Luminescent dirhenium(I)-double-heterostranded helicate and mesocate

Article abstract of DOI:10.1021/ic4023135

The semirigid ligands 1,4-bis(2-(2-hydroxyphenyl)benzimidazol-1-ylmethyl) benzene (H₂-pBC) and 1,3-bis(2-(2-hydroxyphenyl)benzimidazol-1- ylmethyl)-2,4,6-trimethylbenzene (H₂-mBC), containing two hydroxyphenylbenzimidazolyl units as bis-chelating (or bis(bidentate)) NaOH donor, were synthesized and were used to assemble neutral, luminescent heteroleptic, unsaturated double-hetero-stranded, rhenium(I)-based helicate (1) and mesocate (2) with the flexible bis(monodentate) nitrogen donor (1,4-bis(benzimidazol-1-ylmethyl)benzene/1,3- bis(benzimidazol-1-ylmethyl)benzene), and Re₂(CO)₁₀. The photophysical properties of the complexes were studied. Both complexes 1 and 2 exhibit dual emissions in both solution and solid state. In solution, these complexes show both fluorescence and phosphorescence. Complex 1 undergoes a predominantly ligand-centered oxidation, resulting in the generation of phenoxyl radicals.

Full text of DOI:10.1021/ic4023135

Article
pubs.acs.org/IC
Luminescent Dirhenium(I)-Double-Heterostranded Helicate and
Mesocate
Bhaskaran Shankar,† Saugata Sahu,‡ Naina Deibel,§ David Schweinfurth,§,∥ Biprajit Sarkar,§,∥
Palani Elumalai,† Deepak Gupta,† Firasat Hussain,† Govindarajan Krishnamoorthy,‡
and Malaichamy Sathiyendiran*,†
†Department of Chemistry, University of Delhi, Delhi 110 007, India
‡Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039, India
§Institut fur Anorganische Chemie, Universitat Stuttgart, Pfaffenwaldring 55, D-70550 Stuttgart, Germany
̈
̈
∥Institut fur Chemie und Biochemie, Anorganische Chemie, Freie Universitat Berlin, Fabeckstraße 34-36, D-14195 Berlin, Germany
̈
̈
S
* Supporting Information
ABSTRACT: The semirigid ligands 1,4-bis(2-(2-
hydroxyphenyl)benzimidazol-1-ylmethyl)benzene (H2-pBC)
and 1,3-bis(2-(2-hydroxyphenyl)benzimidazol-1-ylmethyl)-
2,4,6-trimethylbenzene (H2-mBC), containing two hydrox-
yphenylbenzimidazolyl units as bis-chelating (or bis-
(bidentate)) N∩OH donor, were synthesized and were used
to assemble neutral, luminescent heteroleptic, unsaturated
double-hetero-stranded, rhenium(I)-based helicate (1) and
mesocate (2) with the flexible bis(monodentate) nitrogen
donor (1,4-bis(benzimidazol-1-ylmethyl)benzene/1,3-bis-
(benzimidazol-1-ylmethyl)benzene), and Re2(CO)10. The photophysical properties of the complexes were studied. Both
complexes 1 and 2 exhibit dual emissions in both solution and solid state. In solution, these complexes show both fluorescence
and phosphorescence. Complex 1 undergoes a predominantly ligand-centered oxidation, resulting in the generation of phenoxyl
radicals.
rarely use. In continuation of our recent work on the synthesis
of rhenium(I)-based metallocycles from flexible ligands,5 we
propose that introducing a hydroxyl or phenolic unit adjacent
to the neutral nitrogen donor would lead to a new type of
chelating ligand, resulting in metallocycles consisting of fac-
Re(CO)3, neutral flexible nitrogen donors, and anionic flexible
N∩O donors.
INTRODUCTION
■
The design and synthesis of metallocycles for use in functional
applications such as molecular sensors, catalysts, dye-sensitized
solar-cells, electroluminescent devices, and biomolecular
imaging have attracted considerable attention.1 One class of
metallocycles, namely rhenium(I)-based complexes consisting
of fac-Re(CO)3, and an imine with/without additional donors,
displayed interesting photophysical and electrochemical proper-
ties.2−5 Because of the stability and intrinsic properties of these
complexes, efforts are being directed toward the synthesis of
new metallocycles. Over the past 15 years, much attention has
focused on using only a few types of building blocks for
rhenium(I)-based metallocycles. For example, neutral donors
have been limited to rigid/flexible ligands containing pyridyl,
imidazolyl, and benzimidazolyl units. RX− (X = O/S; R = H,
alkyl, aryl), biimidazole, bibenzimidazole, 2,2′-bipyrimidine,
indigo, and hydroxybenzoquinone derivatives have been used as
rigid anionic donors.2−5 Few complexes containing bispyr-
idylpyridone and 3-hydroxy-1,2,3-benzotriazine are known.6 It
is now well documented that the fac-Re(CO)3 core needs
neutral ditopic/tritopic/polytopic ligands with neutral two-
electron-coordinating donors and two μ2-XR bridging donor/
dianionic bis-chelating ligands with three-electron-chelating
donors to assemble rhenium(I)-based metallocycles (Figure 1),
but donors other than the above-mentioned building blocks are
Herein, a new type of neutral, flexible bis-chelating N∩OH
donors (1,4-bis(2-(2-hydroxyphenyl)benzimidazol-1-ylmethyl)-
benzene (H2-pBC) and 1,3-bis(2-(2-hydroxyphenyl)-
benzimidazol-1-ylmethyl)2,4,6-trimethylbenzene (H2-mBC),
Figure 1) and their rhenium(I)-based helicate (1) and
mesocate (2) are reported. Self-assembly of the helicate and
mesocate was achieved using Re2(CO)10, flexible bis-chelating
(or bis(bidentate)) H2-pBC/H2‑mBC N∩OH donors, and
bis(monodentate) nitrogen donors in a one-step process. The
ligands and complexes were characterized by elemental analysis,
1
ESI-MS, FT-IR, and H NMR spectroscopy. The molecular
structures of ligands were deduced from DFT calculations, and
complexes were confirmed by single-crystal X-ray crystallog-
raphy. The photophysical and electrochemical properties of
complexes were studied using various techniques.
Received: September 12, 2013
Published: January 6, 2014
© 2014 American Chemical Society
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H2-pBC ligand. A single set of signals was observed for the
aromatic and aliphatic protons of H2-pBC, suggesting the
formation of either a single conformer or conformers that
undergo rapid conversion in solution on the NMR time scale.
The formation of H2-pBC was further confirmed by ESI-MS,
which showed a molecular ion peak at m/z 523.2142 (SI Figure
S2).
Ligand H2-mBC was synthesized using the same procedure
as that for H2-pBC (SI Figures S3 and S4). The molecular
structures of H2-pBC and H2-mBC were optimized using DFT-
calculations. Both ligands adopt anti-conformation modes in
which both the HOPB moieties prefer the intermediate enol
structure (Figure 2).
Synthesis and Characterization of Dinuclear Com-
plexes 1−2. Metallocycle 1 was prepared from Re2(CO)10,
H2-pBC, and 1,4-bis(benzimidazol-1-ylmethyl)benzene (pbenz-
bix) via a one-pot procedure (Scheme 2). The resulting product
is air- and moisture-stable, and moderately soluble in polar
organic solvents. The FT-IR spectrum of 1 in acetone exhibits
strong bands at 2012 (s), 1903 (s), and 1874 (s, br) cm−1,
which are characteristic of fac-Re(CO)3 in an asymmetric
environment (SI Figure S5).2−5 A similar stretching pattern for
the fac-Re(CO)3 unit was observed in an acyclic rhenium
complex containing fac-Re(CO)3 and 2-(1-methyl-1H-benzi-
midazole-2-yl)phenolate with pyridine units in KBr.8 Complex
2 was synthesized similarly to complex 1 using 1,3-bis-
(benzimidazol-1-ylmethyl)benzene (mbenzbix) and H2-mBC
in mesitylene. A similar FT-IR pattern was observed for
complex 2 (SI Figure S6). The mass analysis revealed a signal
that corresponds to a molecular ion at m/z 1401.2047 for [1]+;
1423.1956 for [1+Na]+; 1465.2418 for [2+Na]+; 1481.2153 for
[2+K]+ with an experimental isotope pattern that matches the
calculated values (SI Figures S7−S8).
Figure 1. (A) Stereoelectronic requirement of fac-Re(CO)3 core. (B)
Flexible bis(monodentate) N-donors and flexible bis(bidentate)
N∩OH donors.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Ligands. Ligand H2-
pBC was synthesized from 2-(2-hydroxyphenyl)-1H-benzimi-
dazole (HO-PBH), 1,4-di(bromomethyl)benzene, and NaH in
THF (Scheme 1).7 The isolated ligand is air- and moisture-
Scheme 1. Synthesis of H2-pBC and H2-mBC Ligands
Crystal Structure of Complex 1. Single-crystal X-ray
diffraction analysis showed that complex 1 adopts a helicate
structure (Figure 3). Each Re(I) ion is surrounded by a N∩O
chelating unit (OPB) from the dianionic pBC building unit, a
nitrogen atom from pbenzbix, and three carbonyl groups. Thus,
complex 1 is best regarded as an unsaturated heterostranded
dinuclear double helicate.9 The two ligand strands wrap around
the Re···Re axis in a helical manner. The length of the helix, as
defined by the intramolecular Re···Re distance, is 13.01 Å. The
twist angle of (pBC)2− in complex 1 is 94° and is defined as the
angle between the planes of the two Re−O(Chel)−N(Chel)
units.10
1
stable and soluble in polar organic solvents. The H NMR
spectrum of H2-pBC showed well resolved chemical resonances
corresponding to the hydroxyphenylbenzimidazolyl (HO-PB)
and xylene protons (Supporting Information (SI) Figure S1).
The presence of a singlet at 5.36 ppm for the methylene
protons, the absence of a NH resonance, and the 1:2 xylene
proton to HO-PB units ratio confirmed the formation of the
The helical structure of 1 is further stabilized by intra-
molecular C−H···π interactions between the two strands (p-
phenylene unit to p-phenylene unit, dihedral angle = 90°,
distance = 3.83−4.06 Å). In addition, there are moderate
Figure 2. Optimized structures of ligands.
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Scheme 2. Synthesis of Metallocycles 1 and 2
have been calculated and tabulated.12 To the best of our
knowledge, complex 1 is the first neutral dinuclear rhenium-
based helicate.9
Crystal Structure of Complex 2. Single-crystal X-ray
diffraction analysis showed that compound 2 adopts a M2LL′-
type metallomacrocyclic structure (Figure 4 and SI Figure S10).
Figure 3. (a) Perspective view of 1 with thermal displacement
parameters drawn at 50% probability; hydrogen atoms are omitted for
clarity. (b) In the space fill representation, the carbon atoms of the two
strands pbenzbix and dianionic pBC are differently colored for clarity.
Green = rose = C, white = H, blue = N, red = O, yellow = Re.
Figure 4. Partial packing diagram of 2 showing a one-dimensional
sheet constructed by π···π stacking interactions. Hydrogen atoms are
omitted for clarity.
intramolecular π···π stacking interactions between the p-
phenylene unit of the pbenzbix strand and the two
benzimidazolyl units of the pBC strand (dihedral angle =
∼22°, centroid-to-edge center distance = 3.7 Å) (SI Figure S9).
The (pBC)2− ligand adopts a syn-conformation with anti-
cofacial arrangement of the two OPB units, whereas the
pbenzbix adopts an anti-conformation with cofacial arrange-
ment of the two benzimidazolyl units in complex 1. In the
crystal packing, each helicate is surrounded by four neighboring
helicate molecules that are held together by strong
intermolecular π−π stacking interactions; these intermolecular
interactions occur between the same strands, i.e., pbenzbix to
pbenzbix and pBC to pBC. Each benzimidazolyl ring is nearly
parallel and inverted relative to the benzimidazolyl ring of the
adjacent complex; the centroid-to-centroid distance is 3.5 Å (SI
Figure S9).
The molecular structure of 2 is mesohelicate with both ligand
strands arranged adjacently. The coordination geometry around
the Re centers is a distorted octahedron with a C3N2O
environment. The dianionic ligand and neutral nitrogen donors
adopt a syn-conformation; that is, the two phenolatebenzimi-
dazolyl/two benzimidazolyl units are located on the same sides
and serve as “W-type” chelating/bridging units. The Re···Re
distance in complex 2 is 13.93 Å, which is 0.9 Å longer than
those in helicate 1; this is expected because the helical nature of
the ligand reduces the bridging length of the ligands in complex
1. The major difference between the two ligands in 2 is the
arrangement of the two benzimidazolyl units, which are
orientated inward in mBC and outward in mbenzbix with
respect to the central phenylene core. Complex 2 features weak
intramolecular slipped-cofacial π−π stacking interactions
between the two central phenylene units (SI Figure S10).
In the crystal packing of complex 2, each molecule interacts
with two adjacent molecules that are held together by
intermolecular π−π stacking interactions and CH−π inter-
actions (Figure 4). In one direction, weak π−π stacking
interactions occur between the adjacent benzimidazolyl units of
the OBP units. In another direction, strong CH−π interactions
Similar pBC bis-bidendate ligands that contain pyrazolylphe-
nolate coordinating units have been coordinated to naked metal
ions to generate homoleptic and heteroleptic double helicates;
the resultant complexes are saturated helicates.11 Large
homoleptic helicate molecules have been obtained using
benzimidazolyl as coordinating units, and their helical pitches
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Figure 5. Calculated absorption spectra (maroon) with oscillator strengths (olive) of (A) complex 1 and (B) complex 2. The blue lines represent the
experimental results.
Table 1. Major Absorption Transitions and Corresponding Contributions of Different Molecular Orbitals for Complex 1
absorption transition (nm)
281
oscillator strength (f)
major contributing transitions
character
0.1251
HOMO-9 → LUMO (52%),
HOMO-8 → LUMO+1 (−15%)
HOMO-6 → LUMO+1 (30%)
HOMO-7 → LUMO (40%)
HOMO-2 → LUMO+2 (69%),
HOMO-3 → LUMO+3 (−12%)
HOMO-1 → LUMO (55%),
HOMO → LUMO+1 (−37%)
OPB → pBC (ILCT)
OPB → OPB (ππ*)
Re,CO → pBC (MLCT)
323
327
389
0.1105
0.0644
0.0675
Re,CO → OPB (MLCT)
Re,CO → p-benzbix (MLCT)
Re,CO → p-benzbix, pBC (MLCT)
Re,CO, OPB → pBC (MLCT, ILCT)
Re,CO, OPB → OPB (MLCT)
Table 2. Major Absorption Transitions and Corresponding Contributions of Different Molecular Orbitals for Complex 2
absorption transition (nm)
276
oscillator strength (f)
major contributing transitions
character
OPB → OPB (ππ*)
0.0791
HOMO-9 → LUMO (20%),
HOMO-8 → LUMO+1 (13%),
HOMO-1 → LUMO+10 (−11%)
HOMO-7 → LUMO (36%),
HOMO-7 → LUMO+1 (−16%)
HOMO-1 → LUMO (−21%),
HOMO → LUMO (29%),
318
381
0.0546
0.051
Re,CO, OPB → (MLCT + ILCT)
Re,CO, OPB → OPB (MLCT + ILCT)
HOMO → LUMO+1 (43%)
occur between the methylene units of mbenzbix and the
benzimidazolyl units of mbenzbix.
nature of the different transitions, time-dependent density
functional theory (TDDFT) calculations were performed on
optimized structures of the complexes. The TDDFT study
revealed that the absorption spectra mainly contain three types
of intense absorbances at three different regions (Figure 5).
The transition probabilities of the relatively intense
transitions of complexes 1 and 2 are compiled in Tables 1
and 2, respectively. The transitions are discussed in detail in the
Supporting Information (SI Figures S12−S13).
Emission Spectra. The emission spectra of the complexes
were measured both in a nitrogen purged solution and in the
presence of air (Figure 6). As in the absorption spectra, the
emission spectrum of complex 1 (λmax = 514 nm) is
bathochromically shifted from that of complex 2 (λmax = 482
nm), which suggests that the nature of the spacer that interlinks
the two OPB moieties affects the spectral characteristics of the
complex. The emission spectra are quenched in the presence of
air. Interestingly, the spectra in the presence of air are blue-
shifted compared to those in the nitrogen-purged solutions. To
further elucidate the emission characteristics of the complexes,
time-resolved emissions were measured (Table 3 and SI Figure
The Re−N bond distances of 1 and 2 are very similar and
consistent with values reported for other fac-Re(CO)3
complexes containing benzimidazolyl units.8,13 The Re−Ochel
and Re−Nchel bond lengths are similar to those in previously
reported mononuclear complexes, i.e., [Re(CO)3(L)-
(pyridine)] (H−L = 2-(1-methyl-1H-benzimidazol-2-yl)phenol
or 2-(1-phenyl-1H-benzimidazole-2-yl)phenol), while the Re1−
N(benzimidazolyl) bond length in 1 and 2 is shorter than the Re−
N(pyridine) distances (2.217 Å and 2.230 Å) in the mononuclear
complexes.
Absorption Spectra. The UV−visible absorption spectra
of the Re(I)−carbonyl complexes do not indicate any ground-
state aggregation in DMSO within the concentration range of 5
× 10−5 to 10−3 M. The absorption spectra of both complexes in
DMSO contain strong absorptions between 250 and 300 nm
and tail absorbances with band maxima at ∼360 nm (Figure 5
and SI Figure S11). The transitions correspond to π−π*
transitions, intraligand charge transfer (ILCT), and metal-to-
ligand charge transfer (MLCT). To further understand the
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Figure 7. Calculated triplet emission of (A) complex 1 and (B)
complex 2.
Figure 6. Emission spectra of 1 and 2 in DMSO; λexc = 365 nm.
S14). The emission decays of both complexes are biexponential
in the presence and absence of oxygen. The lifetime of one
species is shorter than 1 ns, whereas the other species has a
much longer lifetime of a few hundreds of nanoseconds. From
the lifetime data, it can be inferred that the complexes emit
from two different excited states. The de-excitation of the
singlet excited state occurs very fast in the femtosecond time
scale, and longer emission occurs from the triplet state in the
rhenium complexes.14 Therefore, the long lifetime component
in the emission of 1 and 2 can be assigned to the 3MLCT state.
The calculated triplet emission energies further substantiate the
conclusion (Figure 7). The highest single occupied molecular
orbital (HSOMO) is mainly localized on the OPB ring of the
ligand center (95%) in both complexes 1 and 2. The short
lifetime component is due to the fluorescence of the ligand.
Similar dual emission due to superimposition of fluorescence of
Figure 8. Emission spectra of complexes 1 and 2 in solid state (λexc
424 nm).
=
3
ligand upon long-lived MLCT phosphorescence was observed
in other rhenium complexes.15 As expected, the phosphor-
escence, which occurs at longer wavelengths, is quenched by
oxygen more than the fluorescence. Accordingly, the emission
spectra of the complexes are blue-shifted in the presence of
oxygen.
Table 4. Excited State Lifetimes (τ, ns) of the Complexes in
Solid State
a
λmon
τ1
τ2
χ2
Solid-State Luminescence. The emission spectra of the
solid-state complexes (Figure 8) red-shifted compared to those
in solution; this observation clearly contradicts the hypsochro-
mic shift that generally occurs due to rigidochromism in the
solid state.16 Time-resolved emissions of the Re(I) complexes
clearly indicate that the complexes also exhibit dual emission in
the solid state (Table 4). However, no short-lived fluorescence
emission was detected during the luminescence decay measure-
ments. The lifetimes of both emitting states are longer than the
phosphorescence of complexes in solution. XRD diffraction
studies unambiguously established the aggregation of the
complexes due to intermolecular π−π interactions and CH−π
interactions between two adjacent metallocycles (Figure 4 and
SI Figure S9). In solid-state excitation spectra, a new band is
observed at ∼420 nm along with a band at 365 nm (SI Figure
complex 1
complex 2
540
530
800 (15%)
600 (26%)
11000 (85%)
2800 (74%)
1.02
1.04
a
λmon = monitoring wavelength (nm).
S15). The additional excitation band substantiates the
aggregation of the complex in the solid state. Additional
phosphorescence is also observed in the solid state as a result of
this aggregation.
Aggregation-induced phosphorescence has been reported for
other metal complexes.17 In iridium complexes, it has been
reported that the energy level of the π* of the ligand is reduced
by π−π stacking between adjacent ligands, which increases the
conjugation.17a Therefore, the triplet energy level of the charge-
transfer state from the metal to the interacting ligand (denoted
Table 3. Excited State Lifetime (τ, ns) of the Complexes in DMSO
complex 1
complex 2
a
λmon
τ1
τ2
χ2
λmon
τ1
τ2
χ2
in presence of N2
in presence of air
514
490
0.80 (24%)
0.70 (40%)
350 (76%)
120 (60%)
1.08
1.04
482
470
1.28 (15%)
0.74 (9%)
176 (84%)
123 (91%)
1.09
1.16
a
λmon - monitoring wavelength (nm).
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3
as MLLCT) decreases and emission occurs from this state in
results, and thus supports a predominantly bridge-centered
the solid. In the Re complex, aggregation should similarly
decrease the energy of the interacting ligand state, resulting in
emission from the 3MLCT and 3MLLCT states. Dual emission
is a rare phenomenon; however, it has also been observed in
some rhenium and iridium complexes.18
oxidation of 1.
Electrochemical and Spectroelectrochemical Inves-
tigation. The redox properties of the metal complexes were
investigated by using cyclic voltammetry. Both 1 and 2 display
an oxidation wave in their cyclic voltammogram in CH2Cl2/0.1
M Bu4NPF6. For complex 1 this process appears at 0.46 V vs
ferrocene/ferrocenium and is quasi-reversible (Figure 9). The
Figure 10. EPR spectrum of oxidized form of 1.
CONCLUSION
■
In conclusion, flexible bis(bidentate) N∩OH donors ligand
were designed and successfully applied in making a new type of
supramolecular architecture. Simultaneous complexation of one
flexible bis(monodentate) ligand, one flexible bis(bidentate)
unit, and two fac-Re(CO)3 cores leads to the self-assembled
dinuclear unsaturated double-heterostranded helicate and
mesocate. This result may provide the way to develop neutral
different shapes and sizes of unsaturated redox active helicates.
The complexes emit both fluorescence and phosphorescence in
solution. On the other hand, they emit dual phosphorescence
due to aggregation-induced structures in the solid state. Our
current work is being directed toward changing the p-xylene to
o-xylene and incorporating various functional groups in
phenylene and benzimidazolyl units in the ligands and further
assembling with fac-Re(CO)3 cores.
Figure 9. Cyclic voltammograms of 1 in CH2Cl2/0.1 M Bu4NPF6 with
increasing the scan rate of the measurement.
intensity of the reduction peak in the second scan improves by
increasing the scan rate of the measurement. For complex 2, the
corresponding redox step appears at comparable potentials.
However, the oxidation process is completely irreversible in the
case of complex 2.
To check the reversibility of the redox and determine the site
of electron transfer in 1, IR spectroelectrochemistry was
performed on complex 1. This method can be put to good use
to determine reversibility, as seen by the regeneration of the
starting spectrum on reversing the redox potential and going
back to the initial potential. Furthermore, the presence of the
CO markers at the rhenium centers in the complex provides an
ideal opportunity to determine the site of electron transfer as
observed by the extent of CO shifts on generating the oxidized
species. Thus, a rhenium-based oxidation process would shift
the starting CO bands by a large extent, since the CO groups
are directly attached to the metal centers.19 On the other hand,
an oxidation step taking place predominantly on the bridging
ligands would result in only a marginal shift of the CO bands.20
In CH2Cl2, complex 1 shows three strong CO stretching
frequencies at 2014, 1904, and 1875 cm−1 (SI Figure S16).
Upon oxidation of 1, the CO bands shift to 2038, 2024, and
1933 cm−1; this marginal shift is certainly not compatible with
oxidation of the rhenium centers. Hence, the oxidation
comprises simultaneous oxidation of the phenoxide moiety of
the bridging ligand to generate two spatially separated phenoxyl
radicals in the oxidized form of 1. Simultaneous oxidation of the
phenoxide groups is expected because they are symmetrical,
spatially well separated, and not conjugated.
EXPERIMENTAL SECTION
■
General Data. Starting materials such as Re2(CO)10, 2-
hydroxybenzaldehyde, 1,2-diaminobenzene, 1,4-bis(bromomethyl)-
benzene, and NaH (60% dispersion in mineral oil) were procured
from commercial sources and used as received. 2-(2-Hydroxyphenyl)-
1H-benzimidazole (HO-PBH),23 1,3-di(bromomethyl)-2,4,6-trime-
thylbenzene,24 pbenzbix, and mbenzbix25 were synthesized by
previously reported methods. Elemental analyses were performed on
a Elementar Analysensysteme GmbH Vario EL-III instrument. FT-IR
spectra were recorded on a Perkin-Elmer FTIR-2000 spectrometer. 1H
NMR spectra were recorded on Jeol JNMECX-400P and Bruker
AMX-400 FT-NMR spectrometers. A Lambda 35 UV/visible
spectrophotometer from PerkinElmer was used to record the
absorption spectra. The mass spectra were performed on a Bruker
MaXis Impact ESI mass spectrometer. Fluorescence spectra were
measured on a Horiba Jobin Yovon Fluoromax4 fluorimeter. Time-
resolved fluorescence were measured by the time correlated single
photon counting method using an Edinburg Instruments Life Spec II
instrument. A picosecond 375 nm laser diode was used as a light
source. The full width of half maxima of the laser diode is 90 ps. EPR
spectra in the X-band were recorded with a Bruker System EMX. IR
spectra were obtained using a Nicolet 6700 FTIR instrument. Cyclic
voltammetry was carried out in 0.1 M Bu4NPF6 solutions using a
three-electrode configuration (glassy-carbon working electrode, Pt
counter electrode, Ag wire as pseudoreference), and a PAR 273
potentiostat and function generator. The ferrocene/ferrocenium (Fc/
Fc+) couple served as internal reference. Spectroelectrochemistry was
The EPR spectrum of the oxidized form of 1 displays a
narrow and isotropic signal centered at g = 2.005 at 295 K
(Figure 10).21,22 This confirms the generation of phenoxyl
radicals, as proposed from the IR spectroelectrochemistry
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performed using an optically transparent thin-layer electrode
(OTTLE) cell.26
Computational Section. The geometry optimizations of H2-pBC
and H2-mBC were carried out in the gas phase using the B3LYP
method with the 6-311+G(d,p) basis set. The initial geometry was
obtained from the X-ray crystal structure coordinates of 1 and 2. The
singlet ground state geometry optimizations of 1 and 2 were carried
out in the gas phase using the B3LYP method. A Stuttgart−Dresden
(SDD) basis set with an effective core potential was used for the
rhenium atom. The 6-311G* basis set was used for all other atoms
except rhenium, using the Gaussian 03 program package.28−31 The
triplet state geometry optimizations of 1 and 2 were carried out in the
gas phase using the UB3LYP method. Geometry optimizations were
attained without any constraints. To exactly inspect the absorption
spectra, UV−visible spectral analysis for vertical excitations from the
ground state was computed using the time dependent density
functional theory (TDDFT) in the gas phase. The TDDFT calculation
was performed, and the B3LYP functional and SDD/6-311G* basis set
for 1 and 2 were used in the optimization step. A total of the lowest
100 singlet excited states and their corresponding oscillator strengths
were determined using a TDDFT calculation for 1 and 2. The
Chemissian and GaussSum programs were used to calculate the
percentage contribution of various groups and the electronic spectral
simulation. The predicted emission wavelengths were obtained by
energy differences between the triplet and singlet optimized states.
Synthesis of 1,4-Bis(2-(2-hydroxyphenyl)benzimidazol-1-
ylmethyl)benzene (H2-pBC). A mixture of NaH (1.0731 g, 0.0268
mol) and 2-(2-hydroxyphenyl)benzimidazole (2.1235 g, 0.01 mol) in
THF (40 mL) was placed in a round-bottom flask and was allowed to
stir for 2 h at 45 °C. Solid 1,4-di(bromomethyl)benzene (1.3305 g,
0.005 mol) was then added to the reaction mixture, which was allowed
to stir continuously for 4 days at 45 °C. The solvent was removed
under reduced pressure. The residue was poured into 100 mL of water.
The solid was extracted from the water solution using chloroform, and
the organic layer was dried over anhydrous sodium sulfate. The ligand
was obtained by removing the solvent under reduced pressure. Yield:
21% (0.626 g). 1H NMR (400 MHz, DMSO-d6): 7.66 (d, 2H, J = 7.44
Hz, H4), 7.36−7.30 (m, 6H, H6,3′,5′), 7.22−7.14 (m, 4H, H5,6′), 6.99
(d, 2H, J = 7.32 Hz, H7), 6.88 (s, 4H, H9, p-phenylene), 6.87−6.82 (m,
2H, H4′), and 5.36 (s, 4H, H8). 13C NMR (100.5 MHz, DMSO-d6):
156.05, 151.9, 142.2, 136.07, 134.98, 131.48, 130.85, 126.88, 122.51,
122.01, 119.2, 118.86, 116.5, 116.32, 110.88, and 47.22 (CH2). ESI-
MS: [M]+ for 523.2142 and [M+Na]+ for 545.1961 Anal. Calcd for
C34H26N4O2·C4H8O: C, 76.75; H, 5.76; N, 9.42. Found: C, 76.95; H,
5.48; N, 9.64.
Synthesis of 1,3-Bis(2-(2-hydroxyphenyl)benzimidazol-1-
ylmethyl)-2,4,6-trimethylbenzene (H2-mBC). The ligand H2-
mBC was obtained by following the same procedure for 1 using
NaH (2.0465 g, 0.0512 mol), 2-(2-hydroxyphenyl)benzimidazole
(4.2115 g, 0.02 mol), and 1,3-di(bromomethyl)-2,4,6-trimethylben-
zene (3.0762 g, 0.01 mol) in THF (40 mL). Yield: 73% (4.301 g). 1H
NMR (400 MHz, DMSO-d6): 10.42 (s, 2H, OH), 7.59 (d, 2H, J =
8.08 Hz, H4), 7.42 (dd, 2H, J = 8.04 Hz, H3′), 7.34−7.39 (m, 2H,
H5′), 7.1 (t, 2H, J = 7.32 Hz, H5), 7.01 (d, 2H, J = 8.08 Hz, H6′), 6.94
(t, 2H, J = 7.34 Hz, H4′), 6.9 (s, 1H, H9), 6.85 (t, 2H, J = 7.7 Hz, H6),
6.50 (d, 2H, J = 8.76 Hz, H7), 5.28 (s, 4H, H8), 2.04 (s, 6H, H11), and
1.74 (s, 3H, H10). 13C NMR (100.5 MHz, DMSO-d6): δ 156.15,
152.57, 142.59, 137.31, 134.3, 131.41, 131.34, 130.93, 130.17, 122.09,
121.32, 119.19, 118.97, 117.45, 116.06, 110.84, 44.75 (CH2), 19.85
(CH3) and 15.33 (CH3). ESI-MS: [M]+ for 565.2697 and [M+Na]+
for 587.2530. Anal. Calcd for C37H32N4O2·H2O: C, 76.27; H, 5.88; N,
9.62. Found: C, 76.44; H, 5.5; N, 9.44.
ASSOCIATED CONTENT
* Supporting Information
■
S
Spectral data of ligands, 1, and 2 and selected molecular orbitals
of 1 and 2. X-ray crystallographic data for 1 and 2 in CIF
format. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: msathi@chemistry.du.ac.in; mvdiran@yahoo.com.
■
Notes
The authors declare no competing financial interest.
Synthesis of [(Re(CO)3)2(pBC)(pbenzbix)] (1). A mixture of
Re2(CO)10 (100.3 mg, 0.1537 mmol), H2-pBC·C4H8O (80.1 mg,
0.1346 mmol), and pbenzbix (51.9 mg, 0.1533 mmol) in mesitylene
(25 mL) was refluxed for 7 h. Yellow product was filtrated at hot
conditions and was washed with distilled mesitylene. Yield: 88%
(188.7 mg). ESI-MS [M]+ for 1401.2047 and [M+Na]+ for 1423.1956.
Crystals obtained from toluene: Anal. Calcd for C62H42N8O8Re2·
C7H8: C, 55.56; H, 3.38; N, 7.51. Found: C, 55.33; H, 3.18; N, 7.83.
Synthesis of [(Re(CO)3)2(mBC)(mbenzbix)] (2). A mixture of
Re2(CO)10 (100.7 mg, 0.1543 mmol), mbenzbix (52.0 mg, 0.1536
mmol), and H2-mBC·H2O (86.6 mg, 0.1486 mmol) in mesitylene (25
mL) was refluxed for 10 h. Yellow precipitate was obtained. The
powder was separated by filtration at hot conditions and washed with
distilled mesitylene. Yield: 77% (172.1 mg). ESI-MS: [M+Na]+ for
1465.2418 and [M+K]+ for 1481.2153. Crystals obtained from
toluene: Anal. Calcd for C65H48N8O8Re2·0.5C7H8: C, 55.31; H,
3.52; N, 7.53. Found: C, 55.04; H, 3.64; N, 7.15.
X-ray Crystallography. A single crystal X-ray structural study of 1
and 2 was performed on an Oxford Diffraction X caliber Sapphire 3
diffractometer equipped with a low-temperature attachment. Data
were collected at low temperature using graphite-monochromator Mo
Kα radiation (λα = 0.71073 Å). The strategy for the data collection was
evaluated by using the CrysAlis PRO software. The data were collected
by the standard phi-omega scan techniques and were scaled and
reduced using CrysAlis RED software.27 The structure was solved by
direct methods using SHELXS-97 and refined by full matrix least-
squares with SHELXL-97, refining on F2. The positions of all the
atoms were obtained by direct methods. All non-hydrogen atoms were
refined anisotropically. The remaining hydrogen atoms were placed in
geometrically constrained positions and refined with isotropic
temperature factors.
ACKNOWLEDGMENTS
■
We thank CSIR, New Delhi, and USIC, University of Delhi, for
financial support and the instrument facility. The authors thank
the central instruments facility (CIF), IIT Guwahati, for the
Life Spec II instrument. We thank Dr. A. Thamaraichelvan,
Thiagarajar College, Madurai, for his continuous research
support.
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