Crystal structure of bis[trans-dichloridobis(propane-1,3-diamine-κ2 N,N′)chromium(III)] dichromate from synchrotron data

In the title organic–inorganic salt, [CrCl2(tn)2]2[Cr2O7] (tn is propane-1,3-diamine), the CrIII ions are coordinated by four N atoms from two tn ligands and two chloride ions in a trans geometry, displaying a distorted octahedral arrangement. The crystal packing is stabilized by N—H⋯Cl and N—H⋯O hydrogen bonds.

The structure of the title compound, [CrCl 2 (tn) 2 ] 2 [Cr 2 O 7 ] (tn = propane-1,3diamine; C 3 H 10 N 2 ), has been determined from synchrotron data. The asymmetric unit contains one Cr III complex cation and half a [Cr 2 O 7 ] 2À anion. In the complex cation, the Cr III ion is coordinated by the four N atoms of two propane-1,3-diamine (tn) ligands in the equatorial plane and by two Cl atoms in a trans configuration, displaying a distorted octahedral coordination sphere. The two six-membered rings in the complex cation have an anti chair-chair conformation with respect to each other. The mean Cr-N(tn) and Cr-Cl bond lengths are 2.09 (1) and 2.320 (2) Å , respectively. The slightly bent dichromate anion is disordered over two sets of sites (occupancy ratio = 0.7:0.3) and has a staggered conformation. The crystal structure is stabilized by intermolecular hydrogen bonds involving the NH 2 groups of the tn ligands as donors and the O atoms of the [Cr 2 O 7 ] 2À anion and chlorido ligands as acceptors.

Chemical context
Propane-1,3-diamine (tn) can act as a bidentate ligand to a central metal ion via its two nitrogen atoms, forming a sixmembered ring. The [CrL 2 (tn) 2 ] + (L = monodentate ligand) cation can adopt either trans or cis geometric isomers. In addition, there are two possible conformations with respect to the six-membered rings in the trans-isomer. The carbon atoms of the two chelate rings of the tn ligands can be located on the same side (syn conformer) or on opposite side (anti conformer) of the equatorial plane . The preference for syn-or anti-conformation in the complex cation is an area of current interest because infrared or electronic absorption spectroscopic methods are not useful in determining the syn or anti conformations of the six-membered chelate rings in these transition metal complexes. The different arrangements of the two six-membered chelate rings of the tn ligands may be dependent on the packing forces and counter-anions in the crystal structure.
The shapes and sizes of counter-anions also play important roles in chemical, biological and environmental processes (Gadre et al., 1992;Fabbrizzi & Poggi, 2013;Santos-Figueroa et ISSN 2056-9890 al., 2013. The dichromate ion is environmentally important due to its high toxicity and its use in industrial processes (Yusof & Malek, 2009;Goyal et al., 2003). Here, we report on the synthesis and structure of [CrCl 2 (tn) 2 ] 2 (Cr 2 O 7 ), (I), in order to determine the conformations of the two sixmembered chelate rings of the tn ligands and of the [Cr 2 O 7 ] 2À anion.
bond angles of the two six-membered chelate rings around the Cr III atom are 90.07 (8) and 91.25 (8) . The other N-C and C-C bond lengths and Cr-N-C, N-C-C and C-C-C angles are also of usual values for tn ligands in chair conformations (Choi & Clegg, 2011;Moon et al., 2012). The [Cr 2 O 7 ] 2À counter-anion is positionally disordered and remains outside the coordination sphere of the Cr III cation. It is of interest to compare the conformation of the [Cr 2 O 7 ] 2À anion with that found in other ionic crystals. The [Cr 2 O 7 ] 2À anion in compound (I) is in a staggered conformation, in contrast to that observed in K 2 Cr 2 O 7 . In the latter, two nearly tetrahedral CrO 4 groups are in an almost eclipsed conformation (Brandon & Brown, 1968 (Moon et al., 2015). A further distortion of the anion is due to its involvement in hydrogen-bonding interactions.

Supramolecular features
The cations and anions in the crystal structure are held together by hydrogen bonds (Table 1) between the NH 2 donor groups of the tn ligand and Cl ligands and O atoms of the dichromate anion as acceptor groups. An extensive array of these contacts generate a three-dimensional network of molecules stacked along the a-axis direction (Fig. 2).

Synthesis and crystallization
The free ligand propane-1,3-diamine was obtained from Aldrich Chemical Co. and used as supplied. All other chemicals were reagent grade materials and used without further purification. As starting materials, trans-[CrCl 2 (tn) 2 ]ClO 4 was prepared as described in the literature (House, 1970;Choi & Clegg, 2011). The crude perchlorate salt (0.117 g) was dissolved in 10 mL of water at room temperature and added 5 mL of water containing 0.05 g of solid K 2 Cr 2 O 7 . The resulting solution was filtered and allowed to stand for two days to give green crystals of the dichromate salt suitable for X-ray structural analysis.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms, with C-H distances of 0.97 Å , and N-H distances of 0.89 Å , and with U iso (H) values of 1.2U eq of the parent atoms. The dichromate anion is positionally disordered over two sets of sites. In a first step, the occupancies of respective pairs, O1SA/O1SB, O2SA/O2SB, O3SA/O3SB and O4SA/O4SB, were refined freely and subsequently fixed at a ratio of 0.7:0.3. The bridging atoms O1SA/O1SB sites were refined using EXYZ/EADP commands; for O3SA, O2SB, O3SB and O4SB atoms ISOR restraints were applied.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.