Synthesis, Structural and Spectroscopic Characterization of the α,ω-Diammonioalkane Hexabromorhodates( III) [H3N(CH2)xNH3]2[H5O2][RhBr6]Br2 (x = 3, 4) – IR Spectra of [H5O2]+ Ions with Weak Solid State Interactions

The reaction of rhodium(III) chloride trihydrate with 1,3-diaminopropane and 1,4-diaminobutane in concentrated hydrobromic acid results in the formation of the bis(α,ω-diammonioalkane) diaquahydrogen(1+) hexabromorhodate(III) dibromides [H 3 N(CH 2 ) x NH 3 ] 2 [H 5 O 2 ][RhBr 6 ]Br 2 (x = 3, 4). Dark red single crystals were obtained by diffusion-controlled crystallization at room temperature. Both compounds crystallize in space group type P1̄ and their structures are closely related. In view of crystal engineering they are inorganic-organic hybrid materials built up from octahedral [RhBr 6 ] 3− , simple Br − and flexible chain-like [H 3 N(CH 2 ) x NH 3 ] 2+ ions with the [H 5 O 2 ] + and further Br − ions incorporated and thus might be considered as addition compounds [H 3 N(CH 2 ) x NH 3 ] 2 [RhBr 6 ]Br ∙ [H 5 O 2 ]Br. Cationic and anionic moieties in the crystal are held together by a complex system of N-H∙∙∙Br and O-H∙∙∙Br hydrogen bonds. Geometric features of O-H∙∙∙Br hydrogen bonding as well as the pronounced disorder of the [H 5 O 2 ] + ions in the diammoniopropane salt indicate the weak association of these species in the solids. Weak association of the ions is also documented by difference ATR-FT-IR spectra of the compounds and related diammonioalkane dibromides that show the stretching modes of the terminal O-H bonds above 3300 cm −1 . Further characteristic bands are recognized at ca. 1635 (s), 985 (vs), and 670 (m) cm −1 .