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Elucidating the role of non-covalent interactions in unexpectedly high and selective CO2 uptake and catalytic conversion of porphyrin-based ionic organic polymers

Here, we present viologen-porphyrin based ionic covalent organic polymers (H2-ICOP and Zn-ICOP) with multiple CO2-philic sites.


Introduction
The rise of atmospheric carbon dioxide (CO2) concentration has become a widespread concern due to its link to the greenhouse effect and threat to the environment. 1 Therefore, viable solutions to efficiently capture and utilization of CO2 (CCU) have received much attention in the past decades. 2,3 Among various CO2 removal technologies, physical The high selective uptake of CO2 is known to arise from large surface areas and noncovalent interactions between CO2 and surfaces of materials. In the past decade, significant effort has been mainly focused on increasing surface areas of materials and the noncovalent interactions between CO2 and surfaces of materials have not attracted enough attentain. 14 One should note that large surface area is one of the parameters, but not the only one, that allows for high CO2 capture. However, materials with the low surface area may still exhibit large CO2 capture capacity with efficient non-covalent interactions with CO2. Therefore, it is vitally important to identify and estimate the strength of these non-covalent interactions in terms of contribution to CO2 capture capacity. 15,16 The CO2 is known to be able to form simultaneous non-covalent interactions via its electron-rich terminal oxygen atoms and its electron-deficient central carbon atom. 16,17 The electron-deficient central carbon atom acts as a Lewis acid (LA) and can form tetrel bonding interaction with anions or lone-pair-possessing atoms such as N, O, S, and F. [18][19][20][21] On the other hand, the electronrich terminal oxygen atom acts as a Lewis base (LB) and can form hydrogen bonding with hydrogen atoms and, as recently reported, halogen bonding with halogen atoms (I, Br, Cl) containing moieties. [22][23][24][25][26][27][28][29] In particular, the electron-rich nitrogen-based functional groups, such as amines, triazoles, triazine cores, imines, tetrazoles, benzimidazoles, azo linkages, and viologens, continues to be a dominant polar functional group to enhance CO2 adsorption capacity and CO2/N2 selectivity thanks to their multiple non-covalent interactions with CO2. 20,27,[30][31][32][33][34][35][36][37][38] Among nitrogen based adsorbents, viologen linked ionic covalent organic polymers (ICOPs) are of particular importance in CCU technologies because they not only have all the advantages of COPs but also possess charged cationic skeletons paired with counterions which provide additional CO2 affinity owing to their electrostatic interactions with CO2 molecules. 35,[39][40][41][42] Consequently, the CO2 uptake capacity of viologen linked ICOPs is found to be significantly higher compared to their neutral counterparts. 34,43,44 Moreover, unlike other porous materials, selective recognition of viologen linked ICOPs can be manipulated by simply controlling the redox state of viologen or the nature of counter-ions for the desired application. 39,41 In recent years, several cationic viologen linked ICOPs with good surface areas has been reported for CCU. 34,39,[43][44][45][46][47][48][49] The general understanding of these studies is that the high CO2 capture capacity of viologen linked ICOPs is credited mostly to tetrel bonding interaction between counter anions and CO2. 50 Commonly, nitrogen-based groups in CCU applications are preferred for their Lewis basic characters to form non-covalent interactions with electron-deficient central carbon (Lewis acid) of CO2. However, in the case of viologen, the quaternary nitrogen (N + ) atom lacks lone electron pairs, hence acts as Lewis acid, consequently, the formation of efficient interaction between cation (N + ) nitrogen and the electron-rich terminal oxygen atoms of CO2, commonly referred to as pnicogen bonds, is highly possible. 19,51 Yet, the pnicogen (OCO2···N + ) interaction is often disregarded, and its contribution to CO2 capture has not been reported.
However, despite their attractive features for CCU applications, only a limited number of viologen linked ICOPs has been reported in the literature mainly due to their low physicochemical stability. This drawback can be overcome by the incorporation of viologens into polymeric systems with suitable rigid and symmetric macrocycles such as porphyrin. 52 The incorporation of porphyrin brings three advantages, first, unstable cationic species get stabilized by extended delocalized polymeric structures. Second, porphyrin is another excellent N containing molecular building block, which can increase CO2 uptake and selectivity through CO2 interaction with pyrroles (-C=N, -NH). Third, the metalation of porphyrin core with Lewis acidic metal such as zinc (II) can promote additional noncovalent interaction with electron-rich terminal oxygen atoms of CO2, known as spodium bonding. 53 In addition to their multiple CO2-philic functional sides, both viologen and porphyrin are also known to exhibit effective CO2 catalytic conversion, which makes them very promising bifunctional materials in CCU applications. [46][47][48][54][55][56] Herein, based on the above considerations, we report viologen-linked porphyrin ICOPs (H2-ICOP and Zn-ICOP) synthesized through the Zincke reaction of H2 and Zn(II) tetrakis (4-aminophenyl) porphyrin with viologen Zincke salt, as presented in Scheme 1.

Synthesis and characterization
The  Figure S1). Washing the solids with the appropriate solvents is found to be crucial to remove unreacted starting materials and any smaller oligomers that may have formed. Therefore, the solids were washed with ethanol and DMSO several times until a colourless solution was obtained. The purification procedure was completed by washing with water and acetone.
Complementary methods were used for the detailed structural characterization of ICOPs. The formation of ICOPs is confirmed by Fourier-transform infrared (FTIR) spectroscopy and solidstate 13 C NMR. In FTIR (Figure 1), the decline of the characteristic -NH2 peak at 3345 cm -1 and cm −1 for H2-ICOP and 1656 cm -1 for Zn-ICOP indicate the loss of 2,4-dinitroaniline group and the formation of viologen-linked ICOPs. Besides, the peaks belonging to the porphyrin ring at 964, 790 cm -1 for H2-ICOP and at 993, and 794 cm -1 for Zn-ICOP confirm the formation of the expected materials. The same structures are obtained for H2-ICOP-dx and Zn-ICOP-dx with identical FTIR as can be shown in Figure S2. The solid-state 13 C NMR spectra ( Figure S3   and staggered morphology with an AB stacking sequence ( Figure S4d).
Owing to their multiple CO2-philic sides, despite their low surface areas, we explored the affinity of H2-ICOP and Zn-ICOP toward CO2 and N2. To evaluate their performance for CO2 capture, we carried out temperature-dependent CO2 uptake measurements at 273, 298, and 323 K up to 1 bar (Figure 4c, d)   To gain further insights from the non-covalent interactions between materials and CO2 molecules, the CO2 isosteric heat of adsorption (Qst) of H2-ICOP and Zn-ICOP was obtained with the Clausius−Clapeyron equation from the CO2 uptake isotherms recorded (Figure 4f). The Qst values for CO2 were found to be 39 and 36 kJ.mol -1 for H2-ICOP and Zn-ICOP, respectively, at zero coverage. This difference of Qst value for the two polymers shows that the non-covalent interactions of the CO2 molecules with the skeleton of H2-ICOP is more efficient than with Zn-ICOP, which may explain the higher CO2 capture capacity of H2-ICOP compare to Zn-ICOP. It is also worth mentioning that the Qst values of the polymers are moderate, not exceeding 50 kJ.mol -1 , which implies that CO2 is physically adsorbed by both ICOPs and the recyclability of the adsorbent at a low energy penalty. respectively. As expected, the H2-ICOP exhibits much higher selectivity compared to Zn-ICOP, which also indicates a powerful CO2 affinity of H2-ICOP.

The catalytic performance in CO2 conversion into cyclic carbonates
To evaluate catalytic activities of viologen-linked porphyrin H2-ICOP and Zn-ICOP, the cycloaddition of CO2 to propylene oxide was chosen as a model reaction and the results were listed in Table 3. The reactions were conducted under 1.0 MPa CO2 pressure and solvent and co-catalyst free condition at 80 °C (for details see ESI).
The compared catalyst Zn-ICOP and H2-ICOP gave the yields of 83% and 54%, respectively. Surprisingly, Zn-ICOP, despite having significantly lower CO2 uptake can be attributed to the synergistic effect of Lewis acid Zn site and a nucleophile counter anion (Cl -). As the zeta potential data revealed that Zn-ICOP possess counter anion (Cl -) on its surface, whereas H2-ICOP does not, which indicates the crucial role of the counter anion (Cl -) in the mechanism of this catalytic reaction. 46,47 Notably, the catalytic activity of Zn-ICOP was achieved with lowered temperature and CO2 pressure, in the solvent-free reaction system, and comparable to that of most of the reported examples (Table 3). Under the optimized reaction conditions, the catalytic activity of Zn-ICOP was further studied over a variety of epoxides. As shown in Table 4, various epoxides are efficiently converted into desired products with moderate yields under mild conditions by the Zn-

ICOP.
With the increase of the size of epoxides, a steady decrease in the yield of cyclic carbonates was observed from PO (83%), 1,2-epoxybutane (42%) to styrene oxide(4%), likely owing to their increasing steric hindrances, except for epichlorohydrin (77%) ( Table   4). The deviation of epichlorohydrin's yield from the trend, despite its larger size, is probably due to additional non-covalent interaction between chlorine functional group and CO2.

3.Conclusion
Two viologen-porphyrin based ionic covalent organic polymers were constructed by a one-pot facile Zincke reaction. The BET surface area analyses reveal that both H2-ICOP Overall, we have demonstrated here that ICOP with multiple CO2-philic sites can indeed exhibit very large CO2 capture capacity, CO2/N2 selectivity, and highly efficient catalytic activity of CO2 cycloaddition of epoxides, owing to efficient non-covalent interactions with CO2, in spite of low surface areas. These results set a useful example for the importance of non-covalent interactions with CO2 for the capture and conversion of CO2 into valueadded products. Therefore, we believe that these findings will prove to be of key importance to the design of bifunctional charged materials for CCU applications.