Bis(μ-pyridine-2,3-dicarboxylato)bis[aqua(3-carboxypyridine-2-carboxylato)indium(III)] tetrahydrate

In the binuclear centrosymmetric title compound, [In2(C7H3NO4)2(C7H4NO4)2(H2O)2]·4H2O, which contains both pyridine-2,3-dicarboxylate and 3-carboxypyridine-2-carboxylate ligands, the InIII atom is six-coordinated in a distorted octahedral geometry. One pyridine ligand is N,O-chelated while the other is N,O-chelated and at the same time bridging to the other via the second carboxyl group. In the crystal, an extensive O—H⋯O hydrogen-bonding network, involving the coordinated and lattice water molecules and the carboxyl groups of the ligands, together with C—H⋯O and π–π interactions [centroid–centroid distance = 3.793 (1) Å], leads to the formation of a three-dimensional structure.

Polycarboxylate ligands are versatile because of their diversity of coordination modes and also because of their ability to initiate self assembly processes by supramolecular interactions (Kondo et al., 1999;Beobide et al., 2006). In the majority of the complexes obtained by proton-transfer methods the metal complex is anionic with the cation derived from the amine used in the synthesis. Among these multicarboxylate ligands, pyridine-2,3-dicarboxylic acid (py-2,3dcH 2 ) has rarely been used under the conditions generally employed in our studies. In the course of this work we prepared the title binuclear indium(II) compound, whose crystal structure we report on herein.
In addition to being a neutral complex, the title compound ( Fig. 1) appears to be the first indium III complex N,O chelated by one py-2,3-dcHion and one py-2,3-dc 2ion, with the latter using the carboxyl group in the 3-position to bridge to a second metal. In the resulting centrosymmetric dimer, the coordination sphere of each metal is completed by a water molecule.
The O 4 N 2 coordination sphere adopts a distorted octahedral geometry with the largest departure being the 75.30 (5)° angles subtended by the chelating ligands (Fig. 1). The average In-O distance of 2.1253 (14) Å is slightly shorter than the average In-N of 2.2478 (17) Å. This can be explained by Pearson's hard and soft acid-base concept (Schlemper et al., 1967).
A final interaction of significance is a complementary π-π stacking interaction (Fig. 2) between the C13═O6 moiety in one half of the dimer and the (N2,C8-C12) ring in the other half (centroid-to-centroid distance = 3.347 (2) Å, angle of the line joining the centroids to the plane of the ring = 74.7 (1)°).

Experimental
A solution of In 2 (SO 4 ) 3 .xH 2 O (34 mg, 0.06 mmol) in water (5 ml) was added dropwise to an aqueous solution of pyridine-2,3-dicarboxylic acid (10 mg, 0.06 mmol) and 2-amino-6-methyl pyridine (13 mg, 0.12 mmol) in a 1:1:2 molar ratio at supplementary materials sup-2 reflux. The solution was cooled to room temperature and upon slow evaporation, X-ray quality crystals were formed which were collected and washed with distilled water.

Refinement
The OH and water H-atoms were located in difference Fourier maps and were refined as riding atoms with U iso (H) = 1.2U eq (O). The C-bound H-atoms were placed in calculated positions and treated as riding atoms: C-H = 0.95 Å with U iso (H) = 1.2U eq (C). Fig. 1. View of the local coordination environments of In III atoms in the title molecule, with 50% probability thermal ellipsoids. Primed atoms are related to the non-primed atoms by the center of symmetry.