Chemical Papers https://doi.org/10.1007/s11696-020-01175-5 ORIGINAL PAPER Cobalt(II) complex‑catalyzed solventless coupling of CO2 and epoxides Harish Chandra Pradhan1 · Somanath Mantri2 · Tungabidya Maharana1 · Alekha Kumar Sutar2,3 Received: 25 October 2019 / Accepted: 24 April 2020 © Institute of Chemistry, Slovak Academy of Sciences 2020 Abstract Present manuscript deals with the comprehensive solventless coupling of carbon dioxide and epoxides to yield cyclic carbonates catalyzed by cobalt complex [Co-HPEED]. Different spectroscopic techniques are used to characterize this cobalt complex, which confirm its square planar geometry. The cobalt complex shows high catalytic activity toward the formation of cyclic carbonates. We found that the types of co-catalysts, temperature and time affect the rate of formation of cyclic carbonates to great extent. Finally, a mechanism appropriate for the formation of cyclic carbonates has been given based on our experimental results. Keywords CO2 · Epoxide · Cobalt complex · Cyclic carbonate · Co-catalyst Introduction Nowadays, scientists are concentrating on bio-renewable resources. Carbon dioxide is a non-toxic, economical and sustainable carbon resource for important industrial processes (Ochiai and Endo 2005). The carbon dioxide amount in air has been increased sharply in last 50 yrs by the use of fossil fuel (Jiang et al. 2019). CO 2 is thermodynamically and kinetically stable (ΔHf° = − 394 kJ mol−1); hence, it is hardly ever used with complete potential (Pulidindi et al. 2014). Thus, a highpotential catalyst or very reactive epoxide is necessary, which can decrease the activation energy and permit the desired reactions to complete (Darensbourg 2007). And also, Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11696-020-01175-5) contains supplementary material, which is available to authorized users. * Tungabidya Maharana mtungabidya@gmail.com * Alekha Kumar Sutar dralekhasutar@gmail.com 1 Department of Chemistry, National Institute of Technology, Raipur, India 2 Catalysis Research Lab, Department of Chemistry, Ravenshaw University, Cuttack, India 3 School of Chemistry, Gangadhar Meher University, Sambalpur, India the cyclic carbonates produced in this process substitute to the classical method (Shaikh and Sivaram 1996). The cyclic carbonates obtained from chemical fixation of CO2 with epoxides have wide application as electrolytes, raw material for polycarbonates, enantiopure aminoalcohol, thermosetting coating, antifoam additive or plasticizer, synthesis of urea derivative, as solvents, biodegradable material (Rokicki 2000). Since last few decades, the metal and nonmetal organocatalysts have intensively been studied for cyclic carbonates, (Hu et al. 2005, 2019; Lu and Darensbourg 2012, Dong et al. 2019) such as compounds of Co (Prajapati et al. 2018; Ji et al. 2018), Al (Yepes et al. 2019), Pd (Trost and Angle 1985), Cr (Kim et al. 2019), Cu (Shi et al. 2018), Ga (Nakano et al. 2011), In (Shibata et al. 2011), Ni (Li et al. 2003), Mn (Yang et al. 2018), Ru (Bu et al. 2010), Sn (Jing et al. 2004) and Zn (Kundu et al. 2019) organocatalysts. Recently, cobalt compounds having Lewis acidic nature have given more priority (Gupta and Sutar 2008). Zhang research group reported the first cycloaddition reaction of racemic propylene oxide and CO2 at 15 bar pressure and 0 °C, with 40% conversion (Lu et al. 2004). In 2006, again Lu et al. reported the Co(III)salen binary system for the cycloaddition of CO2 to propylene oxide at 1 atm pressure (Lu et al. 2006). Coates research group reported that Co(III) salen complex was the effective catalyst for the copolymerization of propylene oxide and CO2 (Cohen et al. 2005). Co(II)salen complexes with binaphthyl backbone were used in the chemical fixation of CO2 to produce cyclic carbonate 13 Vol.:(0123456789) Chemical Papers with TON 800–913 (Shen et al. 2003). Cobalt complex with pyridyl donor showed active catalytic behavior toward CO2 fixation with high TON 800–2025 (Saunders et al. 2012). Schiff base cobalt catalysts have not been referred much till date for synthesis of cyclic carbonates. The aliphatic polycarbonates derived from CO2 are used as an alternative to poly(esters) based on biodegradability and good biocompatibility properties. The poly(propylene carbonate) is used for biomedical applications and approved by the US FDA (Jiang et al. 2013). Cobalt complexes have comparative catalytic characteristic and are also non-toxic (Gowda and Chakraborty 2011). Presently, our research group have reported salen copper complexes, (Routaray et al. 2015a, b) nickel complex (Routaray et al. 2016a, b) for ROP of L-lactide. A very few number of the literature says about catalytic activity of Co(II) salen complexes toward solventless coupling of CO2 and epoxide to produce cyclic carbonate catalyzed by cobalt complex. We, therefore, herein report the catalytic activity of Co(II) ion complex of HPEED (N,N′-bis (1-(2-hydroxyphenyl)ethanone) ethylenediamine) toward solventless chemical fixation of CO2 and epoxides. The main advantage of cobalt complex is its stability and simple synthesis procedure. Finally, a mechanism has been proposed for cyclic carbonate synthesis. Experimental Materials and characterization Schiff base ligand (HPEED) and its cobalt complex (CoHPEED) were synthesized under dry N2 atmosphere by using Schlenk techniques. 1-(2-hydroxyphenyl)ethanone (HPE) was purchased from Sigma-Aldrich, ethylene-1,2-diamine (ED), anhydrous cobalt chloride, triphenyl phosphine (Ph3P), dimethylaminopyridine (DMAP), imidazole (Im), pyridine (Py), triethyl amine (Et3N), propylene oxide and styrene oxide were purchased from E. Merck, India, and carbon dioxide with the purity of 99.9% was used. 1 H and 13C NMR were done on a FT-NMR-Bruker-400 spectrometer, and IR and UV–Vis spectra were recorded by PerkinElmer 1600 FTIR spectrophotometer and Shimadzu 1601 PC UV–Vis spectrophotometer. Thermal gravimetric analysis was observed by PerkinElmer Pyris, Diamond Thermal Analyzer. The thermal analysis was done in N2 atmosphere with heating rate of 10 °C min−1. AAS was recorded by PerkinElmer 3100 atomic absorption spectrometer at λmax of cobalt ion. The composition of compounds can be determined by Heraeus Carlo Ebra 1108 Elemental Analyzer. Vibrating Sample Magnetometer-155 was used to determine the magnetic moment (µeff) of cobalt complex. The molecular weight of samples can be determined by Merck 13 VAPRO 5600, Germany, Vapor Pressure Osmometer. Scanning electron microscope (ZEISS EVO 18) was used for the surface analysis and appearance of Schiff bases and its cobalt complex. The high-pressure reactor BERGHOF, Germany, was used for catalytic part. Bis(1-(2-hydroxyphenyl)ethanone)ethylenediamine cobalt(II) Co-HPEED was synthesized as described in the literature (Moore et al. 1994, Pradhan et al. 2020). The yellowish green colored HPEED Schiff base crystal (Table S1, Figure S1–S4) (Yield: 1.41 g, 84.96 wt%) (Routaray et al. 2016a) refluxed with cobalt chloride at 60ºC for 5 h produced Co-HPEED complex. TGA curve shows that the stability of Co-HPEED is higher than HPEED ligand. The Schiff base shows a weight loss 58.0 wt%, whereas CoHPEED shows 40.1 wt% at 500ºC. Empirical formula of Co-HPEED found to be C18H18CoN2O2 (Co-HPEED calcd: C—60.20; N—7.93; H—5.14%, Found: C—60.89; N—7.39; H—4.91%). The molecular weight of Co-HPEED was 352.93 gmol−1 (Cald 353.28 gmol−1). The peaks for >C=N, >C–O and phenolic OH were observed at 1634 cm−1, 1290 cm−1 and 2950–3300 for HPEED. The difference between IR bands of Schiff base and complex was observed due to formation of complex and also appearance of two fresh bands at 537 cm−1 and 426 cm−1 for Co–O and Co–N bonds of Co-HPEED (Table S1 and Figure S1–S4). And the disappearance of phenolic OH band 2950–3300 cm−1 of HPEED indicates for the formation of Co-HPEED. CoHPEED complex shows hypsochromic shift in π → π* transition from 251 to 210 nm, and for n → π* transition from 320 to 273 nm. One additional band 383 nm for Co-HPEED complex was observed for C → T transition (Table S1). The μeff of Co-HPEED complex is found to be 1.74 BM, indicating paramagnetic nature with square planar geometry having dsp2 hybridization and t62g e1g electronic configuration. SEM images of HPEED and its cobalt complex confirmed the formation of Co-HPEED complex (Figure S5). Coupling of CO2 and epoxides using Co‑HPEED complex Experiments were carried out in a high-pressure reactor (BERGHOF, Germany) having 100 mL PTFE (polytetrafluoroethylene) reactor under stirring (Pradhan et al. 2020). The catalyst Co-HPEED (0.352 g, 1 mmol), epoxides (1000 mmol) and DMAP (0.244 g, 2 mmol) were taken in the reaction vessel without additional organic solvent. The reactor was placed under a fixed pressure of CO2 for 5 min to allow the system to equilibrate, and CO2 was pressurized to a desired pressure at reaction temperature. The pressure remains constant. After completion of reaction, the reactor was allowed to cool to ambient temperature and the unreacted CO2 was slowly vented by opening the outlet valve. The crude mixture was separated using DCM and H2O and Chemical Papers was purified by sodium sulfate. The cyclic carbonate produced was characterized by 1H NMR and 13C NMR. Results and discussion Chemical fixation of CO2 and epoxides using Co‑HPEED complex Table 1 Coupling of CO2 and propylene oxide using Co-HPEED complex Entry Co-catalysts Yield (%)a TONb TOFc (h−1) 1 2 3 4 5 DMAP Imidazole Et3N Ph3P Pyridine 69.0 55.3 54.7 49.5 47.0 642 523 506 457 408 214 174 169 152 136 Based on literature review, it can be assumed that CoHPEED complex was active catalyst toward chemical fixation of CO2. Thus, Co-HPEED complex was used in the coupling reaction of epoxides with CO2 (Scheme 1). Various factors including the types of co-catalysts, temperature, time, types of epoxides and amount of epoxides were considered to develop the optimum conditions and determine the catalytic activity. The nature of co-catalysts has played a significant role in the conversion of PO and CO2 to cyclic propylene carbonate (Bai et al. 2009). To test the catalytic performance using different co-catalysts, a model reaction using Co-HPEED complex was carried out at 100 °C with [propylene oxide]/ [Co-HPEED] = 1000 and at 1.5 MPa pressure (Table 1). Propylene oxide was used as both substrate and solvent. It has been observed that dimethylaminopyridine (DMAP) is the better co-catalyst among pyridine (Py), imidazole (Im), triethyl amine (Et3N) and triphenyl phosphine (Ph3P). Using DMAP as co-catalyst, Co-HPEED showed maximum activity (69.0%) toward coupling of PO and CO2 (Table 1). With comparison to DMAP co-catalyst, other co-catalysts have shown lower catalytic conversions (Table 1) for their lower solubility in propylene oxide (Kilic et al. 2017). By comparing results listed in Tables 1, 2 and 3, for coupling of epoxides with CO 2 , numerous structural variations may be observed. From results, it may be concluded that Co-HPEED is an effective catalyst to produce good to excellent yield and high selectivity for coupling epoxides and CO 2 in the presence of DMAP. By considering different epoxides with CO 2 under same catalytic conditions, the optimal condition may be obtained. In addition to propylene oxide (PO), 1,2-epoxybutane (EB), styrene oxide (SO) and 1,2-epoxyhexane (EH) were used as substrates (Table 2). The activity order of epoxides was SO > EH > EB > PO, due to the presence of electron donating group present at C2 atom of these epoxides (Darensbourg and Yarbrough 2002). Yield (%) can be calculated by matching the ratio of cyclic carbonate to epoxides in the 1H NMR spectrum (Figure S6). The experimental result says that coupling of CO2 with epoxides is a temperature-dependent reaction (Fig. 1). With rise in temperature from 80 to 100 °C, the formation of cyclic carbonate increases from 42.3 to 64.2%, and on further increase in temperature, the yield remains unchanged (Fig. 1). Further, with increase in temperature, the yield (%) almost remains constant (Fig. 1) (Ulusoy et al. 2009). Other substrates such as SO (Figure S7), EB (Figure S8) and EH (Figure S9) also showed same trend. The coupling of CO2 with epoxides has almost completed within first 3 h, getting a yield of 64.2%, 86.8%, 70% and 71.9% for PO (Figure S10), SO (Figure S11), EB (Figure S12) and EH (Figure S13), respectively. With further raise in reaction time, the yields remain almost constant or increase slightly. Therefore, time three hours has been considered as optimum condition. The turnover frequency (TOF) of the reaction has been increased with increase in molar ratios of epoxide/Co-HPEED (Table 3), whereas percentage yield (102 × TOF (h−1)/molar ratio) of cyclic carbonate decreases with rise in molar ratios (Lee et al. 2012). The structure of cyclic propylene carbonate was determined by 1 H NMR (Fig. 2) and 13C NMR (Figure S14) spectroscopy (Castro-Osma et al. 2013). Scheme 1 Coupling of CO2 with epoxides 0.1 mol% Co-HPEED, 0.2 mol% DMAP Reaction conditions: Co-HPEED (4.5 × 10−5 mol), CO2 (1.5 MPa), at 100 °C, time 6 h. [PO]:[co-complex]:[co-catalyst] = 1000:1:2 a Yield (%) was calculated by comparing the ratio of cyclic carbonate to epoxides in the 1H NMR spectrum (Figure S6) b TON = Moles of cyclic carbonate/moles of catalyst at 3 h c TOF = Rate = turnovers/h O O R Epoxides + CO2 O O 6h, 100 0C, 1.5 MPa R Cyclic carbonates 13 Chemical Papers Table 2 Conversion of epoxide and CO2 to cyclic carbonate by Co-HPEED complex Yield (%)a TONb TOFc (h-1) - - - 1000:1:0 -d -d -d CoCl2 1000:1:2 <5 -d -d Co-HPEED 1000:1:0 <5 -d -d 5 1000:1:2 69.0 642 214 6 1000:1:2 89.3 868 289 7 1000:1:2 74.2 700 233 8 1000:1:2 75.1 719 240 Sl No. 1 Complexes HPEED [epoxides]/[Compl ex]/[DMAP] 1000:1:0 2 CoCl2 3 4 Substrate Reaction conditions: Co-HPEED (4.5 × 10−5 mol), CO2 (1.5 MPa), at 100 °C, time 6 h a Yield (%) was calculated by comparing the ratio of cyclic carbonate to epoxides in the 1H NMR spectrum (Figure S6) b TON = Moles of cyclic carbonate/moles of catalyst at 3 h c TOF = Rate = turnovers/h d Data not available Table 3 Coupling of CO2 with different epoxides by Co-HPEED using co-catalyst DMAP ([Co-HPEED]/[DMAP] = 1:2) [Substrate] / [Co-HPEED] TOFa (h-1) Entry [Substrate] / [Co-HPEED] TOFa (h-1) 1 1000 214 9 1000 233 2 2000 426 10 2000 463 3 3000 607 11 3000 658 4 5000 853 12 5000 998 5 1000 289 13 1000 240 6 2000 574 14 2000 475 7 3000 826 15 3000 665 8 5000 1252 16 5000 1033 Entry Epoxides Reaction conditions: At pCO2 = 1.5 MPa, 100 °C and 3 h a TOF(h−1): Moles of cyclic carbonate/moles of catalyst/h 13 Epoxides Chemical Papers for explaining the reaction pathway. For the formation of cyclic propylene carbonates, the catalyst, co-catalyst and CO2 are of the first order (Clegg et al. 2010). Therefore, the rate equation for formation of cyclic carbonate catalyzed by Co-HPEED can be represented by following Eq. 1. Since the concentrations of Co-HPEED, DMAP and CO2 were fixed, Eq. 1 can be expressed as Eq. 2: [ ]a [ ] Rate (r) = K Epoxide [Co - HPEED] CO2 [DMAP] (1) 65 Yield (%) 60 55 50 Co-HPEED (4.5 x 10 -5 mol), CO2 (1.5MPa), 6h. [PO] : [Co] : [DMAP] = 1000: 1: 2 45 80 85 90 95 100 105 110 Temperature/°C Fig. 1 Conversion of PO and CO2 to cyclic propylene carbonate as a function of temperature with Co-HPEED Kinetics analysis of conversion of epoxides and CO2 to produce cyclic propylene carbonate The synthesis of cyclic propylene carbonate was chosen as the model reaction, which provides suitable evidences [ ]a Rate (r) = Kobs Epoxide where Kobs ] [ = K[Co - HPEED] CO2 [DMAP] (2) To calculate the order of epoxide, six sets of reactions were carried out with different time periods at optimal condition, i.e., 3 h. Kinetic study has been carried out considering the conversion of PO and CO 2 to cyclic propylene carbonate with Co-HPEED (4.5 × 10 −5 mol), CO 2 (1.5 MPa), at 100 °C, [propylene oxide]:[CoHPEED]:[DMAP] = 1000:1:2. The straight line plot of ln[M] o/[M] t versus time signifies the first-order kinetics (Figure S15). Apparent rate constant (k app ) was 0.20049 h−1 for PO using Co-HMBED complex. Similar Fig. 2 1H NMR spectrum of cyclic propylene carbonate {Co-HPEED (4.5 × 10−5 mol), 1.5 MPa of CO2, at 100 °C, time 3 h. [propylene oxide]/ [Co]/[DMAP] = 1000/1/2} 13 Chemical Papers observation can be obtained for SO (Figure S16), EB (Figure S17) and EH (Figure S18) with apparent rate constant (k app) 0.43607 h −1, 0.18639 h −1 and 0.18538 h −1, respectively (Routaray et al. 2016a). nucleophilic replacement. Finally, cyclic propylene carbonate has been produced. Conclusions Mechanism of coupling of PO and CO2 to cyclic propylene carbonate Coupling of PO and CO 2 to cyclic propylene carbonate using Co-HPEED catalyst and DMAP as co-catalyst follows four-step mechanism as described in Scheme 2. The compound formed in step 1 reacts with propylene epoxide to produce the intermediate given in step 2. The step 1 is the rate determining step. The Co-HPEED catalyst stabilizes the ring-opening alkoxide intermediate produced from the SN2-type nucleophilic attack on epoxides by the DMAP. A CO 2 molecule consequently reacts to produce a carbonate intermediate in step 3. Lastly, in step 4, cyclic carbonate is produced as a result of SN 2-type The Co-HPEED complex was found to be the most effective catalyst for the conversion of epoxides and carbon dioxide to produce cyclic propylene carbonate in good to excellent yield. Use of co-catalysts plays a vital role for the synthesis of cyclic carbonates. Without co-catalyst, Co-HPEED showed less activity. Co-HPEED complex and co-catalyst DMAP showed the greatest catalytic activity for coupling of CO2 and styrene oxide and were the best one for formation of cyclic carbonates, which is probably due to solubilities of co-catalyst and catalyst in the epoxides. Molar ratios of epoxides to catalyst played a vital role. With increase in molar ratios of epoxide/catalyst, the TOF increases with decrease in yield (%) of cyclic carbonate. 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