Synthesis, structure, and unprecedented solubility of lipophilic borate salts

Article abstract of DOI:10.1016/j.ica.2010.12.039

The multi-gram scale preparation of halide free lipophilic borate salts from inexpensive precursor compounds 3,3′,5,5′-tetra-tert-butyl-2, 2′-biphenol and tetrahydridoboranate salts is reported. The so-called "bortebate" anion is more stable against water and bases than its aluminum analog "altebate", but bortebate formation is significantly slower. Bortebate salts are highly soluble in hydrocarbon solvents, e.g. >35 mmol/L lithium bortebate in pentane at 20 °C. Quantitative salt metathesis reactions between sodium bortebate and halide salts can be easily achieved by precipitation of the sodium halide in methylene chloride.

Full text of DOI:10.1016/j.ica.2010.12.039

Inorganica Chimica Acta 369 (2011) 71–75
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, structure, and unprecedented solubility of lipophilic borate salts
⇑
Michael Wrede, Viktoria Ganza, Geraldt Kannenberg, Frank Rominger, Bernd F. Straub
Organisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 28 December 2010
The multi-gram scale preparation of halide free lipophilic borate salts from inexpensive precursor com-
pounds 3,30,5,50-tetra-tert-butyl-2,20-biphenol and tetrahydridoboranate salts is reported. The so-called
‘‘bortebate’’ anion is more stable against water and bases than its aluminum analog ‘‘altebate’’, but borte-
bate formation is significantly slower. Bortebate salts are highly soluble in hydrocarbon solvents, e.g. >35
mmol/L lithium bortebate in pentane at 20 °C. Quantitative salt metathesis reactions between sodium
bortebate and halide salts can be easily achieved by precipitation of the sodium halide in methylene
chloride.
Dedicated to Prof. Robert G. Bergman
Keywords:
Boron
Anions
Lipophilicity
Sodium
Ó 2010 Elsevier B.V. All rights reserved.
Solubility
1. Introduction
tion of aryl borate esters [9]. We have recently reported aluminate
salts comprising two sterically demanding tert-butylated 2,20-
Salts are very rarely lipophilic in nature. The charge separation
of cations and anions is best stabilized by a polar and hydrophilic
environment. However, lipophilic ions are made possible by a
low charge, large molecular dimensions and a lipophilic molecular
surface. The solubility of salts in aprotic or even unpolar solvents
reflects the lipophilicity of its ionic components [1]. Thus, we con-
sider the solubility of alkali salts in pentane as the ultimate test for
the lipophilicity of an anion.
biphenolate ligands (Fig. 1 down right) [10]. Their low reactivity
against electrophiles such as tritylium cations originates in the ste-
ric shielding of the aluminate center.
Salts of the so-called altebate anion (aluminate with eight tert-
butyl substituents) feature a so far unique solubility in alkanes
[10]. Due to the steric demand of eight tert-butyl groups, the alu-
minate structure is locked in the achiral S4-symmetric conforma-
tion. Most importantly, alkali metal altebates are easily available
in two high yielding steps from cheap chemicals. However, expo-
sure to aqueous acids and bases leads to rapid hydrolysis of the
aluminate. In this article, we report salts of the analogous borate
ester anion ‘‘bortebate’’ (borate with eight tert-butyl substituents)
that displays both higher stability towards water as well as a high-
er solubility of its lithium salt in pentane.
Weakly coordinating anions (WCA) [2] such as [B(C6F5)4]ꢀ
(‘‘BArF20’’) and [B{3,5-C6H3(CF3)2}4]ꢀ (‘‘BArF24’’) are more lipophilic
than chloride or sulfate (Fig. 1 top left) [1,3]. Fluoroalkyl substitu-
ents in silylated tetraphenylborate salts lead to an increased solu-
bility in hexanes, indicative for the applicability of lipophilic salts
in supercritical CO2 (Fig. 1 down left) [4]. The high costs and the
persistence of C–F bonds in the environment, however, are disad-
vantages. A permethylated carborane of the Michl group can be
classified as very lipophilic, halide free, but expensive anion
(Fig. 1 top center) [5]. Krossing’s perfluorated tetraalkoxy alumi-
nate derives its exceptionally low nucleophilicity from the 36 fluo-
rine substituents (Fig. 1 top right) [6]. Spiroborates derived from
2,20-dihydroxybiphenyl have been structurally characterized, and
the relative stability and interconversion of the chiral and achiral
isomers has been discussed [7]. Lipophilic spiroborates have been
successfully tested for wood protection based on their termiticidal
activity and leach resistance [8]. The Waldvogel group has
synthesized halogenated spiroborates by electrochemical oxida-
2. Experimental
2.1. General methodology
Starting materials were used as supplied by Acros-Fisher, Al-
drich Chemical Company without further purification. Reactions
involving air-sensitive reagents were carried out under an atmo-
sphere of dinitrogen or argon using standard Schlenk techniques.
Absolute solvents were dried in an MBRAUN MB SCS-800 solvent
purification system. NMR spectra were recorded using Bruker
DRX-200, Bruker ARX-250, Bruker Avance 300 and Bruker Avance
500 spectrometers. Chemical shifts are reported in ppm relative
to tetramethylsilane for 1H and 13C, and were determined by refer-
ence to 13C and residual 1H solvent peaks (CDCl3 77.0 and 7.26
⇑
Corresponding author. Fax: +49 6221 54 4205.
E-mail address: straub@oci.uni-heidelberg.de (B.F. Straub).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.12.039

72
M. Wrede et al. / Inorganica Chimica Acta 369 (2011) 71–75
R
F3C
F3C
R
CF3
CF3
O
CF3
CF3
F3C
C
B
F3C
CF3
CF3
R
R
B
B
B
O
B
B
B
R
R
Al
O
B
B
F3C
R
R
B
CF3
CF3
F3C
O
B
B
R
R
F3C
F3C
F3C
CF3
R
F3C
CF3
R
Michl's carboranes[5]
R = CH3 or CF3
Krossing's "teflon ball"[6]
[1,3]
Kobayashi's BArF24
Me
Si
Me3C
CMe3
Me
CH2CH2C6F13
C6F13H2CH2C
Si
Me3C
CMe3
CMe3
O
O
O
CH2CH2C6F13
Me
Me
Me
Al
B
Si
O
Me
Me3C
Si
Me
Me3C
CMe3
CH2CH2C6F13
Me
Deelman's fluorophilic tetraphenylborate[4]
our group's "altebate"[10]
Fig. 1. Overview of selected lipophilic arylborates, carboranes, and aluminates with low nucleophilicity.
ppm; d6-acetone 30.83 and 2.05 ppm). 11B chemical shifts are
reported in ppm relative to BF3ꢁOEt2. Melting points were
determined using a Gallenkamp hot-stage microscope and are
uncorrected. An optimized preparation protocol for 3,30,5,50-
tetra-(tert-butyl)-2,20-biphenol has already been reported in the
literature [10,11].
the solvent was removed in vacuo and the mass of the residue was
determined to 1.9723 g (20 °C) and 2.1592 g (22 °C). A sample of
the residue was dissolved in d6-acetone, and found 1H NMR-spec-
troscopically to be >97% pure with a THF/bortebate ratio of
4.0 0.05. The determination of the lithium bortebate solubility
in pentane under an atmosphere of air was not reproducible, pos-
sibly due to partial hydrolysis of the thf adduct of the lithium
counterion.
2.2. Synthesis and characterization
2.2.2. Synthesis of sodium pentakis(tetrahydrofuran) bis[3,30,5,50-
tetra-(tert-butyl)-2,20-diphenolato]borate(III)
2.2.1. Synthesis of lithium tetrakis(tetrahydrofuran) bis[3,30,5,50-tetra-
(tert-butyl)-2,20-diphenolato]borate(III)
Under inert gas conditions, NaBH4 (0.69 g, 18 mmol) was sus-
pended in 15 mL THF. A solution of 3,30,5,50-tetra-(tert-butyl)-
2,20-biphenol (15 g, 37 mmol) in 15 mL THF was slowly added.
The reaction mixture was heated for 8 d under reflux conditions,
and filtered over Celite. Removal of the solvent gave 20 g of a col-
orless powder (17 mol, 92%). 1H NMR (199.92 MHz, d6-acetone,
Under argon atmosphere, LiBH4 (2 M, 6.1 mL, 12 mmol) was di-
luted in 15 mL THF (obtained by distillation from Na/Ph2CO). A
solution of 3,30,5,50-tetra-(tert-butyl)-2,20-biphenol (10 g, 24
mmol) in 15 mL THF was added slowly. The reaction mixture
was heated for 8 d under reflux conditions, and filtered over Celite.
The solvent was removed in vacuo. Excess 3,30,5,50-tetra-(tert-bu-
tyl)-2,20-biphenol was removed by sublimation (6 h, 120 °C,
0.5 mbar). 8.74 g (7.8 mmol, 65%) of a colorless powder were ob-
tained. 1H NMR (500.13 MHz, d6-acetone, 25 °C) d = 7.13 (d, 4H,
4
4
25 °C) d = 7.13 (d, 4H, JH,H = 2.6 Hz, Ar), 7.01 (d, 4H, JH,H = 2.6 Hz,
Ar), 3.62 (m, 20H, THF), 1.79 (m, 20H, THF), 1.31 (s, 36H, Me),
1.24 (s, 36H, Me) ppm. 13C{1H} NMR (50.27 MHz, d6-acetone,
25 °C) d = 155.9 (CAr), 139.3 (CAr), 139.2 (CAr), 133.6 (CAr), 126.0
(CAr), 121.8 (CAr), 68.1 (THF), 35.7 (CCMe3), 34.6 (CCMe3), 32.3
(Me), 31.7 (Me), 26.2 (THF) ppm. 11B{1H} NMR (64.14 MHz, CD2Cl2,
(cmꢀ1) = 2959,
~
m
4
4JH,H = 2.5 Hz, Ar), 7.02 (d, 4H, JH,H = 2.5 Hz, Ar), 3.63 (m, 16H,
THF), 1.79 (m, 16H, THF), 1.31 (s, 36H, Me), 1.24 (s, 36H, Me)
ppm. 13C{1H} NMR (125.77 MHz, d6-acetone, 25 °C) d = 155.9
(CAr), 139.4 (CAr), 139.2 (CAr), 133.6 (CAr), 126.0 (CAr), 121.8 (CAr),
68.1 (THF), 35.7 (CCMe3), 34.6 (CCMe3), 32.3 (Me), 31.7 (Me),
26.2 (THF-C) ppm. 11B{1H} NMR (64.14 MHz, d6-acetone, 25 °C)
~
25 °C) d = 6.44 (s) ppm. Decomp. >89 °C. IR (KBr):
2907, 2869, 1623, 1476, 1464, 1435, 1410, 1390, 1361, 1332, 1282,
1240, 1201, 1132, 1101, 975, 934, 909, 879. Anal. Calc. for
d = 6.45 (s) ppm. Mp. 241 °C. IR (KBr):
m
(cmꢀ1) = 3426, 2959,
C
56H80BNaO4ꢁ2 THF (after THF elimination) (995.24): C, 77.24; H,
9.72. Found: C, 77.30; H, 9.74%. HR-MS (ESIꢀ) m/z (%): calcd.:
2904, 2870, 1637, 1476, 1435, 1411, 1389, 1360, 1282, 1267,
1242, 1102, 1048, 974, 935, 912, 878. (%) 4ꢁTHF (1123.41): Anal.
Calc. for C56H80BLiO4: C, 76.98; H, 10.05. Found: C, 77.23; H,
9.84%. HR-MS (ESIꢀ) m/z ꢀ(%): calcd.: 827.61497 found:
827.61309 (100) [MꢀLiðthfÞ4þ] .
827.61497 found: 827.61412 (100), [MꢀNaðthfÞ5þ]ꢀ.
2.2.3. Synthesis of sodium mono(tetrahydrofuran) bis[3,30,5,50-tetra-
tert-butyl-2,20-diphenolato]borate(III)
The solubility of the product was determined by stirring lithium
bortebate in pentane under an argon atmosphere, and filtering off
the solution by canula transfer into a second Schlenk flask. In a first
experiment, the solution temperature was determined to 20 °C. In
THF elimination from the pentakis(tetrahydrofuran) sodium
bortebate proceeded at 120 °C and 0.1 mbar over a period of 12 h
14 g (16 mmol, 98%) of a colorless powder were obtained. 1H
NMR (250.13 MHz, d6-acetone, 25 °C) d = 7.13 (d, 4H, 4JH,H = 2.5 Hz,
4
a
second experiment, the temperature was 22 °C. 50.0 mL
Ar), 7,02 (d, 4H, JH,H = 2.5 Hz, Ar), 3.62 (m, 4H, THF), 1.78 (m, 4H,
( 0.2 mL) of the saturated solution was measured off into a syringe,
THF), 1.30 (m, 36H, Me), 1.24 (m, 36H, Me) ppm. 13C{1H} NMR

M. Wrede et al. / Inorganica Chimica Acta 369 (2011) 71–75
73
(500.13 MHz, d6-acetone, 25 °C) d = 156.7 (CAr), 140.2 (CAr), 139.9
Me3C
CMe3
CMe3
(CAr), 134.4 (CAr), 126.8 (CAr), 122.6 (CAr), 68.9 (THF), 36.5 (CCMe3),
35.5 (CCMe3), 33.2 (Me), 32.5 (Me), 27.0 (THF) ppm. 11B{H}
(64.14 MHz, d6-acetone, 25 °C) d = 5.74 (s) ppm. Decomp. >218 °C.
~
Me3C
Me3C
HO
CMe3
CMe3
+ MBH4
- 4 H2
2
Me3C
Me3C
M(thf)n
O
O
O
IR (KBr):
m
(cmꢀ1) = 3424, 2959, 2906, 2870, 1467, 1462, 1435,
THF,
8 d, Δ
B
OH
1411, 1390, 1361, 1282, 1265, 1240, 1210, 1201, 1132, 1101,
974, 934, 910, 878. HR-MS (ESIꢀ) m/z (%): calcd.: 827.61497 found:
827.61320 (100) [MꢀNa(thf)+]ꢀ.
O
CMe3
M = Li (n up to 4);
M = Na (n up to 6)
Me3C
CMe3
2.2.4. Synthesis of tetraphenylphosphonium bis[3,30,5,50-tetra-(tert-
butyl)-2,20-diphenolato]borate(III)
Scheme 1. Preparation of thf adducts of alkali metal bortebate salts [9].
Tetraphenylphosphonium bromide (0.53 g, 1.3 mmol) was dis-
solved in 15 mL CH2Cl2. A solution of sodium bortebate (1.3 g,
1.4 mmol) in 10 mL CH2Cl2 was added. Immediately, a colorless so-
lid (NaBr) precipitated. The suspension was filtered over Celite. Re-
moval of the solvent in vacuo gave a colorless powder. The product
was washed with a small amount of pentane. 1.1 g of a colorless
powder (0.94 mmol, 75%) were isolated. 1H NMR (500.13 MHz,
from LiBH4 or NaBH4 took place orders of magnitude more slowly
than the respective altebate formation from LiAlH4 or NaAlH4. The
different rate of H2 formation originates from the increased steric
hindrance of the overall eight tert-butyl groups due to the smaller
boron center, and the lower polarity of the B–H bonds compared to
the more ionic Al–H bonds.
3
CD2Cl2, 25 °C) d = 7.77 (t, 4H, JH,H = 6.6 Hz, Ar), 7.62 (m, 8H, Ar),
4
The S4 symmetric bortebate anion is an achiral meso structure
due to an inner racemate of its two atropisomeric 2,20-biphenolate
ligands. The D2 diastereomer is predicted to be 46.3 kJ molꢀ1 high-
er in energy at the B3LYP/LACV3P**++ level of theory due to the
disadvantageous repulsion of two pairs of tert-butyl groups in
ortho-position to the phenolate oxygen [12]. This value is even
higher than the energetic difference between D2/S4 for the altebate
ion (19.1 kJ molꢀ1) [10]. A strong intramolecular steric repulsion
occurs between two pairs of tert-butyl groups in the D2 structure.
The smaller radius of the lighter earth metal translates into a tigh-
ter packing of the substituents, thus favoring the more sterically
balanced S4 structure. Indeed, X-ray structure determinations
always revealed the S4 symmetry of the bortebate anion (Fig. 2).
At room temperature, the bortebate anion does neither hydro-
lyze in THF/H2O solution nor does addition of aqueous NaOH
cause hydrolytic degradation. The solid thf adducts of lithium
and sodium bortebate can be handled in air. However, addition
7.47 (m, 8H, Ar), 7.10 (d, 4H, JH,H = 2.3 Hz, Ar), 7.01 (d, 4H,
4JH,H = 2.3 Hz, Ar), 1.25 (s, 36H, Me), 1.13 (s, 36H, Me) ppm.
13C{1H} NMR (125.77 MHz, CD2Cl2, 25 °C) d = 154.9 (CAr), 139.2
(CAr), 138.9 (CAr), 136.0 (CAr), 135.9 (CAr), 134.5 (CAr), 134.4 (CAr),
132.5 (CAr), 130.8 (CAr), 130.7 (CAr), 125.7 (CAr), 121.7 (CAr), 117.9
(CAr), 117.2 (CAr), 35.0 (CCMe3), 34.2 (CCMe3), 31.7 (Me), 30.8
(Me) ppm. 11B{1H} NMR (64.14 MHz, CD2Cl2, 25 °C) d = 6.31
(s) ppm. 31P NMR (202.47 MHz, CD2Cl2, 25 °C) d = 22.9 (s) ppm.
~
Mp. >300 °C. IR (KBr):
m
(cmꢀ1) = 2951, 2903, 2867, 1583, 1476,
1463, 1437, 1411, 1388, 1359, 1282, 1243, 1109, 996, 967, 934,
911, 875, 724, 690, 528. Anal. Calc. for C80H100BO4P (1167.43): C,
82.31; H, 8.63. Found: C, 82.44; H, 8.72%. HR-MS (ESI+) m/z (%):
calcd.: 339.12971 found: 339.12978 [PPh4]+. HR-MS (ESIꢀ) m/z
(%): calcd.: 827.61497 found: 827.61432 (100), [MꢀPh4P+]ꢀ.
2.2.5. Synthesis of tetraphenylphosphonium bis[3,30,5,50-tetra-(tert-
butyl)-2,20-diphenolato]aluminate(III)
Tetraphenylphosphonium bromide (0.5 mg, 1.2 mmol) was dis-
solved in 15 mL CH2Cl2. A solution of sodium altebate (1.3 g,
1.4 mmol) in 10 mL CH2Cl2 was added. A colorless solid (NaBr) pre-
cipitated immediately. The suspension was filtered over Celite. Re-
moval of the solvent in vacuo gave a colorless powder. The product
was washed with a small amount of pentane. 1.2 g of a colorless
powder (1.0 mmol, 86%) were obtained. 1H NMR (300.13 MHz,
3
CDCl3, 25 °C) d = 7.69 (t, 4H, JH,H = 6.0 Hz, Ar), 7.37–7.53 (m, 16H,
4
4
Ar), 7.16 (d, 4H, JH,H = 2.5 Hz, Ar), 7.01 (d, 4H, JH,H = 2.5 Hz, Ar),
1.24 (s, 36H, Me), 1.19 (s, 36H, Me) ppm. 13C{1H} NMR
(75.48 MHz, CDCl3, 25 °C) d = 156.0 (CAr), 138.0 (CAr), 137.6 (CAr),
135.9 (CAr), 135.9 (CAr), 134.1 (CAr), 133.9 (CAr), 132.6 (CAr), 130.9
(CAr), 130.7 (CAr), 128.0 (CAr), 121.6 (CAr), 117.7 (CAr), 116.5 (CAr),
35.1 (CCMe3), 34.1 (CCMe3), 31.9 (Me), 30.6 (Me) ppm. 31P NMR
(101.26 MHz, CDCl3, 25 °C) d = 24.2 (s) ppm. Mp. >300 °C. IR
~
Fig. 2. Structural proof of the hexakis(thf) adduct of sodium bortebate by a single-
crystal X-ray structure of mediocre quality (wR2 = 0.306); C black, H gray, O red, Na
yellow, B green [13]. (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
(KBr):
m
(cmꢀ1) = 2951, 2904, 2867, 1463, 1437, 1405, 1386,
1359, 1281, 1242, 1109, 871, 803, 783, 769, 724, 690, 621, 607,
528. Anal. Calc. for C80H100AlO4P (1083.60): C, 81.18; H, 8.52.
Found: C, 81.18; H, 8.49%. HR-MS (ESI+) m/z (%): calcd.:
339.12971 found: 339.12982 [PPh4]+. MS (ESIꢀ) m/z (%): calcd.:
843.58775 found: 843.58690 (100) [MꢀPh4P+]ꢀ.
Table 1
Content of thf ligands in sodium bortebate and sodium altebate, according to
integration in 1H NMR spectra.
3. Results and discussion
T and p
Ratio thf/bortebate
Ratio thf/altebate
The synthesis of the 3,30,5,50-tetra-(tert-butyl)-2,20-biphenol li-
gand can be easily achieved in a high yield 100 g-scale from inex-
pensive 2,4-di-tert-butylphenol and MnO2. The reaction of the
resulting 2,20-biphenol with LiBH4 or NaBH4 in THF takes several
days under reflux conditions (Scheme 1). The bortebate formation
50 °C at 300 mbar
r.t. at 0.1 mbar
60 °C at 0.1 mbar
90 °C at 0.1 mbar
120 °C at 0.1 mbar
7.0
3.2
7.5
2.5 (after 8 h)
1.6 (after 8 h)
0.6 (after 8 h)
0.4 (after 8 h)
1.36 (after 8 h)
0.6 (after 12 h)
0.6 (110 °C, after 8 h)

74
M. Wrede et al. / Inorganica Chimica Acta 369 (2011) 71–75
Me3C
Me3C
CMe3
Me3C
Me3C
CMe3
+ Ph4PBr
- NaBr
CMe3
CMe3
CMe3
CMe3
O
O
O
O
O
O
- THF
Na(thf)1
Ph4P
B
B
CH2Cl2, r.t.
O
O
Me3C
Me3C
Me3C
CMe3
Me3C
CMe3
Scheme 2. Preparation of the tetraphenylphosphonium bortebate.
influence of the size of the anion, and of the possible coordination
of the anion to unsaturated, ‘‘naked’’ sodium appears to be small.
In salt metathesis reactions, alkali metal cations can easily be
exchanged by more lipophilic cations. As an example, addition of
tetraphenylphosphonium bromide to sodium bortebate in CH2Cl2
gives a precipitate of NaBr (Scheme 2).
For a structural comparison of the bortebate and the altebate an-
ion, we obtained single-crystals suitable for X-ray diffraction stud-
ies of the tetraphenylphosphonium salts, respectively. Besides the
shorter B–O bonds (146.7 1 pm) compared to the Al–O bonds
(174.5 1.1 pm), the structural differences between bortebate and
altebate are insignificant (Figs. 3 and 4).
The high solubility of bortebate salts in hydrocarbon solvents is
evidence for its anion’s high lipophilicity. Lithiumtetrakis(tetrahy-
drofuran) bortebate was dissolved in pentane at saturation concen-
trations of 39.5 g/L (20 °C) and 43 g/L (22 °C) under an argon
atmosphere. The pentane solubility of the analogous lithium alte-
bate amounts to a significantly lower value of only 7 g/L (24 °C).
Fig. 3. Single-crystal X-ray structure of the tetraphenylphosphonium bortebate; C
black, H gray, O red, Na yellow, B green P orange [13]. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of
this article.)
4. Conclusion
Bortebate salts display an unprecedented lipophilicity that even
surpasses that of its aluminum analog, reflected in an approxi-
mately sixfold lithium bortebate solubility in pentane. The
synthetic access to bortebates, however, is hampered by the prolon-
gued reaction time of the 2,20-biphenol precursor with BH4ꢀ, and
the required harsh reaction conditions. Bortebates are more resis-
tant towards hydrolysis compared to altebates in alkaline solution.
In our experience, bortebates share the high tendency towards
crystallization with its aluminum sibling.
Acknowledgments
This work was supported by the Deutsche Forschungsgemeins-
chaft DFG (Graduate College 850 and Frontier program), the Fonds
der Chemischen Industrie FCI, the University of Heidelberg, and the
BASF SE (donation of the 2,20-biphenol derivative).
Fig. 4. Single-crystal X-ray structure of the tetraphenylphosphonium altebate; C
black, H gray, O red, Na yellow, Al blue, P orange [13]. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of
this article.)
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2010.12.039.
of aqueous acids leads to instantaneous hydrolysis. This behavior
correlates with boron’s favorite coordination numbers 3 and 4,
and aluminum’s favorite coordination numbers 4–6. Presumably,
a hydroxide nucleophile expands the coordination sphere of alumi-
num in the altebate anion, while it is unable to do so in bortebates.
Tetrahydrofuran ligands of sodium bortebate can be eliminated
by heating to at least 90 °C at 0.1 mbar for two days to a level of 0.6
thf per sodium bortebate. THF elimination proceeds under similar
conditions for bortebate and aluminate salts (Table 1). The relative
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