High boron content carboranyl-functionalized aryl ether derivatives displaying photoluminescent properties

Article abstract of DOI:10.1039/b618771d

The reaction of α,α′-bis(3,5-bis(bromomethyl)phenoxy-p- xylene (3) with 4 equiv of the monolithium salt of 1-Ph-1,2-C₂B ₁₀H₁₁ or 1-Me-1,2-C₂B₁₀H ₁₁ gave the corresponding neutral carboranyl-functionalized aryl ether derivatives closo-4 and closo-5, respectively. These compounds contain four closo clusters that were degraded using basic conditions with KOH in EtOH, affording the corresponding nido-6 and nido-7 as potassium salts. Nido species were also isolated with tetramethylammonium as cation giving compounds nido-8 and nido-9 in good yield. The potassium salts showed good solubility in water and polar solvents. All these compounds were characterized by ¹H, ¹¹B and ¹³C NMR spectroscopy and UV-vis. The electronic data in different solvents indicated a solvatochromic shift for all compounds and a red shift of the absorption maxima for the nido species with respect to the closo derivatives. These neutral and anionic carboranyl-functionalized aryl ether derivatives represent a new family of high boron content luminescent compounds that show strong fluorescence emission in different solvents at room temperature. This phenomenon is very interesting considering the fact that none of the precursors have such a property. The fluorescence emission depends on the cluster substituent (Ph or Me) and the solvent polarity. Additionally, the fluorescence emission intensity was clearly dependent on the solvent polarity; the closo species showed strongest fluorescence intensities in the non-polar solvents, while anionic species were highly emissive in polar solvents. The Royal Society of Chemistry 2007.

Full text of DOI:10.1039/b618771d

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/dalton | Dalton Transactions
High boron content carboranyl-functionalized aryl ether derivatives displaying
photoluminescent properties†
Fre´de´ric Lerouge,a Clara Vin˜as,a Francesc Teixidor,a Rosario Nu´n˜ez,*a Arturo Abreu,b Elba Xochitiotzi,b
Rosa Santillanb and Norberto Farfa´nc
Received 2nd January 2007, Accepted 16th February 2007
First published as an Advance Article on the web 19th March 2007
DOI: 10.1039/b618771d
ꢀ
The reaction of a,a -bis(3,5-bis(bromomethyl)phenoxy-p-xylene (3) with 4 equiv of the monolithium salt
of 1-Ph-1,2-C2B10H11 or 1-Me-1,2-C2B10H11 gave the corresponding neutral carboranyl-functionalized
aryl ether derivatives closo-4 and closo-5, respectively. These compounds contain four closo clusters that
were degraded using basic conditions with KOH in EtOH, affording the corresponding nido-6 and
nido-7 as potassium salts. Nido species were also isolated with tetramethylammonium as cation giving
compounds nido-8 and nido-9 in good yield. The potassium salts showed good solubility in water and
polar solvents. All these compounds were characterized by 1H, 11B and 13C NMR spectroscopy and
UV-vis. The electronic data in different solvents indicated a solvatochromic shift for all compounds and
a red shift of the absorption maxima for the nido species with respect to the closo derivatives. These
neutral and anionic carboranyl-functionalized aryl ether derivatives represent a new family of high
boron content luminescent compounds that show strong fluorescence emission in different solvents at
room temperature. This phenomenon is very interesting considering the fact that none of the precursors
have such a property. The fluorescence emission depends on the cluster substituent (Ph or Me) and the
solvent polarity. Additionally, the fluorescence emission intensity was clearly dependent on the solvent
polarity; the closo species showed strongest fluorescence intensities in the non-polar solvents, while
anionic species were highly emissive in polar solvents.
interest for BNCT. The closo-carboranes have been the chosen
Introduction
molecules for boron delivery to tumors, however they present
Due to their synthetic versatility and the relative ease of
the drawback that they are particularly hydrophobic. In contrast,
chemically modifying them, ortho-carborane derivatives can be
the hydrophilic nido-carborane cages should be presumably more
incorporated into various organic and biochemical structures.1
Furthermore, the o-carborane is very stable to oxidizing agents
adequate systems.
In the course of our studies on carborane clusters we report
herein the high yield synthesis of closo-carborane-containing
aryl ether cores, which present photoluminescent properties.
and strong acids, and exhibits thermal stability to tempera-
tures up to 400 ◦C. For this reason, o-carborane derivatives
have been of interest in a wide range of applications such
Degradation of the closo-carborane moieties allowed us to obtain
as catalysis,2 extraction of radionuclides,3 hybrid materials4 or
a novel type of fluorescent water-soluble high boron content
medical applications.5 The construction of high boron content
macromolecule.
molecules is of considerable current interest6 and a large number
of boron-containing porphyrin derivatives have been synthesized.7
These carboranylporphyrins are fluorescent, can be detected by
fluorescence microscopy, and have been tested for application in
boron neutron capture therapy (BNCT).7 At the same time, the
Results and discussion
Neutral and anionic carboranyl-functionalized aryl ether
introduction of carboranes into different types of dendrimeric
structures, within or at the surface of the molecules, is also being
explored.8,9 Dendrimers can be used as drug delivery systems,
especially those that present water-solubility. In this respect,
the design of water soluble boron rich dendritic systems is of
derivatives
ꢀ
The reaction of a,a -dibromo-p-xylene and 5-hydroxy-
(dimethyl)benzoate refluxed in acetonitrile in the presence
of potassium carbonate gave the tetraester 1 in high yield.10
Subsequently, 1 was reduced with lithium aluminium hydride in
THF to afford the tetra-alcohol (2). The four benzyl alcohols
were converted to the corresponding benzyl bromides using
carbon tetrabromide and triphenylphosphine in THF, affording
aInstitut de Ciencia de Materiales de Barcelona (CSIC), Campus de la UAB,
08193, Bellaterra, Barcelona, Spain. E-mail: rosario@icmab.es; Fax: 34-93-
5805729
bDepto. Qu´ımica, Centro de Investigacio´n y Estudios Avanzados del Instituto
Polite´cnico Nacional, 14-740, CP-07000, Col-San Pedro Zacatenco, Me´xico
D.F., Me´xico
ꢀ
a,a -bis(3,5-bis(bromomethyl)phenoxy-p-xylene (3) in moderate
yield (Scheme 1), employing conditions similar to those described
by Fre´chet et al.11
Grafting of closo-carborane units on 3 was performed in THF
at reflux for 24 h by the substitution of the bromide atoms
cDepto. Qu´ımica Orga´nica, Facultad de Qu´ımica, Universidad Auto´noma
de Me´xico, Ciudad Universitaria, 04510, Me´xico D. F., Me´xico
† The HTML version of this article has been enhanced with colour images.
1898 | Dalton Trans., 2007, 1898–1903
This journal is
The Royal Society of Chemistry 2007
©

View Article Online
this band appears around 2526 cm−1. The 1H NMR spectrum of 3
shows three resonances assigned to aromatic protons at 7.48 ppm
(H-2), 7.37 (H-4ꢀ) and 6.96 ppm (H-2ꢀ), whereas the aliphatic
region shows singlets at 5.11 and 4.45 ppm for the benzyl ether and
benzyl bromide, respectively. The 13C NMR spectrum confirmed
the formation of 3 by the appearance of the benzyl bromide carbon
signal at 33.2 compared to that observed for the precursor 2 at
63.5 ppm. In addition, the 13C NMR spectrum exhibits two signals
at 136.8 (C-1) and 128.2 (C-2) ppm for the central ring, four signals
at 159.5 (C-1ꢀ), 140.1 ppm (C-3ꢀ), 122.6 (C-4ꢀ) and 115.9 ppm (C-
2ꢀ) for the rest of the aromatic carbons, as well as one signal at
70.3 ppm for the benzyl ether carbons. In the 1H NMR spectra of
closo-4 and closo-5 the resonances due to the H-4ꢀ appeared at 6.06
and 6.89 ppm respectively, shifted to lower frequency compared
to 3 (7.37 ppm). The benzyl bromide protons at 4.45 ppm in 3
were also shifted to lower frequency in closo-4 (3.14 ppm) and
closo-5 (3.68 ppm) due to the substitution of Br atoms by the
respective carborane clusters. For closo-5, the singlet at 2.29 ppm
is assigned to the methyl protons of the cluster. The presence of
the clusters in the molecules was also confirmed in the 13C NMR
spectra that show two resonances assigned to the Ccluster at 84.04
and 82.44 ppm for closo-4 and at 78.50 and 76.15 ppm for closo-5.
After degradation, a broad singlet, between −2.03 and −2.45 ppm,
from the B–H–B bridge protons of nido-carborane was noticed.
For compounds nido-7 and nido-9, two very close resonances, at
1.34 and 1.32 ppm, were attributed to the methyl protons of the
cluster. In addition, the most significant difference with regard
to the closo species was observed for resonances assigned to H-4ꢀ
protons; which appear at 5.88 and 5.84 ppm for nido-8, and at
6.78 and 6.82 ppm for nido-9, all with the same relative areas.
Inspection of the 13C NMR spectra for nido species indicated
greater complexity of the system in solution, and no attempts have
been made to assign the different signals. However, it is relevant
to note that in all cases two different resonances attributed to
the benzylic protons bonded to the cluster are observed around
42 ppm. This result seems to indicate the loss of symmetry of the
ꢀ
Scheme 1 Synthesis of a,a -bis(3,5-bis(bromomethyl)phenoxy-p-xylene.
with 4 equiv. of the corresponding monolithium salts of the
1-Ph-1,2-closo-C2B10H11 and 1-Me-1,2-closo-C2B10H11, to afford
compounds closo-4 (Scheme 2) and closo-5, in high yield (82 and
85%, respectively).
Subsequently, partial degradation, or the removal of B(3) or
B(6), in closo-4 and closo-5 was successfully performed using
an excess of KOH in ethanol at reflux for 20 h, affording
compounds nido-6 (Scheme 2) and nido-7, respectively. These
potassium salts were isolated with difficulty due to their hygro-
scopic character, however, it was possible to characterize them
by NMR spectroscopy. Finally, the anions were easily isolated
by precipitation with [Me4N]Cl, to give nido-8 (see Scheme 2) and
nido-9, respectively. Indeed, nido-carborane derivatives bearing K+
cations are soluble in water and polar solvents such as EtOH
or DMSO, while nido-8 and nido-9 bearing [Me4N]+ cations are
completely insoluble in water.
Characterization of carboranyl-containing aryl ether derivatives
All compounds were characterized by means of IR, 1H, 11B,
13C NMR and UV-vis spectroscopy. The IR spectra of closo-
derivatives exhibit a single and strong absorption at 2582 cm−1 as-
signed to the B–H stretching vibration, while for nido compounds
Scheme 2 Incorporation of 1-Ph-1,2-closo-C2B10H11 to the core 3 and degradation reaction to obtain polyanionic aryl ether derivatives.
This journal is The Royal Society of Chemistry 2007 Dalton Trans., 2007, 1898–1903 | 1899
©

View Article Online
Table 1 Electronic absorption data of compounds at room temperature
kmax/nm
show emissions with a kmax in the 363–370 nm range. Thus,
changing the Me group by a Ph group in the cluster leads to a
bathochromic shift of the maximum emission, of about 10 nm,
suggesting that the carborane cage substituent has an influence
on the luminescence properties (see Fig. 3). In addition, upon
excitation of the closo-4 at 320 nm very close emission bands at
374 1 nm were shownwhatever the solventused(seeTable2). The
closo-5 follows the same behaviour with emission maxima at 367
1 nm, upon excitation at 310 nm. This fact indicates that closo
species did not show significant solvent-dependence fluorescence
properties. However, the greatest effect of solvent polarity on the
emission spectra was exhibited for anionic derivatives.
The potassium salt nido-6 showed a ≈ 5 nm red shift between kem
in THF (374 nm) and water (379 nm), being the same behaviour
observed for nido-7 (363 nm in THF and 369 in water). In addition,
the nido-8 and nido-9 species showed an ≈ 8 nm red shift of the
emission bands between THF and DMSO (see Table 2). Thus,
a difference in the Stoke’s shift was observed for a same anionic
compound in different solvents indicating interactions between the
chromophore and its surrounding medium on the excited states.
It is noteworthy that both the nature of the carborane cage and
the solvent polarity significantly influenced the fluorescence phe-
nomenon. Compounds containing closo carboranes are neutral,
whereas those incorporating nido clusters are salts. The charge
of the carboranyl species has an influence on the polarity of
the chromophore leading to a higher sensitivity of the excited
state to solvent effects. Increasing the solvent polarity produces a
correspondingly larger reduction in the energy level of the excited
state, while decreasing the solvent polarity reduces the solvent
effect on the excited state energy level. This effect is much more
apparent in the charged nido-species. Finally the solvent relaxation
effects on fluorescence emission result here in a slight difference in
the size of Stokes shifts.
Compound
THF
Acetone
e/M-1 cm-1
closo-4
nido-6
nido-8
closo-5
nido-7
nido-9
320
332
330
309
340
339
329
342
339
332
349
353
124
123
124
189
190
190
molecule after degradation or the formation of two diastereomers.
The 11B NMR spectrum of closo-4 and closo-5 show two broad
resonances centred at −1.49 and −8.01 ppm, with the ratio 2 : 8.
A completely different spectrum was obtained for nido derivatives
that exhibit five resonances, between −6.29 and −33.74 ppm in
a 2 : 1 : 4 : 1 : 1 ratio, corroborating the degraded nature of the
carborane cage.
Absorption spectra of carboranyl-containing aryl ether derivatives
The absorption measurements were performed in THF and ace-
tone (Table 1). All compounds display a significant solvatochromic
shift, exhibiting bands of absorption maxima in the region 309–
339 nm in THF, and 329–353 nm in acetone. The behaviour of closo
and nido derivatives differed. In comparison with closo-4, the kmax
of nido-6 and nido-8 are 10 nm red shifted. Nevertheless, a 20 nm
red shift was observed between the closo-5 and the related nido
species. This bathochromic shift is the same in both solvents and
could be attributed to changes in the nature of the carborane cage,
before and after degradation. In addition, for the same solvent and
cluster substituent, the absorption maxima shift to lower energy
in the anionic species.
Emission data of carboranyl-containing aryl ether derivatives.
Solvent and cluster dependence of the emission
Solvent dependence of the fluorescence emission intensity
Another interesting aspect of the photophysics of the compounds
under study is the intensity emission dependence on the solvent
polarity. Differences are also observed in terms of PL emission
intensity for each compound, being the strength of the emission
band strongly affected by the solvent. In the neutral closo-4
the strongest fluorescence intensity is observed in the non-polar
solvent, toluene (Fig. 1, left), whereas low intensity emission bands
were exhibited in polar solvents, such as THF or CHCl3. Thus,
the emission bands intensity and PL decreases as the polarity
of the solvent increases. The opposite behaviour was observed
with anionic species, nido-6 and nido-8, in which the strongest
emission intensities are obtained in the most polar solvents, in
H2O (Fig. 1, centre) and DMSO (Fig. 1, right), respectively. The
most drastic effect was shown in THF, where the relative intensity
of the emission bands were substantially reduced and particularly
for nido-6, where quenching of the emission almost occurred.
Additionally, very similar behaviour to that described above was
observed for neutral and anionic methyl-o-carborane derivatives
(see Fig. 2). The closo-5 shows the most intense emission maximum
in toluene, whereas for nido-7 the strongest emission intensity was
observed in water. As for nido-6, in nido-7 the emission band
was weakly detectable in THF, but in this case a new and broad
emission band at 465 nm was also observed.
It is important to emphasize that all of the compounds described
here showed photoluminescence (PL) properties in solution at
room temperature, exhibiting blue emission under ultraviolet
radiation. Such behaviour was completely unpredictable consid-
ering that isolated initial precursors (1-Ph-1,2-C2B10H11, 1-Me-
1,2-C2B10H11 and the core 3) are not fluorescent at rt. Table 2
summarizes the PL emission results in different solvents.
It is worth noting that phenyl-o-carborane derivatives exhibit a
blue emission with a kmax in the 373–381 nm region depending on
the solvent, whereas compounds containing methyl-o-carborane
Table 2 Fluorescence emission data at 5.10−4 M, at room temperature
in solution; after excitation at 320 nm for closo-4, 330 nm for nido-6 and
nido-8, 310 nm for closo-5 and 315 nm for nido-7 and nido-9
kem /nm
Compound
THF
Acetone
Other solvents
closo-4
nido-6
nido-8
closo-5
nido-7
nido-9
373
374
373
366
363
363
374
375
375
368
365
364
375 (Toluene); 374 (CHCl3)
379 (H2O)
381 (DMSO)
367 (Toluene); 368 (CHCl3)
369 (H2O)
370 (DMSO)
1900 | Dalton Trans., 2007, 1898–1903
This journal is
The Royal Society of Chemistry 2007
©

View Article Online
Fig. 1 Fluorescence emission spectra of closo-4 (left), nido-6 (centre) and nido-8 (right) at 5 × 10−4 M in different solvents.
Fig. 2 Fluorescence emission spectra of closo-5 (left), nido-7 (centre) and nido-9 (right) at 5 × 10−4 M in different solvents.
Contrary to the fullerenes, the optical properties of the carbo-
rane derivatives have not been largely studied. Recently, Kunkely
and Volger reported that o-carborane seems to show luminescent
properties only in the solid state at 77 K with electronic transitions
corresponding to the HOMO–LUMO gap, that change with
the carborane cage symmetry.12 However, for the compounds
described here these properties can be observed in solution at
rt. Changing the substituents (phenyl or methyl) may induce
differences in the symmetry and subsequently in the HOMO–
LUMO gap, giving an 8 nm red shift between the closo methyl-
and phenyl-carborane derivatives in toluene (Fig. 3, left). For
anionic species, in which changes in the symmetry were already
observed in the 1H NMR spectra, an 11 nm red shift was
observed between nido-8 and nido-9 in DMSO (Fig. 3, right).
These results were different to those reported by Crespo et al.
for closo- and nido-carboranylphosphines.13 In their case, closo-
carboranylphosphines were not luminescent while the correspond-
ing nido-carborane derivatives exhibited PL properties. This was
attributed to a significant change of the electronic structure in the
cage caused by the loss of one boron atom after degradation, and
they proposed that the transitions are probably associated with
the orbitals of the nido-carborane cage.
In our case, we may propose that specific interactions between
the organic substituents and the carborane cage are at the origin of
the luminescent properties observed, together with changes in the
electronic structure of the carborane. In order to understand the
cause of this photophysical phenomenon efforts are under way to
study these and other closo and nido carborane based derivatives
which contain different organic substituents.
Conclusions
We have described the relatively convenient preparation of neutral
and polyanionic high boron content carboranyl-functionalized
aryl ether derivatives. In the case of nido species, the cation is a key
factor for the solubility. These neutral and anionic carboranyl-
functionalized aryl ether derivatives represent a new family of
high boron content luminescent compounds that show strong
fluorescence emission in different solvents at room temperature.
UV-vis and luminescent spectroscopies indicate that absorption
and emission bands depend on the substituent (Ph or Me) and
electronic nature of the cluster, and the solvent polarity. The
emission intensity is also solvent-dependent. Their properties
and high boron content make these macromolecules interesting
Fig. 3 Excitation and emission spectra of closo-4 and closo-5 in toluene (left), nido-8 and nido-9 in DMSO (right) at room temperature.
This journal is The Royal Society of Chemistry 2007 Dalton Trans., 2007, 1898–1903 | 1901
©

View Article Online
compounds for use in cancer treatments using BNCT. Indeed,
some of these compounds can be dissolved in biological media
and be easily detectable thanks to their luminescence properties.
for 45 min at room temperature. The yellow solution was cooled to
0 ◦C and a mixture of 3 (250 mg, 0.38 mmol) in THF (10 mL) was
added. After refluxing for 24 h, the THF was evaporated in vacuo
to give a yellow residue. The residue was taken up with 10 mL of
diethyl ether and 30 mL of water, the mixture was then transferred
to a separatory funnel. The layers were separated and the organic
phase was extracted with additional diethyl ether (2 × 10 mL).
The combined filtrates were dried over anhydrous MgSO4 and
concentrated in vacuo to give a yellow residue. The solid obtained
was taken up in MeOH, after filtration the suspension gave closo-4
as a light yellow solid. Yield: 380 mg, 82%. 1H NMR ((CD3)2CO):
7.82–7.56 (m, 20H), 7.49 (s, 4H), 6.46 (s, 4H), 6.06 (s, 2H), 5.06 (s,
Experimental
General details
Elemental analyses were performed using a Carlo Erba EA1108
microanalyzer. IR spectra were recorded from KBr pellets on a
Shimadzu FTIR-8300 spectrophotometer. The 1H NMR (300.13
1
1
MHz), 11B{ H} NMR (96.29 MHz) and 13C{ H} NMR (75.47
MHz) spectra were recorded on a Bruker ARX 300 spectrometer.
All NMR spectra were recorded in CDCl3 or CD3COCD3
1
4H), 3.14 (s, 8H). 13C{ H} NMR ((CD3)2CO): 158.5 (C-1ꢀ), 137.3
(C-3ꢀ), 136.8 (C-1), 131.5, 131.2, 130.4, 129.3, 127.6 (C-2), 124.3
solutions at 25 ◦C. Chemical shift values for 11B{ H} NMR
1
(C-4ꢀ), 116.2 (C-2ꢀ), 84.0 (Ccluster), 82.4 (Ccluster), 69.4 (CH2–O), 40.3
1
(CH2–Ccluster). 11B{ H} NMR ((CD3)2CO): −1.5 (2B), −8.0 (8B).
spectra were referenced to external BF3·OEt2, and those for
1H and 13C{ H} NMR were referenced to SiMe4. Chemical
1
IR (KBr, cm−1): 3062 m(Caryl–H), 2927 m(Calkyl–H), 2582 m(B–H).
Elemental analysis calcd (%) for C56H82B40O2: C 55.10, H 6.80;
found: C 54.84; H 7.02.
shifts are reported in units of parts per million downfield from
reference, and all coupling constants are reported in Hz. UV-vis
spectra were recorded using a Shimadzu UV-1700 Pharmaspec
spectrophotometer, using 0.1 cm cuvettes. The concentration of
the compounds was 5 × 10−4 mol L−1. Fluoresence spectra were
measured on Cary Eclipse fluorescence spectrophotometer, using
solutions of compounds at 5 × 10−4 mol L−1.
Preparation of closo-5. The procedure was the same as for
closo-4, using 1-Me-1,2-closo-C2B10H11 (300 mg, 1.89 mmol) in
3 mL of THF, n-Buli 1.6 M (1.35 mL, 2.08 mmol) and 3 (285 mg,
0.45 mmol) in 4 mL THF. After 18 h of reflux, compound closo-
1
5 was isolated as a white solid. Yield: 370 mg, 85%. H NMR
All reactions were performed under an atmosphere of dini-
trogen employing standard Schlenk techniques. THF, Et2O and
toluene were purchased from Merck and distilled from sodium
benzophenone prior to use, and CH3CN was used as received
from J. T. Baker Co. Compounds 1-Ph-1,2-closo-C2B10H11 and 1-
Me-1,2-closo-C2B10H11 were supplied by Katchem Ltd. (Prague)
and used as received. Compounds 1 and 2 were synthesized
according to the literature.10 n-BuLi solution (1.6 M in hexanes)
was purchased from Lancaster or Aldrich. Starting materials:
((CD3)2CO): 7.51 (s, 4H), 6.97 (s, 4H), 6.89 (s, 2H), 5.20 (s, 4H),
1
3.68 (s, 8H), 2.29 (s, 12H). 13C{ H} NMR ((CD3)2CO): 158.8 (C-
1ꢀ), 137.4 (C-3ꢀ), 136.8 (C-1), 127.7 (C-2), 124.9 (C-4ꢀ), 116.7 (C-2ꢀ),
78.5 (Ccluster), 76.2 (Ccluster), 69.5 (CH2–O), 40.3 (CH2–Ccluster), 23.0
1
(CH3). 11B{ H} NMR ((CD3)2CO): −2.2 (1B); −3.8 (1B); −8.3
(8B). IR (KBr, cm−1): 3033 m(Caryl–H), 2943 m(Calkyl–H), 2584 m(B–
H). Elemental analysis calcd (%) for C36H74B40O2 : C 44.51, H 7.68;
Found: C 45.02; H 7.42.
ꢀ
a,a -dibromo-p-xylene; 3,5-dihydroxybenzoic acid; triphenylphos-
Preparation of nido-6. To a solution of KOH (40 mg,
0.656 mmol) in 4 mL of deoxygenated EtOH, closo-4 (100 mg,
0.082 mmol) was added to give a white suspension, which was
stirred for 1 h at room temperature. After this time, the mixture
was refluxed for 20 h. After cooling the mixture, the excess of
KOH was precipitated as potassium carbonate by saturating the
solution with a stream of CO2(g). After filtration the solution
was evaporated to give a white residue. The solid was taken up
in THF, and the suspension was stirred at room temperature for
a few minutes. After filtration, the THF solution was evaporated
phine; carbon tetrabromide; potassium carbonate; lithium hydride
were commercially available from Aldrich and used as received.
a,aꢀ-bis[3,5-bis(bromomehyl)phenoxy]-p-xylene (3). To a solu-
tion of 2 (5.88 g, 14.30 mmol) and CBr4 (28.57 g, 86.05 mmol) in
dry THF (80 mL) was added PPh3 (22.55 g, 86.05 mmol) in two
portions. The reaction mixture was stirred at room temperature
under N2 for 4 h. The pH was adjusted to 7 with Na2CO3 (20 mL),
and brine (80 mL) was added. The aqueous phase was extracted
with CH2Cl2 (3 × 120 mL) the organic phases were collected and
concentrated in a rotatory evaporator. The residue was passed
through a short column of silica gel, washed with hexane and
eluted with a 8 : 2 hexane : ethyl acetate mixture to give 3 as a white
powder. Yield: 5.99 g, 9.05 mmol, 60%. Mp 199–122 ◦C. 1H NMR
(CDCl3) d: 7.48 (4H, H-2, s), 7.37 (2H, H-4ꢀ, s), 6.96 (4H, H-2ꢀ,
s), 5.11 (4H, CH2-O, s), 4.45 (8H, CH2-Br, s). 13C NMR (CDCl3)
d: 159.5 (C-1ꢀ), 140.1 (C-3ꢀ), 136.8 (C-1), 128.2 (C-2), 122.6 (C-4ꢀ)
115.9 (C-2ꢀ), 70.3 (CH2-O), 33.2 (CH2–Br). IR (KBr, cm−1): 3050,
1643, 1595, 1400, 1333, 1301, 1212, 1178, 1040, 778, 698, 552.
Elemental analysis calcd (%) for C24H22Br4O2: C 43.54, H 3.35;
found: C 43.41; H 3.45.
1
to dryness to give nido-6 as a white solid. Yield: 96 mg, 88%. H
NMR ((CD3)2CO): 7.58 (s, 4H), 7.24–6.96 (m, 20H), 6.52 (s, 4H),
5.90 (s, 1H), 5.85 (s, 1H), 4.99 (s, 4H), 2.88 (s, 8H), −2.07 (br s,
1
4H). 13C{ H} NMR ((CD3)2CO): 157.4, 142.6, 142.2, 142.1, 137.5,
132.3, 127.6, 126.6, 125.2, 123.9, 112.8, 69.2, 67.2, 62.9, 42.5, 42.4.
1
11B{ H} NMR ((CD3)2CO): −7.9 (2B), −12.3 (1B), −16,3 (4B),
−32.7 (1B), −35.4 (1B). IR (KBr, cm−1) 2976 m(Caryl–H), 2931
m(Calkyl–H), 2526 m(B–H), 1675, 1406, 1072. The elemental analysis
is not available due to the hygroscopic character of the nido-6.
Preparation of nido-8. The degradation procedure was the
same as for nido-6, however, after cooling the mixture, EtOH was
evaporated to give a white residue which was dissolved in water.
Addition of an excess of a Me4NCl in water solution gave a preci-
pitate. After filtration the solid was dried in vacuo to give nido-8 as
a white powder. Yield: 115 mg, 96%. 1H NMR ((CD3)2CO): 7.60
Preparation of closo-4. To a solution of 1-Ph-1,2-closo-
C2B10H11 (368 mg, 1.66 mmol) in 6 mL of dry THF at 0 ◦C, a
1.6 M solution of n-Buli in hexane (1.33 mL, 1.99 mmol) was added
dropwise. When addition was completed, the mixture was stirred
1902 | Dalton Trans., 2007, 1898–1903
This journal is
The Royal Society of Chemistry 2007
©

View Article Online
(s, 4H), 7.25–6.95 (m, 20H), 6.53 (s, 4H), 5.88 (s, 1H), 5.84 (s, 1H),
Stachow, J. F. Liegeois, B. Pirotte, J. Delarge, A. Demonceau, M.
Fontaine, A. F. No¨els, I. T. Chizvensky, T. V. Zinevich, V. I. Bregadze,
F. M. Dolgushin, A. I. Yanovsky and Y. T. Struchkov, J. Organomet.
Chem., 1997, 536/537, 405; F. Teixidor, M. A. Flores, C. Vin˜as, R.
Sillanpa¨a¨ and R. Kiveka¨s, J. Am. Chem. Soc., 2000, 122, 1963; O.
Tutusaus, S. Delfosse, A. Demonceau, A. F. No¨els, C. Vin˜as, R. Nu´n˜ez
and F. Teixidor, Tetrahedron Lett., 2002, 43, 983; Z Xie, Acc. Chem.
Res., 2003, 36, 1; O. Tutusaus, C. Vin˜as, R. Nu´n˜ez, F. Teixidor, A.
Demonceau, S. Delfosse, A. F. No¨els, I. Mata and E. Molins, J. Am.
Chem. Soc., 2003, 125, 11830.
3 C. Vin˜as, S. Go´mez, J. Bertra´n, F. Teixidor, J.-F. Dozol and H.
Rouquette, Chem. Commun., 1998, 191; C. Vin˜as, S. Go´mez, J. Bertra´n,
F. Teixidor, J.-F. Dozol and H. Rouquette, Inorg. Chem., 1998, 37, 3640;
B. Gru¨ner, J. Plesek, J. Baca, I. Cisarova, J.-F. Dozol, H. Rouquette, C.
Vin˜as, P. Selucky and J. Rais, New J. Chem., 2002, 26, 1519.
4 A. Gonza´lez-Campo, R. Nu´n˜ez, C. Vin˜as and B. Boury, New J. Chem.,
2006, 30, 546; A. Gonza´lez-Campo, B. Boury, F. Teixidor and R. Nu´n˜ez,
Chem. Mater., 2006, 18, 4344.
5 A. H. Solovay, W. Tjarks, B. A. Barnum, F. G. Rong, R. F. Barth,
I. M. Codogni and J. G. Wilson, Chem. Rev., 1998, 98, 1515; M. F.
Hawthorne and A. Maderna, Chem. Rev., 1999, 99, 3421.
6 J. Thomas and M. F. Hawthorne, Chem. Commun., 2001, 1884; Y. Azev,
I. Slepukhina and D. Gabel, Appl. Radiat. Isot., 2004, 61, 1107; L. Ma,
J. Hamdi, F. Wong and M. F. Hawthorne, Inorg. Chem., 2006, 45, 278.
7 M. G. H. Vicente, S. J. Shetty, A. Wickramasinghe and K. M. Smith,
Tetrahedron Lett., 2000, 41, 763; V. I. Bregadze, I. B. Sivaev, D. Gabel
and D. Wo¨hrle, J. Porphyrins Phthalocyanines, 2001, 5, 767; A. R.
Genady, M. E. El-Zaria and D. Gabel, J. Organomet. Chem., 2004,
689, 3242; R. Luguya, F. R. Fronczek, K. M. Smith and M. G. H.
Vicente, Appl. Radiat. Isot., 2004, 61, 1117; J. C. Clark, F. R. Fronczek
and M. G. H. Vicente, Tetrahedron Lett., 2005, 46, 2365; E. Hao and
M. G. H. Vicente, Chem. Commun., 2005, 1306; R. F. Barth, J. A.
Coderre, M. G. H. Vicente and T. E. Blue, Clin. Cancer Res., 2005,
11, 3987; V. Gottumukkala, O. Ongayi, D. G. Baker, L. G. Lomax and
M. G. H. Vicente, Bioorg. Med. Chem., 2006, 14, 1871; J.-Q. Wang,
C.-X. Ren, L.-H. Weng and G.-X. Jin, Chem. Commun., 2006, 162.
8 G. R. Newkome, C. N. Moorefield, J. M. Keith, G. R. Baker and G. H.
Escamilla, Angew. Chem., Int. Ed. Engl., 1994, 33, 666; R. F. Barth,
D. M. Adamns, A. H. Solovay, F. Alam and M. V. Darby, Bioconjugate
Chem., 1994, 5, 58; D. Armspach, M. Cattalini, E. C. Constable, C. E.
Housecroft and D. Phillips, Chem. Commun., 1996, 1823; B. Qualman
and M. M. Kessels, Angew. Chem., Int. Ed. Engl., 1996, 35, 909; H.-J.
Musiol, W. D. Sierralta, P. W. Jungblut, L. Moroder, M. C. Parrott,
E. B. Marchington, J. F. Valliant and A. Adronov, J. Am. Chem. Soc.,
2005, 127, 12081.
5.02 (s, 4H), 3.35 (s, 48H), 2.61–2.47 (m, 8H) −2.03 (br s, 4H).
1
13C{ H} NMR ((CD3)2CO):). 157. 6, 143.4, 143.1, 142.0, 137.3,
132.8, 127.6, 126.0, 125.7, 124.0, 113.1, 70.3, 67.0, 63.2, 45.9, 41.5,
1
41.4. 11B{ H} NMR ((CD3)2CO): −6.3 (2B), −10.9 (1B) −14.8
(4B), −31.1 (1B), −33.7 (1B). IR (KBr, cm−1) 2980 m(Caryl–H),
2931 m(Calkyl–H), 2520 m(B–H). Elemental analysis calcd (%) for
C72H130B36N4O2 ·4H2O: C 56.00, H 8.94; N 3.63. Found: C, 56.11;
H, 8.82; N, 3.54.
Preparation of nido-7. The degradation procedure was the
same as for nido-6, using closo-5 (100 mg, 0.10 mmol) and KOH
(46 mg 0.82 mmol) in 4 mL of EtOH. After 20 h refluxing, and
using the same work up as for nido-6, compound nido-7 was
isolated as a white solid. Yield: 97 mg, 90%. 1H NMR ((CD3)2CO):
7.61 (s, 4H); 7.04 (s, 4H); 6.82 (s, 1H), 6.76 (s, 1H), 5.16 (s, 4H),
1
3.29 (s, 8H), 1.34 (s, 6H), 1.32 (s, 6H), −2.29 (br s, 4H). 13C{ H}
NMR ((CD3)2CO): 159.6, 145.1, 138.4, 128.9, 123.5, 113.2, 70.3,
1
62.9, 55.0, 43.7, 43.6, 23.5. 11B{ H} NMR ((CD3)2CO): −5.9 (s,
1B), −7.2 (s, 2B), −13.4 (s, 2B), −17,0 (s, 2B); −31.7 (s, 1B); −34.2
(s, 1B). IR (KBr, cm−1) 2980 m(Caryl–H), 2931 m(Calkyl–H), 2516 m(B–
H). The elemental analysis is not available due to the hygroscopic
character of the nido-7.
Preparation of nido-9. The procedure was the same as for nido-
1
8. Yield: 113 mg, 90%. H NMR ((CD3)2CO): 7.65 (s, 4H); 7.04
(s, 4H); 6.78 (s, 1H), 6.72 (s, 1H); 5.18 (s, 4H), 3.36 (s, 48H), 3.09
1
(s, 8H), 1.34 (s, 6H); 1.33 (s, 6H); −2.45 (br s, 4H). 13C{ H} NMR
((CD3)2CO): 158.5, 144.2, 137.5, 128.2, 122.6, 112.3, 112.2, 69.4,
1
62.2, 55.2, 54.4, 42.6, 42.5, 22.5. 11B{ H} NMR ((CD3)2CO): −6.5
(s, 3B), −13.3 (s, 2B), −17,4 (s, 2B); −31.7 (s, 1B); −34.2 (s, 1B).
IR (KBr, cm−1) 2980 m(Caryl–H), 2927 m(Calkyl–H), 2515 m(B–H).
Acknowledgements
This work was supported by the Ministerio de Educacio´n y
Ciencia (MAT2004-01108) and Generalitat de Catalunya (CRED-
200400030 and CRED-200500001).
9 R. Nun˜ez, A. Gonza´lez, C. Vin˜as, F. Teixidor, R. Sillanpa¨a¨ and R.
Kiveka¨s, Org. Lett., 2005, 7, 231; R. Nun˜ez, A. Gonza´lez-Campo, C.
Vin˜as, F. Teixidor, R. Sillanpa¨a¨ and R. Kiveka¨s, Organometallics, 2005,
24, 6351–6357.
10 T. K. Vinod and H. Hart, J. Org. Chem., 1991, 56, 5630; P. Rajakumar,
M. Dhanasekaran, S. Selvanayagam, V. Rajakannan, D. Velmurugan
and K. Ravikumar, Tetrahedron Lett., 2005, 46, 995.
11 L. Gilat, A. Adronov and J. M. J. Fre´chet, J. Org. Chem., 1999, 64,
7474; A. Adronov and J. M. J. Fre´chet, Chem. Mater., 2000, 12, 2838.
12 H. Kunkely and A. Vogler, Inorg. Chim. Acta, 2004, 357, 4607.
13 O. Crespo, M. C. Gimeno, P. J. Jones, A. Laguna, J.M. Lo´pez-de-
Luzuriaga, M. Monge, J. L. Pe´rez and M. A. Ramo´n, Inorg. Chem.,
2003, 42, 2061.
Notes and references
1 J. Plesek, Chem. Rev., 1992, 92, 269; R. B. King, Chem. Rev., 2001, 101,
1119; J. F. Valliant, K. J. Guenther, A. S. King, P. Morel, P. Schaffer,
O. O. Sogbein and K. A. Stephenson, Coord. Chem. Rev., 2002, 232,
173.
2 M. F. Hawthorne, Advance in Boron and the Boranes, ed. J. F. Liebman,
A. Grenberg and R. S. Williams, VCH, New York, 1988, p. 225;
F. Teixidor, M. A. Flores, C. Vin˜as, R. Kiveka¨s and R. Sillanpa¨a¨,
Angew. Chem., Int. Ed. Engl., 1996, 108, 2388; A. Felekidis, M. Goblet-
This journal is
The Royal Society of Chemistry 2007
Dalton Trans., 2007, 1898–1903 | 1903
©