Clathrates with Three-Dimensional Host Structures of Hydrogen-Bonded Pentaborate [B5O6(OH)4]- Ions: Pentaborates with the Cations NMe4 +, NEt4 +, NPhMe3 + and pipH+ (pipH+ = Piperidinium)

Abstract

X-ray structure analyses of crystalline [NMe4][B5O6(OH)4] · nH2O with n ≈ 0.25-0.50 (1), [NEt4][B5O6(OH)4] (2), [NPhMe3][B5O6(OH)4] (3), and [pipH][B5O6(OH)4] (4) reveal that these materials are novel clathrates with closely related three-dimensional host structures built up of hydrogen-bonded oligomeric pentaborate [B5O6(OH)4]- ions. The organic cations and water molecules (in 1) occupy as guest species large straight channel-like voids of nearly rectangular cross-section. Compound 1 crystallizes monoclinically with space group P2,/c (Z = 4); the compounds 2,3 and 4, which possess the same host-structure topology, crystallize triclinically with space group P1̄ (Z = 2). 11B-MAS NMR spectra allow the detection of small angular distortions in the pentaborate anions caused by the specific hydrogen bonding in the host frameworks. Upon heating the compounds on a thermobalance in a dynamic inert gas atmosphere dehydration occurs at temperatures of 563 K (1), 543 K (2), 558 K (3) and 523 K (4) before degradation of the organic cations starts at temperatures of 633 K (1), 623 K (2), 623 K (3) and 613 K (4).