Modification at a boron unit: Tuning electronic and optical properties of π-conjugated acyclic anion receptors

Article abstract of DOI:10.1039/c0ob00044b

Substituents at the boron unit of dipyrrolyldiketone boron complexes as π-conjugated acyclic anion receptors play crucial roles for the tuning of solid-state molecular assemblies, anion-binding behaviour and electronic and optical properties. In particular, emission quantum yields can be significantly tunable by boron substituents and pyrrole α-aryl moieties.

Full text of DOI:10.1039/c0ob00044b

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Modification at a boron unit: tuning electronic and optical properties of
p-conjugated acyclic anion receptors†
Hiromitsu Maeda,*a,b Mayumi Takayama,a Kazuki Kobayashic and Hideyuki Shinmoric
Received 26th April 2010, Accepted 17th June 2010
DOI: 10.1039/c0ob00044b
Substituents at the boron unit of dipyrrolyldiketone boron complexes as p-conjugated acyclic anion
receptors play crucial roles for the tuning of solid-state molecular assemblies, anion-binding behaviour
and electronic and optical properties. In particular, emission quantum yields can be significantly
tunable by boron substituents and pyrrole a-aryl moieties.
Introduction
p-Conjugated systems responsive to external chemical stimuli offer
fascinating possibilities as building subunits of tunable electronic
and optical materials. Modifications of the molecular structures
are crucial for controlling their electronic and optical properties.
As examples of stimuli-responsive p-conjugated molecules, we
have reported highly emissive BF2 complexes of 1,3-dipyrrolyl-
1,3-propanediones (e.g., 1a–d), which efficiently bind anions as
chemical stimuli1,2 to induce the conformation changes with the
inversion of pyrrole rings (Fig. 1).3–6 Introduction of appropriate
substituents at the pyrrole rings enables the acyclic anion receptors
to form various anion-responsive supramolecular organized struc-
tures such as crystals, gels5a and amphiphilic vesicles.5d Another
modification of the receptor structures can be achieved by the
replacement of fluorine units in BF2 complexes, which affords
Fig. 1 Anion-binding scheme of dipyrrolyldiketone boron complexes as
catechol-boron-substituted ‘BO2’ complexes such as 2a,b; these
molecules are less emissive than 1a,b.6 Therefore, in order to
p-conjugated acyclic anion receptors 1a–d, 2a–d and 3a–d.
examine the effects of B-substituents for their electronic and
optical properties, we have introduced less electronegative ‘carbon’
moieties—phenyl rings in this case—at the boron unit to provide
‘BC2’ complexes. Singlet oxygen generation can also be controlled
for 2a,b,6 a-phenyl-substituted BO2 complex 2c was obtained as a
reference molecule in 80% yield from diketone by treatment with
BCl3 and catechol. In addition, similar to 1d and 3d, b-ethyl 2d
was synthesized in 3.5% yield from 2b,6 which was the starting
by anion complexation.
material. Chemical identification of these compounds was carried
out by 1H NMR and MALDI-TOF-MS. 11B NMR of 3b in CDCl3
at 20 ◦C exhibits a broad signal at 8.09 ppm in contrast to the fairly
sharp signals of 1b (0.44 ppm) and 2b (8.45 ppm).
Results and discussion
BC2 complexes, diphenylboron-substituted derivatives 3a–c, were
synthesized in 71, 66 and 66% yield by the treatment of the
corresponding diketones with BPh3.7 Similar to 1d,5b b-ethyl 3d
was obtained in 9.0% yield by the iodination of 3b and following
it up with a coupling reaction with phenylboronic acid. On the
other hand, by following the procedures mentioned in literature
Photophysical data are summarised in Table 1. UV/vis absorp-
tion maxima (lmax) of b-ethyl 1b, 2b and 3b in CH2Cl2 are 451, 455
and 448 nm, respectively, whereas those of a-phenyl 1d, 2d and 3d
are observed at 499, 502 and 489 nm, respectively; these lmax values
suggest that substituents at boron units slightly affect the energy
gaps between the ground and excited states. Similarly, lmax values
of 1a, 2a and 3a in CH2Cl2 are 432, 435 and 435 nm, respectively,
and those of a-phenyl 1c, 2c and 3c are 500, 503 and 492 nm,
respectively. Energy levels of HOMO/LUMO corresponding to
the MO located on receptor units estimated by DFT calculations
are, for example, –5.798/–2.714 eV for 1c, –5.160/–2.745 eV for
2c and -5.714/-2.628 eV for 3c, which are correlated with the
electron-withdrawing and electron-donating properties of boron-
substituents.8
aCollege of Pharmaceutical Sciences, Institute of Science and Engineering,
Ritsumeikan University, Kusatsu, 525-8577, Japan. E-mail: maedahir@
ph.ritsumei.ac.jp; Fax: +81 77 561 2659; Tel: +81 77 561 5969
bPRESTO, Japan Science and Technology Agency (JST), Kawaguchi, 332-
0012, Japan
cDivision of Medicine and Engineering Science, Interdisciplinary Graduate
School of Medical and Engineering, University of Yamanashi, Kofu, 400–
8511, Japan
† Electronic supplementary information (ESI) available: Anion-binding
behaviour and CIF files for the X-ray structural analysis of 2d, 3a,b,
3b-I2, 3c, 3c·acetone2, 3d, 3a–c·TBABr and 3d·TBACl. CCDC reference
numbers 759617–759627. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c0ob00044b
In contrast to fairly small distinctions in absorption, interest-
ingly, emission properties such as quantum yields (UF) excited at
each lmax can be dramatically controlled by boron substituents
(Table 1): for example, UF values (and lem) of 1b, 2b and 3b are
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Table 1 Photophysical data (absorption maximum lmax (nm), fluores-
cence emission maximum lem (nm), emission quantum yield UF, and
fluorescence lifetime t (ns)) of 1a–d, 2a,b (references), 2c,d and 3a–d in
a
CH2Cl2
l
max/nm
l
em/nm
UF
t/ns
1a
1b
1c
1d
2a
2b
2c
2d
3a
3b
3c
3d
432 a
451 b
500 c
499 d
435 e
455 e
503
502
435
448
492
451 a
470 b
529 c
535 d
450 e
474 e
530
537
449
464
517
0.96
—
0.98 b
0.95
0.74 (10%), 2.32 (88%)
—
0.98 (16%), 2.24 (78%)
—
0.14 (0.023%), 2.17 (0.077%)
—
0.73 (0.18%), 1.60 (0.12%)
—
0.94 d
0.003
0.001
0.002
0.003
0.009
0.072
0.87
0.23 (3.7%), 2.04 (3.5%)
—
0.70 (11%), 2.13 (83%)
489
512
0.94
a Ref. 4a. b Ref. 4b. c Ref. 5. d Ref. 5b. e Ref. 6
0.98 (470 nm), 0.001 (474 nm) and 0.072 (464 nm), respectively,
and those of 1d, 2d and 3d are 0.94 (535 nm), 0.003 (537 nm)
and 0.94 (512 nm), respectively. b-Unsubstituted receptors exhibit
the similar tendency: UF values (and lem) of 1a, 2a and 3a are
0.96 (451 nm), 0.003 (450 nm) and 0.009 (449 nm), whereas
those of 1c, 2c and 3c are 0.95 (529 nm), 0.002 (530 nm)
and 0.87 (517 nm). These results suggest that the excited states
and quenching processes of diphenylboron-substituted 3a–d are
significantly affected by a-phenyl-substituents, which enhance
UF values; this is in sharp contrast to the highly emissive
BF2 complexes 1a–d and the less emissive catechol-boron 2a–
d, regardless of whether the receptors have a-phenyl moieties
or not (Fig. 2). As speculated from theoretical studies, one of
the quenching processes is presumably intramolecular electron
transfer between core p-units and aryl moieties around the boron.
Further, fluorescence lifetimes (t, ns) by excitation at 399.5 nm
(and contributions for emission efficiencies based on the UF values
excited at each lmax) are 0.74 (10%) and 2.32 (88%) for 1b, 0.98
(16%) and 2.24 (78%) for 1d, 0.14 (0.023%) and 2.17 (0.077%)
for 2b, 0.73 (0.18%) and 1.60 (0.12%) for 2d, 0.23 (3.7%) and
2.04 (3.5%) for 3b and 0.70 (11%) and 2.13 (83%) for 3d. The
relatively larger contributions of shorter lifetimes in 2b,d and 3b
are correlated with their lesser emissive properties.
Fig. 3 Single-crystal X-ray structures (top and side view) of (a) 3a, (b) 3b
(one of the two independent structures), (c) 3c (one of the two independent
structures) and (d) 3d. Atom colour code: brown, pink, yellow, blue, and
red refer to carbon, hydrogen, boron, nitrogen and oxygen, respectively.
˚
by N–H ◊ ◊ ◊ phenyl-p interaction (N ◊ ◊ ◊ p: 3.35 A), whereas b-ethyl
3b exhibits 1-D ordered structures along the b axis and p–p
˚
interaction (3.61 A) between the 1-D columns. a-Phenyl 3c, whose
two phenyl moieties are tilted at 23.5◦ and 38.1◦ to the core p-plane,
˚
forms edge-to-edge stacking dimers (3.48 A). Further, 3d shows
the phenyl ring tilts at 38.6◦ and 45.5◦ and forms stacking dimer
˚
structures at a distance of 3.77 A, whose regular conformation is
in sharp contrast to the corresponding BO2 complex 2d with an
inverted pyrrole ring.
Next, anion-binding behaviour was examined. H NMR spec-
1
tral changes of, for example, 3c in CD2Cl2 (1 mM) at -50 ◦C
upon the addition of Cl- as a tetrabutylammonium (TBA) salt (0
to 1.7 equiv) exhibited down-field shifts of pyrrole NH (9.83 to
12.32 ppm), o-CH (7.66 to 8.15 ppm) and bridging CH (6.53
to 8.58 ppm). This result suggests the formation of receptor–
anion complexes in BC2 complexes as well, as described in
Fig. 1. The exact structures of receptor–anion complexes were
revealed by single-crystal X-ray analyses of 3a·TBABr, 3b·TBABr,
3c·TBABr and 3d·TBACl (Fig. 4). In these cases, two pyrrole rings
are inverted to afford receptor–anion complexes; for example,
in 3c·TBABr, the N(–H) ◊ ◊ ◊ Br-, bridging-C(–H) ◊ ◊ ◊ Br- and o-
C(–H) ◊ ◊ ◊ Br- distances are 3.27/3.31, 3.48 and 3.54/3.62 A,
˚
Fig. 2 Photographs of the CH2Cl2 solutions (1 ¥ 10-3 M, under visible
(top) and UV365 nm (bottom) light) of (a) 1b and 1d, (b) 2b and 2d and (c)
3b and 3d.
respectively, and the a-phenyl moieties are tilted to the core p-
plane at 8.71◦ and 29.04◦, which are much smaller than those of
3c. In this case, the ‘regular’ binding mode in 3a·TBABr is in sharp
contrast to the anion-bridged 1-D chain structures of 1a·TBACl4a
and 1a·TBABr.5e These differences in molecular conformation,
correlated with their assembled structures, are due to the stable
packing modes that are significantly affected by the shapes and
bulkiness of B-substituents. Similar to the former examples,5a,c
receptor–anion complexes are stacked with TBA cations to form
columnar structures. Further, UV/vis spectra of 3a–d along with
Single-crystal X-ray analyses of 3a–d elucidated the exact
structures of the BC2 complexes and their molecular assemblies in
the solid state (Fig. 3). In these receptors, one of the B-phenyl
rings is tilted almost perpendicular to the core plane, as also
observed in the DFT-based optimized structures. Focusing on
the assemblies, b-unsubstituted 3a forms dimers by fairly weak
˚
edge-to-edge stacking (3.74 A) along with the dimeric structures
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Fig. 4 Single-crystal X-ray structures (top and side view) of (a) 3a·TBABr,
(b) 3b·TBABr, (c) 3c·TBABr and (d) 3d·TBACl. Counter cations are
omitted for clarity. Atom colour code: red-brown and yellow-green refer
to bromine and chlorine, respectively.
2c,d in CH2Cl2 are changed by anion complexation: for example,
addition of TBACl to 3a, 3b, 3c and 3d in CH2Cl2 affords small
decreases in absorption with almost no changes of lmax values for
3a, 3b and 3c and subtle shift (+4 nm) for 3d. On the other hand,
fluorescence spectral changes by anions are observed: for example,
Cl- binding of 3b enhances fluorescence quantum yield to 0.40,
in contrast to 3d, which shows almost similar UF value (0.83).
Enhancement of the UF value is possibly because of the changes
in the molecular orbitals by anion binding. Binding constants (Ka)
of the receptors for various anions in CH2Cl2 were examined by
the UV/vis absorption spectral changes (Table 2). In the case
of a-phenyl receptors (1c, 2c and 3c), BC2 complex 3c shows
Ka values that are comparable to those of 1c and 2c. On the
other hand, in the receptors bearing b-ethyl moieties (1b, 2b, 3b,
1d, 2d and 3d), Ka values of 3b and 3d for halide anions are
greater than those of the corresponding BF2 and BO2 complexes.
-
-
Oxoanions such as CH3CO2 and H2PO4 are well bound by the
receptors due to the basicities of anions. Detailed factors that go
to determine the anion-binding affinities, especially related with
B-substituents, are now being examined. Acyclic geometries of the
receptor molecules exhibit the anion selectivities that are much
affected by substituents.
Based on the optical properties of the receptors depending on
the B-substituents and anion complexation, abilities to sensitize
singlet oxygen (1O2) generation were examined by photoirradiation
of anion receptors containing 1,3-diphenylisobenzofuran (DPBF),
and this caused clear absorption spectral changes associated
with DPBF oxidation.9 In this case, a-phenyl b-ethyl receptors
1d, 2d and 3d are used due to their solubility and appropriate
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absorption bands that are less overlapped with that of DPBF. The
max/lem values (nm) (and UF) in toluene are comparable to those
Catechol-substituted boron complex of 1,3-bis(3,4-diethyl-
5-iodopyrrol-2-yl)-1,3-propanedione, 2b-I2
l
in CH2Cl2: 496/527 (0.97) for 1d, 500/531 (0.001) for 2d and
490/520 (0.91) for 3d. Quantum yields (UD) of O2 formation
Following the literature procedure,5b to a CH2Cl2 (35 mL) solution
of 2b6 (130.3 mg, 0.30 mmol) at room temperature was added N-
iodosuccinimide (165.0 mg, 0.72 mmol). The mixture was stirred
at 0 ◦C for 6.5 h. After confirming the consumption of the starting
material by TLC analysis, the mixture was washed with water
and extracted with CH2Cl2, dried over anhydrous Na2SO4, and
evaporated to dryness. The residue was then chromatographed
over a silica gel flash column (eluent: 20% hexane–CH2Cl2) and
recrystallized from CH2Cl2–hexane to afford bisiodo-substituted
1
for 1d, 2d and 3d in toluene were determined to be 0.028, 0.029
1
and 0.065, respectively. Boron complexes exhibit O2 generation
abilities, and B-substituents such as diphenylboron can augment
the efficiency. Less efficient generation of 1O2 even in less emissive
2d suggests that triplet state may not be the main fluorescent
quenching pathway. Further, Cl- complexes of 1d, 2d and 3d,
prepared by the addition of TBACl (35 equiv for samples in
10-6 M enough to obtain >90% Cl- complexes) according to
their Ka values in toluene, afford slightly increased UD values of
0.068, 0.042 and 0.085, respectively. This result suggests that the
photophysical properties can be controlled by external chemical
stimuli.
1
2b-I2 (27 mg, 13%) as ◦a red solid. Rf 0.65 (CH2Cl2). H NMR
(600 MHz, CDCl3, 20 C): d (ppm) 9.39 (m, 2H, NH), 6.86 (m,
2H, catechol-H), 6.80 (m, 2H, catechol-H), 6.43 (s, 1H, CH), 2.80
(m, 4H, CH2), 2.48 (m, 4H, CH2), 1.27 (m, 6H, CH3), 1.21 (m,
6H, CH3). UV/vis (CH2Cl2, lmax[nm] (e, 105 M-1cm-1)): 481.0
(1.14). MALDI-TOF-MS: m/z (% intensity): 684.0 (100), 685.0
(70), 686.0 (32). Calcd for C25H27BI2N2O4 ([M]+): 684.02.
Conclusions
Substituents at the boron unit of p-conjugated acyclic anion
receptors have been found to modulate the electronic and optical
properties, especially fluorescence efficiencies, along with solid-
state assembled structures. Although we found the considerable
differences in the derivatives with various B-substituents, we also
noticed that the properties of these dipyrrolyldiketone boron
complexes such as 1O2 generation behaviour are tunable by anions.
These properties observed in the anion receptors would be useful
for efficient anion sensors and agents for photodynamic therapy
(PDT) by further structure modifications. It is also essential to
pointed out that, in contrast to fluorine moieties in BF2 complexes,
parent catechol and phenyl units in BO2 and BC2 complexes
can be replaced by utility substituents and spacer units to
afford supramolecular assemblies and covalently linked oligomer
systems. Further, the formation of not only boron complexes
but also other metal complexes based on the dipyrrolyldiketone
framework is now under investigation.
Catechol-substituted boron complex of 1,3-(5-phenylpyrrol-2-yl)-
1,3-propanedione, 2c
A dry CH2Cl2 solution (30 mL) of 1,3-di-(5-phenylpyrrol-2-yl)-
1,3-propanedione5a (9.61 mg, 0.027 mmol) was treated with a
CH2Cl2 solution (0.27 mL) of BCl3 (0.265 mg, 0.27 mmol) in
at room temperature under nitrogen and was stirred for 2 h
at the same temperature. The mixture became red. After the
consumption of starting diketone was confirmed by TLC analysis,
catechol (3.85 mg, 0.035 mmol) was added. After 3 h, the mixture
was washed with Na2CO3 aq. and water, dried over anhydrous
Na2SO4, filtered, and evaporated to dryness. The residue was then
chromatographed over a silica gel column (eluent: CH2Cl2) and
recrystallized from CH2Cl2–hexane to afford 2c (10.2 mg, 80%) as
1
a red solid. Rf 0.43 (2% MeOH–CH2Cl2). H NMR (600 MHz,
CDCl3, 20 ◦C): d (ppm) 9.62 (m, 2H, NH), 7.61 (m, 4H, Ar–H),
7.43 (m, 4H, Ar–H), 7.36 (m, 2H, Ar–H), 7.25 (m, 2H, pyrrole-
H), 6.96 (m, 2H, catechol-H), 6.82 (m, 2H, catechol-H), 6.74 (m,
2H, pyrrole-H), 6.62 (s, 1H, CH). UV/vis (CH2Cl2, lmax[nm] (e,
105 M-1cm-1)): 503.0 (1.08). MALDI-TOF-MS: m/z (% intensity):
471.2 (24), 472.2 (100), 473.2 (50). Calcd for C29H21BN2O4 ([M]+):
472.16.
Experimental section
General Procedures
Starting materials were purchased from Wako Chemical Co.,
Nacalai Chemical Co., and Aldrich Chemical Co. and used
without further purification unless otherwise stated. UV-visible
spectra were recorded on a Hitachi U-3500 spectrometer. Fluo-
rescence spectra and quantum yields were recorded on a Hitachi
F-4500 fluorescence spectrometer and a Hamamatsu Quantum
Yields Measurements System for Organic LED Materials C9920-
02, respectively. NMR spectra used in the characterization
of products were recorded on a JEOL ECA-600 600 MHz
spectrometers. All NMR spectra were referenced to solvent.
Matrix-assisted laser desorption ionization time-of-flight mass
spectrometries (MALDI-TOF-MS) were recorded on a Shimadzu
Axima-CFRplus using positive mode. TLC analyses were carried
out on aluminium sheets coated with silica gel 60 (Merck 5554).
Column chromatography was performed on Sumitomo alumina
KCG-1525, Wakogel C-200, C-300, and Merck silica gel 60 and
60H.
Catechol-substituted boron complex of 1,3-bis(3,4-diethyl-5-
phenylpyrrol-2-yl)-1,3-propanedione, 2d
A
two necked flask containing 2b-I2 (10.2 mg,
0.015 mmol), phenylboronic acid (4.5 mg, 0.037 mmol),
tetrakis(triphenylphosphine)palladium(0) (4.1 mg, 0.0035 mmol),
and Na2CO3 (12.5 mg, 0.12 mmol) was flushed with nitrogen
and charged with a mixture of degassed 1,2-dimethoxyethane
(1 mL), and water (0.1 mL). The mixture was heated at 80 ◦C
for 18 h, cooled, then partitioned between water and CH2Cl2.
The combined extracts were dried over anhydrous Na2SO4 and
evaporated. The residue was then chromatographed over a silica
gel column (eluent: 10% hexane–CH2Cl2) and recrystallized from
CH2Cl2–hexane to afford 2d (2.3 mg, 27%) as a red solid. Rf 0.53
(10% hexane–CH2Cl2). H NMR (600 MHz, CDCl3, 20 ◦C): d
1
(ppm) 9.35 (m, 2H, NH), 7.49 (d, J = 7.8 Hz, 4H, phenyl-H),
7.43 (t, J = 7.8 Hz, 4H, phenyl-H), 7.37 (d, J = 7.8 Hz, 2H,
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phenyl-H), 6.83 (m, 2H, catechol-H), 6.76 (m, 2H, catechol-H),
6.64 (s, 1H, CH), 2.88 (q, J = 7.8 Hz, 4H, CH2), 2.62 (q, J =
7.8 Hz, 4H, CH2), 1.37 (t, J = 7.8 Hz, 6H, CH3), 1.19 (t, J =
7.8 Hz, 6H, CH3). UV/vis (CH2Cl2, lmax[nm] (e, 105 M-1cm-1)):
502.0 (1.10). MALDI-TOF-MS: m/z (% intensity): 583.2 (35),
584.2 (100), 585.2 (72). Calcd for C37H37BN2O4 ([M]+): 584.28.
This compound was further characterized by X-ray diffraction
analysis.
8.4 Hz, 4H, phenyl-H), 7.30 (d, J = 7.2 Hz, 4H, phenyl-H), 7.25 (m,
2H, phenyl-H), 6.26 (s, 1H, CH), 2.77 (q, J = 7.8 Hz, 4H, CH2), 2.42
(q, J = 7.8 Hz, 4H, CH2), 1.22 (t, J = 7.8 Hz, 6H, CH3), 1.09 (t, J =
7.8 Hz, 6H, CH3). UV/vis (CH2Cl2, lmax[nm] (e, 105 M-1cm-1)):
470.0 (1.21). MALDI-TOF-MS: m/z (% intensity): 729.1 (35),
730.1 (100), 731.1 (50). Calcd for C31H33BI2N2O2 ([M]+): 730.07.
This compound was further characterized by X-ray diffraction
analysis.
Diphenyl-substituted boron complex of 1,3-dipyrrol-2-yl-1,3-
propanedione, 3a
Diphenyl-substituted boron complex of 1,3-(5-phenylpyrrol-2-yl)-
1,3-propanedione, 3c
BPh3 (73.3 mg, 0.30 mmol) was added to a solution of 1,3-dipyrrol-
2-yl-1,3-propanedione4a (20.0 mg, 0.099 mmol) in dry toluene
(3.0 mL) under nitrogen and was refluxed for 14 h. The solvent
was evaporated to dryness. The residue was then chromatographed
over a silica gel flash column (eluent: CH2Cl2) and recrystallized
from CH2Cl2–hexane to afford 3a (25.6 mg, 71%) as a yellow
solid. Rf 0.67 (CH2Cl2). H NMR (600 MHz, CDCl3, 20 C): d
(ppm) 9.52 (m, 2H, NH), 7.49 (d, J = 7.8 Hz, 4H, phenyl-H),
7.24 (m, 6H, phenyl-H), 7.12 (m, 2H, pyrrole-H), 7.05 (m, 2H,
pyrrole-H), 6.38 (s, 1H, CH), 6.37 (m, 2H, pyrrole-H). UV/vis
(CH2Cl2, lmax[nm] (e, 105 M-1cm-1)): 435.0 (0.75). MALDI-TOF-
MS: m/z (% intensity): 365.1 (16), 366.1 (100), 367.1 (94). Calcd
for C31H35BN2O2 ([M]+): 366.15. This compound was further
characterized by X-ray diffraction analysis.
BPh3 (74.1 mg, 0.33 mmol) was added to a solution of 1,3-di-
(5-phenylpyrrol-2-yl)-1,3-propanedione5a (35.0 mg, 0.099 mmol)
in dry toluene (2.5 mL) under nitrogen and was refluxed for
12 h. The solvent was evaporated to dryness. The residue was
then chromatographed over silica gel flash column (eluent: 50%
hexane–CH2Cl2) and recrystallized from CH2Cl2–hexane to afford
3c (33.6 mg, 66%) as an orange solid. Rf 0.30 (50% hexane–
CH2Cl2). H NMR (600 MHz, CDCl3, 20 C): d (ppm) 9.64 (m,
2H, NH), 7.64 (d, J = 7.8 Hz, 4H, phenyl-H), 7.58 (d, J = 7.2 Hz,
4H, phenyl-H), 7.45 (t, J = 7.8 Hz, 4H, phenyl-H), 7.36 (t, J =
7.8 Hz, 2H, phenyl-H), 7.30 (t, J = 7.2 Hz, 4H, phenyl-H), 7.26
(m, 2H, phenyl-H), 7.12 (m, 2H, pyrrole-H), 6.69 (m, 2H, pyrrole-
H), 6.46 (s, 1H, CH). UV/vis (CH2Cl2, lmax[nm] (e, 105 M-1cm-1)):
492.0 (1.38). MALDI-TOF-MS: m/z (% intensity): 518.2 (100),
519.2 (62), 520.2 (27). Calcd for C35H27BN2O2 ([M]+): 518.22. This
compound was further characterized by X-ray diffraction analysis.
◦
1
◦
1
Diphenyl-substituted boron complex of 1,3-bis-(3,4-diethylpyrrol-
2-yl)-1,3-propanedione, 3b
Diphenyl-substituted boron complex of 1,3-bis(3,4-diethyl-5-
phenylpyrrol-2-yl)-1,3-propanedione, 3d
BPh3 (297.7 mg, 1.25 mmol) was added to a solution of 1,3-bis-
(3,4-diethylpyrrol-2-yl)-1,3-propanedione4b (55.5 mg, 0.25 mmol)
in dry toluene (3.5 mL) under nitrogen and was refluxed for
12 h. The solvent was evaporated to dryness. The residue was
then chromatographed over a silica gel flash column (eluent: 50%
hexane–CH2Cl2) and recrystallized from CH2Cl2–hexane to afford
3b (78.4 mg, 66%) as a yellow solid. Rf 0.33 (50% hexane–CH2Cl2).
1H NMR (600 MHz, CDCl3, 20 ◦C): d (ppm) 9.31 (m, 2H, NH),
7.51 (d, J = 7.8 Hz, 4H, phenyl-H), 7.22 (m, 6H, phenyl-H), 6.88
(d, J = 3.0 Hz, 2H, pyrrole-H), 6.37 (s, 1H, CH), 2.77 (q, J =
7.8 Hz, 4H, CH2), 2.47 (q, J = 7.8 Hz, 4H, CH2), 1.22 (m, 12H,
CH3). UV/vis (CH2Cl2, lmax[nm] (e, 105 M-1cm-1)): 448.0 (0.93).
MALDI-TOF-MS: m/z (% intensity): 477.2 (82), 478.2 (100),
479.2 (37). Calcd for C31H35BN2O2 ([M]+): 478.28. This compound
was further characterized by X-ray diffraction analysis.
A
two necked flask containing 3b-I2 (190.8 mg,
0.26 mmol), phenylboronic acid (70.7 mg, 0.58 mmol),
tetrakis(triphenylphosphine)palladium(0) (54.3 mg, 0.050 mmol),
and Na2CO3 (205.0 mg, 1.9 mmol) was flushed with nitrogen
and charged with a mixture of degassed 1,2-dimethoxyethane
(12 mL), and water (1.2 mL). The mixture was heated at 80 ◦C
for 18 h, cooled, then partitioned between water and CH2Cl2.
The combined extracts were dried over anhydrous Na2SO4 and
evaporated. The residue was then chromatographed over silica
gel column (eluent: 50% hexane–CH2Cl2) and recrystallized from
CH2Cl2–hexane to afford 3d (22.7 mg, 14%) as an orange solid. Rf
0.45 (10% hexane–CH2Cl2). 1H NMR (600 MHz, CDCl3, 20 ◦C):
d (ppm) 9.29 (m, 2H, NH), 7.54 (d, J = 7.8 Hz, 4H, phenyl-H),
7.48 (m, 8H, phenyl-H), 7.39 (t, J = 7.2 Hz, 2H, phenyl-H),
7.27 (m, 4H, phenyl-H), 7.21 (t, J = 7.2 Hz, 2H, phenyl-H),
6.47 (s, 1H, CH), 2.85 (q, J = 7.8 Hz, 4H, CH2), 2.62 (q, J =
7.8 Hz, 4H, CH2), 1.32 (t, J = 7.8 Hz, 6H, CH3), 1.19 (t, J =
7.8 Hz, 6H, CH3). UV/vis (CH2Cl2, lmax[nm] (e, 105 M-1cm-1)):
489.0 (1.01). MALDI-TOF-MS: m/z (% intensity): 629.3 (11),
630.3 (100), 631.3 (21). Calcd for C43H43BN2O4 ([M]+): 630.34.
This compound was further characterized by X-ray diffraction
analysis.
Diphenyl-substituted boron complex of 1,3-bis(3,4-diethyl-5-
iodopyrrol-2-yl)-1,3-propanedione, 3b-I2
Following the literature procedure,5b to a CH2Cl2 (60 mL) solution
of 3b (197.3 mg, 0.41 mmol) at room temperature was added N-
iodosuccinimide (257.4 mg, 1.1 mmol). The mixture was stirred at
room temperature for 3 h. After confirming the consumption of
the starting material by TLC analysis, the mixture was washed with
water and extracted with CH2Cl2, dried over anhydrous Na2SO4,
and evaporated to dryness. The residue was then chromatographed
over a silica gel flash column (eluent: 50% hexane–CH2Cl2) and
recrystallized from CH2Cl2–hexane to afford 3b-I2 (193.8 mg,
64%) as an orange solid. Rf 0.35 (50% hexane–CH2Cl2). 1H NMR
(600 MHz, CDCl3, 20 ◦C): d (ppm) 9.38 (m, 2H, NH), 7.50 (d, J =
Method for X-ray analysis
Crystallographic data are summarised in Table 3. A single crystal
of 2d was obtained by vapour diffusion of octane into a CH2Cl2
solution of 2d. The data crystal was a red prism of approximate
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dimensions 0.50 mm ¥ 0.45 mm ¥ 0.35 mm. Data were collected at
of octane into EtOAc and CH2Cl2 solutions of 3d and 1 equiv
of TBACl. The data crystal was a yellow prism of approximate
dimensions 0.60 mm ¥ 0.40 mm ¥ 0.10 mm. Data were collected at
123 K on a Rigaku RAXIS-RAPID diffractometer with graphite
123 K on a Rigaku RAXIS-RAPID diffractometer with graphite
˚
monochromated Mo-Ka radiation (l = 0.71075 A), structure was
solved by direct methods. A single crystal of 3a was obtained by
vapor diffusion of hexane into a CH2Cl2 solution of 3a. The data
crystal was a yellow prism of approximate dimensions 0.50 mm ¥
0.30 mm ¥ 0.30 mm. Data were collected at 123 K on a Rigaku
RAXIS-RAPID diffractometer with graphite monochromated
˚
monochromated Mo-Ka radiation (l = 0.71075 A), structure was
solved by direct methods. In each case, the non-hydrogen atoms
were refined anisotropically. The calculations were performed
using the Crystal Structure crystallographic software package of
Molecular Structure Corporation.†
˚
Mo-Ka radiation (l = 0.71075 A), structure was solved by direct
methods. A single crystal of 3b was obtained by vapour diffusion
of hexane into a CH2Cl2 solution of 3b. The data crystal was a
yellow prism of approximate dimensions 0.60 mm ¥ 0.40 mm ¥
0.30 mm. Data were collected at 296 K on a Rigaku RAXIS-
RAPID diffractometer with graphite monochromated Mo-Ka
DFT Calculation
Ab initio calculations were carried out by using Gaussian 03
program8 and an HP Compaq dc5100 SFF computer. The
structures were optimized, and the total electronic energies were
calculated at the B3LYP level using a 6-31G** basis set. Molecular
orbitals were determined by single point calculations at the B3LYP
level using a 6-31+G** basis set of the optimized structures at the
B3LYP level using a 6-31G** basis set.
˚
radiation (l = 0.71075 A), structure was solved by direct methods.
A single crystal of 3b-I2 was obtained by vapour diffusion of
hexane into a CH2Cl2 solution of 3b-I2. The data crystal was a
pink prism of approximate dimensions 0.70 mm ¥ 0.30 mm ¥
0.20 mm. Data were collected at 123 K on a Rigaku RAXIS-
RAPID diffractometer with graphite monochromated Mo-Ka
˚
radiation (l = 0.71075 A), structure was solved by direct methods.
Acknowledgements
A single crystal of 3c was obtained by vapour diffusion of hexane
into a CH2Cl2 solution of 3c. The data crystal was a red prism
of approximate dimensions 0.40 mm ¥ 0.30 mm ¥ 0.20 mm.
Data were collected at 123 K on a Rigaku RAXIS-RAPID
diffractometer with graphite monochromated Mo-Ka radiation
This work was supported by Grant-in-Aid for Young Scientists
(B) (No. 21750155) and for Scientific Research in a Priority
Area “Super-Hierarchical Structures” (No. 19022036) from the
Ministry of Education, Culture, Sports, Science and Technology
(MEXT) and Ritsumeikan Global Innovation Research Organi-
zation (R-GIRO) project (2008–2013). We thank Prof. Atsuhiro
Osuka, Dr Shohei Saito, Mr Eiji Tsurumaki and Mr Taro Koide,
Kyoto University, for X-ray analysis and Prof. Hitoshi Tamiaki,
Ritsumeikan University, for various measurements.
˚
(l = 0.71075 A), structure was solved by direct methods. A
single crystal of 3c·acetone2 was obtained by vapour diffusion
of hexane into an acetone solution of 3c. The data crystal was an
orange prism of approximate dimensions 0.50 mm ¥ 0.40 mm ¥
0.40 mm. Data were collected at 123 K on a Rigaku RAXIS-
RAPID diffractometer with graphite monochromated Mo-Ka
˚
radiation (l = 0.71075 A), structure was solved by direct methods.
Notes and references
A single crystal of 3c was obtained by vapour diffusion of
hexane into a CH2Cl2 solution of 3d. The data crystal was a
yellow prism of approximate dimensions 0.30 mm ¥ 0.25 mm ¥
0.10 mm. Data were collected at 123 K on a Rigaku RAXIS-
RAPID diffractometer with graphite monochromated Mo-Ka
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obtained by vapour diffusion of octane into EtOAc solution of
3b and 1 equiv of TBABr. The data crystal was a yellow prism
of approximate dimensions 0.50 mm ¥ 0.30 mm ¥ 0.20 mm.
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diffractometer with graphite monochromated Mo-Ka radiation
˚
(l = 0.71075 A), structure was solved by direct methods. A single
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into EtOAc solution of 3c and 1 equiv of TBABr. The data crystal
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4314 | Org. Biomol. Chem., 2010, 8, 4308–4315
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This journal is
The Royal Society of Chemistry 2010
Org. Biomol. Chem., 2010, 8, 4308–4315 | 4315
©