Synthesis of luminescent ethynyl-extended regioisomers of borate complexes based on 2-(2′-hydroxyphenyl)benzoxazole

Article abstract of DOI:10.1002/chem.201203625

A series of thirteen luminescent tetrahedral borate complexes based on the 2-(2′-hydroxyphenyl)benzoxazole (HBO) core is presented. Their synthesis includes the incorporation of an ethynyl fragment by Sonogashira cross-coupling reaction, with the goal of

Full text of DOI:10.1002/chem.201203625

FULL PAPER
DOI: 10.1002/chem.201203625
Synthesis of Luminescent Ethynyl-Extended Regioisomers of Borate
Complexes Based on 2-(2’-Hydroxyphenyl)benzoxazole
Julien Massue,*[a] Denis Frath,[a] Pascal Retailleau,[b] Gilles Ulrich,*[a] and
Raymond Ziessel*[a]
Abstract: A series of thirteen lumines-
cent tetrahedral borate complexes
based on the 2-(2’-hydroxyphenyl)-
solid state. In all cases, complexation to
boron leads to a donor–acceptor char-
acter that impacts their photophysical
properties. Complexes with a 3- or 5-
substituted fragment display mild to
pronounced internal charge transfer
(ICT), a feature strengthened by the
trary, when the ethynyl module is graft-
ed on the 4-position, narrow, struc-
tured, symmetrical absorption/emission
bands are observed. Moreover, the fact
that protonation has little effect on the
emission maximum wavelength reveals
singlet excited-state decay. Solid-state
emission properties reveal a redshift
compared to solution, explained by
tight packing of the p-conjugated sys-
tems and the high planarity of the dyes.
Subsequent connection of these com-
plexes to other photoactive subunits
(BODIPY, Boranil) provides dyads in
which efficient cascade energy transfer
is observed.
ACHTUNGTRENNUNGbenzACHTUNGTRENNUNGoxazole (HBO) core is presented.
Their synthesis includes the incorpora-
tion of an ethynyl fragment by Sonoga-
shira cross-coupling reaction, with the
goal of extending the conjugation and
consequently redshifting their emission
wavelength. Different regioisomers,
substituted in the 3-, 4-, or 5-position
of the phenolate side of the HBO core,
were studied in order to compare their
photophysical properties. The com-
plexes were characterized by X-ray dif-
fraction and NMR, UV/Vis, and emis-
sion spectroscopy in solution and in the
presence of p-dibutylaminophenyl
ACHTUNGTNERaNUNG cet-
AHCTUNGTRENNUNG
nation of the nitrogen atom of which
leads to a significant blueshift and an
increase in quantum yield. On the con-
Keywords: alkynes · borates · dyes/
pigments · heterocycles · lumines-
cence
Introduction
to microchanges in their environment, they have found in-
terest as sensors for biological materials, toxins, and wastes[3]
and in bioconjugation to proteins,[4] oligosaccharides,[5] syn-
thetic polymers,[6] nanoparticles,[7] and carbon nanotubes.[8]
Coumarins,[9] fluoresceins,[10] cyanines,[11] Rhodamines,[12]
squaraines,[13] and boron dipyrromethene (BODIPY)
dyes[3b,14] are currently among the most popular families of
dyes. They have gained their tremendous popularity because
they meet most of the requirements for organic dyes: sharp
and narrow absorption and fluorescence emission spectra,
chemical and photostability, high absorption coefficients,
high quantum yields, and good solubility. Moreover, there is
the possibility to fine-tune their maximum emission wave-
length by introducing functional groups and/or extending
the conjugation around the fluorescent core. The drawback
of these dyes is that they often exhibit small Stokes shifts,
especially in the case of BODIPY dyes, due to minimal reor-
ganization of the molecules on photoexcitation. One strat-
egy to increase the Stokes shifts is to desymmetrize the mo-
lecular structure of the boron-containing dye. Unlike the di-
pyrrin core present in BODIPY, unsymmetrical structures
would indeed be more prone to have energetically separated
ground and excited states and therefore larger Stokes
shifts.[15]
Organic fluorescent dyes have become indispensable tools
for chemists and biologists in a broad range of technologies
ranging from imaging, medical diagnosis, and photodynamic
therapy[1] to organic electronics and solar-energy conver-
sion.[2] For example, their unique ability to absorb photons
of light at a given wavelength and to re-emit lower-energy
light has proved to be extremely useful in medical imaging,
where these dyes are commonly used to help visualize vari-
ous biological events occurring in organs, tissues, or cells.
Owing to fine-tuning of their luminescence properties due
[a] Dr. J. Massue, D. Frath, Dr. G. Ulrich, Dr. R. Ziessel
Laboratoire de Chimie Molꢀculaire et
Spectroscopies Avancꢀes (LCOSA)
ICEEPS–LCOSA, UMR CNRS 7515
ꢁcole Europꢀenne de Chimie, Polymꢂres et Matꢀriaux (ECPM)
25 rue Becquerel, 67087 Strasbourg Cedex 02 (France)
Fax : (+33)3-68-85-27-6
E-mail: massue@unistra.fr
gulrich@unistra.fr
ziessel@unistra.fr
Homepage: http://www-lmspc.u-strasbg.fr/lcosa
[b] P. Retailleau
Laboratoire de Cristallochimie, ICSN–CNRS
Bꢃt 27-1 avenue de la Terrasse, 91198 Gif-sur-Yvette, Cedex (France)
There is increasing interest in solid-state emitters, which
are key materials in a wide range of technologies including
luminescent sensors, telecommunications, and lasers.[16]
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203625.
Chem. Eur. J. 2013, 19, 5375 – 5386
ꢄ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5375

Solid-state luminescence is usually observed for dyes that
are decorated with peripheral bulky substituents to prevent
concentration annihilation caused by intermolecular interac-
tions.[17] The search for new bright, stable, scalable, and red-
emitting dyes being endless due to the growing demand for
novel structures, it is imperative
ferent ways to modulate their optical and electronic proper-
ties have been explored.
The structural and photophysical properties of different
families of boron complexes have been reported in the liter-
ature, among which neutral, coordinately saturated, N,N (ex-
to design and investigate the
properties of other types of flu-
orescent dyes with original mo-
lecular structures and improved
photophysical properties, both
in solution and in the solid
state. Coordination of a p-con-
jugated ligand to a boronACTHNUTRGNE(UNG III)
center (e.g., BF2, BPh2) rigidi-
fies the molecule, which leads
to high fluorescence yields with
larger Stokes shifts than ob-
served in BODIPY dyes.[18] Dif-
Scheme 1. a) 2-(2’-Hydroxyphenyl)benzoxazole borate complex. b) General structure of the compounds stud-
ied in this work.
cluding BODIPYs),[15,19] N,N,O,O tetradentate,[20] O,N,O tri-
Abstract in French: Cet article prꢀsente une sꢀrie de treize
complexes de bore tꢀtraꢀdriques luminescents basꢀs sur le
motif 2-(2’-hydroxyphenyl)benzoxazole (HBO). Leur syn-
dentate,[21] and N,O bidentate ligands have been described.
The N,O chelating ligands are by far the most studied p-con-
jugated ligands, including a wide range of structures with
single or multiple boron centers, mainly as hydroxyquino-
late,[22] pyrene,[23] perylenetetracarboxylic diimides (PDIs),[24]
and Schiff bases,[25] but some more exotic structures such as
iminoketone[26] and azaoxobenzazulene[27] have also been re-
ported.
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
dꢂonde dꢂꢀmission dans le rouge (basses longueur dꢂonde).
Diffꢀrents rꢀgioisomꢁres, substituꢀs en position 3, 4 ou 5 du
cꢃtꢀ phꢀnol du groupement HBO ont ꢀtꢀ ꢀtudiꢀs afin de com-
parer leurs propriꢀtꢀs spectroscopiques. Les complexes ont
ꢀtꢀ caractꢀrisꢀs par RMN, diffraction des rayons X sur mono-
crystal, spectroscopie UV-Visible et ꢀmission ꢄ la fois en solu-
tion et ꢄ lꢂꢀtat solide. Pour chaque composꢀ, la complexation
au bore entraine un caractꢁre donneur-accepteur de la molꢀ-
cule, ce va qui fortement influer sur les propriꢀtꢀs photophy-
siques. Les complexes fonctionnalisꢀs en position 3 et 5 prꢀ-
sentent un transfert de charge interne (TCI) qui est amplifiꢀ
Following a pioneer paper,[28] we recently presented the
synthesis of 2-(2’-hydroxyphenyl)benzoxazole (HBO) deriv-
atives coordinated to a BF2 fragment (Scheme 1a).[29] These
dyes are very stable in solution in nonpolar solvents and
show bright fluorescence and improved Stokes shifts. Their
emission is mostly located in the UV part of the spectrum,
with maximum emission wavelengths ranging from 385 to
425 nm in toluene. However, these high-energy emissions
impede all kind of in vivo applications due to the high ab-
sorption of biological backgrounds in the UV region.
Furthermore, unsubstituted HBO/HBT borate complex
dyes are poorly soluble in polar solvents such as water, alco-
hols, and DMSO, in which they also undergo boron decom-
plexation. A few requirements must be fulfilled to improve
the stability and redshift the emission of the dyes: 1) in-
crease the electronic density on the phenolic side of the
HBO core, as a prerequisite for improving the chemical sta-
par la prꢀsence du groupement dibutylaminophꢀnACTHNUTRGENNUyG lACHUTTGNRNENUGacetylꢁne
dans la structure des molꢀcules; la protonation de lꢂatome
dꢂazote par de lꢂHCl gazeux entraine un dꢀplacement de la
longueur dꢂonde dꢂꢀmission vers le bleu (longueur dꢂonde
plus ꢀlevꢀe) et une augmentation du rendement quantique. A
lꢂopposꢀ, lorsque lꢂespaceur ꢀthynyle est greffꢀ en position 4,
des bandes dꢂabsorption/ꢀmission
triques sont observꢀes. Par ailleurs, la protonation nꢂa que
peu dꢂeffet sur le maximum de la longueur dꢂonde
ACHTUNGTRENNUNGdꢂꢀmission, mettant en ꢀvidence une dꢀcroissance de lꢂꢀtat
ACHTUNGTNERꢀNUNG troites, structurꢀes et sym-
ACHTUNGTRENNUNGꢀACHTUNGTRENNUNG
ꢀ
bility in polar, protic solvents, in which the B O bond is
prone to hydrolysis; 2) extension of the conjugation to in-
crease the delocalization and therefore lower the LUMO
level of the molecule and consequently bathochromically
shift the fluorescence emission. A few recent papers discuss
the direct influence of dialkyl- and diarylamino groups at
the 5- and 4-positions of the HBO core.[30] However, the
photophysical effects of further extending the conjugation
and of substituents at the 3-position of the HBO core have
not been studied.
excitꢀ de type singulet. Lꢂꢀtude des propriꢀtꢀs dꢂꢀmission ꢄ
lꢂꢀtat solide rꢀvꢁlent un dꢀplacement vers le rouge en compa-
raison des propriꢀtꢀs en solution, qui est expliquꢀ par les em-
pilements importants des systꢁmes conjuguꢀs et la grande pla-
nꢀitꢀ des colorants. La connexion de ces complexes ꢄ dꢂautres
sous-unitꢀs photoactives (BODIPY, Boranil) permet dꢂobte-
nir des dyades oꢅ un transfert dꢂꢀnergie en cascade est obser-
vꢀ.
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Borate Complexes Based on 2-(2’-Hydroxyphenyl)benzoxazole
FULL PAPER
Here we report on our synthetic efforts to redshift the
emission wavelength and improve both the Stokes shifts and
the stability of HBO borate complexes. Our goal was to pre-
pare and characterize three different families of regioiso-
in refluxing ethanol to yield a cyclic carbinolamine, which
underwent oxidative cyclization with 2,3-dichloro-5,6-dicya-
no-1,4-benzoquinone (DDQ) in dichloromethane at room
temperature. Purification by column chromatography af-
forded 5-iodo HBOs 3 and 4 in 87 and 63% yield respec-
tively. These compounds exhibit strong blue-green fluores-
cence in the solid state due to an excited-state intramolecu-
lar proton transfer (ESIPT),[32] as evidenced by a distinctive
ACHTUNGTRENNUNGmers with various electron-donating or electron-withdrawing
groups in order to compare their photophysical features.
Synthesis of the dyes includes connection of a decorated
phenylacetylene fragment to the phenolic side of the HBO
core in the 3-, 4-, or 5-position (Scheme 1b). All compounds
of interest were prepared by Pd-catalyzed Sonogashira
cross-coupling reactions with an appropriate acetylene frag-
ment.
1
downfield H NMR signal for the strongly H-bonded phe-
nolic proton around 11 ppm. Compounds 3 and 4 were then
subjected to Sonogashira cross-coupling reaction conditions.
The optimal protocol was found to involve the use of 5%
The influence of 1) the electronic nature of the extended
fragment and 2) its position on the phenol ring of the HBO
core (3, 4, or 5) on the photophysical properties were scruti-
nized. Finally, we showed that Sonogashira cross-coupling
reactions allow the attachment of these new dyes to photo-
active subunits such as BODIPY or Boranil[31] cores to give
sophisticated molecular cassettes.
[PdCl2ACHTUNGTRENNU(G PPh3)2] catalyst and 10% copper iodide in the pres-
ence of triethylamine as base in toluene at room tempera-
ture overnight. These reaction conditions allowed facile in-
troduction of various acetylene modules at the 5-position of
the phenolic side of the HBO core with yields ranging from
29 to 93% for 5–10 (Scheme 2). Introduction of a methoxy
group at the 4-position of the HBO core, as in 9 and 10, was
carried out with the aim of further increasing the electron
density on the phenolic side of the HBO and hence the sta-
bility of the corresponding HBO borate complexes. Drop-
wise addition of BF3·Et2O to a solution of HBO ligands 5–
10 in anhydrous dichloromethane at room temperature in
the presence of N,N-diisopropylethylamine (DIEA) as base
allowed straightforward boron complexation at the N,O che-
lating site. Pure complexes 11–16 were obtained after purifi-
cation by filtration through a basic Al2O3 column, eluted
Results and Discussion
Synthetic strategies and preparation of the 5-, 4-, and 3-sub-
stituted regioisomers of 2-(2’-hydroxyphenyl)benzoxazole
(HBO) borate complexes are summarized in Schemes 2–4.
Selective mono-iodination of commercially available sali-
cylaldehyde and 4-methoxysalicylaldehyde with an excess of
iodine monochloride in acetic acid leads to 5-iodosalicylal-
dehydes 1 and 2 in good yields (Scheme 2). The other iodo
regioisomers were not observed when the reaction was per-
formed at room temperature. 5-Iodosalicylaldehydes 1 and 2
were then condensed with one equivalent of 2-aminophenol
1
with CH2Cl2. All compounds were characterized by H, 13C,
and 11B NMR as well as MS and elemental analysis.
The synthesis of the 4-substituted HBO borate complexes
is described in Scheme 3. 4-Iodosalicylaldehyde (17) was ob-
tained by monoformylation of 3-iodophenol with para
ACHTUNGTRENNUNGform-
AHCTUNGTRENNUNG
28% yield. 6-Iodosalicylalde-
hyde is also formed as a side
product in the course of the re-
action but easily separated by
column chromatography. Con-
densation of 17 with 2-amino-
phenol in refluxing ethanol fol-
lowed by oxidative cyclization
with DDQ afforded 4-iodo
HBO 18 in 80% yield. Com-
pound 18 was then subjected to
Sonogashira cross-coupling con-
ditions similar to those used to
prepare compounds 5–10 in the
presence of p-tolylacetylene or
p-dibutylaminophenylacetylene.
The HBO ligands 19 and 20
were obtained as yellow pow-
ders in 82 and 75% yield re-
spectively. Finally, boron com-
plexation was achieved by using
similar reaction conditions as
for complexes 11–16 (excess of
Scheme 2. Synthesis of HBO borate complexes 11–16.
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5377

J. Massue, G. Ulrich, R. Ziessel et al.
tramolecular nucleophilic addi-
tion of the phenolate to the
triple bond under the Sonoga-
shira cross-coupling reaction
conditions (PdII, CuI, NEt3),[33]
HBO 24 was first protected
with an acetate group by reac-
tion with acetic anhydride in di-
chloromethane with catalytic
amounts of pyridine. The crude
material was immediately sub-
jected to Sonogashira condi-
Scheme 3. Synthesis of HBO borate complexes 21 and 22.
tions with p-tolylacetylene or p-
dibutylaminoacetylene as acety-
BF3·Et2O in the presence of DIEA in CH2Cl2). Formation
of HBO ligands 19 and 20 and the corresponding BF2 com-
plexes 21 and 22 was monitored by H NMR spectroscopy,
whereby the downfield signal attributed to the H-bonded
phenolic proton disappears on complexation (Figure 1).
lene sources to yield acetate-protected HBOs 25 and 26 in
91 and 66% yield, respectively. Removal of the acetate pro-
tecting group with an excess of K2CO3 in MeOH/THF (1/1)
provided HBOs 27 and 28 in a nearly quantitative yield. Fi-
nally, boron complexation was achieved by using an excess
of BF3·Et2O/DIEA, and the re-
1
sulting crude complexes were
purified by filtration through
basic Al2O3 to afford borate
complexes 29 and 30 in 88 and
71% yield, respectively. The X-
ray structure of complex 29 fur-
ther confirmed connection of
the phenylacetylene fragment
at the 3-position of the HBO
core (see below). It is notewor-
thy that all extended complexes
exhibit improved chemical sta-
bility in protic solvents com-
pared to reported unsubstituted
ones.[29]
Finally, we investigated the
effect of covalent connection of
HBO borate complexes to
other photoactive subunits such
as BODIPY or Boranil. The
Figure 1. 1H NMR spectra in CDCl3 of HBO 20 and corresponding BF2 complex 22.
synthesis of dyads 33 and 36 is
Synthesis of the 3-substituted
HBO borate complexes is de-
scribed in Scheme 4. Selective
mono-iodination at the 3-posi-
tion was achieved by using an
excess of iodine monochloride
on 5-methylsalicylaldehyde in
acetic acid (59% yield of isolat-
ed product). 3-Iodo-5-methyl-
ACHTUNGTRENNUNGsalACHTUNGTRENNUNGicylaldehyde (23) was then
condensed with 2-aminophenol
and the intermediate further
oxidized in the presence of
DDQ to provide HBO 24 in
64% yield. To avoid in situ in- Scheme 4. Synthesis of HBO borate complexes 29 and 30.
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Borate Complexes Based on 2-(2’-Hydroxyphenyl)benzoxazole
FULL PAPER
Figure 2. 11B NMR spectrum of dyad 33 in CDCl3 at 128.38 MHz.
Scheme 5. Synthesis of BODIPY–HBO borate dyad 33 and Boranil–
HBO borate dyad 36.
Figure 3. Top: ORTEP diagram of 21 showing the atom-labeling scheme.
Bottom: View orthogonal to the above diagram to highlight the planarity
of the molecule. Thermal ellipsoids are plotted at the 30% level. Bond
ꢀ
ꢀ
ꢀ
lengths [ꢅ] and angles [8]: B1 N1 1.581(2), B1 O1 1.442(3), B1 F1
ꢀ
ꢀ
ꢀ
1.363(3), B1 F2 1.367(3), N1 C1 1.307(2), N1 C7 1.405(2) ꢅ; N-B-O
108.27(16), F-B-F 110.91(18), N-B-F 108.27(17)/108.05(16), O-B-F
109.94(17)/111.30(17).
presented in Scheme 5. Iodo HBO 3 was treated under So-
nogashira cross-coupling reaction conditions (5% PdII, 10%
CuI, NEt3) in toluene with ethynyl BODIPY derivative 31
to give BODIPY 32 in 64% yield or ethynyl Boranil 34 to
give dyad 35 in 56% yield. Addition of BF3·Et2O in the
presence of DIEA in dichloromethane yielded BODIPY–
HBO borate dyads 33 and Boranil–HBO borate 36 in 83
and 53% respectively. In dyad 33, the complexation reaction
can be monitored by the appearance of two triplets at 1.45
and 0.80 ppm in the 11B NMR spectrum due to the coupling
of the boron centers with two equivalent fluorine nuclei.
group at C10 does not affect the planarity of the molecule.
A very small root mean squared deviation of the HBO plat-
form (0.038 ꢅ for 17 atoms within the heterotetracycle in 21
vs. 0.033 ꢅ for 18 atoms in 29) and a very small atom devia-
tion (0.099 ꢅ for the boron atom in 29 are the most notice-
AHCTUNGTREGaNNUN ble features. The dihedral angle with the tolyl group is sim-
ilar in both complexes [6.26 ꢅ (21) vs. 6.468 (29)]. The
boron center displays a fairly regular tetrahedral geometry
with angles ranging from 108.2(2) to 111.3 (2)8 for 21 and
from 107.7(2) to 111.1 (2)8 for 29. These features are similar
to some previously reported for non-extended HBO borate
The coupling constant of the HBO borate complex (JBꢀF
6.4 Hz) is much smaller than that of the BODIPY (JBꢀF
31.9 Hz) (Figure 2).
=
=
complexes.[29] The B1 O1 and B1 N1 distances are in the
ꢀ
ꢀ
ꢀ
X-ray structures: Single crystals of complexes 21 and 29 suit-
able for X-ray analysis were obtained. The molecular struc-
tures are shown in Figures 3 and 4, respectively. Substitution
of the ethynylmethylbenzene moiety at the C11 position in
HBO borate complex 21 or at C12 for 29 with a methyl
same range for 21 and 29 (1.444 vs. 1.440 ꢅ for B1 O1 and
ꢀ
1.581 vs. 1.579 ꢅ for B1 N1) and are fairly similar to those
in HBO borate complexes reported in the literature.[29]
The arrangement of planar, elongated complex 21, which
ꢀ
crystallized in triclinic space group P1, is strongly governed
Chem. Eur. J. 2013, 19, 5375 – 5386
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5379

J. Massue, G. Ulrich, R. Ziessel et al.
Figure 4. Top: ORTEP diagram of 29 showing the atom-labeling scheme.
Bottom: View orthogonal to the above diagram highlighting the planarity
of the molecule. Thermal ellipsoids are plotted at the 30% level. Bond
ꢀ
ꢀ
ꢀ
lengths [ꢅ] and angles [8]: B1 N1 1.579(2), B1 O1 1.440(3), B1 F1
ꢀ
ꢀ
ꢀ
1.377(2), B1 F2 1.370(2), N1 C1 1.319(2), N1 C7 1.399(2); N-B-O
108.58(13), F-B-F 109.43(14), N-B-F 107.74(15)/108.74(15), O-B-F
111.19(16)/111.06(16)8.
Figure 5. a) A view showing the layered crystal structure of compound
ꢀ
21. b) Perpendicular view to the (212) plane showing dimeric association
by p–p stacking interactions. The molecules, all oriented
along the [221] direction, lie in the (212) plane with a tilt of
10.58, and interact side by side through methyl and aryl hy-
in layers (CH···F interactions as dotted lines) and through p–p columnar
stacking in the crystal structure of compound 21.
ꢀ
ꢀ
drogen and fluorine atoms (C H···F distances of 2.57 and
2.71 ꢅ respectively; Figure 5a). The average interlayer dis-
tance is rather short (3.215 ꢅ) and stacking overlap between
inverted head-to-tail molecules is rather high with centroid–
centroid distances between overlapping phenyl groups of
about 3.72 ꢅ (Figure 5b).
containing a p-dibutylaminophenylacetylene group (14, 16,
22, 30) are shown in Figure 9.
As a general trend, all HBO borate complexes except 21
exhibit rather similar absorption spectra with an unstruc-
tured, large absorption band between 342 and 401 nm and
extinction coefficients in the range 8000–48000mꢀ1 cmꢀ1, de-
pending on the electronic substitution on the phenyl core.
More intense bands are observed below 300 nm, characteris-
tic of the p–p* transitions of the phenyl rings. A noteworthy
exception is dye 21, for which a structured, narrow absorp-
tion band is observed with blueshifted wavelengths and high
extinction coefficients (42000–44000mꢀ1 cmꢀ1). All of the
absorption spectra are centered in the UV region and do
not show drastic solvatochromism. Irradiation in the lower-
energy absorption bands for most of the dyes listed in
Table 1 leads to an intense emission band in the visible
region with maxima ranging between 420 and 571 nm, quan-
tum yields of 6–78%, and Stokes shifts of about 4500 cmꢀ1
This is a significant improvement compared to previously re-
ported non-extended HBO borate complexes, for which
emission wavelengths were all located below 425 nm.[29]
In particular, compared to the unsubstituted HBO borate
complex (see Scheme 1a, labs =349 nm, lem =401 nm, F=
26% in CH2Cl2),[29] delocalization through an ethynyl spacer
leads to a bathochromic shift in absorption and emission re-
gardless of the position at which the fragment is connected
to the HBO core (3, 4, or 5) or its electronic nature. Exten-
Complex 21 is also arranged in layers, which are parallel
to the (123) plane with a short interlayer distance of 3.181 ꢅ
(Figure 6a). Within a layer, the L-shaped molecules form
square dimers around a center of inversion held together by
i
ꢀ
ꢀ
mutual nonclassical C6 H···F2 hydrogen bonds with an H
F distance of 2.46 ꢅ and bordered by adjacent methyl or
aryl groups (see Figure 6b). Along the b crystallographic
axis, the molecules develop strong p–p stacking interactions
with infinite columns of the benzyl groups at the origin of
the unit cell, the centroid–centroid distances of which vary
around 4.0 ꢅ, on one side and columns of the tetracycles at
the center of the unit cell, with shortest centroid–centroid
distance of 3.548 (2) ꢅ between the pentagon at x,y,z and
the C8–C13 six-membered ring at 1ꢀx,ꢀy,1ꢀz on the other
side (Figure 6b).
Photophysical properties: Photophysical data in various sol-
vents for dyes 11–16, 21, 22, 29, 30, 32, 33, and 36 are listed
in Table 1. Absorption and emission properties of complex
16 in various solvents are presented in Figure 7, and a com-
parison of the regioisomers containing a p-tolylacetylene
fragment (12, 21, 29) is shown in Figure 8. Spectra of dyes
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Borate Complexes Based on 2-(2’-Hydroxyphenyl)benzoxazole
FULL PAPER
Table 1. Optical data measured in solution.
Dye labs
[nm]
emax
[mꢀ1 cmꢀ1
lem
D
A
Ff[a]
t
Solvent
G
]
[nm]
[cmꢀ1]
)
[ns]
11
12
13
14
366
9200
9400
10000
7800
8300
9100
43200
39400
10000
43800
21800
21400
9600
15200
11600
25800
48000
13800
41900
43500
33900
30100
29400
32500
37900
14600
14100
16700
19500
21400
18700
16700
15000
63400
78400
427
425
445
452
432
432
520
3800
4000
4100
4500
3800
4200
9600
0.17 2.29 toluene
0.31 2.34 CH2Cl2
0.25 2.47 toluene
0.38 2.92 CH2Cl2
365
376
376
371
365
347
350
368
347
372
370
388
388
395
361
342
382
369
371
390
400
345
401
400
388
383
380
373
332
379
387
362
503
416
0.23
toluene
0.25 2.73 CH2Cl2
0.17 9.31 toluene
[b]
[b]
[b]
[b]
–
–
–
–
CH2Cl2
437
4300
0.18 1.81 CH2Cl2 +HCl(g)
[b]
[b]
[b]
[b]
–
–
–
–
CH3CN
15
16
431
436
473
493
571
419
514
562
421
420
449
512
492
553
506
457
457
444
494
373
536
478
513
513
479
3900
4100
4600
5500
7800
3800
9800
8400
3300
3100
3400
5500
8700
6900
5300
3900
4200
3800
6600
3300
7700
5700
8100
390
0.30 1.65 toluene
0.42 1.74 CH2Cl2
0.37 2.82 cyclohexane
0.50 4.40 toluene
0.18 3.73 CH2Cl2
0.29 1.43 CH2Cl2 +HCl(g)
0.06 0.94 CH3CN
0.09 4.03 THF
0.32 1.05 toluene
0.53 1.00 CH2Cl2
0.78 1.25 toluene
0.73 2.50 CH2Cl2
0.24 1.98 CH2Cl2 +HCl(g)
0.08 1.02 CH3CN
0.44 2.32 THF
0.31 2.97 toluene
0.32 3.02 CH2Cl2
0.52 2.59 toluene
0.18 6.31 CH2Cl2
0.25 2.31 CH2Cl2 +HCl(g)
0.11 5.68 CH3CN
0.40 4.44 THF
21
22
29
30
33
36
0.48 2.43 CH2Cl2
0.50
0.20 0.46 CH2Cl2
Figure 6. a) A view showing the layered crystal structure of complex 29.
b) View perpendicular to the layers showing dimeric association (CH···F
interactions as dotted lines) and also the infinite stacking of the benzyl
groups at the origin of the unit cell on one side, and the HBO platforms
on the other side, along the b axis of complex 29.
3200
[a] Quantum yields determined in solution with quinine sulfate as refer-
ence (F=0.55 in 1n H2SO4, lex =366 nm) for dyes emitting below
480 nm and Rhodamine 6G as reference (F=0.88 in ethanol, lex
=
488 nm) for dyes emitting between 480 and 570 nm. [b] Nonfluorescent.
sion of the conjugation through linking of a trimethylsilyl-
A
published similar BF2 complexes, for which direct connec-
tion of a dimethylamino group at the 5-position of an HBO
core results in bright orange fluorescence at 597 nm.[30a,b] To
confirm the hypothesis of strong ICT in complex 14, absorp-
tion and emission spectra were recorded in CH2Cl2 after
bubbling HCl gas. Since ICT is no longer effective due to
protonation of the p-dibutylphenylamino group, a bright
blue emission can be seen by the naked eye (labs =368 nm,
lem =437 nm, F=18%). The absorption of protonated 14
undergoes both a bathochromic and hypochromic shift
(labs =350 nm, e=39400 for 14 in CH2Cl2 vs. labs =368 nm,
e=10000 for 14 in CH2Cl2 +HCl(g)). Note that 14 is also
emissive in toluene but is quenched in CH3CN.
Introduction of a neighboring methoxyl group in dyes 15
and 16 leads to significant changes in the photophysical
properties. A weak blueshift in absorption and emission as
well as a higher extinction coefficient is observed for 15
versus 12 [labs =370 nm (e=21800mꢀ1 cmꢀ1), lem =436 nm
for 15 in CH2Cl2 compared to labs =376 nm (e=
7800mꢀ1 cmꢀ1), lem =452 nm for 12]. This blueshift is ob-
shift of 16 nm in absorption and 26 nm in emission with re-
spect to the unsubstituted compound and a quantum yield
of 31% in CH2Cl2 (11). Further extending the conjugation
by connecting a phenylacetylene module increases the quan-
tum yield and bathochromically shifts the emission of the re-
sulting borate complexes. For example, connection of the
electron-deficient p-cyanophenylacetylene fragment at the
5-position provides bathochromically shifted dye 13 (labs
365 nm, lem =432 nm, F=25% in CH2Cl2), while the pres-
ence of a p-tolylacetylene module in dye 12 leads to a red-
shift in both absorption (26 nm) and emission (51 nm) with
an improved quantum yield (38%) compared to the unsub-
stituted HBO borate complex. Switching from a methyl to a
strongly electron donating p-dibutylphenylamino group (14)
leads to a completely different photophysical behavior. Dye
14 exhibit a main absorption band at 350 nm, but its fluores-
cence emission is entirely quenched in CH2Cl2, presumably
due to strong internal charge transfer (ICT) occurring in the
complex.[34] These results are in strong contrast to recently
Chem. Eur. J. 2013, 19, 5375 – 5386
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5381

J. Massue, G. Ulrich, R. Ziessel et al.
served in spite of the slight electronic differences of the ter-
minal groups on the ethynyl fragments between 12 (Me)
and 15 (tBu). Like for the p-dibutylaminophenylacetylene
derivatives, drastic changes are observed for 16 compared to
14. Indeed, unlike 14, emission of which was quenched in
CH2Cl2, excitation of 16 at 395 nm leads to a bright yellow-
orange emission centered at 571 nm with F=18%
(Figure 7). A pronounced hypsochromic shift of 152 nm
fragment in the 5- and 3-position, respectively, exhibit simi-
lar photophysical characteristics, such as a large, unstruc-
tured absorption band (labs =376 nm for 12 and 383 nm for
29) and a symmetrical emission band (lem =452 nm for 12
with F=38% and lem =457 nm for 29 with F=32%). Their
emission is mildly to nonsolvatochromic, consistent with
weak ICT. In strong contrast, complex 21 exhibits struc-
tured, narrow, blueshifted absorption bands with increased
extinction coefficients (around 45000mꢀ1 cmꢀ1). The emis-
sion bands fairly mirror the absorption bands and feature re-
duced Stokes shift (around 3000 cmꢀ1), consistent with a
weakly polarized excited state. These experimental observa-
tions are indicative of a singlet-state emission with a signifi-
cant internal cyanine character.[11]
The photophysical data of p-dibutylaminophenylacetylene
complexes 14, 16, 22, and 30 in CH2Cl2 are compiled in
Figure 9. Complex 14, which is not emissive in CH2Cl2,
lights up in the presence of protons, a feature that is clearly
Figure 7. Absorption (gray) and fluorescence emission (black) spectra of
HBO borate complex 16 in various solvents at room temperature.
occurs when HCl gas is bubbled through the solution (lem
=
419 nm) with an improved quantum yield of 29%. Under
these conditions, the solution remains homogeneous without
precipitation of the salt. Furthermore, as seen in Figure 7,
the fluorescence emission of 16 is solvatochromic and
ranges from 473 nm in cyclohexane to 571 nm in CH2Cl2.
This behavior seems consistent with pronounced ICT, but
the value observed in CH3CN is unexpected and not yet
fully understood. The absorption of 16 is centered around
380 nm and does not undergo solvatochromism.
Figure 9. Absorption (gray) and fluorescence emission (black) spectra of
HBO borate complexes a) 14, b) 16, c) 22, and d) 30 in CH2Cl2 before
(c) and after (b) bubbling HCl gas at room temperature.
The absorption and emission spectra of the p-tolylacety-
lene regioisomers 12, 21, and 29 in CH2Cl2 are shown in
Figure 8. Complexes 12 and 29 bearing a p-tolylacetylene
indicative of pronounced ICT along the main molecular axis
(Figure 9a). In complex 16, the presence of a methoxy
group in ortho position seems to prevent some of the nonra-
diative deactivation channels occurring, as highlighted by
bright yellow-orange fluorescence in CH2Cl2. A strong hyp-
sochromic shift in absorption and emission is observed after
bubbling HCl gas through the solution (Figure 9b). The 4-
regioisomer complex 22 exhibits bright green-yellow fluores-
cence at 512 nm with F=72% (Figure 9c). Protonation
with HCl gas leads to a mild blueshift of 20 nm compared to
the other regioisomers with a significant decrease in quan-
tum yield (24%). These experimental observations are con-
sistent with the photophysical behavior of 21 and are indica-
tive of a weakly polarized excited state for complexes substi-
tuted at the 4-position. Note that the absorption of proto-
nated 22 undergoes a pronounced hypsochromic shift (labs
400 nm in CH2Cl2 vs. labs =345 nm in CH2Cl2 +HCl(g)). The
3-regioisomer 30 exhibits rather similar photophysical prop-
=
Figure 8. Absorption (gray) and fluorescence emission (black) spectra of
HBO borate complexes 12 (5-isomer, b), 21 (4-isomer, c) and 29
(3-isomer, g) in CH2Cl2 at room temperature.
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Chem. Eur. J. 2013, 19, 5375 – 5386

Borate Complexes Based on 2-(2’-Hydroxyphenyl)benzoxazole
FULL PAPER
erties to those of complex 16 (Figure 9d) with an unstruc-
tured emission at 373 nm in CH2Cl2 and a broad emission
band centered at 494 nm (F=18%). Protonation with HCl
gas leads to a significant hypsochromic shift and an increase
of the quantum yield (labs =332 nm, lem =373 nm, F=
25%). These results are suggestive of weaker ICT than
those occurring in complexes 14 and 16.
The absorption spectrum of mixed dyad 33 is a linear
combination of those of the individual subunits, likely due
to the orthogonality of the BODIPY and the HBO borate
fragments (Figure 10), which is responsible for the weak
Figure 11. Absorption (dashed gray), excitation (dotted gray), and emis-
sion (black) of Boranil–HBO borate dyad 36 in CH2Cl2 at room tempera-
ture.
Figure 10. Absorption (dashed gray), excitation (dotted gray) and emis-
sion (black) of the BODIPY-HBO borate dyad 33, in CH2Cl2 at room
temperature.
Figure 12. Photographs of borate complexes 21 (top) and 29 (bottom) as
amorphous powders (left), amorphous powders under bench UV lamp
conjugation between the two modules. Excitation in the
lowest-energy band of the HBO module (349 nm) leads to
(irradiation lex =365 nm; middle), and crystals under UV lamp (lex
=
sole emission of the BODIPY fluorophore in CH2Cl2 (lem
=
365 nm; right).
513 nm, F=48%). Note that no residual emission of the
HBO borate fragment is observed, which suggests efficient
energy transfer from the HBO borate complex to the
BODIPY residue. This favorable situation is a consequence
of the pronounced spectral overlap between the emission of
the HBO and the absorption of the second excited state of
the BODIPY fragment. Similar behavior was previously ob-
served with pyrene or perylene as input energy source.[35]
Finally, mixed dyad 36 exhibits two absorption bands cen-
tered at 301 and 416 nm with a high extinction coefficient of
around 80000mꢀ1 cmꢀ1 (Figure 11). Excitation in the lowest-
energy band leads to a unique structured emission band at
479 nm with a quantum yield of 20%.
In order to correlate the photophysics with the strong p
stacking observed in the X-ray structures (Figure 5 and 6),
the solid-state luminescence properties of borate complexes
21 and 29 were investigated. Indeed, as shown in Figure 12,
these complexes exhibit bright luminescence visible to the
naked eye in the solid state under a 365 nm UV lamp, both
as amorphous powders and as crystals (Figure 12). The
highly crystalline powders were dispersed in KBr pellets
and 29 are very similar, for example, an unstructured emis-
sion band at 491 and 464 nm, respectively. These represent
redshifts of 2900 and 330 cmꢀ1 compared to their emission-
maximum wavelengths in CH2Cl2. The emission recorded on
single crystals matches well that recorded on the powders
for 21 and 29. Their quantum yields, calculated from a spec-
trometer fitted with an integrating sphere, were found to be
F=29% for 21 and F=31% for 29. Excitation spectra
were recorded with an emission wavelength set to the maxi-
mum of emission (lex =411 nm for 21 and 419 nm for 29).
Polyexponential decays were observed, presumably due to
the presence of numerous aggregates with average lifetimes
of 5.5 ns for 21 and 4.9 ns for 29.
Solid-state luminescence usually arises from luminescent
dyes carrying peripheral bulky groups that prevent close
packing and subsequent aggregation-induced fluorescence
annihilation.[16] On the other hand, aggregation-induced flu-
orescence is sometimes observed for dyes due to the restric-
tion of intramolecular rotation in the solid state compared
to solution.[36] The substituent groups and planarity of p-con-
jugated dyes play a pivotal role in the crystal packing and
hence in the solid-state fluorescence properties. Some exper-
imental studies provided evidence for solid-state emission
properties correlated to tight packing in p-conjugated sys-
A
(Figure 13). Additionally, the emission profile of single crys-
tals were recorded by using an optical fiber connected to a
spectrometer (Figure 13). The emission-band profiles of 21
Chem. Eur. J. 2013, 19, 5375 – 5386
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5383

J. Massue, G. Ulrich, R. Ziessel et al.
be directed to solubilizing these dyes in buffer solutions, a
prerequisite for their conjugation to biomacromolecules
such as enzymes or DNA.
Experimental Section
General methods and equipment: All reactions were performed under a
dry atmosphere of argon by using standard Schlenk techniques. All
chemicals were received from commercial sources (Aldrich, Alfa Aesar)
and used without further purification. Dichloromethane was distilled
over P2O5 under an argon atmosphere. Thin-layer chromatography
(TLC) was performed on silica gel or aluminum oxide plates coated with
fluorescent indicator. Chromatographic purifications were conducted
with 40–63 mm silica gel or basic aluminum oxide. All mixtures of sol-
vents are given in v/v ratio. The 200 (1H), 300 (1H), 400 (1H), 50.3 (13C),
75.46 (13C), and 100.3 MHz (13C) NMR spectra were recorded at room
temperature with perdeuterated solvents and residual protonated solvent
signals as internal references. The 128 MHz (11B) NMR spectra were re-
corded at room temperature with borosilicate glass as reference. Mass
spectra were measured with a ESI-MS mass spectrometer. Electronic ab-
sorption and emission spectra were measured under ambient conditions
on commercial instruments. UV/Vis spectra were recorded on a dual-
beam grating spectrophotometer with a 1 cm quartz cell. Fluorescence
spectra were recorded with a spectrofluorimeter. Solvents for spectrosco-
py were of spectroscopic grade and used as received. All fluorescence
spectra were corrected. Luminescence lifetimes were measured on a
spectrofluorimeter by using software in time-correlated single-photon
mode coupled to a stroboscopic system. The excitation source was a laser
diode (l=320 nm). The instrument response function was determined by
using a light-scattering solution (LUDOX). The fluorescence quantum
yield (Fexp) was calculated from Equation (1).
Figure 13. Solid-state excitation (gray) and emission recorded on powder
dispersed in KBr pellets (black, continuous) or as crystals (black, dotted)
of HBO borate complexes a) 21 and b) 29 at room temperature.
tems.[37] This is the case for complexes 21 and 29, which are
characterized by strong packing, governed by p–p interac-
tions with interlayer distances of 3.18 and 3.22 ꢅ, respective-
ly. The redshifted emission is probably due to the formation
of dimers in which the transition dipoles are arranged either
in-line or in an oblique manner.[38]
I ODref h2
Iref OD
ð1Þ
Fexp ¼ Fref
h2ref
where I denotes the integral of the corrected emission spectrum, OD the
optical density at the excitation wavelength, and h the refractive index of
the medium. The reference systems used were: quinine, F=55% in
H2SO4 1N, lex =366 nm for dyes emitting below 480 nm; Rhodamine 6G,
F=88% in ethanol, lex =488 nm for dyes emitting between 480 and
570 nm; and cresyl violet, F=55% lex =546 nm in ethanol for dyes emit-
ting above 570 nm.
Conclusion
Synthesis: All reagents were used directly as obtained commercially
unless otherwise noted. 2-Hydroxy-5-iodobenzaldehyde (1),[39] 4-ethynyl-
benzonitrile,[40] p-(dibutylamino)phenylacetylene,[41] BODIPY (31),[42] and
Boranil (34)[31a] were synthesized according to reported procedures. Syn-
thetic protocols for the preparation of salicylaldehydes 2, 17, and 23 and
2-(2’-hydroxyphenyl)benzoxazoles (HBOs) 3–10, 18–20, and 24–28 can be
found in the Supporting Information.
We have synthesized three families of regioisomers of HBO
borate complexes extended on the phenolate side by an
ethynyl fragment at the 3-, 4-, or 5-position. Different pho-
tophysical properties were measured depending on the elec-
tronic nature of the ethynyl module and the position of con-
nection to the HBO core. The 5- and 3-regioisomeric com-
plexes display mild to strong ICT, especially for those in-
cluding a p-dibutylaminophenylacetylene group in their
structure. The emission band, which is strongly solvatochro-
mic, is significantly blueshifted on protonation of the amino
group in CH2Cl2. The 4-isomers display mildly solvatochro-
mic singlet excited-state fluorescence. Moreover, these com-
plexes exhibit fluorescence in the solid state, whereby the
emission can be correlated to tight packing of the p-conju-
gated dyes. Finally, these novel fluorescent modules were at-
tached to other photoactive subunits such as BODIPY, and
strong energy transfer was observed due to favorable spec-
tral overlap between the emission of the HBO fragment and
the absorption of the BODIPY subunit. Further work will
General procedure for the synthesis of HBO borate complexes:
BF3·Et2O (6 equiv) was added by syringed to a stirred solution of the cor-
responding HBO in freshly distilled dichloromethane (0.1 mLmgꢀ1
)
under argon. After 5 min, N,N-diisopropylethylamine (DIEA, 6 equiv)
was added and the resulting mixture stirred at room temperature for 1 h.
The crude solution was then filtered through a column of basic Al2O3,
eluted with CH2Cl2, and the solvents were evaporated in vacuo. Pure
HBO borate complexes were obtained as white to yellow powders after
recrystallization from pentane or cyclohexane.
HBO borate complex 11: Beige powder. 75%. 1H NMR (300 MHz,
CDCl3): d=8.10 (m, 1H, CH Ar), 7.97–8.00 (m, 1H, CH Ar), 7.70–7.76
(m, 2H, CH Ar), 7.59–7.62 (m, 2H, CH Ar), 7.18 (d, 1H, CH Ar, J=
8.7 Hz), 0.27 ppm (s, 9H, CH3); 13C NMR (75.4 MHz, CDCl3): d=159.5,
149.0, 141.1, 130.7, 129.2, 128.3, 127.9, 127.7, 120.6, 117.2, 115.7, 111.8,
106.9, 103.0, 94.8, 0.0 ppm; 11B NMR (128.38 MHz, CDCl3): d=1.38 ppm
(brs); elemental analysis (%) calcd for C18H16BF2NO2Si: C 60.86, H 4.54,
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Borate Complexes Based on 2-(2’-Hydroxyphenyl)benzoxazole
FULL PAPER
N 3.94; found: C 60.74, H 4.84, N 3.70; EI-MS: m/z (%): 355.0 (100),
336.0 (25).
(m, 4H, CH2), 1.23–1.35 (m, 4H, CH2), 0.89 ppm (t, 6H, CH3, J=
7.2 Hz); 13C NMR (75.4 MHz, CDCl3): d=161.4, 159.3, 148.9, 148.6,
134.4, 133.5, 130.9, 127.4, 127.4, 125.3, 123.4, 121.8, 116.9, 111.7, 111.3,
107.5, 104.5, 97.7, 87.3, 50.8, 29.4, 20.4, 14.0 ppm; elemental analysis (%)
calcd for C29H29BF2N2O2: C 71.62, H 6.01, N 5.76; found: C 71.42, H
5.84, N 5.42; EI-MS: m/z (%): 486.1 (100), 467.1 (20).
HBO borate complex 29: Yellow powder. 88%. 1H NMR (300 MHz,
CDCl3): d=7.98–8.01 (s, 1H, CH Ar), 7.71–7.74 (m, 1H, CH Ar), 7.67
(d, 2H, CH Ar, J=10.2 Hz), 7.55–7.58 (m, 2H, CH Ar), 7.51 (d, 2H, CH
Ar, J=8.1 Hz), 7.17 (d, 2H, J=7.8 Hz), 2.38 ppm (s, 6H, CH3); 13C NMR
(75.4 MHz, CDCl3): d=161.7, 158.0, 149.0, 142.0, 138.6, 131.8, 130.8,
129.6, 129.1, 127.6, 127.5, 124.7, 120.2, 117.2, 115.8, 111.7, 115.8, 111.7,
105.9, 95.4, 83.7, 21.6, 20.3 ppm; elemental analysis (%) calcd for
C23H16BF2NO2: C 71.35, H 4.17, N 3.62; found: C 71.28, H 4.04, N 3.58;
EI-MS: m/z (%): 387.1 (100), 368.1(25).
HBO borate complex 30: Yellow powder. 71%. 1H NMR (300 MHz,
CDCl3): d=7.97 (d, 1H, CH Ar, J=1.8 Hz), 7.69–7.72 (m, 1H, CH Ar),
7.53–7.60 (m, 4H, CH Ar), 7.44 (d, 2H, CH Ar, J=6.6 Hz), 6.60 (d, 2H,
CH Ar, J=6.9 Hz), 3.29 (t, 4H, CH2, J=5.7 Hz), 2.34 (s, 3H, CH3), 1.55–
1.62 (m, 4H, CH2), 1.32–1.42 (m, 4H, CH2), 0.97 ppm (t, 6H, CH3, J=
5.4 Hz); 13C NMR (75.4 MHz, CDCl3): d=161.8, 157.7, 149.0, 141.5,
133.3, 130.8, 129.5, 127.5, 127.4, 123.8, 117.0, 116.5, 111.7, 111.4, 111.4,
105.7, 96.9, 82.3, 50.9, 29.4, 20.4, 14.0 ppm; 11B NMR (128.38 MHz,
CDCl3): d=1.54 ppm (brs); elemental analysis (%) calcd for
C30H31BF2N2O2: C 72.01, H 6.24, N 5.60; found: C 71.65, H 5.92, N 5.54;
EI-MS: m/z (%): 500.1 (100), 481.1(10).
HBO borate complex 33: Red powder. 83%. 1H NMR (300 MHz,
CDCl3): d=8.20 (d, 1H, CH Ar, J=2.1 Hz), 8.00–8.03 (m, 1H, CH Ar),
7.76–7.83 (m, 2H, CH Ar), 7.68 (d, 2H, CH Ar, J=8.1 Hz), 7.61–7.64 (m,
2H, CH Ar), 7.32 (d, 2H, CH Ar, J=8.1 Hz), 7.28 (s, 1H, CH Ar), 6.00
(s, 2H, CH Ar), 2.57 (s, 6H, CH3), 1.45 ppm (s, 6H, CH3); 13C NMR
(75.4 MHz, CDCl3): d=161.0, 159.6, 155.9, 149.0, 143.0, 140.7, 135.3,
132.3, 131.2, 130.7, 129.1, 128.8, 128.4, 128.3, 128.0, 127.8, 123.7, 121.5,
120.9, 117.3, 115.3, 111.8, 106.4, 88.9, 31.0, 14.7 ppm; 11B NMR
(128.38 MHz, CDCl3): d=1.45 ppm (JB,F =6.4 Hz), 0.80 (JB,F =31.9 Hz);
elemental analysis (%) calcd for C34H25B2F4N3O2: C 67.48, H 4.16, N
6.94; found: C 67.22, H 3.83, N 6.77; EI-MS: m/z (%): 605.1 (100),
586.1(40), 567.2(10).
HBO borate complex 12: White powder. 92%. 1H NMR (300 MHz,
CDCl3): d=8.14 (d, 1H, CH Ar, J=2.1 Hz), 7.98–8.01 (m, 1H, CH Ar),
7.73–7.79 (m, 2H, CH Ar), 7.57–7.64 (m, 2H, CH Ar), 7.44 (d, 2H, CH
Ar, J=8.1 Hz), 7.23 (d, 1H, CH Ar, J=8.7 Hz), 7.18 (d, 2H, CH Ar, J=
7.8 Hz), 2.39 ppm (s, 3H, CH3); 13C NMR (75.4 MHz, CDCl3): d=166.7,
159.2, 148.9, 140.7, 138.8, 131.5, 130.7, 129.2, 128.4, 127.8, 127.6, 120.6,
119.7, 117.2, 116.0, 111.8, 106.2, 89.8, 86.8, 21.6 ppm; 11B NMR
(128.38 MHz, CDCl3): d=1.43 ppm (brs); elemental analysis (%) calcd
for C22H14BF2NO2: C 70.81, H 3.78, N 3.75; found: C 70.57, H 3.59, N
3.66; EI-MS: m/z (%): 373.1 (100), 354.0 (35).
HBO borate complex 13: Beige powder. 79%. 1H NMR (300 MHz,
CDCl3): d=8.18–8.19 (m, 1H, CH Ar), 7.98–8.02 (m, 1H, CH Ar), 7.75–
7.81 (m, 2H, CH Ar), 7.56–7.68 (m, 6H, CH Ar), 7.24–7.25 ppm (m, 1H,
CH Ar); 13C NMR (100 MHz, CDCl3): d=160.9, 159.8, 148.9, 140.7,
132.2, 132.1, 132.1, 130.6, 129.1, 128.0, 127.8, 127.8, 120.9, 118.5, 117.3,
114.7, 111.8, 106.4, 91.8, 88.0 ppm; 11B NMR (128.38 MHz, CDCl3): d=
1.38 ppm (brs); elemental analysis (%) calcd for C22H11BF2N2O2: C
68.79, H 2.89, N 7.29; found: C 69.18, H 2.72, N 6.93; EI-MS: m/z (%):
384.0 (100), 365.1 (10).
HBO borate complex 14: Yellow powder. 75%. 1H NMR (300 MHz,
CDCl3): d=8.10 (d, 1H, CH Ar, J=1.5 Hz), 7.97–8.00 (m, 1H, CH Ar),
7.73–7.77 (m, 2H, CH Ar), 7.58–7.62 (m, 2H, CH Ar), 7.37 (d, 2H, CH
Ar, J=9 Hz), 7.20 (d, 1H, CH Ar, J=9 Hz), 6.59 (d, 2H, CH Ar, J=
8.4 Hz), 3.29 (t, 4H, CH2, J=7.8 Hz), 1.52–1.63 (m, 4H, CH2), 1.33–1.40
(m, 4H, CH2), 0.97 ppm (t, 6H, CH3, J=7.5 Hz); 13C NMR (100 MHz,
CDCl3): d=161.4, 158.8, 149.0, 140.7, 133.1, 132.9, 130.8, 127.9, 127.7,
127.6, 120.5, 117.2, 111.8, 111.3, 108.1, 106.1, 91.2, 85.3, 50.8, 29.4, 20.3,
14.0 ppm; 11B NMR (128.38 MHz, CDCl3): d=1.41 ppm (brs); elemental
analysis (%) calcd for C29H29BF2N2O2: C 71.62, H 6.01, N 5.76; found: C
71.44, H 5.69, N 5.49; EI-MS: m/z (%): 486.1 (100).
HBO borate complex 15: Off-white powder. 83%. 1H NMR (300 MHz,
CDCl3): d=8.07 (s, 1H, CH Ar), 7.90–7.93 (m, 1H, CH Ar), 7.68–7.71
(m, 1H, CH Ar), 7.52–7.59 (m, 2H, CH Ar), 7.50 (d, 2H, CH Ar, J=
8.7 Hz), 7.39 (d, 2H, CH Ar, J=8.4 Hz), 6.70 (s, 1H, CH Ar, J=8.4 Hz),
3.99 (s, 3H, CH3), 1.34 ppm (s, 9H, CH3); 13C NMR (100 MHz, CDCl3):
d=167.4, 161.8, 151.8, 148.7, 131.4, 130.8, 130.2, 127.3, 127.0, 125.4, 120.0,
116.6, 111.6, 107.7, 101.5, 99.1, 93.5, 82.9, 56.6, 34.9, 31.2 ppm; 11B NMR
(128.38 MHz, CDCl3): d=1.38 ppm (brs); elemental analysis (%) calcd
for C26H22BF2NO3: C 70.13, H 4.98, N 3.15; found: C 70.39, H 5.28, N
3.37; EI-MS: m/z (%): 445.1 (100), 426.2 (15).
HBO borate complex 36: Yellow powder. 56%. 1H NMR (200 MHz,
CDCl3): d=8.18(s, 1H, CH Ar) 8.03 (m, 2H, CH imine+CH Ar), 7.79
(m, 2H, CH Ar), 7.57 (m, 7H, CH Ar), 7.24 (m, 1H, CH Ar), 6.39 (d,
1H, J=8.6 Hz, CH Ar), 6.26 (s, 1H, CH Ar) 3.47 (q, 4H, J=7.0 Hz,
CH2), 1.25 ppm (t, 6H, J=7.0 Hz, CH3); 13C NMR (75.4 MHz, CDCl3):
d=162.3, 157.8, 156.8, 149.2, 143.3, 140.9, 134.2, 132.8, 128.9, 128.1, 123.4,
122.3, 117.4, 112.0, 107.4, 107.2, 106.5, 98.4, 89.1, 88.9, 44.8, 12.9 ppm; ele-
mental analysis (%) calcd for C32H25B2F4N3O3: C 64.36, H 4.22, N 7.04;
found: C 64.09, H 3.92, N 6.82; EI-MS: m/z (%): 597.0 (100), 559.1(35).
HBO borate complex 16: Yellow powder. 88%. 1H NMR (300 MHz,
CDCl3): d=8.03 (s, 1H, CH Ar), 7.89–7.92 (m, 1H, CH Ar), 7.67–7.70
(m, 1H, CH Ar), 7.51–7.55 (m, 2H, CH Ar), 7.39 (d, 2H, CH Ar, J=
8.7 Hz), 6.67 (s, 1H, CH Ar), 6.58 (d, 2H, CH Ar, J=9 Hz), 3.98 (s, 3H,
CH3), 3.29 (t, 4H, CH2, J=7.8 Hz), 1.53–1.64 (m, 4H, CH2), 1.30–1.43
(m, 4H, CH2), 0.97 ppm (t, 6H, CH3, J=7.2 Hz); 13C NMR (75.4 MHz,
CDCl3): d=167.3, 161.4, 148.7, 148.1, 132.9, 130.8, 129.5, 127.2, 126.9,
116.5, 111.5, 111.2, 108.6, 108.4, 101.4, 99.0, 94.9, 81.1, 56.5, 50.7, 29.4,
20.3, 14.0 ppm; 11B NMR (128.38 MHz, CDCl3): d=1.40 ppm; elemental
analysis (%) calcd for C30H31BF2N2O3: C 69.78, H 6.05, N 5.42; found: C
69.57, H 5.75, N 5.18; EI-MS: m/z (%): 516.1 (100), 497.1 (35).
Acknowledgements
HBO borate complex 21: Yellow powder. 65%. 1H NMR (300 MHz,
CDCl3): d=7.97–8.00 (m, 1H, CH Ar), 7.92 (d, 1H, CH Ar, J=8.1 Hz),
7.71–7.77 (m, 1H, CH Ar), 7.54–7.63 (m, 2H, CH Ar), 7.47 (d, 2H, CH
Ar, J=8.4 Hz), 7.46 (d, 1H, CH Ar, J=1.5 Hz), 7.18–7.23 (m, 3H, CH
Ar), 2.39 ppm (s, 3H, CH3); 13C NMR (75.4 MHz, CDCl3): d=159.3,
148.9, 139.6, 133.3, 131.9, 130.8, 129.3, 127.6, 127.6, 125.4, 123.6, 122.7,
119.2, 117.1, 111.7, 105.4, 95.1, 87.9, 21.6 ppm; 11B NMR (128.38 MHz,
CDCl3): d=1.39 ppm (brs); elemental analysis (%) calcd for
C22H14BF2NO2: C 70.81, H 3.78, N 3.75; found: C 70.54, H 3.52, N 3.52;
EI-MS: m/z (%): 373.0 (100), 354.1 (30).
HBO borate complex 22: Orange powder. 75%. 1H NMR (300 MHz,
CDCl3): d=7.85–7.88 (m, 1H, CH Ar), 7.76 (d, 1H, CH Ar, J=8.4 Hz),
7.61–7.64 (m, 1H, CH Ar), 7.43–7.50 (m, 2H, CH Ar), 7.30 (d, 2H, CH
Ar, J=9 Hz), 7.19 (d, 1H, CH Ar, J=4.8 Hz), 7.05 (d, 1H, CH Ar, J=
8.4 Hz), 6.50 (d, 2H, J=9 Hz), 3.22 (t, 4H, CH2, J=8.1 Hz), 1.45–1.56
We thank the Centre National de la Recherche Scientifique (CNRS) for
financial support and the Ministꢂre de lꢆEnseignement Supꢀrieur et de la
Recherche for a MENRT Ph.D. fellowship for D.F.
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Received: October 10, 2012
Revised: December 18, 2012
Published online: February 27, 2013
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