O3−Al2−N2 154.45(10).

Table 1. Catalytic Production of Cyclic Carbonate from CO2 and 1,2-Epoxyhexane (5a) Mediated by Complexes 1−4a Figure 1. Solid state structure of complex 3·2THF. Thermal ellipsoids are drawn at the 50% probability level, and hydrogen atoms and solvent molecules are omitted for clarity. Selected bond lengths [Å] and bond angles [deg]: Al1−O1 1.749(2), Al1−O2 1.752(2), Al1−O3 1.818(2), Al1−O3A 1.853(2), Al1−N1 2.068(2); O1−Al1−O2 118.66(11), O1−Al1−O3 120.65(11), O1−Al1−O3A 94.31(10), O1−Al1−N1 95.04(10), O2−Al1−O3 120.68(11), O2−Al1−O3A 97.65(10), O2−Al1−N1 93.70(10), O3−Al1−O3A 77.02(10), O3− Al1−N1 82.61(9), and O3A−Al1−N1 159.59(10).

coordinated with four donor atoms from one ligand as well as one bridging oxygen atom from the other ligand, adopting trigonal bipyramidal geometry. Bond distances between Al and phenolate oxygen atoms in 3 (Al1−O1 1.749(2) Å and Al1− O2 1.752(2) Å) are found to be comparable, which are significantly shorter than those of Al and bridging alcoholic oxygen atoms (1.818(2) Å). Similar trends have been observed in complex 4. Catalytic Activities. The performances of complexes 1−4 were first examined and compared in the cycloaddition of CO2 to 1,2-epoxyhexane under solvent-free conditions at atmospheric pressure (Table 1). At 100 °C, all complexes efficiently catalyzed the cycloaddition in the presence of 0.6 mol % NBu4Br as cocatalyst and generated the desired cyclic carbonate in 85−99% yield (entry 1−4), which is one of the rare examples of Al-based catalysts that is capable of promoting the

entry

catalyst

Co-catalyst

T (°C)

t (h)

conversionb,c

1 2 3 4 5 6 7 8 9 10 11 12d 13e 14f 15f 16f

1 2 3 4 3 3 3 3 3 3 3 3 3 3 3 3

TBAB TBAB TBAB TBAB

100 100 100 100 100 85 70 100 100 100 100 100 100 100 85 70

18 18 18 18 18 18 18 18 18 12 24 18 18 18 18 18

90 85 96 99 ND 88 85 52 89 82 99 79 66 98 92 88

TBAB TBAB TBAI TOAB TBAB TBAB TBAB TBAB TBAB TBAB TBAB

a

Reaction conditions: 0.3 mol % catalyst and 0.6 mol % cocatalyst. Determined by 1H NMR spectroscopy. cSelectivity for the cyclic carbonate was all >99%. dReaction conditions: 0.2 mol % catalyst and 0.4 mol % cocatalyst. eReaction conditions: 0.3 mol % catalyst and 0.3 mol % cocatalyst. fReaction conditions: 0.3 mol % catalyst and 0.9 mol % cocatalyst. b

cycloaddition at 1 bar CO2 pressure.8h,j,9 The activity decreases in the order of 4 ≈ 3 > 1 > 2, which may be attributed to the more Lewis acidic Al centers in 3 and 4 due to the presence of B


Article

Organometallics electron-withdrawing chloro-substituents (in 3) and the phenyl bridge (in 4). Because of its good activity and easy preparation, complex 3 was employed for further optimization. No conversion was detected in the absence of cocatalyst (entry 5). Lowering the temperature to 85 and 70 °C resulted in lower yields of 88 and 85%, respectively (entry 6 and 7). Different cocatalysts have been tested, and NBu4Br proved to be the best performer (entries 3, 8, and 9), suggesting the importance of balancing good nucleophilic and leaving abilities.4d,11b Reaction time has also been optimized, and 18 h turned to be the optimal choice (entries 3, 10, and 11). Different loadings of precatalyst and cocatalyst were screened, and a near quantitative yield of 98% was obtained in the presence of 0.3 mol % complex 3 and 0.9 mol % NBu4Br (entries 12−14). Lowering the temperature to 85 °C resulted in a good yield of >90% (entries 15−16). Overall, the optimal reaction condition is determined as follows: 0.3 mol % of complex 3, 0.9 mol % of NBu4Br, 85 °C, 18 h, and 1 bar CO2. To explore the scope of the cycloaddition, a variety of monoand disubstituted epoxides were examined, and the results are summarized in Tables 2 and 3, respectively. Cyclic carbonates from monosubstituted epoxides bearing various substituents, including halogen, aryl, alkenyl, ether, morpholine, hydroxyl, and ester groups, were obtained in moderate to excellent yields

Table 3. Synthesis of Disubstituted Epoxides and CO2 Catalyzed by Complex 3 and NBu4Bra

a

Reaction conditions: 0.3 mol % catalyst, 0.9 mol % NBu4Br, 40 h, 10 bar CO2, 85 °C. bIsolated yield. cSelectivity for the cyclic carbonate was all >99%. d78% trans and 22% cis isomers. e79% trans and 21% cis isomers. fOnly cis-carbonate product was formed. g32% poly(carbonate) and 8% poly(ether) were also detected. hReaction conditions: 0.3 mol % catalyst, 40 h, 10 bar CO2, 85 °C. 92% polyether was detected. iReaction conditions: 0.3 mol % catalyst, 3 mol % NBu4Br, 40 h, 10 bar CO2, 85 °C. 10% poly(ether) was detected, and no poly(carbonate) was detected.

Table 2. Cycloaddition of Terminal Epoxides and CO2 Catalyzed by Complex 3 and NBu4Bra

(60 to 97%) (Table 2, entries 1−10). Some substrates containing substituents which might coordinate to the aluminum center and deactivate the catalyst (e.g., 5e−j) gave corresponding cyclic carbonates in 60−97% yields,7a revealing a good tolerance of complex 3 toward different functional groups. Cycloaddition of 1,2,7,8-diepoxyoctane (5l) with 2 equiv of CO2 occurred, which generated the corresponding carbonate 6l in 70% yield, and a higher yield of 90% after prolonged reaction time (entry 11). Moreover, reactions of bulky/internal epoxides (5m−5q) which are generally challenging substrates and usually give rise to unsatisfactory yields4c−e,7a proceeded smoothly at elevated pressure (10 bar) after a prolonged reaction time of 40 h (Table 3, entry 1−5). For substrate 5m, which is composed of 78% trans and 22% cis isomers, the corresponding carbonate was obtained in 79% trans and 21% cis configurations, indicating that the configuration remained almost unchanged during the reaction (entry 1). Similarly, cis-configured epoxides 5n and 5o afforded exclusively ciscarbonates 6n and 6o, which is consistent with previous reports (entries 2 and 3).4d,13 Reaction of cyclohexene oxide (5p) with CO2 deserves some comments (entry 4). In the presence of 0.3 mol % precatalyst 3 and 0.9 mol % NBu4Br at 85 °C and 10 bar of CO2, 52% of cyclic carbonate 6p formed concomitant with 32% poly(carbonate) from copolymerization of 5p and CO2, and 8% poly(ether) resulting from homopolymerization of 5p. Adjusting the amount of NBu4Br leads to different results. In the absence of NBu4Br, no cyclic carbonate or poly(carbonate) was detected, while 92% poly(ether) formed as the only product. In comparison, lowering the catalyst to cocatalyst ratio to 1:10 leads to the formation of 89% cyclic carbonate and 10% poly(ether), and no poly(carbonate). The influence of ammonium salts on product distribution of cyclohexene oxide and CO2 cycloaddition has also been reported by Kleij et al. in a Fe-based catalytic system4j and by Lamberti et al. in bimetallic salalen aluminum complex catalyzed reactions.10 To further explore the potential of aluminum-based catalyst 3, the initial TOF which was generally accepted as a

a Reaction conditions: 0.3 mol % catalyst, 0.9 mol % NBu4Br, 18 h, 1 bar CO2 (balloon), 85 °C. bIsolated yield. cSelectivity for the cyclic carbonate was all >99%. d24 h. e40 h.

C


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measurement for catalyst capacity7b was studied under 1 bar CO24f,7a using 1,2-epoxyhexane 5a as a model substrate. A moderate initial TOF of 127 h−1 was obtained within 0.5 h (Table S1). However, TOF values showed a decreasing trend as the conversion increased, mainly due to the obviously decreased concentration of the substrate. A first-order disappearance of 1,2-epoxyhexane concentration was obtained from the plot of ln([5a]0/[5a]) (Figure 3).

CONCLUSIONS In summary, a series of dinuclear aluminum complexes stabilized by amine-bridged bis(phenolato) ligands have been prepared and characterized. They showed good activities in catalyzing the cycloaddition of terminal epoxides bearing different functional groups and CO2 at 1 bar pressure, and are among the rare examples of Al-based catalysts operative at atmospheric pressure. In addition, generally accepted challenging internal epoxides were also transformed into cyclic carbonates in moderate to good yields at a higher pressure of 10 bar. Further application of aluminum complexes in CO2 fixation is ongoing in our laboratory.

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EXPERIMENTAL SECTION

General Procedures. All manipulations were performed under an argon atmosphere using standard Schlenk techniques, except for ligand precursor synthesis. Solvents (toluene, hexane, and THF) were distilled by refluxing over sodium benzophenoneketyl prior to use. Ligand precursors L1H3, L2H3, and L4H3 were prepared according to optimization of literature methods.14 1,2-Epoxyhexane was dried over CaH2 for 2 days and distilled before use. Other epoxide substrates and carbon dioxide were commercially available and used without further purification. Carbon, hydrogen, and nitrogen analyses were performed by direct combustion with a CarloErba EA-1110 instrument. Aluminum contents were determined by EDTA-based complexometric titration, and procedures are detailed in Supporting Information. The structures of aluminum complexes were determined by X-ray single crystal diffractometry. The structures were solved and refined using SHELEXL-97 programs. L1H3 (R = tBu). A mixture of 2,4-di-tert-butylphenol (20.6 g, 100 mmol), ethanolamine (3 mL, 50 mmol), and paraformaldehyde (3.0 g, 100 mmol) in methanol (6 mL) was heated under reflux for 12 h. After cooling to room temperature, all volatiles were removed under reduced pressure. Pure product was isolated by washing the residue with hexane (25 mL) in 60% yield (14.93 g, 30 mmol). The identity of the product was confirmed by comparison with literature data.14a 1H NMR (CDCl3, 400 MHz): δ 7.20 (s, 1H, ArH), 7.19 (s, 1H, ArH), 6.89 (s, 1H, ArH), 6.88 (s, 1H, ArH), 3.87 (t, J = 5.3 Hz, 2H, OCH2CH2N), 3.74 (s, 4H, NCH2Ar), 2.72 (t, J = 5.3 Hz, 2H, OCH2CH2N), 1.38 (s, 18H, C(CH3)3), 1.26 (s, 18H, C(CH3)3). L2H3 (R = CH3). A mixture of 2,4-dimethylphenol (12.2 g, 100 mmol), ethanolamine (3 mL, 50 mmol), and paraformaldehyde (3.0 g, 100 mmol) was heated neat for 12 h in 85 °C. After reaction, toluene (30 mL) was added to the reaction mixture, and the product was isolated after filtration in 40% yield (6.59 g, 20 mmol). The identity of the product was confirmed by comparison with literature data.14b 1H NMR (CDCl3, 400 MHz): δ 6.84 (s, 2H, ArH), 6.69 (s, 2H, ArH), 3.84 (t, J = 5.3 Hz, 2H, OCH2CH2N), 3.72 (s, 4H, NCH2Ar), 2.68 (t, J = 5.3 Hz, 2H, NCH2CH2O), 2.19 (s, 6H, CH3), 2.17 (s, 6H, CH3). L3H3 (R = Cl). A mixture of 2,4-dichlorophenol (16.3 g, 100 mmol), ethanolamine (3 mL, 50 mmol), and paraformaldehyde (3.0 g, 100 mmol) was heated neat for 18 h in 120 °C. The product was purified by chromatography (eluent: ethyl acetate/hexane = 1:10) and isolated as a reddish brown oil. A mixture of toluene and hexane was added to the oil, and the precipitate was isolated by filtration. The crude product from the precipitate was further purified by recrystallization from CHCl3. Pure product was obtained as colorless crystals (6.58 g, 16 mmol, 32%) 1H NMR (CDCl3, 400 MHz): δ 7.21 (s, 2H, ArH), 6.93 (s, 2H, ArH), 3.87 (t, J = 5.3 Hz, 2H, OCH2CH2N), 3.76 (s, 4H, NCH2Ar), 2.71 (t, J = 5.3 Hz, 2H, NCH2CH2O). 13C NMR (CDCl3, 100 MHz): δ 150.9, 129.1, 128.7, 124.8, 124.5, 121.9 (Ar−C), 59.9 (OCH2CH2N), 56.2 (NCH2Ar), 54.8 (NCH2CH2O). L4H3. 2,4-Di-tert-butylphenol (17.95 g, 87 mmol) was dissolved in methanol (60 mL), and NaOH (3.83 g, 96 mmol) was added to the solution. After stirring for half an hour, 37% aqueous formaldehyde (14 mL, 174 mmol) was added. The resulting mixture was left stirring for 18 h at 25 °C. Then, 300 mL of water and 30 mL of hydrochloric acid were added to the solution until the pH value reached 3−4. The

Figure 3. Semilogarithmic plot of the cycloaddition of 1,2epoxyhexane and CO2 (1 bar) catalyzed by complex 3 and NBu4Br. [5a]0, initial concentration of 1,2-epoxyhexane; [5a], the concentration of 1,2-epoxyhexane. y = 0.1137x + 0.4425; R2 = 0.9817.

A possible mechanism has been proposed (Scheme 3).9b Epoxides are activated through coordination to aluminum Scheme 3. Proposed Mechanism of Cyclic Carbonate Synthesis

centers, which facilitates the attack of halides of the cocatalyst to the less hindered carbon atom of epoxides to generate species B. Carbon dioxide then inserts into the Al−O bond forming carbonate C. Finally, cyclic carbonates form through intramolecular cyclization (Scheme 3). D


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Organometallics

155.3, 150.2, 140.1, 138.2, 135.8, 128.6, 124.4, 122.7, 121.0, 120.5, 119.1 (Ar−C), 35.2, 34.3 (NCH2Ar), 32.0 (C(CH3)3), 29.8 (C(CH3)3). Anal. Calcd for C72H96Al2N2O6: C, 75.89; H, 8.49; N, 2.46; Al, 4.74. Found: C, 76.11; H, 8.46; N, 2.47; Al, 4.72. Catalytic Procedure for the Cycloaddition of CO2 to Epoxides under 1 Bar CO2. In a typical experiment, complex 3 (21.7 mg, 0.05 mmol), NBu4Br (48.1 mg, 0.15 mmol), and 1,2epoxyhexane (2 mL, 16.63 mmol) were added to a 20 mL flask equipped with a magnetic stirring bar under argon. The round bottomed flask was then degassed and filled with CO2. The mixture was heated at 85 °C for 18−24 h. After reaction, a drop of the resulting mixture was analyzed by 1H NMR spectroscopy to determine the yield and selectivity. The final pure product was isolated by column chromatography and confirmed by comparison with literature data. Catalytic Procedure for the Cycloaddition of CO2 to Epoxides under 10 Bar CO2. Experiments were conducted in a 100 mL stainless steel Parr reactor with a stirring bar inside and needle valve for injection. In a typical experiment, complex 3 (21.7 mg, 0.05 mmol) and NBu4Br (48.1 mg, 0.15 mmol) were dissolved by epoxides. The mixture was injected into the reactor which was dried before use. The reactor was pressurized to 10 bar with carbon dioxide and heated at 85 °C for 40 h. After the reaction, a drop of the resulting mixture was analyzed by 1H NMR spectroscopy to determine the yield and selectivity. The final pure product was isolated by column chromatography and confirmed by comparison with literature data. Product 6o which was not synthesized before was characterized by multinuclear NMR spectroscopy. X-ray Crystallographic Structure Determination. Suitable single crystals of complexes 3·2THF and 2×4·9THF were sealed in a thin-walled glass capillary for determination of the single-crystal structures. Intensity data were collected with a Rigaku Mercury CCD area detector in ω scan mode using Mo−Kα radiation (λ= 0.71070 Å). The diffracted intensities were corrected for Lorentz/polarization effects and empirical absorption corrections. The structures were solved by direct methods and refined by fullmatrix least-squares procedures based on |F|2. The hydrogen atoms in these complexes were generated geometrically, assigned appropriate isotropic thermal parameters, and were allowed to ride on their parent carbon atoms. All of the hydrogen atoms were held stationary and included in the structure factor calculation in the final stage of fullmatrix least-squares refinement. The structures were solved and refined using SHELEXL-97 programs.

precipitate was isolated and washed with cool hexane to give 3,5-ditert-butyl-2-hydroxybenzyl alcohol (16.07 g, 68 mmol, 78%). It was subsequently treated with dry hydrochloric acid gas in hexane for 7 h at 50 °C, and all volatiles were removed in vacuo to generate 3,5-ditert-2-hydroxybenzyl chloride (13.76g, 54 mmol, 80%). A mixture of 2hydroxyaniline (2.95 g, 27 mmol) and 3,5-di-tert-2-hydroxybenzyl chloride (13.76 g, 54 mmol) in CH2Cl2 (70 mL) was heated at 50 °C. Triethylamine (9 mL, 65 mmol) was added to the mixture dropwise, and the reaction was stirred for 12 h. Water was added to the reaction to remove triethylamine hydrochloride. The organic layer was dried over sodium sulfate, and the filtrate was concentrated. The product was obtained (9.57 g, 65%) via recrystallization from acetonitrile. The overall yield is 41%. The identity of the product was confirmed by comparison with literature data.14c 1H NMR (CDCl3, 400 MHz): δ 7.21−7.23 (d, J = 8.0 Hz, 1H, ArH), 7.16 (d, 2H, ArH), 6.94 (m, 3H, ArH), 6.82−6.84 (t, 2H, ArH), 4.10 (s, 4H, NCH2Ar), 1.37 (s, 18H, C(CH3)3), 1.24 (s, 18H, C(CH3)3). Complex 1. A mixture of AlMe3 (0.288 g, 4 mmol, in 4 mL THF solution) and L1H3 (1.99 g, 4 mmol, in 15 mL THF solution) was stirred for 5 h at room temperature. After the reaction, all volatiles were removed in vacuo. Twenty milliliters of toluene was added to dissolve the residue. White powders were obtained at room temperature from the toluene solution (1.71 g, 1.64 mmol, 82%). 1 H NMR (C6D6, 400 MHz): δ 7.51 (s, 2H, ArH), 6.78 (s, 2H, ArH), 4.28 (t, J = 5.3 Hz, 2H, OCH2CH2N), 3.31 (br-s, 4H, NCH2Ar), 2.45 (br-s, 2H, NCH2CH2O), 1.52 (s, 18H, C(CH3)3), 1.41 (s, 18H, C(CH3)3). 13C NMR (C6D6, 100 MHz): δ 156.3, 139.8, 138.4, 124.9, 124.6, 121.8 (Ar−C), 58.0 (OCH2CH2N), 56.7 (NCH2Ar), 53.4 (OCH2CH2N), 35.6, 34.7 (Ar−C), 32.4 ((CH3)3C), 30.1 ((CH3)3C). Anal. Calcd for C64H96Al2N2O6: C, 73.67; H, 9.27; N, 2.68; Al, 5.17. Found: C, 73.89; H, 9.24; N, 2.69; Al, 5.15. Complex 2. A mixture of AlMe3 (0.288 g, 4 mmol, in 4 mL THF solution) and L2H3 (1.32 g, 4 mmol, in 15 mL THF solution) was stirred for 5 h at room temperature. After the reaction, all volatiles were removed in vacuo. Two milliliters of toluene and 12 mL of hexane were added to dissolve the residue. White powders were obtained at room temperature after 2 days from the extract (1.41 g, 1.06 mmol, 75%). 1H NMR (CDCl3, 400 MHz): δ 6.86 (s, 2H, ArH), 6.62 (s, 2H, ArH), 4.20 (s, 2H, OCH2CH2N), 3.81 (d, J = 12.4 Hz, 2H, NCHHAr), 3.66 (d, J = 12.4 Hz, 2H, NCHHAr), 2.82 (s, 2H, NCH2CH2O), 2.19 (s, 6H, Ar−CH3), 1.98 (s, 6H, Ar−CH3). 13C NMR (CDCl3, 100 MHz): δ 154.6, 131.7, 128.1, 127.6, 126.2, 120.5 (Ar−C), 57.3 (OCH2CH2N), 56.1 (NCH2Ar), 53.4 (NCH2CH2O), 20.7 (Ar−CH3), 16.2 (Ar−CH3). Anal. Calcd for C40H48Al2N2O6: C, 67.97; H, 6.85; N, 3.96; Al, 7.64. Found: C, 68.17; H, 6.83; N, 3.97; Al, 7.61. Complex 3. A mixture of AlMe3 (0.288 g, 4 mmol, in 4 mL THF solution) and L3H3 (1.64 g, 4 mmol, in 15 mL THF solution) was stirred for 5 h at room temperature. After the reaction, all volatiles were removed in vacuo. Forty-five milliliters of THF was added to dissolve the residue. Colorless crystals were obtained at room temperature after 1 day from the extract (1.34 g, 1.54 mmol, 77%). 1 H NMR (CDCl3, 400 MHz): δ 7.29 (d, 2H, ArH), 6.90 (d, 2H, ArH), 4.32 (t, J = 6.0 Hz, 2H, OCH2CH2N), 3.86 (d, J = 13.4 Hz, 2H, NCHHAr), 3.68 (d, J = 13.4 Hz, 2H, NCHHAr), 2.90 (t, J = 6.0 Hz, 2H, NCH2CH2O). 13C NMR (CDCl3, 100 MHz): δ 152.9, 130.4, 127.9, 125.6, 123.2, 122.6 (Ar−C), 56.8 (OCH2CH2N), 56.3 (NCH2Ar), 53.6 (NCH2CH2O). Anal. Calcd for C32H24Al2Cl8N2O6: C, 44.17; H, 2.78; N, 3.22; Al, 6.20. Found: C, 44.30; H, 2.77; N, 3.23; Al, 6.18. Complex 4. A mixture of AlMe3 (0.288 g, 4 mmol, in 4 mL THF solution) and L4H3 (2.18 g, 4 mmol, in 15 mL THF solution) was stirred for 5 h at room temperature. After the reaction, all volatiles were removed in vacuo. Ten milliliters of THF was added to dissolve the residue. Colorless crystals were obtained at room temperature after 1 day from the extract (1.59 g, 1.4 mmol, 70%). 1H NMR (CDCl3, 400 MHz): δ 7.66−7.64 (dd, J = 8.1 Hz, 1H, ArH), 7.46−7.44 (dd, J = 8.1 Hz, 1H, ArH), 7.28−7.24 (m, J = 7.9 Hz, 1H, ArH), 7.15 (s, 2H, ArH), 7.11−7.07 (d, J = 7.9 Hz, 1H, ArH), 6.84 (s, 2H, ArH), 4.20 (br-s, 4H, NCH2Ar), 1.23 (s, 36H, C(CH3)3). 13C NMR (CDCl3, 100 MHz): δ

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00153. NMR spectra of complexes 1−4 and cyclic carbonates, and a table of X-ray crystallographic data (PDF) X-ray crystallographic data for 3 and 4 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*(D.Y.) E-mail: [email protected]. *(Y.Y.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (Grants 21132002, 21372172, and 21402135), the Major Research Project of the Natural Science of the Jiangsu Higher Education Institutions (14KJA150007), and PAPD are gratefully acknowledged. E


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Organometallics

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