English

• 0.   Introduction

  The water oxidation reaction is a pivotal process in clean energy technologies, governing the efficiency of overall water splitting, carbon dioxide reduction, and fuel cell systems^[1-4]. As the reaction that supplies essential electrons and protons for hydrogen evolution and carbon dioxide reduction, the development of efficient water oxidation catalysts (WOCs) has emerged as a critical research frontier in renewable energy^[5-7]. Nature′s oxygen-evolving complex in photosystem Ⅱ (OEC-PSⅡ) has inspired the design of molecular catalysts, particularly Ru, Ir, and Mn complexes, which demonstrate exceptional water oxidation activity through well-defined active sites (Scheme 1)^[8-10]. However, their homogeneous nature imposes fundamental limitations on recyclability and practical applications. Heterogeneous alternatives such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) have recently gained attention for water oxidation catalysis^[7,11-17]. Yet significant challenges persist: MOFs often demonstrate limited stability under harsh oxidative conditions, while COFs typically lack active sites for optimal catalytic performance. These limitations highlight the need for innovative catalyst architectures that combine atomic-level precision with structural robustness.

  Scheme 1

  

  Scheme 1.  Scheme of engineering molecular WOC in metal covalent organic frameworks (MCOFs) for water oxidation

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  MCOFs have recently emerged as a promising platform that bridges the gap between MOFs and COFs^[18-23]. Generally, COFs incorporating metal ions are referred to as metalated COFs or MCOFs. Notably, for MCOFs, the metal ions can be introduced not only via the metal-containing blocks but also through post-synthetic modification^[18-19]. However, the MCOFs discussed herein have three distinct characteristics that are different from conventional metalated COFs: (ⅰ) the framework is constructed from a metal-complex building unit and an organic building unit linked through covalent bonds; (ⅱ) the metal ions act as templates to direct the controllable arrangement of organic molecular units into a predefined topological architecture; (ⅲ) both the covalently linked organic skeleton and the metal ions are integral components of the overall framework structure. It should also be noted that MCOFs differ significantly from the recently reported covalent metal-organic frameworks (CMOFs), which are synthesized by combining metal clusters with covalent bond formation^[24]. CMOFs should essentially be classified as a subclass of MOFs, as their covalently linked skeletons are periodically interrupted by metal ions.

  Unlike conventional MOFs with their potentially labile metal-linker coordination bonds, MCOFs incorporate strong covalent linkages that enhance durability while maintaining tunable electronic structures. By integrating molecular catalytic units into crystalline frameworks, MCOFs represent a transformative class of materials that synergize the advantages of molecular catalysts, such as designability and high activity, with those of extended frameworks showing structural stability and recyclability (Scheme 1). These materials enable several advantages: (ⅰ) controlled active-site arrangement for cooperative catalysis, (ⅱ) improved stability through covalent bonding and network interpenetration, and (ⅲ) efficient mass and charge transport via ordered porous channels.

  Despite these promising attributes, several fundamental challenges must be addressed to advance MCOF-based water oxidation catalysts. Structure-activity relationships remain poorly understood, and key questions persist concerning how framework porosity affects mass transport and how metal coordination geometry influences reaction mechanisms. This frontier article aims to provide a comprehensive overview of recent strategies for embedding molecular catalysts within MCOFs for water oxidation, with emphasis on material design, mechanistic insights, and catalytic performance. We also outline major remaining challenges and future perspectives to inspire further development in this emerging field.

  1.   Molecular catalysts: foundation and limitation

  The water oxidation reaction plays a pivotal role in artificial photosynthesis, serving as the key bottleneck in overall water splitting and carbon dioxide reduction systems^[6,25-27]. This is due to its kinetically challenging four-electron and four-proton transfer process, which involves O—H bond cleavage and O—O bond formation. These steps result in high energy barriers and intrinsically slow reaction kinetics^[28-29]. Therefore, the development of highly efficient WOCs is essential to overcome these limitations. This role is naturally fulfilled by the OEC-PSⅡ, which contains a Mn₄CaO₅ cluster (Fig.1a). The natural system serves as an excellent source of inspiration for the design of WOC systems. Significant advances have been made in the design of transition metal-based molecular WOCs, with Ru complexes emerging as particularly promising candidates^[30-35]. Some of these complexes exhibited catalytic activities comparable to those of the natural Mn₄CaO₅ oxygen-evolving complex.

  Figure 1

  

  Figure 1.  (a) Oxygen-evolving complex of Mn₄CaO₅ cluster in photosystem Ⅱ that mediates the Kok cycle; (b) Structures of blue dimer, Ru(bda)(pic)₂ and Ru(bda)(isoq)₂, respectively; (c) Schematic representation of general mechanisms for water oxidation catalyzed by metal complexes

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  Effective homogeneous Ru molecular catalysts for water oxidation typically incorporate three essential structural features: a redox-active metal center, an oxidatively robust coordination environment, and at least one labile coordination site (occupied by water or other ligands) to facilitate substrate binding and catalytic turnover. A milestone in this field was achieved in 1982 by Meyer and co-workers with the report of the first well-defined molecular WOC, the dinuclear μ-oxo-bridged complex cis, cis-[(bpy)₂(H₂O)Ru(μ-O)Ru(H₂O)(bpy)₂]⁴⁺ (bpy=2,2′-bipyridine), commonly known as the blue dimer (Fig.1b)^[35-36]. This work demonstrated that a Ru complex could mediate the four-electron and four-proton oxidation of water, establishing a crucial precedent for molecular Ru catalyst design. Subsequently, a variety of molecular Ru-based WOCs have been developed.

  As research on Ru-based molecular catalysts has advanced, the understanding of their water oxidation mechanisms has also deepened. In these systems, the critical O—O bond formation step proceeds via two distinct pathways (Fig.1c). The first is the water nucleophilic attack (WNA) mechanism, in which a water molecule attacks a high valent Ru-oxo species (Ru^Ⅴ=O or Ru^Ⅵ=O), forming a hydroperoxo intermediate (Ru-OOH) that subsequently releases O₂. This pathway exhibits first-order kinetics and a significant KIE, with a value of 2-4. The second mechanism involves interaction between two metal-oxo units (I2M), leading to a peroxo-bridged intermediate via radical coupling. This route is characterized by second-order kinetics and a small KIE (1-1.5)^[33].

  A notable improvement in catalytic performance was achieved through the introduction of anionic ligands, such as carboxylate groups, which stabilize high-valent metal states and provide open coordination sites essential for water binding and O—O coupling. Sun and co-workers pioneered this approach by developing the Ru-bda system (bda=2,2′-bipyridine-6,6′-dicarboxylate)^[31,33]. The resulting complex, [Ru(bda)(pic)₂] (pic=4-picoline), features a distorted coordination geometry that creates an open site for water binding. This enables the formation of a seven-coordinate Ru^Ⅴ=O intermediate that promotes O—O bond formation via the I2M pathway. When the axial ligands were changed from pic to isoq (isoq=isoquinoline), the molecular catalyst [Ru(bda)(isoq)₂] was obtained, which exhibited exceptional catalytic activity with turnover frequencies (TOF) exceeding 300 s^-1, comparable to those of photosystem Ⅱ (Fig.1b).

  These attributes collectively underscore the importance of ligand geometry and electronic properties in dictating both the mechanism and efficiency of molecular water oxidation catalysts. Despite these remarkable achievements, homogeneous molecular Ru catalysts still face fundamental limitations that hinder their practical application: (ⅰ) instability under highly oxidizing conditions, leading to ligand degradation and catalyst decomposition, (ⅱ) aggregation and decomposition pathways that often yield nanoparticles, which may constitute the true catalytic species, and (ⅲ) difficulties in recycling and reusing the catalysts, imposing both economic and practical constraints. These challenges highlight the critical need for strategies that preserve the exceptional catalytic properties of molecular Ru sites while overcoming their inherent instability. This necessitates a transition to stable materials that integrate molecular catalytic principles into robust, heterogeneous frameworks.

  2.   Beyond immobilization: engineering molecular catalysts into MCOFs

  MCOFs have emerged as an effective platform that addresses the fundamental limitations of molecular catalysts while preserving their catalytic excellence. By engineering molecular catalytic units as primary building units into crystalline frameworks, MCOFs successfully merge molecular precision with heterogeneous robustness. The recent demonstration of Ru-based MCOFs confirms that this approach can yield catalysts that are simultaneously highly active, stable, and recyclable.

  It was noted that post-synthetic functionalization is a common method to introduce active sites into crystalline frameworks for various applications^[37-38]. However, the amount and position of the active sites in the structures cannot be precisely controlled. The strategy of engineering molecular catalysts into MCOFs represents a paradigm shift from conventional post-synthetic modification approaches. Rather than simply immobilizing pre-formed catalysts, this methodology involves the de novo design of molecular catalysts as functional building units for framework construction, which can overcome the obstacle of the post-synthetic route. The molecular WOCs typically feature pyridine or carboxylic acid ligands, with the Ru(Ⅱ) metal center often exhibiting unsaturated coordination or binding to solvent molecules. Introducing functional binding groups (e.g., —CHO, —NH₂) into such molecular WOCs can convert them into building units suitable for designing MCOFs. At present, MCOFs are generally synthesized via solvothermal methods, which places high importance on the thermal stability of the metal-complex building units to withstand the harsh solvothermal conditions. Notably, the intrinsic instability of molecular WOCs often results from their unsaturated coordination environment. Hence, developing milder synthetic approaches, such as interfacial synthesis or room- temperature synthesis, could expand the range of molecular WOCs compatible with MCOF fabrication. This approach, exemplified by our work, employs the metal complex Ru(bpy-CHO)₂Cl₂ as a building unit where the catalytic Ru center constitutes an integral part of a node functionalized with reactive aldehyde groups^[39]. These building units were then covalently linked with multidentate amine linkers (ETTA and ETTBA, respectively) through imine linkage to form crystalline, porous three-dimensional MCOFs (RuCOF-100′ and RuCOF-101′, respectively) (Fig.2a).

  Figure 2

  

  Figure 2.  (a) Synthesis and structure of RuCOF-101′; (b) Proposed mechanism of WNA pathways during catalytic water oxidation; (c) Photocatalytic water oxidation of the RuCOF WOC

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  This design strategy addresses the limitations of molecular catalysts through several key advantages. (ⅰ) Structural controllability: active sites are periodically ordered within the framework with precise geometry and density, enabling cooperative effects and eliminating site heterogeneity; (ⅱ) Enhanced stability: the rigid covalent skeleton immobilizes and protects molecular units, preventing deactivating aggregation and decomposition pathways; (ⅲ) Built-in porosity: interconnected mesoporous channels (S_BET > 825 m²·g^-1) ensure effective active site accessibility and facilitate rapid mass transport; (ⅳ) Good recyclability: the heterogeneous nature enables simple catalyst separation and reuse across multiple cycles without performance loss.

  The proof of this concept is demonstrated in the exceptional catalytic performance of Ru-based MCOFs. The activation of the framework by removing the chloride ligands generated coordinatively unsaturated Ru sites (RuCOF-100′ and RuCOF-101′, respectively), which dramatically enhanced their hydrophilicity and created active sites for water molecule binding. These MCOFs demonstrate high oxygen evolution rates (2 305 nmol·g^-1·s^-1 for RuCOF-100′ and 2 830 nmol·g^-1·s^-1 for RuCOF-101′) using a chemical Ce(Ⅳ) oxidant. The detailed mechanistic studies confirmed that the catalytic integrity of the molecular building unit is retained within the framework. The first-order kinetics with the linear dependence of the reaction rate on catalyst concentration and a significant kinetic isotope effect (KIE) value ($ {k}_{\text{H}_{2}\text{O}} $/$ {k}_{\text{D}_{2}\text{O}} $=2.56) provide strong evidence that O—O bond formation proceeds via the WNA pathway, identical to many of their homogeneous analogues (Fig.2b). Cyclic voltammetry reveals well-defined redox waves corresponding to the Ru^Ⅲ/Ru^Ⅱ, Ru^Ⅳ/Ru^Ⅲ, and Ru^Ⅴ/Ru^Ⅳ couples, confirming the accessibility of the required high-valent states. Furthermore, these MCOFs exhibit excellent performance in photocatalytic systems using [Ru(bpy)₃]²⁺ as a photosensitizer (Fig.2c). Notably, the catalytic mechanism of RuCOF-100′ and RuCOF-101′ is similar to that of the classical blue-dimer molecular WOC with the WNA mechanism. However, the instability of the molecular blue-dimer WOC often leads to ligand degradation and catalyst decomposition. In contrast, incorporating its structural motif into MCOFs with a stable framework can enhance the durability of the Ru(Ⅱ) catalytic centers, while the periodic arrangement of the active sites ensures high catalytic activity. The unique covalently connected 3D architecture of these MCOFs enhances framework stability while maintaining active Ru sites and large and open channels essential for the water oxidation reaction. Their versatility has also been extended to challenging organic transformations such as sp³ C—H bond oxidation, highlighting broad catalytic potential. This work establishes that MCOFs serve as active, designable platforms that participate in creating and stabilizing superior catalytic environments.

  Another interesting study by Würthner et al. further demonstrates the effectiveness of this strategy^[40]. Motivated by the high activity WOC of the molecular Ru(bda) catalyst, the research group successfully integrated Ru(bda) molecular WOC into a crystalline, interpenetrated Ru(bda)-COF by using Ru(bda)-dialdehyde and tetra-(4-anilyl)methane as building units (Fig.3a). It should be noted that from a structural view, this material may be more accurately classified as a metal-organic framework (MOF) or a covalent metal-organic framework (CMOF)^[24]. Nevertheless, our focus lies primarily on the general strategy of engineering molecular catalysts to crystalline frameworks. Besides, since this approach also relies on covalent assembly, it remains highly relevant to our discussion.

  Figure 3

  

  Figure 3.  (a) Synthesis and structure of Ru(bda)-COF, and (b) proposed mechanism of WNA and I2M pathways during catalytic water oxidation for Ru(bda)-polymer and Ru(bda)-COF, respectively^[40]

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  This system demonstrated excellent performance in both chemical and photochemical water oxidation, achieving oxygen evolution rates of up to approximately 26 000 μmol·L^-1·s^-1. Notably, the study revealed the paramount importance of framework crystallinity. The Ru(bda)-COF exhibited a 20-30 times higher activity than its amorphous polymeric counterpart with identical chemical connectivity. This significant enhancement was ascribed to a shift in the reaction mechanism facilitated by the periodic arrangement of active sites. Notably, the molecular Ru(bda) WOC catalyst often exhibited the WNA mechanism. Similarly, in the amorphous structure, the disordered distribution of Ru centers restricts water oxidation to the slower WNA pathway. In contrast, the well-defined crystalline Ru(bda)-COF enables a periodic and atomically precise framework that promotes cooperative interactions among multiple active sites, facilitating efficient water oxidation via the I2M mechanism (Fig.3b). This mechanistic transition underscores the essential role of long-range order and well-defined interfaces in enabling cooperative catalysis between Ru centers.

  The observed reaction kinetics are complex and highly dependent on the catalyst structure. RuCOF-100′ and RuCOF-101′ exhibited first-order kinetics consistent with a WNA pathway, while Ru(bda)-COF followed second-order kinetics indicative of an I2M mechanism. These results demonstrated that the framework architecture can fundamentally dictate the reaction pathway. Despite structural differences, both studies successfully integrate molecular Ru WOCs into crystalline porous frameworks, revealing the significant potential of using molecular catalysts as building units for designing high-performance MCOF-based WOCs.

  3.   Conclusions and outlook

  MCOFs represent an appealing platform for water oxidation catalysis, bridging the critical gap between molecular precision and heterogeneous stability. The incorporation of molecular Ru water oxidation catalysts into MCOFs has demonstrated excellent catalytic performance and operational stability. Nevertheless, research on MCOF-based water oxidation is still in its infancy, and it is an appealing and challenging area of research. There are several opportunities and issues for the development of MCOF-based water oxidation catalysts in the future, which are summarized as follows:

  (1) Beyond Ru: extending MCOF construction to earth-abundant first-row transition metals (e.g., Fe, Co, Ni, Cu) represents an essential sustainability goal. A major challenge involves designing ligands that mitigate the inherent lability of these metals while stabilizing high oxidation states within the framework.

  (2) Advanced characterization: developing more advanced characterization techniques is essential for probing active site structure and dynamics during catalysis. Synchrotron-based X-ray total scattering with pair distribution function analysis and X-ray absorption fine structure spectroscopy can provide detailed structural insights. In situ methods (e.g., IR spectroscopy, XPS) and ultrafast spectroscopy (e.g., femtosecond transient absorption) can further elucidate reaction intermediates and mechanisms.

  (3) Device integration: future efforts should shift from powder catalysts to functional devices. Research can focus on fabricating oriented MCOF thin films on conductive substrates and semiconductors to develop efficient electrodes or photoelectrodes for practical applications.

  (4) Reaction coupling: combining water oxidation with other reactions, such as carbon dioxide reduction and organic synthesis, via cascade or tandem systems offers a promising avenue for developing integrated artificial catalytic systems.

  (5) Electrocatalytic applications: electrocatalysis has significant potential for MCOFs in energy conversion. While MCOFs have demonstrated promising results in electrocatalytic CO₂ conversion, their application in the field of electrocatalytic water oxidation remains largely unexplored.

  In summary, the structural modularity and synthetic tunability of MCOFs provide unique opportunities to design next-generation catalysts that combine atomic-level precision with practical robustness. This frontier article aims to offer valuable guidance for the development of MCOF-based catalysts for water oxidation and related reactions, thereby supporting progress in this emerging field.

  
  
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  Scheme 1  Scheme of engineering molecular WOC in metal covalent organic frameworks (MCOFs) for water oxidation

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  Figure 1  (a) Oxygen-evolving complex of Mn₄CaO₅ cluster in photosystem Ⅱ that mediates the Kok cycle; (b) Structures of blue dimer, Ru(bda)(pic)₂ and Ru(bda)(isoq)₂, respectively; (c) Schematic representation of general mechanisms for water oxidation catalyzed by metal complexes

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  Figure 2  (a) Synthesis and structure of RuCOF-101′; (b) Proposed mechanism of WNA pathways during catalytic water oxidation; (c) Photocatalytic water oxidation of the RuCOF WOC

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  Figure 3  (a) Synthesis and structure of Ru(bda)-COF, and (b) proposed mechanism of WNA and I2M pathways during catalytic water oxidation for Ru(bda)-polymer and Ru(bda)-COF, respectively^[40]

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•