Synthesis and characterisation of cyclo-boratetrasiloxane, (RBO)(Me 2SiO)3 (R=nBu, Ar), derivatives

Article abstract of DOI:10.1016/S0277-5387(03)00478-9

Low temperature reactions of dilute solutions of 1,5-dichloro-1,1,3,3,5,5- hexamethyltrisiloxane with boronic acids {RB(OH)₂; R=ⁿBu, C₆H₄Me-2, C₆H₄Me-3, C ₆H₄Me-4, C₆H₄OMe-3, C ₆H₄OMe-4, C₆H₄Br-2, C ₆H₄Br-3, C₆H₄Br-4} in the presence of a twofold excess of Et₃N afforded the 8-membered ring products, cyclo-boratetrasiloxanes, (RBO)(Me₂SiO)₃, in moderate yields. New compounds were colourless oils and were characterised by elemental analysis, NMR (¹H, ¹¹B, ¹³C, ²⁹Si), IR and MS. The cyclo-boratetrasiloxanes are weakly Lewis acidic, with acceptor number (AN) values of ～30, but do not form adducts with amines.

Full text of DOI:10.1016/S0277-5387(03)00478-9

Polyhedron 22 (2003) 3333–3337
www.elsevier.com/locate/poly
Synthesis and characterisation of cyclo-boratetrasiloxane,
(RBO)(Me2SiO)3 (R ¼ nBu, Ar), derivatives
a,
a
b
Michael A. Beckett *, Martin P. Rugen-Hankey , K. Sukumar Varma
a
Department of Chemistry, University of Wales, Bangor, Gwynedd LL57 2UW, UK
b
Pilkington Building Products R and D, European Technical Centre, Lathom, Lancashire L40 5UF, UK
Received 19 June 2003; accepted 28 July 2003
Abstract
Low temperature reactions of dilute solutions of 1,5-dichloro-1,1,3,3,5,5-hexamethyltrisiloxane with boronic acids {RB(OH)2;
R ¼ nBu, C6H4Me-2, C6H4Me-3, C6H4Me-4, C6H4OMe-3, C6H4OMe-4, C6H4Br-2, C6H4Br-3, C6H4Br-4} in the presence of a
twofold excess of Et3N afforded the 8-membered ring products, cyclo-boratetrasiloxanes, (RBO)(Me2SiO)3, in moderate yields.
New compounds were colourless oils and were characterised by elemental analysis, NMR (1H, 11B, 13C, 29Si), IR and MS. The
cyclo-boratetrasiloxanes are weakly Lewis acidic, with acceptor number (AN) values of ꢀ30, but do not form adducts with
amines.
Ó 2003 Elsevier Ltd. All rights reserved.
Keywords: Cyclo-boratetrasiloxanes; B–O–Si linkages; Cyclocondensation; Lewis acidity
1. Introduction
ment of their Lewis acidities by GutmannÕs [30]
method.
Cyclic species containing B–O–Si linkages are rep-
resented in the literature by a number of 6-, 8- and 10-
membered cyclo-borasiloxane ring systems [1–9]. Other
compounds containing the B–O–Si linkage are the
acyclic triorganosilyl esters of orthoboric [10–19], meta-
boric [16–18] and boronic acids [16–20], and the con-
densed borosilicate cage systems [21–24]. In addition to
these examples, which all contain 3-coordinate B cen-
tres, there are also a few cases of compounds with B–
O–Si links in which the B atom is 4-coordinate [25–28].
We are interested in assessing the Lewis acidity at
3-coordinate boron centres in compounds containing B–
O–Si linkages and have previously examined triorg-
anosilyl esters [18,29] and cyclo-boratrisiloxane (Fig. 1a)
and cyclo-diboratetrasiloxane (Fig. 1b) systems [8,29].
Herein, we report the synthesis of some new cyclo-
boratetrasiloxane (Fig. 1c) derivatives, and an assess-
2. Results and discussion
Cyclo-borasiloxane ring systems are synthetically
available by cyclocondensation reactions of appropriate
B and Si containing precursors. Organoboronic acids
have been successfully condensed with dihydroxysilanes,
diethoxysilanes, a; x-dihydroxysiloxanes, a; x-dieth-
oxysiloxanes and a; x-dichlorosiloxanes [1–3,6,7], and
dichlorophenylborane has been condensed with di-
hydroxysilanes [5]. The cyclo-boratetrasiloxanes re-
ported here were prepared by the method of Wannagat
and Eisele [3], first employed in the synthesis of (PhBO)-
(SiMe2O)2, by use of an equimolar mixture of the
appropriate organoboronic acid with 1; 5-dichloro-
1,1,3,3,5,5-hexamethyltrisiloxane, in the presence of two
molar equivalents of NEt3 to act as HCl acceptor, as
shown in Scheme 1. The new compounds 1–9 were
prepared in modest to moderate yields (22–69%) as
colourless oils, after filtration of the reaction mixture to
* Corresponding author. Tel.: +44-1248-351151; fax: +44-1248-
370528.
E-mail address: m.a.beckett@bangor.ac.uk (M.A. Beckett).
0277-5387/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0277-5387(03)00478-9

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M.A. Beckett et al. / Polyhedron 22 (2003) 3333–3337
Fig. 1. Schematic structures of (a) cyclo-boratrisiloxanes, (b) cyclo-diboratetrasiloxanes, and (c) cyclo-boratetrasiloxanes. R ¼ alkyl or aryl.
Scheme 1. Synthesis of cyclo-boratetrasiloxane derivatives, Me6Si3O4BR {R ¼ C6H4Me-2 (1), C6H4Me-3 (2), C6H4Me-4 (3), C6H4OMe-3 (4),
C6H4OMe-4 (5), C6H4Br-2 (6), C6H4Br-3 (7), C6H4Br-4 (8), nBu (9)}.
remove the solid [NEt3H]Cl, and removal of all volatiles
from the filtrate under reduced pressure. Low tempera-
ture reagent addition and dilute reaction mixtures were
essential in obtaining the desired cyclo-boratetrasilox-
anes by limiting polymer formation. Manners and
co-workers [7] have previously prepared the cyclo-
boratetrasiloxane, (PhBO)(SiMe2O)3, in 74% by this
method.
Compounds 1–9 were relatively air-stable but were
susceptible to hydrolysis and hence were stored under
dry N2. Satisfactory elemental (C, H) analysis data
were obtained on all compounds, and mass spectrom-
etry and spectroscopic (NMR, IR) data were consistent
with the 8-membered ring cyclo-boratetrasiloxane for-
mulation. Additionally, the molecular weight of 3, as
determined by freezing point depression of camphor,
was also fully consistent with the 8-membered ring
formulation.
Thermal analysis over the temperature range 20–750
°C was undertaken on 3 in order to ascertain whether
these cyclo-boratetrasiloxanes were possible precursors
to Si–O–B ceramics. The TGA trace revealed significant
(>80%) weight loss from the sample between 180 and
260 °C, with ꢀ100% weight loss by 300 °C. There was no
observable sample residue, and the DSC trace displayed
an endothermic peak at 220 °C, consistent with va-
porisation of the sample. Clearly, 3 was not a potential
thermal precursor to Si–O–B ceramics.
other cyclo-borosiloxanes [3,5,7]. In particular, all
spectra showed intense peaks at Mþ ) CH3 (base peaks
for 1, 2, 3 and 5) with generally relatively weak mo-
lecular ion peaks. All spectra also displayed strong
signals at m/e Mþ ) 57 (Mþ ) SiMe2 + H), and at m/e
207 (Mþ–Me–ArBO). IR spectra of all compounds
were characterised by very strong absorptions in the
1302–1356 cmꢁ1 and 1049–1078 cmꢁ1 regions due to
B–O and Si–O stretching modes, respectively, with
additional strong signals at 2962 (C–H), 1261 (B–O),
and 808 (O–Si–O bend? [31]) cmꢁ1 11B NMR spectra
.
all showed one signal in the range expected [32] for
trigonal CBO2 centres with the butyl derivative 9
having a resonance significantly to higher frequency of
resonances for the aryl derivatives, 1–8. 29Si spectra
show two signals at ca. d )17 separated by ca. 1.5
ppm, with the low frequency signal being approxi-
mately of double intensity and assigned to the Si atoms
linked via O to B. These observations are in accord
with those reported for (PhBO) (SiMe2O)3 [7]. The 29Si
signals for the butyl derivative (9) are at lower fre-
quency by ca. 1.5 ppm relative to the aryl derivatives
1–8. The number of signals and their relative intensities
1
observed in H NMR spectra of 1–9 were all consistent
with their formulations as cyclo-boratetrasiloxanes and
were characterised by two almost coincidental signals
of relative intensity 1:2 centred at ca. 0.2 ppm,
associated with the SiMe2 protons, in proportion to
additional signals associated with the organo-B func-
tionality.
Mass spectra were obtained for all compounds ex-
cept 7 and 9. Molecular ions were observed in all cases
and breakdown patterns were all generally very similar,
and consistent with breakdown patterns reported for
We have previously noted that whereas cyclo-bora-
trisiloxanes were unreactive towards amines, cyclo-

M.A. Beckett et al. / Polyhedron 22 (2003) 3333–3337
3335
diboratetrasiloxanes were sufficiently Lewis acidic to
3.2. Synthesis
form adducts [8,29]. The reaction of the cyclo-bora-
tetrasiloxanes 1–9 with amines was attempted but ad-
ducts were not obtained, indicating that they were
poor Lewis acids. This was further quantified on se-
lected examples by use of GutmannÕs [30,33] method,
which confirmed their weak Lewis acidity and gave
acceptor number (AN) values of 29, 30 and 30, for 1, 3
and 9, respectively. For comparison, we have previ-
ously reported that cyclo-diboratetrasiloxanes had AN
values of 47–62 and cyclo-boratrisiloxanes had values
of 22–28, and an explanation on their relative Lewis
acidities has been offered in terms of competitive O–Si
and O–B p-bonding, and the different B:Si ratios
within the ring systems [8]. The electronic effect of the
Boratetrasiloxanes were all prepared by an adapta-
tion of a literature method [3,7], with the procedure
described below for Me6Si3O4B(C6H4Me-2) (1).
Me6Si3O4B(C6H4Me-2) (1). ortho-Tolylboronic acid,
(C6H4Me-2)B(OH)2, (1.0 g, 7.4 mmol) was dissolved in
anhydrous Et2O (1000 cm3). NEt3 (1.56 g, 15.5 mmol)
was added and the solution was cooled to )78 °C by
means of a dry ice/acetone bath. 1,5-Dichloro-1,1,3,3,5,5-
hexamethyltrisiloxane, ClSiMe2OSiMe2OSiMe2Cl, (2.04
g, 7.4 mmol) was added via a dropping funnel over a 1 h
period, maintaining the stirred reaction temperature at
)78 °C. The reaction mixture was then allowed to warm
to room temperature and was stirred for a further 24 h.
Filtration of the reaction mixture removed [NEt3H]Cl,
and removal of the Et2O from the filtrate, under reduced
pressure, afforded the product as a colourless oil (1.5 g).
Yield 59%. NMR (ppm): d(1H): 7.5 (d, 7.7 Hz, 1H), 7.0
(m, 3H), 2.4 (s, 3H), 0.1 (s, 12H), 0.0 (s, 6H); d(11B):
24.6; d(13C): 144.8, 136.5, 130.4, 130.1, 124.7, 22.8, 0.6,
0.4; d(29Si): )16.5, )17.7. IR (cmꢁ1): 2962s, 1599m,
1439m, 1329brs, 1261s, 1143m, 1074brs, 887m, 857s,
808s, 730m, 653m. MS (EI, 70 eV) m/z (%): 340 (20, Mþ),
325 (100, Mþ ) CH3), 283 (60), 251 (20), 207 (45,
Mþ ) Me ) ArBO), 163 (25), 133 (30). Elemental analy-
sis. Found: C, 45.7; H 7.5%; required for C13H25BO4Si3:
C, 45.9, H, 7.4%.
Me6Si3O4 B(C6H4Me-3) (2). Yield 64%, colourless
oil. NMR (ppm): d(1H): 7.7 (m, 2H), 7.35 (m, 2H), 2.4
(s, 3H), 0.3 (s, 12H), 0.2 (s, 6H); d(11B): 25.1; d(13C):
136.7, 135.8, 134.4, 132.3, 131.7, 127.4, 21.3, 0.6, 0.3;
d(29Si): )16.5, )17.7. IR (cmꢁ1): 2963s, 1423m, 1335s,
1310brs, 1261s, 1214m, 1128m, 1078brs, 885m, 858m,
844m, 808s, 709s. MS (EI, 70 eV) m/z (%): 340 (15, Mþ),
325 (100, Mþ ) CH3), 283 (48, Mþ ) SiMe2 + H), 251
(14), 207 (36, Mþ ) Me ) ArBO), 191 (35), 163 (16), 133
(29). Elemental analysis. Found: C, 45.6; H, 7.5%; re-
quired for C13H25BO4Si3: C, 45.9; H, 7.4%.
Me6Si3O4B(C6H4Me-4) (3). Yield 35%, colourless
oil. NMR (ppm): d(1H): 7.6 (d, 7.7 Hz, 2H), 7.1 (d, 7.7
Hz, 2H), 2.3 (s, 3H), 0.2 (s, 12H), 0.1 (s, 6H); d(11B): 25.2;
d(13C): 141.1, 135.4, 134.7, 128.4, 21.7, 0.7, 0.4; d(29Si):
)16.6, )17.8. IR (cmꢁ1): 2962m, 1610m, 1400m, 1315brs,
1261s, 1141m, 1075brs, 885m, 858m, 807s. MS (EI, 70
eV) m/z (%): 340 (25, Mþ), 325 (100, Mþ ) CH3), 283 (73,
Mþ ) SiMe2 + H), 251 (10), 207 (50, Mþ ) Me ) ArBO),
191 (52), 163 (40), 133 (48). Elemental analysis. Found:
C, 45.7; H, 7.4%; required for C13H25BO4Si3: C, 45.9; H,
7.4%. RastÕs method for molecular weight determina-
tion: 3 (0.021 g) was dissolved in molten camphor (0.4958
g) and allowed to solidify. M. pt 173 °C. Calculated
Molecular weight, 336 g/mol [34].
{–SiMe2OSiMe2–} and
{–SiMe2OSiMe2OSiMe2–}
fragments towards {RBO2} must be very similar and
this is reflected in their AN values. The slightly higher
value (i.e., more acidic nature) of the cyclo-bor-
atetrasiloxane compared to the cyclo-boratrisiloxane
may not be significant but is opposite to that expected
from the increased Si:B ratio. Ring strain may be an
important factor here since adduct formation would
lead to a reduction of the ring angle at B, and this is
more easily accommodated in an 8-membered ring.
Alternatively, it may be that the cyclo-boratrisiloxane
has reduced Lewis acidity, when compared to other
cyclo-borasiloxanes, since it is formally a 6p-electron
system.
3. Experimental
3.1. General
Reactions were carried out under N2 in dried sol-
vents. The boronic acids and Si3Me6O2Cl2 were ob-
tained commercially and used as supplied. IR spectra
were recorded on a Perkin–Elmer FT-IR 1600 spec-
trometer as thin films between NaCl plates. TGA-DSC
analysis on 3 was undertaken on a TA Instruments
SDTQ600 thermal analyser. Multi-element solution
NMR was recorded on a Bruker AC 250 CP/MAS
NMR spectrometer operating at 250.0 MHz for 1H,
80.25 MHz for 11B, 62.90 MHz for 13C, 101.25 MHz
for 31P and 49.66 MHz for 29Si. Chemical shifts (dÞ are
given in ppm with positive values towards high fre-
quency from SiMe4 (1H, 13C, 29Si), BF3OEt2 (11B), and
85% H3PO4 (31P). Elemental analyses were obtained on
a Carlo Erba EA-1108 (C, H, N) instrument using
helium as carrier gas. AN values were obtained as
described previously [33] and were referenced against
PPh3 (d ¼ ꢁ6:0) dissolved in CDCl3 and used a lock.
EI (70 eV) MS spectra were obtained on a Finnigan
4500 instrument.
Me6Si3O4B(C6H4OMe-3) (4). Yield 65%, colourless
oil. NMR (ppm): d(1H): 7.4 (m, 3H), 7.1 (m, 1H), 3.9 (s,
3H), 0.4 (s, 12H), 0.2 (s, 6H); d(11B): 24.8; d(13C): 159.1,

3336
M.A. Beckett et al. / Polyhedron 22 (2003) 3333–3337
136.0, 128.7, 127.7, 120.4, 116.6, 55.1, 0.7, 0.4; d(29Si):
)16.6, )17.6. IR (cmꢁ1): 2961s, 1596m, 1575m, 1485m,
1464m, 1446m, 1421s, 1309brs, 1261s, 1243s, 1182m,
1122s, 1049brs, 944m, 884s, 858s, 805s, 708s, 685m,
673m. MS (EI, 70 eV) m/z (%): 356 (85, Mþ), 341 (60,
Mþ ) CH3), 299 (65, Mþ ) SiMe2 + H), 207 (70,
Mþ ) Me ) ArBO), 91 (100), 73 (88). Elemental analysis.
Found: C, 43.6; H, 7.2%; required for C13H25BO5Si3: C,
43.8; H, 7.1%.
C, 39.4; H, 8.8%; required for C10H27BO4Si3: C, 39.2;
H, 8.9%.
Acknowledgements
We thank the EPSRC and Pilkington Plc for financial
support through a CASE studentship.
Me6Si3O4B(C6H4OMe-4) (5). Yield 69%, colourless
oil. NMR (ppm): d(1H): 7.6 (d 7.7 Hz, 2H), 6.9 (d, 7.7
Hz, 2H), 3.7 (s, 3H), 0.15 (s, 12H), 0.05 (s, 6H); d(11B):
24.7; d(13C): 162.1, 137.1, 126.2, 113.1, 55.1, 0.7, 0.4;
d(29Si): )16.2, )17.7. IR (cmꢁ1): 2961s, 1603s, 1408m,
1316brs, 1261s, 1174m, 1143m, 1075brs, 885m, 858m,
808s, 649m. MS (EI, 70 eV) m/z (%): 356 (100, Mþ), 341
(83, Mþ ) CH3), 299 (100, Mþ ) SiMe2 + H), 207 (74,
Mþ ) Me ) ArBO), 91 (98), 73 (28). Elemental analysis.
Found: C, 43.5; H, 7.2%; required for C13H25BO5Si3: C,
43.8; H, 7.1%.
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oil. NMR (ppm): d(1H): 7.54 (m, 1H), 7.49 (m, 1H), 7.1
(m, 2H), 0.9 (s, 12H), 0.1 (s, 6H); d(11B): 24.3; d(13C):
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Me6Si3O4B(C6H4Br-3) (7). Yield 41%, colourless
oil. NMR (ppm): d(1H): 7.9 (m, 1H), 7.7 (m, 1H), 7.6
(m, 1H), 7.3 (m, 1H), 0.3 (s, 12H), 0.16 (s, 6H); d(11B):
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Me6Si3O4B(C6H4Br-4) (8). Yield 30%, colourless
oil. NMR (ppm): d(1H): 7.5 (d, 7.7 Hz, 2H), 7.35 (d, 7.7
Hz, 2H), 0.1 (s, 12H), 0.0 (s, 6H); d(11B): 24.9 ; d(13C):
136.9, 136.3, 132.4, 130.8, 126.0, 0.6, 0.3; d(29Si): )16.3,
)17.3. IR (cmꢁ1): 2962s, 1586s, 1387s, 1340brs, 1310s,
1262s, 1138s, 1069brs, 1012s, 883m, 856s, 807s, 725m,
643m. MS (EI, 70 eV) m/z (%): 406 (10, Mþ), 391 (58,
Mþ ) CH3), 349 (12, Mþ ) SiMe2 + H), 281 (5), 249 (32),
207 (100, Mþ ) Me ) ArBO), 177 (38), 133 (65), 91 (64).
Elemental analysis. Found: C, 35.4; H, 5.8%; required
for C12H22BBrO4Si3: C, 35.6; H, 5.5%.
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