Highly fluxional blue luminescent aluminum complexes: Al(CH3)(7-azain-2-Ph)2(7-azainH-2-Ph), Al3(μ3-O)(CH3) 3(7-azain-2-Ph)4, and Al3(μ3-O)(CH3)3(7-azain-2-CH 3)4 (7-azain = 7-azaindole anion)

Article abstract of DOI:10.1021/om980435j

The new aluminum complexes Al(CH₃)(7-azain-2-Ph)₂(7-azainH-2-Ph) (1), Al₃(μ₃-O)(CH₃) ₃-(7-azain-2-Ph)₄ (2), and Al₃(μ₃-O)(CH₃)₃(7-azain-2-CH ₃)₄ (3), where 7-azain = 7-azaindole anion, have been synthesized and characterized structurally. The three aluminum atoms in compounds 2 and 3 are bridged by an oxo ligand. Compounds 2 and 3 have an asymmetric structure in the solid state that results in a distinct chemical environment for each aluminum atom and each 7-azain-2-R ligand in the compounds. All compounds emit a blue color, when irradiated by UV light. The blue emission is attributed to the coordinated 7-azain-2-CH₃ and 7-azain-2-Ph ligands. Variable-temperature 1D and low-temperature 2D EXSY ¹H NMR experiments established that compounds 2 and 3 are highly fluxional in solution, attributable to an unusual intramolecular migration of the coordinated 7-azain-2-R ligands.

Full text of DOI:10.1021/om980435j

4666
Organometallics 1998, 17, 4666-4674
High ly F lu xion a l Blu e Lu m in escen t Alu m in u m
Com p lexes: Al(CH3)(7-a za in -2-P h )2(7-a za in H-2-P h ),
Al3(µ3-O)(CH3)3(7-a za in -2-P h )4, a n d
Al3(µ3-O)(CH3)3(7-a za in -2-CH3)4 (7-a za in ) 7-Aza in d ole
An ion )
Shu Gao, Qingguo Wu, Gang Wu, and Suning Wang*
Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada
Received May 26, 1998
The new aluminum complexes Al(CH3)(7-azain-2-Ph)2(7-azainH-2-Ph) (1), Al3(µ3-O)(CH3)3-
(7-azain-2-Ph)4 (2), and Al3(µ3-O)(CH3)3(7-azain-2-CH3)4 (3), where 7-azain ) 7-azaindole
anion, have been synthesized and characterized structurally. The three aluminum atoms
in compounds 2 and 3 are bridged by an oxo ligand. Compounds 2 and 3 have an asymmetric
structure in the solid state that results in a distinct chemical environment for each aluminum
atom and each 7-azain-2-R ligand in the compounds. All compounds emit a blue color, when
irradiated by UV light. The blue emission is attributed to the coordinated 7-azain-2-CH3
and 7-azain-2-Ph ligands. Variable-temperature 1D and low-temperature 2D EXSY 1H NMR
experiments established that compounds 2 and 3 are highly fluxional in solution, attributable
to an unusual intramolecular migration of the coordinated 7-azain-2-R ligands.
In tr od u ction
quinoline)aluminum complexes typically emit in the
yellow-green region. However, with the introduction of
appropriate substituents on the 2- and 4-positions of the
quinoline, the emission energy of 8-hydroxyquinoline
complexes can be shifted to the blue region via mostly
electronic effects.3 Because 8-hydroxyquinoline com-
plexes usually emit in the yellow-green region and
elaborate ligand modification has to be carried out in
order to achieve a blue emission, the application of
8-hydroxyquinoline-based complexes as blue emitters in
electroluminescent displays is therefore rather limited.3
To determine the effect a substituent on the 7-azaindole
ligand has on the properties of aluminum complexes,
we initiated the investigation of aluminum complexes
containing 7-azaindole derivatives, specifically those
that have a subsitituent such as CH3, CF3, and phenyl
at the 2-position (2-R-7-azaindole). We were able to
synthesize 2-phenyl-7-azaindole and 2-methyl-7-azain-
dole (Chart 1) but were unsuccessful in the attempted
synthesis of 2-trifluoromethyl-7-azaindole. Using the
2-R-7-azaindole ligands, we have synthesized three new
aluminum compounds, two of which are trinuclear
complexes and exhibit an unusual dynamic behavior in
solution. The syntheses, crystal structures, luminescent
properties, and dynamic solution behavior of these new
aluminum compounds are presented herein.
We have demonstrated recently that 7-azaindole
forms various complexes readily upon reaction with
trialkylaluminum.1 Depending on the reaction condi-
tions, the resulting 7-azaindole aluminum complexes
display versatile and complex structural features. The
most interesting and important property of these com-
plexes is blue luminescence, because of the potential
application in electroluminescent displays.2 We have
reported that the blue emission from these aluminum
complexes is originated from the coordinated 7-azain-
dole anion, where the aluminum ion plays a key role in
stabilizing the complex and promoting the blue emis-
sion. We have also observed that nonemitting ligands
around the aluminum do not have any significant effect
on the blue emission of the complex, but they do
influence the stability and volatility of the complex. One
factor that could have direct impact on luminescent
properties and other physical properties such as stabil-
ity and volatility of the complex is substituents on the
7-azaindole ligand. In fact, it has been demonstrated
previously in 8-hydroxyquinoline-based aluminum com-
plexes that the introduction of a substituent on the
emitting 8-hydroxyquinoline ligand has a dramatic
effect on emission energy.3 For example, (8-hydroxy-
(1) (a) Ashenhurst, J .; Brancaleon, L.; Hassan, A.; Liu, W.; Schimder,
H.; Wang, S. Organometallics 1998, 17, 3186-3195. (b) Liu, W.;
Hassan, A.; Wang, S. Organometallics 1997, 16, 4257.
(2) (a) Rack, P. D.; Naman, A.; Holloway, P. H.; Sun, S.; Tuenge, R.
T. Mater. Res. Bull. 1996, 21(3), 49. (b) Mauch, R. H.; Velthaus, K.-O.;
Hu¨ttl, B.; Troppenz, U.; Herrmann, R., SID 95 Digest 1995, 720. (c)
Sun, S. S.; Tuenge, R. T.; Kane, J .; Ling, M. J . Electrochem. Soc.
1994, 141(10), 2877. (d) Tsutsui, T. Progress in Electroluminescent
Devices Using Molecular Thin Films In Mater. Res. Bull. 1997, 22(6),
39.
Exp er im en ta l Section
All manipulations were carried out under an atmosphere
of nitrogen using either the standard vacuum line techniques
or inert-atmosphere dryboxes. All solvents were freshly
distilled over the appropriate drying agent under a nitrogen
atmosphere. Trimethylaluminum and 2-amino-3-picoline were
purchased from Aldrich. 2-Aminopicoline was further purified
by distillation. All NMR samples were prepared in the drybox.
2-Phenyl-7-azaindole and 2-methyl-7-azaindole were prepared
(3) (a) Bryan, P.; Lovecchio, F. V.; VanSlyke, S. A. U.S. Patent
5,141,671, 1992. (b) VanSlyke, S. A. U.S. Patent 5,151,629, 1992.
S0276-7333(98)00435-X CCC: $15.00 © 1998 American Chemical Society
Publication on Web 09/15/1998

Blue Luminescent Aluminum Complexes
Organometallics, Vol. 17, No. 21, 1998 4667
solution was stirred and refluxed under nitrogen for 30 min.
After the solution was cooled to room temperature, 2-acet-
amido-3-picoline (3.6 g) was added to the cooled solution and
the mixture was heated at 280 °C for 5 min with stirring. After
the mixture was cooled to room temperature, acetic acid was
added until all solids were dissolved. The solution was then
made basic with sodium hydroxide and extracted with chlo-
roform. The extract was dried over Na2SO4 and concentrated.
The residue was separated by column chromatography on
silica gel with a solution of ethyl acetate/hexane in a 1:1.5 ratio.
The crude product was recrystallized from cyclohexane to give
a crystalline product (0.72 g, yield 23%). Mp: 135-136 °C.
1H NMR (CDCl3; δ, ppm): 11.12 (br, 1H, NH), 8.19 (d, 1H,
CH), 7.80 (d, 1H, CH), 7.01 (m, 1H, CH), 6.15 (s, 1H, CH),
2.50 (s, 3H, CH3).
Al(CH3)(7-a za in -2-P h )2(7-a za in -2-P h -H) (1). 2-Phenyl-
7-azaindole (0.175 g, 0.9 mmol) in 15 mL of toluene was heated
to 60 °C. After all 2-phenyl-7-azaindole was dissolved in
toluene, Al(CH3)3 (0.15 mL, 2 M, 0.3 mmol) was added to the
solution. After the solution was stirred for 20 h at room
temperature, it was concentrated to about 2 mL by vacuum.
Colorless crystals of compound 2 were obtained. Yield: 0.125
g (66%). 1H NMR (CDCl3; δ, ppm, 298 K): 14.76 (s, 1H, NH),
8.40-6.75 (br, m, 24H, Ar H), 6.50 (s, 3H, Ar H), -0.50 (s,
3H, Al-CH3). 13C NMR (CDCl3; δ, ppm, 298 K): 139.3, 129.0,
128.2, 127.9, 127.3, 125.5, 125.2, 114.9, 101.8, 97.6 (aromatic
carbon); -9.0 (br, CH3). Anal. Calcd for C40H31N6Al: C, 71.15;
H, 5.00; N, 13.50. Found: C, 71.11; H, 4.83; N, 13.48.
Al3(CH3)3(µ3-O)(7-a za in -2-P h )4 (2). 2-Phenyl-7-azaindole
(0.078 g, 0.4 mmol) in 10 mL of toluene was heated to 60 °C.
After all 2-phenyl-7-azaindole was dissolved in toluene, Al-
(CH3)3 (0.15 mL, 2 M, 0.3 mmol) was added to the solution
and the solution was stirred for 5 h at room temperature with
water-saturated nitrogen gas being passed through the flask.
After the solution was stirred for an additional 15 h, the
mixture was concentrated to about 2 mL by vacuum. Colorless
crystals of compound 1 were obtained. Yield: 0.140 g (77%).
1H NMR (CDCl3; δ, ppm, 298 K): 7.80-6.20 (m, 36 H, Ar H),
-0.50 (broad, 9H, Al-CH3). Anal. Calcd for C55H45N8OAl3‚
C7H8: C, 73.94; H, 5.30; N, 11.13. Found: C, 73.91; H, 5.38;
N, 10.59. (The sample of 2 was evacuated under vacuum to
remove the solvent molecules before it was sent for elemental
analysis. However, the result of elemental analysis indicated
that there is still one toluene molecule per complex.)
Al3(CH3)(µ3-O)(7-a za in -2-CH3)4 (3). 2-Methyl-7-azaindole
(0.159 g, 1.2 mmol) in 8 mL of toluene was reacted with Al-
(CH3)3 (0.45 mL, 2 M, 0.9 mmol) for 5 h at room temperature
with water-saturated nitrogen gas being passed through the
flask. After the solution was stirred for another 15 h under
nitrogen, it was concentrated to about 2 mL by vacuum.
Colorless crystals of compound 3 were obtained. Yield: 0.142
g (71%). 1H NMR (CDCl3; δ, ppm, 298 K): 7.69-6.10 (m, 16H,
Ar H); 2.34, 2.15 (s, 12H, C-CH3), -0.35, -0.72 (broad, 9H,
Al-CH3). Anal. Calcd for C35H37ON8Al3‚0.5(toluene): C,
64.89; H, 5.76; N, 15.73. Found: C, 64.45; H, 5.74; N, 15.76.
X-r a y Diffr a ction An a lyses. All crystals were obtained
either from concentrated toluene solutions or from solutions
of toluene/hexane. The crystals were sealed in glass capillaries
under nitrogen. 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 and 3 were collected over 2θ ) 3-45° while the data
for 2 was collected over 2θ ) 3-50°. Three standard reflec-
tions were measured every 197 reflections. No significant
decay was observed for any sample during the data collection.
Data were processed on a Pentium PC using the Siemens
SHELXTL software package (version 5.0)5 and corrected for
Lorentz and polarization effects. Neutral atom scattering
Ch a r t 1. 2-Meth yl-7-a za in d ole a n d
2-P h en yl-7-a za in d ole
by a modified procedure reported by Herbert and Wibberley.4
1H NMR spectra were recorded on Bruker AM 400 and ACF
200 spectrometers. 13C NMR spectra were recorded on a
Bruker AM 400 spectrometer at 100.6 MHz. The two-
dimensional (2D) exchange experiment (EXSY) was performed
at 243 K with the standard three-pulse sequence 90°-t1-90°-
mixing-90°-ACQ(t2). The mixing time was 100 ms. A total
of 256 t1 increments were acquired and zero-filled to 512 in
the t1 dimension prior to the 2D Fourier transformation (2D
FT). For each t1, eight scans were accumulated. Sine-bell
window functions were applied in both dimensions. The
spectral width was 4808 Hz, and the digital resolution in the
2D spectrum was 9.39 Hz/point in both dimensions. The total
time to record the 2D free-induction decay (FID) was about
2.5 h. Elemental analyses were performed by Canadian
Microanalytical Service, Delta, British Columbia, Canada.
Excitation and emission spectra were recorded on a Photon
Technologies International QM1 spectrometer.
Syn th esis of 2-P h en yl-7-a za in d ole. (1) P r ep a r a tion of
2-Ben za m id o-3-p icolin e. To a solution of benzoyl chloride
(15 g, 0.107 mol) in 25 mL of pyridine and 35 mL of chloroform
was added 2-amino-3-picoline (12 g, 0.107 mol) with stirring.
After the mixture was refluxed overnight, the reaction mixture
was cooled to room temperature. The resulting precipitate was
removed by filtration. The filtrate was evaporated to dryness,
and the residue was redissolved in ethanol. Yellowish crystals
of 2-benzamido-3-picoline were obtained and dried under
vacuum at 90 °C for 1 h. Yield: 7.50 g (33%). Mp: 130-131
°C. 1H NMR (DMSO-d6; δ, ppm): 9.10 (br, 1H, NH), 8.55 (d,
1H, CH), 8.24 (d, 1H, CH), 8.10 (d, 2H, CH), 7.80-7.40 (m,
4H, CH), 2.40 (s, 3H, CH3).
(2) P r ep a r a tion of 2-P h en yl-7-a za in d ole. Sodium hy-
dride (3.2 g) was added to N-methylaniline (10 g), and the
solution was stirred and heated under reflux with nitrogen
for 30 min and then cooled to room temperature. 2-Benz-
amido-3-picoline (3.3 g) was added to the cooled solution, and
the mixture was heated at 280 °C for 7 min with stirring. After
the mixture was cooled to room temperature, acetic acid was
added to the residue until all solids were dissolved. The
solution was then made basic with sodium hydroxide and
extracted with chloroform. The extract was dried over
Na2SO4 and then concentrated under reduced pressure. After
the addition of hexane (5 mL), the crystals of 2-phenyl-7-
azaindole were collected by filtration and washed with hexane
(1.75 g, yield 58%). 1H NMR (CDCl3; δ, ppm): 12.11 (br, 1H,
NH), 8.29 (d, 1H, CH), 7.94 (d, 1H, CH), 7.84 (d, 2H, CH),
7.54-7.34 (m, 3H), 6.95 (m, 1H, CH), 6.77 (s, 1H, CH).
2-Meth yl-7-a za in d ole. (1) P r ep a r a tion of 2-Aceta m id o-
3-p icolin e. Acetyl chloride (1.96 g, 25 mmol) in 5 mL of
toluene was added to the solution of 2-amino-3-picoline (2.32
g, 21 mmol), pyridine (3 mL), and toluene (15 mL) under
nitrogen at 0 °C with stirring. After the mixture was warmed
to room temperature, the solution was stirred for an additional
1 h. The resulting white solid was collected by filtration and
washed with diethyl ether. Recrystallization from ethanol/
toluene yielded colorless crystals of 2-acetamido-3-picoline
(2.04 g, yield 65%). 1H NMR (DMSO-d6; δ, ppm): 11.27 (br,
1H, NH), 8.39 (d, 1H, CH), 8.25 (d, 1H, CH), 7.54 (m, 1H, CH),
2.43 (s, 3H, CH3), 2.31(s, 3H, CH3).
(2) P r ep a r a tion of 2-Meth yl-7-a za in d ole. Sodium hy-
dride (5.12 g) was added to N-methylaniline (16 g), and the
(5) SHELXTL Crystal Structure Analysis Package; Version 5;
Bruker Axs, Analytical X-ray System; Siemens: Madison, WI, 1995.
(4) Herbert, R. and Wibberley, D. G. J . Chem. Soc. C 1969, 1505.

4668 Organometallics, Vol. 17, No. 21, 1998
Ta ble 1. Cr ysta llogr a p h ic Da ta
Gao et al.
1
2
3
formula
C
40H31N6Al
C55H45N8OAl3/2C7H8
C35H37N8OAl3/0.5C7H8
fw
622.7
1099.2
712.7
space group
a/Å
b/Å
c/Å
P1h
P1h
P1h
11.070(2)
11.447(3)
15.625(3)
92.68(2)
110.32(1)
116.61(1)
1611.1(6)
2
14.008(6)
10.172(1)
14.232(5)
17.715(4)
106.02(2)
99.23(3)
112.68(3)
2987(2)
2
1.22
10.910(3)
17.136(3)
99.03(2)
98.865(5)
91.01(2)
1853.9(7)
2
1.28
R/deg
â/deg
γ/deg
V/Å3
Z
Dc/g cm-3
T/°C
1.28
23
23
23
radiation, λ/Å
Mo KR, 0.710 73
Mo KR, 0.710 73
Mo KR, 0.710 73
µ/cm-1
10.3
11.4
14.5
no. of rflns measd
no. of rflns used (Rini
no. of variables
final Ra (I > 2σ(I))
R (all data)
goodness of fit on F2
4838
4522 (0.016)
425
R1 ) 0.0377, wR2 ) 0.0944
R1 ) 0.0497, wR2 ) 0.1063
1.04
7785
7409 (0.032)
695
R1 ) 0.0589, wR2 ) 0.1572
R1 ) 0.0731, wR2 ) 0.1843
1.05
5106
4770(0.062)
440
R1 ) 0.1649, wR2 ) 0.3810
R1 ) 0.2200, wR2 ) 0.4529
1.16
)
R1 ) ∑|Fo| - |Fc|/∑|Fo|; wR2 ) [∑w[(Fo - Fc2)2]/∑[w(Fo2)2]]1/2, w ) 1/[σ2(Fo2) + (0.075P)2], P ) [Max(Fo2,0) + 2Fc2]/3.
a
2
factors were taken from Cromer and Waber.6 The crystals of
1-3 belong to the triclinic space group P1h. The structures
were solved by direct methods. In the crystal lattices of 2 and
3, there are toluene solvent molecules (2 toluene molecules
per molecule of 2 and 0.5 toluene molecule per molecule of 3).
One of the toluene molecules in 2 displays some degree of
rotational disordering which could not be modeled, while the
toluene molecule in 3 resides on an inversion center and is
disordered over the two sites related by the inversion center.
Crystals of 3 suitable for X-ray diffraction analysis were
difficult to grow and displayed significant twinning. Twinning
and the disorder of the toluene molecule could account for the
poor quality of the structural data of 3 (the diffraction data
for 3 were collected several times by using several different
crystals, but the quality of the resulting data was similar).
The positions for all hydrogen atoms except those attached to
the disordered solvent molecules were calculated, and their
contributions in structural factor calculations were included.
All non-hydrogen atoms in 1-3 were refined anisotropically,
except the carbon atoms of the disordered toluene molecules.
The crystallographic data for compounds 1-3 are given in
Table 1.
compounds were fully characterized by NMR, elemental,
and X-ray diffraction analyses.
Selected bond lengths and angles for 1 and 2 are given
in Table 2. As shown in Figure 1, the aluminum ion in
compound 1 has an approximately tetrahedral geom-
etry, as evidenced by the bond angles around Al(1),
ranging from 101.73(7)° to 117.68(9)°. There are two
7-azain-2-Ph anions bound to Al(1) through the nega-
tively charged indole nitrogen atom. The neutral 7-azain-
2-Ph ligand, however, is bound to Al(1) via the pyridyl
nitrogen atom. The Al(1)-N(indole) bond lengths (1.885-
(2), 1.889(2) Å) are much shorter than that of Al(1)-
N(pyridyl) (1.957(2) Å), reflecting the fact that the
negatively charged indole nitrogen is a better donor
than the neutral pyridyl nitrogen toward the aluminum
ion, consistent with our previous observation.1 Com-
pound 1 has an asymmetric structure with the three
7-azain-2-Ph ligands in an approximately propeller
arrangement. Mononuclear aluminum complexes con-
taining one alkyl ligand are often stabilized by chelating
ligands where the aluminum is either five- or six-
coordinate.7 Examples of four-coordinate monoalkyl-
aluminum complexes where only terminal ligands are
involved similar to compound 1 have been reported
previously.8 We believe that the steric bulkiness of the
ligands in 1 plays a key role in stabilizing the structure
of 1. The phenyl ring is not coplanar with the 7-aza-
indole ring, apparently due to steric interactions. There
is an intramolecular hydrogen bond between the pyridyl
nitrogen atom N(6) and the proton on the indole
nitrogen atom N(1), as indicated by the distances of
N(6)‚‚‚H(1) (1.695 Å) and N(6)‚‚‚N(1) (2.831(3) Å). This
intramolecular hydrogen bond is believed to further
enhance the structural stability of compound 1.
Resu lts a n d Discu ssion
Syn th esis a n d Cr ysta l Str u ctu r es of Al(CH3)(7-
azain -2-P h )2(7-azain H-2-P h ) (1) an d Al3(µ3-O)(CH3)3-
(7-a za in -2-P h )4 (2). Compounds 1 and 2 were obtained
initially as mixtures from a reaction of 2-phenyl-7-
azaindole with Al(CH3)3 in a 1:1 ratio. Both compounds
were then synthesized successfully from independent
stoichiometric reactions. The synthesis of compound 1
is straightforward by simply reacting Al(CH3)3 with
2-phenyl-7-azaindole in a 1:3 ratio in toluene. The
independent synthesis of 2 was somewhat challenging,
due to the involvement of water in the reaction. We
found that the best way to synthesize 2 is by introducing
water-vapor-saturated nitrogen gas into the reaction
mixture of Al(CH3)3 with 2-phenyl-7-azaindole in a 3:4
ratio in toluene. When exposed to air, compound 1
decomposes rapidly, while compound 2 retains its light
yellow crystalline appearance for a few hours. Both
(7) (a) Leman, J . T.; Barron, A. R. Organometallics 1989, 8, 1828.
(b) Lewisnski, L.; Zachara, J .; Mank, B.; Pasynkiewicz, S. J . Orga-
nomet. Chem. 1993, 454, 5. (c) Guilard, R.; Zrinch, A.; Tabard, A.; Endo,
A.; Han, B. C.; Lecomte, C.; Souhassou, M.; Habbou, A.; Ferhat, M.;
Kadish, K. M. Inorg. Chem. 1990, 29, 4476.
(8) (a) Petries, M. A.; Power, P. P.; Dias, R. H. V.; Ruhlandt-Senge,
K.; Waggoner, K. M.; Wehmschulte, R. J . Organometallics 1993, 12,
1086. (b) Healy, M. D.; Ziller, J . W.; Barron, A. R. J . Am. Chem. Soc.
1990, 112, 2949. (c) Power, M. B.; Bott, S. G.; Atwood, J . L.; Barron,
A. R. J . Am. Chem. Soc. 1990, 112, 3446.
(6) Cromer, D. T.; Waber, J . T. International Tables for X-ray
Crystallography; Kynoch Press: Birmingham, U.K., 1974; Vol. 4, Table
2.2A.

Blue Luminescent Aluminum Complexes
Organometallics, Vol. 17, No. 21, 1998 4669
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles
(d eg)
Compound 1
Al(1)-N(3)
Al(1)-N(5)
Al(1)-C(40)
Al(1)-N(2)
N(5)-C(39)
N(5)-C(33)
N(3)-C(26)
N(3)-C(20)
1.885(2)
1.889(2)
1.948(2)
1.957(2)
1.385(2)
1.397(3)
1.383(3)
1.404(2)
N(1)-C(13)
1.361(2)
1.391(3)
1.340(2)
1.354(3)
1.358(2)
1.341(2)
1.333(3)
1.334(3)
N(1)-C(7)
C(39)-N(6)
N(2)-C(13)
N(2)-C(12)
N(6)-C(38)
N(4)-C(25)
N(4)-C(26)
N(3)-Al(1)-N(5)
107.93(7) N(3)-Al(1)-N(2)
108.75(8)
101.73(7)
N(3)-Al(1)-C(40) 112.19(8) N(5)-Al(1)-N(2)
N(5)-Al(1)-C(40) 117.68(9) C(40)-Al(1)-N(2) 107.80(9)
Compound 2
Al(1)-O(1)
Al(1)-N(1)
Al(1)-C(1)
Al(1)-N(6)
Al(1)-N(4)
O(1)-Al(2)
O(1)-Al(3)
N(1)-C(16)
N(1)-C(10)
Al(2)-N(5)
Al(2)-C(2)
Al(2)-N(2)
N(2)-C(15)
N(2)-C(16)
Al(3)-N(7)
Al(3)-N(3)
1.818(2)
1.940(3)
1.953(4)
2.106(3)
2.166(3)
1.766(3)
1.798(2)
1.363(4)
1.407(5)
1.918(3)
1.932(4)
1.942(3)
1.347(5)
1.351(5)
1.886(3)
1.901(3)
Al(3)-C(3)
N(3)-C(29)
N(3)-C(23)
N(4)-C(29)
N(4)-C(28)
N(5)-C(42)
N(5)-C(36)
C(5)-C(6)
N(6)-C(41)
N(6)-C(42)
C(6)-C(7)
N(7)-C(55)
N(7)-C(49)
C(7)-C(8)
N(8)-C(55)
N(8)-C(54)
1.944(4)
1.379(4)
1.409(4)
1.344(4)
1.349(4)
1.377(4)
1.406(5)
1.344(13)
1.346(4)
1.349(5)
1.328(11)
1.381(4)
1.401(4)
1.387(7)
1.329(5)
1.341(5)
F igu r e 1. Diagram showing the molecular structure of 1
with 50% ellipsoids and labeling schemes.
O(1)-Al(1)-N(1) 106.13(12) O(1)-Al(2)-N(5)
99.48(12)
O(1)-Al(2)-C(2) 123.5(2)
O(1)-Al(1)-C(1)
N(1)-Al(1)-C(1)
O(1)-Al(1)-N(6)
N(1)-Al(1)-N(6)
C(1)-Al(1)-N(6)
O(1)-Al(1)-N(4)
N(1)-Al(1)-N(4)
C(1)-Al(1)-N(4)
128.8(2)
124.7(2)
N(5)-Al(2)-C(2) 112.1(2)
86.93(11) O(1)-Al(2)-N(2) 103.45(13)
91.17(13) N(5)-Al(2)-N(2) 100.61(13)
96.9(2)
C(2)-Al(2)-N(2) 114.5(2)
86.12(11) O(1)-Al(3)-N(7) 108.08(12)
88.99(13) O(1)-Al(3)-N(3) 102.09(12)
88.9(2)
N(7)-Al(3)-N(3) 102.71(13)
N(6)-Al(1)-N(4) 172.82(12) O(1)-Al(3)-C(3) 108.3(2)
Al(2)-O(1)-Al(3) 128.28(13) N(7)-Al(3)-C(3) 119.3(2)
Al(2)-O(1)-Al(1) 115.15(12) N(3)-Al(3)-C(3) 114.9(2)
Al(3)-O(1)-Al(1) 116.27(13)
Compound 3
O(1)-Al(2)
O(1)-Al(3)
O(1)-Al(1)
C(1)-N(8)
Al(1)-N(1)
Al(1)-C(34)
Al(1)-N(6)
Al(1)-N(4)
N(1)-C(30)
N(1)-C(32)
Al(2)-N(5)
Al(2)-C(35)
Al(2)-N(2)
N(2)-C(25)
N(2)-C(32)
1.759(7)
1.796(7)
1.823(7)
1.35(2)
Al(3)-N(3)
Al(3)-N(7)
Al(3)-C(33)
N(3)-C(16)
N(3)-C(14)
N(4)-C(16)
N(4)-C(9)
N(5)-C(24)
N(5)-C(22)
N(6)-C(24)
N(6)-C(17)
C(6)-N(7)
N(7)-C(8)
N(8)-C(8)
1.894(8)
1.905(9)
1.933(11)
1.397(14)
1.41(2)
1.293(14)
1.403(14)
1.394(13)
1.400(13)
1.325(13)
1.380(14)
1.34(2)
1.900(10)
1.946(11)
2.121(9)
2.163(9)
1.409(14)
1.410(14)
1.859(9)
1.940(11)
1.942(10)
1.388(14)
1.409(14)
F igu r e 2. Diagram showing the molecular structure of 2
with 50% ellipsoids and labeling schemes. For clarity, all
carbon atoms are shown as ideal spheres.
ligand, resulting from the reaction of H2O with methyl
ligands. The oxo ligand is coplanar with the three
aluminum ions (O(1) deviates from the plane defined
by the three Al atoms by 0.043 Å; the sum of bond angles
around O(1) is 359.7°) with Al-O distances ranging
from 1.766(3) to 1.818(2) Å. A similar bonding mode of
the oxo ligand has been observed in the complexes9a,9b
[(CH3)2Al(µ3-O)Al(CH3)3]22-, [Al3(µ3-O)Cl8]-, [(AlCl2)(µ3-
O)(AlCl3)]22-, and9c [(tBu)2Al]4(µ3-O)2. There are three
methyl groups, which are bound to three aluminum
centers, respectively, with three similar Al-C bond
lengths. The coordination environment around each
aluminum center is, however, different. Al(2) and Al-
(3) are four-coordinate and approximately tetrahedral,
1.41(2)
1.28(2)
Al(2)-O(1)-Al(3)
Al(2)-O(1)-Al(1)
Al(3)-O(1)-Al(1)
O(1)-Al(1)-N(1)
O(1)-Al(1)-C(34)
N(1)-Al(1)-C(34)
O(1)-Al(1)-N(6)
N(1)-Al(1)-N(6)
C(34)-Al(1)-N(6)
O(1)-Al(1)-N(4)
N(1)-Al(1)-N(4)
C(34)-Al(1)-N(4)
N(6)-Al(1)-N(4)
O(1)-Al(2)-N(5)
129.1(4)
114.9(4)
116.0(4)
108.8(4)
131.4(5)
119.4(5)
86.3(3)
90.5(4)
98.3(5)
83.8(3)
90.5(4)
O(1)-Al(2)-C(35)
N(5)-Al(2)-C(35)
O(1)-Al(2)-N(2)
N(5)-Al(2)-N(2)
C(35)-Al(2)-N(2)
C(25)-N(2)-C(32)
C(25)-N(2)-Al(2)
C(32)-N(2)-Al(2)
O(1)-Al(3)-N(3)
O(1)-Al(3)-N(7)
N(3)-Al(3)-N(7)
O(1)-Al(3)-C(33)
N(3)-Al(3)-C(33)
N(7)-Al(3)-C(33)
119.8(5)
118.0(4)
104.6(4)
102.4(4)
108.8(5)
110.7(9)
122.3(7)
123.6(7)
98.5(4)
109.1(4)
107.6(4)
115.8(5)
113.1(5)
111.8(5)
90.0(5)
169.8(3)
101.1(4)
(9) (a) Atwood, J . L.; Zaworotko, M. J . J . Chem. Soc., Chem.
Commun. 1983, 302. (b) Thewalt, U.; Stollmaier, F. Angew. Chem.,
Int. Ed. Engl. 1982, 21, 133. (c) Mason, M. R.; Smith, J . M.; Bott. S.
G.; Barron, A. R. J . Am. Chem. Soc. 1993, 115, 4971.
The structure of compound 2 is shown in Figure 2.
Compound 2 has three aluminum ions sharing one oxo

4670 Organometallics, Vol. 17, No. 21, 1998
Gao et al.
Ch a r t 2. Al3(O)(OR)2(CH3)(7-a za in )4
while Al(1) is five-coordinate and approximately trigo-
nal-bipyramidal with two pyridyl nitrogen atoms N(4)
and N(6) on the axial positions (N(4)-Al(1)-N(6) )
172.8(1)°). The Al(2) center is coordinated by one
carbon, one oxygen, one pyridyl nitrogen, and one indole
nitrogen atom, while Al(3) is bound by one carbon, one
oxygen, and two indole nitrogen atoms. The Al(1)-
N(pyridyl) bond lengths are considerably longer (2.106-
(3), 2.166(3) Å) than the corresponding ones on Al(2) and
Al(3), attributable to the five-coordination and the steric
bulkiness around Al(1). The stoichiometry of compound
2 is closely related to that of Al3(O)(7-azain)4(CH3)(OR)2,
where R is a terminal hexafluoroisopropoxy ligand.1
However, in contrast to the symmetric structure (Chart
2) of Al3(O)(7-azain)4(CH3)(OR)2, where only two distinct
aluminum environments are present, compound 2 has
three distinct aluminum environments. We believe that
the asymmetric arrangement of the aluminum centers
in 2 is caused by the phenyl substituent on the 7-aza-
indole, which increases intramolecular nonbonding
interactions so substantially that the symmetric struc-
ture cannot be achieved (this can be demonstrated
readily by a stick and ball molecular model). It is
conceivable that the analogous compound Al3(O)(7-
azain)4(CH3)3 may have a symmetric structure similar
to that of Al3(O)(7-azain)4(CH3)(OR)2, which would be
good evidence supporting the notion that the asym-
metric structure of 2 is indeed caused by over crowding
of the ligands. Unfortunately, this cannot be confirmed
because we have not been able to synthesize Al3(O)(7-
azain)4(CH3)3 successfully. Again, as observed in 1, the
phenyl portion of the 7-azain-2-Ph ligand in 2 is not
coplanar with the 7-azaindole portion, the dihedral
angle between these two planes ranging from 42.3 to
59.0°, again attributable to steric interactions. The Al-
(1)-Al(2), Al(2)-Al(3), and Al(1)-Al(3) separation dis-
tances are 3.025(4), 3.207(5), and 3.071(5) Å, respec-
tively. Although Al(2) is four-coordinate, the lone pair
of electrons of the noncoordinated pyridyl N(8) atom is
oriented toward Al(2) with a separation distance of
3.215(5) Å. Furthermore, N(8) is situated opposite to
N(5) atom, with the N(5)-Al(2)‚‚‚N(8) angle being
169.4°, raising the question of whether Al(2) will remain
four-coordinate in solution. Not surprisingly, our NMR
investigation on the solution behavior of compound 2
indeed demonstrated unusual dynamic behavior of
compound 2 in solution (vide infra).
F igu r e 3. Diagram showing the molecular structure of 3
with 50% ellipsoids and labeling schemes. All atoms except
aluminum are shown as ideal spheres.
quality of compound 3 is only marginal. The most
serious problems with crystals of 3 are twinning and
low diffraction intensity. We collected data for 3 several
times by using several different crystals, but the data
obtained were similar in quality. The crystal data
presented here were from our best effort. Despite the
poor quality of the data and refinement, the structure
of 3 established by X-ray diffraction analysis is cor-
rect. Selected bond lengths and angles of 3 are given
in Table 2.
As shown in Figure 3, the structure of 3 resembles
that of 2. The three Al atoms are bridged by an oxo
ligand which is coplanar with the plane defined by the
three Al atoms (O(1) is 0.01 Å above the plane; the sum
of angles around O(1) is 360°). The variation of Al-O
bond lengths in 3 follows the same pattern as that in 2,
i.e., O(1)-Al(2) being the shortest and O(1)-Al(1) being
the longest. The Al(1)-Al(2), Al(2)-Al(3), and Al(3)-
Al(1) distances are 3.02(1), 3.21(1), and 3.07(1) Å,
respectively, also similar to those in 2. As observed in
2, the three Al atoms have distinct coordination envi-
ronments: Al(1), distorted trigonal bipyramidal with
two pyridyl nitrogen atoms N(4) and N(6) on the axial
positions (N(4)-Al(1)-N(6) ) 169.8(3)°); Al(2), tetrahe-
dral with a pyridyl and an indole nitrogen atom; Al(3),
tetrahedral with two indole nitrogen atoms. The asym-
metric structure of 3 can again be attributed to steric
interactions caused by the methyl substituent of the
7-azaindole ligand. The lone pair of electrons of the
noncoordinating pyridyl N(8) atom is pointed toward the
Al(2) atom, similar to that in 2. The Al(2)‚‚‚N(8)
distance is, however, much longer (3.99 Å) than that in
2. In addition, the N(8) atom is situated approximately
opposite to N(2) (N(2)-Al(2)‚‚‚N(8) ) 150°), while in 2
N(8) is opposite to N(5). Furthermore, the methyl group
(C(33)) on Al(3) is on the opposite side of the methyl
group C(34) on Al(1) in 3, while in 2 these two methyl
groups are on the same side. The structural difference
between 2 and 3 is most likely caused by steric interac-
tions and packing of the molecules in the crystal lattice.
It is very likely that in solution compounds 2 and 3 have
a similar structure.
Syn t h esis a n d Cr yst a l St r u ct u r es of Al3(µ3-O)-
(CH3)3(7-a za in -2-CH3)4 (3). Compound 3 was synthe-
sized by a procedure similar to that for compound 2 by
reacting Al(CH3)3 with 2-methyl-7-azaindole in a 3:4
ratio in the presence of water-vapor-saturated nitrogen
gas. Compound 3 was fully characterized by elemental,
1H NMR, and X-ray diffraction analyses. The crystal
F lu xion a lity of Com p ou n d s 2 a n d 3 in Solu tion .
As shown in Figures 2 and 3, compounds 2 and 3 exhibit
an asymmetric structure, caused by the difference in

Blue Luminescent Aluminum Complexes
Organometallics, Vol. 17, No. 21, 1998 4671
groups on the aluminum centers would be different,
thus producing three distinct chemical shifts. However,
only one broad Al-CH3 resonance was observed for
compounds 2 and 3 at 298 and 308 K, respectively, an
indication of the presence of an exchange process. As
the temperature decreases, the pattern of chemical
shifts in the Al-CH3 region becomes more complex. At
213 K, three dominant Al-CH3 resonances were ob-
served for both compounds, which is consistent with the
solid-state structure. The four 7-azain-2-CH3 ligands
in compound 3 should have distinct chemical environ-
ments, if 3 retains its crystal structure in solution,
which should result in four distinct C-CH3 resonances
in the 1H NMR spectra. At 298 K, only two broad
resonances of C-CH3 were observed for compound 3,
which became a broad single resonance at temperatures
above 298 K. At 213 K, four dominant C-CH3 reso-
nances were observed, again consistent with the solid-
state structure of 3. The fact that only one Al-CH3 and
one C-CH3 resonance (in 3) were observed above 298
K implies that the dynamic process in solution for
compounds 2 and 3 involves exchange of all Al-CH3 and
all 7-azain-2-R ligands, which is very unusual indeed.
1
One important feature in the H NMR spectra of 2 and
1
F igu r e 4. Al-CH3 region of variable-temperature 1D H
NMR spectra of compound 2 in CDCl3. The impurity peaks
are marked by asterisks.
3 is that at low temperatures, in addition to the
dominant resonances, there are additional resonances
in both the Al-CH3 and C-CH3 (compound 3) regions.
Initially we thought that the minor resonance peaks
belong to impurities. However, after several repeated
measurements on different batches of samples, we were
convinced that these minor peaks are from the samples,
not impurities. The fact that these minor peaks disap-
pear at high temperatures further supports that they
are not from impurities but belong to the sample. One
possible explanation for the minor peaks is that they
are resonances of minor isomers of 2 and 3 that are also
involved in the dynamic process. The presence of minor
peaks is more pronounced in 3 than in 2.
To further understand the mechanism of the exchange
process involved in 2 and 3, we carried out a 1H 2D
EXSY experiment10 on compound 3 (compound 3 was
chosen for 2D EXSY study because it has the C-CH3
resonance in addition to Al-CH3 resonance, allowing
us to monitor the exchange among 7-azain-2-CH3 ligands
as well). The C-CH3 region and the Al-CH3 region of
the 1H 2D EXSY spectrum obtained at 243 K are shown
in Figures 6 and 7, respectively. Although there are
considerable overlaps of peaks in the EXSY spectrum
of 3 and some of the cross-peaks are not well resolved
due to their broadness, some useful information can be
obtained from the spectrum. One important feature in
the C-CH3 region is that cross-peaks are observed
between the peak of the major isomer at 2.70 ppm and
the peaks of a minor isomer at 1.10 and 2.54 ppm,
respectively, an indication of interconversion between
minor and major isomers. Further evidence comes from
the cross-peaks between the peak of the major isomer
at -0.88 ppm and the peaks of a minor isomer at -1.05,
-0.06, and 0.00 ppm in the Al-CH3 region (Figure 7),
again consistent with an exchange between minor and
major isomers of compound 3 in solution. In both the
F igu r e 5. C-CH3 and Al-CH3 regions of variable-
1
temperature 1D H NMR spectra of compound 3 in CDCl3.
The impurity and solvent peaks are marked by asterisks.
coordination environment on Al(1) and Al(2) centers. If
the coordinated pyridyl N(4) atom on the Al(1) center
were ignored, the Al(1) and Al(2) centers would be very
similar (especially in compound 2). One then must
wonder why the two aluminum centers are discrimi-
nated by the pyridyl donors and whether the same
discrimination exists in solution. To address these
questions, we studied the solution behavior of com-
pounds 2 and 3 by 1H NMR spectroscopy. First, we
1
obtained 1D variable-temperature H NMR spectra of
compounds 2 and 3 over the temperature range 328-
213 K. As shown in Figures 4 and 5, compounds 2 and
3 are fluxional in solution. If both compounds retain
their solid-state structures in solution, all three methyl
(10) (a) Derome, A. E. Modern NMR Techniques for Chemistry
Research, Pergamon Press: Oxford, U.K., 1987. (b) Collman, J . P.;
Hartford, S. T.; Franzen, S.; Eberspacher, T. A.; Shoemaker, R. K.;
Woodruff, W. H. J . Am. Chem. Soc. 1998, 120, 1456.

4672 Organometallics, Vol. 17, No. 21, 1998
Gao et al.
Sch em e 1
1
F igu r e 6. C-CH3 region of the 2D H EXSY spectrum of
Al-CH3 signals. Compound 2 appears to contain two
major isomers in a 6:1 ratio in solution, while the third
isomer is only present in trace amount at 218 K.
Compound 3 contains at least three isomers in ap-
preciable amounts at 213 K with an approximate ratio
of 6:2:1. Conclusive assignment of the observed peaks
to the proposed isomers could not be achieved due to
the overlap of peaks. One facile exchange process that
averages the environment of Al(1) and Al(2), ligands 1
and 2, and ligands 3 and 4 in all isomers, is shown in
Scheme 1, where only the forming and breaking of an
Al-N(pyridyl) bond is involved. This process does not,
however, exchange all aluminum centers or all 7-azain-
2-R ligands as implied by the NMR data. To achieve
the exchange of all 7-azain-2-R ligands and all alumi-
num environments within each isomer, additional Al-N
or Al-N′ bond breaking and forming must be involved.
A possible mechanism that involves either Al-N or Al-
N′ bond breaking and forming at each step is given in
Scheme 2. Assuming all three isomers are present,
Scheme 2 shows that there is no one-step pathway to
exchange all Al-CH3 and all C-CH3 groups within each
isomer. In order for each isomer to achieve exchange
with itself, a multistep process must occur where isomer
interconversion is part of the process. For example, the
intramolecular exchange of isomer A would involve first
the migration of ligand 2 from Al(1) and Al(2) to Al(2)
and Al(3) by breaking and forming an Al-N bond, thus
yielding isomer B. The subsequent migration of ligand
4 in isomer B to Al(3) and Al(1) via breaking and
forming an Al-N bond leads to the formation of isomer
C, which in turn converts to the new isomer B via
breaking and forming of an Al-N′ bond, thus achieving
the intramolecular exchange of isomer B. The further
conversion of isomer B to A by breaking and forming
an Al-N bond completes the cycle of all Al-CH3 and
C-CH3 exchanges in isomer A. The mechanism pro-
posed in Scheme 2 is of course not unique but is perhaps
the most facile one, since each step involves breaking
and forming of only one Al-N or Al-N′ bond. Our
understanding of the exchange in compounds 2 and 3
is by no means conclusive, due to the complexity of the
system. Dynamic exchanges have been frequently
observed in dinuclear organoaluminum compounds,
3 in CDCl3 at 243 K.
1
F igu r e 7. Al-CH3 region of the 2D H EXSY spectrum of
3 in CDCl3 at 243 K.
C-CH3 and Al-CH3 regions, there are cross-peaks
within the major isomer and cross-peaks within the
minor isomers. However, due to the broadness of the
peaks, conclusive assignments for all cross-peaks can-
not be achieved. On the basis of the 1D and 2D EXSY
1H NMR data, we can therefore conclude that the
dynamic process displayed by compounds 2 and 3
involves exchanges among different isomers, as well as
within each isomer, and that at 213 K one isomer is
dominant.
Among many possible isomers for compounds 2 and
3, the three isomers that we think most likely to exist
in solution are shown in Scheme 2 as A, B, and C, where
N and N′ represent pyridyl and indole nitrogen atoms,
respectively, and the subscript represents each of the
distinct C-CH3 chemical environments. The solid-state
structure of compounds 2 and 3 is represented by A (the
orientation difference of the Al-CH3 groups in 2 and 3
is ignored to simplify the matter), which we believe is
likely the major isomer in solution at 213 K. If all three
isomers coexist in solution, one would expect to see at
least 12 distinct C-CH3 chemical shifts and 9 distinct

Blue Luminescent Aluminum Complexes
Organometallics, Vol. 17, No. 21, 1998 4673
Sch em e 2
where trans and cis isomerizations are often involved.11
However, the highly fluxional behavior of compounds 2
and 3 is rare in that the process involves exchange of
all ligand environments and all aluminum environ-
ments.
Lu m in escen t P r op er ties of Com p ou n d s 1-3.
Blue luminescence is the other important feature of (7-
azaindole)aluminum complexes. We have observed that
aluminum complexes containing the simple 7-azaindole
ligand typically emit in the 420-440 nm region when
excited by UV. Compounds 2 and 3 demonstrated that
the introduction of a substituent at the 2-position has
a dramatic effect on the structure. We have established
previously that the blue emission of aluminum com-
pounds containing nonsubstituted 7-azaindole ligand is
due to a π* f π* transition localized on the 7-azaindole
ligand.1 Because a methyl group is an electron-donating
substituent while a phenyl group is an electron-accept-
ing12 and sterically demanding group, one could antici-
pate they may have quite different effects on the
electronic properties of the complex, in particular, the
emission energy. We therefore examined the lumines-
cence of compounds 1-3 by measuring their excitation
and emission spectra. The data are shown in Figure 8.
When irradiated by UV, both compounds 1 and 2 have
weak blue luminescence while compound 3 exhibits a
blue emission with a brightness comparable to that of
Al3(O)(7-azain)4(CH3)(OR)2 (quantum yield 0.31, relative
to 9,10-diphenylanthracene). The relatively weak emis-
sion of compounds 1 and 2 could be mostly attributed
to the phenyl substituent, which increases the thermal
vibrations of the emitting 7-azaindole ligand substan-
tially, thus reducing the emission efficiency, in com-
parison to that of the nonsubstituted 7-azaindole. The
other factor that may contribute to the weak emission
of 1 and 2 is the interference of π orbitals on the phenyl
(11) (a) Choquette, D. M.; Timm, M. J .; Hobbs, J . L.; Rahim, M. M.;
Ahmed, K. J .; Planalp, R. P. Organometallics 1992, 11, 529. (b)
Amirkhalili, S.; Hitchcock, P. B.; J enkins, A. D.; Nyathi, J . Z.; Smith,
J . D. J . Chem. Soc., Dalton Trans. 1981, 377. (c) Gosling, K.; Smith,
J . D.; Wharmby, D. H. W. J . Chem. Soc. A. 1969, 1738. (d) Trepanier,
S. J .; Wang, S. Organometallics 1996, 15, 760. (e) Sierra, M. J .; de
Mel, S. J .; Oliver, J . P. Organometallics 1989, 8, 2486.
(12) Dean, J . A. Lange’s Handbook of Chemistry, 3rd ed.; McGraw-
Hill: New York, 1985.

4674 Organometallics, Vol. 17, No. 21, 1998
Gao et al.
ents at the 2-position (the pyridyl ring) of the emitting
8-hydroxyquinoline ligand in aluminum complexes.3
Con clu sion
The introduction of a methyl or phenyl group at the
2-position of the 7-azaindole ligand increases the steric
bulkiness of the ligand and leads to the formation of
two new and asymmetric trinuclear aluminum com-
plexes that are highly fluxional in solution. The unusu-
ally complicated dynamic exchange process in the two
trinuclear complexes is likely caused by an intramo-
lecular migration of the coordinated 2-R-7-azaindole.
There is no significant difference in luminescence
between the nonsubstituted 7-azaindole aluminum com-
plex and the 2-methyl-substituted one. The 2-phenyl-
substituted 7-azaindole complex appears to have a
weaker emission at a wavelength longer than that of
the nonsubstituted 7-azaindole complex, attributable to
both electronic and steric factors.
F igu r e 8. Excitation and emission spectra of 1 (×), 2 (s),
and 3 (0).
ring, which could effectively quench the emission by
intercepting the electrons returning from the excited
state (7-azaindole f phenyl charge transfer). The fact
that the excitation and emission bands of compounds 1
and 2 are much broader than those of 3 and nonsub-
stituted 7-azaindole aluminum compounds, as shown in
Figure 8, appears to further support the presence of
phenyl interference on the excitation and emission
spectra of 7-azaindole. The emission energy (λmax ) 431
nm) of compound 3 is essentially the same as that of
Al3(O)(7-azain)4(CH3)(OR)2 (λmax ) 430 nm), indicating
that the methyl substituent at the 2-position has little
effect on the emission energy of coordinated 7-azaindole.
The emission energy of compound 2 (λmax ) 443 nm)
has shifted considerably toward a longer wavelength,
in comparison to that1 of Al3(O)(7-azain)4(CH3)(OR)2,
which must be caused by the electron-accepting nature
of the phenyl substituent. VanSlyke and co-workers
have observed a similar effect by introducing substitu-
Ack n ow led gm en t. We are grateful to Professor
Stephen Brown and his students for recording the
emission and excitation spectra. We thank the NSERC
of Canada for financial support. G.W. wishes to thank
Queen’s University for a Faculty Initiation Grant.
Su p p or tin g In for m a tion Ava ila ble: Tables giving X-ray
crystallographic data, atomic coordinates, bond lengths and
angles, hydrogen parameters, and anisotropic thermal param-
eters and diagrams showing the complete molecules and
solvent molecules of compounds 2 and 3 and a figure giving
the complete 2D EXSY spectrum of compound 3 obtained at
243 K (27 pages). Ordering information is given on any
current masthead page.
OM980435J