The inorganic–organic hybrid material triethylenetetrammonium hexachloridorhodate(III) chloride

Single crystals of the new title compound [systematic name: 1,4,7,10-tetrazoniadecane hexachloridorhodate(III) chloride], [H3N(CH2)2NH2(CH2)2NH2(CH2)2NH3][RhCl6]Cl, were obtained from the corresponding amine and rhodium trichloride in hydrochloric acid solution by slow crystallization under diffusion-controlled conditions at room temperature. Its solid-state structure is defined by a three-dimensional framework of numerous electrostatic-supported N—H⋯Cl hydrogen bonds between the ionic components of the compound. Within this framework, layered arrangements of the complex ions on one hand and of the protonated amines and chloride ions on the other hand, can be recognized. The octahedral hexachloridorhodate(III) anion resides on a symmetry site, while the triethylenetetrammonium cation and the chloride ion both reside on twofold axes.

Single crystals of the new title compound [systematic name: 1,4,7,10-tetrazoniadecane hexachloridorhodate(III) chloride], [H 3 N(CH 2 ) 2 NH 2 (CH 2 ) 2 NH 2 (CH 2 ) 2 NH 3 ][RhCl 6 ]Cl, were obtained from the corresponding amine and rhodium trichloride in hydrochloric acid solution by slow crystallization under diffusion-controlled conditions at room temperature. Its solid-state structure is defined by a three-dimensional framework of numerous electrostatic-supported N-HÁ Á ÁCl hydrogen bonds between the ionic components of the compound. Within this framework, layered arrangements of the complex ions on one hand and of the protonated amines and chloride ions on the other hand, can be recognized. The octahedral hexachloridorhodate(III) anion resides on a 1 symmetry site, while the triethylenetetrammonium cation and the chloride ion both reside on twofold axes.

Comment
As part of our research on inorganic-organic hybrid materials various alkylammonium hexahalogenidorhodates(III) have been synthesized and structurally characterized with focus on the principles of organization of the organic and inorganic components on the one hand and the hydrogen bonding networks on the other (Frank & Reiß, 1996;Frank & Reiß, 1997;Frank & Bujak, 2002;Frank & Graf, 2004). The aim of the work described in this report was to examine the structural properties of a compound extending the series of known hexachloridorhodates(III) with cations of the general formula H 3 N(CH 2 ) 2 (NH 2 (CH 2 ) 2 ) n NH 3 (n+2)+ [n = 0 (Gillard et al., 1996;Reiß, 1996), n = 1 ].
A light red microcrystalline unresolvable substance is obtained in a fast precipitation reaction by mixing hydrochloric acid solutions of triethylene tetrammonium chloride (n = 2 according to the prementioned formula) and rhodium trichloride. A diffusion controlled crystallization procedure yielded single crystals of sufficient size for singlecrystal structure analysis. The results of elemental analyses and spectroscopic investigations agreed with the for- [RhCl 6 ] 3− and Cl − to be present in the crystal in a ratio of 1:1:1 (Fig. 1). The [RhCl 6 ] 3− ion has a crystallographically imposed 1 symmetry. As expected, the rhodium atom at the centre is coordinated in a nearly ideal octahedral geometry by the six chlorido ligands (Table 1). The complex ion is surrounded by four NH 3 groups and two NH 2 groups of altogether six triethylene tetrammonium cations. All the N-H-Cl hydrogen bonds between these cations and the chlorido ligands of the complex anion have to be considered as weak interactions ( Fig. 1 and Table 2). This is indicated by N -Cl distances varying between 3.165 (2) Å and 3.267 (2) Å (with H -Cl distances from 2.28 Å to 2.45 Å) (Table 2), as well as by the IR frequencies of the N -H stretching modes. The triethylene tetrammonium cation resides on a twofold axis. Its conformation deviates substantially from the ideal all-trans (zigzag chain-like) arrangement. The deviation is primarily described by a torsion of 54.3 (3)° around the bond between C2 and N2, so that the cation's conformation resembles to a stretched 's'.
Apart from that the bond lengths and angles are as expected. The cation has ten weak hydrogen bonds to its environment, so its hydrogen bond donor functions are completely saturated (Fig. 1). The NH 3 group (N1) is connected to two [RhCl 6 ] 3− octahedra by two hydrogen bonds, while the NH 2 group (N2) is connected to a third [RhCl 6 ] 3− octahedron, i. e. taking into account the site symmetry each cation contacts six octahedra. The single Cl − ion (Cl4) resides on a twofold axis and is fixed by hydrogen bonds to two NH 3 and two NH 2 groups, which are positioned in a distorted tetrahedral arrangement (Fig. 1) and belong to four triethylene tetrammonium cations.
In total the arrangement of the ionic components defines a dense inorganic-organic three-dimensional network through extensive electrostatically supported hydrogen bonding. In principle the arrangement of the complex anions agrees well with the one described for diethylene triammonium hexachloridorhodate(III) . The conformational flexibility of the organic cation seems to play a crucial role for the formation of a dense solid. It facilitates the organization of the cations and the single chloride anions into layers that are free of cavities. These cation/chloride layers and layers of the complex supplementary materials sup-2 anions are stacked alternately along the crystallographic a axis (Fig. 2). From another point of view the arrangement of the [RhCl 6 ] 3− ions can be regarded as a distorted face centered cubic packing, if these anions are considered to be pseudo spherical species. A structural fragment, consisting of a central chloride ion and four quarters of surrounding protonated amines, may be considered as a triply positive charged 'pseudo cation' situated in the center of the octahedral holes within the close packing of complex ions (Fig. 3).

Experimental
Mixing hydrochloric acid solutions of triethylene tetrammonium chloride and rhodium trichloride yields a unresolvable microcrystalline powder. To obtain suitable single crystals it is necessary to slow down this precipitation reaction so that controlled growth can be accomplished. For this purpose a three chamber vessel with two lateral chambers and one central chamber, which is separated from the lateral ones by two microporous membranes was used, a setup that guarantees a slow diffusion of the components of the precipitation reaction into the central chamber. The lateral chambers were filled with 5 ml of 20% hydrochloric acid solution of rhodium trichloride and 5 ml of a saturated solution of triethylene tetrammonium in concentrated hydrochloric acid, respectively, while the central chamber contained pure concentrated hydrochloric acid.
Within some days dark red brick-shaped crystals were obtained in the central chamber, that were suitable for X-ray structure analysis and single-crystal ATR-IR and Raman spectroscopy. IR data (ν, cm

Refinement
The atomic coordinates of hydrogen atoms in idealized positions were included in the refinement in riding model approximation. C-H and N-H distances for the CH 2 , NH 2 and NH 3 groups were allowed to refine, the same shifts being applied along the C-H and N-H bonds of a group, respectively, and in additon the torsion angle of the NH 3 group was allowed to refine freely. U iso (H) was set to 1.2 U eq (carrier atom) for the CH 2 groups. A common U iso value was refined for the hydrogen atoms of the NH 2 and the NH 3 group, respectively. Only one significant electron-density maximum (1.33 e Å −3 at 0.87 Å from Rh1) was found in the final difference Fourier map. Fig. 1. : The ionic components of triethylene tetrammonium hexachloridorhodate(III) chloride with their hydrogen bond environment. Hydrogen atoms are drawn with an arbitrary radius and displacement ellipsoids are drawn at the 50% probability level. Dashed lines indicate further hydrogen bonds establishing a three dimensional network. Hydrogen atom labels are omitted for clarity. [Symmetry codes: (i) −x + 1, −y + 2, −z + 1; (ii) −x + 1/2, y + 1/2, −z + 1/ 2; (iii) −x + 1/2, y + 3/2, −z + 1; (iv) −x + 1, −y + 1, −z + 1; (v) −x + 1/2, −y + 1/2, −z + 1.]