Synthesis, structures, and properties of iridium(III) bis-cyclometallated complexes containing three-atom chelates

Article abstract of DOI:10.1016/j.jorganchem.2007.12.009

Heating of [Ir(η²-ppy)₂(MeCN)₂]NO₃ (1, ppy = 2-phenylpyridine) in MeCN under reflux afforded [Ir(η²-ppy)₂(η²-NO₃)] (2). Treatment of 1 with 2-mercaptopyridine (Hmp), 6

Full text of DOI:10.1016/j.jorganchem.2007.12.009

Available online at www.sciencedirect.com
Journal of Organometallic Chemistry 693 (2008) 1510–1517
www.elsevier.com/locate/jorganchem
Synthesis, structures, and properties of iridium(III)
bis-cyclometallated complexes containing three-atom chelates
Wu-Sian Sie a, Jing-Yu Jian a, Tzu-Chih Su a, Gene-Hsiang Lee b, Hon Man Lee c,
Kom-Bei Shiu a,*
a Department of Chemistry, National Cheng Kung University, Tainan 701, Taiwan, ROC
b Instrument Center, National Taiwan University, Taipei 106, Taiwan, ROC
c Department of Chemistry, National Chuang Hua University, Chuang-Hua, Taiwan, ROC
Received 23 September 2007; received in revised form 4 December 2007; accepted 4 December 2007
Available online 15 December 2007
Dedicated to the memory of the late Professor F. Albert Cotton
Abstract
Heating of [Ir(g2-ppy)2(MeCN)2]NO3 (1, ppy = 2-phenylpyridine) in MeCN under reflux afforded [Ir(g2-ppy)2(g2-NO3)] (2). Treat-
ment of 1 with 2-mercaptopyridine (Hmp), 6-methyl-2-hydroxypyridine (Hmhp), 6-chloro-2-hydroxypyridine (Hchp), trimethylacetic
acid (Htma), benzoic acid (Hbz), 2-methylacrylic acid (Hma), and acetic acid (Hac) in the presence of excess Et3N produced [Ir(g2-
ppy)2(g2-XZY)] (XZYꢀ = mpꢀ (3), mhpꢀ (4), chpꢀ (5), acꢀ (6), bzꢀ (7), maꢀ (8), tmaꢀ (9)). Crystal structures of 2, 3, 7, 8, and 9 have
been characterized by X-ray diffraction. The inherent strain contained in the four-member rings, {Ir(g2-XZY)}, is apparently reflected in
the long Ir–X and Ir–Y distances. The absorption and emission properties of nearly all the new complexes except 2 show small variations.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Iridium(III); Cyclometallated complexes; Chelate
1. Introduction
chelates used in the complexes are hard (N, N), (N, O), and
(O, O) donors, whereas all the reported 3-atom chelates are
soft (S, S) and (Se, Se) ligands. This feature is understand-
able in terms of the inherent, much higher, strain contained
in four-member rings (i.e., {Ir(g2-XZY)}), relative to those
in five- or six-member rings, if all rings contain hard
(N, N), (N, O), and (O, O) chelates [8]. Hence, the reported
four-member examples containing only softer (S, S) and
(Se, Se) chelates (ligands, Lewis bases) and soft 5d transi-
tion-metal atom (Lewis acid) are obviously stabilized in
terms of the soft-base–soft-acid interaction according to
the Pearson HSAB (hard and soft acids and bases) theory
[9]. In this paper, however, we wish to report on the facile
syntheses and crystal structures of bis-cyclometallated
Ir(III) complexes with 3-atom (N, O), (N, S), and (O, O)
chelates. Both photophysical and electrochemical proper-
ties of these apparently thermally stable complexes are also
described.
Recently, luminescent cyclometallated d6 transition
metal complexes have been shown to exhibit an enormous
potential for a range of photonic applications. In particu-
lar, organic light emitting devices (OLEDs) [1] based on
the triplet emitters such as Ir(III) [2], Ru(II) [2c,2d,3],
and Os(II) [2c,2d,4] as the organic dopants have been dem-
onstrated with high efficiency. While extensive works have
been done on a series of bis-cyclometallated Ir(III) com-
plexes with 4- or 5-atom chelates [5,6], [(g2-(C^N)2)Ir(g2-
chelate)], the isoelectronic Ir(III) analogues with 3-atom
chelates (XZYꢀ) have received relatively less attention [7].
Note the contrasting feature whereby all the 4- and 5-atom
*
Corresponding author. Tel.: +886 6 2080473; fax: +886 6 2740552.
E-mail address: kbshiu@mail.ncku.edu.tw (K.-B. Shiu).
0022-328X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.12.009

W.-S. Sie et al. / Journal of Organometallic Chemistry 693 (2008) 1510–1517
1511
2. Experimental
were fitted by a single exponential decay. Analytical TLC
was carried out using Merck glass-backed 0.2 mm silica
gel 60 F254 plates. Flash column chromatography was
conducted using Merck silica gel 60 (230–400 mesh).
2.1. General information
All operations were carried out under prepurified N2 by
means of standard Schlenk and vacuum-line techniques.
Solvents were purified by standard procedures and distilled
prior to use. All organic reagents, including 2-mercapto-
pyridine (Hmp), 6-methyl-2-hydroxypyridine (Hmhp),
6-chloro-2-hydroxypyridine (Hchp), 1,1,1-trimethylacetic
acid (Htma), benzoic acid (Hbz), 2-methylacrylic acid
(Hma), and acetic acid (Hac) were purchased from Aldrich
and used without further purification. The bis-cyclometal-
lated iridium precursor complex, [Ir(g2-ppy)2(MeCN)2]-
NO3 (1, ppy = 2-phenylpyridine), was synthesized by the
method reported by Watts et al. [10]. Two different atom
5
5
3
6
3
6
4
6
6
4
6
6
2
1
2
1
4
4
B
B
A
A
B
D
A
C
5
5
N
N
2
2
3
3
Ir
Ir
3
3
N
N
2
2
4
4
5
5
2
1
2
1
1
labeling schemes for assignment of H NMR spectral sig-
3
6
5
3
6
5
4
4
nals for ppy are shown below for complexes with the pres-
1
ence or absence of a molecular C2 axis. H NMR spectra
2.2. [Ir(g2-ppy)2(g2-NO3)] (2)
were acquired on a Bruker AVANCE 300 (300 MHz) spec-
trometer using the deuterated solvent as the lock and the
residual solvent as the internal reference. IR spectra were
recorded on a Perkin–Elmer Spectrum-One or Bruker
TENSOR 27 FT-IR spectrophotometer. Elemental analy-
ses of carbon, hydrogen, and nitrogen were performed on
a Heraeus CHN-OS rapid elemental analyzer by the staff
of the Instrument Center, National Cheng Kung Univer-
sity. The ESI–MS spectra of the iridium complexes dis-
solved in MeCN, mixed with some drops of aqueous
NH3, were measured on a Micromass Quattro Ultima mass
spectrometer by the staff of the Instrument Center,
National Chung Cheng University. UV–vis spectra were
recorded with a Hewlett Packard 8453 instrument at room
temperature using 1-cm path length quartz cell. Cyclic
voltammetry (CV) was carried out in nitrogen-purged
anhydrous 10ꢀ3 M dichloromethane solution at room
temperature with a CHI 600a voltammetric analyzer. Tet-
ramethylammonium tetrafluoroborate (TBABF4) (0.1 M)
was used as a supporting electrolyte. The conventional
three-electrode configuration consists of a platinum work-
ing electrode, a platinum wire auxiliary electrode, and an
Ag/AgNO3 (MeCN) reference electrode with ferroce-
nium/ferrocene (Fc+/0) as the internal standard. Cyclic
votammograms were obtained at scan rate of 100 mV/s.
Photoluminescence (PL) spectra were performed on Hit-
achi F-4500 luminescence spectrophotometer. Excited-state
lifetimes were measured by a home-constructed time-
resolved Laser spectrometer. The instrument consists a
Quanta Ray GCR-170, pulsed Nd:YAG Laser, and used
the third harmonic (355 nm, FWHM = 10 ns) as an excita-
tion source. Emission signals were focused into an ARC
SpectraPro-500 monochrometer. The monochrometer out-
put was sent into a PMT (Hamamatsu, R928). The signal
was then further amplified by a home made fast amplifier
before being sent into a digitizer (LeCory 9350A). Decay
traces were transferred to a personal computer loaded with
the commercial software Origin 3.5. All experimental data
A solution of 1 (0.0505 g, 0.078 mmol) in MeCN (25 ml)
was refluxed for 3 h. The solvent was then pumped off.
Recrystallization from CH2Cl2 at room temperature affor-
ded the yellow product. Yield: 37 mg (84%). Anal. Calc. for
C22H16IrN3O3: C, 46.97; H, 2.87; N, 7.47. Found: C, 46.87;
H, 3.87; N, 8.34%. 1H NMR (CD2Cl2): d 8.80 (d, 2H,
J = 5.8 Hz, H6B), 7.96 (br, 4H, H3B and H4B), 7.60 (d,
2H, J = 7.8 Hz, H6A), 7.37 (br, 2H, H5B), 6.91 (br, 2H,
H5A), 6.72 (br, 2H, H4A), 6.12 (d, 2H, J = 7.6 Hz, H3A).
IR(KBr, cmꢀ1): 1549 (v(N@O)), 1284 (va(NO2)), 1010
(vs(NO2)).
2.3. [Ir(g2-ppy)2(g2-chelate)] (chelateꢀ = mpꢀ (3), mhpꢀ
(4), chpꢀ (5))
To a solution of 1 (ca. 0.16 mmol) in MeCN (15 ml) was
added Hmp, Hmhp, or Hchp (ca. 0.4 mmol) and Et3N (ca.
0.5 ml) stirred at room temperature for 30 min. The result-
ing solution was then heated under reflux overnight (ca.
8 h). The solvent was pumped off to produce the crude
product. The crude product was further purified with a
flash column chromatography and eluted with CH2Cl2. A
clear yellow band was then collected and evaporated to
dryness, affording the yellow crystalline product.
3. Yield: 79 mg (81%). Anal. Calc. for C27H20IrN3S: C,
53.10; H, 3.30; N, 6.88. Found: C, 53.27; H, 3.35; N, 6.86%.
1H NMR (CD2Cl2): d 9.86 (d, 1H, J = 5.4 Hz, H6B or
H6D), 8.03 (d, 1H, J = 5.7 Hz, H6D or H6B), 7.94 (m, 2H,
H4B and H4D) 7.79 (m, 2H, H3B and H3D), 7.65 (m, 2H,
mp), 7.20 (m, 2H, H6A and H6C), 7.16 (t, 1H, J = 7.2 Hz,
H5B or H5D), 7.12 (t, 1H, J = 7.1 Hz, H5D or H5B), 6.95
(t, 1H, J = 7.4 Hz, H5A or H5C), 6.79 (m, 4H with 2H from
mp, 1H as H5C or H5A, and 1H as H4A or H4C), 6.66 (t, 1H,
J = 7.5 Hz, H4C or H4A), 6.35 (d, 1H, J = 7.3 Hz, H3A or
H3C), 6.33 (d, 1H, J = 7.4 Hz, H3C or H3A). ESI–MS:
m/z 612 ((M+1)+).

1512
W.-S. Sie et al. / Journal of Organometallic Chemistry 693 (2008) 1510–1517
4. Yield: 73 mg (75%). Anal. Calc. for C28H22IrN3O: C,
H
3A), 6.00 (br, 1H, CH2), 1.89 (s, 3H, CH3). IR (KBr,
55.25; H, 3.64; N, 6.90. Found: C, 55.27; H, 3.83; N, 6.82%.
1H NMR (CD2Cl2): d 8.97 (d, 1H, J = 5.7 Hz, H6B or
H6D), 8.28 (d, 1H, J = 5.6 Hz, H6D or H6B), 7.94 (m, 2H,
H4B and H4D), 7.81 (m, 2H, H3B and H3D), 7.63 (m, 2H,
H6A and H6C), 7.30(t, 1H, J = 7.9 Hz, H5A or H5C),
7.24(t, 1H, J = 6.2 Hz, H5B or H5D), 7.17(t, 1H,
J = 6.2 Hz, H5D or H5B), 6.89 (t, 1H, J = 7.5 Hz, mhp),
6.87 (m, 2H, H5C or H5A), 6.71 (m, 2H, H4A and H4C),
6.29 (d, 1H, J = 7.5 Hz, mhp), 6.21 (d, 1H, J = 7.1 Hz,
H3A or H3C), 6.14 (d, 1H, J = 7.5 Hz, mhp), 6.06 (d, 1H,
J = 8.6 Hz, H3C or H3A), 2.14 (s, 3H, mhp). ESI–MS:
m/z 609 (M+).
cmꢀ1): 1501 (va (COOꢀ)), 1413 (vs (COOꢀ)). ESI–MS:
m/z 628 ((M+42)+).
9. Yield: 56 mg (58%). Anal. Calc. for C27H25IrN2O2: C,
53.89; H, 4.19; N, 4.66. Found: C, 53.85; H, 4.35; N, 4.63%.
1H NMR (300 MHz, CD2Cl2): d 8.78 (d, 2H, J = 5.7 Hz,
H6B), 7.96 (d, 2H, J = 7.9 Hz, H3B), 7.88 (t, 2H,
J = 7.7 Hz, H4B), 7.60 (d, 2H, J = 7.7 Hz, H6A), 7.34 (t,
2H, J = 6.5 Hz, H5B), 6.85 (t, 2H, J = 7.6 Hz, H5A), 6.67
(t, 2H, J = 7.6 Hz, H4A), 6.11 (d, 2H, J = 7.7 Hz, H3A),
1.18 (s, 9H, (CH3)3C). IR (KBr, cmꢀ1): 1509 (va (COOꢀ)),
1413 (vs (COOꢀ)).
5. Yield: 65 mg (43%). Anal. Calc. for C27H19ClIrN3O:
C, 51.55; H, 3.04; N, 6.68. Found: C, 55.27; H, 3.83; N,
6.82%. 1H NMR (CD2Cl2): d 8.90 (d, 1H, J = 5.6 Hz,
H6B or H6D), 8.42 (d, 1H, J = 7.3 Hz, H6D or H6B), 7.95
(m, 2H, H4B and H4D), 7.85 (m, 2H, H3B and H3D), 7.62
(m, 2H, H6A and H6C), 7.33(t, 1H, J = 7.7 Hz, chp), 7.22
(m, 2H, H5B and H5D), 6.88 (m, 2H, H5A and H5C), 6.72
(m, 2H, H4A and H4C), 6.42 (d, 1H, J = 7.5 Hz, chp),
6.27 (d, 1H, J = 7.8 Hz, H3C or H3A), 6.20 (d, 1H,
J = 8.6 Hz, chp), 6.13 (d, 1H, J = 7.6 Hz, H3C or H3A).
ESI–MS: m/z 629 (M+).
2.5. X-ray crystallography
All single crystals of 2, 3, 7, 8, and 9 were grown from
CH2Cl2/hexane at ambient temperature. The single crystals
of 7 were found in rod or block forms, designated as 7a and
7b, respectively. All crystals were also found suitable for
X-ray analysis after they were mounted on a Nonius Kappa
or a Bruker SMART APEX II CCD diffractometer, each
with graphite-monochromated Mo Ka radiation
˚
(k = 0.71073 A). The h range for data collection is 1.52–
27.50° for 2, 1.73–27.50° for 3, 2.22–27.50° for 7a, 2.33–
27.50° for 7b, 2.71–28.76° for 8, and 2.00–28.72° for 9. Of
the 17572, 19386, 8444, 11721, 13157, and 23362 reflec-
tions collected for 2, 3, 7a, 7b, 8, and 9, 4249, 5061, 2567,
2618, 2679, and 5871 reflections were independent, respec-
tively. All data were corrected for Lorentz and polarization
effects and for the effects of absorption. The structures were
solved by the direct method and refined by least-squares
cycles. All the non-hydrogen atoms were refined anisotrop-
ically. Almost all hydrogen atoms were either found from
difference maps or geometrically located. All calculations
were performed using SHELXTL-97 [11]. A crystallographic
C2 axis is imposed in compounds 7a, 7b, and 8 through
the Ir atom and the C12–C13 bond. In 8, two equivalent
C14 atomic positions were found, each with a 0.5 occu-
pancy of CH2 (i.e., H14D–C14–H14E) and CH3 (i.e.,
C14(H14A)(H14B)- (H14C)). The other essential details
of single-crystal data measurement and refinement are
given in Table 1, and selected distances and angles are listed
in Tables 2 and 3. Since both structures of 7a and 7b are
similar, only the ORTEP of 7a was shown. Thus, the per-
spective views for compounds 2, 3, 7a, 8, and 9 were shown
in Figs. 1–5, respectively.
2.4. [Ir(g2-ppy)2(g2-chelate)] (chelateꢀ = acꢀ (6),
bzꢀ (7), maꢀ (8), tmaꢀ (9))
These complexes were prepared similarly as for the Ir
analogues but using toluene as the solvent in place of
MeCN.
6. Yield: 30 mg (34%). Anal. Calc. for C24H19IrN2O2: C,
51.51; H, 3.42; N, 5.01. Found: C, 51.69; H, 3.32; N, 5.06%.
1H NMR (CD2Cl2): d 8.84 (d, 2H, J = 4.8 Hz, H6B), 7.93
(d, 2H, J = 8.0 Hz, H3B), 7.86 (t, 2H, J = 7.7 Hz, H4B),
7.57 (d, 2H, J = 7.6 Hz, H6A), 7.33 (t, 2H, J =6.3 Hz,
H5B), 6.83 (t, 2H, J = 7.1 Hz, H5A), 6.64 (t, 2H,
J = 7.3 Hz, H4A), 6.07 (d, 2H, J = 7.5 Hz, H3A), 1.98 (s,
3H, CH3). IR (KBr, cmꢀ1): 1524 (va (COOꢀ)), 1411 (vs
(COOꢀ)).
7. Yield: 81 mg (80%). Anal. Calc. for C29H21IrN2O2: C,
56.02; H, 3.40; N, 4.51. Found: C, 56.32; H,3.45; N, 4.53%.
1H NMR (300 MHz, CD2Cl2): d 8.87 (d, 2H, J = 5.4 Hz,
H6B), 8.04 (d, 2H, J = 7.4 Hz, Ph), 7.96 (d, 2H,
J = 8.0 Hz, H3B), 7.85 (t, 2H, J = 7.8 Hz, H4B), 7.62 (d,
2H, J = 7.6 Hz, H6A), 7.53 (m, 1H, Ph), 7.43 (t, 2H,
J = 7.4 Hz, Ph), 7.27 (t, 2H, J = 6.3 Hz, H5B), 6.88 (t,
2H, J = 7.3 Hz, H5A), 6.70 (t, 2H, J = 7.3 Hz, H4A), 6.14
(d, 2H, J = 7.6 Hz, H3A). IR (KBr, cmꢀ1): 1508 (va
(COOꢀ)), 1414 (vs (COOꢀ)).
3. Results and discussion
3.1. Cyclometallated complexes containing (O, O), (N, O),
and (N, S) donor ligands
8. Yield: 67 mg (71%). Anal. Calc. for C26H21IrN2O2: C,
53.32; H, 3.61; N, 4.78. Found: C, 53.34; H, 3.66; N, 4.72%.
1H NMR (CD2Cl2): d 8.79 (d, 2H, J = 4.7 Hz, H6B), 7.93
(d, 2H, J = 7.5 Hz, H3B), 7.85 (t, 2H, J = 7.1 Hz, H4B),
7.58 (d, 2H, J = 7.3 Hz, H6A), 7.31 (t, 2H, J = 4.9 Hz,
H5B), 6.84 (br, 2H, J = 7.4 Hz, H5A), 6.81 (br, 1H, CH2),
6.65 (t, 2H, J = 7.2 Hz, H4A), 6.08 (d, 2H, J = 7.7 Hz,
Treatment of mononuclear [Ir(g2-ppy)2(MeCN)2]NO3
(1) with suitable chelating anions produced substituted
complexes 2–9. Apparently, the nitrate anion in 1 can serve
as a chelate, enabling to replace two lightly ligated MeCN
molecules in MeCN to form [Ir(g2-ppy)2(g2-NO3)] (2). The

W.-S. Sie et al. / Journal of Organometallic Chemistry 693 (2008) 1510–1517
1513
Table 1
Crystallographic data
2
3
7a
7b
8
9
Chem. formula
Cryst. syst.
FW
C22H16IrN3O3
Monoclinic
562.58
C27H20IrN3S
Monoclinic
610.72
C29H21IrN2O2
Monoclinic
621.68
C29H21IrN2O2
Orthorhombic
621.68
C26H21IrN2O2
Orthorhombic
585.65
C27H25IrN2O2
Monoclinic
601.69
T, K
150(2)
C2/c
150(2)
P21/n
150(2)
C2/c
150(2)
Pbcn
150(2)
Aba2
150(2)
P21/n
Space group
˚
a, A
30.0232(4)
9.4160(1)
14.6786(2)
90
116.4784(8)
90
3714.33(8)
8
2.012
9.7746(10)
9.5295(9)
23.659(2)
90.00
93.981(2)
90.00
2198.4(4)
4
1.845
14.6149(11)
12.0898(9)
13.0841(10)
90.00
104.772(2)
90.00
2235.4(3)
4
1.847
12.4189(9)
12.2054(9)
14.9516(11)
90.00
90.00
90.00
2281.3(3)
4
1.810
5.883
0.0214/0.0458
1.035
12.5384(4)
15.0371(5)
11.1258(4)
90.00
90.00
90.00
2097.67(12)
4
1.854
6.391
0.0296/0.0714
1.067
10.7434(3)
23.1210(6)
9.6655(3)
90.00
108.8210(10)
90.00
2272.52(11)
4
1.759
Ft, A
˚
c, A
a, °
b, °
c, °
3
˚
V, A
Z
Dcalc, g cmꢀ3
l, mmꢀ1
R1/wR2
7.219
0.0409/0.1072
1.068
6.189
0.0373/0.0750
1.147
6.003
0.0209/0.0480
1.037
5.902
0.0276/0.0883
0.746
GOF on F2
Table 2
˚
Selected bond lengths (A) and bond angles (°) for 2, 7a, 7b, 8, and 9
2
7a
7b
8
9
Bond lengths
Ir–O
2.254(3)
2.227(3)
2.037(3)
2.040(4)
1.985(4)
1.986(6)
2.240(2)
2.028(2)
1.983(3)
2.239(2)
2.028(2)
1.980(3)
2.237(4)
2.038(4)
1.983(5)
2.283(2)
2.216(2)
2.050(3)
2.021(3)
1.992(3)
1.994(3)
Ir–N
Ir–C
Bond angles
O(1)–Ir–O(2)
N(1)–Ir–N(2)
C(1)–Ir–C(2)
C–Ir–N
57.5(1)
177.8(1)
87.6(2)
80.4(2)
81.3(1)
97.5(2)
97.7(2)
58.54(8)
177.2(1)
88.8(1)
80.9(1)
58.75(7)
177.5(1)
87.7(1)
80.9(1)
59.1(1)
176.7(1)
86.3(2)
81.0(2)
58.15(8)
174.8(1)
85.2(1)
80.6(1)
80.6(1)
97.3(1)
94.3(1)
C–Ir–N0
97.1(1)
97.3(1)
96.5(2)
Fig. 1. Perspective view of [Ir(g2-ppy)2(g2-NO3)] (2).
Table 3
˚
conversion from complex 1 to 2 can be monitored by
Selected bond lengths (A) and bond angles (°) for 3
means of the related spectral data. Following the conver-
3 (X=N, S)
sion, a typical IR N–O stretching band around 831 cmꢀ1
,
Bond lengths
Ir–X
assigned for the free nitrate anion in 1, disappeared gradu-
ally and three new bands of 1549 (v(N@O)), 1284
(va(NO2)), 1010 (vs(NO2)), assigned for the chelating
nitrate, grew [12]. The two ligated MeCN signals also faded
N, 2.127(4)
S, 2.509(1)
2.044(4)
2.035(4)
2.007(5)
Ir–N
Ir–C
1
away in the respective H NMR spectra. Truly, the struc-
1.997(5)
ture was later confirmed by X-ray crystallography, as
described below. A striking difference in solubility between
complexes 1 and 2 in an organic solvent is also observed.
Complex 1 is insoluble in CH2Cl2. Upon formation of
g2-NO3, complex 2 becomes very soluble in this solvent.
As expected, treatment of 1 with the apparently stronger
chelates of mpꢀ, mhpꢀ and chpꢀ in MeCN resulted in
ready substitution of two MeCN ligands and formation
Bond angles
X–Ir–X0
N(1)–Ir–N(2)
C(1)–Ir–C(2)
C–Ir–N
66.3(1)
173.1(2)
90.2(2)
80.8(2)
80.9(2)
94.5(2)
94.0(2)
C–Ir–N0

1514
W.-S. Sie et al. / Journal of Organometallic Chemistry 693 (2008) 1510–1517
Fig. 2. Perspective view of [Ir(g2-ppy)2(g2-mp)] (3).
Fig. 4. Perspective view of [Ir(g2-ppy)2(g2-ma)] (8).
Fig. 5. Perspective view of [Ir(g2-ppy)2(g2-tma)] (9).
Fig. 3. Perspective view of [Ir(g2-ppy)2(g2-bz)] (7).
edge, 7–9 are the first reported structures containing
{Ir(g2-carboxylato)} in the literature, although the chosen
single crystal of 9 was of lower quality as reflected in the
poorer agreement factor (Table 1). Selected bond lengths
and angles for complexes with (O, O) chelates, 2, 7a, 7b,
8, and 9 are compiled in Table 2 and those for 3 with the
(N, S) chelate are compiled separately in Table 3 for
comparison.
of complexes 3–5. It was soon found that the easy conver-
sion can also be performed in toluene to produce
complexes 6–9. Scheme 1 summarizes the new reactions
of complex 1 with (O, O), (N, O), and (N, S) chelates.
All the new products have been fully characterized by spec-
troscopic methods and elemental analyses. The assign-
ments of the 1H NMR signals for the coordinated ppy
ligands of the new complexes, 6, 8, and 9, are straightfor-
ward, for they contain a symmetrical axis of C2 and no
extra aromatic substituents in their chelates [7,8]. However,
the much complicated signals, displayed by complexes, 3–5,
and 7, can be assigned with aids from COSY experiments.
The solid-state structures of 2, 3, 7, 8, and 9 have been
unambiguously established by X-ray crystallography and
shown in Figs. 1–5, respectively. To the best of our knowl-
The geometry around Ir in each of the complexes is
distorted octahedral with two mutually trans pyridyl groups
and the g2-XYZ being opposite to the phenyl rings. The
˚
˚
˚
average Ir–C (1.986 A for 2, 2.002 A for 3, 1.983 A for 7a,
˚
˚
˚
1.980 A for 7b, 1.983 A for 8, and 1.993 A for 9) and
Ir–N(ppy) (2.039 A for 2, 2.040 A for 3, 2.028 A for 7a,
2.028 A for 7b, 2.038 A for 8, and 2.036 A for 9) are normal
˚
˚
˚
˚
˚
˚
by comparison with other Ir(III) bis-ppy complexes
˚
[5h,6a,6l,6m,6n]. The average Ir–O distances (2.241 A for

W.-S. Sie et al. / Journal of Organometallic Chemistry 693 (2008) 1510–1517
1515
with 3-atom XYZ chelates, compared with those in either
complex 10 with a 4-atom chelate or complexes 11–13 with
5-atom chelates, do reflect the much higher ring strain.
N
Ir
O
O
C
C
N
O
(2)
N
3.2. Photophysical study
N
Ir
N
S
C
C
The absorption and photoluminescence data of all the
iridium(III) complexes in CH2Cl2 solutions are summa-
rized in Table 4. Three typical photoluminescence spectra
of 3, 4, and 6 are depicted in Fig. 6. The strong absorption
bands in the ultraviolet region at about 250–280 nm with
distinct vibronic features are assigned to the spin-allowed
heating
- 2 MeCN
N
(3)
+
-
+ mp
- 2 MeCN
Me
N
Ir
C
N
C
C
-NO3-
NO3-
1
intraligand p–p* transitions. The next lower energy in
N
1
the shoulder region of p–p* transitions at about
C
N
Me
+ mhp-
or chp-
(1)
280–350 nm can be ascribed to the typical spin-allowed
metal-to-ligand charge-transfer (1MLCT) transition, while
typical extinction coefficients at peak wavelengths are in
the range 4500–5200 Mꢀ1 cmꢀ1 [13]. The weak shoulders
extending into the visible region at 350–500 nm are
believed to be associated with both spin–orbit coupling
R'
N
Ir
- 2 MeCN
+ RCO2-
-NO3-
N
O
C
C
- 2 MeCN
- NO3-
N
3
enhanced p–p* and 3MLCT (spin-forbidden metal-to-
R' =Me (4), Cl (5)
ligand charge-transfer) transitions, and typical extinction
coefficients at peak wavelengths are in the range 4200–
N
Ir
O
C
C
C
R
4800 Mꢀ1 cmꢀ1
.
O
Highly intensive green luminescence was observed for
our new iridium complexes in CH2Cl2 with kmax at 505–
523 nm and acetone with kmax at 518–524 nm. It appears
that complex 2 is unique, showing kmax at 505 nm in
CH2Cl2 and a 11-nm hypsochromic shift from that found
for [Ir(g2-ppy)2(g2-acac)] (11) (kmax = 516 nm [6a]),
whereas all other complexes, 3–9, show kmax > 516 nm
and a small (2–7 nm) bathochromic shift. Since complex
2 emits light with kmax at 520 nm in acetone, a large solvent
effect is also observed. However, unfortunately as indicated
by the low quantum yield as 0.0001 (Table 4), the intensity
of the emission band from 2 is the lowest among the eight
complexes studied (cf. Fig. 6). Following the shortest
excited-state lifetime of less than 10 ns for 2, compared
with those of 113–253 ns for other complexes (Table 4), it
appears that the excited state of 2 is quenched more easily.
N
R = Me (6), Ph(7), C(Me)CH2(8), CMe3(9)
Scheme 1.
˚
˚
˚
˚
2, 2.240 A for 7a, 2.239 A for 7b, 2.237 A for 8, and 2.283 A
˚
for 9) are significantly longer than those of 2.194 A in
2
2
2
2
˚
[Ir2(g -ppy)4(l:g ,g -ox)] (10) [5h], 2.146 A in [Ir(g -
2
2
2
˚
ppy)2(g -acac)] (11) [6a], 2.169 A in [Ir(g -ppy)2(g -pmo)]
2
2
˚
(12) [6l], and 2.146 A in [Ir(g -ppy)2(g -mao)] (13) [6n],
where oxꢀ is oxalato; acacꢀ is acetylacetonato; pmoꢀ is1-
phenyl-3-methyl-4-isobutyryl-1H-pyrazol-5-olato; and maoꢀ
is 3-methylpentane-2,4-dionato. Quite obviously, the longer
distances between Ir and the donor atoms in our complexes
Table 4
kmax of UV–vis and photoluminescence data of iridium(III) complexes
Absorbancea
Emission
Complex
k
max/nm (e)b
kmax/nm
Uddegassed
s/nsa
2
3
4
5
6
7
8
9
a
254(233.0), 394(88.8), 423(79.0)
264(40.4), 370(5.2), 449(2.3), 485(0.8)
261(45.3), 367(3.7), 398(2.5), 458(1.6)
260(42.7), 372(2.6), 398(1.8), 452(1.3)
262(62.5), 397(6.2), 454(4.9)
262(62.1), 423(6.2)
263(34.3), 400(3.35), 460(2.82)
264(32.0), 365(3.4), 398(2.3), 485(1.8)
505a (520)c
521 (524)
520 (521)
518 (518)
518 (521)
522 (520)
521 (520)
523 (519)
0.0001
0.0029
0.0365
0.0058
0.2338
0.1031
0.1984
0.1335
<10
165
116
223
253
113
118
253
In CH2Cl2 solution (concentration = 10ꢀ5 M).
The unit of e is 103 Mꢀ1 cmꢀ1
b
c
.
In acetone solution.
The quantum yields (U) in degassed CH2Cl2 solution were measured at 298 K with mer-[Ir(g2-ppy)2(g2-acac)] (U = 0.34) as a standard.
d

1516
W.-S. Sie et al. / Journal of Organometallic Chemistry 693 (2008) 1510–1517
HOMO, LUMO, and energy gap data of complexes 4, 5,
9
8
7
6
5
7, and 9 are very close to each other, except the redox
potentials. This feature is apparently supportive to the pre-
vious theoretical results that the ancillary ligands only
influence the HOMO energy levels [5l,6a,14,15]. Further,
the E1/2 data of iridium(III) complexes 5 and 7 exhibit
higher values (451–476 mV) than others (411–440 mV).
This fact implies that despite the similar energy gap of these
complexes, the different substitute groups at different sites
on the chelate ligands have a significant effect on their
molecular orbital energy. When an electron-donating
3
4
6
t
group such as Me to replace Cl of chp, and Bu to replace
450
500
550
600
650
Ph of bz, the E1/2 shifts negatively to 411 mV for 4 from
that of 476 mV for 5, and 440 mV for 9 from that of
451 mV for 7.
Wavelength (nm)
Fig. 6. Emission spectra of 3, 4, and 6 in CH2Cl2 at room temperature.
4. Conclusion
As revealed previously by electrochemistry and theoretical
calculations of related cyclometallated Ir(III) complexes
[5l,6a,14,15], the influence of ancillary ligands of bis-cyclo-
metallated iridium complexes is on the HOMO energy
level. Quite obviously, the chelates (such as g2-nitrate) at
stronger ligand field strength than acac may lower the
HOMO energy levels and the corresponding complexes
show a blue shift, whereas those (such as mp, mhp, chp,
or carboxylate) at weaker ligand field strength than acac
may raise the energy levels and the complexes show a red
shift.
This work reports detailed synthesis, structures, and
electrochemical and photophysical properties of eight
green-emitting iridium(III) metal complexes 2–9 containing
bis-cyclometallated ppy ligands and an g2-chelate in
(O, O), (N, O), or (N, S) donor (Scheme 1). Each of the
iridium complexes is endowed with a four-membered che-
late ring, {Ir(g2-chelate)}. The physical properties of the
complexes support the theoretical results that the ancillary
ligands only influence the HOMO energy levels, reported
previously [5l,6a,14]. However, the absorption and emis-
sion properties of nearly all the new complexes except 2
show small variations (Table 4). It is demonstrated in the
article that a nitrate can serve as an g2-chelate (cf. 2 in
Fig. 1). This complex is unique, emitting light at
3.3. Electrochemical study
The electrochemical behavior of all new iridium(III)
complexes in CH2Cl2 was performed. Complexes 2, 3, 6,
and 8 undergo an irreversible oxidation with Eap ¼ 746,
425, 607, and 588 mV, respectively, whereas the other com-
plexes display a quasi-reversible oxidation (Table 5). The
features can be attributed into the instability of the oxi-
dized species. No reduction processes were observed within
the solvent cathodic potential limit. The energy level of the
HOMO was calculated by following the published proce-
dure [16], while that of the LUMO was evaluated from
the long-wavelength absorption edge using the theory
reported [17]. As shown in Table 5, the summarized
k
max = 505 nm, but suffering the low quantum yield and
the instability of the oxidized intermediate species in
CH2Cl2. Like 1, the complexes containing (N, O) or
(N, S) chelate also suffer the low quantum yields, but the
complexes of carboxylates are different, showing high
quantum yields (0.103–0.234) (Table 4). Based on the elec-
trochemical behavior (cf. Table 5), it appears that either an
t
aryl group (e.g., Ph) or a sterically bulky group (e.g., Bu)
on a carboxylate moiety, f–COꢀ2 g, can stabilize or protect
the oxidized intermediate species so that both 7 and 9 exhi-
bit a quasi-reversible oxidation whereas both 6 and 8
undergo an irreversible oxidation. Further OLED-related
fabrication using some of the complexes reported here is
in progress.
Table 5
Electrochemical data, HOMO, LUMO, and energy gap of iridium(III)
complexes
Complex
E
(mV)a HOMO (eV)b LUMO (eV)c Energy gap (eV)
ox
1=2
5. Supporting information available
4
5
7
9
411
476
451
440
ꢀ5.2
ꢀ5.4
ꢀ5.4
ꢀ5.3
ꢀ2.9
ꢀ3.1
ꢀ3.1
ꢀ3.0
2.3
2.3
2.3
2.3
Crystallographic data for complexes 2, 3, 7a, 7b, 8, and 9
have been deposited with the Cambridge Crystallographic
Data Centre, CCDC Nos. 658877, 658878, and 658880–
658883, respectively, in CIF format. Copies of this infor-
mation may be obtained free of charge from The Director,
CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax:
+44 1223 336033; e-mail: deposit@ccdc.cam.ac.uk).
a
ox
1=2
E
¼ 1=2ðEap þ Ecp) (vs. Ag/Ag+), referenced against ferrocene/fer-
rocenium as internal standard.
b
Data were collected in CH2Cl2 solution containing 0.001 M irid-
ium(III) complexes by cyclic voltammograms.
c
LUMO = HOMO + energy gap.

W.-S. Sie et al. / Journal of Organometallic Chemistry 693 (2008) 1510–1517
1517
(l) J. Li, P.I. Djurovich, B.D. Alleyne, M. Yousufuddin, N.N. Ho,
J.C. Thomas, J.C. Peters, R. Bau, M.E. Thompson, Inorg. Chem.
44 (2005) 1713;
(m) I.R. Laskar, S.-F. Hsu, T.-M. Chen, Polyhedron 25 (2006) 1167;
(n) A.S. Ionkin, W.J. Marshall, B.M. Fish, Organometallics 25
(2006) 1461;
(o) K.K.-W. Lo, C.-K. Chung, N. Zhu, Chem.-Eur. J. 12 (2006)
1500;
(p) Q. Zhao, S. Liu, M. Shi, C. Wang, M. Yu, L. Li, F. Li, T. Yi, C.
Huang, Inorg. Chem. 45 (2006) 6152;
Acknowledgements
The work has been supported financially by the
National Science Council of the Republic of China, Project
NSC 95-2113-M-006-012-MY3. We are also grateful to
Professor I-Jy Chang and her group for measuring the life-
time of the iridium complexes, reported herein.
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