Direct Synthesis from Various Tetraphosphonic Building Blocks of

Direct Synthesis from Various Tetraphosphonic Building Blocks of Homologous Hybrid-Layered Copper(II) Derivatives Incorporating Copper Hydrate Cations Ferdinando Costantino,*,†,‡ Thierry Bataille,† Nathalie Audebrand,† Eric Le Fur,† and Claudio Sangregorio§

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 9 1881-1888

Sciences Chimiques de Rennes (UMR-CNRS 6226, 1-ENSCR) Equipe, “Mate´ riaux Inorganiques: Chimie Douce et Re´ actiVite´ ”, UniVersite´ de Rennes 1, AVenue du General Leclerc, 35042 Rennes Cedex, France, Dipartimento di Chimica, UniVersita` di Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy, and UdR INSTM, UniVersita` di Firenze, Via della Lastruccia 3, 50019 Sesto Fiorentino, Firenze, Italy ReceiVed January 30, 2007; ReVised Manuscript ReceiVed June 26, 2007

ABSTRACT: The solvothermal reaction of three different diamino tetraphosphonic acids of general formula (H2O3PCH2)2N-RN(CH2PO3H2)2, R ) C6H12 (H8L1), 1,4-(CH2)2C6H4 (H8L2), and 1,3-(CH2)2C6H4 (H8L3), with copper acetate has led to three new layered copper phosphonates with similar structures. The three building blocks contribute to the formation of hybrid inorganic/ organic anionic layers constituted of parallel polymeric inorganic units belonging to the layers and spaced by the organic linkers. The formers are built of copper square pyramids and tetrahedral phosphonic groups. The negative charge of the layers is compensated by unusual hexaaqua copper(II) cations placed into the interlayer region and linked to the layers by means of a complicated H-bonds network between water molecules and CPO3 groups in the sheets. Compounds 1 and 2 are isotypical and have the general formula Cu2(H2O)2(H2L1)‚Cu(H2O)6‚2H2O and Cu2(H2O)2(H2L2)‚Cu(H2O)6‚2H2O, respectively. In contrast, in compound 3, with the formula Cu2(H2O)2(H2L3)‚Cu(H2O)3‚3H2O, the interlayer copper ions are three-hydrated and are directly connected to the deprotonated PO groups. The structures of compounds 1 and 3 have been solved from X-ray single-crystal diffraction data, whereas the structure of compound 2 has been refined from X-ray powder diffraction data using a model derived from that of 1. The thermal and magnetic properties of these compounds have been investigated by means of several techniques. Introduction The chemistry of hybrid inorganic-organic materials has seen, during the past decade, an exponential growth due to the high number of possible applications in many fields such as gas storage, catalysis, and ion exchange.1 As an example, the metal-organic frameworks constituted by the assembly of transition metals and polycarboxylate groups possess a high versatility in building up porous materials with remarkable surface area and gas storage properties.2,3 In these compounds, the carboxylic linkers act as spacers of discrete secondary building units leading to rigid and stable inorganic frameworks with an accessible porosity.4-6 On the contrary, the polyphosphonic building blocks usually show a higher structural variability and rarely allow good structural control in engineering “tailor-made” porous materials.7,8 In the field of metal phosphonates, attention has recently been focused on the use of amino bis-methylphosphonic groups with the general formula R-N(CH2PO3H2)2, where R ) n-alkyl, carboxyalkyl, carboxyphenyl, and so on. Such phosphonic acids are versatile ligands because they can be prepared in a relatively simple way and a large variety of organic groups can be chosen.9-13 Di- and tetravalent metal derivatives of these acids showed similar layered structures in which the organic pendants are located between the inorganic sheets with a herringbone interdigitated arrangement. This feature is often dependent on the steric hindrance, the length of the organic moieties attached to the amino diphosphonic group, and the hydrophobic/hydrophilic interactions that occur among them.10,11 Several authors have also prepared metal(II) or (IV) phosphonates using amino tris* To whom correspondence should be addressed. E-mail: [email protected]. † Sciences Chimiques de Rennes. ‡ Universita ` di Perugia. § Universita ` di Firenze.

or tetraphosphonates with the general formula N(R-CH2PO3H2)3 or (H2O3PCH2)2N-R-N(CH2PO3H2)2 to reach new high-dimensional compounds.14-20 In this way, it has been possible to obtain a plethora of compounds varying from closely packed three-dimensional (3D) to pillared-layered or open framework structures depending on the nature of the organic groups and the metal used. As an example, the reaction of many transition metal(II) with (H2O3PCH2)2N-C4H8-N(CH2PO3H2)2 has recently been described, which led to a family of isostructural 3D closely packed pillared compounds.17 The reaction of the same phosphonic acid with Zr(IV) led to a layered compound with a different structure. In this compound, the phosphonic groups were placed inside the layers, contributing to the formation of hybrid inorganic/organic sheets.18 This means that the polyphosphonates tend to have a high structural variability influenced by the flexibility, the presence of noncovalent interactions, and the high number of connecting oxygen atoms. Consequently, the possibility to have similar building units tailored by the organic linkers is usually small. Here, we present the synthesis, structure, magnetic properties, and thermal behavior of three new layered copper(II) R-diamino tetraphosphonates. The layers have the same topology and are constituted of similar inorganic polymeric chains spaced by three different organic linkers, that is, aromatic rings and alkyl chains that have remarkably different lengths, steric requirements, and rigidity. Experimental Section Synthesis of the Phosphonic Acids. All reagents were analyticalgrade commercial products and were used without further purification. The phosphonic acids of formula (H2O3PCH2)2N-R-N(CH2PO3H2)2, R ) C6H12 (H8L1) or C8H8 (H8L2, H8L3), were prepared according to the Moedritzer-Irani synthesis21 by reacting 1,6-diaminohexane, 1,4diamino (para)xylene, and 1,3-diamino (meta)xylene with phosphorous acid and formaldehyde to obtain the respective tetraphosphonic acid.

10.1021/cg070103u CCC: $37.00 © 2007 American Chemical Society Published on Web 08/10/2007

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Table 1. Structural Data and Refinement Details for 1-3 sample code

1 single crystal

2 powder

3 single crystal

empirical formula formula weight wavelength (Å) crystal size (mm) crystal system space group a (Å) b (Å) c (Å) β (°) V (Å3) Z µ (mm-1) calcd density (g cm-3) mo. of data no. of reflections no. of parameters no. of restraints Rpa Rwpb Rint RF2c R1d (I > 2σ/all data) wR2e (I > 2σ/all data) GOFf on F2 peak-hole ratio (e Å-3)

Cu3P4O22N2C10H44 858.65 0.71069 0.2 × 0.2 × 0.5 monoclinic P21/n 11.8015(7) 6.3576(3) 19.398(1) 94.84(3) 1450.2(2) 2 2.492 (Mo KR) 1.962 17197 3359 (I > 2σ) 223 14

Cu3P4O22N2C12H40 878.66 1.54056

Cu3P4O19N2C12H28 818.89 0.71069 0.2 × 0.3 × 0.4 orthorhombic Cmc21 18.094(1) 24.248(1) 6.3656(3)

monoclinic P21/n 12.3279(3) 6.2744(1) 18.4919(4) 91.737(2) 1429.70(6) 2 1.973 4700 1514 90 38 0.09 0.119

0.075

2792.7(2) 4 2.577 (Mo KR) 1.943 15842 2468 (I > 2σ) 195 12 0.102

0.108 0.039/0.066 0.106/0.137 1.093 0.965/-1.451

3.52

0.054/0.063 0.139/0.146 1.047 0.903/-0.748

2 1/2 c 2 2 2 d 2 2 a R ) ∑|I - I |/∑I . b R 2 e p o c o wp ) [∑w(Io - Ic) /∑wIo ] . RF2 ) ∑|Fo - Fc |/∑|Fo |. R1 ) S(|Fobs| - |Fcalcd|)/S|Fobs|. wR2 ) {S[w(Fobs - Fcalcd )2]/ S[w(Fobs2)2]}0.5. f GOF ) [∑w(Io - Ic)2/(No - Nvar)]1/2.

Synthesis of the Copper(II) Phosphonates. The three copper phosphonates with general formula Cu2(H2O)2[(PO3CH2)2-N-R-N(HPO3CH2)2]‚Cu(H2O)x‚nH2O [R ) C6H12 (1), 1,4-(CH2)2C6H4 (H4L2) (2), or 1,3-(CH2)2C6H4 (H4L3) (3); x ) 6, n ) 2 for 1 and 2 and x ) 3, n ) 3 for 3] were prepared by reacting 0.5 mmol of H8L1 (243 mg), H8L2, or H8L3 (256 mg) with a 0.1 M solution (10 mL) of Cu(AcO)2 and urea (5 mmol) in 20 mL of H2O. Two millimoles of sodium hydroxide was used to dissolve the phosphonic acid. After the solutions were mixed (30 mL), rapid precipitation of phosphonates occurred, so that concentrated nitric acid was added dropwise until the solutions were clear and the pH was 3. The clear blue solutions were then maintained at 80 °C for 2 days in closed Teflon bottles. Small blue needle-shaped single crystals of 1 and 3 were filtered off and washed two times with deionized water, while only well-crystallized blue powder was recovered for 2. Physical Measurements. The compositions of 1-3 were evaluated from energy dispersive analyses performed on single crystals with a JSM 6400 spectrometer, equipped with an Oxford Link Isis analyzer with a Varian SpectrAA 10 plus. The mean ratios Cu:P found from five energy dispersive spectroscopy analyses are 1:1.6 for 1, 1:1.7 for 2, and 1:1.4 for 3, respectively. Suitable single crystals of 1 and 3 were mounted on a four-circle Bruker APEX CCD diffractometer, using Mo KR radiation (λ ) 0.71073 Å). Data collection and unit cell parameter refinements were performed by means of the Bruker SMART program.22 Data were processed for reduction through the Bruker integration program SAINT software.23 The SADABS program24 was used for scaling and multiscan absorption correction. Crystallographic data and details on data collection and refinement are listed in Table 1. Structure drawings were carried out with Diamond 3.0e, supplied by Crystal Impact. X-ray powder diffraction data for the structural determination of 2 were obtained with a Siemens D500 diffractometer, using monochromatic Cu KR1 radiation (λ ) 1.5406 Å) selected with an incident beam curved-crystal germanium monochromator. Accurate powder diffraction data for 2 were collected in situ at different temperatures, with a Bruker AXS D5005 powder diffractometer using a diffracted beam graphite monochromator (Cu KR1,2), equipped with an Anton Paar HTK1200 oven camera. Qualitative temperature-dependent X-ray diffraction (TDXD) was performed under flowing air with a powder diffractometer equipped with a curved position-sensitive detector (INEL CPS 120) and a hightemperature attachment from Rigaku. The detector was used in a semifocusing arrangement by reflection (Cu KR1 radiation). With this geometry, the stationary sample is deposited on a flat sample holder

located at the center of the goniometer. The counting time was 45 min for each diffraction pattern. Decompositions were carried out in flowing air with a heating rate of 10 °C h-1 between 17 and 600 °C. (Temperature calibration was carried out with standard materials in the involved temperature range.) Coupled thermogravimetric (TG) and differential thermal analysis (DTA) was performed with a Netzsch STA490C thermoanalyser under a 20 mL min-1 air flux with a heating rate of 5 °C min-1. Magnetic measurements were performed between 2 and 300 K with applied magnetic fields of 1 T using a Cryogenic S600 SQUID magnetometer. The field dependence of the magnetization was measured on the same samples at 2.0 K in field up to 6.5 T. Data were corrected for the magnetism of the sample holder, which was separately determined in the same temperature and field range. The underlying diamagnetism of each sample was estimated from Pascal’s constants. A correction for a temperature-independent paramagnetic contribution χTIP ) 180 × 10-6 emu/mol25 was also applied. The electron paramagnetic resonance spectra of 2 and 3 were recorded with an X-Band Bruker Elexsys E500 spectrometer. Crystal Structure Determination. The structures of 1 and 3 were solved by the direct methods (SIR 97 program)26 and refined by fullmatrix least-squares against F2 using all data, with the SHELXL software package.27 For compound 1, H atoms positions bonded to oxygen atoms were refined by restraining the O-H bond length to 0.93(2) Å, whereas those linked to the C atoms were added geometrically with riding coordinates. Isotropic displacement parameters of hydrogen atoms linked to the carbon atoms were set at a value 1.2 times higher than that of their parent atom and 1.5 times for those linked to the oxygen atoms and were not refined. The structure of compound 3 was initially solved and refined in the monoclinic P21 space group with the following cell parameters: a ) 15.128(9) Å, b ) 6.3656(3) Å, c ) 15.126(9) Å, and β ) 106.54(3)° but the strong similarities of the a and c cell parameters and the high electron residual density found in the difference Fourier map suggested a higher symmetry. For that reason, a double-volume orthorhombic cell with parameters a ) 18.094(1) Å, b ) 24.248(1) Å, and c ) 6.3656(3) Å was found using the Lepage program,28 available in the WinGX programs suite,29 and the successive data refinement was performed in the Cmc21 space group. Hydrogen atoms belonging to the water molecules were not placed, because they could not be found in the difference Fourier map. The hydrogen atoms were refined in the same way of 1, as described above. The structure of compound 2 was refined from laboratory X-ray powder diffraction data starting from that of 1. The positions of the first 20 lines, accurately determined using a

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Figure 1. Rietveld plot of 2; observed, calculated, and differences curves are shown. The inset shows a zoom of the 15-35° 2θ region. modified Pearson VII profile function, were used for indexing using the DICVOL04 program.30 It crystallizes with a monoclinic primitive cell, with parameters shown in Table 1 [M(20) ) 62, F(20) ) 186 (0.0029, 37)].31,32 The systematic absences of the reflections h0l, h + l ) 2n + 1, 0k0, k ) 2n + 1 suggested P21/n as the best group. The strong analogies of the cell parameters and the space group between 1 and 2 (see Table 1) suggested that the two compounds were isostructural. Therefore, an initial structural model for 2 was derived from that of 1 by substituting the atoms of the alkyl chain in 1 with those of the p-xylene aromatic ring and placed in calculated positions, with the aid of the Powdercell program.33 Then, the model was refined using the Rietveld method, implemented in the program GSAS.34 Zero shift, cell parameters, background (Chebycev polynomial function with 12 terms), and profile shapes were first refined. A corrected pseudo Voigt profile function35 (six terms) with two terms for the correction of asymmetry at the low-angle region was used.36 The atomic coordinates and the constrained isotropic thermal factors were then refined. The soft restraints between all of the atoms were imposed as follows: Cu-O ) 1.95(5), P-O ) 1.56(5), P-C ) 1.83(5), C-C ) 1.39(3), and C-N ) 1.48(5) Å, and the bond angle restraints [120(2)°] between the aromatic carbon atoms were also added. The statistical weight of these restraints was maintained to a relatively high value as the refinement proceeded, but it was not possible to set it to zero because of some unrealistic light atom bond distances. At the end of the refinement, the shifts in all parameters were less than their standard deviations. The final Rietveld plot for 2 is shown in Figure 1.

Results Crystal Structure Description. (a) Crystal Structures of 1 and 2. The detailed structure description of the copper(II) tetraphosphonates 1 and 2 is here given only for 1 since 2 is isostructural to 1, except for the nature of the linking organic group (p-xylene instead of linear hexyl chain). The twodimensional (2D) structures can be described as anionic inorganic-organic hybrid sheets in which the tetraphosphonic groups connect the copper atoms. Charged copper(II) hexaaqua cations and water molecules are incorporated between the layers. The layers are stacked in an ABA sequence with an interlayer distance of about 6 Å. Within a sheet, the organic groups connect parallel inorganic units [Cu2(H2O)2(HPO3CH2)4N2]∞ running along the b-axis, as depicted in Figure 2. These units are constituted of two five-coordinated CuO4N square-base pyramids internally linked by two bridging and two terminal tetrahedral PO3C groups belonging to the tetraphosphonic linkers. In 1, the alkyl chains lie in extended conformation, whereas in 2, the aromatic rings are tilted about 20° with respect to the layer plane and they are stacked in a fishbone fashion with minima distances of 5.8 Å from one to each other. The Cu-O distances range from 1.962(3) to 2.003(2) Å, and

Figure 2. Polyhedral representation of 1 and 2 viewed along the b-axis. Tetrahedra are PO3C, and octahedra are Cu(H2O)6.

they are comparable with other Cu(II) amino bis-methylphosphonates previously described.37 The copper atom Cu1 is chelated by oxygen atoms O3 and O6 belonging to two adjacent phosphonic tetrahedra and by the N atom of the ternary amino group. The tetraphosphonic group acts then as endo-tridentate ligand as depicted in Figure 3. The coordination sphere of the copper is completed by one apical water molecule and by oxygen atoms of the adjacent PO groups of the neighboring building unit. The tetraphosphonic groups are constituted of two doubly and two monoconnected phosphonate tetrahedra with copper atoms. Remaining oxygens (O4, O5) point toward the interlayer region and are linked to those belonging to the neighboring sheet by strong H-bonds (O4#1‚‚‚O5 ) 2.615(5) Å, #1 ) -x + 0.5, y + 0.5, -z + 0.5). The orientation of the CuO4N pyramids bridged by the PO3C tetrahedra is nearly perpendicular; that is, the dihedral angle between two adjacent CuO3N bases is 91°. The Cu‚‚‚Cu smallest separation is about 4.3 Å, repeated by symmetry along the inorganic chain, as depicted in Figure 4. The interlayer region is filled by two water molecules and by one Cu(H2O)62+ octahedralcomplex counter cation per formula unit. The charge of the hexaaqua copper cations is compensated by the two deprotonated PO- groups of the phosphonic moiety directed toward the interlayer region. The water molecules of the Cu(H2O)62+ groups are linked via H-bonds to the PO groups and to the free interlayer water molecules. Hydrogen bonds are listed in Table 2, whereas the complete H-bonds network of the octahedral copper hexaaqua ion is depicted in Figure 5. (b) Crystal Structure of 3. Compound 3 is layered, and the topology resembles that of compounds 1 and 2. The layers in 3 are stacked in an ABA sequence and are constituted of inorganic polymeric units spaced by the organic groups along the c-axis. Within the layers, the organic groups are placed at different

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Figure 3. (a) Asymmetric units of 1 and 3. Hydrogen atoms are not labeled for clarity. Ellipsoids are set at 50% probability. (b) Connectivity of the phosphonic group and copper coordination in 1-3.

Figure 4. b-Axis propagation of CuO4N square base pyramids bridged by the exo-bidentate PO3C tetrahedra in 1; the Cu‚‚‚Cu distances are expressed in Å. Table 2. H-Bonds Table for 1a D-H‚‚‚A bond

D-H length (Å)

D‚‚‚A length (Å)

H‚‚‚A length (Å)

DHA angle (deg)

Ow1-Hw1b‚‚‚Ow5#1 Ow5-Hw5b‚‚‚O5#1 Ow2-Hw2a‚‚‚O1#2 Ow4-Hw4b‚‚‚O6#2 Ow4-Hw4a‚‚‚O2#2 Ow2-Hw2b‚‚‚O2#3 Ow5-Hw5b‚‚‚Ow3#4 O3-H3a‚‚‚Ow3#2 Ow4-H4a‚‚‚O2#2 Ow4-H4b‚‚‚O6#2

0.94(4) 0.91(5) 0.88(4) 0.92(4) 0.92(3) 0.92(3) 0.92(4) 0.91(4) 0.88(4) 0.91(3)

2.819(5) 2.991(5) 2.687(4) 2.695(5) 2.774(4) 2.745(4) 2.807(5) 2.688(4) 2.773(4) 2.695(4)

1.91(4) 2.12(4) 1.86(4) 1.84(3) 1.98(4) 1.88(3) 1.88(4) 1.80(4) 1.98(4) 1.82(3)

160(3) 160(4) 153(3) 155(3) 149(3) 155(2) 172(4) 160(4) 148(3) 157(3)

a Symmetry operations: #1 ) -x, -y, -z; #2 ) x - 1, y, z - 1; #3 ) 1.5 - x, -0.5 + y, 0.5 + z; and #4 ) -x + 0.5, y + 0.5, -z + 0.5.

heights with respect to the mean plane of the sheet, as depicted in Figure 6. This feature is attributed to the meta conformation of the xylene rings that makes the N-R-N angle far from the linearity. As a consequence, the distance between two adjacent inorganic chains decreases from about 7 Å, as in the case of 1 and 2, to about 6 Å for 3. The total layer thickness is larger with respect to 1 and 2, and the phosphonate tetrahedra belonging to the same sheet are then closer. The connectivity of Cu atoms with the tetraphosphonic groups is the same as that of the previous compounds; that is, each

Figure 5. H-bond networks of the hexaaqua copper ion with the phosphonic groups in 1. The distances are expressed in Å.

amino bis-phosphonate moiety acts as an endo-tridentate ligand for the copper atoms that are five-coordinated in a square pyramidal environment (see Figure 3b), accepting an apical water molecule. The P1 tetrahedron is linked to two symmetry equivalent copper atoms, Cu1, belonging to the layers and to a third one, Cu2, located in a special position. The Cu‚‚‚Cu intrachain distance is very similar to that of 1 and 2 (Cu1‚‚‚ Cu1#1 ) 4.311(2) Å, #1 ) 0.5 - x, 0.5 - y, 0.5 + z). The P2 tetrahedron interacts with the neighboring symmetry equivalent PO3C group via strong H-bonds (O4‚‚‚O5#4 ) 2.506(6) Å, #4 ) x, -y, z + 0.5). Cu2 bridges two different phosphonic tetrahedra placed around a mirror plane (P1 and P1#3 tetrahedra, #3 ) -x, y, z) belonging to a same tetraphosphonic group [Cu2-O1 ) 1.904(5) Å]. The coordination sphere of Cu2 is completed by three water molecules Ow1, Ow1#3 (#3 ) -x, y, z), and Ow3 in a strongly elongated square pyramidal environment [Cu-Ow3 ) 2.637(4) Å]. The enhanced thickness and the corrugation of the layer induced by the m-xylene groups allow the formation of hydrophilic hemicavities placed between two adjacent layers where disordered water molecules can be

New Layered Copper Tetraphosphonates

Figure 6. Polyhedral representation of 3 viewed along the b-axis. Tetrahedra are PO3.

Figure 7. Coupled TG-DTA curve for 2 (a) and 3 (b).

accommodated, as shown in Figure 6. The shortest distance between the ring is 5.78 Å. Thermal Behavior. Coupled TG-DTA curves for compound 2 and 3 are depicted in Figure 7. TDXD plots are represented in Figure 8. For compound 2, the first two weight losses of 16.6 and 4.8%, observed at 135 and 190 °C, are accompanied by two weak endothermic peaks. They are ascribed to the release of eight (calcd 16.4%) and two (calcd 4.6%) water molecules found in the structure. The TDXD plot (Figure 8a) shows a clear phase transition at ∼55 °C corresponding to the onset of the dehydration. Accurate data collection for the dehydrated phase has been performed at 100 °C using a D5005 diffractometer equipped with an Anton Paar oven camera. The diffraction pattern has

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been successfully indexed using the DICVOL04 program with the following cell parameters: a ) 15.9266(8) Å, b ) 5.9880(1) Å, c ) 12.4895(7) Å, β ) 101.41(1)°, V ) 1167.0(2) Å3 in the P21/n space group, and [M(20) ) 31, F(20) ) 56 (0.09399, 63)]. The comparison with the “as-synthesized” phase (see Table 1) shows that a unit cell contraction occurs after the water molecule losses, corresponding to a decrease of the c-axis and an increase of the a-axis, whereas the b-axis remains nearly the same. Unfortunately, attempts for structural refinement of this phase failed because of a crystallinity loss at high 2θ angles during the data collection. A third phase, observed in the range of 180-240 °C, is expected to be the anhydrous one, according to the TG curve. The very low number of reflections did not allow pattern indexing. Then, three other unresolved weight losses are observed up to 720 °C, associated with three exothermic peaks observed on the DTA curve that can be ascribed to the combustion processes occurring in several steps. The first stage, observed between 240 and 390 °C, corresponds to the amorphous domain (Figure 8a). Then, a poorly crystallized phase appears between 390 and 520 °C, corresponding to the plateau (weight loss of 26.9%) observed on the TG curve in the same temperature range. The weight loss, associated with a weak exothermic peak starting at 520 °C, corresponds to the formation of another unknown phase observed up to 550 °C. It decomposes into R-Cu2P2O7 (ICDD PDF2 No. 44-0182), as also evidenced by the final calculated weight loss for the combustion of the organics (exp. 21.8%, calcd 21.6%). The thermal behavior of compound 3 is shown in Figures 7b and 8b. The complete dehydration proceeds through two stages, leading to a weight loss of 15.3% (calcd 15.4%) at 370 °C. The first transformation occurs at 80 °C and corresponds to the release of five water molecules until 180 °C (weight loss exp. 11.2%, calcd 11.0%). In this temperature range, a strong and continuous shift of diffraction lines is observed on the TDXD plot. From these observations, it is expected that the departure of the weakest bonded water molecules, that is, two free H2O and three H2O bonded to Cu, involves a deep modification of the early structure, followed by the unit cell contraction. The inflection observed in Figure 8b, as well as the vanishing of the diffraction peaks at 185 °C, shows that a dihydrate phase cannot be formed. An amorphous domain is observed between 185 and 490 °C. The degradation of the organic part occurs between 350 and 730 °C, in two stages (weight loss exp. 4.2% and 17.2%, for the two steps; calcd 22.9% for the sum of the steps). The first weight loss of 4.2% is observed until 460 °C. At this temperature, a few diffraction lines emerge from the background. The crystallization is evidenced by the high exothermic peak observed in the range of 440-480 °C. Above this temperature, R-Cu2P2O7 is formed. Magnetic Properties. The temperature dependence of the χT product per formula unit after the data were corrected for diamagnetic and TIP contributions is shown in Figure 9 for samples 2 and 3. For both compounds above 80 K, χT is constant and equal to 1.25 and 1.41 emu K mol-1 for 2 and 3, respectively. These values, corresponding to 0.42 and 0.47 emu K mol-1 of CuII, are in very good agreement with those expected for a S ) 1/2 ion with average gave ca. 2.2, as always observed in similar CuII-containing compounds.38,41 Indeed, the gave values obtained from magnetic measurement, 2.11 and 2.24 for 2 and 3, respectively, were confirmed by room temperature EPR X-band spectra. In the spectrum of 3, the axial signal characteristic of a CuII ion with an elongated square pyramidal coordination is observed, with g⊥ ) 2.12 and g| ) 2.35. On the contrary, for 2, the two components are not resolved,

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Figure 8. TDXD patterns for 2 (a) and 3 (b).

Figure 9. T-dependent magnetic susceptibility for 2 (2) with the best fit (full line) and 3 (9). The inset shows the dependent magnetization, measured at 2 K, for 2 and 3.

resulting in a broad asymmetric line around g ) 2.15. The line broadening reflects the presence of two main species of copper ions, the five-coordinated square-based pyramid and the hexaaqua ion. At low temperature, the χT curves of both samples deviate from the constant value: The downward deviation observed for 2 indicates weak antiferromagnetic interactions while the upward deviation for 3 is indicative of weak ferromagnetic interactions. Such indications are confirmed by the field dependence of the magnetization of the two samples (Figure 9, inset), which display the typical behavior observed for antiferro- and ferromagnetic

interactions. A simple scheme can be assumed to analyze the magnetic behavior observed in compound 2. From structural results, one can see isolated Cu(H2O)62+ complexes (Cu2) and infinite chains of phosphate-bridged copper(II) atoms (Cu1) with copper to copper distances of 4.260(8) Å. Consequently, the magnetic susceptibility data have been equated on the basis of one isolated copper center and an infinite regular chain of copper atoms with a Heisenberg Hamiltonian H ) -JSAiSAi+1.The general expression of the magnetic susceptibility is χ ) χchain + χmono, where χchain ) Ng2β2/kT [(0.25 + 0.074975x + 0.075235x2)/(1.0 + 0.9931x + 0.172135x2 + 0.757825x3)]25 and χmono ) Ng2β2/4kT where x ) |J|/kT and J is the exchange constant. The best fit of the data was obtained for a g value of 2.12, which is very close to the value gave deduced previously, an antiferromagnetic exchange parameter J ) -2.4 cm-1, and an antiferromagnetic exchange parameter J ) -2.4 cm-1. On the contrary, a complex 2D copper network with several pathways for magnetic interactions is observed in compound 3. The complexity of the interaction pattern precluded any attempt to reasonably model the magnetic susceptibility data. A fit to a Curie-Weiss law gave C ) 0.47 emu K mol-1 and a positive Weiss constant of 2.00 K indicating very weak ferromagnetic interactions in the plane. A small decrease of χT was also observed at very low temperature (below 3 K), which can be indicative of very weak antiferromagnetic interplanar interactions. In both compounds, the coupling constants appear to be quite small, especially considering the relatively short

New Layered Copper Tetraphosphonates Scheme 1. Schematic Perpendicular (a) and Parallel (b) Views of the Layers in 1-3, the Reciprocal Disposition of the Organic/Hydrophobic and Inorganic/Hydrophilic Region, and the Copper Hydrate Cations, Shown on the Left of Part b

distances separating the metal ions involved in the exchange. Indeed, similar small values were already reported for some phosphinate-bridged Cu complexes.39,40 For sample 2, we believe that it can be the result of the dihedral angles between the basal Cu planes and the O-P-O bridge. Discussion The three copper phosphonates shown here possess a homologous layered structure in which the five-coordinated copper polyhedra are linked with the same connectivity to the phosphonic groups, designing layers with alternating inorganic and organic regions. Compounds 1 and 2 have very similar cell parameters despite the different conformation and rigidity of the alkyl chain with respect to the p-xylene rings. The b-axis in 1 and 2 and the c-axis in 3 have similar values [6.357(3), 6.274(1), and 6.356(3) Å, respectively] and correspond to the length of the asymmetric part of the inorganic unit in the layer. The a- and c-axes are similar in 1 and 2 [11.802(7) and 19.398(1) Å for 1 vs 12.328(3) and 18.4919(4) Å for 2] while the aand b-axes of 3 [18.094(1) and 24.247(1) Å] are, respectively, about equal to the c-axis and double the a-axis of 1 and 2. The presence of copper(II) hexahydrate complex cations in the interlayer region for 1 and 2 represents an interesting feature since a very limited number of phosphonate-based materials have shown the same characteristic until now.41 In addition, as an example, the copper in two different environment has also been found by Kong and coauthors.42 Scheme 1 shows the alternating disposition of the inorganic units and the organic groups, viewed perpendicularly to (a) and along (b) the layer surface. The structural analogies between the three compounds are clearly evident. The copper hydrate octahedra, in 1 and 2,

Crystal Growth & Design, Vol. 7, No. 9, 2007 1887

are placed in special positions and the water molecules of the coordination sphere are linked via H-bonds to the deprotonated PO groups. On the contrary, in 3, the copper atom is directly linked to the layer through the two PO groups belonging to a same tetraphosphonic unit. The copper coordination sphere is filled by three other water molecules in a square pyramidal environment. The different positioning of the copper atoms in the title compounds can be explained by the design of the layers. Indeed, in 1 and 2, the layers are about planar due to either the flexibility of diaminohexane or the flat character of diamino p-xylene groups. Very likely, the hexaaquacopper cations coming from the solution serve as templates and fill the interlayer space. They cannot be connected to the PO3C oxygen atoms because of the high distance among the phosphonic tetrahedral units. On the contrary, for 3, the m-xylene groups conformation implies that two of the PO3C tetrahedra (namely, P1-O1 and P1#3-O1#3, #3 ) -x, y, z) belonging to the same tetraphosphonic group are closer to each other (about 2.8 Å), thus designing a good coordination site for the linkage of the copper atoms. In addition, a sort of interlayer channel of 6 × 6 Å2, measured between the atomic centers, partially filled by disordered water molecules, and running along the b-axis, is formed because the stacking of the layers places two adjacent hydrophilic hemicavities perfectly aligned along the [100] direction. The propagation of the inorganic chains alternates in opposite directions for 1 and 2, as evidenced by the arrows, while in 3 they are oriented toward the same direction. Scheme 1b shows how the different conformation of the organic groups influences the position of the hydrophobic and the hydrophilic regions and, in our opinion, the copper intercalation. The copper atoms in the layers are chelated by the amino group and by the oxygen atoms belonging to two different phosphonate tetrahedra. This coordination mode has also been found in some metal(II)-amino di- or tetraphosphonate.37,43 Occasionally, some authors have found a different coordination mode in which the oxygen atoms belonging to the phosphonate tetrahedra are bridged to two different metals, whereas the N atoms of the amino groups act as H-acceptors and are protonated by neighboring P-OH groups.9-11,17,19 The Cu-N bond observed in 1-3 could be explained by the good affinity of Cu(II) toward nitrogen. To evaluate some possible applications, ion exchange has been attempted in these compounds, for example, by substituting the copper hydrate ions in 1 and 2 with Ni, Co, and other transition metals with similar ionic radii. All attempts were unsuccessful, probably because the strong H-bonding network links the Cu(H2O)62+ complexes to the layers. Similarly, the presence of H-bonds interactions between two adjacent layers via the oxygen atoms belonging to the pendant phosphonic groups did not show intercalation and/or exfoliation properties. For these reasons, the behavior of these compounds more resembles 3D closely packed than layered structures. Conclusions Three new homologous copper(II) diamino tetraphosphonates have been prepared and fully characterized. To the best of our knowledge, this work shows one of the first examples in which a class of nearly isostructural compounds, built from different tetraphosphonic building blocks, can be obtained from direct synthesis. In the previous papers describing the structures of M(II) or M(IV) tetraphosphonates, a great structural variability was always observed. Cu atoms in 1 and 2 adopt two different coordination modes, that is, square base pyramid for the intralayer Cu1 and octahedron for the interlayer Cu2. The

1888 Crystal Growth & Design, Vol. 7, No. 9, 2007

N-chelating mode of the copper atoms belonging to the layer is common to the three compounds, which probably contributes to the structural isotypism of the sheets. The compounds here described are stable in a good pH range (3-9), show a remarkable thermal stability, and possess low-temperature ferroand antiferromagnetic coupling. The synthesis of the title tetraphosphonic building blocks with other transition metals like Zn, Co, Fe, and Ni is in progress. Furthermore, it could be interesting to check the influence of different coordination environments on the chemical and physical properties of the new materials. Acknowledgment. F.C. thanks the “Ministe`re de l’Education Nationale, de l’Enseignement Supe´rieur et de la Recherche” for financial support (Chap. 3611 art.50, l 1, MR 2005-1620, No. 1558), and we are indebted to Dr. T. Roisnel (Centre de Diffractome´trie X, UMR 6226 CNRS) and G. Marsolier (Universite´ de Rennes 1) for their assistance in single-crystal and powder X-ray diffraction data collection, respectively, Sandra Casale (Centre de Microscopie Electronique a` Balayage et microAnalyse, Universite´ de Rennes 1) for SEM analyses, and Dr. Lorenzo Sorace (L.A.M.M. Laboratory, Firenze) for assistence with EPR measurements. Supporting Information Available: Tables with fractional coordinates, thermal factors, and geometrical parameters for 1-3. X-ray powder pattern of the 100 °C phase of 2. SEM pictures and EDS spectra for 1 and 3. This material is available free of charge via the Internet at http://pubs.acs.org.

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