Propane-1,3-diammonium dichromate(VI)

The title compound, (C3H12N2)[Cr2O7], consists of a discrete dichromate anion with an eclipsed conformation and a propane-1,3-diammonium cation. Both kinds of ions have a mirror plane passing through the bridging O atom and the central methylene C atom of the Cr2O7 2− and C3H12N2 2+ moieties, respectively. Anions and cations are alternately stacked to form columns parallel to the b axis. Ions are linked by intra- and inter-column hydrogen bonds of types N—H⋯O and C—H⋯O, involving O atoms of the dichromate anions as acceptors, and ammonium or methylene groups as donors.

The title compound, (C 3 H 12 N 2 )[Cr 2 O 7 ], consists of a discrete dichromate anion with an eclipsed conformation and a propane-1,3-diammonium cation. Both kinds of ions have a mirror plane passing through the bridging O atom and the central methylene C atom of the Cr 2 O 7 2À and C 3 H 12 N 2 2+ moieties, respectively. Anions and cations are alternately stacked to form columns parallel to the b axis. Ions are linked by intra-and inter-column hydrogen bonds of types N-HÁ Á ÁO and C-HÁ Á ÁO, involving O atoms of the dichromate anions as acceptors, and ammonium or methylene groups as donors.

Propane-1,3-diammonium dichromate(VI) Sonia Trabelsi, Houda Marouani, Salem S. Al-Deyab and Mohamed Rzaigui Comment
Hexavalent chromium is a predominant waste product of several metal finishing, petroleum refining and steel industries (Wani et al., 2007). It exists as chromate in basic and neutral medium and as dichromate in acidic environment.
In presence of 1,3-diaminopropane in water, the chromic acid is condensed into dichromate to form the hybrid title compound, (C 3 H 12 N 2 )Cr 2 O 7 . The observed molecular structure is depicted in Fig. 1. To counter-balance the electric charge of Cr 2 O 7 2-, the used 1,3-diaminopropane has been doubly protonated. The title compound crystallizes in the orthorhombic Pnma space group, so that the dichromate anion and 1,3-diammoniumpropane should be symmetrical with respect to the symmetry plane (m). Owing of the passage of the latter through the bridging atoms O1 and C2 of Cr 2 O 7 and C 3 H 12 N 2 respectively, the asymmetric unit is built by one independent CrO 4 group and the half of a 1,3-diammoniumpropane cation. The main geometrical features of Cr 2 O 7 2agree with those previously observed for this group in other compounds (Akriche & Rzaigui, 2009;Sieroń, 2007;Khadhrani et al., 2006).
The bond lengths and the angles within the cation are comparable with those observed in other 1,3-diammoniumpropane salts such as [C 3 H 12 N 2 ]ZnCl 4 (Kallel et al., 1980) and [C 3 H 12 N 2 ](ClO 4 ) 2 (Pritchard et al., 1992). In this structure, the cations and anions are alternately stacked to form columns parallel to the axis b (Fig. 2). The electrostatic interactions and H-bonds intra and inter columns keep up the three-dimensional network cohesion. The established weak H-bonds (Brown, 1976;Blessing, 1986) of types N-H···O and C-H···O involve oxygen atoms of the dichromate anions as acceptors, and the protonated nitrogen atoms and carbon atoms of 1,3-diammoniumpropane as donors.

Experimental
Single crystals of the title compound were prepared at room temperature by dissolving CrO 3 (0.10 g, 1 mmol) and 1,3-diaminopropane (0.07 g, 1 mmol) in distilled water (20 ml). The resulting solution was stirred during 30 min. and then evaporated slowly at room temperature until the formation of orange prismatic single crystals.

Refinement
All H atoms attached to C and N atoms were fixed geometrically and treated as riding with C-H = 0.97 Å (methylene) and N-H = 0.89 Å. Isotropic displacement parameters for H atoms were calculated as U iso (H) = 1.2U eq (C) for CH 2 groups and U iso (H) = 1.5U eq (N1) for the ammonium group.