The Effect of Oxygen to Salen-Co Complexes for the Copolymerization of PO/CO2

A series of Salen-Co(II) complexes were synthesized to study the effect of O2 on the catalytic performance of Salen-Co complexes for the copolymerization of PO/CO2. The Salen-Co(II) complexes showed low activity on the cyclo-addition of CO2 to PO with the aid of a cocatalyst such as PPNCl. Unexpectedly, with the addition of O2, the activity of Salen-Co(II) complexes was obviously increased and 100% cyclic carbonate was obtained. As the pressure of O2 increased, the activity of the complex also increased. With the existence of O2, the activity of the complexes was influenced by their structures and the pressure of O2, and the complexes with the conjugated structure showed higher activity. The structures of cocatalyst also played a crucial role as for the change of the activity. By altering the electrophilicity of Salen-Co(III), O2 can also be used as cocatalyst for the copolymerization of PO/CO2.


INTRODUCTION
As a kind of greenhouse gas, CO 2 is considered as one of the prime reasons for the climate change and global warming. Reduction of the emissions and the concentration of CO 2 in the environment has become a globe issue that could affect the sustainable development of human society. [1−3] Therefore, the application of CO 2 has received widespread attention. As a source of nontoxic, abundant, inexpensive, and sustainable C1 feedstock, CO 2 has been used in the chemical industry for the production of urea, carboxylic acids, methanol, cyclic carbonates, and polycarbonates. [4−6] Cyclic carbonates can be prepared by cyclo-addition of CO 2 to epoxides and have been widely used as electrolytes for batteries, aprotic solvents, monomers for polymerizations, and intermediates of chemical syntheses, [7−10] while polycarbonates can be prepared by copolymerization of CO 2 with epoxides and show potential applications as packaging materials, coatings, adhesives, and in many other fields. [11−14] Due to the high stability and low reactivity of CO 2 , a large number of catalytic systems such as organocatalysts and metal-based catalysts such as K, Mg, Ca, Fe, Cr, Zn, Al, and rare earth metals have been explored to couple epoxides with CO 2 in the past decades. [15−45] In these studies, Salen-Co catalytic systems showed interesting performance in copolymerization of epoxides and CO 2 .
In previous researches, North and coworkers reported a series of uncomplexed salophen ligands that could activate the reaction as Brønsted acid catalysts and exhibited activity in the cyclo-addition of CO 2 to epoxides. [46] Repo and coworkers reported a series of Co(II)/onium salts that could catalyze cyclo-addition to produce cyclic carbonate in the absence of ligand at high temperature, and the reaction mechanism was also proposed. [47] Unexpectedly, Salen-Co(II) complexes composed of Salen ligands and Co(II) salts showed lower activity in the cyclo-addition reaction of CO 2 to epoxides, [48] while the Salen-Co(III) complex systems showed better performance in the copolymerization of epoxides and CO 2 to produce polycarbonates. [13,14,49−52] There were still many complexes showing activity and specific selectivity of cyclic carbonate in the cyclo-addition reaction. [48,53] The simultaneous formation of cyclic carbonates and polycarbonates in the coupling of epoxides and CO 2 would cause difficulties in product separation. Therefore, it is necessary to develop catalyst systems with high activity and specific selectivity for the copolymerization of epoxides and CO 2 .
Lu and coworkers reported the mechanism of valence change of Salen-Co(III), which indicated that Salen-Co(III) complexes might undergo redox reaction under the combined action of epoxides and water to achieve the conver-sion between Salen-Co(II) and Salen-Co(III) complexes. [54] Although there was only a trace amount of water in the system, it might affect the selectivity of copolymerization via redox reaction of Salen-Co(III) and Salen-Co(II) complexes. Inspired by these reports, in order to obtain catalyst systems with high activity and specific selectivity, the Salen-Co complexes (Scheme 1) were synthesized and the effect of oxygen on these Salen-Co complexes for the copolymerization of propylene oxide (PO) and CO 2 was studied.

EXPERIMENTAL
The synthesis of all complexes was carried out by the method reported in our previous article. [55] PO was purified by distillation after refluxing with CaH 2 for three days under nitrogen atmosphere. The 25 mL steel autoclaves were used as the vessel in all copolymerization. The autoclaves were dried by heating to 120 °C for 24 h, and then cooled to ambient temperature prior to use under dry nitrogen atmosphere. In a typical copolymerization procedure, complexes, co-catalyst, and epoxides in designed ratio were added into an autoclave with a magnetic bar. O 2 and CO 2 were introduced into the autoclave before sealing and then heated to the designed temperature. After the ordered copolymerization time, the autoclave was cooled to ambient temperature and the pressure of autoclaves was reduced slowly and a small amount of the crude product was dissolved in CDCl 3 for 1 H-NMR testing. The crude polymer was purified by dissolution (CH 2 Cl 2 ) and precipitation (acidified methanol), and then dried by vacuum-drying at 40 °C for 48 h. The results of NMR were obtained by Bruker AV 300 M and chemical shifts were calibrated using TMS as an internal standard. The molecular weight of the polymer was determined using Waters 515 GPC with CHCl 3 as solvent and calibrated with polystyrene standards.

RESULTS AND DISCUSSION
In general, most of organometallic catalysts are sensitive to water and oxygen and would lose activity under water or oxygen, while the operation to remove water and oxygen increases the difficulty in practical application. Among metalbased complexes, Salen-Co complexes have relatively high stability. A trace concentration of water could not retard their activity but even have a positive effect on the results of copolymerization, occasionally. [55,56] There have been a few researches about the effect of O 2 on the performance of Salen-Co(III) complex during the copolymerization of PO and CO 2 . As described in the report of Wang and coworkers, O 2 worked as a trigger in the ternary copolymerization of epoxide, CO 2 , and vinyl monomer. The CO 2 -based block copolymers had been synthesized by a one-pot procedure. [57] This result indicated that the Salen-Co(III) complex could maintain activity for copolymerization of PO and CO 2 in the presence of O 2 . Herein, the effect of O 2 on the performance of Salen-Co complexes in copolymerization of PO/CO 2 was studied.
Based on the previous studies, the effect of O 2 on Salen-Co(II) complexes in the cyclo-addition of CO 2 to PO was investigated by complexes 1 and 2. As shown in Table 1, the cyclo-addition reaction could not occur without the presence of cocatalyst, regardless of whether O 2 was added for both complexes (Table 1, entries 1, 2, 14, and 15, and Figs. S1 and S2 in the electronic supplementary informaion, ESI). Using bis(triphenylphosphine) iminium chloride (PPNCl) as cocatalyst, complex 1 could catalyze the cyclo-addition of CO 2 to PO to produce cyclic carbonate with very low activity (Table 1, entries 3, 7, and 16, and Figs. S1 and S2 in ESI). As shown in Figs. 1 and 2, the addition of O 2 showed very interesting performance in the reaction. Due to the participation of O 2 , the activity of complex 1 was more than 30 times higher (TOF from 10 h −1 to 310 h −1 , Table 1, entries 3 and 5 and Fig. S1 in ESI) and that of complex 2 was more than 60 times higher As shown in Fig. 3, with the increase of temperature, the activity of complexes 1 and 2 increased obviously in the presence of O 2 . Moreover, the activity of complex 1 was higher than that of complex 2 under the same conditions; this result could be attributed to the conjugated structure of complex 1 compared with that of complex 2. The conjugated structure played a crucial role in the change of the electrophilicity of the metal center, and this role made complex 1 show higher activity in the cyclo-addition of CO 2 to PO. [55] Investigation concerning the concentration of cocatalyst showed that the increasing amount of cocatalyst resulted in the improved activity of complex 1. The TOF value up to 250 h −1 was observed when employing 2 equivalents of PPN-Cl (Table 1, entries 4 and 6). Same conclusion can be drawn with complex 2 (Table 1, entries 17, 18, and 19).
In order to study the mechanism of the activity change in the cyclo-addition reaction with the existence of O 2 , the effect of cocatalyst was investigated firstly. As shown in Fig. 4 and Table 2, with DMAP as cocatalyst, the Salen-Co(II) complex 1 showed no catalytic activity to the cyclo-addition of CO 2 to PO regardless of whether O 2 was added or not ( Table  2, entries 5 and 6). Using tetrabutylammonium bromide (TBAB) as cocatalyst, the addition of O 2 could obviously improve the activity of complex 1 (Table 2, entries 7 and 8). By using the mixture of TBAB and DMAP as cocatalyst, only a few amount of PO was converted to cyclic carbonate while the decrease of activity was observed with the addition of O 2 (Table 2, entries 9 and 10).
As shown in Fig. 5 and Table 2, different from complex 1, with DMAP as cocatalyst, complex 2 showed very low activity to the cyclo-addition of CO 2 to PO and the activity was improved obviously when O 2 was added ( Table 2,   The stabler intermediates were difficult to activate the cycloaddition reaction. Same as complex 1, when cocatalyst was alternated to TBAB, the addition of O 2 could increase the activity of complex 2 ( Table 2, entries 19 and 20). This indicated that TBAB and PPNCl played identical role in the cyclo-addition of CO 2 to PO using complexes 1 and 2. Compared with DMAP, both PPNCl and TBAB had a stabilizing anion group. Therefore, the cocatalyst with a stabilizing anion is considered more active for the initiation of PO in the coupling reaction with CO 2 . 100% Cyclic carbonate was obtained when 2,4-dinitrophenol and PPNCl were used as cocatalysts ( Table 2, entry 11). These results indicated that there was no formation of new Salen-Co(III) complex which was supposed to generate polycarbonate. The existence of O 2 could not reduce the selectivity of cyclic carbonate by in situ oxidation of Salen-Co(II) complex.
On the basis of the above investigation, we hypothesized the mechanism of cyclo-addition of CO 2 to PO with the presence of O 2 (Fig. 6). PO was activated by Salen-Co(II) complexes at first; then, the cocatalyst as nucleophile ring-opened the activated PO to generate an alkoxide intermediate. With the insertion of CO 2 and elimination of nucleophiles, the cyclic carbonate was obtained. Through this cyclic process, Salen-Co(II) could continuously catalyze the cyclo-addition of   Fig. 6 Proposed mechanism for cyclo-addition of CO 2 to PO with the existence of O 2 by Salen-Co(II). CO 2 to PO. With the existence of O 2 , the electrophilicity of metal Co(II) was improved by oxidation reaction. The improvement of the electrophilicity accelerated the cyclic process of the activation of PO, the insertion of CO 2 , and the elimination of nucleophiles.
Previous studies had found that the Salen-Co(III) complex can catalyze copolymerization of PO and CO 2 in the absence of nucleophilic cocatalyst by altering the the electrophilicity of the Co(III). [55] Based on the above experiments, the electrophilicity of Salen-Co(II) was improved with the existence of O 2 . Therefore, the effect of O 2 on Salen-Co(III) complex in copolymerization of PO and CO 2 was studied. As shown in Table 3, the Salen-Co(III) complex 3 showed very low activity and selectivity in the absence of nucleophilic cocatalyst, but with the addition of O 2 , the activity and selectivity of complexes 3 were improved (Table 3 entries 1 and 2). This result indicates that the existence of O 2 in the copolymerization could improve the electrophilicity of the Salen-Co(III) complexes and O 2 could be used as a cocatalyst in copolymerization. In the copolymerization using PPNCl as cocatalyst, the existence of O 2 improved the activity of copolymerization slightly (Table 3 entries 3, 4, 5, and 6). This result could be attributed to the existence of O 2 that increased the amount of cocatalyst which could improve the activity. [55] Using complexes 1 and 3 together as catalysts, the selectivity of the mixture decreased (Table 3, entries 7 and 8). This result indicated that the effect of oxygen on the catalytic performance of Salen-Co(II) was more obvious. Based on these studies, it can be concluded that the addition of O 2 improved the activity and selectivity of the complexes 3 in the absence of nucleophilic cocatalyst and simultaneously it had no negative effect on the copolymerization using nucleophilic cocatalyst.

CONCLUSIONS
Salen-Co(II) has been investigated with the addition of O 2 in coupling reaction of PO/CO 2 . Salen-Co(II) complexes showed very low activity in the cyclo-addition of CO 2 to PO. With the existence of O 2 , the activity of Salen-Co(II) complexes was significantly increased and 100% cyclic carbonate was formed.
The catalytic performance of the complexes was influenced by the structure, and the complexes with the conjugated structure showed higher activity. The cocatalysts played an important role in the copolymerization, and cocatalysts with a stabilizing anionic group showed higher activity than organic bases cocatalyst. Through the study of cocatalysts, possible reaction mechanisms were proposed. Salen-Co(III) was investigated with the addition of O 2 in copolymerization of PO/CO 2 . By altering the electrophilicity of Salen-Co(III), O 2 could also improve the activity of complex as cocatalyst. Overall, the addition of O 2 had a positive effect on the performance of Salen-Co complexes in the copolymerization of PO and CO 2 .

Electronic Supplementary Information
Electronic supplementary information (ESI) is available free of charge in the online version of this article at http://dx.doi.org/ 10.1007/s10118-020-2451-5.