Three-coordinate organoboron with a BN bond: Substituent effects, luminescence/electroluminescence and reactions with fluoride

Article abstract of DOI:10.1039/b703366d

Five new three-coordinate boron compounds with a BN bond involving an indole or a substituted indole ligand including B(mesityl)₂(indolyl), (1), B(mesityl)₂(2-Me-indolyl), (2), B(mesityl)₂(3-Me- indolyl), (3), B(mesityl)₂ (7-Me-indolyl), (4), and B(mesityl) ₂(3-Ph-indolyl), (5) have been synthesized. The structures of these new compounds were determined by X-ray diffraction analyses. All five compounds are luminescent involving a charge transfer transition between the indolyl π orbital and the boron p_π orbital. The substituent group on the indolyl ring was found to have a subtle impact on the electronic properties of compounds. NMR experiments established that the methyl group at the 7-position of indole is most effective in blocking the rotation of the mesityl group around the B-C bond. The addition of fluoride ions to this group of compounds causes luminescent quenching and an irreversible decomposition of the compounds due to the reaction of the F^- adduct with water molecules. The potential use of this group of compounds as blue emitters in electroluminescent (EL) devices was demonstrated by the successful fabrication of a four-layer EL device where 1 was used as the emitter. The EL device displays a blue emission with a maximum luminescence being 1037 cd m^-2 and a maximum current efficiency about 0.7 cd A^-1 at 5 V. The Royal Society of Chemistry.

Full text of DOI:10.1039/b703366d

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PAPER
www.rsc.org/dalton | Dalton Transactions
=
Three-coordinate organoboron with a B N bond: substituent effects,
luminescence/electroluminescence and reactions with fluoride†
Yi Cui,a Fenghong Li,b Zheng-Hong Lub and Suning Wang*a
Received 6th March 2007, Accepted 12th April 2007
First published as an Advance Article on the web 23rd April 2007
DOI: 10.1039/b703366d
=
Five new three-coordinate boron compounds with a B N bond involving an indole or a substituted
indole ligand including B(mesityl)2(indolyl), (1), B(mesityl)2(2-Me-indolyl), (2),
B(mesityl)2(3-Me-indolyl), (3), B(mesityl)2 (7-Me-indolyl), (4), and B(mesityl)2(3-Ph-indolyl), (5) have
been synthesized. The structures of these new compounds were determined by X-ray diffraction
analyses. All five compounds are luminescent involving a charge transfer transition between the indolyl
p orbital and the boron pp orbital. The substituent group on the indolyl ring was found to have a subtle
impact on the electronic properties of compounds. NMR experiments established that the methyl group
at the 7-position of indole is most effective in blocking the rotation of the mesityl group around the
B–C bond. The addition of fluoride ions to this group of compounds causes luminescent quenching
and an irreversible decomposition of the compounds due to the reaction of the F− adduct with water
molecules. The potential use of this group of compounds as blue emitters in electroluminescent (EL)
devices was demonstrated by the successful fabrication of a four-layer EL device where 1 was used as
the emitter. The EL device displays a blue emission with a maximum luminescence being 1037 cd m−2
and a maximum current efficiency about 0.7 cd A−1 at 5 V.
luminescent three-coordinate boron compounds with the general
Introduction
formula of B(mesityl)2(N-R-indolyl) where R is a substituent
Luminescent organoboron compounds have gained much atten-
group on the indolyl ring and to study the impact of the R
tion recently due mostly to the rapid advancement of organo-
group on the structure and photophysical properties of the boron
boron-based materials in nonlinear optics,1 organic light emitting
compounds and their possible applications in OLEDs and fluoride
sensing. Five new compounds, namely B(mesityl)2(N-indolyl), 1,
B(mesityl)2(N-2-Me-indolyl), 2, B(mesityl)2(N-3-Me-indolyl), 3,
B(mesityl)2(N-7-Me-indolyl), 4, and B(mesityl)2(N-3-Ph-indolyl),
5 have been obtained. The structures and properties of these five
new compounds are presented herein.
diodes (OLEDs)2,3 and sensors.4 Although both four-coordinate
organoboron2 and three-organoboron compounds3 have been
extensively investigated for applications in OLEDs and sensors,4
three-coordinate boron compounds have been found to have a few
advantages including good film forming properties,3 capability of
transporting electrons via the empty pp orbital, and high fluores-
cent sensitivity and selectivity in anion recognition.3,4 Nonetheless,
a key weakness of three-coordinate boron compounds is their poor
chemical stability due to coordination unsaturation. However, as
Experimental
Solvents for reactions were freshly distilled over appropriate drying
agents prior to use. 1H, 13C, 11B and 19F NMR spectra were
recorded on a Bruker Avance 400 or a 600 spectrometer (for
11B NMR, BF3·Et2O was used as the external reference). High
resolution mass spectra (HRMS) were recorded on an Applied
Biosystems/MDS Sciex QSTAR XL QqTOF mass spectrometer
equipped with an electrospray source or measured using a Wa-
ters/Micromass GC-T TOF system equipped with an EI source.
Elemental analyses were performed by Canadian Microanalytical
Service, Delta, British Columbia. UV-Vis spectra were obtained
on a Hewlett Packard 8562A diode array spectrophotometer.
Excitation and emission spectra were recorded on a Photon Tech-
nologies International QuantaMaster Model C-60 spectrometer
at room temperature. Redox potentials were measured in CH2Cl2
solution using 0.1 M NBu4PF6 (TBAF) as the electrolyte on
a CV-50 W BAS Voltammetric Analyzer by using Ag/AgCl
electrode as the reference electrode and platinum electrode as
the working electrode at room temperature. Melting points were
determined with a Fisher–Johns melting point apparatus. Indole,
demonstrated by abundant examples, highly stable triarylboron
compounds can be achieved by introducing substituent groups
such as methyl at the ortho positions of the aryl groups to protect
the boron center from attack by molecules such as H2O. As a
result, previously reported stable three-coordinate organoboron
compounds for OLEDs or sensor applications all contain a
triarylboron core with three B–C bonds.3 Examples of three-
=
coordinate boron compounds that contain a B N bond are
known.5 However, to the best of our knowledge, luminescent three-
=
coordinate boron compounds with a B N bond and their use
in OLEDs and sensor applications have not been explored. It
was the purpose of this investigation to synthesize a new class of
aDepartment of Chemistry, Queen’s University, Kingston, Ontario, K7L 3N6,
Canada
bDepartment of Materials Science and Engineering, University of Toronto,
Toronto, Ontario, M5S 3E4, Canada
† Electronic supplementary information (ESI) available: Fig. 1S–19S and
crystallographic details. See DOI: 10.1039/b703366d
2634 | Dalton Trans., 2007, 2634–2643
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2-methylindole, 3-methylindole, 7-methylindole, phenylboronic
acid (97%), dimesitylboron fluoride and TBAF were purchased
from Aldrich Chemical Co. The intermediates 3-iodoindole6 and
1-(benzenesulfonyl)-3-iodoindole7 were synthesized according to
the reported procedures.
(%) for C26H28BN: C, 85.48; H, 7.33; N, 3.83. Found: C, 85.19; H,
7.79; N, 3.85.
B(mesityl)2(N-2-Me-indolyl) (2). Yield: 67%. Mp: 226 ◦C. 1H
NMR (CDCl3, d, ppm): 7.44 (1H, d, J = 7.6 Hz), 7.09 (1H, dd,
J = 7.2 Hz, 7.6 Hz), 6.84 (1H, dd, J = 7.2 Hz, 8.4 Hz), 6.82 (2H,
s), 6.80 (2H, s), 6.74 (1H, d, J = 8.4 Hz), 2.33 (3H, s), 2.31 (3H,
s), 2.05 (6H, s), 2.00 (6H, s), 1.93 (3H, s). 13C NMR (CDCl3, d,
ppm): 141.3, 141.2, 141.1, 139.3, 139.2, 132.3, 128.8, 128.4, 122.2,
119.0, 114.5, 110.9, 22.1, 22.0, 21.3, 21.3, 16.7. 11B NMR (CDCl3,
d, ppm): 51.5 ppm. HRMS, calcd for C27H30BN, [M]+: 379.2471;
found: 79.2476. Anal. Calcd (%) for C27H30BN: C, 85.49; H, 7.97;
N, 3.69. Found: C, 85.25; H, 8.11; N, 3.65.
Syntheses
Intermediate 1-(benzenesulfonyl)-3-phenylindole. A flask was
charged with 1-(benzenesulfonyl)-3-iodoindole (0.84 g, 2.2 mmol),
phenylboronic acid (0.31 g, 2.4 mmol), Pd(PPh3)4 (78 mg,
0.07 mmol, 3.4% mmol) and Na2CO3 (0.40 g, 3.8 mmol). A
degassed mixture of toluene (45 mL), water (15 mL), and ethanol
(15 mL) in a separated flask was added to the phenylboronic acid
solution via cannula. After the mixture was refluxed for 15 h, the
organic phase and the aqueous phase were separated. The aqueous
phase was extracted with CH2Cl2 and the combined organic phase
was dried over MgSO4, and the residue was purified by a column
chromatograph using hexane–ethyl acetate (2 : 1) as the eluent to
B(mesityl)2(N-3-Me-indolyl) (3). Yield: 71%. Mp: 191 ◦C. 1H
NMR (CDCl3, d, ppm): 7.50 (1H, d, J = 7.2 Hz), 7.22 (1H, dd,
J = 7.2 Hz, 7.6 Hz), 6.97 (1H, dd, J = 7.6 Hz, 8.0 Hz), 6.90 (1H,
s), 6.88–6.83 (5H, m), 2.36 (3H, s), 2.34 (3H, s), 2.27 (3H, s), 2.13
(6H, s), 2.06 (6H, s). 13C NMR (CDCl3, d, ppm): 141.7141.0, 140.5,
138.9, 138.7, 134.4, 129.5, 128.5, 128.3, 123.13, 122.45, 119.27,
118.33, 115.11, 22.47, 21.99, 21.31, 21.21, 9.72. 11B NMR (CDCl3,
d, ppm): 49.7 ppm. HRMS, calcd for C27H30BN, [M]+: 379.2471;
found: 379.2478. Anal. Calcd (%) for C27H30BN: C, 85.49; H, 7.97;
N, 3.69. Found: C,85.33; H, 8.08; N, 3.65.
1
afford 0.62 g the intermediate as white power in 84% yield. H
NMR (CDCl3, d, ppm): 8.11 (1H, d, J = 8.1 Hz), 7.96 (2H, d, J =
6.6 Hz), 7.81 (1H, d, J = 7.8 Hz), 7.74 (1H, s), 7.64 (2H, d, J =
7.2 Hz), 7.76–7.28 (8H, m).
3-phenylindole (3-PI). This ligand was synthesized by a
modified procedure on a related compound.8 A mixture of 1-
(benzenesulfonyl)-3-phenylindole (250 mg, 0.75 mmol), TBAF
(1.0 M in THF, 2.0 mL, 2.0 mmol), and THF (40 mL) was refluxed
for 2 h. After cooling to ambient temperature, the reaction mixture
was evaporated to dryness in vacuo, and H2O (20 mL) was added to
the residue, which resulted in an oil-like suspension. The mixture
was extracted with CH2Cl2 (20 mL × 3) and dried over MgSO4.
The crude product was purified by column chromatography using
hexane–ethyl acetate (1 : 1) as the eluent to afford 120 mg 2-PI as
a white power in 83% yield. 1H NMR (CDCl3, d, ppm): 10.83 (1H,
s), 8.16 (1H, s), 8.03 (1H, d, J = 7.8 Hz), 7.74 (2H, d, J = 7.2 Hz),
7.53–7.43 (3H, m), 7.37–7.25 (3H, m). 13C NMR (CDCl3, d, ppm):
111.5, 136.7, 135.6, 128.8, 127.6, 126.1, 125.8, 122.5, 121.9, 120.4,
119.9, 118.4.
B(mesityl)2(N-7-Me-indolyl) (4). Yield: 54%. Mp: 181 ◦C. 1H
NMR (CDCl3, d, ppm): 7.45 (1H, d, J = 7.8 Hz), 7.15 (1H, dd,
J = 7.2 Hz, 7.8 Hz), 7.07 (1H, d, J = 3.6 Hz), 6.93 (1H, d, J =
7.2 Hz), 6.91 (br) (1H, s), 6.81 (br) (2H, s), 6.66 (br) (1H, s), 6.63
(1H, d, J = 3.6 Hz), 2.34 (3H, s), 2.32 (br) (3H, s), 2.28 (3H, s),
2.16 (br) (3H, s), 1.95 (br) (3H, s), 1.73 (3H, s), 1.57 (3H, s), 1.32
(br) (3H, s). 13C NMR (CDCl3, d, ppm): 141.7, 141.1, 140.3, 139.5,
138.9, 134.6, 132.8, 128.9, 128.4, 126.2, 125.8, 122.8, 118.2, 109.2,
23.4, 22.7, 22.2, 21.3, 21.3, 21.2, 20.1. 11B NMR (CDCl3, d, ppm):
51.9 ppm. HRMS, calcd for C27 H30 BN, [M]+: 379.2471; found:
379.2463. Anal. Calcd (%) for C27H30BN: C, 85.49; H, 7.97; N,
3.69. Found: C, 85.34; H, 8.04; N, 3.70.
B(Mesityl)2(N-3-Ph-indolyl) (5). Yield: 51%. Mp: 233 ◦C. 1H
NMR (CDCl3, d, ppm): 7.86 (1H, d, J = 7.8 Hz), 7.67 (2H, d,
J = 8.4 Hz), 7.45 (2H, dd, J = 7.8 Hz, 7.8 Hz), 7.34 (1H, d, J =
7.2 Hz, 7.8 Hz), 7.28 (1H, s), 7.25 (1H, d, J = 7.8 Hz, 7.8 Hz),
7.04 (1H, d, J = 7.6 Hz, 7.8 Hz), 6.96 (1H, d, J = 7.8 Hz), 6.88
(2H, s), 6.87 (2H, s), 2.37 (3H, s), 2.34 (3H, s), 2.15 (6H, s), 2.09
(6H, s). 13C NMR (CDCl3, d, ppm): 141.8, 141.0, 140.9, 139.2,
139.0, 134.4, 131.7, 129.8, 128.7, 128.6, 128.5, 128.1, 126.9, 125.1,
123.5, 123.0, 119.4, 115.4, 22.5, 22.0, 21.3, 21.2. 11B NMR (CDCl3,
d, ppm): 51.9 ppm. HRMS, calcd for C32H32BN, [M]+: 441.2628;
found: 441.2626. Anal. Calcd (%) for C32H32BN: C, 87.07; H, 7.31;
N, 3.17. Found: C, 87.03; H, 7.44; N, 3.20.
General procedures for the syntheses of B(mesityl)2(N-R-indolyl).
Indole or its derivatives (1 mmol) was reacted with NaH (145 mg,
50% in oil, 3 mmol) in THF for about 1 h at ambient temperature.
The resulting solution was filtered and transferred to the flask
containing 1 mmol of B(mesityl)2F. A bright blue or green
luminescence of the solution was observed immediately. After
being stirred for 12 h, the mixture was separated by column chro-
matography using hexane–methylene chloride (3 : 2) as the eluent.
The white power obtained was further purified by recrystallization
from hexanes–CH2Cl2, producing colorless crystals of the boron
product suitable for single crystal X-ray diffraction analyses.
X-Ray crystallographic analyses
B(mesityl)2(N-indolyl) (1). Yield: 62%. Mp: 216 ◦C. 1H NMR
(CDCl3, d, ppm): 7.54 (1H, d, J = 7.8 Hz), 7.15 (1H, dd, J = 7.2 Hz,
8.4 Hz), 7.10 (1H, d, J = 3.6 Hz), 6.95 (1H, dd, J = 7.2 Hz, 8.4 Hz),
6.85 (1H, d, J = 8.4 Hz), 6.83 (2H, s), 6.82 (2H, s), 6.66 (1H, d,
J = 3.6 Hz), 2.33 (3H, s), 2.30 (3H, s), 2.08 (6H, s), 2.02 (6H,
s). 13C NMR (CDCl3, d, ppm): 141.8, 141.0, 140.1, 139.1, 138.9,
133.4, 132.7, 128.5, 128.4, 123.1, 122.7, 120.4, 115.1, 110.2, 22.4,
21.9, 21.3, 21.2. 11B NMR (CDCl3, d, ppm): 53.6 ppm. HRMS,
calcd for C26H28BN, [M]+: 365.2315; found: 365.2323. Anal. Calcd
Single-crystals of all five boron compounds were mounted on
glass fibers in a brass pin, and data were collected on a Siemens
P4 single-crystal X-ray diffractometer with a SMART CCD-
1000 detector and graphite-monochromated Mo-Ka radiation
operating at 50 kV and 30 mA at 25 ◦C. No significant decay
was observed during the data collection. Data were processed
on a Pentium PC using Siemens SHELXTL software package
(version 5.10).9 Neutral atom scattering factors were taken from
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Cromer and Waber.10 Empirical absorption correction was applied
to all crystals. The crystals of compounds 1, 3 and 4 belong to
the triclinic space group P-1, while compound 2 belongs to the
orthorhombic space group Pna21, and compound 5 belongs to
the monoclinic space group P21/c. The structures were solved by
direct methods. All non-hydrogen atoms were refined anisotropi-
cally. The positions of hydrogen atoms were calculated, and their
contributions in structural factor calculations were included. The
crystal data are summarized in Table 1. Selected bond lengths and
angles are given in Table 2.
Quantum yield measurement
Quantum yields of the five boron compounds were determined
in CH2Cl2 using 9,10-diphenylanthracene as the standard at
298 K (Ur = 0.95).11 The absorbance of all the samples and
the standard at the excitation wavelength is approximately 0.096–
0.109. The quantum yields were calculated using previously known
procedures.12
Fabrication of electroluminescent devices
CCDC reference numbers 639173–639178.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b703366d
The EL devices were fabricated using a K. J. Lesker OLED
cluster tool with six high-vacuum process chambers.
Table 1 Crystallographic data for compounds 1–5
1
2
3
4
5
Formula
FW
C26H28BN
365.30
P-1
C27H30BN
379.33
Pna2(1)
22.398(2)
8.1607(7)
12.2170(10)
90
90
90
2233.1(3)
4
1.128
C27H30BN
379.33
P-1
C27H30BN
379.33
P-1
C32H32BN
441.40
P2(1)/c
11.435(1)
18.515(2)
11.949(1)
90
92.268(1)
90
2528.0(4)
4
Space group
˚
a/A
8.215(1)
11.616(1)
12.231(1)
105.341(2)
94.143(2)
106.575(2)
1064.9(2)
2
8.635(4)
11.606(6)
12.262(7)
99.281(9)
98.716(9)
107.487(9)
1130.3(10)
2
8.499(1)
10.100(1)
14.424(2)
100.384(2)
106.956(2)
103.731(2)
1108.2(3)
2
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
Z
Dc/g cm−3
l/mm◦−1
1.139
0.064
56.44
1.115
0.063
50.00
1.137
0.064
56.56
1.160
0.066
56.50
0.064
56.50
2hmax
/
Reflns measured
7481
4761 (0.0164)
259
14 894
5067 (0.0275)
281
6724
3961 (0.0326)
301
7919
5020 (0.0228)
262
17 595
5892 (0.0336)
369
Reflns used (Rint
No. variables
Final R
)
[I > 2r(I)]a,b
R1 = 0.055
wR2 = 0.141
R1 = 0.096
wR2 = 0.156
0.917
R1 = 0.038
wR2 = 0.080
R1 = 0.069
wR2 = 0.087
0.900
R1 = 0.055
wR2 = 0.159
R1 = 0.084
wR2= 0.176
0.983
R1 = 0.053
wR2 = 0.120
R1 = 0.139
wR2 = 0.142
0.832
R1 = 0.050
wR2 = 0.111
R1 = 0.095
wR2 = 0.123
0.905
R (all data)
GOF on F2
a R1 = R[|Fo| − |Fc|]/R|Fo|. b wR2 = {R[w(Fo − Fc2)]/R(wFo2)}1/2. x = 1/[r2(Fo2) + (0.075P)2], where P = [max.(Fo2,0) + 2Fc2]/3.
2
Table 2 Selected bond lengths, (dihedral) angles of 1–5
Dihedral angle between the indolyl
Bond angle/◦
and the BC2 plane/◦
˚
Compd
Bond length/A
1
N–B
B–C(9)
B–C(18)
1.444(2)
1.566(3)
1.587(2)
N–B–C(9)
N–B–C(18)
C(9)–B–C(18)
119.17(14)
115.99(15)
124.81(15)
24.2
2
3
4
5
N–B
B–C(9)
B–C(18)
1.452(2)
1.572(3)
1.575(3)
N–B–C(9)
N–B–C(18)
C(9)–B–C(18)
118.90(14)
118.98(15)
122.12(15)
31.3
24.2
35.6
25.5
N–B
B–C(9)
B–C(18)
1.442(3)
1.573(3)
1.574(3)
N–B–C(9)
N–B–C(18)
C(9)–B–C(18)
119.00(19)
116.66(18)
124.25(18)
N–B
B–C(9)
B–C(18)
1.457(3)
1.572(3)
1.574(3)
N–B–C(9)
N–B–C(18)
C(9)–B–C(18)
122.73(18)
113.35(18)
123.23(18)
N–B
B–C(9)
B–C(18)
1.444(2)
1.570(2)
1.584(2)
N–B–C(9)
N–B–C(18)
C(9)–B–C(18)
117.76(15)
118.79(15)
123.43(15)
2636 | Dalton Trans., 2007, 2634–2643
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N,Nꢀ-bis(1-Naphthyl)-N,Nꢀ-diphenylbenzidine
(NPB)
and
complexes, the crystal structures were determined by single-crystal
X-ray diffraction analyses.
bathocuproine (BCP) were employed as the hole transport
and hole block layers, respectively. Current–voltage (I–V) and
Luminance–voltage (L–V) characteristics were determined
in ambient atmosphere using a HP 4140B pA meter and a
Minolta LS-110 meter, respectively. Electroluminescent spectra
were recorded using an USB2000-UV-Vis miniature fiber optic
spectrometer.
Crystal structures
The crystal structures of 1–5, projected down the indolyl plane, are
shown in Fig. 1. The boron centers in all five compounds display
a typical trigonal planar geometry. While the B–C bond lengths
in all five compounds are similar to those of the four-coordinate
boron compounds,2 the B–N bond lengths are in the range of
Results and discussion
˚
˚
1.440–1.457 A (1.447 A in average), which are substantially shorter
than the B–N single bond lengths observed in the four-coordinate
Syntheses and characterization
2
˚
boron compounds (∼1.634 A in average). The short B–N bond
The 3-phenylindole compound was synthesized by employing a
literature procedure8 for related compounds, which involved firstly
iodination of indole at the 3 position, subsequent sulfonylation by
PhSO2Cl to protect the nitrogen site, followed by Suzuki–Miyaura
coupling with phenylboronic acid and subsequent desulfonylation
using TBAF (1M in THF) as shown in Scheme 1. The indolyl
substituted three-coordinate boron compounds were synthesized
by stoichiometric reactions of B(Mesityl)2F with the correspond-
ing sodium salt of indole and derivatives obtained from in situ
deprotonation of the corresponding starting materials by NaH
at room temperature as shown by the representative procedure
for compound 5 in Scheme 1. All five compounds were fully
characterized by NMR (1H, 13C and 11B), HRMS and elemental
analyses. 1H NMR experiments indicated that all five compounds
are stable in solution upon extended exposure. All five compounds
are thermally stable with mp close to or above 200 ◦C.
=
lengths observed in 1–5 are consistent with a B N double bond
where the two lone pairs of electrons on the indolyl nitrogen atom
are involved in one r bond and one p bond with the boron center.
=
This is further supported by the fact that the observed B N bond
lengths in our compounds are comparable with those of previously
=
known three-coordinate boron compounds that contain a B N
double bond.5d,5g Although the sum of the bond angles around
boron centers are close to 360◦, the three angles around boron
Scheme 1
The 11B chemical shifts for the five compounds are 53.6 ppm,
51.5 ppm, 49.7 ppm, 51.9 ppm, 51.9 ppm, respectively. These val-
ues are in the region typical for three-coordinate organoborons.3
Furthermore, the 11B NMR data confirm that the substituent
group R has a subtle impact on the electron density of the boron
center. Compared to the non-substituted indolyl, the methyl group
at the 3-position appears to be most effective in enhancing the
electron density on the boron center while the methyl group at
the 7-position is the least effective and comparable to that of
the phenyl group at the 3-position, which can be attributed to
both steric and electronic effects of the ligand. To examine the
impact of the substituent group on the structure of the new boron
Fig. 1 A diagram showing the molecular structure of 1–5 with 50%
thermal ellipsoids.
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centers are not all 120◦. The C–B–C bond angles are all greater
than the C–B–N angles which can be attributed to the two bulky
mesityl groups. The C–B–N angles are all less than 120◦ with
the exception of compound 4, which displays two quite different
N–B–C bond angles (N–B–C(9) = 122.68(2)◦ and N–B–C(18) =
113.91(2)◦), clearly attributable to the steric interactions between
the 7-methyl group of the indolyl and the methyl groups of the
mesityl groups. The methyl groups at C2 and C7 position of the
indolyl ring appears to have the most significant impact on the
conjugation between the indolyl ring and the boron center as
reflected by the much greater dihedral angles between the indolyl
plane and the BC2 plane for compounds 2 (31.3◦) and 4 (35.6◦),
compared to those of compounds 1, 3 and 5 (24.2◦, 24.2◦, and
25.5◦, respectively). Between 2-methylindolyl and 7-methylindolyl,
the latter clearly has a greater steric interaction with the mesityl
groups as indicated by the dihedral angle of 4.
All five compounds display some degree of intermolecular inter-
actions in the solid state, as shown by the crystal packing diagrams
and the short contact distances (see ESI†). For compounds 1 and
3, weak p stacking interactions between two mesityl groups from
two neighboring molecules, and between two neighboring indolyl
rings are evident. For compound 2, weak interactions between
the indolyl rings appear to be present (edge-to-edge interaction).
For compound 5, there is p-stacking between two neighboring
phenyl rings. In addition, edge-to-edge interactions between two
indolyl rings are also evident. For compound 4, the only significant
intermolecular interactions are between two neighboring mesityl
groups.
indolyl ring and the phenyl group at the C3 position do not have
significant impact on the rotation barrier of the mesityl groups
around the B–C bond. The 1H NMR spectrum of compound 4 is,
however, unique as shown by Fig. 2. There are 7 methyl peaks in
the 1H NMR spectrum of 4 at ambient temperature, one of which
belongs to the methyl on the indolyl group, and the remainder be-
long to the six methyl groups of the two mesityls (four for o-methyl
and two for p-methyl). The fact that four well resolved o-methyl
groups are observed for 4 at ambient temperature indicates that the
rotation barrier of the mesityl group around the B–C bond is much
greater than those of compounds 1–3 and 5. This is consistent with
the crystal structure of 4 where the methyl group at C7 position
of the indolyl ring clearly imposes a large steric blocking effect on
the mesityl group. To estimate the rotation barrier of the mesityl
groups in 4, variable temperature 1H NMR spectra were recorded
in CDCl3 and the data for the methyl region is shown in Fig. 2.
At 253 K, the four broad o-methyl peaks become sharp. As the
temperature increases, the four o-methyl signals become broad and
one pair of them coalesces at ∼328 K (the coalescence temperature
for the other pair is a few degrees higher and the pairs were assigned
by COSY NMR). Using the variable temperature NMR data, the
rotation barrier of the mesityl group about B–C bonds in 4 was
estimated to be 15.9 kcal mol−1.13 This rotation barrier was also
confirmed by using the chemical shifts of the two pairs m-H atoms
of the mesityls which display distinct coalescence temperatures.
This value is similar to the value (16.6 kcal mol−1) observed for
B(Mes)2(dbp-NPB) where the boron center is protected by six o-
methyl groups and is among the biggest rotation barrier known for
three-coordinate boron compounds.3e Nonetheless, the rotation
barrier of the mesityl groups in 4 is still not sufficient to prevent the
enantiomer inter-conversion in solution at ambient temperature
due to the apparent slow exchange of the o-methyl groups as shown
Structure and dynamic behavior in solution
The crystal structures of compounds 1–5 illustrate that these
molecules have no symmetry, hence are chiral in the solid state. As
a result, the two mesityl groups are distinct and the four o-methyl
groups in these molecules have distinct chemical environments. In
solution, if the rotation of the mesityl group around the B–C bond
occurs readily, the four o-methyl groups should be reduced to
two distinct groups (one group for each mesityl), provided that
=
there is no free rotation of the indolyl ring around the B N
bond. To establish the structure of 1 in solution, we carried out
1
a series of NMR experiments. The H and 13C spectra of 1 (the
methyl region) at ambient temperature in CDCl3 clearly display
two distinct and well resolved o-methyl chemical shifts and two
p-methyl chemical shifts corresponding to the two mesityl groups
(see ESI†), confirming that a rapid rotation of the mesityl group
around the B–C bond is present at room temperature but there is
=
no free rotation around the B N bond. The rapid rotation of the
mesityl group around the B–C bond in 1 is consistent with previous
observations that three-coordinate boron compounds with two
mesityl groups usually have a rotation barrier about 10 kcal mol−1.3
=
The restricted rotation around the B N bond is consistent with
=
the previous observation that the B N bond in three-coordinate
boron compounds has a very large rotation barrier.5
To investigate the impact of the substituent group of the indolyl
on the dynamic behavior of the boron compounds, we also
examined the NMR spectra of 2–5 (see ESI†). The chemical shift
patterns of the o-methyl and p-methyl groups of the mesityls in
2, 3 and 5 at ambient temperature closely resemble that of 1, an
indication that the methyl substituent at C2 and C3 positions of the
Fig. 2 Variable temperature 1H NMR of 4 in CDCl3 (the methyl region).
2638 | Dalton Trans., 2007, 2634–2643
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Table 3 UV-Vis data for 1–5
electron delocalization and decreases the HOMO–LUMO energy
gap.
Compds
UV-Visa (kmax/nm) (e/M−1 cm−1
)
To gain a better understanding of the electronic properties,
molecular orbital calculations were performed for all five com-
pounds using the geometric parameters from X-ray diffraction
analysis as the initial inputs (B3LYP-6-311G** basis set, Gaussian
98).14 Geometry optimization was carried out for all five structures.
Orbital surface diagrams show that the HOMO level for all
compounds is a p orbital with contributions nearly exclusively
from the indolyl ring. Nonetheless, there is also significant
contribution from the pp orbital of the boron atom in the HOMO
level. Noteworthy is the apparent pp–pp overlap between the
N and the B atom, consistent with the presence of a p bond
between these two atoms. The LUMO level is an antibonding p*
orbital with contributions from the entire molecule. However, the
contribution from the boron p orbital is dominant. The HOMO–
LUMO diagrams for 5 are shown in Fig. 4 as a representative
example. Diagrams for all other compounds are provided in the
ESI.† The MO calculation results provide support for the lowest
electronic transition in these five boron compounds being a p to
p* transition involving both indolyl and the boron center. Because
this transition involves orbitals that are dominated by two different
parts of the molecule, it can be described as a charge transfer.
Although the calculated HOMO–LUMO gap does not match the
experimental values (which is expected since the HOMO–LUMO
gap is only an approximation of the energy difference between the
ground state and the first excited state), the general trend follows
the experimental observation.
1
2
232 (2.70 × 104), 254 (2.83 × 104), 272 (3.09 × 104),
310 (sh)b (1.48 × 104)
230 (2.09 × 104), 262 (1.73 × 104), 280 (1.93 × 104),
320 (sh)b (8.96 × 103)
3
4
5
230 (1.86 x104), 272 (2.07 × 104), 316 (8.26 × 103)
230 (1.92 × 104), 274 (1.84 × 104), 320 (7.35 × 103)
232 (3.46 × 104), 274 (2.56 × 104), 328 (1.29 × 104)
a Concentration: [M] = 2.27–3.96 × 10−5 M. b Shoulder peak.
in the NMR spectrum at ambient temperature. The NMR data of
1–5 illustrate that the most effective way to achieve the maximum
rotation barrier for the mesityl groups is to introduce a suitable
substituent group at the 7-position.
Absorption spectra and molecular orbital calculations
As shown in Fig. 3 and Table 3, all five compounds in CH2Cl2 at
ambient temperature exhibit several intense absorption bands in
the UV region which do not change significantly with solvents.
The high energy absorption band at k < 280 nm is assigned
to p–p* transitions, while the low energy absorption band in
the region of 300–380 nm can be attributed to charge transfer
transitions from the indolyl ring to the boron center, based on
the results of molecular orbital calculations and our earlier work
on linearly conjugated three-coordinate boron compounds. Using
the absorption edge, the optical energy gaps for compounds 1–5
were estimated to be 3.39, 3.18, 3.24, 3.26 and 3.16 eV, respectively.
Among the five compounds, 1 has the largest energy gap while 2
and 5 have the smallest gap. The electron donating methyl group in
2–4 clearly is responsible for the decreasing of the HOMO–LUMO
gap, compared to 1. Among the methyl substituent molecules 2–4,
4 has the largest energy gap, which is consistent with the steric
interaction between the methyl at the 7-position of the indolyl
ring and the o-methyl groups of the mesityl rings that effectively
reduced the degree of conjugation between the boron center and
the indolyl ring as revealed by the crystal structure. The relatively
small energy gap (3.16 eV) of 5 can be attributed to the conjugation
of the phenyl group with the indole ring, which increases the
Fig. 4 Diagrams showing the HOMO and LUMO levels of compound 5.
Electrochemical properties
Cyclic voltammetry (CV) diagrams for all five compounds were
recorded. All five compounds exhibited irreversible oxidation
peaks while no reduction potentials were observed within the
solvent limit (see ESI†). The oxidation peak can be attributed
Fig. 3 UV-Vis absorption spectra of compounds 1–5 in CH2Cl2.
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Table 4 HOMO/LUMO energy levels and energy gaps for 1–5
1
2
3
4
5
Eox/Va
1.63
1.46
3.18/4.14
1.51
3.25/4.38
1.58
3.26/4.24
1.49
Optical DEb/MO DE/eV
Exp. LUMOc/MO LUMO/eV
Exp. HOMOd/MO HOMO/eV
3.39/4.44
−2.54/−1.42
−5.93/−5.85
3.18/4.16
−2.61/−1.50
−5.79/−5.66
−2.58/−1.52
−2.56/−1.36
−2.62/−1.47
−5.76/−5.66
−5.81/−5.74
−5.88/−5.71
a Measured in CH2Cl2 using Bu4N[PF6] as the electrolyte and AgCl/Ag as the reference electrode. b Recorded in CH2Cl2 with a concentration ∼10−6 M.
c From the optical energy gap and the HOMO energy. d From the oxidation potential, calibrated using the oxidation potential and the absolute HOMO
energy level of FeCp2.
to the oxidation of the indolyl group. Although three-coordinate
organoboron compounds are known to display reversible or quasi-
reversible reduction peaks,3,15 the lack of a reversible or quasi-
reversible reduction peak for all five compounds is not surprising
since the pp orbital on the boron center in these compounds
is occupied by the electrons from the nitrogen atom of the
indolyl ring. The electrochemical behavior of compounds 1–5 are
similar to three-coordinate boron compounds with pyrazine or
bipyridylidene.5g,5h Using the oxidation potentials with the fer-
rocene oxidation potential as the standard for the vacuum energy
level and the optical energy gap, the HOMO and LUMO energy
levels for all five compounds were obtained and summarized in
Table 4. Compound 2 has the highest HOMO level, followed by
compound 5 while compound 1 has the lowest HOMO level. The
trend of the observed HOMO level is consistent with the results
of MO calculations. The LUMO levels of 1–4 are comparable to
those of previously reported three-coordinate boron compounds
with a BC3 environment.
transition supported by MO calculation results. The solvent-
dependent emission spectra of compound 1 are provided in Fig. 6
as a representative example.
Luminescence
Fig. 6 Solvent dependent emission spectra of 1 (∼10−6 M).
When irradiated by UV light, all five compounds emit a bright
blue color with kmax from 423 nm to 458 nm in the solid state
while in solution the emission color ranges from blue–green to
green, depending on the solvents used. In CH2Cl2, for example,
the kem maxima ranges from 462 nm (1) to 500 nm (5) as shown
by Fig. 5. The emission energy of 1–5 shifts to a lower energy with
increasing solvent polarity, which is consistent with the presence
of a polarized excited state and the charge transfer electronic
Consistent with the trend of the optical energy gaps as
determined by UV-Vis spectra, 1 has the highest emission energy
(462 nm, 2.68 eV) while 2 and 5 have the lowest emission energy
in CH2Cl2 (500 nm, 2.48 eV) or in the solid state at ambient
temperature. All five compounds except 3 have a quantum yield
above 20% (Table 5) Compared to the BC3 type compounds
where an electron-donating nitrogen heterocycle or a diarylamino
group is separated from the boron center by a phenyl or a
biphenyl group such as B(mesityl)2{p-C6H4-(N-7-azaindolyl)} or
B(mesityl)2{p-C6H4-NPh(1-naph)} we reported previously,3e,3f the
quantum efficiencies of 1–5 are much lower.
Table 5 Quantum yield and emission data for compounds 1–5
Emission (kmax/nm)
Compds
Quantum yield/(Uem,%)a
In CH2Cl2
Film
1
2
3
4
5
27
22
13
20
26
462
501
485
476
500
423
458
437
439
452
a Using 9,10-diphenylanthracene as the standard in CH2Cl2 at RT. ([M] =
∼10−6 M).
Fig. 5 The emission spectra of compounds 1–5 in CH2Cl2 (∼10−6 M).
2640 | Dalton Trans., 2007, 2634–2643
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=
Electroluminescence
The reactivity of the B N bond toward fluoride ions
Compound 1 was selected as a representative example for eval-
uating performance as an emitter in electroluminescent devices
by fabricating a four layer EL device with the structure ITO/
CuPc(25 nm)/NPB(45 nm)/1(20 nm)/BCP(20 nm)/LiF(1 nm)/
Al(100 nm), where NPB (N,Nꢀ-bis(1-naphthyl)-N,Nꢀ-diphenyl-
benzidine) functions as a hole transport layer and BCP (batho-
cuproine) functions as an electron transport layer. As shown in
Fig. 7, the EL produced a blue emission with kmax at 445 nm. The
turn-on voltage of the EL device is ∼4.5 V and the maximum
brightness and current efficiency are ∼1037 Cd m−2 at 10 V and
0.7 cd A−1 at 5 V as shown in Fig. 8. The EL results demonstrate
Luminescent three-coordinate organoboron compounds with
three aryl groups such as mesityl, duryl, or 9-anthryl have been
demonstrated recently to be highly selective and sensitive sensors
towards fluoride ions due to the reversible binding of F− to the
boron center.4 Although compounds 1–5 have a trigonal planar
geometry around the boron center, the pp orbital of the boron is
shared by a pair of electrons from the nitrogen atom. It is therefore
not obvious if compounds 1–5 can bind to fluoride ions. To obtain
an answer to this, we examined the reaction of 1 with NBu4F
(TBAF) by first using fluorescent spectroscopy. As shown in
Fig. 9 and 10, the fluorescence intensity of 1 in CH2Cl2 at 463 nm
decreases gradually upon addition of TBAF and reaches the mini-
mum after ∼2 equiv. of F− is added. In addition to the diminished
emission intensity, the emission maximum also shifts from 463 nm
to ∼400 nm with the addition of F−. At first glance, the emission
quenching can be simply attributed to the formation of the fluoride
adduct A shown in Scheme 2, which effectively removes the p bond
between B and N atoms and blocks the indolyl to boron charge
transfer. Cl− and Br− ions were found to have little impact on the
luminescence of 1, which is not surprising since previous work
on the BC3 type of compounds has shown consistently that once
the boron center is sterically protected by four or more methyl or
=
that three-coordinate boron compounds with a B N bond such as
compound 1 are promising emitters for electroluminescent devices.
Although the brightness of the EL device based on 1 is not as high
as some of the devices based on the BC3 type compounds such as
B(mesityl)2{p-C6H4-NPh(1-Naph)} reported previously,3e,3f due
to the inherently low emission quantum efficiency of 1, the
simplicity of this class of compounds and their easy syntheses,
compared to the BC3 type compounds, still make them attractive
for potential EL applications where high brightness is not required.
Fig. 7 EL spectrum of the device.
Fig. 9 The titration diagram of 1 with TBAF (1.2 × 10−4 M) in CH2Cl2
(kex = 315 nm).
Fig. 8 L and J characteristics of the EL device.
Fig. 10 The Stern–Volmer plot of the TBAF titration of 1.
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In summary, we have demonstrated that the substituent group
on the indolyl ring has significant impact on the electronic prop-
erties and structural rigidity of the corresponding B(mesityl)2(N-
R-indolyl) complexes. We have shown that a substituent group
at the C7 position is most effective in steric blocking and the
enhancement of the B–C(mesityl) bond rotation barrier. We have
established that this group of compounds are blue emitters and
are capable of acting as emitters in OLEDs. In addition, we have
shown that fluoride ions can bind selectively to the boron center
of this group of molecules, leading to an effective quenching of
the indolyl–boron charge transfer emission, however, the fluoride
adduct formed is unstable in ambient air due to its irreversible
reactions with water molecules.
Acknowledgements
Scheme 2
We thank the Natural Sciences and Engineering Research Council
of Canada for financial support.
other substituents at the ortho positions, only fluoride ions among
the halide family are small enough to access the boron center.4
However, we have observed that unlike the previously reported
BC3 compounds where the emission intensity can usually be fully
restored after the F− titration experiment by simply using water
to extract the fluoride ion, the emission intensity of 1 cannot be
recovered by using water extraction.
References
1 (a) Z. Yuan, C. D. Entwistle, J. C. Collings, D. Albesa-Jove, A. S.
Batsanov, J. A. K. Howard, N. J. Taylor, H. M. Kaiser, D. E. Kaufmann,
S. Y. Poon, W. Y. Wong, C. Jardin, S. Fathallah, A. Boucekkine, J. F.
Halet and T. B. Marder, Chem.–Eur. J., 2006, 12, 2758; (b) Z. Yuan, N. J.
Taylor, R. Ramachandran and T. B. Marder, Appl. Organomet. Chem.,
1996, 10, 305; (c) Z. Yuan, J. C. Collings, N. J. Taylor and T. B. Marder,
J. Solid State Chem., 2000, 154, 5; (d) Z. Yuan, N. J. Taylor, T. B.
Marder, I. D. Williams, S. K. Kurtz and L. T. Cheng, Chem. Commun.,
1990, 1489; (e) R. Stahl, C. Lambert, C. Kaiser, R. Wortmann and R.
Jakober, Chem.–Eur. J., 2006, 12, 2358.
2 (a) S. Wang, Coord. Chem. Rev., 2001, 215, 79; (b) Q. D. Liu, M. S.
Mudadu, R. Thummel, Y. Tao and S. Wang, Adv. Funct. Mater., 2005,
15, 143; (c) Y. Cui, Q. D. Liu, D. R. Bai, W. L. Jia, T. Ye and S. Wang,
Inorg. Chem., 2005, 44, 601; (d) Y. Qin, I. Kiburu, S. Shah and F. Ja¨kle,
Org. Lett., 2006, 8, 5227; (e) Q. G. Wu, M. Esteghamatian, N. X. Hu, Z.
Popovic, G. Enright, Y. Tao, M. D’Iorio and S. Wang, Chem. Mater.,
2000, 12, 79; (f) H. Y. Chen, Y. Chi, C. S. Liu, J. K. Yu, Y. M. Cheng,
K. S. Chen, P. T. Chou, S. M. Peng, G. H. Lee, A. J. Carty, S. J. Yeh
and C. T. Chen, Adv. Funct. Mater., 2005, 15, 567.
3 (a) C. D. Entwistle and T. B. Marder, Angew. Chem., Int. Ed., 2002,
41, 2927; (b) C. D. Entwistle and T. B. Marder, Chem. Mater., 2004,
16, 4574; (c) Y. Shirota, J. Mater. Chem., 2005, 15, 75; (d) W. L. Jia,
X. D. Feng, D. R. Bai, Z. H. Lu, S. N. Wang and G. Vamvounis, Chem.
Mater., 2005, 17, 164; (e) W. L. Jia, M. J. Moran, Y. Y. Yuan, Z. H. Lu
and S. N. Wang, J. Mater. Chem., 2005, 15, 3326; (f) W. L. Jia, D.-R.
Bai, T. McCormick, Q. D. Liu, M. Motala, R. Y. Wang, C. Seward, Y.
Tao and S. Wang, Chem.–Eur. J., 2004, 10, 994; (g) A. Sundararaman,
K. Venkatasubbaiah, M. Victor, L. N. Zakharov, A. L. Rheingold and
F. Ja¨kle, J. Am. Chem. Soc., 2006, 125, 16554; (h) F. Jakle, Coord. Chem.
Rev., 2006, 250, 1107.
4 (a) S. Yamaguchi, T. Shirasaka and K. Tamao, Org. Lett., 2000, 2,
4129; (b) S. Yamaguchi, T. Shirasaka, S. Akiyama and K. Tamao,
J. Am. Chem. Soc., 2002, 124, 8816; (c) S. Yamaguchi, S. Akiyama and
K. Tamao, J. Organomet. Chem., 2002, 652, 3; (d) S. Yamaguchi, S.
Akiyama and K. Tamao, J. Am. Chem. Soc., 2001, 123, 11372; (e) M.
Melaimi, S. Sole, C. W. Chiu, H. D. Wang and P. Gabbai, Inorg. Chem.,
2006, 45, 8136; (f) C. W. Chiu and F. P. Gabbai, J. Am. Chem. Soc.,
2006, 128, 14248; (g) X. Y. Liu, D. R. Bai and S. Wang, Angew. Chem.,
Int. Ed., 2006, 45, 5475.
To understand this, we examined the NMR spectra of the 1 : 1
mixture of TBAF and compound 1 using reagent grade CD2Cl2
1
under air. The H NMR of the fresh mixture shows two distinct
indolyl species, neither of which belong to compound 1 or the
free indole. The spectrum rapidly changes with time and the final
spectrum shows one dominating indolyl species and one minor
indolyl species (∼10 : 1 ratio). In addition, two distinct sets of
mesityl groups are also observed, one of which can be assigned
to {B(Mesityl)2}2O (C in Scheme 2), based on the 1H NMR
spectrum of the isolated compound C (the structure of C was also
confirmed by a single-crystal X-ray diffraction analysis, which
is similar to the one reported earlier.16). 19F NMR of the fresh
mixture showed a signal at −156 ppm which is consistent with
the F− bound species A. However, this signal diminishes rapidly
with time and is replaced by a new signal at −136 ppm which can
be assigned to the fluoride adduct of C (species B in Scheme 2),
due to its similarity to the 19F NMR spectrum of the mixture of
TBAF and C. 11B NMR data of the mixture of TBAF and 1 also
confirmed the formation of species C which has a characteristic
11B chemical shift at ∼50.0 ppm. Based on the NMR data, we
therefore concluded that the fluoride adduct A is unstable toward
water molecules from TBAF (which contains water molecules), air
or the solvent, and decomposes irreversibly to form species such as
B and C. The presence of species B and C were further confirmed
−
by the observation of m/z peaks at 265 {B(mesityl)2O} and 285
−
{B(mesityl)2(F)(OH)} in the mass spectrum of the mixture of
TBAF and 1. Because similar decomposition was not observed in
the presence of chloride or bromide (in the absence of fluorides,
compounds 1–5 are stable toward air and water), the binding of
the fluoride ion to the boron center is key for the spontaneous
hydrolysis of compound 1. The absence of the free indole signals
5 (a) N. M. D. Brown, F. Davidson and J. W. Wilson, J. Organomet.
Chem., 1980, 192, 133; (b) D. R. Brittelli and D. F. Eaton, J. Phys. Org.
Chem., 1989, 2, 89; (c) D. C. Pestana and P. P. Power, Inorg. Chem.,
1991, 30, 528; (d) K. Okada, R. Suzuki and M. Oda, J. Chem. Soc.,
Chem. Commun., 1995, 2069; (e) C. Cornelissen-Gude and W. Rettig,
J. Phys. Chem. A, 1999, 103, 4371; (f) M. Glogowsk, P. J. Grisdale, J. L.
Williams and T. H. Regan, J. Organomet. Chem., 1973, 54, 51; (g) A.
Lichtblau, H. D. Hausen, W. Schwarz and W. Kaim, Inorg. Chem., 1993,
32, 73; (h) A. Lichtblau, W. Kaim, A. Schulz and T. Strahl, J. Chem.
Soc., Perkin Trans., 1992, 1497.
1
in H NMR spectra is attributed to the formation of a hydrogen
bond between the fluoride and the indole molecule as depicted in
Scheme 2, which was confirmed by the spectral similarity of 1 +
TBAF and that of indole + TBAF (see ESI†).
2642 | Dalton Trans., 2007, 2634–2643
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6 (a) V. Bocchi and G. Palla, Synthesis, 1982, 1096; (b) B. Witulski, N.
Buschmann and U. Bergstrasser, Tetrahedron, 2000, 56, 8473.
7 (a) Y. Liu and G. W. Gribble, Tetrahedron Lett., 2000, 41, 8717; (b) S. F.
Liu, Q. G. Wu, H. L. Schmider, H. Aziz, N. X. Hu, Z. Popovic and
S. N. Wang, J. Am. Chem. Soc., 2000, 122, 3671; (c) O. Ottoni, R. Cruz
and R. Alves, Tetrahedron, 1998, 54, 13915.
8 A. Yasuhara and T. Sakamoto, Tetrahedron Lett., 1998, 39, 595.
9 Bruker AXS SHELXTL NT Crystal Structure Analysis Package,
Analytical X-ray System, 1999, Version 5.10, Madison W.I.
10 D. T. Cromer and J. T. Waber, International Tables for X-ray Crystal-
lography, Kynoch Press, Birmingham, AL, 1974, vol. 4, Table 2.2A.
11 S. L. Murov, I. Carmichael and G. L. Hug, Handbook of Photochem-
istry, Mekker, New York, 2nd edn, 1993.
14 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, Jr., R. E.
Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels,
K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R.
Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski,
G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick,
A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V.
Ortiz, A. G. Baboul, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I.
Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-
Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe,
P. M. W. Gill, B. G. Johnson, W. Chen, M. W. Wong, J. L. Andres, M.
Head-Gordon, E. S. Replogle and J. A. Pople, GAUSSIAN 98 (Revision
A.6), Gaussian, Inc., Pittsburgh, PA, 1998.
12 N. J. Demas and G. A. Crosby, J. Am. Chem. Soc., 1970, 29, 7262.
13 Dudley H. Williams and I. Fleming, Spectroscopic Methods in Organic
Chemistry, McGraw-Hill Book Company (UK) Limited, London,
1987, 4th edn.
15 H. Doi, M. Kinoshita, K. Okumoto and Y. Shirota, Chem. Mater.,
2003, 15, 1080.
16 C. J. Cardin, H. E. Parge and J. W. Wilson, J. Chem. Res., Synop., 1983,
93.
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