ISSN 0023-1584, Kinetics and Catalysis, 2016, Vol. 57, No. 6, pp. 821–825. © Pleiades Publishing, Ltd., 2016. Original Russian Text © O.M. Chukanova, G.P. Belov, 2016, published in Kinetika i Kataliz, 2016, Vol. 57, No. 6, pp. 827–832.

Effect of the Ligand Nature in Cobalt Complexes on the Selectivity of the Reaction of Carbon Dioxide and Propylene Oxide O. M. Chukanova* and G. P. Belov Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia *e-mail: [email protected] Received January 26, 2016

Abstract⎯The reaction of carbon dioxide with propylene oxide in the presence of the (salen)CoCl or (TPP)CoCl (salen = bis(3,5-di-tert-butyl-salicylidene)-1,2-diaminocyclohexane, TPP = 5,10,15,20-tetraphenylporphyrin) catalyst and the PPNCl (bis(triphenylphosphine)iminium chloride) cocatalyst has been carried out at 20–60°С and a СО2 pressure of 0.6 MPa to investigate the effect of the ligand nature on the reaction rate and selectivity. The change in the reaction rate and selectivity in relation to the temperature and cocatalyst/catalyst ratio has been studied. The activation energy of the copolymerization of СО2 with propylene oxide catalyzed by the (salen)CoCl complex have been obtained. Keywords: carbon dioxide, catalysis, cobalt complexes, propylene carbonate, copolymerization, epoxides DOI: 10.1134/S0023158416050074

INTRODUCTION Carbon dioxide is a promising renewable and cheap source of carbon; however, today only a few chemical processes involving a quite stable СО2 molecule into the synthesis of organic compounds are known [1]. In recent decades, catalytic syntheses of aliphatic polycarbonates or cyclic carbonates from СО2 and epoxides have attracted great interest all over the world due to the discovery of efficient catalysts for these processes [2–4]. Cyclic carbonates can be used as aprotic solvents, intermediates for fine chemical synthesis, and electrolytic solvents in lithium-ion batteries. They are characterized by a high boiling point, low vapor pressure, biodegradability, and low toxicity, so they can be assigned to the class of “green” solvents [5]. The other product of the reaction between СО2 and epoxides, an aliphatic polycarbonate, can be assigned to the class of new, biocompatible and biodegradable polymers. Polycarbonates possess good adhesive properties and can be used as components of composite materials [6]. The extraordinary growth of interest in the copolymerization of СО2 with epoxides is due to the discovery of efficient catalytic systems containing organometallic complexes of Co or Cr with N- and O-containing ligands, as well as cocatalysts belonging to quaternary ammonium salts or bases [2–4, 6]. With these catalytic systems, the reaction can be carried out under mild conditions, even at room temperature and atmospheric pressure [4, 7]. The composition and configuration of the complex has a significant effect on its activity and process selec-

tivity. Co and Cr complexes can be efficient both in the polycarbonate and cyclic carbonate syntheses, depending on the reaction conditions. Only sparse information about the effect of certain synthesis conditions on the reaction selectivity in the presence of various catalytic systems can presently be found in literature. Generally, studies are performed under the same reaction conditions using a series of ligands with a similar composition and different substituents. For example, the effect of substituents in the ligands of the salen (N2O2) type on the activity of cobalt complexes in the copolymerization of СО2 with propylene oxide at 22°С and a pressure of 5.5 MPa is considered in [8], while their effect in the copolymerization of СО2 with cyclohexene oxide in the presence of a cocatalyst at 25 and 60°С and a pressure of 1.5 MPa is reported in [9, 10]. In [11], data on the change in the rate of the reaction between СО2 and propylene oxide catalyzed by cobalt porphyrin complexes with various substituents under the same conditions—25°С and 3 MPa СО2— are presented. It is problematic to compare activity and selectivity data obtained for different complexes under different reaction conditions. The main aim of this work was to study the effect of the ligand nature on the catalytic behavior of complexes under similar reaction conditions. Cobalt complexes with known ligands, such as salen ((R,R)-N,N'-bis(3,5-di-tert-butyl-salicylidene)1,2-cyclohexanediamine) and TPP (5,10,15,20-tetraphenylporphyrin), namely, (salen)CoCl and (TPP)CoCl, which allow carrying out the reaction between СО2 and propylene oxide (PO) under mild

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conditions, were examined in the study. Earlier [12, 13], we studied in detail the kinetics of the copolymerization of СО2 with propylene oxide in the presence of the (salen)CoDNP complex (DNP = 2,4-dinitrophenolate) and determined the pre-exponential factor and copolymerization activation energy. In this work, we compared the earlier determined values of the reaction activation parameters with the value of the apparent activation energy of the copolymerization process that takes place in the presence of the (salen)CoCl complex containing a different counter ion. EXPERIMENTAL Chemicals and Methods of Investigation Cobalt(III) complexes were synthesized by the oxidation of (salen)Co(II) and (TPP)Co(II) (98%, Aldrich) according to procedures presented in [9] and [11], respectively. The cocatalyst—bis(triphenylphosphine)iminium chloride (PPNCl, 97%)—was obtained from Strem Chemicals. rac-Propylene oxide (Sigma-Aldrich) was dried by distillation from CaH2. CO2 (99.8%) was obtained from Linde Gas. 1H NMR spectra were recorded on a Bruker AVANCE III spectrometer (Bruker, Germany) in CDCl3. Attenuated total reflectance (ATR) IR spectra were recorded on an ALPHA spectrometer (Bruker Optics, Germany). ATR, cm–1: (salen)CoCl 687 w, 747 m, 783 s, 833 s, 868 m, 1005 m, 1038 m, 1172 s, 1203 m, 1255 s, 1271 m, 1322 s, 1361 s, 1391 m, 1407 m, 1437 s, 1459 s, 1529 m, 1635 s, 2868 m, 2953 s; (TPP)CoCl 704 s, 755 s, 794 s, 836 m, 1005 s, 1074 s, 1179 m, 1352 s, 1441 m, 1490 m, 1599 m, 3026 w, 3055 w. Copolymerization of СО2 with Propylene Oxide The copolymerization procedure was similar to the procedure described earlier [13]. A complex (0.025– 0.035 mmol, 1 equiv) was dissolved in rac-PO (4–5 mL, 2000–2300 equiv). Thereafter, PPNCl was added in an argon atmosphere, and the reaction mixture was stirred for 15 min. The solution was transferred into a predried 0.1-L reactor in a СО2 atmosphere. The pressure in the reactor was increased by admitting СО2, and the reactor was connected to a thermostat preheated to a required temperature. The pressure in the system stabilized 5–6 min after the beginning of the process and was constant during the reaction. Kinetic studies were performed by recording the variation of the СО2 pressure in a calibrated vessel. The reaction was terminated by reduction of pressure and temperature to normal conditions. A small fraction of the solution was taken for determining the composition of the reaction products by means of NMR spectroscopy. 1H NMR (CDCl , 500 MHz), ppm: poly(propylene 3 carbonate) (PPC) 1.33, 1.35 (d, 3H, CH3), 4.11–4.30 (m, 2H, CH2), 5.01 (m, 1H, CH); propylene carbonate (PC) 1.49, 1.51 (d, 3Н, СН3), 4.02, 4.03, 4.05 (dd,

1Н, СН2), 4.55, 4.56, 4.58 (dd, 1Н, СН2), 4.86 (m, 1Н, CH). To determine the reaction selectivity, the ratio of the number of moles of propylene oxide in PPC to the total quantity of propylene oxide in the two products (РС and РРС), f, was calculated from the spectra: f = nPOPPC/nPO(PPC + PC). The polymer product was precipitated with МеОН (30 mL) with the addition of a 5% HCl solution in МеОН (5 mL) and was vacuum-dried at 120–140°С for 6 h. RESULTS AND DISCUSSION The reaction of СО2 and propylene oxide was conducted in the presence of a known catalytic system consisting of the (salen)CoCl or (TPP)CoCl) organometallic complex and the PPNCl cocatalyst: The reaction yields two products, namely, PC and PPC: O

CH3 O

O CH3

+ CO2

O +

O

O

O

n

CH3 PC

PPC

To reveal the differences between the catalytic behaviors of the complexes, we studied the temperature and cocatalyst/catalyst ratio effects on the reaction rate and selectivity. Below, the results obtained for the different complexes are considered separately. Reaction of СО2 and Propylene Oxide in the Presence the (Salen)CoCl/PPNCl Catalytic System The reaction kinetics was studied by recording СО2 consumption during the process. Typical dependences of СО2 uptake versus time for this catalytic system are presented in Fig. 1. The reaction rate (mol СО2/h) was measured at a propylene oxide conversion of <25%. The reaction starts after a certain induction period whose duration may vary with catalyst concentration [12]. The variation of the reaction rate with temperature was studied at 20–60°С and a constant PPNCl/(salen)CoCl molar ratio of 1. Under these conditions, the reaction products mainly contain PPC (the parameter f calculated from NMR spectra is 0.96–0.97). With an increasing temperature, the reaction rate grows, while the selectivity remains unchanged. Therefore, the effective activation energy of copolymerization (Ea) can be estimated from the temperature dependence of the reaction rate. To calculate the value of Ea, it was necessary to take into account that the concentration of СО2 dissolved in KINETICS AND CATALYSIS

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N

N Cl N Co N N

N

Co O Cl O

tBu tBu

823

tBu tBu

(salen)CoCl

(TPP)CoCl Ph

Ph +

Ph P N P Ph Cl− Ph

PPNCl

Ph Scheme.

propylene oxide changes with an increasing temperature. These data are presented in [13]. The reaction rate was calculated via the equation w = (d[CO2/dt) × ([CO2] × [(salen)CoCl])–1. An Arrhenius dependence of w on the temperature is presented in Fig. 2. The effective activation energy was 42 ± 2.4 kJ/mol. Comparing the results obtained for the (salen)CoCl and (salen)CoDNP [13] catalysts, note, that, in the latter case, Ea was higher, namely, 45.7 kJ/mol. There are only a few works in which Ea values for the copoly-

merization of СО2 with epoxides are presented. The activation energy for the synthesis of PPC and poly(cyclohexene carbonate) in the presence of the (salen)CrCl complex without a cocatalyst is 67.6 and 46.9 kJ/mol, respectively [14]. The computational procedure was not described in detail, so it is difficult to make a conclusion about the causes of the difference in these values. Possibly, use of the cocatalyst leads to a decrease in the activation energy of copolymerization. The nature of the metal in the complex can also have an effect on the energetic parameters of the reaction. Only one work has been devoted to ln W

nCO 2 / nCo , mol/mol 2

7

600 1 6 300

0

20

40

60 Time, min

Fig. 1. СО2 uptake as a function of time at (1) 30 and (2) 50°С. Reaction conditions: 0.6 MPa, [PPNCl]/[(salen)CoCl] = 1, [(salen)CoCl] = (6.8–7.8) × 10–3 mol/L. KINETICS AND CATALYSIS

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5 3.0

3.1

3.2

3.3 3.4 T –1 × 103, K–1

Fig. 2. Rate of the СО2 and propylene oxide copolymerization as a function of temperature in the Arrhenius coordinates. Reaction conditions: 0.6 MPa, [PPNCl]/[(salen)CoCl] = 1, [(salen)CoCl] = (6.8–7.8) × 10–3 mol/L.

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TOF, h–1

f 1.0

900 0.9

600

decrease in the PPC yield: f decreases from 0.97 to 0.86 as [PPNCl]/[(salen)CoCl] is increased from 0.5 to 3. A decrease in the РРС content in the products caused by an increase in the [PPNCl]/[(salen)CoDNP] ratio was also observed in the copolymerization of СО2 with PO carried out under slightly different conditions: 25°С and 1.5 MPa [16]. Next, we will discuss the characteristic features of the reaction between СО2 and propylene oxide catalyzed by a cobalt porphyrin complex.

300

0.8 0

1

2 3 [PPNC]/[(salen)CoCl]

Fig. 3. Effect of the cocatalyst/catalyst ratio on the reaction rate and selectivity. Reaction conditions: 0.6 MPa, 50°С, [(salen)CoCl] = (6.8–7.8) × 10–3 mol/L.

nCO 2 / nCo , mol/mol 2 1 200

Reaction of СО2 and Propylene Oxide in the Presence of the (TPP)CoCl/PPNCl Catalytic System Typical time dependences of the СО2 consumption observed in the presence of the (TPP)CoCl/PPNCl catalytic system are given in Fig. 4. As in the case of the (salen)CoCl/PPNCl complex, the reaction proceeds for a long time at a constant rate. However, slight differences were observed as well: the induction periods that were observed for the first catalytic system were absent in the case of (TPP)CoCl. With an increasing temperature, the reaction rate passes through a maximum at Т = 35°С. TOF changes with temperature as follows (0.6 MPa, [PPNCl]/[(TPP)CoCl] = 1): T, °C TOF, h–1

100

30 114.2

35 132.1

45 86.0

60 0*

* There is no reaction over 1 h.

0

30

60

90

120 Time, min

Fig. 4. СО2 uptake as a function of time at (1) 30 and (2) 35°С. Reaction conditions: 0.6 MPa, [PPNCl]/[(ТРР)CoCl] = 1, [(ТРР)CoCl] = (6.8–7.8) × 10–3 mol/L.

detailed investigation of the kinetics of the copolymerization of СО2 with cyclohexene oxide in the presence of a dizinc complex [15]. According to that work, the activation energy is 96.8 kJ/mol. To compare the activities of various catalytic systems, the reaction rate is estimated in terms of the catalyst turnover frequency TOF (h–1), which was determined as the number of moles of PO (or СО2) consumed in the reaction per mole of catalyst per hour. Hereinafter, reaction rates are presented in terms of TOF. The cocatalyst/catalyst ratio is known to have an effect on the reaction selectivity [2–4]. The dependence of the reaction rate and selectivity (f, see EXPERIMENTAL) on the cocatalyst/catalyst ratio is presented in Fig. 3. An increase in the cocatalyst/catalyst ratio is accompanied by an increase in the reaction rate and by a

It was reported [11, 17] how the PO conversion over the same catalytic system changes with an increasing temperature at a СО2 pressure of 3 and 2 MPa, respectively. According to [11], the PO conversion over 18 h takes its maximum value between 40 and 50°С and decreases on passing to 60°С. In [17], a substantial decrease in the PPC yield and PO conversion was observed as the temperature was changed from 25 to 60°С over 5 h. The following possible causes of the observed trend in the presence of porphyrin complexes were considered [16]: the formation of Co(II), which is inactive in the reaction, and the coordination of the cyclic carbonate forming in the reaction to the active site, which prevents the coordination and insertion of the epoxide. Additional studies are required for making more definite conclusions about the causes of this phenomenon. The unusual behavior of the reaction rate and selectivity was observed as the cocatalyst/catalyst ratio was varied. These data are presented in Fig. 5. The maximum reaction rate is achieved at [PPNCl]/[(TPP)CoCl] = 1. At the same ratio, the reaction products contain the largest amount of РРС (f = 0.82). Increasing this ratio to 2 leads to a decrease in the polymer content of the product (f = 0.3). In [17], it was also noted that the РРС selectivity in the presence of this catalytic system reaches the maximum value at catalyst/cocatalyst = 1. However, the reaction KINETICS AND CATALYSIS

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TOF, h–1

f

150 1.0 100

825

complex. The ability of porphyrin complexes to catalyze the reaction of СО2 with epoxides under very mild conditions (room temperature and low СО2 pressures) can also be assigned to the advantages of porphyrin complexes. The selectivity of the process depends substantially on the cocatalyst to catalyst ratio, and temperature variations do not affect significantly the composition of the products.

0.5 50

0

1

2 [PPNCl]/[(ТРР)CoCl]

Fig. 5. Effect of the cocatalyst/catalyst ratio on the reaction rate and selectivity. Reaction conditions: 0.6 MPa, 35°С, [(ТРР)CoCl] = (6.8–7.8) × 10–3 mol/L.

was carried out under different conditions (25°С, 2 MPa), and the РРС selectivity was higher, namely, 99%. As the catalyst/cocatalyst ratio is varied in the 0.5–2 range, the selectivity changes slightly from 97 to 94%. These data differ from the data obtained in this work, possibly due to the difference between the reaction conditions. At the same time, results presented in different works under similar conditions may vary substantially as well. For example, the reaction rate at 25°С and 5 MPa in terms of TOF is 86 h–1 according to [17] and 28 h–1 according to [18] for the same catalytic system. This shows that catalytic systems should be compared using similar synthesis conditions within one study. To summarize the above data, it should be noted that the ligand nature has a substantial effect on the behavior of the catalytic system, although there are common features for both catalysts as well. Both with the salen complex and with the porphyrin complex, the cyclic carbonate formation selectivity grows with an increasing cocatalyst concentration. Other conditions being equal (30°С, 0.6 MPa СО2, cocatalyst/catalyst = 1), the reaction rate is four times higher in the case of the salen complex; however, the relative fraction of the cyclic carbonate is lower than in the reaction conducted in the presence of the porphyrin complex. The activation energy of the copolymerization of CO2 with propylene oxide over the (salen)CoCl/PPNCl catalytic system is 42 kJ/mol. This value is somewhat smaller than the value obtained earlier for the (salen)CoDNP/PPNCl catalyst [13]. Despite the higher reaction rate in the case of the use of the salen ligand, it is easier to vary the cyclic carbonate formation selectivity by using the porphyrin complex. For example, at a cocatalyst/catalyst ratio of 2, the relative fraction of propylene oxide in the РРС polycarbonate (f) is 0.9 in the case of the salen complex and 0.3, in the case of the porphyrin KINETICS AND CATALYSIS

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ACKNOWLEDGMENTS This work was supported by the Presidium of the Russian Academy of Sciences, basic research program no. 25: “Fundamental Aspects of Carbon Energetics Chemistry.” REFERENCES 1. Maeda, C., Miyazaki, Y., and Ema, T., Catal. Sci. Technol., 2014, no. 4, p. 1482. 2. Klaus, S., Lehenmeier, M.W., Anderson, C.E., and Rieger, B., Coord. Chem. Rev., 2011, vol. 255, p. 1460. 3. Kember, M.R., Buchard, A., and Williams, C.K., Chem. Commun., 2011, vol. 47, p. 141. 4. Lu, X.-B. and Darensbourg, D.J., Chem. Soc. Rev., 2012, vol. 41, p. 1462. 5. Schaffner, B., Schaffner, F., Verevkin, S.P., and Borner, A., Chem. Rev., 2010, vol. 110, p. 4554. 6. Taherimehr, M. and Pescarmona, P.P., J. Appl. Polym. Sci., 2014, vol. 131, no. 21, app. 41141. 7. Lu, X.-B., Ren, W.-M., and Wu, G.-P., Acc. Chem. Res., 2012, vol. 45, no. 10, p. 1721. 8. Cohen, C.T. and Coats, G.W., J. Polym. Sci., Part A: Polym. Chem., 2006, vol. 44, no. 17, p. 5182. 9. Shi, L., Lu, X.-B., Zhang, R., Peng, X.-J., Zhang, C.-Q., Li, J.-F., and Peng, X.-M., Macromolecules, 2006, vol. 39, no. 17, p. 5679. 10. Niu, Y. and Li, H., Colloid Polym. Sci., 2013, vol. 291, no. 9, p. 2181. 11. Anderson, C.E., Vagin, S.I., Xia, W., Jin, H., and Rieger, B., Macromolecules, 2012, vol. 45, no. 17, p. 6840. 12. Chukanova, O.M., Perepelitsina, E.O., and Belov, G.P., Polym. Sci., Ser. B: Polym. Chem., 2014, vol. 56, no. 5, p. 547. 13. Chukanova, O.M., Bukhovets, E.V., Perepelitsina, E.O., and Belov, G.P., Polym. Sci., Ser. B: Polym. Chem., 2015, vol. 57, no. 3, p. 224. 14. Darensbourg, D.J., Chem. Rev., 2007, vol. 107, no. 6, p. 2388. 15. Jutz, F., Buchard, A., Kember, M.R., Fredriksen, S.B., and Williams, C.K., J. Am. Chem. Soc., 2011, vol. 133, no. 43, p. 17395. 16. Lu, X.-B., Shi, L., Wang, Y.-M., Zhang, R., Zhang, Y.-J., Peng, X.-J., Zhang, Z.-C., and Li, B., J. Am. Chem. Soc., 2006, vol. 128, no. 5, p. 1664. 17. Qin, Y., Wang, X., Zhang, S., Zhao, X., and Wang, F., J. Polym. Sci., Part A: Polym. Chem., 2008, vol. 46, no. 17, p. 5959. 18. Chatterjee, C., Chisholm, M.H., El-Khaldy, A., McIntosh, R.D., Miller, J.T., and Wu, T., Inorg. Chem., 2013, vol. 52, no. 8, p. 4547.

Translated by E. Boltukhina