Mechanistic investigation of improved syntheses of iridium(III)-based OLED phosphors

Article abstract of DOI:10.1021/om300325y

Treatment of [IrCl₃(tht)₃] (tht = tetrahydrothiophene) with a stoichiometric amount of PPh₃ gave the monosubstitution product [Ir(tht)₂(PPh₃)Cl₃] (5), whose synthesis, particularly that leading to the effective preparation of OLED phosphors, was studied and optimized to achieve the best product yields. Thus, the independent treatment of 5 with 2,4-difluorophenylpyridine (dfppyH) or with variable amounts of benzyldiphenylphosphine (bdpH) gave rise to the formation of the cyclometalation products [Ir(dfppy)(tht)(PPh ₃)Cl₂] (7), [Ir(bdp)(bdpH)(tht)Cl₂] (8), and [Ir(bdp)(PPh₃)(tht)Cl₂] (10), depending on the stoichiometry and conditions employed. Upon further treatment with 5-pyridyl-3-trifluoromethyl-1H-pyrazole (fppzH), these Ir(III) complexes 7, 8, and 10 were capable of yielding the phosphors [Ir(dfppy)(fppz)₂] (1), [Ir(bdp)₂(fppz)] (4), and [Ir(bdp)(fppz)₂] (2), respectively. The general mechanism en route to their formation was studied and discussed.

Full text of DOI:10.1021/om300325y

Article
pubs.acs.org/Organometallics
Mechanistic Investigation of Improved Syntheses of Iridium(III)-
Based OLED Phosphors
Cheng-Huei Lin,† Yuan-Chieh Chiu,† Yun Chi,*,† Yu-Tai Tao,*,‡ Liang-Sheng Liao,*,§
Meu-Rurng Tseng,∥ and Gene-Hsiang Lee⊥
†Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan
‡Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan
§Institute of Functional Nano & Soft Materials, SooChow University, Suzhou 215123, People's Republic of China
∥Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu 31040, Taiwan
⊥Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan
S
* Supporting Information
ABSTRACT: Treatment of [IrCl3(tht)3] (tht = tetrahydrothio-
phene) with a stoichiometric amount of PPh3 gave the
monosubstitution product [Ir(tht)2(PPh3)Cl3] (5), whose
synthesis, particularly that leading to the effective preparation
of OLED phosphors, was studied and optimized to achieve the
best product yields. Thus, the independent treatment of 5 with
2,4-difluorophenylpyridine (dfppyH) or with variable amounts
of benzyldiphenylphosphine (bdpH) gave rise to the formation
of the cyclometalation products [Ir(dfppy)(tht)(PPh3)Cl2] (7),
[Ir(bdp)(bdpH)(tht)Cl2] (8), and [Ir(bdp)(PPh3)(tht)Cl2]
(10), depending on the stoichiometry and conditions employed.
Upon further treatment with 5-pyridyl-3-trifluoromethyl-1H-
pyrazole (fppzH), these Ir(III) complexes 7, 8, and 10 were
capable of yielding the phosphors [Ir(dfppy)(fppz)2] (1), [Ir(bdp)2(fppz)] (4), and [Ir(bdp)(fppz)2] (2), respectively. The
general mechanism en route to their formation was studied and discussed.
[(C∧N)2Ir(μ-Cl)]2.24 Subsequent treatment of this class of
1. INTRODUCTION
intermediates with any potentially anionic chelate (L∧X)H
affords the heteroleptic [(C∧N)2Ir(L∧X)] in high yields (eqs 1
and 2).25
Luminescent Ir(III) metal complexes, particularly those with
chelating cyclometalates, play a key role in the recent
development of optoelectronic technologies such as organic
light-emitting diodes (OLEDs),1−9 light-emitting electrochem-
ical cells,10 biological labels,11−13 chemical sensors,14,15 and
solid-state organic lighting applications.16 The attraction of
these complexes comes from their higher chemical stability due
to strong metal−ligand bonding interactions, as well as their
longer excitation lifetimes and higher emission quantum
yields.17 Furthermore, the strong spin−orbit coupling induced
by the central Ir(III) metal ion promotes an efficient
intersystem crossing from the singlet to the triplet excited
state manifold, which then facilitates strong electrolumines-
cence by harnessing both singlet and triplet excitons of the as-
fabricated optoelectronic devices.18−20 As a result, syntheses of
Ir(III) complexes have been extensively examined in the past
two decades.21−23
IrCl3·nH2O + (C∧N)H (2 equiv) → [(C∧N)2Ir(μ‐Cl)]2
(1)
[(C∧N)2Ir(μ‐Cl)]2 + (L∧X)H → [(C∧N)2Ir(L∧X)]
(2)
Ir(acac)3 + (C∧N)H (3 equiv) → [Ir(C∧N)3]
(3)
Alternatively, the tris-substituted cyclometalate complexes
[Ir(C∧N)3] can be obtained from either treatment of the
aforementioned [(C∧N)2Ir(μ-Cl)]2 with more (C∧N)H
chelate26 or by simply heating Ir(acac)3 (acacH = acetylace-
tone) with 3 equiv of (C∧N)H at high temperature (eq 3).27,28
The disadvantage of the Ir(acac)3 reagent is its lower synthetic
yield from IrCl3·nH2O, which then raises the cost for acquiring
this starting material. Third, the COD reagent [Ir(COD)(μ-
Cl)]2 (COD = 1,4-cyclooctadiene), due to its +1 formal
Conventionally, the syntheses are achieved with the
employment of three distinctive iridium source reagents: i.e.
IrCl3·nH2O, Ir(acac)3, and [Ir(COD)(μ-Cl)]2. It has been
reported that the nitrogen-containing heteroaromatics (C∧N)
H can readily react with IrCl3·nH2O in high-boiling protic
solvents to afford a dicyclometalated dimer of the formula
Received: April 19, 2012
Published: May 25, 2012
© 2012 American Chemical Society
4349
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Organometallics
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Preparation of [Ir(tht)2(PPh3)Cl3] (5). A mixture of IrCl3(tht)3
(500 mg, 0.89 mmol) and triphenylphosphine (PPh3, 256 mg, 1.0
mmol) in decalin (15 mL) was heated to reflux for 2 h. After the
reaction mixture was cooled to room temperature, the solvent was
removed under vacuum and the residue was dissolved in CH2Cl2 and
then filtered through a pad of Celite. The filtrate was collected and
concentrated on a rotavap to give a crude product. Further
crystallization from a mixture of CH2Cl2 and hexane at room
temperature gave yellow crystals (594 mg, 0.81 mmol, 91%). Single
crystals of 5 were obtained from a mixture of ethyl acetate and hexane
at room temperature.
oxidation state and greater tendency to form strong bonds with
carbon-based σ-donors, was reported to be particularly useful
for synthesizing cyclometalated Ir(III) complexes that possess a
five-membered N-heterocyclic carbene framework.29,30 Addi-
tionally, a more recent development was the preparation of tris-
heteroleptic complexes, for which employment of [Ir(COD)(μ-
Cl)]2 allowed the isolation of [(ppy)(dfppy)Ir(acac)] (ppyH =
2-phenylpyridine and dfppyH = 2,4-difluorophenylpyridine) in
good yield over two steps (eqs 4 and 5), giving a significant
improvement over methods that use IrCl3·nH2O as the starting
material.31
Spectral Data of 5: MS (FAB, 193Ir) m/z 702 (M − Cl)+; 1H NMR
(400 MHz, CDCl3, 294 K) δ 7.80−7.71 (m, 6H), 7.42−7.32 (m, 9H),
3.80−3.69 (m, 4H), 2.63−2.52 (m, 4H), 2.38−2.22 (m, 4H), 1.98−
1.86 (m, 4H); 31P{1H} NMR (202 MHz, CDCl3, 294 K) δ −21.5 (s,
1P). Anal. Calcd for C26H31Cl3IrPS2: C, 42.36; H, 4.24. Found: C,
42.42; H, 4.39.
[Ir(COD)(μ‐Cl)]2 + ppyH + dfppyH
→ [(ppy)(dfppy)Ir(μ‐Cl)]2
(4)
Selected Crystal Data of 5: C27.50H34.50Cl3IrO1.50PS2; Mr = 782.69;
[(ppy)(dfppy)Ir(μ‐Cl)]2 + acacNa
monoclinic; space group P21/c; a = 13.1972(7) Å, b = 10.2905(5) Å, c
= 21.7022(11) Å, β = 95.0332(11)°, V = 2935.9(3) Å3; Z = 4; ρcalcd
=
→[(ppy)(dfppy)Ir(acac)]
(5)
1.771 Mg m−3; F(000) = 1546; crystal size 0.38 × 0.33 × 0.05 mm3;
λ(Mo Kα) = 0.710 73 Å; T = 150(2) K; μ = 5.040 mm−1; 20 309
reflections collected, 6741 independent reflections (Rint = 0.0431),
GOF = 1.029, final R1(I > 2σ(I)) = 0.0289 and wR2(all data) =
0.0742.
Recently, a fourth reagent, IrCl3(tht)3 (tht = tetrahydrothio-
phene) has been employed in synthesizing a class of true blue
emitting Ir(III) complexes such as [Ir(dfppy)(fppz)2] (1;
fppzH = 3-trifluoromethyl-5-(2-pyridyl)pyrazole), from which
an OLED with maximum external quantum efficiency (EQE) of
8.5% and CIE(x,y) coordinates of 0.16 and 0.18 was obtained.32
Unfortunately, the reported reaction affords the desired
isomeric Ir(III) complexes only in lower yield (≤20%). Herein,
we describe a new methodology which allowed better control
of the stereochemistry and afforded the desired product in
higher yields. Moreover, the same strategy was equally
applicable for the scale-up preparation of the true blue
phosphor [Ir(bdp)(fppz)2] (2; bdpH = benzyldiphenylphos-
phine) and functional derivatives that have shown a maximum
EQE of up to 11.7% and CIE(x,y) coordinates of 0.16 and
0.11.33 For both synthetic reactions, key intermediates en route
to the desired products were characterized by NMR spectros-
copy and single-crystal X-ray diffraction methods. Apparently,
the present investigation is valuable for the efficient synthesis of
relevant Ir(III) phosphors that cannot be prepared by
traditional methods.
Preparation of [Ir(tht)(PPh3)2Cl3] (6). Method 1. A mixture of
IrCl3(tht)3 (400 mg, 0.71 mmol) and PPh3 (392 mg, 1.5 mmol) in
decalin (15 mL) was heated to reflux for 6 h. After the reaction
mixture was cooled to room temperature, the solvent was removed
under vacuum and the solid residue was triturated with hexane (50
mL) and collected by filtration. Further purification was done by
washing with CH2Cl2 to give a yellow powder (396 mg, 0.44 mmol,
61%).
Method 2. A procedure similar to that described for method 1 was
conducted. A mixture of 5 (100 mg, 0.136 mmol) and PPh3 (39 mg,
0.15 mmol) in decalin (10 mL) was heated to reflux for 0.5 h.
Complex 6 was obtained in 24% yield (29 mg, 0.032 mmol).
Spectral Data of 6: MS (FAB, 193Ir) m/z 787 [M − (tht, Cl)]+; 1H
NMR (400 MHz, CDCl3, 294 K) δ 7.64 (t, J = 9.0 Hz, 6H), 7.52 (t, J
= 9.0 Hz, 6H), 7.36−7.20 (m, 5H), 7.20−7.08 (m, 13H), 3.65 (br,
2H), 2.33 (br, 2H), 2.20 (br, 2H), 1.85 (br, 2H); 31P{1H} NMR (202
MHz, CDCl3, 294 K) δ −25.4 (d, JPP = 13.5 Hz, 1P), −30.0 (d, JPP
=
13.5 Hz, 1P). Anal. Calcd for C40H38Cl3IrP2S: C, 52.72; H, 4.20.
Found: C, 52.09; H, 4.20.
Preparation of [Ir(dfppy)(tht)(PPh3)Cl2] (7). Method 1. A
mixture of 5 (154 mg, 0.21 mmol) and dfppyH (44 mg, 0.23
mmol) in decalin (10 mL) was heated to reflux for 1 h. After
evaporation of solvent, the residue was dissolved in CH2Cl2 and the
solution was then filtered through a pad of Celite. The filtrate was
concentrated on a rotavap, followed by crystallization from a mixture
of CH2Cl2 and hexane at room temperature, giving pale yellow crystals
(31 mg, 0.039 mmol, 19%).
Method 2. A mixture of IrCl3(tht)3 (100 mg, 0.18 mmol), dfppyH
(37 mg, 0.20 mmol), and PPh3 (49 mg, 0.187 mmol) in decalin (8
mL) was heated to reflux for 1.5 h. After the reaction mixture was
cooled to room temperature, the solution was concentrated, the
residue was dissolved in CH2Cl2, and this solution was then filtered
through a pad of Celite. After concentration and recrystallization,
complex 7 was obtained in 39% yield (55 mg, 0.068 mmol).
2. EXPERIMENTAL SECTION
General Procedures. All reactions were performed under an
argon atmosphere, and solvents were distilled from appropriate drying
agents prior to use. Commercially available reagents were used without
further purification unless otherwise stated. All reactions were
monitored using precoated TLC plates (0.20 mm with fluorescent
indicator UV254). Mass spectra were obtained on a JEOL SX-102A
instrument operating in electron impact (EI) or fast atom bombard-
1
Spectral Data of 7: MS (FAB, 193Ir) m/z 804 (M + 1)+; H NMR
(400 MHz, CDCl3, 294 K) δ 9.16 (d, J = 4.8 Hz, 1H), 7.95 (d, J = 8.4
Hz, 1H), 7.51−7.40 (m 7H), 7.30−7.16 (m, 3H), 7.15−7.05 (m, 7H),
6.63 (t, J = 6.6 Hz, 1H), 6.45−6.35 (m, 1H), 3.59 (br, 1H), 2.77 (br,
1H), 2.64 (br, 1H), 2.01 (br, 3H), 1.82 (br, 1H), 1.70 (br, 1H);
19F{1H} NMR (470 MHz, CDCl3, 294 K) δ −107.1 (m, 1F), −111.9
(t, J = 10.8 Hz, 1F); 31P{1H} NMR (202 MHz, CDCl3, 294 K) δ
−15.8 (s, 1P). Anal. Calcd for C33H29Cl2F2IrNPS: N, 1.74; C, 49.31;
H, 3.64. Found: N, 2.23; C, 48.62; H, 3.90.
1
ment (FAB) mode. H and 13C NMR spectra were recorded on a
Varian Mercury-400 or an INOVA-500 instrument. Elemental analysis
was carried out with a Heraeus CHN-O Rapid elemental analyzer. The
Ir(III) metal reagent [IrCl3(tht)3] was synthesized using the literature
method.34
Selected Crystal Data of 7: C33H29Cl2F2IrNPS; Mr = 803.70;
orthorhombic; space group P212121; a = 10.6610(8) Å, b =
4350
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Organometallics
Article
13.0231(10) Å, c = 21.6671(16) Å, V = 3008.2(4) Å3; Z = 4; ρcalcd
=
of Celite. The filtrate was collected and concentrated on a rotavap to
give a crude product. Crystallization from CH2Cl2 and hexane at room
temperature gave yellow crystals of 9 (62 mg, 0.063 mmol, 56%).
Method 2. A mixture of IrCl3·3H2O (106 mg, 0.30 mmol) and
bdpH (170 mg, 0.62 mmol) in degassed 2-methoxyethanol (15 mL)
was heated at 120 °C for 12 h. After the mixture was cooled to room
temperature, fppzH (64 mg, 0.30 mmol) and Na2CO3 (320 mg, 3.0
mmol) were added and the mixture was stirred at room temperature
for 3 h. An excess of water was added to induce precipitation, which
was collected by filtration and washed with methanol and ether in
sequence. The residue was purified by silica gel column chromatog-
raphy using CH2Cl2 as the eluent. Further crystallization from CH2Cl2
and hexane at room temperature gave yellow crystals of 9 (120 mg,
0.12 mmol, 40%). Single crystals suitable for an X-ray diffraction study
were obtained from a mixture of CH2Cl2 and hexane at room
temperature.
1.775 Mg m−3; F(000) = 1576; crystal size 0.25 × 0.25 × 0.20 mm3;
λ(Mo Kα) = 0.710 73 Å; T = 150(2) K; μ = 4.776 mm−1; 17 246
reflections collected, 6851 independent reflections (Rint = 0.0524),
GOF = 1.043, final R1(I > 2σ(I)) = 0.0671 and wR2(all data) =
0.1580.
Preparation of [Ir(dfppy)(fppz)2] (1). Method 1. A mixture of 7
(151 mg, 0.19 mmol), 5-pyridyl-3-trifluoromethyl-1H-pyrazole
(fppzH, 87 mg, 0.41 mmol), and Na2CO3 (100 mg, 0.94 mmol) in
decalin (8 mL) was heated to reflux for 12 h. After the reaction
mixture was cooled to room temperature, the solvent was removed
and the residue was purified by silica gel column chromatography
using a 2/1 mixture of ethyl acetate and hexane as the eluent. Further
crystallization from a mixture of CH2Cl2 and hexane at room
temperature gave pale yellow crystals of 1 (83 mg, 0.10 mmol, 55%).
Method 2. A mixture of IrCl3(tht)3 (150 mg, 0.27 mmol), PPh3 (73
mg, 0.28 mmol), and dfppyH (54 mg, 0.28 mmol) in decalin (15 mL)
was heated at 190 °C for 6 h. After the reaction mixture was cooled to
room temperature, fppzH (120 mg, 0.56 mmol) and Na2CO3 (142
mg, 1.3 mmol) were added and the mixture was heated to reflux for
another 24 h. After chromatographic separation and recrystallization,
complex 1 was obtained in 32% yield (69 mg, 0.086 mmol).
Spectral Data of 9: MS (FAB, 193Ir) m/z 992 (M + 1)+, 956 (M −
Cl+); 1H NMR (500 MHz, CDCl3, 294 K) δ 8.26 (d, J = 7.0 Hz, 1H),
7.65 (t, J = 9.0 Hz, 2H), 7.43 (t, J = 9.0 Hz, 1H), 7.37−7.30 (m, 3H),
7.21 (d, J = 7.0 Hz, 1H), 7.16−7.08 (m, 5H), 7.06 (d, J = 7.0 Hz, 1H),
7.04 (d, J = 7.0 Hz, 1H), 7.02 (s, 1H), 7.00−6.90 (m, 3H), 6.84 (t, J =
7.5 Hz, 2H), 6.82−6.77 (m, 4H), 6.68 (t, J = 7.5 Hz, 2H), 6.63 (t, J =
7.5 Hz, 2H), 6.42−6.35 (m, 5H), 4.98 (dd, J = 15.0, 10.0 Hz, 1H),
4.28 (dd, J = 15.0, 10.0 Hz, 1H), 3.73 (dd, J = 16.5, 10.0 Hz, 1H), 3.02
(dd, J = 16.5, 10.0 Hz, 1H); 19F{1H} NMR (470 MHz, CDCl3, 294 K)
δ −60.33 (s, 3F); 31P{1H} NMR (202 MHz, CDCl3, 294 K) δ 2.70 (d,
J = 16.5 Hz, 1P), −17.33 (d, J = 16.5 Hz, 1P). Anal. Calcd for
C47H38ClF3IrN3P2: N, 4.24; C, 56.94; H, 3.86. Found: N, 4.23; C,
56.77; H, 4.16.
1
Spectral Data of 1: MS (FAB, 192Ir) m/z 808 (M + 1)+; H NMR
(400 MHz, d6-acetone, 294 K) δ 8.26 (d, JHH = 8.8 Hz, 1H), 8.18−
8.03 (m, 4H), 7.94−7.90 (m, 2H), 7.44−7.39 (m, 3H), 7.32 (td, JHH
=
5.6, 1.6 Hz, 1H), 7.23 (s, 1H), 7.21 (s, 1H), 7.16 (td, JHH = 6.2, 1.2 Hz,
1H), 6.63 (td, JHH = 8.2, 2.4 Hz, 1H), 5.82 (dd, JHH = 8.8, 2.4 Hz, 1H);
19F NMR (470 MHz, d6-acetone, 294 K) δ −111.8 (s, 1F), −109.6 (s,
1F), −60.7 (s, 3F), −60.6 (s, 3F). Anal. Calcd for C29H16F8IrN7: N,
12.15; C, 43.18; H, 2.00. Found: N, 12.03; C, 43.35; H, 2.02.
Preparation of [Ir(bdp)(bdpH)(tht)Cl2] (8). Method 1. A
mixture of 5 (100 mg, 0.14 mmol) and bdpH (81 mg, 0.29 mmol)
in decalin (6 mL) was heated to reflux for 1 h. After the reaction
mixture was cooled to room temperature, the solvent was removed
under vacuum and the residue was filtered through a pad of Celite.
The filtrate was collected and concentrated on a rotavap to give a
crude product. Crystallization from a mixture of CH2Cl2 and hexane at
room temperature gave pale yellow crystals (92 mg, 0.10 mmol, 75%).
Method 2. A mixture of IrCl3(tht)3 (200 mg, 0.36 mmol), bdpH
(206 mg, 0.75 mmol), and PPh3 (98 mg, 0.37 mmol) in decalin (10
mL) was heated to reflux for 12 h. After the mixture was cooled to
room temperature, the solvent was removed under vacuum and the
residue was dissolved in CH2Cl2 and filtered through a pad of Celite.
After concentration and recrystallization, complex 8 was obtained in
61% yield (195 mg, 0.22 mmol).
Selected Crystal Data of 9: C47H38ClF3IrN3P2; Mr = 991.39;
monoclinic; space group P21/n; a = 12.4793(4) Å, b = 20.8875(7) Å, c
= 15.9283(5) Å, β = 109.143(1)°, V = 3922.3(2) Å3; Z = 4; ρcalcd
=
1.679 Mg m−3; F(000) = 1968; crystal size 0.36 × 0.15 × 0.12 mm3;
λ(Mo Kα) = 0.710 73 Å; T = 150(2) K; μ = 3.608 mm−1; 29 888
reflections collected, 9005 independent reflections (Rint = 0.0444),
GOF = 1.034, final R1(I > 2σ(I)) = 0.0276 and wR2(all data) =
0.0595.
Preparation of [Ir(bdp)2(fppz)] (4). A mixture of 9 (120 mg, 0.12
mmol) and silver trifluoromethanesulfonate (AgOTf, 40 mg, 0.16
mmol) in 2-methoxyethanol (10 mL) was heated at 120 °C for 1 h in
the dark. After the mixture was cooled to room temperature, the
mixture was filtered through a pad of Celite and mixed with water to
induce precipitation. The collected precipitate was next purified by
silica gel column chromatography using CH2Cl2 as the eluent.
Crystallization from CH2Cl2 and hexane at room temperature gave
white crystals (38 mg, 0.04 mmol, 33%).
Trapping of [Ir(bdp)(PPh3)(tht)Cl2] (10). Method 1. A mixture
of 5 (100 mg, 0.14 mmol) and bdpH (41 mg, 0.15 mmol) in decalin
(8 mL) was heated to reflux for 1 h. After the mixture was cooled to
room temperature, the solvent was removed under vacuum, and
hexane was added (20 mL), the resulting precipitate was collected by
filtration. Further purification was done by washing with CH2Cl2 and
hexane to give a pale yellow powder (101 mg), the major component
of which was believed to be [Ir(bdp)(PPh3)(tht)Cl2] (10).
Method 2. A mixture of 6 (100 mg, 0.11 mmol) and bdpH (33 mg,
0.12 mmol) in decalin (8 mL) was heated to reflux for 2 h. A pale
yellow powder (106 mg) was obtained as a mixture of products.
Preparation of [Ir(bdp)(fppz)2] (2). Method 1. A mixture of 5
(150 mg, 0.20 mmol) and bdpH (62 mg, 0.22 mmol) in decalin (10
mL) was heated at 180 °C for 4.5 h. After the mixture was cooled to
room temperature, fppzH (92 mg, 0.428 mmol) and Na2CO3 (108
mg, 1.02 mmol) were added and the mixture was heated to reflux for
another 16 h. After cooling and evaporation of solvent, the residue was
purified by silica gel column chromatography with a 1/1 mixture of
ethyl acetate and hexane as eluent. Further crystallization from mixed
CH2Cl2 and hexane at room temperature gave white crystals of 2 (63
mg, 0.071 mmol, 35%).
Spectral Data of 8: MS (FAB, 193Ir) m/z 868 (M − Cl)+; 1H NMR
(500 MHz, CD2Cl2, 233 K) δ 8.81 (br, 1H), 8.03 (d, J = 7.5 Hz, 1H),
7.71 (br, 2H), 7.39−7.29 (m, 4H), 7.29−7.19 (m, 4H), 7.19−7.11 (m,
4H), 7.09 (t, J = 6.5 Hz, 2H), 7.01 (t, J = 7.5 Hz, 1H), 6.95 (t, J = 7.5
Hz, 1H), 6.79 (t, J = 7.5 Hz, 1H), 6.78−6.71 (m, 2H), 6.65 (t, J = 7.5
Hz, 2H), 6.34 (t, J = 9.0 Hz, 2H), 6.05 (d, J = 7.5 Hz, 2H), 4.79 (dd, J
= 15.0, 10.0 Hz, 1H), 3.79−3.69 (m, 1H), 3.63 (dd, J = 15.0, 10.0 Hz,
1H), 3.51 (dd, J = 17.5, 11.5 Hz, 1H), 3.10−3.02 (m, 1H), 2.01−1.91
(m, 2H), 1.86 (dd, J = 17.5, 11.5 Hz, 1H), 1.72−1.56 (m, 4H);
31P{1H} NMR (202 MHz, CD2Cl2, 233 K) δ 4.63 (d, JPP = 16.2 Hz,
1P), −14.8 (d, JPP = 16.2 Hz, 1P). Anal. Calcd for C42H41Cl2IrP2S: C,
55.87; H, 4.58. Found: C, 55.84; H, 4.64.
Selected Crystal Data of 8: C84H82Cl4Ir2P4S2; Mr = 1805.70;
monoclinic; space group C2/c; a = 43.3581(17) Å, b = 8.8949(3) Å, c
= 19.2344(7) Å, β = 103.151(2)°, V = 7223.5(5) Å3; Z = 4; ρcalcd
=
1.660 Mg m−3; F(000) = 3600; crystal size 0.28 × 0.25 × 0.25 mm3;
λ(Mo Kα) = 0.710 73 Å; T = 150(2) K; μ = 4.022 mm−1; 27 074
reflections collected, 8308 independent reflections (Rint = 0.0318),
GOF = 1.120, final R1(I > 2σ(I)) = 0.0245 and wR2(all data) =
0.0625.
Preparation of [Ir(bdp)(bdpH)(fppz)Cl] (9). Method 1. A
mixture of 8 (100 mg, 0.12 mmol), fppzH (26 mg, 0.12), and
Na2CO3 (58 mg, 0.55 mmol) in decalin (8 mL) was heated for 0.5 h at
160 °C. After the mixture was cooled to room temperature, the solvent
was removed under vacuum and the residue was filtered through a pad
Method 2. A mixture of 6 (160 mg, 0.18 mmol) and bdpH (53 mg,
0.19 mmol) in decalin (10 mL) was heated at 180 °C for 3 h. After the
mixture was cooled to room temperature, fppzH (79 mg, 0.37 mmol)
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and Na2CO3 (93 mg, 0.88 mmol) were added and the mixture was
heated to reflux for another 16 h. After chromatographic separation
and recrystallization, complex 2 was obtained in 35% yield (54 mg,
0.06 mmol).
Scheme 2. Synthetic Procedures for the Ir(III) Metal
Complex 1
a
Method 3. A mixture of IrCl3(tht)3 (155 mg, 0.28 mmol), PPh3 (78
mg, 0.30 mmol), and bdpH (83 mg, 0.301 mmol) in decalin (10 mL)
was heated at 180 °C for 6 h. After the mixture was cooled to room
temperature, fppzH (120 mg, 0.56 mmol) and Na2CO3 (150 mg, 1.4
mmol) were added and the mixture was heated to reflux for 12 h. After
chromatographic separation and recrystallization, complex 2 was
obtained in 44% yield (109 mg, 0.12 mmol).
Single Crystal X-ray Diffraction Studies. Single-crystal X-ray
diffraction data were measured on a Bruker SMART Apex CCD
diffractometer using Mo Kα radiation (λ = 0.710 73 Å). Data
collection was executed using the SMART program. Cell refinement
and data reduction were performed with the SAINT program. The
structure was determined using the SHELXTL/PC program and
refined using full-matrix least squares. Selected crystallographic data
and refinement parameters are summarized in the section following
their synthetic procedures and spectral data.
a
Reagents and conditions: (i) PPh3, decalin, 196 °C, 0.5 h; (ii)
dfppyH, decalin, 196 °C, 1 h; (iii) fppzH, Na2CO3, decalin, 196 °C, 12
h.
3. RESULTS AND DISCUSSION
Improved Synthesis of 1. In the 1960s, Chatt and co-
workers reported that PEt3 can readily coordinate to
chloroiridic acid, H2IrCl6, in forming the stable yellow complex
[IrCl3(PEt3)3]3, which was the first example of an Ir(III) atom
accommodating up to three phosphine donors.35 Subsequently,
it was noted that, upon treatment of IrCl3·nH2O with
dimethyl(1-naphthyl)phosphine at high temperature, the
phosphine ligand underwent both metal coordination and C−
H activation at the naphthyl pendant, giving a distinctive class
of cyclometalated complexes.36 Bearing these precedents in
mind, we then synthesized the novel Ir(III) complex [Ir-
(bdp)2(OAc)] (3) by treatment of [Ir(tht)3Cl3] with bdpH in
the presence of sodium acetate, NaOAc (Scheme 1).37,38 The
In terms of purification and characterization, high-quality
crystals of 5 can be easily obtained by recrystallization at room
temperature; therefore, an X-ray diffraction study was used for
revealing its molecular structure. In contrast, complex 6 is much
less soluble in organic solvents, such that its identification is
solely based on elemental analysis and multinuclear NMR
spectroscopy. Furthermore, the retention of two and one
weakly coordinated tht ligand on both complexes, particularly
for 5, confirms their inherent potential to serve as an alternative
synthetic reagent for other Ir(III) phosphors. Figure 1 depicts
Scheme 1. Cyclometalation of Benzyldiphenylphosphine on
a
an Ir(III) Metal Center
a
Reagents and conditions: (i) bdpH, NaOAc, decalin, 196°C, 2 h; (ii)
fppzH, decalin, 196 °C, 12 h.
isolation of 3 then triggered the preparation of a phosphine-
containing Ir(III) phosphor; one such example is [Ir-
(bdp)2(fppz)] (4), with two cyclometalated phosphorus
ancillaries.
Figure 1. ORTEP diagram of 5 with ellipsoids shown at the 40%
probability level. Selected bond distances (Å): Ir−P(1) = 2.2946(9),
Ir−S(1) = 2.3562(9), Ir−S(2) = 2.3557(9), Ir−Cl(1) = 2.3779(9), Ir−
Cl(2) = 2.3577(9), Ir−Cl(3) = 2.4174(8).
Recently, in sharp contrast to the triphenyl phosphite that
yields double cyclometalation,39,40 a bulky phosphine ligand
such as PPh3, was found to react readily with the
aforementioned Ir(III) source complex [Ir(tht)3Cl3], giving
two isolable products, namely [Ir(tht)2(PPh3)Cl3] (5) and
[Ir(tht)(PPh3)2Cl3] (6), in a stepwise fashion (Scheme 2). The
monosubstitution product 5 was obtained as the major product
for the reaction using a stoichiometric amount of PPh3, while
the disubstitution product 6 was isolated by addition of 2 equiv
of PPh3 to [Ir(tht)3Cl3] or by treatment of the monosub-
stitution product 5 with a another 1 equiv of PPh3 under similar
conditions.
the ORTEP diagram of 5. As can be seen, the three chloride
ligands adopt a meridional arrangement around the Ir(III)
atom. The PPh3 is coordinated trans to the unique chloride,
while the remaining axial sites are occupied by two tht ligands.
For the bond distances, the Ir−Cl(3) length of 2.4174(8) Å is
notably longer than those of the trans-substituted chlorides, i.e.
Ir−Cl(1) = 2.3779(9) and Ir−Cl(2) = 2.3577(9) Å, confirming
the greater trans influence of PPh3 exerted on the central
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chloride, while both tht ligands on 5 adopt a puckered
arrangement with an average Ir−S distance of 2.356 Å, which is
similar to that of Ir(III) complexes bearing tht or other
thioether ligands such as dithiane.38,41
Subsequently, a new Ir(III) complex of the formula
[Ir(dfppy)(tht)(PPh3)Cl2] (7) was then synthesized by heating
the monosubstituted PPh3 adduct 5 with 2-(2,4-
difluorophenyl)pyridine (dfppyH) (cf. Scheme 2) or, alter-
natively, by heating a mixture of IrCl3(tht)3, dfppyH, and PPh3
in a one-pot fashion in refluxing decalin. Figure 2 shows the
study the associated reaction sequences, i.e. phosphine addition
versus cyclometalation, we then conducted the reaction of
[Ir(tht)3Cl3] with 3 equiv of bdpH in the absence of NaOAc
with the hope of synthesizing the substitution products at
different stages.45 To our surprise, only the Ir(III) complex
[Ir(bdp)(bdpH)(tht)Cl2] (8), which contains one coordinated
bdpH, together with one cyclometalated bdp chelate, was
isolated in lower yield (≤24%). Alternatively, upon switching
from [Ir(tht)3Cl3] to 5 as the Ir(III) source reagent, the
respective reaction with bdpH afforded 8 in a much improved
yield of 75%, while using the one-pot strategy, i.e. employing a
mixed solution of [Ir(tht)3Cl3], 1 equiv of PPh3, and 2 equiv of
bdpH, afforded the same product 8 in a comparable yield of
61%. These control reactions hinted that either the formation
of 5 or the temporal coordination of PPh3 is the key step
leading to the efficient formation of diphosphine complex 8
(Scheme 3).
a
Scheme 3. Synthetic Route to the Ir(III) Metal Complex 4
Figure 2. ORTEP diagram of 7 with ellipsoids shown at the 40%
probability level. Selected bond distances (Å): Ir−P(1) = 2.306(3),
Ir−S(1) = 2.407(3), Ir−N(1) = 2.091(10), Ir−C(1) = 1.972(12), Ir−
Cl(1) = 2.489(3), Ir−Cl(2) = 2.377(3).
ORTEP diagram of 7, for which the dfppy chelate and two
chlorides assume a square disposition in a way similar to that
for the analogous Ir(III) complex [Ir(ppy)(Cl)2(PPh3)2],42
leaving the remaining axial sites occupied by one PPh3 and one
tht ligand. The Ir−Cl(1) distance of 2.489(3) Å is significantly
longer than the Ir−Cl(2) distance of 2.377(3) Å, showing the
difference induced by the greater trans influence imposed by
the carbon donor versus the pyridyl donor.43 Importantly, the
trans PPh3−Ir−tht ligand arrangement in 7 is notably different
from the trans PPh3−Ir−Cl arrangement observed in 5, despite
the possession of all three types of PPh3, tht, and chloride
ligands. Thus, this change in ligand arrangement was not
controlled by the kinetics but by the more influential
thermodynamic stability of the individual structure.
Finally, heating a mixture of 7 and fppzH in a 1:2 ratio gave
the previously reported blue-emitting derivative 1 in 55% yield.
An attempt to synthesize 7 directly from IrCl3(tht)3 without
isolation, followed by addition of fppzH and heating in one pot,
had given formation of identical product 1 in 32% yield.
Remarkably, this recorded yield is much higher than the ∼20%
yield of its original preparation that used no PPh3 additive.32
Thus, this single-pot strategy represents the optimized synthesis
of the blue-emitting complex 1 and associated phosphors.44
Alternative Synthetic Pathway to 4. As reported in the
previous section, the reaction of [Ir(tht)3Cl3] with varying
amounts of PPh3 gave formation of the mono- and
disubstitution products 5 and 6 (Scheme 2), while treatment
of [Ir(tht)3Cl3] with at least 2 equiv of bdpH in the presence of
NaOAc led to the isolation of 3 (Scheme 1), for which the
formation of cyclometalated bdp chelates was apparently
caused by the added NaOAc promoter.37 In an attempt to
a
Reagents and conditions: (i) 2 equiv of bdpH, decalin, 196 °C, 1 h;
(ii) fppzH, Na2CO3, decalin, 160 °C, 0.5 h; (iii) AgOTf,
MeOCH2CH2OH, 120 °C, 1 h.
The molecular geometry of 8 was unequivocally confirmed
by single-crystal structure analyses. As shown in Figure 3, the
two chlorides have a cis disposition, while the cyclometalated
bdp chelate is located trans to these chlorides, and the
remaining axial sites are occupied by the monodentate bdpH
and one tht ligand. As usual, the Ir−C distance in the benzyl
cyclometalate is short (ca. 2.064(3) Å), which points to
enhanced covalent bonding character. The respective metalla-
cycle is virtually coplanar, with the phenyl substituents on the
phosphorus atom residing both above and below this plane.
Variations of the Ir−Cl and Ir−P distances are somehow
relevant to the trans effect discussed earlier. Remarkably, the
Ir−P(1) distance of 2.2423(8) Å in 8 is notably shorter that the
Ir−P(1) distance of 2.2946(9) Å in 5. Both phosphorus atoms
are trans with respect to the chloride, meaning that the
shortening of this Ir−P distance in 8 is probably caused by the
formation of a metallacycle, providing strong evidence with
regard to its enhanced stability.
Application of 8 in synthesis was next probed with fppzH in
the presence of Na2CO3. This reaction afforded the substitution
product [Ir(bdp)(bdpH)(fppz)Cl] (9), for which the tht and
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original approach [Ir(tht)3Cl3] → 3 → 4 shown in Scheme 1.37
The disadvantage of the former lies in the extreme conditions
required for the final cyclometalation of coordinated bdpH in 9
in comparison to the high-yield generation of 3, with which the
cyclometalation was easily executed in the presence of added
NaOAc.
Improved Synthesis of 2. According to our previous
report, blue-emitting phosphors such as 2 can be synthesized in
18−23% yield from the stepwise treatment of IrCl3(tht)3 with 1
equiv of bdpH, followed by heating in the presence of 2 equiv
of fppzH and excess Na2CO3.33 Unfortunately, this yield is
much too low for scale-up industrial application and, thus, an
alternative method is urgently needed, since its analogues are
known to be superior for making true blue emitting OLEDs
with decent efficiencies. Encouraged by the isolation of 8 and
its capability of serving as a synthetic reagent, we initiated the
preparation of [Ir(bdp)(PPh3)(tht)Cl2] (10), in which the bdp
chelate is expected to remain intact throughout the reaction,
while the weakly coordinated PPh3 would act as both stabilizer
and leaving group at the same time.
Figure 3. ORTEP diagram of 8 with ellipsoids shown at the 40%
probability level. Selected bond distances: Ir−P(1) = 2.2423(8), Ir−
P(2) = 2.3136(7), Ir−S(1) = 2.4411(8), Ir−Cl(1) = 2.4521(7), Ir−
Cl(2) = 2.4358(7), Ir−C(1) = 2.064(3) Å.
One simple way to synthesize this intermediate 10 is via
heating of 5 with an equal ratio or slight excess of bdpH in
decalin (Scheme 4). However, after precipitating the mixture of
one adjacent chloride were replaced by the incoming fppz
chelate. The alternative, much simpler approach is to add 2
equiv of bdpH to IrCl3·3H2O in 2-methoxyethanol, followed by
addition of fppzH in the presence of Na2CO3, which afforded 9
in 40% yield, calculated using IrCl3·3H2O as the limiting
reagent. It appears that the chloride trans to the cyclometalated
benzyl group in 8 was the preferable site for substitution. Since
the aforementioned trans chloride is, in principle, more labile
than the other chloride ligand, cf. Ir−Cl(1) = 2.4521(7) and
Ir−Cl(2) = 2.4358(7) Å (Figure 3), we thus speculate that the
transformation from 8 to 9 is under kinetic control.
a
Scheme 4. Synthetic Route to the Ir(III) Complex 2
a
Figure 4 depicts the ORTEP diagram of 9, in which the
remaining chloride still exhibits an identical bond distance, cf.
Reagents and conditions: (i) 1 equiv of bdpH, decalin, 196 °C, 1−2
h; (iii) 2 equiv of fppzH, Na2CO3, decalin, 196 °C, 12 h.
products by addition of hexane, we observed two sets of 31P
NMR signals at δ 3.62 and −14.93 with JPP = 17.3 Hz and δ
−3.44 and −16.05 with JPP = 16.2 Hz, all with nearly equal
intensities. In accordance with a previous report,46 the signals at
δ 3.62 and −14.93 can be identified as the coordinated bdpH
and cyclometalated bdp chelate of 8. This leaves the second set
of signals at δ −3.44 and −16.05 to the coordinated PPh3 and
cyclometalated bdp chelate of 10. Attempts to execute the
reaction at lower temperature and shorter reaction time failed
to cause any substantial change of this 31P NMR signal ratio,
which implied that the higher nucleophilicity of bdpH made the
selective formation of 10 less desirable: i.e., showing rapid
displacement of ligated PPh3. Fortunately, upon switching the
Ir(III) source to 6, which possesses two coordinated PPh3
groups, we were able to obtain a mixture that showed the 8:10
product ratio increased to 1:2.5, confirming both the spectral
assignment and our speculation. Without further isolation,
these mixtures of products were capable of reacting with added
fppzH and Na2CO3 in decalin solution, giving the desired
Ir(III) complex 2 in 35% yield. Furthermore, the one-pot
procedure that employed IrCl3(tht)3 even increased the
isolated yield to 44%, representing a significant improvement
versus that reported in the literature.33
Figure 4. ORTEP diagram of 9 with ellipsoids shown at the 30%
probability level. Selected bond distances: Ir−P(1) = 2.2598(8), Ir−
P(2) = 2.3078(8), Ir−N(1) = 2.118(2), Ir−N(2) = 2.111(2), Ir−
C(10) = 2.076(3), Ir−Cl(1) = 2.4312(7) Å.
Ir−Cl(1) = 2.4312(7) Å, implying the retention of the original
bonding character and confirming the proposal of kinetic
control. In fact, this last chloride could be removed with a
scavenger, AgOTf, affording the expected cyclometalated
product 4 in moderate yield (Scheme 3). Overall, this
tranformation [Ir(tht)3Cl3] → 5 → 8 → 9 → 4 is an
alternative, but less desirable, synthetic protocol versus the
CONCLUSION
■
In summary, the present work reports a nonconventional
method of synthesizing several Ir(III) phosphors that have been
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OLEDs.32,33 The syntheses started with the conversion of
[IrCl3(tht)3] to 5 bearing one PPh3 and two tht ligands. After
that, treatment of 5 with dfppyH afforded 7 or, with a variable
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weakly coordinated PPh3 and tht ligands are believed to be
responsible for the successful preparation of these OLED
phosphors in better yields.
ASSOCIATED CONTENT
* Supporting Information
■
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S
CIF files giving X-ray crystallographic data for the Ir(III) metal
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AUTHOR INFORMATION
Corresponding Author
*E-mail: ychi@mx.nthu.edu.tw (Y.C.); ytt@chem.sinica.edu.tw
(Y.-T.T.); lsliao@suda.edu.cn (L.-S.L.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank both the National Science Council of Republic of
China and the National Natural Science Foundation of China
for financial support.
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