Synthesis, structure, and optical properties of iridium(III) complex with 1-benzyl-2-phenylbenzimidazole and 4,4'-dicarboxy-2,2'-bipyridyne

Article abstract of DOI:10.1134/S0036023617080046

A new cyclometalated iridium(III) complex [Ir(L)₂(Hdcbpy)] (1) has been synthesized, where L is 1-benzyl-2-phenylbenzimidazole and Hdcbpy is monoprotonated 4,4′-dicarboxy-2,2′-bipyridine. The structure of complex 1 has been determined by X-ray

Full text of DOI:10.1134/S0036023617080046

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2017, Vol. 62, No. 8, pp. 1085–1089. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © S.I. Bezzubov, A.A. Bilyalova, I.V. Kuznetsova, K.G. Pavlov, Yu.M. Kiselev, V.D. Dolzhenko, 2017, published in Zhurnal Neorganicheskoi Khimii, 2017,
Vol. 62, No. 8, pp. 1087–1091.
PHYSICAL METHODS
OF INVESTIGATION
Synthesis, Structure, and Optical Properties of Iridium(III) Complex
with 1-Benzyl-2-Phenylbenzimidazole
and 4,4'-Dicarboxy-2,2'-Bipyridyne
S. I. Bezzubova, *, A. A. Bilyalovab, I. V. Kuznetsovab, K. G. Pavlovb,
Yu. M. Kiselevb, and V. D. Dolzhenkob
aKurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow, 119991 Russia
bDepartment of Chemistry, Moscow State University, Moscow, 119991 Russia
*e-mail: bezzubov@igic.ras.ru
Received October 12, 2016
Abstract—A new cyclometalated iridium(III) complex [Ir(L)2(Hdcbpy)] (1) has been synthesized, where L is
1-benzyl-2-phenylbenzimidazole and Hdcbpy is monoprotonated 4,4′-dicarboxy-2,2′-bipyridine. The
structure of complex 1 has been determined by X-ray diffraction. The optical properties of complex 1 have
been studied, and the quantum yield of luminescence has been measured.
DOI: 10.1134/S0036023617080046
Recently, cyclometalated complexes (CMCs) of 1-benzyl-2-phenylbenzimidazole and Hdcbpy is mono-
iridium(III) have been considered to be photosensitiz- protonated 4,4′-dicarboxy-2,2′-bipyridine.
ers (PSs) for solar cells [1] and hydrogen generation
photocatalysts [2]. In both cases, a complex should
have noticeable light absorption in the visible region of
EXPERIMENTAL
its spectrum and some other characteristics [3]. The
use of 2-arylbenzimidazole derivatives allowed us to
obtain a number of iridium(III) complexes possessing
the properties required for PSs [4–6]. The regularities
of the effect of the nature of both benzimidazole and
auxiliary ligands on the electronic structure of iridium
complexes and their optical and electrochemical prop-
erties were elucidated. In addition, the most promising
complexes were successfully tested in solar cells.
Commercially available pure-for-analysis reagents
were used in the work without additional purification.
The solvents were distilled and dried according to
standard procedures. 1-H-2-Phenylbenzimidazole [8]
and 4,4′-dicarboxy-2,2′-bipyridine (H2dcbpy) [9]
were prepared according to known procedures. The
synthesis was carried out in an argon atmosphere; the
purification of substances and other manipulations
were carried out in air.
On the basis of the data obtained, we assume that,
1-Benzyl-2-phenylbenzimidazole (L) was prepared
varying substituents in the benzimidazole moiety of by adding benzyl bromide (0.288 g, 1.6 mmol) to a sus-
the ligands, we will be able to increase the molar pension of 1-H-2-phenylbenzimidazole (0.294 g,
absorption coefficients of the corresponding irid- 1.5 mmol) and NaOH (0.120 g, 3 mmol) in DMSO
ium(III) complexes in the visible region of the spec- (1 mL) followed by stirring the mixture in an argon
trum. However, this modification of ligands is difficult atmosphere at 40°C. After 1.5 h, DMSO (0.5 mL) was
to obtain because N-phenyl derivatives with various added, because the mixture became very viscous and
substituents in the benzimidazole moiety are difficult did not allow to be mixed. The mixture turned yellow
to synthesize. Since the nature of the substituent at the 3 h after the reaction; water (10 mL) was added
nitrogen atom of 2-arylbenzimidazole mainly affects therein, and the precipitate was filtered off. The pre-
the solubility of the corresponding iridium(III) com- cipitate was dissolved in ethanol on heating; the
plex and has no effect on its other properties [6], irid- obtained solution was evaporated to dryness, and the
ium(III) CMCs with N-benzylated 2-arylbenzimidaz- resulting precipitate was redissolved in CH2Cl2 (1 mL).
oles derived from N–H benzimidazoles in one stage
[7] are of interest. Here, we present the results of the
synthesis and studies of the structure and optical prop-
erties of complex [Ir(L)2(Hdcbpy)] (1), where L is
Under slow evaporation, colorless needle crystals and
yellow oil precipitated, the latter was removed with
ether. The remaining crystals were dried in vacuum at
50°C for 2 h. Yield, 180 mg (41%).
1085

1086
BEZZUBOV et al.
Table 1. Crystallographic data and characteristics of data
refinement for complex 1
A red powder was obtained. Yield, 17.1 mg (21% based
on iridium trichloride).
1H NMR (DMSO-d6), δ, ppm: 5.30 s (4 H), 5.64 d
(2 H, J = 8.3 Hz), 5.85−5.99 m (4 H), 6.27 d (2 H,
J = 7.0 Hz), 6.74 t (2 H, J = 7.4 Hz), 6.81−7.19 m
(9 H), 7.43 d (2 H, J = 8.4 Hz), 7.52−7.61 m (5 H),
7.99 d (2 H, J = 5.7 Hz), 8.22 d (2 H, J = 5.6 Hz),
9.00 s (2 H).
Parameter
Value
Molecular formula
FW
C52H37Ir N6O4 · H2O
1890.89
Crystal size, mm
Syngony
0.25 × 0.20 × 0.08
Monoclinic
Space group
P21 c
1
The H and 31P NMR spectra were recorded at
a, Å
12.5122(10)
13.1980(11)
28.035(2)
101.415(1)
4538.0(6)
25°C on a Bruker Avance 400 spectrometer. The
chemical shifts are given in ppm relative to signals
from the remained solvent. The electronic absorption
spectra were measured on a SF2000 spectrophotome-
ter in quartz cell (1 cm). The luminescence spectra
were recorded on a Perkin Elmer LS55 spectrometer.
The quantum yield of luminescence was determined
relative to Rhodamine 6G.
b, Å
c, Å
β, deg
V, Å3
Z
Ρ
4
calcd, g/cm3
1.493
μ, mm–1
2.997
F(000)
2040
2.14–25.05
–14 ≤ h ≤ 14, –15 ≤ k ≤ 15,
Single crystals of compound 1 were obtained by
slowly evaporating a solution of the complex in a
methanol/methylene chloride (1/1 vol/vol) mixture.
The experimental data array was collected on a Bruker
SMART APEX II diffractometer at 150 K (MoKα radi-
ation, λ = 0.71073 Å, graphite monochromator) in the
ω scan mode. Absorption was taken into account from
the measured intensities of equivalent reflections [10].
The structure was solved by a direct method and refined
by the full-matrix anisotropic least squares method with
respect to F2 for all non-hydrogen atoms [11]. Hydro-
gen atoms were placed in the calculated positions and
refined according to the “rider” model. A cavity (V =
126 Å3) was found in the crystal of complex 1. The use
of the SQUEEZE procedure [12] showed that this cav-
ity contained five electrons, which is extremely small
for a completely occupied solvent molecule. It was not
possible to clarify whether a water molecule was pres-
ent in this cavity. Crystallographic data, details of the
experiment and refinement of the structure of 1 are
given in Table 1. The full tables of atomic coordinates,
bond lengths, and valence angles are deposited with
the Cambridge Structural Database: CCDC 1509611
(http://www.ccdc.cam.ac.uk).
θ range, deg
Index intervals
–33 ≤ l ≤ 33
31767
8026
Number of all reflections
Number of unique reflections
Data completeness for θ, %
Number of parameters
99.9
577
1.001
F2
R1 for I > 2σ(I)
wR2 (all data)
Δρmax/Δρmin, e/Å3
0.0375
0.0765
1.360/–1.042
1H NMR (CDCl3), δ, ppm: 5.47 s (2 H), 7.12 d
(2 H, J = 6.7 Hz), 7.20–7.26 m (2 H), 7.28–7.37 m
(4 H), 7.43–7.51 m (3 H), 7.70 m (2 H), 7.88 d (1 H, J =
8.0 Hz).
Complex [Ir(L)2(Hdcbpy)] (1) was synthesized in
two steps. In the first stage, a mixture of 2-ethoxyetha-
nol and water (10 mL, 3/1 vol/vol) was added to IrCl3 ·
3H2O (27.9 mg, 0.079 mmol) and L (90 mg, 0.306 mmol).
The resulting mixture was heated under reflux with
stirring under argon for 24 h, cooled, and 3 mL of
water was added to the obtained mixture. The precipi-
tate formed was filtered off, washed repeatedly with
alcohol and acetone, and dried in vacuum at 50°C for
12 h. The resulting dimeric complex [Ir(L)2Cl]2 (20 mg)
was not characterized and used in the following syn-
thetic stages without additional purification.
RESULTS AND DISCUSSION
Alkylation of N−H benzimidazoles is usually car-
ried out in the presence of sodium hydride or other
bases on heating [7], but we succeeded in simplifying
substantially the procedure. 1-Benzyl-2-phenylben-
zimidazole (L) was obtained in good yield by the reac-
tion between 1-H-2-phenylbenzimidazole and benzyl
In the second stage, the resulting dimer (15.0 mg,
0.009 mmol) and H2dcbpy (4.4 mg, 0.018 mmol) in a
mixture of MeOH and CH2Cl2 (1/1 vol/vol, 10 mL) bromide in DMSO in the presence of sodium hydrox-
ide as the base (Scheme 1). The isolated ligand was
introduced into the cyclometalation reaction with
iridium trichloride using the standard procedure [13]
to obtain a dimer; the addition of 4,4′-dicarboxy-2,2′-
bipyridine (H2dcbpy) to the obtained dimer afforded a
previously unknown Ir(III) complex. Although a satu-
rated methanolic solution of NH4PF6 was used to iso-
was heated under reflux with stirring under argon in
the dark for 16 h. The resulting mixture was cooled, a
saturated solution of NH4PF6 in methanol (1 mL) was
added, and the obtained mixture was stirred for 20 min.
The resulting red precipitate was recrystallized from a
1/1 vol/vol mixture of methylene chloride and metha-
nol. The product was dried in vacuum at 50°C for 2 h.
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SYNTHESIS, STRUCTURE, AND OPTICAL PROPERTIES OF IRIDIUM(III) COMPLEX
1087
late the complex in order to obtain the cationic- spectrum. The composition and structure of the
anionic compound [Ir(L)2(H2dcbpy)][PF6], zwitter-
obtained compound were determined by the com-
1
ionic complex 1 with a half-dissociated dicarboxylic
bined data obtained by the H NMR and X-ray dif-
acid precipitated, with no signals in its 31P NMR fraction methods.
O
N
N
OH
O
N
N
N
N
N
NH
H
ii
Ir+
i
N
O−
L
1
Scheme 1. Synthesis of 1-benzyl-2-phenylbenzimidazole (L) and complex 1. Reaction conditions: (i) benzyl bromide, KOH,
DMSO; (ii) the first stage is IrCl3 · 3H2O, 2-ethoxyethanol/water (3/1 vol/vol), the second stage is 4,4′-dicarboxy-2,2′-
bipyridine (H2dcbpy), CH2Cl2/CH3OH (2/1 vol/vol), then an excess of a saturated solution of NH4PF6 in CH3OH.
A molecule of complex 1 is built in a manner typi- formed by two carbon atoms and two nitrogen atoms
cal for these compounds [5, 6]. The central iridium of two cyclometalated benzimidazoles and two nitro-
atom is located in a distorted octahedral environment gen atoms of substituted bipyridine (Fig. 1). The bond
C(11)
C(12)
C(10)
C(13)
C(9)
C(14)
C(16)
C(8)
C(17)
C(15)
N(2)
C(18)
O(3)
C(7)
C(20)
C(5)
C(4)
C(19)
C(44)
C(42)
C(43)
C(6)
N(1)
C(1)
C(41)
O(4)
C(45)
C(48)
C(3)
N(5)
Ir(1)
C(46)
C(47)
C(2)
C(21)
C(22)
O(1)
C(23)
C(24)
N(6)
C(52)
C(40)
N(3)
C(50)
O(2)
C(49)
C(51)
C(26)
C(39)
C(27)
C(25)
C(38)
N(4)
C(35)
C(37)
C(28)
C(29)
C(36)
C(30)
C(34)
C(33)
C(32)
C(31)
Fig. 1. Molecular structure of complex 1. Hydrogen atoms are omitted (except for the COOH group). The ellipsoids of atomic
thermal vibrations are given with the 50% probability level.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 62 No. 8 2017

1088
BEZZUBOV et al.
Fig. 2. Crystal packing of complex 1.
lengths and angles in the coordination environment of chloride and methanol; however, 1 dissolves in a
the iridium atom are within the limits typical for these 1/1 vol/vol mixture of these solvents.
complexes (Table 2). The nitrogen atoms of dicar-
In solution, complex 1 is intensely reddened by
noticeable light absorption in the range of 450−600 nm
(Fig. 3). This absorption band is due to charge transfer
from metal to dcbpy and that from benzimidazole
ligands to dcbpy, which we repeatedly observed and
interpreted based on the DFT/TDDFT calculations of
other Ir(III) complexes [4–6]. Strong absorption bands
in the region of 350–400 nm are also caused by a charge
transfer of the same nature. Absorption at 300 nm cor-
responds to π → π* intraligand electronic transitions.
The molar absorption coefficients of complex 1 in the
visible region are comparable to those for the above-
boxybipyridine are more distant from the central atom
as compared with the nitrogen atoms of benzimidaz-
oles because of the trans effect of Ir−C covalent bonds.
In crystal 1, the molecules are packed by several
hydrogen bonds, some of which are formed by a sol-
vated water molecule acting as a proton donor and
forming a bridge (Fig. 2). The presence of a cavity not
filled with a solvent in the crystal 1 indicates the high
strength of the framework formed by the hydrogen
bonds. Apparently, it is a strong package that causes
the low solubility of the complex in pure methylene
6
Luminescence spectrum
Excitation spectrum
Absorption spectrum
5
15
4
10
3
2
5
0
450
500
650
1
Wavelength, nm
250
350
450
550
650
750
850
0
250 300 350 400 450 500 550 600 650
Wavelength, nm
Wavelength, nm
Fig. 3. Electronic absorption spectrum (a 1/1 vol/vol
Fig. 4. Luminescence, excitation, and absorption spectra
of complex 1 (CH Cl , 25°C).
CH Cl /MeOH mixture, 25°C) of complex 1.
2
2
2
2
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SYNTHESIS, STRUCTURE, AND OPTICAL PROPERTIES OF IRIDIUM(III) COMPLEX
1089
Table 2. Selected interatomic bonds (d) and angles (ω) in the structure of 1
Bond
d, Å
Bond
d, Å
Ir(1)–C(1)
Ir(1)–C(21)
Ir(1)–N(1)
2.009(5)
2.027(6)
2.040(5)
Ir(1)–N(3)
Ir(1)–N(5)
Ir(1)–N(6)
2.039(4)
2.135(4)
2.143(4)
Angle
ω, deg
Angle
ω, deg
C(1)Ir(1)C(21)
C(1)Ir(1)N(3)
C(21)Ir(1)N(1)
N(3)Ir(1)N(1)
C(1)Ir(1)N(5)
C(21)Ir(1)N(5)
89.8(2)
N(3)Ir(1)N(5)
N(1)Ir(1)N(5)
C(1)Ir(1)N(6)
C(21)Ir(1)N(6)
N(3)Ir(1)N(6)
N(3)Ir(1)N(6)
101.16(17)
87.58(17)
169.45(19)
100.64(19)
90.28(16)
98.46(17)
93.43(19)
93.1(2)
168.88(17)
92.60(19)
177.60(18)
Table 3. Optical properties of complex 1
λ
abs, nm (ε × 10−3, M−1 cm−1 a
)
λex, nmb
478
Measured in a CH Cl /MeOH mixture (1/1 vol/vol) at 25°C.
λ
lum, nmb
655
Quantum yield, %c
5
306 (49), 370 (13), 430 (2.7), 510 (1.3)
a
b
c
2
2
Measured in CH Cl at 25°C.
2
2
Measured in CH Cl at 25°C relative to Rhodamine 6G.
2
2
Standard error: 1 nm for λ
,
1% for ε
, 5% for quantum yield.
max
max
mentioned related complexes based on N-phenylben- the former is much easier to modify, including the ben-
zimidazoles (1000–3000 M‒1 cm‒1, Table 3).
zylation method developed in the present work.
The study of the luminescence properties of com-
plex 1 in CH2Cl2 at room temperature showed that,
firstly, the complex emits in the orange region of the
spectrum (with a maximum at 655 nm) with a relative
luminescence quantum yield of 5% (relative to
Rhodamine 6G) and, secondly, the excitation spec-
trum of 1 almost coincides (in positions of bands) with
its absorption spectrum (Fig. 4).
This means that almost all electronic transitions
that transfer molecule 1 to the excited singlet state
upon light absorption transform it into the radiative
triplet state (it has been found that iridium(III) CMCs
emit from the triplet level [13]). For comparison, pre-
viously we observed the opposite picture exemplified
with iridium(III) CMCs with N-phenylbenzimidaz-
oles: the absorption spectrum (especially in the visible
region) did not coincide with the excitation spectrum.
This property of complex 1 that was found here can be
considered as an advantage of N-benzylbenzimidaz-
oles over other related compounds as ligands for irid-
ium(III) CMCs, since the complex under light
absorption should transform as efficiently as possible
from the excited singlet state to the triplet state to be
applied as a PS. For heavy ions (for example, Ir3+), the
triplet state has a significantly longer lifetime, which
means that the probability of an electronic transition
from the excited PS molecule to a semiconductor is a
key step at the operation of Grätzel solar cells [1–3].
ACKNOWLEDGMENTS
This work was supported by the Russian Foundation
for Basic Research, grant no. 16-33-00604mol_a.
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Translated by V. Avdeeva
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 62 No. 8 2017