Synthesis and Thermal Studies of Two Phosphonium Tetrahydroxidohexaoxidopentaborate(1-) Salts: Single-Crystal XRD Characterization of [iPrPPh3][B5O6(OH)4]·3.5H2O and [MePPh3][B5O6(OH)4]·B(OH)3·0.5H2O

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 (<300 °C). BET analysis of the anhydrous and pyrolysed materials indicated they were non-porous with surface areas of 0.2–2.75 m2/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.


Thermal Studies
Organic cation polyborates are known to thermally decompose in air with the formation of glassy B2O3 at 800 °C [10,34]. The closely related tetraphenylphosphonium pentaborate salt, [PPh4][B5O6(OH)4] . 1.5H2O, is reported to be thermally decomposed in a similarly manner [19]. In previous studies, water is lost at lower temperatures with the formation of 'anhydrous' pentaborates and this is followed at higher temperatures by oxidation of the cation, gaseous evolution, and the formation of darkened intumesced solids. At higher temperatures again, these solids shrink down to form glassy residual materials of B2O3 [10,24]. The thermal decomposition of 1 and 2 was studied by TGA/DSC analysis in air over the temperature range 20-800 °C.

Thermal Studies
Organic cation polyborates are known to thermally decompose in air with the formation of glassy B 2 O 3 at 800 • C [10,34]. The closely related tetraphenylphosphonium pentaborate salt, [PPh 4 ][B 5 O 6 (OH) 4 ]·1.5H 2 O, is reported to be thermally decomposed in a similarly manner [19]. In previous studies, water is lost at lower temperatures with the formation of 'anhydrous' pentaborates and this is followed at higher temperatures by oxidation of the cation, gaseous evolution, and the formation of darkened intumesced solids. At higher temperatures again, these solids shrink down to form glassy residual materials of B 2 O 3 [10,24]. The thermal decomposition of 1 and 2 was studied by TGA/DSC analysis in air over the temperature range 20-800 • C.
The data for 1 were consistent with the initial loss of 5  Figure S10 for TGA plots). The TGA plot of 2 had a similar profile, with loss of 4.0 × H 2 O in the first stage (100-275 • C) and the formation of anhydrous [MePPh 3 ][B 6 O 9.5 ]. Since 2 is a 1:1 B(OH) 3 /pentaborate(1-) co-crystal, this material is formulated as an anhydrous hexaborate [18,33,34]. This endothermic dehydration step for 2 is a two-stage process involving the loss of interstitial H 2 O and partial condensation/cross-linking of the pentaborate B-OH groups (2.0 × H 2 O, 100-150 • C), with further condensation/cross-linking of the pentaborate B-OH groups (2.0 × H 2 O, 150-275 • C). This two-stage water loss is qualitatively very similar to that observed for [PPh 4 ][B 5 O 6 (OH) 4 ]·1.5H 2 O [19].
It was anticipated that, upon further heating (275-800 • C), 1 and 2 would leave, after oxidation of the cations during the exothermic second stages, with glassy residues com-prised of 2.5 or 3.0 equivalents of B 2 O 3 , respectively. However, the residual masses from 1 and 2 were higher than calculated, indicating that they both contained additional, non-B 2 O 3 material. It has been noted that phosphonium salts, with simple non-polar substituents, generally decompose cleanly with little residue [37], and our previous studies on the thermal decomposition of phosphonium polyborates are consistent with this [18,19]. However, some phosphonium salts are also known to decompose with residual material [37]. The additional residual material from 1 and 2 possibly arises through the incorporation of phosphorus and/or an organic char slowing down the oxidation process.
Porosity data (BET analysis [38]) of organic pentaborates salts and their thermally derived anhydrous, pyrolysed and residual glasses have been reported and the results indicated that they were non-porous [20][21][22]. Compounds 1 and 2 both possess unusual solid-state pentaborate structures (see sc-XRD studies, Section 2.4), with 1 layered and 2 having its cations stacked in channels. We were, therefore, interested in obtaining porosity data on the thermally derived intermediate materials from these phosphonium pentaborates to see if these structural modifications are influential. Thus, samples of 'anhydrous' and 'pyrolysed' materials were obtained from 1 and 2 by heating ca. 0.5 g samples in a furnace in air for 24 h at 300 • C and 625 • C, respectively. These materials had surface areas of 0.2-2.75 m 2 /g and were essentially non-porous, with similar values to those obtained for materials derived thermally from [PPh 4 ][B 5 O 6 (OH) 4 ]·1.5H 2 O [39] and organic pentaborates [20][21][22], again suggesting that the intumesced solids have 'foamlike' gas-encapsulated macroporous structures [40].

Spectroscopic Studies
IR and NMR ( 1 H, 13 C, 11 B and 31 P) data for 1 and 2 are reported in the experimental section. These spectroscopic data are in agreement with the expected data for the anions and cations found in 1 and 2.
Compounds 1 and 2 are insoluble in organic solvents but 'dissolve' in H 2 O with decomposition of the pentaborate(1-) anion by the borate equilbria processess that are also involved in their formation [25][26][27]. The cations in 1 and 2 are not affected by this, and their presence was confirmed as substituted phosphonium cations by 1 H, 13 C and 31 P spectroscopic analysis. Thus, 31 P spectra of 1 and 2 both show only one signal at the expected chemical shift for their phosphonium cations [42]. The 11 B spectra of 1 and 2 (in D 2 O) show three signals, corresponding to the tetrahedral boron centre of [B 5 O 6 (OH) 4 4 ] − (ca. +18 ppm) in the form of 'signature spectra', as was previously observed [43]. These signals arise from the equilbrium concentrations of the borate anions present from the 'disolution' of the original pentaborate(1-) anion.

X-ray Crystallography
There are two independent isopropyltriphenylphosphonium(1+) cations, two independent tetrahydroxidohexaoxidopentaborate(1-) anions, and seven waters of crystallization within the unit cell of 1. The asymmetric unit cell of 2 contains two independent tetrahydroxidohexaoxidopentaborate(1-) anions and two independent methyltriphenylphosphonium(1+) cations. Additionally, 2 also contains two independent B(OH) 3 molecules and a single disordered H 2 O of crystallization. These crystallographic studies are in agreement with the formulation of 1 and 2 as ionic phosphonium(1+)/pentaborate(1-) salts, as indicated by their spectroscopic and thermal analysis. The co-crystallization of B(OH) 3 is not uncommon in recrystallized samples of pentaborate(1-) salts containing bulky cations [18,[33][34][35][36]. Drawings of the structures of 1 and 2 showing atomic numbering are shown in Figures 2 and 3, respectively. Selected crystallographic information is available in the experimental section and full details can be found in the Supplementary Information. ) salts, as indicated by their spectroscopic and thermal analysis. The co-crystallization of B(OH)3 is not uncommon in recrystallized samples of pentaborate(1-) salts containing bulky cations [18,[33][34][35][36]. Drawings of the structures of 1 and 2 showing atomic numbering are shown in Figure 2 and Figure 3, respectively. Selected crystallographic information is available in the experimental section and full details can be found in the Supplementary Information.   ) salts, as indicated by their spectroscopic and thermal analysis. The co-crystallization of B(OH)3 is not uncommon in recrystallized samples of pentaborate(1-) salts containing bulky cations [18,[33][34][35][36]. Drawings of the structures of 1 and 2 showing atomic numbering are shown in Figure 2 and Figure 3, respectively. Selected crystallographic information is available in the experimental section and full details can be found in the Supplementary Information.   The tetrahydroxidohexaoxidopentaborate(1-) anion is crystallographically wellknown [10] and has the gross structure of two fused, slightly puckered ('planar') boroxole (B 3 O 3 ) rings sharing a spiro 4-coordinate boron centre; all other boron atoms are 3-coordinate, and are bound solely to oxygen atoms within the rings, or to exo hydroxido groups (see Figure 1). B-O bonds lengths and OBO and BOB angles within these com- The [ i PrPPh 3 ] + and [MePPh 3 ] + cations in 1 and 2 are also well-known crystallographically [45,46] with P-C distances ranging from 1.795 (2)

·1.5H 2 O, has an interesting supramolecular giant structure composed of interpenetrating networks of complex H-bonded anion-anion interactions and cation-cation interactions
involving multiple embraces of their aromatic rings [19]. Aromatic embraces are known to be strong stabilizing interactions [47,48] and are likely to be responsible (together with H-bonding interactions) for the crystallized self-assembly [28][29][30][31] of this compound. We examined the structures of 1 and 2 to see if similar aromatic interactions occur in these compounds, and details, together with their H-bonding interactions, are described below.
Molecules 2023, 28, x FOR PEER REVIEW 8 of 12 Figure 5. The centrosymmetric paired [ i PrPPh3] + cations (containing P1) in 1 display vertex-to-face interactions phenyl ring interactions, in addition to an offset face-to-face interactions (not highlighted). Similar interactions also occur in centrosymmetric paired cations containing P31.
Compound 2 is a further example of a co-crystallized phosphonium pentaborate salt with one B(OH)3 and 0.5 (disordered) H2O per cation/anion. The supramolecular structure of 2 also displays anion-anion H-bond interactions and cation-cation aromatic embraces, but the details of these stabilizing interactions differ from those observed in 1 and [PPh4][B5O6(OH)4] . 1.5H2O and are described below.
All hydroxyl groups of the two independent B(OH)3 and the two independent pentaborate(1-) anions are used as H-bond donor centres. The anion containing B1 forms two donor H-bonds to two α-sites (O9H9 … O11′ and O10H10 … O6′) of two adjacent pentaborates (one containing B11 and one containing B1) and both these interactions are R2 2 (8) [49] with the ring involving the O10H10 donor centrosymmetric (reciprocal). The anion containing B1 also forms two donor H-bonds to two adjacent B(OH)3 molecules: O8H8 … O32, and O7′H7′ … O31. The anion containing B11 forms three donor H-bonds to three adjacent anions at two α-sites (O17H17 … O4′ and O18H18 … O13′, reciprocal) and one β-site, (O19H19 … O7′). This O10H19 … O7′ interaction is part of a two larger R4 4 (12) ring interactions with both these rings including both B(OH)3 molecules ( Figure 6). The fourth pentaborate donor interaction is to the disordered H2O (O20H20 … O44), and overall the anion can be represented as α,α,β,ω [21]. The hydroxido groups of the two B(OH)3 molecules are arranged asymmetrically to maximise their acceptor/donor H-bond interactions. The B(OH)3 containing B31 forms a R2 2 (8) 'pincer' ring with the B(OH)3 containing B21, and likewise this B(OH)3 forms a 'pincer' R2 2 (8) interaction with the pentaborate containing B11 (Figure 6a). These interactions allow for the two co-crystallised B(OH)3 molecules to function as 'spacer' units to expand the lattice and replace what would otherwise be a simpler pentaborate/pentaborate R2 2 (8) interaction [18,[33][34][35][36]. A view along the a axis of 2 (along the plane shown in Figure 6a) is shown in Figure  6b. This view reveals a stacked anionic lattice (rectangular and honeycomb-like) with channels that are occupied by the cations; interestingly, each cationic stack is occupied by A view along the a axis of 2 (along the plane shown in Figure 6a) is shown in Figure 6b. This view reveals a stacked anionic lattice (rectangular and honeycomb-like) with channels that are occupied by the cations; interestingly, each cationic stack is occupied by either cations containing solely P1 or P21 and adjacent cationic stacks in the arrangement, as shown in Figure 6b. Cations are arranged as centrosymmetric pairs within the stacks with P1· · · P1 and P21· · · P21 distances of 6.253(2) Å and 6.255(2) Å, respectively. The [MePPh 3 ]I (2.50 g, 6.2 mmol) was dissolved in H 2 O (50 mL). To this solution, excess Dowex 550A monosphere (OH − form) was added and the suspension was stirred for 24 h. The ion-exchange resin was removed by filtration and MeOH (50 mL) was added to the filtrate. B(OH) 3 (1.91 g, 30.9 mmol) was added to the resulting solution, which was then heated for 1 h. The solvent was removed by rotary evaporation to yield an orange solid as the crude product, which was dried at 110 • C for 24 h (2.84 g, 98%). NMR.

Conclusions
Two substituted aryl phosphonium pentaborate salts were synthesized by templated crystallization from aqueous solution primed with B(OH )3 and appropriate aryl phospho-