Four novel ionic assemblies combining polyoxometalates (POMs) and the cationic photochromic spiropyran SPPy+ (1',3',3'-trimethyl-6-nitro-8-pyridiniomethylspiro[2H-[1]benzopyran-2,2'-indoline]), namely, (SPPy)2[W6O19]·2CH3CN, (SPPy)2.6(H)0.4[PW12O40]·CH3CN, (SPPy)2.1(H)1.9[β-Mo8O26]·CH3CN, and (SPPy)3.7(H)0.3[β-Mo8O26], were synthesized and extensively characterized using complementary techniques, including structural characterization of (SPPy)2[W6O19]·2CH3CN by X-ray diffraction. In particular, solid-state 13C NMR spectroscopy was used for the first time on POM/Spiro compounds to assess the proportion of the merocyanine form in these supramolecular materials. Strikingly, in contrast to the deeply colored (SPPy)2.6(H)0.4[PW12O40]·CH3CN and (SPPy)2.1(H)1.9[β-Mo8O26]·CH3CN assemblies, which are nonphotochromic in the solid state at room temperature, (SPPy)2[W6O19]·2CH3CN and (SPPy)3.7(H)0.3[β-Mo8O26] exhibit strong photocontrast under soft-light irradiation. These observations highlight the crucial role of both the nature of the POM and the synthesis conditions on the optical properties of the final materials. The solid-state photochromic properties of these species have been fully investigated, and the coloration and fading kinetic parameters have been determined. Importantly, once discolored, (SPPy)2[W6O19]·2CH3CN and (SPPy)3.7(H)0.3[β-Mo8O26] turn back to a colored state in the dark, making them the first POM/Spiro hybrid materials to exhibit solid-state negative photochromism.

Achieving efficient photocatalytic CO2 reduction is a current complex challenge, requiring the development of strategies that optimize not only the capture of photons but also the photoinduced charge separation and electron transfer processes. In this pursuit, we have immobilized polyoxometalates (POMs), specifically [SiW12O40]4(SiW12) and [W10O32]4- (W10), within the Zr-based porphyrinic metal-organic framework (MOF) MOF-545 catalytic material with the purpose of maximizing its CO2 photoreduction activity. The resulting SiW12@MOF-545 and W10@MOF-545 composites were fully characterized by various techniques (IR spectroscopy, powder X-ray diffraction, N2 adsorption isotherms, HADDF-STEM) to confirm the POM's incorporation via impregnation. Highresolution TEM images of sections of W10@MOF-545 crystals prepared by ultramicrotomy confirm the location of POMs inside the MOF channels. These characterizations were complemented by simulations in order to locate the POM into the MOF's cavities and identify host/guest interactions. In photocatalytic conditions, i.e. under visiblelight irradiation and in CH3CN/TEOA 20:1 solution, the two SiW12@MOF-545 and W10@MOF-545 composites reduced CO2 to formate with 100 % selectivity at rates of 669 and 1238 mu mol gMOF first 2 h. Remarkably, W10@MOF-545 showed around a 3-fold increase in activity compared to its POM-free counterpart. DFT calculations suggest that both POM guests can accept photoexcited electrons from the porphyrin linkers of MOF-545, allowing increased lifetime of the photogenerated holes in the MOF upon illumination, thus boosting TEOA oxidation by the porphyrinic MOF for subsequent CO2 reduction. Moreover, the calculations unveil the origin of the observed superior overall catalytic activity of W10@MOF-545 over SiW12@MOF-545 due to stronger thermodynamic driving force for charge separation, providing rational guidelines for future design of efficient photocatalysts.

Incorporation of Eu polyoxometalates and carbon dots into a Zn-MOF matrix represents an efficient strategy to build luminescent ratiometric thermometers.

A reduced polyoxometalate (POM) functionalized with bisphosphonate (BP) ligands possessing antitumoral properties was used to synthesize metallic gold nanostars (AuNSs) with high reproducibility through a silver-assisted seed growth method. Notably, this study is the first to employ a reduced polyoxometalate as a dual-function reducing and capping agent in the synthesis of gold nanostars. The resulting AuNS@POM composites, characterized by a plasmon band located at 744 nm, combine in a single hybrid nanocomposite alendronate ligands, Mo(VI) ions, and Au-0 centers. The colloidal stability of AuNS@POM was demonstrated in a biological culture medium, and cell experiments were performed to study its antitumoral activity on U87 and MCF7 cell lines from glioblastoma and liver cancer, respectively. First, it was established that the antitumoral activity of the BP POM is enhanced due to the Mo(V) to Mo(VI) oxidation process that occurs during AuNS formation. Second, it has been shown that in the presence of the composite, cellular metabolic activity decreases drastically under the effect of irradiation at 808 nm, even leading to almost complete cell death. AuNS@POM therefore exhibits strong antitumor chemo-photothermal activity, paving the way for the development of a type of dual therapeutic agent operating under near-infrared irradiation.

We report the use of Zr-based metal-organic frameworks (MOFs) MOF-545 and MOF-545(Cu) as supports to prepare catalysts with uniformly and highly dispersed Ni nanoparticles (NPs) for CO2 hydrogenation into CH4. In the first step, we studied the MOF support under catalytic conditions using operando diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, ex situ characterizations (PXRD, XPS, TEM, and EDX-element mapping), and DFT calculations. We showed that the high-temperature conditions undoubtedly confer a potential for catalytic functionality to the solids toward CH4 production, while no role of the Cu could be evidenced. The MOF was shown to be transformed into a catalytically active material, amorphized but still structured with dehydroxylated Zr-oxoclusters, in line with DFT calculations. In the second step, Ni@MOF-545 catalysts were prepared using either impregnation (IM) or double solvent (DS) methods, followed by a dry reduction (R) route under H-2 to immobilize Ni NPs. The highest catalytic activity was obtained with the Ni@MOF-545 DS R catalyst (595 mmol(CH4) g(Ni)(-1) h(-1)) with 100% CH4 selectivity and 60% CO2 conversion after similar to 3 h. The higher catalytic activity of Ni@MOF-545 DS R is a result of much smaller (similar to 5 nm) and better dispersed Ni NPs than in the IM sample (20-40 nm), the latter exhibiting sintering. The advantages of the encapsulation of Ni NPs by the DS method and of the use of a MOF-545-based support are discussed, highlighting the interest of designing yet-unexplored Zr-based MOFs loaded with Ni NPs for CO2 hydrogenation.

The photocatalytic activity for CO2 reduction of a series of Ni-substituted polyoxometalates (POMs) differing in nuclearity, shape and size, has been investigated under visible light irradiation, with [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine) as photosensitizer and triethanolamine as sacrificial donor. The tetrabutylammonium salt of the Ni4 tetranuclear species was found to exhibit the highest CO production and its stability under photocatalytic conditions was demonstrated. The catalytic performance was significantly lower for the alkaline salt due to the separation of the POM from its counter-ions occurring only for the tetrabutylammonium salt. Photophysical experiments evidenced a bimolecular electron transfer from the reduced photosensitizer [Ru(bpy)3]+ to the Ni4 POM, the former arising from the reductive quenching of the [Ru(bpy)3]2+ excited state by triethanolamine. This was further supported by DFT calculations, which also showed that the Ni4 POM accumulates at least two electrons and four protons to carry out the CO2 reduction catalytic process.

Merging the rich chemistry of Ce(IV) polyoxometalates (POMs) with that of 3d polyanions remains a challenge due to the strong competition between these highly oxophilic lanthanide cations and 3d metallic ions for coordination to lacunary molecular metal oxides. We report herein the characterization of an unprecedented water stable hexameric CeIV/CoII POM (Ce12Co6) made of two {(SiW9)2Ce6} units connected to a {(SiW10)2Co6(PO4)2} core. In addition, the pentameric CeIV/NiII compound Ce6Ni8, where two {PW9Ni3W} and a {PW10Ni2} fragments are grafted on a {(PW9)2Ce6} moiety, has been obtained. Magnetic studies of Ce6Ni8 revealed ferromagnetic interactions between the NiII centers constituting the {Ni3PW10} fragments, in agreement with the geometry of such a trinuclear cluster. Related insoluble barium salts of Ce12Co6 and Ce6Ni8 were also prepared, allowing their solid-state electrochemical investigations and showing in particular that in Ce12Co6, both the cobalt, cerium, and silicotungstate moieties are electroactive. Finally, photophysical studies demonstrate the formation of long-lived reduced POMs photosensitized by [Ru(bpy)3]2+, suggesting that Ce12Co6 and Ce6Ni8 could be used as efficient reservoirs of reduction equivalents for photocatalytic reactions.

Anthracene-,benzophenone-, and anthraquinone-based UVphotoinitiatorshave been, respectively, covalently grafted on the [Mo6O19](2-) polyoxometalate (POM) platform,affording highly colored charge transfer organo-imido Lindqvist complexes.It has been evidenced that photosystems combining N-methyldiethanolamine (MDEA) as the electron donor and these hybridPOMs promote free-radical photopolymerization of acrylate monomerderivatives under irradiation in the visible range, while in similarconditions, no polymerization was observed considering as photoinitiatorsa mixture of the organic and POM precursors. After 800 s, acrylateconversion yields up to 90% under 405 nm LED irradiation and 50% under470 nm LED irradiation were thus obtained considering the POM-imidoanthraquinone(POM-AQ) compound. This shows that such organo-imidoLindqvist species can represent new easy-to-synthesize, efficientvisible-light photoinitiators. Moreover, due to the presence of thePOM, coatings prepared using the POM-AQ/MDEA/soybeanoil epoxidized acrylate photosystem exhibit excellent mechanical properties,with very good flexibility, resistance to brittle fracture, and adherenceto the steel. Finally, POM-AQ was shown to have a remarkablecapacity to generate singlet oxygen species under visible light irradiation,and the antibacterial activity against Staphylococcusaureus under solar light of the related POM-derivedsurface has been demonstrated.

An entry from the Inorganic Crystal Structure Database, the world’s repository for inorganic crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the joint CCDC and FIZ Karlsruhe Access Structures service and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

The immobilization of polyoxometalates (POMs) near catalytic centers of metal-organic frameworks (MOFs) has been reported as an advantageous strategy to boost their photocatalytic activity toward strategic reactions such as CO2 reduction (CO2RR) or hydrogen evolution (HER), although the reasons for such enhancement are still poorly understood. Unveiling the role of POM guests in the reaction mechanisms is therefore a key step toward the development of the next generation of multicomponent catalytic materials with optimal photocatalytic performances. Here, we elucidate the remarkable role of encapsulated [PW12O40](3-) (PW12) polyoxometalates in boosting the photocatalytic activity of the Rh-functionalized UiO-67 MOF toward CO2RR and HER by combining theoretical density functional theory and microkinetic modeling approaches with experimental photophysical and spectroscopic techniques. First, we characterized in detail the reaction mechanism for CO2RR and HER catalyzed by the PW12-containing Rh-functionalized MOF, using [Ru(bpy)(3)](2+) as the photosensitizer (PS) and triethanolamine (TEOA) as the sacrificial electron donor in acetonitrile. Our results reveal that the encapsulated POMs act as efficient electron reservoirs, which quench [Ru(bpy)(3)](+)-the photogenerated reduced form of the PS -and transfer the electrons to the Rh catalytic sites of the MOF. Notably, this is shown to favor the regeneration of the oxidized PS over its unproductive degradation, boosting the turnover numbers of the photocatalytic system. Such a mechanism can explain not only the higher formate and H-2 product yields in the POM-containing catalyst but also the experimentally observed higher impact on the HER pathway than that on the CO2RR one, as the source of protons is generated in the reductive quenching of the photoexcited PS by TEOA. Finally, our computational exploration was extended to a whole variety of POMs, which allowed establishing relationships between their redox potentials and the activity of the related POM-containing catalytic materials. The optimal activity is reached when both the ability of the POM to accept electrons and that of its reduced form to reduce the Rh catalyst are simultaneously maximized, leading to a volcano plot whereby POMs with a moderate redox potential display the highest impact on photocatalytic performances.

Efficient, selective and recyclable heterogeneous catalysts for photocatalytic CO2 reduction to CO under visible light irradiation are readily prepared by immobilization of cobalt molecular catalysts into Zr(iv)-based MOFs.

A complete picture of the reaction mechanism driving the photocatalytic reduction of CO2into formate promoted by the Zr-based porphyrinic MOF-545 in CH3CN/TEOA solutions is provided for the first time by combining experimental and computational approaches.

Three crystalline heterometallic molybdenum(V) phosphates have been synthesized under hydrothermal conditions. They all contain {M[P4Mo6O28(OH)(3)](2)}(16-) [M = Mn(II) or Co(II)] polyoxometalate ( POM) units, with the M ions sandwiched between two {P4Mo6V} rings. In the presence of Fe(II) ions in the reaction medium, a three-dimensional (3D) Fe-Mn compound built from the connection of Mn(P4Mo6)(2) units to Fe(II) and Fe(III) centers by extra phosphate ions is obtained. Alternatively, the introduction of [Ru(bpy)(3)](2+) complexes in the synthetic medium prevents the formation of such high-dimensional compounds. In the two Ru(bpy)-Mn and Ru(bpy)-Co hybrids, chains are indeed formed, whereby the Mn(P4Mo6)(2) or Co(P4Mo6)(2) anions are bridged by Mn(II) or Co(II) ions, respectively. The charge of these anionic chains is compensated by neighboring [Ru(bpy)(3)](2+) complexes. Among these three compounds, only Fe-Mn and Ru(bpy)-Mn are active for the heterogeneous photocatalytic reduction of CO2 into CH4 as the major product and CO (yield in CH4 of 1440 and 600 nmol g(-1) h(-1) with selectivity in CH4 equal to 92.6 and 85.2%, respectively, under 8 h irradiation) in water, in the presence of triethanolamine (TEOA) as an electron donor and [Ru(bpy)(3)](2+) as a photosensitizer. A density functional theory (DFT) analysis allowed for proposing a reaction mechanism involving the formation of a solvated electron via photoionization of a one-electron reduced [Ru-II(bpy)(2)(bpy(center dot-))](+) complex as the key step to reduce CO2 to CO2(center dot-). The latter can then coordinate to the peripheral M(II) ions to yield CO through electron- and proton-transfer steps involving reduced POMs and protons generated in the photooxidation of the sacrificial donor. Concerning the nonactive compound, Ru(bpy)-Co, DFT calculations revealed that the Co(II) dimers present in the structure may spontaneously take the extra electron out of CO2(center dot-) to form a Co-Co bond, releasing CO2 back. Finally, preliminary results suggest that the reduction of CO to CH4 could be photochemically accomplished by the POM-based materials in the presence of TEOA, with no mechanistic requirement for the participation of [Ru(bpy)(3)](2+).

A temperature sensor based on lanthanide molecular species immobilized in a lanthanide metal–organic framework exhibits high relative thermal sensitivity in the physiological domain.

A cationic boron dipyrromethene (BODIPY) derivative (1+) has been successfully combined with two polyoxometalates (POMs), the Lindqvist-type [W6O19]2- and the β-[Mo8O26]4- units, into three new supramolecular fluorescent materials (1)2[W6O19]·2CH3CN, (1)2[W6O19], and (1)4[Mo8O26]·DMF·H2O. The resulting hybrid compounds have been fully characterized by a combination of single-crystal X-ray diffraction, IR and UV-vis spectroscopies, and photoluminescence analyses. This self-assembly approach prevents any π-π stacking interactions not only between the BODIPY units, responsible for aggregation-caused quenching (ACQ) effects, but also between the BODIPY and the POMs, avoiding intermolecular charge-transfer effects. Noticeably, the POM units do not only act as bulky spacers, but their negative charge density drives the molecular arrangement of the 1+ luminophore, strongly modifying its fluorescence in the solid state. As a consequence, the 1+ cations are organized into dimers in (1)2[W6O19]·2CH3CN and (1)2[W6O19], which are weakly emissive at room temperature, and in a more compact layered assembly in (1)4[Mo8O26]·DMF·H2O, which exhibits a red-shifted and intense emission upon similar photoexcitation.

The last few years have seen huge growing interest in the heterogenisation of molecular catalysts since it allows combining the advantages of homogeneous and heterogeneous catalyses. Besides bringing recyclability, the immobilisation of the catalyst may increase its stability while allowing tuning its selectivity. In this respect, Metal-Organic Frameworks (MOFs) attract evergrowing interest as a platform for their confinement within their pores or channels. In this review, Cat@MOF composites wherein molecular catalysts (Cats) are immobilised into MOFs through non-covalent interactions with their host are reviewed thoroughly. Polyoxometalates (POMs) and other metal-based complexes as immobilised molecular species are covered. In the first part, the different synthetic methods and analytical tools are described. A critical analysis of the various physico-chemical methods available to characterise the Cat@MOF composites is provided - particular attention being paid toward their pertinence to the investigation of the content, the position and the stability of the catalyst within the MOF. Besides, the focus is on non-conventional techniques such as the Pair Distribution Function (PDF) method and a section is dedicated to the contribution of DFT calculations. In the second part, the applications of these materials in the fields of catalysis, including oxidation and reduction reactions, acid-base catalysis, and photo- and electrocatalysis, are detailed.