Bis(4-aminopyridinium) dichromate(VI)

The asymmetric unit of the title salt, (C5H7N2)2[Cr2O7], contains four independent cations and two independent dichromate anions. The crystal structure consists of discrete dichromate anions with an eclipsed conformation stacked in layers parallel to (010) at y = 1/4 and y = 3/4. These layers are linked via 4-aminopyridinium cations by N—H⋯O and weak C—H⋯O hydrogen bonds, forming a three-dimensional supramolecular network. In addition, π–π interactions are present in this structure; the shortest distance separating mean planes through 4-aminopyridinium cations is 3.679 (6) Å.


Comment
Usually, pyridine can serve as an efficient bioisostere of benzene in drug design, and considerable interest has been shown in pyridine derivatives in the field of modern agrochemistry and medicinal chemistry, because substitution of the benzene by pyridine may result in good biological activity and low toxicity of molecules containing the pyridyl moiety (Patani & LaVoie, 1996;Ma & Huang, 2003). In this work, we report the preparation and the structural investigation of a new organic dichromate, (C 5 H 7 N 2 ) 2 ·Cr 2 O 7 , (I).
The asymmetric unit of (I) is composed of two independent dichromate anions and four independent 4-aminopyridinium cations ( Fig. 1). The structure of the compound consists of discrete dichromate ions with an eclipsed conformation stacked in layers parallel to the (010)  In this atomic arrangement four independent 4-aminopyridinium cations are present. Examination of the organic cations shows that the bond distances and angles show no significant difference from those obtained in oxalate and picrate salts involving the same organic group (Fun et al., 2009;Ramesh et al., 2010). Each 4-aminopyridinium cations is planar, with a maximum deviation of 0.0024 (2) Å. The shortest inter-planar distance between nearby pyridine rings is 3.679 (6) Å, which is much shorter than 3.80 Å, indicating the formation of π-π interactions ( Fig. 3, Janiak, 2000).
The established H-bonds of types N-H···O and C-H···O involve O atoms of the dichromate anions as acceptors, and the protonated N atoms and carbon atoms of 4-aminopyridinium as donors.

Experimental
Single crystals of the title compound were prepared at room temperature by dissolving CrO 3 (0.10 g, 1 mmol) and 4aminopyridine (Fampridine, 0.09 g, 1 mmol) in distilled water (20 ml). The resulting solution was stirred during 1 h, filtered and then evaporated slowly at room temperature until the formation of orange prismatic single crystals.

Refinement
All H atoms were located in a difference map. Nevertheless, they were geometrically placed and refined using a riding model, with C-H = 0.93 Å, N-H = 0.86 Å, and with U iso (H) = 1.2U eq (carrier C or N).  Projection of (I) along the a axis. The H-atoms not involved in H-bonding are omitted.