Syntheses, Characterization, and Application of Tridentate Phenoxyimino-Phenoxy Aluminum Complexes for the Coupling of Terminal Epoxide with CO2: From Binary System to Single Component Catalyst

A series of binuclear aluminum complexes 1–3 supported by tridentate phenoxyiminophenoxy ligands was synthesized and used as catalysts for the coupling reaction of terminal epoxide with carbon dioxide. The aluminum complex 1, which is catalytically inactive toward the coupling of epoxide with CO2 by itself, shows moderate activity in the presence of excess nucleophiles or organic bases at high temperature. In sharp contrast to complex 1, bifunctional complexes 2 and 3, which incorporate tertiary amine groups as the built-in nucleophile, are able to efficiently transform terminal epoxide with CO2 to corresponding cyclic carbonates as a sole product by themselves at 100 ◦C. The number of amine groups on the ligand skeleton and the reaction temperature exert a great influence on the catalytic activity. The bifunctional complexes 2 and 3 are also active at low carbon dioxide pressure such as 2 atm or atmospheric CO2 pressure. Kinetic studies of the coupling reactions of chloropropylene oxide/CO2 and styrene oxide/CO2 using bifunctional catalysts under atmospheric pressure of CO2 demonstrate that the coupling reaction has a first-order dependence on the concentration of the epoxide.


Introduction
With the rapid development of the industry as well as the ever-growing human activity, a huge amount of CO 2 has been released to the air, which has caused severe environmental problems [1][2][3][4]. The utilization of CO 2 as feedstock to produce valuable chemicals is a promising way to solve this problem, but this conversion is limited due to the inertness of CO 2 [5]. The catalytic coupling reaction with epoxides represents one of the promising processes employing CO 2 to produce organic cyclic carbonate, which has found widespread applications in many respects [6][7][8][9][10]. Generally, the catalyst capable of achieving the transformation is comprised of nucleophile and Lewis acid. Thus, organic compounds and metal-based complexes have been employed as the Lewis acid for the coupling reaction, with the aid of nucleophiles to constitute the binary catalytic system. Usually, metal-based complexes display higher reactivity than organocatalysts for the coupling reaction. As a result of the diversity of the metal across the periodic table and the organic ligands of different structures, miscellaneous metal complexes based on aluminum , magnesium [33][34][35][36], chromium [37][38][39][40][41][42], cobalt [43][44][45][46][47][48], iron [49][50][51][52][53], and rare earth metals [54,55] have been innovated for the coupling reaction of CO 2 with epoxides. Of many metal complexes, aluminum complexes have been drawing attention due to their low price, easy availability, and the superior selectivity for the cyclic carbonate over polymeric products [56]. It is well established that the electronic effects and steric hindrance around the aluminum center that can be finely tuned are crucial for the catalytic activity and the product selectivity. For example, optically active carbonate can be produced from the coupling of racemic epoxide with CO 2 when the chiral complexes are employed [44,45,57].
In addition to binary catalytic system, there is a tendency to employ a bifunctional catalyst in which nucleophile and Lewis metal complexes are constructed into one molecule as the one-component catalyst for the coupling of CO 2 with epoxides in the past decade [58][59][60][61][62][63][64][65][66][67][68]. Quaternary ammonium halide as a nucleophile is usually integrated into the metal complex to construct an ionic bifunctional catalyst. The pre-association of the nucleophilic halide to the metal complex via electrostatic attraction would effectively reduce the translational entropy penalty at the ring-opening step and the cyclization step as compared with the binary catalytic system [69]. Thus, the bifunctional catalyst loading for the coupling reaction is significantly reduced, and usually, a higher catalytic reactivity is achieved than that with the corresponding binary catalytic system. However, the incorporation of the quaternary onium salts into the metal complex impairs the solubility of the bifunctional catalyst. Another drawback is the thermostability of the ammonium halide-functionalized catalyst at elevated temperature [59], which undergoes ammonium salt decomposition pathways, including Zaitsev and Hoffman type eliminations [70][71][72] and retro-Menschutkin reactions [73][74][75][76]. Hence, it is important and necessary to develop bifunctional catalyst of high efficiency and stability at elevated temperature.
In this work, we present the syntheses of a series of aluminum complexes supported by novel tridentate phenoxyimino-phenoxy ligands and their catalytic behaviors for the coupling of terminal epoxides with CO 2 . Compared with tetradentate salen-type ligand stabilized metal complexes, which are extensively employed for the coupling of epoxide with CO 2 , aluminum complexes supported by tridentate ligands for this transformation are rare. As a result of the low coordination number of tridentate ligands, the resultant complexes are expected to have a more acidic metal center, which is supposed to benefit the activation of the epoxide ring and to facilitate the ring-opening. Moreover, an amine group was incorporated onto the ligand framework as the built-in nucleophile to construct neutral bifunctional complexes 2 and 3, in order to avoid the decomposition of ammonium halide moiety under elevated temperature. Systematic investigation of the catalytic behaviors of the aluminum complexes 2 and 3 reveals that the neutral and ionic bifunctional complexes are highly active toward the coupling of epoxide to CO 2 , whereas the unfunctionalized aluminum complex 1 is catalytically inactive by itself.

Syntheses and Characterization of Complexes 1-3
The tridentate ligand precursors H 2 L 1 -H 2 L 3 (see Scheme 1) were readily synthesized by reacting the substituted salicylaldehyde with substituted aminophenol in 1:1 molar ratio (see the experimental section in the supporting material). The three ligand precursors were identified by proton nuclear magnetic resonance ( 1 H NMR) and 13 C-nuclear magnetic resonance ( 13 C NMR) (Figures S1-S3 in the supporting material). Subsequent syntheses of complexes 1-3 were conducted by the alkyl-elimination reaction of Et2AlCl with the corresponding ligand precursors at low temperature, as illustrated in Scheme 1. In a typical procedure, a tetrahydrofuran (THF) solution of H2L 1 was added dropwise to a stirred hexane solution of AlEt2Cl at -30 °C to afford a yellow Scheme 1. The syntheses of complexes 1-3. Subsequent syntheses of complexes 1-3 were conducted by the alkyl-elimination reaction of Et 2 AlCl with the corresponding ligand precursors at low temperature, as illustrated in Scheme 1. In a typical procedure, a tetrahydrofuran (THF) solution of H 2 L 1 was added dropwise to a stirred hexane solution of AlEt 2 Cl at −30 • C to afford a yellow solution. The reaction was slowly warmed up to room temperature and stirred for another 4 h. After the removal of all the volatiles under reduced pressure, yellow solids were separated, which was followed by washing with cold hexane three times and drying to constant weight under vacuum. The structure of the isolated solids was characterized by the NMR technique with CDCl 3 as the solvent. The 1 H NMR spectrum shows no resonances of hydroxy groups or ethyl groups bound to aluminum. In the low field, the resonance of 8.06 ppm assignable to the imino moiety was observed, which slightly shifted to the low field when compared with that in free ligand precursor H 2 L 1 (see Figure S4 in the supporting material). Two distinct resonances of 1.32 ppm and 0.84 ppm for the tBu groups were found (1.49 and 1.35 ppm in the H 2 L 1 ), indicating chelation of the tridentate ligand to aluminum. Moreover, the upfield shift of the imino group in the H 2 L 1 from 8.72 to 8.06 ppm in the isolated complex suggested the coordination of nitrogen of the imino group to the aluminum center. Remarkably, no solvent molecules were detected in the 1 H NMR spectrum, suggesting that the isolated complex is THF-free.
In addition to the characterization by NMR, mass spectrometry (MS) was employed to characterize the isolated complex. A very weak peak at m/z = 350.1566 was detected, which corresponds to the cationic species 1a having the formula of C 21 H 25 O 2 NAl, as shown in Scheme 2. Interestingly, a strong signal was observed at m/z = 414.2219, which was assigned to the cationic species 1b with the formula of C 23 H 33 O 4 NAl. We supposed that the species 1b was generated by the coordination of two methanol molecules to the cationic species 1a. Thus, it is reasonable to interpret the weak signal of 1a and the strong peak of 1b on the mass spectrum. In the light of the strong signal of 1b on the MS, no solvent detection on the 1 H NMR spectrum, and the coordination number of 5 usually adopted by aluminum complex, we expected that complex 1 should be a binuclear structure, with each aluminum center being surrounded by a phenoxyimino-phenoxy ligand and two chlorides, as presented in Scheme 2.  Similarly, treatment of the ligand precursors H2L 2 and H2L 3 with AlEt2Cl under low temperature resulted in the formation of bifunctional complexes 2 and 3. The characterization of complex 2 by 1 H NMR showed the successful alkyl elimination between AlEt2Cl and H2L 2 . Moreover, there are no THF molecules detected on the 1 H NMR spectrum, suggesting that complex 2 is THF-free. The MS spectrum of complex 2 shows a m/z peak positioning at 397.1085. We tentatively ascribed this signal to the structure 2a, which probably is an ethanol coordinated species, as illustrated in Figure 1. Meanwhile, a peak at m/z = 783.3226 was detected, which is ascribed to the cationic species 2b. Although a tertiary amine group is introduced into the molecule, the inner coordination of the amine group to the Al center is impossible because of the orientation of the tertiary amine group. Taking into account the coordination saturation of the aluminum, it is Similarly, treatment of the ligand precursors H 2 L 2 and H 2 L 3 with AlEt 2 Cl under low temperature resulted in the formation of bifunctional complexes 2 and 3. The characterization of complex 2 by 1 H NMR showed the successful alkyl elimination between AlEt 2 Cl and H 2 L 2 . Moreover, there are no THF molecules detected on the 1 H NMR spectrum, suggesting that complex 2 is THF-free. The MS spectrum of complex 2 shows a m/z peak positioning at 397.1085. We tentatively ascribed this signal to the structure 2a, which probably is an ethanol coordinated species, as illustrated in Figure 1. Meanwhile, a peak at m/z = 783.3226 was detected, which is ascribed to the cationic species 2b. Although a tertiary amine group is introduced into the molecule, the inner coordination of the amine group to the Al center is impossible because of the orientation of the tertiary amine group. Taking into account the coordination saturation of the aluminum, it is reasonable to assume the complex 2 adopts binuclear structure with dangling tertiary amine groups. Similar results were also observed for complex 3, as indicated by the 1 H NMR and MS, which should also adopt a binuclear structure with dangling amine groups via chloride bridges.
Similarly, treatment of the ligand precursors H2L 2 and H2L 3 with AlEt2Cl under low temperature resulted in the formation of bifunctional complexes 2 and 3. The characterization of complex 2 by 1 H NMR showed the successful alkyl elimination between AlEt2Cl and H2L 2 . Moreover, there are no THF molecules detected on the 1 H NMR spectrum, suggesting that complex 2 is THF-free. The MS spectrum of complex 2 shows a m/z peak positioning at 397.1085. We tentatively ascribed this signal to the structure 2a, which probably is an ethanol coordinated species, as illustrated in Figure 1. Meanwhile, a peak at m/z = 783.3226 was detected, which is ascribed to the cationic species 2b. Although a tertiary amine group is introduced into the molecule, the inner coordination of the amine group to the Al center is impossible because of the orientation of the tertiary amine group. Taking into account the coordination saturation of the aluminum, it is reasonable to assume the complex 2 adopts binuclear structure with dangling tertiary amine groups. Similar results were also observed for complex 3, as indicated by the 1 H NMR and MS, which should also adopt a binuclear structure with dangling amine groups via chloride bridges.

Catalytic Performance of Complexes 1-3 in the Coupling Reactions of Terminal Epoxide with CO2
To begin with, we examined the catalytic performance of complex 1 for the coupling of propylene oxide (PO) with CO2 (see Scheme 3). Unfortunately, complex 1 was catalytically inactive by itself, which was in agreement with the previous literature reports [21,22]. When it was paired with a nucleophile or organic base, complex 1 showed moderate activity for the PO/CO2 coupling (see Tables S1-S4 in the supporting information).

Catalytic Performance of Complexes 1-3 in the Coupling Reactions of Terminal Epoxide with CO 2
To begin with, we examined the catalytic performance of complex 1 for the coupling of propylene oxide (PO) with CO 2 (see Scheme 3). Unfortunately, complex 1 was catalytically inactive by itself, which was in agreement with the previous literature reports [21,22]. When it was paired with a nucleophile or organic base, complex 1 showed moderate activity for the PO/CO 2 coupling (see Tables S1-S4 in the supporting information). For instance, complex 1 in combination with tetrabutylammonium bromide (TBAB, 40 eq) catalyzed the PO/CO 2 coupling to produce propylene carbonate (PC) as the sole product at 80 • C under 20 atm CO 2 in 10 h with 91% PO conversion (see entry 8 in Table S1). There was no detection of any polymeric products as evidenced by the 1 H NMR spectrum of the reaction mixtures. It was also interesting to find that the complex 1/TBAB (40 eq) system displayed a zeroth-order dependence on the PO throughout the coupling (see Table S2 and Figure S7, in the supporting material). This result is analogous to Han's report in which polymeric ionic liquid was employed as the catalyst for the PO/CO 2 coupling reaction [77]. When other organobases such as 4-dimethylaminopyridine (DMAP) or 1-methylimidazole (1-MeIm) were employed as the cocatalysts, high cocatalyst loading was required for effective transformation (see Tables S3 and S4  For instance, complex 1 in combination with tetrabutylammonium bromide (TBAB, 40 eq) catalyzed the PO/CO2 coupling to produce propylene carbonate (PC) as the sole product at 80 °C under 20 atm CO2 in 10 h with 91% PO conversion (see entry 8 in Table  S1). There was no detection of any polymeric products as evidenced by the 1 H NMR spectrum of the reaction mixtures. It was also interesting to find that the complex 1/TBAB (40 eq) system displayed a zeroth-order dependence on the PO throughout the coupling (see Table S2 and Figure S7, in the supporting material). This result is analogous to Han's report in which polymeric ionic liquid was employed as the catalyst for the PO/CO coupling reaction [77]. When other organobases such as 4-dimethylaminopyridine (DMAP) or 1-methylimidazole (1-MeIm) were employed as the cocatalysts, high cocata lyst loading was required for effective transformation (see Tables S3 and S4 in  It has been reported by North and Pasquale that quaternary ammonium halide would decompose at high temperature to generate tertiary amine and halohydrocarbon [78]. The in situ generated tertiary amine, together with the remaining ammonium halide salts, assists the metal complex to convert propylene oxide and CO2 to cyclic carbonate Scheme 3. This is a figure. Schemes follow the same formatting. It has been reported by North and Pasquale that quaternary ammonium halide would decompose at high temperature to generate tertiary amine and halohydrocarbon [78]. The in situ generated tertiary amine, together with the remaining ammonium halide salts, assists the metal complex to convert propylene oxide and CO 2 to cyclic carbonate. Recently, Kim and his coworkers reported the coupling of epoxide with CO 2 catalyzed by a series of tertiary amines with moderate catalytic activity [79]. These results enlightened us to design amine-functionalized complex 2 as the single-component catalyst for the coupling of epoxide with CO 2 . The catalytic results were compiled in Table 1. It was satisfying that the bifunctional complex 2 by itself succeeded in transforming the CO 2 and PO to PC. Analysis of the reaction mixture showed that PC was exclusively produced without the formation of any polymeric products as evidenced by 1 H NMR spectrum. Although complex 2 displayed very low activity at 80 • C (entries 1 and 2, in Table 1), however, the catalytic activity was considerably enhanced when the reaction was conducted at 100 • C. For instance, 68% of PO was converted to PC under 10 atm CO 2 in 10 h (entry 3, Table 1). In addition to the reaction temperature, increasing the CO 2 pressure accelerates the coupling reaction. Then, 94% of PO was transformed to PC at 20 atm CO 2 pressure within 10 h (entry 4, Table 1), suggesting that high CO 2 pressure, namely, the high CO 2 concentration facilitated the coupling reaction. Subsequent examination of the catalytic performance of complex 2 in other terminal epoxides/CO 2 coupling reactions confirmed again the significance of the reaction temperature and CO 2 pressure. The 1,2-butylene oxide (BO) conversion of 7% at 80 • C under 10 atm CO 2 was improved to 49% when the reaction was performed at 100 • C under 20 atm of CO 2 pressure (entries 5 and 6, Table 1). Similar trends were also observed for the coupling of CO 2 with epichlorohydrin (or chloropropylene oxide, abbreviated as CPO) or styrene oxide (SO). The CPO conversion of 12% at 80 • C under 10 atm of CO 2 in 10 h was enhanced to 36%, 52%, and 90% within 5, 10, and 24 h, respectively, when the reaction temperature was simply increased to 100 • C (entries 7, 9-11, Table 1). As the CO 2 pressure was increased to 20 atm, the conversion of CPO rose to 40% in 5 h and 62% in 10 h under 20 atm CO 2 (entries 12 and 13, Table 1). As for the SO/CO 2 coupling, the reaction was quite sluggish  Table 1). High SO conversion can only be achieved after a long reaction time (entry 17, Table 1).
Complex 2 was capable of transforming CPO/CO 2 and SO/CO 2 to cyclic carbonate under atmospheric CO 2 pressure. The results were collected in Table 2, and the kinetic curves (conversion vs. reaction time) were depicted in Figure 2. As expected, the rate of the coupling of CO 2 with either CPO or SO under 1 atm CO 2 was extremely slow, which was partly due to the low CO 2 pressure. As can be seen from Figure 2, there is a surge in the CPO conversion after 36 h (the blue curve). This might be interpreted by the poor solubility of complex 2 in CPO. Therefore, the metal complex 2 was not well dispersed in the epoxide, leading to a heterogeneous system. Thus, the effective concentration of complex 2 was lower than the theoretical value at the beginning. With the proceeding of the coupling reaction, highly polar cyclic carbonate was generated, which in turn acted as solvent to dissolve complex 2. Thus, the effective concentration of complex 2 was continuously increased in the course of the reaction until the full dissolution of complex 2. This would accelerate the coupling, as is shown in Figure 2 (the blue curve). A similar situation took place in the case of SO/CO 2 coupling as well, where the reaction rate gradually increased with the reaction time, as indicated by the kinetic curve (the red curves in Figure 2).  The success of complex 2 as a single-component catalyst motivated us to examine the catalytic performance of complex 3, which has more tertiary amine groups on the ligand skeleton for the coupling of epoxides toward CO2. The catalytic results were collected in Table 3. As anticipated, complex 3 displayed higher activity than complex 2 did under the same conditions, which was tentatively attributed to the incorporation of one more amine group. Similar to complex 2, complex 3 exhibited low activity at 80 °C. For example, 14% of PO was transformed to PC under 20 atm CO2 in 10 h (entry 1, Table 3). However, the catalytic activity was dramatically improved by increasing the temperature The success of complex 2 as a single-component catalyst motivated us to examine the catalytic performance of complex 3, which has more tertiary amine groups on the ligand skeleton for the coupling of epoxides toward CO 2 . The catalytic results were collected in Table 3. As anticipated, complex 3 displayed higher activity than complex 2 did under the same conditions, which was tentatively attributed to the incorporation of one more amine group. Similar to complex 2, complex 3 exhibited low activity at 80 • C. For example, 14% of PO was transformed to PC under 20 atm CO 2 in 10 h (entry 1, Table 3). However, the catalytic activity was dramatically improved by increasing the temperature to 100 • C, with the 64% conversion of PO in 5 h, and 99% in 10 h (entries 2 and 3, Table 3). The significance of reaction temperature was also observed in the BO/CO 2 coupling. The BO conversion of 5% at 80 • C was dramatically increased to 76% when the reaction temperature was increased to 100 • C at 10 atm CO 2 pressure (entries 4 and 5, Table 3). Similarly, enhancing the CO 2 pressure to 20 atm gave rise to 95% of BO conversion in 10 h (entry 6, Table 3), which was much higher than that of the coupling reaction catalyzed by complex 2 (entry 6, Table 1). Complex 3 displayed high catalytic activity for the CPO/CO 2 and SO/CO 2 coupling. Taking the CPO/CO 2 coupling as an example, complex 3 converted 92% of CPO in 5 h at 100 • C under 20 atm of CO 2 to produce the corresponding carbonate (entry 7, Table 3), and a higher conversion rate of 98% was achieved in 10 h (entry 8, Table 3). When SO was used as the substrate, 50% of the SO was converted to the corresponding cyclic carbonate in 10 h at 100 • C under 20 atm of CO 2 (entry 9, Table 3). Nearly complete conversion of SO was achieved by extension of the reaction time to 24 h (entry 10, Table 3).
Complex 3 also displayed high activity toward CPO/CO 2 coupling under low CO 2 pressure. For example, 85% of CPO was converted to cyclic carbonate at 100 • C within 5 h under constant 2 atm CO 2 pressure (entry 1, Table 4). Prolonging the reaction time to 10 h gave rise to the nearly complete conversion of CPO (entry 2, Table 4). Complex 3 was still active under atmospheric CO 2 pressure, but the activity dropped sharply. For example, the conversion of CPO was only 28% in 5 h and 55% in 10 h (entries 1 and 2, Table 4), owing to the low concentration of CO 2 dissolved in CPO. By further extending the reaction time, high conversion of CPO was achieved (entries 3-6, Table 4). However, for SO/CO 2 coupling, complex 3 displayed very low activity. The conversion of SO was only 26% in 10 h when the reaction was performed at 100 • C, maintaining the CO 2 pressure at constant 2 atm (entry 7, Table 4). Lower SO conversion was expected, as the reaction was carried out at atmospheric CO 2 pressure. Analogous to CPO/CO 2 coupling, a high conversion of SO can be achieved by extending the reaction time (entries 8-14, Table 4). Kinetic studies of the coupling reactions of CPO/CO 2 and SO/CO 2 at atmospheric CO 2 pressure by complex 3 are plotted in Figure 3. It is clear that the CPO/CO 2 coupling is faster than the SO/CO 2 coupling under the same conditions (the blue curves in Figure 3). It is worth noting that the coupling of CPO/CO 2 has a first-order dependence on CPO concentration throughout the coupling reaction, as illustrated in Figure 3 (the red dash line, -ln(1-Conversion) vs. time). This may indicate a better solubility of complex 3 in CPO than that of complex 2, which was tentatively ascribed to the incorporation of more amine groups on the ligand framework. Differing from CPO/CO 2 coupling, the kinetic curve (-ln(1-Conversion) vs. time) of SO/CO 2 coupling shows an increase in the slope during the early stage of the reaction (the red dot curve in Figure 3), reflecting the heterogeneous nature of the system owing to the poor solubility of complex 3 in SO. As the reaction proceeded, more SC was produced, which in turn helped to dissolve complex 3, increasing the catalyst concentration and speeding up the coupling reaction. After some time, complex 3 was totally dissolved, forming a homogeneous system and maintaining constant concentration. Thus, a linear relationship was observed at the late stage of the coupling reaction, demonstrating a first-order dependence on SO concentration. The linear relation at the late stage also suggested the high stability of complex 3 under elevated temperature such as 100 • C.  Figure 3), reflecting the heterogeneous nature of the system owing to the poor solubility of complex 3 in SO. As the reaction proceeded, more SC was produced, which in turn helped to dissolve complex 3, increasing the catalyst concentration and speeding up the coupling reaction. After some time, complex 3 was totally dissolved, forming a homogeneous system and maintaining constant concentration. Thus, a linear relationship was observed at the late stage of the coupling reaction, demonstrating a first-order dependence on SO concentration. The linear relation at the late stage also suggested the high stability of complex 3 under elevated temperature such as 100 °C.
General procedures: All reactions sensitive to air and moisture were carried out in a glovebox filled with dry argon. Proligands H 2 L 1 -H 2 L 3 were synthesized according to the literature methods and structurally identified by 1 H NMR and 13 C NMR [24]. The aluminum complexes 1-3 were prepared by equimolar reaction of the proligands with AlEt 2 Cl in a glovebox filled with argon. The structures of complexes 1-3 were characterized by 1 H NMR, 13 C NMR, and MS. The coupling of epoxide with CO 2 was carried in a stainless-steel autoclave equipped with a magnetic stirring bar. The reaction mixture was analyzed by the 1 H NMR spectra with CDCl 3 as a solvent. The 1 H and 13 C NMR spectra were recorded using Bruker AVANCE III 500 MHz spectrometer (Billerica, MA, USA). Mass spectra of complexes 1-3 were obtained in the electrospray positive mode (ESI+) on Thermo LTQ Orbitrap XL spectrometer (Waltham, MA, USA), samples were diluted in methanol or ethanol, at DUT Chemistry Analysis & Research Centre, Dalian University of Technology (Dalian, China).

Coupling Reaction of Epoxide with CO 2 by Aluminum Complex 1
The typical procedure for the coupling of PO with CO 2 by complex 1 under elevated pressure is as follows. Complex 1 and cocatalyst were dissolved in PO in a Schlenk tube. Then, the solution was transferred via syringe into the pre-dried autoclave under CO 2 atmosphere. Then, the autoclave was pressurized with CO 2 and heated. After the designated time, the autoclave was cooled in an ice bath. The excess of the CO 2 was vented out. The conversion of PO was determined by GC analysis and 1 H NMR of the reaction mixture, and the yield was calculated based on isolated propylene carbonate. The results showed that the GC result was consistent with that by weight analysis of the PC.

Coupling Reaction of Epoxide with CO 2 by Bifunctional Aluminum Complexes
The typical procedure for the coupling of PO with CO 2 by complex 2 or 3 under elevated pressure is as follows. Complex 2 (38.7 mg, 50 µmol) was first placed in the predried autoclave. After the internal atmosphere of the autoclave was displaced by CO 2 three times, PO was injected, which was followed by charging CO 2 to 20 atm. Then, the autoclave was heated for the designated time. The analysis was the same as the above method.
The typical procedure for the coupling of CO 2 with CPO by complex 2 or 3 at atmospheric CO 2 pressure is as follows. Complex 2 (38.7 mg, 50 µmol) was placed in a 10 mL round-bottom flask in a glovebox. The flask was taken out, which was equipped with a balloon. The flask was vacuumed and recharged with CO 2 . After the injection of a prescribed amount of CPO, the flask was heated and maintained at 100 • C. The samples were taken out periodically and analyzed by Agilent GC to determine the conversion of CPO.

Conclusions
We have demonstrated the synthesis of a series of aluminum complexes 1-3. Structural characterization by NMR and MS suggested the formation of binuclear structures via the chloride. Although complex 1 can be activated by either a nucleophile or an organic base to enable the coupling of terminal epoxide with CO 2 , the excess use of cocatalyst unambiguously hampers its practical application. In contrast, the bifunctional complexes 2 and 3 containing tertiary amine as the built-in nucleophile are highly active catalysts by their own for the coupling of CO 2 with terminal epoxides. For bifunctional complexes 2 and 3, high temperature facilitates the rapid transformation. It is found that the incorporation of more amine groups in the metal complex greatly enhances the catalytic activity. The bifunctional complexes are even active under low CO 2 pressure at the expense of catalytic activity to some extent. This work may shed light on the design of metal-based catalyst of high activity for the coupling of terminal epoxide with CO 2 .