Diastereoselective formation of chiral tris-cyclometalated iridium (III) complexes: Characterization and photophysical properties

Article abstract of DOI:10.1021/ja048655d

Chiral, facial tris-cyclometalated Ir(III) complexes, fac-Δ-Ir(pppy) ₃, fac-Λ-Ir(pppy)₃, fac-Λ-IrL (where pppy is (8R, 10R)-2-(2′-phenyl)-4,5-pinenopyridine and L is a tripodal ligand comprising three pppy moieties connected through

Full text of DOI:10.1021/ja048655d

Published on Web 07/09/2004
Diastereoselective Formation of Chiral Tris-Cyclometalated
Iridium (III) Complexes: Characterization and Photophysical
Properties
Christine Schaffner-Hamann,† Alexander von Zelewsky*,† Andrea Barbieri,‡
Francesco Barigelletti,‡ Gilles Muller,§ James P. Riehl,§ and Antonia Neels|
Contribution from the Department of Chemistry, UniVersity of Fribourg, Pe´rolles,
CH-1700 Fribourg, Switzerland, Istituto ISOF-CNR, Via P. Gobetti 101, 40129 Bologna, Italy,
Department of Chemistry, UniVersity of Minnesota, Duluth, Minnesota 55812-3020, and Institute
of Chemistry, UniVersity of Neuchaˆtel, AV. BelleVaux 51, CH-2000 Neuchaˆtel, Switzerland
Received March 9, 2004; E-mail: Alexander.vonzelewsky@unifr.ch
Abstract: Chiral, facial tris-cyclometalated Ir(III) complexes, fac-∆-Ir(pppy)3, fac-Λ-Ir(pppy)3, fac-Λ-IrL (where
pppy is (8R,10R)-2-(2′-phenyl)-4,5-pinenopyridine and L is a tripodal ligand comprising three pppy moieties
connected through a mesityl spacer) have been synthesized and characterized. In IrL, NMR and CD studies
indicate that only one diastereomer is formed, with the Λ configuration at the metal center, whereas
enantiopure pppy yields the fac-Λ- and the fac-∆-stereoisomer in a ratio 2:3. fac-Λ-IrL was structurally
characterized using X-ray crystallography. The luminescence properties including CPL, of the three
complexes and their sensitivity to dioxygen were examined.
Introduction
longer excited-state lifetimes (τ on microsecond time scale) and
higher luminescence efficiencies (Φ ≈ 10-1-10-2) in fluid
The optical properties of transition metal complexes, espe-
cially the d6 Ru(II), Os(II), Rh(III), and Ir(III) species, have
been studied by a wide variety of spectroscopic approaches.1,2
In particular, the luminescent and photophysical properties of
related tris-chelate Ir(III) complexes have been the object of
detailed studies.1,3,4 Ir(III) complexes can be prepared with either
diimine ligands or cyclometalated ligands, such as 2-phenylpy-
ridine (ppy)3-5 and the tris-chelate complexes with this formally
monoanionic ligand are isoelectronic with the cationic tris-
diimine complexes of Ru(II) and Os(II). Compared to Ru(II)
and Os(II) complexes, the diimine d6 Ir(III) complexes exhibit
solutions, at room temperature.1-6 The luminescence yields are
found to be even higher for Ir(ppy)3 and related complexes, in
some cases with Φ > 0.5.5-7 Also for this reason, Ir(III)
cyclometalated complexes are receiving a great deal of attention
as efficient phosphor dopant in polymer matrix for applications
in the area of organic light emitting diode devices (OLEDs).7
Recently,8 we described the synthesis of a chiral tripodal ligand
based on bipyridine units and the study of its coordination with
ruthenium (II) and iron (II). It was demonstrated that this ligand
enables the formation of octahedral complexes with predeter-
mined configuration at the metal center with complete stereo-
selectivity. Bis-cyclometalated complexes of rhodium (III) were
also described using chiral ligands,9 but less attention has been
devoted till now to iridium (III) tris-cyclometalated complexes
in diastereoselective synthesis. Here, we describe the formation,
† University of Fribourg.
‡ Istituto ISOF-CNR.
§ University of Minnesota Duluth.
| University of Neuchaˆtel.
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9
10.1021/ja048655d CCC: $27.50 © 2004 American Chemical Society
J. AM. CHEM. SOC. 2004, 126, 9339-9348
9339

A R T I C L E S
Schaffner-Hamann et al.
characterization, absorption, and luminescence properties (in-
cluding circularly polarized luminescence, CPL) of facial
mononuclear Ir(III) complexes comprising three chiral cyclo-
metalating didentate ligands, which are derivatives of phenyl-
pyridine. Two diastereoisomers of the tris-didentate complex
with (8R,10R)-2-(2′-phenyl)-4,5-pinenopyridine (pppy), as well
as a single stereoisomer of a related tripodal ligand (L) are
investigated.
decay profiles against time was accomplished by using software
provided by the manufacturer. The lifetime values are obtained
with an estimated uncertainty of 10%. The circularly polarized
luminescence and total luminescence spectra were recorded on
an instrument described previously, operating in a differential
photon-counting mode.14 The excitation source was a 450-watt
xenon continuous wave arc lamp of a SPEX Fluorolog 2
spectrofluorimeter. The dispersion of the excitation and emission
monochromators (SPEX 1681B) was 4 nm/mm. The measure-
ments were performed in dry acetonitrile solutions at concentra-
tions of 1 × 10-4 M at 295 K.
Experimental Section
General. NMR spectra were recorded on a Varian Gemini-
300 (for 300 MHz NMR) or on a Bruker Advance DRX400
(for 400 MHz NMR) spectrometers. Chemical shifts are given
in ppm using the solvent as internal standard. Mass spectral
data (ESI and HRMS) were acquired on a Bruker FTMS 4.7 T
BioApex II. Circular dichroism (CD) spectra were recorded on
a Jasco J-715 spectropolarimeter. Electrochemical measurements
were carried out at room temperature using a PAR 273A
electrochemical analysis system with 270 research electrochem-
istry software. Cyclic voltammograms were obtained in buty-
ronitrile, using a microcell equipped with a stationary platinum
disk electrode with tetra-n-butylammonium hexafluorophosphate
(0.1 M) as base electrolyte. Ru(bpy)32+ was used as the standard,
taking the oxidation potential as +1.26 V, vs SCE.10 Half-wave
potentials were calculated as an average of the cathodic and
anodic peak. UV/Visible spectra for acetonitrile or CH2Cl2
solutions were measured on a Perkin-Elmer Lambda 40
spectrometer. The steady-state luminescence spectra for air-
equilibrated and freeze-pump-thaw degassed dilute (ca. 2 ×
10-5 M) acetonitrile solutions of the investigated complexes
were measured using a Perkin-Elmer LS 50B spectrometer or
a Spex Fluorolog spectrometer, equipped with a Hamamatsu
R928 tube; an excitation wavelength of 350 nm was used.
Luminescence quantum yields at room temperature (Φ) were
evaluated by comparing wavelength-integrated intensities (I)
with reference to [Ru(bpy)3]Cl2 (Φr ) 0.028 in air-equilibrated
water11), and quinine sulfate (Φr ) 0.546 in 1 N H2SO412) as
standards and by using the following equation13
Materials. Oxygen- or water-sensitive reactions were con-
ducted under a positive pressure of argon in oven-dried
glassware, using Schlenk techniques. Unless otherwise stated,
commercial grade reagents (spectroscopic grade when needed)
were used without further purification. 1,3,5-tris-bromomethyl-
benzene,15 (8R,10R)-2-(2′-phenyl)-4,5-pinenopyridine (pppy),9c
2-(2′-hydroxy)ethyl-2-tetrahydro-2H-pyran ether16 were prepared
according or analogous to the literature methods. Ir(acac)3 was
purchased from Strem Chemicals.
X-ray Crystallography. Suitable orange single crystals of
fac-Λ-IrL were obtained by slow diffusion of acetone solution
of the complex in water at room temperature. Data were
measured at -120 °C using Mo-KR radiation (λ ) 0.710 73
Å) with a Stoe Mark II-Imaging Plate Diffractometer System
(Stoe & Cie, 2002)17 equipped with a graphite-monochromator.
The structure was solved by direct methods using the program
SHELXS-97.18 The refinement and all further calculations were
carried out using SHELXL-97.19 One O-C-C-O fragment is
disordered over 2 positions and both parts (half occupied) are
refined with constrained bond distances to their theoretical
values (C-O ) 1.41; C-C ) 1.52). Two acetone molecules,
one occupies a special position, were found per asymmetric unit.
Both of them are half occupied and refined isotropically. The
hydrogen atoms were included in calculated positions and treated
as riding atoms using SHELXL-97 default parameters. The
non-H atoms were refined anisotropically, using weighted full-
matrix least-squares on F2. An absorption correction was applied
using DIFABS (Tmin ) 0.313, Tmax ) 0.748).
Precursor 1. In a 150 mL flask fitted with condenser,
dropping funnel and argon inlet was flushed with argon and
then charged with 2-(2′-hydroxy)ethyl-2-tetrahydro-2H-pyran
ether (1.9 g, 0.01 mol), ground sodium hydroxide (0.4 g, 0.01
mol), tetrabutylammonium hydrogenosulfate (0.13 g, 0.4 mmol)
and dry THF (70 mL). The mixture was then stirred at room
temperature during 1 h. 1,3,5-tris-bromomethyl-benzene (0.9 g,
0.0025 mol) in THF (20 mL) was added slowly and the mixture
heated to reflux for 3 h. A white precipitate appeared. The
solution was then cooled, filtered and the solvent was removed
to leave a yellow oil. This oil was dissolved in CH2Cl2, and the
organic layer was washed with water. The organic phase was
dried over MgSO4, filtered and evaporated to dryness. The
Arn2I
Φ ) Φr
Anr2Ir
where A and n are absorbance values (ca. 0.1) at the employed
excitation wavelength and refractive index of the solvent,
respectively. Measurements at 77 K were conducted by employ-
ing capillary tubes immersed in liquid nitrogen and hosted within
homemade quartz dewars. Band maxima and relative lumines-
cence intensities were obtained with uncertainty of 2 nm and
20%, respectively. The luminescence lifetimes were obtained
with an IBH 5000F single-photon equipment by using a 375
nm nanoled excitation source. Analysis of the luminescence
(9) (a) Ghizdavu, L.; Kolp, B.; Von Zelewsky, A.; Stoeckli-Evans, H. Eur. J.
Inorg. Chem. 1999, 1271-1279. (b) Ghizdavu, L.; Von Zelewsky, A.;
Stoeckli-Evans, H. Eur. J. Inorg. Chem. 2001, 993-1003. (c) Ghizdavu,
L.; Lentzen, O.; Schumm, S.; Brodkorb, A.; Moucheron, C.; Kirsch-De
Mesmaeker, A. Inorg. Chem. 2003, 42, 1935-1944.
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Earths; Gschneidner, K. A., Bunzli, J.-C. G., Pecharsky, V. K., Eds.; North-
Holland Publishing Company: Amsterdam, 2004, Vol. 34, in press.
(15) Newkome, G. R.; Yao, Z.; Baker, G. R.; Gupta, V. K.; Russo, P. S.;
Saunders: M. J. J. Am. Chem. Soc. 1986, 108, 849-850.
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Soc. 1976, 98, 6536-6544. (b) Juris, A.; Balzani, V.; Belser, P.; Von
Zelewsky, A. HelV. Chim. Acta 1981, 64, 2175-2182. (c) Sutin, N.; Creutz,
C. AdV. Chem. Ser. 1978, 168, 1-27.
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Moore, S. S.; Cram, D. J. J. Am. Chem. Soc. 1977, 99, 2564-2571.
(17) Stoe & Cie 2002. X-Area V1.17 & X-RED32 V1.04 Software. Stoe & Cie
GmbH, Darmstadt, Germany.
(11) Nakamaru, K. Bull. Chem. Soc. Jpn. 1982, 55, 2697-2705.
(12) Meech, S. R.; Phillips, D. J. Photochem. 1983, 23, 193-217.
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9
9340 J. AM. CHEM. SOC. VOL. 126, NO. 30, 2004

Chiral Tris-Cyclometalated Iridium (III) Complexes
A R T I C L E S
residue was purified by column chromatography (SiO2, eluent:
hexane/EtOAc 1:1 to EtOAc) to give 1 as a colorless oil (1.05
g, 60%). 1H NMR (300 MHz, CDCl3): δ 7.21 (s, 3H, HR); 4.61
(m, 3H, Ha′); 4.53 (s, 6H, He); 3.83 (m, 6H, He′); 3.70-3.40
(m, 24H, Ha,b,c,d); 1.90-1.42 (m, 18H, Hb′,c′,d′). 13C NMR (100
MHz, CDCl3): δ 139.1; 126.6; 99.3; 73.5; 71.1; 70.0; 67.1; 62.6;
60.8; 31.0; 25.8; 19.9. ESI-MS calcd. for C36H60NaO12 [M+Na+]
707.40, found 707.41.
(dd, 3H, H4′), 7.27 (s, 3H, HR), 4.57 (s, 6H, He), 3.66-3.22 (m,
18H, Hb,c,d), 3.17 (ddd, 6H, H7), 2.86 (m, 3H, H10), 2.58 (ddd,
3H, H9exo), 2.30-2.25 (m, 3H, H8,aendo), 1.85 (m, 3H, Haexo
)
1.44 (s, 9H, H13), 1.28 (d, 3H, H9endo), 0.65 (s, 9H, H12). 13C
NMR (100 MHz, CDCl3): δ 156.3; 149.6; 146.2; 141.3; 140.3;
139.0; 129.0; 128.8; 127.1; 126.7; 119.8; 73.6; 70.8; 70.0; 69.7;
45.3; 44.0; 41.4; 38.3; 34.1; 28.9; 26.7; 21.5. HRMS calcd. for
C75H88N3O6([M+H]+): 1126.66676, found: 1126.66570. UV-
vis (λ in nm (ꢀ, M-1 cm-1); CH2Cl2, 7.6 × 10-5 M): 256
Precursor 2. To a solution of 1 (1.05 g, 1.53 mmol) dissolved
in 50 mL of a mixture CH2Cl2/MeOH (1:1) were added 2 drops
of HCl 37%. The solution was stirred at room-temperature
overnight. NaHCO3 was then added and the solvents were
removed. The crude product was taken up into EtOAc, filtered
and evaporated to dryness to leave an oil whose purity was good
(47100); 281 (32900). [R]D ) -130 °, 25 °C, 0.195 g.L-1
.
Complexes fac-∆/Λ-Ir(pppy)3. Ir(acac)3 (0.06 g, 0.12 mmol)
and (8R,10R)-2-(2′-phenyl)-4,5-pinenopyridine (0.10 g, 0.40
mmol) were dissolved in degassed glycerol (5 mL) and the
solution was heated at 190 °C during 24 h. Once the solution is
cooled, it was diluted with CH2Cl2 and water. The organic layer
was washed with water, dried over MgSO4 and filtered.
Following removal of the solvent, the residue was purified by
flash chromatography on a silica column using CH2Cl2/hexane
1:1 as eluent to yield 39% of a mixture of pure fac-Λ-Ir(pppy)3
and fac-∆-Ir(pppy)3 in the ratio 2:3. The two diastereoisomers
were then separated by silica preparative plate (eluent Hexane/
EtOAc 5:1) to yield fac-Λ-Ir(pppy)3 (18 mg) and fac-∆-Ir-
(pppy)3 (25 mg) as bright yellow powders. fac-Λ-Ir(pppy)3:
1H NMR (400 MHz, acetone d6): δ 7.86 (s, 3H, H3); 7.67 (dd,
3H, H6′); 7.19 (s, 3H, H6); 6.88 (dd, 3H, H3′); 6.78 (ddd, 3H,
H5′); 6.72 (ddd, 3H, H4′); 3.15 (m, 6H, H7); 2.72 (m, 3H, H9exo);
2.52 (m, 3H, H10); 2.30 (m, 3H, H8); 1.32 (s, 9H, H13); 1.26 (d,
9H, H9endo); 0.47 (s, 9H, H12). 13C NMR (100 MHz, acetone
d6): δ 165.3; 161.3; 145.2; 145.1; 143.0; 141.4; 137.4; 128.8;
123.6; 119.5; 118.5; 44.8; 40.3; 39.3; 32.8; 32.4; 25.6; 21.1.
HRMS calcd. for C54H54Ir N3 [M]+: 937.39414, found:
937.39397. UV-vis (λ in nm (ꢀ, M-1 cm-1); CH2Cl2, 1.9 ×
10-5 M): 242 (53 700); 286 (39 200); 342 (12 100); 382 (sh,
9200); 408 (sh, 6800); 452 (sh, 3100); 487 (sh, 1500). Emission
(CH3CN, 2 × 10-5 M): excitation 350 nm, emission 505 nm.
CD (CH2Cl2 λ in nm (∆ꢀ) 1.9 × 10-5 M): 297 (83); 281 (-20).
fac-∆-Ir(pppy)3: 1H NMR (400 MHz, acetone d6): δ 7.86 (s,
3H, H3); 7.67 (dd, 3H, H6′); 7.16 (dd, 3H, H3′); 6.86-677 (m,
9H, H4′,5′,6); 3.16 (m, 6H, H7); 2.61 (m, 3H, H9exo); 2.33 (m,
6H, H8,10); 1.37 (s, 9H, H13); 1.15 (d, 3H, H9endo); 0.74 (s, 9H,
H12). 13C NMR (100 MHz, acetone d6): δ 165.5; 131.1; 145.7;
145.0; 142.1; 141.3; 137.6; 129.1; 123.8; 119.7; 118.8; 44.9;
40.2; 39.5; 33.0; 31.5; 25.6; 21.6. HRMS calcd. for C54H54Ir
N3 [M]+: 937.394 14, found: 937.394 08. UV-vis (λ in nm
(ꢀ, M-1 cm-1); CH2Cl2, 2.3 × 10-5 M): 242 (42 400); 285
(31 900); 334 (sh, 9600); 385 (sh, 7400); 409 (sh, 5200); 456
(sh, 1800); 488 (sh, 1000). Emission (CH3CN, 2 × 10-5 M):
excitation 350 nm, emission 507 nm. CD (CH2Cl2 λ in nm (∆ꢀ)
1.3 × 10-5 M): 297 (-92); 281 (29).
1
enough (>95% by NMR) to be used without purification. H
NMR (300 MHz, CDCl3): δ 7.22 (s, 3H, HR); 4.55 (s, 6H, He);
3.71-3.57 (m, 24H, Ha,b,c,d). 13C NMR (100 MHz, CDCl3): δ
139.0; 126.7; 73.4; 73.0; 70.9; 70.0; 62.2. ESI-MS calcd. for
C21H36NaO9 [M+Na+] 451.23, found 451.26.
Precursor 3. A solution of the triol 2 (0.615 g, 1.42 mmol)
in 50 mL of anhydrous CH2Cl2, in the presence of freshly
distilled triethylamine (5 mL) was cooled to -5 °C under argon.
A solution of mesyl chloride (0.8 mL, 10 mmol) in 10 mL of
anhydrous CH2Cl2 was added dropwise to the previous solution.
The temperature was maintained below 0 °C as the reaction is
very exothermic. After 3 h stirring at -5 °C, the mixture was
brought to room temperature. The reaction mixture was washed
with H2O and dried over MgSO4. Due to the instability of the
compound on column, it was used without purifications in the
1
next step. H NMR (300 MHz, CDCl3): δ 7.23 (s, 3H, HR);
4.51 (s, 6H, He); 4.36 (m, 6H, Ha); 3.75 (m, 6H, Hb); 3.63 (m,
12H, Hc,d).
Precursor 4. Crude 3 was dissolved in acetonitrile (80 mL)
and NaI (9 g, 60 mmol) was added. The yellow mixture was
then heated at 80 °C for 8 h. After the solvent was removed,
the residue was taken up into H2O/CH2Cl2. Extraction with CH2-
Cl2 and drying over MgSO4 left, after evaporation of the solvent,
a brown oil which was filtered on alumina (eluent hexane/EtOAc
1:1) to give the product as a colorless oil (0.463 g, 43% from
1
the triol). H NMR (300 MHz, CDCl3): δ 7.23 (s, 3H, HR);
4.56 (s, 6H, He); 3.77-3.63 (m, 18H, Hb,c,d); 3.27 (t, 6H, Ha).
13C NMR (100 MHz, CDCl3): δ 139.0; 126.7; 73.6; 72.4; 70.7;
70.0; 3.3. ESI-MS calcd. for C21H33I3NaO6 [M+Na+] 784.93,
found 784.96.
Ligand (-)-(7S,10R)-L. To a freshly prepared solution of
LDA (7 mmol in THF) cooled to -20 °C, was added a degassed
solution of (8R,10R)-2-(2′-phenyl)-4,5-pinenopyridine (1.2 g,
4.8 mmol in anhydrous THF) over 40 min. The reaction medium
was then warmed to 0 °C for 3 h. The solution turned
progressively dark red; then 4 (0.41 g, 0.53 mmol) dissolved in
dry THF (10 mL) was added. The mixture was stirred overnight,
and the reaction was quenched with water (2 mL). After
evaporation of the THF in a vacuum, the residue was taken up
in a CH2Cl2/H2O mixture and the organic layer was dried over
MgSO4 and filtered. Following removal of the solvent, the
residue was purified by column chromatography (SiO2, hexane/
EtOAc 5:1 to 1:1) and then by silica preparative plate (hexane/
EtOAc 3:1). 5 was obtained as a white powder in 56% yield
fac-∆-IrL. Ir(acac)3 (13 mg, 0.027 mmol) and (-)-
(7S,10R)-L (30 mg, 0.027 mmol) were dissolved in degassed
ethyleneglycol (300 mL) heated at 140 °C and the solution was
then brought to 175 °C during 24 h and under vigorous stirring.
The solvent was then evaporated by trap-to-trap and the residue
was purified by silica preparative plate (eluent Hexane/EtOAc
2:1) and then by alumina preparative plate (eluent Hexane/CH2-
Cl2 1:2) to yield IrL as a bright yellow powder (6 mg, 18%)
1H NMR (400 MHz, acetone d6): δ 7.94 (s, 3H, H3), 7.66 (d,
3H, H6′), 7.26 (s, 3H, H6), 7.05 (dd, 3H, H3′), 6.75 (ddd, 3H,
H5′), 6.68 (m, 6H, H4′,R), 4.43 (m, 6H, He), 3.74-3.53 (m, 18H,
1
(0.336 g). H NMR (400 MHz, CDCl3): δ 8.23 (s, 3H, H3),
7.97 (d, 6H, H2′,6′), 7.65 (s, 3H, H6), 7.47 (dd, 6H, H3′,5′), 7.39
9
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A R T I C L E S
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Scheme 1. Syntheses of the Model Complexes Fac-∆-Ir(pppy)3 and Fac-Λ-Ir(pppy)3
Scheme 2. Synthesis of the Ligand L
H
b,c,d), 3.29 (ddd, 3H, H7), 2.65 (m, 6H, H9exo,aendo), 2.41 (m,
3H, H10), 2.26 (m, 3H, H8), 2.05 (m, 3H, Haexo), 1.54 (d, 3H,
9endo), 1.39 (s, 9H, H13), 0.43 (s, 9H, H12). HRMS calcd. for
procedure5a using (8R,10R)-2-(2′-phenyl)-4,5-pinenopyridine
(pppy) as the cyclometalating ligand precursor (Scheme 1).
As demonstrated by Thompson,4d the reaction temperature
strongly affects the facial/meridional product ratio of the
reaction. When temperature is ∼190 °C, the facial isomer is
the predominant product.
H
C75H84N3O6Ir [M]+: 1315.598 39, found: 1315.601 35. UV-
vis (λ in nm (ꢀ, M-1 cm-1); CH2Cl2, 4.5 × 10-6 M): 245
(48 200); 288 (39 700); 348 (9900); 379 (sh, 9300); 408 (sh,
6400); 453 (sh, 2200); 487 (sh, 1100). Emission (CH3CN, 2 ×
10-5 M): excitation 350 nm, emission 508 nm. CD (CH2Cl2 λ
in nm (∆ꢀ) 4.5 × 10-6 M): 299 (71), 283 (-14).
The use of the enantiomerically pure (8R,10R)-pppy renders
the Λ and ∆ isomers of the tris-cyclometalated species diaste-
reoisomeric. Separation with ordinary chromatographic methods
becomes therefore possible. Preparative thin-layer chromatog-
raphy gave the fac-Λ/∆ isomers in a ratio 2:3. The absolute
configuration at the metal center of the major product was
Results and Discussion
Synthesis. Tris-cyclometalated Ir(III) complexes fac-Λ/∆-
Ir(pppy)3 were prepared in an analogous way to literature
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9342 J. AM. CHEM. SOC. VOL. 126, NO. 30, 2004

Chiral Tris-Cyclometalated Iridium (III) Complexes
A R T I C L E S
Figure 1. 1H NMR spectra in acetone d6 of (a) ligand pppy, (b) complex fac-∆-Ir(pppy)3, and (c) complex fac-Λ-Ir(pppy)3 (400 MHz, 298 K).
clearly identified as the ∆ isomer: its CD spectrum shows a
positive Cotton effect at high energy and a negative one at lower
energy which is in full agreement with absolute configuration
determined by X-ray crystallography in related complexes with
similar ligands.20
The tripodal ligand L is obtained in five steps according to
a divergent strategy in which the coordinating moieties pppy
are connected to the tripode 4 during the last step as shown in
Scheme 2.
alcoholate was then added a solution of 1,3,5-tris-bromomethyl-
benzene. The tris-THP compound 1 was isolated in 60% yield.
The THP protective groups were cleaved by acidic treatment
and the resulting triol 2 was converted to the tris-mesylate 3
and subsequently to the tris-iodo compound 4. 4 was then
reacted with 3 equiv. of the anion of pppy (formed by using
LDA). The ligand L was obtained in 56% yield.
The complex IrL was synthesized from the reaction of Ir-
(acac)3 with 1 equiv. of L in a large volume of ethylenglycol
heated at 175 °C. After purification, the complex was obtained
in 18% yield.
The tridentate unit 4 is prepared from 1,3,5-tris-bromomethyl-
benzene in four steps. HO(CH2)2O(CH2)2OTHP was reacted
with NaOH in the presence of NBu4HSO4 and to the resulting
Polarity. The relative Rf values of the complexes fac-Λ-Ir-
(pppy)3, fac-∆-Ir(pppy)3 and IrL (TLC SiO2, hexane/EtOAc 3:1)
are 0.87, 0.80, and 0.17, respectively. Thus, IrL is by far the
most polar of the three complexes, whereas fac-∆-Ir(pppy)3,
(20) (a) Hayoz, P.; von Zelewsky, A.; Stoeckli-Evans, H. J. Am. Chem. Soc.
1993, 115, 5111-5114; Mu¨rner, H. R.; Stoeckli-Evans, H.; von Zelewsky,
A. Inorg. Chem. 1996, 35, 3931-3935.
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A R T I C L E S
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1
Figure 2. Aromatic region of the H NMR spectra in acetone d6 of (a) ligand L and (b) complex fac-Λ-IrL (400 MHz, 298 K).
fac-Λ-Ir(pppy)3, respectively have very similar polarities.
Despite this, IrL has the highest solubility in several solvents
(hexane, methanol, ether, benzene, etc). This is most probably
due to the molecular size. In general, it can be stated, that these
coordination compounds behave in many respects as organic
molecules of low polarity.
1
Characterization. NMR. Figure 1 illustrates the H NMR
spectra for the free ligand pppy, fac-∆-Ir(pppy)3 and fac-Λ-Ir-
(pppy)3 complexes. These spectra, as well as the 13C NMR
spectra, show, in each case, only one set of signals, owing to
the high symmetry of these species: therefore the complexes
have facial configuration (C3 symmetry) and the three ligands
are thus equivalent. Upon complexation with Ir(III), the signal
Figure 3. CD spectra of fac-∆-Ir(pppy)3 and fac-Λ-Ir(pppy)3 (298 K, in
CH2Cl2).
corresponding to the proton 2′ in the free pppy disappears
indicating that the cyclometalation occurred.
The 1H NMR spectrum of the complex IrL is consistent with
a facial structure too, but more important, it shows the presence
of only one diastereomer. Only one set of signals is observed.
A comparison of the aromatic region of the spectra of the free
ligand L and the IrL is shown in Figure 2.
tripodal bipyridine ligand (L′), only the Λ isomer was obtained
in [RuL′]2+, as well as in [FeL′]2+
.
Circular Dichroism, Electrochemistry, and Photophysical
Properties. Circular Dichroism. Neither the chiral didentate
ligand nor the tripodal ligand show detectable activities in the
range 250-600 nm, whereas upon cyclometalation to Ir(III), a
strong Cotton effect is observed in all cases. Figure 3 displays
the CD spectra of the Λ and ∆ diastereoisomers of Ir(pppy)3.
The near mirror symmetry of these spectra indicates that the
entire CD activity in this spectral range is mainly determined
by the chiral configuration at the metal center. The Λ- and
∆-complexes are behaving as pseudo-enantiomers in this respect.
The CD spectrum of IrL (Figure 4) shows clearly that the
metal center has a Λ configuration and thus it confirms the
L shows a set of signals in the aromatic part with chemical
shifts ranging from 7.28 to 8.21 ppm. In the complex, these
signals lie in the range 6.67-7.94 ppm. A doublet at 7.05 and
a triplet at 6.68 ppm are assigned to the protons of the phenyl
ring ortho and meta to the metalated carbon atom, respectively
3′ and 4′. The protons are high-field shifted compared to L:
∆δ ) 0.42 ppm for 3′ and ∆δ ) 0.71 ppm for 4′. Upon
coordination, the protons 6 are upfield shifted (∆δ ) 0.64 ppm)
because the deshielding effect of the metal center on the proton
ortho to the nitrogen atoms is counterbalanced by the shielding
effect of the others pppy units. An important point is that the
spectrum of IrL is very similar to that of fac-Λ-Ir(pppy)3. In
fact, it was predicted that the Λ isomer should be formed in
the IrL complex, since in our previous work8 with analogous
1
analogy between the H NMR spectra of fac-Λ-Ir(pppy)3 and
fac-Λ-IrL.
Electrochemistry. The cyclic voltammograms of the com-
plexes fac-Λ-Ir(pppy)3, fac-∆-Ir(pppy)3 and fac-Λ-IrL were
recorded in butyronitrile. They display reversible couples at
9
9344 J. AM. CHEM. SOC. VOL. 126, NO. 30, 2004

Chiral Tris-Cyclometalated Iridium (III) Complexes
A R T I C L E S
Figure 4. CD spectra of fac-Λ-IrL (298 K, in CH2Cl2).
Figure 5. Absorption spectrum and normalized luminescence spectra of
fac-∆-Ir(pppy)3 in degassed acetonitrile at 298 K (solid line), and at 77 K
(dashed line).
Table 1. Absorption and Electrochemical Data of the Studied
Complexes
absorptiona
λ (nm) {ꢀ, 103 M-1cm-1
redox (V/SCE)b
Table 2. Luminescence and Photophysical Properties of the
ox
complex
}
E
1/2
Iridium Complexesa
fac-Λ-Ir(pppy)3 242 (53.7), 286 (39.2), 342 (12.1),
382 (9.2), 408(6.8) 452 (3.1), 487 (1.5)
fac-∆-Ir(pppy)3 242 (42.4), 285 (31.9), 334 (9.6),
385 (7.4), 409 (5.2) 456 (1.8), 488 (1.0)
0.52
0.57
0.49
298 K
τair 105× k
77 K
λmax
(nm) 102× Φair (ns) (s-1
τ
105× k 105× knr λmax
τ
r
r
)
Φ
(µs) (s-1
)
(s-1
)
(nm) (µs)
fac-Λ-IrL
245 (48.2), 288 (39.7), 348 (9.9),
Λ-Ir(pppy)3 505
∆-Ir(pppy)3 507
0.9
1.0
1.0
19.4 4.6 0.64 1.4 4.6
20.5 4.9 0.64 1.6 4.1
24.5 4.1 0.51 1.2 4.2
2.6 507 2.1
2.3 511 2.3
4.1 507 1.9
379 (9.3), 408 (6.4) 453 (2.2), 487 (1.1)
Λ-IrL
508
a In CH2Cl2 or acetonitrile solvents, at 298 K. b In butyronitrile, in the
conditions used (explored window < -1.80 V), the reduction of the
phenylpyridyl-based derivatives is not observed.
a Acetonitrile solvent, air-equilibrated and degassed cases as indicated.
Excitation wavelength was 350 nm for luminescence spectra and 375 nm
for luminescence lifetimes.
+0.52, +0.57, and +0.49 V/SCE, respectively. The data are
reported in Table 1. These potentials are assigned to metal-
centered IrIV/IrIII reduction couples. They indicate the relative
ease of oxidation in the various species. The pineno-phenylpy-
ridine ligand shifts all redox potentials to a more negative
(cathodic) potential, when compared to the parent complex Ir-
(ppy)3 (E1/2 ) 0.77 V/SCE).5a In the conditions used (window
of measurements: +1.60 < E < -1.80 V), it is not possible to
observe the reduction waves, which are very probably localized
on the pppy ligands.21
Optical Absorption. The data for the absorption spectra are
summarized in Table 1, together with the oxidation potentials.
In Figure 5, the absorption spectrum and luminescence profiles
for fac-∆-Ir(pppy)3, are illustrated.
The absorption properties are discussed with reference to
results for facial isomers of the Ir(III)-phenylpyridine complexes
reported in the literature.1,3-6,22 The spectra show intense bands
appearing between 240 and 350 nm which are assigned to spin
allowed πfπ* transitions in the pppy ligand. At lower energies
(into the visible region from 350 to 500 nm), weaker bands are
observed. The assignment of these bands is more difficult,
because their low intensity (ꢀ e 104 M-1 cm-1, see Table 1
and Figure 5) is not compatible with spin allowed πfπ*
transitions. On the other hand, MLCT transitions are also
unlikely in this energy region, based on the fact that ligand
centered reduction, if any, occurs at very negative potentials
(see Table 1).21 In addition, careful inspection of the absorption
profile in Figure 5, reveals structured features in the visible
region, whereas MLCT bands are always structureless.1,23 It
seems reasonable therefore to admit a mixed (LC-CT) character
to these bands, by considering the possibility of intraligand CT
transitions involving the phenyl and pyridyl rings of pppy ligand,
as suggested by recent results for Ir(III) diphenylpyridine
complexes.24 Furthermore, for the tail extending further toward
lower energy (ꢀ e 103 M-1 cm-1), spin-orbit effect due to the
metal center could be involved, leading to direct singlet-to-triplet
transitions. The bands of fac-Λ-IrL are very close in energy to
that of fac-Λ-Ir(pppy)3, indicating that there are no important
differences in coordination geometry in the two complexes. Both
complexes exhibit strong LC absorption in the 300-nm region,
indicating again a substantial similarity between bridged and
unbridged pppy ligand.
Luminescence. Figure 5 compares the luminescence spectral
profiles for complex fac-∆-Ir(pppy)3 in degassed acetonitrile
at room temperature and at 77 K; the excitation wavelength
was 350 nm. Quite similar spectra were obtained also for fac-
Λ-Ir(pppy)3 and fac- Λ-IrL.
The luminescence profiles in Figure 5 exhibit a vibronic
resolution which is typical for the family of Ir(III)-ppy
complexes; 1,4-6,22 owing to the known temperature effect, the
profiles at 77 K are better resolved than those at room
temperature. It is noteworthy that the band maximum for the
77 K spectrum in Figure 5 is slightly red-shifted with respect
to that at room temperature. This is in contrast with cases where
3
the luminescence is of MLCT nature, as for example for the
extensively studied Ru(II)-polypyridine complexes,23 or for 3LC
states with some admixture of 3MLCT character, as for example
Ir(III)-terpyridine complexes.25 Actually, for MLCT emitters,
a typical blue-shifting is observed on going from room tem-
perature (fluid solvent) to 77 K (frozen solvent).23 On the basis
(21) Kulikova, M. V.; Balashev, K. P.; Kvam, P. I.; Songstad, J. Russ. J. Gen.
Chem. 2000, 70, 163-170.
(22) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee, H.-E.;
Adachi, C.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E. J. Am. Chem.
Soc. 2001, 123, 4304-4312.
(23) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; Von
Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85-277.
(24) Polson, M.; Fracasso, S.; Bertolasi, V.; Ravaglia, M. Scandola, F. Inorg.
Chem. 2004, 42.
9
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A R T I C L E S
Schaffner-Hamann et al.
[Ir(tpy)2]3+ 25
.
To notice that for acetonitrile solvent, the diffusion
constant is kdiff ≈ 2.0 × 1010 M-1 s-1, as evaluated from
equation below30
8RT
3η
kdiff
)
1000
(2)
where R is the gas constant and η ) 0.345 × 10-3 Pa s30 is the
solvent viscosity. The fact that for our Ir-tris-cyclometalated
complexes, the observed quenching rate constant is approxi-
mately equal to the diffusion constant, indicates that the
quenching step within the collision associate is highly efficient,
eqs 3 and 430
kde
*Ir ) O2yk z*IRO2
98products
(3)
(4)
diff
k-diff
k
diffkde
kq )
k-diff + kde
where *Ir stands for the luminescent triplet of the iridium
complex, *Ir‚‚‚O2 is the collision associate, k-diff is the rate for
break up of the collision associate into the original species, and
Figure 6. Circularly polarized luminescence (upper curve) and total
luminescence (lower curve) spectra of the two Ir(III) complexes in dry
MeCN at 295 K, upon excitation at 388 nm.
kde is the rate constant for product formation from the associate.
Thus, for kde . k-diff, kq ≈ kdiff. This is in contrast with what
happens for most cases of luminescence quenching in several
metal complexes, where kq was found to be well lower than
of these characteristics, the luminescence spectra for fac-∆-Ir-
(pppy)3 (and for fac-Λ-Ir(pppy)3 and fac-Λ-IrL as well) are
assigned to (largely) 3LC excited states. Table 2 collects
luminescence data for both freeze-pump-thaw degassed, and
air-equilibrated cases at room temperature and at 77 K in frozen
solvent.
The complexes are highly luminescent in oxygen-free solvent,
with luminescence quantum yields Φ ≈ 0.5-0.6, and lifetimes
τ ≈ 1.2-1.6 µs. kr and knr values are in the range 4-5 × 105
and 2-4 × 105 s-1, respectively, in line with previous results
for complexes of the [Ir(ppy)3] family.4-6,22 In air-equilibrated
solvent, a remarkable quenching effect is registered, with Φair
≈ 0.01 and τair ≈ 20 ns, a 60-fold reduction in luminescence
features with respect to degassed solutions. The luminescence
of the complexes is quenched by dioxygen with a rate that can
be drawn from the data of Table 2, on the basis of the equation
below.26
kdiff, and where energy transfer (occurring with efficiencies lower
than unit) was the predominant mechanism associated with kde.28
The energy content of *Ir is ca. 2.4 eV, as estimated from
the emission profiles, and the quenching mechanism for the
couple *Ir/O2 could involve an energy transfer or an electron-
transfer process.28 Energy transfer would lead to a singlet oxygen
state (1O2), and for the present cases would be exothermic by
about 1-1.5 eV (1.6 and 1.0 eV are the energy levels of the
two available excited states of the oxygen species).31,32 In
conclusion, when energy transfer takes place, kq is found no
more than 2-4 × 109 M-1 s-1 in acetonitrile,25,28,29,32 while it
is found an outstanding kq ) 2 × 1010 M-1 -1
s
in our complexes.
For these, the driving force for an electron-transfer step within
the couple *Ir/O2, can be evaluated to be ∆G° ≈ -1.1 eV (2.4
eV is the energy content of *Ir, oxidation of our Ir complexes
and reduction of oxygen are ∼+0.53 and -0.78 V vs SCE,
respectively). Interestingly, the reorganization energy for elec-
tron transfer in acetonitrile, λ, is between 0.6 and 1 eV, as
evaluated according to current approaches.33 Thus, it seems that
the activation energy for electron transfer within the *Ir/O2
couple, eq 533
τ
Φ
)
) 1 + kqτ[O2]
(1)
τair Φair
The evaluated value for kq[O2] for the three investigated
complexes is 4.9 ( 0.2 × 107 s-1. Given that for air-equilibrated
2
∆G0
λ
acetonitrile, [O2] ) 1.9 × 10-3 27
,
the evaluated average
λ
4
∆G* ) 1 +
(5)
(
)
quenching rate constant is kq ) 2.6 ( 0.2 × 1010 M-1 s-1. This
appears to be among the highest values ever reported for
quenching of luminescence of transition metal complexes by
dioxygen; for comparison purposes, kq is ca. 2 × 109 for both
can be close to zero (i.e., photoinduced electron transfer within
*Ir/O2 could be barrierless). In conclusion, the occurrence of
[Ru(bpy)3]2+ 28
,
and [Os(bpy)3]2+ 29
,
and 1.1 × 108 M-1 s-1, for
(29) The luminescence lifetime for [Os(bpy)3]2+ in degassed and air-equilibrated
acetonitrile is 65 and 50 ns, respectively.
(25) Collin, J.-P.; Dixon, I. M.; Sauvage, J.-P.; Williams, J. A. G.; Barigelletti,
F.; Flamigni, L. J. Am. Chem. Soc. 1999, 121, 5009-5016.
(26) Lakowicz, J. R. Principle of Fluorescence Spectroscopy; Plenum Publish-
ers: New York, 1999; Vol. 8.
(30) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry;
New York, 1993, pages 207-209.
(31) Halliwell, B.; Gutteridge, J. M. C. Free Radical in Biology and Medicine;
Clarendon: Oxford, 1982.
(27) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry;
New York, 1993.
(32) Abdel-Shafi, A. A.; Beer, P. D.; Mortimer, R. J.; Wilkinson, F. Phys. Chem.
Chem. Phys. 2000, 2, 3137-3144.
(33) (a) Marcus, R. A.; Sutin, N. Biochim Biophys. Acta 1985, 811, 265-322.
(b) Wasielewski, M. R. In Photoinduced Electron Transfer; Part A; Fox,
M. A., Chanon, M. Eds.; Elsevier: Amsterdam, 1988, 161-206.
(28) (a) Demas, J. N.; Harris, E. W.; McBride, R. P. J. Am. Chem. Soc. 1977,
99, 3547-3551. (b) Abdel-Shafi, A. A.; Beer, P. D.; Mortimer, R. J.;
Wilkinson, F. HelV. Chim. Acta 2001, 84, 2784-2795.
9
9346 J. AM. CHEM. SOC. VOL. 126, NO. 30, 2004

Chiral Tris-Cyclometalated Iridium (III) Complexes
A R T I C L E S
Figure 7. Stereopair of the X-ray structure of fac-Λ-IrL. The hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (deg) are:
Ir-N1, 2.140(5); Ir-N2, 2.133(5); Ir-N3, 2.120(5); Ir-C1, 2.012(7); Ir-C30, 2.026(8); Ir-C53, 2.001(6); C53-Ir-N3, 79.5(2); C30-Ir-N2, 79.5(3);
C1-Ir-N1, 79.6(2); C53-Ir-N2, 169.8(2); C1-Ir-N3, 167.0(2); C30-Ir-N1, 169.3(3).
electron transfer might provide a possible explanation for the
very high kq obtained experimentally.
the observed spectra, but “cause” CPL or CD due to their
influence on chiral coordination. To our best knowledge, this
is the first example of CPL from iridium (III) complexes and,
although the signals here are weak, they are measurable.
X-ray Structure Analysis. The crystal structure of the
iridium complex fac-Λ-IrL confirms the formation of the neutral
1:1 complex. The compound crystallizes in the chiral space
group P21212 (Flack parameter x ) -0.003(7)) with one neutral
complex molecule and 1.5 acetone molecules (partially oc-
cupied) per asymmetric unit. A view of the complex is shown
in Figure 7. The structure shows nicely the coiling of the tris-
cyclometaling ligand L around the Ir(III) center. The three
cyclometalating moieties coordinate in an octahedral coordina-
tion geometry. The complex has approximate C3-symmetry (not
crystallographic) with Ir-N and Ir-C bond lengths in the range
2.120(5)-2.140(5) and 2.001(6)-2.026(8), respectively. These
values are similar to those reported for the fac-Ir(tpy)3 (tpy )
2-(para-tolyl)pyridine) complex.35 The bite angles within each
chelate are around 79.5° and the torsional angles N-C-C-C
range between 6.9(14)° and 11.1(8)° indicating a small defor-
mation from planarity of the phenylpyridine units.
Circularly Polarized Luminescence (CPL) Measurements.
The circularly polarized luminescence (∆I) and total lumines-
cence (I) spectra measured for fac-∆-Ir(pppy)3 and fac-Λ-Ir-
(pppy)3 in acetonitrile solutions (at 295 K) are shown in Figure
6.
The degree of circularly polarized luminescence is given by
the luminescence dissymmetry ratio
IL - IR
∆I
glum
)
)
(6)
1
1
I
(IL - IR)
2
2
where IL and IR refer, respectively, to the intensity of left and
right circularly polarized emissions. As usual for most chiral
organic chromophores and transition metal complexes,34 |glum
|
that were obtained are small: -0.0032 ( 0.0004 for fac-∆-Ir-
(pppy)3 and +0.0028 ( 0.0004 for fac-Λ-Ir(pppy)3 as deter-
mined at the maximum emission wavelength. Although the glum
values are very small (a value equal to ∼0.003 corresponding
to light that is only 0.3% circularly polarized), opposite CPL
signals were measured for the two compounds having opposite
configuration at the metal center. Indeed, it is usually assumed
that the sign and the magnitude of the CPL signals are mainly
determined by the ∆ or Λ arrangement of the ligands. Remote
asymmetric centers on the ligands have little direct effect on
Conclusion
This report represents a further step in the systematic
development of stereoselective synthesis of coordination com-
pounds. It shows clearly that the introduction of chiral centers
in the ligands, in this case the pinene groups attached to the
pyridine ring of phenylpyridine, leads to only modest stereo-
selectivity with respect to helical configuration of the octahedral
(34) (a) Riehl, J. P.; Richardson, F. S. Chem. ReV. 1986, 86, 1-16. (b) Peacock,
R. D.; Stewart, B. J. Chem. Soc., Chem. Commun. 1982, 295-296. (c)
Gunde, K. E.; Credi, A.; Jandrasics, E.; von Zelewsky, A.; Richardson, F.
S. Inorg. Chem. 1997, 36, 426-434. (d) Field, J. E.; Muller, G.; Riehl, J.
P.; Venkataraman, D. J. Am. Chem. Soc. 2003, 125, 11 808-11 809.
(35) Garces, F. O.; Dedian, K.; Keder, N. L.; Watts, R. J. Acta Crystallogr.
1993, C49, 1117-1120.
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A R T I C L E S
Schaffner-Hamann et al.
Ir(III)-center in a tris-didentate cyclometalated complex. Con-
necting these three ligands by a C3-symmetric moiety (derived
from mesitylene) yields a chiral tripode ligand that coordinates
in a completely stereoselective way to the Ir(III)-center. The
introduction of the pinene groups into the ligand has, beside
the stereo control, the benefit of rendering these uncharged
molecules highly soluble in a large range of solvents. Their
photophysical properties are an additional feature of this
interesting class of compounds.
National Science Foundation and the FIRB project RBNE019H9K
“Molecular Manipulation for Nanometric Devices” by MIUR
for financial support. This work is dedicated to the memory of
Jean-Marc Kern.
Supporting Information Available: Tables of crystal data,
atomic coordinates, bond distances, bond angles and anisotropic
displacement parameters for fac-Λ-IrL, as well as the CIF file.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. We thank Freddy Nydegger (University
of Fribourg) for the electrospray mass spectra, the Swiss
JA048655D
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9348 J. AM. CHEM. SOC. VOL. 126, NO. 30, 2004