Syntheses, structures, and fluxionality of blue luminescent Zinc(II) complexes: Zn(2,2',2-tpa)Cl2, Zn(2,2',2-tpa)2(O2CCF3)2, and Zn(2,2',3-tpa)4(O2CCF3)2 (tpa = tripyridylamine)

Article abstract of DOI:10.1021/ic991436m

Three novel Zn(II) complexes containing either 2,2',2_-tripyridylamine (2,2',2_-tpa) or 2,2',3_-tripyridylamine (2,2',3_-tpa) have been synthesized and structurally characterized. Compound 1, Zn(2,2',2_-tpa)Cl₂, has a tetrahedral geometry while compounds 2, Zn(2,2',2_-tpa)₂(O₂CCF₃)₂, and 3, Zn(2,2',3_-tpa)₄(O₂CCF₃)₂, have an octahedral geometry. The 2,2',2_-tpa ligand in 1 and 2 functions as a bidentate ligand, chelating to the zinc center, while the 2,2_,3_-tpa ligand in 3 functions as a terminal ligand, binding to the zinc center through the 3-pyridyl nitrogen atom. All three compounds emit a blue color in solution and in the solid state. The emission maxima for the three compounds in solution are at λ = 422, 426, and 432 nm, respectively. The blue luminescence of the complexes is due to a π* → π transition of the tpa ligand as established by an ab initio calculation on the free ligand 2,2',2_-tpa and complex 1. Compounds 1 and 2 are fluxional in solution owing to an exchange process between the coordinate and noncoordinate 2-pyridyl rings of the 2,2',2_-tpa ligand. Compound 2 is also fluxional owing to a cis-trans isomerization process, as determined by variable-temperature ¹H NMR spectroscopic analysis.

Full text of DOI:10.1021/ic991436m

Inorg. Chem. 2000, 39, 2397-2404
2397
Syntheses, Structures, and Fluxionality of Blue Luminescent Zinc(II) Complexes:
Zn(2,2′,2′′-tpa)Cl2, Zn(2,2′,2′′-tpa)2(O2CCF3)2, and Zn(2,2′,3′′-tpa)4(O2CCF3)2
(tpa ) Tripyridylamine)
Wenyu Yang, Hartmut Schmider, Qingguo Wu, You-sheng Zhang, and Suning Wang*
Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada
ReceiVed December 16, 1999
Three novel Zn(II) complexes containing either 2,2′,2′′-tripyridylamine (2,2′,2′′-tpa) or 2,2′,3′′-tripyridylamine
(2,2′,3′′-tpa) have been synthesized and structurally characterized. Compound 1, Zn(2,2′,2′′-tpa)Cl2, has a tetrahedral
geometry while compounds 2, Zn(2,2′,2′′-tpa)2(O2CCF3)2, and 3, Zn(2,2′,3′′-tpa)4(O2CCF3)2, have an octahedral
geometry. The 2,2′,2′′-tpa ligand in 1 and 2 functions as a bidentate ligand, chelating to the zinc center, while the
2,2′′,3′′-tpa ligand in 3 functions as a terminal ligand, binding to the zinc center through the 3-pyridyl nitrogen
atom. All three compounds emit a blue color in solution and in the solid state. The emission maxima for the three
compounds in solution are at λ ) 422, 426, and 432 nm, respectively. The blue luminescence of the complexes
is due to a π* f π transition of the tpa ligand as established by an ab initio calculation on the free ligand
2,2′,2′′-tpa and complex 1. Compounds 1 and 2 are fluxional in solution owing to an exchange process between
the coordinate and noncoordinate 2-pyridyl rings of the 2,2′,2′′-tpa ligand. Compound 2 is also fluxional owing
1
to a cis-trans isomerization process, as determined by variable-temperature H NMR spectroscopic analysis.
Introduction
prepare. We have demonstrated that 7-azaindole or 2,2′-
dipyridylamine can produce a bright blue luminescence when
Luminescent organic and coordination compounds have been
an active research area for decades because of their various
potential applications in materials sciences. The most notable
recent interest in luminescent compounds concerns their ap-
plications in electroluminescent displays.1-3 In electrolumines-
cence, electrical energy is converted to light via a medium of
luminescent materials.1 Electroluminescent devices using various
compounds including organic polymers, small organic mol-
ecules, and coordination compounds have been fabricated
successfully either by a spin-coating or a vacuum sublimation
process. To achieve full-color electroluminescent displays, three
color components, i.e., green, blue, and red, must be available.
Stable blue luminescent compounds that are useful in electro-
luminescent devices are still rare and very challenging to
deprotonated and bound to either an aluminum ion or a boron
center.4 Blue electroluminescent devices using 7-azaindole-
boron compounds have been fabricated successfully.3f However,
many of our previously reported aluminum or boron compounds
based on 7-azaindole or 2,2′-dipyridylamine are not stable
enough for electroluminescent devices. The instability of these
compounds is due mostly to the fact that 7-azaindole or 2,2′-
dipyridylamine in the complexes is negatively charged and hence
susceptible to moisture in air. Our recent efforts have therefore
focused on (1) stabilizing the charge on the negatively charged
ligand and (2) using neutral ligands. In this paper, we report
the results of our investigation on blue luminescent zinc(II)
complexes using neutral 2,2′,2′′-tripyridylamine (2,2′,2′′-tpa) and
2,2′,3′′-tripyridylamine (2,2′,3′′-tpa) as the light-emitting ligands.
(1) (a) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (b)
Tang, C. W.; VanSlyke, S. A.; Chen, C. H. J. Appl. Phys. 1989, 65,
3611. (c) Shirota, Y.; Kuwabara, Y.; Inada, H.; Wakimoto, T.; Nakada,
H.; Yonemoto, Y.; Kawami, S.; Imai, K. Appl. Phys. Lett. 1994, 65,
807. (d) Hamada, Y.; Sano, T.; Fujita, M.; Fujii, T.; Nishio, Y.; Shibata,
K. Jpn. J. Appl. Phys. 1993, 32, L514. (e) Bulovic, V.; Gu, G.;
Burrows, P. E.; Forrest, S. R. Nature 1996, 380, 29.
(2) (a) Adachi, C.; Tokito, S.; Tsutsui, T.; Saito, S. Jpn. J. Appl. Phys.
1988, 27, L713. (b) Adachi, C.; Tsutsui, T.; Saito, S. Appl. Phys. Lett.
1990, 56, 799. (c) Tao, X. T.; Suzuki, H.; Wada, T.; Sasabe, H.;
Miyata, S. Appl. Phys. Lett. 1999, 75, 1655. (d) Shen, Z.; Burrows, P.
E.; Bulovic, V.; Borrest, S. R.; Thompson, M. E. Science 1997, 276,
2009. (e) Aziz, H.; Popovic, Z. D.; Hu, N.-X.; Hor, A.-M.; Xu, G.
Science 1999, 283, 1900.
(3) (a) Hu, N.-X.; Esteghamatian, M.; Xie, S.; Popovic, P.; Ong, B.; Hor,
A.-M.; Wang, S. AdV. Mater. 1999, 11, 1460. (b) Hamada, Y.; Sano,
T.; Fujii, H.; Nishio, Y.; Takahashi, H.; Shibata, K. Appl. Phys. Lett.
1997, 71, 3338. (c) Hamada, Y.; Sano, T.; Fujita, M.; Fujii, T.; Nishio,
Y.; Shibata, K. Chem. Lett. 1993, 905. (d) Baldo, M. A.; Lamansky,
S.; Burrows, P.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett.
1999, 75, 5. (e) Wu, Q.; Esteghamatian, M.; Hu, N.-X.; Popovic, Z.;
Enright, G.; Tao, Y.; D’Iorio, M.; Wang, S. Chem. Mater., in press.
(f) Wu, Q.; Esteghamatian, M.; Hu, N.-X.; Popovic, Z. D.; Enright,
G.; Breeze, S. R.; Wang, S. Angew. Chem., Int. Ed. Engl. 1999, 38,
985.
Experimental Section
All reactions were carried out under a nitrogen atmosphere. All
starting materials were purchased from Aldrich Chemical Co. and used
without further purification, unless otherwise stated. Tetrahydrofurnan,
hexane, and toluene were distilled from sodium and benzophenone
under a nitrogen atmosphere, and dichloromethane was distilled from
P2O5 under a nitrogen atmosphere. 1H NMR spectra were recorded on
Bruker Avance 300 and 400 MHz spectrometers. The Canadian
Microanalytical Service Ltd., Delta, British Columbia, performed the
elemental analyses. TLC was also carried out on SiO2 (silica gel F254,
Whatman), and column chromatography was also carried out on silica
(silica gel 60, 70-230 mesh). Melting points were determined on a
Fisher-Johns melting point apparatus, and excitation and emission
spectra were recorded on a Photon Technologies International Quan-
taMaster model C-60 spectrometer.
(4) (a) Hassan, A.; Wang, S. J. Chem. Soc. Chem. Commun. 1998, 211.
(b) Liu, W.; Hassan, A.; Wang, S. Organometallics 1997, 16, 4257.
(c) Ashenhurst, J.; Brancaleon, L.; Hassan, A.; Liu, W.; Schmider,
H.; Wang, S.; Wu, Q. Organometallics 1998, 17, 3186-3195. (d)
Hassan, A.; Wang, S., J. Chem. Soc., Chem. Commun. 1998, 339.
10.1021/ic991436m CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/04/2000

2398 Inorganic Chemistry, Vol. 39, No. 11, 2000
Yang et al.
Synthesis of Zn(CF3COO)2. A 2.70 g (0.033 mol) sample of ZnO
was placed in a 100 mL flask, and 15.00 g of CF3COOH (excess) was
added. The mixture was stirred for 2 h at ambient temperature. After
all ZnO reacted with CF3COOH, excess CF3COOH was removed under
vacuum. The product was washed with diethyl ether and dried in a
vacuum at 50 °C overnight, giving 9.30 g (0.032 mmol) of Zn(CF3-
COO)2 (98% yield).
mounted on glass fibers. All data were collected on a Siemens P4 single-
crystal diffractometer with graphite-monochromated Mo KR radiation,
operating at 50 kV and 35 mA at 23 °C. The data for 1-3 were
collected over 2θ ranges of 3-47, 3-50, and 3-45°, respectively. The
data for 2,2′,2′′-tpa and 2,2′,3′′-tpa were collected over 2θ ranges of
3-68 and 3-45°, respectively. Three standard reflections were
measured every 197 reflections. No significant decay was observed
for any of the samples. Data were processed on a Pentium PC using
the Siemens SHELXTL software package5 (version 5.0) and corrected
for absorption, Lorentz, and polarization effects. Neutral-atom scattering
factors were taken from Cromer and Waber.6 The crystals of 1, 2, and
2,2′,3′′-tpa belong to the monoclinic space groups P21/c, P2/c, and P21/
n, respectively, uniquely determined by systematic absences. The crystal
of 3 belongs to the triclinic space group P1h, while the crystals of 2,2′,2′′-
tpa belong to the rhombohedral space group R3. All structures were
solved by direct methods. Some of the CF3 groups in compounds 2
and 3 display a typical 2-fold rotational disorder, which was modeled
and refined successfully. All non-hydrogen atoms were refined aniso-
tropically, except those of some of the disordered CF3 groups. The
positions for all hydrogen atoms in compounds 1-3 were calculated,
and their contributions were included in the structure factor calculations.
All hydrogen atoms in the free ligands were located directly from
difference Fourier maps and refined successfully. The crystallographic
data for compounds 1-3 and the ligands are given in Table 1.
Synthesis of 2,2′,2′′-Tripyridylamine (2,2′,2′′-tpa). A 4.00 g (0.023
mol) sample of 2,2′-dipyridylamine, 5.54 g (0.035 mol) of 2-bromopy-
ridine, 2.08 g (0.037 mol) of potassium hydroxide, and 120 mg of cupric
sulfate (catalyst) were placed in a 100 mL flask. The mixture was heated
to 180 °C and stirred for 6 h. The reaction was monitored by TLC.
After the reaction mixture was cooled to ambient temperature, dichlo-
romethane and water were added to dissolve the solids. The aqueous
phase was discarded, and the organic phase was washed with distilled
water until the pH was 7. Na2SO4 was used to dry the organic phase.
The product was isolated by using a chromatographic column with
ethanol as the eluant. Colorless crystals of 2,2′,2′′-tpa were obtained
1
in 48% yield from an ethanol/hexane solution. Mp: 129-131 °C. H
3
NMR (δ, ppm; CDCl3, 298 K): 8.43 (1H, d, J ) 5.1 Hz), 7.68 (1H,
3
3
3
dd, J ) 7.5 Hz), 7.11 (1H, d, J ) 6.9 Hz), 7.08 (1H, dd, J ) 5.1
Hz)
Synthesis of 2,2′,3′′-Tripyridylamine (2,2′,3′′-tpa). A 3.03 g (0.018
mol) sample of 2,2′-dipyridylamine, 3.08 g (0.019 mol) of 3-bromopy-
ridine, 1.40 g (0.025 mol) of potassium hydroxide, and 0.075 g of cupric
sulfate (catalyst) were placed in a 100 mL flask. The mixture was heated
to 180 °C and stirred for 5 h. After the reaction mixture was cooled to
ambient temperature, dichoromethane and water were added to dissolve
the solids. The aqueous phase was discarded. The organic phase was
washed with distilled water until the pH was 7 and dried over Na2SO4.
The product was isolated by using a chromatographic column and a
mixed-solvent eluant (ethyl acetate/ethanol, 1:1). Colorless crystals of
2,2′,3′′-tripyridylamine were obtained in 57% yield from a solution of
Results and Discussion
Syntheses and Crystal Structures. (a) Ligands. 2,2′-
Dipyridylamine has no emission in the visible region. When it
is coordinated to a metal center such as Zn(II) or Al(III) as a
neutral ligand, weak emission at λmax <400 nm is observed.7
The near-UV emission makes 2,2′-dipyridylamine complexes
unsuitable as blue emitters for electroluminescent applications.
Aluminum or boron complexes with deprotonated 2,2′-dipy-
ridylamine, albeit producing bright blue emission at ∼470 nm,
are in general too unstable to be useful in electroluminescent
devices.4 Because the HOMO-LUMO gap of the ligand can
be affected by substituents, complexes containing properly
modified 2,2′-dipyridylamine may have an emission in the blue
region with an improved efficiency. We therefore carried out
the syntheses of modified 2,2′-dipyridylamine (dpa) ligands by
replacing the proton on the nitrogen atom of dpa with an
aliphatic or aromatic group R. Among the new modified dpa
ligands, only 2,2′,2′′-tripyridylamine (2,2′,2′′-tpa) and 2,2′,3′′-
tripyridylamine (2,2′,3′′-tpa) yield blue luminescence when
coodinated to an Al(III) ora Zn(II) ion. Therefore, our investiga-
tion focused on these two ligands. The syntheses of these two
ligands were carried out by using Ullmann condensation
methods,8 where copper(II) and KOH are used as catalysts
(Scheme 1).
1
ethanol/ethyl acetate/hexane. Mp: 103-105 °C. H NMR (δ, ppm;
3
CDCl3, 298 K): 8.35 (2H, d, J ) 4.8 Hz), 8.46 (1H, s), 8.44 (2H, d,
3
3
3J ) 4.8 Hz), 7.63 (2H, dd, J1 ) 8.4 Hz, J2 ) 7.2 Hz), 7.55 (1H, d,
3J ) 8.1 Hz ), 7.34 (1H, dd, 3J1 ) 8.1 Hz, 3J2 ) 4.8 Hz), 7.04 (2H, d,
3
3
3J ) 8.4 Hz), 7.01 (1H, dd, J1 ) 7.2 Hz, J2 ) 4.8 Hz).
Synthesis of Zn(2,2′,2′′-tpa)Cl2 (1). A 55 mg (0.40 mmol) quantity
of ZnCl2 was dissolved in methanol. A 100 mg (0.40 mmol) sample of
2,2′,2′′-tpa in dissolved in methanol was then added to the ZnCl2
solution at ambient temperature. Colorless solids appeared at once.
Colorless crystals of 1 (71 mg, 0.19 mmol) were obtained in 46% yield
by recrystallization from chloroform and hexane. Mp: 265-267 °C.
3
1H NMR (δ, ppm; CDCl3, 298 K): 8.64 (1H, d, J ) 5.4 Hz), 7.90
3
3
3
(1H, dd, J1 ) 8.4 Hz, J2 ) 5.1 Hz), 7.62 (1H, d, J ) 8.4 Hz), 7.39
(1H, dd, 3J1 ) 5.1 Hz, 3J2 ) 5.4 Hz). Anal. Calcd for C15H12N4ZnCl2:
C, 46.85; H, 3.15; N, 14.57. Found: C, 46.55; H, 3.18; N, 14.51.
Synthesis of Zn(2,2′,2′′-tpa)2(CF3COO)2 (2). A 200 mg quantity
of Zn(CF3COO)2 (0.69 mmol) and 343 mg of 2,2′,2′′-tpa (1.38 mmol)
were dissolved in THF, and the mixture was stirred for 1 h at ambient
temperature. After concentration of the solution, a small amount of
hexane was added. Colorless crystals of 2 were obtained in 84% yield.
Mp: 179-180 °C. 1H NMR (δ, ppm; CDCl3, 298 K): 8.57 (1H, d, 3J
The structures of both ligands were examined by a single-
crystal X-ray diffraction analysis. As shown in Figure 1, the
2,2′,2′′-tpa ligand is nonplanar and has a crystallographically
imposed C3 symmetry. The central nitrogen atom N(1) is
coplanar with the three carbon atoms to which it binds (the
maximum deviation of the nitrogen atom from the NC3 plane
is 0.028 Å), while the three pyridyl rings have a propeller
arrangement with a dihedral angle of 35.3° between the central
3
3
3
) 3.9 Hz), 7.78 (1H, dd, J1 ) 8.4 Hz J2 ) 7.2 Hz), 7.11 (1H, d, J
) 8.4 Hz), 7.08 (1H, dd, 3J ) 7.2 Hz). Anal. Calcd for Zn(CF3COO)2-
(2,2′,2′′-tpa)2: C, 51.82; N, 14.22; H, 3.07. Found: C, 51.73; N, 14.27;
H, 3.08.
Synthesis of Zn(2,2′,3′′-tpa)4(CF3COO)2 (3). A 29 mg (0.10 mmol)
quantity of Zn(CF3COO)2 and 99 mg (0.40 mmol) of 2,2′,3′′-tpa were
dissolved in THF, and the mixture was stirred for 1 h at ambient
temperature. After concentration of the solution and the addition of a
small amount of hexane, colorless crystals of compound 3 were obtained
(5) SHELXTL: crystal structure analysis package, Version 5; Siemens:
Madison, WI, 1995.
1
in 77% yield. Mp: 112-114 °C. H NMR (δ, ppm; CDCl3, 298 K):
3
8.56 (1H, s), 8.50 (1H, br), 8.39 (2H, d, 3J ) 3.9 Hz), 7.76 (1H, d, J
(6) Cromer, D. T.; Waber, J. T. International Tables for X-ray Crystal-
lography; Kynoch Press: Birmingham, U.K., 1974; Vol. 4, Table 2.2A.
(7) (a) Ho, K. Y.; Yu, W. Y.; Cheung, K. K.; Che, C. M., J. Chem. Soc.,
Chem. Commun. 1998, 2101. (b) Ho, K. Y.; Yu, W. Y.; Cheung, K.
K.; Che, C. M. J. Chem. Soc., Dalton Trans. 1999, 1581.
(8) (a) Goodbrand, H. B.; Hu, N. X. J. Org. Chem., 1999, 64, 670. (b)
Lindley, J. Tetrahedron 1984, 40, 1433. (c) Fanta, P. E. Synthesis
1974, 1.
3
3
) 8.4 Hz), 7.69 (2H, dd, J ) 7.2 Hz), 7.48 (1H, dd, J ) 7.5 Hz),
3
3
3
7.10 (2H, dd, J1 ) 7.2 Hz, J2 ) 3.9 Hz), 7.05 (2H, d, J ) 7.2 Hz).
Anal. Calcd for Zn(CF3COO)2(2,2′,3′′-tpa)4: C, 59.84; H, 3.77; N,
17.45. Found: C, 60.26; H, 3.86; N, 16.76.
X-ray Crystallographic Analysis. All crystals were obtained either
from CH2Cl2/hexane solutions or from THF/hexane solutions and were

Blue Luminescent Zinc(II) Complexes
Inorganic Chemistry, Vol. 39, No. 11, 2000 2399
Table 1. Crystallographic Data for the Ligands and Compounds 1-3
2,2′,2′′-tpa
2,2′,3′′-tpa
1
2
3
formula
fw
space group
a/Å
b/Å
c/Å
C15H12N4
248.3
R3
15.912(2)
15.912(2)
4.134(1)
90
90
120
906.5(5)
3
1.364
0.86
C15H12N4
248.3
P21/n
8.758(12)
11.598(6)
24.985(12)
90
94.72(5)
90
2529(4)
8
1.304
0.82
45
C15H12N4Cl2Zn
384.3
P21/c
13.939(2)
8.437(3)
14.193(3)
90
103.96(1)
90
1619.9(7)
4
1.577
18.46
47
C34H24N4F6O4Zn
787.98
P2/c
19.292(10)
10.466(2)
18.574(4)
90
113.709(18)
90
3434(2)
4
1.524
7.99
50
0.710 73
23
C64H48N16F6O4Zn
1284.5
P1h
10.222(7)
12.375(3)
13.744(7)
105.13(5)
108.34(5)
105.50(3)
1472.7(13)
1
1.448
5.01
45
0.710 74
23
R/deg
â/deg
γ/deg
V/Å3
Z
Dc/g cm-3
µ/cm-1
2θ
λ/Å
68
0.710 73
23
0.710 73
23
0.710 73
23
temp/°C
no. of reflns measd
no. of reflns used (Rint)
no. of variables
final R values [I > 2σ(I)]:
R1,awR2b
447
372 (0.092)
74
3581
3326 (0.030)
439
2514
2404 (0.021)
199
6114
5909 (0.035
503
3870
3756 (0.040)
409
0.0685, 0.1293
0.0608, 0.1417
0.0470, 0.1132
0.0737, 0.1858
0.0806, 0.1529
R values (all data):
0.0688, 0.1296
0.1036, 0.1739
0.0675, 0.1244
0.1425, 0.2364
1.058
0.1732, 0.1927
1.036
R1,a wR2b
goodness-of-fit on F2
1.326
1.027
1.069
a R1 ) ∑|Fo| - |Fc|/∑|Fo|. b wR2 ) [∑w(fo - Fc2)2/∑w(Fo )2]1/2; w ) 1/[σ2(Fo ) + (0.075P)2], where P ) [Max(Fo , 0) + 2Fc2]/3.
2
2
2
2
Scheme 1
NC3 plane and the pyridyl plane, attributable to steric interac-
tions between the pyridyl rings. All molecules of 2,2′,2′′-tpa
are stacked along the unique c axis with an intermolecular
separation distance of 4.134(1) Å. In contrast, 2,2′,3′′-tpa
Figure 2. ORTEP diagram showing the structure of the 2,2′,3′′-tpa
ligand with 50% thermal ellipsoids and the atom-labeling scheme.
crystallizes in the monoclinic crystal system. There are more
than one stacking directions for the molecules of 2,2′,3′′-tpa in
the crystal lattice. Two independent molecules of 2,2′,3′′-tpa
planar, with a maximum deviation from the NC3 plane of 0.013
Å. The dihedral angles between the pyridyl ring and the NC3
plane in 2,2′,3′′-tpa vary from 27.6 to 60.2°, with an average
are present in the asymmetric unit. Both molecules are nonpla-
of 38.6°, significantly larger than those in 2,2′,2′′-tpa. The
nar, as is the case for 2,2′,2′′-tpa. The central NC3 portion is
presence of a hydrogen atom at the 2- and 6-positions in the
3-pyridyl ring likely increases the steric interaction, leading to
an increase in the dihedral angle in comparison to that of 2,2′,2′′-
tpa. The structure for one of the 2,2′,3′′-tpa molecules in the
asymmetric unit is shown in Figure 2.
Complexes. 2,2′,2′′-tpa and 2,2,′3′′-tpa ligands react readily
with a variety of Zn(II) compounds to form various complexes.
Three examples of Zn(II) complexes with these two ligands are
presented herein. They are Zn(2,2′,2′′-tpa)Cl2 (1), obtained from
the reaction of 2,2′,2′′-tpa with ZnCl2 in a 1:1 ratio, Zn(2,2′,2′′-
tpa)2(O2CCF3)2 (2), obtained from the reaction of 2,2′,2′′-tpa
with Zn(O2CCF3)2 in a 2:1 ratio, and Zn(2,2′,3′′-tpa)4(O2CCF3)2
(3), obtained from the reaction of 2,2′,3′′-tpa with Zn(O2CCF3)2
in a 4:1 ratio. All three compounds are air stable. The structures
of all three compounds weer determined by single-crystal X-ray
diffraction analyses. Important bond lengths and angles are listed
in Table 2.
Figure 1. ORTEP diagram showing the structure of the 2,2′,2′′-tpa
ligand with 50% thermal ellipsoids and the atom-labeling scheme.

2400 Inorganic Chemistry, Vol. 39, No. 11, 2000
Yang et al.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
Compounds 1-3
Compound 1
Zn(1)-N(1)
Zn(1)-N(2)
2.057(4)
2.058(4)
Zn(1)-Cl(2)
Zn(1)-Cl(1)
2.2011(15)
2.2076(16)
N(1)-Zn(1)-N(2)
88.39(14) N(1)-Zn(1)-Cl(1) 110.58(12)
N(1)-Zn(1)-Cl(2) 111.91(11) N(2)-Zn(1)-Cl(1) 110.64(11)
N(2)-Zn(1)-Cl(2) 111.96(11) Cl(2)-Zn(1)-Cl(1) 119.20(6)
Compound 2
Zn(1)-O(1)
Zn(1)-N(2)
Zn(1)-N(1)
2.127(5)
2.167(6)
2.167(7)
Zn(2)-O(3)
Zn(2)-N(5)
Zn(2)-N(6)
2.110(5)
2.156(6)
2.182(5)
O(1)-Zn(1)-O(1A)
O(1)-Zn(1)-N(2)
O(1A)-Zn(1)-N(2)
N(2)-Zn(1)-N(2A)
O(1)-Zn(1)-N(1)
O(1A)-Zn(1)-N(1)
N(2)-Zn(1)-N(1)
N(2A)-Zn(1)-N(1)
87.0(3) O(3)-Zn(2)-O(3A)
175.0(2) O(3)-Zn(2)-N(5)
91.3(2) O(3A)-Zn(2)-N(5)
83.9(3)
93.7(2)
86.8(2)
90.8(3) N(5A)-Zn(2)-N(5) 179.2(3)
88.5(3) O(3)-Zn(2)-N(6)
90.4(3) N(5A)-Zn(2)-N(6)
86.9(2) N(5)-Zn(2)-N(6)
94.2(2) O(3)-Zn(2)-N(6A)
175.7(2)
94.9(2)
84.6(2)
Figure 4. ORTEP diagram showing the structure of compound 2 with
50% thermal ellipsoids and the atom-labeling schemes.
92.05(19)
N(1)-Zn(1)-N(1A) 178.4(3)
one of the independent molecules is shown here. The zinc center
is six-coordinate with an octahedral geometry. Two 2,2′,2′′-tpa
ligands chelate to the zinc center in a fashion similar to that
observed in compound 1. Two trifluoroacetate ligands are
coodinated to the zinc center as terminal ligands and in a cis
geometry. The crystals of the trans geometric isomer of 2 were
not obtained. The Zn-N bond lengths (2.156-2.182 Å) are
much longer than those of compound 1 (2.057-2.058 Å),
obviously resulting from the increased number of ligands in
compound 2. The Zn-O bond lengths are normal.9 The
noncoordinated nitrogen atom N(4) is 4.824 Å from the zinc
center. The central NC3 portion of the 2,2′,2′′-tpa ligand in
compound 2 is planar. One of the pyridyl rings (N(4)) is nearly
coplanar with the NC3 plane (dihedral angle ) 7.3°), while the
remaining two pyridyl rings have dihedral angles of 65.3 and
62.6°, respectively, with the NC3 plane. There appears to be
some π-π stacking between the pyridyl rings of the two 2,2′,2′′-
tpa ligands, as indicated by the C(11)-N(2A) distance of 3.536
Å. Many examples of zinc(II) complexes with an octahedral
geometry have been observed previously.9a,10 In principle,
2,2′,2′′-tpa can function not only as a bidentate chelating ligand
but also as a tridentate chelating ligand where all three pyridyl
groups bind to the same central atom. It is therefore interesting
that only the bidentate bonding mode is observed in compound
1, where the zinc center is only four-coordinate. The bidentate
binding mode of 2,2′,2′′-tpa in 2 could be explained by the
coordination saturation around the zinc atom.
Compound 3
2.131(6) Zn(1)-N(1)
2.270(7)
Zn(1)-O(1)
Zn(1)-N(5)
2.138(6)
93.9(3)
O(1)-Zn(1)-O(1A) 180.000(1) O(1)-Zn(1)-N(5)
O(1)-Zn(1)-N(1) 92.6(2)
O(1)-Zn(1)-N(1A) 87.4(2)
N(1)-Zn(1)-N(1A) 180.000(2) N(5A)-Zn(1)-N(5) 180.000(1)
N(1)-Zn(1)-N(5A) 90.2(2)
O(1A)-Zn(1)-N(5) 86.1(3)
N(1)-Zn(1)-N5) 89.8(2)
Figure 3. ORTEP diagram showing the structure of compound 1 with
50% thermal ellipsoids and the atom-labeling scheme.
The structure of compound 3 is shown in Figure 5. Compound
3 has a crystallographically imposed inversion center of
symmetry. There are four 2,2′,3′′-tpa ligands in 3. However, in
contrast to 2,2′,2′′-tpa, the 2,2′,3′′-tpa ligands in 3 function as
terminal ligands and bind to the zinc (II) center by the 3-pyridyl
nitrogen atoms only. The steric interactions between the four
2,2′,3′′-tpa ligands must be responsible for the terminal bonding
mode. The two trifluoroacetate ligands are coordinated to the
Zn(1) center as terminal ligands in a trans fashion, leading to
the six-coordinate octahedral geometry of the zinc center. The
two noncoordinating 2-pyridyl rings in the 2,2′,3′′-tpa ligand
have an orientation similar to that in the free ligand; i.e., the
As shown in Figure 3, two of the 2-pyridyl rings in 2,2′,2′′-
tpa chelate to the Zn(II) center in compound 1. The N(1)-
Zn(1)-N(2) angle of 88.39(14)° is much less than a typical
tetrahedral bond angle (109°). Compound 1 is therefore a
somewhat distorted tetrahedron. The amino nitrogen atom N(3)
is coplanar with the three carbon atoms to which it binds (C(5),
C(10), and C(11); the sum of bond angles around N(3) is 360°),
as in the free ligand. One of the pyridyl rings (N(4)) is nearly
coplanar with the central NC3 plane (dihedral angle ) 3.4°),
while the remaining two pyridyl rings have dihedral angles of
63.1 and 65.0°, respectively, with the NC3 unit. The Zn-N and
Zn-Cl bond lengths in 1 are typical.9 The noncoordinating N(4)
atom is 4.33 Å from the zinc center.
(9) (a) Prince, R. H. In ComprehensiVe Coordination Chemistry; Wilkin-
son, G., Gillard, R. D., McCleverty, J. A., Eds.; Pergamon Press: New
York, 1987; Vol. 5, Chapter 56.1. (b) Auf der Heyde, T. P. E. Acta
Crystallogr, 1984, B40, 582. (c) Kerr, M. C.; Preston, H. S.; Ammon,
H. L.; Huheey, J. E.; Stewart, J. M. J. Coord. Chem. 1981, 11, 111.
(d) Smith, H. W. Acta Crystallogr. 1975, B31, 2701.
The structure of compound 2 is shown in Figure 4. Compound
2 has a crystallographically imposed 2-fold rotation symmetry.
There are two independent molecules in the asymmetric unit.
The structures and bond lengths and angles for these two
independent molecules are essentially identical. Therefore, only
(10) Mikami-Kido, M.; Saito, Y. Acta Crystallogr. 1982, B38, 452.

Blue Luminescent Zinc(II) Complexes
Inorganic Chemistry, Vol. 39, No. 11, 2000 2401
Scheme 2
Figure 5. ORTEP diagram showing the structure of compound 3 with
50% thermal ellipsoids and the atom-labeling scheme.
solid state. As the temperature increased, all chemical shifts
became broad and coalesced at about 230 K, consistent with
the presence of an exchange process between the coordinate
and noncoordinate 2-pyridyl rings. At 298 K, only one set of
2-pyridyl chemical shifts were observed, attributable to a fast
exchange at this temperature. The proposed exchange mecha-
nism is shown in Scheme 2. The behavior of 1 in THF-d8 is
similar to that in CD2Cl2. Similar exchange phenomena involv-
ing tripod ligands such as HOC(2-Py)(1-methylimidazole)2 have
been reported previously.11
The observed structure for compound 2 in the solid state is
the cis isomer where all three 2-pyridyl rings of 2,2′,2′′-tpa are
not equivalent: there are one noncoordinating 2-pyridyl and
two coordinating 2-pyridyls, one in the basal plane and the other
in the axial position. Therefore, if compound 2 retains the cis
structure in solution, three sets of chemical shifts (12 peaks) in
a 1:1:1 ratio due to the three distinct 2-pyridyl groups should
1
be observed. The examination of compound 2 by H NMR
spectroscopy over the temperature range of 180-298 K using
either THF-d8 or CD2Cl2 as the solvent revealed that compound
2 is highly dynamic in solution. The behaviors of compound 2
in THF and dichloromethane are very similar. The variable-
Figure 6. 1H NMR spectra of compound 1 in CD2Cl2 at 183-298 K.
1
nitrogen atoms are oriented away from each other. Two
structural isomers, trans and cis, are possible for compound 3.
However, only the trans isomer was observed for compound 3,
again attributable to steric interactions. The central NC3 unit of
2,2′,3′′-tpa in 3 is planar. The smallest dihedral angle between
a pyridyl ring and the NC3 plane is 12.3° (N(8)).
temperature H NMR spectra of 2 in THF-d8 are shown in
Figure 7. At 298 K, only one set of chemical shifts due to
2-pyridyl are present, indicative of exchange. At 183 K, 13 peaks
with nonequal intensities are observed, an indication that the
cis isomer is not the only species present in solution. In the
trans isomer of 2, all coordinate 2-pyridyl rings are in the basal
plane. Therefore, two sets of chemical shifts (8 peaks) in a 2:1
ratio due to the coordinate and noncoordinate 2-pyridyl rings
should be present for the trans isomer. If cis and trans isomers
coexist in solution, 20 peaks should be present in the 1H NMR
spectrum of 2. Accidental peak overlaps could reduce the
number of peaks observed, which we believe is the case for
compound 2; i.e., the observed 13 peaks with nonequal
intensities are due to the coexistence of cis and trans isomers.
To assign the chemical shifts to the corresponding isomers, we
attempted 2D exchange NMR (EXSY) and COSY experiments
at 183 K. However, due to the extensive peak overlaps,
conclusive peak assignments were unsuccessful. Nevertheless,
the NMR spectra shown in Figure 7 indicated that it is most
Fluxional Behavior of Compounds 1 and 2. In compound
1, two of the 2-pyridyl rings in the 2,2′,2′′-tpa ligand are chelated
to the Zn(1) center in an approximately symmetric fashion, while
the remaining 2-pyridyl ring is not coordinated. If compound 1
retains the same structure in solution, two sets of distinct
chemical shifts due to the coordinate and noncoordinate
1
2-pyridyl rings should be observed in the H NMR spectrum
of compound 1. However, the 1H NMR spectrum of 1 at ambient
temperature shows only one set of 2-pyridyl chemical shifts,
indicative of the presence of some dynamic exchange in solution.
To confirm this, we examined the 1H NMR spectrum of 1 over
the temperature range of 183-298 K using CD2Cl2 as the
solvent. The variable-temperature NMR spectra of 1 are shown
in Figure 6. At 183 K, two sets of 2-pyridyl chemical shifts in
a 2:1 ratio assigned to the coordinate and noncoordinate pyridyl
groups were observed, thus confirming that, at 183 K, the
structure of compound 1 in solution is the same as that in the
(11) (a) Canty, A. J.; Minchin, N. J.; Healy, P. C.; White, A. H. J. Chem.
Soc., Dalton Trans. 1982, 1795. (b) Byers, P. K.; Canty, A. J.; Minchin,
N. J.; Patrick, J. M.; Skelton, B. W.; White, A. H. J. Chem. Soc.,
Dalton Trans. 1985, 1183.

2402 Inorganic Chemistry, Vol. 39, No. 11, 2000
Yang et al.
Figure 8, in contrast to the free ligands, compounds 1-3 in
solution each have a broad emission band (bandwidth at half-
height ) 80-90 nm) with λmax ) 422, 426, and 432 nm,
respectively. Previously reported 2,2′-dipyridylamine Zn(II)
complexes emit mostly at λmax <400 nm in solution and in the
solid state.7 The replacement of a proton on the amino group
in 2,2′-dipyridylamine by a pyridyl group apparently resulted
in a decrease of the HOMO-LUMO energy gap of the 2,2′,2′′-
tpa and 2,2′,3′′-tpa ligand in complexes 1-3. To understand
the origin of the red shift of the emission energy from the free
ligand to the complex, we performed ab initio calculations for
the free ligand, 2,2′,2′′-tpa, and compound 1 at the restricted
Hartree-Fock (RHF) level using a standard split-valence
polarized (6-31G*) basis set for the free ligand and an
unpolarized (6-31G*) basis set for the zinc atom and employing
the Gaussian 98 suite of programs.12 The geometric parameters
were obtained from crystal structural data. In Figure 9 are plotted
the energies of the highest occupied molecular orbitals (HO-
MO’s) and lowest unoccupied molecular orbitals (LUMO’s) for
the free ligand and compound 1, together with the surfaces on
which these orbitals attain values of (0.05 au. As one may
see, the HOMO of the free ligand (which has a C3 symmetry)
is a π orbital with contributions from all three 2-pyridyl rings
and a substantial contribution from the lone pair of the amino
nitrogen atom. The HOMO of the complex is also a π orbital
localized on the noncoordinating pyridyl ring (N(4)) and the
lone pair of the amino nitrogen atom, owing to the low
symmetry (C1). However, the nodal structure of the HOMO of
the ligand is not compatible with that of the complex. The
energy of the HOMO level of the complex is much lower than
that of the free ligand. The LUMO of the free ligand is a π*
orbital with contributions from all three pyridyl rings, while
the LUMO of the complex is a π* orbital consisting of mostly
atomic orbitals from one of the coordinated pyridyl rings. There
is essentially no contribution from the amino nitrogen in the
LUMO’s of the free ligand and the complex. The observed
emission of the free ligand and the complex can be therefore
attributed to the transition between π (HOMO) and π* (LUMO)
levels. The LUMO energy level of the complex is again much
lower than that of the free ligand. The decrease in the energies
of both HOMO and LUMO levels in the complex relative to
those of the free ligand is caused by the coordination of the
pyridyl rings to the zinc center, which makes the nitrogen atoms
of the pyridyl rings more electronegative, thus lowering the
energy of the molecular orbitals. The net effect is that the
HOMO-LUMO energy gap of the complex is smaller than that
of the free ligand, as shown in Figure 9. Consequently, the
emission energy of the complex has a red shift in comparison
to that of the free ligand. Due to the exaggerated gap in Hartree-
Fock calculations, the HOMO-LUMO energy gap difference
does not at all correspond to the experimental values. However,
the qualitative trend is retained.
Figure 7. 1H NMR spectra of compound 2 in THF-d8 at 183-298 K.
Figure 8. THF solution excitation and emission spectra of 2,2′,2′′-tpa
(thin solid lines), 2,2′,3′′-tpa (dashed lines), compound 1 (solid squares,
excitation; empty squares, emission), 2 (solid triangles, excitation; empty
triangles, emission), and 3 (dark solid lines).
likely that compound 2 exists as both cis and trans isomers in
solution. The dynamic behavior of 2 can be attributed to both
isomer interconversion and exchange of coordinate and non-
coordinate 2-pyridyl rings.
The four 2,2′,3′′-tpa ligands in the trans isomer of compound
3 are essentially equivalent. Therefore, only one set of chemical
1
shifts due to 2,2′,3′′-tpa should be observed in the H NMR
spectrum of 3, if 3 exists as the trans isomer only. Indeed,
variable-temperature 1H NMR spectra for compound 3 in THF-
d8 or CD2Cl2 in the temperature range of 183-298 K show
only one set of chemical shifts due to the 2,2′,3′′-tpa ligand
which do not change with temperature. We therefore concluded
that compound 3 exists as the trans isomer only and is not
fluxional in solution. The nonfluxional behavior of 3 in solution,
in contrast to those of 1 and 2 can be attributed to the fact that
there are four 2,2′,3′′-tpa ligands in 3, which makes the molecule
very crowded, thus favoring the sterically less demanding trans
isomer.
Luminescent Properties of Compounds 1-3. 2,2′,2′′-tpa
and 2,2′,3′′-tpa ligands have no emission in the visible region.
In solution, these two ligands have sharp emission bands at λmax
) 347 and 360 nm, respectively (Figure 8). When these ligands
were bound to a zinc center, blue luminescence was observed.
Because ZnCl2 and Zn(O2CCF3)2 are not luminescent, the
observed luminescence of the complexes is attributed to the
coordinate 2,2′,2′′-tpa and 2,2′,3′′-tpa ligands. As shown in
(12) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone,
V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.;
Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R.
L. D.; Fox, J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,
A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.;
Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.6; Gaussian,
Inc.: Pittsburgh, PA, 1998.

Blue Luminescent Zinc(II) Complexes
Inorganic Chemistry, Vol. 39, No. 11, 2000 2403
Figure 9. HOMO and LUMO orbitals of 2,2′,2′′-tpa and compound 1.
In addition to red-shifting the emission energy, the formation
of complexes also significantly enhances the emission efficien-
cies of the ligands. The quantum yields for 2,2′,2′′-tpa and
2,2′,3′′-tpa were determined to be 0.026 and 0.024, respectively,
relative to that of 9,10-diphenylanthracene. In contrast, the
quantum yields for compounds 1-3 were determined to be
0.054, 0.22, and 0.10, respectively. The chelating mode of the
ligand and the absence of heavy atoms such as chlorine are
believed to be responsible for the relatively high quantum yield
of compound 2. A similar enhancement of quantum yield by
coordination has been observed previously.4,13 The excitation
and emission spectra of compounds 1-3 in the solid state are
shown in Figure 10. Despite the similarity of their emission
bands in solution, the emission bands of the three compounds
in the solid state are quite different. Compound 1 has a relatively
narrow emission band with λmax ) 395 nm (bandwidth at half-
height ) 77 nm), while compound 2 has a very broad emission
band (bandwidth at half-height ) 120 nm) with λmax ) 385
nm. Although the emission maxima of 1 and 2 in the solid state
are no longer in the blue region, the emission bands of both
compounds cover much of the blue region, giving the observed
blue luminescence. Compound 3 has a broad emission band
(bandwidth at half-height ) 100 nm) with λmax ) 470 nm in
the solid state, giving the appearance of a whitish-blue
luminescence. The deviation of the solid-state emission spectra
from those of solutions is likely caused by crystal packing
differences of compounds 1-3 in the solid state. It is however
surprising that compounds 1 and 2 have a blue shift while
compound 3 has a red shift from solution to the solid state. In
Figure 10. Excitation and emission spectra in the solid state of 1
(crosses), 2 (empty squares), and 3 (solid lines).
the crystal lattice of 1, there are π-π stackings between the
noncoordinating pyridyl rings of neighboring molecules, with
the shortest atomic separation distance being 3.885 Å. There is
however no intermolecular stacking in the crystal lattice of 2.
In the crystal lattice of 3, there is an extensive π-π stacking
between the coordinate 3-pyridyl ring and one of the noncoor-
dinate 2-pyridyl rings, with the shortest atomic separation
distance being 3.582 Å. The observed emission energy red shift
of 3 from solution to the solid state could be caused by the
π-π interactions between 2-pyridyl and 3-pyridyl rings. Further
research is required to better understand this phenomenon.
Conclusion
(13) (a) Photochemistry and Photophysics of Coordination Compounds;
Yersin, H., Vogler, A., Eds.; Springer-Verlag: Berlin, 1987. (b)
Concepts of Inorganic Photochemistry; Adamson, A. W., Fleischauer,
P. D., Eds.; John Wiley & Sons: New York, 1975.
We demonstrated here that 2,2′,2′′-tpa and 2,2′,3′′-tpa ligands
form a variety of Zn(II) complexes which have interesting
structural features and display rare blue luminescence. The

2404 Inorganic Chemistry, Vol. 39, No. 11, 2000
Yang et al.
coordination of the 2,2′,2′′-tpa and 2,2′,3′′-tpa ligands to a Zn-
(II) center causes a shift of emission energy from UV to blue
and an increase in emission efficiency. Zn(II) complexes of
2,2′,2′′-tpa are fluxional in solution owing to the intramolecular
exchange of coordinate and noncoordinate 2-pyridyl rings. There
is a significant difference between the emission spectra of
compounds 1-3 in solution and in the solid state, the cause of
which is yet to be understood. Compounds 1-3 have the
potential to be used as blue emitters in electroluminescent
devices. Efforts are being taken by our group to investigate the
electroluminescent properties of compounds 1-3 and the results
will be reported elsewhere.
Acknowledgment. We thank the Natural Sciences and
Engineering Council of Canada and the Xerox Research
Foundation for financial support.
Supporting Information Available: Lisings of X-ray experimental
details, atomic coordinates, thermal parameters, and bond distances and
angles. This material is available free of charge via the Internet at
http://pubs.acs.org.
IC991436M