Facile synthesis and characterization of phosphorescent Pt(N ^C^N)X complexes

Article abstract of DOI:10.1021/ic100740e

In order to investigate the ground state and excited state properties of Pt(N^{^}C^{^}N)X, we have prepared a series of Pt complexes, where N^{^}C^{^}N aromatic chelates are derivatives of m-di(2-pyridinyl)benzene (dpb) and X are monoanionic and monodentate ancillary ligands including halide and phenoxide. Facile synthesis of platinum m-di(2-pyridinyl)benzene chloride and its derivatives, using controlled microwave heating, was reported. This method not only shortened the reaction time but also improved the reaction yield for most of the Pt complexes. Two Pt(N^{^}C^{^}N)X complexes have been structurally characterized by X-ray crystallography. The change of functional group does not affect the structure of the core Pt(N^{^}C^{^}N)Cl fragment. Both molecules pack as head-to-tail dimers, each molecule of the dimer related to the other by a center of inversion. The electrochemical studies of all Pt complexes demonstrate that the oxidation process occurs on the metal-phenyl fragment and the reduction process is associated with the electron accepting groups like pyridinyl groups and their derivatives. The maximum emission wavelength of the Pt(N^{^}C^{^}N)X complexes ranges between 471 and 610 nm, crossing the spectrum of visible light. Most of the Pt complexes are strongly luminescent (Φ = 0.32-0.63) and have short luminescence lifetimes (τ = 4-7 μs) at room temperature. The lowest excited state of the Pt(N ^{^}C^{^}N)X complexes is identified as a dominant ligand-centered ³π-π* state with some ¹MLCT/³MLCT character, which appears to have a larger ¹MLCT component than their bidentate and tridentate analogs. This results in a high radiative decay rate and high quantum yield for Pt(dpb)Cl and its analogs. However, the excited state properties of the Pt(N^{^}C ^{^}N)X complexes are strongly dependent on the nature of the electron-accepting groups and substituents to the metal-phenyl fragment. A rational design will be needed to tune the emission energies of the Pt(N ^{^}C^{^}N)X complexes over a wide range while maintaining their high luminescent efficiency.

Full text of DOI:10.1021/ic100740e

11276 Inorg. Chem. 2010, 49, 11276–11286
DOI: 10.1021/ic100740e
Facile Synthesis and Characterization of Phosphorescent Pt(N∧C∧N)X Complexes
Zixing Wang,† Eric Turner,† Vanessa Mahoney,† Sijesh Madakuni,† Thomas Groy,‡ and Jian Li*,†
†School of Materials, Arizona State University, Tempe, Arizona 85287, United States, and
‡Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287, United States
Received April 16, 2010
In order to investigate the ground state and excited state properties of Pt(N∧C∧N)X, we have prepared a series of
Pt complexes, where N∧C∧N aromatic chelates are derivatives of m-di(2-pyridinyl)benzene (dpb) and X are
monoanionic and monodentate ancillary ligands including halide and phenoxide. Facile synthesis of platinum m-di-
(2-pyridinyl)benzene chloride and its derivatives, using controlled microwave heating, was reported. This method not
only shortened the reaction time but also improved the reaction yield for most of the Pt complexes. Two Pt(N∧C∧N)X
complexes have been structurally characterized by X-ray crystallography. The change of functional group does not
affect the structure of the core Pt(N∧C∧N)Cl fragment. Both molecules pack as head-to-tail dimers, each molecule of
the dimer related to the other by a center of inversion. The electrochemical studies of all Pt complexes demonstrate that
the oxidation process occurs on the metal-phenyl fragment and the reduction process is associated with the electron
accepting groups like pyridinyl groups and their derivatives. The maximum emission wavelength of the Pt(N∧C∧N)X
complexes ranges between 471 and 610 nm, crossing the spectrum of visible light. Most of the Pt complexes
are strongly luminescent (Φ = 0.32-0.63) and have short luminescence lifetimes (τ = 4-7 μs) at room temperature.
The lowest excited state of the Pt(N∧C∧N)X complexes is identified as a dominant ligand-centered 3π-π* state with
some 1MLCT/3MLCT character, which appears to have a larger 1MLCT component than their bidentate and tridentate
analogs. This results in a high radiative decay rate and high quantum yield for Pt(dpb)Cl and its analogs. However, the
excited state properties of the Pt(N∧C∧N)X complexes are strongly dependent on the nature of the electron-accepting
groups and substituents to the metal-phenyl fragment. A rational design will be needed to tune the emission energies of
the Pt(N∧C∧N)X complexes over a wide range while maintaining their high luminescent efficiency.
Introduction
effort has been directed toward the development of Pt com-
plexes as efficient phosphorescent emitters for displays3,5 and
lighting applications.6,7 The strong spin-orbital coupling of
the heavy metal ions like Ir(III) and Pt(II) ensures a high
quantum yield of triplet emission for these cyclometalated
metal complexes.8-12 The phosphorescent OLEDs fabricated
with this class of emitters can utilize all of the electro-generated
Square planar platinum (Pt) complexes have attracted a
great deal of interest from academia and industry due to their
potential application in many areas such as chemosensors,1
photocatalysts,2 organic light-emitting diodes (OLEDs),3 and
photovoltaic devices.4 In recent years, an intensive research
*Author to whom correspondence should be sent, e-mail: jian.li.1@
asu.edu.
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pubs.acs.org/IC
Published on Web 11/22/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 24, 2010 11277
singlet and triplet excitons and achieve 100% photon/electron
conversion efficiency.13 Moreover, the square planar coordi-
nation geometry of Pt(II) complexes offers additional photo-
chemical reactivity over six-coordinated octahedral Ir(III)
complexes.14 The mechanism of excimer formation due to
the potential π-π and metal-metal (Pt-Pt) interactions has
been explored to fabricate a single-doped white OLED for
lighting applications.6,7 Thus, a better understanding of the
ground state and excited state properties of Pt(II) complexes
will allow us to tune their optical properties and explore the
related applications more effectively.
and its derivatives highly desirable as emissive dopants for
OLEDs.17b Efficient blue, green, and white OLEDs have been
fabricated using classes of emitters like Pt(N∧C∧N)Cl.7,17
However, the reason why Pt(dpb)Cl is more emissive than its
0
bidentate analog platinum (2-phenylpyridinato-N,C2 ) acetyl-
acetonate [(ppy)Pt(acac)] and tridentate analog platinum
(2-phenyl-bipyridinyl) chloride [Pt(phbpy)Cl] is not well-under-
stood. Williams and co-workers have studied the photophysical
properties of Pt(dpb)Cl and its derivatives, where the 4 postion
of the centered phenyl ring was modified with different aryl
substituents.18 The emission spectra of this class of Pt complexes
can be shifted to a shorter or longer wavelength in a certain
range with the use of electron-withdrawing or electron-donating
aryl substituents, while the quantum yields and luminescent
lifetimes are largely unaffected. Nonetheless, these structural
modifications leave the core segment of Pt complexes intact by
keeping the same electron-accepting group (i.e., pyridinyl
groups) and the planar geometry of the molecules. Several
interesting questions remain regarding excited state properties
of Pt(N∧C∧N)Cl: to what extent can the excited state properties
be controlled, and how do the changes of the ancillary ligands
(i.e., replacing chloride) influence the excited state properties of
Pt complexes?
It has been well documented that luminescence from
cyclometalated Ir(III) and Pt(II) complexes originates from
the admixture of ligand-centered (3LC) states and metal-
to-ligand-charge-transfer (MLCT) states through spin-orbit
coupling.8-12,15 Employing different cyclometalating ligands
enables the excited state energy of Pt complexes to be varied
over a wide spectral range, i.e., turning the emission color
from red to blue.16-19 However, the nature and energy level
of excited states of Pt complexes could also be strongly
affected by the change in the complex system. Compared
with Pt(II) complexes with bipyridines and their analogues,
which are typically nonemissive at room temperature,14c,20
the cyclometalated Pt complexes are emissive in solution at
room temperature with a less pronounced d-d quenching
influence due to the existence of the cyclometalated carbon.16,21
In addition to bidentate coordination like C∧N [e.g., 2-phenyl-
pyridine (ppy)] and C∧C, Pt complexes with three classes of
tridentate cyclometalating ligands have also been investigated,
which include N∧C∧N aromatic chelate derivatives [e.g.,
m-di(2-pyridinyl)benzene (dpb)],17,18 C∧N∧N aromatic chelate
derivatives [e.g., 2-phenyl-bipyridine (phbpy)],22,23 and C∧N∧C
aromatic chelate derivatives [e.g., m-bis(phenyl)pyridine
(bppy)].24 The use of different classes of aromatic chelates will
affect not only the photophysical properties of the single
molecule but also the functionality of aggregates. For example,
the complex consisting of a Pt ion, chloride, and m-di(2-
pyridinyl)benzene, i.e., Pt(dpb)Cl, has been reported to have a
high luminescent efficiency (around 0.6) and comparably short
luminescent lifetime (around 10 μs),17a which makes Pt(dpb)Cl
In order to investigate the ground state and excited state
properties of Pt(N∧C∧N)X, we have prepared a series of Pt
complexes, where X are monoanionic and monodentate
ancillary ligands including halide and phenoxide. The struc-
tures and abbreviations for the cyclometalated Pt complexes
reported here are listed in Figure 1. Microwave-assisted
heating was also explored to perform the Stille cross-coupling
reactions as well as metal coordination reactions of Pt
complexes. Thus, the one-week preparation of Pt complexes
following literature reports17,25 can be shortened to one day
adopting this new synthetic method. The electrochemical and
photophysical properties of the Pt(N∧C∧N)X complexes are
discussed in detail. The electrochemical studies of all Pt
complexes demonstrate that the highest occupied molecular
orbital (HOMO) localizes on the metal-phenyl fragment, and
the lowest unoccupied molecular orbital (LUMO) occupies
dominantly on the electron accepting groups like pyridinyl
groups and their derivatives. The lowest excited state of
Pt(N∧C∧N)X complexes is identified as a dominant ligand-
centered 3π-π* state with some 3MLCT/1MLCT character.
The excited state properties of the Pt(N∧C∧N)X complexes
can be strongly affected by changing the electron-accepting
groups and molecular geometry.
(13) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. J. Appl.
Phys. 2001, 90, 5048.
(14) (a) Miskowski, V. M.; Houlding, V. H. Inorg. Chem. 1991, 30, 4446.
(b) Houlding, V. H.; Miskowski, V. M. Coord. Chem. Rev. 1991, 111, 145.
(c) Miskowski, V. M.; Houlding, V. H.; Che, C. M.; Wang, Y. Inorg. Chem. 1993,
32, 2518.
(15) Rausch, A. F.; Murphy, L.; Williams, J. A. G.; Yersin, H. Inorg.
Chem. 2009, 48, 11407.
(16) Brooks, J.; Babayan, Y.; Lamansky, S.; Djurovich, P. I.; Tsyba, I.;
Bau, R.; Thompson, M. E. Inorg. Chem. 2002, 41, 3055.
(17) (a) Williams, J. A. G.; Beeby, A.; Davies, E. S.; Weinstein, J. A.;
Wilson, C. Inorg. Chem. 2003, 42, 8609. (b) Sotoyama, W.; Satoh, T.; Sawatari,
N.; Inoue, H. Appl. Phys. Lett. 2005, 86, 153505.
(18) Farley, S. J.; Rochester, D. L.; Thompson, A. L.; Howard, J. A. K.;
Williams, J. A. G. Inorg. Chem. 2005, 44, 9690.
(19) Che, C.-M.; Kwok, C.-C.; Lai, S.-W.; Rausch, A. F.; Finkenzeller,
W. J.; Zhu, N.; Yersin, H. Chem.;Eur. J. 2010, 16, 233.
(20) Aldridge, T. K.; Stacy, E. M.; McMillin, D. R. Inorg. Chem. 1994, 33,
722.
(21) Balashev, K. P.; Puzyk, M. V.; Kotlyar, V. S.; Kulikova, M. V.
Coord. Chem. Rev. 1997, 159, 109.
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Chem. Commun. 1990, 513.
Experimental Section
The microwave reactor (Discover, CEM Corp.) was used to
perform the microwave flash heating reaction. The UV-visible
spectra were recorded on a Cary 5G UV-vis-NIR spectro-
meter (Varian). Steady state emission experiments at room
temperature were performed on a Horiba Jobin Yvon Fluoro-
Log-3 spectrometer. Quantum efficiency measurements were
carried out at room temperature in a solution of dichloro-
methane. Before the emission spectra were measured, the solu-
tions were thoroughly bubbled by nitrogen inside of a glovebox
with the content of oxygen less than 0.1 ppm. Solutions of
coumarin47(coumarin 1; Φ=0.73, excited at360nm),26 coumarin
(23) (a) Lu, W.; Zhu, N.; Che, C.-M. Chem. Commun. 2002, 900. (b) Che,
C.-M.; Zhang, J.-L.; Lin, L.-R. Chem. Commun. 2002, 2556. (c) Che, C.-M.; Fu,
W.-F.; Lai, S.-W.; Hou, Y.-J.; Liu, Y.-L. Chem. Commun. 2003, 118.
(24) Yam, V. W.-W.; Tang, R. P.-L.; Wong, K. M.-C.; Lu, X.-X.;
Cheung, K.-K.; Zhu, N. Chem.;Eur. J. 2002, 8, 4066.
ꢁ
(25) Cardenas, D. J.; Echavarren, A. M.; Ramırez de Arellano, M. C.
´
Organometallics 1999, 18, 3337.
(26) Jones, G., II; Jackson, W. R.; Choi, C.-Y.; Bergmark, W. R. J. Phys.
Chem. 1985, 89, 294.

11278 Inorganic Chemistry, Vol. 49, No. 24, 2010
Wang et al.
Figure 1. Structural formula and abbreviations used for the Pt(N∧C∧N)X complexes. The ancillary ligand X is chloride unless noted.
6(Φ=0.78, excitedat420nm),27 andrhodamineB(Φ=0.70,
excited at 510 nm)28 in ethanol were used as references for Pt
complexes with a selected emission range. The equation Φs=
Φr[(ηs2ArIs)/(ηr2AsIr)] was used to calculate the quantum
yield, where Φs is the quantum yield of the sample, Φr is the
quantum yield of the reference, η is the refractive index of
the solvent, As and Ar are the absorbances of the sample and
the reference at the wavelength of excitation, and Is and Ir are
the integrated areas of emission bands.29 Phosphorescence
lifetime measurements were performed on the same spectro-
meter with a time correlated single photon counting method
using a LED excitation source. NMR spectra were recorded
on a Varian Gemini-300, 400, or 500 MHz spectrometer with
TMS as the internal reference, and chemical shifts were
referencedtoresidualprotiatedsolvent. Massspectrawerere-
cordedonanAppliedBiosystems Voyager-DESTR MALDI-
TOF mass spectrometer. The Microanalysis Laboratory at
Zhejiang University performed all elemental analysis on the
Vario EL III Elemental Analyzer.
Table 1. Crystal Data and Summary of Intensity Data Collection and Structure
Refinement for Pt-4 and Pt-13
Pt-4
Pt-13
empirical formula
fw
temp, K
C16H9ClF2N2Pt
497.79
298(2)
C24H13ClF2N2Pt
597.90
298(2)
wavelength, A
cryst syst
space group
unit cell dimensions
a (A)
b (A)
c (A)
R (deg)
β (deg)
0.71073
monoclinic
P21/n
0.71073
triclinic
P1
8.3088(12)
9.5462(14)
17.574(3)
90.00
94.747(3)
90.00
1389.2(4)
4
2.380
10.311
928
2.430 to 25.096
7.7675(12)
10.9813(17)
11.7112(18)
83.038(3)
71.048(2)
75.310(2)
913.1(2)
2
2.175
7.864
568
2.602 to 27.563
γ (deg)
volume, A3
Z
dcalc, Mg/m3
abs coeff, mm-1
F(000)
X-Ray Crystallography. X-ray diffraction data were collected
on a Bruker SMART APEX CCD diffractometer with graphite-
monochromated Mo KR radiation (λ = 0.71073 A) at 298(2) K
for both Pt-4 and Pt-13. A sphere of diffraction data was
collected up to a resolution of 0.84 A for Pt-4 and 0.77 A for
Pt-13, and the intensity data were processed using the SAINT
program. The cell parameters for the Pt complexes were
obtained from the least-squares refinement of spots (from 1818
collected frames for each compound) using the SAINT program.
Absorption corrections were applied by using SADABS.30 All
calculations for the structure determination were carried out using
the SHELXTL package (version 6.14).31 Initial Pt atomic positions
were located by Patterson methods using XS, and the remaining
structure of Pt-4 was found using difference maps and refined by
least-squares methods using SHELXL-97 with 2451 independent
reflections within the range of θ = 2.33-25.03° (completeness
100.0%). The Pt atomic positions for Pt-13 were determined by
θ range for
data collection, deg
reflns collected
indep reflns
refinement method
10867
2463
full-matrix
least-squares on F2
7174
3204
full-matrix
least- squares on F2
3204/0/271
1.131
data/restraints/params 2451/0/199
goodness-of-fit on F2
final R indices
1.120
0.0288
0.0397
[I > 2σ(I)]
R indices (all data)
0.0323
0.0435
Patterson synthesis using XS, and the remaining atoms were found
in the difference maps. The structure was refined by least-squares
methods using SHELXL-97 on 3204 reflections within the range
of θ = 1.92-25.00° (completeness 99.9%). Calculated hydrogen
positions were input and refined in a riding manner along with
the attached carbons. A summary of the refinement details and
resulting factors are given in Table 1.
(27) Reynolds, G. A.; Drexhage, K. H. Opt. Commun. 1975, 13, 222–225.
(28) Arbeloa, F. L.; Ojeda, P. R.; Arbeloa, I. L. J. Luminesc. 1989, 44, 105.
(29) DePriest, J.; Zheng, G. Y.; Goswami, N.; Eichhorn, D. M.; Woods,
C.; Rillema, D. P. Inorg. Chem. 2000, 39, 1955.
(30) Blessing, R. H. Acta Crystallogr. 1995, A51, 33.
(31) Sheldrick, G. M. SHELXTL, version 6.14; Bruker Analytical X-ray
System, Inc.: Madison, WI, 2003.
Electrochemistry. Cyclic voltammetry and differential pulsed
voltammetry were performed using a CH Instrument 610B elec-
trochemical analyzer. Anhydrous DMF (Aldrich) was used as the
solvent under a nitrogen atmosphere, and 0.1 M tetra(n-butyl)-
ammonium hexafluorophosphate was used as the supporting

Article
Inorganic Chemistry, Vol. 49, No. 24, 2010 11279
electrolyte. A silver wire was used as the pseudo-reference electrode.
A Pt wire was used as the counter electrode, and glassy carbon was
used as the working electrode. The redox potentials are based on
the values measured from differential pulsed voltammetry and are
reported relative to a ferrocenium/ferrocene (Fcþ/Fc) redox couple
used as an internal reference (0.45 V vs SCE).32 The reversibility of
reduction or oxidation was determined using cyclic voltammetry.33
As defined, if peak anodic and peak cathodic currents have an
equal magnitude under the conditions of fast scan (100 mV/s or
above) and slow scan (50 mV/s), then the process is reversible; if the
magnitudes in peak anodic and peak cathodic currents are the
same in fast scan but slightly different in slow scan, the process is
defined as quasi-reversible; otherwise, the process is defined as
irreversible.
1. Platinum [2,6-Di(2-pyridinyl-KN)-4-fluorophenyl-KC] Chloride
[Pt-2]. 1H NMR (500 MHz, CDCl3): δ 7.25 (d, J = 10.0 Hz, 2H),
7.34 (ddd, J1 = 1.5 Hz, J2 = 5.5 Hz, J3 = 8.0 Hz, 2H), 7.66 (dd,
J1 = J2 = 8.0 Hz, 2H), 7.99 (ddd, J1 = 1.5 Hz, J2 = J3 = 8.0 Hz,
2H), 9.39 (dd, J1 = 5.5 Hz, J2 = 21.0 Hz, 2H). HRMS (MALDI-
TOF), m/z calcd for [C16H10ClFN2Pt]: 479.0164. Found: 478.9798.
Calcd for [Mþ-Cl]: 444.0476. Found 444.0356. Anal. Calcd. for
C16H10ClFN2Pt: C, 40.05; H, 2.10; N, 5.84. Found: C, 39.16; H,
2.20; N, 5.89.
2. Platinum [2,6-Di(2-pyridinyl-KN)-3-fluorophenyl-KC] Chloride
[Pt-3]. 1H NMR (500 MHz, CDCl3): δ 6.84 (dd, J1 = 8.5 Hz, J2 =
11.5 Hz, 1H), 7.23 (ddd, J1 =1.0Hz, J2 =5.5Hz,J3 =7.5Hz, 1H),
7.29 (ddd, J1 = 2.0 Hz, J2 = 5.5 Hz, J3 = 7.0 Hz, 1H), 7.40 (dd,
J1 = 4.0 Hz, J2 = 8.5 Hz, 1H), 7.58 (dd, J1 = 5.5 Hz, J2 = 8.0 Hz,
1H), 7.89-7.96 (m, 3H), 9.23 (dd, J1 = 5.5 Hz, J2 = 21.0 Hz, 1H),
9.33 (dd, J1 = 5.5 Hz, J2 = 21.0 Hz, 1H). HRMS (MALDI-TOF),
m /z calcd for [C16H10ClFN2Pt]: 479.0164. Found: 478.9971. Calcd
for [Mþ-Cl]: 444.0476. Found: 444.0448. Anal. Calcd. for C16H10-
ClFN2Pt: C, 40.05; H, 2.10; N, 5.84. Found: C, 39.75; H, 2.25;
N, 5.96.
Synthesis. General Procedure for the Stille Coupling Follow-
ing Literature Methods25. A mixture of substituted m-dibromo-
benzene (10 mmol), 2-(tri-n-butylstannyl)pyridine (11.4 g,
30 mmol), Pd(PPh3)2Cl2 (70 mg, 0.1 mmol), and LiCl (2.55 g,
60 mmol) in toluene (70 mL) was stirred under reflux for 3 days
under a nitrogen atmosphere. After cooling to room tempera-
ture, the reaction mixture was filtered. The filtrate was poured
into water then extracted with dichloromethane. The combined
organic phase was washed with brine and dried over MgSO4.
The solution was rotary-evaporated todryness. The crude product
mixture was then flash chromatographed on a silica column
using a mixture of hexanes/ether (4:1) to give the product.
General Procedure for Microwave-Accelerated Stille Coupling
Reaction. Under a nitrogen atmosphere, a mixture of substituted
m-dibromobenzene (1 mmol), 2-(tri-n-butylstannyl)pyridine
(1.0 mL, 3.0 mmol), Pd(PPh3)2Cl2 (7 mg, 0.01 mmol), CuO (0.24 g,
3 mmol), and DMF (4 mL) was placed in a 10 mL Pyrex pressure
vessel. The tube was capped and rapidly heated by the microwave
reactor (2450 MHz, 200W). The reaction mixture was kept at
160 °C for 15 min by controlling the flow rate of cooling air. After
cooling to room temperature, the reaction mixture was poured into
50 mL of ethyl acetate and was filtered. The filtrate was washed by
water and dried over MgSO4. The solution was rotary-evaporated
to dryness. The crude product mixture was then flash chromato-
graphed on a silica column using a mixture of hexane/ether (4:1) to
give the product.
3. Platinum [3,5-Difluoro-2,6-di(2-pyridinyl-KN)-phenyl-KC]
1
Chloride [Pt-4]. H NMR (500 MHz, CDCl3): δ 6.67 (t, J =
11.0 Hz, 1H), 7.30 (ddd, J1 = 1.5 Hz, J2 = 6.0 Hz, J3 = 7.5 Hz,
2H), 7.89 (d, J = 7.5 Hz, 2H), 7.96 (ddd, J1 = 1.5 Hz, J2 = 7.5
Hz, J3 = 7.5 Hz, 2H), 9.31 (ddd, J1 =1.0 Hz, J2 = 6.0 Hz, J3 =
21.0 Hz, 2H). HRMS (MALDI-TOF), m/z calcd for [C16H9-
ClF2N2Pt]: 497.0070. Found: 496.9369. Calcd for [Mþ-Cl]:
462.0382. Found: 462.0216. Anal. Calcd. for C16H9ClF2N2Pt:
C, 38.61; H, 1.82; N, 5.63. Found: C, 38.10; H, 1.91; N, 5.74.
4. Platinum [3,4,5-Trifluoro-2,6-di(2-pyridinyl-KN)-phenyl-KC]
Chloride [Pt-5]. 1H NMR (500 MHz, CDCl3): δ 7.30 (ddd, J1 =
1.5 Hz, J2 = 5.5 Hz, J3 = 7.5 Hz, 2H), 7.90 (d, J = 8.0 Hz, 2H),
7.96 (ddd, J1 = 1.5 Hz, J2 = 7.5 Hz, J3 = 8.0 Hz, 2H), 9.32 (dd,
J1 = 5.5 Hz, J2 = 21.5 Hz, 2H). HRMS (MALDI-TOF), m/z
calcd for [C16H8ClF3N2Pt]: 514.9976. Found: 514.9945. Calcd for
[Mþ-Cl]: 480.0287. Found: 480.0506. Anal. Calcd. for C16H8ClF3-
N2Pt: C, 37.26; H, 2.66; N, 5.70. Found: C, 40.50; H, 3.10; N, 5.68.
5. Platinum[2,6-Di(2-pyridinyl-KN)-4-cyano-phenyl-KC] Chloride
[Pt-6]. 1H NMR (500 MHz, CDCl3): δ 7.45 (ddd, J1 = 1.5 Hz, J2 =
6.0 Hz, J3 = 7.5 Hz, 2H), 7.73 (s, 2H), 7.79 (d, J = 8.0 Hz), 8.08
(ddd, J1 = 1.5 Hz, J2 = 7.5 Hz, J3 = 8.0 Hz, 2H), 9.45 (dd, J1 = 6.0
Hz, J2 = 22.0 Hz, 2H). HRMS (MALDI-TOF), m/z calcd for
[C17H10ClN3Pt]: 486.0211. Found: 486.0336. Calcd for [Mþ-Cl]:
451.0522. Found: 451.0664. Anal. Calcd. for C17H10ClN3Pt: C,
41.94; H, 2.07; N, 8.63. Found: C, 39.80; H, 1.95; N, 8.40.
The 1H NMR data of corresponding ligands (L2-L10) following
the previous synthetic method are presented in the Supporting
Information. The ligands of L11-L13 adopt different synthetic
procedures, which are also reported in the Supporting Information.
General Procedure for Metal Coordination Reaction Following
Literature Methods25. A mixture of substituted m-di(2-pyridi-
nyl)benzene (1 mmol), K2PtCl4 (0.41 g, 1 mmol), and acetic acid
(60 mL) was stirred under reflux for 3 days in a nitrogen atmo-
sphere. After cooling to room temperature, the reaction mixture
was filtered. The precipitate was washed with methanol, water,
ethanol, and ether. The crude product was further purified by
recrystallization in DMSO/methanol or train sublimation.
General Procedure for Microwave-Accelerated Metal Coordi-
nation Reaction. Under a nitrogen atmosphere, a mixture of
K2PtCl4, m-di(2-pyridinyl)benzene (1 mmol), and 3 mL of
AcOH/H2O (9:1, v/v) was placed in a 10 mL Pyrex pressure
vessel. The tube was capped and rapidly heated by the micro-
wave reactor (2450 MHz, 200 W). The reaction mixture was kept
at 160 °C for 30 min by controlling the flow rate of cooling air.
After cooling to room temperature, the reaction mixture was
filtered. The precipitate was washed with methanol, water,
ethanol, and ether. The crude product was further purified by
recrystallization in DMSO/methanol or train sublimation. The
reaction yields of Pt complexes through two different methods
are presented in Table 3.
6. Platinum [2,6-Di(2-pyridinyl-KN)-4-acetylphenyl-KC]
Chloride[Pt-7]. 1H NMR (500 MHz, CDCl3): δ 2.68 (s, 3H),
7.36 (ddd, J1 = 1.5 Hz, J2 = 5.5 Hz, J3 = 7.5 Hz, 2H), 7.83 (d,
J = 8.0 Hz, 2H), 8.03, (ddd, J1 = 1.5 Hz, J2 = 7.5 Hz, J3 =
8.0 Hz, 2H), 8.07 (s, 2H), 9.40 (ddd, J1 = 1.0 Hz, J2 = 5.5 Hz,
J3 = 22.0 Hz, 2H). HRMS (MALDI-TOF), m/z calcd for
[C18H13ClN2OPt]: 503.0364. Found: 503.0198. Calcd for
[Mþ-Cl]: 468.0676. Found: 468.0892. Anal. Calcd. for C18H13-
ClN2OPt: C, 42.91; H, 2.60; N, 5.56. Found: C, 42.06; H, 2.76;
N, 5.60.
7. Platinum [2,6-Di(2-pyridinyl-KN)-4-methylphenyl-KC] Chlor-
ide [Pt-8]. 1H NMR (500 MHz, CDCl3): δ 2.37 (s, 3H), 7.28 (dd,
J1 =2.0Hz,J2 =7.5Hz,2H),7.29(s,2H),7.66(d,J=8.0Hz,2H),
7.93 (ddd, J1 = 2.0 Hz, J2 = 7.5 Hz, J3 = 8.0 Hz, 2H), 9.34 (ddd,
J1 = 1.0 Hz, J2 = 6.0 Hz, J3 = 22.0 Hz, 2H). HRMS (MALDI-
TOF), m/z calcd for [C17H13ClN2Pt]: 475.0415. Found: 475.0691.
Calcd for [Mþ-Cl]: 440.0726. Found: 440.1626. Anal. Calcd. for
C17H13ClN2Pt: C, 42.91; H, 2.75; N, 5.89. Found: C, 42.79; H, 2.71;
N, 5.47.
8. Platinum [2,6-Di(2-pyridinyl-KN)-4-methoxyphenyl-KC]
Chloride [Pt-9]. 1H NMR (500 MHz, CDCl3): δ 3.90 (s, 3H), 7.10
(s, 2H), 7.28 (ddd, J1= 1.5 Hz, J2 = 6.0 Hz, J3 = 8.0 Hz, 2H),
7.65 (d, J = 8.0 Hz, 2H), 7.94 (ddd, J1 = 1.5 Hz, J2 = J3 = 8.0
Hz, 2H), 9.34 (ddd, J1 = 1.0 Hz, J2 = 6.0 Hz, J3 = 21.0 Hz, 2H).
(32) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877.
(33) Harris, D. C. Quantitative Chemical Analysis, 6th ed.; W. H. Freeman
and Company: New York; pp 394-396.

11280 Inorganic Chemistry, Vol. 49, No. 24, 2010
Wang et al.
HRMS (MALDI-TOF), m/z calcd for [C17H13ClN2OPt]:
491.0364. Found: 491.0523. Calcd for [Mþ-Cl]: 456.0676.
Found: 456.0828. Anal. Calcd. for C17H13ClN2OPt: C, 41.50;
H, 2.66; N, 5.70. Found: C, 40.80; H, 2.79; N, 5.68.
Scheme 1. Synthesis of m-Di(2-pyridinyl)benzene and Pt(N∧C∧N)Cl
Complexes
9. Platinum [2,6-Di(2-pyrimidinyl-KN)-phenyl-KC] Chloride
[Pt-10]. 1H NMR (500 MHz, CDCl3): δ 7.29 (dd, J1 = 5.0 Hz,
J2 = 5.5 Hz, 2H), 7.36 (t, J = 8.0 Hz, 1H), 7.87 (d, J = 7.5 Hz,
2H), 8.94 (dd, J1 = 5.0 Hz, J2 = 2.5 Hz, 2H), 9.45 (ddd, J1 = 2.5
Hz, J2 = 5.0 Hz, J3 = 20.5 Hz, 2H). Anal. Calcd. for
C14H9ClN4Pt: C, 36.26; H, 1.96; N, 12.08. Found: C, 36.16;
H, 2.10; N, 12.47.
10. Platinum [2,6-di(3-isoquinolyl-KN)-phenyl-KC] chloride
1
[Pt-11]. H NMR (500 MHz, CDCl3): δ 7.27 (t, J = 7.5 Hz,
1H), 7.49 (d, J = 7.5 Hz, 2H), 7.63 (ddd, J1 = 8.0 Hz, J2 = 7.0
Hz, J3 = 1.5 Hz, 2H), 7.79 (ddd, J1 = 8.0 Hz, J2 = 7.0 Hz, J3 =
1.5 Hz, 2H), 7.87 (d, J = 8.0 Hz, 2H), 7.97 (dd, J1 = 4.0, J2 =
7.0 Hz, 2H), 8.07 (d, J = 8.0 Hz, 2H), 10.07 (dd, J = 23 Hz, 2H).
Anal. Calcd. for C24H15ClN2Pt: C, 51.31; H, 2.69; N, 4.99.
Found: C, 50.73.97; H, 2.31; N, 5.27.
2H), 8.68 (d, J = 8.5 Hz, 2H), 9.16 (d, J1 = 6.5 Hz, 2H). Anal.
Calcd. for C30H18F2N2OPt: C, 54.96; H, 2.77; N, 4.27. Found: C,
55.7; H, 3.25; N, 3.81.
11. Platinum [2,6-Di(1-isoquinolyl-KN)-phenyl-KC] Chloride
[Pt-12]. 1H NMR (500 MHz, CDCl3): δ 7.44 (t, J = 8.0 Hz, 1H),
7.68 (d, J = 6.5 Hz, 2H), 7.74 (ddd, J1 = 1.5 Hz, J2 = 8.5 Hz,
J3 = 8.0 Hz, 2H), 7.80 (ddd, J1 = 1.5 Hz, J2 = 8.5 Hz, J3 = 6.5
Hz, 2H), 7.95 (d, J = 8.0 Hz, 2H), 8.29 (d, J = 8.0 Hz, 2H), 8.99
(d, J = 8.5 Hz, 2H), 9.52 (dd, J1 = 6.5 Hz, J2 = 18.5 Hz, 2H).
Anal. Calcd. for C24H15ClN2Pt: C, 51.31; H, 2.69; N, 4.99.
Found: C, 50.23.97; H, 2.93; N, 5.04.
Results and Discussion
ꢁ
Synthesis and Characterization. Cardenas et al. reported
a two-step synthesis for Pt(N∧C∧N)Cl,25 using the Stille
cross-coupling reaction34 followed by a coordination reac-
tion. However, each step required at least 3 days to com-
plete. Thus, a week of preparation was needed to synthesize
a new Pt complex. In high-throughput chemistry, it is
important to decrease the reaction time as well as to
effectuate the purification procedure. It is well documented
that microwave flash heating can dramatically reduce reac-
tion time (from days and hours to minutes and seconds).
Specifically, facile syntheses of cyclometalated metal com-
plexes including [Ru(bpy)3]2þ, [Ir(bpy)3]3þ, and Ir(ppy)3
have previously been explored.35-37 Thus, it is worthy to
explore if the microwave irradiation method can expedite
the process of the Stille cross-coupling reaction38 and the
metal coordination reaction (Scheme 1).
12. Platinum [3,5-Difluoro-2,6-di(1-isoquinolyl-KN)-phenyl-
1
KC] Chloride [Pt-13]. H NMR (500 MHz, CDCl3): δ 6.91 (t,
J = 12 Hz, 1H), 7.69-7.74 (m, 4H), 7.84 (ddd, J1 = 1.0 Hz, J2 =
8.0 Hz, J3 = 8.0 Hz, 2H), 7.92 (d, J = 8.0 Hz, 2H), 8.51 (d, J =
8.5 Hz, 2H), 9.43 (dd, J1 = 6.5 Hz, J2 = 18.5 Hz, 2H). HRMS
(MALDI-TOF), m/z calcd for [C24H13ClF2N2Pt]: 597.0383.
Found: 597.0388. Calcd for [Mþ-Cl]: 562.0695. Found:
562.0765. Anal. Calcd. for C24H13ClF2N2Pt: C, 48.21; H, 2.19;
N, 4.69. Found: C, 47.97; H, 2.41; N, 4.82.
General Procedure for Complex Conversion from Pt-Cl to Pt-
OPh. Under nitrogen, 1 mmol of the platinum complex was
slowly added to a suspension of phenol (2 mml), KOH (1.5
mmol), and acetone (25 mL). After the mixture was stirred for
2 h, 50 mL of ether was added to the reaction mixture to gene-
rate more solids. The precipitate was filtrated off and washed
by water, acetone, and ether. The yellow-to-red products with
a yield of 40-60% were obtained from recrystallization in
dichloromethane/acetone.
The palladium-catalyzed Stille cross-coupling reac-
tions of 1,3-dibromobenzene derivatives with 2-(tri-n-
butylstannyl)pyridine were previously carried out in the
presence of lithium chloride (LiCl) in refluxed toluene for
3 days with a 75% yield.25 The initial set of microwave
experiments was carried out using 1,3-dibromobenzene,
2-(tri-n-butylstannyl)pyridine (3 equiv), LiCl (6 equiv),
and a catalytic amount of bis(triphenylphosphine)palla-
dium(II) chloride (5% equiv). The reactions were per-
formed under an atmosphere of nitrogen in sealed Pyrex
vessels. This preliminary survey was carried out in toluene,
a 9:1 (v/v) mixture of toluene/N,N-dimethylforma-
mide (DMF), and pure DMF to evaluate and optimize
the solvent conditions used in the microwave heating.
However, the expected product was barely detectable
under any of these conditions. When copper oxide
13. Platinum [3,5-Difluoro-2,6-di(2-pyridinyl-KN)-phenyl-KC]
Phenoxide [Pt-4-OPh]. 1H NMR (500 MHz, DMSO-d6): δ 6.37
(td, J1 = 7.0 Hz, J2 = 1.0 Hz, 1H), 6.84-6.86 (m, 2H), 6.95-6.98
(m, 2H), 7.14 (t, J = 11.5 Hz, 1H), 7.53 (ddd, J1 = 1.5 Hz, J2 =
6.0 Hz, J3 = 7.5 Hz, 2H), 7.96 (d, J = 7.5 Hz, 2H), 7.82 (ddd,
J1 = 1.5 Hz, J2 = 7.5 Hz, J3 = 7.5 Hz, 2H), 8.47 (ddd, J1 =1.0
Hz, J2 = 6.0 Hz, J3 = 15.0 Hz, 2H). Anal. Calcd. for C22-
H14F2N2OPt:C, 47.57;H, 2.54; N, 5.04. Found:C, 47.16; H, 2.60;
N, 5.21.
14. Platinum [2,6-Di(1-isoquinolyl-KN)-phenyl-KC] Phenoxide
1
[Pt-12-OPh]. H NMR (500 MHz, CDCl3): δ 6.58 (t, J = 7.0
Hz, 1H), 7.14-7.17 (m, 2H), 7.19-7.22 (m, 2H), 7.43 (t, J =
8.0 Hz, 1H), 7.61 (d, J = 6.5 Hz, 2H), 7.76 (ddd, J1 = 1.5 Hz, J2 =
8.5 Hz, J3 = 8.0 Hz, 2H), 7.83 (ddd, J1 =1.5Hz, J2 =8.5Hz, J3 =
6.5 Hz, 2H), 7,92 (d, J= 8.0 Hz, 2H), 8.31 (d, J= 8.0 Hz, 2H), 8.82
(dd, J1 = 6.5 Hz, J2 = 18.5 Hz, 2H), 9.52 (d, J = 8.5 Hz, 2H).
Anal. Calcd. for C30H20N2OPt: C, 58.16; H, 3.25; N, 4.52. Found:
C, 57.71; H, 3.71; N, 4.68.
(34) (a) Stille, J. K. Angew. Chem., Int. Ed. Engl. 1986, 25, 508. (b) Michell,
T. N. Synthesis 1992, 803. (c) Ritter, K. Synthesis 1993, 735. (d) Farina, V.; Roth,
G. P. In Advances in Metal-Organic Chemistry; Liebeskind, L. S., Ed.; JAI Press
Inc: Greenwich, CT, 1996; Vol. 5, p 1.
(35) Tastumura-Noue, T.; Tanabe, M.; Minami, T.; Ohashi, T. Chem.
Lett. 1994, 23, 2443.
(36) Yoshikawa, N.; Musuda, Y.; Tastumura-Noue, T. Chem. Lett. 2000,
29, 1206.
(37) (a) Saito, K.; Matsusue, N.; Kanno, H.; Hamada, Y.; Takahashi, H.;
Matsumura, T. Jpn. J. App. Phys. 2004, 43, 2733. (b) Konno, H.; Sasaki, Y.
Chem. Lett. 2003, 32, 252.
15. Platinum [3,5-Difluoro-2,6-di(1-isoquinolyl-KN)-phenyl-
KC] Phenoxide [Pt-13-OPh]. 1H NMR (500 MHz, CDCl3): δ
7.08-7.13 (m, 4H), 7.28-7.32 (m, 2H), 7.65 (ddd, J1 = 1.5 Hz,
J2 =7.0Hz, J3 = 8.0 Hz, 2H), 7.87 (ddd, J1 =1.5Hz, J2 =7.0Hz,
J3 = 8.0 Hz, 2H), 7.99 (d, J = 8.0 Hz, 2H), 8.05 (d, J = 8.5 Hz,
(38) Larhed, M.; Hoshino, M.; Hadida, S.; Curran, D. P.; Hallberg, A.
J. Org. Chem. 1997, 62, 5583.

Article
Inorganic Chemistry, Vol. 49, No. 24, 2010 11281
Table 2. The Stille Cross-Coupling Reaction Yields under Two Conditionsa
Table 3. The Metal Coordination Reaction Yields under Two Conditionsa
a Method A: Substituted m-dibromobenzene (1 mmol), 2-(tri-n-bu-
tylstannyl)pyridine (3.0 equiv), CuO (3.0 equiv), Pd(PPh3)2Cl2 (2%
equiv), DMF, 150 °C (sealed tube), microwave heating for 15 minutes.
Method B: Substituted m-dibromobenzene (1 mmol), 2-(tri-n-butylstan-
nyl)pyridine (3.0 equiv), LiCl (6.0 equiv), Pd(PPh3)2Cl2 (2% equiv),
toluene, reflux for 3 days.
a Method A: Substituted m-di(2-pyridinyl)benzene (1 mmol),
K2PtCl4 (0.42 g, 1 mmol), AcOH/H2O (9:1, v/v), 160 °C (sealed tube),
microwave heating for 30 minutes. Method B: Substituted m-di(2-
pyridinyl)benzene (1 mmol), K2PtCl4 (0.42 g, 1 mmol), AcOH, reflux
for 3 days.
(CuO) replaced LiCl as the base, the reaction yield
improved only if pure DMF was used as the solvent.39
The failure of other solvent combinations can be partially
attributed to the low microwave absorption of toluene.40
To study the full scope of the procedure, a series of
m-di(2-pyridinyl)benzene derivatives was prepared under
both conditions (methods A and B in Table 2). All
reactions provided fair to good yields of the isolated
products after a workup procedure including filtration,
extraction, and subsequent flash column chromatogra-
phy. The change of substituents (from the electron donat-
ing group to the electron withdrawing group) did not
influence the reaction outcome.
A rich chemistry of Pt ions with the cyclometalating
tridentate ligands has been approached, including platinum
complexes of N∧N∧C-binding ligands,23c,41 C∧N∧C-binding
ligands,24 and a N∧C∧N-coordinating tridentate ligand.17,18
Generally, the metal coordination of these complexes will
take several days. For example, Pt(dpb)Cl can be obtained in
a good yield after a 3-day reaction between dpb and potas-
sium tetrachloroplatinate (K2PtCl4) in a refluxed solution of
acetic acid.17a Here, microwave-assisted synthesis of plati-
num complexes was performed as follows: a suspension of
K2PtCl4 (1 mmol), dpb (1 mmol), and 3 mL of AcOH/H2O
(9:1, v/v) was sealed in a 10 mL Pyrex vessel. The vessel was
set up in the microwave reactor and irradiated at different
temperatures and power settings and for various durations.
After cooling to room temperature, the product precipitated
from the solution and was further purified through recrys-
tallization in dimethyl sulfoxide/methanol or train sublima-
tion. Our experimental results showed that a high reaction
yield (80%) can be realized in a 30 min reaction at 160 °C
using 200 W. This reaction yield is much higher than the
literature reported synthesis of cyclometalated Ir complexes
using microwave heating.37
The reactions of substituted dpb were studied next to
determine the effect of the substituents (Table 3). When an
electron-donating group was introduced on the phenyl ring,
the yield of complexes decreased significantly (entries 6, 8,
and 11). Conversely, the yield increased when an electron-
withdrawing group, such as fluorine and a cyano group, was
introduced (entries 2, 4, 5, 7, and 12). A similar trend was
also observed under the conditions of normal oil heating. A
hindering group such as isoquinolin-1-yl did not affect the
(39) Gibbs, R. A.; Krishnan, U.; Dolence, J. M.; Poulter, C. D. J. Org.
Chem. 1995, 60, 7821.
(40) Kappe, C. O. Angew. Chem., Int. Ed. 2004, 43, 6250.
(41) Lai, S.-W.; Chan, M. C. W.; Cheung, T.-C.; Peng, S.-M.; Che, C.-M.
Inorg. Chem. 1999, 38, 4046.

11282 Inorganic Chemistry, Vol. 49, No. 24, 2010
Wang et al.
Scheme 2. Proposed Mechanism for the Metal Coordination Reaction
Figure 3. ORTEP drawings of (a) Pt-4, and (b) Pt-13 in a dimeric form.
The thermal ellipsoids for the image represent a 25% probability limit.
The hydrogen atoms are omitted for clarity. The dotted line indicates the
shortest distance between two Pt atoms, and the solid line with double
arrows indicates the shortest distance between the Pt atom and the least-
squares plane of Pt-N-C-N.
Figure 2. ORTEP drawings of (a) Pt-4 and (b) Pt-13. The thermal
ellipsoids for the image represent a 25% probability limit. The hydrogen
atoms are omitted for clarity.
Table 4. Selected Bond Distances and Plane-to-Plane Distance (A) for Pt Complexes
Discussed in the Paper
reaction yield. A high coordination reaction yield was
observed under both conditions (entry 12).
Pt-C
Pt-N1
Pt-N2
Pt-Cl
Pt Pt Pt plane
A possible reaction mechanism has been proposed in order
to uncover the influence of substituents on the yield of metal
coordination reactions. Typically, a ligand coordinates
to a metal center, and a proximal C-H bond is activated,
forming a chelate ring.42 The coordination reaction can be
formally considered as a multicentered pathway involving a
nucleophile-assisted electrophilic reaction (Scheme 2).42,43
Here, the nucleophile is a chloride ion. The substituents can
affect the coordination reactions by influencing the C-H
bond activation. When the electron-withdrawing group was
introduced onto the phenyl ring, the electron density at the
C-H bond was decreased. This increased the C-H bond
acidity, resulting in more active bond cleavage.
3 3 3
3 3 3
3.40
Pt-125 1.907(8) 2.033(6) 2.041(6) 2.417(2)
4.85
3.42
5.00
Pt-4 1.910(6) 2.028(5) 2.043(5) 2.4124(15)
Pt-13 1.918(8) 2.009(7) 2.021(7) 2.412(2)
3.38
3.47
provided in Table 4. Both Pt-4 and Pt-13 molecules have a
distorted square planar geometry. Moreover, the change of
functional group does not affect the structure of the core
Pt(N∧C∧N)Cl fragment. The bond lengths of Pt-C, Pt-N1,
Pt-N2, and Pt-Cl are similar for all of these three com-
plexes. Similar to the literature reported Pt-1 molecules,25
both Pt-4 and Pt-13 molecules pack as head-to-tail dimers,
each molecule of the dimer related to the other by a center of
inversion (Figure 3). The dimers have a plane-to-plane
separation of 3.38 A for Pt-4 and 3.47 A for Pt-13, indicative
of moderate π-π interactions (Figure 3). However, Pt-4 and
Pt-13 have different levels of metal-metal interaction due to
different molecular geometries. The shortest Pt-Pt distance
(3.42 A) of Pt-4 is much smaller than those of Pt-1 (4.85 A)25
and Pt-13 (5.00 A). The difference can be attributed to the
unflattened structure of Pt-13 created by changing pyridinyl
groups to isoquinonyl groups.
Single crystals of Pt-4 and Pt-13 were prepared by slow
sublimation. Molecular plots of Pt-4 and Pt-13 are shown in
Figures 2 and 3. The crystallographic data are also given in
Table 1. The selected bond lengths, plane-to-plane dis-
tances, and Pt-Pt distances for Pt-1,25 Pt-4, and Pt-13 are
(42) Ryabov, A. D. Chem. Rev. 1990, 90, 403.
(43) Jawad, J. K.; Puddephatt, R. J.; Stalteri, M. A. Inorg. Chem. 1982, 21,
332.

Article
Inorganic Chemistry, Vol. 49, No. 24, 2010 11283
Table 5. Redox Properties of Pt(N∧C∧N)X Complexesa
the bidentate C∧N ligands. For selected Pt(N∧C∧N)X
complexes, the change of ancillary ligands from chloride
to phenoxide appeared to have a small influence on the
electrochemical properties of Pt complexes. The oxida-
tion and reduction values of Pt complex phenoxides are
comparable to their chloride analogs.
Pt(N∧C∧N)X
E1/2Ox (V)
E1/2Red (V)
ΔE1/2 (V)
Pt-1
Pt-2
Pt-3
Pt-4
Pt-5
Pt-6
Pt-7
Pt-8
0.41
0.47
0.44
0.50
0.51
0.59
0.44
0.40
0.41
0.5
0.31
0.34
0.56
0.36
0.50
0.58
-2.18
-2.08
-2.09
-2.07
-2.07
-1.99
-2.07
-2.18
-2.16
-1.92
-2.08
-1.84
-1.74
-2.09
-1.83
-1.82
2.59
2.55
2.53
2.60
2.58
2.58
2.53
2.58
2.57
2.42
2.39
2.18
2.28
2.45
2.33
2.40
Electronic Spectroscopy. The room temperature (RT)
absorption and emission spectra and low-temperature (77 K)
emission spectra were recorded for all Pt(N∧C∧N)X com-
plexes (Table 6). A comparison of absorption features among
Pt-1, Pt-4, Pt-9, and Pt-12 is shown in Figure 4. All of the
Pt complexes exhibit very strong absorption bands below
300 nm (ε>1ꢀ 104 Lmol-1cm-1) due to 1π-π* transitions
localized on N∧C∧N ligands (LC), together with a set
of intense bands in the region 320-460 nm (ε > 2 ꢀ 103 L
mol -1cm-1), which are attributed to MLCT transitions
involving both the cyclometalating ligands and platinum
metal ions. Much weaker energy absorption bands between
440 and 600 nm (ε < 200 L mol-1 cm-1) are identified as a
triplet transition (T1) on the basis of the small energy shift
between absorption and emission at room temperature. The
structural changes of Pt complexes have a strong influence on
the absorption bands associated with 1MLCT and T1 transi-
tions. Compared with Pt-1, adding electron-withdrawing
groups to the phenyl ring (e.g., Pt-4) leads to an increase in
1MLCT and T1 transition energies. On the other hand,
adding electron-donating groups to the phenyl ring (e.g.,
Pt-9) or replacing with lower energy electron-accepting
groups (e.g., Pt-12) will decrease the energies of 1MLCT
Pt-9
Pt-10
Pt-11
Pt-12
Pt-13
Pt-4-OPh
Pt-12-OPh
Pt-13-OPh
a Redox measurements were carried out in DMF solution. For all of
the Pt complexes reported here, the oxidation process is irreversible and
the first reduction process is reversible or quasi-reversible. The redox
values are reported relative to Fcþ/Fc.
Electrochemistry. The electrochemical properties of
Pt(N∧C∧N)X complexes (1-13) were examined using
cyclic voltammetry, and the values of redox potentials
were determined using differential pulsed voltammetry
(Table 5). All of the electrochemical data reported here
were measured relative to an internal ferrocenium/ferro-
cene reference (Fcþ/Fc). For all of the Pt complexes, they
have one irreversible oxidation potential between 0.3 and
0.6 V and their first reduction potential between -1.8 and
-2.2 V. The redox values of Pt-1 are slightly different than
the literature values (E1/2OX =0.43VandE1/2red =-2.03 V)
due to the different choice of solvent systems.18 The redox
properties of Pt complexes are strongly affected by the
structural modification of m-di(2-pyridinyl)benzene ligands.
Incorporating the electron-withdrawing functional groups
(like fluorine, cyano, and acetyl group) on the phenyl rings
gives an increase in the oxidation potential and a decrease in
the reduction potential, while introducing the electron-
donating groups (like methyl and methoxyl groups) on
the same location will lead to an increase in the reduction
potential.18,44 As the conjugated π systems of electron
accepting groups increase, the complexes including Pt-11,
Pt-12, and Pt-13 demonstrate a marked decrease in the
reduction potential.
The electrochemical data support the fact that reduc-
tion occurs mainly on the ligands (i.e., pyridinyl group
and its analogs) and oxidation is localized on the metal
center, which is consistent with most literature reports
on cyclometalated Pt complexes.16 Due to the potential
effects of the solvent on the square planar Pt(III) ions, the
oxidation process of Pt(II) complexes is typically
irreversible.45 Compared with Pt(C∧N)(acac) with similar
cyclometalated ligands, the Pt(N∧C∧N)Cl complexes
showed a marked decrease in their reduction potential.
For example, the reduction value of Pt(dpb)Cl is more
than 0.2 V lower than (ppy)Pt(acac) (-2.41 V vs Fc/
Fcþ).16 This can be possibly attributed to the extended
conjugation of tridentate N∧C∧N ligands compared with
1
and T1 transitions. However, while MLCT transition
energies (for Pt-9 and Pt-12) decrease with minor
changes in intensity, the decrease in the T1 absorption
energies coincides with a clear decrease in the extinction
coefficient (Figure 4, inset).
The room temperature and 77 K emission spectra of
Pt-1, Pt-4, Pt-9, and Pt-12 are shown in Figure 5. The
emission spectra of these Pt complexes are affected
strongly due to the structural changes. In general, the
introduction of the electron-withdrawing groups on the
phenyl ring, associated with an increase of oxidation poten-
tial, shifts the emission spectra to shorter wavelengths. On the
other hand, adding the electron-donating groups on the
phenyl rings will not only decrease the oxidation potential
but also lower the emission energies of Pt complexes. Simi-
larly, the red-shifted emission spectrum of Pt-12 due to the
replacement of isoquinonyl groups concurred with a drop in
the reduction potential. Such substitution effects have been
observed for previously reported tricyclometalated iridium
complexes and bidentate platinum complexes.16
Most of the Pt(N∧C∧N)Cl complexes, i.e., keeping the core
segment of Pt-1, are strongly luminescent (Φ = 0.32-0.63)
and have short luminescence lifetimes (τ = 4-7 μs) at room
temperature, similar to values reported for Pt-1.17,25 Excep-
tions are complexes with isoquinolyl or quinolyl groups. For
example, the Φ values of Pt-12 and Pt-13 are less than 0.10,
and the τ of Pt-11 is more than 40 μs. The radiative (kr) and
nonradiative decay (knr) rates can be calculated from the
room temperature Φ and τ data.46 The kr values of the
(46) The equations kr = Φ/τ and knr = (1 - Φ)/τ were used to calculate
the rates of radiative and nonradiative decay, where Φ is the quantum
efficiency and τ is the luminescence lifetime of the sample at room tempera-
ture.
(44) Kulikova, M.; Balashev, K. P.; Kvam, P. I.; Songstad, J. Russ. J. Gen.
Chem. 2000, 70, 163.
(45) Kvam, P.-I.; Puzyk, M. V.; Balashev, K. P.; Songstad, J. Acta Chem.
Scand. 1995, 49, 335–343.

11284 Inorganic Chemistry, Vol. 49, No. 24, 2010
Wang et al.
Figure 4. The comparison of absorption spectra of Pt-1, Pt-4, Pt-9, and
Pt-12 in CH2Cl2 at room temperature. The T1 absorption transitions are
shown in the inset.
Figure 5. Room temperature (top) and 77 K emission (bottom) spectra
of Pt-1, Pt-4, Pt-9, and Pt-12. 2-MeTHF was used for low temperature
measurement instead of CH2Cl2.
Pt(N∧C∧N)Cl complexes range between 1.2 ꢀ 105 and 5 ꢀ
103 s-1, which are strongly structure-dependent. The deriva-
tives of Pt-1 maintain very high kr values, although it appears
to decrease 1-2 fold by adding an electron-donating group to
the phenyl rings. Moreover, the kr values are around 1 order
of magnitude lower, while the pyridinyl groups are replaced
by isoquinolyl or quinolyl groups (e.g., Pt-11 and Pt-12). The
knr values span a narrower range of values (2.9 ꢀ 105 to 2 ꢀ
104 s-1) and tend to increase as the emission energy decreases,
indicating an observation of energy gap law.47,48 All of the
Pt(N∧C∧N)Cl complexes are intensely emissive at low tem-
peratures (77 K), and most have short luminescence life-
times (τ = 5-12 μs), except Pt-11 has a luminescent lifetime
exceeding 100 μs. Although it is not clear why Pt-11 has such

Article
Inorganic Chemistry, Vol. 49, No. 24, 2010 11285
a long luminescent lifetime at room temperature due to both
low kr and knr values, the distinction of Pt-11, Pt-12, and Pt-
13 from the rest of Pt(N∧C∧N)Cl complexes suggests that the
luminescent properties of the Pt(N∧C∧N)Cl complexes could
be significantly modified by changing the nature of electron-
accepting groups even though the structure of the core
segment Pt-N-C-N is largely unaffected (Figure 2).
All of the Pt(N∧C∧N)X complexes have the structured
luminescent spectra displaying resolved vibronic progres-
sions. Two prominent vibronic features are presented in the
emission spectra (Figure 5). The intensity ratio of the first
major vibronic transition to the highest energy peak
(Eem(0-0)) is a measure of vibronic coupling between the
ground and excited states (Huang-Rhys factor, SM) and
is proportional to the degree of structural distortion that
occurs in the excited state relative to the ground state.49-52
The dominant vibrational mode associated with the excited
state distortion (pωM) can be obtained from the energy
difference (in cm-1) of these vibronic transitions at 77 K,
whereas the SM value can be estimated from the peak heights.
As illustrated in Figure 5, the values of the vibronic spacing
for all of the selected Pt complexes are the same within
experimental error (pωM ≈ 1300 cm-1). For three Pt com-
plexes with the same core segment (i.e., Pt-1, Pt-4, and Pt-9),
the Huang-Rhys factors also increase monotonically with
decreasing Eem(0-0), and the SM value of Pt-9 is larger than
those of Pt-1 and Pt-4.
Ground-State and Excited-State Properties of Pt-
(N∧C∧N)X. The lowest energy excited state of square
planar d8 metal complexes has been studied intensely
during the past two decades. However, it has been focused
on how to raise the energy level of the metal-centered
(d-d) excited state and how to prevent potential quench-
ing of the luminescence of Pt complexes.16,21-24 It lacks a
systematic and detailed study to correlate the nature of
the excited state of Pt complexes (e.g., 3MLCT, 3LC, and
1,3ILCT) with their luminescent properties. Although it is
well documented that the excited state of a Pt(N∧C∧N)X
Figure 6. The comparison of absorption spectra (top) and emission spectra
(bottom) of Pt-1, ppyPt(acac), ppyPt(py)Cl, and Pt(pbpy)Cl in CH2Cl2 at
room temperature. The T1 absorption transitions are shown in the inset.
Table 7. Emission Properties of Pt-1 and Pt-12 and Their Analogs at Room
Temperature
λ
max (nm) τ (μs) ΦPL kr (105 s-1
)
knr (105 s-1
)
Pt-117a
490
484
485
563
592
592
7.2
2.6
1.8
0.5
3.5
4
0.60
0.15
0.002
0.025
0.072
0.08
0.83
0.57
0.011
0.5
0.21
0.2
0.55
3.2
5.5
19.5
2.7
2.3
ppyPt(acac)16
ppyPt(py)Cl
Pt(phbpy)Cl41a
Pt-12
3
like Pt-1 has a majority of LC characters mixed with
1
some MLCT/3MLCT components,15,18 it is still very
piqPt(acac)
valuable to explore why Pt-1 is more emissive than most
of the literature reported Pt complexes.
The emission properties of Pt-1 have been compared with
Table 7. The high quantum yield of Pt-1 can be attributed to
its high radiative decay rate and low nonradiative decay
rate. Although both ppyPt(acac) and ppyPt(py)Cl are
similar to Pt-1 in terms of emission energies, structured
emission spectra, and well-resolved triplet absorption spec-
tra, they appeared to have higher MLCT transition energies
than Pt-1 (Figure 6a), associated with higher reduction
values. The potential larger energy difference between the
MLCT state and 3LC state could result in a weaker mixing of
both singlet and triplet MLCT components into the excited
state for ppyPt(acac) and ppyPt(py)Cl. This analysis is
supported by the fact that Pt-1 has a higher kr value, a higher
extinction coefficient of triplet absorption, and less pro-
nounced vibronic features of the emission spectrum (smaller
Huang-Rhys factor; Figure 6 and Table 7).54 A similar trend
was observed for the comparison between red-emitting Pt-12
and its analog platinum(2-phenyl-isoquinolyl-N,C20) acety-
lacetonate [piqPt(acac)],55 although both complexes have
those of its analogs, i.e., ppyPt(acac),16 platinum (2-phenyl-
0
pyridinato-N,C2 ) (pyridine) chloride [ppyPt(py)Cl],53 and
Pt(phbpy)Cl23 (Scheme S1 shows the structure of these
complexes), which have similar coordination spheres con-
sisting of two pyridinyl rings, one cyclometalated phenyl
ring, and one chloride. The absorption and room tempera-
ture emission spectra of these four complexes are shown in
Figure 6, and their photophysical data are presented in
(47) Caspar, J. V.; Westmoreland, T. D.; Allen, G. H.; Bradley, P. G.;
Meyer, T. J.; Woodruff, W. H. J. Am. Chem. Soc. 1984, 106, 3492.
(48) Damrauer, N. L.; Boussie, T. R.; Devenney, M.; McCusker, J. K.
J. Am. Chem. Soc. 1997, 119, 8253.
(49) Caspar, J. V.; Westmoreland, T. D.; Allen, G. H.; Bradley, P. G.;
Meyer, T. J.; Woodruff, W. H. J. Am. Chem. Soc. 1984, 106, 3492.
(50) Rillema, D. P.; Blanton, C. B.; Shaver, R. J.; Jackman, D. C.; Boldaji,
M.; Bundy, S.; Worl, L. A.; Meyer, T. J. Inorg. Chem. 1992, 31, 1600.
(51) Allen, G. H.; White, R. P.; Rillema, D. P.; Meyer, T. J. J. Am. Chem.
Soc. 1984, 106, 2613.
(52) Damrauer, N. L.; Boussie, T. R.; Devenney, M.; McCusker, J. K.
J. Am. Chem. Soc. 1997, 119, 8253.
(53) Mdleleni, M. M.; Bridgewater, J. S.; Watts, R. J.; Ford, P. C. Inorg.
Chem. 1995, 34, 2334.
(54) Li, J.; Djurovich, P. I.; Alleyne, B. D.; Yousufuddin, M.; Ho, N. N.;
Thomas, J. C.; Peters, J. C.; Bau, R.; Thompson, M. E. Inorg. Chem. 2005,
44, 1713.

11286 Inorganic Chemistry, Vol. 49, No. 24, 2010
Wang et al.
of Pt(N∧C∧N)X complexes through Pt-X interaction, even
though the excited state localizes mainly in the Pt-N-C-N
fragment.15,17 The study of Pt(N∧C∧N)(OPh) complexes
provides an alternative route to developing halogen-free
phosphorescent emitters which can be used to improve the
operational stability of blue and white OLEDs.17b,57
Conclusion
Facile synthesis of platinum m-di(2-pyridinyl)benzene
chloride and its derivatives, using controlled microwave
heating, was reported. This method not only shortened the
reaction time but also improved the reaction yield for most of
the Pt complexes. Moreover, the reaction yield is dependent on
the identity of substituents on the centered phenyl rings.
A possible reaction mechanism for the metal coordination
reaction was proposed as a multicentered pathway of nucleo-
phile-assisted electrophilic reaction. Such a high-throughput
synthetic method will help us build a database for this class of
Pt complexes and enable us to explore their potential utilities in
OLEDs and other applications thoroughly.
The electrochemical and photophysical properties of a
series of Pt(N∧C∧N)X complexes are discussed in detail.
The electrochemical studies of all Pt complexes demonstrate
that the oxidation process occurs on the metal-phenyl frag-
ment and the reduction process is associated with the elec-
tron-accepting groups like pyridinyl groups and their
derivatives. The lowest excited state of Pt(N∧C∧N)X com-
plexesis identified asa dominant ligand-centered3π-π* state
with some MLCT character, which appears to have a larger
1MLCT component than Pt(C∧N)(LX) and Pt(C∧N∧N)Cl
analogs. This results in a high radiative decay rate and high
quantum yield for Pt(dpb)Cl and its analogs. However, the
excited state properties of the Pt(N∧C∧N)X complexes are
strongly dependent on the nature of the electron-accepting
groups and substituents to the metal-phenyl fragment. A
rational design will be needed to tune the emission energies of
the Pt(N∧C∧N)X complexes over a wide range while main-
taining their high luminescent efficiency. Most of the Pt-
(N∧C∧N)X complexes reported here are highly emissive and
stable to sublimation, making them ideal as phosphorescent
emitters for OLEDs. Devices fabricated with Pt-4 and Pt-8
have an external quantum efficiency of >10% for mono-
chromatic and white phosphorescent OLEDs.7,17b
Figure 7. The comparison of emission spectra of Pt-4, Pt-4-OPh, Pt-13,
and Pt-13-OPh in CH2Cl2 at room temperature.
lower quantum yields than their green-emitting analogs
(Figure S1). Compared with Pt-1, Pt(phbpy)Cl has different
3
excited state characteristics, i.e., MLCT, which results in
emission and absorption spectra with much fewer features.23
The low quantum yield of Pt(phbpy)Cl can be attributed to a
comparably low radiatve decay rate and a much faster non-
radiative decay process due to possible d-d quenching.17a,56
The excited state properties of the Pt(N∧C∧N)X com-
plexes are strongly dependent on the chemical structures.
The change of functional groups on the phenyl ring of Pt-1
will not only modify the emission energy but also the nature
of the excited state. For example, adding electron donating
groups to the phenyl rings (e.g., Pt-9) lowers the radiative
decay rate and increases the Huang-Ryes factor, indicating
a smaller component of 1MLCT/3MLCT character in the
lowest excited state. Replacing the pyridinyl group with
isoquinonyl groups also leads to a 10-fold drop in the
radiative decay rate, which results in a much lower quantum
yield for Pt-12 and Pt-13 than most literature reported
Pt(N∧C∧N)Cl complexes.17 The comparison of photophysi-
cal properties between Pt-12 and piqPt(acac) indicates that
the nature of electron-accepting groups could strongly influ-
ence the excited state properties of both Pt(N∧C∧N)Cl and
Pt(C∧N)(acac) complexes (Figure S1).
Three Pt(N∧C∧N)(OPh) complexes were examined using
similar N∧C∧N-coordinating ligands (Table 6 and Figure 7).
The change of ancillary ligands does not modify the oxidation
or reduction potentials of corresponding Pt complexes sig-
nificantly, and both Pt-Cl and Pt-OPh complexes have
similar emission spectra. However, Pt-OPh complexes have
around one-half the quantum yields of their Pt-Cl analogs
due to comparably lower kr values (for Pt-12-OPh and Pt-13-
OPh) and higher knr values (for Pt-4-OPh). This indicates that
the ancillary ligands can influence the excited state properties
Acknowledgment. The authors thank the National
Science Foundation (CHE-0748867) for partial support
of this work. V.M. and J.L. thank Fulton Undergraduate
Research Initiative (FURI) for its generous support.
Supporting Information Available: 1H NMR spectra, experi-
mental details, and characterization of new compounds reported.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(55) The structure and synthesis of platinum (2-phenyl-isoquinolyl-N,
C20) (acetylacetonate) [piqPt(acac)] is provided in the Supporting Informa-
tion.
(56) Lai, S.-W.; Lam, H.-W.; Lu, W.; Cheung, K.-K.; Che, C.-M.
Organometallics 2002, 21, 226–234.
(57) Giebink, N. C.; D’Andrade, B. W.; Weaver, M. S.; Mackenzie, P. B.;
Brown, J. J.; Thompson, M. E.; Forrest, S. R. J. Appl. Phys. 2008, 103,
044509.