Synthesis of facial cyclometalated iridium(iii) complexes triggered by tripodal ligands

Article abstract of DOI:10.1039/c2dt12309f

The tripodal ligands composed of the 1,3,5-trisubstituted cyclohexyl moiety as a molecular scaffold and 2-phenylpyridyl moieties as a coordination site were designed. The homoleptic cyclometalated fac-Ir(C^N)₃ complexes could be obtained by the reaction of IrCl₃·nH₂O with the designed tripodal ligands. The single crystal X-ray structure determination confirmed the fac configuration and a distorted octahedral geometry with three intramolecular cyclometalated 2-phenylpyridyl ligands surrounding the iridium metal center. Also, the cyclohexyl scaffold was found to serve as a flexible scaffold to induce the fac configuration. The thus-obtained homoleptic cyclometalated fac-Ir(C^N)₃ complexes exhibited a broad emission band in the emission spectra at 298 K.

Full text of DOI:10.1039/c2dt12309f

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PAPER
Synthesis of facial cyclometalated iridium(III) complexes triggered by tripodal
ligands†
Toshiyuki Moriuchi,*a Lisheng Mao,‡a Hsyueh-Liang Wu,§a Satoshi D. Ohmura,a Masami Watanabeb and
Toshikazu Hirao*a
Received 1st December 2011, Accepted 30th May 2012
DOI: 10.1039/c2dt12309f
The tripodal ligands composed of the 1,3,5-trisubstituted cyclohexyl moiety as a molecular scaffold
and 2-phenylpyridyl moieties as a coordination site were designed. The homoleptic cyclometalated
fac-Ir(C^N)3 complexes could be obtained by the reaction of IrCl3·nH2O with the designed tripodal
ligands. The single crystal X-ray structure determination confirmed the fac configuration and a distorted
octahedral geometry with three intramolecular cyclometalated 2-phenylpyridyl ligands surrounding the
iridium metal center. Also, the cyclohexyl scaffold was found to serve as a flexible scaffold to induce the
fac configuration. The thus-obtained homoleptic cyclometalated fac-Ir(C^N)3 complexes exhibited a
broad emission band in the emission spectra at 298 K.
Introduction
Results and discussion
Luminescent iridium(III) complexes with cyclometalated ligands
such as 2-phenylpyridine are attracting widespread interest due
to their unique photophysical properties and promising electro-
luminescence applications.1–5 The emission properties of these
complexes have been demonstrated to be dependent on the
cyclometalating ligands. Homoleptic cyclometalated Ir(C^N)3
complexes have two stereoisomers: facial (fac) and meridional
(mer) isomers. These fac and mer isomers of cyclometalated
Ir(C^N)3 complexes exhibit markedly different photophysical
properties.3 Generally, the fac isomer has a higher emission
quantum yield than the mer isomer. The utilization of a tripodal
ligand having three C^N type coordination sites is considered to
be a reliable strategy for the selective formation of the fac isomer
based on the regulation of scrambling coordination.4 In this
context, we embarked upon the design of homoleptic cyclometa-
lated fac-Ir(C^N)3 (where C^N is a substituted 2-phenylpyridine)
complexes by using tripodal ligands.
1,3,5-Trisubstituted cyclohexane was focused on as a molecular
scaffold for the design of the tripodal ligands. The advantage in
the use of 1,3,5-trisubstituted cyclohexane as a molecular scaf-
fold depends on regulation of the direction of substituents. Tripo-
dal ligands 1a and 1b, wherein 2-phenylpyridyl moieties are
connected to the cyclohexyl scaffold through the ether spacer,
were prepared by the reaction of cis,cis-1,3,5-cyclohexanetriol
with the corresponding 5-(bromomethyl)-2-phenylpyridine
derivative (Fig. 1). The introduction of 2-phenylpyridyl moieties
into the cyclohexyl scaffold through the ethylene spacer afforded
the tripodal ligand 1c, which was synthesized by the reaction of
cis,cis-1,3,5-tris(bromomethyl)cyclohexane with 2-phenyl-5-
lithiomethylpyridine. The homoleptic cyclometalated fac-
Ir(C^N)3 complexes 2a–c could be obtained by the reaction of
IrCl3·nH2O with the tripodal ligands 1a–c, respectively, and
were fully characterized by spectral data. The formation of un-
desired polymer complexes resulted in the low yields. Further
structural information was obtained by single-crystal X-ray struc-
ture determination (Table 1). The single crystal X-ray structure
determination of 2a confirmed the fac configuration and a dis-
torted octahedral geometry with three intramolecular cyclometa-
lated 2-phenylpyridyl moieties surrounding the iridium metal
center, wherein the cyclohexyl scaffold adopts in a triaxial chair
conformation, as depicted in Fig. 2a. Selected bond distances
and angles of 2a are shown in Table 2. Each σ-donor C atom in
the 2-phenylpyridyl moiety arranges in a trans disposition to the
N atom of the adjacent 2-phenylpyridyl moiety. The intramole-
cular formation of the fac configuration resulted in large defor-
mation with regard to the phenyl and pyridyl rings in the
complex, resulting in the dihedral angles between the least-
squares plane of the phenyl and pyridyl rings (10.9(4)° for Aph–
Apy; 12.0(4)° for Bph–Bpy; 21.4(4)° for Cph–Cpy; 14.3(4)° for
aDepartment of Applied Chemistry, Graduate School of Engineering,
Osaka University, Yamada-oka, Suita, Osaka 565-0871, Japan.
E-mail: moriuchi@chem.eng.osaka-u.ac.jp,
hirao@chem.eng.osaka-u.ac.jp; Fax: +81-6-6879-7415;
Tel: +81-6-6879-7413
bIdemitsu Kosan Co., Ltd., Kamiizumi, Sodegaura, Chiba 299-0293,
Japan. Fax: +81-438-75-7217; Tel: +81-438-75-3733
†Electronic supplementary information (ESI) available: Difference
NOE of 2c. CCDC 828763 for 2a and 828764 for 2b. For ESI and crys-
tallographic data in CIF or other electronic format see DOI:
10.1039/c2dt12309f
‡Current address: Shanghai Institute of Organic Chemistry, Chinese
Academy of Sciences, 345 Lingling Road, Shanghai 200032, P.R.
China. Fax: +86-21-61453998; Tel: +86-21-61453998 ex. 2362.
§Current address: Department of Chemistry, National Taiwan Normal
University, NO. 88, Sec. 4, Tingzhou Rd., Taipei 116, Taiwan, R.O.C.
Fax: +886-2-29324249; Tel: +886-2-77346142.
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Dph–Dpy; 15.3(4)° for Eph–Epy; 9.6(4)° for Fph–Fpy). Two
symmetry-independent molecules, which are conformational
enantiomers, exist in the asymmetric unit (Fig. 2a) and are con-
nected through π–π interaction with an interplanar distance of
ca. 3.3 Å between the pyridyl moieties (Fig. 2b).
The single crystal X-ray structure determination of 2b with the
ether spacer linked tripodal ligand also revealed the fac configur-
ation with a distorted octahedral geometry around the iridium
metal center, wherein the cyclohexyl scaffold adopts a triaxial
chair conformation as observed in 2a (Table 1 and Fig. 3a).
Selected bond distances and angles of 2b are shown in Table 3.
The distortion of the 2-phenylpyridyl moiety was observed due
to the intramolecular formation of the fac configuration, resulting
in the dihedral angles between the least-squares plane of the
phenyl and pyridyl rings (16.5(3)° for Aph–Apy; 16.9(3)° for
Bph–Bpy; 13.6(2)° for Cph–Cpy). In the crystal packing, each
molecule is connected through π–π interaction with an interpla-
nar distance of ca. 3.3 Å between the pyridyl moieties (Fig. 3b).
Nuclear Overhauser effect (NOE) of 2c in CD2Cl2 at 25 °C
provided interesting information for the conformation of the
cyclohexyl scaffold. Irradiation of the cyclohexyl proton at the
1-position showed no enhancement of the cyclohexyl axial
proton at the 2-position (see ESI†). Irradiation of the cyclohexyl
axial proton at the 2-position enhanced the pyridyl proton at the
6-position (see ESI†). In comparison to the triaxial chair
Table 1 Crystallographic data for 2a and 2b
2a
2b
Formula
Formula weight 865.45
Crystal system
Space group
a, Å
b, Å
c, Å
C42H36N3O3Ir·0 5CH2Cl2 C42H33N3O3F3Ir·2CH2Cl2
1046.82
Monoclinic
P21/n (No. 14)
16.0320(5)
14.8771(5)
16.3333(5)
92.186(1)
3892.8(2)
4
Monoclinic
P21/n (No. 14)
16.7268(4)
13.2938(5)
31.8075(8)
103.2250(7)
6885.2(4)
8
β, °
V, Å3
Z
D
calcd, g cm−1
1.670
1.786
μ(Mo Kα), cm−1 40.129
37.752
T, °C
−150
−150
λ(Mo Kα), Å
0.71075
0.059
0.71075
0.046
a
R1
b
wR2
0.189
0.147
a R1 = Σ||Fo| − |Fc||/Σ|Fo|. b wR2 = [Σw(Fo2 − Fc /Σw(Fo ) ]
.
2
2 2 1/2
Fig. 1 Tripodal ligands 1a–c and homoleptic cyclometalated fac-
Ir(C^N)3 complexes 2a–c.
Fig. 2 (a) Molecular structures of two independent molecules in the asymmetric unit of 2a, which are conformational enantiomers and (b) a dimer
structure through π–π interaction between the 2-phenylpyridyl moieties in the crystal packing of 2a.
9520 | Dalton Trans., 2012, 41, 9519–9525
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conformation of 2a possessing the ether spacer linked tripodal
ligand, the homoleptic cyclometalated fac-Ir(C^N)3 complex 2c
possessing the ethylene spacer linked tripodal ligand was found
to form the triequatorial chair conformation, which might be
caused by steric hindrance between the hydrogen atoms of the
ethylene spacer. The cyclohexyl scaffold was found to induce
the fac configuration.
The electronic absorption spectra of the homoleptic cyclo-
metalated fac-Ir(C^N)3 complexes 2a–c in dichloromethane
1
solution are shown in Fig. 4. The spin-allowed π–π* transitions
of the 2-phenylpyridyl moiety were observed around
230–320 nm. The absorption bands around 320–510 nm might
be assigned to both spin-allowed and spin-forbidden MLCT tran-
sitions.3a,b The emission spectra of the homoleptic cyclometa-
lated fac-Ir(C^N)3 complexes 2a–c in dichloromethane solution
(excitation at 350 nm) are depicted in Fig. 5. The emission of 2a
possessing the ether spacer linked tripodal ligand is characterized
by a relatively broad spectrum with the emission peak around
512 nm probably resulting from the 3MLCT state with significant
Table 2 Selected bond lengths (Å), angles (°) and dihedral angles (°)
between the least-squares planes of the pyridyl and phenyl rings for 2aa
2a
Bond lengths
Ir1–N1A
Ir1–N1B
Ir1–N1C
Ir1–C14A
Ir1–C14B
Ir1–C14C
2.123(9)
2.136(10)
2.141(8)
2.056(11)
2.040(11)
2.032(10)
Ir2–N1D
Ir2–N1E
Ir2–N1F
Ir2–C14D
Ir2–C14E
Ir2–C14F
2.128(9)
2.122(9)
2.130(10)
2.030(12)
2.035(11)
2.047(11)
3
contribution of the ligand-centered π–π* state. Almost the same
Bond angles
Table 3 Selected bond lengths (Å), angles (°) and dihedral angles (°)
between the least-squares planes of the pyridyl and phenyl rings for 2b
N1A–Ir1–N1B
N1A–Ir1–N1C
N1A–Ir1–C14A
N1A–Ir1–C14B
N1A–Ir1–C14C
N1B–Ir1–N1C
N1B–Ir1–C14A
N1B–Ir1–C14B
N1B–Ir1–C14C
N1C–Ir1–C14A
N1C–Ir1–C14B
N1C–Ir1–C14C
C14A–Ir1–C14B
C14A–Ir1–C14C
C14B–Ir1–C14C
93.2(4)
89.6(3)
80.4(4)
98.8(4)
165.8(4)
92.0(4)
171.4(4)
80.4(4)
96.4(4)
93.7(4)
169.0(4)
79.6(4)
94.8(5)
90.9(4)
93.2(5)
N1D–Ir2–N1E
N1D–Ir2–N1F
N1D–Ir2–C14D
N1D–Ir2–C14E
N1D–Ir2–C14F
N1E–Ir2–N1F
N1E–Ir2–C14D
N1E–Ir2–C14E
N1E–Ir2–C14F
N1F–Ir2–C14D
N1F–Ir2–C14E
N1F–Ir2–C14F
C14D–Ir2–C14E
C14D–Ir2–C14F
C14E–Ir2–C14F
91.2(4)
89.1(4)
80.7(4)
168.6(4)
91.6(4)
94.1(4)
94.4(4)
79.2(4)
173.0(5)
166.9(4)
97.7(4)
79.6(4)
93.7(5)
92.4(5)
98.6(5)
2b
Bond lengths
Ir1–N1A
Ir1–N1B
Ir1–N1C
2.140(6)
2.142(6)
2.131(6)
Ir1–C14A
Ir1–C14B
Ir1–C14C
2.020(8)
2.031(6)
2.034(8)
Bond angles
N1A–Ir1–N1B
N1A–Ir1–N1C
N1A–Ir1–C14A
N1A–Ir1–C14B
N1A–Ir1–C14C
N1B–Ir1–N1C
N1B–Ir1–C14A
N1B–Ir1–C14B
90.5(3)
91.1(3)
79.7(3)
96.2(3)
166.0(3)
92.8(3)
166.7(3)
79.4(3)
N1B–Ir1–C14C
N1C–Ir1–C14A
N1C–Ir1–C14B
N1C–Ir1–C14C
C14A–Ir1–C14B
C14A–Ir1–C14C
C14B–Ir1–C14C
99.8(3)
96.4(3)
169.3(3)
79.0(3)
92.6(3)
91.4(3)
95.0(3)
Dihedral angles
Apy–Aph
Bpy–Bph
10.9(4)
12.0(4)
21.4(4)
Dpy–Dph
Epy–Eph
Fpy–Fph
14.3(4)
15.3(4)
9.6(4)
Cpy–Cph
Dihedral angles
Apy–Aph
Bpy–Bph
16.5(3)
16.9(3)
Cpy–Cph
13.6(2)
a Two independent molecules exist in the asymmetric unit.
Fig. 3 (a) Molecular structure of 2b and (b) a dimer structure through π–π interaction between the 2-(4-fluorophenyl)pyridyl moieties in the crystal
packing of 2b.
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emission spectrum compared to 2a was observed in the emission
spectrum of 2c possessing the ethylene spacer linked tripodal
ligand. The homoleptic cyclometalated fac-Ir(C^N)3 complex 2b
possessing the ether spacer linked tripodal ligand also exhibited
a broad emission band around 484 nm. Hypsochromical shift of
the emission peak of 2b compared to that of 2a is due to the
inductive effect of the fluorine substitution. Photophysical pro-
perties of 2a–c were listed in Table 4. The quantum yields of
the homoleptic cyclometalated fac-Ir(C^N)3 complexes 2a–c in
dichloromethane solution are 37, 61 and 32%, respectively, in
line with previous results for fac-Ir(C^N)3 complexes.3,4 Lower
quantum yields of 2a–c in more polar acetonitrile and methanol
solution than in less polar dichloromethane solution supports the
3MLCT excited state.5
Conclusions
In conclusion, the tripodal ligands composed of the 1,3,5-trisub-
stituted cyclohexyl moiety as a molecular scaffold and 2-phenyl-
pyridyl moieties as a coordination site were designed to
demonstrate the formation of homoleptic cyclometalated fac-
Ir(C^N)3 complexes based on the regulation of ligand scram-
bling. The single-crystal X-ray structure determination confirmed
the fac configuration and a distorted octahedral geometry with
three intramolecular cyclometalated 2-phenylpyridyl ligands sur-
rounding the iridium metal center. Also, the cyclohexyl scaffold
was found to induce the fac configuration. Further investigations
including the tuning of the emission properties are now in
progress.
Experimental
General methods
All reagents and solvents were purchased from commercial
sources and were further purified by standard methods, if necess-
ary. Infrared spectra were obtained with a JASCO FT/IR-480
Plus spectrometer. 1H NMR and 13C NMR spectra were recorded
on a JEOL JNM-ECP 400 (400 and 100 MHz, respectively)
spectrometer with tetramethylsilane as an internal standard.
Mass spectra were run on a JEOL JMS-700 mass spectrometer.
Fig. 4 Electronic absorption spectra of 2a–c in dichloromethane
(2.5 × 10−5 M) at 298 K.
Synthesis of tripodal ligand 1a
A 100 mL two-necked bottle with a magnetic stirrer was charged
with 2-chloro-5-methylpyridine (5.5 mL, 50 mmol), phenylboro-
nic acid (7.32 g, 60 mmol) and Pd(PPh3)4 (1.73 g, 3 mol%)
under argon. Toluene (130 mL), ethanol (20 mL) and potassium
carbonate (2.0 M in water, 55 mL) were added using a syringe
through a rubber septum. The reaction mixture was stirred and
heated under reflux for 48 h. It was then cooled and poured into
a separatory funnel. The organic layer was separated and the
aqueous phase was extracted twice with dichloromethane. The
combined organic extracts were dried over anhydrous MgSO4,
filtered and concentrated in vacuo. The residue was purified by
column chromatography on silica gel eluting with hexane and
ethyl acetate to give 5-methyl-2-phenylpyridine as a white solid
Fig. 5 Emission spectra (λex = 350 nm) of 2a–c in dichloromethane
(2.5 × 10−5 M) at 298 K.
(6.85 g, 81% yield). To
a suspension of 5-methyl-2-
Table 4 Photophysical properties of 2a–c at 298 K
CH2Cl2
CH3CN
CH3OH
2
λema(nm)
Φ
λema(nm)
Φ
λema(nm)
Φ
τairb(ns)
2a
2b
2c
512
484
511
0.37
0.61
0.32
513
484
515
0.09
0.13
0.08
512
483
516
0.04
0.06
0.02
51.1(1.0)c
62.2(1.2)c
50.9(1.2)c
a λex = 350 nm. b Measured with 1.0 × 10−4 M dichloromethane solution (λex = 350 nm) at room temperature under air by the time-correlated single-
photon-counting technique (error 0 1 ns) monitoring λem
9522 | Dalton Trans., 2012, 41, 9519–9525
.
c The data in parentheses are χ2.
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phenylpyridine (1.69 g, 10 mmol) and NBS (2.14 g, 12 mmol)
in 50 mL of carbon tetrachloride, 0.024 g of benzoyl peroxide
was added. The mixture was refluxed for 3 h and then cooled
overnight. The succinimide precipitate was filtered off and the
filtrate was rotary evaporated to dryness. The residue was
purified by column chromatography on silica gel eluting with
hexane and ethyl acetate to give 5-(bromomethyl)-2-phenylpyri-
dine (1.32 g, 53% yield). To a mixture of 5-(bromomethyl)-2-
phenylpyridine (1.17 g, 4.70 mmol), cis,cis-1,3,5-cyclohexane-
triol dihydrate (0.24 g, 1.43 mmol) and 60% sodium
hydride (1.7 g, 42.5 mmol) under argon, DMF (10 mL)
was added using a syringe through a rubber septum. The
mixture was stirred at room temperature for 24 h and then
poured into ice water. The organic phase was separated by
extracting three times with dichloromethane. The solvents were
removed in vacuo and the residue was purified by column
chromatography on silica gel eluting with hexane and ethyl
acetate to afford the desired tripodal ligand 1a as a pale yellow
solid (0.67 g, 74% yield).
septum. The mixture was stirred at room temperature for 24 h
and then poured into ice water. The organic phase was separated
by extracting three times with dichloromethane. The solvents
were removed in vacuo and the residue was purified by column
chromatography on silica gel eluting with hexane and ethyl
acetate to afford the desired tripodal ligand 1b as a pale yellow
solid (0.25 g, 61% yield).
1b: IR (KBr) 3042, 2954, 2869, 1600, 1568, 1513, 1479,
1
1354, 1229, 1082 cm−1; H NMR (400 MHz, CD2Cl2) δ 8.61
(dd, 3H, J = 2.2, 0.7 Hz), 8.05–8.00 (m, 6H), 7.76 (dd, 3H, J =
8.2, 2.2 Hz), 7.71 (dd, 3H, J = 8.2, 0.7 Hz), 7.19–7.13 (m, 6H),
4.62 (s, 6H), 3.41 (tt, 3H, J = 11.4, 4.1 Hz), 2.54 (dt, 3H, J =
11.4, 4.1 Hz), 1.37 (dt, 3H, J = 11.4, 11.4 Hz); 13C NMR
1
(100 MHz, CD2Cl2) 163.9 (d, JF–C = 248.1 Hz), 155.9, 149.3,
3
136.6, 135.8, 133.0, 129.0 (d, JF–C = 8.4 Hz), 120.0, 115.9 (d,
2JF–C = 22.1 Hz), 73.3, 68.0, 38.6 ppm; HRMS (FAB) m/z calcd
for C42H36N3O3F3 (M+ + H), 688.2787; found, 688.2780.
1a: IR (KBr) 3034, 2940, 2860, 1600, 1563, 1475, 1446,
Synthesis of tripodal ligand 1c
1
1350, 1076 cm−1; H NMR (400 MHz, CD2Cl2) δ 8.63 (s, 3H),
8.03–8.00 (m, 6H), 7.76–7.75 (m, 6H), 7.49–7.45 (m, 6H),
7.43–7.39 (m, 3H), 4.63 (s, 6H), 3.42 (tt, 3H, J = 11.4, 4.0 Hz),
2.55 (dt, 3H, J = 11.4, 4.0 Hz), 1.38 (dt, 3H, J = 11.4, 11.4 Hz);
13C NMR (100 MHz, CD2Cl2) 156.9, 149.3, 139.5, 136.5,
133.0, 129.3, 129.0, 127.1, 120.3, 73.3, 68.0, 38.6 ppm; HRMS
(FAB) m/z calcd for C42H39N3O3 (M+ + H), 634.3070; found,
634.3058.
Hydrogen chloride was bubbled into a suspension of cis,cis-
1,3,5-cyclohexanetricarboxylic acid (1.0 g, 4.6 mmol) in 20 mL
of dry 1-propanol for 2 h. The suspension was heated under
reflux for 1 h and then cooled to room temperature. Volatiles
were removed under reduced pressure and the residue was
treated with ice water. The oily product was extracted with ether.
After being dried over anhydrous MgSO4, followed by filtration,
the filtrate was concentrated to remove volatiles, affording tripro-
pyl cis,cis-1,3,5-cyclohexanetricarboxylate quantitatively. A sol-
ution of tripropyl cis,cis-1,3,5-cyclohexanetricarboxylate (1.5 g,
4.5 mmol) in dry THF (20 mL) was added dropwise to a suspen-
sion of LiAlH4 (1.0 g) in dry THF (10 mL). The mixture was
refluxed for 24 h and cooled to 0 °C, the salts were decomposed
with water and ethanol. After filtration followed by rotary evap-
oration, cis,cis-1,3,5-cyclohexanetrimethanol was obtained
(0.7 g, 89% yield). Triphenylphosphine (1.9 g, 7.2 mmol) and
cis,cis-1,3,5-cyclohexanetrimethanol (0.4 g, 2.3 mmol) were dis-
solved in dry DMF(10 mL). Bromine (1.9 g, 11.9 mmol) was
added dropwise to the stirred mixture below room temperature.
The reaction was stirred at ambient conditions for 24 h. Removal
of the DMF under reduced pressure gave an orange-red oil. The
purification by column chromatography led to isolation of
cis,cis-1,3,5-tris(bromomethyl)cyclohexane in 55% yield (0.46 g).
To a solution of 2-phenyl-5-lithiomethylpyridine in THF (3 mL),
which was generated from 2-phenyl-5-methylpyridine (0.51 g,
3.0 mmol), cis,cis-1,3,5-tris(bromomethyl)cyclohexane (0.36 g,
1.0 mmol) was added dropwise in THF (2 mL) at 0 °C. The
reaction was warmed to room temperature and stirred for 24 h.
Then, the mixture was poured into ice water and the product was
extracted with ether three times. The residue was purified by
column chromatography on silica gel eluting with hexane and
ethyl acetate to afford the desired tripodal ligand 1c as a pale
yellow solid (0.23 g, 25% yield).
Synthesis of tripodal ligand 1b
A 200 mL two-necked bottle with a magnetic stirrer was charged
with 2-chloro-5-methylpyridine (2.7 mL, 25 mmol), 4-fluoro-
phenyl boronic acid (4.2 g, 30 mmol) and Pd(PPh3)4 (0.86 g,
3 mol%) under argon. To the mixture, toluene (75 mL), ethanol
(10 mL) and potassium carbonate (2.0 M in water, 28 mL) were
added successively and the whole mixture was refluxed for 48 h.
Then, it was cooled to room temperature. The aqueous layer was
separated and extracted three times with dichloromethane. The
combined organic layer was dried over anhydrous Na2SO4,
filtered and evaporated to give a red oil which solidified upon
standing. The crude product was purified by column chromato-
graphy on silica gel eluting with hexane and ethyl acetate to give
2-(4-fluorophenyl)-5-methylpyridine. Repeated precipitation at
−78 °C provided pure 2-(4-fluorophenyl)-5-methylpyridine
(2.48 g, 53%). To a 100 mL round bottle with a magnetic stirrer
were added 2-(4-fluorophenyl)-5-methylpyridine (1.0 g,
5.3 mmol), NBS (1.14 g, 6.4 mmol), benzoyl peroxide (13 mg,
0.05 mmol) and carbon tetrachloride (20 mL). The mixture was
heated to reflux for 5 h, then it was cooled and the resulting
solid was filtered off. The filtrate was evaporated to give a pale
yellow solid, which was purified by column chromatography on
silica gel eluting with hexane and ethyl acetate to give 5-(bromo-
methyl)-2-(4-fluorophenyl)pyridine (0.91 g, 65%). To a mixture
of 5-(bromomethyl)-2-(4-fluorophenyl)pyridine (0.53 g,
2.0 mmol), cis,cis-1,3,5-cyclohexanetriol dihydrate (0.1 g,
0.6 mmol) and 60% sodium hydride (0.8 g, 20 mmol) under
argon, DMF (2 mL) was added using a syringe through a rubber
1c: IR (KBr) 3029, 2914, 2848, 1596, 1561, 1473, 1446,
1390, 1019 cm−1; 1H NMR (400 MHz, CD2Cl2) δ 8.52 (dd, 3H,
J = 2.4, 0.7 Hz), 8.01–7.98 (m, 6H), 7.68 (dd, 3H, J = 8.2,
0.7 Hz), 7.58 (dd, 3H, J = 8.2, 2.4 Hz), 7.47–7.43 (m, 6H),
7.40–7.36 (m, 3H), 2.72–2.68 (m, 6H), 1.91 (d, 3H, J = 12.1 Hz),
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Preparation of facial cyclometalated iridium(III) complex 2c
1.64–1.57 (m, 6H), 1.43–1.32 (m, 3H), 0.68 (dt, 3H, J = 12.1,
12.1 Hz); 13C NMR (100 MHz, CD2Cl2) 155.0, 150.1, 139.8,
137.2, 136.8, 129.0, 128.9, 126.9, 120.2, 40.1, 39.3, 37.1,
30.4 ppm; HRMS (FAB) m/z calcd for C45H45N3 (M+ + H),
628.3692; found, 628.3697.
To a two-necked round bottom flask were added tripodal ligand
1c (0.12 g, 0.19 mmol), IrCl3·nH2O (0.071 g, 0.24 mmol), silver
trifluoroacetate (0.18 g, 0.81 mmol), 1,2-dichlorobenzene
(20 mL) and then water (1 mL). The mixture was heated to
reflux under argon for 5 h. After cooling to room temperature,
the reaction mixture was passed through a pad of Celite. The
filtrate was collected and the volatiles were removed in vacuo.
The residue was purified by column chromatography on silica
gel eluting with hexane and dichloromethane to afford the
desired facial cyclometalated iridium(III) complex 2c as a yellow
solid (7.4 mg, 5% yield).
Preparation of facial cyclometalated iridium(III) complex 2a
To a two-necked round bottom flask were added tripodal ligand
1a (0.32 g, 0.5 mmol), IrCl3·nH2O (0.179 g, 0.6 mmol), silver
trifluoroacetate (0.44 g, 2.0 mmol), 1,2-dichlorobenzene
(40 mL) and then water (2 mL). The mixture was heated to
reflux under argon for 5 h. After cooling to room temperature,
the reaction mixture was passed through a pad of Celite. The
filtrate was collected and the volatiles were removed in vacuo.
The residue was purified by column chromatography on silica
gel eluting with hexane and dichloromethane to afford the
desired facial cyclometalated iridium(III) complex 2a as a yellow
solid (16.0 mg, 4% yield).
2c: IR (KBr) 3036, 2929, 2849, 1604, 1580, 1485, 1436,
1
1393, 1029 cm−1; H NMR (400 MHz, CD2Cl2) δ 7.86 (d, 3H,
J = 8.3 Hz), 7.65–7.61 (m, 3H), 7.54 (dd, 3H, J = 8.3, 2.2 Hz),
7.35–7.31 (m, 3H), 6.92–6.87 (m, 6H), 6.82 (d, 3H, J = 2.2 Hz),
2.54–2.48 (m, 3H), 2.22–2.15 (m, 3H), 2.00–1.93 (m, 3H),
1.70–1.62 (m, 3H), 1.56–1.47 (m, 3H), 1.22 (d, 3H, J = 12.3
Hz), 0.67 (dt, 3H, J = 12.3, 12.3 Hz); 13C NMR (100 MHz,
CD2Cl2) 164.2, 160.4, 148.0, 144.6, 137.8, 137.6, 136.8, 129.7,
124.2, 120.2, 119.6, 39.0, 37.8, 35.1, 29.2 ppm; HRMS (FAB)
m/z calcd for C45H42N3Ir (M+), 817.3008; found, 817.3024.
2a: IR (KBr) 3033, 2941, 2848, 1604, 1579, 1480, 1429,
1
1395, 1135 cm−1; H NMR (400 MHz, CD2Cl2) δ 7.71 (d, 3H,
J = 8.0 Hz), 7.55 (d, 3H, J = 1.8 Hz), 7.55–7.52 (m, 3H), 7.48
(dd, 3H, J = 8.0, 1.8 Hz), 7.00–6.97 (m, 3H), 6.84–6.80 (m,
6H), 4.49 (d, 3H, J = 9.1 Hz), 3.85 (d, 3H, J = 9.1 Hz), 3.75 (br,
3H), 2.38 (d, 3H, J = 15.0 Hz), 1.55 (dt, 3H, J = 15.0, 4.0 Hz);
13C NMR (100 MHz, CD2Cl2) 165.1, 158.4, 147.8, 143.5,
136.4, 135.4, 133.3, 128.8, 122.8, 119.0, 118.0, 72.0, 67.1,
31.0 ppm; HRMS (FAB) m/z calcd for C42H36N3O3Ir (M+),
823.2386; found, 823.2397.
Proton magnetic resonance nuclear Overhauser effect
measurements
A sample was prepared under argon. Nuclear Overhauser effect
experiments were performed with 2 s irradiation of a freeze–
pump–thaw degassed CD2Cl2 solution of 2c. The 400 MHz
1H NMR spectra were recorded at 25 °C. Nuclear Overhauser
enhancements were obtained by saturation of the desired
resonance. Irradiation of the cyclohexyl proton at the 1-position
showed no enhancement of the cyclohexyl axial proton at the
2-position (see ESI†). Irradiation of the cyclohexyl axial proton
at the 2-position enhanced the pyridyl proton at the 6-position
(see ESI†).
Preparation of facial cyclometalated iridium(III) complex 2b
To a two-necked round bottom flask were added tripodal ligand
1b (50 mg, 0.07 mmol), IrCl3·nH2O (21 mg, 0.07 mmol), silver
trifluoroacetate (62 mg, 0.28 mmol), 1,2-dichlorobenzene
(40 mL) and then 2-ethoxyethanol (10 mL). The mixture was
heated to 140 °C under argon for 24 h. After cooling to room
temperature, the reaction mixture was passed through a pad of
Celite. The filtrate was collected and the volatiles were removed
in vacuo. The residue was purified by column chromatography
on silica gel eluting with hexane, ethyl acetate and dichloro-
methane to afford the desired facial cyclometalated iridium(III)
complex 2b as a yellow solid (7.2 mg, 12% yield).
Physical measurements
UV-vis spectra were obtained using a Hitachi U-3500 spectro-
photometer in dichloromethane solution under nitrogen at
298 K. Emission spectra were recorded using a Shimadzu
RF-5300PC spectrofluorophotometer and were not corrected for
instrument response. Emission measurements were carried out in
deaerated solvent at 298 K. Quinine sulfate in 1 N H2SO4 (ϕ =
0.55) was used as the standard for quantum yield measurements.
Emission lifetimes were measured with 1.0 × 10−4 M dichloro-
methane solution (λex = 350 nm) at room temperature under air
by the time-correlated single-photon-counting technique; error
0.1 ns; monitoring λem on a Hamamatsu FL920S instrument
equipped with a pulsed H2 light source.
2b: IR (KBr) 3044, 2946, 2857, 1589, 1552, 1484, 1448,
1
1254, 1182, 1124 cm−1; H NMR (400 MHz, CD2Cl2) δ 7.73
4
(d, 3H, J = 8.0 Hz), 7.62 (dd, 3H, J = 8.7 Hz, JF–H = 5.8 Hz),
7.60 (d, 3H, J = 2.0 Hz), 7.58 (dd, 3H, J = 8.0, 2.0 Hz), 6.68
3
(dd, 3H, J = 2.5 Hz, JF–H = 10.7 Hz), 6.64 (ddd, 3H, J = 8.7,
3
2.5 Hz, JF–H = 8.7 Hz), 4.56 (d, 3H, J = 9.2 Hz), 3.93 (d, 3H,
J = 9.2 Hz), 3.83 (br, 3H), 2.46 (d, 3H, J = 15.2 Hz), 1.62 (dt,
3H, J = 15.2, 4.0 Hz); 13C NMR (100 MHz, CD2Cl2) 165.1,
X-ray structure analysis
1
3
164.7 (d, JF–C = 251.1 Hz), 162.1 (d, JF–C = 5.3 Hz), 148.9,
141.0, 137.0, 134.4, 125.8 (d, 3JF–C = 9.1 Hz), 122.4 (d, 2JF–C
16.7 Hz), 119.2, 107.7 (d, JF–C = 23.6 Hz), 73.1, 68.2,
32.1 ppm; HRMS (FAB) m/z calcd for C42H33N3O3F3Ir (M+),
877.2103; found, 877.2127.
=
All measurements for 2a and 2b were made on a Rigaku
R-AXIS-RAPID diffractometer with graphite monochromated
Mo Kα radiation. The structures of 2a and 2b were solved by
direct methods and expanded using Fourier techniques. The non-
2
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hydrogen atoms were refined anisotropically. The H atoms were
placed in idealized positions and allowed to ride with the C
atoms to which each was bonded. Crystallographic details are
given in Table 1. Crystallographic data (excluding structure
factors) for the structures reported in this paper have been depos-
ited with the Cambridge Crystallographic Data Centre as sup-
plementary publication no. CCDC 828763 for 2a and CCDC
828764 for 2b.†
Acknowledgements
We thank Dr. G. Fukuhara and Prof. Y. Inoue at Osaka Univer-
sity for the measurement of emission lifetimes. Thanks are due
to the Analytical Center, Graduate School of Engineering, Osaka
University.
Notes and references
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This journal is © The Royal Society of Chemistry 2012
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