James L. Timmis

Two substituted phosphonium tetrahydoxidohexaoxidopentaborate(1-) salts, [ⁱPrPPh₃][B₅O₆(OH)₄]·3.5H₂O (1) and [MePPh₃][B₅O₆(OH)₄]·B(OH)₃·0.5H₂O (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 (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 H₂O/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 ⁱPr groups oriented towards the pentaborate(1-)/H₂O 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.

The synthesis and characterization of a triborate salt, [H₂N(CH₂CH₂)₂NH₂][B₃O₃(OH)₄]₂ (1), and four pentaborate salts, [H₂N(CH₂CH₂)₂NH][B₅O₆(OH)₄] (2a), [MeHN(CH₂CH₂)₂NH][B₅O₆(OH)₄] (2b), [MeHN(CH₂CH₂)₂NMe][B₅O₆(OH)₄] (2c) and [Me₂N(CH₂CH₂)₂NMe₂][B₅O₆(OH)₄]₂ (2d) are described. TGA and DSC analysis (in air, 25–1000 °C) indicate that triborate 1 decomposes to B₂O₃via a multistage process, with the first stage (2N(CH₂CH₂)₂NH₂][B₆O₁₀]. The pentaborates (2a–2d) are thermally decomposed to B₂O₃via a 2 stage process involving polymeric [NMC][B₅O₈]. The anhydrous polyborates were amorphous. BET analysis of materials derived from the thermolysis of 1 at 250, 400, 600, and 1000 °C, were all non-porous (surface area 2 g⁻¹). A single-crystal X-ray diffraction study of 1 showed that it contains isolated triborate(1−) anions in a structure comprised of alternating cationic and anionic layers held together via extensive H-bonds. Single-crystal XRD structural studies on pentaborate salts 2c and 2d are also reported.

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 (2 g⁻¹.

[C₆H₅CH₂NH₃][B₅O₆(OH)₄] was obtained as colourless crystals in high yield from a MeOH/ H₂O (1:1) solution of benzylamine and boric acid (1:5). A single-crystal X-ray study confirmed that the solid-state structure was comprised of a supramolecular H-bonded pentaborate anion lattice, templated by the benzylammonium cations which occupy positions within the lattice cavities. Each pentaborate anion formed 4 H-bonds to 4 neighbouring pentaborate anions at α,α,α,β acceptor sites. Additionally, each cation H-bonds to 3 pentaborate anions. Crystals were monoclinic, P2₁/c, with a = 9.3511(2) Å, b = 14.5157(4) Å, c = 10.4670(2) Å, β = 90.778(2)°, T = 120 K, V = 1420.64(6) ų, and Z = 4. TGA/DSC analysis showed that [C6H₅CH₂NH₃][B₅O₆(OH)₄] thermally decomposed in air at 800 °C to 2.5B₂O₃, via a low-temperature (200–250 °C) dehydration step to a condensed pentaborate.

The synthesis, structural characterization (XRD), and thermal properties of nine non-metal cation (NMC) pentaborate anion salts, [NMC][B₅O₆(OH)₄] (1a–1i) is described (NMC = [NH₃CMe₂(CH₂OH)]⁺ (a), [O(CH₂CH₂)₂NH₂]⁺ (b), [NH₃CMe(CH₂OH)₂]⁺ (c), [2-(2-CH₂CH₂OH)PyH]⁺ (d), [(CH₂)₄NH(CH₂CH₂OH)]⁺ (e), [(CH₂)₅NH(CH₂CH₂OH)]⁺ (f), [2-MeImid]⁺ (g), [Me₃NCMe₂(CH₂OH)]⁺ (h), [O(CH₂CH₂)₂NMe₂]⁺ (i). Single-crystal X-ray diffraction studies on all compounds show that they contain isolated pentaborate anions, H-bonded together in a supramolecular array, with the cations occupying the cavities within the network. Compound 1c was obtained as a partial hydrate (0.16H₂O). TGA and DSC analysis (in air, 25–1000 °C) indicate that compounds 1a–1i thermally decompose via a 2 stage process to B₂O₃. The first stage (5O₈] (2a–2i)}. Five condensed pentaborates (2a–c, 2e, 2g) were synthesised and characterized by powder XRD and BET analysis. These condensed pentaborates were amorphous. The isolated pentaborates intumesced at ∼600 °C (occupying ∼10 times their original volume), and then contracted back to black glassy B₂O₃ solids at 1000 °C. The intumescent materials (3a), (3b), (3e), (3g), and a final B₂O₃ sample (4b) were synthesised and isolated and their porosities determined. BET surface area analysis on the isolated pentaborates (1a–c, 1e, 1g), the condensed pentaborates (2a–c, 2e, 2g), intumesced materials (3a, 3b, 3e, 3g), and B₂O₃ (4b) showed that they were all ‘non-porous’ (2 g⁻¹).

The title compound, C(36)H(30)NP(2) (+)·I(-), was obtained accidently from crystallization of a reaction mixture containing [(Ph(3)P)(2)N]OH and B(OH)(3), which was contaminated with MeI. There are two independent [(Ph(3)P)(2)N](+) cations and two I(-) anions within the asymmetric unit. The central PNP angles are non-linear [137.6 (2) and 134.4 (2)°] and the phenyl substituents on P centres adopt different conformations within these two cations.

The reaction of metal alkoxides M(OR)₄ (M = Ti, Zr; R = organyl) with (MeO)₃B₃O₃ (1 : 0.67) in dry propan-2-one at room temperature led to gels which when dried and calcined in air for 24 h at 500–1000 °C afforded bi-phased mixed-oxide materials formulated as 4TiO₂·3B₂O₃ and ZrO₂·B₂O₃ in high ceramic yields and purity; the B₂O₃ phases of these materials were amorphous. The materials remained amorphous upon calcination at lower temperatures. The TiO₂ phase of the 4TiO₂·3B₂O₃ was crystalline when calcined at higher temperatures with either anatase (600 °C) or rutile (>800 °C) being obtained. The ZrO₂ phase of the ZrO₂·B₂O₃ was crystalline when calcined at higher temperatures and was obtained as a metastable tetragonal phase (800 °C). In a similar reaction, Al(OⁱPr)₃ (2 : 1) gave a bi-phased aluminium borate–boron oxide (Al₁₈B₄O₃₃·7B₂O₃) after calcination at >700 °C. The dried gels and oxide materials were all characterized by elemental analysis, TGA–DSC, and powder XRD.