Synthesis and fluorescence emission of neutral and anionic di- and tetra-carboranyl compounds

Article abstract of DOI:10.1039/c1dt10309a

A new family of photoluminescent neutral and anionic di-carboranyl and tetra-carboranyl derivatives have been synthesized and characterized. The reaction of α,α′-bis(3,5-bis(bromomethyl)phenoxy-m-xylene with 4 equiv. of the monolithium salt of 1-Ph-1,2-C₂B₁₀H ₁₁ or 1-Me-1,2-C₂B₁₀H₁₁ gives the neutral tetracarboranyl-functionalized aryl ether derivatives closo-1 and closo-2, respectively. The addition of the monolithium salt of 1-Ph-1,2-closo-C₂B₁₀H₁₁ to α,α, ′-dibromo-m-xylene or 2,6-dibromomethyl-pyridine gives the corresponding di-carboranyl derivatives closo-3 and closo-4. These compounds, which contain four or two closo clusters, were degraded using the classical method, KOH in EtOH, affording the corresponding nido species, which were isolated as potassium or tetramethylammonium salts. All the compounds were characterized by IR, ¹H, ¹¹B and ¹³C NMR spectroscopy, and the crystal structure of closo-3 was analysed by X-ray diffraction. The carboranyl fragments are bonded through CH₂ units to different organic moieties, and their influence on the photoluminescent properties of the final molecules has been studied. All the closo- and nido-carborane derivatives exhibit a blue emission under ultraviolet excitation at room temperature in different solvents. The fluorescence properties of these closo and nido-derivatives depend on the substituent (Ph or Me) bonded to the C_cluster, the solvent polarity, and the organic unit bearing the carborane clusters (benzene or pyridine). In the case of nido-derivatives, an important effect of the cation is also observed. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1dt10309a

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PAPER
Synthesis and fluorescence emission of neutral and anionic di- and
tetra-carboranyl compounds†
Fre´de´ric Lerouge,a Albert Ferrer-Ugalde,‡a Clara Vin˜as,a Francesc Teixidor,a Reijo Sillanpa¨a¨,e Arturo Abreu,b
Elba Xochitiotzi,c Norberto Farfa´n,c Rosa Santilland and Rosario Nu´n˜ez*a
Received 23rd February 2011, Accepted 10th May 2011
DOI: 10.1039/c1dt10309a
A new family of photoluminescent neutral and anionic di-carboranyl and tetra-carboranyl derivatives
have been synthesized and characterized. The reaction of a,a¢-bis(3,5-bis(bromomethyl)phenoxy-
m-xylene with 4 equiv. of the monolithium salt of 1-Ph-1,2-C2B10H11 or 1-Me-1,2-C2B10H11 gives the
neutral tetracarboranyl-functionalized aryl ether derivatives closo-1 and closo-2, respectively. The
addition of the monolithium salt of 1-Ph-1,2-closo-C2B10H11 to a,a,¢-dibromo-m-xylene or
2,6-dibromomethyl-pyridine gives the corresponding di-carboranyl derivatives closo-3 and closo-4.
These compounds, which contain four or two closo clusters, were degraded using the classical method,
KOH in EtOH, affording the corresponding nido species, which were isolated as potassium or
tetramethylammonium salts. All the compounds were characterized by IR, 1H, 11B and 13C NMR
spectroscopy, and the crystal structure of closo-3 was analysed by X-ray diffraction. The carboranyl
fragments are bonded through CH2 units to different organic moieties, and their influence on the
photoluminescent properties of the final molecules has been studied. All the closo- and nido-carborane
derivatives exhibit a blue emission under ultraviolet excitation at room temperature in different solvents.
The fluorescence properties of these closo and nido-derivatives depend on the substituent (Ph or Me)
bonded to the Ccluster, the solvent polarity, and the organic unit bearing the carborane clusters (benzene
or pyridine). In the case of nido-derivatives, an important effect of the cation is also observed.
great potential of these systems can be increased by a carbon
substitution reaction allowing the 1,2-closo-C2B10H12 carborane
Introduction
to be grafted on macromolecules such as dendrimers,9,10 or
porphyrins,11 among others. Also of importance, is the C–C bond
in the o-carborane derivatives. This interesting and unusual bond
displays both s- and p-characteristics12 that are variable depending
on the substituent.1c,13
The twelve-vertex 1,2-closo-C2B10H12 icosahedral carboranes are
among the most widely studied boranes. This is mainly due
to the fact that 1,2-closo-C2B10H12 or their derivatives present
exceptional characteristics,1,2 such as low nucleophilicity, chemical
inertness, thermal stability,3 electron-withdrawing properties,4 and
low toxicity in biological systems.5 These properties have stimu-
lated the development of a wide range of potential applications,
such as catalysis, materials science and even medicine.6–8 The
Recently, several papers reported the photoluminescent proper-
ties of different systems incorporating o-carboranes within their
structures, for example: p-conjugated C3-symmetrical structures,14
p-phenylene-ethynylene-based polymers,15 or polyfluorene conju-
gated polymers.16 These studies show that the clusters interact with
the aromatic p-conjugation influencing the emission properties. It
appears that the introduction of o-carborane induces aggregation-
based emission (AIE),17 influencing the photoluminescent prop-
erties of polyfluorene, whereas the variable C–C bond acts as
a quencher to fluorescence from phenylene-ethylene segments.
Nevertheless, the presence of o-carborane clusters in p-conjugated
C3-symmetric systems separarted by a methylene moiety prevents
p–p stacking and enhances fluorescence intensity.14b
aInstitut de Ciencia de Materiales de Barcelona (ICMAB-CSIC), Campus
de la UAB, 08193, Bellaterra (Barcelona), Spain. E-mail: rosario@
icmab.es; Fax: +34-93-5805729
bUniversidad Polite´cnica de Pachuca, Carretera Pachuca Cd. Sahagu´n, Km
20, Zempoala, Ex-Hacienda de Santa Barbara, C. P. 43830, Hidalgo,
Me´xico
cFacultad de Qu´ımica, Departamento Qu´ımica Orga´nica, Universi-
dad Nacional Auto´noma de Me´xico (UNAM), Ciudad Universitaria,
04510, Me´xico D. F., Me´xico
dDepartamento de Qu´ımica, Centro de Investigacio´n y de Estudios Avanzados
del Instituto Polite´cnico Nacional, Aptdo. Postal 14-740, CP-07000, San
Pedro Zacatenco, Me´xico D.F., Me´xico
In a previous work,18 we reported high-boron content neutral
and anionic tetracarboranyl-functionalized aryl-ether derivatives
that exhibited fluorescence emission in different solvents at room
temperature. We proposed that specific interactions between
the organic substituents and the cluster induce changes in the
eDepartment of Chemistry, University of Jyva¨skyla¨, FIN-40014, Jyva¨skyla¨,
Finland
† CCDC reference number 802843. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1dt10309a
‡ Enrolled in the UAB PhD Program.
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Scheme 1 Synthesis of neutral and anionic tetra-carboranyl compounds.
electronic structure of the carborane cage. We also pointed out that
solvent polarity influences the emission properties. Thus, pursuing
our work dealing with the preparation, functionalization and
photoluminescent properties of carboranyl-containing derivatives,
we report herein the preparation of a new set of compounds, in
which o-carboranyl units have been bonded to different cores. As a
first step we have investigated the influence of a meta-substitution
in the central benzene of the tetracarboranyl-containing aryl-ether
molecule instead of a para-substitution. We have also synthesized
smaller compounds consisting of two carborane clusters bonded
to benzene and pyridine rings in meta positions. A comparative
study of the photoluminescent properties for all compounds has
been carried out in different solvents at room temperature. The
crystal structure of one of these compounds has been analysed by
X-ray diffraction.
monolithium salts of 1-Ph-1,2-closo-C2B10H11 or 1-Me-1,2-closo-
C2B10H11 on compound 1b in THF, the mixture was refluxed
16 h to 24 h to afford closo-1 or closo-2, in 61 and 70% yields,
respectively (Scheme 1). In a similar way, by addition of the
monolithium salt of 1-Ph-1,2-closo-C2B10H11 in THF to a,a,¢-
dibromo-m-xylene (2) or 2,6-bis(bromomethyl)pyridine (3), and
after refluxing 16 h, the carboranyl derivatives closo-3 or closo-4
were obtained in 75% and 78% yields, respectively (Scheme 2).
Compound closo-3 was crystallized from a solution of acetone
to give monocrystals suitable for X-ray diffraction analysis.
Subsequently, partial degradation of closo-1 and closo-2 was
achieved using an excess of KOH in ethanol at reflux for 20 h; the
Results and discussion
Synthesis of starting compounds
a,a-Bis(3,5-bis(bromomethyl)phenoxy-m-xylene (1b) was pre-
pared from tetra-alcohol a,a-bis(3,5-bis(hydroxymethyl)phenoxy-
m-xylene (1a)19 by treatment with CBr4 and PPh3 in THF at room
temperature for 16 h, following similar conditions to those previ-
ously reported by us.18 In addition, 2,6-bis(bromomethyl)pyridine
(3) was prepared by reaction of 2 equiv. of PBr3 on the diol 2,6-
dihydroxymethyl-pyridine in CHCl3 at 40 ◦C for 3 h, according to
a published procedure.20
Synthesis of neutral and anionic di- and tetra-carboranyl
compounds
Grafting of closo-carborane units to aryl-ether derivatives was
performed following a procedure previously described by our
group.18 Briefly, after adding 4 equiv. of the freshly prepared
Scheme 2 Synthesis of neutral and anionic di-carboranyl compounds.
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Table 1 Supramolecular interactions between neighboring carboranes in the crystal packing of compound closo-3
∠ CH ◊ ◊ ◊ H (◦)
∠ BH ◊ ◊ ◊ H (◦)
˚
˚
d H–H (A)
d C–B (A)
C(8)H(8) ◊ ◊ ◊ H(5A)B(5)
C(8)H(8) ◊ ◊ ◊ H(15A)B(15)
C(3)H(3B) ◊ ◊ ◊ H(10)B(10)
C(10)H(10) ◊ ◊ ◊ H(20A)B(20)
2.327
2.304
2.538
2.413
3.968
3.946
3.756
3.667
147.51
140.64
111.79
122.45
131.78
139.12
124.38
114.42
obtained potassium salts were isolated affording nido-1 and nido-
2, respectively. Finally, the anions were isolated by precipitation
with [Me4N]Cl, to give the tetramethylammonium salts nido-3 and
nido-4 (Scheme 1). Partial degradation of closo-3 and closo-4 gave
the respective nido-5 and nido-6 as the potassium salts that after
precipitation with [Me4N]Cl led to nido-7 and nido-8 (Scheme 2).
Although the potassium salts were difficult to isolate due to their
hygroscopic character, we were able to characterize them by NMR
spectroscopy. The potassium salts are soluble in water and polar
solvents such as EtOH or DMSO.
very interesting features were observed in the 2.7 to 2.5 ppm
region of their 1H NMR spectra that are assigned to the methylene
protons bonded to the clusters. The loss of symmetry was clearly
evidenced by the observation of AB systems for the diastereotopic
benzyl protons. The coupling constant values varying from 14.9
to 16.8 Hz are consistent with a geminal position. All of these
observations were confirmed by inspection of the 13C NMR spectra
for nido species that show a greater complexity of the system, and
for that reason no attempts to assign the different signals have been
made. However, it is relevant to note that in all cases two different
resonances attributed to the benzylic protons bonded to the cluster
were observed around 42 ppm. The loss of symmetry can be
easily explained by the two possibilities of partial degradation in
C2B10H12 derivatives, either in B(3) or B(6). When the o-carboranyl
fragment is singly bonded to a platform as is the current case, the
partial degradation leads to two enantiomers.
The 11B NMR spectrum of closo-2 shows three broad resonances
centred at -3.55, -5.15 and -9.27 ppm, with the ratio 1 : 1 : 8. closo-
1, closo-3 and closo-4 showed two signals around -2 and -9 ppm
with the ratio 2 : 8. A completely different spectrum was obtained
for nido derivatives that exhibit four to seven resonances, in the
range -6.05 to -35.60 ppm, corroborating the decapitation of the
carborane cage.
Characterization of di- and tetra-carboranyl compounds
1
All compounds were characterized by means of IR, H, 11B and
13C NMR, and UV-vis spectroscopies. The IR spectra of closo-
derivatives exhibit a strong absorption at 2580 cm-1 assigned to
the B–H stretching vibration, while for nido compounds this band
appears around 2516 cm-1 whatever the organic moieties, giving a
first evidence of the effective degradation.
The 1H NMR spectrum of starting 1b shows four resonances at
7.53, 7.47, 7.05 and 6.97 ppm assigned to the aromatic protons,
whereas two singlets at 5.11 and 4.45 ppm are observed in the
aliphatic region attributed to the benzyl ether and benzyl bromide
protons, respectively. Compound 2 exhibits three aromatic signals
at 7.39, 7.31 and 7.29 ppm in the 1H NMR spectrum and only one
for the benzyl bromide protons at 4.44 ppm, whereas 3 presents
only two resonances for the pyridine unit at 7.72 and 7.39 ppm,
and the CH2 protons appear as a singlet at 4.56 ppm. In all
cases, grafting of carborane units was established by a clear shift
of benzyl bromide protons in the starting compound to lower
frequencies: 2.97 ppm for closo-1, 3.69 ppm for closo-2, 3.20 ppm
for closo-4 and 3.04 for closo-3. Compounds containing the 1-Ph-
1,2-C2B10H10 exhibit additional signals in the 6 to 7 ppm range
relative to the aromatic protons. For closo-2 a singlet centred at
2.30 ppm integrating for 12H evidences the grafting of 1-Me-1,2-
closo-C2B10H11 on the organic core. The presence of the clusters
The Ortep plot of closo-3 with the labelling system used is
presented in Fig. 1. closo-3 has a pseudo-twofold axis through
C5–C8. The C1–C2 and C11–C12 bond lengths are 1.707(7) and
˚
1.706(7) A, respectively. All other bonding parameters are normal.
The crystal packing is dominated by very weak BH ◊ ◊ ◊ HB and
BH ◊ ◊ ◊ HC contacts. The supramolecular structure is defined by
two types of intermolecular interactions,21 CH ◊ ◊ ◊ HB between two
parallel displaced columns (see Fig. 2), which are summarized
in Table 1. The methylene groups (atoms C3 and C10) interact
with two different BH carborane fragments within the same
column. Interactions between columns concurrently occur among
the aromatic proton H8 and the H15A and H5A hydrides coming
from two different carboranes of two different molecules, leading
to void channels (Fig. 2b). In the unit cell of closo-3 there are 8
1
in the molecules was also confirmed by 13C{ H} NMR spectra
3
˚
voids each having a volume of 19 A . The amount of the voids is
that show two resonances, between 84 and 82 ppm, assigned to
the Ccluster for phenyl-o-carborane derivatives. A shift of those
signals to higher fields, 78.52 and 76.17 ppm, was observed for
closo-2. After partial degradation, 1H NMR spectra of all of
the nido compounds exhibit a broad singlet, between -1.92 and
-2.62 ppm attributed to the B–H–B bridge proton. Compounds
nido-2 and nido-4 exhibited two very close resonances in the 1.34
and 1.32 ppm region, integrating 6H each that were attributed to
the methyl protons of the cluster, indicating a loss of symmetry.
Other evidence consistent with this symmetry loss is observed
when resonances assigned to H-4/H-2 protons appear at 6.79 and
6.85 ppm for nido-2, and at 6.74 and 6.80 ppm for nido-4. In the
case of the nido compounds bearing phenyl-o-carborane units,
only about 2.5% of the total volume of the unit cell (Fig. 2b). Fig. 3
shows a perspective view of the packing forms by intermolecular
interactions between adjacent molecules from groups of columns
in a zig-zag array.
Photophysical properties of di- and tetra-carboranyl compounds
Emission results at room temperature show that phenyl-o-
carborane derivatives exhibit maximum emission intensities (lem
)
between 369 and 371 nm reasonably independent of the solvent.
Conversely, compounds containing methyl-o-carborane exhibit
a lem in a wide range, 333–363 nm, depending on the solvent
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Fig. 1 Ortep plot of solid state structure of closo-3. Thermal ellipsoids are drawn at 20% level.
Fig. 2 a) A perspective of the packing in the crystal lattice of compound
closo-3, through the crystallographic ab plane (a) and ac plane (b).
(Table 2). It is important to emphasize that isolated initial
carboranyl precursors 1-Ph-1,2-C2B10H11 and 1-Me-1,2-C2B10H11
exhibited no emission at r.t. If lem of closo-1 is compared with
lem of closo-2, a bathochromic shift of 30 nm is observed in THF
Table 2 Fluorescence emission data at 5 ¥ 10-4 M, in solution at room
temperature
lem (nm)
Fig. 3 A perspective of the crystal packing of closo-3 highlighting the
zig-zag array.
Compound
THF
Other solvent
closo-1
closo-2
nido-1
nido-2
nido-3
nido-4
369a
333a
370b
339b
369a
338a
371a (Toluene)
363a (Toluene)
373a (H2O)
and around 10 nm in toluene (Fig. 4). The significant fluorescence
lem solvent dependence of closo-2 and the solvent independence
of closo-1 can be attributed to an interaction of C–H from the
methyl group with the solvent, induced by the strong electron-
withdrawing character of the o-carboranyl moiety.
362b (H2O)
371a (DMSO)
366b (DMSO)
a After excitation at 310 nm. b After excitation at 300 nm.
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Fig. 4 Fluorescence emission for closo-2 (left) and closo-1 (right) in THF.
Table 3 Fluorescence emission data at 5 ¥ 10-4 M, in solution at room
After partial degradation of the carborane cages in closo-1
and closo-2, the emission properties of the corresponding nido
compounds are in the range 338–373 nm, depending on the solvent
(Table 2). Both nido-1 and nido-3 show a weak red shift, 2–
3 nm, comparing their emissions in THF and other solvents. The
same behaviour is observed with nido-2 and nido-4 with a larger
shift, 23 nm and 28 nm, respectively (Table 2). The difference
in the Stoke’s shift for the anionic compounds in different
solvents indicates that interactions between the chromophore
and the medium on the excited states take place. 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.22
In fact, the behaviour of these meta-substituted aryl-ether
derivatives carrying either methyl or phenyl-o-carboranes units
is similar to that found previously for para-substituted aryl-ether
derivatives.18 These results corroborate the low influence of the
organic core configuration on the emission properties of the final
carboranyl-containing compounds.
This minor influence of the aryl-ether substitution on the core
indicates that the photoluminescence is related with the peripheral
part of the molecule bearing the carboranyl fragments bonded to
one benzene ring through a CH2 bridge. This prompted us to
synthesize smaller dicarboranyl-containing molecules similar to
the existing peripheral fragments in closo-1 and closo-2, and nido-
1 to nido-4. In this way, closo-3 was synthesized and its emission
temperature
lem (nm)
Stokes shift (nm)
Compound THF Other solvent
THF Other solvent
closo-3
closo-4
nido-5
nido-6
nido-7
nido-8
371a 370a (Toluene), 373a (CHCl3) 61
60 (toluene)
62 (toluene)
32 (H2O)
89 (H2O)
32 (DMSO)
65 (DMSO)
343a 372a (Toluene), 375a (CHCl3) 33
348b 415 (H2O)c
353a 482 (H2O)d
341b 342a (DMSO)
340a 365b (DMSO)
48
43
41
30
a After excitation at 310 nm. b After excitation at 300 nm. c After excitation
at 383 nm. d After excitation at 393 nm.
bands compared to those of closo-1 (Table 3). As expected, and
according to the results obtained for closo-1, both display very
similar lem whatever the solvent used, with a maximum of emission
around 370 nm (Fig. 5). However, the fluorescence emission
intensity is dependent on the solvent reaching its maximum in
toluene, with lower intensities in CHCl3 and THF, which is similar
to the results described in a previous work.18 If the intensities are
compared with a common solvent, the tetracarboranyl-containing
closo-1 shows a higher intensity than the dicarboranyl-containing
closo-3, probably due to the fact that the former carries twice the
numbers of luminescent units.
To complete this study closo-4 has been synthesized. It shows a
similar structure to closo-3, however in this case the benzene ring
has been replaced by a pyridine ring carrying two carborane units.
We expected that the existence of one N atom in the aromatic ring
Fig. 5 Emission spectra of closo-1 (left) and closo-3 (right) in several solvents.
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could induce some differences in the photoluminescent properties,
as weak hydrogen bonding interactions with the solvent could be
produced.
When measurements are performed in H2O, the results obtained
with nido-5 and nido-6 are very different from those observed in
THF. The excitation wavelengths are red shifted from 300 and
310 nm in THF to 383 and 393 nm in H2O (Table 3), and the
emission bands are located in the 415–482 range (Fig. 7). This red
shift of the excitation and emission bands is in agreement with a
bathochromic effect due to the higher polarity of H2O compared
to THF.
Certainly the behaviour of closo-4 is clearly affected by the
solvent: a) a stronger fluorescence intensity combined with a lower
lem was observed in THF as compared to toluene or CHCl3
(Table 3, Fig. 6); b) it was also observed a 42 nm shift from
343 nm in THF to 385 nm in acetone (Fig. 6). The smaller Stokes
shift observed for closo-4 in THF compared with closo-3 (Table 3)
along with the highest emission intensity (Fig. 6) could be due
to interactions between the pyridine ring and THF. Indeed, since
closo-3 does not exhibit such properties, it can be interpreted that
the pyridine acts as a stronger dipole than the benzene ring in
closo-3, leading to a different behaviour depending on the solvent
polarity.22
Fig. 7 Normalized fluorescence emission spectra of nido-1 (exc 300 nm),
nido-5 (exc 383 nm) and nido-6 (exc 393 nm) in H2O.
Finally nido-5 exhibits a lower Stokes shift in H2O (30 nm) than
in THF (48 nm), whereas nido-6 behaves differently with a Stokes
shift of 89 nm (Table 3). This difference is also observed between
nido-7 and nido-8 in DMSO. In both cases the highest Stokes shift
is observed with the pyridine derivatives, indicating that the dipole
moment of these molecules are higher in the excited state than in
the ground state, providing an increase of the Stokes shifts with
the solvent polarity.22
Fig. 6 Fluorescence emission spectra of closo-4 in different solvents.
As described earlier, the carborane cages in closo-3 and closo-
4 can be partially degraded leading to the corresponding nido
species. The benzene derivatives nido-5 and nido-7 show the same
behaviour in THF as the pyridine derivatives nido-6 and nido-8.
The nido-5 exhibits a maximum emission at 348 nm that shifts
to 341 nm for nido-7, whereas their Stokes shifts follow the same
trend, from 48 to 41 nm (Table 3). Concerning the pyridine species,
nido-6 and nido-8, a 13 nm decrease of lmax and the Stokes shift
is observed (Table 3). For these compounds the only difference is
the cation (K+ or [NMe4]+) associated with the nido cage. The
emission properties in THF of nido derivatives with the same
cation, that is nido-5 and nido-6 or nido-7 and nido-8, are roughly
the same with a band in the 348–353 nm range and 340–341 nm,
respectively (Table 3). In these examples, the larger cation seems to
lead to a lower lmax and a lower Stokes shift. These results indicate
a lower influence of the aromatic units (benzene or pyridine)
when carrying nido cages and a significant effect of the cation
on the fluorescence in THF. Nevertheless, such behaviour is not
observed in the case of more complex structures such as nido-1
and nido-3 or nido-2 and nido-4, because the maximum of emission
remains the same in THF independent of the cation (Table 3). In
these, the participation of the ether units of the aryl-ether core
in weak interactions with the cations could be the reason for
this observation. These interactions would compete with the nido-
carborane/cation interaction, thus masking the cation influenced
fluorescence observed for nido-5 to nido-8.
Conclusions
A new family of neutral and anionic di-carboranyl and tetra-
carboranyl derivatives have been synthesized and characterized.
The carboranyl fragments are bonded through CH2 units on
different organic moieties, and their influence on the photolumi-
nescent properties of the final molecules has been studied. All the
closo- and nido-carborane derivatives exhibit a blue emission under
ultraviolet excitation at room temperature in different solvents.
The fluorescence studies in these closo-derivatives indicate that
the maximum of emissions depend on several factors such as: the
substituent (Ph or Me) bonded to the Ccluster, the solvent polarity,
and the organic unit bearing the carborane clusters (benzene or
pyridine). In the case of nido-derivatives, an important effect of
the cation is also observed. Nevertheless, few differences on the
maxima emission bands are observed between tetra-carboranyl
closo and nido-species, whereas a major influence of the cluster
nature is observed for di-carboranyl compounds. This last effect
is attributed to the participation of the ether units of the aryl-
ether core in weak interactions with the cations. It is important
to notice that the common fragment in all these compounds is
the CH2 bridge between the carborane cage and the aromatic ring,
which seems to prevent p–p stacking. In our opinion this is the key
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of the photoluminescence properties especially since compounds
without this moiety do not display such a feature.
yellow solution was cooled to 0 ◦C and a suspension of 1b (200 mg,
0.30 mmol) in a dry toluene/diethyl ether (2 : 1) mixture (6 mL) was
added. After refluxing for 24 h, the orange mixture was quenched
with 15 mL of water and transferred to a separatory funnel. The
layers were separated and the aqueous layer was extracted with
additional diethyl ether (2 ¥ 6 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 hexane, after
filtration the suspension gave 224 mg (0.18 mmol, yield 61%) of a
yellow solid. 1H NMR ((CD3)2CO): 7.67 (d, 8H, 3Jortho = 7.32 Hz),
7.44 (m, 16H), 6.33 (s, 4H), 5.88 (s, 2H), 4.98 (s, 4H), 2.97 (s, 8H).
13C NMR ((CD3)2CO): 158.44, 137.41, 136.84, 131.51, 131.22,
130.35, 129.33, 128.77, 126.95, 126.37, 124.36, 116.20, 84.00,
82.40, 69.51, 40.31. 11B NMR ((CD3)2CO): -1.51 (s, 2B), -8.09 (s,
8B). IR (KBr, cm-1): 3062 n(C–Haryl), 2931 n(C–Halky), 2584 n(B–
H), 1596, 1072. Elemental Analysis Calcd (%) for C56H82B40O2: C
55.10, H 6.72; Found: C 55.53; H 6.86.
Experimental
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
1
1
(300.13 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 solutions at 25 C. Chemical shift values for 11B{ H}
1
NMR spectra were referenced to external BF3·OEt2, and those for
1H and 13C{ H} NMR were referenced to SiMe4. Chemical shifts
1
are reported in units of parts per million downfield from reference,
and all coupling constants are reported in Hertz. Fluorescence
spectra were measured on a Cary Eclipse Fluorescence Spec-
trophotometer, using solutions of compounds at 5 ¥ 10-4 mol L-1.
All reactions were performed under an atmosphere of dinitrogen
employing standard Schlenk techniques. THF, Et2O and toluene
were purchased from Merck and distilled from sodium benzophe-
none prior to use, and CH3CN was 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 1a19 and 2,6-bis(bromomethyl)pyridine
(3)20 were synthesized according to the literature. n-BuLi solution
(1.6 M in hexane) was purchased from Lancaster or Aldrich. Start-
ing materials: a,a,¢-dibromo-m-xylene (2); 3,5-dihydroxybenzoic
acid; 1,3-dihydroxymethyl-pyridine, triphenylphosphine; carbon
tetrabromide; potassium carbonate; lithium hydride and phos-
phorous tetrabromide were commercially available from Aldrich
and used as received.
Synthesis of closo-2
To a solution of methyl-o-carborane (300 mg, 1.89 mmol) in 3 mL
of dry diethyl ether at 0 ◦C, was added a 1.6 M solution of n-BuLi
in hexane (1.35 mL, 2.08 mmol). When addition was completed,
the mixture was stirred 45 min at room temperature. The light
yellow solution was cooled to 0 ◦C and a suspension of 1b (299 mg,
0.45 mmol) in a dry toluene/diethyl ether (2 : 1) mixture (9 mL) was
added. After refluxing for 16 h, the yellow mixture was quenched
with 15 mL of water and transferred to a separatory funnel. The
layers were separated and the aqueous layer 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 hexane, after
filtration the white suspension gave 295 mg (0.30 mmol, yield 70%)
1
Synthesis of a,a¢-bis[3,5-bis(bromomethyl)phenoxy]-m-xylene (1b)
of a light yellow solid. H NMR ((CD3)2CO): 7.60 (s, 1H), 7.46
(s, 3H); 6.98 (s, 4H), 6.90 (s, 2H), 5.22 (s, 4H), 3.69 (s, 8H), 2.30
(s, 12H). 13C NMR ((CD3)2CO): 158.77, 137.45, 137.38, 128.74,
127.07, 126.64, 125.05, 116.68, 78.52, 76.17, 69.67, 40.26, 22.99.
11B NMR ((CD3)2CO): -3.55 (s, 1B), -5.15 (s, 1B), -9.27 (s, 8B). IR
(KBr, cm-1): 2943 n(C–Halkyl), 2584 n(B–H), 1596, 1054. Elemental
Analysis Calcd (%) for C36H74B40O2: C 44.51, H 7.62; Found: C
45.02; H 7.42.
To a solution of 1a (3.435 g, 8.4 mmol) and CBr4(16.71 g,
50.2 mmol) in dry THF was added PPh3 (22.55 g, 86.05 mmol)
in two portions. The reaction mixture was stirred at room
temperature under N2 for 16 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 ¥ 100 mL). The organic phases
were collected and concentrated under vacuum. The residue was
passed through a short column of silica gel, washed with hexane
and eluted with a 85 : 15 hexane : ethyl acetate mixture to give 1b
as a◦white powder (3.023 g, 4.56 mmol) yield 55.0%, mp 139–
143 C. IR (KBr, cm-1): 3445, 2883, 1722, 1594, 1445, 1334, 1300,
1211, 1178, 1040, 697, 551. MS, m/z (%) [M+, 662 (1)]: 583 (3), 385
(50), 383 (100), 381 (49), 303 (15), 223 (15), 183 (30), 104 (27). 1H
NMR (270 MHz, CDCl3): 7.53 (1H, H-6, s), 7.47 (3H, H-2,H-3,
H-4, s), 7.05 (2H, H-4¢, s), 6.97 (4H, H-2¢, s), 5.11 (4H, CH2O, s),
4.45 (8H, CH2Br, s). 13C NMR (67.94 MHz, CDCl3): 159.3 (C-1¢),
139.6 (C-1), 137.1 (C-3¢), 129.2 (C-3), 127.5 (C-2), 126.8 (C-6),
122.4 (C-4¢), 115.7 (C-2¢), 70.1 (CH2O), 33.0 (CH2Br).
Synthesis of closo-3
To a solution of phenyl-o-carborane (369 mg, 1.66 mmol) in 4 mL
of dry THF at 0 ◦C, was added a 1.6 M solution of n-BuLi in
hexane (1.33 mL, 2.00 mmol). When addition was completed, the
mixture was stirred 45 min at room temperature. The light red
◦
solution was cooled to 0 C and a solution of a,a,¢-dibromo-m-
xylene (2) (200 mg, 0.75 mmol) in dry THF (4 mL) was added.
After refluxing for 18 h, THF was distilled off to give a dark orange
residue. The solid was quenched with 30 mL of water and 15 mL
of ether were added, the resulting mixture was transferred to a
separatory funnel. The layers were separated and the aqueous
layer was extracted with additional diethyl ether (2 ¥ 6 mL).
The combined filtrates were dried over anhydrous MgSO4 and
concentrated in vacuo to give a yellow residue. Recrystallization in
acetone gave closo-3 as white crystals (316 mg, 0.58 mmol, yield
Synthesis of closo-1
To a solution of phenyl-o-carborane (294 mg, 1.33 mmol) in 3 mL
of dry diethyl ether at 0 ◦C, was added a 1.6 M solution of n-BuLi
in hexane (0.98 mL, 1.46 mmol). When addition was completed,
the mixture was stirred 45 min at room temperature. The light
1
3
75%). H NMR (CDCl3): 7.71 (d, 4H, Jortho 7.3 Hz), 7.49 (m,
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 7541–7550 | 7547
©

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6H), 7.12 (dd, 1H, 3Jortho 7.7 Hz), 6.71 (d, 2H, 3Jortho 7.7 Hz), 6.29
(s, 1H), 3.04 (s, 4H). 13C NMR (CDCl3): 136.32, 132.58, 132.44,
131.88, 131.72, 130.47, 130.03, 129.36, 84.53, 82.64, 41.78. 11B
NMR (CDCl3): -2.66 (s, 2B), -9.29 (s, 8B). IR (KBr, cm-1): 3060
n(C–Haryl), 2931 n(C–Halky), 2580 n(B–H), 1596, 1072. Elemental
Analysis Calcd (%) for C24H38B20: C 53.06; H 7.00; Found: C 53.23;
H 7.18.
68.14, 66.29, 62.24, 55.20, 42.83, 41.97. 11B NMR ((CD3)2CO):
-6.05 (s, 2B), -14.55 (s, 5B), -31.76 (s, 1B), -34.21 (s, 1B). IR
(KBr, cm-1): 3031 n(C–Haryl), 2920 n(C–Halky), 2518 n(B–H), 1591,
1483, 1029. Elemental Analysis Calcd (%) for C72H130B36N4O2: C
58.65 H 8.82; N 3.80. Found: C, 58.11; H, 8.82; N, 3.54.
Synthesis of nido-2
The degradation procedure was the same as for nido-1, using closo-
2 (100 mg, 0.10 mmol) and KOH (46 mg 0.82 mmol). Compound
nido-2 was isolated as a white solid. Yield: 95 mg, 88%.1H NMR
((CD3)2CO): 7.71 (s, 1H), 7.49 (d, 2H, 3Jortho 7.3 Hz), 7.43 (dd, 1H,
3Jortho 7.3 Hz), 7.02 (s, 4H), 6.85 (s, 1H), 6.79 (s, 1H), 5.18 (s, 4H),
3.63 (t, 8H, THF), 3.06 (s, 8H), 1.79 (t, 8H, THF), 1.34 (s, 6H),
1.32 (s, 6H), -2.28 (s, 4H). 13C NMR ((CD3)2CO): 158.57, 144.10,
138.09, 128.44, 127.25, 123.90, 122.58, 112.15, 69.50, 68.07, 61.96,
54.11, 42.80, 42.70, 26.15, 22.51. 11B NMR ((CD3)2CO): -6.05 (s,
1B), -7.19 (s, 2B), -13.38 (s, 2B), -16.41 (s, 1B), -17.38 (s, 1B),
-31.74 (s, 1B), -34.20 (s, 1B). IR (KBr, cm-1): 3033 n(C–Haryl), 2931
n(C–Halkyl), 2518 n(B–H), 1627, 1404, 1008. Elemental Analysis
Calcd (%) for C36H74B36K4O2·2(C4H8O): C 43.03; H 7.33. Found:
C 42.75; H 7.52.
Synthesis of closo-4
Using the same procedure as described for closo-3, using phenyl-
o-carborane (368, 1.66 mmol), n-Buli (1.26 mL, 2.00 mmol) and 3
(200 mg, 0.75 mmol), was obtained closo-4 as a light yellow solid
(320 mg, 0.58 mmol, yield 78%). 1H NMR (CDCl3): 7.76 (d, 4H,
3Jortho 7.3 Hz), 7.46 (m, 7H), 6.75 (d, 2H, Jortho 7.7 Hz), 3.20 (s,
3
4H). 13C NMR (CDCl3): 155.86, 137.75, 132.49, 131.81, 131.72,
129.96, 124.45, 84.68, 81.47, 43.74. 11B NMR (CDCl3): -3.65 (s,
2B), -10.09 (s, 8B). IR (KBr, cm-1): 3060 n(C–Haryl), 2931 n(C–
Halky), 2581n(B–H), 1593, 1065. Elemental Analysis Calcd (%) for
C23H37B20N: C 50.76; H 6.80; Found: C 51.02; H 6.92.
Synthesis of nido-1
closo-1 (100 mg, 0.082 mmol) was added to a solution of KOH
(40 mg, 0.656 mmol) in 4 mL of deoxygenated EtOH. The white
suspension was stirred for 1 h at room temperature and refluxed
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 to dryness to give nido-1 as a
Synthesis of nido-4
The procedure was the same as for nido-3, using closo-2. Yield:
1
119 mg, 95%. H NMR ((CD3)2CO): 7.73 (s, 1H), 7.58 (d, 2H,
3Jortho 7.5 Hz), 7.47 (dd, 1H, 3Jortho 7.5 Hz), 7.02 (s, 4H), 6.80 (s, 1H),
6.74 (s, 1H), 5.20 (s, 4H), 3.36 (s, 48H), 3.09 (s, 8H), 1.34 (s, 6H),
1.33 (s, 6H), -2.41 (s, 4H). 13C NMR ((CD3)2CO): 159.18, 144.87,
138.18, 129.41, 126.43, 124.04, 123.53, 113.42, 70.26, 55.82, 43.37,
43.24, 23.88. 11B NMR ((CD3)2CO): -5.92 (s, 1B), -7.06 (s, 2B),
-13.52 (s, 2B), -17.43 (s, 2B), -31.81 (s, 1B), -34.17 (s, 1B). IR
(KBr, cm-1): 3033 n(C–Haryl), 2929 n(C–Halkyl), 2515 n(B–H), 1589,
1483, 1026. Elemental Analysis Calcd (%) for C52H122B36N4O2: C
50.95; H 9.96; N 4.57. Found: C, 51.25; H, 10.13; N, 4.40.
1
white solid. Yield: 95 mg, 87%. H NMR ((CD3)2CO): 7.47 (s,
1H), 7.40 (m, 3H), 7.10 (m, 8H), 7.01 (m, 12H), 6.20 (s, 1H),
6.17 (s, 1H), 5.87 (s, 4H), 3.63 (t, 16H, THF), 4.89 (s, 4H), 2.61
2
2
(d, 2H, Jgem 16.6 Hz), 2.57 (d, 2H, Jgem 16.6 Hz), 2.52 (d, 2H,
2Jgem 16.6 Hz), 2.47 (d, 2H, Jgem 16.6 Hz), 1.79 (t, 16H, THF),
2
-2.26 (s, 4H). 13C NMR ((CD3)2CO): 157.16, 141.93, 138.08,
Synthesis of nido-5
132.22, 129.04, 127.84, 127.27, 127.05, 126.57, 125.96, 124.95,
123.74, 112.98, 69.24, 68.07, 67.29, 63.14, 42.75, 42.24, 26.15. 11
B
The degradation procedure was the same as for nido-1, using closo-
3 (100 mg, 0.18 mmol) and KOH (40 mg 0.72 mmol). Compound
nido-5 was isolated as a white solid. Yield: 90 mg, 84%. 1H NMR
((CD3)2CO): 7.22 (s, 4H), 6.94 (m, 7H), 6.85 (s, 2H), 5.99 (d, 1H),
NMR ((CD3)2CO): -6.47 (s, 2B), -14.35 (s, 5B), -30.36 (s, 1B),
-33.41 (s, 1B). IR (KBr, cm-1): 3031 n(C–Haryl), 2931 n(C–Halkyl),
2522 n(B–H), 1658, 1407, 1006. Elemental Analysis Calcd (%) for
C56H82B36K4O2·4(C4H8O): C 53.34; H 7.03. Found: C 54.29; H
6.59.
2
2
3.63 (t, 8H, THF), 2.59 (d, 1H, Jgem 14.9 Hz), 2.57 (d, 1H, Jgem
14.9 Hz), 2.46 (d, 1H, 2Jgem 14.9 Hz), 2.45 (d, 1H, 2Jgem 14.9 Hz),
1.79 (t, 8H, THF), -1.95 (s, 2H). 13C NMR ((CD3)2CO): 144.62,
143.77, 141.96, 141.22, 135.33, 135.02, 133.75, 132.84, 129.12,
128.52, 127.98, 127.47, 127.09, 126.84, 126.01, 69.18, 68.07, 65.22,
44.96, 43.21, 26.15. 11B NMR ((CD3)2CO): -7.81 (s, 1B), -8.82 (s,
1B), -12.44 (s, 1B), -16.44 (s, 2B), -17.85 (s, 2B), -32.93 (s, 1B),
-35.58 (s, 1B). IR (KBr, cm-1) 3055 n(C–Haryl), 2923 n(C–Halky),
2518 n(B–H), 1753, 1602, 1442. Elemental Analysis Calcd (%) for
C24H38B18K2·2(C4H8O): C 51.30; H 7.30. Found: C 50.92; H 7.45.
Synthesis of nido-3
Elaboration of nido-3 followed the same experimental route as
nido-1, until the mixture was cooled to r.t. EtOH was evaporated
to give a white residue which was dissolved in water. Addition of
an excess of Me4NCl in water solution gave a precipitate. After
filtration the solid was dried in vacuo to give nido-3 as a white
powder. Yield: 113 mg, 94%. 1H NMR ((CD3)2CO): 7.65 (s, 1H),
3
7.51 (m, 3H), 7.25 (d, 8H, Jortho 7.5 Hz), 7.01 (m, 12H), 6.52
Synthesis of nido-6
(s, 4H), 5.93 (s, 1H), 5.89 (s, 1H), 5.04 (s, 4H), 3.34 (s, 48 H),
2
2
2.61 (d, 2H, Jgem 14.9 Hz), 2.57 (d, 2H, Jgem 14.9 Hz), 2.52 (d,
The degradation procedure was the same as for nido-1, using closo-
4 (100 mg, 0.18 mmol) and KOH (40 mg 0.72 mmol). Compound
nido-6 was isolated as a white solid. Yield: 100 mg, 91%.1H NMR
((CD3)2CO): 7.49 (t, 1H, 3J 7.7 Hz), 7.37 (d, 1H, 3J 7.7 Hz), 7.31
2
2
2H, Jgem 14.9 Hz), 2.47 (d, 2H, Jgem 14.9 Hz), -2.13 (s, 4H).
13C NMR ((CD3)2CO): 159.36, 143.13, 139.01, 133.52, 128.10,
127.74, 127.27, 127.05, 126.57, 125.96, 124.85, 124.11, 113.05,
7548 | Dalton Trans., 2011, 40, 7541–7550
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©

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(d, 1H, 3J 7.7 Hz), 7.22 (m, 4H), 6.82 (m, 6H), 3.63 (t, 4H, THF),
2.80 (d, 1H, 2Jgem 16.8 Hz), 2.75 (d, 1H, 2Jgem 16.8 Hz), 2.61 (d, 1H,
Acknowledgements
2
2Jgem 16.8 Hz), 2.55 (d, 1H, Jgem 16.8 Hz), 1.79 (t, 4H, THF),
This work has been supported by Generalitat de Catalunya
(2009/SGR/00279), Ministerio de Ciencia Innovacio´n
(CTQ2010-16237), Mobility Program UNAM-CSIC and
CONACYT-Me´xico. A. Ferrer-Ugalde thanks the AGAUR
(Generalitat de Catalunya) for a FPI grant.
-1.92 (s, 2H). 13C NMR ((CD3)2CO): 160.35, 159.95, 141.99,
134.34, 132.25, 132.16, 126.34, 126.33, 125.19, 125.07, 120.18,
68.07, 67.17, 65.20, 44.98, 44.81, 26.15. 11B NMR ((CD3)2CO):
-7.97 (2B), -12.23 (1B), -14.20 (1B), -17,50 (3B), -32.53 (1B),
-35.25 (1B). IR (KBr, cm-1): 3056 n(C–Haryl), 2927 n(C–Halky),
2518 n(B–H), 1627, 1400, 1006. Elemental Analysis Calcd (%) for
C23H37B18K2N·(C4H8O): C 48.25; H 6.69; N 2.08. Found: C 48.24;
H 6.49; N 2.32.
e
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2.57 (d, 1H, Jgem 14.9 Hz), 2.48 (d, 1H, Jgem 14.9 Hz), 2.47 (d,
1H, 2Jgem 14.9 Hz), -1.98 (s, 2H). 13C NMR ((CD3)2CO): 143.53,
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The procedure was the same as for nido-3, using closo-4. Yield:
1
3
116 mg, 95%. H NMR ((CD3)2CO): 7.27 (t, 1H, J 7.7 Hz),
3
3
7.07 (d, 1H, J 7.7 Hz), 7.01 (d, 1H, J 7.7 Hz), 6.67 (m, 4H),
2
6.42 (m, 6H), 2.82 (s, 24H), 2.75 (d, 1H, Jgem 16.8 Hz), 2.65 (d,
2
2
2
1H, Jgem 16.8 Hz), 2.54 (d, 1H, Jgem 16.8 Hz), 2.43 (d, 1H, Jgem
16.8 Hz), -2.62 (s, 2H). 13C NMR ((CD3)2CO): 159.53, 158.92,
141.90, 132.06, 125.23, 124.09, 120.22, 67.17, 65.20, 55.64, 43.82,
42.91. 11B NMR ((CD3)2CO): -8.67 (2B), -13.73 (1B), -15.20 (1B),
-18.00 (3B), -32.97 (1B), -35.58 (1B) IR (KBr, cm-1): 3044 n(C–
Haryl), 2926 n(C–Halkyl), 2518 n(B–H), 1634, 1412, 946. Elemental
Analysis Calcd (%) for C31H61B18N3: C 55.48; H 9.10; N 6.26.
Found: C 54.70; H 9.15; N 6.10.
X-ray crystallographic study of closo-3
The crystals were obtained in acetone. Crystallographic data were
collected at 173 K with a Nonius-Kappa CCD area detector
diffractometer, using graphite-monochromatized Mo-Ka radia-
˚
tion (l = 0.71073 A). The data sets were corrected for absorption
using SADABS.23a The structure was solved by direct methods by
use of the SHELXS-97 program.23b The full-matrix, least-squares
refinements on F2 were performed using SHELXL-97 program.23b
The CH and BH hydrogen atoms were included at fixed dis-
tances with the fixed displacement parameters from their host
atoms.
Crystal data for closo-3: C24H38B20, Mr = 542.74, monoclinic,
˚
˚
˚
C2/c, a = 33.7783(14) A, b = 7.6998(2) A, c = 26.8231(13) A, b =
116.055(3), V = 6267.3(4) A , Z = 8, rcalc = 1.150 g cm-3, m(Mo-
3
˚
Ka) = 0.055 mm-1, F(000) = 2256, qmax = 25.25◦, 10454 reflections,
5650 independent reflections (Rint = 0.104), R1 = 0.125, wR2 = 0.298
for 397 parameters with reflections I > 2s(I). CCDC 802843.†
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 7541–7550 | 7549
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7550 | Dalton Trans., 2011, 40, 7541–7550
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