4-(4-Pyridyl)pyridinium bis(pyridine-2,6-dicarboxylato)chromium(III) tetrahydrate

The title compound, (C10H9N2)[Cr(C7H3NO4)2]·4H2O or (4,4′-bipyH)[Cr(pydc)2]·4H2O (where 4,4′-bipy is 4,4′-bipyridine and pydcH2 is pyridine-2,6-dicarboxylic acid), was synthesized by the reaction of chromium(III) chloride hexahydrate with pyridine-2,6-dicarboxylic acid and 4,4′-bipyridine in a 1:2:4 molar ratio in aqueous solution. This compound is composed of an anionic complex, [Cr(pydc)2]−, protonated 4,4′-bipyridine as a counter-ion, (4,4′-bipyH)+, and four uncoordinated water molecules. The anion is a six-coordinate complex with a distorted octahedral geometry around the CrIII atom, formed by two tridentate pyridine-2,6-dicarboxylate, pydc2−, groups. Intermolecular O—H⋯O, N—H⋯O and C—H⋯O hydrogen bonds, and C—O⋯π stacking interactions (with distances of 3.3390 (13) and 3.4575 (13) Å) connect the various components into a supramolecular structure.

Intermolecular O-H···O, N-H···O and C-H···O hydrogen bonds with D···A ranging from 2.6736 (14) Å to 3.4235 (18) Å (Table 1), ion pairing and C-O···π stacking interactions seem to be effective in the stabilization of the crystal structure, resulting in the formation of an interesting supramolecular structure.

S2. Experimental
A solution of CrCl 3 .6H 2 O (133 mg, 0.5 mmol) in water (5 ml) was added to an aqueous solution of pyridine-2,6-dicarboxylic acid (167 mg, 1 mmol) and 4,4′-bipyridine (312 mg, 2 mmol) in water (10 ml) in a 1:2:4 molar ratio and refluxed for an hour. Purple crystals of the title compound were obtained after allowing the mixture to stand for two weeks at room temperature.

S3. Refinement
The H-atoms were included in calculated positions and treated as riding atoms, with the exception of H atoms on the water molecules. The latter were located in a low theta Fourier map and refined by a constrained rigid type geometry, where O-H = 0.85 Å and C-H = 0.95 Å with U iso (H) = 1.2U eq (parent O or C-atom).

Figure 1
The molecular structure of the title compound, displacement ellipsoids drawn at the 50% probability level.

4-(4-Pyridyl)pyridinium bis(pyridine-2,6-dicarboxylato)chromium(III) tetrahydrate
Crystal data (C 10   Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq