Synthesis, thermal properties and structural characterisation, of the tetraphenylphosphonium pentaborate salt, [PPh4][B5O 6(OH)4]·1.5H2O

Article abstract of DOI:10.1016/j.ica.2011.11.001

[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) , b = 13.3180(3) , c = 16.2055(4) , α = 91.1950(10)°, β = 91.180(2)°, γ = 105.456(2)°, T = 120(2) K, V = 2768.66(11) ³ 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][B ₅O₈].

Full text of DOI:10.1016/j.ica.2011.11.001

Inorganica Chimica Acta 383 (2012) 199–203
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Synthesis, thermal properties and structural characterisation, of the
tetraphenylphosphonium pentaborate salt, [PPh4][B5O6(OH)4]Á1.5H2O
b
b
a
c
Michael A. Beckett a, , Peter N. Horton , Michael B. Hursthouse , James L. Timmis , K. Sukumar Varma
⇑
a School of Chemistry, Bangor University, Bangor LL57 2UW, United Kingdom
b School of Chemistry, Southampton University, Southampton SO17 1BJ, United Kingdom
c Pilkington Building Products R and D, European Technical Centre, Lathom, Lancashire L40 3UF, United Kingdom
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 August 2011
Received in revised form 26 October 2011
Accepted 2 November 2011
Available online 10 November 2011
[Ph4P][B5O6(OH)4]Á1.5H2O (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,
b = 91.180(2)°,
a = 91.1950(10)°,
c
= 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].
Keywords:
Polyborate
Pentaborate
Ó 2011 Elsevier B.V. All rights reserved.
X-ray structure
Supramolecular
Tetraphenylphosphonium
1. Introduction
supplied. NMR spectra were obtained on a Bruker Avance-500
using XWIN-NMR 3.5. 11B, 13C and 31P, 1H NMR spectra were ob-
Inorganic borate minerals are of great industrial importance,
with applications as flame retardant [1], luminescence [2], piezo-
electric [3], second harmonic generation [4], and non-linear optical
materials [5,6]. Metal borates have a richly diverse structural
chemistry [7–9], and non-metal cation based borates may serve
as thermal precursors to new porous materials [10–12]. In the
solid-state, non-metal cation pentaborate salts form H-bonded
supramolecular anionic giant structures containing the cations
within the cavities of the lattice [13–15]. The [Ph4P]+ cation is often
used to aid the crystallisation of large inorganic anions [16], and
tained at 500 MHz (1H), 160 MHz (11B), 125 MHz (13C), 202 MHz
(
31P), dissolved in D2O. Fourier transform Infrared spectra (FTIR)
were obtained as KBr pellets on a Perkin-Elmer 100 FTIR spectrom-
eter over 450–4000 cmÀ1. TGA and DSC analysis was performed
between 10 and 800 °C (in air) on an SDT Q600 V4.1 Build 59
instrument using Al2O3 crucibles, with a ramp temperature rate
of 10 °C minÀ1. X-ray crystallography was carried out at the EPSRC
National Crystallography service at the University of Southampton.
CHN analysis was carried out at OEA laboratories Ltd., Callington,
Cornwall.
the structures which result often contain
a supramolecular
network made up of multiple structure directing phenyl-phenyl
embraces [17,18]. This manuscript details the synthesis and X-
ray characterisation of the tetraphenylphosphonium pentaborate
salt, [PPh4][B5O6(OH)4]Á1.5H2O (1), and discusses the anion–anion
and cation–cation structure directing effects present. The thermal
properties of 1 are also described.
2.2. Preparation of tetraphenylphosphonium pentaborate (1)
Tetraphenylphosphonium bromide (1.00 g, 2.38 mmol) was dis-
solved in distilled water (15 ml), to which excess Dowex 550A
monosphere ion exchange resin (OHÀ form) was added. The
solution was stirred for 24 h, the ion exchange resin removed by
filtration, and methanol (15 ml) was added to the filtrate. Boric
acid (0.74 g, 11.92 mmol) was added to the H2O/MeOH solution
which was gently warmed for 1 h. The solvent was concentrated
and crystals of 1 (0.56 g, 42%) precipitated from solution. The crys-
tals were removed by filtration and allowed to dry in a desiccator.
Re-crystallization of the crude product from distilled water yielded
crystals suitable for single-crystal X-ray diffraction studies.
2. Experimental
2.1. Material and analyses
All chemicals were obtained commercially from Sigma Aldrich
Chemical Company or Alfa Aesar Chemicals and were used as
⇑
1:
C24H27B5O11.5P. Anal. Calc.: C = 49.31%, H = 4.67%. Found:
Corresponding author.
E-mail address: m.a.beckett@bangor.ac.uk (M.A. Beckett).
C = 49.45%, H = 4.65%. m.p. 250 °C. NMR: (d 1H/ppm): 7.57
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.11.001

200
M.A. Beckett et al. / Inorganica Chimica Acta 383 (2012) 199–203
(4H, s), 7.77 (1H, dd); (11B/ppm) 1.2, 13.0, 17.7: (13C/ppm) 117.45,
118.12, 130.08, 134.62, 135.23, 135.25;
31P/ppm) 22.7. IR:
max/cmÀ1 3476, 3416.1, 3069, 2346, 1638.2, 1617, 1442, 1361,
Table 1
Crystallographic and structure refinement parameters for 1.
(
m
Empirical formula
Formula weight
T (K)
C24H27B5O11.5
584.48
120(2)
P
1107, 1026, 925, 782, 760, 721, 693, 662, 529.
Crystal system
Space group
triclinic
P1
2.3. X-ray crystallography
Merohedral twin ratio:
18.86(7):18.35(11):30.87(7):31.92(11)
Crystallisation of 1 from distilled water yielded crystals in the
triclinic space group P1. Numerical details of the solution and
refinement of its X-ray structure are described in Table 1. A suitable
crystal was selected and data collected on a Bruker Nonius Kap-
paCCD Area Detector at the window of a Bruker Nonius FR591 rotat-
Twin law
a (Å)
b (Å)
(0À10À10000À1)
13.3174(3)
13.3180(3)
16.2055(4)
91.1950(10)
91.180(2)
105.456(2)
2768.66(11)
4
c (Å)
a
(°)
b (°)
ing anode (Mo Ka = 0.71073 Å) driven by COLLECT [19] and DENZO
c
(°)
[20] software at 120 K. The structures were determined in SHELXS-97
[21] and refined using SHELXL-97 [21]. All non-hydrogen atoms were
refined anisotropically, with all non-water hydrogen atoms placed
geometrically using standard riding models. The water hydrogen
atoms were restrained to a fixed geometry but with free rotation
of the water molecule. The crystal used was a merohedral, inversion
twin (matrix 0À10À100001) in the following ratio (18.86(7):
18.35(11):30.87(7):31.92(11)).
V (Å3)
Z
Dc (g cmÀ3
)
1.402
0.160
1212
l
(mmÀ1
)
F(000)
Crystal size (mm)
2h max (°)
Reflections collected
0.14 Â 0.10 Â 0.05
54.96
47412
Independent reflections 22986 [0.0444]
[Rint
]
Completeness (%)
Data/restraints/
parameters
99.3
22986/965/1577
3. Results and discussion
Goodness-of-fit on F2
Final R indices
1.098
3.1. Synthesis
R1 = 0.0667, wR2 = 0.1194
[F2 > 2 (F2)]
r
The title compound, [Ph4P][B5O6(OH)4]Á1.5H2O (1), was ob-
tained as analytically pure colourless crystals in high yield from
a MeOH/H2O solution of [PPh4][OH] and B(OH)3 in the molar ratio
1:5. The salt was characterized by NMR (11B, 13C, and 1H) and IR
spectroscopy, and by a single-crystal XRD study. Spectroscopic
data are in agreement with the single-crystal XRD structure (see
below) and with previously reported NMC pentaborates [12,13].
R indices (all data)
R1 = 0.0979, wR2 = 0.1368
0.370, À0.310
Max, min difference
(eÅÀ3
)
C of the phenyl rings and P31 range from 1.790(11) to 1.821(14) Å
(av. C-P31 1.805 Å) and, when all P–C bonds considered the range
increases to 1.761(11)–1.821(11) Å. Bond angles about P31 range
from 107.1(5) to 111.4(5)° (av. 109.5°) with angles about all P cen-
tres in the range 106.4(4)–112.3(5)°, confirming tetrahedral P
atoms. These data, and C–C bond lengths and C–C–C bond angles
within the cation’s phenyl rings are in accordance with previously
reported bond distances and angles for the [PPh4]+ cation [22].
The pentaborate anion, as illustrated in Fig. 1, contains one 4-
coordinate B centre and four 3-coordinate B centres, with all B
atoms bound to either four or three O atoms, respectively. The
B–O distances to the 4-coordinate B1 range from 1.457(9) to
1.499(9) Å (av. 1.471 .Å) with corresponding data for all 4-
coordinate B centres of 1.427(10)–1.508(15) Å (av. 1.468 Å). The
OBO angles at B1 range from 108.2(7) to 110.5(6)° (av. 109.45°)
3.2. Crystal structure of [PPh4][B5O6(OH)4]Á1.5H2O
Compound 1 is ionic, comprised of discrete [PPh4]+ cations with
[B5O6(OH)4]À anions, has 1.5 molecules of interstitial water per for-
mula unit, and displays supramolecular networks of intercon-
nected anions (H-bonds) and cations (phenyl embraces). The
structure is best refined in the space group P1, although there ex-
ists high degrees of pseudo-symmetry (ꢀ90%) for inversion cen-
tres, 2-fold rotations and translations, meaning the structure can
ꢀ
be solved to a reasonable degree in either P1 or C2, but with extra
disorder observed instead. The reasons for these apparent symme-
try anomalies lie in the supramolecular networks described below.
There are four independent cations and four independent anions
per unit cell, and a total of six interstitial H2O molecules. One of
the anions, together with the H2O molecules associated with it, is
disordered. The anion containing B131 [occupancy factor
0.675(5)], is disordered over two sites with the alternative anion
containing B31 [occupancy factor 0.325(5)]. Two of the interstitial
water molecules (O201 and O202) have an occupancy factor of 1,
whereas the other four water molecules are disordered over eight
sites with an occupancy of 0.325(5) [O203, O204, O205, and O206]
and 0.675(5) [O303, O304, O305, and O306] and are associated
with either the pentaborate anion containing B31 or B131, respec-
tively. A drawing of the formula unit of 1 (containing B1 and P31) is
shown in Fig. 1. The anions containing B11, B21 and B131 (and
B31) are very similar in structure to the one containing B1, and
the cations containing P1, P61 and P91 are very similar in structure
to the cation containing P31.
with corresponding data for all 4-coordinate
B centres of
105.0(12)–112.1(7)° (av. 109.45°) consistent with (distorted) sp3
hybridized tetrahedral B centres. The B–O bond-lengths at trigonal
B centres within the anion containing B1 range from 1.331(9) to
1.401(9) Å (av. 1.366 Å) with corresponding data for all 3-
coordinate B centres of 1.322(13)–1.434(9) Å (av. 1.367 Å). These
bond distances are generally significantly shorter than those
involving 4-coordinate B centres. OBO at trigonal B centres and O
ring angles (BOB) within the anion containing B1 range within
116.5(8)–123.9(7)° (av. 120.0°) and 119.0(6)–125.3(7)° (av.
122.5°) respectively. The variation in these bond-lengths and an-
gles are in accord with bond lengths and angles that have been pre-
viously reported in pentaborate [12–14,23,24] and related [25–31]
structures.
As noted above, the supramolecular structure of 1 consists of two
interpenetrating networks of interconnecting anions (H-bonds) and
cations (phenyl embraces). A common feature of pentaborate struc-
tures is an H-bonded anion-anion lattice with cations situated
within the cavities. Each pentaborate anion is able to form 4 donor
Molecular dimensions within the cations and anions (exempli-
fied with data for ions illustrated in Fig. 1) are discussed before the
crystal supramolecular architecture. The distance between the ipso

M.A. Beckett et al. / Inorganica Chimica Acta 383 (2012) 199–203
201
Table 2
Details of hydrogen bonds for [PPh4][B5O6(OH)4]Á1.5H2O.
D–H. . .A
d(D–H)
d(H. . .A)
d(D. . .A)
\(DHA)
O7–H7. . .O21i
0.84
0.84
0.84
0.84
0.84
0.84
0.84
0.84
0.84
0.84
0.84
0.84
0.84
0.84
0.84
0.84
0.84
0.84
0.84
0.84
0.84
0.84
0.84
0.84
0.84
0.85
0.85
0.85
0.85
0.85
0.85
0.85
0.85
0.85
0.85
0.85
0.85
0.85
0.85
0.85
0.85
0.85
0.85
0.85
1.88
2.06
2.14
2.19
2.27
2.44
1.89
1.94
1.91
2.11
2.12
2.14
1.9
1.89
2.08
2.18
2.55
2.11
2.1
2.714(9)
2.776(15)
2.709(10)
2.835(8)
3.059(11)
2.919(8)
2.723(16)
2.756(11)
2.736(8)
2.778(11)
2.84(2)
173
O8–H8. . .O33ii
142.8
124.6
133.8
157.3
117.4
170.5
164.5
166.8
136.7
143.7
145.2
158
169.7
137.3
121.4
123.9
133.6
134.6
131.5
129.5
162.6
165.8
112.3
141.6
124.8
153.9
124.5
144.8
157.9
172.4
107
O8–H8. . .O133ii
O9–H9. . .O139
O9–H9. . .O206
O10–H10. . .O201
O17–H17. . .O31iii
O17–H17. . .O131iii
O18–H18. . .O23ii
O19–H19. . .O30
O20–H20. . .O303
O20–H20. . .O203
O27–H27. . .O1iv
2.87(3)
2.697(9)
2.716(9)
2.754(9)
2.717(10)
3.098(10)
2.75(3)
O28–H28. . .O13v
O29–H29. . .O10vi
O30–H30. . .O304
O30-H30. . .O204
O37–H37. . .O11vii
O38–H38. . .O3v
Fig. 1. ORTEP drawing of the cation containing P31 and the anion containing B1 of
[PPh4][B5O6(OH)4] (1) showing numbering scheme. The unit cell of 1 contains four
independent cations and four independent anions, one of which is disordered. The
numbering schemes for the other cations (containing P1, P61, and P91) and anions
(containing B11, B21, and B31/B131) follow a similar pattern.
2.75(3)
O39–H39. . .O206
O40–H40. . .O20
2.1
2.733(18)
2.707(16)
2.695(16)
2.749(17)
3.021(8)
2.779(11)
2.700(9)
2.875(5)
2.652(9)
2.811(13)
2.964(8)
3.279(14)
2.489(19)
2.945(7)
2.810(15)
2.839(9)
2.505(15)
2.647(11)
2.865(8)
2.728(13)
2.858(6)
2.616(9)
2.892(7)
2.853(8)
2.974(8)
2.09
1.88
1.93
2.6
2.07
2.12
2.09
2.08
2.07
2.16
2.43
2.1
2.11
2.11
2.11
2.09
2.1
O137–H137. . .O11vii
O138–H138. . .O3v
O139–H139. . .O305
O140–H140. . .O306
O201–H21A. . .O19viii
O201–H21B. . .O25viii
O202–H22A. . .O9
O202–H22B. . .O39
O202–H22B. . .O135
O203–H23A. . .O35
O203–H23B. . .O202
O204–H24A. . .O15
O204–H24B. . .O40
O205–H25A. . .O5
O205–H25B. . .O29viii
O206–H26A. . .O205
O303–H33A. . .O140
O303–H33B. . .O202
O304–H34A. . .O15
O304–H34B. . .O306
O305–H35A. . .O5
O305–H35B. . .O29viii
O306–H36A. . .O20
167.6
139.3
143
109.2
122.2
146.3
130.3
154.9
117.8
167.7
146.7
156.6
2.12
2.1
2.07
2.11
2.06
2.1
2.17
Symmetry transformations used to generate equivalent atoms:
(i) x, y + 1, z + 1; (ii) x, y + 1, z; (iii) xÀ1, y, z; (iv) x, yÀ1, zÀ1.
(v) x, yÀ1, z; (vi) xÀ1, yÀ1, zÀ1; (vii) x + 1, y, z; (viii) x + 1, y + 1, z + 1.
Fig. 2. A view of the supramolecular [B5O6(OH)4]À anion network in 1.
H-bonds to neighbouring pentaborates with acceptor sites desig-
nated as
a
, b and
c
depending on distance from the central tetrahe-
ique lattice structure which is expanded and adapted to accept the
interpenetrating cation lattice.
dral boron atom [13,14]. A commonly occurring H-bonding motif,
encountered in ‘herringbone’ and ‘brickwall’ structures, is desig-
The crystal structure of 1 also displays a supramolecular net-
work of non-covalent phenyl embraces between [Ph4P]+ cations.
A view of this cation network, in the same orientation as the anion
network, is shown in Fig. 3. The three main types of phenyl em-
braces which have been reported for [PPh4]+ cations are offset
face-to-face, edge-to-face, and vertex-to-face [17,18,33]. The net-
work in 1 has every Ph group of every cation forming an offset
face-to face embrace with a neighbouring cation. Thus, the four
phenyl groups on P1 embrace phenyl groups on P31, P31’, P91
and P91’ with the other embraces as follows: P31 phenyls with
phenyls on P1, P1’, P61, P61’; P61 phenyls with phenyls on P31,
P31’, P91 and P91’; P91 phenyls with phenyls on P61, P61’, P1
and P1’. The prime notation indicates a cation in an adjacent unit
cell. All phenyl interactions are between 3.90(1) and 4.49(1) Å,
and the interaction between a phenyl ring of P1 with a phenyl ring
on P31 is shown in Fig. 4. P. . .P separations range from 7.33(1) to
8.33(1) Å, and are consistent with strong phenyl embrace interac-
tions and are significantly shorter than 13.6 Å which is twice the
van der Waals radius when the cation is regarded as a sphere [34].
nated a,a,a,b [12]. With compound 1 the ‘interstitial’ H2O molecules
form additional H-bonds between anions and the situation is
complicated further by anion/H2O disorder. The four H-bond donor
interactions from the O7, O8, O9, and O10 sites on the borate anion
containing B1 can be designated
s.o.f. acceptor site in parentheses for disordered interactions. The
other pentaborates follow similar numbering scheme and
their interactions are ), ,b,H2O(H2O) (for anion containing
B11), ,b,H2O(H2O) for b21, and ,H2O,H2O (for B131). The alter-
native anion to that containing B131 (i.e. containing B31) may be
designated ,H2O,b. A view of the anion/H2O H-bonding network
a,a(a),b(H2O),H2O, with the lower
a
a(a a
a
,a
a,a
a,a
is illustrated in Fig. 2. Details of H-bond interactions can be found
in Table 2. An alternative system for designating H-bond interac-
tions has been devised by Etter [32]. This system describes all the
a
interactions in 1 as reciprocal pairs, R22(8); the b interactions are
not the simple C(8) chains as observed in the herringbone and brick-
wall structures, but include the interstitial H2O molecules. The
inclusion of H2O molecules within the anionic lattice produces a un-

202
M.A. Beckett et al. / Inorganica Chimica Acta 383 (2012) 199–203
was analysed using TGA/DSC, and results are shown in Fig. 5. The
first event is when there is a small weight loss ca. 3–4% around
60–70 °C, corresponding to the loss of interstitial water in the
structure (calc. 3.3%). The second weight loss event (ca. 5–6%) com-
mences in the temperature range of 200–250 °C, and corresponds
to the dehydration and crosslinking of the pentaborate anion net-
work to form an anhydrous pentaborate (calc. 6.1%), [Ph4P][B5O8].
There is then a steady weight loss until 800 °C which is the decom-
position/oxidation of the cation leaving 2.5 molar equivalents of
boron oxide (B2O3) which has a percentage weight remaining of
ca. 32–33% (calc. 29.8%).
4. Conclusions
Tetraphenylphosphonium pentaborate, [PPh4][B5O6(OH)4]Á1.5
H2O crystallized in high yield form an aqueous methanol solution
of [PPh4][OH] and B(OH)3 (1:5). The solid-state structure is
comprised of interpenetrating supramolecular networks of inter-
connected pentaborate anions (H-bonds) and tetraphenylphos-
phonium cations (phenyl embraces). TGA/DSC analysis revealed
that [PPh4][B5O6(OH)4]Á1.5H2O is thermally decomposed in air at
800 °C to 2.5B2O3.
Fig. 3. A view of the supramolecular [PPh4]+ cation network in 1.
Acknowledgements
We would like to thank EPSRC and Pilkington Plc for financial
support for the project.
Appendix A. Supplementary material
CCDC 832938 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/j.ica.2011.
11.001.
Fig. 4. A drawing showing an offset phenyl embrace between a phenyl on P1 and
P31. The P1. . .P31 separation is 7.438 Å.
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