Synthesis, Structure, Optical, and Electrochemical Properties of Iridium(III) Complexes with 2-Arylphenantroimidazoles and Dibenzoylmethane

Article abstract of DOI:10.1134/S0036023619020037

New cyclometalated neutral iridium(III) complexes [Ir(L)₂(dbm)] have been synthesized, where L is 2-arylphenanthroimidazoles with various electron-donor or acceptor substituents, dbm is dibenzoyl-methane. The compositions and structures of the

Full text of DOI:10.1134/S0036023619020037

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2019, Vol. 64, No. 2, pp. 207–215. © Pleiades Publishing, Ltd., 2019.
Russian Text © A.A. Bilyalova, S.V. Tatarin, P. Kalle, D.E. Smirnov, I.S. Zharinova, Yu.M. Kiselev, V.D. Dolzhenko, S.I. Bezzubov, 2019, published in Zhurnal Neorganicheskoi Khimii,
2019, Vol. 64, No. 2, pp. 172–180.
COORDINATION
COMPOUNDS
Synthesis, Structure, Optical, and Electrochemical Properties
of Iridium(III) Complexes with 2-Arylphenantroimidazoles
and Dibenzoylmethane
A. A. Bilyalovaa, b, S. V. Tatarinb, P. Kallea, b, D. E. Smirnova, b, I. S. Zharinovaa, b,
Yu. M. Kiselevb, V. D. Dolzhenkob, and S. I. Bezzubova, *
aKurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow, 119991 Russia
bMoscow State University, Moscow, 119991 Russia
*e-mail: bezzubov@igic.ras.ru
Received March 14, 2018; revised April 10, 2018; accepted July 6, 2018
Abstract⎯New cyclometalated neutral iridium(III) complexes [Ir(L)2(dbm)] have been synthesized, where
L is 2-arylphenanthroimidazoles with various electron-donor or acceptor substituents, dbm is dibenzoyl-
methane. The compositions and structures of the ligands and complexes have been studied by X-ray diffrac-
tion and high-resolution mass spectrometry. In the absorption spectra of the complexes, a bathochromic shift
of the absorption maxima is observed with an increase in the electron donor properties of the ligands. All the
complexes show the reversible redox behavior; redox potentials fall in the range of 0.8–1.6 V. The combina-
tion of the obtained results allows us to consider the synthesized compounds as potential photosensitizers in
solar cells.
Keywords: cyclometalated iridium complexes, absorption spectra
DOI: 10.1134/S0036023619020037
In the past two decades, cyclometalated irid- ligands, which can lead to a noticeable decrease in the
redox potentials of the iridium complexes.
ium(III) complexes are widely used first as stable and
brightly luminescent components of organic light-
emitting diodes [1–4]. The unique photophysical
properties of these compounds attract an increased
interest for their use as photocatalysts for the genera-
tion of hydrogen [5, 6] and photosensitizers in solar
cells [7–11]. In the latter case, the dye complex should
have a noticeable light absorption in the visible region
of the spectrum and the reversible redox behavior [12].
The synthesis and comprehensive study of Ir(III)
complexes with “antenna” 2-arylbenzimidazole
ligands containing electron-donor or acceptor substit-
uents together with the analysis of the literature data
allowed us to determine general rules of the effect of
the nature of “antenna” ligands on the electronic
structure of iridium complexes and their optical and
electrochemical properties [13–16]. It was shown that
the properties of the complexes can be varied within
wide limits; however, it seems that a pronounced
effect can be achieved only by replacing the ordinary
“anchor” ligand, namely 4,4'-dicarboxy-2,2'-bipyri-
dine. In this connection, aromatic β-diketones [17]
In the present work, 2-arylphenanthroimidazoles
as “antenna” ligands and dibenzoylmethane as a
model β-diketone were used for the synthesis of the
target compounds. 2-Arylphenanthroimidazoles were
used because their conjugated system is significantly
expanded as compared to 2-arylbenzimidazoles; as a
result, we can expect an increase in the extinction
coefficients in absorption spectra [18, 19].
EXPERIMENTAL
We used commercially available reagents of pure
for analysis grade (Russian State Standard) or higher,
which is used without additional purification. Aniline
was distilled before using in the ligand synthesis.
Dibenzoylmethane (dbm) was recrystallized from
benzene. The solvents were distilled and dried by rou-
tine procedures. Ligands 1,2-diphenylphenanthroim-
idazole (LH), 1-phenyl-2-(4-chlorophenyl)phenan-
throimidazole (LCl), 1-phenyl-2-(4-methoxyphe-
nyl)phenanthroimidazole (LOMe), and 1-phenyl-2-
(3,4-dimethoxyphenyl)phenanthroimidazole (LOMe2)
seem promising, since they act as negatively charged were prepared according to Scheme 1 [19].
207

208
BILYALOVA et al.
NH2
+
R
O
O
R
O
H
N
N
AcOH
t°, Ar
O
CH3
NH4O
R = 4-Cl
Ligand = LCl LH LOMe
Scheme 1. Synthesis of 2-arylphenanthroimidazoles and interpretation of ligand abbreviations.
H
4-OMe 3,4-OMe
LOMe2
LCl to prepare colorless crystals. Yield, 65%. 1H NMR
spectrum, δ, ppm (CDCl3): 3.80 (s, 3H), 6.82 (d, J =
8.0 Hz, 1H), 7.17 (m, 1H), 7.25 (m, 1H), 7.48–7.67 (m,
9H), 7.74 (t, J = 8.0 Hz, 1H), 8.71 (d, J = 8.3 Hz, 1H),
8.77 (d, J = 8.2 Hz, 1H), 8.87 (d, J = 8.0 Hz, 1H).
1,2-Diphenylphenanthroimidazole (LH). A mixture
of phenanthrenquinone (0.515 g, 2.5 mmol), benzal-
dehyde (0.25 mL, 2.5 mmol), ammonium acetate
(0.783 g, 10.2 mmol), and aniline (1 mL, 11.0 mmol)
was boiled under argon atmosphere in glacial acetic
acid (15 mL) for 3 h. After cooling, cold water was
added to the solution. The dark green precipitate was
filtered off and washed with water until neutral pH was
obtained. The filter residue was dissolved in a mini-
mum amount of acetone and heated. A small amount of
water was added to the boiling solution, and the result-
ing white precipitate was filtered off and washed with
a mixture of water and ethyl alcohol (1 : 1 v/v). The sub-
stance was purified using column chromatography
(SiO2, CH2Cl2 : petroleum ether (1 : 1 v/v) → CH2Cl2)
1-Phenyl-2-(3,4-dimethoxyphenyl)phenanthroimid-
azole (LOMe2). A mixture of phenanthrenequinone
(0.513 g, 2.5 mmol), 3,4-dimethoxybenzaldehyde
(0.395 g, 2.4 mmol), ammonium acetate (0.782 g,
10.1 mmol), and aniline (1 mL, 11.0 mmol) was boiled
in glacial acetic acid (15 mL) in an argon atmosphere
for 3 h. In the following, the synthesis was carried out
similarly to LCl to prepare a white powder. Yield, 75%.
1H NMR spectrum, δ, ppm (CDCl3): 3.73 (s, 3H),
3.88 (s, 3H), 6.79 (d, J = 7.9 Hz, 1H), 7.14 (m, 1H),
7.16–7.21 (m, 2H), 7.27 (m, 1H), 7.48–7.57 (m, 3H),
7.60–7.69 (m, 4H), 7.75 (t, J = 7.8 Hz, 1H), 8.72 (d,
J = 8.3 Hz, 1H), 8.78 (d, J = 8.4 Hz, 1H), 8.89 (d, J =
8.3 Hz, 1H).
1
to form a white powder. Yield, 64%. H NMR spec-
trum, δ, ppm (CDCl3): 7.19 (d, J = 7.8 Hz, 1H), 7.25–
7.35 (m, 4H), 7.49–7.55 (m, 3H), 7.57–7.69 (m, 6H),
7.75 (t, J = 8.1 Hz, 1H), 8.72 (d, J = 8.2 Hz, 1H), 8.78
(d, J = 8.2 Hz, 1H), 8.89 (d, J = 8.3 Hz, 1H).
Complexes [Ir(L)2(dbm)] (1–4) were synthesized
in two stages. In the first stage, IrCl3 · 3H2O (0.050 g,
0.14 mmol) and an excess of 2-arylphenanthroimidaz-
ole L (0.50 mmol) were boiled in a mixture of 2-
ethoxyethanol : water (3 : 1 vol/vol) for 24 h under
argon (T = 135°C). Then distilled water was added to
the hot solution; the precipitate was filtered off and
washed with H2O and ethanol. The product was
extracted with dichloromethane, the solvent was evap-
orated, and the residue was dried in vacuum at 50°C
for 12 h. The obtained dimeric complexes [Ir(L)2Cl]2
were not characterized and used without further puri-
fication.
In the second stage, the dimer (0.03 mmol), diben-
zoylmethane (16.8 mg, 0.075 mmol), and K2CO3
(33.1 mg, 0.24 mmol) were mixed in acetonitrile
(10 mL) and heated under reflux with stirring in dark
under argon for 16 h. After cooling, a precipitate was
formed, which contained the target product, a slight
excess of diketone and potassium carbonate, and
impurities from the starting dimer. The precipitate was
filtered off, washed with water, dissolved in CH2Cl2,
1-Phenyl-2-(4-chlorophenyl)phenanthroimidazole
(LCl). A mixture of phenanthrenequinone (0.517 g,
2.5 mmol), 4-chlorobenzaldehyde (0.337 g, 2.4 mmol),
ammonium acetate (0.783 g, 10.2 mmol), and aniline
(1 mL, 11.0 mmol) was boiled in glacial acetic acid
(15 mL) in an argon atmosphere for 3 h. The resulting
red solution was cooled and cold water was added. The
dark green precipitate was filtered off, washed with
water, and recrystallized several times from acetone:
water (2 : 1 v/v) to form a white powder. Yield, 70%.
1H NMR spectrum, δ, ppm (CDCl3): 7.17 (d, J = 8.0
Hz, 1H), 7.24–7.31 (m, 3H), 7.48–7.55 (m, 5H),
7.59–7.68 (m, 4H), 7.75 (t, J = 8.2 Hz, 1H), 8.71 (d,
J = 8.0 Hz, 1H), 8.77 (d, J = 8.2 Hz, 1H), 8.85 (d, J =
8.1 Hz, 1H).
1-Phenyl-2-(4-methoxyphenyl)phenanthroimidazole
(LOMe). A mixture of phenanthrene quinone (0.513 g,
2.5 mmol), 3,4-dimethoxybenzaldehyde (0.395 g,
2.4 mmol), ammonium acetate (0.782 g, 10.1 mmol),
and aniline (1 mL, 11.0 mmol) was boiled in glacial
acetic acid (15 mL) in an argon atmosphere for 3 h. In
the following, the synthesis was carried out similarly to
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SYNTHESIS, STRUCTURE, OPTICAL
209
Table 1. Crystallographic data, details of data collection, and characteristics of data refinement for the structures of LOMe
and complex 3
LOMe
3
Molecular formula
FW
C28H20N2O
400.46
0.08 × 0.15 × 0.50
Orthorhombic
Pbca
C71H49IrN4O4 · CH2Cl2
1299.27
Crystal size, mm
Crystal system
Space group
a, Å
0.35 × 0.35 × 0.40
Orthorhombic
Pbca
8.5356(6)
21.897(8)
22.12(2)
23.090(10)
11184(12)
b, Å
18.8806(13)
24.9462(16)
4020.3(5)
c, Å
V, Å3
Z
ρ
8
8
calc, g/cm3
1.323
1.543
μ, mm–1
0.081
1680
2.541
5232
F(000)
θ range, deg
Index intervals
2.16–27.00
‒10 ≤ h ≤ 10
‒24 ≤ k ≤ 24
‒31 ≤ l ≤ 31
2.04‒25.00
‒3 ≤ h ≤ 26
‒3 ≤ k ≤ 26
‒3 ≤ l ≤ 27
Number of reflections:
collected
27214
4390
100
281
1.038
15449
9813
99.9
748
0.973
unique
Completeness of data collection to θ, %
Number of parameters
GOOF to F2
R1 for I > 2σ(I)
wR2 (all data)
Δρmax/Δρmin, e/Å3
0.0406
0.0961
0.225/‒0.204
0.0351
0.0744
0.797/‒0.628
resolution mass spectrum (ESI) m/z: [M]+ for
and used for chromatography (SiO2, eluent CH2Cl2 :
petroleum ether (1 : 1 vol/vol) → CH2Cl2). The first
fraction was an excess of diketone; the second colored
fraction is the product; the remaining impurities were
retained on the sorbent. The product was dried in a
vacuum for 10–12 h.
Complex [Ir(LCl)2(dbm)] (1). Yield, 34%. 1H NMR
spectrum, δ, ppm (DMSO-d6): 6.06 s (1H), 6.18 m
(4H), 6.59 m (2H), 6.90 m (2H), 7.11 m (2H), 7.21–
7.33 m (8H), 7.40 m (2H), 7.52 m (2H), 7.66 d (4H, J =
8.0 Hz), 7.73 d (2H, J = 7.9 Hz), 8.05 m (8H), 8.63 d
(2H, J = 8.4 Hz), 8.78 d (2H, J = 8.3 Hz), 9.02 d (2H,
C69H45IrN4O2+ calcd. 1154.3194, found 1154.3201.
Complex [Ir(LOMe)2(dbm)] (3). Yield, 47%.
1H NMR spectrum, δ, ppm (DMSO-d6): 3.31 s (6H),
6.01 s (1H), 6.26 m (4H), 6.49 m (2H), 6.82 t (2H, J =
7.9 Hz), 7.06 t (2H, J = 7.9 Hz), 7.22–7.32 m (8H),
7.38 m (2H), 7.51 t (2H, J = 8.0 Hz), 7.63 d (4H, J =
8.2 Hz), 7.69 d (2H, J = 7.9 Hz), 7.93–8.07 m (8H),
8.65 d (2H, J = 8.3 Hz), 8.80 d (2H, J = 8.4 Hz), 9.05 d
(2H, J = 8.2 Hz). High resolution mass spectrum
(ESI) m/z: [M]+ for C71H49IrN4O4+ calcd. 1214.3405,
J = 8.3 Hz). High resolution mass spectrum (ESI) found 1214.3419.
m/z: [M]+ for C69H43IrN4O2Cl2+ calcd. 1222.2393,
found 1222.2397.
Complex [Ir(LOMe2)2(dbm)] (4). Yield, 20%.
1H NMR spectrum, δ, ppm (DMSO-d6): 3.02 s (6H),
3.19 s (6H), 5.93 s (2H), 6.05 s (1H), 6.51 s (2H), 6.86
t (2H, J = 7.8 Hz), 7.15 d (2H, J = 7.7 Hz), 7.31 t (6H,
J = 7.7 Hz), 7.38 m (2H), 7.43–7.53 m (4H), 7.66 d
(4H, J = 7.9 Hz), 7.76 d (2H, J = 7.9 Hz), 7.95 –8.03 m
(8H), 8.66 d (2H, J = 8.3 Hz), 8.81 d (2H, J = 7.9 Hz),
9.10 d (2H, J = 8.0 Hz). High resolution mass spec-
Complex [Ir(LH)2(dbm)] (2). Yield, 54%. 1H NMR
spectrum, δ, ppm (DMSO-d6): 5.98 s (1H), 6.34 d
(2H, J = 7.7 Hz), 6.53 t (2H, J = 7.8 Hz), 6.58 t (2H,
J = 7.6 Hz), 6.79 t (2H, J = 8.0 Hz), 7.06 m (4H),
7.24–7.31 m (8H), 7.38 m (2H), 7.52 d (2H, J = 8.0 Hz),
7.62 d (4H, J = 8.1 Hz), 7.69 d (2H, J = 7.9 Hz), 7.94–
8.05 m (6H), 8.10 m (2H), 8.65 d (2H, J = 8.2 Hz),
trum (ESI) m/z: [M]+ for C73H54IrN4O6+ calcd.
8.80 d (2H, J = 8.1 Hz), 9.01 d (2H, J = 8.3 Hz). High 1275.3695, found 1275.3654.
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BILYALOVA et al.
C(26)
C(27)
C(25)
C(19)
C(28)
C(20)
C(21)
C(24)
C(18)
C(17)
C(23)
N(2)
C(8)
C(22)
C(6)
C(16)
C(15)
C(7)
C(2)
C(1)
C(5)
C(9)
C(10)
C(14)
C(13)
N(1)
O(1)
C(3)
C(4)
C(11)
C(12)
Fig. 1. Molecular structure of 1-phenyl-2-(4-methoxyphenyl)phenanthroimidazole (LOMe). The ellipsoids of thermal vibra-
tions of atoms are shown with a 50% probability.
1H NMR spectra were recorded at 25°C on a
eter at a temperature of 100 K using MoKα radiation (λ =
Bruker Avance 400 spectrometer, chemical shifts are 0.71073 Å, graphite monochromator) in the ω-scanning
given in ppm relative to the signals of the solvent resi- mode. The data were corrected for absorption by mea-
dues. High-resolution mass spectra were measured on
a Brukermicro TOF-QTM ESI-TOF mass spectrom-
eter (electrospray ionization) equipped with a time-
of-flight detector. Electronic absorption spectra of
solutions of complexes in dimethylformamide (DMF)
were measured on an SF-2000 spectrophotometer in
quartz cells (1 cm) at room temperature. Cyclic vol-
tammograms were measured on an Ecotest-VA
polarograph in a three-electrode cell with a carbon
glass working electrode, a platinum auxiliary elec-
trode, and a standard silver chloride reference elec-
trode. The complexes were dissolved in a 0.01 M solu-
tion of (n-Bu4N)ClO4 in DMF saturated with argon.
The measurements were carried out at a potential
sweep of 25 mV/s in the alternating mode (modulation
amplitude 30 mV, frequency 20 Hz); ferrocene was
used as an external standard.
suring the intensities of the equivalent reflections [20].
Single crystals of complex 3 were obtained by slow
evaporation of a solution of the complex in methy-
lene chloride. Single-crystal X-ray diffraction analy-
sis of complex 3 was performed on an Enraf-Nonius
CAD4 automatic diffractometer at 150 K using MoKα
radiation (λ= 0.71073 Å, graphite monochromator). The
experimental data were corrected for the Lorentz factors
and polarization. The data were corrected for absorption
by measuring the intensities of equivalent reflections
[20]. Both structures were determined by a direct method
and refined by the full-matrix anisotropic least-squares
method for F2 for all non-hydrogen atoms [21]. Hydro-
gen atoms were located in calculated positions and
refined using the “rider” model. Main crystal data,
details of data collection, and characteristics of structure
refinement are listed in Table 1. The crystallographic
data have been deposited with the Cambridge Structural
Single crystals of the LOMe ligand were obtained by
its recrystallization from ethanol. The experimental data Database: CCDC nos. 1828295 (LOMe ligand) and
were collected on a Bruker SMART APEX II diffractom- 1828296 (complex 3) (http://www.ccdc.cam.ac.uk).
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SYNTHESIS, STRUCTURE, OPTICAL
211
1H NMR and high resolution mass spectroscopy data.
Structural formulas of the iridium(III) complexes
RESULTS AND DISCUSSION
The compositions and structures of the ligands and
complexes 1–4 were determined according to the obtained here are shown in Scheme 2.
N
N
N
N
Cl
Cl
O
O
Ir
Ir
O
O
N
N
N
N
1
2
N
N
H3CO
H3CO
N
N
H3CO
H3CO
O
O
Ir
Ir
O
O
H3CO
H3CO
N
N
N
N
3
4
Scheme 2. Structures of iridium(III) complexes 1–4 synthesized in this work.
1
The structures of ligand LOMe and complex 3 were
studied by X-ray diffraction. In the ligand molecule
(Fig. 1), the phenanthrene and imidazole fragments
are in the same plane, while the 4-methoxyphenyl ring
is inclined to this plane by an angle of ~23°. In turn,
the N-phenyl ring is located almost orthogonal (angle
~88°) to the phenanthroimidazole moiety.
The coordination environment of the iridium atom
in complex 3 is a strongly distorted octahedron (Fig. 2,
Table 2) formed by the carbon and nitrogen atoms of
two 2-arylphenanthroimidazoles and two oxygen
atoms of deprotonated dibenzoylmethane. In this
structure, the Ir–O bond lengths are significantly
In the H NMR spectra, both the coordinated
cyclometalated ligands are chemically identical and
give the same set of signals with signals of deproton-
ated dibenzoylmethane superimposed. In the mass
spectra, in addition to the signals of complexes 1–4,
peaks corresponding to the [Ir(L)2]+ fragments are
observed, i.e. those of cyclometalized residues of the
complexes obtained probably when dibenzoylmethane
is released in the course of ionization. This indicates a
significantly higher bond strength between the Ir(III)
cation and cyclometalated ligands as compared to
additional O- or N-donor ligands.
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212
BILYALOVA et al.
C(47)
C(46)
C(48)
C(45)
C(40)
C(41)
C(37)
C(49)
C(51)
C(44)
C(42)
C(43)
C(39)
C(38)
C(50)
C(56)
N(4)
C(52)
C(68)
C(55)
C(36)
C(34)
C(53)
C(69)
C(4)
N(3)
Ir(1)
C(54)
O(3)
C(67)
C(65)
C(33)
C(35)
C(2)
C(70)
C(71)
O(1)
C(66)
C(64)
C(63)
C(29)
C(3)
C(5)
C(61)
C(60)
C(62)
C(31)
C(30)
O(2)
C(32)
O(4)
C(1)
C(26)
C(57)
N(1)
C(25)
C(27)
C(28)
C(59)
C(58)
C(24)
C(7)
C(6)
C(8)
N(2)
C(9)
C(22)
C(15)
C(23)
C(21)
C(14)
C(13)
C(16)
C(10)
C(20)
C(17)
C(12)
C(19)
C(11)
C(18)
Fig. 2. Molecular structure of complex 3. Hydrogen atoms are omitted for clarity. The ellipsoids of thermal vibrations of atoms
are shown with a 50% probability.
shorter (by ~0.04 Å) as compared to those in the pre- angle of ~12° with the plane of the imidazole ring,
viously studied Ir(III) complex with 1,2-diphenylben- which is inclined by ~26° with respect to the 4-
zimidazole and dbm [22], while the Ir–N bond methoxyphenyl ring. In both the coordinated ligands,
the N-phenyl rings are practically orthogonal (the
angle is ~85°) to the phenanthroimidazole moieties, as
in the free ligands. Unlike the Ir(III) complex with
1,2-diphenylbenzimidazole, where the molecule has
C2 symmetry, complex 3 does not possess symmetry
due to the extremely strong geometric differences
between the chemically identical cyclometalated
ligands. In general, X-ray diffraction studies show that
lengths, on the contrary, are longer by ~0.03 Å. In
addition, the cyclometalated ligands are not com-
pletely flat. In one of them the phenanthrene fragment
is inclined relative to the imidazole by ~7°, while the
latter is inclined by ~6° with respect to the 4-methoxy-
phenyl ring. In the second ligand coordinated to irid-
ium, the deviations are even more pronounced: the
phenanthrene fragment is not flat, and two benzene
rings form an angle of ~24°. In turn, the plane most the molecule of complex 3 exists in a tensed state and
closely corresponding to the phenanthrene fragment seems to be less stable than, for example, analogous
(according to the least-squares method) forms an complexes with 2-arylbenzimidazole derivatives.
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SYNTHESIS, STRUCTURE, OPTICAL
213
Table 2. Selected bond lengths and bond angles in the
structure of 3
Since cyclometalated ligands of all synthesized com-
plexes contain a phenanthrene fragment, it can be
argued with high probability that the molecules of
these compounds are tense and have relatively low sta-
bility, which is consistent with the mass spectrometry
data obtained.
Bond
d, Å
Bond
d, Å
Ir(1)–C(1)
Ir(1)–C(29)
Ir(1)–N(1)
1.989(5)
1.982(5)
2.061(4)
Ir(1)–N(3)
Ir(1)–O(3)
Ir(1)–O(4)
2.091(5)
2.138(4)
2.129(4)
The crystal packing of complex 3 is formed due to
numerous weak directed interactions of the type C–
H···π and π ···π. The solvent molecule of dichloro-
methane is retained in the structure by weak C–H···π
contacts with phenanthrene fragments of two neigh-
boring molecules of the complexes.
Angle
ω, deg
Angle
ω, deg
С(1)Ir(1)C(29) 92.7(2)
N(1)Ir(1)O(3)
N(1)Ir(1)O(4)
99.99(17)
78.05(15)
С(1)Ir(1)N(3)
C(1)Ir(1)O(4)
96.8(2)
88.19(18) N(1)Ir(1)N(3) 176.02(16)
83.49(16)
С(1)Ir(1)O(3) 174.20(18) N(3)Ir(1)O(3)
The dried complexes showed marked solubility
only in DMF or DMSO. Electronic absorption spec-
tra of complexes 1–4 were measured in DMF at 25°C
(Fig. 3) and interpreted according to the quantum
chemical calculations of such complexes [13, 14]. The
electronic transitions π → π* localized on cyclometa-
lated ligands, cause intense absorption of the com-
plexes in the UV region (Table 3). The absorption
bands in the range of 350–450 nm are caused by
charge transfer from the metal to the ligands and
between the phenanthroimidazole ligands and diben-
zoylmethane (ε ≈ 10000–30000 L mol–1 cm–1). The
extinction coefficients are higher as compared to those
for the corresponding complexes with 2-arylbenzim-
idazoles. In this case, the bands undergo a significant
bathochromic shift when chlorine is replaced by
methoxy groups in the aryl fragment of the ligands.
The long-wavelength absorption bands (>450 nm) of
the complexes have molar absorption coefficients fall-
ing in the range 300–2000 L mol–1 cm–1, which may
be due to the non-planar structure of the ligands and,
as a result, the violation of the conjugation and a
decrease in the probability of separate electronic tran-
sitions with charge transfer.
С(29)Ir(1)N(1) 97.80(19) N(3)Ir(1)O(4) 104.24(16)
С(29)Ir(1)O(3) 93.00(18) С(29)Ir(1)O(4) 175.52(17)
According to the data of alternating cyclic voltam-
metry (Fig. 4, Table 3), complexes 1–3 demonstrate
the reversible redox behavior, and two oxidation-
reduction waves are observed for each of these com-
pounds. Both waves undergo a noticeable effect from
varying substituents in cyclometalated ligands. Thus,
the introduction of a chloride atom into the aryl frag-
ment of ligands leads to a significant increase in redox
potentials. In turn, the methoxy group in the same
position has only a minor effect on the potentials. It
seems that the methoxy group is a mesomeric donor
and, at the same time, an inductive acceptor. When
the methoxy group is in the meta position to the met-
alized carbon atom, both its effects become less pro-
nounced decreasing its influence on the molecular
orbitals of the complex. On the contrary, the introduc-
tion of the second methoxy group into complex 4 (in
the para-position to the metallic carbon) significantly
7
12000
10000
8000
6000
4000
6
5
1
3
2
4
2000
0
4
3
2
1
0
430 450 470 450 510 530 550 570 590
λ, nm
300
320
340
360
380
400
420
440
460
480
500
λ, nm
Fig. 3. Electronic absorption spectra of complexes 1–4 (DMF, 25°C).
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 64 No. 2 2019

214
BILYALOVA et al.
Table 3. Optical and redox characteristics of complexes 1–4
abs, nm (ε × 10–3 , mol–1 cm–1 a
)
Eox , Vb
Complex
λ
1
2
3
4
313 (64), 362 (31), 405 (12), 490 (0.6)
310 (59), 367 (28), 415 (19), 516 (0.3)
311 (54), 360 (30), 410 (23), 485 (1.0), 518 (0.5)
322 (51), 368 (29), 436 (21), 515 (0.3)
1.24, 1.60
1.05, 1.30
1.03, 1.26
0.82, 1.05с, 1.22, 1.34с
a
b
Measured in DMF at 25°C.
Measured in DMF saturated with argon at 25°С. Electrolyte 0.01 М (n-Bu N)ClO , potential sweep 25 mV/s, frequency 10 Hz,
4
4
amplitude 30 mV. Ferrocene was used as an external standard before measuring the complexes. The values of the potentials are given
relative to the standard hydrogen electrode.
с
Irreversible oxidation.
Standard error 1 nm for λ
, 10 mV for E .
ox
max
reduces the redox potential. At the same time, irre- teristics comparable to those for effective photosensi-
versible oxidation-reduction waves appear for com- tizers.
plex 4. This is explained by the fact that an increase in
the electron donor properties of the ligands leads to a
ACKNOWLEDGMENTS
decrease in the fraction of d orbitals of iridium in the
occupied boundary molecular orbitals of the complex
and the electrons are lost during oxidation by irrevers-
ibly oxidizing ligands. We have repeatedly observed
this fact and interpreted it on the basis of the quantum
chemical calculations [13–15]. Most importantly, the
replacement of 4,4'-dicarboxy-2,2'-bipyridine in the
iridium complexes with dibenzoylmethane has signifi-
cantly reduced the redox potential of the complexes.
The study was supported by the Russian Science
Foundation (project no. 17-73-10084). The X-ray dif-
fraction studies were performed at the Shared Facility
Center of the Kurnakov Institute.
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