Olivier Oms

Nishith Maity

Jérôme Marrot

Jiahao Yu

Eric Rivière

William Shepard

Youven Benseghir

Khadija Talbi

Anne Dolbecq

Minh-Huong Ha-Thi

Jose-Ramon Galan-Mascaros

Pierre Mialane

Structure and Electronic Properties of Large Oligomeric Heterometallic 3d/Ce IV Polyoxometalates

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 Ce IV /Co II POM (Ce12Co6) made of two {(SiW9)2Ce6} units connected to a {(SiW10)2Co6(PO4)2} core, which stability in aqueous media has been evidenced. In addition, the pentameric Ce IV /Ni II 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 Ni II centers constituting the {Ni3PW10} fragments, in agreement with the geometry of such 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 an efficient reservoir of reduction equivalents for photocatalytic reactions.

INTRODUCTION

The chemistry of 3d-4f heterometallic polyoxometalates (POMs) has attracted remarkable interest over the last decade, not only because of the enormous variety of new structures and topologies of these compounds that could result, but also because of their many potential applications, for example in magnetism and catalysis. [START_REF] Das | Heterometallic 3d-4f polyoxometalates : An emerging field with structural diversity to multiple applications[END_REF][START_REF] Liu | A brief review of the crucial progress on heterometallic polyoxotungstates in the past decade[END_REF] However, very few molecular POMs in which both 3d and 4f metal ions are incorporated within the polyoxometalate framework have been reported to date, the number of 1D to 3D species consisting of 3d homometallic entities connected by 4f cations and 4f homometallic molecular oxides connected by 3d metal ions also remaining limited. [START_REF] Das | Heterometallic 3d-4f polyoxometalates : An emerging field with structural diversity to multiple applications[END_REF] The difficulty in obtaining a large library of heterometallic molecular POMs can be explained considering that (i) 3d and 4f cations compete for coordination with lacunary POMs, and as a result pure 3d or 4f species can be obtained concomitantly rather than the targeted 3d-4f POMs, and (ii) the combination of rare earth and 3d cations with lacunary POMs often lead to the precipitation of amorphous materials that are not easy to characterize. However, the (photo)  -(electro) [START_REF] Li | A New Supramolecular Assembly Based on Triple-Dawson-Type Polyoxometalate and 3d-4f Heterometallic Clusters[END_REF] -catalytic [START_REF] Das | A self-assembled heterometallic {Co7-Ho1} nanocluster: 3d-4f trimeric Keggin-type silicotungstate [HoCo7Si3W29O108(OH)5(H2O)4] 18-and its catalytic and magnetic applications[END_REF] properties of some of the successfully isolated compounds could be investigated, suggesting that the presence of both electron acceptor and donor metallic centers in a single POM framework can confer interesting properties. Nevertheless, most of the obtained species have been synthesized for magnetic purposes, as exemplified by the isolation of the POM-based {Dy III Mn III 4} [START_REF] Wu | Heterometallic appended {MMn III 4} cubanes encapsulated by lacunary polytungstate ligands[END_REF] and {Dy III 30Co II 8} 7 compounds, both of which were found to be single-molecule magnets. This could explain why very few species incorporating the diamagnetic, electron acceptor Ce IV ion have been characterized, while the chemistry of polyoxometalates incorporating this ion has proved to be very rich as evidenced in particular by the works of Duval and coll.. This group has indeed shown that large oligomeric homometallic POMs based on {Ce IV 3}, [START_REF] Duval | Influence of the pH on the Condensation of Tetravalent Cerium Cations in Association with [α-SiW9O34] 10-Leading to the Formation of a Ce6O4(OH)4 Core[END_REF] {Ce IV 4} [START_REF] Dufaye | Bottom-up synthesis of functionalized {Ce4(SiW9O34)2(L)2} polyoxometalates[END_REF] or {Ce IV 6} 10,11

building units and with tremendously various topologies can be obtained. Very interestingly, Kortz and coll. also evidenced that large, water stable, cyclic peroxo-Ce IV POM species can be isolated. [START_REF] Qasim | Peroxo-Cerium(IV)-Containing Polyoxometalates[END_REF] In contrast, the two 3d-4f heterometallic POMs obtained by Reinoso et al. in presence of this ion only incorporate one Ce IV center. In the {Ce IV Cu II 3} molecular compound [(Ce IV (O2CCH3))Cu II 3(H2O)(B--GeW9O34)2] 11-, resulting from the addition of a Ce IV salt to the well-known [Cu II 4(H2O)2(B--GeW9O34)2] 12-Weakley-type POM, a {Ce IV Cu II 3} tetranuclear cluster is sandwiched between two {GeW9} units, a Cu II center being substituted by a Ce IV one during the synthetic process. [START_REF] Reinoso | New Type of Heterometallic 3d-4f Rhomblike Core in Weakley-Like Polyoxometalates[END_REF] Alternatively, the {Ce IV Mn IV 6} [(-P2W16O57(OH)2)(Ce IV Mn IV 6O9(CH3COO)8)] 8-molecular POM, which consists in a divacant Dawson unit capped by a planar Mn6 hexagon enclosing a cerium cation, was prepared by Fang et al. by reacting the pre-formed [Ce IV Mn IV 6O9(O2CCH3)9(NO3)(H2O)2] heterometallic precursor with the vacant [-P2W15O56] 12-Dawson species. [START_REF] Fang | A polyoxometalate-based manganese carboxylate cluster[END_REF] Also, we can note that an additional heterometallic Ce/Mn compound has been characterized. However, this last compound consists in the co-crystallization of the cerium {(A--SiW9O34)2Ce IV 4O2(CH3COO)2} and manganese {(A--SiW9O34)Mn III 3Mn IV O3(CH3COO)3} tetranuclear POMs, [START_REF] Fang | A co-crystal of polyoxometalates exhibiting single-molecule magnet behavior: the structural origin of a large magnetic anisotropy[END_REF] highlighting the great difficulty to gather in a single molecule 3d and cerium(IV) clusters.

Herein we report for the first time on the synthesis and characterization of large Ce IV /3d heterometallic POM systems. The hexameric Ce12Co6 and pentameric Ce6Ni8 molecular POMs have been obtained from one-pot reactions performed under mild conditions. These two compounds have been fully characterized in the solid state and their behavior in solution investigated. Their magnetic properties have been studied. Electrochemical investigations have also been carried out, while photophysical studies have been performed to investigate potential electron transfer reaction between these heterometallic species and a photosensitizer, namely [Ru(bpy)3] 2+ .

EXPERIMENTAL SECTION

Synthesis. Na10[A--SiW9O34]•12H2O and Na9[A--PW9O34]•7H2O have been synthesized as previously described. [START_REF] Ginsberg | Inorganic Syntheses[END_REF] All other chemicals were used as purchased without purification.

Rb24Na8[{(SiW9O34)Ce3O2(OH)2(H2O)}2{SiW10O37Co2(OH)(P O4)}]2Co(H2O)2Co(H2O)4•85H2O (Ce12Co6). Na10[A-- SiW9O34]
•12H2O (500 mg, 0.19 mmol) and CoCl2•6H2O (38 mg, 0.16 mmol) were dissolved in 10 mL of an aqueous solution of 0.125 M NaOAc (pH 7, acidified with 1 M HCl). Then, (NH4)2Ce(NO3)6 (200 mg, 0.36 mmol) and Na3PO4•12H2O (50 mg, 0.15 mmol) were added as solids, and the pH adjusted to 6.0 with 1 M NaOH. The suspension was heated to 80°C for 20 min, affording a dark orange solution which was cooled to room temperature. After centrifugation, RbCl (150 mg, 1.24 mmol) was added and the mixture stirred at room temperature for 15 min. The obtained solid was filtrated (m = 230 mg), dissolved in 5 mL of 0.125 M NaOAc (pH 7) and let to slowly evaporate. Brown crystals were collected after one night and washed with a 1 M RbCl aqueous solution, ethanol and ether (44 mg, yield 7% vs. W). M.W. = 20 281 g.mol -1 . IR: ν (cm -1 ) = 1050 (w), 1009 (w), 991 (w), 936 (m), 859 (s), 668 (s), 498 (s). EDX: W:Co:Ce:Rb:Na; calculated 56:6:12:24:8; experimental 56:5.4:12.6:22.0:6.8.

Ba16[{(SiW9O34)Ce3O2(OH)2(H2O)}2{SiW10O37Co2(OH)(PO4)} ]2Co(H2O)2Co(H2O)4•108H2O (BaCe12Co6). Crystals of Ce12Co6 (230 mg, 1.13 10 -5 mmol) were dissolved in 20 mL of 0.125 M NaOAc (pH 6, acidified with 1M HCl) and added to a 20 mL BaCl2•2H2O (1 g, 4.09 mmol) pH 6 aqueous solution. The obtained precipitate was filtrated and washed with water, ethanol and ether (180 mg, yield = 76%). M.W. = 20 680 g.mol -1 . IR: ν (cm -1 ) = 1040 (sh), 1008 (w), 993 (w), 936 (m), 864 (s), 662 (s), 495 (s). EDX: W:Co:Ce:Ba; calculated 56:6:12:16; experimental 56:5.1:12.1:18.

Rb20Na3[{PW9O34Ni3(H2O)3(OH)3(WO3)}2{(PW10Ni2(H2O)2O 36(OH)2)((PW9O34)2Ce6(OH)4O4)}]•60H2O (Ce6Ni8): Na9[A--PW9O34]•7H2O (500 mg, 0.19 mmol) and NiCl2•6H2O (129 mg, 0.54 mmol) were dissolved in 10 mL of a 1 M NaOAc (pH = 7) solution. (NH4)2Ce(NO3)6 (100 mg, 0.18 mmol) was added as a solid, and the pH adjusted to 7.0 with 1M NaOH. The suspension was heated to 80°C for 30 min, affording a yellow solution which was cooled to room temperature. RbCl (100 mg, 0.83 mmol) was then added and the mixture stirred at room temperature for 1 h. The solution was centrifuged and let to stand at room temperature. The solution was centrifuged again the day after and then let to evaporate slowly. Yellow crystals were collected one day later and washed with a 1 M RbCl aqueous solution, ethanol and ether (86 mg, yield 15% vs. W). M.W. = 16 409 g.mol -1 . IR: ν (cm -1 ) = 1101 (w), 1036 (m), 935 (m), 874 (m), 810 (m), 676 (s), 484 (s). EDX: W:Ni:Ce:Rb:Na; calculated 48:8:6:20:3; experimental 48:9.3:6.6:20.1:2.4.

Ba11.5[{PW9O34Ni3(H2O)3(OH)3(WO3)}2{(PW10Ni2(H2O)2O36( OH)2)(PW9O34)2Ce6(OH)4O4}]•80H2O (BaCe6Ni8). Ce6Ni8 (100 mg, 6.09 10 -3 mmol) was dissolved in 5 mL of water and added to a 5 mL BaCl2•2H2O (200 mg, 0.82 mmol) pH 6 aqueous solution. The obtained precipitate was filtrated and washed with water, ethanol and ether (80 mg, yield = 79%). M.W. = 16 570 g.mol -1 . IR: ν (cm -1 ) = 1099 (w), 1043 (m), 937 (m), 876 (m), 813 (m), 672 (s), 479 (s). EDX: W:Ni:Ce:Ba; calculated 48:8:6:11.5; experimental 48: 9.9:5.7:12.2.

Physical methods. Infrared (IR) spectra were recorded on a Nicolet 30 ATR 6700 FT spectrometer. Thermogravimetry analyses (TGA) were performed on a Mettler Toledo TGA/DSC 1, STARe System apparatus under air flow (50 mL min -1 ) at a heating rate of 5°C min -1 up to 700°C. UV-vis spectra were recorded on a Perkin Elmer Lambda 750 UV/Vis/NIR spectrometer using a quartz cuvette with a 1 cm long optical pathway. EDX experiments were performed with a Jeol JSM-5800LV Scanning Microscope equipped with an integrated EDX system, which allows, playing on the applied voltage, to analyze samples up to 2 m from the surface and not only at the surface of the crystals. Magnetic data were collected using a Quantum Design MPMS XL7 SQUID magnetometer. Magnetic susceptibility has been recorded under a magnetic field of 0.1 T.

Nanosecond transient absorption measurements were performed with the setup described previously. [START_REF] Fatima | Selective population of triplet excited states in heavy-atom-free BODIPY-C60 based molecular assemblies[END_REF] Briefly, an OPO pumped by a pulsed nanosecond Nd:YAG laser (EK-SPLA) was used to excite the samples. The laser pulse width was ~ 4 ns (energy of about 1-2mJ/pulse) and the pulse frequency was 10Hz. A white light super continuum laser (STM-2-UV LEUKOS) was used as a probe light. The probe laser was operated at a repetition rate of 20 Hz. The probe beam was split by means of a beam splitter that directs one part of beam to the sample cell while the other is directed towards the reference, to monitor spectral energy distribution fluctuations. The transmitted light from the sample enters into the center of the optical fiber (round-to-linear) bundle that goes to a spectrograph (SPEX 270M, Jobin-Yvon) for analysis. The dispersed white light from the spectrograph is detected by an intensified charge coupled device (ICCD) camera PIMAX-4 (Princeton Instrument). Upon varying the delay time between pump and probe, the transient absorption spectra with and without pump can be recorded are sample spectra with and without pump respectively. All the measurements were performed under argon saturation using quartz cuvette of dimensions 10 mm x 4 mm.

For the electrochemical characterization in the solid state, the barium salts BaCe12Co6/GPO and BaCe6Ni8/GPO (GPO = graphene oxide) were integrated into carbon paste electrodes. These were prepared by ball milling of a mixture of paraffin oil (20 mg), graphite powder (80 mg), and BaCe12Co6 or BaCe6Ni8 (20 mg), respectively. The composites were introduced in a pocket electrode (0.07 cm 2 surface area and 4 mm depth). All electrochemical experiments were performed under ambient conditions with a Bio-Logic VMP3 multichannel potentiostat and implemented with a three-electrode configuration using carbon rod as counter electrode, Ag/AgCl (3 M KCl) as reference electrode, and the pocket working electrode. 1 M H2SO4 (pH 0.1) was used as working electrolyte solution. All potentials were measured vs. Ag/AgCl and converted to the reversible hydrogen electrode (RHE) reference scale using ERHE = EAg/AgCl + 0.21 + 0.059 pH (V). Cyclic voltammetry (CV) experiments were carried out with 50 mV s -1 scan rate.

For Ce6Ni8, X-ray Diffraction Intensity data collection was carried out with a Bruker D8 VENTURE diffractometer equipped with a PHOTON 100 CMOS bidimensional detector using a high brilliance IµS microfocus X-ray Mo K monochromatized radiation ( = 0.71073 Å). Despite numerous attempts, achieving good resolution factors for the structure of Ce12Co6 using this diffractometer has proven challenging. In a final effort, crystals of Ce12Co6 were subjected to synchrotron diffraction single-crystal data collection with a microfocused X-rays on the Proxima 2A beamline (Synchrotron SOLEIL, France,  = 0.72936 Å), [START_REF] Duran | PROXIMA 2A -A New Fully Tunable Micro-focus Beamline for Macromolecular Crystallography[END_REF] allowing to obtain a good model of the Ce12Co6 structure. The absorption corrections were based on multiple and symmetryequivalent reflections in the data sets using the SADABS program [START_REF] Sheldrick | Program for Scaling and Correction of Area Detector Data[END_REF] based on the method of Blessing. [START_REF] Blessing | Empirical Correction for Absorption Anisotropy[END_REF] The structures were solved by direct methods and refined by full-matrix least-squares using the SHELX-TL package. [START_REF] Sheldrick | Software Package for the Crystal Structure Determination[END_REF] The hydrogen atoms were theoretically located on the basis of the conformation of the supporting atoms. The crystallographic data are gathered in Table S1. CCDC -2268425 (Ce6Ni8) and 2268426 (Ce12Co6) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

RESULTS AND DISCUSSION

Synthesis and Characterizations. Ce12Co6 has been obtained via a one-pot reaction by mixing at 80°C in a sodium acetate aqueous solution the trivacant POM [A--SiW9O34] 10-, sodium phosphate and the Co II and Ce IV precursors. The powder obtained after addition of rubidium chloride to the solution cooled to room temperature was recrystallized in sodium acetate (pH 7), affording single crystals. X-ray diffraction studies (see Table S1 for crystallographic data) revealed that Ce12Co6 is a hexameric compound made of two {(SiW9)2Ce IV 6} units connected to a {(SiW10Co II 2)2Co II 2(PO4)2} fragment (Figure 1). In each [(SiW9O34)2Ce6O4(OH)4(H2O)2] 8-unit, six octacoordinated Ce IV centers forming an octahedron are sandwiched between two [A--SiW9O34] 10-trivacant POMs, the cerium ions being linked via both oxo and hydroxo ligands (dCe-O(H) = 2.10 -2.51 Å).

Besides, the {[SiW10O37Co2(OH)(PO4)]2Co(H2O)2Co(H2O)4} fragment is made of a central Co II ion located on a pseudo inversion center and connected to two [-SiW10O37(OH)Co2] 7-Keggin polyoxometalates constituted of two {W3O13} and two {W2CoO13} units around an {SiO4} tetrahedron, the cobalt ions being connected via a vertex. In addition, two phosphate groups not only bind these cobalt centers but also cerium cations of the {(SiW9)2Ce IV 6} fragments. Finally, a sixth cobalt ion is connected both to a {SiW10Co2} unit and a phosphato group, affording the hexameric Ce12Co6 POM represented at the bottom of Figure 1. EDX experiments confirm the metallic composition of the reported heterometallic POM. Its infrared spectrum (Figure 2a) presents the characteristic W=O and W-Ob-W intense bands at wavenumbers below 1000 cm -1 , while the bands located at 1050 cm -1 and 1009 cm -1 can be attributed to P-O vibrations. All these characterizations, combined to TGA experiments (Figure 2b), allow to propose the Rb24Na8[{(SiW9O34)Ce3O2(OH)2(H2O)}2{SiW10O37Co2(OH)(P O4)}]2Co(H2O)2Co(H2O)4•85H2O formula for Ce12Co6.

Ce12Co6 is poorly soluble in water but can be solubilized in a pH 7 0.125M sodium acetate aqueous solution. Infrared spectrum of the compound dissolved in this medium and reprecipitated with cesium chloride after 30 min at room temperature is analogous to that of pristine Ce12Co6 crystals (Figure 2a), suggesting that this heterometallic POM is stable in aqueous solution. This is supported by electronic absorption spectroscopy experiments, the visible spectrum of Ce12Co6 dissolved in 0.125 M sodium acetate (pH 7) showing no evolution after 14 hours at room temperature (Figure 2c). Solid-state electronic absorption spectrum of Ce12Co6 (Figure S1) is reminiscent to that obtained in aqueous solution, this last exhibiting d-d transitions at  = 554 nm and 541 nm, respectively, supporting again the stability of this complex in aqueous solution. Preliminary magnetic studies have been performed on a powdered sample of Ce12Co6. Related DC susceptibility curves recorded at H = 1000 and 10 000 Oe are represented in Figure S2. When the sample is cooled, the χMT value diminishes from χMT = 21.4 cm 3 mol -1 K at 295 K to χMT = 2.85 cm 3 mol -1 K at 2 K. The 300-30 K χMT behavior is characteristic of Co II spin-orbit coupling phenomena, the paramagnetic ions in Ce12Co6 possessing a high-spin ground state 4 T1 splitted into six anisotropic Kramers doublets. However, at low temperature, the observed decrease can also be due to magnetic anisotropy and exchange interactions between the lowest Kramers doublets of the Co II centers characterized by effective spins S′=1/2, these doublets being the only ones significantly populated at low temperature (typically T < 30K). Concerning the exchange interactions, they could in fact be both of ferromagnetic and antiferromagnetic natures considering that Co-O-Co angles in the 91.5-98.8 and 116.9-126.5° ranges are found in the hexanuclear cobalt cluster constituting Ce12Co6. Nevertheless, a complete treatment considering the Hamiltonian adapted to the problem would be necessary to confirm this assumption. Besides, it could be noticed that the χMT value determined at room temperature for Ce12Co6 is in good agreement with those previously reported for Co(II) POMs. For example, a χMT value of 37.5 cm 3 mol -1 K was found at 300K for the decanuclear Co(II) compound [Co II 10Co III (OH)5(H2O)5(W6O24)(PW9O34)3] 22-reported by Duan et al., [START_REF] Duan | Large Magnetic Polyoxometalates Containing the Cobalt Cubane '[Co III Co3 II (OH)3(H2O)6-m(PW9O34)] 3-' (m = 3 or 5) as a Subunit[END_REF] leading to χMT = 3.7 cm 3 mol -1 K per cobalt ion vs. 3.6 cm 3 mol -1 K at 295 K for Ce12Co6.

The cesium salt of Ce12Co6 obtained above is slightly soluble in water. However, it is known that POM barium salts are fully insoluble in aqueous media. [START_REF] Azmani | Understanding polyoxometalates as water oxidation catalysts through iron vs. cobalt reactivity[END_REF] In view to investigate the solid-state electrochemical properties of Ce12Co6 (see below), its alkaline counter-cations have been replaced by Ba 2+ ions by simply adding BaCl2 to a sodium acetate solution of the title compound, affording BaCe12Co6 as a fully water-insoluble powder. Infrared spectra of Ce12Co6 and BaCe12Co6 are identical (Figure S3). EDX experiments indicate that all the rubidium and sodium cations have been replaced by Ba 2+ . TGA measurement of BaCe12Co6 (Figure 2b) indicates a weight loss of 11.2% at 700°C -a temperature at which the compound is fully decomposed into oxides -, allowing to determine its molecular weight, which is in agreement with the water content of 10.4% measured at 280°C and allowing to propose Ba16[{(SiW9O34)Ce3O2(OH)2(H2O)}2{SiW10O37Co2(OH)(PO4) }]2Co(H2O)2Co(H2O)4•108H2O as formula.

Ce6Ni8 has been obtained by heating in water at pH 7 for 30 min the Ni II , Ce IV and [PW9O34] 9-precursors. After cooling down to room temperature, rubidium chloride was added to the yellow solution and single crystals were isolated from the slowly evaporated mother solution. X-Ray diffraction studies (Table S1) revealed that Ce6Ni8 is a pentameric compound (Figure 3). Such {PW9Ni3W} fragment is analogous to that found in the [Ni3(H2O)3PW10O39H2O] 7-compound previously characterized. [START_REF] Gómez-García | Novel Polyoxotungstate Containing a triangulo Ni II 3 Cluster with Ferromagnetic Exchange Interactions and an S = 3 Ground State[END_REF] Overall, it follows that the Ce6Ni8 molecular pentameric compound adopts a pseudo trigonal bipyramidal arrangement. To the best of our knowledge, despite the tremendously huge number of POMs structurally characterized to date, such arrangement is unprecedented in polyoxometalate chemistry. EDX measurements confirm the W:Ce:Ni metallic ratio in Ce6Ni8. TGA experiments (Figure S4) indicate a weight loss of 8.1% at 700°C. Considering that the metallic oxides are entirely formed at this temperature, this leads to propose the formula Rb20Na3[{PW9O34Ni3(H2O)3(OH)3(WO3)}2{(PW10Ni2(H2O)2O 36(OH)2)((PW9O34)2Ce6(OH)4O4)}]•60H2O for Ce6Ni8. The calculated weight loss due to the departure of water molecules (7.5%) is in good agreement with the weight loss observed at 250°C (7.6%). In addition to the W=O (935 cm -1 ), W-Ob-W (874 cm -1 ) and W-Oc-W (810 cm -1 ) bands, the FT-IR spectrum of Ce6Ni8 (Figure S5) exhibits two bands at 1101 and 1036 cm -1 which correspond to P-O antisymmetric stretching modes of the phosphotungstate groups.

The magnetic properties of Ce6Ni8 have been investigated and interpreted considering the {PW10Ni2} and the two {PW9Ni3W} units as three isolated fragments. Also, the Ni II centers in {PW10Ni2} have been considered as magnetically non-interacting. Figure 4 evidences that the MT curve exhibits a continuous increase upon cooling from 300 to 10 K, after which MT rapidly decreases. This is characteristic of a ferromagnetic behavior, in agreement with Ni-O-Ni angles in the 93.5-101.9° range in {PW9Ni3W}, [START_REF] Bertrand | Magnetic exchange in transition metal complexes. V. Ferromagnetic spin coupling in a tetranuclear nickel(II) cluster[END_REF] while the sudden decrease in MT below 10 K is due to the zero-field splitting (ZFS) effect. The following Hamiltonian has thus been considered:

Ĥ = -2J (Ŝ1Ŝ2 + Ŝ2Ŝ3 + Ŝ3Ŝ1+Ŝ'1Ŝ'2 + Ŝ'2Ŝ'3 + Ŝ'3Ŝ'1) + ∑ 𝑫(Ŝ 𝒛 𝟐 -Ŝ(Ŝ + 𝟏))
𝟖 𝒊=𝟏 with S1 = S2 = S3 = S'1 = S'2 = S'3 = 1 associated to the local spin of the Ni II centers constituting the two {PW9Ni3W} units, and the axial ZFS Hamiltonian being relative to the eight Ni II centers present in Ce6Ni8. The MT = f(T) and the variation of the magnetization M with the applied magnetic field H in the 0 -5.5 T range at 2, 3, 4, 6 and 8 K (Figure S6) curves were thus fitted, the best fit parameters obtained being J = +3.8 cm -1 , g = 2.27 and D = +6.7 cm -1 (Res = 0.019). [START_REF]𝑅𝑒𝑠 = (𝑀 𝑒𝑥𝑝 -𝑀 𝑐𝑎𝑙𝑐 𝑛 𝑖=1 ) 𝑒𝑥𝑝 - 𝑐𝑎𝑙𝑐 𝑛 𝑖=1[END_REF] The determined parameters are in excellent agreement with those extracted from the magnetic data of the related [Ni3(H2O)3PW10O39H2O] 7-POM (J = +3.9 cm -1 , g = 2.33 and D = +5.8 cm -1 ), validating our model. As some small deviations between the fits and the experimental MT = f(T) and M = f(H) curves are observed at low temperatures (T < 6K), we have attempted to introduce a J' exchange parameter relating the magnetic interaction between the two Ni II centers constituting the {PW10Ni2} unit. The best fit parameters obtained were g = 2.276, D = +6.62 cm -1 , J = +3.89 cm -1 and J' = -0.349 cm -1 . However, only a slight improvement of the low temperature fit was observed (Figure S7). Also, considering the {Ni3} trinuclear units as isosceles triangles did not significantly improve the fit for T < 6K.

The complex Ce6Ni8 is well soluble in water. Infrared spectrum of the compound dissolved in water and re-precipitated with cesium chloride is analogous to that of pristine Ce6Ni8 crystals (Figure S5). Both the solution (Figure S8) and solid state (Figure S9) electronic spectra evidenced two d-d centered bands in the visible range, with  = 715 and 749 nm and  = 721 and 767 nm, respectively. Recording UV-Vis spectra of Ce6Ni8 in solution as a function of time (Figure S8) has highlighted a slight decrease of the absorbance (ca. 6.5% after 1 h) at max = 715 nm. Nevertheless, as the observed Ce6Ni8 evolving in solution is slow, a water insoluble salt of Ce6Ni8 has been obtained by readily reprecipitating the alkaline salt of Ce6Ni8 with Ba 2+ , as performed for Ce12Co6. The IR spectrum of the obtained BaCe6Ni8 is strictly analogous to that of Ce6Ni8 (Figure S5). Finally, its full formula was determined after TGA (Figure S10) and EDX experiments.

Cyclic voltammetry (CV) experiments in 1 M H2SO4 solutions were then conducted to investigate the redox behaviors of BaCe12Co6 and BaCe6Ni8, respectively (Figure 5). At negative potentials, both BaCe12Co6 and BaCe6Ni8 functionalized electrodes show multi-electron reduction processes, assigned to multiple electron uptake by {XW9} (X = Si, P) moieties. [START_REF] Arenas | Electrochemical redox processes involving soluble cerium species[END_REF][START_REF] Swenson | Electrochemical properties of polyoxometalate composite materials containing multiple redox centers[END_REF][START_REF] Boussema | Dawson-type polyoxometalate nanoclusters confined in a carbon nanotube matrix as efficient redox mediators for enzymatic glucose biofuel cell anodes and glucose biosensors[END_REF] The current density reaches maximum values at lowest potential suggesting the concomitant appearance of hydrogen evolution albeit, at very high overpotential. The overall process appears irreversible, since the electron oxidation waves are not observable in the scans towards positive potentials. When positive potentials over 1 V vs RHE are applied, both electrodes show a reversible Ce III/IV wave. This redox wave appears at significantly high oxidation potentials when compared with aqueous Ce III/IV , which must be due to electronic withdrawing effects imposed by the POM structure. The major difference between both electrodes resides in the appearance of an additional quasi-reversible Co III /Co II redox peak in BaCe12Co6, before and independently of the Ce III/IV event. [START_REF] Yu | Sustainable oxygen evolution electrocatalysis in aqueous 1 M H2SO4 with earth abundant nanostructured Co3O4[END_REF] To investigate potential electron transfer reaction between these heterometallic species and a photosensitizer (PS), nanosecond transient absorption measurements were performed in samples containing the POM, [Ru(bpy)3] 2+ as PS and ascorbate as electron donor by photoexcitation at 460 nm, which correspond to the absorption band of the PS. Transient absorption spectra (TAS) in the presence of Ce12Co6 together with those of the control experiment in the absence of the POM were recorded at different delay times in pH = 7 acetate solutions (Figure 6). In the absence of POM, photoinduced electron transfer between photoexcited PS and ascorbate produced the reduced PS ([Ru(bpy)3] + ) with characteristic transient absorption band at ca. 500 nm rising until 1µs (Figure 6a). At longer delay time, the decay of this transient band reflects the charge recombination between the reduced PS and oxidized ascorbate. The data were fitted globally using a bi-exponential function and the resulted kinetics at selected wavelengths and decays associated difference spectra (DADS) are presented in Figure 6b and Figure S11a, respectively. The first DADS has an associated lifetime of about 170 ns and exhibits a bleaching signal at 460 nm corresponding to the excited state of the PS. The second DADS associated to 38 µs lifetime exhibits absorption maxima at 500 nm and a negative signal at 460 nm, representing the reduced PS. In the presence of POM, new broad absorption features, tentatively attributed to reduced POM, [START_REF] Black | Rapid photoinduced charge injection into covalent polyoxometalate-bodipy conjugates[END_REF] appeared at longer delay times, from hundreds µs and lives for almost 10 ms (Figure 6c). This is consistent with previous studies which suggested broad and shapeless spectra of reduced POM appearing at particularly longer time delays. [START_REF] Black | Rapid photoinduced charge injection into covalent polyoxometalate-bodipy conjugates[END_REF][START_REF] Smortsova | Time-Resolved Spectroscopy and High-Efficiency Light-Driven Hydrogen Evolution of a {Mo3S4}-Containing Polyoxometalate-Based System[END_REF] In the similar manner, the experiments were also performed under identical conditions using Ce6Ni8, [Ru(bpy)3] 2+ as the photosensitizer and ascorbate as the electron donor in water (Figure S13-S14). The results showed the signature of reduced POM with a lifetime of 732 µs. The detailed discussion of the experiments with Ce6Ni8 can be found in the supplementary information. Overall, these results support the possibility of POM reduction photosensitized by [Ru(bpy)3] 2+ for both reported heterometallic POMs.

CONCLUSIONS

We have thus shown here for the first time that it is possible to build oligomeric polyoxometalates associating in a single molecule high-nuclearity cerium(IV) and 3d-substituted POM clusters. Moreover, it has been possible to readily obtain these species under mild conditions via a one-pot synthetic strategy and starting from simple precursors. In the cobalt complex Ce12Co6, two {(SiW9)2Ce6} units connect a {(SiW10)2Co6(PO4)2} core, resulting in a hexameric compound. In contrast, a {(PW9)2Ce6} entity constitutes the central unit of the pentameric species Ce6Ni8, which adopts a unique pseudo trigonal bipyramidal arrangement, the cerium POM cluster being connected to a {PW10Ni2} and two {PW9Ni3W} fragments. UV-vis and IR studies showed that these high-nuclearity species are stable in aqueous media. Insoluble barium salts of Ce12Co6 and Ce6Ni8 have also been prepared, and solid-state electrochemical investigations evidenced that in Ce12Co6 the cobalt, cerium and silicotungstate moieties are electroactive. Moreover, photophysical studies demonstrate the formation of long-lived reduced POM photosensitized by [Ru(bpy)3] 2+ for both Ce12Co6 and Ce6Ni8, which can thus potentially be used as efficient electron reservoirs. The photocatalytic properties of these new species are currently under study.

ASSOCIATED CONTENT (c)

  using Equation: ΔA = 𝑙𝑜𝑔 10 ( where 𝑠 𝑟𝑒𝑓 𝑝 and 𝑠 𝑟𝑒𝑓 0 are reference spectra with and without pump respectively while 𝑠 𝑠𝑎𝑚𝑝𝑙𝑒 𝑝 and 𝑠 𝑠𝑎𝑚𝑝𝑙𝑒 0

Figure 1 .

 1 Figure 1. Polyhedral representation of Ce12Co6 and of its constituting units; blue octahedra, WO6; orange polyhedra, CeO8; brown tetrahedra, SiO4; green tetrahedra, PO4; purple octahedra, CoO6.

6 Figure 2 .

 62 Figure 2. a) IR spectra of pristine Ce12Co6 crystals (green line) and Ce12Co6 dissolved in 0.125 M NaOAc pH 7 and re-precipitated with CsCl (red line); b) TGA measurements of Ce12Co6 (green line) and BaCe12Co6 (blue line); c) UV-Vis spectra of Ce12Co6 (0.125 M NaOAc pH 7, C = 4.2 10 -4 mol.L -1 ) recorded every hour in the 0-14 h range.

Figure 3 .

 3 Figure 3. Polyhedral representation of Ce6Ni8 and of its constituting units; blue octahedra, WO6; orange polyhedra, CeO8; purple tetrahedra, PO4; green octahedra, NiO6. Six octacoordinated Ce IV centers sandwiched between two [A--PW9O34] 9-POMs form an octahedron highly reminiscent of that found in Ce12Co6 (see above), the cerium ions being also connected via oxo and hydroxo ligands (dCe-O(H) = 2.19 -2.58 Å). One of the cerium ions of the {(PW9O34)2Ce6(OH)4O4} unit ensures a connection to an {-PW10Ni2} entity in which the nickel centers belong to two adjacent {NiW2O13} trimetallic triads, the 3d ions being bridged via two {Ni-(O-W-O)2-Ni} and a {Ni-(O-P-O)2-Ni} fragments (dNi•••Ni = 4.76 Å). The resulting trimeric unit

  27

Figure 4 .

 4 Figure 4. Temperature times magnetic susceptibility per mole of Ce6Ni8 as a function of temperature between 300 and 2 K.

Figure 5 .

 5 Figure 5. CV curves of (a) BaCe12Co6/graphene oxide (GPO) and (b) BaCe6Ni8/GPO in 1 M H2SO4.

Figure 6 .

 6 Figure 6. Nanosecond transient absorption spectra and selected kinetic measurements after excitation at 460 nm of [Ru(bpy) 3 ] 2+ (51 µM) and ascorbate (100 mM) in acetate

  Kinetics of bleaching signal (450 nm), absorption signals of [Ru(bpy)3] + (500 nm) and of reduced POM (650 nm) are shown in Figure 6d. Decay of [Ru(bpy)3] + absorption band at 500 nm is more rapid in the presence of POM, and its disappearance together with the growth of absorption bands at 650 nm of reduced POM (inset of Figure 6d) indicate an electron transfer process from reduced PS to POM. The decays were fitted globally using tri-exponential function yielding DADS shown in Figure S11b. The first and the second time components of 120 ns and 1.4 µs are attributed to decay time of excited and reduced PS, respectively. The longest time component of 1500 µs reflects the lifetime of the reduced POM, corresponding to its recombination with oxidized ascorbate. The mechanism of the formation of reduced POM in [Ru(bpy)3] 2+ /ascorbate/POM transient absorption experiment is summarized in Figure S12.

.mol -1.K) T (K)

Supporting Information: Complementary crystallographic, TGA, IR, magnetic, photophysical and electrochemical data (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS

This work was supported by the Ministère de l'Enseignement Supérieur et de la Recherche, the Université Paris-Saclay and the Université de Versailles Saint-Quentin and by a grant of the Agence Nationale de la Recherche (ANR-21-CE50-0024). ICIQ-CERCA is supported by the MCIN/AEI/10.13039/501100011033 (CEX2019-000925-S). JRGM thanks the support from the Spanish Ministry of Science and Innovation (PID2021-124796OB-I00), from the Generalitat de Catalunya (2021-SGR-1154), and from the CERCA Programme/Generalitat de Catalunya. J.Y. thanks the China Scholarship Council (CSC) for predoctoral fellowships (File No. 201806270234).

AUTHOR INFORMATION Corresponding Author

Pierre Mialane -Institut Lavoisier de Versailles (ILV), UMR 8180, Université Paris-Saclay, Université de Versailles Saint-Quentin en Yvelines, 45 Avenue des Etats-Unis, 78035 Versailles cedex, France. E-mail: pierre.mialane@uvsq.fr

Authors

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

Synopsis

Unprecedented water stable hexameric Ce IV /Co II and pentameric Ce IV /Ni II large polyoxometalates have been synthesized and their magnetic properties investigated. Solid-state electrochemical investigations showed that in the Ce/Co system both the cobalt, cerium and silicotungstate moieties are electroactive, while photophysical studies demonstrate the formation of long-lived reduced POMs photosensitized by [Ru(bpy)3] 2+ , suggesting that these species could be used as an efficient reservoir of reduction equivalents for photocatalytic reactions.