Cationic tetranuclear macrocyclic CaCo₃ complexes as highly active catalysts for alternating copolymerization of propylene oxide and carbon dioxide

Abstract

We found that a cationic hetero tetranuclear complex including a calcium and three cobalts exhibited high catalytic activity toward alternating copolymerization of propylene oxide (PO) and carbon dioxide (CO₂). The tertiary anilinium salt [PhNMe₂H][B(C₆F₅)₄] was the best additive to generate the cationic species while maintaining polymer selectivity and carbonate linkage, even under 1.0 MPa CO₂. Density functional theory calculations clarified that the reaction pathway mediated by the cationic complex is more favorable than that mediated by the neutral complex by 1.0 kcal mol⁻¹. We further found that the flexible ligand exchange between Ca and Co ions is important for the alternating copolymerization to proceed smoothly.

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Introduction

Alternating copolymerization of carbon dioxide (CO₂) and epoxide to produce polyalkylenecarbonates (PACs) has attracted much interest because not only are PACs biodegradable and biocompatible materials (Fig. 1a), but they also contain significant amounts of CO₂ upon treatment with commercially available inexpensive comonomers such as propylene oxide (PO), for which the content of CO₂ reaches 43% wt of the resulting polypropylenecarbonate (PPC).^1–3 Thus, such an efficient CO₂ capturing system has prompted many researchers to develop more efficient catalysts, including mononuclear salen complexes,^3j,4 mononuclear porphyrin and corrole complexes,⁵ homo- and hetero-dinuclear complexes,^6–8 and hetero-multinuclear complexes,⁹ with improved catalytic activity, polymer selectivity, and carbonate linkage. Among these complexes, dinuclear complexes together with tetranuclear complexes supported by alkoxide bridged skeletons (Fig. 1b) are investigated most often because of their advantages for catalysing the alternating copolymerization, even under lower CO₂ pressure.^6–9

Fig. 1 Multinuclear complexes catalysed alternating copolymerization of epoxide and CO₂.

A notable feature of the hetero-multinuclear complex active for alternating copolymerization is that two metal ions exist in proximal positions to cooperatively activate both epoxides and CO₂, for which the turnover limiting step is established to be a ring-opening step by nucleophilic attack of the carbonate species to the coordinating epoxide (Scheme 1a).¹⁰ These multinuclear complexes, however, are only effective for reactive epoxides such as cyclohexene oxide (CHO), and do not work well for less reactive epoxides such as PO. In 2020, Williams reported the first example of an aryloxide-bridged hetero dinuclear Co(III)K(I) complex-catalysed alternating copolymerization of PO and CO₂, though the catalyst system requires high CO₂ pressure (2–3 MPa).^8g In this context, we found a new hetero tetranuclear complex including a Ca ion and three divalent Co ions by which alternating copolymerization of PO proceeded selectively under the lowest CO₂ pressure (1.0 MPa) in hetero-multinuclear complexes (Scheme 1b). In addition, we report the remarkable additive effects of tertiary anilinium salts with non-coordinating anions, such as [PhNMe₂H][B(C₆F₅)₄], to improve the catalytic activity of the tetranuclear complex by generating the corresponding cationic hetero tetranuclear complex (Scheme 1c).

Scheme 1 Key transition state structures of (a) neutral multinuclear catalysts, (b) cationic multinuclear catalysts. (c) This work: alternating copolymerization of PO and CO₂ by cationic multinuclear species in the presence of anilinium salt.

Results and discussion

First, we synthesized macrocyclic tetranuclear CaM₃ complexes 1_CaM through the formation of a macrocyclic mononuclear calcium complex, 1_Ca, which was prepared by treating Ca(OAc)₂·H₂O and three equivalents of 1,4-diformyl-2,3-dihydroxylbenzene, followed by the addition of three equivalents of (1R,2R)-(−)-1,2-cyclohexanediamine in MeOH at 70 °C. Complex 1_Ca had three vacant salen motifs around the calcium ion as characterized by its ¹H NMR spectrum and mass spectrum. In the ¹H NMR spectrum in CD₃CN, a singlet signal of imine protons and a singlet signal of aromatic protons were observed at 8.43 and 6.87 ppm, respectively, and the integral ratio of the imine and aromatic signals was 1 : 1, consistent with the condensation of two aldehydes of 1,4-diformyl-2,3-dihydroxylbenzene with two amine moieties of (1R,2R)-(−)-1,2-cyclohexanediamine to form a macrocyclic ligand. In high resolution mass spectroscopy, the molecular ion peak (m/z = 771.3186) was observed for [1Ca + H]+ (C₄₂H₄₇N₆O₆Ca: m/z (calcd) = 771.3183). Various divalent first-row transition metal ions (M = Mn, Fe, Co, Ni, Cu, and Zn) were introduced to 1_Ca to give the corresponding macrocyclic tetranuclear complexes 1_CaM (Scheme 2);¹¹ it is noteworthy that tetranuclear complexes including Mn, Fe, and Co were inaccessible by the previously reported one-step template method using lanthanide ions and divalent first-row transition metal ions such as Ni, Cu, and Zn.¹² The structure of complex 1_CaZn was determined by a preliminary X-ray single crystal analysis, where one of two acetate ligands of 1_CaZn bridged Ca and Zn ions in a κ²-fashion, and the other acetate ligand coordinated onto the Zn ion in a κ¹-fashion (Fig. S2†).

Scheme 2 Preparation of hetero tetranuclear complexes 1_CaM.

Alternating copolymerization mediated by CaM₃ complexes

We tested the catalytic performance of these newly prepared tetranuclear complexes 1_CaM (0.01 mmol) for alternating copolymerization of propylene oxide (PO; 60 mmol, neat) and CO₂ (1.0 MPa) at 50 °C for 18 h, and found that only complex 1_CaCo exhibited catalytic activity. The turnover number (TON) of 1_CaCo reached 240 to afford poly(propylenecarbonate) (PPC) with high polymer selectivity (PS, 92%) and high carbonate linkage (CL, >99%) in a bimodal manner due to the inevitable contamination of a small amount of water, whose molecular weights and polydispersity indexes (PDI) were M_n = 4.9 kg mol⁻¹ (PDI = 1.07) and 11.6 kg mol⁻¹ (PDI = 1.08), respectively (Table 1, entry 1).¹³ In sharp contrast, other 1_CaM complexes of Mn, Fe, Ni, Cu, and Zn were inactive (Table 1, entries 2–6). A notable finding was that the catalytic activity of the tetranuclear CaCo₃ complexes was sensitive to the ligand backbone: complex 2_CaCo having a 1,2-diphenylethylene tether between two nitrogen atoms exhibited slightly higher catalytic activity than 1_CaCo, though with lower polymer selectivity (entry 7).

Table 1 Screening of catalysts

Entry	Catalyst	TON^a	PS^a [%]	CL^a [%]	M n [kg mol^−1]	PDI^b
a Determined by ^1H NMR. n.d. = not detected. b Determined by GPC analysis using polystyrene standards.
1	1CaCo	240	92	>99	4.9/11.6	1.07/1.08
2	1CaMn	n.d.	—	—	—	—
3	1CaFe	n.d.	—	—	—	—
4	1CaNi	n.d.	—	—	—	—
5	1CaCu	n.d.	—	—	—	—
6	1CaZn	n.d.	—	—	—	—
7	2CaCo	370	75	>99	8.3/19.6	1.06/1.06

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With the best tetranuclear complex 1_CaCo in hand, we looked into the additive effects of protic cocatalysts for alternating copolymerization of PO and CO₂ (Table 2). N,N-Dimethylanilinium tetrakis(pentafluorophenyl)borate, [PhMe₂NH][B(C₆F₅)₄], dramatically improved the TON value to 1840 without decreasing the PS and CL (entries 2 vs. 1), while other anilinium salts, such as [PhMeNH₂][B(3,5-(CF₃)₂C₆H₃)₄] (entry 3, TON = 600, PS = 99%, CL = 87%), [PhNH₃][B(3,5-(CF₃)₂C₆H₃)₄] (entry 4, TON = 560, PS = 99%, CL = 50%), and [Ph₂MeNH(OEt₂)][B(3,5-(CF₃)₂C₆H₃)₄] (entry 5, TON = 790, PS = 83%, CL = 63%), resulted in only moderate catalytic activity and lower CL. In contrast, the addition of trialkyl ammonium salt, [ⁿBu₃NH][PF₆] was ineffective for increasing the catalytic activity (entry 6, TON = 110, PS = 77%, CL = >99%). A strong proton source, [(Et₂O)₂H][B(3,5-(CF₃)₂C₆H₃)₄], enhanced the TON value to 1470; however, the PS and CL decreased to 83% and 31%, respectively (entry 7). When we used tritylium tetrakis(pentafluorophenyl)borate, [Ph₃C][B(C₆F₅)₄], the TON value was high (1340) but the PS and the CL were lower than that for [PhMe₂NH][B(C₆F₅)₄] (entry 8), and when 2 equiv. of [Ph₃C][B(C₆F₅)₄] was added, only homo polymerization of PO proceeded with a high TON value (2910), and no PPC was obtained (entry 9).¹⁴ In the absence of 1_CaCo, [PhMe₂NH][B(C₆F₅)₄] acted as a catalyst for the homo polymerization of PO (entry 10). The addition of quaternary ammonium salts, such as [ⁿBu₄N]Cl, or iminium salts, such as [PPN]Cl, both of which are frequently used as cocatalysts to improve the nucleophilicity of reaction intermediates, alkoxide species and carbonate species, by generating anionic active species,^5,15 resulted in lower catalytic activity of 1_CaCo and a lower PS (TON = 460, PS = 17% for [ⁿBu₄N]Cl, entry 11; TON = 380, PS = 34% for [PPN]Cl, entry 12). On the basis of the above results, we drew three conclusions regarding the first example of the alternating copolymerization of epoxide and CO₂ catalyzed by cationic hetero-multinuclear complex:¹⁶ (i) the catalytically active cationic species was in situ-generated through removal of the AcO moiety bound to metals by an acidic proton source or the trityl cation, while a dicationic species was inactive for alternating copolymerization but active for ring-opening polymerization of PO (Scheme 3), (ii) the acidity of anilinium salts is quite important for generating the catalytically active cationic species: low acidic ammonium salts are unable to produce the cationic species, while strong acids, such as [Ph₂MeNH][B(3,5-(CF₃)₂C₆F₃)₄] and [(Et₂O)₂H][B(3,5-(CF₃)₂C₆F₃)₄], promoted homo polymerization of PO, and (iii) N,N-dimethylaniline suppressed the ring-opening polymerization of PO to keep the CL level high. Accordingly, we selected [PhMe₂NH][B(C₆F₅)₄] as the best additive.

Table 2 Screening of catalysts and additives

Entry	Additive	TON^a	PS^a [%]	CL^a [%]	M n [kg mol^−1]	PDI^b
a Determined by ^1H NMR. b Determined by GPC analysis using polystyrene standards. c 2 equiv. of [Ph3C][B(C6F5)4] to catalyst 1CaCo was added. d No catalyst was added.
1	—	240	92	>99	4.9/11.6	1.07/1.08
2	[PhMeNH2][B(C6F5)4]	1840	95	98	40.0/85.7	1.11/1.04
3	[PhMeNH2][B(3,5-(CF3)2C6H3)4]	600	99	87	5.9	1.30
4	[PhNH3][B(3,5-(CF3)2C6H3)4]	560	99	50	—	—
5	[Ph2MeNH(OEt2)][B(3,5-(CF3)2C6H3)4]	790	83	63	8.4/22.1	1.13/1.10
6	[^nBu3NH][PH6]	110	77	>99	—	—
7	[(Et2O)2H][B(3,5-(CF3)2C6H3)4]	1470	83	31	6.9	1.51
8	[Ph3C][B(C6F5)4]	1340	87	84	21.1/50.1	1.14/1.06
9	[Ph3C][B(C6F5)4]^c	2910	99	<1	29.6	1.45
10^d	[PhMe2NH][B(C6F5)4]	2880	96	<1	12.5	1.58
11	[PPN]Cl	460	17	>99	—	—
12	[^nBu4N]Cl	380	34	>99	—	—

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Scheme 3 Catalytic species derived from complex 1_CaCo.

Plausible reaction mechanism

We propose a plausible mechanism as outlined in Scheme 4. Treatment of less catalytically active neutral species A with [PhMe₂NH][B(C₆F₅)₄] affords a highly active cationic species B, from which one of two acetate moieties of A is liberated to release acetic acid. In the initiation stage, a coordinating PO is ready to open its epoxy ring due to activation by electronically positive cationic species B, and nucleophilic attack of the remaining acetate moiety gives cobalt-alkoxide species C. The cobalt-alkoxide species C isomerized to calcium-alkoxide species C′, and then, a CO₂ molecule coordinates to the Co center to produce intermediate D, where the alkoxide moiety reacts with CO₂ to generate carbonate species E. Following ring-opening reaction of PO and CO₂ insertion propagates the polycarbonate chain.

Scheme 4 Plausible reaction mechanism of alternating copolymerization of PO and CO₂ mediated by complex 1_CaCo with anilinium borate.

To clarify the additive effects of [PhNMe₂H][B(C₆F₅)₄], we performed a reaction pathway search based on density functional theory calculations. Because of the high calculation cost of our very large complexes, we calculated a model reaction using simplified Ca(II)Co(II)₃ complexes and ethylene oxide (EO) using simple function and basis (B3LYP function with a basis-set of 6-31G(d,p) for H, C, N, O and LANL2DZ for Ca and Co) and the IEFPCM option with ε = 12.42 (ε of ethylene oxide).^17,18 The structure of a neutral complex A1 was constructed based on the crystal structure of 1_CaZn, and the structure of a cationic complex B1 is constructed by removing one κ¹-acetate ligand of A1, in which an ethylene oxide (EO) coordinating to a Co ion was moved to a Ca ion to occupy a coordination site of a large Ca ion.

In the cationic pathway shown in Scheme 5, model complex B1 is stabilized by the dissociation of an EO and forms B1-EO; further isomerization by the transfer of coordinating EO on Ca ions to Co ions affords complex B2-EO. Subsequent nucleophilic attack of the acetate ligand leads to a ring-opening reaction of EO through a transition state TS_BC2-EO to give Co-alkoxide intermediate C2-EO, and the activation energy of this step is 24.7 kcal mol⁻¹. The formation of Ca-alkoxide C1-EO from B1-EO through B1′-EO and TS_BC1-EO requires slightly higher activation energy (25.3 kcal mol⁻¹) than TS_BC2-EO. Next, the coordination of CO₂ on the Co-alkoxide species generates unstable intermediate D2-EO, while the coordination of CO₂ on Ca-alkoxide species affords much more stable intermediate D1-EO, suggesting that the CO₂ insertion proceeds via the Ca-alkoxide intermediate D1-EO. The CO₂ insertion from D1-EO goes barrierlessly to afford E1-EO, which is then isomerized to E1′-EO to start the propagation. Without the dissociation of an EO, the ring-opening step to give both Ca- and Co-alkoxides (TS_BC1 and TS_BC2) requires much higher energies than TS_BC2-EO, and thus these pathways are eliminated.

Scheme 5 Energy diagram for alternating copolymerization by cationic CaCo₃ complexes.

On the other hand, the activation barrier of the most favorable neutral pathway is 25.7 kcal mol⁻¹ for A2-EO→TS2_AF-EO, which is 1.0 kcal mol⁻¹ higher than that of the cationic pathway (Scheme 6). Dissociation of one EO from A1 affords A1-EO, and the coordination mode change of the κ¹-acetate ligand to the κ²-coordination mode and the transfer of coordinating EO on Ca ions to Co ions yields the most stable intermediate A2-EO. Like the cationic pathway, transition state TS_AF2-EO, which generates Co-alkoxide intermediate F2-EO, has lower activation energy than TS_AF1-EO for generating the Ca-alkoxide intermediate F1-EO. In addition, CO₂ coordination onto Ca-alkoxide species G1-EO is favored compared with CO₂ coordination onto Co-alkoxide species G2-EO by 4.5 kcal mol⁻¹.

Scheme 6 Energy diagram for alternating copolymerization by neutral CaCo₃ complexes.

According to these results, we concluded that in the CaCo₃ tetranuclear catalyst, an electronically positive cationic species strongly activates epoxide, allowing for smooth progression of the ring-opening step. Furthermore, flexible ligand exchange between Ca and Co ions is important to promote the CO₂ insertion step to give PPC with high CL. Though the energy difference of activation barriers is small, just looking at the number of the difference in the activation energies of the cationic and neutral pathways, the reaction rate of the cationic pathway was estimated to be 5.4 times faster than that of the neutral pathway, consistent with the experimental results on catalytic activities of complex 1_CaCo (TON = 240) and 1_CaCo with [PhNMe₂H][B(C₆F₅)₄] (TON = 1840): cationic species advanced the alternating copolymerization 7.7 times faster than neutral species.

Conclusions

We found new tetranuclear macrocyclic catalyst 1_CaCo for alternating copolymerization of CO₂ and PO, and remarkable additive effects of [PhMe₂NH][B(C₆F₅)₄] to improve the catalytic activity of 1_CaCo while maintaining high polymer selectivity and high carbonate linkage, even under 1.0 MPa of CO₂. On the basis of control experiments, using 1 or 2 equiv. of [Ph₃C][B(C₆F₅)₄], together with density functional theory calculations, we concluded that the reaction of 1_CaCo with [PhMe₂NH][B(C₆F₅)₄] afforded catalytically active cationic species, which strongly activated PO as a Lewis acid to reduce the activation energy. In addition, flexible ligand exchange between Ca and Co ions is important to reduce the energy for CO₂ insertion steps to obtain PPC with high CL.

Data availability

The authors declare that all data supporting the findings of this study are available from the corresponding author upon reasonable request.

Author contributions

H. N. conducted DFT calculations, and he wrote the manuscript with the assistance of K. M. and J. O. S. M. conducted most of the experiments.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by JSPS KAKENHI 20K152770 as a Grant-in-Aid for Young Scientists and 21K18205 as a Grant-in Aid for Challenging Research. DFT calculation was achieved through the use of PC cluster for large-scale visualization at the Cybermedia Center, Osaka University. We thank Dr Tobias Schindler for his experimental contribution in the early stage of the project.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3sc00974b

‡ Present address: Interdisciplinary Research Center for Catalytic Chemistry (IRC3), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Japan.

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