Metallosalen covalent organic frameworks for heterogeneous catalysis

Metallosalen covalent organic frameworks (M(salen)‐COFs) have garnered significant attention as promising candidates for advanced heterogeneous catalysis, including organocatalysis, electrocatalysis, and photocatalysis, due to their unique structural advantages (combining molecules catalysts and crystalline porous materials) and tunable topological network. It is essential to provide a comprehensive overview of emerging designs of M(salen)‐COFs and corresponding advances in this field. Herein, this review first summarizes the reported metallolinkers and the synthesis methods of M(salen)‐COFs. In addition, the review enumerates the excellent M(salen)‐COF based heterogeneous catalysts and discusses the fundamental mechanisms behind the outstanding heterogeneous catalytic performance of M(salen)‐COFs. These mechanisms include the pore enrichment effect (enhancing local concentration within porous materials to promote catalytic reactions), the three‐in‐one strategy (integrating enrichment, reduction, and oxidation sites in one system), and the incorporation of a built‐in electric field (implanting a built‐in electric field in heterometallic phthalocyanine covalent organic frameworks). Furthermore, this review discusses the challenges and prospects related to M(salen)‐COFs in heterogeneous catalysis.


| INTRODUCTION
Covalent organic frameworks (COFs) are crystalline, periodic, and porous organic materials formed by reversible covalent bonds between light elements such as C, O, B, N, S, and Si and so forth.3][4] In 2005, Yaghi's group reported the first COFs based on boronic ester linkages. [5]This discovery has trigged subsequent research focusing on the structural design and functional exploration.Regarding structural design, considerable attention has been dedicated to COF linkage invention.By now, at least 32 examples of COFs based on various linkage types have been reported, significantly enriching the diversity of COF structures. [6,7]The exploration of different linkages has also contributed to improved chemical stability of COF materials.For instance, COFs based on polyphenylene ether linkages, designated as JUC-505 and JUC-506 by Fang's group, exhibited superior chemical stability compared to the existing metal organic frameworks (MOFs), zeolites, and other COF materials. [8][17] Many works have indicated that COFs with metal coordination sites can serve as supramolecular ligands or carriers, enabling the anchoring metal atoms, ions, or nanoparticles to enhance the function of COFs. [6]Since metal species typically do not interfere with the construction of COFs, their impact on the crystallinity and topology of COFs is minimal.
][32][33][34][35] The structural design of M(salen)-COFs plays a critical role in determining their performance.Hence, it is imperative to conduct a thorough analysis of the structures of M(salen)-COFs, with a specific emphasis on the metal linker types, common heterogeneous catalytic applications, and a detailed investigation into the catalytic mechanisms.This is vital for the synthesis of innovative M(salen)-COFs.As illustrated in Figure 1, since Wang's group first reported salen based COFs in 2017, [36] M(salen)-COFs have been extensively developed.In terms of structural dimension, M(salen)-COF has evolved from its original two-dimensional form to a three-dimensional configuration.In terms of topology, it has shifted from the initially prevalent hcb network to dia and sql topological networks, featuring heteropore networks. [31,36,37]n this review, we focus on the synthetic strategy of M (salen)-COFs and summarize the metallolinkers and typical heterogeneous catalysis, and the underlying catalytic mechanism of M(salen)-COFs to date.In addition, we have assessed the challenges and prospects for the future development of M(salen)-COFs.We are of the opinion that M(salen)-COFs have the potential to revolutionize the field of heterogeneous catalytic technologies, thus contributing to a cleaner and more sustainable future.

| DESIGNED SYNTHESIS OF M (SALEN)-COFS 2.1 | Metallolinkers for different topological M(salen)-COFs
Metallolinkers play a crucial role in the construction of M(salen)-COFs, enabling the incorporation of metal nodes into the framework.Acting as emerging metallolinkers, M(salen) typically functions as a two-connected metallolinkers (Figure 2A,B,D,F).From a topological perspective, the coordination network stemming from M(salen) typically results in various distinct topological structures, including hcb, sql, dia, kgm, fxt, hxl, sod, cds, pcu, acs, and bcu. [38]However, currently, only M (salen)-COFs with hcb, sql, and dia topological networks have been reported.There is still much work to be done in exploring the design and synthesis of new topological M(salen)-COFs.From an experimental synthesis perspective, achieving the unreported topological structures of M(salen)-COFs is feasible.
As shown in Figure 2, there are numerous possibilities for metal nodes incorporating M(salen) segments, including common two-coordinate units (Figure 2A,B,D,F), four-coordinate unit (Figure 2E), [39] as well as three-coordinate units (Figure 2C). [30]Therefore, it is entirely possible to synthesize other unreported topological networks.The strategic design of topological networks for emerging M(salen)-COFs holds practical significance.By integrating M(salen) units as metallolinkers into the framework, they play a pivotal role in shaping the overarching architecture of COFs (or MOFs).These M(salen) units function as pivotal nodes, facilitating connections between organic linkers and offering coordination sites for other metal ions.Consequently, this approach can pave the way for the development of tailored materials with specific characteristics and diverse applications, including catalysis, [35] gas storage, [30,33] and sensing. [31]The outcomes achieved are contingent on the choice of metal within the M(salen) complex and the complementary organic linkers in the structure.The key to addressing these challenges lies in the thoughtful selection of alternative organic ligands and the implementation of intelligent synthetic strategies.For example, Gu's group achieved the successful synthesis of a Ni(salen)-COF (named Ni-COF) incorporating both hcb and kgm topological structures through the strategic combination of four-coordinate 1, 2, 4, 5-benzenetetraamine (BTA) and twocoordinate 2, 5-dihydroxy-1, 4-benzenedicarboxaldehyde (HBC) organic ligands. [32]The prepared Ni-COF, which features a composite hcb and kgm topological network and includes heavy metal atoms, further augments the spin-orbit coupling phenomenon.This observation aligns with the predictions of the Haldane model, suggesting the potential emergence of a Dirac cone at the K point within this specific structural context.This study provides a paradigm for the design and synthesis of conductive COFs.

| Synthetic strategies for M (salen)-COFs
Currently, the synthesis of M(salen)-COF primarily relies on two strategies.As shown in Figure 3, one approach involves the pre-synthesis of Salen-COF, followed by a subsequent metalation step to coordinate the targeted M(salen)-COFs.The other method employs a one-pot synthesis strategy to directly synthesize M(salen)-COFs.In 2017, Wang's group reported their pioneering work in the synthesis of Salen-COFs. [36]As shown in Figure 4, The obtained Salen-COFs was first synthesized via Schiff base condensation between F I G U R E 3 Typical synthetic strategies in the reported M(salen)-COFs, including (A) kill two birds with one stone and (B) one-pot synthesis.
In several subsequent reports of M(salen)-COFs, some cases have adapted this method for the designed synthesis of M(salen)-COFs.As described in Figure 3A, the conventional method for this strategy involves the condensation reaction of salicylaldehyde based ligands with ethylenediamine (EDA) through a Schiff base reaction, forming the Salen structure in a merged form.Subsequently, the Salen pocket coordinates with metal ions to form the desired M(salen)-COFs.In general, the crystalline structure of M(salen)-COF can be preserved after the process of metalation.For example, Wang's group reported the two-step synthesis of Zn(salen)-COFs.The obtained Salen-COF was first synthesized via Schiff base condensation between 1, 3, 5-tris[(5-tert-butyl-3formyl-4-hydroxyphenyl)ethynyl] benzene and EDA in a single step.Subsequently, high crystalline Zn(salen)-COFs was obtained from the metalation with Salen-COF and different metal ions (Zn 2+ , Co 2+ , Ni 2+ Mn 2+ , and Cu 2+ ).
Another alternative way to synthesized M(salen)-COF is one-pot synthesis.As shown in Figure 3B, the one-pot synthesis involves the simultaneous reaction of a ligand containing a salicylaldehyde group, EDA, and metal salt in the same reaction system, resulting in the synthesis of Zn (salen)-COF.Subsequently, through metal ion exchange, M (salen)-COFs can be further prepared.Cui's group reported the one-pot synthesis of chiral Zn(salen)-COFs.The obtained Zn(salen)-COF was synthesized via Schiff base condensation between 1, 3, 5-tris(3ʹ-tert-butyl-4ʹ-hydroxy-5ʹ-formylphenyl) benzene, EDA and Zn(OAc) 2 . [40]The resulting CCOF-4 exhibited enhanced chemical stability in 1 M HCl and 9 M NaOH and allowed for the metalation of multiple metals into the framework during the process of metal ion exchange.In contrast to the above-mentioned strategy, M(salen)-COF prepared via the one-pot method inevitably retains Zn elements within the framework and allows for two continuous rounds of metal ion modification.This work provides an example for the synthesis of multiple-metal Salen-COFs.In fact, some works have also reported the direct synthesis of non-zinc-based M(salen)-COFs materials.As shown in Figure 3B, M(salen)-COFs (M = Co, Ni, Pt, and Pd) have also been reported.The studies collectively provide strong guidance for the design and synthesis of M(salen)-COFs, directly advancing the research on M(salen)-COFs. [33]ecently, we successfully developed one-step synthesis for preparing M(salen)-COFs.This approach offers several benefits: it enables the production of 2.61 g of highly crystalline M(salen)-COFs in a single step, and the synthesis does not require the removal of oxygen from the solvent.This method simplifies the synthesis process of M(salen)-COFs and provides an efficient and practical approach for the preparation of M(salen)-COFs. [41]| TYPICAL APPLICATIONS FOR M(SALEN)-COFS

| M(salen)-COFs for organocatalysis
As a newly emerging metallo-covalent organic frameworks, M(salen)-COFs process the following unique advantages used as organocatalysts: (1) More than 12 different metals have been reported, and they also possess the potential to be incorporated into COFs.Abundant metal elements can be selected as metal active The synthesis of M/Salen-COF.Reproduced with permission. [36]Copyright 2017, American Chemical Society.
sites for organic reactions, including noble metal (Pt and Pd), [33] rare earth metals (Eu) and transition metals (Zn, Fe, Co, Ni, Cu, etc). [29](2) Chiral organic functional group was incorporated into COFs, further endow COFs with chirality. [40]When they are used in organic catalytic reactions, especially in asymmetric synthesis, they can enhance reaction activity, thereby simultaneously regulating reaction activity and enantioselectivity.(3) The reported M-COFs with various metal coordination sites, such as porphyrin, titanium phthalocyanine, bipyridine, and catechols, tend to predominantly form structures with a single metal active site.However, through judicious structural design, M(salen) has the capability to exhibit multiple configurations of active centers, which include monometallic, bimetallic, and even trimetallic synergistic metal active sites.Very recently, Niu's group discovered a binuclear synthetic Zn(salen)derived catalyst. [39]The reported binuclear catalyst transformed intermolecular nucleophilic reactions into intramolecular nucleophilic reactions, not only promotes the depolymerization of polyethylene terephthalate (PET) plastic under ambient conditions but also enables efficient chemical recycling of PET under mild conditions.This work also offers valuable insights for the structural design of M-COFs where multiple metals synergistically participate in reactions.
Up to now, even though M-COFs materials based on the above-mentioned dinuclear metal catalysts have been developed, the structural advantages of their bimetallic sites still need further exploration.Wang's group reported the first case for M(salen)-COFs based organocatalysts. [36]As mentioned in Figures 2A and 4, a Salenfunctionalized COFs were constructed via a "kill two birds with one stone" approach.When Co(salen)-COF was exampled in the Henry reaction, it behaved comparable catalytic activity and better recycle stability than homogeneous catalysts, Co(salen).In this work, the authors realized that the Henry reaction might not be the optimal method for evaluating the catalytic performance of achiral metallosalen COFs based organocatalysts.Meanwhile, the catalytic tests of M(salen)-COF in other typical reactions have not yielded satisfactory results.Additionally, by introducing chiral centers or altering the type of metal ligands, there is potential to enhance the catalytic performance of M(salen)-COF.
In 2017, Cui's group have constructed a series of chiral metallosalen COFs via one-pot synthesis (Figure 5). [40]he obtained CCOF-3 and CCOF-4 demonstrated a multifunctional organocatalysts for asymmetric cyanation of aldehydes, Diels-Alderreaction, alkeneepoxidation, epoxidering-opening reaction, and related sequential reactions.This work fully demonstrated the advantages of chiral M(salen)-COFs in organic catalytic reactions: adaptability to various substrates, excellent recyclability, and enantioselectivity up to 97%.It is noteworthy that both the chiral nature of M(salen)-COFs and the active metal center have a significant impact on organic catalytic reactions, including those involving carbon dioxide.In the aforementioned work, when different transition metals are chosen as the active centers, the types of reactions they can efficiently catalyze also vary.This conclusion is also corroborated by another example.Singh's group reported the synthesis of a chiral Mn lll -Salen-COFs via rapid microwave-promoted condensation and postsynthetic metal exchange reaction. [42]The obtained chiral Mn lll -Salen-COFs demonstrated high conversion for various chiral epoxides up to 72% ee.It should be underscored that chiral M(salen) homogeneous catalysts have demonstrated exceptional catalytic efficacy in the realms of asymmetric catalysis as well as photo-involved organic catalytic processes.Recently, Gilmour's group showed that chiral Al(salen) complexes can be used for the efficient photochemical deracemization of cyclopropyl ketones. [43]ccording to this work, the use of chiral Al(salen) complexes enables simultaneous regulation of reactivity and enantioselectivity by acting as photocatalysts in the photochemical deracemization of cyclopropyl ketones, further achieving up to 98:2 enantiomeric ratio.The chiral M(salen) complexes concurrently exhibit roles as both photocatalysts and chiral catalysts, thereby epitomizing a synergistic approach in catalytic processes.The adoption of this strategy holds significant merit for the design of M(salen)-COFs based photocatalysts intended for organic synthesis, given that their flexibly designed framework facilitates exacting donor-acceptor recognition motifs, pivotal for enantioselective photocatalyst design.
Another significant application of M(salen)-COFs in organic catalytic reactions is its catalysis of the reaction between epoxides and CO 2 . [44]This type of catalytic reaction also falls under an important topic: the chemical conversion of CO 2 .In 2013, Deng's group reported the first case for M(salen) based conjugated microporous polymers (CMPs). [45]The obtained Co(salen)-CMPs displayed excellent CO 2 adsorption capacities and exceptionally high catalytic activities toward the reaction between CO 2 and epoxides (propylene oxide).In this work, M(salen)-CMPs (M = Co and Al) combined the dual functions of pore enrichment and catalysis, achieving dual effects as well.Under ambient conditions, Co (salen)-CMPs promoted the coupling reaction with an 81.5% yield.Additionally, the synthesis of M(salen) CMPs facilitated the development of a class of porous materials incorporated with M(salen).Subsequent research led to the successful fabrication of M(salen)-COFs, with an indepth exploration of their capability to catalytically convert CO 2 into beneficial chemicals.In the reaction of catalytically converting CO 2 and epoxides into cyclic carbonates, the pore structure of M(salen)-COFs plays a pivotal role for the catalytic conversion of CO 2 under ambient conditions.Liu's group used Co(salen)-COFs as a heterogeneous catalyst for efficient cycloaddition of CO 2 and epoxides to cyclic carbonates, whose basic structure is similar as the first case reported by Deng's group. [35,45]The conversation of CO 2 to PC decreased approximately 10% under comparable conditions.Co (salen)-COFs behaved apparent mesopore structure with the pore size distribution at 2.06 nm.However, the pore size distribution of Co(salen)-CMPs mainly located at 0.5 nm, behaved a nanometer-sized pores.This result indicated the framework type containing M(salen) also affects the performance of this reaction.Yang's group also reported a series of M(salen)-COFs (M = Zn, Co, Mn, and Cu) via facile solvothermal synthesis under air. [46]his synthetic approach adeptly circumvents the intricacies associated with the requisite deoxygenation steps during the fabrication of M(salen)-COFs.In the cycloaddition reactions of epoxides with CO 2 , when compared with amorphous Co(salen) based porous organic frameworks catalysts, Co(salen)-COFs showed enhanced activity and recycling stability.
Meanwhile, it is essential to correctly understand the impact of the active metal type in M(salen) and M(salen)-COFs on the cycloaddition reactions of epoxides with CO 2 .Deng's group have proposed a catalytic mechanism for the cycloaddition of epoxide to carbon dioxide catalyzed by M(salen) (M = Co, Zn, and Al) based on density functional theory calculations. [47]This calculations results revealed that the highest reaction barrier for Co-Salen catalyst is lower than that of M(salen) (M = Zn and Al).The results revealed that M(salen) (M = Co, Zn, and Al) catalyzed reactions can proceed at ambient temperature and pressure, corroborating the experimental findings.This mechanism offered insights for designing innovative catalysts for such reactions.Based on this work, alterations in the central metal species of M(salen) did not markedly improve the performances of the CO 2 cycloaddition reaction.Yang's work has demonstrated that the type of M(salen)-COFs influences deeply on the The synthetic strategies of CCOF-3 (R = H) and CCOF-4 (R = CMe 3 ).Reproduced with permission. [40]Copyright 2017, American Chemical Society.performances of the cycloaddition reaction.The combination of Co(salen)-COF with Tetrabutylammonium bromide (TBAB) efficiently catalyzes the cycloaddition reaction, achieving a 91% conversion rate and exhibiting 99% selectivity towards propylene carbonate (PC). [46]owever, when the central metal of Co(salen)-COF was switched from cobalt to copper, the conversion of the catalytic reaction between CO 2 and epoxides dropped sharply by nearly 70%.Compared to the homogeneous M (salen) catalyst, the advantage of M(salen)-COFs lies in their unique layered crystalline structures inherent in their design, which effectively isolate the active sites and constrain their mobility, further enhanced cooperation between adjacent active sites in neighboring layers.
[50][51] However, the cost of industrial chemical fixation remains constrained by the high temperatures and pressures required in the catalytic process.Consequently, designing an efficient catalyst that operates at ambient temperature and pressure could offer a viable solution to reduce the current cost of carbon dioxide chemical fixation.M(salen)-COFs present a feasible scheme for the resource utilization of carbon dioxide catalytic conversion under ambient conditions.Nevertheless, the M(salen)-COF-based catalysts encompass a vast library of materials.With such abundant possibilities, there is no doubt that a better-performing material can be designed and developed.
Based on previous work, Deng's group utilized digital material design techniques to construct a structural database containing more than 10 994 types of M(salen)-COFs. [52]As shown in Figure 6, we selected 28 metal elements as the metal center of M(salen).Moreover, typical topological network, including hcb, sql, kgm, and dia network were constructed to build COF database.The 10 994 M(salen)-COF structures were optimized by molecular dynamic methods and the excess adsorption amounts of CO 2 for those COF structures were simulated.Subsequently, the average densities of CO 2 for those COFs with the high and low uptake were analyzed.For example, Hg 148 , Ti 107 , Os 117 , Pt 117 , Bi 107 -based Salen-COF exhibits the low excess adsorption amounts of 0.277, 0.297, 0.306, 0.309, and 0.312 mol g −1 .Meanwhile, Zn 113 , Fe 107 , Co 136 , Cr 107 , Mo 107 -based Salen-COF exhibits the high excess adsorption amounts of 3.990, 3.576, 2.488, 3.715, and 3.749 mol g −1 .By observing the enriched local concentration of CO 2 inside special pore structures, the "pore enrichment effect" was found and utilized as a basis for synthesizing COFs catalysts predicted to process the best CO 2 cycloaddition reaction activity.According to the simulated results, we found that there is a pore enrichment effect within the pore of M(salen)-COF.The pore enrichment effect refers to the specific adsorption of small gas molecules within the pore channel and interior functional groups of porous materials.These interactions significantly enhance the local concentration of CO 2 inside the pore channel, thereby promoting the catalytic reaction of CO 2 within the porous material.
Based on the simulation results, we synthesized Zn (salen)-COF with the theoretically predicted optimal pore enrichment effect (named Zn-Salen-COF-SDU113) and tested its performance in catalyzing the cycloaddition reaction between CO 2 and terminal epoxides under ambient conditions.As shown in Figure 7A,B, the Zn(salen)-COF was synthesized via Schiff base condensation reaction between TTHEPB ligands, EDA, and the following metalation with Zn(OAc) 2 .The identified Zn (salen)-COF processes a typical hcb topological network.This honeycomb feature was also observed by highresolution TEM images in Figure 7C,D.Above observation confirmed the identified Zn(salen)-COF was successfully synthesized.Figure 7E showed that at ambient temperature and pressure, Zn(salen)-COF achieved a remarkable CO 2 conversion yield of 97.3% and a TON value of 760.16, comparable to the best-known catalysts for similar reactions reported previously.Notably, Zn (salen)-COF pioneered the catalytic reaction between CO 2 and 2, 3-epoxybutane among porous materials under ambient conditions.Moreover, as shown in Figure 7F, the synthesized Zn(salen)-COF can maintain its stability for at least five cycles toward the CO 2 /propylene oxide coupling reaction.The experimental results validated the promoting role of pore enrichment effect in catalytic reactions within porous materials.This work also provided an effective strategy for designing low-cost catalysts for carbon dioxide chemical fixation.

| M(salen)-COFs for electrocatalysis
The utilization of water splitting for sustainable hydrogen production offers an appealing approach to realizing a carbon-neutral future in hydrogen energy. [53,54]However, the practical implementation and upscaling of this technology necessitate the presence of highly efficient electrocatalysts, which play a pivotal role in evolving cathodic hydrogen generation and anodic oxygen production.[57] The outstanding performance mentioned previously can be mainly attributed to the following advantages exhibited by MOFs, M-COFs, or M-CMPs when employed as electrocatalysts: (1) abundant coordination sites increase the content of coordinating metal species; (2) numerous unsaturated-coordinated metal active sites endow higher catalytic activity to porous materials; (3) two-dimensional structural properties inherent in COFs facilitate the complete exposure of catalytic centers, thereby promoting the efficient advancement of catalytic reactions.These intrinsic structural attributes render porous materials as highly promising candidates for electrocatalysis. [58,59]M(salen)-COFs inherit the structural advantages mentioned above.In this section, this review provides a summary of recent advancements in M(salen)-COF based electrocatalysts for The principle for constructing M(salen)-COF database and typical COFs with high uptake and low uptake.Reproduced with permission. [52]Copyright 2020, Springer Nature.
water splitting and CO 2 reduction reaction, while also emphasizing the benefits of these porous electrocatalysts.
[62][63] It offers a sustainable and environmentally friendly method for generating hydrogen.To enhance the activity of HER, efficient catalysts are frequently utilized, resulting in accelerating rates of hydrogen production and improving energy conversion efficiency.While platinum-based catalysts demonstrate optimal Gibbs free energy (ΔG *H ) and rapid reaction kinetics during the HER process, offering both efficient hydrogen generation and longterm stability, their relatively high cost due to platinum's precious metal nature presents a significant drawback.Furthermore, the limited availability of platinum resources raises concerns about potential supply constraints. [64]Additionally, Pt-based catalysts can occasionally promote the production of methane instead of pure hydrogen during the HER process. [65]As a result, the quest for excellent electrocatalytic HER catalysts necessitate finding a delicate balance between achieving high activity and stability while also considering the associated costs and resource limitations, addressing the drawbacks associated with Pt-based catalysts.Typically, HER occurs at the surface of electrode and mass transport within the electrocatalyst takes place through pore channel.M-COFs possess permanent pore structure (E) The yield of PC depending on the reaction time and (F) recycling stability of Zn(salen)-COF at atmospheric pressure and room temperature.Reproduced with permission. [52]Copyright 2020, Springer Nature.and a significant specific surface area, which facilitates electron transfer and offers ample space for the exposure of active sites.Furthermore, COFs can achieve enhanced electrocatalytic activity and stability during the HER through theoretical and experimental design.
In terms of electrocatalytic HER, there have been relatively few reports on M(salen)-COFs, and the structureperformance relationship remains unclear.Additionally, M (salen)-COFs-based electrocatalysts have encountered challenges such as poor electrical conductivity and a lack of inherent active sites.Therefore, enhancing the electrical conductivity of COFs-based electrocatalysts and exploring the influence of metal species and their coordination environment on active sites in HER are crucial for the development of high-performance M(salen)-COFs based electrocatalysts for HER electrocatalysis.
Very recently, Qiao's group reported a series of crystalline, stable M(salen)-COF (M = Zn, Cu, Ni, Co, Fe, and Mn) based electrocatalysts. [66]This work achieved significant progress by synthesizing highly crystalline Zn(salen)-COF through the condensation reaction of 1,3,5-tris(4′-hydroxy-5′-formylphenyl) benzene and ethylenediamine.The metal ions in Zn (salen)-COF were exchanged to modulate the band structure, resulting in a series of M(salen)-COF complexes, including Cu, Ni, Co, Mn, and Fe.Furthermore, the intricate integration of conductive poly(3,4ethylenedioxythiophene) (PEDOT) into the 1D pore channels of M(salen)-COF through in situ solid-state polymerization significantly improved the performance for the electrocatalytic hydrogen evolution reaction.The obtained PEDOT@Mn-Salen-COF EDA demonstrated excellent HER performance with an overpotential of 150 mV and a Tafel slope of 43 mV dec −1 , marking a noteworthy achievement in M(salen)-COF based HER electrocatalysts.This work holds significance in advancing the development of highly efficient COF-based electrocatalysts for the HER process.
In electrocatalytic applications, M-COF based electrocatalysts often suffer limitations due to their poor conductivity and a lack of intrinsic active sites.To overcome these challenges, several strategies have been developed: One approach is to carbonize the COFs material, producing carbon-based materials with abundant metal active sites.This approach presents both advantages and challenges.While it effectively enhances conductivity, it may degrade the inherent properties of M-COFs, resulting in the loss of their distinct pore structure and functional groups.Nevertheless, it establishes a fundamental basis for the design and synthesis of high-performance single-atom catalysts.
For example, Shan's group successfully synthesized Co(salen)-COF derived Co nanoparticles embedded in nitrogen-doped carbon. [67]The synthesized Co@N-C-800 exhibited an ORR half-wave potential (E 1/2 ) of 0.85 V, surpassing the benchmark Pt/C (E 1/2 = 0.84 V), and an overpotential (E η ) of 0.35 V at a current density of 10 mA cm −2 for OER, comparable to IrO 2 catalysts (E η = 0.36 V), while also demonstrating enhanced peak power density and cycling stability in Zn-air batteries compared to Pt/C-based counterparts.This softconfinement conversion strategy not only yields uniform metal nanoparticles embedded in porous carbons but also offers a remarkable noble metal-free multifunctional electrocatalyst suitable for ORR, OER, HER, and Zn-air batteries, achieved the effect of killing three birds with one stone.This approach was similarly employed in a study by Cui's group (Figure 8). [67]In their research, they fabricated a N-doped carbon catalyst embedded with Cu nanoparticles, approximately 30 nm in size, utilizing the spatial confinement properties of a nitrogen-rich Cu (salen)-COF.The optimized Cu NPs/N-C-800 displayed superior electrocatalytic ORR activity.Furthermore, the Zn-air battery prepared with Cu NPs/N-C-800 demonstrated a heightened peak power density of 163.5 mW cm −2 and maintained long-term cycling stability for 118 h.Distinctively, Qiao's group skillfully introduced conductive PEDOT into the pore structure of M(salen)-COFs via the in situ solid-state polymerization method (Figure 9), perfectly achieving the construction of an efficient electrocatalyst based on the weakly conductive M(salen)-COF. [68]bove progress clearly demonstrates that M(salen)-COF possesses several advantages.These include its capability to finely adjust coordination environments, continuous energy band structure modulation, modifiable pore structure, well-defined active sites, and its high crystallinity.These attributes make it an outstanding choice for exploring the structure-activity relationship of electrocatalysts.
The electrocatalytic OER is a critical half-cell reaction in water splitting.OER is a coupled reaction involving four electrons and four protons.The energy required for this (namely the high overpotential) results in the actual oxygen evolution potential being higher than the theoretical decomposition voltage of water splitting (1.23 V). [69] To enhance the efficiency of water electrolysis for hydrogen production, the design and synthesis of efficient OER catalysts are crucial.Currently, the most efficient OER catalysts are based on precious metals such as iridium and ruthenium oxides (e.g., IrO 2 and RuO 2 ).[71][72][73] Over the past few decades, a variety of porous materials, including MOFs, COFs, covalent triazine frameworks (CTFs), and CMPs have | 97 demonstrated remarkable efficiency in the electrocatalytic OER.For example, Tang's group reported NiCo bimetal-organic nanosheets. [74]When loaded on glassy carbon electrodes, NiCo-UMOFNs reached a current density of 10 mA cm −2 at an overpotential of 250 mV.When loaded on copper foam, its overpotential decreased to 189 mV.This study not only elucidates the fundamental processes of structural transformations during the OER, but also offers vital guidelines for the design of highly active OER electrocatalysts.Additionally, Kurungot's group presented a stable and efficient OER electrocatalyst. [57]The obtained Co-TpBpy-COF retains exceptional stability over 1000 cycles and 24 h of OER durability.Furthermore, we also presented a M F I G U R E 8 (A) Schematic diagram of the synthesis of Cu-Salen-COF and Cu NPs/N-C materials.(B) Open-circuitvoltage, specific capacity plot at a current density of 10 mA cm −2 , discharge polarization curve and power density curves, galvanostatic charging/discharging cycling curves of ZABs.Reproduced with permission. [67]Copyright 2022, American Chemical Society.ZAB, zinc-air battery.
(salen)-CMP based electrocatalyst. [75]The obtained Fe (salen)-CMP-3 exhibited an overpotential of only 238 mV at 10 mA cm −2 and low Tafel slope of 63 mV dec −1 during the OER process.This work demonstrated CMPs incorporating M(salen) can be used as OER electrocatalysts with outstanding performances.Thomas's group presented an efficient methods for producing crystalline and hierarchical M-COF-based electrocatalysts. [74]The synthesized macro-microporous Co-COF based electrocatalyst exhibited an overpotential 170 mV lower than that of the benchmark RuO 2 catalyst (Figure 10).This work confirms the significance of mass transfer in the electrocatalytic reactions involving metal-containing porous crystalline materials and provides a reference for the design and synthesis of macroporous M(salen)-COFs.
Besides the previously described strategy of carbonizing M-COFs to yield metal-containing porous carbon materials, an alternative approach involves the direct synthesis of conductive M-COFs for electrocatalytic applications.This method maintains enhanced conductivity while preserving the inherent structure of M-COFs.From a topological standpoint, the construction of conductive M(salen)-COFs presents significant challenges.Based on the COF database from our prior research, a 2-connected M(salen) unit typically forms M (salen)-COFs characterized by triangular and hexagonal pores, tetragonal pores, or a mix of triangular and hexagonal pores.Hence, the judicious selection of organic ligand configurations is crucial for the effective synthesis of conductive M(salen)-COFs.
Zang's group reported the synthesis of π-conjugated M (salen)-COFs nanosheets that demonstrated outstanding photocatalytic HER performances. [76]Cu-salphen-HDCOFs were synthesized via a Schiff-based reaction involving the well-known 2, 3, 6, 7, 10, 11-hexaiminotriphenylene (HATP), and 2, 6-diformylphenol (DFP) under conventional solvothermal conditions.As shown in Figure 11A, the sheet-like Cu-salphen-HDCOF microcrystal was derived through a crystallization and metalation process.By modulating the type of salicylaldehyde-based organic ligands, it is possible to further construct π-conjugated heteroporous COFs.Gu's group achieved the construction of heteroporous M(salen)-COFs. [77]As shown in Figure 11B,C, heteroporous COFs with hexagonal and quadrilateral pores were synthesized using HATP and 4, 6dihydroxy-5-methyl-1, 3-diformylbenzene (DMDB).Incorporating Ni(OAc) 2 enables the synthesis of Ni-Salphen-COF under analogous reaction conditions.This modification not only reduced the synthesis step for M(salen)-COFs but also minimized the loss of crystallinity in M(salen)-COF during metalation.The two examples of M(salen)-COFs did not exhibit any direct evidence of conductivity, despite The synthesis of M(salen)-COF EDA based electrocatalysts for the HER process.Reproduced with permission. [68]Copyright 2022, Wiley-VCH GmbH.possessing π-conjugated skeletons and long-range ordered channels.During the OER tests, the M(salen)-COF powder was used directly for electrocatalysis.According to the reported results, the prepared Ni-Salphen-COF electrode exhibits a small overpotential of 284 mV at the current density of 10 mA cm −2 .Additionally, by incorporating Fe ions into M(salen)-COF, the prepared NiFe-Salen-COF electrode exhibits enhanced catalytic activity, as evidenced by its minimal Tafel slope of 53 mV dec −1 .The improved Tafel slope indicates that the NiFe-Salphen COF electrode possesses faster reaction kinetics for water oxidation compared to IrO 2 catalysts.
This work provides direct evidence that such strong πconjugated frameworks are conductive.As presented in Figure 11E, the as-synthesized Ni-COF exhibits an electrical conductivity of 1.2 S cm −1 at room temperature, surpassing most previously reported COFs.
This study shows the structural benefits of employing M(salen) with a square-planar metal-coordination configuration as a linker in the synthesis of conductive M-COFs.This work also offers fundamental insights into conductive M(salen)-COFs.Researchers can justify the design of a conductive framework while incorporating variability in the quantity and ratio of functionality through strategic selection of building blocks.This approach can potentially give rise to Ni-COF with emerging properties that are greater than the sum of the individual M(salen) and π-conjugated ligands.For instance, these M(salen)-COFs proved suitable for CO 2 cycloaddition reactions.Liang's group presented analogous M(salen)-functionalized porous organic polymers (M = Mn, Co, and Ni). [78]The synthesized Co(salen)-POP achieved an impressive 96% yield and 99% selectivity in just 30 min.This also indicates that π-conjugated M (salen)-COFs are applicable for heterogeneous catalytic  [74] Copyright 2019, American Chemistry Society.
reactions.According to previous research, these M (salen)-COFs materials possess high specific surface areas and porosity, thus providing numerous reactive sites.Additionally, their π-conjugated structures can interact strongly with reagents/reactants, effectively catalyzing organic reactions.Moreover, these π-conjugated M(salen)-COFs possess robust thermal stability and chemical stability, enabling their use under severe reaction conditions, even in 3 M KOH, thereby affording them high potential significance in industrial applications. [32]dditionally, COFs can be mechanically mixed with conductive agents, resulting in composite electrocatalysts.With the addition of conductive materials, the electrocatalytic performances for the prepared electrocatalysts have been promoted.This strategy is primitive yet straightforward, effectively addressing the problem of poor conductivity in COFs when used as electrocatalysts.Deng's group have reported the acetylene black assembly M(salen)-COF based electrocatalysts and described a methodology for computationally screening M(salen)-COFs toward the electrocatalytic oxygen evolution reaction. [29]In this work, we screened over 100 catalyst models comprising basic catalytic units of M-N x O y (with M denoting 3d transition metals) by integrating density functional theory and machine learning.According to the predicted results, the overpotentials of 11 best performing model catalysts, including Fe- Utilizing renewable energy sources, such as solar and wind power, for electrocatalytic CO 2 RR, serving as an effective method for storing renewable energy.The electrocatalytic process allows CO 2 to be converted into valuable chemicals or fuels, promoting carbon recycling, and mitigating greenhouse gas emissions.Moreover, this method yields economically significant chemicals and fuels, including methanol, ethylene, and ethanol.Consequently, it presents a viable alternative for advancing a low-carbon energy system in the future, diminishing our reliance on fossil fuels.
Metallo-covalent organic frameworks were regarded as promising catalysts for electrochemical CO 2 conversions due to the combination of molecular and heterogeneous catalysts.Yaghi's group reported metal porphyrin based covalent organic frameworks. [79]The prepared Co-367-Co, which combines the advantages of cobalt porphyrin and π-conjugated COFs, exhibited a high Faradaic efficiency (90%) and turnover numbers of 290 000, which represents a 26-fold improvement compared to the cobalt porphyrin complex.
It should be mentioned that M(salen) complex have been confirmed to be a good molecular catalyst toward electrochemical carbon dioxane reduction.Verma's group introduced Ni(II) and Cu(II) salen complexes as electrocatalysts for CO 2 electroreduction for the first time. [77]In this study, M(salen) complexes proved effective in reducing the overpotential required for CO 2 RR.Additionally, a possible mechanism for the formation of reaction products using M(salen) catalysts was proposed.According to the research, during the electrocatalytic CO 2 RR process, the Ni(salen) and Cu(salen) complexes undergo multi-electron reduction, further producing C1 and C2 hydrocarbons as well as CO [80] .The Ni-Salphen complex exhibited a total faradaic efficiency of 74% at 1.5 V. Additionally, Zeng's group revealed that the high spin of 3d electrons in Co 2+ enhances the activation of CO 2 over Co(salophen) based catalysts. [81]Both Co-(salophen) and Co(salen) exhibit lateral coordination with CO 2 at the Co sites.Moreover, the bond strength between CO 2 and the Co site in Co-salophen is enhanced, leading in increased electrocatalytic activity.The synthesized Co (salophen)-Br demonstrated a faradaic efficiency of 98.5% for CO at −0.70 V versus the reversible hydrogen electrode.This efficiency is superior to those observed for Co(salophen)-Cl (64.8%) and Co(salophen)-I (81.8%).The above results clearly imply that M(salen)-COF is a highly promising electrocatalyst for CO 2 RR.
Lan's group introduced the pioneering instance of M (salen)-COF based electrocatalysts for CO 2 RR. [82]In this study, they developed a strategy to integrating bimetallic active sites into light-sensitive COFs and successfully constructed light-assisted electrocatalysts that are highly efficiency for CO 2 RR.Within this study, salphen pockets were incorporated into phthalocyanine-based COFs.The synthesis of NiPc-DFP-Co-COF was accomplished through a Schiff base condensation reaction involving 2, 3, 9, 10, 16, 17, 23, 24-octaaminophthalocyaninato nickel (II) (denoted as NiPc-8NH 2 ), and 2, 6-diformylphenol (denoted as DFP), followed by metalation with M(OAc) 2 (M = Co and Ni, Figure 12A).The resulting NiPc-DFP-Co-COF demonstrated a remarkable Faradic efficiency of nearly 100% for CO formation across a potential range of −0.7 to −1.1 V under light irradiation (Figure 12B,C).NiPc-DFP-Co-COF perfectly integrates the dual advantages of M(salphen) and NiPc functional heterometallic units, further demonstrating synergistic effects in the electrocatalytic CO 2 RR process.The resulting built-in electric field can effectively reduce the rate-determining energy barrier and enhance the electron density, thereby enhancing the activity of electrocatalytic CO 2 RR.It is worth noting that NiPc-DFP-Co-COF displays prominent π-conjugated structural features, aligning with the synthesis approach of the conductive Ni-COF mentioned-above.This serves as a further illustration of M(salen) integrated with a π-conjugated system to produce conductive M(salen)-COFs.

| M(salen)-COFs for photocatalysis
To date, a variety of technologies have been developed to convert CO 2 into hydrocarbons or high-value chemicals.In addition to the above-mentioned technologies for the organocatalytic and electrocatalytic CO 2 , photocatalytic CO 2 reduction is also considered as one of the most promising solutions to address global energy and environmental issues. [83,84]Photocatalytic CO 2 reduction can achieve the production of solar fuels and high-value chemicals under ambient conditions, such as methanol, ethanol, hydrocarbons, and other products. [85,86]ery recently, M(salen)-COFs were regarded as a promising candidate for photocatalytic process. [37,76,87]his is mainly attributed to the following structural properties of M(salen)-COFs.First, the permanent pore structure of M(salen)-COFs has unique gas adsorption characteristics, further resulting in the effect of pore enrichment.For instance, Mastalerz's group reported a series of three-dimensional isostructural metal salphen organic frameworks.The as-synthesized Cu-MaSOF 100  [82] Copyright 2023, Chinese Chemical Society.has a high O st of 31.2 kJ mol −1 for CO 2 and an uptake of 14.9 wt.% at 1 bar and 273 K, which exhibits a comparable selectivity of CO 2 /N 2 (S IAST = 52). [33]he pore enrichment effect is also vital for photocatalytic CO 2 RR.Zhang's group perfectly explained the effect CO 2 enrichment on photocatalytic CO 2 reeducation reaction and developed a three-in-one strategy of one-step diluted CO 2 reduction. [87]They reported a photocatalytic system that combines CO 2 enrichment, CO 2 reduction, and water oxidation active sites, which can be directly used for the reduction and conversion of low concentration CO 2 .First, as shown in Figure 13A, Zn-S-COF was firstly synthesized via Schiff base condensation reaction between 1, 3, 5-triazine-2, 4, 6-tris(4′-hydroxy-5′-formylphenyl)benzene, (1R, 2R)-(−)-1, 2-diaminocyclohexane, and Zn 2+ .Subsequently, the functional species, [Emim]BF 4 , was loading into the pore of Zn-S-COF.Thus, the resulted [Emim]BF 4 @Zn-S-COF processes enhanced selective absorption ability for diluted CO 2 , further improved the concentration of CO 2 inner pore channel (Figure 13B).As mentioned above, M (salen) complexes have been proved to be a promising candidate for photocatalytic CO 2 reduction reaction.Meanwhile, triazine based covalent organic frameworks has been confirmed to be effective to water oxidation and CO 2 reduction reaction. [88]Given the considerations, the synthesized [Emim]BF 4 @Zn-S-COF exhibits a CO 2 to CO conversion rate of 105.88 μmol g −1 h −1 under visible light irradiation.Moreover, under a diluted CO 2 concentration of 15%, it demonstrates a reduction rate of 126.51 μmol g −1 h −1 when driven by natural sunlight (Figure 13C).This study provides a novel strategy for the enrichment and in-situ conversion of industrial CO 2 emissions.
In evaluating the potential of porous materials for the capture and catalytic conversion of dilute carbon dioxide, it is crucial to consider the practical demands of industrial settings.Within chemical processing, gas absorption techniques for CO 2 are well-established and may present a more economical alternative compared to the synthesis of ionic liquids or porous frameworks.The comprehensive approach introduced by this work provides significant insights for academia.The implications and merits of converting low-concentration CO 2 deserve continued scrutiny.
F I G U R E 13 (A) Schematic illustration of the one-pot synthesis of Zn-S-COF and (B) the proposed three-in-one strategy.(C) Performance of photocatalytic CO 2 reduction to CO with diluted CO 2 (15% CO 2 and 85% N 2 ) for as-synthesized M(salen)-COFs.Reproduced with permission. [87]Copyright 2023, Wiley-VCHGmbH.
In fact, π-conjugated NiPc-M(salen) based frameworks is also favor to photocatalysis.Because of one of the processes of photocatalytic reaction involves the problem of the transfer efficiency of electrons after the separation of photogenerated electrons and holes. [82,89]ang's group reported an isostructural π-conjugated NiPc-based porous organic polymers and investigated its performance for photocatalytic CO 2 RR. [90]As shown in Figure 14A, a series of metal-containing POPs was synthesized via Schiff base condensation reaction between NiPc-8NH 2 , DFP and the following metalation with M(OAc) 2 (M = Co and Ni).As shown in Figure 14B, the synthesized NiPc-Co-POP exhibits apparent feature of isolated single metal atoms.Furthermore, NiPc-Co-POP shows apparent higher photocatalytic activity than NiPc-Ni-POP due to the calculated results NiPc-Co-POP (1.18 eV) delivered lower free energy than NiPc-Ni-POP (1.85 eV) at the rate-determining step of *COOH generation (Figure 14C,E).Compared with molecular catalyst, including (NH 2 ) 8 NiPc, Ni-salphen, NiPc-Ni-POP demonstrated better photocatalytic CO 2 RR activity and achieved remarkable CO production ability (7.77 mmol g −1 ) with a high selectivity of 96%.Significantly, through a comparison of M(salen) with other metal coordination units within the same framework, this study confirmed that M-N 2 O 2 -coordinated M(salen) or M-Salphen-based catalysts are more active than M-N 4 -coordinated NiPc-based catalysts (Figure 14D).These findings emphasize the distinct structural benefits of M(salen)-COF in contrast to other metal-containing COFs.
Additionally, through the fine-tuning of the connection of central metal and organic ligands, the rational design and adjustment of catalytic active sites (M-N 2 O 2 ) can be easily achieved.Since the type of central metal and organic ligands can be used to construct new M (salen)-COF photocatalysts, their spectral absorption range can also be adjusted through the design of metal sites and organic ligands or metallization.Moreover, to establish a well-organized framework favors charge separation and transfer is also an effective strategy for solar light conversion.
Photocatalytic hydrogen evolution reaction is the process of using photocatalysts to split water and produce hydrogen gas under light illumination. [90,91]This technology holds promise as a significant avenue for renewable energy production in the future, especially considering the potential of hydrogen as a clean energy carrier.Both photocatalytic hydrogen evolution reactions and electrocatalytic hydrogen evolution reactions have their unique advantages and limitations.4] Lotch's group developed crystalline hydrazone-based COFs and presented the pioneering work for COF-based visible light induced HER photocatalysts. [95]In the past decade, COFs have been widely explored for various photocatalytic applications.M(salen)-COFs can also serve as ideal platforms for HER photocatalysis.First, M(salen)-COFs are synthesized by employing predetermined metal salen units and other functional ligands, which favors the integrating of targeted functions.Second, altering the metal species and coordinated environments is beneficial for adjusting the selectivity and the types of products.Third, similar with other categories of COFs, π-conjugated characteristics and a well-defined long-range structural order endow the separation of photoexcited electrons and holes, while also enhancing charge carrier mobility. [96][99] The homogeneous cobalt-salen catalyst achieved a TON value of ~64 700 when combining CdS nanorods. [98]These studies suggest the potential of M (salen)-COFs as an efficient photocatalyst for hydrogen evolution reactions.Very recently, we proposed an effective strategy for photocatalytic hydrogen evolution reaction by integrating a metal salen molecular catalyst with a light harvester (pyrene) into covalent organic framework. [37]As shown in Figure 15A, M/Zn(salen)-COF were synthesized via Schiff base condensation between 5, 5ʹ, 5″, 5‴-(pyrenec-1, 3, 6, 8-tetrayl)tetrakis-2-hydroxybenzaldehyde (Py) and EDA in the presence of Zn(OAc) 2 •2H 2 O (Figure 15A, step i).Lately, the M/Zn (salen)-COF was obtained after the metalation with the corresponding metal acetate (Figure 15A, step ii).The synthesized M(salen)-COF exhibited a typical sql topology.The photocatalytic hydrogen evolution performance of M(salen)-COF exhibits apparent differences under the same catalytic conditions.As shown in Figure 15B,C, the optimized 6.81 wt.% Co(salen)-COF exhibited excellent hydrogen evolution activity with a production rate of 1378 μmol g −1 h −1 , which is superior to reported nonnoble metal-based COF photocatalysts.The superior photocatalytic activity can be assigned to the close connection between the Pyrene and the M(salen) in M (salen)-COFs and the formation of a conjugated system inside the COF that promotes the transfer and utilization of photogenerated charges.
This study indicates the importance of the conjugated linkage between light-harvesting moiety and photocatalytic active center for effective photocatalytic hydrogen evolution reaction, and further demonstrated the significance of heterogenization of molecular catalysts in covalent organic frameworks for photocatalysis.PROSPECTS We herein offer an overview of the recent developments of M(salen)-COFs, including their designed synthesis and their application in heterogeneous catalysis for water splitting and CO 2 conversion.However, it can be deduced from the existing work that research on M (salen)-COFs, encompassing structural design and performance exploration, is in its early stages.M(salen)-COFs perfectly inherit and enhance the dual benefits of both COFs and M(salen) complexes.While there has Reproduced with permission. [89]Copyright 2021, Wiley-VCHGmbH.
been advancement in the realm of heterogeneous catalysis with M(salen)-COFs, they still present certain challenges that require further investigation.
Design and synthesis of novel M(salen)-COFs structure remain essential but challenging.As discussed, up to date, only M(salen)-COFs exhibiting hcb, sql, dia, and heteropore topological networks have been reported, leaving numerous potential topological networks for M(salen)-COFs unexplored.Consequently, the structure-property relationships between these unexplored networks remain unclear.102][103][104] The structural determination of M(salen)-COFs currently relies on the combined use of techniques such as powder Xray diffraction (PXRD), high-resolution TEM, or singlecrystal electron diffraction.These methods have certain limitations, namely the stability challenges posed by highenergy electron beams and high vacuum pressures during the synthesis and testing of fragile M(salen)-COFs in these techniques.In terms of PXRD techniques, the strategy of using computer structure simulations and validating corresponding structure with PXRD often struggles to accurately depict the detailed stacking arrangement of synthesized two-dimensional M(salen)-COFs.Thus, acquiring the single-crystal structure of M(salen)-COFs will directly reveal the enigma regarding the intricate stacking patterns in two-dimensional M-COFs.
The synthesis of conductive M(salen)-COFs is still in need of further development.Currently, only a few potential conductive M(salen)-COFs have been reported, and only one case of M(salen)-COFs has provided evidence of conductivity. [32]The intriguing electronic structural properties, such as topological insulators, have not been thoroughly investigated behind it.Due to the low-temperature synthesis process of M(salen)-COFs, which only requires reactions in solution, it is possible to attempt the synthesis of high-quality M(salen)-COFs thin films based on the unique electronic structure mentioned above by optimizing organic ligand design.
One of the promising directions for advancing the development of M(salen)-COFs involves the creation of comprehensive databases encompassing their electronic properties, gas adsorption performance, and other relevant attributes. [105,106]This initiative leverages artificial intelligence techniques, including machine learning and automation and robotics technology, to enable the possibility of predictive modeling and automated synthesis of M(salen)-COFs.
The M(salen) complex, renowned for its outstanding catalytic performance as a homogeneous catalyst, has been the subject of extensive research.109] Consequently, M(salen)-COFs display remarkable Reproduced with permission. [37]Copyright 2023, Wiley-VCH GmbH.chemical stability and reusability, positioning them as promising materials for use in organic catalysis, electrocatalysis, and photocatalysis.The heterogenization of M (salen) molecular catalysts within covalent organic frameworks (COFs) provides a platform for improved stability, recyclability, controllability, activity, and expanded application scope.This methodology is applicable not only to M (salen) complexes but also extends to other prominent homogeneous catalysts, including N-heterocyclic carbenes and M(cyclen)-based catalysts, for the heterogenization in M-COFs.
In conclusion, M(salen)-COFs present a promising avenue for the design of high-performance heterogenous catalysts.Their tunable topological networks, remarkable catalytic activities, and potential for sustainable energy conversion make them attractive candidates for addressing pressing environmental and energy challenges in the coming years.With ongoing advancements in this field, M(salen)-COFs have the potential to revolutionize the field of heterogeneous catalytic technologies.

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I G U R E 7 (A and B) The synthesis of Zn(salen)-COF.(C and D) Transmission electron microscope (TEM) images for Zn(salen)-COF.

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I G U R E 10 (A) Schematic representation of macro-TpBpy-COF based electrocatalysts for the OER process.(B and C) Synthesis of macro-TpBpy-Co.(D) SEM image of macro-TpBpy.(E) OER performance of macro-COF-Co, including OER polarization curves and (F) corresponding Tafel plots.Reproduced with permission.
and Cu-N 2 O 2 -1, are smaller than 0.6 eV.After considering the feasibility of experimental synthesis, we integrated the Fe-N 2 O 2 , Co-N 2 O 2 , and Ni-N 2 O 2 moieties into COFs to prepare COF-based electrocatalysts for evaluating OER performance.Subsequently, a series of M-N 2 O 2 -salen-COFs (M = Zn, Fe, Co, Ni, and Cu) were synthesized via traditional solvothermal method and metalation.Notably, with the addition of acetylene black, the Ni-N 2 O 2 -salen-COF electrode showed an overpotential of 335 mV at a current density of 10 mA cm −2 and maintained electrochemical stability for at least 65 h.This work fully demonstrates the structural charm of M(salen)-COFs as OER electrocatalysts, further providing research insights for the efficient design of novel porous materials for electrocatalysis.

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I G U R E 12 (A) Schematic illustration of the synthesis of NiPc-DFP-Co COF.Evaluation of the electrocatalytic CO 2 RR performance: (B) linear sweep voltammetry curves, (C) FE CO , (D) Tafel plot and (E) j CO and turnover frequency for NiPc-DFP-Co COF at different potentials under dark and light conditions.Reproduced with permission.

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I G U R E 14 (A) The stepwise synthesis procedure of NiPc-MPOP and (B) corresponding HAADF images.(C) Comparison of CO 2 photoreduction activity of NiPc-2HPOP, NiPc-Ni-POP, and NiPc-Co-POP.(D) Schematic representation for the high CO 2 reduction activity of Ni-N 2 O 2 species compared to that of Ni-N 4 .(E) Calculated free energy diagrams for CO 2 RR of NiPc-2HPOP, NiPc-Ni-POP, and NiPc-Co-POP.

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I G U R E 15 (A) Schematic illustration of the stepwise synthesis procedure of M/Zn(salen)-COF and (B) photocatalytic H 2 production on Zn-Salen-COF and M/Zn(salen)-COF (M = Fe, Co, Ni).(C) Photocatalytic H 2 production on Co/Zn-Salen-COF with different Co contents.