Synthesis and Thermal Studies of Two Phosphonium Tetrahydroxidohexaoxidopentaborate(1-) Salts: Single-Crystal XRD Characterization of [(i)PrPPh(3)][B(5)O(6)(OH)(4)]·3.5H(2)O and [MePPh(3)][B(5)O(6)(OH)(4)]·B(OH)(3)·0.5H(2)O †

Two substituted phosphonium tetrahydoxidohexaoxidopentaborate(1-) salts, [(i)PrPPh(3)][B(5)O(6)(OH)(4)]·3.5H(2)O (1) and [MePPh(3)][B(5)O(6)(OH)(4)]·B(OH)(3)·0.5H(2)O (2), were prepared by templated self-assembly processes with good yields by crystallization from basic methanolic aqueous solutions primed with B(OH)(3) and the appropriate phosphonium cation. Salts 1 and 2 were characterized by spectroscopic (NMR and IR) and thermal (TGA/DSC) analysis. Salts 1 and 2 were thermally decomposed in air at 800 °C to glassy solids via the anhydrous phosphonium polyborates that are formed at lower temperatures (<300 °C). BET analysis of the anhydrous and pyrolysed materials indicated they were non-porous with surface areas of 0.2–2.75 m(2)/g. Rhe recrystallization of 1 and 2 from aqueous solution afforded crystals suitable for single-crystal XRD analyses. The structure of 1 comprises alternating cationic/anionic layers with the H(2)O/pentaborate(1-) planes held together by H-bonds. The cationic planes have offset face-to-face (off) and vertex-to-face (vf) aromatic ring interactions with the (i)Pr groups oriented towards the pentaborate(1-)/H(2)O layers. The anionic lattice in 2 is expanded by the inclusion of B(OH)(3) molecules to accommodate the large cations; this results in the formation of a stacked pentaborate(1-)/B(OH)(3) structure with channels occupied by the cations. The cations within the channels have vf, ef (edge-to-face), and off phenyl embraces. Both H-bonding and phenyl embrace interactions are important in stabilizing these two solid-state structures.