Blue luminescent 2-(2′-pyridyl)benzimidazole derivative ligands and their orange luminescent mononuclear and polynuclear organoplatinum(II) complexes

Article abstract of DOI:10.1021/ic0487530

Five new 2-(2′-pyridyl)benzimidazole derivative ligands, 1,4-bis[2-(2′-pyridyl)benzimidazolyl]benzene (1,4-bmb), 4,4′-bis[2-(2′-pyridyl)benzimidazolyl]biphenyl (bmbp), 1-bromo-4-[2-(2′-pyridyl)benzimidazolyl]benzene (Brmb), 1,3-bis-[2-(2′-pyridyl)benzimidazolyl]benzene (1,3-bmb), and 1,3,5-tris[2-(2′-pyridyl)benzimidazolyl]benzene (tmb), have been synthesized by Ullmann condensation methods. The corresponding mononuclear and polynuclear Pt^II complexes, Pt₂(1,4-bmb)Ph₄ (1), Pt₂(bmbp)Ph₄ (2), Pt(Brmb)Ph₂ (3), Pt ₂(1,3-bmb)Ph₄ (4), and Pt₃(tmb)Ph₆ (5), have been obtained by the reaction of the appropriate ligand with [PtPh₂(SMe₂)]_n. The structures of the free ligands 1,4-bmb, bmbp, and tmb, as well as the complexes 1-3, were determined by single-crystal X-ray diffraction. All ligands display fluorescent emissions in the purple/blue region of the spectrum at ambient temperature and phosphorescent emissions in the blue/green region at 77 K, which are attributable to ligand-centered π → π* transition. No ligand-based emission was observed for the Pt^II complexes 1-5. All Pt^II complexes display orange/red emissions at 77 K in a frozen solution or in the solid state, attributable to metal-to-ligand charge transfers (MLCT). Variable-temperature ¹H NMR experiments establish that complexes 1, 4, and 5 exist in isomeric forms in solution at ambient temperature due to the hindered rotation of the square PtC₂N₂ planes in the complexes.

Full text of DOI:10.1021/ic0487530

Inorg. Chem. 2005, 44, 1332−1343
Blue Luminescent 2-(2′-Pyridyl)benzimidazole Derivative Ligands and
Their Orange Luminescent Mononuclear and Polynuclear
Organoplatinum(II) Complexes
Qin-De Liu, Wen-Li Jia, and Suning Wang*
Department of Chemistry, Queen’s UniVersity, Kingston, Ontario K7L 3N6, Canada
Received September 6, 2004
Five new 2-(2
bis[2-(2 -pyridyl)benzimidazolyl]biphenyl (bmbp), 1-bromo-4-[2-(2
[2-(2 -pyridyl)benzimidazolyl]benzene (1,3-bmb), and 1,3,5-tris[2-(2
′
-pyridyl)benzimidazole derivative ligands, 1,4-bis[2-(2
-pyridyl)benzimidazolyl]benzene (Brmb), 1,3-bis-
-pyridyl)benzimidazolyl]benzene (tmb), have been
′-pyridyl)benzimidazolyl]benzene (1,4-bmb), 4,4′-
′
′
′
′
synthesized by Ullmann condensation methods. The corresponding mononuclear and polynuclear PtII complexes,
Pt2(1,4-bmb)Ph4 (1), Pt2(bmbp)Ph4 (2), Pt(Brmb)Ph2 (3), Pt2(1,3-bmb)Ph4 (4), and Pt3(tmb)Ph6 (5), have been obtained
by the reaction of the appropriate ligand with [PtPh2(SMe2)]n. The structures of the free ligands 1,4-bmb, bmbp,
and tmb, as well as the complexes 1−3, were determined by single-crystal X-ray diffraction. All ligands display
fluorescent emissions in the purple/blue region of the spectrum at ambient temperature and phosphorescent emissions
in the blue/green region at 77 K, which are attributable to ligand-centered π f π* transition. No ligand-based
emission was observed for the PtII complexes 1 5. All PtII complexes display orange/red emissions at 77 K in a
−
frozen solution or in the solid state, attributable to metal-to-ligand charge transfers (MLCT). Variable-temperature
1H NMR experiments establish that complexes 1, 4, and 5 exist in isomeric forms in solution at ambient temperature
due to the hindered rotation of the square PtC2N2 planes in the complexes.
Introduction
film-forming properties.8 Most of previously reported star-
burst molecules are based on 2,2′-diphenylamine or carbazole
derivatives9 due to the excellent hole transport properties of
2,2′-diphenylamino and carbazolyl units. Recent investiga-
tions on phosphorescent emitters for OLEDs have generated
much interest in emitter materials containing heavy-metal
centers.3 PtII and IrIII are among the most widely investigated
Since Tang and VanSlyke developed the first multilayer
thin film devices based on tris(8-hydroxy quinolinato)-
aluminum,1 there has been much research effort on the
investigation of new luminescent organic/organometallic
compounds for use in the improvement of organic light-
emitting devices (OLEDs).2-6 It has been demonstrated that
one of the key factors to obtain a stable OLED is the good
film-forming capability of the deposited layers.7 Starburst
and long linear-shaped organic compounds with large
conjugation systems have been found to often possess
relatively high glass transition temperature (Tg) and very good
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* Author to whom correspondence should be addressed. E-mail: wangs@
chem.queensu.ca.
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1332 Inorganic Chemistry, Vol. 44, No. 5, 2005
10.1021/ic0487530 CCC: $30.25
© 2005 American Chemical Society
Published on Web 01/26/2005

Luminescent Organoplatinum(II) Complexes
metal centers because heavy metal centers with d8 or d6
electronic configurations can either enhance ligand-centered
phosphorescent emission3 or produce metal-to-ligand charge-
transfer (MLCT) emission. As part of our research efforts
to investigate the coordination chemistry of starburst organic
compounds and develop new phosphorescent metal com-
plexes, we reported recently a series of linear and starburst
2,2′-dipyridylamine derivative ligands10 and the luminescent
properties of their PtII complexes.11 Although the free ligands
based on 2,2′-dipyridylamine derivatives were demonstrated
to be useful blue emitters in OLEDs,10b their PtII complexes
were found to only display weak green phosphorescent
emission at 77 K. The lack of phosphorescence by the PtII
complexes at ambient temperature could be caused by the
low rigidity of the six-membered chelate ring of the 2,2′-
dipyridylamio group, which leads to the loss of energy by
thermal vibrational decay.12 To enhance the rigidity of the
chelate, we synthesized a series of 2-(2′-pyridyl)benzimida-
zole derivative ligands. Similar to 2,2′-dipyridylamine, 2-(2′-
pyridyl)benzimidazole also possesses two neutral nitrogen
donors that can chelate to a metal center and an NH site
that can be coupled to a carbon atom of different aryl groups,
forming various interesting ligands. In contrast to the 2,2′-
dipyridylamino group, the 2-(2′-pyridyl)benzimidazolyl group
forms a five-membered chelate ring with a metal center, thus
resulting in a relatively rigid chelate complex. In addition,
the 2-(2′-pyridyl)benzimidazolyl group is more conjugated
than 2,2′-dipyridylamino, which may result in a relatively
low energy emission that is red-shifted, compared to that of
the corresponding 2,2′-dipyridylamino complexes. Herein we
report the results of our investigation on the new 2-(2′-
pyridyl)benzimdazole derivative ligands and their corre-
sponding PtII complexes.
Experimental Section
All starting materials were purchased from Aldrich Chemical
Co. and used without further purification. All solvents used in
syntheses and spectroscopic measurements were distilled over
appropriate drying reagents. 1H NMR and 13C NMR were recorded
on a Bruker Advance 500 MHz spectrometer operated at 500 MHz
for 1H and 125 MHz for 13C. Excitation and emission spectra were
obtained with a Photon Technologies International QuantaMaster
model C-60 spectrometer. Emission lifetime was measured on a
Photon Technologies International phosphorescent lifetime spec-
trometer, Timemaster C-631F equipped with a xenon flash lamp,
and digital emission photon multiplier tube using a band pathway
of 5 nm for excitation and 2 nm for emission. UV-vis spectra
were recorded on a Hewlett-Packard 8452A diode array spectro-
photometer. Cyclic voltammetry was performed using a BAS CV-
50W analyzer with scan rates of 100 mV s-1. The electrolytic cell
used was a conventional three-compartment cell, in which a Pt
working electrode, a Pt auxiliary electrode, and Ag/AgCl reference
electrode were employed. The CV measurements were performed
at room temperature using 0.10 M tetrabutylammonium hexafluo-
rophosphate (TBAP) as the supporting electrolyte. The ferrocenium/
ferrocene couple was used as the internal standard. Elemental
analyses were performed by Canadian Microanalytical Service Ltd.,
Delta, British Columbia, Canada. The syntheses of the ligands 1,4-
bmb, bmbp, Brmb, 1,3-bmb, and tmb were achieved by a modified
Ullmann condensation method reported previously.13 The PtII
starting material [PtPh2(SMe2)]n (n ) 2 or 3) was prepared using
a procedure reported in the literature.14
Syntheses of 1,4-Bis[2-(2′-pyridyl)benzimidazolyl]bezene (1,4-
bmb) and 1-Bromo-4-[2-(2′-pyridyl)benzimidazolyl]benzene
(Brmb). A mixture of 1,4-dibromobenzene (0.50 g, 2.12 mmol),
2-(2′-pyridyl)benzimidazole (0.99 g, 5.0 mmol), CuI (0.081 g, 0.43
mmol), 1,10-phenanthroline (0.16 g, 0.88 mmol), and Cs2CO3 (2.9
g, 8.9 mmol) was suspended in 3.0 mL of DMF. The mixture was
refluxed for 24 h and then cooled to room temperature. The resulting
brown sticky residue was extracted with CH2Cl2 (50 mL × 4), and
the organic extracts were combined, dried over MgSO4, and purified
by column chromatography (1:1 THF/hexane) to obtain the crude
product of 1,4-bmb as light yellow powder. Recrystallization of
the crude product in CHCl3/hexanes afforded 1,4-bmb as colorless
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M.; Fujii, T.; Nishio, Y.; Shibata, K. Chem. Lett. 1993, 905.
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Tao, Y.; D’Iorio, M.; Wang, S. Chem. Mater. 2000, 12, 79. (b) Liu,
Q. D.; M. S. Mudadu; H. Schmider; R. Thummel; Y. Tao; S. Wang.
Organometallics 2002, 21, 4743. (c) Jia, W. L.; Bai, D. R.; McCormick,
T.; Liu, Q. D.; Motala, M.; Wang, R. Y.; Seward, C.; Tao, Y.; Wang
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1
crystals (0.40 g, 35%). Mp: 229-230 °C. H NMR (CD2Cl2, δ,
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2002, 12, 2612. (b) Ogawa, H.; Ohnishi, K.; Shirota, Y. Synth. Met.
1997, 91, 243. (c) Utsumi, H.; Nagahama, D.; Nakano, H.; Shirota,
Y. J. Mater. Chem. 2000, 10, 2436. (d) Noda, T.; Ogawa, H.; Noma,
N.; Shirota, Y. J. Mater. Chem. 1999, 9, 2177. (e) Shirota, Y.;
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F.; Heuer, H. W.; Jonas, F.; Karbach, A.; Kirchmeyer, S.; Thurm, S.;
Wehrmann, R. Synth. Met. 2000, 111-112, 139.
ppm): 8.46 (d, 2H, J ) 4.7 Hz, py), 8.31 (d, 2H, J ) 7.9 Hz, py),
7.91 (dd, 2H, J ) 6.5, 1.8 Hz, benzimidazolyl), 7.88 (td, 2H, J )
7.8, 1.7 Hz, py), 7.48 (s, 4H, Ph), 7.37-7.44 (m, 6H, benzimida-
zolyl), 7.35 (ddd, 2H, J ) 7.5, 4.8, 1.0 Hz, py). 13C NMR (CD2Cl2,
δ, ppm): 151.0, 149.9, 149.0, 143.3, 138.2, 138.0, 137.0, 128.6,
125.0, 124.3, 124.2, 123.5, 120.5, 111.2. Anal. Calcd for
C30H20N6: C, 77.57; H, 4.31; N, 18.10. Found: C, 77.83; H, 4.14;
N, 18.20. Column chromatography also gave rise to the monosub-
stituted product, Brmb, which was further purified by recrystalli-
zation in CHCl3/hexanes (0.24 g, 32%). Mp: 168-170 °C. 1H NMR
(CD2Cl2, δ, ppm): 8.37 (d, 1H, J ) 4.6 Hz, py), 8.28 (d, 1H, J )
7.9 Hz, py), 7.87 (d, 1H, J ) 8.0 Hz, benzimidazolyl), 7.84 (td,
1H, J ) 7.8, 1.8 Hz, py), 7.66 (d, 2H, J ) 8.7 Hz, Ph), 7.38 (td,
(10) (a) Yang, W. Y.; Chen, L.; Wang, S. Inorg. Chem. 2001, 40, 507. (b)
Pang, J.; Tao, Y.; Freiberg, S.; Yang, X. P.; D’Iorio, M.; Wang, S. J.
Mater. Chem. 2002, 12, 206.
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3781.
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R. Spectrochemical Analysis; Prentice Hall: Englewood Cliffs, NJ,
1988; Chapter 12.
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Chem. 2002, 648, 302.
Inorganic Chemistry, Vol. 44, No. 5, 2005 1333

Liu et al.
1H, J ) 7.5, 1.2 Hz, benzimidazolyl), 7.34 (td, 1H, J ) 8.0, 1.2
Hz, benzimidazolyl), 7.29 (ddd, 1H, J ) 7.5, 4.8, 1.0 Hz, py), 7.24-
7.27 (m, 3H, Ph and benzimidazolyl). 13C NMR (CD2Cl2, δ,
ppm): 150.8, 149.8, 149.0, 143.3, 138.1, 137.6, 137.0, 132.7, 129.6,
124.9, 124.3, 124.2, 123.5, 120.4, 111.1, 110.2. HRMS: calcd for
M+ (C18H11N3Br), m/z 348.0136; found, m/z 348.0131.
1H, J ) 6.7 Hz, benzimidazolyl), 7.10 (t, 1H, J ) 7.1 Hz,
benzimidazolyl), 6.93 (t, 4H, J ) 7.2 Hz, Ph), 6.91 (t, 4H, J ) 6.8
Hz, Ph), 6.83 (t, 2H, J ) 7.2 Hz, Ph), 6.78 (t, 2H, J ) 7.1 Hz, Ph),
6.50 (d, 1H, J ) 8.0 Hz, benzimidazolyl), 6.48 (d, 1H, J ) 8.0 Hz,
benzimidazolyl). The 13C NMR spectrum could not be obtained
due to poor solubility. Anal. Calcd for C54H40N6Pt2: C, 55.77; H,
3.47; N, 7.23. Found: C, 55.89; H, 3.48; N, 7.12.
Synthesis of 4,4′-Bis[2-(2′-pyridyl)benzimidazolyl]biphenyl
(bmbp). Bmbp was synthesized in the same manner as described
for 1,4-bmb. In the presence of CuI (0.049 g, 0.26 mmol), 1,10-
phenanthroline (0.095 g, 0.63 mmol), and Cs2CO3 (1.75 g, 5.4
mmol), the reaction of 4,4′-diiodobiphenyl (0.52 g, 1.29 mmol) with
2-(2′-pyridyl)benzimidazole (0.60 g, 3.0 mmol) afforded bmbp as
colorless crystals (0.47 g, 68%). Mp: >300 °C. 1H NMR (CD2Cl2,
δ, ppm): 8.45 (d, 2H, J ) 5.0 Hz, py), 8.22 (d, 2H, J ) 7.8 Hz,
py), 7.97 (d, 2H, J ) 8.0 Hz, benzimidazolyl), 7.80-7.83 (m, 6H,
Ph and py), 7.47 (d, 4H, J ) 8.3 Hz, ph), 7.41 (ddd, J ) 8.1, 5.7,
2.5 Hz, benzimidazolyl), 7.33-7.37 (m, 4H, benzimidazolyl), 7.27
(dd, 2H, J ) 7.5, 5.5 Hz, py). 13C NMR (CD2Cl2, δ, ppm): 150.9,
149.8, 149.4, 143.1, 140.2, 138.0, 137.7, 136.9, 128.3, 128.2, 125.1,
124.5, 124.2, 123.7, 120.7, 111.3. Anal. Calcd for C36H24N6‚
0.1CHCl3: C, 78.47; H, 4.40; N, 15.21. Found: C, 78.64; H, 4.46;
N, 15.11.
Synthesis of Pt2(bmbp)Ph4 (2). In the same manner as described
for 1 except layering toluene instead of hexane upon the solution,
the reaction of bmbp (50 mg, 0.093 mmol) and [PtPh2(SMe2)]n (76
mg, 0.185 mmol Pt) afforded complex 2 as orange crystals (103
1
mg, 89% yield). H NMR (DMSO-d6, δ, ppm): 8.33 (m, 6H, py
and central Ph), 8.09 (t, 2H, J ) 7.7 Hz, py), 8.02 (d, 2H, J ) 8.1
Hz, central Ph), 7.61 (t, 2H, J ) 7.5 Hz, py), 7.58 (d, 4H, J ) 7.3
Hz, Ph), 7.47 (d, 4H, J ) 7.0 Hz, Ph), 7.36 (t, 2H, J ) 8.0 Hz,
benzimidazolyl), 7.18 (d, 2H, J ) 8.2 Hz, benzimidazolyl), 7.13
(d, 2H, J ) 8.0 Hz, py), 7.09 (t, 2H, J ) 7.5 Hz, benzimidazolyl),
6.93 (t, 4H, J ) 7.5 Hz, Ph), 6.91 (t, 4H, J ) 7.5 Hz, Ph), 6.83 (t,
2H, J ) 7.4 Hz, Ph), 6.77 (t, 2H, J ) 7.2 Hz, Ph), 6.48 (d, 2H, J
) 8.2 Hz, benzimidazolyl). The 13C NMR spectrum could not be
obtained due to poor solubility. Anal. Calcd for C60H44N6Pt‚
CH2Cl2: C, 55.33; H, 3.47; N, 7.23. Found: C, 54.85; H, 3.48; N,
7.12.
Syntheses of 1,3,5-Tris[2-(2′-pyridyl)benzimidazolyl]benzene
(tmb) and 1,3-Bis[2-(2′-pyridyl)benzimidazolyl]benzene (1,3-
bmb). Tmb and 1,3-bmb were obtained in the same manner as
described for 1,4-bmb except that reaction was kept refluxing for
72 h instead of 24 h. In the presence of CuI (0.11 g, 0.58 mmol),
1,10-phenantholine (0.21 g, 1.17 mmol), and Cs2CO3 (4.13 g, 12.7
mmol), the reaction of 1,3,5-tribromobenzene (0.60 g, 1.9 mmol)
with 2-(2′-pyridyl)indolyl (1.3 g, 6.7 mmol) afforded tmb and 1,3-
bmb. These two compounds were separated by column chromato-
graph using THF/hexane/MeOH (1:1:0.05) as the eluent, and the
crude products were recrystallized from CH2Cl2/hexanes to afford
tmb (0.50 g, 40%) and 1,3-bmb (0.28 g, 32%) as colorless crystals.
Tmb: mp 277-279 °C; 1H NMR (CD2Cl2, δ, ppm) 8.49 (d, 3H, J
) 4.5 Hz, py), 8.30 (d, 3H, J ) 7.9 Hz, py), 7.90 (d, 3H, J ) 8.0
Hz, benzimidazolyl), 7.83 (t, 3H, J ) 7.7 Hz, py), 7.51 (s, 3H,
Ph), 7.37 (t, 3H, J ) 8.1 Hz, benzimidazolyl), 7.33 (dd, 3H, J )
8.0, 4.9 Hz, py), 7.29 (t, 3H, J ) 7.9 Hz, benzimidazolyl), 7.21 (d,
3H, J ) 8.1, benzimidazolyl); 13C NMR (CD2Cl2, δ, ppm) 150.6,
149.7, 149.1, 143.1, 139.7, 138.0, 137.2, 127.1, 125.1, 124.6, 124.4,
123.9, 120.8, 110.9. Anal. Calcd for C42H27N9‚0.25CH2Cl2: C,
74.74; H, 4.08; N, 18.57. Found: C, 74.68; H, 4.55; N, 18.43. 1,3-
Synthesis of Pt(Brmb)Ph2 (3). In the same manner as described
for 1 except layering toluene instead of hexane upon the solution,
the reaction of Brmb (20 mg, 0.057 mmol) and [PtPh2(SMe2)]n (25
mg, 0.060 mmol Pt) afforded complex 3 as orange crystals (35
1
mg, 88% yield). H NMR (DMSO-d6, δ, ppm): 8.31 (d, 1H, J )
5.0 Hz, py), 8.09 (d, 1H, J ) 8.0 Hz, py), 8.01 (d, 2H, J ) 8.6 Hz,
Ph-Br), 7.81 (d, 2H, J ) 8.6 Hz, Ph-Br), 7.59 (dd, 1H, J ) 7.5,
5.4 Hz, py), 7.55 (d, 2H, J ) 7.5 Hz, Ph), 7.45 (d, 2H, J ) 7.8 Hz,
Ph), 7.31 (t, 1H, J ) 8.2 Hz, benzimidazolyl), 7.12 (d, 1H, J ) 8.4
Hz, benzimidazolyl), 7.05 (t, 1H, J ) 8.2 Hz, benzimidazolyl), 7.02
(d, 1H, J ) 8.1 Hz, py), 6.91 (t, 2H, J ) 7.2 Hz, Ph), 6.90 (t, 2H,
J ) 7.4 Hz, Ph), 6.81 (t, 1H, J ) 7.2 Hz, Ph), 6.76 (t, 1H, J ) 7.3
Hz, Ph), 6.44 (d, 1H, J ) 8.4 Hz, benzimidazolyl). The 13C NMR
spectrum could not be obtained due to poor solubility. Anal. Calcd
for C30H22N3BrPt‚1.5C7H8: C, 58.06; H, 4.06; N, 5.02. Found: C,
57.93; H, 4.13; N, 4.73.
Synthesis of Pt2(1,3-bmb)Ph4 (4). In the same manner as
described for 1 except layering toluene instead of hexane upon the
solution, the reaction of 1,3-bmb (30 mg, 0.064 mmol) and [PtPh2-
(SMe2)]n (76 mg, 0.129 mmol Pt) afforded complex 4 as orange
crystals (66 mg, 89% yield). 1H NMR (DMSO-d6, δ, ppm): 8.25-
8.35 (m, 6H, py and central Ph), 8.19 (t, 1H, J ) 8.0 Hz, py), 8.12
(t, 1H, J ) 7.7 Hz, py), 7.62 (m, 2H, py), 7.53 (d, 4H, J ) 7.0 Hz,
Ph), 7.39-7.44 (m, 5H, Ph and py), 7.36 (d, 2H, J ) 7.5 Hz,
benzimidazolyl), 7.33 (t, 2H, J ) 7.7 Hz, benzimidazolyl), 7.07 (t,
2H, J ) 7.3 Hz, benzimidazolyl), 6.89 (m, 8H, Ph), 6.80 (t, 2H, J
) 7.2 Hz, Ph), 6.76 (t, 2H, J ) 7.3 Hz, Ph), 6.45 (d, 1H, J ) 8.0
Hz, benzimidazolyl), 6.43 (d, 1H, J ) 8.0 Hz, benzimidazolyl).
The 13C NMR spectrum could not be obtained due to poor solubility.
Anal. Calcd for C54H40N6Pt2‚0.25CH2Cl2‚0.5C7H8: C, 56.38; H,
3.62; N, 6.83. Found: C, 56.30; H, 3.56; N, 6.76.
1
bmb: mp 190-192 °C; H NMR 8.43 (d, 2H, J ) 5.8 Hz, py),
8.27 (d, 2H, J ) 7.9 Hz, py), 7.87 (d, 2H, J ) 8.1 Hz,
benzimidazolyl), 7.85 (t, 2H, J ) 7.8 Hz, py), 7.65 (t, 1H, J ) 7.9
Hz, Ph), 7.48 (d, 2H, J ) 7.9 Hz, Ph), 7.30-7.39 (m, 7H, py,
benzimidazolyl, ph), 7.25 (d, 2H, J ) 7.9 Hz, benzimidazolyl);
13C NMR (CD2Cl2, δ, ppm) 150.8, 149.9, 149.0, 143.2, 139.2,
138.2, 137.6, 137.0, 130.9, 127.5, 125.0, 124.3, 124.2, 120.4, 111.1;
HRMS calcd for (M - H)+ (C30H20N6) m/z 464.174 9, found m/z
464.172 9.
Synthesis of Pt2(1,4-bmb)Ph4 (1). A mixture of 1,4-bmb ligand
(30 mg, 0.064 mmol) and [PtPh2(SMe2)]n (53 mg, 0.129 mmol Pt)
was dissolved in 10 mL of CH2Cl2. Then 5 mL of hexane was
successfully layered upon the solution. Slow evaporation of the
solvent and diffusion of hexane into the CH2Cl2 layer afforded
Synthesis of Pt3(tmb)Ph6 (5). In the same manner as described
for 1 except layering toluene instead of hexane upon the solution,
the reaction of tbp (50 mg, 0.076 mmol) and [PtPh2(SMe2)]n (95
mg, 0.23 mmol Pt) afforded complex 5 as orange crystals (110
1
complex 1 as orange crystals after 2 days (68 mg, 91% yield). H
1
NMR (DMSO-d6, δ, ppm): 8.38 (d, 1H, J ) 4.7 Hz, py), 8.36 (d,
1H, J ) 4.5 Hz, py), 8.28 (s, 2H, central Ph), 8.27 (s, 2H, central
Ph), 8.26 (t, 1H, J ) 8.1 Hz, py), 8.20 (t, 1H, J ) 8.2 Hz, py),
7.67 (m, 2H, py), 7.58 (d, 4H, J ) 6.7 Hz, Ph), 7.39-7.48 (m, 9H,
Ph, benzimidazolyl, and Py), 7.20 (d, 1H, J ) 8.3 Hz, py), 7.13 (t,
mg, 85% yield). H NMR (DMSO-d6, δ, ppm, 298 K): 8.96 (s,
2H), 8.85 (s, 1H), 8.30-8.37 (m, 4H), 8.14 (t, 2H, J ) 8.0 Hz),
7.35-7.82 (m, 24 H), 7.07-7.13 (m, 3H), 6.88-6.91 (m, 12H),
6.74-6.82 (m, 6H), 6.47 (d, 1H, J ) 8.2 Hz), 6.43 (d, 2H, J ) 8.2
Hz). The 13C NMR spectrum could not be obtained due to poor
1334 Inorganic Chemistry, Vol. 44, No. 5, 2005

Luminescent Organoplatinum(II) Complexes
Table 1. Crystal Data for 1,4-bmb, bmbp, tmb, and Complexes 1-3
1,4-bmb
bmbp
tmb
1
2
3
formula
C30H20N6‚2CHCl3 C36H24N6‚2CHCl3 C42H27N9‚1.5CH2Cl2 C54H40N6Pt2‚2CH2Cl2‚ C60H44N6Pt2‚2CH2Cl2 C30H22N3BrPt‚2C7H8
2hexane
fw
703.26
monoclinic
P21/c
9.4513(18)
20.766(4)
8.6353(16)
90
110.685(3)
90
1585.5(5)
2
779.35
monoclinic
P21/c
6.158(3)
32.742(18)
9.152(5)
90
96.325(12)
90
1834.0(17)
2
785.11
monoclinic
I2/a
22.002(5)
13.385(3)
27.273(7)
90
90.178(6)
90
8032(3)
8
1505.30
monoclinic
P21/n
12.571(3)
18.355(4)
13.389(3)
90
91.310(4)
90
3088.6(13)
2
1409.04
triclinic
P1h
833.77
monoclinic
P21/c
10.348(3)
19.768(6)
18.316(5)
90
93.892(5)
90
3738.0(18)
4
cryst system
space group
a/Å
b/Å
c/Å
9.6917(15)
11.5099(18)
13.659(2)
91.300(2)
106.758(2)
108.803(2)
1369.9(4)
1
R/deg
â/deg
γ/deg
V/Å3
Z
Dc/g cm-3
µ/mm-1
2θmax/deg
no. of rflcns measd
no. of rflcns used
Rint
1.473
0.576
56.60
11 445
3810
1.411
0.506
56.66
12 318
4190
1.299
0.272
56.60
27 981
9254
1.619
4.744
56.62
19 713
7275
1.708
5.342
56.58
9900
6271
0.0138
352
1.570
4.857
56.68
26 434
8753
0.0345
230
0.0664
226
0.0375
545
0.0663
358
0.1997
394
no. of params
final R [I > 2σ(I)]
R1a
0.0394
0.0756
0.0719
0.1544
0.0686
0.1878
0.0572
0.1167
0.0761
0.2053
0.0822
0.1611
wR2b
R (all data)
R1a
0.1281
0.0900
0.2415
0.2097
0.793
0.1442
0.2225
0.923
0.1788
0.1436
0.810
0.0912
0.2184
1.053
0.2913
0.2019
0.748
wR2b
goodness of fit on F2 0.757
a R1 ) ∑[|Fo| - |Fc|]/∑|Fo|. b wR2 ) {∑[w(Fo - Fc )]/∑(wFo )}1/2. w ) 1/[σ2(Fo ) + (0.075P)2], where P ) [max(Fo ,0) + 2Fc ]/3.
2
2
2
2
2
2
Table 2. Selected Bond Lengths (Å) and Angles (deg) for Complexes
solubility. Anal. Calcd for C78H57N9Pt3‚1.5CH2Cl2‚2C7H8: C, 55.67;
1-3
H, 3.67; N, 6.25. Found: C, 55.44; H, 3.70; N, 6.11.
Complex 1
X-ray Crystallographic Analysis. Single crystals of 1,4-bmb,
bmbp, and tmp ligands and the complexes 1-3 were obtained and
characterized by X-ray diffraction analysis. Crystals were mounted
on glass fibers for data collection. Data were collected on a Siemens
P4 single-crystal X-ray diffractometer with a Smart CCD-1000
detector and graphite-monochromated Mo KR radiation, operating
at 50 kV and 35 mA at 25 °C. The data collection ranges of 2θ are
3.92-56.60° for 1,4-bmb, 4.64-56.66° for bmbp, 5.02-56.60° for
tmb, 4.40-56.62° for complex 1, 3.04-56.86° for complex 2, and
3.14-56.58° for complex 3. No significant decay was observed
for all of the samples. Data were processed on a PC using the Bruker
SHELXTL software package15 (version 5.10) and were corrected
for Lorentz and polarization effects. Ligands 1,4-bmb and bmbp
and complex 3 crystallize in the monoclinic space group P21/c,
and the molecules of 1,4-bmb and bmbp possess an inversion center.
The crystals of tmp belong to the body-centered monoclinic space
group I2/a. Complexes 1 and 2 crystallize in monoclinic space group
P21/n and triclinic space group P1h, respectively, and their molecules
possess an inversion center. All non-hydrogen atoms except the
solvent molecules and a few carbon atoms (C7 and C12) in complex
3 were refined anisotropically. Complex 3 crystallized as very thin
needle crystals, which diffract poorly, greatly reducing the quality
of its crystallographic data. As a result, one of the coordinated
phenyl groups (C7-C12) in 3 had to be refined as a rigid body to
obtain meaningful Pt-C bond distances. All hydrogen atoms except
those of the disordered solvent molecules were calculated, and their
contributions in structural factor calculations were included. The
crystallographic data are given in Table 1. Selected bond lengths
and angles for complexes 1-3 are listed in Table 2.
Pt(1)-C(1)
2.019(13)
2.156(8)
87.4(4)
172.4(4)
100.0(4)
Pt(1)-C(7)
1.974(13)
2.129(8)
95.1(4)
177.0(4)
77.4 (3)
Pt(1)-N(1)
Pt(1)-N(2)
C(1)-Pt(1)-C(7)
C(1)-Pt(1)-N(2)
C(7)-Pt(1)-N(2)
C(1)-Pt(1)-N(1)
C(7)-Pt(1)-N(1)
N(1)-Pt(1)-N(2)
Complex 2
Pt(1)-C(1)
Pt(1)-N(1)
C(1)-Pt(1)-C(7)
C(1)-Pt(1)-N(2)
C(7)-Pt(1)-N(2)
1.974(13)
2.130(10)
88.0(4)
172.0(4)
97.3(4)
Pt(1)-C(7)
Pt(1)-N(2)
C(1)-Pt(1)-N(1)
C(7) - Pt(1)-N(1)
N(1)-Pt(1)-N(2)
1.997(11)
2.109(9)
97.6(4)
174.0(4)
76.8(3)
Complex 3
Pt(1)-C(1)
Pt(1)-N(1)
C(1)-Pt(1)-C(7)
C(1)-Pt(1)-N(2)
C(7)-Pt(1)-N(2)
1.914(18)
2.132(16)
87.2(6)
171.5(7)
99.6(6)
Pt(1)-C(7)
Pt(1)-N(2)
C(1)-Pt(1)-N(1)
C(7)-Pt(1)-N(1)
N(1)-Pt(1)-N(2)
1.950(9)
2.088(15)
96.8(6)
175.9(5)
76.3(7)
Results and Discussion
Syntheses. All new 2-(2′-pyridyl)benzimidazole derivative
ligands were synthesized in good yield using the modified
Ullmann condensation procedures depicted in Scheme 1,
where an appropriate aryl halide reacts with 2-(2′-pyridyl)-
benzimidazole in the presence of CuI catalyst, 1,10-phen-
athroline, and Cs2CO3. The size of the cation of the base
does have an effect on the yield of the coupling reaction of
the aryl halide and 2-(2′-pyridyl)benzimidazole,13 as indicated
by the much higher yield of the reaction using Cs2CO3 than
K2CO3 (the latter only afforded trace product). The high
reactivity of aryl iodide resulted in the isolation of the
bisubstituted product bmbp from the reaction of 4,4′-
diiodobiphenyl with benzimidazole in good yield (Scheme
1, top). In contrast, due to the relatively low reactivity of
(15) SHELXTL NT Crystal Structure Analysis Package, version 5.10; Bruker
Axs Analytical X-ray System: Madison, WI, 1999.
Inorganic Chemistry, Vol. 44, No. 5, 2005 1335

Liu et al.
Scheme 1
Scheme 2
aryl bromide, the reaction of 1,4-dibromobenzene or 1,3,5-
tribromobenzene with 2-(2′-pyridyl)benzimidazole gave rise
to a mixture of two products (Scheme 1, middle and bottom),
which can be separated by column chromatograph. For 1,4-
dibromobenzene, the bisubstituted product bmb and the
monosubstituted product BrmB were obtained. For 1,3,5-
tribromobenzene, the trisubstituted product tmb and the
bisubstituted product 1,3-bmb where a bromine atom was
replaced by a hydrogen were isolated. Due to the steric effect,
the reaction of 1,3,5-tribromobenzene with 2-(2′-pyridyl)-
benzimidazole requires a long reaction time (reflux for 72
h) to produce the trisubstituted product. The bromine atom
was probably lost due to thermal cleavage of the C-Br bond
under the reaction conditions. All ligands are fully character-
1
ized by H and 13C NMR, elemental analyses, or high-
resolution mass spectroscopy. Single-crystal X-ray diffraction
analyses were performed for the 1,4-bmb, bmbp, and tmb
ligands.
The PtII complexes 1-5 were synthesized by using [PtPh2-
(SMe2)]n as the starting material.14 The reactions of
[PtPh2(SMe2)]n with the corresponding ligand in a stoichio-
metric amount afforded the corresponding complexes in good
yield. The relatively labile bridging ligand SMe2 in [PtPh2-
(SMe2)]n was replaced by the chelating 2-(2′-pyridyl)-
benzimidazolyl moiety in the resulting complexes. The
synthetic routes for complexes 1-5 are summarized in
1
Scheme 2. All complexes were characterized by H NMR
single-crystals of complexes 4 and 5 for X-ray diffraction.
However, no suitable crystals for these two compounds were
obtained.
Crystal Structures. Ligands. The crystal structure of 1,4-
bmb is shown in Figure 1. The molecule of 1,4-bmb
possesses an inversion center. The central phenyl ring is not
coplanar with the benzimidazole ring, as indicated by the
dihedral angle of 54.0° between the two planes. The pyridyl
ring is also not coplanar with the benzimidazole ring, and
spectroscopy and elemental analyses. Due to their poor
solubility in common organic solvents, 13C NMR spectra for
the complexes could not be obtained. The 1H NMR spectra
of complexes 1, 4, and 5 at ambient temperature are very
complex, indicating either the presence of structural isomers
or a slow exchange process. To establish the structures of
the complexes, single-crystal X-ray analyses for complexes
1-3 were carried out. Many attempts were made to obtain
1336 Inorganic Chemistry, Vol. 44, No. 5, 2005

Luminescent Organoplatinum(II) Complexes
Figure 1. Molecular structure of bmb with 50% thermal ellipsoids and
labeling schemes. All hydrogen atoms are omitted for clarity.
Figure 3. Molecular structure of tmb with 50% thermal ellipsoids and
labeling schemes showing the cuplike structure of the molecule. All
hydrogen atoms are omitted for clarity.
ladderlike structure as shown in Figure 2b. The distance
between the stacked plane is about 3.57 Å.
The crystal structure of tmb is shown in Figure 3. The
three pyridylbenzimidazolyl groups in tmb are almost
perpendicular to the central phenyl ring, as indicated by the
dihedral angles of 58.8, 73.1, and 86.6°, respectively. As a
result, the molecule of tmb has a cuplike shape. As shown
in Figure 3, two of the 2-(2′-pyridyl)benzimidazolyl groups
are oriented to the same side of the central benzene ring while
the third one is oriented to the opposite side of the benzene
ring, perhaps to minimize steric interactions. If tmb retains
the same structure in solution, the 1H NMR spectrum of tmb
should show two sets of chemical shifts of the 2-(2′-pyridyl)-
benzimidazolyl. However, at ambient temperature, only one
set of chemical shifts due to 2-(2′-pyridyl)benzimidazolyl
was observed in the NMR spectrum of tmb, indicating that
the three 2-(2′-pyridyl)benzimidazolyl groups have a sym-
metric arrangement in solution at ambient temperature. The
molecules of tmb in the crystal lattice are paired up through
π-π stacking involving one of the 2-(2′-pyridyl)benzimi-
dazolyl groups (see Supporting Information). The distance
between the stacked planes is about 3.66 Å.
Figure 2. (a) Molecular structure of bmbp with 50% thermal ellipsoids
and labeling schemes. All hydrogen atoms are omitted for clarity. (b)
Ladderlike double chain structure formed by π-π stacking in bmbp.
Complexes 1-3. The crystal structure of complex Pt2(1,4-
bmb)Ph4 (1) is shown in Figure 4a. The molecule of complex
1 possesses an inversion center. Each PtII center in 1 is
bidentately chelated by a 1,4-bmb ligand via the 2-(2′-
pyridyl)benzimidazolyl unit and further coordinated by two
phenyl ligands, forming a typical square-planar geometry.
The square plane of the Pt center is somewhat distorted, as
indicated by the bond angle of N(1)-Pt(1)-N(2) (77.4(3)°).
The Pt-C and Pt-N bond lengths are typical for PtII
complexes. The Pt center is coplanar with the 2-(2′-pyridyl)-
benzimidazolyl plane. The structure of 1 supports that the
environment around the Pt center in 1 has a much higher
rigidity than that of the PtII complexes with the 2,2′-
dipyridylamino derivative ligands, where the six-membered
chelate ring is often not coplanar.11 The PtC2N2 plane is
almost perpendicular to the central benzene ring as indicated
by the dihedral angle of ∼86.9°. The intramolecular Pt(1)-
the dihedral angle between the two planes is 34.9°. Each
benzimidazolyl group of 1,4-bmb has π-π stacking with
another benzimidazolyl group of an adjacent molecule, hence
forming a 1D π-π stacking chain structure (see Supporting
Information). The distance between the stacked planes is 3.61
Å.
The crystal structure of bmbp is similar as that of 1,4-
bmb as shown in Figure 2a. The molecule of bmbp also
possesses an inversion center. The central biphenyl rings are
coplanar. The benzimidazolyl ring is not coplanar with the
biphenyl rings, as indicated by the dihedral angle of 69.2°.
The pyridyl ring is almost coplanar with the benzimidazolyl
ring, as shown by the dihedral angle of 15.1°. In the crystal
lattice, each molecule of bmbp forms π-π stacking with
two neighboring molecules via the pyridyl and the benzimi-
dazolyl rings, resulting in the formation of an interesting 1D
Inorganic Chemistry, Vol. 44, No. 5, 2005 1337

Liu et al.
Figure 5. (a) Molecular structure of complex 2 with 50% thermal ellipsoids
and labeling schemes. All hydrogen atoms are omitted for clarity. (b) Parallel
1D chain structure formed by intermolecular π-π stacking in complex 2.
Figure 4. (a) Molecular structure of complex 1 with 50% thermal ellipsoids
and labeling schemes. All hydrogen atoms are omitted for clarity. (b) Viewed
along a axis, the packing diagram showing the honeycomb structure in
complex 1. Crystal solvent molecules are omitted for clarity.
Pt(1A) separation distance is 13.391(4) Å. The shortest
intermolecular Pt-Pt distance is 9.093(4) Å. There is no
significant π-π stacking interaction. When viewed along a
axis, an interesting honeycomb structure was observed for
1 (Figure 4b), which traps CH2Cl2 and hexane solvent
molecules in the cavities.
The crystal structure of Pt2(bmbp)Ph4 (2) resembles that
of 1 as shown in Figure 5a. The molecule of 2 also possesses
an inversion center. The Pt center in 2 has a coordination
environment similar to that in 1. The central biphenyl unit
is coplanar but is nearly perpendicular to the PtC2N2 plane
(dihedral angle 84.5°). Due to the presence of the biphenyl
group in 2, the intramolecular Pt-Pt distance is very long
(17.421(4) Å). The shortest Pt-Pt distance in 2 is intermo-
lecular separation between Pt(1) and Pt(1′) (6.738(4) Å).
Despite the similarity of the molecular structures of 1 and
2, their extended structures are quite different. There is no
π-π stacking interaction in 1. However, a π-π stacked 1D
chain structure was observed in crystal lattice of 2. As shown
in Figure 5b, the two 2-(2′-pyridyl)benzimidazolyl units of
2 form π-π stacking with other two 2-(2′-pyridyl)benzimi-
dazolyl units from two neighboring molecules, leading to
the formation of a 1D chain structure. All the 1D chains are
parallel with each other with the distance between the stacked
plane being 3.69 Å.
Figure 6. Molecular structure of complex 3 with 50% thermal ellipsoids
and labeling schemes. All hydrogen atoms are omitted for clarity.
perpendicular to the bromobenzene plane. The shortest
intermolecular Pt-Pt separation distance in 3 is 4.873(6) Å.
The molecules of 3 form a π-π stacked dimer structure via
their 2-(2′-pyridyl)benzimidazolyl units (see Supporting
Information) with the distance between the stacked planes
being 3.50 Å.
The common and important feature observed for the
structures of 1-3 is that the coordination plane of the Pt
center (or the plane of the 2-(2′-pyridyl)benzimidazolyl
group) is nearly perpendicular to the benzene ring that is
attached to the 2-(2′-pyridyl)benzimidazolyl group. We
believe that this is caused by the steric interaction between
the ortho hydrogen atom (i.e. H(16) in 1 and 2) of the pyridyl
ring and the hydrogen atoms of the central benzene ring (for
1) or biphenyl rings (for 2). For the free ligands, this
interaction can be readily avoided by the free rotation of the
pyridyl ring to bring the ortho hydrogen away from the
central benzene or biphenyl rings. For the complexes,
however, the pyridyl can no longer rotate freely due to its
The crystal structure of complex Pt(Brmb)Ph2 (3) is shown
in Figure 6. The environment around the Pt center in 3
resembles those of 1 and 2. Complex 3 could be regarded
as half of the molecule of 2. Again the PtC2N2 plane is almost
1338 Inorganic Chemistry, Vol. 44, No. 5, 2005

Luminescent Organoplatinum(II) Complexes
Chart 1
benzimidazolyl groups of 1 at ambient temperature can be
attributed to the coexistence of the anti and syn isomers in
solution. We believe that both isomers are also likely present
in the solid state. The anti isomer is likely to form single
crystals more readily than the syn isomer, hence making it
possible to identify the anti structure by X-ray diffraction.
The fact that both isomers are observed in solution at ambient
temperature indicates that the energy barrier for the inter-
conversion of the two isomers must be fairly large. Indeed,
with the variable-temperature 1H NMR spectral data, we have
estimated that the activation energy for the exchange process
of anti and syn structures to be ∼69 kJ/mol by using a
previous established method.16 This large energy barrier is
clearly attributable to the hindered rotation of the 2-(2′-
pyridyl)benzimidazolyl group with respect to the central
benzene ring, due to the ortho hydrogen steric interaction
and the chelation of the 2-(2′-pyridyl)benzimidazolyl to the
Pt atom.
Although the crystal structure of 2 is similar to that of 1,
no dynamic exchange phenomenon was observed for 2 in
solution. This is not surprising since the presence of the
central biphenyl unit in 2 allows the free rotation around
the C-C bond of the biphenyl, thus a facile interconversion
of the syn and anti isomers.
Figure 7. Variable-temperature 1H NMR spectra of complexes 1 (top)
and 5 (bottom) in DMSO-d6 showing the change of one of the proton signals
on the 2-(2′-pyridyl)benzimidazolyl group.
binding to the Pt center, which is believed to be responsible
for the distinct solution behavior of the complexes.
Complex 4 where the two 2-(2′-pyridyl)benzimidazolyl
groups at 1,3 positions are chelated by two PtII centers
displays a dynamic exchange phenomenon in solution,
similar to that of 1. Again, on the basis of NMR data, both
syn and anti isomers of 4 coexist in solution at ambient
temperature, which undergo a slow interconversion. The
activation energy for the interconversion process was esti-
mated to be ∼65 kJ/mol.
Structures and Fluxionality of PtII Complexes in
Solution. The crystal structure of 1 has an inversion center.
Therefore, if 1 retains the same structure in solution, one
would expect to see only one set of chemical shifts due to
the 2-(2′-pyridyl)benzimidazolyl group. However, surpris-
ingly, as shown in Figure 7, top, two distinct sets of well-
resolved peaks in ∼1:1 ratio, corresponding to two different
groups of 2-(2′-pyridyl)benzimidazolyl, were observed in the
1H NMR spectrum of 1 in DMSO-d6 at 25 °C. As the
temperature is increased, the peaks become broad. At 40 °C,
the two sets of chemical shifts merge into one set. This
temperature-dependent spectral change is clearly caused by
some dynamic exchange process of 1 in solution. Although
the crystal structure of 1 is centrosymmetric (i.e. with the
two 2-(2′-pyridyl)benzimidazolyl units on the opposite side
of the central benzene ring, shown as the anti structure in
Chart 1), a second structural isomer with a C2 symmetry
where both 2-(2′-pyridyl)benzimidazolyl groups are on the
same side of the benzene ring (shown as the syn structure
in Chart 1) is also possible for 1. The 2-(2′-pyridyl)-
benzimidazolyl units in either the anti or the syn structure
are chemically equivalent. Therefore, one distinct set of
chemical shift is expected for each of the isomers. The
observed two sets of chemical shifts of the 2-(2′-pyridyl)-
Complex 5 is a very crowded molecule with three 2-(2′-
pyridyl)benzimidazolyl groups at 1,3,5 positions chelated to
three PtII centers. Free rotation of the 2-(2′-pyridyl)benzimi-
dazolyl groups with respect to the central benzene ring is
therefore unlikely, on the basis of the same arguments for 1
and 4. On the basis of the arrangement of three 2-(2′-pyridyl)-
benzimidazolyl groups, there are two possible structures of
5 in solution, A and B as depicted in Chart 2. In structure
A, all of the PtC2N2 planes are oriented to the same side of
the central phenyl ring with an approximate C3 symmetry,
hence resulting in only one set of chemical shifts of the 2-(2′-
pyridyl)benzimidazolyl groups in the NMR spectra. In
structure B, one of the 2-(2′-pyridyl)benzimidazolyl groups
(or the PtC2N2 planes) is oriented in the opposite direction
from the other two, forming an “two-down-one-up” struc-
(16) Williams, D. H.; Fleming, I. Spectroscopic Methods in Organic
Chemistry, 4th ed.; McGraw-Hill Co. Ltd.: London, 1987;
Chapter 3.
Inorganic Chemistry, Vol. 44, No. 5, 2005 1339

Liu et al.
Chart 2
Table 3. Electrochemical Data (E1/2 Values for Reductions and
Irreversible Oxidation Waves) for Complexes 1-5
compd
Eox (V)
Ered (V)
compd
Eox (V)
Ered (V)
1b
2b
3a
1.16, 1.30
1.11
0.98
-1.33
-1.36
-1.46
4b
5b
1.18
1.15
-1.34
-1.30
Figure 8. UV-vis absorption spectra of bmbp, Brmb, and tmb in CH2Cl2.
The absorption spectra of 1,4-bmb and 1,3-bmb are similar to that of bmbp.
a In CH3CN solution containing 0.10 M (TBA)PF6 at 293 K, E1/2 for
FeCp2+/FeCp2 (versus AgCl/Ag) ) 0.45 V. b In DMF solution containing
0.10 M (TBA)PF6 at 293 K, E1/2 for FeCp2+/FeCp2 (versus AgCl/Ag) )
0.45 V.
Table 4. UV-Vis Absorption Data for the Ligands and the Complexes
compd
abs (nm) (ꢀ, M-1 cm-1) in CH2Cl2, 298 Ka
1,4-bmb
230 (63 000), 300 (73 000)
bmbp
230 (51 000), 296 (70 000)
Brmb
1,3-bmb
tmb
1
2
3
4
5
232 (29 000), 302 (28 000)
232 (65 000), 306 (79 000)
230 (78 000), 300 (91 000)
234 (66 000), 324 (37 000), 456 (6400)
240 (73 000), 322 (42 000), 444 (6300)
234 (38 000), 322 (18 000), 440 (2900)
234 (73 000), 322 (42 000), 458 (6600)
234 (94 000), 324 (50 000), 478 (9100)
ture, similar to the crystal structure of the free ligand tmb.
Because of the presence of an approximate mirror plane
symmetry in B, relating units a and a′, two sets of chemical
shifts of the 2-(2′-pyridyl)benzimidazolyl groups with a 2:1
1
1
ratio should be observed in the H NMR spectra. H NMR
experiments revealed that the molecule of complex 5 adopts
mainly the structure B in solution at ambient temperature.
As shown in Figure 7, bottom, at ambient temperature, well-
resolved peaks corresponding to two sets of 2-(2′-pyridyl)-
benzimidazolyl groups in 2:1 ratio are observed. The weak
peaks could be attributed to the presence of a small amount
of structure A. As the temperature increases, the spectrum
gradually becomes broad. At 80 °C, all peaks start to merge
into one set, which can be attributed to the interconversion
of the 2-(2′-pyridyl)benzimidazolyl groups of a and a′ types
with that of b and the interconversion of the major isomer
B with the minor isomer A. The activation energy for the
exchange process of 5 was estimated to be >70 kJ/mol.
Electrochemistry. Cyclic voltammograms recorded for the
complexes 1-5 in either DMF or CH3CN all display one
reversible reduction wave between -1.20 and -1.50 V
(Table 3). On the basis of the previous studies by Kaim and
others17 on Pt(diimine)R2 and by Eisenberg18 on Pt(diimine)-
(CtCR)2 where similar reversible reduction waves were
observed and attributed to one electron reduction of the
diimine ligand, we assigned the reversible reduction wave
to one-electron reduction of the benzimidazolyl chelate ligand
in the complexes. The reduction potential shows considerable
variation for the five complexes. Since the PtPh2 portion is
the same for all complexes, this variation can be only
attributed to the variation of the chelate ligand. Complexes
a [M] ) 4.8 × 10-6-2.9 × 10-5 M in CH2Cl2.
1 and 4 with the 1,4- and 1,3-disubstituted ligands, 1,4-bmb
and 1,3-bmb, respectively, display similar reduction potential
(-1.33 and -1.34 V) while complex 2 with the disubstituted
biphenyl ligand bmbp has a more negative reduction potential
(-1.36 V). Complex 5 with the trisubstituted ligand tmb has
the most positive reduction potential (-1.30 V). For the
benzene derivative ligands, the data indicate that as the
number of 2-pyridylbenzimidazolyl units increase in the
ligand, it becomes easier to reduce the complex.
All complexes display an irreversible oxidation wave. For
complexes that contain the benzene derivative ligands (1, 4,
5), the variation of this oxidation potential is very small. On
the basis of the previous reports on related diimine Pt(II)
complexes,17 we assigned this oxidation wave to the oxida-
tion of the Pt(II) center.
Absorption Spectra. The absorption spectra of the free
ligands 1,4-bmb, bmbp, Brmb, 1,3-bmb, and tmb are similar
(Figure 8). Two absorption bands with λmax ≈ 230 and 300
nm were observed for all free ligands, which are attributed
to the π f π* transition centered on the 2-(2′-pyridyl)-
benzimidazolyl units. This is supported by the extinction
coefficients of the absorption bands. As shown in Table 4
and Figure 8, the ꢀ values of 1,4-bmb, bmbp, and 1,3-bmb
are approximately twice of that of Brmb and tmb has a ꢀ
value about three times of that of Brmb. Clearly the ꢀ value
is linearly dependent on the number of 2-(2′-pyridyl)-
benzimidazolyl units present in the molecule. For the benzene
derivative ligands 1,4-bmb, 1,3-bmb, Br-bmb, and tmb, the
(17) Klein, A.; Kaim, W. Organometallics 1995, 14, 1176 and references
therein.
(18) (a) Hissler, M.; Connick, W. B.; Geiger, D. K.; McGarrah, J. E.; Lipa,
D.; Lachicotte, R. J.; Eisenberg, R. Inorg. Chem. 2000, 39, 447. (b)
Paw, W.; Cummings, S. D.; Mansour, M. A.; Connick, W. B.; Gieger,
D. K.; Eisenberg, R. Coord. Chem. ReV. 1998, 171, 125. (c) McGarrah,
J. E.; Kim, Y. J.; Hissler, M.; Eisenberg, R. Inorg. Chem. 2001, 40,
4510.
1340 Inorganic Chemistry, Vol. 44, No. 5, 2005

Luminescent Organoplatinum(II) Complexes
Figure 11. Emission spectra of tmp. (The phosphorescent emission
spectrum was obtained using a time-resolved phosphorescence spectrometer.)
Figure 9. UV-vis absorption spectra of complexes 2, 3, and 5 in CH2Cl2.
The absorption spectra of complexes 1 and 4 are similar to that of 2.
presence of some degree of communication among the 2-(2′-
pyridyl)benzimidazolyl units. On the basis of the electro-
chemical data, this low energy shift can be attributed mostly
to the decrease of the LUMO energy level associated with
the increase of the number of 2-(2′-pyridyl)benzimidazolyl
units.
Luminescent Properties. At ambient temperature, all free
ligands have purple/blue emission with λmax ≈ 365-415 nm
in CH2Cl2 solution, which are assigned to fluorescent
emissions originating from a ligand-centered π f π*
transition. In the solid state, the fluorescent emissions of the
free ligands are red-shifted to 386-431 nm, which is
attributable to intermolecular interactionssmost likely π-π
stacking interactions, as seen in the crystal structures. No
phosphorescent emission bands were observed for the free
ligands at ambient temperature. However, at 77 K, in addition
to the intense fluorescent bands, new low-energy emission
bands (λmax ) ∼500 nm) with well-resolved vibrational
intervals (∼1300-1400 cm-1) were observed for all ligands.
Using a time-resolved phosphorescence spectrometer, we
were able to completely remove the fluorescent emission
bands and determine the decay lifetimes of these low energy
emission bands to be in the range of 7.0(1)-13.2(6) µs,
indicating phosphorescent emission. The phosphorescent
emission spectrum and the full emission spectrum of tmb
are shown in Figure 11 as a representative example. The
complete luminescent data are given in Table 5. The full
emission and phosphorescent spectra of 1,4-bmb, bmbp,
Brmb, and 1,3-bmb are provided in the Supporting Informa-
tion.
The PtII complexes 1-5 do not display any detectable
emission in solution at ambient temperature. At 77 K, the
frozen solution (CH2Cl2, THF, or CH3CN) of complexes 1-5
exhibit an intense emission band with λmax at ∼537-601
nm (Table 5). The emission spectra of complex 5 along with
the free ligand’s phosphorescent emission spectrum are
shown in Figure 12 to illustrate the impact of the Pt(II)
coordination. The emission spectra of complexes 1-4 are
provided in the Supporting Information. No significant shift
of the emission maxima was observed when different
solvents were used. The lack of emission of the Pt(II)
complexes in solution at ambient temperature is most likely
caused by the thermal quenching of the solvent molecules.
Figure 10. Low-energy region of the UV-vis absorption spectra of
complex 2 in various solvents.
absorption band at ∼300 nm shows a small shift toward the
longer wavelength as the number of substituent groups
increases.
The absorption spectra of complexes 1-5 have similar
features (Figure 9). Besides the intense absorption bands in
UV region (λmax ≈ 232 and 322 nm), which are assigned to
the ligand-centered π f π* transition, all of the complexes
have a broad and weak absorption band that covers the region
of 410-550 nm (λmax ) 445-480 nm), resulting in the
orange-red appearance of complexes 1-5. These absorption
bands display the typical negative solvatochromism, i.e., a
high-energy shift in a more polar solvent, as demonstrated
by the absorption spectra of complex 2 in various solvents
(Figure 10), which is consistent with the assignment of
MLCT transitions (dPt f π*) involving the Pt center and
the 2-(2′-pyridyl)benzimidazolyl unit.17,18 Again, the ꢀ values
of the absorption bands for complexes 1-5 show a linear
dependence on the number of 2-(2′-pyridyl)benzimidazolyl
units present in the molecule. For example, the ꢀ values of
the MLCT bands for complexes 1, 2, and 4 are approximately
twice of that of 3 and 5 has a ꢀ value about three times of
that of 3. The absorption maxima of the MLCT bands shift
to lower energy as the number of 2-(2′-pyridyl)benzimida-
zolyl units increases in the complex, an indication of the
Inorganic Chemistry, Vol. 44, No. 5, 2005 1341

Liu et al.
Table 5. Luminescence Data for the Ligands and the Complexes
compd
excitatn wavelength, nm
emission bands λmax, nma
decay lifetime τ, µs
conditns
1,4-bmb
331
336
335
335
343
373
335
335
332
344
334
334
334
340
335
335
338
330
335
335
466
424
466
420
466
420
465
432
466
409
365
386
CH2Cl2/298 K
solid/298 K
CH2Cl2/77 K
CH2Cl2/77 Kc
CH2Cl2/298 K
solid/298 K
CH2Cl2/77 K
CH2Cl2/77 Kc
CH2Cl2/298 K
solid/298 K
CH2Cl2/77 K
CH2Cl2/77 Kc
CH2Cl2/298 K
solid/298 K
CH2Cl2/77 K
CH2Cl2/77 Kc
CH2Cl2/298 K
solid/298 K
CH2Cl2/77 K
CH2Cl2/77 Kc
solid/298 K
CH2Cl2/77 K
solid/298 K
CH2Cl2/77 K
solid/298 K
379, 470, 505, 542 (sh)b
469, 502, 534 (sh)
415
426
388, 471, 503, 543 (sh)
467, 499, 534 (sh)
373
400
370, 471, 505, 543 (sh)
469, 501, 535 (sh)
365
431
370, 470, 500, 540 (sh)
466, 498, 534 (sh)
390
9.2(5), 8.4(3)
13.2(6), 10.0(3)
8.9(5), 8.7(3)
7.2(1), 7.0(1)
bmbp
Brmb
1,3-bmb
tmb
397
371, 472, 509, 543 (sh)
470, 503, 539
582
537, 575 (sh)
587
552, 583 (sh)
592
550, 584 (sh)
611
8.2(3), 7.5(2)
7.9(4)
7.5(2), 6.8(3)
8.0(3)
7.8(3), 7.0(2)
7.5(2)
7.1(2), 6.5(1)
8.1(4)
1
2
3
4
5
CH2Cl2/77 K
solid/298 K
CH2Cl2/77 K
solid/298 K
560, 600 (sh)
621
601, 631 (sh)
7.9(4), 7.2(2)
8.2(2)
8.4(2), 7.6(3)
CH2Cl2/77 K
a [M] ) 5.9 × 10-5-3.4 × 10-5 M. b sh ) shoulder. c Obtained using a time-resolved phosphorescence spectrometer.
emissions is independent of excitation wavelength and is
somewhat red-shifted from the corresponding emission band
of the frozen solutions, likely a consequence of intermo-
lecular interactions in the solid state. The decay lifetime of
the emission bands of complexes 1-5 were determined to
range from 6.5(1) to 8.4(2) µs (Table 5).
Conclusions. The syntheses of five new luminescent
organic molecules based on the 2-(2′-pyridyl)benzimidazolyl
chromophore have been achieved. These new ligands have
been found to bind to the PtII center readily via N,N-chelation
to form the corresponding mononuclear, dinuclear, and
trinuclear PtII complexes. X-ray crystal structural data
indicate that the 2-(2′-pyridyl)benzimidazolyl unit has little
conjugation with the benzene or the biphenyl groups to which
it is directly attached to. Instead, this unit is usually
perpendicular to the benzene or the biphenyl group to
minimize steric interaction between ortho hydrogen atoms.
As a consequence of the different orientation of the 2-(2′-
pyridyl)benzimidazolyl units with the respect to the central
benzene ring and the restricted rotation, structural isomers
for complexes 1, 4, and 5 were observed in solution. For
the free ligands, because the ortho hydrogen interactions can
be minimized by the free rotation of the pyridyl group, no
structural isomers were observed in solution. The emission
of the free ligands at ambient temperature and at 77 K is
dominated by fluorescence while the emission of the PtII
complexes involves phosphorescence only. The orange-red
phosphorescent emission of the PtII complexes has been
found to be most likely MLCT transitions involving the PtII
center and the 2-(2′-pyridyl)benzimidazolyl chelate. Although
the Pt(II) complexes do not emit in solution at ambient
Figure 12. Emission spectra of complex 5 and the phosphorescent
spectrum of the tmb ligand at 77 K.
As shown by the data in Table 5 and Figure 12, the
phosphorescent emission band of the free ligand is at a much
higher energy than that of the corresponding PtII complex,
an indication that the emission of the complex is likely not
a ligand-based transition. Furthermore, the emission energy
of the complexes (1, 2, 4, and 5) changes in the same manner
as the MLCT absorption band does, i.e., shifting to lower
energy as the number of 2-(2′-pyridyl)benzimidazolyl units
increases in the complex. By taking into consideration of
the absorption data and the electrochemical data, we suggest
that the emission of the complexes at 77 K originates most
likely from a MLCT excited state. The solids of all the PtII
complexes display orange-red emission with λmax ≈ 582-
621 nm at ambient temperature. The energy of these
1342 Inorganic Chemistry, Vol. 44, No. 5, 2005

Luminescent Organoplatinum(II) Complexes
packing diagrams, crystallographic data in CIF format, complete
excitation and emission spectra, variable-temperature 1H NMR
spectra for 1 and 4, and tables of atomic coordinates, thermal
parameters, bond lengths and angles, and hydrogen parameters. This
material is available free of charge via the Internet at
http://pubs.acs.org.
temperature, they do emit in the solids at ambient temper-
ature, which renders them potentially useful as phosphores-
cent emitters for OLEDs.
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council for financial support.
Supporting Information Available: Complete listing of crystal
data for the free ligands and complexes 1-3, including lattice
IC0487530
Inorganic Chemistry, Vol. 44, No. 5, 2005 1343