Crystal and species engineering with semi-flexible nitrogen containing organic cations: Generation of [H13O6] ions with the unfavoured unbranched topology by enclosing hydrochloric acid in a porous inorganic-organic hybrid material

The reaction of rhodium(III) chloride trihydrate with 1,4-diazacycloheptane in concentrated hydrochloric acid results in the formation of tris(1,4-diazoniacycloheptane) hexaaquahydrogen(1+) bis(hexachlororhodate(III)) chloride, [C5H14N2](3)[H13O6][RhCl6](2)Cl (1). Dark red crystals of 1 are obtained by diffusion-controlled crystallization at room temperature. Slow evaporation of the mother liquor over a period of several days yields a few tiny crystals of the bis(1,4-diazoniacycloheptane) hexachlororhodate(III) chloride hydrate, [C5H14N2](2)[RhCl6]Cl.1.75 H2O (2), as red thin squared plates. In the context of crystal engineering, compounds 1 and 2 are inorganic-organic hybrid materials built up from octahedral [RhCl6](3-), simple Cl- and semi-flexible heterocyclic 1,4-diazoniacycloheptane ions, incorporating either the [H13O6](+) and further Cl- ions or portions of simple water molecules. Both compounds crystallize in the space group type P2(1)/c. Compound 1 contains isolated [H13O6](+) ions with a linear chain-like configuration enclosed in the cavities of the inorganic-organic framework. The presence of a strong central (OHO)-H-...-O-... hydrogen bond within the [H13O6](+) ions in 1 is confirmed by the short (OO)-O-... separation of 2.47 Angstrom and by characteristic IR absorption bands at 1626 (s), similar to 1250 (m) and 668 (m) cm(-1). During the thermal decomposition, compound 1 looses at first five equivalents of water and one equivalent of hydrochloric acid in a two-step process at 37 degreesC and 67 degreesC. This is followed by the decomposition of the 1,4-diazoniacycloheptane cations and the hexachlororhodate(III) anions, starting at 190 degreesC and proceeding intensified at 240 degreesC.