Charged cyclometalated iridium(III) complexes that have large electrochemical gap

Article abstract of DOI:10.1016/j.ica.2011.10.052

Bis-cyclometalated cationic Ir(III) diimine complexes [Ir(C^N) ₂(N^N)](PF₆) with 1-phenylpyrazoles (C^N) and 1-(4′-tert-butyl-2′-pyridyl)pyrazole (N^N) are white solids that have absorption onset below 425 nm and electrochemical gap of up to 3.5 V.

Full text of DOI:10.1016/j.ica.2011.10.052

Inorganica Chimica Acta 383 (2012) 316–319
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Note
Charged cyclometalated iridium(III) complexes that have large electrochemical gap
⇑
⇑
Nail M. Shavaleev , Rosario Scopelliti, Etienne Baranoff, Michael Grätzel, Mohammad K. Nazeeruddin
École Polytechnique Fédérale de Lausanne, Laboratory of Photonics and Interfaces, CH-1015 Lausanne, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 July 2011
Received in revised form 12 October 2011
Accepted 13 October 2011
Available online 20 October 2011
Bis-cyclometalated cationic Ir(III) diimine complexes [Ir(C^N)2(N^N)](PF6) with 1-phenylpyrazoles (C^N)
and 1-(40-tert-butyl-20-pyridyl)pyrazole (N^N) are white solids that have absorption onset below 425 nm
and electrochemical gap of up to 3.5 V.
Ó 2011 Elsevier B.V. All rights reserved.
Keywords:
Iridium
Cyclometalation
Complex
Pyrazole
Electrochemistry
1. Introduction
Two new complexes, 3 (3H and 3F), were prepared by reaction
of the neutral ligand with a cyclometalated Ir(III) precursor
Cyclometalated iridium(III) complexes are finding increasing
application in electro-luminescent devices as emitters [1–8] and
as electronic materials [9–11]. Recently, He et al reported that
Ir(III) complexes with 1-(20-pyridyl)pyrazole (1, Chart 1) emit
[Ir(C^N)2(l-Cl)]2 (Scheme 1). The complexes were purified by col-
umn chromatography (on silica) and by re-crystallization; they
were characterized by elemental analysis, ESI+ MS, and NMR
spectroscopy.
blue-green phosphorescence from a predominantly ligand cen-
tered
and optical properties of 1 could be tuned by replacing a pyridine
in the cyclometalating ligand with a pyrazole. We reasoned that
Fig. 1 shows the X-ray molecular structure of complex 3F. The
metal ion is in a distorted octahedral [Ir(C^N)2(N^N)]+ coordination
environment with the two nitrogen atoms of the cyclometalating
ligands in trans-position to each other. The Ir–N bonds to the
cyclometalating ligand are shorter than those to the neutral ligand
(Table 1). The same trend has been previously noted in the struc-
tures of 1 and 2H (Chart 1), and it has been attributed to the strong
trans-influence of an anionic phenyl [12,14]. For the neutral ligand
in 3F, the Ir–N bond to pyrazole is shorter by 0.051 Å than that to
pyridine (Table 1); in 1, however, this difference is less than
0.012 Å [12].
3
(p–
p⁄) excited state [12]. We considered that the redox
the pyrazole is a weaker
the pyridine [13], and that the pyrazole ligands have
p
-acceptor and a weaker
r
p
-donor than
–
p⁄ transi-
tions at higher energy than the analogous pyridine derivatives
[14]. Here, we describe a successful application of this approach
to access cationic bis-cyclometalated Ir(III) complexes that have a
high-energy absorption onset and a large electrochemical gap.
The ligands in 3F are nearly planar: the dihedral angle between
their constituent rings is less than 7°. In the solid state, the com-
2. Results and discussion
plexes participate in intermolecular p–p aromatic stacking that in-
Scheme 1 shows a new neutral ligand L that was prepared by a
non-catalyzed C–N coupling reaction of pyrazole with 2-chloro-4-
tert-butylpyridine in DMSO, in the presence of potassium tert-
butoxide as a base [15]. The pyridyl ring was modified with a bulky
tert-butyl group to improve solubility of the Ir(III) complexes and
to reduce the intermolecular interaction between them in the solid
state.
volves only the cyclometalating ligands. The strongest interaction
occurs by the overlap of 2,4-difluorophenyl rings at inter-planar
distance of 3.245 Å. The bulky neutral ligand does not participate
in
p–p aromatic stacking. Inter-metallic communication is likely
to be negligible because of the long Ir–Ir distance (8.812 Å).
The redox properties of 3 were studied by cyclic voltammetry in
DMF and acetonitrile (Fig. 2 and Fig. S1, Supporting Information).
The electrochemical potentials (relative to Fc+/Fc) and electro-
⇑
Corresponding authors. Tel.: +41 21 693 6124; fax: +41 21 693 4111.
E-mail addresses: nail.shavaleev@epfl.ch (N.M. Shavaleev), mdkhaja.nazeerud-
ox
chemical gaps (defined as
D
E = E1/2 ꢀ Ered) of the complexes were
found to be solvent-independent (Table 2).
din@epfl.ch (Md.K. Nazeeruddin).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.10.052

N.M. Shavaleev et al. / Inorganica Chimica Acta 383 (2012) 316–319
317
Table 1
R
R
Selected bond lengths (Å) in complex 3Fa.
R
R
N
C^N
Ir–C
N^N
2
2
N
N
N
Ir–N(pz)b
Ir–N(py)b
Ir–N(pz)b
2.111(5)
Ir
PF6
Ir
PF6
2.029(6)
2.036(7)
2.024(6)
2.033(5)
2.162(6)
N
N
N
N
a
Each row corresponds to one ligand in the complex.
N(py) and N(pz) are nitrogen atoms of pyridine and pyrazole, respectively.
b
1: R = H (1H); F (1F)
2: R = H (2H); F (2F)
Chart 1. Previously reported Ir(III) complexes [12,14].
3H
c
a
b
Cl
I
N
Ir
N
N
N
O
R
3F
R
N
N
2
N
d
N
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
E/V vs Fc+/Fc
PF6
N
N
N
L
Fig. 2. Cyclic voltammograms of the complexes in CH3CN (0.1 M NBu4PF6, 100 mV/s).
The unit on the vertical axis is 10
Information.
lA. CVs in DMF are provided in the Supporting
3: R = H (3H); F (3F)
Table 2
Redox properties of Ir(III) complexesa.
Scheme 1. Synthesis of the ligand and Ir(III) complexes: (a) H2O2, acetic acid, under
air, 80 °C; (b) POCl3, ‘dry finger’, under air, 130 °C; (c) pyrazole, KO-t-Bu, dry DMSO,
under Ar, 140 °C; (d) [Ir(C^N)2(l-Cl)]2, CH2Cl2/CH3OH, under Ar, 40 °C.
Complex
Solvent
E1/2ox/V
E
red/V
D
E/Vb
3H
CH3CN
DMF
CH3CN
DMF
CH3CN
CH3CN
CH3CN
CH3CN
0.94 (98)c
ꢀ2.31d
ꢀ2.33d
ꢀ2.24d
ꢀ2.24d
ꢀ2.19
ꢀ2.15
ꢀ1.89
ꢀ1.83
3.25
3.23
3.50
0.91 (146)c
3F
1.26 (112)c
e
1Hf
1Ff
0.88
1.20
0.95
1.25
3.07
3.35
2.84
3.08
2Hg
2Fg
a
Relative to Fc+/Fc. On glassy carbon working electrode, in the presence of 0.1 M
(NBu4)PF6, at scan rate 100 mV/s. Estimated error: 50 mV. The anodic/cathodic
peak separation for the standard, Fc+/Fc couple [16], was 68–88 mV in CH3CN; 83–
88 mV in DMF.
b
c
ox
D
E = E1/2 ꢀ Ered
.
Quasi-reversible process; the anodic/cathodic peak separation is given in
brackets.
d
Irreversible process; the reduction peak potential is reported.
The process was outside of the electrochemical window of the solvent (>1.0 V
e
relative to Fc+/Fc).
f
From reference [12].
From reference [14].
g
ing Information). The oxidation potential undergoes a positive shift
by 320 mV on fluorination of the phenyl ring of the cyclometalat-
ing ligand (Table 2). The first reduction is irreversible; its potential
undergoes a positive shift by 70–90 mV on fluorination (Table 2).
The redox properties of 3 were compared to those of 1 and 2
[12,14]. The oxidation potential is similar for the complexes that
have either a 2,4-difluorophenyl (3F ꢁ 1F ꢁ 2F ꢁ 1.2 V) or a phenyl
(3H ꢁ 1H ꢁ 2H ꢁ 0.9 V) as an aryl substituent (Table 2). The
300 mV difference in the potential between these two series re-
flects the strong electron-withdrawing properties of the fluorine
and suggests that the oxidation is localized on a metal-aryl
fragment.
Fig. 1. Structure of 3F (50% probability ellipsoids; H atoms, PF6 anion, and co-
crystallized diethyl ether molecule omitted; ORTEP). Heteroatoms are shown as
octant ellipsoids: Ir, black; N, shaded; F, clear.
The first oxidation process in 3 is quasi-reversible. Although
both the forward and the return waves are observed, their peak
separation is wider compared to that for the standard (Fc+/Fc cou-
ple [16]) and strongly depends on the scan rate (Table S2, Support-

318
N.M. Shavaleev et al. / Inorganica Chimica Acta 383 (2012) 316–319
The reduction potential of the complexes appears to be deter-
Ir(III) complexes [Ir(C^N)2(N^N)]+ that have high-energy absorp-
tion onset and large electrochemical gap. A facile modification of
these ligands opens opportunities for the development of func-
tional Ir(III) complexes.
mined by the neutral ligand: (3 ꢁ ꢀ2.3 V) < (1 ꢁ ꢀ2.2 V) <
(2 ꢁ ꢀ1.9 V). We consider the pyridyl ring of ligand L to be the main
electron acceptor in 3. The negativeshift of thereduction potentialof
3 by 90–120 mV with respect to that of 1 [12] arises from the pres-
ence of an electron-donating tert-butyl group in the neutral ligand
and from the replacement of a pyridine with a pyrazole, a weaker
3. Experimental
p
-acceptor [13], in the cyclometalating ligand.
3H has an electrochemical gap of 3.2 V that on fluorination in-
Purification, crystal growth, and handling of all compounds
were carried out under air. All products were stored in the dark.
Chemicals from commercial suppliers were used without purifica-
tion. Chromatography was performed on a column with an i.d. of
30 mm on silica gel 60 (Fluka, Nr 60752). The progress of reactions
and elution of products were followed on TLC plates (silica gel 60
F254 on aluminum sheets, Merck).
creases to 3.5 V in 3F (Table 2). To our knowledge, the value of
the electrochemical gap in 3F is the highest reported for a cationic
bis-cyclometalated Ir(III) complex with a neutral diimine ligand
(Table 2) [12,14,17,18].
The complexes 3 are white solids and give colorless solutions in
polar organic solvents. The electronic absorption spectra of 3 in
dichloromethane do not have strong transitions in the visible range
(Table 3, Fig. 3 and Fig. S2, Supporting Information). The absorption
3.1. Synthesis of the ligand L
onset, defined as a wavelength above which
e < 0.1%emax, is ob-
The reaction was performed under argon. Pyrazole (440 mg,
6.46 mmol, excess, Aldrich) and potassium tert-butoxide
(714 mg, 6.36 mmol, excess, Acros) were dissolved at RT in dry
and degassed DMSO (3 mL, Acros, 99.8%, ExtraDry, over Molecular
Sieves, AcroSealÒ). 2-Chloro-4-tert-butylpyridine (1 g, 5.89 mmol)
[23,24] was added. The reaction mixture was stirred at 140 °C for
9 h to give orange solution. It was cooled to RT. It was extracted
with water and ether. Organic layer was washed with water to ex-
tract DMSO. Purification by chromatography (run twice; silica,
2 ꢂ 25 g) removed the starting material (hexane/CH2Cl2; 3/1–1/1)
and recovered the product (0.5% CH3OH in CH2Cl2). The product
was isolated as colourless oil that crystallized to a pale yellow solid
after a week of standing: 923 mg (4.59 mmol, 78%). Anal. Calcd for
served at 425 nm in 3H; it is blue-shifted to 400 nm in the fluori-
nated complex 3F (Fig. 3). The shorter wavelength of an
absorption onset (Table 3) corresponds to a larger value of the elec-
trochemical gap (Table 2); therefore, we assign the lowest energy
transition in the complexes (k > 350 nm;
e
< 103 Mꢀ1 cmꢀ1; Fig. 3)
to the (metal and aryl)-to-pyridyl charge transfer.
3 do not display luminescence at 400–700 nm in 10ꢀ4 M dichlo-
romethane solutions at room temperature under near-UV excita-
tion; the upper limit of their quantum yield is 0.05%. We
attribute the lack of luminescence of 3 to the quenching of a li-
gand-centered or charge-transfer excited state by proximate non-
emissive d–d metal states; the same deactivation mechanism has
been suggested for the cyclometalated Ir(III) complexes with 1-
phenylpyrazoles [19–22].
C12H15N3 (MW 201.27): C, 71.61; H, 7.51; N, 20.88. Found: C,
72.13; H, 7.54; N, 20.71. 1H NMR (400 MHz, CDCl3; py = pyridine;
pz = pyrazole): d = 8.58 (dd, J = 2.8, 0.8 Hz, 1H, pz), 8.32 (dd,
J = 5.6, 0.4 Hz, 1H, py), 8.00 (m, 1H, py), 7.76 (m, 1H, pz), 7.20 (m,
1H, py), 6.47 (m, 1H, pz), 1.39 (s, 9H, tert-butyl) [25,26]. 13C NMR
(100 MHz, CDCl3): d = 163.54 (C, py), 152.03 (C, py), 148.02 (CH,
py), 141.97 (CH, pz), 127.36 (CH, pz), 119.04 (CH, py), 109.52
(CH, py), 107.78 (CH, pz), 35.38 (tert-butyl, C), 30.72 (tert-butyl,
CH3) [25,26]. GC-EI+ MS: m/z 201 (M+, 100%), 186 ({MꢀCH3}+, 95%).
In conclusion, 1-(40-tert-butyl-20-pyridyl)pyrazole and 1-(20,40-
R2-phenyl)pyrazole (R = H or F) form cationic bis-cyclometalated
Table 3
Optical absorption of new compoundsa.
Compound
kmax/nm (
e
/103 Mꢀ1 cmꢀ1
)
konset/nmb
L
3H
3F
253 (11), 281 (10), 305 (0.9, sh)
258 (39), 310 (12, sh), 377 (1.1, sh)
253 (44)
330
425
400
3.2. Synthesis of the complexes 3
a
In CH2Cl2, at room temperature, in the range 250–500 nm. Estimated errors:
1 nm for kmax 5 nm for konset 5% for
The reactions were performed under argon and in the absence
of light. The solvents were de-oxygenated by bubbling with Ar,
;
;
e.
b
Defined as a wavelength above which
e
< 0.1%emax.
but they were not dried. Cyclometalated precursor [Ir(C^N)2(l-
Cl)]2 [27] was dissolved in CH2Cl2 (40 mL) at RT. After addition of
CH3OH (5 mL) and ligand L (excess), the reaction mixture was stir-
red at 40 °C overnight to give colourless or pale brown solution. It
was evaporated to dryness. The work-up and further synthetic de-
tails are provided below. The complexes are air- and moisture-sta-
ble white solids; to the naked eye they appear to be non-emissive
at RT under excitation with 365 nm light in the solid state, in
CH2Cl2 solution, or on silica TLC plate.
3
2
3H: The reaction was performed with [Ir(ppz)2(
0.097 mmol; ppz = 1-phenylpyrazole-N ,C ) and (50 mg,
l
-Cl)]2 (100 mg,
2
2
0
L
1
0.25 mmol, excess). Purification by chromatography (silica, 15 g)
removed the impurities (1–3% CH3OH in CH2Cl2) and recovered
the product as colourless eluate (4–6% CH3OH in CH2Cl2). The tar-
get fractions were evaporated. Dry residue was dissolved in CH3OH
(4 mL), and the complex was precipitated by KPF6 (305 mg,
1.66 mmol, in 10 mL of water, excess, Alfa Aesar). More water
(15 mL) was added. The resulting suspension was stirred
(30 min) and filtered. The complex was washed with water and
ether. It was re-crystallized by pouring its solution in CH2Cl2
(2 mL) to ether (30 mL). The product was filtered and washed with
3F
3H
0
350
375
400
Wavelength / nm
425
450
Fig. 3. Absorption spectra of the complexes 3H (8.63 ꢂ 10ꢀ5 M) and 3F
(9.28 ꢂ 10ꢀ5 M) in CH2Cl2. Additional absorption spectra are provided in the
Supporting Information.

N.M. Shavaleev et al. / Inorganica Chimica Acta 383 (2012) 316–319
319
ether. White solid: 124 mg (0.15 mmol, 77%). Anal. Calcd for
30H29F6IrN7P (MW 824.78): C, 43.69; H, 3.54; N, 11.89. Found:
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/
j.ica.2011.10.052.
C
C, 43.23; H, 3.45; N, 12.27. 1H NMR (400 MHz, CD2Cl2): d = 8.69
(d, J = 2.8 Hz, 1H), 8.19–8.15 (m, 2H), 7.91 (d, J = 1.2 Hz, 1H), 7.87
(d, J = 6.0 Hz, 1H), 7.42 (d, J = 2.0 Hz, 1H), 7.37–7.31 (m, 3H),
7.14–6.99 (m, 4H), 6.94–6.84 (m, 2H), 6.82 (dd, J = 3.2, 2.0 Hz,
1H), 6.63–6.60 (m, 2H), 6.37–6.31 (m, 2H), 1.43 (s, 9H, tert-butyl).
19F NMR (376 MHz, CD2Cl2): d = ꢀ73.1 (d, JPꢀF = 710 Hz, 6F, PF6).
31P NMR (162 MHz, CD2Cl2): d = 144.5 (septet, PF6, JPꢀF = 710 Hz).
ESI+ TOF MS: m/z 680.21 ({MꢀPF6}+, 100%).
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d = 9.51 (d, J = 3.2 Hz, 1H), 8.63 (m, 2H), 8.43 (d, J = 1.6 Hz, 1H),
7.71 (d, J = 6.0 Hz, 1H), 7.61 (d, J = 2.0 Hz, 1H), 7.55 (dd, J = 6.0,
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M.K.N. thanks World Class University program, funded by the Min-
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Appendix A. Supplementary material
[27] M. Nonoyama, J. Organomet. Chem. 86 (1975) 263.
CCDC 833460 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The