Jonathan Nicolas Dumazert

Romain Coulon

Guillaume H V Bertrand

Stéphane Normand

Laurence Méchin

Matthieu Hamel

Compensated bismuth-loaded plastic scintillators for neutron detection using low-energy pseudo-spectroscopy

Keywords: Neutron detection, Compensation, Modified plastic scintillators

Introduction

Detecting neutron radiation is vital in numerous fields of nuclear instrumentation, from flow monitoring on industrial infrastructures [START_REF] Mattingly | Physics design of fissile mass-flow monitoring system[END_REF] to dose rate assessment for radioprotection [START_REF] Hankins | Progress in Personal Neutron Dosimetry[END_REF], and notably to address radiological threats in homeland security [START_REF]Chemical, Biological, Radiological or Nuclear (CBRN) Detection: a Technological Overview, NATO Parliamentary Assembly[END_REF]. Alternative technologies to conventional helium-3 detectors are actively searched for due to the worldwide strategical issues surrounding tritium and the repeatedly announced shortage of the isotope [START_REF]Managing Critical Isotopes, Weaknesses in DOE's Management of Helium-3 Delayed the Federal Response to a Critical Supply Shortage[END_REF]. In this context, the interest in the loading of plastic scintillators with organometallic complexes has risen, as compatible with the deployment of sensitive, scalable and cost-effective detectors [START_REF] Pichat | [END_REF]. Bismuth has thus been extensively studied [6], [7] as a dopant for low and medium-energy gamma-ray pseudo-spectroscopy. Now the use of gadolinium, which exhibits the largest crosssection (48,890 b) available among stable elements for the capture of incident thermal neutrons, in conjunction with bismuth-loaded plastic scintillators opens a potential for innovation. A technical conundrum lies within the separation of the scintillation signal due to the prompt gamma-rays most significantly emitted after the (n,γ) neutron radiative capture from the scintillation signal attributable to ambient gamma-rays and falling into the same energy range. To address this limitation, the authors propose a two-scintillator neutron detection system using a dual gadolinium/terbium detection and compensation scheme.

Related work

Neutron detection represents a long-term technological challenge due to the indirectly ionizing nature of the radiation: the secondary particles generated after an interaction between a neutron and an atom of the sensor, which carry a charge, actually form the signature of the neutron presence. It is also imperative, when monitoring neutron activity, to discriminate this signature against the signal generated by secondary particles generated through the interaction between background photon radiation and the electrons of the sensor atoms. Among the detectors specifically dedicated to thermal neutrons, helium-3 counters, which exploit a (n,p) type reaction with a thermal cross-section equal to 5327 b, are the most widespread [8]. Given the massive market value fluctuations of helium-3, alternative technologies are investigated, notably among scintillators [9].

In order to increase their sensitivity to thermal neutron radiations, these plastics have to be loaded with neutron adsorbing dopants, mainly boron-10 or lithium-6 [10], [11], [12]. While these isotopes, as alpha particles emitters after neutron capture, allow a discrimination by the difference in deposited energy densities between neutron and photon-generated pulses (the background photon signature being essentially formed of low-energy electrons and photons), their absorption thermal cross-section, respectively equal to 3840 and 950 b, is significantly lower than the one of their helium-3 competitor. A real time neutron radioactivity monitoring system based on these isotopes would additionally require the electronic embedment of online Pulse Shape Discrimination (PSD) algorithms.

As already mentioned, natural gadolinium presents a 48,890 b cross-section for the radiative (n,γ) capture of thermal neutrons [START_REF] Abdushukurov | Gadolinium foils as converters of thermal neutrons in detectors of nuclear radiation[END_REF] at 25 meV (ambient temperature), hence qualifying it as a sensitive neutron radiation converter. Two main approaches have been explored in order to take advantage of this capture efficiency. The insertion of gadolinium inside the sensor was investigated in the fields of inorganic scintillation, as in Lithium-Gadolinium-Borate crystals [START_REF] Williams | [END_REF] and gadolinium-loaded hafnium oxide [START_REF] Blasy | Neutron detection utilizing gadolinium doped hafnium oxide films[END_REF], and modified-plastic scintillation, for instance by the loading of a polystyrene matrix with gadolinium tris-tetramethylheptanedionate complexes [START_REF] Bertrand | [END_REF]. The most common approach nevertheless remains, out of feasibility and reproducibility considerations, the covering of a photon radiation sensor with a gadolinium layer, therefore exploiting the electron and photon cascade after thermal neutron capture as a signature for the detection. Gadolinium-covered gas detectors have been extensively studied by Abdushukurov et al. [17] while Masaoka et al. [18] have developed a micro-strip gas chamber equipped with a gadolinium converter for neutron-position sensitive detection associated with scattering experiments.

With the aim of developing robust and portable detectors for personal operational dosimetry, semiconductor-based technologies are privileged, due notably to their high electron stopping power and photon attenuation, together with superior spectroscopic performance. Vitale et al. [START_REF] Vitale | Solid-State Neutron Detector Detector with Gadolinium Converter[END_REF] have thus validated the patented concept of a solid-state neutron detector with a gadolinium converter through neutron beam exposures, while gadolinium covers have been inserted into MOSFET components as reported by Lee et al. [START_REF] Lee | [END_REF]. Pixelated gadolinium-covered cadmium telluride diodes have similarly been assessed by Miyake et al. [21] with the purpose of taking advantage of both the cadmium and the gadolinium high cross-sections for thermal neutron captures.

As the essential part of the exploitable prompt photon and electron signature reads under 200 keV for portable sensors [START_REF] Dumazert | Compensated gadolinium-loaded plastic scintillators for thermal neutron detection and counting[END_REF], a major technical difficulty lies within the separation of the scintillation signal attributable to recoil electrons in the plastic matrix, generated after an interaction with electrons or photons present in the (n,γ) cascade, from secondary electrons due to an interaction with background photons of comparable energy. This issue is addressed in reference work by the implementation of various two-sensor compensation schemes. Kandlakunta et al. [START_REF] Kandlakunta | [END_REF] have introduced a head-to-tail two-silicon semiconductor system to quantify the internal conversion electrons produced after a neutron radiative capture. Both semiconductors are covered with a 25 µm gadolinium foil, and a High Density Polyethylene (HDPE) layer stops the internal conversion electrons at the entrance of one of the two sensors. The substraction of both sensor responses therefore only restitutes the electron signature of incident thermal neutron captures. Aoyama et al. [24] have described another two-silicon semiconductor compensation scheme, in which two distinct covers are used: a gadolinium cover for the first detector and a tin one, chosen for its atomic number (Z = 50) reasonably close to the one of gadolinium (Z = 64), for the second detector. A 2 mm Lucite layer is used to isolate the electron responses of both sensors to incident radiations, the substraction of both spectral responses aiming at highlighting 71 keV internal conversion electrons emitted after neutron radiative captures. This paper describes the investigation of a plastic-scintillator based alternative to semiconductors in two-sensor compensation schemes. In order to maximize the detection efficiency of the system, the entire photon and electron capture signature below 200 keV is exploited, which no longer requires a 10 keV energy resolution (like in the isolation of internal conversion electrons). In these conditions, the pseudo-spectroscopic performance of moderatesize bismuth-loaded plastic scintillators offers sufficient detection efficiency and energy resolution at low and medium energies. Additionnally, the detection of the prominent gammaray and internal conversion electron signature below 200 keV is enhanced by the closest compensation available of the interactions between the gadolinium converter and incident photon radiations, which is ensured by a terbium-based correction. The concomitant gain in robustness and sensitivity will allow us, after the adequate treatment of the counting signals, to present quantitative figures of merit for the detection of incident neutron radiations.

Principle of the detection method

The principle of the neutron radiation detection system hereby presented is based on three key features:

1) The use of a natural gadolinium cover on scintillator sensor to convert incident thermal neutron radiation into a photon (prompt gamma-rays and X-rays) and electron (internal conversion and auger electrons) source term;

2) a two-identical bismuth-loaded plastic scintillator system, where:

-the first scintillator, named detection scintillator, is covered with a gadolinium foil in which incident thermal neutron absorption occurs, while both the scintillator and the foil interact with incident photon radiation (and more marginally incident fast neutrons); -the second scintillator, named compensation scintillator, is covered with a terbium foil of the same thickness as the gadolinium one. Both the scintillator and the foil interact with the photon and fast neutron part of the incident radiation, while thermal neutron absorption in terbium is negligible.

As, contrary to reference work [START_REF] Kandlakunta | [END_REF], [24], we aim at exploiting the photon part of the neutron capture signature, the cover of the compensation detector must present the closest behavior to gadolinium (atomic number Z = 64) regarding photoelectric effects, Compton diffusions and X-ray emissions, which no element ensures more finely than terbium (Z = 65). In order to select energy windows of interest when studying the response to radiations, the sensors must allow for low-energy pseudo-spectroscopy through the enhancement of photoelectric and Compton interactions, hence the incorporation of bismuth in the scintillators;

3) a unit for the compensation treatment of the signals collected on both channels, which isolates the signature of the interaction between thermal neutrons and gadolinium in the detection scintillator by subtracting the compensation scintillator response (incident photon and fast neutron radiations) to the total detection scintillator response (incident photon, fast and slow neutron radiations).

Photon and electron signature of thermal neutron radiative capture by gadolinium-157 and gadolinium-155 isotopes

Natural gadolinium (Gd) exists in the form of a combination of isotopes among which gadolinium-155 (14.8% proportion, 60,900 b cross-section) and mostly gadolinium-157 (15.65%, 254,000 b) present an exceptionally high interaction probability with thermal neutrons [START_REF] Abdushukurov | Gadolinium foils as converters of thermal neutrons in detectors of nuclear radiation[END_REF].

The prompt source term associated with the de-excitation of a gadolinium nucleus after the absorption of a thermal neutron is subdivided into a prompt photon source term and a prompt electron source term. The complete equations of the nuclear reactions are presented in Eqs. ( 1) and ( 2), where the sum of the gamma rays labeled  and the X-rays noted RX forms the photon source term, and the sum of internal conversion electrons noted CIe -and the Auger electrons labeled Ae -forms the electron source term.

𝐺𝑑 + 𝑛 → 𝐺𝑑 * + 𝛾 + 𝑅𝑋 + 𝐶𝐼𝑒 + 𝐴𝑒

(1)

𝐺𝑑 + 𝑛 → 𝐺𝑑 * + 𝛾 + 𝑅𝑋 + 𝐶𝐼𝑒 + 𝐴𝑒 (2)
As we primarily aim at deploying small and medium-volume scintillators (1-10 cm height and diameter), the exploitable signature is limited to the most significant electron terms and few hundred keV photon terms, which will significantly interact inside the sensors and thus induce a measurable scintillation signal. For the 𝐺𝑑 (𝑛, 𝛾) radiative capture, the significant  are emitted at 79.51 keV (0.773 per neutron capture hereafter abbreviated nc) and 181.931 keV (1.39/nc) [25], together with the significant 𝐶𝐼𝑒 associated with the lowest energy ray at 29.27 keV (0.348/nc) [26] and accompanying 𝑅𝑋 [25] between 5 and 50 keV. For the 𝐺𝑑 radiative capture, the significant  are emitted at 88.967 keV (0.266/nc) and 199.213 keV (0.389/nc) [25], together with the significant 𝐶𝐼𝑒 associated with the lowest energy ray ray at 38.73 keV (0.107/nc) [26] and accompanying 𝑅𝑋 [25] between 5 and 50 keV.

A characteristic spectral distribution of the statistical signature of a thermal neutron flux into a gadolinium-covered bismuth-loaded plastic scintillator is simulated using the probabilistic MCNPX2.7 Monte Carlo code for particle transport [START_REF]MCNPX User Manual[END_REF]. The program simulates a complete history of every generated corpuscle, for source terms as well as for diffusion products, until its energy falls below a 1 keV threshold.

The modeled natural gadolinium converter takes the simplified form of a 100% pure 7.901 g/cm 3 density foil whose thickness is set equal to 250 µm. The bismuth-loaded plastic scintillator is represented by a 1.1 g/cm 3 84.6 wt% carbon, 7.1 wt% hydrogen, 6.2 wt% oxygen, 1.3 wt% nitrogen and 0.8 wt% bismuth, ℎ = 2.9 cm height and 𝑟 = 3.3 cm radius perfect cylinder using the MCNPX2.7 benz.01t material card to account for the styrene structure of the matrix. The diffusers described in paragraph III.B. are neglected in this first-order model.

For gadolinium-155 and 157 isotopes respectively, a photon and electron source term following a homogenous spatial distribution is generated inside the converter using the IAEA database [25] for the totality of the prompt gamma-rays and the BRICC database [26] for the main internal conversion electrons with associated X rays and Auger electrons. Eventually, using the deposition pulse height tally 8 of the MCNPX2.7 code, the spectrum of photon and electron actual energy deposition inside the bismuth-loaded plastic scintillator is calculated. These rates are respectively labeled 𝜏 , and 𝜏 , for the de-excitation source terms associated with gadolinium-157 and 155, and expressed in deposition probability per source term (d.t -1 ).

The overall  typical spectrum for the signature deposition for the detection reads

𝜏 = . , . , (3)
with the α and β weighting coefficients being computed as the products of isotopic importance and cross-section.

The spectral distribution displayed in Fig. 1 confirms that the energy deposition from the deexcitation signature predominantly lies under 200 keV, with peaks corresponding to the couple of gamma-rays, internal conversion electrons and X rays highlighted supra.

Detail of the system

The detection and compensation part of the device is represented by two same-geometry equalvolume plastic scintillators, which may be set side by side as presented in Fig. 2 or head to tail. The detection scintillator (1) is covered with gadolinium (3) under the form of a metal, oxide or alloy foil or deposit, whereas the compensation scintillator ( 2) is covered with terbium (4) (or failing that any alternative lanthanide with the closest atomic number to the one of gadolinium), with the same possible options. Both scintillators are covered with diffusers (5) such as white paint or Teflon so that as many scintillation photons as possible are collected. Both channels are isolated by a suitably dimensioned attenuator (typically lead) thickness (6) inserted in between them so that X and gamma rays generated inside (3) and forming the exploited signature of a neutron capture induce as few false counts in (2) as possible. Both scintillator plus converter ensembles may optionally be isolated from photon background radiation by a lead or steel shield (7), in order to maximize the signal-to-noise ratio, completed by a moderating material for incident neutrons such as a thickness of polyethylene, and a mumetal shield against static magnetic fields. The conversions of the scintillation photons from (1) and ( 2) are respectively performed at the entrances of the photomultipliers ( 8) and ( 9). The acquisition and compensation treatment of the electronic signals from ( 8) and ( 9) is achieved by dedicated electronics (10) connected to a software and interface (11). The treatment of scintillation pulses includes amplitude triggering, bin-to-bin subtraction of the energy spectra acquired on the detection channel (channel 1) and the compensation channel (channel 2), integration of the spectral difference over energy windows of interest in order to isolate the signature of thermal neutron captures in the detection scintillator. The construction and parameterization of the hypothesis test for neutron detection is thoroughly described in Section 6.

Elaboration of a laboratory prototype 4.1. Preparation of bismuth-loaded plastic scintillators

The loading of plastic scintillators remains arduous as far as turbidity, self-absorption, stability and homogeneity may be concerned [START_REF] Cherepy | Bismuth-loaded plastic scintillators for gamma spectroscopy and neutron active interrogation[END_REF]. For the assessment of the detection method described supra, bismuth-loaded plastic scintillators are prepared as described by Bertrand et al. [START_REF] Bertrand | [END_REF] and comprise:

1. a reticulated polystyrene matrix; triphenyl bismuth (BiPh3) as a bismuth-rich organometallic compound.

The topological representation of these molecules is presented in Fig. 3. The choice of a styrenebased matrix is firstly driven by commercial standard, cost-effectiveness and radiation hardness [START_REF] Britvich | [END_REF]. It is secondly noteworthy that, though bismuth has been successfully inserted in polyvinylcarbazole (PVK) matrices [30], these are challenging in terms of scalability, process (high temperature being required for polymerization), temperature and mechanical resilience.

An extensive study of tricarboxylate and triaryl organometallic compounds in polystyrenebased scintillators [31] has shown that BiPh3 significantly enhances photoelectric effects below 200 keV, which is critical for low-energy pseudo-spectroscopy, without inducing any dramatic scintillation quenching when inserted up to 17 wt%. A proof of scalabity up to 155 cm 3 for a 5 wt% bismuth load was additionally established in [START_REF] Bertrand | [END_REF], which will be exploited in future developments. All the components were mixed in a round bottom flask. The resulting solution was put under a nitrogen atmosphere and then frozen using liquid nitrogen. Multiple freeze-pump-thaw cycles were performed to achieve total degassing. The solution was subsequently poured into a cylindrical glass mold containing a small quantity of initiator. The filled mold was purged with nitrogen, sealed and placed into an oven at 40-45 °C for 30 days. Once complete polymerization was observed, the mold was cooled down to room temperature and shattered to free the plastic piece.

The quantitative composition of bismuth-loaded plastic scintillators is described in Table 1: based on the conclusions of [START_REF] Bertrand | [END_REF], the authors have inserted the largest assessed quantity of primary fluorophore for given monomer and organometallic dopant quantities in order to maximize the light yield. A 0.8 wt% bismuth load was selected to allow for a scale-up from the reference samples (2 cm 3 as exploited for neutron detection in [START_REF] Dumazert | Compensated gadolinium-loaded plastic scintillators for thermal neutron detection and counting[END_REF]) without any dramatic degradation of the homogeneity nor of the optical qualities of the prototype sensor. The scintillating samples are cut into a cylindrical shape and polished so that they meet the desired dimensions, i.e. 3.3 cm diameter and 2.9 cm height. Their lateral surface is ultimately covered with a reflecting white paint to limit the escape of scintillation photons.

The most crucial figure of merit for the employability of the synthetized scintillators as radioactivity sensors lies within their light yield, labeled 𝑅 and expressed in scintillation photon per megaelectronvolt (Ph/MeV). This yield is determined by a relative method, using the acquisition and measurement setup described in the next paragraph, where the high voltage is set equal to 1700 V, successively with our 0.8 wt% bismuth-loaded plastic scintillator and an identical shape and volume reference EJ-200 whose yield is known to equal 𝑅 = 10000 ± 100 𝑃ℎ/𝑀𝑒𝑉. Both samples are exposed to a cesium-137 source of activity 𝐴 = 34MBq for 10 min. The ratio of the channel numbers corresponding to 80% of the Compton maxima provides a value for the relative yield of our sample to the reference. A light yield 𝑅 = 8500 ± 200 𝑃ℎ/𝑀𝑒𝑉 is derived for the 0.8 wt% bismuth-loaded plastic scintillator, qualifying it as a decent candidate for gamma-ray detection. The ultraviolet (UV) responses of the scintillators are depicted in Fig. 4, while their spectral responses to the cesium-137 exposure are superimposed in Fig. 5.

Acquisition and measurement setup

On both the detection and compensation channel, the uncovered face of the plastic scintillator is coupled to a bialkali photocathode of a Hamamatsu™ H1949-51 model photomultiplier tube (PMT) powered by a CAEN N147 model 4CH HV programmable power supply. A custom made electronic card developed at the CEA LIST [32] and devoted to the processing of short scintillation pulses (rise and decay time within the nanosecond time scale) digitizes the signal at a frequency equaling 200 Msample s -1 , performs a filtering of the signal over 7 samples in order to maximize the signal-to-noise ratio (SNR) and a differentiation to correct base line fluctuations, triggers the pulse and determines its maximal amplitude to store the event into a 8-bit pulse height histogram. The PMT is isolated from ambient light and static noise sources inside an opaque Faraday cage.

Energy calibration of the plastic scintillators

In order to implement a counting compensation over energy windows of interest below 200 keV, it is necessary, for the bismuth-loaded samples, to establish a correspondence between every channel in which counts are displayed and an energy value. An energy calibration of the scintillators was performed by making use of several gamma radiation sources available in the laboratory, positioned at a distance 𝑑 = 1.5 cm of the scintillator surface along the symmetry axis of the scintillator:

-an americium-241 source whose photoelectric peak is observed at 59 keV; -a cobalt-57 source whose photoelectric peak is observed at 122 keV;

-an europium-152 source with one photoelectric peak observed at 122 keV, and two Compton edges respectively at 198 keV and 245 keV.

The experimental points are joined by line segments. The three counting spectra are superimposed on Fig. 6, whereas Fig. 7 presents the energy calibration curve. Fig. 6. Energy spectra in presence of americium-241, cobalt-57 and europium-152 sources. Fig. 7. Energy calibration curve of bismuth-loaded plastic scintillator.

Hypothesis test for neutron detection

From the counts 𝑁 and 𝑁 , collected on channel 1 (gadolinium-covered detection scintillator) and 2 (terbium-covered compensation scintillator) respectively, and integrated over a temporal window  (empirically identified as sufficient to achieve precise and stable count rate estimations), it is easy to compute the average count rates noted  and  , by Eq. ( 4) and Eq. ( 5), as well as their associated standard deviations 𝜎  and 𝜎  according to Eq. ( 6) and Eq. (7).

 = (4)
 = (5)
𝜎  =  (6)
𝜎  =  (7)
The neutron detection operation takes the form of a classical hypothesis test with a null hypothesis H0 representing the absence of any detected neutron activity and the non-null hypothesis H1 representing the detection of a neutron activity. The test compares the difference of the count rates estimated on channel 1 and 2 respectively to a significance level which is proportional to the standard deviation of the difference.

If:

 - > 𝑡  𝜎  + 𝜎  (8)
Then H1 is accepted.

Else H0 is accepted with a confidence level 1 -𝐾.

The coverage factor 𝑡 in hypothesis test Eq. ( 8) for neutron detection governs the thermal neutron sensitivity 𝑆 versus false detection probability K compromise. The distributions 𝐾 = 𝑓(𝑡 ) admits the well known distribution represented in Fig. 8. To ensure a 1 -𝐾 = 95% 𝑐onfidence level, the coverage factor is set equal to 𝑡 = 1.96. Eventually, in case H1 is accepted, a rate labeled  , attributable to thermal neutrons incident on the detection channel and captured in the gadolinium cover, is estimated over the temporal window  as

 =  - ± 𝜎  + 𝜎  (9)
Fig. 8. Distribution of the false detection probability K as a function of the coverage factor 𝑡 .

Results

In order to assess the performance of the detection scheme described throughout the previous sections, three counting measurements were performed on channels 1 and 2:

-radiation measurement over 𝜏 = 900 s in presence of a cesium-137 source of activity 𝐴 = 34 MBq positioned at a distance 𝑑 = 16 cm of the detection ensemble along its symmetry axis; -radiation measurement over 𝜏 = 3600 s in presence of a californium-252 source of activity 𝐴 = 784 kBq positioned at a distance 𝑑 = 16 cm of the detection ensemble with 𝑒 = 5 cm High Density Polyethylene (HDPE) neutron moderator in between;

-radiation measurement over 𝜏 = 3600 s in presence of a californium-252 source of activity 𝐴 = 784 kBq positioned at a distance 𝑑 = 16 cm of the detection ensemble with 𝑒 = 5 cm Permali HB 780 boron wood neutron moderator and absorber in between.

Both gadolinium and terbium covers are 2.5×2.5 cm surface 250 µm thick 99.9% pure metal foils from Strem Chemicals [33], [34]. The scintillators are isolated by a 5 cm thick lead shield.

No lead castle against background photon radiation was added to the setup. The high voltage supplying the photomultipliers is set equal to 2000 V.

The average count rates  and  computed over selected energy windows of interest are listed in Table 2. The relevant quantities for the detection test as well as the result of the said test are reported in Table 2. The experimental setup (detection channel) is depicted in Fig. 9 in presence of a californium-252 source plus boron wood exposure.

Discussion

The neutron detection test for countings over the three considered thresholds results in a nondetection (null hypothesis acceptance) in presence of the cesium-137 source (typically used for the qualification neutron detectors as it is representative of long-lived fission products present in the fuel cycle). The gamma dose rate at the detector level is measured equal to 17 µSv/h with a commercial Thermo Electron ESM FH 40G-L 10 radiameter, hence confirming the robustness of a photon response compensation up to "controlled area" standards, defined by legislation [START_REF] Légifrance | Arrêté du 15 mai 2006 relatif aux conditions de délimitation et de signalisation des zones surveillées et contrôlées et des zones spécialement réglementées ou interdites compte tenu de l'exposition aux rayonnements ionisants, ainsi qu'aux règles d'hygiène, de sécurité et d'entretien qui y sont imposées[END_REF] from 7.5 to 25 µSv/h.

A sealed californium-252 source is both a neutron and a photon radiation emitter. The neutron emission is described by a Watt spectrum and associated to spontaneous fissions [START_REF] Cf | Table de radionucléides[END_REF], which account for 3.09% of a californium-252 source activity. The mean number of neutrons emitted per fission equals  = 3.77 𝑛/𝑓 . This neutron emission is accompanied by a background of prompt and delayed gamma-rays (X and gamma rays associated to alpha disintegrations are neglected due to low emission probabilities for energies above 20 keV). The resultant background distribution is modeled [START_REF] Hayes | Preliminary Benchmarking Efforts and MCNP Simulation Results for Homeland[END_REF]

The expectation for the energy of a background californium-252 gamma ray reads 𝐸 = 875 𝑘𝑒𝑉, which leads, in a first-order approach, to consider the compensation of a cesium-137 source (gamma emitter at 662 keV) as representative of the californium-252 photon background compensation. The gamma dose rate at the detector level in presence of the HDPE moderated californium-252 is additionally measured equal to 1.2 µSv/h with the radiometer, thus one order of magnitude below the cesium-137 background which was robustly compensated supra.

Now Table 2 shows that in presence of the HDPE moderated californium-252 source, the detection hypothesis is accepted when the [59-198 keV] and [122-198 keV] energy ranges are taken into account, with neutron rates reading  = 0.39 ± 0.16 cps and  0.29 ± 0.13 cps respectively. The detection hypothesis is however rejected for countings over a 198 keV threshold, which is consistent with the neutron capture signature distribution as presented in Fig. 1.

The neutron HDPE moderator was ultimately replaced by an identical thickness boron wood neutron moderator and absorber. The substitution firstly results in a reduction of the count rates recorded on the total considered spectra from 59 keV to saturation, for both channels considered independently. This reduction is explained by the difference in densities of the moderators, respectively 𝜌 = 0.95 g/cm 3 for HDPE and 𝜌 = 1.35 g/cm 3 for boron wood, and the proportionality between these densities and Compton effects in the moderating materials, responsible for the attenuation of the gamma-ray background incident on the sensors. As an illustration, the ratios of count rates recorded on [198 keV; max], with HDPE moderation and boron wood moderation read

 ,  , 1.40 ± 0.01  ,  , = 1
.40 ± 0.01 on channel 1 and 2 respectively, in agreement with the ratio of the moderator densities =1.42, hence leading to an approximate 42% expectation of the photon background reduction. In the meantime, the substitution of HDPE by Permali boron wood results in the rejection of the detection hypothesis on the three countings ranges, therefore canceling the detection response over the two first energy ranges obtained for absorption-free moderation of incident neutron with HDPE. This measurement confirms that the photon background of the californium-252 was indeed compensated in the HDPE measurement, and that neutron captures in gadolinium were indeed responsible for a significant neutron rate displayed over [59 keV; max] and [122 keV; max].

It follows that the measurement campaign on gadolinium-and terbium-covered plastic scintillators highlights the potential for a robust compensation technique dedicated to neutron radiation detection, associated with selective counting at low and medium-energy allowed by bismuth-loading. In order to benchmark the thermal neutron detection scheme based on the physical sensors and the numerical parameters we have introduced, with reference helium-3 proportional counters, a thermal neutron sensitivity estimate labeled 𝑆 and expressed in count per thermal 𝑛 times square centimeter (𝑐/(𝑛 .cm -2 ) or cps/nv) has to be calculated. The spontaneous fission rate of the californium-252 source reads

Conclusion

The neutron measurement by compensation carried out with two gadolinium-and terbiumcovered bismuth-loaded plastic scintillators has allowed for the unambiguous detection of a neutron activity in the photon background of a moderated californium-252 source (estimated about 1.2 µSv/h with a standard radiameter, which matches the typical constraints of a "monitored area" in the context of radioprotection applications). Thus the concept of the neutron detection scheme introduced in this paper has been experimentally validated: this scheme has proven itself robust to a "controlled area" cesium-137 photon background, while being portable and more cost-effective than silicon-based compensation systems. In order to increase the count rates, as well as the precision associated to them, future works will be turned towards a scale-up of the loaded scintillating samples, with all the known challenges associated to such a process: homogeneity and turbidity issues with the loading, self-absorption of the scintillator. The implementation of the neutron counting scheme, including the smoothing on the acquired signals and the hypothesis test for detection, on a field-programmable gate array (FPGA) is currently under development at the CEA, LIST. Whereas preliminary simulations have shown that ani biases result, given the relatively small dimensions of the sensors, in less than 2% distortion of the estimated count rate, this systematic bias will have to addressed by dedicated algorithms when scale-ups will be considered. Research work will additionnally be carried out on the choice of an optimal matrix and on the structuration of the scintillatorphotomultiplier interface to maximize the extraction of scintillation photons.

Fig. 1 .

 1 Fig. 1. Simulated spectral distribution of the capture signature in a 0.8 wt% bismuth-loaded plastic scintillator.

Fig. 2 .

 2 Fig. 2. Principle sketch of the detection system.

2 . 2 , 5 -

 225 diphenyloxazole (PPO) as a primary fluorophore; 3.1,4-Bis((2-methylstyryl)benzene) (Bis-MSB) as a secondary fluorophore;4.

Fig. 3 .

 3 Fig. 3. Topological representation of molecules.

Fig. 4 .

 4 Fig. 4. UV responses of 0.8 wt% bismuth-loaded plastic scintillator (left) and reference EJ-200 (right).

Fig. 5 .

 5 Fig. 5. Energy spectra of 0.8 wt% bismuth-loaded plastic scintillator and reference EJ-200 in presence of a cesium-137 source.



  = 979.13 ± 0.74  = 130.22 ± 0.11  = 98.61 ± 0.10  = 980.62 ± 0.74  = 129.83 ± 0.11  = 99.04 ± 0.10  - = -1.49  - = 0.39  - = -0.43 𝑡  𝜎  + 𝜎  = 798.34 ± 0.67  = 88.00 ± 0.09  = 64.78 ± 0.08  = 800.39 ± 0.67  = 87.71 ± 0.09  = 65.05 ± 0.08  - = -2.05  - = 0.29  - = -0.27 𝑡  𝜎  + 𝜎  = 1.85 𝑡  𝜎  + 𝜎  = 0.25 𝑡  𝜎  + 𝜎  = 0.21 H0 accepted H1 accepted H0 accepted  = 0.29 ± 0.13 [198 keVmax]  = 468.31 ± 0.51  = 36.46 ± 0.05  = 26.06 ± 0.05  = 468.25 ± 0.51  = 36.48 ± 0.06  = 26.06 ± 0.05  - = 0.06  - = -0.02  - = 0.00 𝑡  𝜎  + 𝜎  = 1.41 𝑡  𝜎  + 𝜎  = 0.16 𝑡  𝜎  + 𝜎  = 0.14 H0 accepted H0 accepted H0 accepted Table 2. Average count rates in count per second (cps) over energy ranges of interest and results of the hypothesis test for neutron detection.

Fig. 9 .

 9 Fig. 9. Experimental measurement setup (detection channel).

7 

 7 10 f/s expressed in fission per second, and the neutron emission rate therefore equals 𝐴 , = 3.77  𝐴 , = 1.4  10 𝑛/𝑠 expressed in neutron per second. The thermal neutron fluence incident on the detection scintillator at the exit of the HDPE moderator, i.e. the neutron fluence integrated between 1 meV and 50 meV (cadmium absorption cut-off), labeled  and expressed in thermal neutron per generated neutron and square centimeter (𝑛 (𝑛 ⁄ . cm 2 )), is evaluated by simulation using the average surface flux tally 2 of the MCNPX2.7 code. The estimate reads  = 1.6  10 𝑛 (𝑛. ⁄ cm 2 ), from which an estimate of the thermal neutron flux incident on a sensor, labeled  and expressed in thermal neutron per square centimeter and per second (𝑛 /(cm 2 s)), is derived as  =   𝐴 , = 5.8 𝑛 /(cm 2 s). The thermal neutron sensitivity estimate is eventually calculated as 𝑆 =   (11) equaling 𝑆 = 0.067 ± 0.028 cps / nv and 𝑆 = 0.050 ± 0.022 cps / nv for countings over [59-198 keV] and [122-198 keV] respectively.

Table 1 .

 1 Composition of bismuth-loaded plastic scintillators

	mmonomers (g)	mPPO (g)	mBis-MSB (mg)	mdopant (mg)	wt% (Bi)
	13.44	4.14	5.4	420 (BiPh3)	0.8

  by a piecewise defined function p of photon energy 𝐸 , so that 𝑝 𝐸 = 𝐷 𝑐 𝐸 , where D is a normalization constant and c reads

		6.6 + 𝑒 . , 20 𝑘𝑒𝑉 < 𝐸 < 500 𝑘𝑒𝑉
	𝑐 𝐸 =	20.2𝑒 .	+ 𝑒 . , 500 𝑘𝑒𝑉 < 𝐸 < 1.4 𝑀𝑒𝑉
		7.2𝑒 .	+ 𝑒 . , 1.4 𝑀𝑒𝑉 < 𝐸 < 10.4 𝑀𝑒𝑉


Olivier Oms

Shu Yang

William Salomon

Jérôme Marrot

Anne Dolbecq

Eric Rivière

Antoine Bonnefont

Laurent Ruhlmann
email: lruhlmann@unistra.fr

Pierre Mialane
email: pierre.mialane@uvsq.fr

Heteroanionic Materials Based on Copper Clusters, Bisphosphonates and Polyoxometalates: Magnetic Properties and Comparative Electrocatalytic Studies

Three compounds associating for the first time polyoxotungstates, bisphosphonates and copper ions have been structurally characterized. They consist in heteropolyanionic monodimensional materials where [Cu6(Ale)4(H2O)4] 4-(Ale = alendronate = [O3PC(O)(C3H6NH3)PO3] 4-) complexes alternate with polyoxometalate (POM) units. In Na12[{SiW9O34Cu3(Ale)(H2O)}{Cu6(Ale)4(H2O)4}]•50H2O (SiW9CuAle), the polyoxometalate core consists in a {SiW9Cu3} monomer capped by a pentacoordinated Ale ligand, while sandwich-type Keggin {(SbW9O33)2Cu3(H2O)2.5Cl0.5} and Dawson {(P2W15O56)2Cu4(H2O)2} complexes are found in Na8Li29[{(SbW9O33)2Cu3(H2O)2.5Cl0.5}2{Cu6(Ale)4(H2O)4}3]•163H2O (SbW9CuAle) and Na20[{(P2W15O56)2Cu4(H2O)2}{Cu6(Ale)4(H2O)4}]•50H2O (P2W15CuAle), respectively. A comparative magnetic study of the SiW9CuAle and SbW9CuAle compounds has allowed to fully quantify the Cu II superexchange interactions either for the POM and non-POM subunits,

evidencing that while the paramagnetic centers are antiferromagnetically coupled in the polyoxometalate units, both antiferromagnetic and ferromagnetic interactions coexist in the {Cu6(Ale)4(H2O)4} cluster. All the studied compounds present a good efficiency upon the reduction of HNO2 or NO2 -, the POM acting as a catalyst. However, it has been found that SbW9CuAle is inactive toward the reduction of nitrates, highlighting that both the {(SbW9O33)2Cu3} unit and the {Cu6(Ale)4(H2O)4} cluster do not act as electrocatalysts for this reaction. In contrast, SiW9CuAle and P2W15CuAle have shown a significant activity upon the reduction of NO3 -and thus both at pH 1 and pH 5, evidencing that the chemical nature of the polyoxometalate is a crucial parameter even if it acts as pre-catalyst. Moreover, comparison of the activities of P2W15CuAle and [(P2W15O56)2Cu4(H2O)2] 16-evidenced that if the {Cu6(Ale)4(H2O)4} cluster do not act as electrocatalysts, it act as a co-factor, significantly enhancing the catalytic efficiency of the active POM.

Introduction

Among polyoxometalates (POMs), 1 polyoxotungstates (POTs) and especially the Dawson or Keggin type POT derivatives occupy a specific place as they represent perfect candidates for the multi-step elaboration of POM-based materials. In particular, starting from well-known vacant polyoxotungstate precursors [START_REF]Inorganic Syntheses[END_REF] able to incorporate from one to twenty 3d centers, a vast family of 3d-incorporating inorganic compounds with a tremendous variety of compositions and structures has been obtained. 3 Organic-inorganic hybrid 3d metal-POTs have also been isolated using a large range of secondary oxygen ((poly)carboxylate) [START_REF]Trends in Polyoxometalate Researchs[END_REF] and nitrogen (ethylenediamine, (poly)pyridine, azide,…) 5 containing ligands. As they represent a class of well-insulated complexes of controlled nuclearity and topology, these inorganic or hybrid materials constitute ideal models for magnetic investigations. 6 Moreover, these last years, several POM compounds behaving as Single-Molecule Magnets have been characterized. 7 3dcontaining polyoxotungstates have also been widely investigated for their catalytic and electrocatalytic properties. [START_REF] Hill | Special issue on polyoxometalates with catalytic and electrocatalytic properties[END_REF] For example, the ability of the [Co II 4(H2O)2(PW9O34)2] 10-complex to act as a robust water oxidation catalyst has been demonstrated. 9 The epoxidation of alkenes by Zn II , Fe III or Ni II /Fe III POTs has also been studied. 10 Besides, the electrocatalytic activity of these materials has been investigated, focusing mainly on the NOx reduction. Especially, a huge number of 3d-POTs have been found to reduce nitrites electrochemically. 11 In contrast, much less substituted POTs have been found to be active toward nitrate electroreduction. In short, it has been found that some POTs incorporating Cu II12 or Fe III13 centers can be highly active for the reduction of NO3 -. More recently, it has been shown that a POT incorporating Ni II centers and bisphosphonate (BP) ligands is also able to efficiently reduce nitrate into nitrogen. 14 Herein we report on a series of BP/POT/Cu II compounds. The reactivity of three different vacant POTs ([A--SiW9O34] 10-, [-SbW9O33] 9-and [-P2W15O56] 12-, respectively) with the [O3PC(OH)(C3H6NH2)PO3] 4-alendronate ligand (Ale) and Cu II ions has been thoroughly studied, and three new compounds structurally characterized. Surprisingly, they all contain chains built of two anionic fragments where POM unitsthe POM being purely inorganic or incorporating the alendronate ligandand copper/alendronate multinuclear clusters alternate.

The magnetic properties of two of these compounds have been thoroughly studied, allowing quantifying the magnetic exchange interactions both within the POM complexes and the copper/alendronate entities. The abilities of these three compounds to electrochemically reduce nitrites and nitrates have been evaluated and compared, highlighting that the nature of the POM plays a crucial role in the electrocatalytic activity even if these materials can act as preelectrocatalysts since the reduction of the nitrate ions occurs at a potential lower than the one of the Cu(II/I) reduction. Also, the ability of the copper/alendronate entity to act as a co-factor is discussed.

Experimental section

Synthesis. All reagents were purchased and used without further purification except alendronic Then, alendronic acid (93 mg, 3.73 10 -4 mole) is added and the pH readjusted to 7.5 with 1M

NaOH. The solution is stirred for two hours at room temperature, and let slowly evaporating. [START_REF] Sheldrick | program for scaling and correction of area detector data[END_REF] based on the method of Blessing. [START_REF] Blessing | [END_REF] The structures were solved by direct methods and refined by full-matrix least-squares using the SHELX-TL package. [START_REF] Sheldrick | SHELX-TL version 5.03, Software Package for the Crystal Structure Determination[END_REF] In the three reported structures, there is a discrepancy between the formulae determined by elemental analysis and that deduced from the crystallographic atom list because of the difficulty in locating all the disordered water molecules and alkali ions. Disordered water molecules and counter-ions were thus refined with partial occupancy factors. The hydrogen atoms were theoretically located on the basis of the conformation of the supporting atoms. The crystallographic data are gathered in Table 1. CCDC numbers: 1431904 (SiW9CuAle), 1431905 (SbW9CuAle) and 1431906 (P2W15CuAle).

Magnetic measurements. Magnetic measurements on powder were carried out with a Quantum Design SQUID Magnetometer with an applied field of 1000 G using powder samples pressed in pellets to avoid preferential orientation of the crystallites. The independence of the susceptibility value with regard to the applied field was checked at room temperature. The susceptibility data were corrected from the diamagnetic contributions as deduced by using Pascal's constant tables. The susceptibility times temperature as a function of temperature curves have been simulated using MAGPACK. 21

Electrochemistry. The solution were deaerated thoroughly for at least 30 minutes with pure argon and kept under a positive pressure of this gas during experiments.

Electrochemical measurements were carried out on a PARSTAT 2273 potentiostat at room temperature under argon atmosphere. A three-electrode system was used with a glassy carbon (GC, Tokai Japan) electrode as a working electrode, a platinum wire as auxiliary electrode and the reference electrode was the saturated calomel electrode, which was electrically connected to the solution by a junction bridge filled with electrolyte. The buffer solutions were prepared from the following solution, 1 mol.L -1 CH3COOLi + CH3COOH (pH 5) and 0.5 mol L -1 Na2SO4 + H2SO4 (pH 1).

Results and discussion

Synthesis and structures of SiW9CuAle, SbW9CuAle and P2W15CuAle.

Na12[{SiW9O34Cu3(Ale)(H2O)}{Cu6(Ale)4(H2O)4}]•50H2O (SiW9CuAle) can be obtained in good yield (64%) in water (pH = 7.5) at room temperature from the reaction of [A--SiW9O34] 10-, CuCl2 and alendronate. SiW9CuAle results from the co-crystallization of two distinct polyanionic units, the polyoxometalate/copper/alendronate [SiW9O34Cu3(Ale)(H2O)] 8- complex (Figure 1a) and the copper/alendronate [Cu6(Ale)4(H2O)4] 4-cluster (Figure 1b). In SiW9CuAle, all the alendronate ligands are tetraanionic, the global charge of the compound being compensated by twelve sodium cations. In the {SiW9O34Cu3(Ale)(H2O)} POM, the 3d metal cations fill the vacancies of the {SiW9} precursor, the three Cu II ions forming a pseudoisosceles triangle (dCu••• Cu = 3.67 and 3.84-3.85 Å respectively). Each paramagnetic center is connected to three oxygen atoms of the silicotungstate ligand and an alendronate ligand caps the {SiW9Cu3} fragment, connecting two copper ions of two edges of the triangle via one O-P-O bridges, respectively. In the third edge, the d 9 centers are directly connected via the deprotonated alkoxo group of the Ale ligand and via a long O-P-C-P-O bridge. All the 2porbitals of the Ale bridging oxygen atoms are directed toward the Cu II dx²-y² magnetic orbitals.

We can note that while the [SiW9O37{Cu(H2O)}3] 10-species has been postulated, 22 it has not been possible so far to isolate this monomeric complex. However, several {SiW9}/Cu II compounds have been characterized, including one monomeric {SiW9Cu4} hybrid compound, 4f

four dimeric {(SiW9)2Cun} (n = 5, 23 6 24 or 8 25 ) species and a tetrameric {(SiW9)4Cu14} complex. 26 3a and4a, respectively. For SbW9CuAle, MT continuously decreases from 300 K (MT = 10.98 cm 3 .mol -1 .K, the MT value calculated for 24 non-interacting Cu II centers being 10.89 cm 3 .mol -1 .K assuming g = 2.2) to 2 K (MT = 6.82 cm 3 .mol -1 .K), indicating that overall weak antiferromagnetic interactions are predominant. In contrast, the MT = f(T) curve related to SiW9CuAle is characteristic of overall strong antiferromagnetic interactions, MT decreasing from 300 K (MT = 3.57 cm 3 .mol -1 .K, the MT value calculated for 9 noninteracting Cu II centers being 4.08 cm 3 mol -1 K assuming g = 2.2) to 20 K (MT = 3.20 cm 3 .mol - 1 .K). At low temperature, the MT product increases from 20 K to 4 K (MT = 3.38 cm 3 .mol - 1 .K), highlighting the presence of weak ferromagnetic interactions. Finally, MT decreases from 4 to 2 K (MT = 3.30 cm 3 .mol -1 .K). The quantification of the magnetic exchange interactions in SbW9CuAle and SiW9CuAle is not an easy task considering that for these materials the measured MT products are the resultant of the contributions of trinuclear POM units and hexanuclear {Cu6Ale4} BP complexes.

Moreover, SbW9CuAle is made of three crystallographically independent {Cu6Ale4} fragments and two crystallographically independent POM units (see above). Nevertheless, attempts to fit the data have been effectuated. Starting with SbW9CuAle, it has been considered that (i) the two POMs and the three {Cu6Ale4} units constituting this material are magnetically equivalent, respectively and (ii) the {Cu6Ale4} and the {(SbW9)2Cu3} units are magnetically isolated. Also, in the {(SbW9)2Cu3} fragment, it has been assumed that the copper centers form an equilateral triangle. Besides, a two-J model has been used for the modelization of the {Cu6Ale4} complex. This led to the coupling schemes represented on Figures 3c and3d, which correspond to the following Hamiltonian:

Ĥ(1) = -2J(ŜCuaŜCub + ŜCubŜCuc + ŜCucŜCua) -2J3(ŜCudŜCue + ŜCugŜCuh) -2J4(ŜCueŜCuf + ŜCudŜCuf + ŜCufŜCug + ŜCufŜCuh + ŜCugŜCui + ŜCuhŜCui + ŜCuiŜCud + ŜCuiŜCue) (1)
A good fit to the experimental MT data in the 300 -2 K temperature range has been obtained

for J = -6.80 cm -1 , J3 = +2.40 cm -1 and J4 = -0.37 cm -1 (TIP = 231 10 -6 cm 3 .mol -1 per Cu II center, g = 2.06 and R = 3.3 10 -5 ). [START_REF]MTcalc-MTobs)2/(MTobs)[END_REF] The calculated | J | value is slightly higher than those previously determined for pure {(XW9)2Cu3} (X = As, Sb, Ge) POM species. 30 This can be due to the Focusing now on the {Cu6Ale4} cluster, it is found that both ferromagnetic and antiferromagnetic magnetic interactions coexist in this fragment. It is well known that while several parameters affect the amplitude of the magnetic interactions in {Cu(OX)2Cu} (X = H, CH3, Ph...) dimers with dx²-y² copper orbitals directed toward the bridging ligands, the major factor controlling the exchange coupling is the Cu-O(X)-Cu θ bridging angle. In OH-bridged complexes, based both on experimental results 31 or theoretical calculations, 32 a linear correlation between J and θ has been proposed, with a crossover point at θ ~ 97.5° below which the magnetic behaviour changes from antiferromagnetic to ferromagnetic coupling. In contrast,

for OPh-bridged complexes a crossover point has been theoretically predicted at θ ~ 77.5°, 33 highlighting the huge importance of the X substituent borne by the oxygen atom. In the {Cu6Ale4} cluster, the Cud and Cue and the Cuh and Cug centers are dibridged by oxygen atoms of {P-C(O)-P} BP groups, respectively. Such magnetic core has not been previously reported.

However it can be mentioned that the {Cu-O(CP2)-Cu} fragment is characterized by θ angles in the 87.8-90.6° range. Such very low bridging angles must highly favour ferromagnetic interactions. Also, the two connected {CuO4} fragments are not planar, with a dihedral angle of 65.2-66.9°, thus reducing the overlap of the magnetic orbitals of the copper centers via the p-orbitals of the bridging oxygens. Both these factors explain the sign of the J3 = +2.40 cm -1 constant determined. 34 Finally, weak antiferromagnetic interactions (J4 = -0.37 cm -1 ) have been found between the copper centers connected by {O-P-O} bridges. The energy diagram deduced from the J3 and J4 values indicate that the ground state characterizing {Cu6Ale4} is a triplet, with a S = 0 first excited state very close in energy and located at 0.74 cm -1 . The magnetic behavior of SiW9CuAle has been interpreted considering the POM {SiW9Cu3} fragment and the {Cu6Ale4} cluster as magnetically isolated. In {SiW9Cu3}, the copper centers form an isosceles triangle, necessitating the introduction of an additive exchange parameter compared to the model used for the {SbW9Cu3} magnetic unit. The coupling scheme associated to SiW9CuAle is represented on Figures 3d and4c, and leads to the following Hamiltonian: The best fit to the experimental MT data in the 300 -2 K temperature range has been obtained

Ĥ(2) = -2J1ŜCuaŜCub -2J2(ŜCubŜCuc + ŜCucŜCua) -2J3(ŜCudŜCue + ŜCugŜCuh) -2J4(ŜCueŜCuf + ŜCudŜCuf + ŜCufŜCug + ŜCufŜCuh + ŜCugŜCui + ŜCuhŜCui + ŜCuiŜCud + ŜCuiŜCue) (2)
for J1 = -255 cm -1 , J2 = -55.0 cm -1 , J3 = +1.56 cm -1 and J4 = -0.11 cm -1 (TIP = 110 10 -6 cm 3 .mol - 1 per Cu II center, g = 2.19 and R = 8.6 10 -6 ). [START_REF]MTcalc-MTobs)2/(MTobs)[END_REF] Considering the {SiW9Cu3} fragment, the dx²-y² orbitals of the Cua and Cub atoms interact via the p-orbital of the deprotonated O(C) atom of the BP ligand, with a low dihedral angle of 12.1° and a large θ Cu-O-Cu bridging angle of 134.1°. This leads to a strong antiferromagnetic interaction (J1 = -255 cm -1 ), slightly higher than those found for the few previously reported single alkoxo-bridged Cu II dimers. For example, a coupling constant of -191.5 cm -1 has been found for the [Cu2(bdmap)(acac)(NH3)] 2+ (bdmap = 1,3-bis(dimethylamino)-2-propanolato; acac = acetylacetonate) complex (θ =135.8°). 35 However, in SiW9CuAle, the copper centers are bridged by an additional O-Si-O groups. The coupling constant associated with the interactions involving the magnetic metal centers connected via O-P-O bridges is also negative, but as expected weaker (J2 = -55.0 cm -1 ).

While the MT behaviour at high temperature is mainly due to the {SiW9Cu3} unit, at lower temperature, the increase observed between 25 and 3 K is related to the ferromagnetic interactions between respectively the Cud and Cue and the Cuh and Cug centers (J3 = +1.56 cm - 1 ) belonging to the {Cu6Ale4} fragment (Figure 4b). In contrast, we can note that for SbW9CuAle, the low temperature MT vs. T behaviour is due both to the {Cu6Ale4} (J3 = +2.40 cm -1 and J4 = -0.37 cm -1 ) and the {SiW9Cu3} (J = -6.80 cm -1 ) fragments. As a consequence, on the SbW9CuAle MT vs. T plot, the ferromagnetic coupling in {Cu6Ale4} is masked by the weak antiferromagnetic coupling in the {SbW9Cu3} fragment; a deconvolution of the MT vs.

T curve is thus needed for visualizing the ferromagnetic exchange in SbW9CuAle (Figure 3b).

In addition, for SiW9CuAle, the MT decrease below 3K is correlated to the small antiferromagnetic interactions between the O-P-O bridged Cu II ions of {Cu6Ale4} (J4 = -0.11 cm -1 ). Importantly, we can note that the magnetic coupling constants characterizing the {Cu6Ale4} unit and determined in SbW9CuAle and SiW9CuAle are in good agreement.

Electrochemistry. The redox potentials of P2W15CuAle, SbW9CuAle and SiW9CuAle in a pH 5 and pH 1 medium are gathered in Table 2 and compared to that of the previously reported fully inorganic [(SbW9O33)2{Cu(H2O)}3] 12-(Sb2W18Cu3) POM 16 considered here as a reference.

The cyclic voltammogram (CV) of P2W15CuAle in a pH 5 medium is shown in Figure 5A. In the potential domain explored, it consists of three waves with peaks potential located at -0.14, -0.18 and -0.57 V vs. SCE, respectively. The last reversible wave, denoted III in Figure 5A, is attributed to the redox processes of the W centers (W VI /W V couple). To the reduction waves I and II is associated a single oxidation peak at -0.02 V vs. SCE with a characteristic shape usually encountered for the oxidation of Cu 0 species. 24b Sb2W18Cu3, SbW9CuAle, SiW9CuAle and P2W15Cu4 show at pH 5 (see SI, Figures SI2-6) similar redox behaviour with two successive reduction waves associated to the reduction of Cu II to Cu I and the reduction of Cu I to Cu 0 . On potential reversal, a large oxidation current wave is observable which display the characteristic shape for the reoxidation of deposited Cu 0 on the electrode surface. Again for the first couple, Cu II /Cu I , the cathodic peak current (ipc) is proportional to the square root of the scan rate which indicates that the electrochemical processes are diffusion controlled. For the compound P2W15Cu4 the first redox waves attributed to the W VI /W V couple is observed nearly at the same potential than for P2W15CuAle (Table 2) At variance, for SbW9CuAle and SiW9CuAle, the W VI /W V reversible waves III are not observable while two ill-defined waves are observed at -0.59 V and -0.81 V vs. SCE for the parent compound Sb2W18Cu3. Controlled potential coulometry at a potential just below the peak potential of the second reduction process for Sb2W18Cu3, SbW9CuAle and SiW9CuAle indicate that all the Cu II centers are actually reduced with the exchange of 6.1, 48.2 and 18.2 electrons per molecule respectively.

At pH 1 all the compounds present similar redox behaviour with again two consecutive reduction waves related to the reduction of Cu II to Cu I and the reduction of Cu I to Cu 0 (see ESI, Figures SI6-10). In the reverse scan, a large reoxidation current wave is still noticeable. The only change is a small anodic shift of ca. 70-80 mV for the Cu II /Cu I couple and of ca. 50-100 mV for the Cu I /Cu 0 couple, showing that these compounds are easier to reduce at lower pH.

These small shifts may be due to the nature of the buffer used (1 M CH3COOLi + CH3COOH at pH 5 vs. 0.5 M Na2SO4 + H2SO4 at pH 1). Another possible explanation is the influence of the pKa of the aqua Cu(II) centers (Cu II -OH2) that can be easily deprotonated, giving the hydroxo forms Cu(II) centers (Cu II -OH) when the pH increases as shown in previous work. 37 Electrocatalysis of NOx reduction. Nitrate reduction was chosen to investigate the electrocatalytic capacities of each complex studied in this work. The nitrite reduction, NO2 - being a possible chemical intermediate in the reduction of nitrate, has also been studied.

Reduction of Nitrate.

The CVs were measured as a function of the excess parameter  ( = CNO 3 -/C POM) with the same potential scan rate (2 mV.s -1 ). As is apparent in Figure 6 at pH 5 for P2W15CuAle, the addition of even modest quantities of nitrate induced a large cathodic current enhancement starting at a potential below -0.75 V (only after the first W wave). These observations show an efficient reduction of nitrate by the reduced species of P2W15CuAle. In the explored potential domain, no reduction of nitrate could be detected on the glassy carbon electrode in the absence of polyoxometalate complexes (see SI, Figure SI11). The relative electrocatalytic activities of the reported POMs can be determined calculating the kinetic current Jkinetic (NO 3 -), using the following formula:

Jkinetic (NO 3 -) = (IPOM+NO 3 --IPOM)/S
where IPOM+NO 3 -is the reduction current of the POM in the presence of nitrate, IPOM the reduction current for the POM alone and S the geometrical surface of the electrode. In the current experiment, for P2W15CuAle and at pH 5, the Jkinetic (NO 3 -) measured at -0.90 V are -43.0 and -100.0 A.cm -2 for excess parameters  C NO 3 - 0 /C POM 0 ) of 10 and 30, respectively.

SiW9CuAle, in the same conditions, provides the Jkinetic (NO 3 -) values of -99.2 and -209.6 A.cm -2 for  = 10 and 30, respectively (see SI, Table SI1 and Figure SI12).

For

P2W15Cu4, using the same conditions at pH 5, the Jkinetic (NO 3 -) measured at -0.90 V are lower: -8.7 and -22.7 A.cm -2 for excess parameters

 C NO 3 - 0 /C POM 0
) of 10 and 30, respectively (see SI, Table SI1, Figures SI13-14). Similar electrocatalytic behavior were previously published by Nadjo et al. for this compound. 38 It thus should be noticed that the Jkinetic (NO 3 -) obtained for P2W15CuAle are significantly higher than the ones obtained for the corresponding POM without the alendronate complexes, which might be due to the higher number of Cu atoms in the P2W15CuAle compound (4 in P2W15Cu4 versus 10 in P2W15CuAle).

Surprisingly, the compounds Sb2W18Cu3 and SbW9CuAle do not present significant cathodic enhancements upon the addition of nitrate in solution (see SI, Table SI1, Figures SI15-16). In corresponding to the copper deposition on the glassy carbon electrode. This suggests that the chemical nature of the POM entities adsorbed on the surface of the Cu 0 nanoparticles strongly affects the electrocatalytic activity for the reduction of nitrate on the Cu 0 nanoparticles. Furthermore, it is worth pointing out that the catalytic current with Cu-containing polyoxometalates may not be due solely to the amount of deposited Cu 0 (which is higher for SbW9CuAle than for P2W15CuAle), but should also be influenced by the type of POM used.

Thus the electrocatalytic reduction of nitrate must be traced successively to the properties of the electrodeposited copper and to those of the reduced W centres.

At pH 1, P2W15CuAle gives the Jkinetic (NO 3 -) values of -145.3 and -397.5 A.cm -2 for  = 10 and 30, respectively, showing an increase of current of the reduction of the nitrate at low pH (see SI, Figure SI17). SiW9CuAle under the same conditions provides the same behaviour with Jkinetic (NO 3 -) values of -276.5 and -537.0 A.cm -2 for  = 10 and 30, respectively, indicating an increase of more than two times of the current for the reduction of nitrate for SiW9CuAle at low pH (see SI, Table SI1, and Figure SI18). Again, Sb2W18Cu3 and SbW9CuAle do not present a cathodic enhancement upon the addition of nitrate in solution at pH 1 (see SI, Table SI1 andFigures SI19 andSI20).

Reduction of Nitrite.

All the four complexes have shown a good efficiency upon the reduction of nitrite. The results for Jkinetic (HNO 2 /NO 2 -) values are gathered for comparison in the Supporting Information (Table SI1, Figures SI21-28). Figure 7 indicates that upon reduction, P2W15CuAle catalyses the reduction of nitrite at pH 5. The electrocatalytic reduction of nitrite is found to occur just before the deposition of Cu 0 (wave II), already at the first reversible process of the reduction of Cu II /Cu I (wave I).

Jkinetic (NO 2 -) measured at -0.15 V (peak I) and for  = 10 and 30 were -30.3 and -40.7 A.cm -2 , respectively (see SI, Table SI1). In contrast, at this potential and in the same medium, no significant electrocatalytic activity toward the reduction of NO3 -was detected. It is remarkable that in the presence of NO2 -, a significant reduction current is obtained at the potential of the peak I, while no Cu 0 oxidation peak is obtained during the reversed anodic scan when the cathodic potential limit is set at -0.1 V vs. SCE (Figure 8B). This shows that the NO2 -is able to reoxidize the electrochemically generated Cu I POMAle species, leading to the regeneration of the Cu II POMAle species which seem to be stable when the potential is kept above -0.1 V vs. SCE. It can be concluded that this POM is a good electrocatalyst for the reduction of nitrite in more acidic solution. We can note that at pH 1, the active species should be HNO2 and/or NO.

As a matter of fact, the equilibrium HNO2 + H2O  H3O + + NO2 -(pKa = 3.3 at 18°C) takes place. Also, even if HNO2 disproportionates in a fairly acidic solution, with 3HNO2  H3O + + NO3 -+ 2NO, the rate of this reaction is known to be low. P2W15CuAle at pH 1 presents the larger cathodic current enhancement (Figure 8) in a potential domain of first wave (Cu II /Cu I couple) compared to SiW9CuAle, SbW9CuAle and P2W15Cu4. The corresponding calculated Jkinetic (HNO 2 ) for  = 10 and 30 at -0.07 V were -49.9 and -103.2 A.cm -2 respectively (see SI, Table SI1). SiW9CuAle. The ability of the three reported compounds to electrocatalytically reduce nitrite and nitrate has been studied. All the studied compounds present a good efficiency upon the reduction of HNO2 or NO2 -. In contrast, it has been found that the purely inorganic complex Sb2W18Cu3 is inactive toward the reduction of nitrate, highlighting that the formation of a Cu 0 film at the electrode is not sufficient for electrocatalytically reducing NO3 -. The SbW9CuAle complex is also inactive toward nitrate reduction. As this compound is made of a {(SbW9)2Cu3} fragment, similar to that found in Sb2W18Cu3, and [Cu6(Ale)4(H2O)4] 4-units, this evidences that the copper/alendronate cluster is also inactive. In contrast, SiW9CuAle and P2W15CuAle have shown a good efficiency as pre-catalysts upon the reduction of NO3 -either at pH 1 and pH 5, showing that the chemical nature of the POM is a crucial parameter for the catalytic reduction of nitrate by POM species. Interestingly, comparison of the activities of P2W15CuAle and [(P2W15O56)2Cu4(H2O)2] 16-evidenced that if the {Cu6(Ale)4(H2O)4} cluster do not act as electrocatalysts, it act as a co-factor, significantly enhancing the catalytic efficiency of the active POM. Moreover, it must be possible to introduce functional organic or inorganic fragments into the SiW9CuAle POM by pre-functionalization of the bisphosphonate via its amino group, as previously shown for cobalt 38 and nickel 14 derivatives. In particular, the grafting of an anchoring organic group onto the bisphosphonate ligand constituting SiW9CuAle, allowing to prepare covalently functionalized electrodes, is under study.

Graphic for the Table of Content

Three 1D compounds where purely inorganic or hybrid copper-substituted polyoxometalates alternate with anionic copper-alendronate complexes have been characterized. Despite the presence of different sub-units, the magnetic interactions characterizing these materials have been fully quantified. Furthermore, while all these compounds are efficient electrocatalysts for the reduction of nitrites, only two of them present a good activity as pre-catalysts for the reduction of nitrates, highlighting the crucial role of the nature of the polyoxometalate. The role of the copper-alendronate entity as a co-factor is also evidenced.

  , 16 Na10[A--SiW9O34]•xH2O (SiW9), 2 Na12[-P2W15O56]•24H2O (P2W15), 2 and Na9[-SbW9O33]•19.5H2O (SbW9),17 which have been synthesized according to 10 -4 mole) are dissolved in 10 mL of water and the pH adjusted to 7.5 with 1M NaOH.

Figure 1 .

 1 Figure 1. Ball and stick representations of a) the monomeric {SiW9O34Cu3(Ale)(H2O)} Keggin building unit found in SiW9CuAle; b) the {Cu6(Ale)4(H2O)4} unit common to SiW9CuAle, SbW9CuAle and P2W15CuAle and c) the 1D structure of [{SiW9O34Cu3(Ale)(H2O)}{Cu6(Ale)4(H2O)4}] 12-. Blue octahedra, WO6; purple tetrahedra, SiO4; green spheres, Cu; orange spheres, P; black spheres, C; blue spheres, N; red spheres, O. Hydrogen atoms have been omitted for clarity.

  Focusing now on the [Cu6(Ale)4(H2O)4] 4-complex, it is observed that this unit contains two {Cu2(Ale)2(H2O)2} pairs where all the copper atoms are in square pyramidal distorted environment. The basal planes of the copper ions are each made of two bridging alkoxo ligands (dCu•••O = 1.93-1.99 Å, Cu-O-Cu = 88.8-90.1°) and two P-O oxygen atoms (dCu•••O = 1.93-1.95 Å). These pairs are connected by two copper centers via eight O-P-O bridges, affording a cyclic hexanuclear species. Again, all the bridging oxygen atoms reside in the xy planes. While the coordination sphere of one Cu II ion of each {Cu2(Ale)2(H2O)2} pair is completed by a terminal water molecule (dCu•••O = 2.36-2.37 Å), the other one is connected to a terminal O=W oxygen atom (dCu•••O = 2.29-2.34 Å) of the {W6} crown of the [SiW9O34Cu3(Ale)(H2O)] 8-moiety,affording a -POM-{Cu6}-POM-{Cu6}-infinite molecular wire (Figure1c). It can be noticed that only one pure alendronate/copper cluster has been previously fully characterized,27 and its structure is completely different from that of the [Cu6(Ale)4(H2O)4] 4-one found in SiW9CuAle. Indeed, in the neutral dinuclear [Cu2(Ale)4(H2O)2] complex previously reported, the copper centers are connected by two Ale ligands via two O-P-O and two O-P-C-P-O bridges, all the alkoxo groups of the Ale ligands remaining protonated. This can be explained considering the much lower pH (pH = 2) of the reacting media compared to the one fixed for the synthesis of SiW9CuAle (pH = 7.5). The IR spectrum of SiW9CuAle shows intense O-P-O symmetric and asymmetric stretching bands in the 1000-1200 cm -1 region. Above 1200 cm -1 , the bands related to the organic backbone are observed, while the intense bands due to the W=O and W-O vibrations are observed below 1000 cm -1 . SiW9CuAle is soluble in water. It can be recovered after dissolution by addition of NaCl, the obtained product possessing exactly the same IR spectrum as the pristine one, strongly suggesting that both the {SiW9O34Cu3(Ale)(H2O)} and the {Cu6(Ale)4(H2O)4} fragments are stable in solution. Na8Li29[{(SbW9O33)2Cu3(H2O)2.5Cl0.5}2{Cu6(Ale)4(H2O)4}3]•163H2O (SbW9CuAle) has been obtained my mixing [-SbW9O33] 9-, CuCl2 and Ale in water (room temperature, pH = 8). Addition of NaCl afforded a precipitate which can be recrystallized in a 1M LiCl aqueous solution, leading to a mixed Na/Li salt. In SbW9CuAle, one [(SbW9O33)2Cu3(H2O)2Cl] 13-and one [(SbW9O33)2Cu3(H2O)3] 12-POM units (Figure SI1) co-crystallize with three crystallographically independent [Cu6(Ale)4(H2O)4] 4-complexes analogous to that found in SiW9CuAle. The {Sb III 2W18Cu3} POM cluster has been previously described 16 and consists in three Cu II centers sandwiched between two {Sb III W9} units and connected via long O-W-O-W-O bridges (dCu•••Cu = 4.72-4.84 Å). As for the SiW9CuAle compound, two copper centers of each [Cu6(Ale)4(H2O)4] 4-cluster are connected to O=W oxygen atoms (dCu•••O = 2.17-2.31 Å) of the {W6} crown of the POM. However, in opposition to SiW9CuAle, in SbW9CuAle, the anionic units form a -POM-{Cu6}-POM-{Cu6}2-POM-{Cu6}-POM-{Cu6}2-chain (Figure 2a), with a POM:[Cu6(Ale)4(H2O)4] 4-ratio of 2:3 compared to 1:1 in SiW9CuAle. The third compound Na20[{(P2W15O56)2Cu4(H2O)2}{Cu6(Ale)4(H2O)4}]•50H2O (P2W15CuAle) has been synthesized in conditions similar to those used for SiW9CuAle but replacing the {SiW9} precursor with the {P2W15} one. P2W15CuAle is made of the [(P2W15O56)2Cu4(H2O)2] 16-POM, which can be described as a rhomb-like tetranuclear Cu II cluster sandwiched between two {P2W15} units, 28 and of the [Cu6(Ale)4(H2O)4] 4-complex. In the solid state, [(P2W15O56)2Cu4(H2O)2] 16-clusters alternate with [Cu6(Ale)4(H2O)4] 4-units, forming a -POM-{Cu6}-POM-{Cu6}-1D material (Figure 2b), the [Cu6(Ale)4(H2O)4] 4- complexes and the POMs being connected via oxygen atoms belonging to the {W3} caps of the Dawson units (dCu•••O = 2.21-2.23 Å). Finally, to date, attempts to isolate separately the POM and the {Cu6Ale4} clusters by adding various counter-cations to an aqueous solution of SbW9CuAle have failed, a combination of the two anionic species exhibiting an IR spectrum strictly analogous to that of SbW9CuAle being systematically obtained.

Figure 2 .

 2 Figure 2. Ball and stick representations of the 1D structures of a) [{(SbW9O33)2Cu3(H2O)2.5Cl0.5}2{Cu6(Ale)4(H2O)4}3] 37- (SbW9CuAle) and b) [{(P2W15O56)2Cu4(H2O)2}{Cu6(Ale)4(H2O)4}] 20-(P2W15CuAle). Blue octahedra, WO6; purple spheres, Sb; green spheres, Cu; orange spheres, P; black spheres, C; blue spheres, N; red spheres, O.

Figure 3

 3 Figure 3. a) Thermal behaviour of MT for SbW9CuAle at 1000 Oe in the 2 -300 K range. The solid red line above the experimental data is the theoretical curve derived from the Hamiltonian 1 (see text); b) deconvolution of the theoretical thermal behaviour of MT for SbW9CuAle highlighting the contributions of the POM (blue) and {Cu6Ale4} (green) fragments; c) the coupling scheme used for modelizing the magnetic behaviour of the {(SbW9O33)2Cu3} units in SbW9CuAle; d) the coupling scheme used for modelizing the magnetic behaviour of the {Cu6(Ale)4(H2O)4} units in SbW9CuAle and SiW9CuAle.

  approximations mentioned above. In particular, in SbW9CuAle, the intramolecular Cu•••Cu distances are in the 4.72-4.84 Å range, while the shortest Cu•••Cu distances between the copper ions of the {(SbW9)2Cu3} and those of the {Cu6Ale4} units are of 5.52 Å. These relatively short intermolecular copper-copper distances can induce weak interactions, not taken into account in our model.

Figure 4 .

 4 Figure 4. a) Thermal behaviour of MT for SiW9CuAle at 1000 Oe in the 2 -300 K range. The solid red line above the experimental data is the theoretical curve derived from the Hamiltonian 2 (see text); b) deconvolution of the theoretical thermal behaviour of MT for SiW9CuAle highlighting the contributions of the POM (blue) and {Cu6Ale4} (green) fragments. c) the coupling scheme used for modelizing the magnetic behaviour of the {SiW9O34Cu3(Ale)(H2O)}unit in SiW9CuAle.

Figure 5 .

 5 Figure5. A) Cyclic voltammogram of 0.1 mM P2W15CuAle in a pH 5 buffer solution (1 M CH3COOLi + CH3COOH). v = 2 mV.s -1 . B) Variation of scan rates (from inner to outer curve) : 10,20, 30, 40, 50, 60, 70, 80, 90, and 100 mV s -1 . The potential reversal is fixed after the Cu II to Cu I redox process. (Inset) Variation of the cathodic peak current intensity as a function of the square root of the scan rate. The working electrode was glassy carbon, and the reference electrode was Saturated Calomel Electrode (SCE). * Reoxidation peak of the deposited Cu 0 on electrode surface.

Figure 6 .

 6 Figure 6. A) Electrocatalysis of NO3 -reduction in the presence of 0.1 mM P2W15CuAle in a pH 5 medium, v = 2 mV.s -1 . B) Magnification of A) to highlight the reduction of nitrate ions. * Reoxidation peak of the deposited Cu 0 on electrode surface.

  and SiW9CuAle, show an efficient reduction of nitrate but only after the first W wave of the POM subunit, and after the step II (couple Cu I /Cu 0 )

Figure 7 .

 7 Figure7. A) Electrocatalysis of NO2 -reduction in the presence of 0.1mM P2W15CuAle in a pH 5 medium. v = 2 mV.s -1 , the working electrode was glassy carbon and the reference electrode was the Saturated Calomel Electrode (SCE). B) Electrocatalysis of NO2 -reduction with reversal potential fixed after the Cu 2+ /Cu + redox process. *Reoxydation peak of the deposited Cu 0 on electrode surface.

Figure 8 .

 8 Figure 8. A) Electrocatalysis of NO2 -reduction in the presence of 0.1 mM P2W15CuAle in a pH 1 medium, v = 2 mV.s -1 , the working electrode was glassy carbon and the reference electrode was SCE. B) Electrocatalysis of NO2 -reduction with reversal potential fixed after the Cu II /Cu I redox process. * Reoxydation peak of the deposited Cu 0 on electrode surface.

  SI) available: CVs of P2W15CuAle, Sb2W18Cu3, SbW9CuAle and SiW9CuAle in a pH 5 and pH 1 medium. Table of the value of the catalytic efficiency CAT at different values of the excess parameter  𝐶 𝑁𝑂 3 1 medium. This material is available free of charge via the Internet at http://pub.acs/org. ACKNOWLEDGMENT This work was supported by the Ministère de l'Enseignement Supérieur et de la Recherche, the CNRS, the Université de Versailles Saint Quentin en Yvelines, the Université de Strasbourg, the Université Paris XI and a public grant overseen by the French National Research Agency (ANR) as part of the "Investissements d'Avenir" program n°ANR-11-IDEX-0003-02 and the Labex CHARMMMAT ANR-11-LABX-0039. We also thank the Labex CSC of the University of Strasbourg for financial support.

  Elemental analyses of the solids were performed by the "Service de microanalyses ICSN CNRS", in Gif sur Yvette (France), and by the "Service d'Analyse du CNRS", in Vernaison (France). FT-IR spectra were recorded in the 4000-400 cm -1 range on a

	calc. (found) for P12W30Cu10C16H148N4O196Na20 (M. W. = 10515.4 g.mol -1 ): P 3.53 (3.67), W
	for SiW9O120Cu9C20H147N5P10Na12 (M. W. = 5218.4 g.mol -1 ): W 31.71 (29.72), Cu 10.96 52.45 (50.07), Cu 6.04 (6.05), C 1.83 (2.10), H 1.42 (1.74), N 0.53 (0.53), Na 4.37 (4.28). IR
	(11.06), P 5.93 (5.89), Na 5.29 (5.44), C 4.60 (4.64), H 2.84 (2.51), N 1.34 (1.18). IR (ν/cm -1 ): (ν/cm -1 ): 1622 (m), 1494 (m), 1395 (w), 1118 (m), 1081 (s), 1051 (s), 1015 (m), 933 (s), 877
	1626 (m), 1511 (m), 1472 (m), 1395 (w), 1109 (m), 1054 (s), 1115 (m), 980 (m), 967 (m), 950 (s), 727 (s), 687 (s), 577 (s), 558 (s).
	(m), 930 (m), 911 (m), 878 (s), 804 (s), 671 (s), 559 (s), 509 (s).
	Preparation Physical Measurements.	of
	Na8Li29[{(SbW9O33)2Cu3(H2O)2.5Cl0.5}2{Cu6(O3PC(O)(C3H6NH3)PO3)4(H2O)4}3]•163	H2O
	(SbW9CuAle): Na9[-SbW9O33]•19.5H2O (570 mg, 2 10 -4 mole) and alendronic acid (80 mg,
	3.21 10 Preparation of Na20[{(P2W15O56)2Cu4(H2O)2}{Cu6(O3PC(O)(C3H6NH3)PO3)4(H2O)4}]•50H2O
	(P2W15CuAle): CuCl2•2H2O (60 mg, 4.46 10 -4 mole) and alendronic acid (45 mg, 1.80 10 -4
	mole) are dissolved in 20 mL of water. Then, Na12[-P2W15O56]•24H2O (750 mg, 1.74 10 -4
	mole) is added as a solid and the pH adjusted to 7 with 1M NaOH. The solution is stirred for
	two hours at room temperature. Finally, NaCl (40 mg) is added and the solution is left to

After one week, green needle crystals suitable for X-Ray diffraction studies are collected and washed with ethanol and ether (m = 250 mg; yield = 64% based on Ale). Anal. calc. (found) -4 mole) are dissolved in 10 mL of water. Then , CuCl2•2H2O (108 mg, 6.33 10 -4 mole) is added and the pH adjusted to 8 with 1M NaOH. The solution is stirred for one hour at room temperature. The addition of 750 mg of NaCl leads to the precipitation of a green powder which is redissolved in 20 mL of 1M LiCl. After one week, green needle crystals suitable X-Ray diffraction studies are collected and washed with ethanol and ether (m = 200 mg; yield = 43% based on Cu). Anal. calc. (found) for Sb4W36Cu24P24C48N12H444ClNa8Li29O396 (M. W. = 17322.5 g.mol -1 ): W 38.21 (37.30), Cu 8.80 (8.82), P 4.29 (4.32), C 3.33 (3.41), H 2.58 (2.19), N 0.97 (0.93), Na 1.06 (1.00), Li 1.16 (1.13).

IR (ν/cm -1 ): 1625 (m), 1493 (m), 1396 (w), 1115 (m), 1049 (s), 932 (s), 864 (s), 682 (s), 566 (s), 505 (s). crystallize. After two weeks, green rod crystals suitable for X-Ray diffraction studies are collected and washed with ethanol and ether (m = 152 mg; yield = 32% based on Ale). Anal.

Nicolet 30 ATR 6700 FT spectrometer. Single-crystal X-Ray intensity data collections were carried out with a Bruker Nonius X8 APEX 2 diffractometer equipped with a CCD bidimensional detector using Mo Kmonochromatized radiation ( = 0.71073 Å). The absorption corrections were based on multiple and symmetry-equivalent reflections in the data sets using the SADABS program

Table 1 .

 1 Crystallographic data for SiW9CuAle, SbW9CuAle and P2W15CuAle.

		SiW9CuAle	SbW9CuAle	P2W15CuAle
	Empirical formula	SiW9O120Cu9	Sb4W36Cu24P24C48	P12W30Cu10C16H148N4
		C20H147N5P10Na12	N12H468ClNa8Li29O396	O196Na20
	Formula weight, g	5218.4	17346.2	10515.4
	Crystal system	monoclinic	triclinic	monoclinic
	Space group	P21/c	P-1	P21/c
	a / Å	23.3560(8)	22.384(3)	32.256(3)
	b / Å	22.4995(8)	32.340(4)	28.454(3)
	c /Å	28.2725(9)	34.221(4)	27.131(2)
	 / °	90	75.951(4)	90
	 / °	99.6330(17)	78.016(5)	93.400(4)
	 / °	90	78.695(5)	90
	V / Å 3	14647.7(9)	23229(5)	24857(9)
		4	2	4
		200	200	200
	calc / g cm -3	2.370	2.304	2.654
	 / mm -1	8.577	10.359	14.838
	Data / Parameters	42833 / 1636	82318 / 4165	43911 / 2165
	Rint	0.0984	0.0954	0.1015
	GOF	1.072	1.031	1.037
	R (>2(I))