Tris(6-carboxypyridine-2-carboxylato)terbium(III) 2.75-hydrate

In the title compound, [Tb(C7H4NO4)3]·2.75H2O, the Tb3+ atom is coordinated by three tridentate 6-carboxypyridine-2-carboxylate ligands and lies on a crystallographic threefold rotation axis. The coordination polyhedron around TbIII adopts a distorted tricapped trigonal–prismatic geometry. Disordered water molecules with partial occupancy are also present in the crystal, one of which is associated with each of the carboxylate O atoms of the complex unit.

In the title compound, [Tb(C 7 H 4 NO 4 ) 3 ]Á2.75H 2 O, the Tb 3+ atom is coordinated by three tridentate 6-carboxypyridine-2carboxylate ligands and lies on a crystallographic threefold rotation axis. The coordination polyhedron around Tb III adopts a distorted tricapped trigonal-prismatic geometry. Disordered water molecules with partial occupancy are also present in the crystal, one of which is associated with each of the carboxylate O atoms of the complex unit.
Financial support from the University of Geneva and the SNF is gratefully acknowledged.

Comment
Since the first structural investigation of tris(dipicolinato)ytterbium complex (Albertsson, 1970), a number of different lanthanide complexes with dipic (= hydrogen 2,6-pyridinedicarboxylate) have been reported. Their brief overview can be found, e.g. in the report of the Hamacek group . Recently, we have reported on the self-assembly of a trinuclear luminescent europium complex with bis(6-methoxycarbonyl-2-carbonylpyridine)amine (L) .
In an attempt to synthesize the analogous terbium(III) compound with L using the same procedure, small transparent crystals were isolated from the resulting DMSO solution. However, these X-ray quality crystals had a different shape than expected (cubic crystals for Eu 3 L 3 ). Structural studies reveal the formation of a partially hydrated tris(dipicolinato -1 )terbium complex, the title compound [Tb(dipic) 3 ] . 2.75H 2 O (I) (Fig. 1). Accordingly, the presence of dipicolinate anions is explained by the complete hydrolysis of both ester and amide functions of the ligand L. In addition to crystallography, the obtained crystalline material was analysed using spectroscopic methods. Fig. 2 shows the typical emission spectrum of [Tb(dipic) 3 ] 3with characteristic 5 D 4 -> 7 F j transitions (D'Aléo et al., 2007(D'Aléo et al., , 2008. The luminescent lifetime at room temperature was found to be 1.45 ms, which is a somewhat lower value compared to the D'Aleo's value (2.02 ms) probably due to additional quenching of surrounding water molecules. The IR spectrum on Fig. 3 shows stretching O-H vibrations at about 3400 cm -1 and water bending vibrations at 1637 cm -1 .
From the bond lengths, the valence of the Tb atom was calculated to be 3.15. Taking into account the oxidation state of the Tb atom and in the absence of any other charged species in the crystals, the ligand has to be partially protonated.
However, the quality of the data does not allow the localion of the position of this extra hydrogen on each of the ligands.
Apart from the complex, additional partial water molecules are present in the crystal, one of which (O17) is associated with the carboxyl oxygens of the ligand (details of the refinement are given elsewhere).
In the complex, the Tb III cation is nine-coordinated by three monoanionic dipic 1ligands and lies on the threefold rotation axis. The Tb atom lies 0.081 Å from the plane defined by the three nitrogen donors, so that these four atoms are nearly coplanar (Fig. 4). The two planes defined by the three O2 and three O9 donors, respectively, form a discrete tricapped trigonal prism with the distance to the central terbium atom equal to 1.581 Å and 1.635 Å, respectively. The asymmetric unit comprises one dipicolinate ligand and 1/3 of a Tb atom and the partial water O17 (S.O.F = 0.50 Concerning the crystal packing, the [Tb(dipic) 3 ] units are arranged in the plane around disordered partial water molecules, occupying the available spaces, which resemble channels (Fig. 5a). The underlayer is shifted along the b axis in order to optimize hydrogen bonding interactions (Fig. 5b). A comparison of structural data in Table 2 shows that the choice of the counter-ion [absent in the case of (I)] has a significant influence on the final crystal packing (seven space groups for nine tris(dipicolinato)terbium complexes), and also on the coordinate bonds within the complex (various Tb-N and Tb-O distances).

Experimental
To a solution of 60.8 mg (0.18 mmol) of the pyridine-containing ligand L and 152.5 mg (0.18 mmol) of Tb(Otf) 3 . 13.7H 2 O in 5 ml DMF was added NaH (26 mg 3.5 eq). The mixture was stirred for three hours under a nitrogen atmosphere, filtered and then evaporated to dryness. The residue was dissolved in DMSO, filtered and water was allowed to diffuse into this solution. The IR spectrum of the isolated solid was measured at room tempeature with a Perkin-Elmer Spectrum 1 (equipped with a Specac Golden Gate ATR accessory). The phosphorescence spectrum was obtained under the same conditions with a Perkin-Elmer Lambda 900 (λ exc = 273 nm).

Refinement
In the absence of any other element that could satisfactorily fit the solvent peaks in the Fourier difference map, the solvent density was attributed to partially occupied water molecules. One of them was included in the model. Its occupancy was refined to 0.45 with U iso fixed to 0.05 and then fixed to 0.5 while refining the anisotropic displacement parameters. The other possible water molecules that were seen in the solvent density had low occupancies and large anisotropic displacement parameters. The Squeeze/bypass procedure (Sluis et al., 1990) was therefore used to take care of the extra electron density in the channel. 26 Electrons were found in a void of 202 Å 3 . This is compatible with the presence of 2.5 extra water molecules per unit-cell that were added to the formula. Concerning the charge of the complex, a model with a protonated COOH ligand making a neutral complex is the most probable since infrared measurement and the synthesis conditions do not suggest the presence of an oxonium ion. Although some density is found in the final difference Fourier map around O2 and O4, the geometry of both COOgroup and especially their symmetry and bond length do not allow to conclude unambiguously on the position of the extra hydrogen in the ligand. As a consequence, it was not included in the model. Short O17-O contacts [2.84 (2) Å to O10 and 2.85 (2) Å to O4] indicate possible hydrogen bonds between O17 and O4 and O10. O4 and O10 are potential candidates to accommodate the extra proton on the ligand and can act as donors for these hydrogen bonds. If not protonated, they would suit as acceptors for hydrogen bonds involving the hydrogen of the water molecule containing O17. The extra water molecules present in the structure that are not included in the model may also participate to the hydrogen-bonding network.