Reaction Mechanisms of Single Metal Site Catalysts Supported on Covalent Organic Frameworks

Reaction Mechanisms of Single Metal Site Catalysts Supported on Covalent Organic Frameworks Aswin Gopakumar,[a] Manuel A. Ortuño,*[b] and Julio Lloret-Fillol*[a, c] Wiley VCH Dienstag, 16.07.2024 2499 / 360106 [S. 1/17] 1 ChemCatChem 2024, e202400100 (1 of 16) © 2024 The Author(s). ChemCatChem published by Wiley-VCH GmbH ChemCatChem www.chemcatchem.org Review doi.org/10.1002/cctc.202400100 Covalent organic frameworks (COFs) have become a versatile platform to immobilize a wide variety of single-atom metal catalysts. The resulting post-synthetic modified materials present a spectrum of valuable properties ranging from homogeneous to heterogeneous systems, such as well-defined catalytic sites, selectivity, recyclability, and stability. In this minireview, we discuss selected contributions that provide experimental and computational details on reaction mechanisms (e.g., via EXAFS, TEM, and DFT) catalyzed by single-atom metals embedded within the COF structure. When applicable, we highlight the different behaviour between molecular (homogeneous) and COF-supported (heterogeneous) sites regarding catalytic performance. With this survey, we aim to decipher the key features that aid in seeing COFs as not merely passive supports but as active items in catalysis. 1. Introduction Reticular chemistry relies on the controlled assembly of synthetic subunits to design well-defined porous materials.[1] This approach allows for fine-tuning material properties, such as chemical composition, surface area, and pore size. Among the different families of porous materials, here we focus on covalent organic frameworks (COFs).[1] COFs are a class of crystalline, lightweight, and porous materials constructed through strong covalent bonding of organic building blocks (subunits). These frameworks are notable for their highly ordered structures, which can be two-dimensional or three-dimensional, tailored through the reticular synthesis of carefully chosen building blocks.[2,3] This unique molecular architecture facilitates precise spatial arrangement and robustness, making COFs ideal for a variety of applications such as gas storage, separation processes, and catalysis. Moreover, the large access to potential organic building blocks allows for fine-tuning of physical and chemical properties to meet specific needs, making them very versatile materials. These properties contributed to its rapid development in the last decade dramatically impacting all fields of chemistry,[4,5] and particularly, heterogeneous catalysis.[6,7] Covalent organic frameworks can be catalysts themselves[8] or act as catalyst supports,[9,10] hosting nanoparticles or anchoring molecular species. The precise control exerted into the framework allows for designing chemical environments suitable for post-synthetic modification and functionalization.[11] As such, these materials have become a great asset in preparing single-site metal catalysts.[12] These syntheses typically involve two pathways – (i) post-synthetic solvothermal metalation of the COF utilizing different metal precursors, or (ii) using, in the synthesis of the COF, metal complexes directly as COF subunits (Figure 1). When excluding building subunits that already contain metal atoms, such as porphyrins and phthalocyanines, the metalation usually occurs at the edges of the framework where functional groups are available, rather than the corners where aromatic units reside (Figure 1). Both pathways have advantages and problems, the former can lead to uncoordinated metal precursors while the latter can only be employed when metal complexes are stable under the usually demanding COF synthetic routes. However, regardless of the synthetic process, it is desirable to have a high reproducibility on the metal incorporation within the COF, including reproducibility on catalytic site density and distribution. Furthermore, the strategy should anticipate control over the metal complex coordination sphere resulting during the synthesis. Regarding the coordination and relative position of the well-defined metal centers within the framework, they can be classified as either corners or edges. Corners refer to the core structural units within the COF, typically consisting of multifunctional organic molecules acting as vertices where building blocks converge, thus defining the geometry of the framework. Edges, on the other hand, comprise the molecules that connect these corners, influencing the porosity and topology of the COF. This distinction underscores the local chemical environment as a pivotal factor in controlling the exposition of active catalyst sites. Therefore, here we will focus the attention on reported COFs where the metal sites are well-exposed. Moreover, such immobilization of molecular-like species into a solid framework merges the high selectivity and tunability of homogeneous catalysts with the stability and recyclability of heterogeneous systems (Figure 2). More interestingly, the COF does not frequently play an innocent role, thus [a] Dr. A. Gopakumar, Prof. J. Lloret-Fillol Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology (BIST) Avinguda Països Catalans 16, 43007, Tarragona, Spain E-mail: [email protected] [b] Dr. M. A. Ortuño Centro Singular de Investigación en Química Biolóxica e Materiais Moleculares (CIQUS), Universidade de Santiago de Compostela Jenaro de la Fuente s/n, 15782 Santiago de Compostela, Spain E-mail: [email protected] [c] Prof. J. Lloret-Fillol Institution for Research and Advanced Studies (ICREA) Passeig Lluís Companys 23, 08010, Barcelona, (Spain) Supporting information for this article is available on the WWW under https://doi.org/10.1002/cctc.202400100 © 2024 The Author(s). ChemCatChem published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution Non-Commercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. Figure 1. Schematic post-synthetic and direct metalation of COFs. Wiley VCH Dienstag, 16.07.2024 2499 / 360106 [S. 2/17] 1 ChemCatChem 2024, e202400100 (2 of 16) © 2024 The Author(s). ChemCatChem published by Wiley-VCH GmbH ChemCatChem Review doi.org/10.1002/cctc.202400100 18673899, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cctc.202400100 by Consorcio Interuniversitario D, Wiley Online Library on [05/08/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License new synergies between the metal active site and the support can be exploited to enhance catalysis. Indeed, the growth of COFs as platforms for single-atom metal catalysts is clearly demonstrated by their coverage in recent years. For instance, Guan et al.[13] reported a comprehensive review of the synthesis and applications of metalated COFs; Salemi et al.[14] collected Pd catalysts immobilized into COFs; Daliran et al.[15] gathered porous materials as catalysts for CH bond activation and functionalization; and Wang et al.[16] as well as López-Magano et al.[17] surveyed COF materials for photocatalytic applications. However, less attention has been devoted to the thorough examination of reaction mechanisms within these materials. It is typically assumed that the same or equivalent mechanism applies to molecular (homogeneous) and COF-supported (heterogeneous) catalysts, but that is not necessarily the case. For instance, from the phenomenological point of view, there are examples of frameworks that significantly modify the catalytic activity of the metal site, control the product selectivity, or shut down deactivation pathways.[18,19] Further understanding of these fascinating alterations of catalytic behaviour is essential to further exploit them. To this end, efforts are delving deeper into the mechanistic features of COFs, and a combination of both reaction experimental and computational techniques is often necessary (chemical reactivity, kinetics, IR, UV-Vis, EXAFS, TEM, and DFT, among others). In this minireview, we have selected recent contributions of single metal site catalysts anchored to COFs, focusing on those where the metal sites are at the edges of connecting subunits (Figure 1), which provide detailed insights into the role of the framework in the reaction mechanism. We have also considered COFs based on triazine units (CTFs) despite some of them presenting low crystallinity, and we have excluded COFs based on macrocyclic MN4metal sites, such as porphyrins and phthalocyanines, which usually are at the corners of the frames and have been covered somewhere else.[20–24] We present the main conclusions and discuss the role of the framework in the catalytic process. 2. Mechanistic Studies on Single Metal Sites in COFs Here, we collect selected contributions where detailed mechanistic information has been obtained through experimental and/or computational techniques. We divide them according to the type of reaction under study: (i) photo/electro CO2 reduction, (ii) oxygen evolution and reduction, and (iii) organic transformations. 2.1. CO2Reduction CO2photoand electroreduction is a topic of great interest nowadays. The fact that there are plenty of value-added CO2 reduction products implies a significant number of potential suitable catalysts and their fundamental understanding is of paramount interest. Single metal site COFs are seen as one of the latest generations of materials that can facilitate CO2 Prof. Julio Lloret-Fillol is currently an ICREA professor and group leader at ICIQ (Spain) since 2014, where he is involved in homogeneous and heterogeneous photoand electrocatalysis research. He obtained his PhD from the University of Valencia (Spain) in 2006 and then moved to the University of Heidelberg (Germany) as a MEyC and later as a Marie Curie postdoctoral fellow before joining as a Ramon y Cajal group leader at the University of Girona and then at ICIQ. Dr Aswin Gopakumar is a Marie Curie postdoctoral fellow at ICIQ (Spain) since 2023, where he is involved in heterogeneous photo and electrocatalysis research. He obtained his PhD from EPFL (Switzerland) in heterogeneous catalytic CO2reduction research in 2018. Afterwards, he moved to the University of Antwerp (Belgium) as an IOF postdoctoral fellow until mid-2022. In late 2022, he joined ICIQ for another postdoctoral stay. Dr Manuel A. Ortuño is currently a Distinguished Researcher at CIQUS in Universidad de Santiago de Compostela (Spain) since 2020, where he is involved in the simulation of homogeneous and porous chemical systems for catalysis. He obtained his PhD from the Universitat Autònoma de Barcelona (Spain) in 2014, and then moved to the University of Minnesota (USA) in 2015 and ICIQ (Spain) in 2018 for postdoctoral stays. Figure 2. Heterogenization of molecular species in solid frameworks. Wiley VCH Dienstag, 16.07.2024 2499 / 360106 [S. 3/17] 1 ChemCatChem 2024, e202400100 (3 of 16) © 2024 The Author(s). ChemCatChem published by Wiley-VCH GmbH ChemCatChem Review doi.org/10.1002/cctc.202400100 18673899, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cctc.202400100 by Consorcio Interuniversitario D, Wiley Online Library on [05/08/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License reduction through electrochemical and photochemical methods. Figure 3 shows various single-site metal-loaded COFs (MCOFs) used in CO2reduction reactions (CO2RRs). Lu et al. reported a series of metal-loaded (Co, Ni, and Zn) based on DQTP (2,6-diaminoanthraquinone-2,4,6-triformylphloroglucinol, Figure 3a) for the photocatalytic CO2RR to produce HCOOH or CO using [Ru(bpy)3]Cl2(bpy=bipyridine) as photosensitizer and triethanolamine as an electron donor.[25] The Co variant exhibits a high selectivity towards CO production with a rate of 1020 μmol h1g1whereas the Zn variant was found to selectively produce HCOOH with a production rate of 153 μmolh1g1. The Ni variant produces intermediate selectivity and yields. The study further investigates the integration of earth-abundant metals into COFs for photocatalytic CO2RR. Using density functional theory (DFT), it focuses on the interaction between Zn and the DQTPCOF structure. It reveals that Zn binds with quinone oxygen atoms in the COF, leading to stronger bonding and altered interlayer distances, indicative of coordination bonding. This mechanism is crucial for enhancing the semiconductor properties of the COFs, thereby improving their efficiency in photocatalytic CO2RR. Transition metals such as Co(II), Ni(II), and Zn(II) were successfully embedded into the frameworks, significantly affecting physical properties such as pore size and surface area, with metal content quantified around 6–7 wt%. The integration of metals was experimentally confirmed through X-ray photoelectron spectroscopy (XPS) and supported by DFT calculations, which identified potential binding sites and indicated strong interlayer interactions. Stability assessments included thermogravimetric analysis (TGA) and microscopic analyses (SEM and TEM), which demonstrated robustness and preservation of the COF structure post metalloading. Furthermore, photocatalytic performance tests indicated that these materials, particularly DQTP COFCo, maintained high catalytic activity and structural integrity even after several recycling processes, underscoring their durability and functional stability. Likewise, Zhong et al. proposed single Ni sites within the COF derived from triformylbenzene and diaminobipyridine (Figure 3b) that could achieve a CO production rate of 4057 μmolg1in a 5 h reaction with a 96% selectivity over H2 evolution under photocatalytic conditions.[26] For the photocatalytic studies, [Ru(bpy)3]Cl2was the photosensitizer and triethanolamine was the electron donor employed. In addition, bpy was also added to the reaction. The authors proposed based on DFT calculations that (N4)Ni (N2from the COF and N2 from the bpy) was the active site, but no direct spectroscopic evidence was reported. Ni-TpBpy was synthesized by treating TpBpy with Ni(ClO4)2·6H2O in acetonitrile, resulting in a Ni content of 0.3 mmolg1. Ni ions were confirmed via Ni(ClO4)2 treatment, evidenced by XPS and energy-dispersive X-ray spectroscopy (EDX) mapping which showed Ni2+peaks and homogeneous Ni distribution. Metal binding sites are experimentally validated using HAADF-STEM, displaying bright spots indicative of atomically dispersed Ni centers. The framework‘s stability is demonstrated through repeated photocatalytic cycles, maintaining structural integrity and chemical consistency, as shown by minimal changes in Fourier transform infrared (FTIR) and XPS spectra after use, confirming both the robustness and efficiency of Ni-TpBpy in catalytic applications. Das et al. reported a novel Co(II)-loaded COF based on triformylphluroglucinol and trihydrazinotriazine (Figure 3c) that can fix CO2to amines to generate N-formylated products under visible light with a TON of 114.[27] This catalyst system offers excellent catalytic activity, selectivity, and recyclability at least up to 5 times for the sustainable one-pot synthesis of Nformylated products from CO2. The Co(II)@Tp-TH COF photocatalyst features a cobalt(II) loading of 4.9 wt% as determined by atomic absorption spectroscopy (AAS). Metal binding sites within this material were confirmed using X-ray photoelectron spectroscopy, which showed specific binding energies indicative of cobalt interacting with nitrogen atoms in the framework, and FTIR, where shifts in NH stretch bands suggested interactions between cobalt ions and nitrogen groups. The stability of the catalyst was evidenced by its thermal robustness up to 370°C, as shown by TGA, and its ability to maintain structural integrity and performance after multiple uses, confirming its suitability for practical applications in catalysis. Cheng et al. reported another Re-loaded COF (ReCOF) based on trimethyltriazine and bis(formyl)bipyridine (Figure 3d) that can give photocatalytic CO production rate (190.6 μmolg1h1with about 100% selectivity) and O2evolution (90.2 μmolg1h1) among all the porous catalysts in CO2 reduction with H2O as sacrificial agents.[28] DFT calculations show the catalytic action of Re-complex in COFs, creating a ligand-to-metal charge-transfer channel. Photon absorption triggers charge transfer from the triazine ring to Re-bpy in ReCOF, efficiently separating HOMO and LUMO. This separation aids CO2reduction via excited electrons moving to Re-bpy, and H2O oxidation through holes in the triazine ring. For CO2 reduction in ReCOF, CO2is activated by Re, forming *COOH through a proton-coupled electron transfer. The formation of *COOH, the key step, has a 0.71 eV energy barrier. *COOH then transforms into *CO, releasing CO. In H2O oxidation, the process starts with H2O dissociating into *OH, facilitated by N in the triazine ring. *OH turns into *O, which combines with H2O to form *OOH, the crucial step with a 1.73 eV barrier. O2is finally produced after proton removal from *OOH. The MCOF features a metal loading of 13.5% by weight, quantified using ICP-OES. Experimental confirmation of metal binding sites within the COF was achieved using FTIR spectroscopy, solidstate 13C NMR, and XPS, which verified the structural integration and chemical state of the rhenium complexes. The material demonstrated excellent stability and durability, maintaining high catalytic efficiency over 50 h of operation and retaining about 90% of its initial activity after multiple cycles, showcasing its robustness in photocatalytic applications for CO2reduction. Zhang et al. reported a Cu(II)-loaded COF based on tris(aminophenyl) triazine and bipyridine dicarboxaldehyde (Figure 3e, Cu-BpyCOF) that can demonstrate significant improvements in photocatalytic CO2reduction.[29] The Cu-BpyCOF enhanced CH4yield to 70.0 μmolg1and CO yield to 6.4 μmolg1, compared to 45.9 μmolg1and 13.0 μmolg1, respectively, for pristine BpyCOF. The best CH4evolution rate was achieved with 10 mg of Cu-BpyCOF. When using DMF as a Wiley VCH Dienstag, 16.07.2024 2499 / 360106 [S. 4/17] 1 ChemCatChem 2024, e202400100 (4 of 16) © 2024 The Author(s). ChemCatChem published by Wiley-VCH GmbH ChemCatChem Review doi.org/10.1002/cctc.202400100 18673899, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cctc.202400100 by Consorcio Interuniversitario D, Wiley Online Library on [05/08/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Figure 3. Single-site metal-loaded COFs used in CO2RRs. n=0, 1, 2, etc. Wiley VCH Dienstag, 16.07.2024 2499 / 360106 [S. 5/17] 1 ChemCatChem 2024, e202400100 (5 of 16) © 2024 The Author(s). ChemCatChem published by Wiley-VCH GmbH ChemCatChem Review doi.org/10.1002/cctc.202400100 18673899, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cctc.202400100 by Consorcio Interuniversitario D, Wiley Online Library on [05/08/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License solvent, CO production reached 40.8 μmolg1with nearly 100% selectivity, and up to 90.0 μmolg1with 66.4% selectivity at a 50% DMF ratio. Cu-BpyCOF showed a higher CO2uptake of 14.7 cm3g1than BpyCOF (9.2 cm3g1). DFT calculations revealed that the LUMO of Cu-BpyCOF mainly localized on CuBpy moieties, facilitating charge separation and enhancing photocatalytic performance. This study highlights the pivotal role of single Cu sites in CO2adsorption and reduction, offering nearly 100% selectivity for CO or CH4by modifying the reaction media. Triethylamine, serving as both hole sacrificial agent and proton transfer catalyst, further boosts the CO2reduction efficiency, underscoring the potential of COF-based photocatalysts with non-noble metal single sites for selective CO2 reduction. The Cu-BpyCOF material incorporates single Cu sites with a metal loading of about 8.3 wt%, confirmed using techniques such as FT-IR, solid-state NMR, XPS, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and XAS, which validated the coordination of Cu to nitrogen sites and detailed the electronic and atomic environment. Stability assessments through TGA showed the framework‘s thermal stability up to 500°C, and PXRD confirmed the maintenance of its crystalline structure post-metal incorporation. Photostability tests indicated minimal Cu leaching, with the structure and catalytic performance of the COF remaining robust across multiple cycles of CO2reduction, demonstrating good operational stability. A Re-loaded COF based on tetrakis(4-formylphenyl)pyrene and bis(cyanomethyl) bipyridine ligands (Figure 3f) was investigated by Fu et al. that can achieve a maximum rate of 1040 mmolg1h1for CO production with 81% selectivity.[30] CO production rates were further increased up to 1400 mmolg1h1, with an improved selectivity of 86% when a photosensitizer was added. Photophysics of the system was studied by transient absorption spectroscopy (TAS). The COF was chemically modified by loading with 18 wt% of the rhenium complex [Re(CO)5Cl], which ligated half of the available bipyridine sites within the framework. This metal integration was experimentally verified using PXRD and STEM combined with EDX, which confirmed the uniform distribution and structural incorporation of Re. Further evidence was provided by FTIR spectroscopy, indicating characteristic CO-stretching bands of the Re complex. The material demonstrated good stability under photocatalytic conditions, retaining crystallinity and functionality over extended periods of visible light illumination, although a decline in crystallinity and activity was noted after 50 h, pointing to challenges in long-term stability. In solution, excitation upon irradiation forms a metal-tobipyridine excited state (3MLCT) which is then quenched by an electron donor.[31] However, the COF differs from the homogenous catalysts. The authors proposed a pyrene excitation to a bipyridine-based LUMO. The presence of the Re center within the COF leads to the formation of a long-lived charge-separated (non-emissive) state. Bandomo et al. reported a single-site MnCOF based on (pyrenetetrayl)tetraaniline and bipyridyl dialdehyde (Figure 3g) that shows a low CO2RR onset potential of 190 mV and high current densities (>12 mA·cm2, at 550 mV overpotential) in water (Figure 4).[32] TOFCO and TONCO values are as high as 1100 h1and 5800 (after 16 h), respectively, which are more than 10-fold higher than those obtained for the equivalent Mnbased molecular catalyst reported until then. IR and extended X-ray absorption fine structure (EXAFS) spectra were used to identify the coordination of the Mn resulting equivalent within the COF to the homogeneous complex in solution {fac-Mn- (N2)(CO)3S; S=solvent}. Catalytic intermediates were characterized and in situ monitoring of electrochemically active COF by ATR-IR spectroelectrochemistry. Although ATR-IR-SEC can serve to elucidate metal CO2RR intermediates, this was the first time employed in the study of a COF, and it is still rarely employed. In addition, DFT calculations were also essential to identify the key intermediates. Noteworthy, the authors proposed that the COF imposes mechanical constraints on the {fac-Mn(CO)3S} centers, offering a strategy to avoid forming the detrimental dimeric Mn0Mn0, which is a resting state typically observed for the homologous molecular complex. The absence of dimeric species correlates with catalytic enhancement. The COFbpyMn demonstrates a high Mn loading of approximately 85%, confirmed using ICP-OES. The metal binding sites within the COF were meticulously characterized through FTIR, XPS, and XAS, ensuring accurate metal incorporation and coordination. Additionally, the stability of the material was validated through TGA and PXRD, which established its robustness and consistent catalytic activity, further evidenced by stable performance in extended cyclic voltammetry (CV) tests. These attributes underscore the material‘s effectiveness for electrocatalytic applications. Recently, the same group reported another Mn-based COF specifically COFbpyMn (with bipyridine linker, as in the previously mentioned one) and COFPTMn (with phenanthroline linker), tailored for electrocatalytic CO2reduction.[33] COFbpyMn stands out with superior electrochemical CO2reduction activity due to its high crystallinity and porosity, significantly enhancing catalytic efficiency over its molecular analogs non-covalently immobilized on carbon electrodes. COFPTMn, which employs a larger phenanthroline linker, demonstrates heightened catalytic activity at near-neutral pH but is hindered by reduced crystallinity and porosity, impacting overall performance. The electrochemical testing reveals that COFPTMn achieves a CO2 reduction onset potential of 1.0 V vs SCE and produces CO as the predominant product with a Faradaic efficiency of 30% for CO and 15% for formate. These frameworks stabilize Mn(I) radical anion intermediates, effectively preventing Mn0Mn0 dimer formation through the electronic and steric features of the COF structure. This setup promotes a highly efficient CO2 reduction mechanism influenced by the COFs’structural properties, resulting in enhanced catalytic activity. The metal loading was quantitatively determined using ICP-OES, revealing that 38 mg of Mn per gram of COF was incorporated, accounting for the metalation of approximately 36% of the available phenanthroline sites. The metal binding sites were experimentally confirmed via ATR-FTIR spectroscopy, which displayed characteristic CO stretching frequencies at 2027 and 1922 cm1, indicative of {Mn(CO)3Br} moieties in a facial coordination geometry within the COF backbone. Stability assessments through TGA showed two major decomposition steps, with the Wiley VCH Dienstag, 16.07.2024 2499 / 360106 [S. 6/17] 1 ChemCatChem 2024, e202400100 (6 of 16) © 2024 The Author(s). ChemCatChem published by Wiley-VCH GmbH ChemCatChem Review doi.org/10.1002/cctc.202400100 18673899, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cctc.202400100 by Consorcio Interuniversitario D, Wiley Online Library on [05/08/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License initial step likely due to the release of CO ligands from Mn centers, and PXRD patterns confirmed the crystalline integrity post-metalation, though with reduced crystallinity compared to other analogs. These elements highlight the COF’s structural design tailored for enhanced CO2reduction performance. Zou et al. presented a study on enhancing CO2photoreduction using one-dimensional COFs synthesized by the Schiff base condensation of pyrene tetrayltetraaniline (PyTTA) and 2,2’bipyridine-5,5’-dicarbaldehyde (2D COFs, Figure 3g) and bis[p-(formyl)phenyl] phenanthroline (1D COFs, Figure 3h) supporting Co(II) sites.[34] They found that 1DCOFs, especially PyTTACOF, have superior CO2photoreduction capabilities compared to 2D versions. This enhancement is linked to the edge microstructures in 1DCOFs, which stabilize the structure, minimize energy loss during exciton dissociation, and facilitate effective charge transfer. Specifically, the 1D-PyTTACOF, when combined with Co ions, achieved an impressive CO evolution rate of 1003 μmolg1in 8 h under visible light. This research not only offers a solution to charge transfer issues in COFs but also underscores the importance of controlling dimensionality Figure 4. DFT energies for the (a) endergonic dimerization mechanism process within the COF material and (b) optimized structures for COFbpyMn2A1 and COFbpyMn2-A3; comparison of CVs at 100 mV·s1(c, e) and ATR-IR absorbance potential dependent spectra of COFbpyMnjNT (d) and the model 1jNT (f) in water (0.5 M NaHCO3) under Ar at room temperature; (g) schematic representation of the ATR-IR-SEC cell and (h) the proposed catalytic cycle for COFbpyMn in the based on computational models and spectroelectrochemical data. Reprinted with permission from Ref. [32] Copyright 2021, American Chemical Society. Wiley VCH Dienstag, 16.07.2024 2499 / 360106 [S. 7/17] 1 ChemCatChem 2024, e202400100 (7 of 16) © 2024 The Author(s). ChemCatChem published by Wiley-VCH GmbH ChemCatChem Review doi.org/10.1002/cctc.202400100 18673899, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cctc.202400100 by Consorcio Interuniversitario D, Wiley Online Library on [05/08/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License to boost photocatalytic efficiency. Additionally, DFT was employed to compare the electronic properties of 1DPyTTACOF to its 2D counterpart. These calculations show a distinct distribution in the HOMO and LUMO of the 1DPyTTACOF, contributing to its enhanced photocatalytic activity. Calculations of binding energy and systemic energy, based on the dihedral angle between interlayer benzene rings, provide further understanding of the stability and interactions within the COF structure. These theoretical findings corroborate the experimental results, emphasizing the pivotal role of edge microstructures in advancing CO2photoreduction. The presence of cobalt was validated through XPS and EXAFS, confirming the CoN/O coordination within the frameworks. Stability assessments revealed that 1D PyTTACOF displayed superior chemical and thermal stability, maintaining structural integrity and photocatalytic activity even after exposure to harsh chemical environments and repeated usage cycles. This robustness, combined with effective metal incorporation, highlights the potential of these COFs, particularly the 1D PyTTACOF, in photocatalytic applications for CO2reduction. Qiu et al. designed imine-linked COFs based on triazinetriyl trianiline and triformylphloroglucinol (Figure 3i) as the electron bridge that links the photocatalyst and the robust metal Co(II) active sites for photocatalytic CO2RR.[35] When integrated with defective g-C3N4, the composite generated 37.3 mmol·h1of CO with 98.8% selectivity over H2evolution under visible light irradiation, which greatly outperformed other non-noble metal species as co-catalysts. The coupling of Co with COF ligands was analyzed with DFT, revealing that NCoO (0.54 eV) has a lower formation energy compared to NCoN (1.43 eV) and OCoO (0.83 eV). This supports the experimental findings where C=CN and C=O groups in the bare COF preferentially form stable six-membered chelating rings with Co(II). The metal binding sites are confirmed through XPS, TEM, HAADF-STEM, and supported by DFT simulations, indicating preferential binding of Co to specific nitrogen and oxygen sites. The stability of these materials is demonstrated through consistent performance over multiple cycles of CO2photoreduction, highlighting their potential for practical photocatalytic applications. Yang et al. reported a Ni-modified COF composed of Nacylhydrazone-linked electron-donor and electron-acceptor dyads (HCOFNi, Figure 3j) that can generate 5694 μmolg1of CO with 96% selectivity over H2evolution in 2 h under visible light irradiation, which greatly outperforms that of the typical imine-linked counterpart (ICOFNi).[36] DFT calculations were conducted through the carboxyl intermediates step. The CO2 molecule presents the bent configuration after interacting with Ni centers, showing that the activation of CO2molecules could be realized on Ni active sites. Notably, the Ni–C bond length (3.074 Å) for HCOFNi is shorter than that of ICOFNi (3.277 Å), indicative of stronger binding interaction between CO2molecules and Ni active sites in HCOFNi. The adsorption energy of CO2on Ni is 0.58 eV for HCOFNi, which is more negative than that of ICOFNi (0.28 eV). Such a large difference in the adsorption behaviour of CO2over Ni sites is mainly attributed to local microenvironments enabled by the linkages of HCOF and ICOF. It is generally accepted that stronger CO2adsorption is favourable for the activation of CO2 and the stabilization of metal-CO2adducts. HCOFNi was characterized for its metal content, metal binding sites, and stability using various techniques. Metal loading was determined to be 0.050 mmolg1using ICP-AES. The metal binding sites were confirmed through XRD and FTIR, which indicated the incorporation and coordination of nickel ions within the framework. XPS further supported these findings by showing changes in binding energies. Stability assessments demonstrated that the material maintained its catalytic activity and selectivity over several cycles, with no significant changes observed in the FTIR spectrum, XRD pattern, or the highresolution Ni 2p XPS spectrum, indicating the material‘s good durability under the tested conditions. Yang et al. reported a Zn-Salen-based COF (ZnSCOF) made from diaminocyclohexane and triazine tris(hydroxyformylphenyl)benzene (Figure 3k) that can drive a CO2-to-CO conversion rate of 105.88 μmolg1h1under diluted CO2(15%) atmosphere.[37] Moreover, the natural sunlight-driven diluted CO2reduction rate also reaches 126.51 μmolg1in 5 h. Further experiments and theoretical calculations reveal that the triazine ring in the ZnSCOF promotes the activity of H2O oxidation and CO2reduction sites, and the loaded ILs provide an enriched CO2atmosphere, realizing the efficient photocatalytic activity in diluted CO2reduction. The ZnSCOF contains approximately 7.20 wt% zinc, determined through ICPOES. Experimental confirmation of the zinc-binding sites was achieved using XAS, including X-ray absorption near-edge structure (XANES) and EXAFS analyses. These studies revealed that zinc is stably coordinated by two nitrogen and two oxygen atoms, forming a Zn-N2O2coordination configuration. Stability assessments via TGA showed that ZnSCOF is thermally stable up to 280°C and maintains chemical integrity after immersion in various solvents and across a range of pH conditions for 7 days, as evidenced by consistent PXRD patterns post-testing. Zhu et al. introduced a π-conjugated van der Waals heterostructure, which combines polymeric carbon nitride (pCN) with a Ni(II)-containing Salphen-based COF based on trithiocyanuric acid, hydroxyterephthalaldehyde, benzenetetraamine tetrahydrochloride (NiCOF, Figure 3l), revealing enhanced photocatalytic activity.[38] The heterostructure exhibited a faster exciton annihilation decay lifetime compared to neat polymeric carbon nitride (p-CN), indicating efficient electron transfer. Photocatalytic tests showed a hydrogen generation rate of 2.3 mmolg1h1and CO generation of 6.2 μmolg1·h1, outperforming neat p-CN by approximately 127 times for H2 and 3 times for CO. This indicates a promising approach for solar fuel production using such heterostructures. DFT calculations reveal a work function of 4.69 eV for p-CN and 4.36 eV for NiCOF. This difference in work functions drives electrons from NiCOF to p-CN, creating a built-in electric field that enhances charge separation efficiency. DFT also indicates that single Ni sites in NiCOF are effective for proton reduction, vital for the heterostructure‘s improved photocatalytic performance. The metal loading was executed with Ni at a weight percentage of 3%, optimizing the photocatalytic efficiency of the material. The metal binding sites, specifically the single-atom Ni sites, Wiley VCH Dienstag, 16.07.2024 2499 / 360106 [S. 8/17] 1 ChemCatChem 2024, e202400100 (8 of 16) © 2024 The Author(s). ChemCatChem published by Wiley-VCH GmbH ChemCatChem Review doi.org/10.1002/cctc.202400100 18673899, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cctc.202400100 by Consorcio Interuniversitario D, Wiley Online Library on [05/08/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License were confirmed experimentally through XAS and EXAFS analyses, which revealed NiN and NiO bonding configurations. Stability assessments demonstrated that the p-CN/NiCOF heterostructure maintained structural integrity and catalytic performance over repeated photocatalytic cycles, supported by unchanged XRD and FTIR post-testing. These factors illustrate a well-synthesized material capable of efficient charge separation and enhanced photocatalytic activity under visible light. Ai et al. introduced Co(II)-infused covalent organic framework (CoCOF), synthesized with biphenyl tetracarbaldehyde and (benzothiadiazolediyl)bis[benzenamine] ligands. This CoCOF effectively converts CO2to CO, achieving a high rate of 2423 μmolg1·h1and over 99% selectivity under visible light.[39] The CO2reduction involves stages of *CO2and *COOH formation, followed by CO release (with * denoting the adsorption site). Energy requirements for *CO2formation are 0.21 eV for the bare COF and 0.42 eV for CoCOF. The transition from *CO2to *COOH in CoCOF is more energy-efficient (0.21 eV) compared to the bare CO (1.31 eV). This indicates that CoCOF, with the added CoO, enhances charge migration and CO2reduction efficiency. Both CoCOF and bare COF require similar energy for the *COOH to *CO transition, but the bare COF is more efficient in converting *CO to CO due to a lower energy barrier. The bare COF is effective in generating *CO2and releasing *CO but less efficient in overall catalysis, mainly due to a high energy barrier in the *CO2to *COOH transition, a critical step in the process. Conversely, CoCOF displays superior photocatalytic activity as CoO integration significantly lowers this energy barrier, facilitating the reaction. He et al. reported a Ni(bpy)32+complex supported on COFs synthesized via condensation of benzotrithiophene tricarbaldehyde with diethoxyterephthalohydrazide, dibutylterephthalohydrazide, and dihexylterephthalohydrazide (designated as BtECOF, BtBCOF, and BtHCOF) that can enhance the photocatalytic CO2RR.[40] These COFs, synthesized with varying alkyl chain lengths, showed distinct electron transport efficiencies. Notably, integrating Ni(bpy)32+into COFs markedly improved their photocatalytic performance. The strength of hydrogen bonds was found to be pivotal in facilitating electron transfer to the Ni complex. The standout performer was the BtECOF with Ni(bpy)32+, achieving a CO production of 2900 μmolg1in 4 h, with an 88% selectivity for CO over H2. This preference occurs probably due to weak heteroatom-hydrogen bonds (X···HC, X=S, N, O), as seen in and similar bonds in two other Ni(bpy)32+ @COFs. Hydrogen bond strength decreases with longer alkyl chains in COFs. BtECOF has the strongest hydrogen bonds, with a dominant N···HC bond. Atomic dipole corrected Hirshfeld charges (ADCH) of Ni atoms in Ni(bpy)32+@COFs follows the order: Ni(bpy)32+@BtECOF <Ni(bpy)32+@BtBCOF <Ni(bpy)32+@BtHCOF, correlating with hydrogen bond strength. The coordination structure of Ni(bpy)32+in BtECOF was confirmed experimentally, forming a supramolecular system containing Ni(bpy)32+. DFT calculations determined that the Ni(bpy)32+ion prefers to locate near the knot position in the COF, with confirmed weak interactions through heteroatom-hydrogen bonds. The diminution of steric groups on COF or metal complex can optimize catalytic performance, enhancing hydrogen bond interactions. The addition of Ni complex in the supramolecular system does not significantly change the growth kinetics of the excited state but facilitates decay kinetics, indicating efficient intermolecular electron transfer. The presence and distribution of Ni within the COF were confirmed using HAADF-STEM, which showed atomically dispersed Ni atoms. XPS and EXAFS further defined the chemical states and local coordination environment of Ni, primarily coordinating with nitrogen atoms. Stability assessments indicated that the materials are robust, maintaining their structure and chemical integrity post-reaction as evidenced by SEM, TEM, and XPS analyses, along with being thermally stable up to 350°C. These methods collectively confirmed the effective integration and stability of the metal within the COF, supporting its potential in photocatalytic applications. A DFT study by Xie et al. elucidated a PtCOF photocatalytic system constructed by embedding Pt single atom in TFPTTMTCOF (TFPT=tris(formylphenyl)triazine, TMT=trimethyltriazine) that can have an enhanced CO2photoreduction ability.[41] Pt can be effectively integrated into COF structures through NPtC2linkages, resulting in a narrower band gap and enhanced light absorption. The Pt atom donates electrons to the COF, exhibiting an oxidation state, and its d orbitals, hybridized with the p orbitals of carbon and nitrogen, span the top of the valence band and the bottom of the conduction band. This PtCOF shows a high CO2adsorption capacity with more negative adsorption energy, significantly activating CO2 by elongating its C=O bonds and altering its linear structure. The key step in reducing CO2to CO on COF involves converting CO2to *COOH, which has an energy barrier of 1.41 eV. With Pt incorporation, this barrier drops significantly to 0.72 eV. However, further reduction of CO on PtCOF faces a high barrier of 2.14 eV. These insights highlight the improved photocatalytic activity and selectivity for CO2reduction in COF photocatalysts with single Pt atoms (Figure 5). The performance comparison of various MCOFs that can catalyze CO2RR is summarized in Table S1(SI). Figure 5. (a) Free energy diagram and (b) optimized structures for CO2 reduction on Pt-loaded COF. Reprinted with permission from Ref. [41] Copyright 2023, Elsevier. Wiley VCH Dienstag, 16.07.2024 2499 / 360106 [S. 9/17] 1 ChemCatChem 2024, e202400100 (9 of 16) © 2024 The Author(s). ChemCatChem published by Wiley-VCH GmbH ChemCatChem Review doi.org/10.1002/cctc.202400100 18673899, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cctc.202400100 by Consorcio Interuniversitario D, Wiley Online Library on [05/08/2024]. 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Mater. Chem. A 2022,10, 11514–11523. Manuscript received: January 16, 2024 Revised manuscript received: June 2, 2024 Accepted manuscript online: June 2, 2024 Version of record online: ■■■,■■■■ Wiley VCH Dienstag, 16.07.2024 2499 / 360106 [S. 16/17] 1 ChemCatChem 2024, e202400100 (16 of 16) © 2024 The Author(s). ChemCatChem published by Wiley-VCH GmbH ChemCatChem Review doi.org/10.1002/cctc.202400100 18673899, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cctc.202400100 by Consorcio Interuniversitario D, Wiley Online Library on [05/08/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License REVIEW In this minireview, we survey and discuss the key mechanistic features of selected single metal site catalysts supported on covalent organic frameworks (COFs), with a focus on photoand electrocatalytic CO2reduction, oxygen evolution reaction, and organic transformations. Dr. A. Gopakumar, Dr. M. A. Ortuño*, Prof. J. Lloret-Fillol* 1 – 17 Reaction Mechanisms of Single Metal Site Catalysts Supported on Covalent Organic Frameworks Wiley VCH Dienstag, 16.07.2024 2499 / 360106 [S. 17/17] 1 18673899, 0, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cctc.202400100 by Consorcio Interuniversitario D, Wiley Online Library on [05/08/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License