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Volume 70 
Part 1 
Pages 28-36  
February 2014  

Received 19 September 2013
Accepted 28 December 2013
Online 16 January 2014

Metal-organic frameworks based on uranyl and phosphonate ligands

aCentro de Ciências e Tecnologias Nucleares, Instituto Superior Técnico, Universidade de Lisboa, 2695-066 Bobadela LRS, Portugal,bCICECO, Departamento de Química, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal,cQOPNA, Departamento de Química, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal, and dDepartment of Organic Chemistry, Ghent University, B-9000 Ghent, Belgium
Correspondence e-mail: filipe.paz@ua.pt

Three new crystalline metal-organic frameworks have been prepared from the reaction of uranyl nitrate with nitrilotris(methylphosphonic acid) [H6nmp, N(CH2PO3H2)3], 1,4-phenylenebis(methylene)diphosphonic acid [H4pmd, C6H4(PO3H2)2], and (benzene-1,3,5-triyltris(methylene))triphosphonic acid [H6bmt, C6H3(PO3H2)3]. Compound [(UO2)2F(H3nmp)(H2O)]·4H2O (I) crystallizes in space group C2/c, showing two crystallographically independent uranyl centres with pentagonal bipyramidal coordination geometries. While one metal centre is composed of a {(UO2)O3([mu]-F)}2 dimer, the other comprises an isolated {(UO2)O5} polyhedron. Compound [(UO2)(H2pmd)] (II) crystallizes in space group P21/c, showing a centrosymmetric uranyl centre with an octahedral {(UO2)O4} coordination geometry. Compound [(UO2)3(H3bmt)2(H2O)2]·14H2O (III) crystallizes in space group [P\bar 1], showing two crystallographically independent uranyl centres. One uranyl centre is a {(UO2)O5} pentagonal bipyramid similar to that in (I), while the other is a {(UO2)O4} centrosymmetric octahedron similar to that in (II). Compounds (I) and (III) contain solvent-accessible volumes accounting for ca 23.6 and 26.9% of their unit-cell volume, respectively. In (I) the cavity has a columnar shape and is occupied by disordered water molecules, while in (III) the cavity is a two-dimensional layer with more ordered water molecules. All compounds have been studied in the solid state using FT-IR spectroscopy. Topological studies show that compounds (I) and (III) are trinodal, with 3,6,6- and 4,4,6-connected networks, respectively. Compound (II) is instead a 4-connected uninodal network of the type cds.

1. Introduction

Metal-organic frameworks (MOFs) are nowadays typically prepared using hydrothermal synthetic approaches with the aim of obtaining crystalline products, in particular suitable large crystals for single-crystal diffraction. Most metal-organic assemblies involving f-block elements (Andrews & Cahill, 2013[Andrews, M. B. & Cahill, C. L. (2013). Chem. Rev. 113, 1121-1136.]) make use of polycarboxylate ligands, particularly those built around an aromatic skeleton which are, by themselves, a particularly notable subset (Thuéry et al., 2013[Thuéry, P., Masci, B. & Harrowfield, J. (2013). Cryst. Growth Des. 13, 3216-3224.]). Our research group has been employing multipodal phosphonate-based organic linkers for the construction of MOFs with, mainly, rare-earth cations (Rocha et al., 2009[Rocha, J., Paz, F. A. A., Shi, F. N., Ferreira, R. A. S., Trindade, T. & Carlos, L. D. (2009). Eur. J. Inorg. Chem. pp. 4931-4945.]; Cunha-Silva, Ananias et al., 2009[Cunha-Silva, L., Ananias, D., Carlos, L. D., Almeida Paz, F. A. & Rocha, J. (2009). Z. Kristallogr. 224, 261-272.]; Cunha-Silva, Lima et al., 2009[Cunha-Silva, L., Lima, S., Ananias, D., Silva, P., Mafra, L., Carlos, L. D., Pillinger, M., Valente, A. A., Paz, F. A. A. & Rocha, J. (2009). J. Mater. Chem. 19, 2618-2632.]; Soares-Santos et al., 2010[Soares-Santos, P. C., Cunha-Silva, L., Paz, F. A., Ferreira, R. A., Rocha, J., Carlos, L. D. & Nogueira, H. I. (2010). Inorg. Chem. 49, 3428-3440.]; Almeida Paz et al., 2004[Almeida Paz, F. A., Shi, F. N., Klinowski, J., Rocha, J. & Trindade, T. (2004). Eur. J. Inorg. Chem. pp. 2759-2768.], 2005[Almeida Paz, F. A., Rocha, J., Klinowski, J., Trindade, T., Shi, F. N. & Mafra, L. (2005). Prog. Solid State Chem. 33, 113-125.]; Shi, Paz, Girginova, Amaral et al., 2006[Shi, F. N., Paz, F. A. A., Girginova, P. I., Amaral, V. S., Rocha, J., Klinowski, J. & Trindade, T. (2006). Inorg. Chim. Acta, 359, 1147-1158.]; Shi, Paz, Girginova, Rocha et al., 2006[Shi, F. N., Paz, F. A. A., Girginova, P. I., Rocha, J., Amaral, V. S., Klinowski, J. & Trindade, T. (2006). J. Mol. Struct. 789, 200-208.]; Vilela, Mendes et al., 2013[Vilela, S. M. F., Mendes, R. F., Silva, P., Fernandes, J. A., Tomé, J. P. C. & Paz, F. A. A. (2013). Cryst. Growth Des. 13, 543-560.]; Shi et al., 2008[Shi, F. N., Trindade, T., Rocha, J. & Paz, F. A. A. (2008). Cryst. Growth Des. 8, 3917-3920.]; Silva et al., 2011[Silva, P., Vieira, F., Gomes, A. C., Ananias, D., Fernandes, J. A., Bruno, S. M., Soares, R., Valente, A. A., Rocha, J. & Paz, F. A. (2011). J. Am. Chem. Soc. 133, 15120-15138.]; Cunha-Silva et al., 2007[Cunha-Silva, L., Mafra, L., Ananias, D., Carlos, L. D., Rocha, J. & Almeida Paz, F. A. (2007). Chem. Mater. 19, 3527-3538.]; Silva et al., 2012[Silva, P., Fernandes, J. A. & Almeida Paz, F. A. (2012). Acta Cryst. E68, m294-m295.]; Vilela et al., 2012[Vilela, S. M. F., Ananias, D., Gomes, A. C., Valente, A. A., Carlos, L. D., Cavaleiro, J. A. S., Rocha, J., Tomé, J. P. C. & Almeida Paz, F. A. (2012). J. Mater. Chem. 22, 18354-18371.]; Vilela, Firmino et al., 2013[Vilela, S. M. F., Firmino, A. D. G., Mendes, R. F., Fernandes, J. A., Ananias, D., Valente, A. A., Ott, H., Carlos, L. D., Rocha, J., Tomé, J. P. C. & Almeida Paz, F. A. (2013). Chem. Commun. 49, 6400-6402.]). The phosphonate group constitutes a suitable rigid building block for the construction of robust multi-dimensional networks because the existence of three O atoms placed at the vertices of a tetrahedron mimics well the basic building unit of zeolites and zeo-type materials. These units are, in this way, structurally and thermally more robust than carboxylates, ultimately leading to stronger connections with larger cations such as uranyl. Phosphonate-based MOFs are, nevertheless, considerably fewer in the literature than their carboxylate-based counterparts. This is due in part to the fact that phosphonates tend to form densely packed layered structures that are not porous. In addition, because of their lower solubility, metal phosphonates tend to be less crystalline than carboxylates. This often means that even if a phosphonate-based MOF is obtained, it must be structurally characterized by powder X-ray diffraction (PXRD) instead of single-crystal methods (Adelani et al., 2011[Adelani, P. O., Oliver, A. G. & Albrecht-Schmitt, T. E. (2011). Cryst. Growth Des. 11, 3072-3080.]). Notwithstanding, phosphonates have become a prominent family of ligands among those used in the synthesis of uranyl-organic frameworks (UOFs). Indeed, several groups have exploited the high affinity of phosphonic acids for uranyl to generate a large variety of original architectures, from one-dimensional nanotubules up to three-dimensional frameworks (Thuéry, 2013[Thuéry, P. (2013). Cryst. Growth Des. 52, 435-447.]).

[Scheme 1]

The richness in the structural chemistry of uranyl arises from the extremely flexible coordination chemistry of the uranyl cation, UO22+, commonly adopting the UO7 pentagonal bipyramidal geometry, but also yielding the less common UO6 tetragonal bipyramid and the UO8 hexagonal bipyramid (Alsobrook et al., 2011[Alsobrook, A. N., Alekseev, E. V., Depmeier, W. & Albrecht-Schmitt, T. E. (2011). Cryst. Growth Des. 11, 2358-2367.]). In this current work, we have employed UO2(NO3)2·6H2O as the source of uranyl and three organic polyphosphonate linkers already used by us, namely 1,4-phenylenebis(methylene)diphosphonic acid (H4pmd) (Vilela, Mendes et al., 2013[Vilela, S. M. F., Mendes, R. F., Silva, P., Fernandes, J. A., Tomé, J. P. C. & Paz, F. A. A. (2013). Cryst. Growth Des. 13, 543-560.]; Shi et al., 2008[Shi, F. N., Trindade, T., Rocha, J. & Paz, F. A. A. (2008). Cryst. Growth Des. 8, 3917-3920.]), nitrilotris(methylphosphonic acid) (H6nmp) (Silva et al., 2011[Silva, P., Vieira, F., Gomes, A. C., Ananias, D., Fernandes, J. A., Bruno, S. M., Soares, R., Valente, A. A., Rocha, J. & Paz, F. A. (2011). J. Am. Chem. Soc. 133, 15120-15138.], 2012[Silva, P., Fernandes, J. A. & Almeida Paz, F. A. (2012). Acta Cryst. E68, m294-m295.]; Cunha-Silva et al., 2007[Cunha-Silva, L., Mafra, L., Ananias, D., Carlos, L. D., Rocha, J. & Almeida Paz, F. A. (2007). Chem. Mater. 19, 3527-3538.]) and (benzene-1,3,5-triyltris(methylene))triphosphonic acid (H6bmt) (Vilela et al., 2012[Vilela, S. M. F., Ananias, D., Gomes, A. C., Valente, A. A., Carlos, L. D., Cavaleiro, J. A. S., Rocha, J., Tomé, J. P. C. & Almeida Paz, F. A. (2012). J. Mater. Chem. 22, 18354-18371.]; Vilela, Firmino et al., 2013[Vilela, S. M. F., Firmino, A. D. G., Mendes, R. F., Fernandes, J. A., Ananias, D., Valente, A. A., Ott, H., Carlos, L. D., Rocha, J., Tomé, J. P. C. & Almeida Paz, F. A. (2013). Chem. Commun. 49, 6400-6402.]). Three new structures have been isolated and characterized, [(UO2)2F(H3nmp)(H2O)]·4H2O (I), [(UO2)(H2pmd)] (II) and [(UO2)3(H3bmt)2(H2O)2]·14H2O (III), with phase purity being confirmed using a combination of elemental analyses and FT-IR spectroscopy.

2. Experimental

2.1. Materials and methods

The organic ligand nitrilotris(methylenephosphonic acid) [H6nmp, N(CH2PO3H2)3, 97%, Fluka] is commercially available. 1,4-Phenylenebis(methylene)diphosphonic acid (H4pmd; Vilela, Mendes et al., 2013[Vilela, S. M. F., Mendes, R. F., Silva, P., Fernandes, J. A., Tomé, J. P. C. & Paz, F. A. A. (2013). Cryst. Growth Des. 13, 543-560.]) and (benzene-1,3,5-triyltris(methylene))triphosphonic acid (H6bmt; Vilela et al., 2012[Vilela, S. M. F., Ananias, D., Gomes, A. C., Valente, A. A., Carlos, L. D., Cavaleiro, J. A. S., Rocha, J., Tomé, J. P. C. & Almeida Paz, F. A. (2012). J. Mater. Chem. 22, 18354-18371.]) were prepared according to published procedures.

Elemental analysis (CHN) was performed on a CE Instruments EA1110 automatic analyser. To guarantee complete combustion of the samples, a couple of milligrams of V2O5 were added during the analysis, and the results were corrected to reflect the addition of this substance. FT-IR spectra (range 4000-350 cm-1) were collected as KBr pellets (Sigma-Aldrich, FT-IR grade) using a Bruker Tensor 27 spectrometer by averaging 200 scans at a maximum resolution of 4 cm-1.

2.2. Hydrothermal synthesis

Crystals of compounds (I)-(III) were isolated by mixing UO2(NO3)2·6H2O and the appropriate organic ligand in deionized water (see below for the employed amounts). The reaction mixtures were then transferred into Teflon-lined 26 ml autoclaves that were placed at 363 K for a period of 15 h. A few drops of HF were added to the reaction mixtures to promote the isolation of phase-pure compounds (I) and (III). The addition of this mineralizing agent was not necessary for the preparation of crystalline compound (II).

2.2.1. [(UO2)2F(H3nmp)(H2O)]·4H2O (I)

150.0 mg of UO2(NO3)2·6H2O (0.299 mmol) and 59.8 mg of the ligand H6nmp (0.200 mmol) composed the initial reaction mixture. Elemental analysis: calc (%): C 3.81, H 2.03, N 1.48; found: C 3.99, H 2.38, N 1.47.

2.2.2. [(UO2)(H2pmd)] (II)

150.0 mg of UO2(NO3)2·6H2O (0.299 mmol) and 79.6 mg of the ligand H4pmd (0.299 mmol) composed the initial reaction mixture. Elemental analysis: calc (%): C 17.99, H 1.89; found: C 18.20, H 2.03.

2.2.3. [(UO2)3(H3bmt)2(H2O)2]·14H2O (III)

150.0 mg UO2(NO3)2·6H2O (0.299 mmol) and 106.0 mg of the ligand H6bmt (0.294 mmol) composed the initial reaction mixture. Elemental analysis: calc (%): C 11.93, H 3.11; found: C 12.10, H 3.32.

2.3. Single-crystal X-ray diffraction

Single crystals of (I) and (III) were harvested from the crystallization vials and immediately immersed in highly viscous FOMBLIN Y perfluoropolyether vacuum oil (LVAC 140/13, Sigma-Aldrich) to avoid degradation caused by evaporation of the solvent. Single crystals of (II) were harvested from the crystallization vials and immersed in silicon grease. All crystals were mounted on Hampton Research CryoLoops with the help of a Stemi 2000 stereomicroscope equipped with Carl Zeiss lenses (Kottke & Stalke, 1993[Kottke, T. & Stalke, D. (1993). J. Appl. Cryst. 26, 615-619.]). Data were collected on a Bruker X8 Kappa APEX-II CCD area-detector diffractometer (graphite-monochromated Mo K[alpha] radiation, [lambda] = 0.7107 Å) equipped with an Oxford Cryosystems Series 700 Cryostream monitored remotely using the software interface Cryopad (Oxford Cryosystems, 2006[Oxford Cryosystems (2006). Cryopad. Oxford Cryosystems Ltd, Oxford, England.]).

Some O atoms of the crystallization water molecules [all in compound (I) and two in compound (III)] were refined with a common isotropic displacement parameter. The remaining non-H atoms were successfully refined using anisotropic displacement parameters. H atoms bound to C were placed at idealized positions and allowed to ride during refinement with Uiso(H) = 1.2Ueq(C). H atoms bound to O or N [in (I)] belonging to the organic ligands were located from difference Fourier maps and refined with the O-H and N-H distances restrained to 0.95 (1) Å. In some cases, additional anti-bumping restraints were added, namely P...H greater than 2.00 (1) Å in (I), and H...H greater than 2.16 (1) Å for atoms belonging to different water molecules in (III). The H atom bound to O8 in (I) was included as disordered over two different positions with assigned site occupancies of 0.5. H atoms belonging to the anisotropically refined water molecules in (III) were also located from difference Fourier maps and refined with the O-H and H...H distances restrained to 0.95 (1) and 1.55 (1) Å, respectively. The H atoms of the water molecules in (I) could not be located, and the atom sites in the structure are therefore 10 H atoms per formula unit (80 per unit cell) short of the stated compound formula.

The final difference Fourier synthesis for the three compounds showed: for (I), the highest peak (1.52 e Å-3) located 0.94 Å from U1 and the deepest hole (-1.04 e Å-3) 1.00 Å from U2; for (II), the highest peak (1.91 e Å-3) and the deepest hole (-2.74 e Å-3) located 0.48 and 0.77 Å from U1, respectively; for (III), the highest peak (0.64 e Å-3) located 0.17 Å from O2W and the deepest hole (-0.76 e Å-3) 0.86 Å from U2. Details of the crystallographic data collection and structure refinement are summarized in Table 1[link].1

Table 1
Experimental details

Experiments were carried out with Mo K[alpha] radiation using a Bruker X8 Kappa CCD APEX-II diffractometer. Absorption was corrected for by multi-scan methods, SADABS (Bruker, 2008[Bruker (2008). APEX2, SADABS, SAINT-Plus and SHELXTL. Bruker AXS, Madison, Wisconsin, USA.]). H atoms were treated by a mixture of independent and constrained refinement.

  (I) (II) (III)
Crystal data
Chemical formula C3H11FNO14P3U2·4H2O C8H10O8P2U C9H14O13P3U1.50·7H2O
Mr 945.16 534.13 906.27
Crystal system, space group Monoclinic, C2/c Monoclinic, P21/c Triclinic, [P\bar {1}]
Temperature (K) 150 296 150
a, b, c (Å) 25.8297 (10), 8.7560 (3), 17.7432 (7) 10.9522 (13), 6.4691 (7), 9.2802 (10) 8.7982 (3), 11.8876 (4), 12.6446 (4)
[alpha], [beta], [gamma] (°) 90, 99.951 (2), 90 90, 96.333 (9), 90 95.219 (2), 106.752 (2), 96.980 (2)
V3) 3952.5 (3) 653.50 (13) 1245.89 (7)
Z 8 2 2
[mu] (mm-1) 16.71 12.69 10.03
Crystal size (mm) 0.05 × 0.03 × 0.01 0.03 × 0.02 × 0.01 0.02 × 0.02 × 0.01
 
Data collection
Tmin, Tmax 0.489, 0.921 0.702, 0.939 0.825, 0.906
No. of measured, independent and observed [I > 2[sigma](I)] reflections 76 767, 5310, 4724 8989, 2475, 1556 54 948, 4560, 4004
Rint 0.074 0.055 0.064
(sin [theta]/[lambda])max-1) 0.685 0.769 0.602
 
Refinement
R[F2 > 2[sigma](F2)], wR(F2), S 0.026, 0.059, 1.06 0.034, 0.060, 1.00 0.021, 0.041, 1.08
No. of reflections 5310 2475 4560
No. of parameters 255 91 380
No. of restraints 5 1 32
[Delta][rho]max, [Delta][rho]min (e Å-3) 1.52, -1.04 1.91, -2.74 0.63, -0.76
Computer programs: APEX2 (Bruker, 2008[Bruker (2008). APEX2, SADABS, SAINT-Plus and SHELXTL. Bruker AXS, Madison, Wisconsin, USA.]), SAINT-Plus (Bruker, 2008[Bruker (2008). APEX2, SADABS, SAINT-Plus and SHELXTL. Bruker AXS, Madison, Wisconsin, USA.]), SAINT-Plus, SHELXS97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]), SHELXL97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]), DIAMOND (Brandenburg, 2009[Brandenburg, K. (2009). DIAMOND. Crystal Impact GbR, Bonn, Germany.]), SHELXTL (Bruker, 2008[Bruker (2008). APEX2, SADABS, SAINT-Plus and SHELXTL. Bruker AXS, Madison, Wisconsin, USA.]).

3. Results

3.1. Crystal structure of [(UO2)2F(H3nmp)(H2O)]·4H2O (I)

Compound (I) crystallizes in space group C2/c, with the asymmetric unit being composed of a whole residue of nitrilo­tris(methylenephosphonic acid) (H3nmp3-), two crystallographically independent uranyl centres, a coordinated water molecule, a coordinated fluoride anion and four water molecules distributed among ten distinct crystallographic locations with fractional site occupancies (Fig. 1[link]a). The presence of fluoride in the structure was proven by the combination of data arising from different characterization methods: on the one hand, the usage of a few drops of HF in the synthesis is crucial (see §2[link]), since elimination of this step fails to lead to the same compound; on the other hand, both the elemental analysis and the performed crystallographic studies unequivocally show that these anions are present.

[Figure 1]
Figure 1
(a) Molecular units in (I). Non-H atoms are represented as displacement ellipsoids drawn at 50% probability, while H atoms are represented as small spheres with arbitrary radii. Atom O8 is bonded to two H atoms, each with site occupancy 0.50; (b) and (c) coordination geometries around U2 and U1, respectively. Symmetry codes are defined in Table 2[link].

The coordination geometries around the metal centres resemble slightly distorted pentagonal bipyramids, with the oxo groups occupying the apical positions (see Table 2[link] for details). The equatorial positions of the coordination polyhedron around U1 are occupied by a coordinated water molecule and four O atoms from the organic ligands. The bond lengths are 1.764 (4) and 1.769 (4) Å for U1 = O, 2.510 (4) Å for U1-Owater and in the range 2.323 (4)-2.368 (4) Å for U1-Ophosphonate. The mean plane formed by the equatorial atoms and the metal centre [largest deviation of 0.245 (4) Å for O4] subtends angles with the axial U=O bonds of 88.45 (16) and 89.26 (16)°. The equatorial cis O-U1-O angles range from 65.24 (13) to 78.94 (13) ° (Fig. 1[link]c). The coordination environment around U2 resembles a centrosymmetric bimetallic cluster bridged by two crystallographically equivalent fluoride anions. The remaining three equatorial positions are occupied by O atoms belonging to the organic ligands. The bonding distances are 1.767 (4) and 1.770 (4) Å for U2=O, between 2.307 (3) and 2.355 (3) Å for U2-Ophosphonate, and 2.336 (3) and 2.351 (3) Å for U2-F. The U...U distance in the dimer is 3.9829 (3) Å. The mean plane formed by the equatorial atoms and the metal centre [largest deviation of 0.037 (3) Å for F1] subtends angles with the axial U=O bonds of 88.48 (15) and 89.44 (15)°. The equatorial cis O-U1-O angles range from 63.63 (11) to 79.44 (12)° (Fig. 1[link]b). The organic residue H3nmp3- is coordinated to six metal centres, with one of the phosphonate groups coordinated to the metal as a [mu]3-O,O',O''-ligand, and the two PO3H groups as [mu]2-O,O'-ligands. The remaining charge-balancing proton is bonded to the central N1 atom.

Table 2
Selected bond lengths and angles (Å, °) for the U6+ coordination environments in (I)

U1-O10 1.764 (4) U2-O13 1.767 (4)
U1-O11 1.769 (4) U2-O12 1.770 (4)
U1-O4 2.323 (4) U2-O5 2.307 (3)
U1-O1 2.331 (4) U2-O6iii 2.311 (3)
U1-O9i 2.354 (4) U2-F1 2.351 (3)
U1-O2ii 2.368 (4) U2-F1iv 2.336 (3)
U1-O1W 2.510 (4) U2-O7i 2.355 (4)
    U2···U2iv 3.9829 (3)
       
O10-U1-O11 177.74 (19) O13-U2-O12 179.02 (17)
O10-U1-O4 84.27 (17) O13-U2-O5 88.77 (15)
O11-U1-O4 97.91 (17) O12-U2-O5 91.88 (15)
O10-U1-O1 95.15 (17) O13-U2-O6iii 89.78 (15)
O11-U1-O1 86.01 (16) O12-U2-O6iii 91.04 (16)
O4-U1-O1 75.54 (13) O5-U2-O6iii 79.54 (12)
O10-U1-O9i 93.82 (17) O13-U2-F1iv 90.48 (14)
O11-U1-O9i 86.23 (17) O12-U2-F1iv 88.57 (15)
O4-U1-O9i 73.50 (13) O5-U2-F1iv 146.73 (11)
O1-U1-O9i 146.64 (13) O6iii-U2-F1iv 133.72 (11)
O10-U1-O2ii 85.63 (17) O13-U2-F1 88.66 (14)
O11-U1-O2ii 92.70 (17) O12-U2-F1 91.14 (15)
O4-U1-O2ii 151.51 (14) O5-U2-F1 149.54 (11)
O1-U1-O2ii 78.94 (13) O6iii-U2-F1 70.11 (11)
O9i-U1-O2ii 133.83 (13) F1iv-U2-F1 63.63 (11)
O10-U1-O1W 88.73 (18) O13-U2-O7i 90.02 (15)
O11-U1-O1W 89.24 (17) O12-U2-O7i 89.43 (16)
O4-U1-O1W 137.52 (13) O5-U2-O7i 76.57 (12)
O1-U1-O1W 146.92 (13) O6iii-U2-O7i 156.11 (12)
O9i-U1-O1W 65.24 (13) F1iv-U2-O7i 70.17 (11)
O2ii-U1-O1W 68.60 (13) F1-U2-O7i 133.77 (11)
       
Symmetry codes: (i) [x, -y+1, z+{1\over 2}]; (ii) [-x+1, y, -z+{1\over 2}]; (iii) [-x+{1\over 2}, y-{1\over 2}, -z+{1\over 2}]; (iv) [-x+{1\over 2}, -y+{1\over 2}, -z+1].

As the structure does not show any noticeable void volume, we conclude that all of the water molecule sites in the crystal structure have been found. These molecules are, however, included with fractional site occupancies, and their H atoms could not be located from difference Fourier maps. This means that the water molecules are loosely located and suffer from spatial and conformational disorder, and for this reason the hydrogen-bond network cannot be described completely. The interaction N1-H1Z...F1 is the only hydrogen bond with an unequivocal connection between a donor and an acceptor. While the O3 atom donates its H atom to a pair of close, and fractionally occupied, water molecules, atom O8 appears to be connected to two half-occupied H atoms, with these being donated to two pairs of close and fractionally occupied water molecules (see Table 3[link] and Fig. 1[link] for details). The water molecules occupy a continuous column parallel to the c direction of the unit cell (Fig. 2[link]). This column has a cross section of ca 10 × 5 Å2, accounting for ca 23.6% of the total volume of the unit cell (ca 932 Å3).

Table 3
Hydrogen-bond geometry (Å, °) for (I)

D-H...A D-H H...A D...A D-H...A
N1-H1Z...F1v 0.95 (1) 1.81 (3) 2.705 (5) 157 (5)
O3-H3Z...O6W 0.96 (1) 2.19 (2) 3.139 (13) 175 (6)
O3-H3Z...O7W 0.96 (1) 1.70 (2) 2.643 (13) 171 (5)
O8-H8Y...O4W 0.95 (1) 1.88 (4) 2.802 (10) 163 (13)
O8-H8Y...O5W 0.95 (1) 1.62 (3) 2.560 (13) 171 (14)
O8-H8X...O8W 0.95 (1) 1.90 (2) 2.850 (11) 176 (13)
O8-H8X...O9W 0.95 (1) 2.48 (3) 3.415 (17) 170 (12)
Symmetry code: (v) [-x+{1\over 2}, y+{1\over 2}, -z+{1\over 2}].
[Figure 2]
Figure 2
Crystal packing of (I) viewed along the [001] direction. The metal centres are represented as green or yellow polyhedra, depending on whether their coordination environment contains F atoms. H atoms of the water molecules were not located and, for this reason, the water molecules appear as isolated O atoms.

3.2. Crystal structure of [(UO2)(H2pmd)] (II)

Compound (II) crystallizes in space group P21/c, with the asymmetric unit being composed of half a 1,4-phenylenebis(methylene)diphosphonic acid ligand (H2pmd2-), a U6+ metal centre occupying an inversion centre, and one oxo ligand (Fig. 3[link]a). The coordination geometry around the centrosymmetric metal centre resembles an octahedron squeezed along the axis defined by the crystallographically equivalent oxo ligands (Fig. 3[link]b). The remaining positions of the coordination environment are occupied by four O atoms from the organic ligands. The bond lengths are 1.781 (3) Å for U1=O, and 2.269 (4) and 2.278 (3) Å for U1-Ophosphonate. While the equatorial cis octahedral O-U1-O angles range from 88.95 (14) to 91.05 (14)°, the trans octahedral angles are exactly 180° due to the location of the metal centre at the inversion centre (see Table 4[link] for details). The centrosymmetric organic residue H2pmd2- is coordinated to four metal centres with each PO3H group acting as a [mu]2-O,O' ligand between two such centres. Compound (II) does not show any porosity, and its supramolecular interactions are limited to the O2-H2Z...O4 hydrogen bond and to C-H...[pi] interactions (see Table 5[link] and Fig. 4[link] for details).

Table 4
Selected bond lengths and angles (Å, °) for the U6+ coordination environments in (II)

U1-O4 1.781 (3) U1-O1 2.278 (3)
U1-O3ii 2.269 (4)    
       
O4-U1-O4i 180 O3ii-U1-O1 88.95 (14)
O4-U1-O3ii 90.25 (15) O3iii-U1-O1 91.05 (14)
O4-U1-O3ii 89.75 (15) O4-U1-O1i 90.07 (15)
O3ii-U1-O3iii 180 O1-U1-O1i 180
O4-U1-O1 89.93 (15)    
Symmetry codes: (i) -x+1, -y, -z+2; (ii) [x, -y+{1\over 2}, z+{1\over 2}]; (iii) [-x+1, y-{1\over 2}, -z+{3\over 2}].

Table 5
Hydrogen-bond geometry (Å, °) for (II)

D-H...A D-H H...A D...A D-H...A
O2-H2Z...O4iv 0.83 (1) 1.88 (2) 2.703 (5) 168 (5)
C1-H1B...Cg 0.97 2.86 3.588 (5) 133
Cg is the centroid of the ring composed by the atoms C2vi, C3vi, C4vi, C2vii, C3vii and C4vii. Symmetry codes: (iv) x, y+1, z; (vi) [x, -y+{1\over 2}, z-{1\over 2}]; (vii) [-x+2, y+{1\over 2}, -z+{3\over 2}].
[Figure 3]
Figure 3
(a) Molecular units in (II). Non-H atoms are represented as displacement ellipsoids at 50% probability, while H atoms are represented as small spheres with arbitrary radii; (b) coordination geometry around the metal centre. Symmetry codes are defined in Table 4[link], and: (v) -x+2, -y, -z+2.
[Figure 4]
Figure 4
Crystal packing of (II) viewed along the [010] direction. Metal centres are represented as rose-coloured squeezed octahedra. O-H...O and C-H...[pi] interactions are depicted as green and blue dashed lines, respectively.

3.3. Crystal structure of [(UO2)3(H3bmt)2(H2O)2]·14H2O (III)

Compound (III) crystallizes in space group [P\bar 1], with the asymmetric unit being composed of a whole residue of (benzene-1,3,5-triyltris(methylene))triphosphonic acid (H3bmt3-), one and a half crystallographically independent uranyl centres, a coordinated water molecule, and seven water molecules distributed over nine distinct crystallographic sites (Fig. 5[link]a). Notably, even though the crystallization of (III) could only be achieved with the addition of HF to the reaction mixture [as for compound (I), see §2[link]], fluoride anions do not seem to be included in the crystal structure. Elemental analysis on representative amounts of the isolated compound strongly point to the absence of fluoride anions (see §2[link]). In addition, no crystallographic evidence was found for the presence of fluoride anions.

[Figure 5]
Figure 5
(a) Molecular units in (III). Non-H atoms are represented as displacement ellipsoids at 50% probability, while H atoms are represented as small spheres with arbitrary radii; (b) and (c) coordination geometries around U2 and U1, respectively. Symmetry codes as in Table 6[link].

The coordination environments around the two metal centres are different: while that of U1 resembles a squeezed centrosymmetric octahedron similar to that in compound (II) (Fig. 5[link]b), the coordination environment of U2 bears strong similarities to the pentagonal bipyramid around U1 in compound (I) (Fig. 5[link]c). The bond lengths around U2 are 1.765 (3) and 1.769 (3) Å for U2=O, 2.472 (3) Å for U2-Owater and in the range 2.316 (3)-2.392 (3) Å for U2-Ophosphonate. The mean plane formed by the equatorial atoms and the metal centre [largest deviation 0.205 (4) Å for O8] subtends angles with the axial U=O bonds of 89.37 (15) and 89.80 (15)°. The equatorial cis O-U2-O angles range from 66.10 (10) to 77.17 (10)°. For the U1 environment, the bond lengths are 1.774 (3) for U1=O, and 2.294 (4) and 2.307 (3) Å for U1-Ophosphonate. The equatorial cis octahedral O-U1-O angles range from 88.08 (11) to 91.92 (11)° (see Table 6[link] for details). The organic residue H3bmt3- is coordinated to six metal centres, with all of the PO3H groups coordinating to the metal as [mu]2-O,O'-ligands.

Table 6
Selected bond lengths and angles (Å, °) for the U6+ coordination environments in (III)

U1-O10 1.774 (3) U2-O8ii 2.316 (3)
U1-O1 2.294 (3) U2-O4iii 2.324 (3)
U1-O5i 2.307 (3) U2-O9iv 2.326 (3)
U2-O12 1.765 (3) U2-O2 2.392 (3)
U2-O11 1.769 (3) U2-O1W 2.472 (3)
       
O10-U1-O10v 180 O11-U2-O9iv 93.96 (12)
O10-U1-O1 91.00 (12) O8ii-U2-O9iv 76.92 (10)
O10-U1-O1v 89.00 (12) O4iii-U2-O9iv 77.17 (10)
O1-U1-O1v 180 O12-U2-O2 91.82 (12)
O10-U1-O5i 89.89 (12) O11-U2-O2 87.43 (12)
O1-U1-O5i 88.08 (11) O8ii-U2-O2 132.79 (10)
O10-U1-O5iii 90.11 (12) O4iii-U2-O2 73.10 (10)
O1-U1-O5iii 91.92 (11) O9iv-U2-O2 150.20 (10)
O5i-U1-O5iii 180 O12-U2-O1W 85.23 (13)
O12-U2-O11 179.23 (14) O11-U2-O1W 94.61 (13)
O11-U2-O8ii 85.65 (13) O8ii-U2-O1W 68.03 (10)
O12-U2-O4iii 90.71 (13) O4iii-U2-O1W 138.80 (11)
O11-U2-O4iii 88.91 (13) O9iv-U2-O1W 143.08 (11)
O8ii-U2-O4iii 153.07 (10) O2-U2-O1W 66.10 (10)
O12-U2-O9iv 86.61 (12)    
Symmetry codes: (i) x+1, y, z; (ii) -x, -y, -z+1; (iii) -x, -y+1, -z+1; (iv) x, y, z-1; (v) -x+1, -y+1, -z+1.

The water molecules occupy a continuous zigzag layer parallel to the (100) planes of the unit cell, occupying ca 26.9% of the total cell volume (ca 335 Å). The considerable number of donor and acceptor atoms contributes to the existence of a complex network of hydrogen bonds, showing rings with graph-set motifs R44(8) (two), R44(8), R53(10), R86(16), R87(17) and R128(23) (Grell et al., 1999[Grell, J., Bernstein, J. & Tinhofer, G. (1999). Acta Cryst. B55, 1030-1043.]; see Table 7[link] and Fig. 6[link] for additional details).

Table 7
Hydrogen-bond geometry (Å, °) for (III)

D-H...A D-H H...A D...A D-H...A
O3-H3Z...O6Wvi 0.94 (1) 1.69 (1) 2.625 (4) 168 (4)
O6-H6Z...O3Wiii 0.94 (1) 1.68 (1) 2.609 (5) 168 (4)
O7-H7Z...O4W 0.94 (1) 1.76 (1) 2.696 (4) 171 (5)
O1W-H1X...O6Wvi 0.95 (1) 1.80 (1) 2.747 (4) 173 (4)
O1W-H1Y...O4Wii 0.95 (1) 1.86 (2) 2.752 (5) 156 (5)
O2W-H2X...O7Wvii 0.95 (1) 2.18 (6) 3.002 (9) 144 (8)
O2W-H2Y...O11 0.95 (1) 2.15 (5) 3.010 (8) 150 (9)
O3W-H3X...O8W 0.95 (1) 1.85 (1) 2.799 (7) 175 (4)
O3W-H3Y...O10 0.95 (1) 2.67 (4) 3.336 (5) 128 (4)
O3W-H3Y...O1v 0.95 (1) 2.59 (3) 3.333 (5) 136 (3)
O4W-H4X...O3viii 0.95 (1) 1.95 (2) 2.841 (5) 156 (4)
O4W-H4Y...O5 0.95 (1) 2.12 (2) 3.031 (4) 161 (4)
O5W-H5X...O10 0.95 (1) 2.07 (2) 2.989 (5) 163 (4)
O5W-H5Y...O7W 0.95 (1) 1.87 (1) 2.823 (7) 174 (3)
O6W-H6X...O5W 0.94 (1) 1.73 (2) 2.645 (5) 164 (3)
O6W-H6Y...O2 0.95 (1) 1.93 (1) 2.875 (4) 175 (4)
O7W-H7X...O2W 0.95 (1) 1.81 (2) 2.741 (10) 163 (6)
O7W-H7Y...O9ix 0.95 (1) 2.04 (2) 2.972 (6) 166 (6)
O8W-H8X...O7W 0.95 (1) 1.99 (3) 2.927 (8) 167 (9)
O8W-H8Y...O3Wx 0.96 (1) 2.07 (5) 2.901 (8) 145 (7)
Symmetry codes: (ii) -x, -y, -z+1; (iii) -x, -y+1, -z+1; (v) -x+1, -y+1, -z+1; (vi) -x+1, -y, -z+1; (vii) -x+1, -y, -z; (viii) x-1, y, z; (ix) x+1, y, z-1; (x) -x+1, -y+1, -z.
[Figure 6]
Figure 6
Crystal packing of (III) viewed along the [100] direction. Metal centres are represented as rose or yellow polyhedra, for octahedral or pentagonal bipyramidal geometries, respectively. O-H...O interactions are depicted as dashed green lines. Crystallization water molecules with low site occupancy (O9W and O10W) have been omitted for clarity.

3.4. FT-IR spectroscopy

FT-IR spectroscopy on representative amounts of (I)-(III) show the most important diagnostic bands attributed to the functional groups composing the organic ligands (Fig. 7[link]). Due to the low wavenumbers in which the U-F vibrational modes appear (typically below 300 cm-1) these bands could not be used to establish unequivocally the absence of fluoride anions in the structure of compound (III).

[Figure 7]
Figure 7
FT-IR spectra of (I)-(III) collected in the 400-4000 cm-1 range.

The FT-IR spectra of (I) and (III) show very broad bands in the range 3600-3200 cm-1 attributed to the [nu](O-H) stretching modes of the coordinated water molecules. The in-plane deformation [delta](H2O) bands of the water molecules are also clearly visible in the range ca 1650-1600 cm-1. All of these bands are, as expected, absent from the spectrum of (II). The bands found between ca 3200-2800 cm-1 are attributed to the typical symmetric and asymmetric [nu](C-H) stretching vibrational modes, being much more visible for (II) due to the absence of the aforementioned broad band from the water molecules. The absence of water in (II) also permits observation in much more detail of the typical [nu](PO-H) stretching vibrational mode at ca 3160 cm-1 (Fig. 7[link]).

As we have reported previously for lanthanide-containing MOF structures with H4pmd ligands (Vilela et al., 2014[Vilela, S. M. F., Ananias, D., Fernandes, J. A., Silva, P., Gomes, A. C., Silva, N. J. O., Rodrigues, M. O., Tomé, J. P. C., Valente, A. A., Ribeiro-Claro, P., Carlos, L. D., Rocha, J. & Almeida Paz, F. A. (2014). J. Mater. Chem. C, doi: 10.1039/c3tc32114b.]), the combination of [delta](CH2) and [nu](Ph) vibrational modes in the ca 1520-1400 cm-1 spectral region is markedly visible in the spectrum of (II) due to the appearance of sharp strong bands which are typically absent from the remaining materials, particularly compound (I). The presence of this organic ligand is also notable as a strong broad band peaking at ca 870 cm-1, attributed to a combination of the [rho](CH2) and [gamma](CH) modes (Vilela et al., 2014[Vilela, S. M. F., Ananias, D., Fernandes, J. A., Silva, P., Gomes, A. C., Silva, N. J. O., Rodrigues, M. O., Tomé, J. P. C., Valente, A. A., Ribeiro-Claro, P., Carlos, L. D., Rocha, J. & Almeida Paz, F. A. (2014). J. Mater. Chem. C, doi: 10.1039/c3tc32114b.]). Below ca 1100 cm-1, all spectra are strongly dominated by a multitude of bands arising from the phosphonate groups which overlap with the typical [nu]3 asymmetric stretching vibrational mode of the uranyl, UO22+, group. Particularly, between 1100 and 900 cm-1, various bands are attributed to the [nu](PO3) modes of these groups, and at ca 580 and 554 cm-1, two markedly visible bands in the spectrum of (II) arise from the combination of the [delta](PO3) and [gamma](Ph) modes.

4. Topological studies

The frameworks (I)-(III) can be conveniently described by employing a topological approach (Baburin et al., 2005[Baburin, I. A., Blatov, V. A., Carlucci, L., Ciani, G. & Proserpio, D. M. (2005). J. Solid State Chem. 178, 2452-2474.]; Blatov et al., 2004[Blatov, V. A., Carlucci, L., Ciani, G. & Proserpio, D. M. (2004). CrystEngComm, 6, 377-395.]). This simplification approach, purely based on mathematical concepts, consists of reducing each network into objects that structurally could be envisaged as central nodes and connecting bridges (between two or more individual nodes). All topological studies have been performed using the software package TOPOS (Blatov, 2012[Blatov, V. A. (2012). Struct. Chem. 23, 955-963.]). From the structures of (I)-(III) a number of common structural features can be discerned among the frameworks: (i) they are assembled from one or two crystallographically independent uranyl-based metal centres; (ii) they contain a single organic residue [or a fragment in the case of (II)] in their asymmetric units; (iii) the organic ligands make physical connections between the uranyl-based metal centres.

Alexandrov et al. (2011[Alexandrov, E. V., Blatov, V. A., Kochetkov, A. V. & Proserpio, D. M. (2011). CrystEngComm, 13, 3947-3958.]) have considered that an organic ligand is of crucial structural importance in a framework when it establishes links between two or more metal centres ([mu]n). This idea could be extended to other structurally important aggregates of atoms, such as the {(UO2)O3([mu]-F)}2 dimer present in (II) and formed by the crystallographically independent U2 centres (see detailed structure description above). Therefore, for the performed topological studies, the centroids of each organic ligand and of the {(UO2)O3([mu]-F)}2 dimer were taken as network nodes, along with the remaining uranyl metal centres. Two of the compounds are, in this way, trinodal: (I) is a 3,6,6-connected network with overall point symbol {43}2{47·67·8}2{48·66·8}, of which the {(UO2)O3([mu]-F)}2 dimer and the organic ligands are both six-connected nodes (Fig. 8[link]a); (III) is instead a 4,4,6-connected network (point symbol: {42·84}{44·62}2{49·66}2) having only the organic ligand as a six-connected node (Fig. 8[link]c). While (III) has a known topology deposited within the TOPOS database (4,4,6T19), (I) corresponds to a new topological type. Searches in the Reticular Chemistry Structure Resource (RCSR; O'Keeffe et al., 2008[O'Keeffe, M., Peskov, M. A., Ramsden, S. J. & Yaghi, O. M. (2008). Acc. Chem. Res. 41, 1782-1789.]) and in EPINET (Hyde et al., 2006[Hyde, S. T., Delgado Friedrichs, O., Ramsden, S. J. & Robins, V. (2006). Solid State Sci. 8, 740-752.]) reveal that the nodal connectivity of compound (I) is, to the best of our knowledge, unprecedented among MOF structures.

[Figure 8]
Figure 8
Topological representations of (I)-(III).

Compound (II) is a much simpler network having only two nodes which are topologically identical: both the organic ligand and the uranyl group are 4-connected nodes with exactly identical coordination shells up to N10 (= 1489) (Fig. 8[link]b). In this way, this network is 4-connected uninodal with point symbol {45·8} and topology identical to that of the CdSO4 compound (type cds).

5. Conclusions

Three new MOF compounds based on uranyl groups coordinated to polyphosphonate-bearing ligands were isolated from hydrothermal synthesis and structurally characterized in the solid state. Compound (I) shows two different uranyl environments with pentagonal bipyramidal geometries, from which one comprises a centrosymmetric bimetallic cluster formed by two F- bridges. In contrast to the previously prepared materials with lanthanides and the ligand H6nmp, which comprised two-dimensional coordination polymers (Silva et al., 2011[Silva, P., Vieira, F., Gomes, A. C., Ananias, D., Fernandes, J. A., Bruno, S. M., Soares, R., Valente, A. A., Rocha, J. & Paz, F. A. (2011). J. Am. Chem. Soc. 133, 15120-15138.]; Cunha-Silva et al., 2007[Cunha-Silva, L., Mafra, L., Ananias, D., Carlos, L. D., Rocha, J. & Almeida Paz, F. A. (2007). Chem. Mater. 19, 3527-3538.]), compound (I) comprises a highly porous three-dimensional network.

The coordination geometry around the uranyl group in (II) resembles a squeezed octahedron. The structure comprises layers of inorganic uranyl phosphonates alternating with organic layers composed by the hydrocarbon part of the ligand. This resembles the packing observed in the previously prepared lanthanide materials with the same ligand (Vilela, Mendes et al., 2013[Vilela, S. M. F., Mendes, R. F., Silva, P., Fernandes, J. A., Tomé, J. P. C. & Paz, F. A. A. (2013). Cryst. Growth Des. 13, 543-560.]; Shi et al., 2008[Shi, F. N., Trindade, T., Rocha, J. & Paz, F. A. A. (2008). Cryst. Growth Des. 8, 3917-3920.]). In contrast to these compounds (II) is fully dehydrated.

Compound (III) is based on two uranyl environments with different coordination geometries, one being a pentagonal bipyramid and the other a squeezed octahedron. In relation to the previously prepared compounds with H6bmt and lanthanides (Vilela, Firmino et al., 2013[Vilela, S. M. F., Firmino, A. D. G., Mendes, R. F., Fernandes, J. A., Ananias, D., Valente, A. A., Ott, H., Carlos, L. D., Rocha, J., Tomé, J. P. C. & Almeida Paz, F. A. (2013). Chem. Commun. 49, 6400-6402.]; Vilela et al., 2012[Vilela, S. M. F., Ananias, D., Gomes, A. C., Valente, A. A., Carlos, L. D., Cavaleiro, J. A. S., Rocha, J., Tomé, J. P. C. & Almeida Paz, F. A. (2012). J. Mater. Chem. 22, 18354-18371.]), compound (III) exhibits a considerably larger cavity occupied by a larger number of water molecules. While compounds (II) and (III) have known topological types, compound (I) corresponds to an unprecedented topological type among MOF structures.

Acknowledgements

We thank Fundação para a Ciência e a Tecnologia (FCT, Portugal), the European Union, QREN, FEDER, COMPETE and Laboratório Associado Centro de Investigação em Materiais Cerâmicos e Compósitos, CICECO (Pest C-CTM/LA0011/2013), the research unit QOPNA (PEst-C/QUI/UI0062/2013) for their general funding scheme. We further thank FCT for the R&D project PTDC/QUI-QUI/098098/2008 (FCOMP-01-0124-FEDER-010785), and for specific funding towards the purchase of a single-crystal diffractometer. We are also grateful to FCT for the PhD grant No. SFRH/BD/66371/2009 (to SMFV) and the post-doctoral grants SFRH/BPD/47087/2008 (to BM) and SFRH/BPD/63736/2009 (to JAF).

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Acta Cryst (2014). B70, 28-36   [ doi:10.1107/S2052520613034781 ]