Cycloaddition of CO2 to Epoxides Using a Highly Active Co(III) Complex of Tetraamidomacrocyclic Ligand
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- Ghosh, A., Ramidi, P., Pulla, S. et al. Catal Lett (2010) 137: 1. doi:10.1007/s10562-010-0325-0
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Synthesis of various cyclic carbonates with yield up to 100% and turn over frequency (TOF) of 351 h−1 using CO2 and epoxides and a cobalt (III) complex of tetraamidomacrocyclic ligand is described. The catalyst was characterized by single crystal X-ray crystallography. A study of reaction conditions indicates that 2 MPa pressure of CO2 without any co-solvent is sufficient to achieve the desired product.
KeywordsCyclic carbonatesCO2-epoxidesCo(III)-Tetraamidomacrocyclic ligand catalystCo-catalysts
Both homogeneous and heterogeneous reaction conditions were employed to develop cyclic carbonates. For their industrial production, reactions are generally catalyzed by metal or organic halide salts, such as potassium iodide or tetraethylammonium bromide [13, 14]. Over the years, numerous homogeneous catalysts have been reported in the literature for their ability to synthesize cyclic carbonates with varying yields and TOFs. These include quaternary ammonium salts , alkali and alkaline earth metal salts [16, 17], tin and antimony compounds , various transition metal salts [19–21], and ionic liquids  etc.
In order to increase the selectivity, yields and reaction rates for catalytic cyclic carbonate synthesis, several metal complexes of well-defined ligands that act as Lewis acids along with Lewis base co-catalysts were introduced by various groups. For example, metal porphyrins and phthalocyanines are attractive catalysts since they are cheap, easily available, and highly reactive catalysts [23–27]. Metal complexes of salen ligands are other types of well defined metal catalysts which have been studied extensively for the activation of CO2 for synthesizing polycarbonates [7, 28] and cyclic carbonates. Among many salen complexes, those derived from aluminum, nickel, copper, zinc, magnesium, cobalt, and chromium are noteworthy [29–34]. In terms of reactivity, aluminum and chromium salen complexes are very active in producing the cyclic carbonates. M. North and co-workers developed one of the most highly reactive catalysts that can operate at relatively mild reaction conditions. This catalyst is a μ-oxo-dimeric Al-salen complex which produces cyclic carbonates in high yields at a low temperature and 1 atm of CO2 pressure [30, 31].
The synthesis of cyclic carbonates from CO2 and similar epoxides has been applied on an industrial scale and a number of the works cited above report on the catalyst development and its reaction mechanisms . However, the activity, stability, and recovery of the catalysts still need to be improved. In many cases, metal-based homogenous catalysts, besides being active at high pressure and temperature, are moisture sensitive (salen complexes)  and oxygen (low-valent metal complexes) . These catalysts (metal porphyrin and phthalocyanine complexes) agglomerate with each other and need to be used in large quantities and require co-solvents to achieve good activity . Heterogeneous catalysts have the inherent problem of lower activity and they also need the use of co-solvents . Considering these facts, there is still room to develop novel and efficient homogenous catalysts for synthesizing cyclic carbonates under relatively mild reaction conditions.
In this present work, we report the use and detailed study of a cobalt-tetraamidomacrocyclic complex (1, Fig. 1) as a new catalyst for the synthesis of aromatic and aliphatic cyclic carbonates in excellent yields, high selectivity, and fast reaction rates. Structurally the catalyst resembles porphyrin or salen complexes but has four deprotonated N atoms which act as strong σ donor ligands to coordinate to Co(III) ion. Although similar complexes have been known for some time [38–40], but no catalytic activity has been tested using such complexes in cyclic carbonate synthesis. We inferred that the catalyst would provide the following advantageous characteristics: (a) much more robustness than any salen complexes, especially hydrolytic stability; (b) possesses planar four coordinate geometry (as revealed by the crystal structure of the cobalt complex), and thus leaves two axial positions still available for binding, a crucial feature for allowing the docking of epoxide on the Lewis acidic metal center and facilitate the reaction with carbon dioxide; (c) can easily be synthesized; (d) is uncreative towards oxygen and moisture, and thus can be handled without any specialized equipment, and (e) shows no agglomeration (unlike metal phthalocyanine or porphyrin complexes) and has flexible solubility including in many epoxides. Therefore, the use of 1 may be beneficial to investigate in detail for the synthesis of cyclic carbonates.
Chemicals were purchased from Aldrich chemical company, USA and Acros chemical company and used without further purifications unless otherwise mentioned. Propylene epoxide, cyclohexene epoxide were purified using CaH2. CO2 was obtained from Airgas (99.9%). All high pressure reactions were carried out in 100 mL Parr reactor connected to a 4842 controller. 1H and 13C-NMR spectra were obtained using 200 and 600 MHz Bruker instruments equipped with a 5 mm triple resonance inverse probe. The spectra were collected at 25 °C and chemical shifts are in ppm relative to TMS as external standard unless otherwise stated. Infrared spectra were obtained using a Shimadzu FT-IR Affinity-1 spectrophotometer. Electrospray ionization mass spectra (ESI-MS) were obtained using an Agilent 100 series MSD VL spectrometer. Gas Chromatography Mass spectra (GC/MS) were obtained using an Agilent technologies 6890N network GC system and equipped with Agilent Technologies 5975 inert XL mass selective detector. Elemental analysis was performed at Midwest Microlab LLC., Indianapolis.
2.2 Synthesis of Catalyst
The details of catalyst synthesis and characterizations are presented in the supplementary information. Both lithium and tetraphenylphosphonium salts of the catalyst were used in the reaction and characterization purposes as required.
2.3 Synthesis of Cyclic Carbonates
A representative method for synthesis of cyclic carbonate (Propylene carbonate (PC)) using propylene oxide (PO) and co-catalyst 4-dimetheylaminopyridine (DMAP) is described below. Propylene Oxide (2.08 g, 0.036 mol), 1 (15 mg, 0.035 mmol) and co-catalyst DMAP (8.6 mg, 0.07 mmol) were added into a 100 mL stainless steel Parr high pressure reactor. The reactor was pressurized with CO2 (Purity 99.9%, Airgas) and the temperature of the reactor was quickly increased and maintained at 120 °C. The final pressure of the reaction vessel was read as 2 MPa. The reaction mixture was allowed to stir in these conditions for 3 h. The reactor was quickly brought to a low temperature using an ice water bath and pressure was released slowly. The reaction mass was collected using dichloromethane for a complete transfer and the catalyst was separated by passing through a small plug of silica gel (2 g, 60–100 mesh) and eluting with dichloromethane. The solvent and excess epoxide were removed at reduced pressure. The yield was calculated with respect to the authentic sample by the external standard method from the GC peak. The isolated yield was calculated by taking the weight of the isolated product. The product was characterized by IR, 1H-NMR and GC-MS. The reaction yield was also calculated at different time intervals.
In accordance to the above-mentioned conditions, the amount of DMAP was varied from 1 to 2.5 mol equivalents with respect to 1 while the effect of the co-catalyst was studied. The pressure of CO2 was optimized at 2 MPa based upon the experiments carried in the range of 0.068–4.8 MPa. Similarly, the study of temperature was executed in the range of 70–120 °C.
After filtration through a short column of silica and washing with dichloromethane the silica adsorbed catalyst was further used to perform the synthesis of PC. Reusability tests were performed for 3 times consecutively.
3 Results and Discussion
3.1 Catalyst Synthesis and Structure Determination
The tetraamidomacrocyclic ligand and its cobalt complex (1, Fig. 1) were synthesized following the literature procedure of similar cobalt complexes with readily available starting materials (see supporting information for details) . The tetraamidomacrocyclic ligand was characterized by 1H-NMR, electrospray ionization mass spectrometer (ESI-MS, m/z 373.3 (M-H+, 100)), infrared spectroscopy (IR) and elemental analysis. Using n-butyllithium as a strong base, under an inert atmosphere, the ligand was deprotonated and reacted with cobalt(II) chloride in tetrahydrofuran. Finally, the desired cobalt(III) complex was obtained under air exposure. The cobalt complex is negatively charged and both the lithium and the tetraphenylphosphonium versions of the cobalt complex were obtained. The tetraphenylphosphonium salt was obtained by the metathesis of 1 with tetraphenylphosphonium chloride in water. ESI-MS, elemental, UV/Vis studies were performed to characterize the cobalt complex. The cobalt complex was found to be highly air and moisture stable. In fact, a high hydrolytic stability of the complex was observed by dissolving the complex in dilute sulphuric acid solution and finally analyzing the solution using ESI-MS and UV/Vis studies. All these properties are suitable for achieving a robust homogeneous catalyst for cyclic carbonate synthesis.
3.2 Catalytic Activity
The catalyst and co-catalyst were found to be soluble in all epoxides thus no co-solvents were used for this reaction. Cyclic carbonates were found to form from CO2 and epoxides using 0.1 mol% catalyst and 0.2 mol% of co-catalyst with respect to epoxides. The required pressure of CO2 was found to be 2 MPa, which is notably lower than many literature-reported values for this reaction. A blank reaction at similar conditions in absence of the catalyst and co-catalyst showed little product formed. This not only indicates the need for both catalyst and co-catalyst to be present for an optimum activity but also that the reactor has no influence in the reaction. The products were confirmed by respective molecular ion peaks in GC/MS. The formation of the product was also supported by the broad infra-red peak at ~1800 cm−1 (see supplementary information). The absence of any other carbonate peaks in the IR spectra eliminates the possible formation of polycarbonate during the reaction and thus indicating high selectivity. Gas chromatrography of the reaction mixture shows the presence of the product (RT 6.6 min, PC) as the sole component other than epoxide (RT 1.2 min, PO), suggesting a high reaction selectivity. The reaction yield was also calculated at different time intervals.
Formation of PC from PO and CO2 using 1 and various co-catalysts
Isolated yieldc (%)
4-Dimethylamino pyridine (DMAP)
1/Tetraphenyl phosphonium bromide
Formation of cyclic carbonates using different epoxides
We attempted to reuse the catalyst after adsorbing onto silica gel. Though a diminishing trend in efficiency after each recycle (TONrecycle : 5701st, 4892nd and 3013rd) is observed, the selectivity of the product remained unaltered indicating a retention of identity of the catalyst.
In conclusion, we have found that the cobalt complex of tetraamidomacrocyclic ligand is an efficient catalyst for the production cyclic carbonate at TOF of 351 h−1. The catalyst is air stable, easily synthesized and environmentally benign metal complex and found to be active to produce cyclic carbonates without any added co-solvent. The catalyst is capable of producing cyclic carbonates using epoxide and CO2 at 120 °C and 2 MPa of pressure. Efficiency to form cyclic carbonates was studied using both aromatic and aliphatic epoxides and the catalyst showed activity even at relatively low CO2 pressure (~0.7 MPa). The effectiveness of the catalyst is believed to be governed by the unique electronic and geometric properties offered by the tetraamidomacrocyclic ligand. Although the outcome of the catalyst recovery method is not satisfactory, the retention of product selectivity leaves us further opportunity to develop alternative techniques. Further studies are needed for designing other catalysts for better efficiency and recovery to synthesize cyclic carbonate and polycarbonates.
AG likes to thank UALR faculty start up grant and Department of Energy (Grant number DE-FG36-06GO86072) for financial assistance to complete the work. AG and SLC also like to thank Arkansas State Technology Authority for a summer scholarship (09-EPSCoR-0072) grant for financial support.