Diaquatris(nitrato-κ2 O,O′){2,2′-[pyridine-2,6-diylbis(methyleneoxy)]dibenzaldehyde-κO 1}dysprosium(III)–2,2′-[pyridine-2,6-diylbis(methyleneoxy)]dibenzaldehyde (1/1)

The title compound, [Dy(NO3)3(C21H17NO4)(H2O)2]·C21H17NO4, may be considered as an organic–metalorganic 1:1 co-crystal, in which the two dialdehyde molecules act as a ligand and as an organic moiety, respectively. The DyIII atom coordinates nine O atoms from the organic ligand, bidentate nitrate ions and water molecules, approximating a square-face-tricapped trigonal–prismatic geometry. The coordinated dialdehyde is not planar: the uncoordinated oxybenzaldehyde group is twisted by 39.96 (4)° from the rest of the ligand. In contrast, the free organic moiety is almost planar, with an r.m.s. deviation of 0.15 Å. In the crystal, segregated stacks of dialdehyde are formed in the [100] direction. For the complex, the shortest π–π contact is found at 3.781 (2) Å, and for the free ligand, at 3.785 (2) Å. The crystal structure is further stabilized by O—H⋯O and O—H⋯N hydrogen bonds in which coordinated water molecules are the donor groups.

The title compound, [Dy(NO 3 ) 3 (C 21 H 17 NO 4 )(H 2 O) 2 ]ÁC 21 H 17 -NO 4 , may be considered as an organic-metalorganic 1:1 cocrystal, in which the two dialdehyde molecules act as a ligand and as an organic moiety, respectively. The Dy III atom coordinates nine O atoms from the organic ligand, bidentate nitrate ions and water molecules, approximating a squareface-tricapped trigonal-prismatic geometry. The coordinated dialdehyde is not planar: the uncoordinated oxybenzaldehyde group is twisted by 39.96 (4) from the rest of the ligand. In contrast, the free organic moiety is almost planar, with an r.m.s. deviation of 0.15 Å . In the crystal, segregated stacks of dialdehyde are formed in the [100] direction. For the complex, the shortestcontact is found at 3.781 (2) Å , and for the free ligand, at 3.785 (2) Å . The crystal structure is further stabilized by O-HÁ Á ÁO and O-HÁ Á ÁN hydrogen bonds in which coordinated water molecules are the donor groups.

Related literature
For the X-ray structure of the free ligand and other rare-earth complexes based on this ligand, see : Rodríguez De Luna et al. (2010). For isotypic complexes, see: Garza Rodríguez (2010). For the nomenclature of 9-coordinated metal centers, see: IUPAC (2005).

Experimental
Crystal data [Dy(NO 3   revealed that the by-product, which crystallizes in space group P1, is isoformular, although the coordination number is reduced to 9, because one L ligand, present in the asymmetric unit, is not coordinated to the metal. The resulting formula is then [Ln III L(NO 3 ) 3 (H 2 O) 2 ].L, which may be seen as an organic-metalorganic system.
So far, we have detected the presence of this new complex with Ln = Ho III , Tb III and Dy III , on the basis of unit-cell parameters (Garza Rodríguez, 2010). The X-ray characterization is however complicated by the very low yield and the poor quality of single crystals we have obtained. The present report is for Ln = Dy III , which gave a suitable refinement.
The asymmetric unit contains one complex and one free ligand (Fig. 1). The Dy III atom is bonded to the monodentate L ligand, three bidentate nitrate ions, and two water molecules, forming nine Dy-O bonds in the range 2.320 (2)-2.460 (2) Å. The resulting coordination geometry approximates a square-face-tricapped trigonal prismatic polyhedron (polyhedral symbol in the IUPAC nomenclature: TPRS-9; IUPAC, 2005), with distortions from the ideal D 3h symmetry induced by the nitrate bite angles (Fig. 1, inset). The organic ligand is not planar, and the peripheral ring, C15···C21/O3/O4 is twisted by 39.96 (4)° from the rest of the ligand. The free ligand is more planar, and presents a conformation reminiscent of that

Experimental
The title compound was obtained by mixing 2,2′-[pyridine-2,6-diyl-bis(methyleneoxy)]dibenzaldehyde (L, 50 mg in 15 ml of acetonitrile) and Dy(NO 3 ) 3 .5H 2 O (100 mg in 2 ml of acetonitrile), at room temperature. The mixture was refluxed for 5 h and then cooled to room temperature. After evaporation of the solvent, a few crystals of the complex were collected.

Refinement
C-bound H atoms were placed in idealized positions, with C-H bond lengths fixed to 0.95 (aromatic CH) or 0.99 Å (methylene CH 2 ). In the case of coordinated water molecules, H atoms were clearly detected in a difference map, and refined freely.