A Novel Three-Dimensional Copper(II) 1,2-Ethylenediphosphonate Framework with Channel-like Voids

Turquoise monoclinic single crystals of the novel three-dimensional Cu2[μ8-O3P(CH2)2PO3]·3.2H2O coordination polymer were prepared using the silica gel method. Space group C2/m (no. 12) with a = 1483.6(2), b = 668.44(8), c = 436.30(6) pm, β = 93.28(2)°. The Cu2+ cation is coordinated by four oxygen atoms stemming from the 1,2-ethylenediphosphonate dianions in a square planar manner and two water molecules in the axial positions. The connection between the Cu2+ cations and the [CPO3] units from the 1,2-ethylenediphosphonate dianions leads to layers parallel to (100), which are linked by the ethylene groups to a three-dimensional framework with channel-like voids. The channel-like voids accommodate water molecules not bound to Cu2+ and extend parallel to [001] with an opening of about 550 pm × 260 pm. Magnetic measurements reveal an antiferromagnetic behavior due to a superexchange coupling between Cu2+ ions through an oxygen bridge. The UV/Vis spectrum reveals three d–d transition bands at 694, 774, and 918 nm. The compound can be fully dehydrated by thermal treatment and rehydrated by storage in ambient air.

93. 28(2)°. The Cu 2+ cation is coordinated by four oxygen atoms stemming from the 1,2ethylenediphosphonate dianions in a square planar manner and two water molecules in the axial positions. The connection between the Cu 2+ cations and the [PO 3 C] units from the 1,2ethylenediphosphonate dianions leads to layers parallel to (100), which are linked by the ethylene groups to a three-dimensional framework with channel-like voids. The voids accommodate water molecules not bound to Cu 2+ and extend parallel along [001] with an opening of about 550  260 pm. Magnetic measurements reveal an antiferromagnetic behaviour due to a superexchange coupling between Cu 2+ ions through an oxygen bridge. The UV-Vis spectrum reveals three dd transition bands at 694, 774, and 918 nm. The compound can be fully dehydrated by thermal treatment and rehydrated by storage in ambient air.

Introduction
Metal organophosphonates are of great interest due to their potential application e.g. as gas-phase sensor [ 1 ], catalyst [ 2 , 3 ], and ion exchanger [ 4 ]. Using organodiphosphonates as ligands, various porous structures can be synthesized [5−10]. The connection between Cu 2+ and anions of alkylene-and arylenediphosphonic acids leads to coordination polymers with one-, two and threedimensional structural features [ 11 − 18 ]. The structure depends on the chain lengths and shape of the organic unit. Cu(II)-alkylenediphosphonates possess mostly threedimensional frameworks, in which compounds with e.g. butylene and pentylene units have an open threedimensional framework accommodating water molecules [11]. On the other hand, depending on the synthesis conditions the reaction between Cu 2+ and 1,4butylenediphopshonic acid can also result in a layer-like coordination polymer [16]. The structures of these coordination polymers can be modified using additional Ndonor ligands [ 19 − 24 ]. Moreover, transition metal organodiphosphonates have interesting magnetic properties with ferromagnetic and/or antiferromagnetic interactions between the metal atoms [12−15,19,25−30] .
Herein, we report on a novel zeolite-like threedimensional copper 1,2-ethylenediphosphonate with channel-like voids accommodating water molecules.
Symmetry code: #2: x, -y, z-1; #3: x, y, z-1; #6: -x, -y, -z The Cu 2+ polyhedra ribbons and the [PO 3 C] tetrahedra are linked by common corners forming infinite layers parallel to (100). The phosphorous atoms are located above and below the Cu 2+ polyhedra ribbons. These layers are stacked in …ABAB… sequence along the [100] direction (Fig. 3). The layers are connected by the C−C bonds of the ethylene groups, leading to a three-dimensional framework with small channel-like voids along [001] ( Fig. 5 and 4). The largest and narrowest openings of the channel-like voids are approximately 550 × 260 pm with van der Waals radii [33] of the framework atoms taken into account excluding the weakly bonded water molecule O(w1), which can be easily removed by thermal treatment and is not essential for the stability of the copper 1, 2    TGA/DTA studies (Fig. 7) were carried out in air from room temperature to 1000 °C at a heating rate of 10 K min −1 . The loss of water molecules occurs stepwise in two endothermal processes. The first step is finished at approximately 138 °C and the second one at 240 °C. The weight loss of 14.8 % corresponds to the release of all water molecules (calc. 15.5 %). The dehydrated compound is stable up to 320 °C. This is obviously due to the small contribution of the weakly bound water molecule (O(w1)) to the bond valence sum of Cu 2+ . Thus making it unnecessary to reconstruct the coordination sphere during dehydration, which causes crystal structures of many coordination polymers often to collapse on heating. A small increase of weight between 320 and 360 °C is followed by a steep weight loss during a highly endothermic process. This can be associated to the decomposition of the 1,2ethylenediphosphonate anion, which was similarly observed in other organodiphosphonate complexes [11,18]. Between 370 °C and 450 °C an increase in weight occurred. XRD patterns of samples annealed between 380 and 450 °C showed an amorphous fraction and only reflections of CuO. The following weight loss leads to the formation of Cu 2 P 2 O 7 and is finished at about 660 °C. The colourless residue was identified as phase pure Cu 2 P 2 O 7 by X-ray powder diffraction. The entire weight loss up to 1000 °C of 19.2 % is in good agreement with the calculated one of 18.8 % with Cu 2 P 2 O 7 as the final product. . The dehydrated sample was split into two fractions. One fraction was stored in water at room temperature for 24 h and the other one was stored at ambient air for 7 days leading to a nearly complete rehydration and the colour turned back to turquoise in both cases. Thermogravimetric analysis of the rehydrated samples up to 1000 °C showed a total weight loss of 16.9 % and 17.0 %, respectively, indicating that the rehydrated samples contained 2.7 H 2 O rather than 3.2 H 2 O per formula unit. Fig. 8 shows the XRD patterns of the dehydrated and rehydrated samples compared with the asprepared material Cu 2 [µ 8 -O 3 P(CH 2 ) 2 PO 3 )]⋅3.2H 2 O. There is no significant change in the patterns indicating that the crystal structure of the host has not been changed during the dehydration and rehydration process. Furthermore, the compound can be partially dehydrated by storage in a desiccator over sulfuric acid. After 7 days the colour had turned to blue and a weight loss of 9.6 % was observed corresponding to the loss of 2 H 2 O per formula unit. A longer storing time in the desiccator did not lead to further weight losses.   [ 36 ]. A precise inspection shows that the polyhedron deviates only slightly from D 4h symmetry (three transition bands), therefore the energy levels of the d xz and d yz orbitals are very close to each other, resulting in almost identical transition bands [37,38]. According to Billing and Hathaway [36]   The magnetic susceptibility of Cu 2 [µ 8 -O 3 P(CH 2 ) 2 PO 3 )]⋅3.2H 2 O was measured between 3 and 300 K (Fig. 11). The compound shows a paramagnetic behaviour. The change of the susceptibility in the range 35−300 K followed the Curie-Weiss law with a Curie constant of C = 11.77·10 −6 m 3 K mol −1 . The observed Weiss temperature of Θ = -19.45 K suggests an antiferromagnetic interaction. From the Curie constant the magnetic moment is calculated as µ mag = 2.74 µ B per formula unit, which is close to the calculated spin-only value for two isolated Cu 2+ ions (two unpaired electrons) of 2.83 µ B . Figure 12 shows the magnetic moment depending on temperature. It can be seen that the magnetic moment decreases drastically below approximately 100 K indicating an antiferromagnetic coupling between Cu 2+ ions. A direct exchange (δ bond) between neighbouring Cu 2+ can be certainly excluded because of the long Cu Cu distance of 334.2(1) pm. The antiferromagnetic behaviour can be explained by superexchange coupling between Cu 2+ through the phosphonate oxygen atom O(1), which connects two copper ions (see Fig. 1 and 4). The coupling through the bridging water molecule O(w1) can be excluded because of its considerably longer distance to the Cu 2+ ions. Between 100 K and 300 K the magnetic moment changes only slightly and is close to the theoretical value indicating that there is no antiferromagnetic coupling.

Conclusions
In summary, we reported on the synthesis and crystal structure of a novel three-dimensional Cu(II)-1,2ethylenediphosphonate coordination polymer with channels accommodating water molecules. Thermal investigations reveal that the dehydrated compound is stable between 240 and 320 °C. The dehydrated sample can be rehydrated upon storage in ambient air. Magnetic measurements show an antiferromagnetic interaction between the copper ions by a superexchange through the phosphonate oxygen atoms.

Experimental Section
Single crystals of Cu 2 [µ 8 -O 3 P(CH 2 ) 2 PO 3 )]⋅3.2H 2 O were grown in a silica gel matrix [39]. To a mixture of 6 ml 2M HNO 3 , 3 ml water, and 6 ml of 0. ATR Fourier transformed infrared (IR-ATR) measurements were carried out at room temperature with a resolution of 2 cm −1 using a Bruker Alpha FT-IR spectrometer equipped with diamond ATR unit. Thermoanalytic measurements with a heating rate of 10 K/min were performed in flowing air using a Netzsch STA 449 device. Temperature dependent magnetizations were measured at µ 0 H = 0.3 T in the temperature range of 3 to 300 K using a Quantum Design PPMS 9. The X-ray powder diffraction patterns were recorded at room temperature on a Bruker D8-Advance diffractometer, equipped with a one-dimensional silicon strip detector (LynxEye TM ) and operating with Cu-Kα radiation. The diffuse reflectance UV-Vis spectrum was obtained using a Perkin Elmer UV-Vis spectrometer Lambda 19. BaSO 4 was used as a white standard. X-ray single crystal structure determination was performed on a Siemens P4 four-circle diffractometer (MoKα, graphite monochromator) in a theta range up to 27.98°. Numerical absorption corrections have been applied. The phase problem was solved by direct methods. Full matrix least squares refinement employing |F| 2 made use of the SHELXTL program suite [40]. Hydrogen atom positions of water molecules have not been taken into account. The C bound hydrogen atom was located in a Difference Fourier map and has been refined with an isotropic displacement parameter. Crystallographic data are given in Table 3

-Ethylenediphosphonate Framework with Channel-like Voids
A novel Cu(II) 1,2-ethylenediphosphonate coordination polymer has been synthesized and structurally characterized. The three-dimensional framework has channel-like voids accommodating water molecules.