Halogenido ligand exchange synthesis, spectroscopic properties and thermal behaviour of the inorganic–organic hydrogen-bonded network solid [4,4′-H2bipy][H7O3][RhBr6] containing discrete and weakly associated [H7O3]+ ions

Dark-red single crystals of 4,4′-bipyridinium triaquahydrogen(1+) hexabromidorhodate(III) [4,4′-H2bipy][H7O3][RhBr6] (1) have been synthesized by a diffusion-controlled ligand exchange process from rhodium(III) chloride trihydrate and 4,4′-bipyridine dissolved in hydrochloric and hydrobromic acid, respectively. 1 could be considered as an inorganic–organic hydrogen-bonded network solid built up from the inorganic isolated hexabromidorhodate [RhBr6]3− octahedra, organic 4,4′-bipyridinium(2+) [4,4′-H2bipy]2+ and triaquahydrogen(1+) [H7O3]+ cations with nearly symmetrical O⋯O distances. The oppositely charged components in the structure of 1 are bound together by an intricate system of weak N(O)–H⋯Br hydrogen bonds. The ATR-FTIR spectrum of 1 confirms the existence of the weakly associated [H3O7]+ cations showing characteristic broad bands at ca. 3380, 1680, 1150 and 600 cm−1. 1 was found to be a long-time stable at ambient conditions material. It contains a definite amount of acid that is released in a controlled manner – upon heating 1 in a first step releases water forming [4,4′-H2bipy][H5O2][RhBr6] (3) and then, in a second step, [H3O]Br is lost. A comparison of ionic arrangements in the structure of 1 and in 4,4′-bipyridinium dibromide, [4,4′-H2bipy]Br2 (2) shows that both crystal structures contain almost linear rods with alternating inorganic and organic components linked through weak hydrogen bonds.

The diffusion-controlled reaction of rhodium(III) chloride trihydrate and 4,4′-bipyridine dissolved in hydrochloric and hydrobromic acid, respectively, via ligand exchange affords the hydrobromic acid incorporating inorganic–organic hydrogen-bonded network solid [4,4′-H2bipy][H7O3][RhBr6] that contains isolated [H7O3]+ ions and is remarkably stable at ambient conditions.Figure optionsDownload as PowerPoint slide