Syntheses, structural characterization, and first electroluminescent properties of mono-cyclometalated platinum(II) complexes with Greater than classical π-π stacking and Pt-Pt distances

Article abstract of DOI:10.1021/om049211t

A series of mono-cyclometalated Pt(II) complexes (9, 10,13,15, and 17) bearing 2-(3,5-bis(trifluoromethyl)phenyl)-4-methylpyridine (1) as the same chromophoric NC cyclom-etalated ligand and the second nonchromophoric bidenate ligands with increasing bulkiness were synthesized by the following steps. The palladium-catalyzed cross-coupling reaction between 2-chloro-4-methylpyridine (2) and 3,5-bis(trifluoromethyl)phenylboronic acid (3) was used to prepare 2-(3,5-bis(trifluoromethyl)phenyl)-4-methylpyridine (1). Di-tert-butyl(trim- ethylsilyl)anylmethylphosphine, tert-Bu₂P-CH₂-SiMe ₃ (5), was used as a ligand for the Suzuki coupling. The cyclometalation of 1 by K₂PtCl₄ (7) was carried out in (MeO)_sP=O (8) with the formation of a chloro-bridged dimer NCPt(μu-Cl)₂PtC N (9). The reactions of 9 with 2,2,6,6-tetramethylheptane-3,5-dionate(11), 1,1,1,3,3,3-hexafluoro-2-pyra2ol-1- ylmethylpropan-2-ol (12), 2-[(diphenylphosphanyl)methyl)-1,1,1,3,3,3- hexafluoropropan-2-ol (14), and 2-[(di-tert-butylphosphanyl)methyl]-1,1,1,3,3,3- hexafluoropropan-2-ol (16) afforded square-planar Pt(II) complexes 10, 13, 15, and 17, correspondingly. According to X-ray analyses, the plane-to-plane distances of the π-π stacked complexes (10, 13, 15, and 17) increase from 3.478 A for 10 to 4.203 A for 15. The value of 4.203 A for aromatic-aromatic π-π interaction is greater than the usual range of 3.3-3.8 A for these phenomena. Pt-Pt distances increase from 5.076 A for 13 to 8.123 A for 17. The geometry of complex 17 deviates greatly from square planar and is best described as "bowl-shaped". The effect of an increased π-π stacking distance on the electroluminescent properties of these Pt compounds is studied. The typical yellow luminescence band associated with the excimers of Pt compounds is absent for complex 15; however, it shows a green 540 nm electroluminesce band instead. An efficient LED constructed using compound 15 shows green electroluminescence with a peak efficiency of 6 cd/A. DuPont contribution #8551.

Full text of DOI:10.1021/om049211t

Organometallics 2005, 24, 619-627
619
Syntheses, Structural Characterization, and First
Electroluminescent Properties of Mono-cyclometalated
Platinum(II) Complexes with Greater than Classical
π-π Stacking and Pt-Pt Distances†
Alex S. Ionkin,* William J. Marshall, and Ying Wang
DuPont Central Research & Development, DuPont Displays, Experimental Station,
Wilmington, Delaware 19880-0328
Received October 12, 2004
A series of mono-cyclometalated Pt(II) complexes (9, 10, 13, 15, and 17) bearing 2-(3,5-
bis(trifluoromethyl)phenyl)-4-methylpyridine (1) as the same chromophoric N∧C cyclom-
etalated ligand and the second nonchromophoric bidenate ligands with increasing bulkiness
were synthesized by the following steps. The palladium-catalyzed cross-coupling reaction
between 2-chloro-4-methylpyridine (2) and 3,5-bis(trifluoromethyl)phenylboronic acid (3) was
used to prepare 2-(3,5-bis(trifluoromethyl)phenyl)-4-methylpyridine (1). Di-tert-butyl(trim-
ethylsilyl)anylmethylphosphine, tert-Bu2P-CH2-SiMe3 (5), was used as a ligand for the Suzuki
coupling. The cyclometalation of 1 by K2PtCl4 (7) was carried out in (MeO)3PdO (8) with
the formation of a chloro-bridged dimer N∧CPt(µ-Cl)2PtC∧N (9). The reactions of 9 with
2,2,6,6-tetramethylheptane-3,5-dionate (11), 1,1,1,3,3,3-hexafluoro-2-pyrazol-1-ylmethylpro-
pan-2-ol (12), 2-[(diphenylphosphanyl)methyl]-1,1,1,3,3,3-hexafluoropropan-2-ol (14), and
2-[(di-tert-butylphosphanyl)methyl]-1,1,1,3,3,3-hexafluoropropan-2-ol (16) afforded square-
planar Pt(II) complexes 10, 13, 15, and 17, correspondingly. According to X-ray analyses,
the plane-to-plane distances of the π-π stacked complexes (10, 13, 15, and 17) increase
from 3.478 Å for 10 to 4.203 Å for 15. The value of 4.203 Å for aromatic-aromatic π-π
interaction is greater than the usual range of 3.3-3.8 Å for these phenomena. Pt-Pt distances
increase from 5.076 Å for 13 to 8.123 Å for 17. The geometry of complex 17 deviates greatly
from square planar and is best described as “bowl-shaped”. The effect of an increased π-π
stacking distance on the electroluminescent properties of these Pt compounds is studied.
The typical yellow luminescence band associated with the excimers of Pt compounds is absent
for complex 15; however, it shows a green 540 nm electroluminesce band instead. An efficient
LED constructed using compound 15 shows green electroluminescence with a peak efficiency
of 6 cd/A.
Introduction
energy of about 2 kJ mol-1 for a typical aromatic-
aromatic π-π interaction.13 There are three different
patterns of the stacked arrangements in monoaromatic
systems: a perfect face-to-face alignment of atoms; offset
or slipped packing; and T-shaped conformation, which
is C-H-π interaction (Figure 1).
Aromatic-aromatic or π-π interactions are impor-
tant noncovalent intermolecular forces.1-3 They play an
important role in the binding and conformations from
DNA-metal complexes4,5 to the area of electrolumines-
cence in OLED technology.6-12 Calculations give an
The face-to-face π-π interaction, where most of the
ring-plane area overlaps, is a rare phenomenon. The
usual π-π interaction is an offset or slipped stacking.
The ring normal and the vector between the ring
centroids form an angle of around 20° up to centroid-
centroid distances of 3.8 Å.1 The T-shaped mode of π-π
interaction is known for benzene. The stacked structures
become increasingly favorable with increasing arene
size.14 Larger systems such as pyrene or coronene show
† DuPont contribution #8551.
* To whom correspondence should be addressed. E-mail:
alex.s.ionkin@usa.dupont.com.
(1) Janiak, C. J. Chem. Soc., Dalton Trans. 2000, 21, 3885.
(2) Roesky, H. W.; Andruh, M. Coord. Chem. Rev. 2003, 236, 91.
(3) Jones, G. B. Tetrahedron 2001, 57, 7999.
(4) Yang, F.; Fanwick, P. E.; Kubiak, C. P. Inorg. Chem. 2002, 41,
4805.
(5) Hunter, C. A.; Singh, J.; Thornton, J. M. J. Mol. Biol. 1991, 218,
837.
(6) Brooks, J.; Babayan, Y.; Djurovich, P. I.; Adamovich, V.; Tsyba,
I.; Bau, R.; Forrest, S.; D’Andrade, B.; Thompson, M. E. Abstracts of
Papers, 224th ACS National Meeting, Boston, MA, 2002; American
Chemical Society: Washington, DC, 2002.
(10) Lai, S.-W.; Chan, M. C. W.; Cheung, K.-K.; Peng, S.-M.; Che,
C.-M. Organometallics 1999, 18, 3991.
(7) Sakai, K.; Kurashima, M.; Akiyama, N.; Satoh, N.; Kajiwara,
T.; Ito, T. Acta Crystallogr. 2003, C59, m349.
(8) Lai, S.-W.; Chan, M. C. W.; Cheung, K.-K.; Che, C.-M. Inorg.
Chem. 1999, 38, 4262.
(9) Lai, S.-W.; Cheung, T.-C.; Chan, M. C. W.; Cheung, K.-K.; Peng,
S.-M.; Che, C.-M. Inorg. Chem. 2000, 39, 255.
(11) Lai, S.-W.; Chan, M. C. W.; Cheung, K.-K.; Peng, S.-M.; Che,
C.-M. Organometallics 1999, 18, 3327.
(12) Liu, Z.-H.; He, C.; Duan, C.-Y.; You, X.-Z. Inorg. Chem.
Commun. 1999, 2, 279.
(13) Jorgensen, W. L.; Severance, D. L. J. Am. Chem. Soc. 1990,
112, 4768.
10.1021/om049211t CCC: $30.25 © 2005 American Chemical Society
Publication on Web 01/14/2005

620 Organometallics, Vol. 24, No. 4, 2005
Ionkin et al.
Figure 1. Three general conformations in π-π interactions of monoaromatic systems.
a typical π-π stacking interaction (2 kJ mol-1), the
steric bulk of the second auxiliary ligand of square-
planar Pt(II) complexes bearing the same cyclometa-
lated phenyl-pyridine ligand was gradually increased
to break the π-π stacking above 3.8 Å, and the changes
in emission spectra were recorded. The results are
discussed in the following sections.
Results and Discussion
Figure 2. Perfect and twisted alignments of 2-phenylpy-
ridines in π-π stacking.
1.1. Synthesis of Mono-cyclometalated Pt(II)
Complexes and Structural Studies. 2-(3,5-Bis(trif-
luoromethyl)phenyl)-4-methylpyridine (1) was selected
as a ligand for cycloplatination for two reasons: it gives
blue emissive bis-cyclometalated Ir(III) complexes, and
it gives one of the most intense blue emissive bis-
cyclometalated Ir(III) complexes.21 The palladium-
catalyzed Suzuki cross-coupling reaction between 2-chlo-
ro-4-methylpyridine (2) and 3,5-bis(trifluoromethyl)-
phenylboronic acid (3) was used to prepare 2-(3,5-bis-
(trifluoromethyl)phenyl)-4-methylpyridine (1) (Scheme
1).
the slipped π-π stacking with the hydrogen atoms
roughly over the ring centers.13,15-17 The introduction
of nitrogen into aromatic systems increases the tendency
to stack. The 2-phenylpyridine ligands are important
features of the cyclometalated complexes of platinum
and iridium for luminescent studies,18,19 where the
phenomenon of π-π stacking correlates with a yellow
band in the photo- or electroemission.20 Bicyclic systems,
such as 2-phenylpyridine, have several additional align-
ments in π-π stacking (Figures 2 and 3). The perfect
phenyl-to-phenyl and pyridine-to-pyridine alignments
are again rare. The alignments with twisted orienta-
tions of two phenyl-pyridine rings (second picture in
Figure 2) are most common.
The synthetic protocol involves Pd2dba3 (4)/di-tert-
butyl(trimethylsilyl)anylmethylphosphine (5) as the
catalyst in the presence of cesium fluoride (6) in 1,4-
dioxane as solvent.22
The cyclometalation of 2-(3,5-bis(trifluoromethyl)-
phenyl)-4-methylpyridine (1) with potassium tetrachlo-
roplatinate (7) was carried out in trimethyl phosphate
(8). The electrophilic reactions, like cyclometalation, in
trimethyl phosphate take place at lower temperatures
in many difficult cases.23 The Pt(II)-bridged dichloride
dimer 9 precipitated from the trimethyl phosphate
solution, simplifying the purification step (Scheme 2).
Platinum(II) [4,6-bis(trifluoromethyl)-2-(4-methyl-2-
pyridinyl-κN)phenyl-κC](2,2,6,6-tetramethyl-3,5-hep-
tanedionato-κO,κO′) (10) was prepared by the reaction
of complex 9 and lithium 2,2,6,6-tetramethylheptane-
3,5-dionate (11) (Scheme 3).
Two additional alignments resulted upon the rotation
of the perfect alignment of 2-phenylpyridine systems
(shown in Figure 2) by 180° around the pyridine-to-
pyridine π-π stacking and around phenyl-to-phenyl
π-π stacking (Figure 3).
This report describes the successful breaking of π-π
stacking of mono-cyclometalated Pt(II) compounds with
more than a 3.8 Å plane-to-plane distance, resulting in
the elimination of the yellow band in the emission of
OLED devices. Because of the relatively low energy for
(14) Goddard, R.; Haenel, M. W.; Herndon, W. C.; Kruger, C.;
Zandler, M. J. Am. Chem. Soc. 1995, 117, 30.
(15) Robertson, J. M.; White, J. G. J. Chem. Soc. 1947, 358.
(16) Kitaigorodski, A. I. Molecular Crystals and Molecules; Academic
Press: New York, 1973.
A crystal of complex 10 suitable for X-ray analysis
was grown from pentane. The ORTEP drawing of 10 is
shown in Figure 4.
Complex 10 represents the classical case of the
twisted π-π stacking of cyclometalated platinum com-
(17) Desiraju, G. R.; Gavezzotti, A. J. Chem. Soc., Chem. Commun.
1989, 621.
(18) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee,
H.-E.; Adachi, C.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E. J.
Am. Chem. Soc. 2001, 123, 4304.
(19) Tamayo, A. B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.;
Tsyba, I.; Ho, N. N.; Bau, R.; Thompson, M. E. J. Am. Chem. Soc. 2003,
125, 7377.
(21) Grushin, V.; Petrov, V. A. PCT Int. Appl. 2003, WO 2003069961.
(22) Ionkin, A. S.; Marshall, W. J. J. Organomet. Chem. 2004, 689,
1057.
(20) Brooks, J.; Babayan, Y.; Lamansky, S.; Djurovich, P. I.; Tsyba,
I.; Bau, R.; Thompson, M. E. Inorg. Chem. 2002, 41, 3055.
(23) Cowley, A. H.; Norman, N. C.; Pakulski, M. Inorg. Synth. 1990,
27, 235.

Mono-cyclometalated Platinum(II) Complexes
Organometallics, Vol. 24, No. 4, 2005 621
Figure 3. Pyridine-to-pyridine and phenyl-to-phenyl alignments of 2-phenylpyridines in π-π stacking.
Scheme 1
Scheme 2
Figure 4. ORTEP drawing of the π-π stacked platinum
complex 10. This is an example of the classical twisted
phenyl-pyridine alignment in π-π stacking. The plane-to-
plane distance is 3.478 Å. The Pt-Pt distance is 5.084 Å.
Scheme 3
pounds. The plane-to-plane distance is 3.478 Å. The two
phenyl-pyridine rings are twisted by 46.6° relative to
each other (Figure 5). The centroid-centroid distance
between the two pyridine fragments is 4.410 Å. The ring
normal and the vector between pyridine ring centroids
form an angle of 37.9°. The centroid-centroid distance
between two phenyl fragments is 3.820 Å. The ring
normal and the vector between phenyl ring centroids
form an angle of 20.6°. The Pt-Pt distance is 5.084 Å
(Table 1). There is very little distortion from the square-
planar geometry of 10 (second row of Table 2). The
phenyl and pyridine moiety of the phenyl-pyridine
ligand of 10 is only slightly bowed relative to each other
with an angle of 5.3° (first row of Table 2).
Figure 5. Outstanding features of the cyclometalated ring
system in complexes 10, 13, 15, and 17: the bow formed
by phenyl and pyridine moieties of the phenyl-pyridine
ligand (left picture) and the deviation of platinum from
square-planar geometry (right picture).
Scheme 4
To increase the steric bulk on the side of the auxiliary
ligand, 1,1,1,3,3,3-hexafluoro-2-pyrazol-1-ylmethylpro-
pan-2-ol (12) was used instead of lithium 2,2,6,6-
tetramethylheptane-3,5-dionate (11) in the ligand ex-
change reaction of starting complex 9 (Scheme 4). In
contrast to acetylacetonate complexes, like 10, 1,1,1,3,3,3-
hexafluoro-2-pyrazol-1-ylmethylpropan-2-ol (12) forms
a nonplanar six-membered ring with the metal. The
pyrazol moiety and two CF3 groups have a tendency to
orient themselves outside the plane of the six-membered
ring and push the second molecule participating in π-π
stacking farther away.

622 Organometallics, Vol. 24, No. 4, 2005
Ionkin et al.
between pyridine ring centroids form an angle of 11.9°.
The phenyl fragments are located too far apart (more
than 10 Å) to participate in a π-π interaction. The Pt-
Pt distance has been found to be close to the value found
for compound 10: 5.076 Å (Table 1). However, the
distortion from square-planar geometry has increased
for 13. The nitrogen atom (N8) deviated by 0.383 Å from
the mean plane of the trans atoms (O-Pt-N and
C-Pt-N) (second row of Table 2). The phenyl and
pyridine moieties of the phenyl-pyridine ligand of 13 are
bowed relative to each other with an angle of 10.3°.
To separate the π-π stacking of the aromatic rings
even further, 2-[(diphenylphosphanyl)methyl]- 1,1,1,3,3,3-
hexafluoropropan-2-ol (14) was used instead of 1,1,1,3,3,3-
hexafluoro-2-pyrazol-1-ylmethylpropan-2-ol (12) in the
reaction with platinum precursor 9 (Scheme 5). The
rationale behind this experiment was that two phenyl
groups at the phosphorus atom in addition to two
trifluoromethyl groups will push the π-π stacking
planes beyond energetically favorable interaction.
According to the results of the X-ray analysis (Figure
7), complex 15 is the trans-N,P-isomer. This is the
closest to the mode of phenyl-to-phenyl alignment of
π-π stacking, but the distance is long. The plane-to-
plane distance in compound 15 was found to be 4.203
Å. This value is out of the range of 3.3-3.8 Å accepted
for the phenomenon of π-π stacking.1 The Pt-Pt
distance was found to be 7.028 Å. The centroid-centroid
distance between the two phenyl fragments is 4.237 Å,
which is also above the normal range found in π-π
stacking.1 The ring normal and the vector between the
pyridine ring centroids form an angle of 7.3°. The
pyridine fragments are located too far apart (more than
10 Å) to participate in a π-π interaction. The distortion
from square-planar geometry is moderate for 15. The
nitrogen atom (N1) deviates by 0.191 Å from the mean
plane of the trans atoms (O-Pt-P and C-Pt-N)
(second row of Table 2). The phenyl and pyridine moiety
Figure 6. ORTEP drawing of the π-π stacked platinum
complex 13. This is the case of pyridine-to-pyridine align-
ment in π-π stacking. The plane-to-plane distance is 3.504
Å. The Pt-Pt distance is 5.076 Å.
The single crystal of complex 13 suitable for X-ray
analysis was grown from pentane. The ORTEP drawing
of 13 is shown in Figure 6.
Platinum(II) [4,6-bis(trifluoromethyl)-2-(4-methyl-2-
pyridinyl-κN)phenyl-κC],[R,R-bis(trifluoromethyl)-2-pyra-
zol-1-ylmethylpropan-2-olato-κN1,κO] (13) was isolated
as the trans-N,N-isomer. There is pyridine-to-pyridine
orientation in π-π stacking of cyclometalated platinum
complex 13. The plane-to-plane distance is 3.504 Å,
which is slightly higher than for complex 10. The
centroid-centroid distance between the two pyridine
fragments is 3.581 Å. The ring normal and the vector
Table 1. π-π Stacking Parameters for Compounds 10, 13, 15, and 17
10 13 15
17
plane-to-plane distance, Å
Pt-Pt distances, Å
3.478
5.084
3.504
5.076
4.203
7.028
3.533
8.123
orientation of two molecules relative
to each other in the crystal cell
centroid-centroid contacts
between two pyridine fragments, Å
centroid-centroid contacts
between two phenyl fragments, Å
pyridine/pyridine displacement:
angle between the
twisted phenyl-pyridine
alignment (46.6°)
4.410
pyridine-to-pyridine
alignment (180°)
3.581
phenyl-to-phenyl
alignment (180°)
>10 Å
pyridine-to-pyridine
alignment (180°)
3.784
3.820
37.9
>10 Å
4.237
>10 Å
11.9
>10 Å to measure
21.0
ring-centroid vector and the ring
normal to one of the pyridine planes
phenyl/phenyl displacement:
angle between the ring-centroid
vector and the ring normal to one
of the phenyl planes
20.6
>10 Å to measure
7.3
>10 Å to measure
Table 2. Distortion of the Ring System in 10, 13, 15, and 17
10 13 15
17
θ (deg), dihedral angle between
the mean planes of the six-membered
rings of the phenyl-pyridine ligand
deviations (Å) from the mean plane
of the trans atoms (O-Pt-O, O-Pt-N,
or O-Pt-P)
5.3
10.7
7.1
19.7
N1 ) -0.068,
C12 ) 0.112,
N27 ) -0.167,
C38 ) 0.085
N8 ) -0.383,
C19 ) 0.017
N1 ) -0.191,
C12 ) -0.048
N1) 0.866,
C12 ) -0.006

Mono-cyclometalated Platinum(II) Complexes
Organometallics, Vol. 24, No. 4, 2005 623
Figure 8. ORTEP drawing of the π-π stacked platinum
complex 17. This is the case of pyridine-to-pyridine align-
ment in π-π stacking. The plane-to-plane distance is 3.533
Å. The Pt-Pt distance is 8.123 Å.
Figure 7. ORTEP drawing of the π-π stacked platinum
complex 15. This is the closest to the mode of phenyl-to-
phenyl alignment of π-π stacking but far way. The plane-
to-plane distance is 4.203 Å. The Pt-Pt distance is 7.028
Å.
Scheme 5
Scheme 6
Figure 9. ORTEP drawing of the platinum complex 17
showing “bowl-shaped” conformation. Complex 17 is greatly
distorted. The angle between the mean planes of the six-
membered rings of the phenyl-pyridine ligand of 17 is 19.7°.
geometry around the platinum atom and a deviation of
the phenyl-pyridine ligand from planarity. The angle
between the mean planes of the six-membered rings of
the phenyl-pyridine ligand of 17 is 19.7°. The geometry
of 17 deviates greatly from square planar (Figures 9 and
5). The nitrogen atom (N1) again deviates by 0.866 Å
from the mean plane of the trans atoms (O-Pt-P and
C-Pt-N) (second row of Table 2). It would be best to
describe the geometry as “bowl-shaped” not square
planar.24
A Cambridge Structural Database (CSD) search has
been performed using QUEST3D routine to evaluate
Pt-Pt distances of the mono-cyclometalated π-π stacked
Pt complexes containing a parent 2-phenylpyridine
moiety. The results of the search are as follows: the
smallest Pt-Pt distance was 3.528 Å25 and the longest
Pt-Pt distance was 8.679 Å.26 The average Pt-Pt
of the phenyl-pyridine ligand of 15 are bowed relative
to one another with an angle of 7.1°.
To spread the Pt-nuclei farther apart in the lattice,
the phenyl groups at the phosphorus in 15 were replaced
by tert-butyl groups as in complex 17 (Scheme 6). The
reaction between platinum precursor 9, 2-[(di-tert-
butylphosphanyl)methyl]-1,1,1,3,3,3-hexafluoropropan-
2-ol (16), and tetrabutylammonium hydroxide in a
solution of THF afforded the desired complex 17.
According to the results of the X-ray analysis (Figures
8 and 9), complex 17 is the trans-N,P-isomer. The Pt-
Pt distance in complex 17 is the longest in this study
at 8.123 Å. There is the pyridine-to-pyridine alignment
of π-π stacking in complex 17. The plane-to-plane
distance is 3.533 Å. However, the main feature of
compound 17 is the distortion of the square-planar
(24) Corey, S.; Jun, P.; Suning, W. Eur. J. Inorg. Chem. 2002, 1390.
(25) Chassot, L.; Muller, E.; Zelewsky, von A. Inorg. Chem. 1984,
23, 4249.
(26) DePriest, J.; Zheng, G. Y.; Goswami, N.; Eichhorn, D. M.;
Woods, C.; Rillema, D. P. Inorg. Chem. 2000, 39, 1955.

624 Organometallics, Vol. 24, No. 4, 2005
Ionkin et al.
Scheme 7
distance was 6.72 Å among the eight found struc-
tures.27-31 Thus, complexes 15 and 17 from this inves-
tigation possess above average Pt-Pt distances. It
appears that Pt-Pt measurement is the simplest test
of the bulkiness of the Pt complex. However, it could
not be used alone without analyzing the distortion of a
molecule or π-π stacking; for example, complex 17 in
our study has the longest Pt-Pt distance of 8.123 Å,
but because of substantial distortion (Table 2) of the
cyclometalated moiety of the complex and the deviation
from planarity of the square Pt geometry, complex 17
is a mediocre photoemitter.
Several patterns of aromatic-aromatic π-π interac-
tions and different Pt-Pt distances in complexes 10, 13,
15, and 17 were detected by X-ray analysis. Tables 1
and 2 summarize the π-π stacking parameters and
distortion values for compounds 10, 13, 15, and 17. The
initial correlations with OLED performance made from
them will be discussed in the next section.
1.2. Electroluminescent and Photoluminescent
Properties of Mono-cyclometalated Pt(II) Com-
plexes. Organometallic compounds containing Ir and
Pt atoms represent a new class of electroluminescent
materials for light-emitting diodes.18,32-44 Highly ef-
ficient devices can be fabricated from these emitters
either as neat film36-38 or in a guest-host system. The
luminescent states of these compounds usually contain
strong metal-to-ligand charge transfer (MLCT) charac-
ter. The spin-orbital interaction of the heavy Ir, Pt
atom introduces mixed singlet/triplet character in the
MLCT emitting state. The triplet character allows the
electron-hole recombination statistics to approach the
theoretical limit of 100% instead of the 25% limit with
the singlet state. The metal-to-ligand charge transfer
nature of the emitting state allows the facile tuning of
the luminescent wavelength by the substitution of donor
and acceptor groups on the organic ligands. Emitters
with luminescence wavelength ranging from near blue
to deep red have been synthesized.18,34-44
While most of the high-efficiency organometallic emit-
ters are based on Ir compounds, Pt-based emitters have
also been reported.45,46 LEDs built with Pt compounds
as the emitter usually show luminescence in the blue
region plus additional yellow-orange luminescence in
the 580-600 nm region. The blue luminescence is
attributed to the monomer emission, and the yellow-
orange emission is generally attributed to the excimer
emission.45,46 The facile formation of excimers for the
Pt compounds is presumably due to their square-planar
geometry, which promotes intermolecular stacking.
Electroluminescent devices were made using com-
pounds 10, 15, and 17 as the emitters. The device
configuration consists of ITO as the anode, MPMP (300
Å) as the hole transport material, Pt emitters, DPA (100
Å) as the electron transport material, AlQ(300 Å) as the
electron injection material, and LiF/Al as the cathode.
Molecular structures of the materials used are shown
in Scheme 7.
Table 3 summarizes the electroluminescent and pho-
toluminescent properties of these Pt compounds. Com-
pound 15 shows green electroluminescence at 540 nm,
while compound 10 gives an additional yellow peak at
580 nm. Compound 17 shows an electroluminescence
peak at 560 nm, which may be the convolution of the
540 and 580 nm peak. Compound 15 shows a reasonable
efficiency of 6 cd/A without any further device optimiza-
tion. The onset voltage is relatively high, 19.5 V at 100
cd/m2, which may be further reduced by optimizing
device structure. The voltage dependence of the elec-
troluminescence efficiency and radiance is shown in
Figure 10.
Figure 11 shows the absorption, photoluminescence,
and electroluminescence spectra of compound 15. As can
be seen, in methylene chloride solution the Pt compound
emits blue light with a peak at 405 nm, while the
electroluminescence peak is at 540 nm. We note all
three Pt compounds show the same 405 nm emission
in solution (Table 1), reflecting the fact that they all
have the same emissive phenylpyridine ligand. The
detailed photophysics of Pt compounds can be extremely
complicated and is not pursued in this paper. Our
tentative interpretation is that the 405 nm peak may
be attributed to the Pt monomer emission, basically
from the phenylpyridine ligand with some perturbation
by the Pt atom. The 540 nm electroluminescence band
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Mono-cyclometalated Platinum(II) Complexes
Organometallics, Vol. 24, No. 4, 2005 625
Figure 10. Voltage dependence of the electroluminescence
efficiency (cd/A) and radiance (cd/m2) of compound 15.
Figure 12. Comparison of the electroluminescence spectra
of compounds 15 and 10.
distance than compound 10. For example, the plane-to-
plane distance is 4.203 Å for 15 compared to 3.478 Å
for 10; the Pt-Pt distance is 7.028 Å for 15 compared
to 5.084 Å for 10. The larger intermolecular distance is
presumably due to the bulkier 2-[(diphenylphosphanyl)-
methyl]-1,1,1,3,3,3-hexafluoropropan-2-ol (14) ligand
incorporated in complex 15. This increased intermo-
lecular distance did not eliminate the aggregation effect
of Pt compounds in the solid-state electroluminescence
spectrum, as evidenced by the appearance of the 540
nm electroluminescence band, which is different from
the monomer band at 405 nm. However, the increased
separation does seem to either eliminate the 580-600
nm yellow band or cause it to shift to higher energy. It
is known that the excimer emission wavelength is
sensitive to the packing geometry of the molecules as
well as the intermolecular distance. In general, in-
creased intermolecular separation will destabilize the
excimer and cause the emission wavelength to shift to
higher energy. Qualitatively, this is consistent with the
current observation. Further detailed photophysical
studies are needed to gain a complete quantitative
understanding of the problem.
Figure 11. Absorption (dashed line), photoluminescence
(PL), and electroluminescence (EL) spectra of compound
15. The photoluminescence spectrum was taken in meth-
ylene chloride solution using 350 nm excitation.
is due to intermolecular interaction of the Pt compounds
in the solid state. Its peak position is different from the
580-600 nm excimer emission generally observed for
other Pt compounds.45-48
Summary
For compound 10, the electroluminescence spectrum
either is red-shifted or shows an additional peak in the
580 nm region. The comparison of the electrolumines-
cence spectra of 10 and 15 is shown in Figure 12.
Although films deposited by thermal evaporation are
usually amphorous or polycrystalline, the single-crystal
structural data can still be useful in terms of providing
a guide to the natural tendency of how molecules pack
together in the solid state. On the basis of the X-ray
data summarized in Tables 1 and 2, we note that
compound 15 in general shows a larger intermolecular
The π-π stacking of mono-cyclometalated Pt(II) com-
plexes was broken above 3.8 Å plane-to-plane distances
by increasing the steric bulk of the second auxiliary
ligand in complexes bearing the same cyclometalated
phenyl-pyridine ligand. By doing so, this did not elimi-
nate the long-wavelength electroluminescence band in
OLED devices, but it did change its emission wave-
length to higher energies (540 nm). Our tentative
conclusion is that the steric bulkiness did not totally
eliminate the intermolecular interaction of the Pt
compounds; rather, it weakens the intermolecular in-
Table 3. Electroluminescent Device Data and Photoluminescent Properties of Pt(II) Complexes
Pt(II)
complex
EL peak
efficiency, cd/A
EL peak
radiance, cd/m2
electroluminescence
peaks, nm
photoluminescence
peaks, nm
PL quantum yields,
in methylene chloride
10
15
17
1.2
6
0.15
200
2000
130
540+580
540
560
405
405
405
0.0089
0.017
0.0069

626 Organometallics, Vol. 24, No. 4, 2005
Ionkin et al.
Platinum(II) [4,6-Bis(trifluoromethyl)-2-(4-methyl-2-
pyridinyl-KN)phenyl-KC],[r,r-bis(trifluoromethyl)-2-pyra-
zol-1-ylmethylpropan-2-olato-KN1,KO] (13). Platinum(II)
di-µ-chlorobis[4,6-bis(trifluoromethyl)-2-(4-methyl-2-pyridinyl-
κN)phenyl-κC] (9) (4.0 g, 0.000374 mol), 3.71 g (0.0150 mol) of
1,1,1,3,3,3-hexafluoro-2-pyrazol-1-ylmethylpropan-2-ol (12), 7.76
g of a 55-60% solution of tetrabutylammonium hydroxide in
water, and 20 mL of THF were refluxed for 2 h under argon
atmosphere. The reaction mixture was poured into 200 mL of
the water and extracted by 200 mL of diethyl ether twice. The
extracts were dried over magnesium sulfate overnight. The
solvent was removed in a rotavapor, and the residue was
purified by chromatography on silica gel with the eluent
petroleum ether/ethyl ether at 10:0.5. Yield of platinum(II)
[4,6-bis(trifluoromethyl)-2-(4-methyl-2-pyridinyl-κN)phenyl-
κC],[R,R-bis(trifluoromethyl)-2-pyrazol-1-ylmethylpropan-2-
olato-κN1,κO] (13) was 3.10 g (55.56%) as a yellow solid with
teraction and shifts the aggregate emission to shorter
wavelength.
Experimental Section
General Procedures. All the operations related to cata-
lysts were carried out under an argon atmosphere using
standard Schlenk techniques. Anhydrous solvents were used
in the reactions. Solvents were distilled from drying agents
or passed through alumina columns under an argon or
nitrogen atmosphere. 3,5-Bis-trifluoromethylphenylboronic
acid, 2-chloro-4-methylpyridine, Pd2dba3, cesium fluoride, 1,4-
dioxane, lithium 2,2,6,6-tetramethylheptane-3,5-dionate, tri-
methyl phosphate, 55-60% solution of tetrabutylammonium
hydroxide in water, and potassium tetrachloroplatinate(II)
were purchased from Aldrich.
2-(3,5-Bis(trifluoromethyl)phenyl)-4-methylpyridine (1).
3,5-Bis(trifluoromethyl)phenylboronic acid (3) (15.0 g, 0.05815
mol), 7.42 g (0.05816 mol) of 2-chloro-4-methylpyridine (2),
17.43 g (0.1148 mol) of cesium fluoride (6), 0.53 g (0.000579
mol) of tris(dibenzylideneacetone)dipalladium(0) (4), 0.33 g
(0.00142 mol) of di-tert-butyl(trimethylsilyl)methylphosphine
(5), and 100 mL of 1,4-dioxane were stirred at room temper-
ature for 12 h. The reaction mixture was filtered, and the
solvent was removed under vacuum. The resulting mixture
was purified by chromatography on silica gel with the eluent
petroleum ether/ethyl ether at 10:0.5. Yield of 2-(3,5-bis-
(trifluoromethyl)phenyl)-4-methylpyridine (1) was 16.18 g
1
no mp until 200 °C. H NMR (500 MHz, CD2Cl2): δ 2.50 (s,
3H, Me), 4.80 (s, 2H, CH2), 6.40-8.90 (m, 8H, arom-H). 19F
NMR (377 MHz, CD2Cl2): δ -57.19 (s, 3F, CF3), - 63.35 (s,
3F, CF3), - 77.21 (s, 6F, CF3). Anal. Calcd for C21H13F12N3-
OPt (MW: 746.41): C, 33.79; H, 1.76; N, 5.63. Found: C, 34.02;
H, 2.00; N, 5.62.
Platinum(II) [4,6-Bis(trifluoromethyl)-2-(4-methyl-2-
pyridinyl-KN)phenyl-KC], [3-(diphenylphosphino)-1,1,1-
trifluoro-2-(trifluoromethyl)-2-propanolato-KO,KP] (15).
(1.77 g, 0.00166 mol) Platinum(II) di-µ-chlorobis[4,6-bis(tri-
fluoromethyl)-2-(4-methyl-2-pyridinyl-κN)phenyl-κC] (9) 1.21
g (1.77 g, 0.00166 mol), 0.0033 mol of 2-[(diphenylphosphanyl)-
methyl]-1,1,1,3,3,3-hexafluoropropan-2-ol (14), 1.72 g of a 55-
60% solution of tetrabutylammonium hydroxide in water, and
20 mL of THF were refluxed for 2 h under argon atmosphere.
The reaction mixture was poured into 200 mL of the water
and extracted by 200 mL of diethyl ether twice. The extracts
were dried over magnesium sulfate overnight. The solvent was
removed in a rotavapor, and the residue was purified by
chromatography on silica gel with the eluent petroleum ether/
ethyl ether at 10:0.5. Yield of platinum(II) [4,6-bis(trifluoro-
methyl)-2-(4-methyl-2-pyridinyl-κN)phenyl-κC],[3-(diphenyl-
phosphino)-1,1,1-trifluoro-2-(trifluoromethyl)-2-propanol-
ato-κO,κP] (15) was 3.10 g (55.56%) as a yellow solid with no
1
(91%) as a colorless liquid. H NMR (500 MHz, C6D6): δ 2.56
(s, 3H, Me), 7.11 (s, 1H, arom-H), 7.51 (s, 1H, arom-H), 7.90
(s, 1H, arom-H), 8.45-8.55 (m, 3H, arom-H). 19F NMR (377
MHz, CDCl3): δ -63.35 (s, 3F, CF3), - 63.36 (s, 3F, CF3). Anal.
Calcd for C14H9F6N: C, 55.09; H, 2.97; N, 4.59. Found: C,
55.01; H, 3.12; N, 4.44.
Platinum(II) Di-µ-chlorobis[4,6-bis(trifluoromethyl)-
2-(4-methyl-2-pyridinyl-KN)phenyl-KC] (9). 2-(3,5-Bis(tri-
fluoromethyl)phenyl)-4-methylpyridine (1) (12.00 g, 0.0376
mol), 18.27 g (0.0376 mol) of potassium tetrachloroplatinate-
(II) (7), and 200 mL of trimethyl phosphate (8) were stirred
at 90 °C for 18 h under a flow of nitrogen. The resultant
precipitate was filtered and dried under 1.0 mm vacuum. The
yield of the dimer 9 was 15.7 g (78.11%) as a yellow powder.
The above crude chloride-bridged dimer was used without
further purification in the next steps according to established
practice in cyclometalated platinum research.20
1
mp until 200 °C. H NMR (500 MHz, CD2Cl2): δ 2.45 (s, 3H,
Me), 2.80 (s, 2H, CH2), 7.10-9.10 (m, 15H, arom-H). 19F NMR
(377 MHz, CD2Cl2): δ -59.12 (s, 3F, CF3), - 63.53 (s, 3F, CF3),
- 78.04 (s, 6F, CF3). 31P NMR (500 MHz, CD2Cl2): δ 23.42 (t,
1JPPt, 2082.05 Hz). Anal. Calcd for C30H20F12NOPPt (MW:
864.52): C, 41.68; H, 2.33; N, 1.62. Found: C, 41.70; H, 2.48;
N, 1.82.
Platinum(II) [4,6-Bis(trifluoromethyl)-2-(4-methyl-2-
pyridinyl-KN)phenyl-KC](2,2,6,6-tetramethyl-3,5-
heptanedionato-KO,KO′) (10). Platinum(II) di-µ-chlorobis-
[4,6-bis(trifluoromethyl)-2-(4-methyl-2-pyridinyl-κN)phenyl-
κC] (9) (3.72 g, 0.00348 mol), 6.62 g (0.0347) of lithium 2,2,6,6-
tetramethylheptane-3,5-dionate (11), and 30 mL of THF were
refluxed for 2 h under argon atmosphere. The reaction mixture
was poured in 200 mL of water and extracted by 200 mL of
diethyl ether twice. The extracts were dried over magnesium
sulfate overnight. The solvent was removed in a rotavapor and
the residue was purified by chromatography on silica gel with
the eluent petroleum ether/ethyl ether at 10:0.5. Yield of
platinum(II) [4,6-bis(trifluoromethyl)-2-(4-methyl-2-pyridin-
yl-κN)phenyl-κC],(2,2,6,6-tetramethyl-3,5-heptanedion-
ato-κO,κO′) (10) was 2.14 g (45.05%) as a yellow solid with no
Platinum(II) [4,6-Bis(trifluoromethyl)-2-(4-methyl-2-
pyridinyl-KN)phenyl-KC],[3-(di-tert-butylphosphino)-1,1,1-
trifluoro-2-(trifluoromethyl)-2-propanolato-KO,KP] (17).
Platinum(II) di-µ-chlorobis[4,6-bis(trifluoromethyl)-2-(4-meth-
yl-2-pyridinyl-κN)phenyl-κC] (9) (4.0 g, 0.00374 mol), 4.88 g
(0.0150 mol) of 2-[(di-tert-butylphosphanyl)methyl]-1,1,1,3,3,3-
hexafluoropropan-2-ol (16), 7.76 g of a 55-60% solution of
tetrabutylammonium hydroxide in water, and 40 mL of THF
were refluxed for 2 h under argon atmosphere. The reaction
mixture was poured into 200 mL of the water and extracted
by 200 mL of diethyl ether twice. The extracts were dried over
magnesium sulfate overnight. The solvent was removed in a
rotavapor, and the residue was purified by chromatography
on silica gel with the eluent petroleum ether/ethyl ether at
10:0.5. Yield of platinum(II) [4,6-bis(trifluoromethyl)-2-(4-
methyl-2-pyridinyl-κN)phenyl-κC],[3-(di-tert-butylphosphino)-
1,1,1-trifluoro-2-(trifluoromethyl)-2-propanolato-κO,κP] (17) was
1
mp until 200 °C. H NMR (500 MHz, CD2Cl2): δ 1.11 (s, 9H
t-Bu), 1.20 (s, 9H t-Bu), 2.40 (s, 3H, Me), 5.90 (s, 1H, H-Cd),
7.00-9.10 (m, 5H, arom-H). 19F NMR (377 MHz, CD2Cl2): δ
-59.41 (s, 6F, CF3), - 63.31 (s, 6F, CF3). Anal. Calcd for
C25H27F6NO2Pt (MW: 682.56): C, 43.99; H, 3.99; N, 2.05.
Found: C, 44.13; H, 4.08; N, 2.31.
1
1.73 g (28.04%) as a yellow solid with no mp until 200 °C. H
NMR (500 MHz, CD2Cl2): δ 1.10 (s, 18H, Me), 2.30 (s, 3H,
Me), 2.50 (s, 2H, CH2), 7.00-8.60 (m, 5H, arom-H). 19F NMR
(377 MHz, CD2Cl2): δ -54.53 (s, 3F, CF3), -63.30 (s, 3F, CF3),
-77.55 (s, 6F, CF3). 31P NMR (500 MHz, CD2Cl2): δ 47.29 (t,
(47) Lu, W.; Mi, B.-X.; Chan, M. C. W.; Hui, Z.; Zhu, N.; Lee, S.-T.;
Che, C.-M. Chem. Commun. 2002, 206.
(48) Lu, W.; Mi, B.-X.; Chan, M. C. W.; Hui, Z.; Che, C.-M.; Zhu, N.;
Lee, S.-T. J. Am. Chem. Soc. 2004, 126, 4958.

Mono-cyclometalated Platinum(II) Complexes
Organometallics, Vol. 24, No. 4, 2005 627
Table 4. Summary of Crystal Data, Data Collection, and Structural Refinement Parameters of
10, 13, 15, and 17
10
13
15
17
empirical formula
fw
C
25H27F6NO2Pt
C21H13F12N3OPt
746.43
gold, irreg. block
triclinic
P1h
C30H20F12NOPPt
C26H28F12NOPPt
682.57
864.53
824.55
cryst color, form
cryst syst
space group
a (Å)
gold, rod
gold, irreg. block
lt.yellow, irreg. block
monoclinic
triclinic
monoclinic
C2/c
P1h
P2(1)/c
14.633(3)
10.3459(10)
10.9225(10)
11.2004(11)
83.1732(18)
84.3240(17)
65.3389(16)
1140.38(19)
2
9.6483(8)
8.1234(13)
b (Å)
25.866(6)
12.0183(9)
14.108(2)
c (Å)
26.110(6)
13.4404(11)
24.683(4)
R (deg)
90
91.3790(15)
90.000(3)
â (deg)
93.702(4)
107.0130(14
91.664(3)
γ (deg)
90
98.4040(15)
90.000(3)
V (Å3)
9862(4)
1470.6(2)
2827.6(8)
Z
16
2
4
density (g/cm3)
1.839
2.174
1.952
1.937
abs µ (mm-1
F(000)
)
5.759
6.271
4.928
5.12
5312
708
832
1600
cryst size (mm)
temp (°C)
0.25 × 0.06 × 0.06
0.25 × 0.25 × 0.20
0.28 × 0.23 × 0.20
0.14 × 0.20 × 0.20
-100
-100
-100
-100
scan mode
ω
ω
ω
ω
detector
Bruker-CCD
Bruker-CCD
28.3
Bruker-CCD
Bruker-CCD
θmax (deg)
28.33
28.28
28.3
no. obsrvd reflns
no. unique reflns
Rmerge
31086
6591
9734
45504
11534
4999
6653
6908
0.0386
0.0179
0.0138
0.0362
no. params
645
354
416
391
Sb
1.046
1.042
1.054
1.086
R indices [I>2σ(I)]a
R indices (all data)a
max diff peak, hole
(e/Å3)
wR2 ) 0.088, R1 ) 0.041
wR2 ) 0.098, R1 ) 0.070
1.805,-1.247
wR2 ) 0.055, R1 ) 0.023
wR2 ) 0.056, R1 ) 0.025
1.414,-1.244
wR2 ) 0.046, R1 ) 0.018
wR2 ) 0.047, R1 ) 0.020
0.562,-1.244
wR2 ) 0.053, R1 ) 0.024
wR2 ) 0.058, R1 ) 0.034
2.060,-0.915
2
2
2
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|, wR2 ) {∑[w(Fo - Fc )2]/∑[w(Fo2)2]}1/2 (sometimes denoted as Rw2). b GooF ) S ) {∑[w(Fo - Fc )2]/(n -
p)}1/2, where n is the number of reflections and p is the total number of refined parameters.
1JPPt, 1945.8 Hz). Anal. Calcd for C26H28F12NOPPt (MW:
824.54): C, 37.87; H, 3.42; N, 1.70. Found: C, 37.93; H, 3.57;
N, 1.89.
The OLED samples were characterized by measuring their
(1) current-voltage (I-V) curves, (2) electroluminescence
radiance versus voltage, and (3) electroluminescence spectra
versus voltage. The I-V curves were measured with a Keithley
Source measurement unit model 237. The electroluminescence
radiance (in the unit of cd/m2) versus voltage was measured
with a Minolta LS-110 luminescence meter, while the voltage
was scanned using the Keithley SMU. The electroluminescence
spectrum was obtained by collecting light using an optical
fiber, through an electronic shutter, dispersed through a
spectrograph, and then measured with a diode array detector.
All three measurements were performed at the same time and
controlled by a computer. The efficiency of the device at certain
voltage is determined by dividing the electroluminescence
radiance of the LED by the current density needed to run the
device. The unit is in cd/A.
OLED Device Fabrication and Characterization. OLED
devices were fabricated by the thermal evaporation technique.
The base vacuum for all of the thin film deposition was in the
range of 10-6 Torr. The deposition chamber was capable of
depositing eight different films without the need to break the
vacuum. Patterned indium tin oxide (ITO) coated glass sub-
strates from Thin Film Devices, Inc. were used. These ITOs
are based on Corning 1737 glass coated with 1400 Å ITO
coating, with sheet resistance of 30 ohms/square and 80% light
transmission. The patterned ITO substrates were then cleaned
ultrasonically in aqueous detergent solution. The substrates
were then rinsed with distilled water followed by 2-propanol
and then degreased in toluene vapor.
The cleaned, patterned ITO substrate was then loaded into
the vacuum chamber, and the chamber was pumped down to
10-6 Torr. The substrate was then further cleaned using an
oxygen plasma for about 5 min. After cleaning, multiple layers
of thin films were then deposited sequentially onto the
substrate by thermal evaporation. Patterned metal electrodes
(Al or LiF/Al) or bipolar electrodes were deposited through a
mask. The thickness of the film was measured during deposi-
tion using a quartz crystal monitor. The completed OLED
device was then taken out of the vacuum chamber and
characterized immediately without encapsulation.
Acknowledgment. The authors wish to thank Kurt
Adams for funding X-ray research.
Supporting Information Available: The crystallographic
information files (CIF) of compounds 10, 13, 15, and 17 are
available free of charge via the Internet at http://pubs.acs.org.
OM049211T