Crystal structure of bis(acetato-κ2 O,O′)diaqua[1-(pyridin-2-ylmethylidene-κN)-2-(pyridin-2-yl-κN)hydrazine-κN 1]terbium(III) nitrate monohydrate

The Tb3+ ion is nine-coordinated in a distorted tricapped trigonal-prismatic geometry by the three N atoms of the tridentate 1-(pyridin-2-ylmethylidene)-2-(pyridin-2-yl)hydrazine ligand, four carboxylate O atoms of two chelating acetate groups and two O atoms of the coordinated water molecules. In the crystal, the complex cations are linked by pairs of O—H⋯O hydrogen bonds into dimers. These dimers, nitrate anions and non-coordinating water molecules are joined by O—H⋯O and N—H⋯O hydrogen bonds into a three-dimensional structure.

In the title compound, [Tb(C 2 H 3 O 2 ) 2 (C 11 H 10 N 4 )(H 2 O) 2 ]NO 3 ÁH 2 O, the Tb 3+ ion is nine-coordinated in a distorted tricapped trigonal-prismatic geometry by the three N atoms of the tridentate 1-(pyridin-2-ylmethylidene)-2-(pyridin-2-yl)hydrazine ligand, four carboxylate O atoms of two chelating acetate groups and two O atoms of the coordinating water molecules. The organic hydrazine ligand is disordered over two orientations with a refined occupancy ratio of 0.52 (3):0.48 (3). All bond lengths in the coordination environment of the Tb 3+ ion are slightly larger than those observed in the isostructural Y 3+ and Er 3+ complexes. In the crystal, the complex cations are linked by pairs of O-HÁ Á ÁO hydrogen bonds into dimers. These dimers, nitrate anions and non-coordinating water molecules are joined by O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds into a three-dimensional structure.

Chemical context
As a result of their various architectures and numerous applications (Binnemans, 2005), lanthanide complexes have attracted significant attention, and the synthesis of new complexes of this type has became relevant. Both mononuclear and polynuclear lanthanide complexes reveal specific properties as molecular magnets (Cristó vã o & Hnatejko, 2015), luminescence materials (Lahoud et al., 2016) and preparates for medical biology (Zhang et al., 2014). Used as ligands, Schiff bases together with carboxylate anions display large versatility in forming coordination compounds with metal ions and can generate a wide variety of coordination types. Considerable interest is afforded to the development of polydentate ligands containing different (hard and soft) N, O or S binding sites, designed to yield special topological structures (Binnemans, 2005). By appropriate design, the molecular structure of the ligand can be modified in order to coordinate metal ions in diverse modes resulting in specific architectures. The coordination mode also depends on the adopted synthetic procedures. In this context, for synthesis of the terbium(III) complex, the Schiff base 1-(pyridin-2-ylmethylidene)-2-(pyridin-2-yl)hydrazine (HL), which provides three softdonating N atoms from two pyridine rings and the imino function, was used together with acetate anions, which provide hard-donating O atoms, as co-ligands (Neves et al., 1992;ISSN 2056-9890 Schwingel et al., 1996Gregó rio et al., 2015). The ligand HL and acetate groups were used in our previous attempts to prepare new mono-and binuclear lanthanide(III) complexes (Ndiaye-Gueye, Dieng, Thiam, Sow et al., 2017;. In the present study, mixing of the HL ligand, sodium acetate and hexahydrated terbium nitrate yields a nine-coordinated mononuclear complex of Tb 3+ .

Structural commentary
The crystallographic study shows a 1:1:2 ratio of HL/Tb/ acetate in the resulting cationic complex when these components were mixed at room temperature in ethanol with a 1:1:3 ratio. The asymmetric unit comprises a Tb 3+ ion coordinated by one tridentate HL ligand, two chelating acetate ions, two coordinating water molecules, one non-coordinating nitrate anion and one non-coordinating water molecule (Fig. 1). The Schiff base acts as a tridentate ligand with three donating N atoms, forming two five-membered chelate rings (TbNCCN and TbNNCN). The Tb 3+ ion is nine-coordinated and its environment can be described as a distorted tricapped trigonal prism with slanted base faces N1, N2, O2 and O3, O5W, O6W. The Tb-O(Ac) bond lengths lie within the range 2.401 (3)-2.476 (3) Å (Table 1) and are comparable to the average value of 2.46 (6) Å for analogous structures from the Cambridge Structural Database (CSD Version 5.38, November 2016; Groom et al., 2016). The Tb-OW bond lengths involving O atoms of the coordinating water molecules of 2.357 (3) and 2.362 (3) Å are also well comparable with the known values [average 2.41 (5) Å from CSD]. In the title structure, the bonds Tb-N differ in length: the distance involving the imino N atom is shorter than those involving the pyridine N atoms: 2.542 (4) Å vs 2.574 (4) and 2.588 (4) Å (Table 1). The same relations between the Tb-N(imine) and Tb-N(Py) bond lengths were observed in the structures of {N,N 0 -cyclohexane-1,2-diylbis[1-(pyridin-2-yl)methanimine]}-tris(nitrato)terbium (Chen et al., 2013) and {(2,9-diformylphenanthroline)bis[(2pyridyl)hydrazone]}bis(nitrato)terbium nitrate (Carcelli et al., 2005), though the absolute values of Tb-N distances of the same kind in these three structures are different. The distances Tb-O(Ac), Tb-OW and Tb-N in the title structure are slightly larger (by 0.03-0.04 Å ) than the corresponding distances observed in isostructural Y 3+ and Er 3+ complexes we recently reported (Ndiaye-Gueye, . These observations can be correlated with the decrease in the unit-cell volume: 1060.5 (2) Å 3 for Tb 3+ vs 1051.3 (2) Å 3 for Y 3+ and 1049.6 (2) Å 3 for Er 3+ . The bond lengths in the disordered chain C-CH N-NH-C bridging two pyridine rings are 1.484 (14) and 1.513 (17) Å for C-C, 1.293 (17) and 1.319 (13) Å for C N, 1.393 (13) and 1.396 (13) Å for N-N and 1.411 (13) and 1.417 (12) Å for N-C. These bonds are slightly longer than observed for this ligand in other complexes. This may be related to the disorder detected for this chain. The dihedral angle formed by the planes of two terminal pyridine rings is 11.0 (4) .

Supramolecular features
The crystal structure is stabilized by hydrogen bonds giving rise to a three-dimensional network ( Table 2). The complex cations are linked into centrosymmetric dimers by pairs of An ORTEP view of the title compound, showing some of hydrogen bonds as dashed lines. Displacement ellipsoids are plotted at the 50% probability level. O-HÁ Á ÁO hydrogen bonds between one of two coordinating water molecules (O5W) and the acetate O1 atom in an R 2 2 (8) manner. The second coordinating water molecule (O6W) acts as hydrogen-atom donor, forming hydrogen bonds with the non-coordinating water molecule and the nitrate anion, as shown in Fig. 1. The acetate O atoms act as acceptors in the hydrogen bonds with the HN groups of adjacent complex cation. Furthermore, the non-coordinating water molecule forms hydrogen bonds to the nitrate anions. There are also some C-HÁ Á ÁO contacts, which contribute to the crystal architecture and may be considered as weak hydrogen bonds (Fig. 2, Table 2).

Figure 2
The packing showing the hydrogen bonds as dashed lines.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms of the water molecules were located in difference-Fourier maps. The O-H distances involving the O5W and O7W water molecules were restrained to 0.82 (2) Å , those involving O6W were constrained using the AFIX 7 instruction. Other H atoms (CH and CH 3 groups) were positioned geometrically and refined using a riding model with U iso (H) = 1.2U eq (C) (1.5 for CH 3 groups). The chain bridging the two pyridine rings was found to be disordered. This disorder may be explained by the fact that the sequence of atoms C(py)-CH N-NH-C(py) overlaps with the sequence C(py)-NH-N=CH-C(py), meaning two orientations for the ligand. For the refinement, we assumed that the C atom of CH group from one chain is situated nearby the N atom of NH group from the second chain, and the same relates inversely, whereas the imino N atoms of both chains occupy the same position. The occupancy factors were refined to a 0.52 (3)