B3O3Ph3(7-azaindole): Structure, luminescence, and fluxionality

Article abstract of DOI:10.1021/om990053t

A 7-azaindole adduct of boroxine, B₃O₃Ph₃-(7-azaindole), 1, was obtained from the reaction of PhB-(OH)₂ with 7-azaindole. The crystal structure of 1 shows that the 7-azaindole ligand is bonded to the boroxine molecule through a B-N bond and an H?O hydrogen bond. Compound 1 is fluorescent in solution and the solid state. There is, however, a dramatic difference in the emission maximum of the solution (λ_max = 368 nm) and solid (λ_max = 400 nm) spectra. The solution behavior of 1 was examined by ¹H NMR spectroscopic methods, which established that compound 1 is highly fluxional in solution, attributable to an intermodular 7-azaindole ligand dissociation/association process with an activation energy of 34 ± 1.5 kJ mol^-1. To determine the role of the hydrogen bond in the dynamic process of 1, the structure and solution behavior of B₃O₃Ph₃(Py), 2, was also examined. In solution compound 2 undergoes an intermodular exchange process similar to that of compound 1, with an activation energy of 39 ± 1.5 kJ mol^-1. The relatively small activation energy in 1 could be attributed to the hydrogen bond and the reduced base strength of 7-azaindole, relative to that of pyridine.

Full text of DOI:10.1021/om990053t

Organometallics 1999, 18, 2553-2556
2553
B O P h (7-a za in d ole): Str u ctu r e, Lu m in escen ce, a n d
3
3
3
F lu xion a lity
Q. G. Wu, Gang Wu, Lorenzo Brancaleon, and Suning Wang*
Department of Chemistry, Queen’s University, Kingston, Ontario, K7L 3N6 Canada,
and Steacie Institute for Molecular Science, National Research Council, 100 Sussex Drive,
Ottawa, Ontario, K1A 0R6 Canada
Received J anuary 27, 1999
Summary: A 7-azaindole adduct of boroxine, B3O3Ph3-
luminescent properties of boroxine-based compounds in
the visible region have not been investigated. We
therefore report a new 7-azaindole compound, B O Ph -
(7-azaindole), 1, was obtained from the reaction of PhB-
OH)2 with 7-azaindole. The crystal structure of 1 shows
(
3
3
3
that the 7-azaindole ligand is bonded to the boroxine
molecule through a B-N bond and an H‚‚‚O hydrogen
bond. Compound 1 is fluorescent in solution and the
solid state. There is, however, a dramatic difference in
the emission maximum of the solution (λmax ) 368 nm)
and solid (λmax ) 400 nm) spectra. The solution behavior
(7-azaindole) (1), which is not only luminescent but also
highly fluxional in solution. The synthesis, structure,
luminescence, and fluxionality of 1 are presented herein.
Exp er im en ta l Section
1
of 1 was examined by H NMR spectroscopic methods,
All syntheses were carried out under a nitrogen atmosphere.
Solvents were freshly distilled prior to use. 7-Azaindole and
PhB(OH) were purchased from Aldrich Chemical Co. NMR
2
spectra were recorded on a Bruker AM 400 spectrometer.
Elemental analyses were performed by Canadian Microana-
lytical Service, Delta, British Columbia. Excitation and emis-
sion spectra were recorded on a Photon Technologies Inter-
national QM1 spectrometer.
which established that compound 1 is highly fluxional
in solution, attributable to an intermolecular 7-azaindole
ligand dissociation/ association process with an activa-
-
1
tion energy of 34 ( 1.5 kJ mol . To determine the role
of the hydrogen bond in the dynamic process of 1, the
structure and solution behavior of B3O3Ph3(Py), 2, was
also examined. In solution compound 2 undergoes an
intermolecular exchange process similar to that of
compound 1, with an activation energy of 39 ( 1.5 kJ
Syn th eses. B O (P h ) (7-a za in ) (1). 7-Azaindole (0.118 g,
3
3
3
1 mmol) was added to phenylboron dihydroxide (0.366 g, 3
mmol) in 10 mL of toluene at 23 °C. The mixture was stirred
and heated at 80 °C for 5 h. After the solution was cooled to
room temperature, it was concentrated to about 2 mL by
vacuum. After the solution was kept for several days at 23
-
1
mol . The relatively small activation energy in 1 could
be attributed to the hydrogen bond and the reduced base
strength of 7-azaindole, relative to that of pyridine.
°
C, colorless crystals of compound 1 were obtained. Yield: 0.28
In tr od u ction
1
g (65%). H NMR (CD
7
Hz, 1H, 7-azain), 8.02 (dd, J ) 6.8 Hz, J ) 3.4 Hz, 6H, Ph),
7
2
Cl
2
, ppm, 25 °C): δ 10.52 (s, 1H,
3
3
-azain), 8.85 (d, J ) 5.7 Hz, 1H, 7-azain), 8.24 (d, J ) 7.7
We reported recently that 7-azaindole is a good blue
emitter when bound to aluminum or boron centers.
The usefulness of 7-azaindole boron compounds in
electroluminescence has been demonstrated by a blue
electroluminescent device using our boron compound B2-
3
4
1
,2
3
.47 (d, J ) 3.6 Hz, 1H, 7-azain), 7.40-7.38 (m, 9H, Ph), 7.36
3 3
(
t, J ) 6.8 Hz, 1H, 7-azain), 6.65 (d, J ) 3.6 Hz, 1H, 7-azain).
1
3
C NMR (CD Cl , ppm, 25 °C): δ 136.61 (7-azain), 134.42 (7-
2
2
azain), 133.77 (Ph), 130.09 (Ph), 127.99 (Ph), 127.10 (7-azain),
116.19 (7-azain), 102.41 (7-azain). 11B NMR (CDCl
/CH Cl
400 MHz, ppm, relative to BF ‚O(C H ) , 25 °C): δ 20.53 (s,
(
O)(7-azain)2R2, R ) ethyl or phenyl, as the emitting
3
2
2
,
2
b
layer. In search of new luminescent boron compounds,
we initiated the investigation on 7-azaindole derivatives
of boroxines. The attractiveness of boroxines lies in the
fact that they are generally stable chemically and
3
2
5 2
3
B). Anal. Calcd for C25
Found: C, 69.64; H, 4.93; N, 6.50.
(P h ) (P yr ) (2). Compound 2 was obtained by a pro-
cedure similar to that for compound 1 in 56% yield. H NMR
21 2 3 3
H N O B : C, 69.84; H, 4.92; N, 6.52.
B
3
O
3
3
1
3
thermally, making them potentially good candidates
for electroluminescent applications. Although boronic
acids that contain an aromatic fluorophore have been
3
4
(CD
2
Cl
2
, ppm, 25 °C): δ 9.07 (dd, J ) 6.6 Hz, J ) 1.5 Hz,
3 4 3
2
7
7
H, Py), 8.07 ( J ) 6.1 Hz, J ) 2.5 Hz, 6H, Ph), 8.01 (tt, J )
4
3
.7 Hz, J ) 1.5 Hz, 1H, Py), 7.63 (t, J ) 7.5 Hz, 2H, Py),
4
found recently to be effective as fluorescent probes, the
13
2 2
.43-7.39 (m, 9H, Ph). C NMR (CD Cl , 400 MHz, ppm, 25
°C): δ 143.87 (Py), 140.76 (Py), 133.76 (Ph), 129.74 (Ph), 127.55
1
1
(
1) (a) Liu, W.; Hassan, A.; Wang, S. Organometallics 1997, 16, 4257.
b) Ashenhurst, J .; Brancaleon, L.; Hassan, A.; Liu, W.; Schmider, H.;
(Ph), 125.54 (Py). B NMR (CDCl
relative to BF ‚O(C , 25 °C): δ 20.47 (s, 3B). Anal. Calcd
for C23 : C, 70.68; H, 5.16; N, 3.58. Found: C, 70.53;
H, 5.10; N, 3.62.
X-r a y Diffr a ction An a lyses. All crystals were obtained
from concentrated toluene solutions and mounted on glass
capillaries. All data were collected on a Siemens P4 single-
crystal diffractometer with graphite-monochromated Mo KR
radiation, operated at 50 kV and 40 mA at 23 °C. The data
for 1 were collected over 2θ 3-50°, while the data for 2 were
collected over 2θ 3-48°. Three standard reflections were
measured every 197 reflections. No significant decay was
3 2 2
/CH Cl , 400 MHz, ppm,
(
3
2 5 2
H )
Wang, S.; Wu, Q. Organometallics 1998, 17, 3186. (c) Gao, S.; Wu, Q.;
Wu, G.; Wang, S. Ibid. 1998, 17, 4666.
2) (a) Hassan, A.; Wang, S. J . Chem. Soc., Chem. Commun. 1998,
11. (b) Wu, Q.; Esteghamatian, M.; Hu, N. X.; Popovic, Z.; Enright,
G.; Breeze, S. R.; Wang, S. Angew. Chem., Int. Ed. 1999, 38, 985.
3) (a) Washburn, R. M.; Levens, E.; Albright, C. F.; Billig, F. A.
H
3 3
20NO B
(
2
(
Org. Synth. 1959, 39, 3. (b) Snyder, H. R.; Konecky, M. S.; Lennarz,
W. J . J . Am. Chem. Soc. 1958, 80, 3611. (c) Brown, H. C.; Cole, T. E.
Organometallics 1985, 4, 816. (d) Cotton, F. A.; Wilkinson, G. Advanced
Inorganic Chemistry, 5th ed.; J ohn Wiley & Sons: New York, 1988;
pp 202-203.
(
4) Cooper, C. R.; Spencer, N.; J ames, T. D. J . Chem. Soc., Chem.
Commun. 1998, 1365.
1
0.1021/om990053t CCC: $18.00 © 1999 American Chemical Society
Publication on Web 06/02/1999

2
554 Organometallics, Vol. 18, No. 13, 1999
Notes
Ta ble 1. Cr ysta llogr a p h ic Da ta
1
2
formula
fw
space group
a/Å
b/Å
c/Å
C25H21N2O3B3
429.9
C23H20NO3B3
390.8
P21/n
P21/c
11.8267(11)
14.243(2)
13.7878(12)
90
100.272(4)
90
14.671(4)
11.562(8)
13.062(7)
90
103.65(3)
90
R/ deg
â/ deg
γ/ deg
3
V/Å
Z
2285.4(5)
4
2153(2)
4
-
3
Dc, g cm
1.25
1.21
-
1
µ, cm
0.80
0.77
2
θmax, deg
50
48
reflns measured
reflns used (Rint)
no. of variables
final R [I > 2σ(I)]
4275
3509
4008 (0.040)
299
3341 (0.024)
271
a
R1 ) 0.0675
R1 ) 0.0574
wR2 ) 0.1344
R1 ) 0.0858
wR2 ) 0.1585
1.099
b
wR2 ) 0.1235
F igu r e 1. Diagram showing the molecular structure of 1
with 50% thermal ellipsoids and labeling schemes.
R (all data)
R1 ) 0.1910
wR2 ) 0.1806
1.009
goodness-of-fit on F2
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for 1
a
R1 ) ∑|Fo| - |Fc|/∑|Fo|. wR2 ) [∑w[(Fo2 - Fc ) ]/∑[w(Fo ) ]]1/2.
b
2
2
2 2
2
2
2
2
2
w ) 1/[σ (Fo ) + (0.075P) ], where P ) [max(Fo , 0) + 2Fc ]/3.
O(2)-B(2)
O(2)-B(3)
O(3)-B(3)
O(3)-B(1)
O(1)-B(2)
1.372(6)
1.383(5)
1.348(5)
1.478(6)
1.345(5)
O(1)-B(1)
N(1)-B(1)
C(14)-B(1)
C(20)-B(3)
C(8)-B(2)
1.456(6)
1.666(7)
1.580(7)
1.547(6)
1.548(6)
observed for either crystal during the data collection. Data
were processed on a Pentium PC using Siemens SHELXTL
software package (version 5.0) and corrected for Lorentz and
5
polarization effects. Neutral atom scattering factors were taken
from Cromer and Waber. The crystals of 1 and 2 belong to
B(2)-O(2)-B(3)
B(3)-O(3)-B(1)
B(2)-O(1)-B(1)
O(1)-B(2)-O(2)
O(1)-B(2)-C(8)
O(2)-B(2)-C(8)
O(3)-B(3)-O(2)
121.5(4)
O(2)-B(3)-C(20) 118.5(4)
O(1)-B(1)-O(3) 112.5(4)
6
122.8(4)
123.0(4)
120.3(4)
120.8(5)
118.9(4)
119.5(4)
the monoclinic crystal system. The space groups P2
1
/n and
O(1)-B(1)-C(14) 113.8(4)
O(3)-B(1)-C(14) 113.1(4)
P2 /c were determined uniquely for compounds 1 and 2,
1
O(1)-B(1)-N(1)
O(3)-B(1)-N(1)
103.5(4)
104.6(4)
respectively, on the basis of systematic absences. The struc-
tures were solved by direct methods. The positions for all
hydrogen atoms were calculated, and their contributions in
structural factor calculation were included. All non-hydrogen
atoms in 1 and 2 were refined anisotropically. The crystal-
lographic data for compounds 1 and 2 are given in Table 1.
C(14)-B(1)-N(1) 108.3(4)
O(3)-B(3)-C(20) 122.0(4)
The basic features of the structure of 1 resemble those
of previously known B3Ph3(O)3L compounds, where L
is a simple Lewis donor. Two of the boron atoms (B(2),
7
Resu lts a n d Discu ssion
B(3)) have a trigonal planar geometry while the third
boron atom (B(1)) has a tetrahedral geometry. The
B(2)-O(2), B(2)-O(1), B(3)-O(3), and B(3)-O(2) dis-
tances ranging from 1.345(5) to 1.383(5) Å are much
shorter than those of B(1)-O(1) and B(1)-O(3) (1.456-
Str u ctu r e of 1 in th e Solid Sta te a n d in Solu tion .
Our initial intention in carrying out the reaction of PhB-
(OH)2 with 7-azaindole was to make new boron com-
plexes where the 7-azaindole anion (7-azain) functions
as a bridging ligand in a manner similar to that in R2B2-
(6), 1.478(6) Å), consistent with the geometry around
2
each boron center. Similarly, the B(2)-C(8) and B(3)-
C(20) bond lengths (1.548(6), 1.547(6) Å) are somewhat
shorter than that of B(1)-C(14) (1.580(7) Å). The B(1)-
N(1) bond length (1.666(7) Å) is similar to those previ-
(
O)(7-azain)2 compounds. However, regardless of the
reaction conditions and the amount of 7-azaindole used,
we found that compound 1 with the formula of B3O3-
(Ph)3(7-azain-H) was always obtained as the only prod-
7
ously reported. However, unlike the previously reported
uct. Compound 1 can be regarded as a 7-azaindole
adduct of the boroxine compound B3O3Ph3. The forma-
tion of the boroxine compound from the reaction of PhB-
structures of B3Ph3(O)3L where no internal hydrogen
bonds are present, the 7-azaindole ligand in 1, in
addition to the formation of a B-N bond, forms an
(OH)2 is not surprising because it has been well estab-
8
internal hydrogen bond with the oxygen atom, O(3),
lished that RB(OH)2 can cyclize readily to form the
corresponding boroxine compound when either heated
as evident by the distance of N(2)‚‚‚O(3), 2.792 Å.
Niedenzu and Bielawski reported a pyrazole adduct,9a
B3O3Ph3(Hpz), in 1986. Two different structures were
proposed for B3O3Ph3(Hpz) by Niedenzu’s and Yalpani’s
3
or Lewis bases are present in solution. Compound 1
displays an unusual purple luminescence in the solid
state when irradiated by UV light. In solution, it
appeared to be highly dynamic. To fully understand the
properties of compound 1, we determined the structure
of 1 by a single-crystal X-ray diffraction analysis.
The molecular structure of 1 is shown in Figure 1.
Selected bond lengths and angles are given in Table 2.
9
a,b
groups,
on the basis of their solution NMR studies.
(7) (a) Yalpani, M.; Boese, R. Chem. Ber. 1983, 116, 3347. (b)
Beckett, M. A.; Strickland, G. C.; Varma, K. S. Polyhedron 1995, 14,
2623. (c) Beckett, M. A.; Strickland, G. C.; Varma, K. S.; Hibbs, D. E.;
Hursthouse, M. B.; Malik, K. M. A. J . Organomet. Chem. 1997, 535,
3
3.
(
8) (a) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry,
(
5) SHELXTL crystal structure analysis package, Version 5; Bruker
Axs, Analytical X-ray System: Madison, WI, 1995.
6) Cromer, D. T.; Waber, J . T. International Tables for X-ray
Crystallography; Kynoch Press: Birmingham, Al, 1974; Vol. 4, Table
.2A.
5th ed.; J ohn Wiley & Sons: New York, 1988; pp 92-93. (b) Hamilton,
W. C.; Ibers, J . A. Hydrogen Bonding in Solids; W. A. Benjamin, Inc.:
New York, 1968.
(9) (a) Bielawski, J .; Niedenzu, K. Inorg. Chem. 1986, 11, 1771. (b)
Yalpani, M.; K o¨ ster, R. Chem. Ber. 1988, 121, 1553.
(
2

Notes
Organometallics, Vol. 18, No. 13, 1999 2555
Sch em e 1
1
F igu r e 2. Variable-temperature H NMR spectra of 1 in
CD Cl (left) and the simulated spectra for the phenyl
2
2
chemical shift at 7.39 ppm (right).
We believe that it is very likely that in the solid state
Niedenzu’s compound has a hydrogen bond structure
similar to that of 1.
Due to the formation of the hydrogen bond, each of
the three boron atoms in 1 in the solid state has a
distinct environment. In solution, one can anticipate
that the hydrogen bond could switch rapidly between
O(3) and O(1), resulting in a similar chemical environ-
ment for both B(2) and B(3). Hence, chemical shifts due
to two types of phenyl groups should be observed in the
F igu r e 3. Diagram showing the plot of ln(k) vs 1/T and
the linear fit for compounds 1 and 2.
less of the ratio of 1 and noncoordinated 7-azaindole,
indicative of the presence of a rapid exchange between
coordinated and noncoordinated 7-azaindole ligands. In
addition, the NMR spectra show that as the concentra-
tion of 7-azaindole increases in solution, the chemical
shifts due to 7-azaindole shift toward those of free
7-azaindole, further supporting the presence of the
intermolecular exchange process shown in Scheme 1.
Because the exchange process of 1 is between two
nonequally populated sites (one tetrahedral boron site
and two trigonal planar boron sites), the free energy for
the exchange process therefore cannot be obtained by
1
13
H and C NMR spectra of 1 in solution. However,
contrary to our anticipation, only one set of chemical
1
13
shifts due to phenyls was observed in H and C NMR
spectra of 1 at ambient temperature, indicating that
compound 1 must be fluxional in solution. Variable-
1
13
temperature H and C NMR confirmed the presence
of a dynamic process. As shown in Figure 2, the two
peaks at 8.02 (protons at 2,6 positions) and 7.39 ppm
1
0
(protons at 3, 4, 5 positions, marked by *) due to the
using k ) 2.22 ∆ν and the Eyring equation. We
1
0
phenyl groups broaden as the temperature decreases.
At 180 K, two broad peaks with a 2:1 ratio due to the
protons at 3, 4, 5 positions were observed, which is in
agreement with the presence of two types of phenyl
environments.
There are two possible mechanisms responsible for
the dynamic behavior of 1. The first mechanism involves
an intramolecular migration of the 7-azaindole to all
three boron sites. The second mechanism is an inter-
molecular process involving the dissociation of the
therefore performed a line shape analysis for the
1
phenyl peak located at 7.39 ppm in the H NMR spectra
of 1 over the temperature range 260-180 K. The
simulated spectra are shown in Figure 2. From the data
of line shape analyses, we obtained the exchange rate
k for several temperatures. From the slope of the plot
of ln(k) versus 1/T the activation energy Ea of 34 ( 1.5
-
1
kJ mol was obtained (Figure 3).
A similar dynamic process was also observed in
B3R3O3L, where L is a simple terminal ligand.7
A
7
7
-azaidole ligand from 1 and the reattachment of
-azaindole ligand to the boroxine B3O3Ph3, averaging
quantitative kinetic analysis on some of the previously
known B3R3O3L compounds was reported by Beckett
7
b,c
the environment around the three boron centers. If the
exchange process is an intramolecular process, the
chemical shifts of the coordinated 7-azaindole ligand
should not be affected by the presence of free 7-azaindole
ligand in solution. Consequently two sets of 7-azaindole
chemical shifts, the coordinated ones and the uncoor-
dinated ones, should be observed in the NMR spectra
of the mixture of 1 and 7-azaindole. To distinguish
between the intra- and intermolecular exchange mech-
anisms, we examined a series H NMR spectra of 1 in
the presence of varied amount of 7-azaindole. The H
NMR spectra of the solution of 1 and 7-azaindole from
a 1:0 ratio to a 1:8 ratio were examined. Only one set of
chemical shifts due to 7-azaindole was observed, regard-
and co-workers.
However, in their analysis, the
equation k ) 2.22 ∆ν was used in the calculation of the
exchange rate constant and the subsequent derivation
of free energy, which is in error because the exchange
is not on two equally populated sites.
To compare the exchange behavior of 1 with that of
simple Lewis base adducts, B3R3O3L, we synthesized
1
1
B3R3O3(Py) (2). The structure of 2 is very similar to
those of B3R3O3L molecules reported earlier. The solu-
tion behavior of 2 is very similar to that of 1, i.e., a rapid
1
1
(
10) G u¨ nther, H. NMR Spectroscopy, 2nd ed.; J ohn Wiley & Sons:
Chichester, 1992.
11) The detail of the structure of 2 can be found in the Supporting
Information.
(

2
556 Organometallics, Vol. 18, No. 13, 1999
Notes
dissociation/coodination of pyridine. The line shape
analysis for the phenyl peak at 7.41 ppm in the variable-
1
temperature H NMR data established that the activa-
tion energy for the exchange process of 2 is 39 ( 1.5 kJ
-
1
mol . The fact that the activation energy for the
exchange process of B3R3O3(Py) is lower than that of
B3R3O3(7-azainH) is somewhat surprising because 7-aza-
indole is a much bigger ligand than pyridine; hence one
would expect the exchange process for 1 could be slower
and the activation energy would be higher than that of
2
. The activation energy discrepancy between 1 and 2
could be explained by an intermediate involving a
hydrogen bond in the exchange process of 1 which
effectively decreases the energy barrier for compound
1
, in comparison to that of compound 2, where such an
intermediate is not possible. The alternative explanation
is that 7-azaindole is a weaker base than pyridine due
to both steric and electronic factors, resulting in a
relatively low activation energy in compound 1.
F igu r e 4. Emission spectra of compound 1 in solution
(dashed line) and solid state (solid line).
Lu m in escen ce of 1. The free 7-azaindole ligand and
compound 2 have no emission in the visible region in
both solution and solid state. We have demonstrated
previously that the coordinated 7-azaindole anion does
produce blue luminescence in aluminum and boron
solution (λ ) 357 nm). Based on our NMR studies of
the solution behavior of 1, the blue shift of the emission
band in solution can be attributed to the dissociation/
coordination exchange process of the 7-azaindole ligand.
The emission band in solution therefore contains con-
tributions from both compound 1 and the free 7-azain-
dole ligand. Compound 1 is stable in air and can be
sublimed readily at 110 °C and 0.08 mmHg, making it
a potential candidate for applications in organic elec-
troluminescent devices, which is being investigated in
our laboratory.
1
,2
complexes. Because the 7-azaindole ligand in 1 is
neutral, we did not anticipate any emission by this
compound in the visible region. It is therefore to our
surprise when we observed that compound 1 emits a
purple color when irradiated by UV light in the solid
state. The emission maximum of 1 in the solid state was
determined to be at λ ) 400 nm. The formation of a
hydrogen bond and a B-N bond between the 7-azain-
dole and the B3Ph3O3 unit must have played an impor-
tant role in promoting the luminescence because a
similar luminescence has not been observed in com-
plexes where 7-azaindole coordinates to Al(III) or B(III)
Ack n ow led gm en t. We thank the Natural Science
and Engineering Research Council of Canada for finan-
cial support.
as a terminal ligand through the pyridyl nitrogen site
only.12 An interesting phenomenon is the blue shift of
Su p p or tin g In for m a tion Ava ila ble: Crystal data, tables
of atomic coordinates, thermal parameters, a complete list of
bond lengths and angles, hydrogen parameters, and a diagram
showing the structure of 2. This material is available free of
charge via the Internet at http://pubs.acs.org.
the emission band in solution, as shown in Figure 4
.
The emission maximum of 1 in solution is at λ )
3
65 nm, very close to that of free 7-azaindole ligand in
(12) Wu, Q. and Wang, S. Unpublished results.
OM990053T