Improvement in phosphorescence efficiency through tuning of coordination geometry of tridentate cyclometalated platinum(II) complexes

Article abstract of DOI:10.1021/ic101109h

A series of tridentate cyclometalated platinum(II) complexes (C ^∧*)PtL (L = Cl or acetylide) featuring a fused five-six-membered metallacycle were synthesized. The structure of the complexes was confirmed by X-ray crystallography. In contrast to the C ^∧N^∧N platinum complexes with a fused five-five-membered metallacycle, the platinum coordination in C ^∧N^*N complexes is much closer to a square planar geometry. The photophysical properties of the complexes were studied. The geometrical change from C^∧N^∧N to C ^∧N^*N led to a substantial improvement in phosphorescence efficiency of the complexes with an acetylide ligand in solution at room temperature. For example, the quantum yield of (C^∧N ^*N)PtCCPh was measured to be 56%, demonstrating a big jump from 4% reported for (C^∧N^∧N)PtCCPh.

Full text of DOI:10.1021/ic101109h

8922 Inorg. Chem. 2010, 49, 8922–8928
DOI: 10.1021/ic101109h
Improvement in Phosphorescence Efficiency through Tuning of Coordination
Geometry of Tridentate Cyclometalated Platinum(II) Complexes
Deepak Ravindranathan,† Dileep A. K. Vezzu,† Libero Bartolotti,† Paul D. Boyle,‡ and Shouquan Huo*,†
†Department of Chemistry, East Carolina University, Greenville, North Carolina 27858, and
‡Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695
Received June 1, 2010
A series of tridentate cyclometalated platinum(II) complexes (C∧N*N)PtL (L = Cl or acetylide) featuring a fused five-
six-membered metallacycle were synthesized. The structure of the complexes was confirmed by X-ray crystallography.
In contrast to the C∧N∧N platinum complexes with a fused five-five-membered metallacycle, the platinum coordination
in C∧N*N complexes is much closer to a square planar geometry. The photophysical properties of the complexes were
studied. The geometrical change from C∧N∧N to C∧N*N led to a substantial improvement in phosphorescence
efficiency of the complexes with an acetylide ligand in solution at room temperature. For example, the quantum yield of
(C∧N*N)PtCCPh was measured to be 56%, demonstrating a big jump from 4% reported for (C∧N∧N)PtCCPh.
Introduction
demonstrated by the room temperature emission from cis-
Pt(thpy)2 (Hthpy = 2-(2-thienyl)pyridine)4a-c and (ppy)Pt-
(acac) (Hppy = 2-phenylpyridine).4d The development of
highly efficient tridentate N∧C∧N (based on 1,3-dipyridylben-
zene) platinum complexes5 rep∧resents an effective geometrical
switch from weakly emissive C N∧N (based on 6-phenyl-2,20-
bipyridine) cyclometalated platinum complexes.6 A high quan-
tum yield of 60% in dichloromethane was reported for the
N∧C∧N platinum complex derived from 1,3-dipyridylben-
zene.5b The use of N∧C∧N platinum complexes as highly effi-
cient triplet emitters in organic light emitting diode (OLED)
devices has been demonstrated.5f-i However, a further modi-
fication on the N∧C∧N platinum complex by designing a fused
six-six-membered metallacycle led to a very low quantum yield
(1.6%).7 On the other hand, the room temperature emission
quantum efficiency of [Ru(N∧N∧N)2]2þ complexes could be
Phosphorescent platinum complexes have recently attracted
a great deal of attention because of their potential applications
in various areas from organic electronics1 to biological applica-
tions.2 A key factor in the development of phosphorescent
materials is the emission quantum efficiency. Geometrical and
electronic optimizations have been performed to enhance the
phosphorescence of platinum complexes.3 For example, the use
of cyclometalating ligands can induce larger d orbital splitting
and reduce the possibility of nonradiative d-d transition as
*Author to whom correspondence should be addressed. E-mail: huos@
ecu.edu.
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Published on Web 09/02/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 19, 2010 8923
Chart 1. Tridentate C∧N*N Platinum Complexes and the Quantum Yield in Dichloromethane Compared to the C∧N∧N Complexes
Scheme 1. Synthesis of Tridentate Cyclometalated Platinum(II) Complexesa
a Reagents and conditions: (a) Pd(dba)2 (2%), DPPF (2%), NaOtBu (1.2 equiv), toluene, reflux. (b) Pd(OAc)2 (2%), PPh3 (8%), DME, 2 M K2CO3
(aq), reflux. (c) K2PtCl4 (1 equiv), AcOH, reflux. (d) alkyne, CuI, triethylamine, dichloromethane, room temperature.
improved (up to 7%) by introducing a fused six-six-membered
metallacycle.8
fused five-six-five-membered metallacycle, where an “X∧Y”
(X, Y = C or N) denotes a bidentate coordination to the plati-
num to form a five-membered metallacycle and “X*Y” denotes
a coordination to form a six-membered metallacycle.11 Com-
pared with the tridentate C∧N∧N or N∧C∧N platinum com-
plexes, the platinum in the tetradentate complexes displayed a
coordination closer to a square planar geometry, while the
tridentate C∧N∧N or N∧C∧N complexes displayed a planar
but significantly distorted square geometry because of the
strained fused five-five-membered chelation. Herein reported
are a series of highly luminescent tridentate cyclometalated
platinum complexes featuring a fused five-six-membered me-
tallacycle, (C∧N*N)PtL (L = Cl, acetylide).
The phosphorescence efficiency of C∧N∧N type of cyclo-
metalated platinum complexes has been reported to be
generally low6,9 (<10%) with only a few exceptions.10 For
example, the quantum yield of (C∧N∧N)PtCl (Chart 1) was
reported to be 2.5%.6b Replacing the chloride with a phenyl-
acetylide ((C∧N∧N)PtCCPh, CCPh = phenylacetylide) did
not lead to much improvement in quantum yield (4%).9a
Recently, we have reported highly phosphorescent tetraden-
tate C∧N*N∧C and N∧C*C∧N platinum complexes with a
€
€
(8) (a) Abrahamsson, M.; Jager, M.; Kumar, R. J.; Osterman, T.;
€
Eriksson, L.; Persson, P.; Becker, H.-C.; Johansson, O.; Hammarstrom, L.
Results and Discussion
€
J. Am. Chem. Soc. 2006, 128, 12616–12617. (b) Abrahamsson, M.; Jager, M.;
Kumar, R. J.; Osterman, T.; Persson, P.; Becker, H.-C.; Johansson, O.;
The newly designed C∧N*N complexes 1-5 are shown in
Chart 1. All complexes are strongly emissive in the solid state.
Compounds 2, 3, and 5 are highly emissive in solution at
room temperature, and the quantum yields are compared
with that of the C∧N∧N platinum complex as shown in
€
€
Hammarstrom, L. J. Am. Chem. Soc. 2008, 130, 15533–15542.
(9) (a) 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, 4958–4971. (b) Lanoe, P.-H.; Fillaut,
€
J.-L.; Toupet, L.; Williams, J. A. G.; Bozec, H. L.; Guerchais, V. Chem. Commun.
2008, 4333–4335. (c) Schneider, J.; Du, P.; Wang, X.; Brennessel, W. W.;
Eisenberg, R. Inorg. Chem. 2009, 48, 1498–1506. (d) Schneider, J.; Du, P.;
Jarosz, P.; Lazarides, T.; Wang, X.; Brennessel, W. W.; Eisenberg, R. Inorg.
Chem. 2009, 48, 4306–4316.
(11) Vezzu, D. A. K.; Deaton, J. C.; Jones, J. S.; Bartolotti, L.; Harris,
C. F.; Marchetti, A. P.; Kondakova, M.; Pike, R. D.; Huo, S. Inorg. Chem.
2010, 49, 5107–5119.
(10) Kui, S. C.; Sham, I. H. T.; Cheung, C. C. C.; Ma, C.-W.; Yan, B.;
Zhu, N.; Che, C.-M.; Fu, W.-F. Chem.;Eur. J. 2007, 13, 417–435.

8924 Inorganic Chemistry, Vol. 49, No. 19, 2010
Ravindranathan et al.
Figure 1. ORTEP drawing of 1 and 2. Ellipsoids are at the 50% probability level and hydrogen atoms were omitted for clarity.
alkynes in the presence of CuI and triethylamine.9a All new
Table 1. Selected Bond Lengths (A) and Angles (deg) for 1
and 2
compounds were characterized by 1H and 13C NMR, MS,
and elemental analysis.
complex 1
complex 2
X-ray Crystal Structure. The molecular structure of the
new complexes was confirmed by X-ray crystal structure
determination of compounds 1 and 2 (Figure 1). The
crystal data and structure refinement details are provided
in Supporting Information (Table S1), and selected bond
lengths and angles are listed in Table 1. The four angles
around the platinum range from 82° to 95° with the sum
of 360°, indicating planar coordination geometry. Trans
C(1)-Pt(1)-N(3) angle is 174.82 (6)° and 173.18 (6)° for 1
and 2, respectively, while the trans C-Pt-N angles in
(C∧N∧N)PtCl6a and (C∧N∧N)PtCCPh9a were 160.2(9)° and
160.5(8)°, respectively, and similar angles have been reported
for other platinum complexes with a fused five-five-membered
metallacycle.5b As expected, the complexes with a fused five-
six-membered metallacycle have a platinum coordination
geometry much closer to a square. The Pt(1)-C(1) bond
lengths for 1 (1.986(2) A) and 2 (1.983(2) A) are almost
identical, which are similar to those found in the C∧N∧N
platinum complexes.6,9 The Pt(1)-N(3) bond (2.118(1) A)
that is trans to the Pt(1)-C(1) bond is longer than the
Pt(1)-N(1) bond (1.999(1) A) in 1, which can be attributed
to the trans effect of the carbon donor. The Pt(1)-C(23) bond
in 2 is 1.959 (2) A. The phenyl group of the acetylide in 2 is
twisted from the platinum coordination plane. The N-phenyl
group is nearly perpendicular to the coordination plane.
DFT Calculations. Density functional theory (DFT)
calculations were performed on the compounds 1, 2, 4, and5
to examine the electronic character of the complexes. The
selected molecular orbital energies and the contribution of
platinum to the MOs are provided in Supporting Informa-
tion (Table S2). The orbital density for the HOMOs and
LUMOs is depicted in Figure 2. The HOMO of 1is localized
mainly on the platinum metal (54%) and the rest on the
chlorine (10%) and the phenylpyridine group, while the
HOMO of 4 is localized mainly on the platinum (54%) and
chlorine atoms (17%). The contribution from the fluorine-
substituted phenyl ring is negligible. On the other hand, the
HOMOs of the acetylide complexes 2 and 5 are localized on
the platinum (26-27%) and the triple bond (34%). The
LUMOs of all the compounds are mainly localized on the
phenylpyridine ring and more concentrated in the pyridine
ring (60-63%).
Pt1-C1
Pt1-N1
Pt1-N3
Pt1-Cl1
N2-C11
N2-C12
N2-C17
C1-Pt1-N1
C1-Pt1-N3
N1-Pt1-N3
C1-Pt1-Cl1
N1-Pt1-Cl1
N3-Pt1-Cl1
1.9856(16)
1.9995(14)
2.1181(14)
2.3029(4)
1.396(2)
1.413(2)
1.459(2)
82.80(6)
174.82(6)
92.12(5)
91.44(5)
174.22(4)
93.65(4)
Pt1-C1
Pt1-N1
Pt1-N3
Pt1-C23
N2-C11
N2-C12
N2-C17
C23-Pt1-C1
C23-Pt1-N1
C1-Pt1-N1
C23-Pt1-N3
C1-Pt1-N3
N1-Pt1-N3
1.9834(19)
2.0397(14)
2.1050(16)
1.9594(18)
1.405(2)
1.409(2)
1.461(2)
91.62(8)
174.08(7)
82.46(7)
95.02(7)
173.18(6)
90.89(6)
Chart 1. As shown in Chart 1, a geometrical change from the
fused five-five-membered to five-six-membered metallacycle
has profound effects on the phosphorescence efficiency.
It should be mentioned that a terpyridine-like ligand involving
a five-six-membered chelation has been used to make lumi-
nescent platinum complexes, although the quantum yields
remained low (0.2% in dichloromethane).12 The low quan-
tum yield might be attributed to the lack of a strong ligand
field effect induced by the cyclometalating ligand and the
acetylide present in 2, 3, and 5.
Synthesis. The synthesis of the complexes is shown in
Scheme 1. The C∧N*N ligands L1 and L2 were prepared
by three steps of Pd-catalyzed cross-coupling reactions. The
C-N coupling13 of aniline with 2-bromopyridine produced
N-phenylpyridin-2-amine, which cross coupled with 2,6-dib-
romopyridine under the same reaction conditions to give the
bromide intermediate, 6-bromo-N-phenyl-N-(pyridin-2-yl)-
pyridin-2-amine. The tridentate cyclometalating ligands L1
and L2 were then prepared in good yields by the Pd-cata-
lyzed C-C cross coupling14 of the bromide intermediate
with phenylboronic acid and 2,4-difluorophenylboronic
acid, respectively. The reaction of the ligands L1 and L2
with K2PtCl4 in acetic acid gave the desired cyclometalated
platinum complexes 1 and 4 in high yields. The acetylide
complexes 2, 3, and 5 were prepared by treating 1 and 4 with
(12) Hu, Y.-Z.; Wilson, M. H.; Zong, R.; Bonnefous, C.; McMillin, D. R.;
Thummel, R. P. Dalton Trans. 2005, 354–358.
(13) Yang, J.-S.; Lin, Y.-H.; Yang, C.-S. Org. Lett. 2002, 4, 777–780.
(14) Lohse, O.; Thevenin, P.; Waldvogel, E. Synlett 1999, 45–48.

Article
Inorganic Chemistry, Vol. 49, No. 19, 2010 8925
Figure 2. Calculated orbital density for the HOMOs (top) and LUMOs (bottom) of 1, 2, 4, and 5.
Table 2. Photophysical Data for Complexes 1-5
298 Ka
77 Kb
complex
λ
abs/nma (ε ꢀ 103 M-1 cm-1
)
λem/nm
τ/μsd
je
λem/nm
τ/μs
solidc λem/nm (τ/μs)
1
2
3
4
5
330 (18.8), 345 (18.6), 372 (9.42)
332 (14.4), 347 (13.8), 378 (8.87)
333 (17.9), 346 (17.4), 379 (9.07)
324 (12.7), 343 (12.7), 369 (7.56)
327 (14.7), 345 (13.3), 373 (9.47)
491, 529, 561
488, 526, 558
489, 527, 559
475, 512, 542
473, 508, 538
30
23
21.3
35.8
27.4
506 (12.2), 537 (12.7), 570 (12.8)
492 (10.7), 527 (10.9), 562 (10.9)
501 (10.3), 525 (10.6), 560 (10.0)
485 (9.7), 539 (15.4), 581 (16.4)
482 (9.8), 531 (16.0), 568 (16.2)
495, 531
495, 532
9.2
9.7
56
47
480, 514
3.3
22
a Measurements were carried out in a solution of dichloromethane. b Measurements were carried out in a solution of 2-methyltetrahydrofuran. c Solid
state studies were measured with pure powder samples. d Measured in deoxygenated dichloromethane. e Quantum yields were measured in deoxygenated
dichloromethane with a deoxygenated solution of (N∧C∧N)PtCl5b in dichloromethane (j = 60) as the reference.
Figure 3. Absorption spectra of 1-5 in dichloromethane.
Figure 4. Normalized emission spectra of complex 1-5 in 2-methyl-
tetrahydrofuran (77 K). λex = 350 nm.
1
Photophysical Properties. Photophysical data are sum-
marized in Table 2. The absorption spectra of 1-5 are
shown in Figure 3. The emission spectra of 1-5 in frozen
glass are shown in Figure 4 and those of 2, 3, and 5 in
dichloromethane solution in Figure 5. For comparison, the
absorption, fluorescence emission, and phosphorescence
emission spectra of the ligands L1 and L2 were recorded
and provided in Supporting Information (Figure S1). The
assignment of absorption spectra of the complexes can be
made by analogy with other cyclometalated platinum com-
plexes. High energy bands with stronger absorbance can be
assigned as ligand based π-π* transitions, while the low
energy absorptions that were not presented in the absorption
spectra of the corresponding ligands can be assigned as
charge transfer bands. The lowest charge transfer bands
may be assigned as mixed MLCT/LLCT/ILCT (1) or
MLCT/LLCT transitions (2-5) based on the HOMO and
LUMO characters of the complexes. The solvent effect was
found to be moderate on the lowest absorption band for 1
and 2 (Supporting Information, Tables S3 and S4). The
fluorine substituent caused blue shift of electronic spectra as
observed in other complexes.4d,11

8926 Inorganic Chemistry, Vol. 49, No. 19, 2010
Ravindranathan et al.
remained largely unchanged as the concentration of the
complexes increased from 1 ꢀ 10-6 M to 1 ꢀ 10-4 M. The
perpendicular N-phenyl ring and the twisted acetylide
groups may prevent any Pt-Pt or π-π interactions that
are considered to be the main cause of self-quenching in
square planar platinum complexes, which might be another
factor contributing to the high quantum efficiency. Solvent
effect on the emission of 1 and 2 was small (Supporting
Information, Table S3 and S4). The emission spectra of 2, 3,
and 5 displayed vibrational side bands. The lifetimes of the
emission of 2 and 3 are nearly the same (9.2 and 9.7 μs,
respectively), while the emission of 5 has a much shorter
lifetime of 3.3 μs. The radiative decay rates constant esti-
mated from the quantum yield and lifetimes for 2, 3, and 5
were 6.1, 4.9, and 6.7 ꢀ 104 s-1, respectively. The sum of
nonradiative decay rate constants for 2, 3, and 5 were
estimated to be 4.8, 5.5, and 23.6 ꢀ 104 s-1, respectively.
The lower quantum efficiency observed for 5 can be attrib-
uted to the faster nonradiative decay (4 times faster than
those for 2 and 3), although such nonradiative pathways are
not clear. In contrast, the nonradiative decay reported for the
Figure 5. Emission spectra of 2, 3, and 5 in dichloromethane at room
temperature. λex = 350 nm.
All new complexes are emissive in frozen glass. The spectra
in frozen glass are highly structured (Figure 4) and display a
vibronic progression of 1450-1520 cm-1 that is the typical
range of aromatic ring stretching. The emissions in frozen
glass of 2-MeTHF have lifetimes in the range of 21.3-35.8 μs.
These features suggest that the emissions are based on ligand-
centered triplet transitions. All three complexes 1-3 derived
from the ligand L1 display identical band shape and have
nearly the same energy, indicating that the emissions may
originate from similar ligand-centered excited states. Com-
pared with the phosphorescence of ligand L1 (see Support-
ing Information, Figure S1), the emissions of the complexes
are red-shifted, which has also been observed in other
cyclometalated platinum complexes.4 The complexes 4
and 5 exhibit a spectroscopic characteristic similar to those
of 1-3 except for the blue shift of the emission energy.
Complexes 1-5 also emit intensely in solid state. The emis-
sion spectra obtained with pure powder sample are pro-
vided in Supporting Information (Figures S2 and S3).
In dichloromethane at room temperature, the emis-
sions of compounds 1 and 4 were very weak and the
quantum yields were not estimated. However, when the
chloride was replaced with an acetylide, the resulted
complexes 2, 3, and 5 emitted intensely in dichloro-
methane (Figure 5). This may be due to the stronger
acetylide ligands, which further raise the energy of non-
radiative d-d transitions. Distortion of the triplet states
may not be responsible for the difference in emission
efficiency between 1 and 2, since the compound 2 has
more distorted triplet geometry compared to the ground
state (Supporting Information, Figure S4). Given the fact
that at 77 K both the chloride and acetylide complexes
emitted intensely and displayed essentially same emission
spectra, the lack of room temperature emission of the
chloride complexes in solution might be due to thermally
accessible nonemissive metal-centered transitions.
(C∧N∧N)PtCCPh (2.5 ꢀ 106 s-1 9a
) is much faster. Further
studies may be required for unveiling other factors that affect
the phosphorescence efficiency.15 The emission features of 2,
3, and 5 including lifetimes and the spectral band shape
suggest a ligand-centered emission with MLCT admixture in
dichloromethane at ambient temperature.
In summary, we have demonstrated that, by tuning
coordination geometry of tridentate cyclometalated pla-
tinum complexes from a fused five-five-membered me-
tallacycle to the five-six-membered metallacycle that is
closer to ideal square planar geometry, a substantial
increase in emission quantum efficiency could be
achieved. Since other similar geometrical variations such
as (N∧N*C)PtL, (N∧C*N)PtL, and (C∧N*C)PtL can be
envisaged for tridentate complexes and the ligand design
and synthesis exemplified in this report is representative
of simplicity and modularity (Scheme 1), this study
should open a new avenue to a large number of highly
luminescent platinum complexes potentially useful for
various applications as mentioned before. Currently, we
are working on the N∧N*C and N∧C*N platinum com-
plexes and the results will be reported in due course.
Experimental Section
Synthesis. All reactions involving moisture- and/or oxygen-
sensitive organometallic complexes were carried out under
nitrogen atmosphere and anhydrous conditions. Tetrahydro-
furan (THF) and 2-methyltetrahydrofuran were distilled from
sodium and benzophenone under nitrogen before use. All other
anhydrous solvents were purchased from Aldrich Chemical Co.
and were used as received. All other reagents were purchased
from chemical companies and were used as received. Mass
spectra were measured on a Waters spectrometer. NMR spectra
were measured on a Mercury VX 300 or a Varian 500 spectro-
meter. Spectra were taken in CDCl3 or CD2Cl2 using tetra-
The quantum yield of 56% measured for compound 2
represents a significant improvement when compared
with 4% of (C∧N∧N)PtCCPh. Such a jump in quantum
yield is quite remarkable, since the geometric change from
C∧N∧N to C∧N*N does not seem to increase the rigidity
of the molecule. Possibly, the C∧N*N ligand exerted a
stronger field as the square geometry was restored in the
C∧N*N platinum complexes. Concentration quenching
was not observed since the lifetimes of the emission
1
methylsilane as standard for H NMR chemical shifts and the
solvent peak (CDCl3, 77.0 ppm; CD2Cl2, 53.8 ppm) as standard
for 13C NMR chemical shifts. Elemental analyses were per-
formed at Atlantic Microlab, Inc. in Norcross, Georgia.
(15) (a) Tong, G.-M.; Che, C.-M. Chem.;Eur. J. 2009, 15, 7225–7237. (b)
Ma, B.; Djurovich, P. I.; Thompson, M. E. Coord. Chem. Rev. 2005, 249, 1501–
1510.

Article
Preparation of N-Phenylpyridin-2-amine16. General Proce-
Inorganic Chemistry, Vol. 49, No. 19, 2010 8927
Hz, 1C), 104.05 (t, JC-F = 52.1 Hz, 25 Hz, 1C). MS: m/z calcd
for C22H15N3 F2: 359.12. Found 360.14 (M þ Hþ). Anal. Calcd
for C22H15N3 F2: C, 73.53; H, 4.21; N, 11.69. Found C, 73.66; H,
4.11; N, 11.50.
dure. To a 250 mL dry, nitrogen flushed flask were charged
2-bromopyridine (3.16 g, 20 mmol), sodium tert-butoxide
(2.31 g, 24 mmol), Pd(dba)2 (0.46 g, 0.8 mmol), DPPF (0.44 g,
0.8 mmol), aniline (2.73 mL, 30 mmol), and anhydrous toluene
(75 mL). The mixture was refluxed for 24 h. After cooling to
room temperature, 50 mL of ethyl acetate was added and stirred
for a while. The precipitate formed was filtered and the filtrate
was evaporated. The crude mixture was purified by chromato-
graphy on silica gel with pure dichloromethane first to remove
excess aniline and then a mixture of dichloromethane and ethyl
acetate (v/v = 10:1) and recrystallization from hexane and
dichloromethane to provide white crystals. 2.16 g, yield 89%.
6-Bromo-N-phenyl-N-(pyridin-2-yl)pyridin-2-amine. This com-
pound was prepared from N-phenylpyridin-2-amine and 2,6-dibro-
mopyridine following the general procedure described above and
purified by chromatography on silica gel with a mixture of hexane
and ethyl acetate (v/v = 5:1), light brown solid, 1.16 g, 89%.1H
NMR (500 MHz, CDCl3) δ 8.31 (ddd, J = 5 Hz, 2 Hz, 1 Hz, 1H),
7.58 (dt, J= 7.5 Hz, 2 Hz, 1H), 7.38 (t, J= 7.5 Hz, 2H), 7.33 (t, J=
7.5 Hz, 1H), 7.27-7.23 (m, 1H), 7.22-7.18 (m, 2H), 7.11 (d, J =
7.5 Hz, 1H), 7.05 (d, J= 8 Hz, 1H), 6.96-6.94 (m, 1H), 6.79 (d, J=
8.5 Hz, 1H); 13C NMR (75 MHz, CDCl3) δ 157.86, 157.35, 148.29,
144.19, 139.70, 139.31, 137.47, 129.73, 127.56, 126.14, 121.19,
118.76, 117.53, 114.08. MS: m/z calcd for C16H12N3Br: 325.02.
Found 326.07 (MþHþ). Anal. Calcd for C16H12N3Br: C, 58.91; H,
3.71; N,12.88. Found C, 59.42; H, 3.72; N, 12.73.
Preparation of Platinum Complex 1. To a 100 mL dry,
nitrogen flushed flask was charged ligand L1 (323 mg, 1 mmol),
K2PtCl4 (415.5 mg, 1 mmol), and glacial acetic acid (50 mL). The
mixture was degassed and refluxed under nitrogen for 21 h.
After cooling to room temperature, the yellow and orange
precipitates were collected by filtration and washed with water,
and dried in air. The crude material was purified by flash
chromatography on silica gel with dichloromethane. The com-
pound was further recrystallized from dichloromethane and
hexanes to give yellow solid, 490 mg, yield 65%. 1H NMR
(500 MHz, CD2Cl2) δ 9.90 (dd, J = 6 Hz, 2 Hz, 1H), 8.13 (dd,
J = 8 Hz, 1.5 Hz, 3JPt-H = 16 Hz, 1H), 7.70-7.65 (m, 4H), 7.60
(t, J = 7.5 Hz, 1H), 7.52 (d, J = 7 Hz, 1H), 7.48-7.43 (m, 3H),
7.22 (dt, J = 7.5 Hz, 1.5 Hz, 1H), 7.11 (t, J = 7.5 Hz, 1H), 7.04 (t,
J = 6 Hz, 1H), 6.80 (d, J = 8.5 Hz, 1H), 6.58 (d, J = 8.5 Hz, 1H);
13C NMR (75 MHz, CD2Cl2) δ 166.43, 150.43, 150.39, 149.12,
144.94, 142.79, 141.76, 138.08, 137.64, 135.55, 131.59, 130.75,
139.91, 129.91, 129.26, 123.92, 123.21, 118.58, 117.40, 115.56,
113.04. MS: m/z calcd for C24H19N4Pt (acetonitrile complex):
558.13; found 558.13; Anal. Calcd for C22H16N3ClPt: C, 47.79;
H, 2.92; N, 7.60. Found C, 47.64; H, 2.92; N, 7.63.
Preparation of Complex 2. To a 50 mL dry, nitrogen flushed
flask was charged complex 1 (100 mg, 0.18 mmol), phenylace-
tylene (59.5 μL, 0.54 mmol), Et3N (1.65 mL), CuI (2.8 mg), and
dichloromethane (17 mL). The mixture was stirred at room
temperature in the absence of light for 12 h. The reaction
mixture was then evaporated and the crude product was purified
by flash chromatography on silica gel with dichloromethane as
eluting solvent to provide yellow crystals, 650 mg, 65%. 1H
NMR (500 MHz, CD2Cl2) δ 10.23 (dd, J = 6 Hz, 2 Hz, 1H), 8.43
(dd, J = 8 Hz, 1.5 Hz, 3JPt-H = 26 Hz, 1H), 7.78-7.65 (m, 4H),
7.63-7.60 (m, 2H), 7.53-7.51 (m, 3H), 7.45 (d, J = 7.5 Hz, 2H),
7.30 (t, J = 7.5 Hz, 2H), 7.24-7.17 (m, 2H), 7.11 (dt, J = 7.5 Hz,
1 Hz, 1H), 6.98 (dt, J = 7 Hz, 1.5 Hz, 1H), 6.75 (d, J = 8.5 Hz,
1H), 6.56 (d, J = 9 Hz, 1H); 13C NMR (75 MHz, CD2Cl2) δ
165.14, 153.62, 150.97, 150.72, 145.06, 143.34, 142.77, 139.15,
138.20, 138.01, 134.12, 131.95, 131.80, 130.98, 130.11, 129.52,
128.36, 125.48, 123.71, 123.68, 118.83, 118.03, 115.08, 112.96,
105.40, 101.97. MS: m/z calcd for C30H21N3Pt: 618.14; found
619.17 (M þ Hþ); Anal. Calcd for C30H21N3Pt: C, 58.25; H,
3.42; N, 6.79. Found C, 57.64; H, 3.34; N, 6.82.
Preparation of Ligand L1. Representative Procedure. To a
25 mL dry, nitrogen flushed flask were charged 6-bromo-N-
phenyl-N-(pyridin-2-yl)pyridin-2-amine (652 mg, 2 mmol), phe-
nylboronic acid (292 mg, 2.4 mmol), triphenylphosphine (42 mg,
0.16 mmol), and DME (7 mL). A homogeneous solution was
formed. To this solution was added 2 M K2CO3 (2.5 mL, 5
mmol). After the mixture was purged with nitrogen, Pd(OAc)2
(8.9 mg, 0.04 mmol) was added. The mixture was refluxed for 6 h
then cooled to room temperature. After cooling to room tem-
perature, 15 mL of ethyl acetate was added and stirred for a
while. The precipitate formed was filtered and the filtrate was
transferred into a separating funnel and the organic layer was
separated and retained. The aqueous phase was extracted with
ethyl acetate (3 ꢀ 25 mL). The combined organic layers were
washed with water (50 mL) and brine (50 mL) and dried over
MgSO4. Filtration and evaporation produced dark brown oil,
which was purified by chromatography on silica gel with a
mixture of hexane and ethyl acetate (v/v = 3:1) and recrystalli-
zation from hexane and dichloromethane to provide white
Complex 3. This compound was prepared following the
general procedure for complex 2, yellow crystals, 31% yield.
1H NMR (500 MHz, CD2Cl2) δ 10.32 (dd, J = 6 Hz, 2 Hz, 1H),
8.41 (dd, J = 7.5 Hz, 1.5 Hz, 3JPt-H = 27 Hz, 1H), 7.69-7.57
(m, 6H), 7.48 (d, J = 7.5 Hz, 1H), 7.43-7.41 (m, 2H), 7.18 (dt,
J = 7.5 Hz, 1.5 Hz, 1H), 7.07 (dt, J = 7.5 Hz, 1.5 Hz, 1H), 6.93
(dt, J = 7 Hz, 1.5 Hz, 1H), 6.71 (d, J = 9 Hz, 1H), 6.52 (d, J = 8
Hz, 1H), 2.6 (t, J = 7 Hz, 2H), 1.68-1.64 (m, 2H), 1.59-1.56 (m,
2H), 1.39-1.36 (m, 4H), 0.93 (t, J = 7 Hz, 3H); 13C NMR (75
MHz, CD2Cl2) δ 165.16, 153.70, 150.86, 150.59, 145.05, 143.38,
143.26, 139.41, 137.77, 137.71, 131.88, 130.95, 129.99, 129.71,
123.53, 123.30, 118.47, 117.85, 114.95, 112.82, 100.43, 89.04,
32.15, 31.39, 29.41, 23.19, 22.12, 14.35. MS: m/z calcd for
C30H29N3Pt: 626.20; found 627.24 (M þ H). Anal. Calcd for
C30H29N3Pt: C, 57.50; H, 4.66; N, 6.71. Found C, 57.42; H, 4.59;
N, 6.74.
1
crystals, 340 mg, yield 54%. H NMR (500 MHz, CDCl3) δ
8.34 (dd, J = 5 Hz, 2 Hz, 1H), 7.84-7.82 (m, 2H), 7.61-7.55 (m,
2H), 7.41-7.32 (m, 5H), 7.27-7.22 (m, 4H), 7.18 (d, J = 8.5 Hz,
1H), 6.94-6.92 (m, 1H), 6.87 (d, J = 8.5 Hz, 1H); 13C NMR (75
MHz, CDCl3) δ 158.04, 157.61, 155.49, 148.32, 144.91, 139.04,
138.15, 137.09, 129.51, 128.73, 128.50, 127.59, 126.64, 125.57,
117.99, 117.39, 114.46, 114.06. MS: m/z calcd for C22H17N3:
323.14. Found 324.17 (MþHþ). Anal. Calcd for C22H17N3: C,
81.71; H, 5.30; N, 12.99. Found C, 81.91; H, 5.32; N, 13.11.
Ligand L2. This compound was prepared following the
1
representative procedure, yellow solid, 0.713 g, yield 98%. H
NMR (500 MHz, CDCl3) δ 8.33 (dd, J = 5 Hz, 2 Hz, 1H),
7.75-7.71 (m, 1H), 7.59-7.53 (m, 2H), 7.43-7.36 (m, 3H),
7.25-7.20 (m, 3H), 7.09 (d, J = 8 Hz, 1H), 6.93-6.90 (m, 2H),
6.84-6.80 (m, 2H); 13C NMR (75 MHz, CDCl3) δ 162.87 (dd,
JC-F = 249.4 Hz, 12 Hz, 1C), 160.71 (dd, JC-F = 251.4 Hz, 12
Complex 4. Following the representative procedure for com-
plex 1, this complex was prepared in 49% yield. 1H NMR (500
MHz, CD2Cl2) δ 9.84 (dd, J = 6 Hz, 1.5 Hz, 1H), 7.84-7.82 (m,
1H), 7.75 (dd, J = 9 Hz, 2.5 Hz, 1H), 7.72-7.66 (m, 4H), 7.61 (t,
J = 8 Hz, 1H), 7.48 (d, J = 7.5 Hz, 2H), 7.05 (dt, J = 7 Hz, 1 Hz,
1H), 6.85 (d, J = 7.5 Hz, 1H), 6.69-6.61 (m, 2H); 13C NMR (75
MHz, CD2Cl2) δ 150.53, 149.58, 142.83, 138.70, 138.51, 134.13,
131.84, 130.95, 130.25, 118.97, 117.60, 117.40, 117.02, 116.74,
Hz, 1C), 157.94, 157.53, 150.35 (d, JC-F = 3 Hz, 1C), 148.29,
144.75, 137.99, 137.19, 132.03 (dd, JC-F = 9.5 Hz, 4.5 Hz, 1C),
129.49, 127.44, 125.58, 123.31 (dd, JC-F = 11 Hz, 3.5 Hz, 1C),
118.13, 117.95, 117.27, 114.90, 11.56 (dd, JC-F = 20.5 Hz, 3.5
(16) Katritzky, A. R.; Huang, T.-B.; Voronkov, M. V. J. Org. Chem.
2001, 66, 1043–1045.

8928 Inorganic Chemistry, Vol. 49, No. 19, 2010
Ravindranathan et al.
115.94, 99.91 (t, JC-F = 54 Hz, 27 Hz, 1C) (only partial carbon
signals were observed because of very poor solubility). MS: m/z
calcd for C24H17F2N4Pt (acetonitrile complex): 594.11; found
594.13; Anal. Calcd for C22H14N3F2ClPt: C, 44.34; H, 2.40; N,
7.14. Found C, 44.19; H, 2.26; N, 6.95.
(Q.Y. 60%)5b was used as a reference. The optical density of both
sample and reference solutions was maintained below 0.1 AU at
and above the excitation wavelength. Emission spectra in a frozen
glass were recorded in 2-MeTHF at 77 K. Solid-state emission
spectra were recorded using a powder sample holder. Phosphores-
cence lifetime measurements were performed on the same fluori-
meter equipped with variable high rep rate pulsed xenon source for
excitation and were limited to lifetimes >0.4 μs. Delayed phos-
phorescent emission spectra of the ligands were measured using a
gated analog detector with R928 red extended PMT in 2-MeTHF
at 77 K. The radiative and nonradiative rate constants were
calculated from the luminescent quantum yield (φ) and lifetime
(τ) of the phosphorescent complex according to eqs 1 and 2,
respectively,
Complex 5. This compound was prepared following the
general procedure for complex 2, yellow crystals, 82% yield.
1H NMR (500 MHz, CD2Cl2) δ 10.19 (dd, J = 6 Hz, 2 Hz, 1H),
8.07 (dd, J = 9.5 Hz, 2.5 Hz, 3JPt-H = 30 Hz, 1H), 7.89 (dd, J =
8 Hz, 3 Hz, 1H), 7.72-7.66 (m, 4H), 7.62 (t, J = 7 Hz, 1H), 7.51
(d, J = 8 Hz, 2H), 7.47 (d, J = 6.5 Hz, 2H), 7.30 (t, J = 7.5 Hz,
2H), 7.20 (t, J = 7.5 Hz, 1H), 6.99 (dt, J = 7 Hz, 1 Hz, 1H), 6.79
(d, J = 8.5 Hz, 1H), 6.64-6.59 (m, 2H); 13C NMR (75 MHz,
CD2Cl2) δ 162.98 (dd, JC-F = 248.4 Hz, 12.5 Hz, 1C), 161.33 (d,
JC-F = 7.5 Hz, 1C), 160.24 (dd, JC-F = 257.6 Hz, 12.5 Hz, 1C),
kr ¼ φτ- 1
ð1Þ
153.60, 150.75, 150.53, 147.12 (d, JC-F = 7.5 Hz, 1C), 143.23,
138.61, 138.19, 131.92, 131.75, 131.01, 130.14, 129.15, 128.42,
125.75, 120.86, 120.65, 118.85, 118.10, 116.88, 116.58, 115.37,
105.01, 101.59, 99.52 (t, JC-F = 54.6 Hz, 27.5 Hz, 1C). MS: m/z
calcd for C30H19N3F2Pt: 654.12; found 655.16 (M þ Hþ); Anal.
Calcd for C30H19N3F2Pt: C, 55.05; H, 2.93; N, 6.42. Found C,
54.78; H, 2.78; N, 6.23.
knr ¼ krðφ- 1 - 1Þ
ð2Þ
where kr and knr are the radiative and nonradiative rate constant,
respectively.
DFT Calculations. The electronic structure calculations were
X-ray Crystallography. Data Collection and Processing. The
crystals were prepared using a solvent diffusion method. The
sample was mounted on a nylon loop with a small amount of
Paratone N oil. All X-ray measurements were made on a
Bruker-Nonius Kappa Axis X8 Apex2 diffractometer at a
temperature of 110 K. The data collection strategy was a
number of ω and j scans. The frame integration was performed
using SAINT.24 The resulting raw data was scaled and absorp-
tion was corrected using a multiscan averaging of symmetry
equivalent data using SADABS.25 The structure was solved by
direct methods using the XS program.26 All non-hydrogen
atoms were obtained from the initial solution. The hydrogen
atoms were introduced at idealized positions and were allowed
to ride on the parent atom. The structural model was fit to the
data using full matrix least-squares based on F2. The calculated
structure factors included corrections for anomalous dispersion
from the usual tabulation. The structure was refined using the
XL program from SHELXTL,27 and graphic plots were pro-
duced using the NRCVAX crystallographic program suite.
performed on complexes 1, 2, 4, and 5 using DMol3,17
a
numerical based density functional computer program.18,19
The orbitals for each atom in the molecule are represented by
a double numerical plus double polarization basis set that was
developed in our laboratory. For platinum, there were both f
and g polarization functions. For all calculations, we used
the Becke-Tsuneda-Hirao exchange-correlation energy-density
functional,20,21 a 20 Bohr cutoff and a fine integration grid. The
SCF convergence was set to 1.0 ꢀ 10-8 and the geometry was
considered converged when the gradient was less than 5.0 ꢀ
10-4 hartree/Bohr. Relativity is introduced into the calculations
with the scattering theoretic approach for all electron scalar
relativistic corrections introduced by Delley.22 Since the experi-
ments were performed in dichloromethane, we employed the
solvation model COSMO23 (with a dielectric constant of 9.08) to
simulate the possible effect that solvent might have on the
present calculations.
Photophysical Experiments. Absorption spectra were re-
corded on a Shimadzu 2445 UV/vis spectrophotometer, using
1 cm path-length quartz cuvettes. The steady-state emission
spectra were measured using a PTI QM-4CW system and an
excitation source with the spectral bandpass of 2 nm was used.
Quantum yields were measured using comparative method at room
temperature in a dichloromethane solution. The solution was
deoxygenated by purging with argon gas for 20 min. A long necked
1 cm quartz cuvette was used for measurements. The cuvette was
cooled with a dry ice-acetone bath to prevent solvent loss. A
deoxygenated solution of (N∧C∧N)PtCl in dichloromethane
Acknowledgment. Financial support from ECU and the
Research Corporation for Science Advancement is gratefully
acknowledged. D.R. is the recipient of the undergraduate
research award of ECU. P.D.B. thanks the Department of
Chemistry of North Carolina State University and the State of
North Carolina for funding the purchase of the Apex2 diffrac-
tometer.
Supporting Information Available: Crystallographic data in
CIF format for 1 and 2. DFT calculation results. Solvent effects.
Electronic spectra of the ligands. Solid emission spectra and
proton NMR spectra of complexes 1-5. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
(17) DMol3; Accelrys Inc.; San Diego, CA.
(18) (a) Delley, B. J. Chem. Phys. 1990, 92, 508–517. (b) Delley, B. J. Chem.
Phys. 2000, 113, 7756–7764.
(19) Delley, B. DMol a Standard Tool for Density Functional Calcula-
tions: Review and Advances. In Modern Density Functional Theory: A Tool
for Chemistry; Seminario, J. M., Politzer, P., Eds.; Elsevier Science Publishing:
Amsterdam, 1995; Vol. 2.
(20) Becke, A. D. J. Chem. Phys. 1988, 88, 2547–2553.
(21) Tsuneda, T; Suzumura, T; Hirao, K. J. Chem. Phys. 1999, 110,
10664–10678.
(24) Bruker-Nonius, SAINT, version 2009.9; Bruker-Nonius: Madison, WI
53711, USA, 2009.
(25) Bruker-Nonius, SADABS, version 2009.9; Bruker-Nonius: Madison,
WI 53711, USA, 2009.
(26) Bruker-AXS, XS, version 2009.9; Bruker-AXS: Madison, WI 53711,
(22) Delley, B. Int. J. Quantum Chem. 1998, 69, 423–433.
(23) Klamt, A.; Schuurmann, G. J. Chem. Soc. Perkin Trans. 2 1993, 799–
805.
USA, 2009.
(27) Bruker-AXS, XL, version 2009.9; Bruker-AXS, Madison, WI 53711,
USA, 2009.