Templated heptaborate and pentaborate salts of cyclo-alkylammonium cations: structural and thermal properties

Abstract

The synthesis and characterization of a series of cyclo-alkylammonium pentaborate salts {[cyclo-C_nH_2n−1NR₃][B₅O₆(OH)₄] (R = H, n = 3, 5–7 (1–4); R = Me, n = 6 (5))} are reported. Compounds 1, 2 and 5 have been further characterized by single-crystal XRD studies. Attempted recrystallization of 3 and 4 yielded small crops of the unexpected heptaborate salts, [cyclo-C₆H₁₁NH₃]₂[B₇O₉(OH)₅]·3H₂O·B(OH)₃ (6) and [cyclo-C₇H₁₃NH₃]₂[B₇O₉(OH)₅]·2H₂O·2B(OH)₃ (7) which were also characterized crystallographically. All compounds show extensive supramolecular H-bonded anionic lattices templated by the cations. H-bond interactions are described in detail. TGA–DSC analysis of the pentaborates 1–5 showed that they thermally decomposed in air at 800 °C to 2.5B₂O₃, in a 2 step process involving dehydration (<250 °C) and oxidative decomposition (250–600 °C). BET analysis of materials derived from the pentaborates had internal porosities of <1 m² g⁻¹.