Enhanced Carrier Separation in Visible-Light-Responsive Polyoxometalate-Based Metal-Organic Frameworks for Highly Efficient Oxidative Coupling of Amines

Linker Engineering of High-Nuclearity {V12@P8W48}-Based Metal-Organic Frameworks for Green-Light-Driven Oxidative Coupling of Amines

The development of long-wavelength visible-light-responsive and reusable photocatalysts for organic transformation is of significant interest. Herein, we report the design and synthesis of high-nuclearity {V12@P8W48}-based metal-organic frameworks, POMOF1 and POMOF2, as heterogeneous photocatalysts for long-wavelength light-triggered oxidation. Linker engineering, by tuning from visible-light-inactive triazole (L1) to a photosensitive anthraquinone-derived ligand (L2), not only leads to the generation of porous 1D open channels within POMOF2 but also imparts a strong peak absorption centered at 500 nm. Moreover, the integration of {V12@P8W48} and Cu2+ ions together with L2 into POMOF2 enables the continued and broad absorption ranging from the ultraviolet to near-infrared region. Consequently, POMOF2 exhibited excellent activity in the green-light-driven oxidative coupling of benzylamines, affording a series of imines with high conversions of up to 99% under mild conditions. In contrast, POMOF1 could barely promote the reaction under the same conditions, further confirming the advantage of linker modulation. POMOF2 is stable and can be reused for three cycles with little loss of catalytic activity and structural integrity. This work highlights the potential of linker engineering as an efficient approach for designing long-wavelength photocatalysts, which can further push forward photoredox catalysis.

Bionic Artificial Leaves Based on AIE-Active Supramolecular Hydrogel for Efficient Photocatalysis

A novel hydrogel-based biomimetic artificial leaf is fabricated by integrating host-guest interactions with covalent bonding. Specifically, a water-soluble tetraphenylethylene-embedded pillar[5]arene (m-TPEWP5), which exhibits aggregation-induced emission (AIE) property, is synthesized as the host molecule. An amphiphilic guest G is introduced to form a stable complex (HGSM) via non-covalent interactions. Subsequent copolymerization of HGSM with gelatin methacryloyl (GelMA) yields a hydrogel network (HGGelMA), which not only exhibits AIE characteristics but also enables encapsulation of the acceptor eosin Y (ESY), thereby resulting in the construction of an artificial light-harvesting system HGGelMA⊃ESY that serves as a biomimetic leaf. To emulate natural photosynthesis more closely and optimize the utilization of the collected energy, two organic reactions are performed within this artificial leaf: dehalogenation of bromoacetophenone derivatives and coupling of benzylamine. These reactions demonstrate remarkable catalytic activity and recycling ability during the photocatalytic process.

[Ru(bpy)3]2+ Derivatives-Incorporated POM@MOFs with Good Photocatalytic Activity for Visible-Light-Driven Oxidative Coupling of Amines to Imines

Two novel POM@MOFs, {H3Zn2.5(H2O)10[Ru(dcbpy)3][PMo11VIMoVO40]}·2H2O (RuZn-PMo) and {H3Ni2.5(H2O)12[Ru(dcbpy)3][PMo11VIMoVO40]}·5H2O (RuNi-PMo), have been synthesized through a traditional hydrothermal method. They are composed of [Ru(bpy)3]2+-derived hexa-carboxylate and Keggin-type anion [PMo11VIMoVO40]4-. In addition, their structures were well characterized by various spectroscopic methods. Under the irradiation of visible light (λ > 400 nm), RuZn-PMo and RuNi-PMo as heterogeneous photocatalysts showed efficient photocatalytic performance in the coupling reaction of amines, with TONs of 451 and 454, respectively. Moreover, RuZn-PMo exhibited excellent reusability and stability after three continuous reaction cycles. Besides, EPR measurements were performed to elucidate the reaction mechanism.

Enhanced Oxidative Coupling of Thiols to Disulfides Using the Visible-Light-Responsive POM@MOF Constructed with Ru Metalloligands

The photocatalytic oxidative coupling of thiols to disulfides by using visible light represents an economically viable and environmentally sustainable strategy. A novel POM@MOF photocatalyst (Ru-CdS-SiW) was synthesized through the encapsulation of Keggin-type [SiW12O40]4- within a MOF composed of Ru metalloligands and {Cd4S2O16} clusters. In this structure, the incorporation of POMs to the MOFs reduced the charge transport distance, facilitated the separation and transfer of photogenerated charges and holes, and prevented the recombination of electron-hole pairs. The Ru-CdS-SiW catalyst demonstrated exceptional catalytic performance, achieving a 98.1% yield in the S-S bond formation from 4-methylthiophenol coupling with an apparent quantum yield of 4.8% at 440 nm. Through comprehensive exploratory experiments and electron paramagnetic resonance (EPR) measurements, we elucidated the mechanism underlying the photoinduced oxidative coupling of thiols. Notably, this catalytic reaction operates under mild visible-light conditions and exhibits remarkable recyclability, presenting significant potential for applications in sensitive systems, such as protein disulfide bond formation.

Highly Efficient Yolk-Shell Photocatalyst Constructed by Integration of Ni2P and Cu2O Nanoparticles to Defective Metal-Organic Frameworks for Visible-Light-Driven Amine Oxidation

Realizing the directional migration of photogenerated carriers plays an important role in improving the photocatalytic performance. Meanwhile, light-driven oxidative coupling of benzylamine under ambient conditions with an inexpensive catalyst is highly desirable for the industrial field. Herein, via in situ synthesis, defect engineering, and photodeposition, a yolk-shell nanostructured photocatalyst, Ni2P@OH-NH2-UiO-66@Cu2O, featuring nickel phosphide (Ni2P) nanoparticles (NPs) trapped inside a defect engineered metal-organic framework (MOF, namely OH-NH2-UiO-66) and Cu2O NPs adhering on the surface of MOFs, has been rationally fabricated for the achievement of spatial separation of oxidation/reduction cocatalyst in photocatalytic reaction systems. The yolk-shell structure can effectively avoid the aggregation of the Ni2P and Cu2O NPs. Remarkably, the separation of electron collector Ni2P and hole collector Cu2O regulates the directional movement of the photogenerated carriers and effectively improves the electron-hole separation efficiency to generate abundant reactive superoxide radicals (•O2-) and hydroxyl radicals (•OH). Ni2P@OH-NH2-UiO-66@Cu2O achieves a conversion of 99% for the oxidative coupling of benzylamine into imine within 1 h at ambient temperature under visible-light irradiation. The present study provides an economical method to construct a MOF-based yolk-shell photocatalyst for the oxidative coupling of amines.

Visible-Light-Responsive Tetranuclear Ir-Based Polyoxometalates Achieve Photocatalytic Baeyer-Villiger Oxidation of Ketones

Synthesizing efficient photocatalysts with a broad-spectrum response is crucial for improving solar energy utilization. In this work, we have constructed two examples of tetrameric Ir-based polyoxometalates by introducing an Ir ion. The introduction of Ir ions lowers the band gap energy, and the light absorption range is extended into the visible region. Both displayed satisfactory reactivity for the visible-light-catalyzed Baeyer-Villiger reaction of cyclohexanone, especially compound 1, which reacted up to 95.1% yield for 3 h with TON and TOF values of 951 and 510 h-1, respectively. Meanwhile, 1 also presents excellent cyclic and structural stability, and the yield can still reach 92.2% after five cyclic reactions.

Ru Metalloligands Participate in the Construction of POM@MOF for Enhancing the Visible Photoinduced Baeyer-Villiger Oxidation Reaction

Directed synthesis of high-efficiency visible photoinduced Baeyer-Villiger oxidation catalysts is of primary significance. Here, the isopolymolybdate anion [β-Mo8O26]4- is for the first time encapsulated with the photosensitive metalloligand [Ru(bpy)2(H2dcbpy)]2+ (bpy = 2,2'-bipyridine; H2dcbpy = 2,2'-bipyridine-5,5'-dicarboxylic acid) to synthesize polyoxometalate@metal-organic frameworks, {(CdDMF)2[Ru(bpy)2(dcbpy)]3([β-Mo8O26])}·5DMF (Ru-Mo8). The composite photocatalyst Ru-Mo8 not only has a light absorption of 700 nm but also shortens the photogenerated electron transfer distances and accelerates charge and proton transfer. Ru-Mo8 can perform the Baeyer-Villiger oxidation with high selectivity and up to 96.7% yield under visible light (λ > 400 nm) irradiation. The turnover number and turnover frequency of the reaction were computed to be 967 and 548 h-1, respectively, and the apparent quantum yield was 6.84% by 425 nm. Simultaneously, the radical mechanism of Baeyer-Villiger oxidation of Ru-Mo8 in the O2/benzaldehyde system under visible light (λ > 400 nm) irradiation was proposed.

Self-Assembled 8-Ti-Containing Polyoxomolybdate for the Photocatalytic Hydrogen Evolution

An S-shaped 8-Ti-containing polyoxomolybdate (NH4)Cs2Na6H3[Ti8(GeMo9O34)2(GeMo5O23)2]·44H2O (1) has been prepared under the one-pot hydrothermal method and further characterized. According to single-crystal X-ray, compound 1 consists of two {GeMo9O34} and two {GeMo5O23} segments, which are linked with a {Ti8O34} cluster. Moreover, the {GeMo5O23} fragment is rare and the first discovery, which enriches the lacunary Keggin-type family and offers the possibility of obtaining new structures. Among all of the reported polyoxomolybdates, compound 1 has the highest nuclearity of Ti centers. From the photocatalytic hydrogen evolution studies, compound 1 can be used as the heterogeneous catalyst with an H2 evolution rate of 2392.6 μmol g-1 h-1 under minimally optimized conditions.

Intensifying Photocatalytic Baeyer-Villiger Oxidation of Ketones with the Introduction of Ru Metalloligands and Bimetallic Units in POM@MOF

The synthesis of visible light-responsive and efficient photocatalysts toward green Baeyer-Villiger oxidation organic synthesis is of extraordinary significance. In this work, we have synthesized two examples of visible light responsive crystalline polyoxometalate@metal-organic framework materials Ru-NiMo and Ru-CoMo by introducing Ru metalloligands and {CdM3O12} bimetallic units (M = Ni or Co). This is the first report of metalloligand-modified polyoxometalate@metal-organic framework materials with bimetallic nodes, and the materials form a three-dimensional framework directly through coordination bonds between {CdM3O12} bimetallic units and metalloligands. In particular, Ru-NiMo can achieve efficient photocatalytic conversion of cyclohexanone to ε-caprolactone in yields as high as 95.5% under visible light excitation in the range of λ > 400 nm, achieving a turnover number and turnover frequency of 955 and 440 h-1, respectively, which are the best known photocatalysts for Baeyer-Villiger oxidation, while apparent quantum yield measured at 485 nm is 4.4%. Moreover, Ru-NiMo exhibited excellent structural stability and recyclability, producing a 90.8% yield after five cycles of recycling.

Photocatalytic Reduction of Nitrobenzene to Aniline by an Intriguing {Ru(C6H6)}-Based Heteropolytungstate

In this paper, we have successfully synthesized a structurally novel heteropolytungstate via coordination of four {Ru(C6H6)} and trivacant {TeW9O33} clusters, formulated as Cs4Na2H2[Te2W20O72(H2O){(C6H6)Ru}4]·12H2O (1). Compound 1 inherited the strong absorption of [Ru(C6H6)Cl2]2 in the visible region and {TeW9O33} in the UV region, providing a good basis for photocatalysis. As expected, compound 1 showed good photocatalytic activity in the visible-light-driven reduction of nitrobenzene using N2H4·H2O as a reductant with a yield of 99.8%, a high turnover number (TON = 330), and a high turnover frequency (TOF = 24 h-1). The cyclic experiment of nitrobenzene reduction indicated that compound 1 was an effective and stable heterogeneous catalyst. Finally, the nitrobenzene reduction pathway was affirmed using condensation with azobenzene as a reaction intermediate based on control experiments.

Visible-Light-Responsive Polyoxometalate@Metal-Organic Frameworks Involving Ir Metalloligands for Highly Selective Photocatalytic Oxidation of Sulfides to Sulfoxide

The synthesis of highly efficient visible-light-responsive photocatalysts is fundamental to solving the problems of low efficiency and poor selectivity in photocatalytic organic synthesis reactions. We synthesized a crystalline polyoxometalate @metal-organic framework material {Zn4 (H2 O)8 [Ir(ppy)2 (dcbpy)]4 [SiW12 O40 ]} ⋅ 4H2 O (Ir-SiW) by self-assembly of Ir metalloligands with POMs. The introduction of Ir metalloligands extends the light absorbing range to visible light, improving the efficient utilization of solar energy. The transfer of photogenerated electrons from Ir metalloligands to SiW12 was observed under visible light irradiation, which boosted the carrier separation efficiency. The synergistic effect of the two components increased the photocatalytic thioether oxidation activity, and the product methyl phenyl sulfoxide for 2.5 h under visible light irradiation (λ >400 nm) reached 99.5 %, which was higher than those of other POM-based photocatalysts. Meanwhile, the yield of methyl phenyl sulfoxide was still higher than 97 % after three cycles, demonstrating the high stability and reusability of Ir-SiW.

Three-Motif Molecular Junction Type Covalent Organic Frameworks for Efficient Photocatalytic Aerobic Oxidation

Covalent organic frameworks (COFs), with the features of flexible structure regulation and easy introduction of functional groups, have aroused broad interest in the field of photocatalysis. However, due to the low light absorption intensity, low photoelectron conversion efficiency, and lack of suitable active sites, it remains a great challenge to achieve efficient photocatalytic aerobic oxidation reactions. Herein, based on reticular chemistry, we rationally designed a series of three-motif molecular junction type COFs, which formed dual photosensitizer coupled redox molecular junctions containing multifunctional COF photocatalysts. Significantly, due to the strong light adsorption ability of dual photosensitizer units and integrated oxidation and reduction features, the PY-BT COF exhibited the highest activity for photocatalytic aerobic oxidation. Especially, it achieved a photocatalytic benzylamine conversion efficiency of 99.9% in 2.5 h, which is much higher than that of the two-motif molecular junctions with only one photosensitizer or redox unit lacking COFs. The mechanism of selective aerobic oxidation was studied through comprehensive experiments and density functional theory calculations. The results showed that the photoinduced electron transfer occurred from PY and then through triphenylamine to BT. Furthermore, the thermodynamics energy for benzylamine oxidation on PY-BT COF was much lower than that for others, which confirmed the synergistic effect of dual photosensitizer coupled redox molecular junction COFs. This work provided a new strategy for the design of functional COFs with three-motif molecular junctions and also represented a new insight into the multifunctional COFs for organic catalytic reactions.

Multiple therapeutic mechanisms of pyrrolic N-rich g-C3N4 nanosheets with enzyme-like function in the tumor microenvironment

Nanozyme-based synergistic catalytic therapies for tumors have attracted extensive research attention. However, the unsatisfactory efficiency and negative impact of the tumor microenvironment (TME) hinder its clinical applications. In this study, we provide an easy method to prepare transition metals loaded onto pyrrolic nitrogen-rich g-C3N4 (PN-g-C3N4) for forming metal-N4 sites. This N-rich material effectively transfers electrons from g-C3N4 to metal-N4 sites, promotes the oxidation-reduction reaction of metals with different valence states, and improves material reusability. Under TME conditions, copper ions loaded onto PN-g-C3N4 (Cu-PN-g-C3N4, CPC) can produce ·OH through a Fenton-like reaction for tumor inhibition. This Fenton-like reaction and tumor cell inhibition can be improved further by a photodynamic effect caused by light irradiation. We introduced upconversion nanoparticles (UCNPs) into CPC to obtain nano-enzymes (UCNPs@Cu-PN-g-C3N4, UCPC) for effectively penetrating the tissue, which emits light corresponding to the UV absorption region of CPC when excited with 980 nm near-infrared (NIR) light. The nanoplatform can reduce H2O2 concentration upon exposure to NIR light; this induces an increase in dissolved oxygen content and produces a higher supply of reactive oxygen species (ROS) for destroying tumor cells. Owing to the narrow bandgap (1.92 eV) of UCPC under 980 light irradiation, even under the condition of hypoxia, the excited electrons in the conduction band can reduce insoluble O2 through a single electron transfer process, thus effectively generating O2•-. Nanoenzyme materials with catalase properties produce three types of ROS (·OH, O2•- and 1O2) when realizing chemodynamic and photodynamic therapies. An excellent therapeutic effect was established by killing cells in vitro and the tumor-inhibiting effect in vivo, proving that the prepared nanoenzymes have an effective therapeutic effect and that the endogenous synergistic treatment of multiple treatment technologies can be realized.

Enhanced catalysis of a vanadium-substituted Keggin-type polyoxomolybdate supported on the M3O4/C (M = Fe or Co) surface enables efficient and recyclable oxidation of HMF to DFF

In the reaction of oxidizing 5-hydroxymethylfurfural (HMF), attaining high efficiency and selectivity in the conversion of HMF into DFF presents a challenge due to the possibility of forming multiple products. Polyoxometalates are considered highly active catalysts for HMF oxidation. However, the over-oxidation of products poses a challenge, leading to decreased purity and yield. In this work, metal-organic framework-derived Fe3O4/C and Co3O4/C were designed as carriers for the vanadium-substituted Keggin-type polyoxomolybdate H5PMo10V2O40·35H2O (PMo10V2). In this complex system, spinel oxides can effectively adsorb HMF molecules and cooperate with PMo10V2 to catalyze the aerobic oxidation of HMF. As a result, the as-prepared PMo10V2@Fe3O4/C and PMo10V2@Co3O4/C catalysts can achieve efficient conversion of HMF into DFF with almost 100% selectivity. Among them, PMo10V2@Fe3O4/C exhibits a higher conversion rate (99.1%) under milder reaction conditions (oxygen pressure of 0.8 MPa). Both catalysts exhibited exceptional stability and retained their activity and selectivity even after undergoing multiple cycles. Studies on mechanisms by in situ diffuse reflectance infrared Fourier transform spectroscopy and X-ray photoelectron spectroscopy revealed that the V5+ and Mo6+ in PMo10V2, together with the metal ions in the spinel oxides, act as active centers for the catalytic conversion of HMF. Therefore, it is proposed that PMo10V2 and M3O4/C (M = Fe, Co) cooperatively catalyze the transformation of HMF into DFF via a proton-coupled electron transfer mechanism. This study offers an innovative approach for designing highly selective and recyclable biomass oxidation catalysts.

Incorporating a D-A-D-Type Benzothiadiazole Photosensitizer into MOFs for Photocatalytic Oxidation of Phenylsulfides and Benzylamines

Oxidation and removal of highly toxic sulfides and amines are particularly important for environmental and human security but remain challenging. Here, incorporating an excellent photosensitizer, donor-acceptor-donor (D-A-D)-type 4,4'-(benzo[c][1,2,5]thiadiazole-4,7-diyl)dibenzoic (H2L), into metal-organic frameworks (MOFs) has been manifested to promote the charge separation, affording four three-dimensional (3D) MOFs (isostructural 1-Co/1-Zn with Co2/Zn2 units, and 2-Gd/2-Tb with Gd/Tb-cluster chains) as photocatalysts in the visible light-driven air-O2-mediated catalytic oxidation and removal of hazardous phenylsulfides and benzylamines. Impressively, structure-property correlation illustrated that the transition metal centers assembled in MOFs play an important role in the photocatalytic activity, and we can conclude that 1-Zn can be a robust heterogeneous catalyst possessing good light adsorption and fast charge separation in oxidation removal reactions of both benzylamines and phenylsulfides under visible light irradiation and room temperature with excellent activity/selectivity, stability, and reusability.

Fabrication of Polyoxometalate-Based Metal-Organic Frameworks Integrating Paddlewheel Rh2(OAc)4 for Visible-Light-Driven Oxidative Coupling of Amines

The development of visible-light-responsive, environmentally friendly, and reusable photocatalysts for organic oxidation reactions is of vital significance. Herein, four polyoxometalate-based metal-organic frameworks (POMOFs) were synthesized and systematically characterized by assembling the paddlewheel complex Rh2(OAc)4 and various polyoxometalates (POMs). Single-crystal X-ray diffraction analysis revealed that the four POMOFs were isomorphic and possessed rare structural features among the POMOFs, with POMs as nodes and Rh2(OAc)4 as linkers. As expected, the activities of the four POMOFs for the photocatalytic oxidative coupling of benzylamine were better than that of Rh2(OAc)4 or POMs individually, which was ascribed to the synergistic effect between them, and the intrinsic reasons for the difference in the activity were explained via electrochemical measurements. In particular, the product imine yield reached 96.1% with NaRh-SiW12 as the catalyst and a turnover number and a turnover frequency of 480.5 and 120.5 h-1, respectively, while the product yield remained as high as 92% after three repetitions, evidencing its high stability. Moreover, the higher activities of the four POMOFs for the selective epoxidation of various alkenes reaffirm the synergistic effect between Rh2(OAc)4 and POMs.