trans-Dichloridobis(propane-1,3-diamine-κ2 N,N′)chromium(III) perchlorate

In the title compound, [CrCl2(C3H10N2)2]ClO4, the CrIII atom is coordinated equatorially by four N atoms of two propane-1,3-diamine (tn) ligands and axially by two mutually trans Cl atoms, thus displaying a slightly distorted octahedral geometry with no crystallographically imposed symmetry. The two six-membered chair chelate rings in the complex cation are in an anti conformation with respect to each other. The Cr—N bond lengths range from 2.0831 (18) to 2.0917 (19) Å, and the Cr—Cl bond lengths are 2.3148 (6) and 2.3135 (6) Å. The perchlorate anions have slightly distorted tetrahedral geometries. Weak intermolecular hydrogen bonds involving the tn ligand NH groups as donors, and chloride ligands and anion O atoms as acceptors are observed.

In the title compound, [CrCl 2 (C 3 H 10 N 2 ) 2 ]ClO 4 , the Cr III atom is coordinated equatorially by four N atoms of two propane-1,3-diamine (tn) ligands and axially by two mutually trans Cl atoms, thus displaying a slightly distorted octahedral geometry with no crystallographically imposed symmetry. The two sixmembered chair chelate rings in the complex cation are in an anti conformation with respect to each other. The Cr-N bond lengths range from 2.0831 (18) to 2.0917 (19) Å , and the Cr-Cl bond lengths are 2.3148 (6) and 2.3135 (6) Å . The perchlorate anions have slightly distorted tetrahedral geometries. Weak intermolecular hydrogen bonds involving the tn ligand NH groups as donors, and chloride ligands and anion O atoms as acceptors are observed.
There are also two possible conformations with respect to the six-membered chelate rings (present as chairs) in the trans geometric isomer: the carbon atoms of these rings in the two tn ligands can be located on the same side (syn conformer) or on opposite side (anti conformer) of the equatorial plane. In the crystal structures of trans-[Cr(Me 2 tn) 2 Cl 2 ]Cl and trans-[Cr(Me 2 tn) 2 Br 2 ] 2 Br 2 .HClO 4 .6H 2 O (Me 2 tn = 2,2-dimethylpropane-1,3-diamine), both syn and anti conformational isomers are found together (Choi et al., 2002;Choi et al., 2007), while trans-[Cr(Me 2 tn) 2 Cl 2 ]ClO 4 (Choi et al., 2008) has only the anti conformer, as do trans-[Cr(tn) 2 F 2 ]ClO 4 (Vaughn & Rogers, 1985) and trans-[Cr(tn) 2 Cl 2 ] 3 [Fe(CN) 6 .6H 2 O (Kou et al., 2001). The preference for syn or anti conformation of chelate rings in trans complex cations with tn or Me 2 tn ligands is thus subtle and worthy of further study. Infrared and electronic absorption spectroscopic methods are not useful in distinguishing such syn and anti conformations in these metal complexes. Structural studies of bromido-containing chromium(III) complexes are relatively rare compared to those with chlorido ligands. Therefore we attempted to prepare trans-[Cr(tn) 2 Br 2 ]ClO 4 by a literature method (Couldwell & House, 1972); its UV-visible and IR spectra are nearly the same as those of trans-[Cr(tn) 2 Cl 2 ]ClO 4 (House, 1970), and it was only with a crystal structure analysis that we established that the product was actually the dichlorido rather than the dibromido complex. We report here the structure of trans-[Cr(tn) 2 Cl 2 ]ClO 4 (I) which provides further information on the conformation of the two six-membered chelate rings.
In the title complex (I), the chromium(III) ion is coplanar with the four coordinating N atoms and adopts an octahedral geometry, in which the four nitrogen atoms of two tn ligands occupy the equatorial sites and the two chlorine atoms coordinate axially in a trans configuration. The two six-membered rings have their usual stable chair conformations, and they are exclusively in the anti conformation with respect to each other in the unique cation of the asymmetric unit ( Fig. 1).
The Cr-N distances (Table 1) are in the range 2.0831 (18)-2.0917 (19) Å, typical for Cr-N bonds involving primary amines (Choi et al., 2002;Choi et al., 2007). The Cr-Cl distances [2.3135 (6) and 2.3148 (6) Å] are very close to the values 2.3179 (9) and 2.3212 (4) Å found in trans-[Cr(Me 2 tn) 2 Cl 2 ]ClO 4 (Choi et al., 2008), and typical generally of Cr-Cl bond lengths in the Cambridge Structural Database (Allen, 2002), but shorter than the 2.4743 (10) Å for Cr-Br bond lengths in trans-[Cr(en) 2 Br 2 ]ClO 4 (Choi et al., 2010). The assignment of the axial ligands as Cl rather than the Br intended and expected from the synthesis is also clearly correct from the satisfactory refinement of anisotropic displacement parameters, demonstrating an appropriate electron density. The internal geometry of the tn ligands is typical for these in chair conformations (Vaughn, 1981 The ligand propane-1,3-diamine was obtained from Aldrich Chemical Co. and was used as supplied. All other chemicals were reagent grade materials and were used without further purification. We intended to prepare trans-[Cr(tn) 2 Br 2 ]ClO 4 as described in the literature (Couldwell & House, 1972) but obtained instead trans-[Cr(tn) 2 Cl 2 ]ClO 4 , as demonstrated by this crystal structure analysis.
CrCl 3 .6H 2 O (5.4 g) was dissolved in DMSO (25 ml) and the solution was boiled for 10 min. A mixture of 1,3-propanediamine (3 ml) and DMSO (15 ml) was added and boiling was continued for 2 min. After cooling to 60°C, the solution was poured into well stirred acetone (300 ml). The precipitate was filtered off and washed with acetone, then dissolved in aqueous HBr (20 ml, 48%) and the solution was heated on a steam bath for 15 min. and filtered. The filtrate was heated on a steam bath for a further 15 min. Aqueous HClO 4 (5 ml, 60%) was added to the solution. The resulting green crystals were collected and washed with ethanol. The infrared spectrum (nujol) was consistent with the crystallographically determined structure. The chloro ligands in the title compound are clearly retained from the chromium(III) chloride starting material, and were not substituted as intended by Br in the reaction with HBr.

Refinement
The crystal was a non-merohedral twin with a 23.45 (6)% contribution of the minor component according to the refinement; because of the twinning, merging of symmetry-equivalent data could not be performed prior to refinement. The twin law is 1 0 0 / 0 -1 0 / -0.2 0 -1, corresponding to a 180° rotation about the a axis. Hydrogen atoms were located in a difference map and refined freely with individual isotropic displacement parameters. Fig. 1. The structure of the complex cation and anion (displacement ellipsoids are drawn at the 50% probability level).