Poly[hexaaquabis(μ3-naphthalene-2,6-dicarboxylato)(μ2-naphthalene-2,6-dicarboxylato)diholmium(III)]

The crystal structure of the title compound, [Ho2(C12H6O4)3(H2O)6]n, contains binuclear centrosymmetric {Ho2O2(CO2)4(H2O)6} cores interconnected via the naphthalene-2,6-dicarboxylate (NDC2−) bridging ligands into a two-dimensional neutral plane net, ∞2[Ho2(NDC)3(H2O)6], exhibiting a typical (4,4)-topology. Interactions between adjacent layers are assured by a series of C—H⋯π contacts and a number of strong and highly directional O—H⋯O hydrogen bonds involving the coordinated water molecules and neighbouring coordinated carboxylate groups. One NDC2− bridging ligand has its centroid located at a crystallographic centre of inversion.


Comment
In less than twenty years, the field of Crystal Engineering involving the synthesis and characterization of multi-dimensional metal-organic frameworks (also known as coordination polymers) has grown immensely to become one of the most active research areas in inorganic chemistry. These worldwide efforts are motivated by the new and often striking structural features obtained by varying the metal centres and the bridging organic ligands, and by the prospect of making materials with direct industrial applications. Following our efforts in the hydrothermal synthesis and structural characterization of highly crystalline materials of this kind, (Cunha-Silva, Cunha-Silva, Shi et al., 2007;Shi et al., 2007;Mafra et al., 2006;Shi et al., 2006;Paz, Rocha, Klinowski et al., 2005;Almeida Paz, Shi, Mafra et al., 2005;Shi et al., 2005;Paz & Klinowski, 2004;Paz & Klinowski, 2003;Almeida Paz et al. 2002a, 2002b, 2002c, we report here the low temperature crystal structure at 180 (2) K of a two-dimensional lanthanide-organic framework containing residues of naphthalene-2,6-dicarboxylic acid (H 2 NDC), [Ho 2 (NDC) 3 (H 2 O) 6 ], which is analogous to that reported by Deluzet et al. (2003) (Zheng, Sun et al., 2004;Zheng, Wang et al., 2004;Paz & Klinowski, 2003;Wang et al., 2002;Min & Lee, 2002).
The structure of the title compound, I, contains a single crystallographically independent metallic centre, Ho1, coordinated to three water molecules (O1W, O2W and O3W) and four NDC 2bridging ligands (Figure 1a), with a {HoO 8 } coordination geometry resembling a highly distorted dodecahedron (Figure 1 b). The Ho-O bond lengths were found in the 2.252 (3)-2.461 (4) Å range,in good agreement with those of related materials as revealed by a search in the CSD.
The three crystallographically independent carboxylate groups coordinate to the Ho 3+ centres in distinct coordination fashions as shown in Figure 1a. Notably, the C8 carboxylate group is coordinated via a typical syn,syn-µ 2 -bridging coordination fashion leading to the formation of binuclear centrosymmetric anionic [Ho 2 (NDC) 6 (H 2 O) 6 ] 6unit ( Figure 1 plane nets close pack in a parallel fashion (not along a principal axis of the unit cell) to produce the crystal structure. Along supplementary materials sup-2 the [010] crystallographic direction the packing occurs in an orderly ABAB··· fashion (Figure 3 b). Connections between adjacent layers are mainly assured by strong and highly directional O-H···.O hydrogen bonds involving the O2W and O3W coordinated water molecules from one layer and the coordinated carboxylate groups from the neighbouring layer ( Figure   4 and Table in the main paper summarizing the geometrical parameters of the hydrogen bonding interactions). Moreover, these connections are reinforced by weak C-H···π interactions between coordinated NDC 2residues belonging to adjacent layers (not shown). It is important to stress that, within each ∞ 2 [Ho 2 (NDC) 3 (H 2 O) 6 ] layer, O1W is also engaged in strong O-H···.O hydrogen bonds which reinforce the connections between neighbouring binuclear units ( Figure 4).
To a solution of HoCl 3 .6H 2 O (1.054 g, 2.778 mmol) in distilled water (6.88 g), naphthalene-2,6-dicarboxylic acid (0.100 g, 0.463 mmol) and triethylamine (0.097 g, 0.959 mmol) were added and the mixture was stirred thoroughly for 5 minutes at ambient temperature. The suspension, with a molar composition of 6.01 Ho 3+ : 1.00 H 2 NDC: 2.07 TEA: 137 H 2 O, was transferred to a Parr teflon-lined stainless steel vessel (ca 21 cm 3 ) and placed for 8 h at 145 °C in a preheated oven. Before opening, the reaction vessel was allowed to cool slowly to ambient temperature at a rate of 10 ° per hour over a period of 14 h. The isolated crystalline material was mainly composed of crystals of the title compound which were preserved in a portion of the mother liquor before being manually selected under a polarized microscope for subsequent crystal mounting on a glass fibre.
A small amount of colourless plate-like crystals, which could not be physically separated from the title compound, were also investigated and revealed to be isostructural with the frameworks reported by Zheng, Sun et al. (2004). The crystal data for this material will be the subject of a separate communication.

Refinement
A slightly smeared-out electron density was found surrounding the carbon atoms of one bridging naphthalene-2,6-dicarboxylate ligand. However, the quality of the data set did not allow a sensible modelling of this disorder over, at least, two istinct crystallographic positions. C3, C4, C9 and C10 atoms were instead refined using anisotropic displacement parameters which define a typical prolate thermal motion for these atoms.
H atoms associated with the water molecules were clearly visible in difference Fourier maps and were included in the final structural model with the O-H and H···H restrained to 0.95 (1) and 1.55 (1) Å, respectively, in order to ensure a chemically reasonable geometry for these moieties. These H atoms were allowed to ride on their parent O atoms with U iso fixed at 1.5×U eq (O). H atoms bound to carbon were instead placed at idealized positions and allowed to ride on their parent atoms with U iso fixed at 1.2×U eq (C). All C-H distances are of 0.95 Å.    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 > σ(F 2 ) is used only for calculating Rfactors(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Ho1