Synthesis, thermal properties and structural characterisation, of the tetraphenylphosphonium pentaborate salt, [PPh4][B5O6(OH)4]·1.5H2O

[Ph4P][B5O6(OH)4]·1.5H2O (1) was obtained as colourless crystals in high yield from a MeOH/H2O solution of B(OH)3 and [PPh4][OH] (5:1). A single-crystal X-ray study confirmed that the solid-state structure was comprised of supramolecular networks of interconnected anions (H-bonds) and cations (phenyl embraces). Crystals are triclinic, P1¯ with a = 13.3174(3) Å, b = 13.3180(3) Å, c = 16.2055(4) Å, α = 91.1950(10)°, β = 91.180(2)°, γ = 105.456(2)°, T = 120(2) K, V = 2768.66(11) Å3 and Z = 4. TGA/DSC analysis showed that 1 thermally decomposed in air at 800 °C to 2.5B2O3, via a low temperature dehydration step to a condensed pentaborate, [PPh4][B5O8].

The solid-state structure of [Ph4P][B5O6(OH)4]·1.5H2O is comprised of interpenetrating supramolecular networks of H-bonded anions (shown) and phenyl embraced cations, with no direct (other than van der Waals) cation/anion interactions.Figure optionsDownload as PowerPoint slideHighlights
► Thermally, [Ph4P][B5O6(OH)4]·1.5H2O, is decomposed in air at 800 °C to 2.5B2O3.
► This proceeds via a low temperature dehydration step to a condensed pentaborate, [PPh4][B5O8].
► The solid-state structure of [Ph4P][B5O6(OH)4]·1.5H2O is comprised of two supramolecular inter-penetrating networks.
► The anionic network is through H-bonds.
► The cationic network is through phenyl embraces.