Synthesis, redox properties, and electronic coupling in the diferrocene aza-dipyrromethene and azaBODIPY donor-acceptor dyad with direct ferrocene-α-pyrrole bond

Article abstract of DOI:10.1021/ic500526k

3,3′-Diferrocenylazadipyrromethene (3) and corresponding difluoroboryl (azaBODIPY) complex (4) were synthesized in several steps from ferrocenecarbaldehyde, following the well-explored chalcone-type synthetic approach. The novel diiron complexes, in which ferrocene groups are directly connected to the α-pyrrolic positions were characterized by a variety of spectroscopic techniques, electrochemistry, spectroelectrochemistry, and X-ray crystallography, while their electronic structure, redox properties, and UV-vis spectra were correlated with the density functional theory (DFT) and time-dependent DFT calculations.

Full text of DOI:10.1021/ic500526k

Article
pubs.acs.org/IC
Synthesis, Redox Properties, and Electronic Coupling in the
Diferrocene Aza-dipyrromethene and azaBODIPY Donor−Acceptor
Dyad with Direct Ferrocene−α-Pyrrole Bond
Christopher J. Ziegler,*,† Kullapa Chanawanno,† Abed Hasheminsasab,† Yuriy V. Zatsikha,‡,§
Eranda Maligaspe,‡ and Victor N. Nemykin*,‡
†Department of Chemistry, University of Akron, Akron Ohio 44325-3601, United States
‡Department of Chemistry and Biochemistry, University of Minnesota Duluth, 1039 University Drive, Duluth, Minnesota 55812,
United States
§Institute of Organic Chemistry, National Academy of Sciences of Ukraine, 5 Murmanska str., 02660 Kyiv, Ukraine
S
* Supporting Information
ABSTRACT: 3,3′-Diferrocenylazadipyrromethene (3) and
corresponding difluoroboryl (azaBODIPY) complex (4) were
synthesized in several steps from ferrocenecarbaldehyde,
following the well-explored chalcone-type synthetic approach.
The novel diiron complexes, in which ferrocene groups are
directly connected to the α-pyrrolic positions were charac-
terized by a variety of spectroscopic techniques, electro-
chemistry, spectroelectrochemistry, and X-ray crystallography,
while their electronic structure, redox properties, and UV−vis
spectra were correlated with the density functional theory (DFT) and time-dependent DFT calculations.
Scheme 1. Preparation of the Target Azadipyrromethene
3 and azaBODIPY 4
INTRODUCTION
■
a
The preparation of new functional materials that are highly
efficient for solar energy conversion is of fundamental interest.1
Donor−acceptor dyads or donor−antennae−acceptor triads that
can generate long-lived highly energetic charge-separated (CS)
states have been proposed for applications as soft light-harvesting
materials.2 Because of their well-known absorption properties in the
visible electromagnetic spectrum, a large variety of porphyrins and
their analogues coupled with organometallic or organic donor groups
have been tested as donor−acceptor dyads for organic photovoltaics
(OPVs) and dye-sensitized solar cells (DSSCs).3 Ferrocene-
substituents were found to be potentially useful as electron-donating
groups for light-harvesting assemblies. Not surprisingly, photo-
physical as well as redox properties of ferrocene-porphyrins,
subphthalocyanines, phthalocyanines, and tetraazaporphyrins
have also been intensively investigated in recent years.4 More
recently, dipyrromethenes, azadipyrromethenes, and their borylated
derivatives BODIPYs and azaBODIPYs have attracted significant
attention because of their specific photophysical properties.5 During
the past decade, several ferrocene-(aza)dipyrromethenes and
ferrocene-(aza)BODIPY conjugates in which ferrocene substituent
is connected to the chromophoric unit via spacer located at
α-, β-pyrrolic, or meso-position were reported.6 To the best of our
knowledge, however, no reports are available on azadipyrromethenes
and azaBODIPYs with ferrocene groups connected directly to the
heterocyclic chromophore. Thus, in this paper, we report preparation
and characterization of the ferrocenyl-containing azadipyrromethene
and azaBODIPY donor−acceptor dyads 3 and 4 (Scheme 1) with
direct ferrocene-α-pyrrole bond.
a
Reagents and conditions: (i) PhCOCH3, NaOH/EtOH, rt/24 h,
yield 68%; (ii) CH3NO2, NEt3/EtOH, heat/72 h, yield 70%; (iii)
NH4OAc, EtOH, heat/72 h, yield 4.4%; (iv) BF3·Et2O DIPEA/
CH2Cl2 heat/24 h, yield 47%.
RESULTS AND DISCUSSION
■
The precursor to the azadipyrromethene modified with ferrocene
at the α-pyrrolic positions is the chalcone 1, which can be
synthesized via an aldol condensation with ferrocenecarbaldehyde.
Received: March 7, 2014
© XXXX American Chemical Society
A
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The chalcone 1 was further modified with nitromethane by using a
Michael addition to afford compound 2, which is then reacted with
ammonium acetate in refluxing ethanol to produce azadipyrro-
methene 3. The corresponding ferrocene-containing azaBODIPY
4 can be generated by reaction of 3 with BF3·Et2O in the presence
of a base (DIPEA). Organometallic azadipyrromethene 3 and
azaBODIPY 4 are stable in the solid state in air and can be purified
by conventional chromatographic methods, but slowly degrade
upon prolonging standing in solution under ambient atmosphere.
Structures of the target compounds 3 and 4 were confirmed by the
X-ray diffraction analysis (Figure 1 and Tables S1−2) and further
Figure 2. Experimental (CH2Cl2) and PCM-TDDFT predicted UV−vis
spectra of 3 (top) and 4 (bottom).
In agreement with the other ferrocene-BODIPY dyads,
complexes 3 and 4 have two major bands in the visible region
with the lower energy band being broader compared to the
higher energy band (Figure 2). These bands in ferrocenyl-
containing azadipyrromethene derivative 3 (546 and 673 nm)
were observed at significantly higher energies compared to those
in azaBODIPY 4 (623 and 850 nm). Steady-state fluorescence
measurements in 3 and 4 are indicative of complete quenching in
both systems and agree well with the standard quenching
mechanism in ferrocene-containing compounds.7 According to
this mechanism, electron-transfer from the low-spin iron(II)
center in ferrocene to the photoexcited azadipyrromethene or
azaBODIPY core is predominantly responsible for the
fluorescence quenching.
The redox properties of diiron compounds 3 and 4 were
studied using electrochemical CV and DPV methods (Figure 3
Figure 1. X-ray crystal structure of 3 (top) and two crystallographically
independent molecules of 4 (bottom).
elucidated by 1H and 13C NMR spectroscopy (Supporting
Information), UV−vis spectra (Figure 2), and elemental analyses.
Experimental crystal structures of 3 and 4 are shown in Figure 1,
while their detailed descriptions are provided in Supporting
Information Tables S1 and S2. In the X-ray crystal structures of
both complexes 3 and 4, ferrocene groups were found in a syn
conformation. The torsion angles between the ferrocene and
pyrrole fragments vary between 7.63° and 20.25° in 3, while they
are significantly smaller in the case of complex 4 (1.25−4.54°).
The torsion angles between the phenyl and pyrrole groups follow
the same trend: they are larger in complex 3 (19.81−21.25°)
compared to those in complex 4 (12.07−20.38°). Because of the
larger ferrocene to pyrrole torsion angle, the crystallographic
Fe−Fe distance in complex 3 is smaller (6.827 Å) than those in 4
(7.125−7.401 Å).
Figure 3. Room-temperature CV data on compounds 3 (left) and 4
(right) in CH2Cl2/0.05 M TBAF system.
and Supporting Information Table S3). In both cases two reversible
oxidations of ferrocene substituents were observed. The difference
in potential between the first and the second oxidations waves is
340 and 460 mV for complexes 3 and 4, respectively, in CH2Cl2/
0.05 M TBAF system (TBAF = tetrabutylammonium tetrakis
B
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(pentafluorophenyl)borate, which is indicative of their electronic
coupling (Figure 3).8 Although electronic coupling in one
diferrocene BODIPY system has been previously described,6c the
magnitude of electronic communication between the ferrocene
units in compounds 3 and 4 is unprecedented. Oxidation of the
azaBODIPY core was not observed for both compounds 3 and 4
within electrochemical window, while irreversible (compound 3)
and reversible (compound 4) reduction was observed at −1.42 and
−1.15 V, respectively.
To clarify the nature of redox-active species in oxidized forms
of 3 and 4, we have conducted spectroelectrochemical oxidation
and reduction experiments (Figure 4 and Supporting Information
Figure 5. DFT-PCM (TPSSh/6-311G(d)) calculated orbital energies
for complexes 3 and 4 with pictorial representation of the frontier MOs.
predominantly ferrocene-centered, while HOMO−6andHOMO−7
are π-orbitals centered on heterocyclic core. In addition, HOMO
and HOMO−4 in 3 and 4 have substantial (∼20−40%)
π-character. The LUMO in both compounds is predominantly
heterocycle-centered π*-orbital and energetically well separated
from the LUMO+1. Such electronic structure results in
appearance of numerous low-energy ferrocene-to-chromophore
MLCT transitions predicted by the TDDFT calculations (Figure 2).
MLCT transitions, which originate from almost pure ferrocene
MOs (HOMO−1 to HOMO−3) to LUMO are predicted to have
low intensities, while similar transitions from the mixed ferrocene-
chromophore HOMO and HOMO−4 to LUMO are predicted to
be intense (Supporting Information). TDDFT calculations also
correctly predict low-energy shifts of the major bands in UV−vis
spectra going from 3 to 4 (Figure 2). The major reason for such a
shift is the substantial stabilization of LUMO in 4 compared to 3.
CONCLUSIONS
■
In conclusion, we have synthesized and characterized the first
azadipyrromethene and azaBODIPY complexes with ferrocene
substituents directly connected to the α-pyrrolic carbon atom
position. The ferrocene units in these compounds are electroni-
cally coupled to each other. The DFT and TDDFT calculations
on target systems 3 and 4 are in reasonable agreement with the
experimental data and suggesting of the predominantly
ferrocene-centered MOs in the HOMO region and chromo-
phore π*-character LUMO.
Figure 4. Room-temperature spectroelectrochemical oxidation of 4 at
first (top) and second (bottom) oxidation potentials in CH2Cl2/0.15 M
TBAF system.
Figures S5 and S6). In the case of azaBODIPY 4, during the first
oxidation process, both major bands in visible region lose their
intensity and three new bands at 639, 760, and 996 nm appear in
the spectrum. In addition, new broad NIR band appears in the
spectrum (Figure 4). This band spans between ∼1700 and 2650
nm and is centered outside of the accessible range for our
spectrometer. The appearance of the NIR bands in 4+ is clearly
suggestive of its mixed-valence character and resembles the
ferrocenyl-containing mixed-valence porphyrins discussed ear-
lier.4l−o During the second oxidation, all NIR bands lose their
intensities and new intensive band at 697 nm appears in the visible
region (Figure 4). Similar behavior has been also observed for 3 →
3+ → 32+ transformation under spectroelectrochemical conditions
(Supporting Information Figure S6). It is important to note that
42+ cation generated under spectroelectrochemical oxidation
conditions can be easily reduced back to the neutral 4.
The electronic structure and the vertical excitation energy calc-
ulations for compounds 3 and 4 (Figures 2 and 5 and Supporting
Information) correlate well with the experimental results. First,
DFT calculations suggest that the energies and electronic
structures of syn and anti conformations of 3 and 4 are very
close to each other. In agreement with the electrochemical
data, HOMO−HOMO−5 MOs in complexes 3 and 4 are
EXPERIMENTAL SECTION
■
Materials. Chemicals were obtained commercially and used without
further purification. Basic alumina (50−200 μm) was purchased
from Sorbent Technologies. The tetrabutylammonium tetrakis-
(pentafluorophenyl)borate (TBAF) was used in anhydrous DCM
for electrochemical studies, after preparation according to literature
procedures.8a
Instrumentation. UV−vis data were obtained on Jasco-720
spectrophotometer. Electrochemical measurements were conducted
using a CH Instruments electrochemical analyzer utilizing a three-
electrode scheme with platinum working, auxiliary, and Ag/AgCl
reference electrodes in a 0.05 M solution of TBAF in DCM with redox
potentials corrected using an internal standard (decamethylferrocene,
FcH*) in all cases. The redox potentials were then corrected to
ferrocene using appropriate oxidation potentials for FcH*/FcH*+
versus FcH/FcH+ in the CH2Cl2/0.05 M TBAF system. NMR spectra
were recorded on a Varian Mercury 300 MHz and Varian NMRS
500 MHz spectrometers with a 500 MHz frequency for protons and
125 MHz for carbon. Chemical shifts were reported in parts per million
(ppm) with respect to residual solvent peaks as internal standard
C
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(1H CDCl3, δ = 7.26 ppm, DMSO-d6, δ = 2.50 ppm; 13C CDCl3, δ =
77.36 ppm, DMSO-d6, δ = 39.7 ppm) and referenced to tetramethyl-
silane [Si(CH3)4]. FTIR spectra were recorded on a Nicolet iS5 spectro-
meter using NaCl disks. Elemental analyses were performed by Atlantic
Microlab of Norcross, GA 30091. Electrospray MS (ES-MS; positive
mode) spectra were recorded using a Bruker HCT-ultra ETD II Ion
Trap mass spectrometer.
for 4·0.26H2O·1.8(CH2Cl2) C40H30Fe2N3BF2(H2O)0.26(CH2Cl2)1.8: C,
57.73; H, 3.96; N, 4.83%. Found: C, 57.82; H, 4.13; N, 4.64%.
X-ray Crystallography. X-ray intensity data for compound 3 was
measured on a CCD-based X-ray diffractometer system equipped with a
Mo-target X-ray tube (Mo Kα radiation, λ = 0.71073 Å) operated at
2000 W power. Crystals were mounted on a cryoloop using Paratone oil
and placed under a steam of nitrogen at 100 K. The detector was placed
at a distance of 5.009 cm from the crystal. A specimen of 3 approximate
dimensions 0.22 mm × 0.13 mm × 0.09 mm was used for the X-ray
crystallographic analysis. Data were acquired using three sets of Omega
scans at different Phi settings. The frame width was 0.5°. The structure
was solved and refined using the Bruker SHELXTL Software Package,17
using the space group P2(1)/c, with Z = 4 for the formula unit, C40H31Fe2N3.
Hydrogen atoms were assigned ideal positions and refined isotropically as
riding atoms.
Computational Aspects. All computations were performed using
Gaussian 09 software running under Windows or UNIX OS.9 Molecular
orbital contributions were compiled from single point calculations using
the VMOdes program.10 In all calculations, TPSSh hybrid (10% of
Hartree−Fock exchange)11 exchange correlation functional was used
because it was found in a set of model gas-phase calculations that it is
superior over standard GGA (BP86)12 and hybrid B3LYP13 exchange-
correlation functionals. Indeed, use of hybrid B3LYP exchange-correlation
functional results in HOMO, which is purely azaBODIPY centered MO in
disagreement with experimental data, while calculated using BP86
exchange-correlation functional vertical excitation energies for MLCT
transitions were found to be severely underestimated. In all calculations,
Wachter’s full-electron basis set for iron and 6-311G(d) basis set for all
other atoms were employed.14 Solvent effects were modeled using PCM
approach.15 In all TDDFT calculations, the lowest 40 excited states were
calculated to cover experimentally observed transitions in UV−vis region.
Synthesis. Compound 1. This compound has been previously
synthesized.16 An ethanolic solution of 1.00 g of acetylferrocene
(4.4 mmol) and 0.44 mL of benzaldehyde (4.4 mmol) were mixed in a
250 mL Erlenmeyer flask and 2 mL of a 30% aqueous NaOH solution
was added dropwise after mixing. The reaction mixture turned dark
orange upon addition of the base. After 24 h of reaction, an excess
amount of cold water was added with vigorous stirring to the flask until a
solid precipitate was observed. The solid was collected, washed with cold
water, and air-dried. The purity of the crude product was determined at
this point using TLC and 1H NMR spectroscopy. The spectral characteristics
were in good agreement with those found in the literature. Yield: 1.3 g
(68%). 1H (CDCl3): δ (ppm) 7.82 (d, 1H, J = 15 Hz, trans H); 7.67 (s, 2H,
H on benzene); 7.43 (s, 3H, H on benzene); 7.15 (d, 1H, J = 15 Hz, trans
H); 4.93 (s, 2H, Cp on ferrocene); 4.61 (s, 2H, Cp on ferrocene); 4.23
(s, 5H, Cp on ferrocene).
Compound 2. Compound 1 (1.00 g, 2.4 mmol), nitromethane
(0.64 mL, 12 mmol), and triethylamine (3.01 mL, 21.6 mmol) were
combined in 30 mL of ethanol and heated to reflux for 72 h. The ethanol
was removed, and the resulting oil extracted with CH2Cl2 (50 mL) and
water (50 mL). The organic layer was washed with DI water (2 × 50 mL),
and the organic fractions were dried over anhydrous magnesium sulfate.
The solvent was removed, and the resulting brown oily product used in
the next step without further purification. Yield: 0.81 g (70%).
Compound 3. An ethanolic solution of 2 (2 g, 4.14 mmol) and
ammonium acetate (9.57 g, 124.2 mmol) was refluxed for 48 h. After the
solution was cooled to room temperature, the solvent was removed and
crude product was dissolved in CH2Cl2 and washed with water. The
CH2Cl2 layer was collect, dried over Na2SO4, and evaporated to dryness.
The collected dark solution was purified by column chromatography
using basic alumina and CH2Cl2 as the mobile phase. The pure product
is purple solid. Yield 0.12 g (4.35%, based on 1). Purple crystals were
obtained from CH2Cl2 solution. 1H NMR (CDCl3) δ: 8.09 (d, 2H, J =
6 Hz), 7.35−7.43 (m, 3H), 6.82 (s, 1H), 4.89 (s, 2H, Cp on ferrocene),
4.61 (s, 2H, Cp on ferrocene), 4.23 (s, 5H, Cp on ferrocene). Anal.
Calcd for 3·0.7H2O C40H31Fe2N3(H2O)0.7: C, 70.86; H, 4.82; N, 6.20%.
Found: C, 70.73; H, 4.66; N, 6.24%.
X-ray intensity data for 4 was collected on a CCD-based diffract-
ometer with dual Cu/Mo ImuS microfocus optics (Cu Kα radiation, λ =
1.54178 Å). Crystals were mounted on a cryoloop using Paratone oil and
placed under a steam of nitrogen at 100 K. A specimen of 4 approximate
dimensions 0.26 mm × 0.32 mm × 0.59 mm, was used for the X-ray
crystallographic analysis. The integration of the data using a triclinic
unit cell yielded a total of 35640 reflections to a maximum θ angle of
62.99° (0.87 Å resolution), of which 9559 were independent (average
redundancy 3.728) The structure was solved and refined using the
Bruker SHELXTL Software Package,17 using the space group P1, with
̅
Z = 4 for the formula unit, C40H30BF2Fe2N3. Hydrogen atoms were
assigned ideal positions and refined isotropically as riding atoms. Crystal
data and structure refinement parameters for 3 and 4 are summarized in
Supporting Information Table S2. CCDC reference numbers: CCDC
986211 (3) and CCDC 986212 (4).
ASSOCIATED CONTENT
* Supporting Information
Crystallographic and DFT data for target compounds. This
material is available free of charge via the Internet at http://pubs.
acs.org.
■
S
AUTHOR INFORMATION
Corresponding Authors
*E-mail: ziegler@uakron.edu.
*E-mail: vnemykin@d.umn.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
C.J.Z. acknowledges the University of Akron for support of this
research. Generous support by the NSF CHE-1110455 and NSF
MRI CHE-0922366 grants, Minnesota Supercomputing In-
stitute, and University of Minnesota Grant-in-Aid to VNN is
greatly appreciated.
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