Tetrakis(μ-3-azaniumylbenzoato)-κ3 O:O,O′;κ3 O,O′:O;κ4 O:O′-bis[tetraaquaneodymium(III)] hexachloride tetrahydrate

The structure of the title compound, [Nd2(C7H7NO2)4(H2O)8]Cl6·4H2O, consists of dimeric cationic units related by an inversion centre. The two NdIII atoms are linked by two bridging bidentate carboxylate groups and two bidentate chelating bridging carboxylate groups, with an Nd⋯Nd separation of 4.1259 (4) Å. Each NdIII atom is nine-coordinated by five O atoms from the carboxylate groups of the zwitterionic azaniumylbenzoate ligands and four from water molecules. They adopt a distorted tricapped trigonal–prismatic arrangement. The dihedral angle between the mean planes of the benzene ring and the carboxlate groups are 7.7 (6) and 24.4 (5)°. The two carboxylate groups are almost perpendicular to one another with a dihedral angle of 84.0 (7)°, while the two benzene rings are inclined to one another by 81.8 (2)°. The molecular packing is stabilized by O—Hwater⋯Cl, O—Hwater⋯N, N—H⋯Cl, N—H⋯O, and O—Hwater⋯O hydrogen bonds and π–π stacking interactions [centroid–centroid distance = 3.500 (3) Å] between symmetry-related benzene rings. All of the Cl− anions and the uncoordinated water molecules are disordered over two sets of sites with different occupancy ratios.

The structure of the title compound, [Nd 2 (C 7 H 7 NO 2 ) 4 -(H 2 O) 8 ]Cl 6 Á4H 2 O, consists of dimeric cationic units related by an inversion centre. The two Nd III atoms are linked by two bridging bidentate carboxylate groups and two bidentate chelating bridging carboxylate groups, with an NdÁ Á ÁNd separation of 4.1259 (4) Å . Each Nd III atom is nine-coordinated by five O atoms from the carboxylate groups of the zwitterionic azaniumylbenzoate ligands and four from water molecules. They adopt a distorted tricapped trigonal-prismatic arrangement. The dihedral angle between the mean planes of the benzene ring and the carboxlate groups are 7.7 (6) and 24.4 (5) . The two carboxylate groups are almost perpendicular to one another with a dihedral angle of 84.0 (7) , while the two benzene rings are inclined to one another by 81.8 (2) . The molecular packing is stabilized by O-H water Á Á ÁCl, O-H water Á Á ÁN, N-HÁ Á ÁCl, N-HÁ Á ÁO, and O-H water Á Á ÁO hydrogen bonds andstacking interactions [centroid-centroid distance = 3.500 (3) Å ] between symmetryrelated benzene rings. All of the Cl À anions and the uncoordinated water molecules are disordered over two sets of sites with different occupancy ratios.

Comment
In recent years, much research has been done on lanthanide coordination compounds with some organic ligands, which have chelated structures and exhibit photophysical properties for the application in luminescence probes for chemical or biological macromolecules and the active center for molecular based luminescent materials (Yan et al., 1997;Scott et al., 1992). Especially lanthanide complexes with aromatic carboxylic acids show higher thermal and luminescent stability for practical applications than other lanthanide complexes because they readily form dimer or infinite chain polymeric structures (Ma et al., 1994).We report herein on the preparation and crystal structure of the title compound.
The molecular structure of the title compound consists of dimeric units related by an inversion centre (Fig. 1 Nd···Nd intradimer separation is 4.1259 (4) Å indicates that the metal···metal distances are primarily governed by the nature and mode of the coordination of the bridging groups (Sun et al., 2002). The carboxylate group shows a distortion from the molecular plane; the dihedral angle between the mean-planes of the benzene ring (C2-C7; plane 1) and the carboxlate group (O2/C1/O3 i ) is 7.7 (6)°, and that between the mean-planes of benzene ring (C9-C14; plane 2) and the O1/C8/O4 carboxlate group is 24.4 (5)°. The two carboxylate groups are almost perpendicular to one another with a dihedral angle of 84.0 (7) °, and planes 1 and 2 are inclined to one another by 81.8 (2) ° compared with the corresponding value found in the complex In the crystal hydrogen bonds involving the free and the coordinated water molecules, the ammonium group NH 3 and the Cl atoms build up a three dimensionnal network (Fig. 2, Table 1). There is also slipped π -π stacking interactions between the symetry related C9-C14 phenyl ring (Table 2). Both hydrogen-bonding and π-π interactions combine to stabilize the three-dimensional network.

Experimental
NdCl 3 (0.25 g, 1mmol) was dissolved in an aqueous solution of NaOH (0.5 M, 25 ml) with constant stirring. 3-aminobenzoic acid (0.14 g, 1 mmol) was added to the mixture and the pH was adjusted to ca. 3 using 4M HCl. The mixture was refluxed supplementary materials sup-2 at 353K for about 1 h and then cooled to room temperature. Slow evaporation of the solvent at room temperature lead to the formation of prismatic purple crystals of the title compound.

Refinement
The chloride anion Cl3 is disordered over three sites, Cl3A, Cl3B and Cl3C, which were refined with occupancies of 0.75, 0.15 and 0.10, respectively. The water molecule O6W is also disordered over two positions (O6WA and O6WB), which were refined with occupancy factors 0.72/0.28, so no H-atoms could be reliably defined. All H atoms attached to C and N atoms were fixed geometrically and treated as riding with C-H = 0.93 Å and N-H = 0.89 Å with U iso (H) = 1.2U eq (C) or U iso (H) = 1.5U eq (N). The H-atoms of the coordinated water molecules were initially refined using distance restraints [O-H = 0.85 (2) Å, and H···H = 1.40 (2) Å] with U iso (H) = 1.5U eq (O). However, in the last cycles of refinement, they were treated as riding on their parent O atoms.
Figures Fig. 1. The molecular structure of the title compound, with the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level [Symmetry code: (i) -x + 1, -y + 1, -z + 1; Hydrogen atoms have been omitted for clarity].

Special details
Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell esds are taken into account in the estimation of distances, angles and tor-

sion angles
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.