Redetermination of [Pr(NO3)3(H2O)4]·2H2O

The structure of the title compound, tetraaquatris(nitrato-κ2 O,O′)praseodymium(III) dihydrate, was redetermined. The structure models derived from the previous determinations [Rumanova et al. (1964 ▶). Kristallografiya, 9, 642–654; Fuller & Jacobson (1976 ▶). Cryst. Struct. Commun. 5, 349–352] were confirmed, but now with all H atoms unambiguously located, revealing a complex O—H⋯O hydrogen-bonding network, extending throughout the whole structure. In the title compound, the coordination environment of the PrIII atom can best be described as a distorted bicapped square antiprism defined by three bidentate nitrate anions and four water molecules. Additionally, two lattice water molecules are observed in the crystal packing. The title compound is isotypic with several other lanthanide-containing nitrate analogues.

The structure of the title compound, tetraaquatris(nitrato-2 O,O 0 )praseodymium(III) dihydrate, was redetermined. The structure models derived from the previous determinations [Rumanova et al. (1964). Kristallografiya, 9, 642-654;Fuller & Jacobson (1976). Cryst. Struct. Commun. 5, 349-352] were confirmed, but now with all H atoms unambiguously located, revealing a complex O-HÁ Á ÁO hydrogen-bonding network, extending throughout the whole structure. In the title compound, the coordination environment of the Pr III atom can best be described as a distorted bicapped square antiprism defined by three bidentate nitrate anions and four water molecules. Additionally, two lattice water molecules are observed in the crystal packing. The title compound is isotypic with several other lanthanide-containing nitrate analogues.
Data collection: CrysAlis PRO (Agilent, 2010); cell refinement: CrysAlis PRO; data reduction: CrysAlis PRO; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2008); software used to prepare material for publication: PLATON (Spek, 2009 The title compound was serendipitously obtained in low yield as an undesired product of an experiment aimed at obtaining a Pr III -containing coordination polymer, with dicarboxylate ligands as the connecting moieties, following an earlier successful synthesis of a vanadium metal-organic framework with the same type of linkers (Liu et al., 2012).
However, in both of the latter structures, the "position of elements of H were undetermined". In the now determined structure, these hydrogen atoms could unambiguously be located from difference Fourier electron density maps, revealing an extended hydrogen bonding network.
The asymmetric unit consists of one Pr III cation, three nitrate anions, and in total six water molecules. The Pr III cation is ten-coordinated by the oxygen atoms of three bidentate nitrate anions and four water molecules. Additionally, two lattice water molecules are included in the second coordination sphere of the praseodymium cation. The coordination polyhedron around Pr III can be best described as a distorted bicapped square antiprism ( Figure 1). The Pr-O distances range from 2.5677 (16) to 2.7307 (15) Å and from 2.4287 (17) to 2.4578 (17) Å for the coordinating nitrate groups and water molecules, respectively ( Table 1). The coordinating nitrate groups are positioned on the same side of the polyhedron, whereas the coordinating water molecules are positioned on the opposite side.
When searching the Cambridge Structural Database (CSD, Version 5.33) (Allen, 2002), another Pr III -complex is found (CSD reference code VAFLOD), containing three nitrate anions and four water molecules in its coordination sphere.
However, this complex shows a totally different, approximately twofold symmetry (Weakley, 1989). Other structures of metal-organic complexes of Pr III , coordinated by only nitrate and water molecules are found, showing different coordination assemblies, i.e. a 12-coordinated Pr III atom with five nitrate and two aqua ligands, balanced by two additional counter ions (CSD reference code QERRIP) (Rohde & Urland, 2006), a complex with three nitrate and three aqua ligands (CSD reference code CUKMUQ) (Weakley, 1984), and even the formation of dimers (CSD reference code BUPFIB) (Weakley, 1982) have been previously reported.
In the reported structure, a complex, extended hydrogen bonding network is formed throughout the whole structure, stabilizing the crystal packing, i.e. in total 16 different hydrogen bonds are observed between the coordinating water molecules, nitrate anions and lattice water molecules (Figure 2). In total, eight different symmetry equivalent water or nitrate oxygen atoms are involved in the hydrogen bond network. In fact, the praseodymium complexes are all interconnected through these solvent water molecule hydrogen bonds (Figure 3) show the following hydrogen bonds: O10 is hydrogen bonded to the symmetry-equivalent water molecule oxygen atom O14 and nitrate O8. Water O11 is hydrogen bonded to the symmetry equivalent waters O14 and O15. Water O12 is hydrogen bonded to the symmetry equivalent nitrate O4 and forms an intramolecular hydrogen bond with water O15.
Water O13 shows two bifurcated hydrogen bonds to the symmetry equivalent nitrates O7 and O9 and to O4 and O8. The solvent water molecule O14 forms an intramolecular hydrogen bond with nitrate O9 and a bifurcated hydrogen bond to the symmetry equivalent nitrates O3 and O5. Solvent water molecule O15 forms a bifurcated hydrogen bond to two different symmetry equivalent nitrate O3 atoms and another hydrogen bond to a symmetry equivalent nitrate O6. Details of the hydrogen-bonding geometry are given in Table 2.

Experimental
The title compound was serendipitously obtained in low yield as a product of an experiment aimed at obtaining a Pr IIIcontaining coordination polymer, with dicarboxylate ligands as the connecting moiety, following an earlier successful synthesis of a vanadium metal-organic framework with the same type of linkers (Liu et al., 2012).
In the synthesis, 0.1 mmol Pr(NO 3 ) 3 . xH 2 O, along with 0.3 mmol dicarboxylic acid and four drops of 0.6 M aqueous HNO 3 was dissolved in 5 ml of a 1:1 mixture of 1:1 MeOH/H 2 O. After seven days of heating the mixture to 363 K, the title compound was isolated as colourless crystals, suitable for single-crystal X-ray diffraction analysis.

Refinement
All hydrogen atoms were located in a difference Fourier electron density map and further refined with isotropic temperature factors fixed at 1.5 times U eq of the parent atoms, applying a restraint value of 0.84 (2) Å for the O-H distances.