Integrated experimental and theoretical insights into CO₂ fixation: tetraazamacrocyclic catalysts in ionic liquids for cyclic carbonate formation

Abstract

The electrochemical cycloaddition of carbon dioxide to epoxides was investigated using tetraazamacrocyclic metal complexes as electrocatalysts in ionic liquids under mild conditions. The process was carried out in the absence of additional organic solvents, employing Ni(cyclam)Cl₂ and Co(cyclam)Cl₂Cl as catalysts, which facilitated the activation of CO₂. The electrosynthesis was conducted in 1-butyl-3-methylimidazolium-based ionic liquids, which not only acted as solvents but also played a crucial role in promoting epoxide ring opening and stabilizing reaction intermediates. Electrochemical experiments using propylene oxide, styrene oxide, and epichlorohydrin demonstrated that the nature of the epoxide substituent significantly impacts the formation of cyclic carbonates. The highest yields were obtained when BMImBr was used as the reaction medium, while other ionic liquids such as BMImBF₄ and BMImTFSI resulted in negligible conversion. Spectroelectrochemical studies provided additional insights into the reaction mechanism, confirming the role of halide anions in facilitating carbonate formation. Furthermore, density functional theory (DFT) calculations were performed to explore the interaction between Ni(cyclam) complexes and CO₂. Theoretical results indicate that the trans-I isomer of [Ni(cyclam)]⁺ favors CO₂ coordination and activation, which aligns with the experimental findings. Computational analysis also supported the importance of ionic liquid composition in stabilizing key reaction intermediates. This study highlights the potential of electrocatalytic methodologies for the sustainable conversion of CO₂ into high-value chemicals, contributing to the development of greener and more efficient synthetic strategies.

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Sustainability spotlight

The increasing concentration of CO₂ in the atmosphere highlights the need for efficient carbon capture and utilization strategies. Electrochemical approaches offer a sustainable alternative by enabling the direct transformation of CO₂ into value-added products under mild conditions. In this work, we investigate tetraazamacrocyclic complexes as electrocatalysts for the electrosynthesis of cyclic carbonates in ionic liquids, demonstrating enhanced catalytic performance and selectivity. The combination of experimental and theoretical studies provides key mechanistic insights into CO₂ activation and carbonate formation, contributing to the design of more efficient and sustainable electrocatalytic systems. This study aligns with the UN Sustainable Development Goals (SDGs) 7 (Affordable and Clean Energy), 9 (Industry, Innovation, and Infrastructure), and 13 (Climate Action) by advancing energy-efficient CO₂ conversion technologies and promoting greener synthetic methodologies.

1 Introduction

The atmospheric concentration of carbon dioxide (CO₂) has risen from pre-industrial levels of ∼280 ppm to over 415 ppm today, driving global climate change, ocean acidification, and ecosystem disruption.^1–3 Addressing these challenges demands not only drastic emission cuts but also innovative approaches to capture, store, and valorize CO₂ in the form of value-added chemicals.

Electrochemical CO₂ fixation has emerged as a promising route because it can directly couple renewable electricity to chemical synthesis under mild conditions, minimizing thermal inputs and aligning with the tenets of green chemistry.^4–9 By modulating the applied potential, one can selectively activate CO₂ toward diverse products without resorting to harsh reagents or extreme pressures.

Among the many CO₂ conversion pathways, the cycloaddition of CO₂ to epoxides to yield cyclic carbonates is particularly attractive. Cyclic carbonates serve as high-performance aprotic solvents, electrolytes in lithium-ion batteries, monomers for polycarbonate production, and fine-chemical intermediates in pharmaceutical syntheses.^10–15 Their broad industrial utility underscores the importance of developing sustainable, scalable methods for their manufacture.

Traditional syntheses of cyclic carbonates often rely on phosgene or its equivalents (di- or polyols) under elevated temperatures and pressures, raising safety and environmental concerns.^16–19 Photocatalytic systems—such as cobalt–phthalocyanine/TiO₂ hybrids—have demonstrated near-quantitative yields under UV or visible irradiation,²⁰ yet require specialized catalysts and light sources that can limit practicality.

Electrochemical approaches using simple salts (e.g. alkali halides, quaternary ammonium salts) or non-macrocyclic transition-metal complexes have shown proof-of-concept for CO₂–epoxide cycloaddition, but often suffer from low selectivity, long reaction times (>10 h), and the need for large overpotentials (>−2.4 V vs. Ag/AgCl).^16–26 Superbase-based deep eutectic solvents have improved yields at moderate temperatures but still fall short of fully ambient operation.²⁷

Ionic liquids (ILs) offer a “green” reaction medium with negligible vapor pressure, wide electrochemical windows, and tunable solvation of charged intermediates. ILs such as BMImBr, BMImBF₄, and BMImTFSI have repeatedly been shown to stabilize metal-alkoxide species and facilitate epoxide ring opening, dramatically enhancing cyclic-carbonate formation under mild potentials.^18,28–38 In particular, halide-rich ILs (e.g. BMImBr) excel: the Br⁻ anion both nucleophilically opens the epoxide and stabilizes ensuing intermediates, leading to higher yields than fluorinated or sulfonyl ILs.^39,40

Despite these advances, two key gaps remain. First, most electrocatalytic systems to date employ non-macrocyclic catalysts, leaving the influence of well-defined ligand architectures on CO₂ activation largely unexplored. Second, while individual reaction steps have been probed spectroelectrochemically or theoretically, a fully integrated experimental–computational mechanism under truly ambient temperature and pressure is still lacking.

Herein, we introduce tetraazamacrocyclic complexes—Ni(cyclam)Cl₂ and Co(cyclam)Cl₂Cl—as robust electrocatalysts for CO₂ cycloaddition to epoxides in BMImBr. We combine cyclic voltammetry, controlled-potential electrolysis, FT-IR spectroelectrochemistry, and density-functional-theory (DFT) calculations to dissect the roles of metal isomerism, IL composition, applied potential, and substrate structure in driving high yields of propylene carbonate, styrene carbonate, and epichlorohydrin carbonate.⁴¹

Our integrated approach not only achieves quantitative conversion (100%) of propylene oxide at −1.8 V vs. Ag/AgCl and ambient conditions but also delivers a unified mechanistic framework for CO₂ activation in macrocyclic metal complexes. These insights furnish design principles for next-generation electrocatalysts and IL media, advancing sustainable CO₂ utilization in green chemistry and industrial processes.

2 Materials and methods

2.1 Materials

All reagents and solvents used were of analytical grade or higher. The macrocyclic complexes [Ni(cyclam)Cl₂] and [Co(cyclam)Cl₂]Cl were synthesized following previously reported procedures.^42,43 Elemental analysis confirmed that, for [Ni(cyclam)Cl₂], the found percentages were 36.84% C, 7.25% H, and 17.1% N (predicted: 36.4% C, 7.28% H, 17.0% N); for [Co(cyclam)Cl₃]Cl, the found values were 34.12% C, 6.91% H, and 15.1% N (predicted: 32.84% C, 6.63% H, 15.33% N). Propylene oxide, styrene oxide, and epichlorohydrin were purchased from Sigma-Aldrich and used without further purification. Carbon dioxide (extra pure, 99.99%) was obtained from INDURA, Chile. The ionic liquids BMImBF₄ (1-butyl-3-methylimidazolium tetrafluoroborate), BMImTFSI (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, >98.5%), and BMImBr (1-butyl-3-methylimidazolium bromide) were also purchased from Sigma-Aldrich. Prior to use, all ionic liquids were dried under vacuum at 80 °C for 24 hours, and their water content was verified using a Karl Fisher titrator (all were below 1% w/w). Solvents were handled under air-free conditions using syringes and cannulas.⁴⁴

2.2 Cyclic carbonate preparation

The electrosynthesis of cyclic carbonates was performed in a one-compartment (undivided) cell using a three-electrode configuration. All electrolysis experiments were carried out at 25 °C and atmospheric pressure. A glassy carbon plate (SPI West Chester, PA, USA; geometric area: 3.25 cm²) was used as the working electrode, a platinum wire served as the counter electrode, and an Ag/AgCl (saturated) electrode specially conditioned for non-aqueous media was employed as the reference electrode.⁴⁵ Initially, a solution containing 78 mmol of the ionic liquid and the appropriate catalyst (macrocyclic complex) was prepared and sonicated for uniform mixing. Next, the solution was transferred into the electrochemical cell, and 10 mmol of the chosen epoxide was added. The mixture was stirred for 15 minutes before inserting the electrodes. Two glass tubes were then introduced into the cell: one immersed in the solution near the working electrode to allow the continuous introduction of carbon dioxide, and a second positioned approximately 4 cm above the solution to serve as an exhaust for pressure equilibration. Carbon dioxide was bubbled continuously during the electrolysis (conducted over 8 to 24 hours) while a constant potential of either −1.4 V or −1.8 V (vs. Ag/AgCl) was applied under constant stirring. After electrolysis, the reaction mixture was extracted with diethyl ether and concentrated by rotary evaporation. The resulting colorless liquid was collected and analyzed by ¹H NMR spectroscopy.

2.3 Spectroelectrochemical techniques

FT-IR spectroelectrochemical experiments were carried out using a PTFE spectroelectrochemical cell equipped with a ZnSe ATR crystal on a Pike Veemax III system. In a typical experiment, a solution containing 10 mmol of propylene oxide and 0.8 mmol of Ni(cyclam)Cl₂ in 80 mmol of BMImBr was electrolyzed at −1.8 V (vs. Ag/AgCl). Spectra were recorded at 0, 30, 90, and 240 seconds to monitor changes in vibrational bands corresponding to epoxide ring deformation and cyclic carbonate formation.

2.4 Instrumentation

Water titrations were performed using a CRISON Titromatic KF 2S-2B. ¹H NMR spectra were recorded on a BRUKER AVANCE III HD 400 MHz spectrometer. FT-IR measurements were conducted on a Thermo Nicolet iS10 instrument. Routine electrochemical measurements were performed using a CH Instruments 760C potentiostat, while controlled potential electrolysis was carried out with a BASi PWR-3 potentiostat.

3 Theoretical analysis

Theoretical calculations were performed to explore the geometry of [Ni(cyclam)]⁺ and its interaction with CO₂, as no crystallographic structure is available. [Ni(cyclam)]⁺ and [Ni(cyclam)(CO₂)]⁺ were fully optimized at the density functional theory (DFT) level using the Gaussian16 computational package,⁴⁶ implementing the Becke's three parameters nonlocal hybrid exchange potential with the nonlocal correlation functional of Lee, Yang, and Parr (B3LYP)^47–49 without any symmetry restriction. The Gaussian-type orbitals, 6–311G(d,p), were used for light atoms,⁵⁰ while the Stuttgart-Dresden effective core potential⁵¹ was implemented for transition metal elements. A tight SCF convergence criteria (10⁻⁸ a. u.) was set in all calculations with the Grimme dispersion correction (D3).⁵² Harmonic vibrational frequencies were calculated to ensure that the optimized geometries were minima on the potential energy surface. The PCM solvation model⁵³ was implemented to simulate an implicit solvent (water) with a dielectric constant ε = 78.3553. The CO₂ binding energy (E_bind) was defined according to the following expression: E_bind = E_{[Ni(cyclam) - (CO2)]+} – (E_{[Ni(cyclam)]+} + E_[CO2]), which represents the energy difference between the complex and the optimized energies of the free CO₂ molecule and [Ni(cyclam)]⁺. In addition, based on the experimental results, the structure of the cyclic carbonate propylene with the ionic liquid BMImBr was optimized to identify the zones more prone to react with the activated CO₂.

4 Results and discussion

4.1 Experimental results

To elucidate the electrocatalytic performance of the tetraazamacrocyclic complexes, a series of experiments were conducted using cyclic voltammetry, controlled potential electrolysis, and spectroelectrochemical techniques. These investigations examined the influence of ionic liquid composition, applied potential, and substrate structure on the formation of cyclic carbonates from CO₂ and epoxides. The following section details the key experimental findings, which form the basis for a comprehensive mechanistic interpretation. All experiments reported herein were conducted at constant room temperature (25 °C) and atmospheric pressure.

4.1.1 Electrochemical behavior. To investigate the electrocatalytic properties of cyclam-type macrocyclic nickel complexes in the formation of cyclic carbonates, preliminary cyclic voltammetry experiments were conducted in an ionic liquid medium containing epoxide and Ni(cyclam)Cl₂. Fig. 1 illustrates the electrochemical behavior of the nickel complex in 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF₄), which functions both as the solvent and supporting electrolyte. The voltammogram of the neat ionic liquid reveals no significant electrochemical activity, confirming the absence of redox-active impurities and validating the electrochemical window used.

Fig. 1 Electrochemical behavior of BMImBF₄ used as solvent (dashed black line), a solution containing 9 mM of Ni(cyclam)Cl₂ and N_2(sat) (red), addition of propylene oxide (10 mM) saturated with N₂ (green) and saturated with CO₂ (blue). Scan rate: 100 mV s⁻¹, WE: glassy carbon, CE: Pt coil and RE: Ag/AgCl_(sat).

In contrast, the cyclic voltammogram of the nickel complex (depicted in red) shows two well-defined redox processes with half-wave potentials (E_1/2) at 0.42 V and −1.40 V. These correspond to the Ni(II)/Ni(III) and Ni(II)/Ni(I) redox couples, respectively.⁵⁴ These results confirm the electrochemical accessibility of both oxidation and reduction states of the nickel center in this macrocyclic environment.

Upon the addition of propylene oxide to the solution containing Ni(cyclam)Cl₂, the cyclic voltammogram (shown in green) remains similar in overall profile to that of the nickel complex alone. The redox couple around 0.47 V is only slightly shifted and displays a modest increase in current (∼1 μA), while the redox process at −1.4 V becomes more irreversible. Specifically, the anodic current decreases, and the cathodic current increases, suggesting a potential interaction between the reduced form of the complex and the epoxide. The broadening of this signal further supports the presence of such an interaction.

The effect of introducing carbon dioxide to the system is illustrated by the voltammogram shown in blue. At positive potentials, a shoulder emerges in both the anodic and cathodic waves, while at negative potentials, the previously reversible Ni(II)/Ni(I) redox couple is replaced by an irreversible process, peaking around −1.8 V. This transformation in electrochemical behavior indicates a significant interaction between the nickel complex and CO₂ under reductive conditions.

Taken together, these observations suggest that the irreversible signal observed in the presence of CO₂ likely corresponds to a catalytic reaction involving the activated CO₂ species and the epoxide, mediated by the reduced form of the nickel cyclam complex. These findings highlight the potential of such macrocyclic systems for facilitating electrochemical transformations relevant to carbon capture and conversion in ionic liquid media.

4.1.2 Electrocatalytic synthesis of cyclic carbonates. To evaluate the electrocatalytic activity of the complexes in the electrochemical activation of carbon dioxide and the subsequent electrosynthesis of cyclic carbonates, controlled potential electrolysis was performed under the previously studied conditions.

For these experiments, 78 mmol of ionic liquid (15 mL) and 10 mmol of epoxide were used to obtain the corresponding carbonate, with the catalyst representing 1% of the epoxide concentration. Styrene oxide is less reactive than aliphatic epoxides due to resonance stabilization of its benzylic alkoxide intermediate, which both raises the barrier to ring opening and enables rearrangement pathways that yield benzaldehyde as a detectable by-product under our electrolysis conditions.

Table 1 presents the different parameters varied in each experiment, including time and applied potential varied in each experiment. Additionally, relevant data such as charge and yield are provided.

Table 1 Optimization of reaction parameters in the electrosynthesis of styrene carbonate from styrene oxide and CO₂ in ionic liquids^a

Exp.	Ionic liquid	Catalyst	Time (h)	Potential (volts)	Charge (coulomb)	F/mol	Yield %
a Reaction conditions: 1% cat mol., 78 mmol ionic liquid, 10 mmol styrene oxide.
I	BMImBF4	—	6	−1.4	22	2.3 × 10^−2	0
II	BMImTFSI	—	6	−1.4	18	1.8 × 10^−2	0
III	BMImBF4	Ni(cyclam)Cl2	8	−1.4	33	3.5 × 10^−2	0
IV	BMImTFSI	Ni(cyclam)Cl2	8	−1.4	29	3.0 × 10^−2	0
V	BMImBF4	Ni(cyclam)Cl2	24	−1.4	108	1.1 × 10^−1	0
VI	BMImTFSI	Ni(cyclam)Cl2	24	−1.4	110	1.1 × 10^−1	0.33
VII	BMImBF4	Ni(cyclam)Cl2	24	−1.8	105	1.1 × 10^−1	0
VIII	BMImBr	Ni(cyclam)Cl2	24	−1.8	171	1.77 × 10^−1	59.3

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A comparison of the yield values in experiments I and II indicates that no product is formed in the absence of the metal complex. This indicates that BMImBF₄ under the studied conditions, does not exhibit catalytic activity for this type of reaction.

Electrolysis performed using the [Ni(cyclam)Cl₂] complex as an electrocatalyst, applying the previously established CO₂ reduction potential (−1.4 V) resulted in no conversion to the desired product. Although CO₂ is activated for cycloaddition with the epoxide, the abundance of cations and anions in the reaction medium is insufficient to stabilize the epoxy ring opening. As a result, the cycloaddition reaction remains incomplete.

To assess whether the previously observed negative results are due to slow reaction kinetics, the effect of electrolysis time on conversion yield was investigated. As the reaction time increased from 8 to 24 hours, no detectable conversion to cyclic carbonate was observed in BMImBF₄, while only trace amounts (0.33%) were detected in BMImTFSI. This comparison suggests that the absence of product is more likely related to the thermodynamic constraints of the reaction rather than a kinetic limitation.

Experiment VII was conducted by increasing the applied electrochemical potential during the electrolysis to −1.8 V to enhance carbon dioxide activation and promote its subsequent cycloaddition with the epoxide. However, the results remained negative, with 0% conversion observed.

It is important to highlight that the proposed mechanism for this type of reaction³⁹ relies on the formation of an alkoxide intermediate through epoxide ring opening. The role of the ionic liquid as a stabilizer of the alkoxide species has been identified as a crucial step in the process. Previous reports³⁹ suggest that the anion of the ionic liquid plays a more significant role than cation, with halides (e.g. Br−) demonstrating superior performance compared to fluorinated anions (BF₄⁻).

A key finding was obtained in experiment VIII, where a conversion of 59.3% was achieved, confirming that the nature of the anion is critical for the successful electrosynthesis of cyclic carbonate from epoxides. This observation is in strong agreement with previously reported data.

Although our work focused on Br⁻, Gallardo-Fuentes et al. report that in BMImBr the CO₂-propylene oxide cycloaddition yield reaches 72.8%, versus 41.1% in BMImBF₄ and 26.1% in BMImTFSI.³⁹ These trends correlate with the conjugate-acid pK_a values (HBr ≈ −8, HBF₄ ≈ −0.4, HTFSI ≈ 1.7), indicating that a more basic, nucleophilic anion promotes the key epoxide-opening step. By analogy, one would expect Cl⁻ or I⁻ to follow a similar pattern—driven by their respective acidities—though Br⁻ offers the optimal balance of ring-opening ability and intermediate stabilization under our conditions.

To investigate the effect of epoxide structure on the electrosynthesis of cyclic carbonates, two additional epoxides-epichlorohydrin and propylene oxide- were used as starting materials. The results obtained from the series of three epoxides in the electrosynthesis of cyclic carbonates are presented below.

Table 2 presents the results of the controlled potential electrolysis (−1.8 V) of the different epoxides, the metal complex and carbon dioxide over a 24-hour period. In general, the results for the epoxide conversion are favorable in all cases, with conversion yields ranging from 17.7% to 100%. It is important to note that the conversion values reported correspond exclusively to the formation of cyclic carbonates, as indicated in the caption of Table 2. Although benzaldehyde was identified as a detectable by-product in reactions involving styrene oxide, it was not quantified in this study. Therefore, the reported data reflect selective product yields rather than total epoxide consumption.

Table 2 Conversion to cyclic carbonates in BMImBr at 24 hours of electrolysis at −1.8 V^a

Exp.	Complex	Epoxide	Charge/C	F/mol	%Conversion
a Reaction conditions: 1% cat. mol., 78 mmol ionic liquid, 10 mmol epoxide at 24 h of electrolysis at −1.8V vs. Ag/AgCl(sat). Epoxide 1: propylene oxide, 2: epichlorohydrin, 3: styrene oxide.
1	—	1	91	0.094	93.0%
2	Ni(cyclam)Cl2		290	0.30	100%
3	[Co(cyclam)Cl2]Cl		198	0.20	98.2%
4	—	2	90	0.093	90.7%
5	Ni(cyclam)Cl2		114	0.12	83.0%
6	[Co(cyclam)Cl2]Cl		176	0.18	99.2%
7	—	3	92	0.095	17.7%
8	Ni(cyclam)Cl2		171	0.18	59.3%
9	[Co(cyclam)Cl2]Cl		103	0.11	22.4%

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In the cases of electrolysis performed without the addition of a catalyst (Experiments 1, 4, and 7), propylene oxide and epichlorohydrin achieve yields higher than 90%, while styrene oxide reached only 17.7%. The lower reactivity of styrene oxide can be attributed to the stabilization of the benzyl alkoxide intermediate through resonance delocalization (Fig. 2). This effect reduces the nucleophilicity of the alkoxide oxygen, making it less reactive toward the cycloaddition of carbon dioxide, which explains the significantly lower yield observed compared to the other epoxides.

Fig. 2 Resonance stabilization of the alkoxide generated in styrene oxide ring opening.

The results obtained for propylene oxide vary considerably in terms of the amount of charge developed. When Ni(cyclam)Cl₂ (Experiment 1) complex is used, charge increases almost threefold from the blank (Experiment 1) and conversion rate increases from 93% to 100%. Considering the increase in charge and the slight increase in conversion rate; the reaction time could be too long in Experiment 2 and probably a complete conversion could be achieved at shorter times, which would further reinforce the idea that the nickel complex studied could be used as a catalyst for the cycloaddition of carbon dioxide to propylene oxide.

A further analysis of Experiment 2 and 3 shows that charge decreases from 290 to 198C when Ni(cyclam)Cl₂ is replaced with [Co(cyclam)Cl₂]Cl. As expected, the yield in Experiment 2 is slightly higher than Experiment 3, being in good concordance with a higher charge.

Epichlorohydrin electrolysis shows, in all three systems (Experiment 4, 5 and 6), conversions higher than 85%, reaching 99.2% in Experiment 6. Considering the electrocatalytic activity of the cobalt complex against the reduction of carbon dioxide and the intrinsic activity of the ionic liquid, the yield increase shown by Co(cyclam)⁺³ could be related to its ability to stabilize the alkoxide intermediary, rather than by the ability to activate carbon dioxide for its subsequent addition to epichlorohydrin.

Electrolysis carried out with styrene oxide shows significantly lower yields than the other two epoxides, being 17.7% when a catalyst is not used (Experiment 7) and 22.4% when the cobalt complex is used (Experiment 9). The most interesting result is obtained with the nickel catalyst, in which conversion increases to about 60%. It is also found that the oxide readily reacts with carbon dioxide, forming benzene acetaldehyde (see ESI†), whereby the low yield obtained by electrolysis without catalyst and with the cobalt complex could be explained by the generation of the previously mentioned by-products. On the other hand, the higher yield obtained with nickel catalyst suggests that it could have a higher specificity towards carbonate formation.^25,55

4.1.3 Spectroelectrochemical analysis. FT-IR spectroelectrochemical measurements were carried out in order to observe spectroscopic evidence that could shed light about the reaction mechanism. A as a model, propylene oxide and Ni(cyclam)Cl₂ in a CO₂/BMImBr saturated solution was electrolyzed during 240 seconds at −1.8V vs. Ag/AgCl.

Fig. 3 shows the FTIR/ATR spectra of the different species in solution during the electrosynthesis of propylene carbonate. A fast analysis of the spectra shows an evident increase in the bands at 1570, 1167 and 1466 cm⁻¹ (♣), all of them appear as the most important changes in the spectra, and are associated to the solvent structure, particularly to deformation of the imidazolium ring.^56,57 Increase in absorbance over time is related to polarization of the electrodes due to lack of convection in the spectroelectrochemical cell. Prior to application of −1.8 V vs. Ag/AgCl (black line) propylene oxide spectrum can be observed, showing characteristics bands at 825, 1265 and 1406 cm⁻¹, corresponding to epoxide ring deformation,⁵⁸ a band at 944 cm⁻¹ can also be observed, related to –CH₃ rocking.⁵⁹

Fig. 3 FTIR/ATR Spectroelectrochemical measurements carried out using a propylene oxide (10 mmol) and Ni(cyclam)Cl₂ (0.8 mmol) in BMImBr (80 mmol) solution. Spectra of the solution saturated with CO₂ were taken at 0 seconds (black line), 30 and 90 seconds (dashed line) and at 240 seconds (blue line). Electrolysis was carried out at −1.8V vs. Ag/AgCl. WE: glassy carbon, CE: Pt coil, RE: Ag/AgCl_(sat).

When the electrosynthesis starts, a series of changes can be observed all along the spectra. A first group of bands, associated to epoxide ring deformation (●), reduces its absorbance as time passes. As a result, bands are not observed in the last spectrum recorded, meaning that as the reaction takes place, the epoxide ring opens up, forming an alkoxide. A second group of bands can also be detected as the reaction takes place (♦), these signals can be assigned to the cyclic carbonate structure and are mainly associated to vibration modes of the cycle formed between the alkoxide and the activated carbon dioxide. A summary of the most significative FTIR bands can be found in Table 3.

Table 3 Assignments of the infrared spectra of electrolyzed propylene oxide

cm^−1	Description	Ref.	cm^−1	Description	Ref.
825	C–O–C ring deformation	58	1167	Imidazole H–C–C & H–C–N bending	56 and 57
871	Symmetric carbonate ring vibration	59	1265	C–O–C ring deformation	58
944	–O–CO–C– symmetric stretching	59	1406	–O–CH2 wagging	59
1002	Alkoxide stretching	60	1453	Imidazole ring C–H stretch	56 and 57
1056	–C–O– ring stretch	59	1466	C–O–C ring deformation	58
1104			1570	Imidazole (ring stretching)	56 and 57

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4.2 Theoretical results

In parallel with the experimental studies, density functional theory (DFT) calculations were performed to probe the molecular interactions between the [Ni(cyclam)]⁺ complex and CO₂. This computational work includes the optimization of trans-I and trans-III isomers, evaluation of CO₂ binding energies, and analysis of frontier molecular orbitals. These theoretical insights complement the experimental data and provide a deeper understanding of the catalytic process at the molecular level.

Given that the reactivity of [Ni(cyclam)]⁺ in CO₂ activation is highly dependent on its structural configuration, particular attention was paid to the equilibrium between its trans-I and trans-III isomers.

[Ni(cyclam)]⁺ can exist in equilibrium as trans-I and trans-III isomers. It is reported that in solution, 85% corresponds to the structure trans-III.⁶¹Fig. 4 shows the optimized geometries; according to our computational calculations, trans-III is approx. 7.3 kcal mol⁻¹ more stable than structure trans-I, which has a distorted cycle configuration that influences the planarity of the system. Both structures were subsequently explored for the coordination of CO₂.

Fig. 4 Optimized geometries of the trans-I and trans-III isomers of [Ni(cyclam)]⁺. Color code: carbon atoms are gray, hydrogen in white, nitrogen in blue, and nickel in green.

Different positions of the CO₂ molecule, considering perpendicular and parallel orientation to the cycle, were built to study the interaction with [Ni(cyclam)]⁺. In both isomers, the most stable structures corresponded to the parallel alignment. Table 4 summarizes the E_bind of the most stable [Ni(cyclam)(CO₂)]⁺ isomers with some critical bonds and angles. It is important to state that E_bind carries an error due to the non-consideration of explicit solvent, counteranions, and entropy. The relative energies between the two isomers are relevant to these calculations.

Table 4 Angles and bond lengths of the optimized [Ni(cyclam)(CO₂)]⁺ structures. The N–Ni distance corresponds to an average of the four lengths

Isomer + CO2	N–Ni (Å)	N–Ni–N angles (°)		Ni-CO2 (Å)	E bind (kJ mol^−1)
trans-I	2.091	84.9	94.2	2.09	−34.873
trans-III	2.099	85.1	94.7	2.13	−29.989

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The most negative value and thus more favorable bonding interaction was detected for the structure trans-I (Fig. 5a). This evidence is consistent with the complexes formed with CO that prefer a trans-I isomer interaction than trans-III.⁶² In the case of CO_2, the oxygen atoms are repelled from the amino groups of the cycle, leading to bending in the O–C=O angle in approx. 35° from the planarity. Fig. 5c shows the most stable configuration of [Ni(cyclam)(CO₂)]⁺trans-III, only 4.88 kJ mol⁻¹ less stable than trans-I.

Fig. 5 The optimized structures of [Ni(cyclam)(CO₂)]⁺ (a) trans-I and (c) trans-II. (b) and (d) graphical representations of the HOMO and LUMO orbitals of each isomer. Color code: carbon atoms are gray, hydrogen in white, nitrogen in blue, oxygen in red and nickel in green.

In trans-I, the carbon atom belonging to the CO₂ molecule is located at 2.09 Å from nickel, and hydrogen bonds also mediate its interaction between the oxygen atoms of carbon dioxide and the amino groups of the cycle with an average distance of 2.48 Å. In the trans-III structure, the Ni-C distance is slightly longer (2.13 Å). The HOMO and LUMO frontier orbitals, alpha and beta, were plotted for both systems (Fig. 5b and d). The interaction between CO₂ and the ligand is mainly stabilized through the LUMOs of CO₂ and the β-HOMOs of [Ni(cyclam)]⁺, primarily the dz² of the nickel-metal center. This evidence is in accordance with the work reported recently by Masood and coworkers,⁶³ who described the [Ni(cyclam)(CO₂)]⁺trans-III structure at a similar level of theory.

Once the CO₂ is coordinated to [Ni(cyclam)]^+, the complex can be attacked by high electronic density centers to produce new molecules like cyclic carbonates. In order to identify these zones, we calculated the dual descriptor of reactivity (DDR) on the trans-III structure, which is the most abundant in solution according to experimental reports.⁶¹ The DDR has been defined as a robust indicator of unambiguously nucleophilic and electrophilic sites.⁶⁴ DDR is a local reactivity descriptor proposed by Morell and coworkers,^65,66 and is described in terms of the derivative of the Fukui function concerning the number of electrons.^67–69 As shown in Fig. 6a, the DDR plot shows two regions: the yellow zone where nucleophilic attacks can occur, and the red region, where electrophilic reactions can occur. The carbon atom located at the CO₂ has an important density depletion; this electrophilic region can be attacked by a center with high availability of electron density.

Fig. 6 (a) Dual descriptor of reactivity plot of [Ni(cyclam)(CO₂)]⁺trans-III, (b) the optimized structure of BMImBr and propylene oxide, and its (c) electrostatic potential surface.

According to the experimental evidence, ionic liquids play a key role in opening epoxide rings. The best performance system propylene oxide stabilized by BMImBr ionic liquid was optimized by DFT. Fig. 6b shows the optimized structure and relevant bond distances. The electrostatic potential surface (EPS) was also plotted for this system, reflecting a high-density accumulation in the oxygen atom of the propylene oxide, which can therefore act as a nucleophile and attack the activated carbon atom of the [Ni(cyclam)(CO₂)]⁺ complex, achieving the conversion of epoxide into carbonate.

5 Mechanistic insight

Based on the integrated experimental observations and theoretical calculations, a coherent mechanism for the electrosynthesis of cyclic carbonates is proposed. The DFT results indicate that the [Ni(cyclam)]⁺ complex exists in a dynamic equilibrium between its trans-I and trans-III isomers, with the slightly less stable trans-I form offering a more favorable site for CO₂ coordination. Computed binding energies and reactivity descriptors suggest that once CO₂ is activated by the reduced complex, it becomes highly susceptible to nucleophilic attack by the epoxide. In addition, the dual descriptor analysis reveals distinct electrophilic regions on the coordinated CO₂, findings that correlate with the experimental evidence of epoxide ring opening and subsequent cyclic carbonate formation. Collectively, these insights support a mechanism in which the reduced [Ni(cyclam)]⁺ complex first activates CO₂, thereby facilitating nucleophilic attack by the epoxide—an event further promoted by the stabilizing effect of bromide anions—ultimately driving the formation of cyclic carbonates.

Considering the previously discussed evidence and relevant literature reports, a plausible electrosynthesis mechanism for cyclic carbonates is depicted in Fig. 7. In the first step, the carbon dioxide molecule is activated by the reduced metal complex. Subsequently, the activated CO₂ reacts with the epoxide, which has been opened by the ionic liquid. The final step involves the addition of another CO₂ molecule, leading to ring closure and formation of the desired product. It is worth noting that previous mechanistic studies on metal–salen complexes, particularly those involving Cr(III), have shown that the metal center can coordinate to the oxygen atom of the epoxide ring, thereby increasing the electrophilicity of the adjacent carbon and facilitating nucleophilic attack. This interaction has been proposed as a key step in asymmetric ring-opening reactions catalyzed by such complexes, contributing to both reactivity and selectivity.⁷⁰

Fig. 7 Mechanism for the electrosynthesis of cyclic carbonates in BMImBr using Ni (cyclam)Cl₂ as catalyst in homogeneous medium.

6 Conclusion

The electrosynthesis of cyclic carbonates was carried out using cyclam-type metal complexes as electrocatalysts, achieving high conversion yields under optimized conditions. The best performance was obtained using Ni(cyclam)Cl₂ in the presence of BMImBr, applying −1.8 V for 24 hours, which highlights the crucial role of the ionic liquid anion in stabilizing key reaction intermediates. In contrast, BMImBF₄ and BMImTFSI were ineffective under the same conditions, further supporting the importance of halide-based ionic liquids for promoting the cycloaddition reaction.

The conversion of propylene oxide to propylene carbonate reached 100% when Ni(cyclam)Cl₂ was used as a catalyst, demonstrating that electrosynthesis under ambient conditions can provide an efficient alternative to traditional catalytic approaches. The proposed mechanism suggests that CO₂ activation occurs through the electrochemical reduction of the metal center, followed by a nucleophilic attack from the epoxide. The presence of Br⁻ anions in the ionic liquid facilitates the stabilization of the alkoxide intermediate, enabling the formation of the cyclic carbonate.

To further understand the catalytic process, density functional theory (DFT) calculations were performed, providing insight into the electronic structure of [Ni(cyclam)]⁺ complexes, the binding energy of CO₂, and the differences in reactivity between trans-I and trans-III isomers. The results indicate that CO₂ coordination is more favorable in the trans-I configuration, and that specific electronic properties of the complex play a key role in enhancing reactivity. Additionally, computational analysis of the interaction between the epoxide and the ionic liquid helped to rationalize the experimental observations regarding reaction efficiency and selectivity.

Overall, the combination of electrochemical, spectroelectrochemical, and computational studies provides a comprehensive understanding of the factors influencing the electrocatalytic conversion of CO₂ into cyclic carbonates. These findings contribute to the development of more sustainable methodologies for CO₂ utilization, offering potential applications in green chemistry and industrial processes.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

The authors would like to express their gratitude to FONDECYT Regular Projects 1221179 and 1220107, FONDEQUIP EQM150020, EQM190016, and EQM150101, Millennium Institute of Green Ammonia as Energy Vector ICN2021_023 (MIGA) and FONDECYT 11220512. This work was partially funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany's Excellence Strategy – EXC-2193/1 – 390951807.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5su00100e

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