Testing the Limits of the BOPHY Platform: Preparation, Characterization, and Theoretical Modeling of BOPHYs and Organometallic BOPHYs with Electron-Withdrawing Groups at β-Pyrrolic and Bridging Positions

Article abstract of DOI:10.1002/chem.201702597

Several BOPHY derivatives with and without ferrocene fragments, and with electron-withdrawing ester groups appended to the β-pyrrolic positions have been prepared and characterized by NMR, UV/Vis near-infrared (NIR), high-resolution mass spectrometry, and fluorescence spectroscopy, as well as X-ray crystallography. The redox properties of new BOPHYs were probed by electrochemical (cyclic and differential pulse voltammetry) and spectroelectrochemical methods. In an attempt to prepare BOPHY derivatives with a cyano group at the bridging position using a similar approach for BODIPY cyanation, adducts from the nucleophilic attack of the cyanide anion on the bridging position in BOPHY have been isolated and characterized by spectroscopic methods. Oxidation of such adducts, however, resulted in formation of either the starting BOPHYs, or partial extrusion of the BF₂ fragment from the BOPHY core, which was confirmed by spectroscopy and X-ray crystallography. DFT and TDDFT calculations on all target materials correlate well with the experimental data, and suggest the dramatic reduction of the nitrogen atom basicity at the hydrazine bridge of the BOPHY upon introduction of the cyano group at the bridging-carbon atom.

Full text of DOI:10.1002/chem.201702597

A Journal of
Accepted Article
Title: Testing limits of BOPHY platform: preparation, characterization,
and theoretical modeling of BOPHYs BOPHYs with electron-
withdrawing groups at b-pyrrolic and bridging positions
Authors: Victor N. Nemykin, Yuriy V. Zatsikha, Dion B. Nemez,
Rebecca L. Davis, Simarpreet Singh, David E. Herbert, Alex
J. King, and Christopher J. Ziegler
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Testing limits of the BOPHY platform: preparation, characterization, and theoretical
modeling of BOPHYs and organometallic BOPHYs with electron-withdrawing groups at -
pyrrolic and bridging positions
Yuriy V. Zatsikha,[a] Dion B. Nemez,[a] Rebecca L. Davis,[a]
Simarpreet Singh,[a] David E. Herbert,[a] Alex J. King,[b] Christopher J. Ziegler,[c]* and Victor N.
Nemykin[a]*
[a] Department of Chemistry, University of Manitoba, Winnipeg, MB R3T 2N2, Canada
[b] Department of Chemistry & Biochemistry, University of Minnesota Duluth, Duluth, USA
[c] Department of Chemistry, University of Akron, Akron OH 44325-3601, USA
*E-mail: Viktor.Nemykin@umanitoba.ca (V.N.N.). ORCID 0000-0003-4345-0848
*E-mail: ziegler@uakron.edu (CJZ).
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ABSTRACT
Several BOPHY derivatives with and without ferrocene fragments and with electron-withdrawing
carbethoxy groups appended to the -pyrrolic positions have been prepared and characterized by
NMR, UV-Vis-NIR, APCI MS, and fluorescence spectroscopy as well as X-ray crystallography.
The redox properties of new BOPHYs were probed by electrochemical (CV and DPV) and
spectroelectrochemical methods. In attempt for the preparation of the BOPHY derivatives with
cyano group at bridging position using similar to recently communicated BODIPY cyanation
approach (Chem. Commun. 2016, 52, 11563-11566 and ChemistrySelect, 2016, 1, 1462–1466),
adducts of the nucleophilic attack of the cyanide anion on the bridging position in BOPHY have
been isolated and characterized by spectroscopic methods. Oxidation of such adducts, however,
resulted in formation of either starting BOPHYs or partial extrusion of the BF2 fragment from the
BOPHY core, which was confirmed by spectroscopy and X-ray crystallography. DFT and TDDFT
calculations on all target materials correlate well with the experimental data and are suggestive of
the dramatic reduction of the nitrogen atom basicity at the hydrazine bridge of BOPHY upon
introduction of the cyano group at the bridging carbon atom.
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INTRODUCTION
Search for organic chromophores with an intense and broad absorption in the near-infrared (NIR)
region attracts a significant attention because of the potential use of such compounds in light
harvesting, (bio)imaging, and photodynamic therapy applications.[1] Initially, the search for such
systems was limited to natural and synthetic dyes chosen from porphyrins,[2] phthalocyanines,[3]
and their extended -conjugated analogues.[4] Recently, several chromophores of non-macrocyclic
nature emerged in this field of research. In particular, the boron-containing dipyrromethene
(BODIPY)[5] as well as azadipyrromethene (aza-BODIPY)[6] compounds were documented to
have desired tunable photophysical properties in the NIR region. It was shown that the reduction
of the HOMO-LUMO gap and broad absorption in the NIR region in BODIPYs and aza-BODIPYs
can be achieved by (i) introduction of the electron-donating groups at -pyrrolic position(s),[7] (ii)
introduction of the electron-withdrawing groups at meso-position of BODIPY chromophore,[8] and
(iii) extension of the core BODIPY’s and aza-BODIPY’s -system via additional conjugation with
aromatic fragments.[9] In general, the first and the third approaches lead to destabilization of the
HOMO, which reduces the first oxidation potential and thus stability of the respective BODIPYs
and aza-BODIPYs, while second synthetic methodology stabilizes the LUMO energy and thus
results in more stability toward oxidative stress.[8] Following the success of BODIPYs and aza-
BODIPYs, we introduced in 2014 a new tetracyclic -conjugated bis(difluoroboron)-1,2-
bis{(pyrrol-2-yl)methylene}hydrazine (BOPHY) chromophore that exhibits an outstanding
fluorescence quantum yield.[10] Following our initial report, several research groups have proposed
a variety of synthetic methodologies, which allow tuning of the optical, photophysical, and redox
properties of the parent BOPHY chromophore by introducing variants bearing specifically
designed electron-withdrawing or electron-donating groups located at - and(or) -pyrrolic
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positions of the -conjugated core.[11] Similar to BODIPYs and aza-BODIPYs, a variety of
coupling reactions, nucleophilic substitution reactions, or the Knoevenagel condensation were
used for such functionalization.[10, 11] We have shown that diferrocene-containing BOPHY has
desirable for light-harvesting applications intense and broad absorption in the visible-to-NIR
region, which originates from numerous predominantly metal-to-ligand charge-transfer (MLCT)
and core-centered * transitions.[12] In order to expand absorption of the diferrocene-containing
BOPHY in the NIR region without compromising its stability, we attempted cyanation reaction at
the bridging (meso) carbon atom at the BOPHY core using a recently developed synthetic
methodology for cyanation reaction of the BODIPY core.[8] This approach leads to unexpected
chemical transformations discussed in this paper that allowed us to test the limits of BOPHY
platform.
Results and Discussion
We have explored two synthetic strategies for incorporation of the cyano group at the bridging
position of the BOPHY core. The first one explored nucleophilic addition of cyanide anion to a
new, BOPHY core 4 substituted with phenyl and carbethoxy fragment at -pyrrolic positions
(Scheme 1). This new BOPHY core was prepared using known pyrrole 1,[13] which was formylated
to form pyrrolecarboxaldehyde 2. The aldehyde 2 was converted into Schiff base 3 upon treatment
with hydrazine hydrate in ethanol solution. Finally, interaction of 3 with BF3·Et2O resulted in
formation of new BOPHY 4 (Supporting Information). In the next step, BOPHY 4 was reacted
with potassium cyanide in the presence of crown ether using a previously reported protocol for
preparation of meso-cyano BODIPYs.[8] Although nucleophilic substitution product 5 is stable and
can be characterized by the regular spectroscopic methods (Supporting Information), further
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oxidation did not result in desired BOPHY product 6. Instead, we observed formation of the
starting BOPHY 4 and cyanated product 7.
Scheme 1. The synthesis of compounds 4 - 7.
the asymmetric nature of 7 was confirmed by NMR spectroscopy (presence of two sets of
signals for substituents at the pyrrolic fragments, one NH signal, and one C(bridging)-H signal,
Supporting Information), while its ultimate structure was established by X-ray crystallography
(Figure 1).
C(13)
N(4)
C(14)
C(12)
C(1)
C(22)
C(11)
C(4)
N(5)
C(15)
C(2)
C(21)
C(20)
O(3)
C(23)
C(25)
C(3)
N(3)
C(10)
O(1)
C(16)
F(2)
B(1)
C(17)
C(18)
N(1)
C(19)
C(26)
N(2)
F(1)
C(24)
C(8)
O(2)
C(5)
C(7)
O(4)
C(6)
C(9)
C(27)
C(31)
C(30)
C(28)
C(29)
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Figure 1. ORTEP of compound 7 with ellipsoids shown at 50% probability level with side-chain
atom labels omitted for clarity. Selected bond distances (Å) and angles (°): B(1)-F(1) 1.3731(16),
B(1)-F(2) 1.3947(16), B(1)-N(1) 1.5290(17), B(1)-N(2) 1.5859(17), C(1)-N(4) 1.1461(16), C(1)-
C(2) 1.4543(17), C(2)-N(3) 1.3131(15), C(2)-C(3) 1.4212(16), C(3)-N(1) 1.3941(15), C(3)-C(4)
1.4038(16), C(4)-C(5) 1.4165(17), C(5)-C(6) 1.4078(18), C(6)-N(1) 1.3529(16), N(2)-N(3)
1.3902(13), C(17)-N(2) 1.3178(15), C(17)-C(18) 1.3956(17), C(18)-N(5) 1.3922(15), C(18)-
C(19) 1.4187(16), C(19)-C(20) 1.4081(17), C(20)-C(21) 1.4116(17), C(21)-N(5) 1.3383(16);
F(1)-B(1)-F(2)110.09(10), F(1)-B(1)-N(1) 113.41(11), F(2)-B(1)-N(1) 110.90(11), F(1)-B(1)-
N(2) 108.62(10), F(2)-B(1)-N(2) 107.21(10), N(1)-B(1)-N(2) 106.33(10).
In agreement with the spectroscopic data, X-ray crystallography confirmed the
introduction of a cyano group at the bridging carbon position. However, the BF2 fragment adjacent
to the cyano group in 7 is extruded from the BOPHY core. Treatment of cyano-adduct intermediate
salt with oxidative agent (Magic Blue, DDQ, NBS, Pb(OAc)4, or Br2) in DCM solution
instantaneously leads to formation of intense red solution, which is stable enough to be
characterized by UV-Vis spectroscopy (Figure 2). However, this compound is not stable for a
prolonged time in solution and transformation into a brown solution occurs within one hour. The
decomposition of this product can be also observed by qualitative TLC. It was found that during
the elution process, the starting red spot splits into two new spots of highly fluorescent yellow
color (dye 4) with higher Rf, and brown weakly fluorescent compound (dye 7) with lower Rf. All
attempts for isolation of the target 6 appeared to be unsuccessful and resulted in the formation of
a mixture of compounds 4 and 7.
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1.2
0.8
0.4
0.0
1.2
0.8
0.4
0.0
4
7
6
12
13
300
400
500
600
700
300 400 500 600 700 800 900
Wavelength, nm
Wavelength, nm
Figure 2. Normalized absorption spectra of compounds 4, 6, 7 (left) and 12, 13 (right) in DCM.
The low stability of the cyano modified BOPHY 6 is not overly surprising as one might expect
significant decrease in the basicity of the hydrazine nitrogen atom adjacent to the cyano group in
6. This, in turn, would reduce -donating properties of the nitrogen atom near the cyano group and
weaken the N-B bond in BOPHY 6. In order to increase the basicity of the hydrazine-type nitrogen
atoms and thus improve our chances for preparation of the bridging-cyano BOPHYs, we first
conducted a test cyanation reaction with diferrocene-containing BODIPY 11.[7a] The reaction went
smoothly and produced a compound prepared earlier by a different method,[8a] target cyanogroup-
containing BODIPY 10 in 80% yield (Scheme 2 and Supporting Information). In addition to testing
the methodology, transformation of BODIPY 11 to 10 allowed us to improve a total yield of NIR
absorbing BODIPY 10 when prepared from 8 as compared to our prior method for preparation of
10 from 8 and 9 (Scheme 2). Encouraged by these results, we introduced electron-donating
ferrocene fragments to new BOPHYs 12 and 13 using Knoevenagel condensation reaction with
ferrocene carboxaldehyde (Scheme 3).
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Scheme 2. An alternative synthetic route for dye 11 preparation through the direct cyanation
reaction of dye 10.
The most electron-rich BOPHY 13 then was tested in the cyanation reaction with potassium
cyanide in the presence of a crown ether. Again, we were able to isolate and characterize
nucleophilic addition reaction product 14. All attempts to oxidize this intermediate with variety of
oxidants (DDQ, “magic blue,” Pb(OAc)4, or NBS) to form a target BOPHY 15, however, resulted
in recovery of the starting BOPHY 13 and small quantities of degradation products. Thus, unlike
in the case of BODIPY 11, which was found to be an excellent starting material for the cyanation
reaction, the BOPHY core is less susceptible for such chemical transformation and tends to either
remove cyano group from adducts 5 or 14 during oxidation reaction or extrude BF2 fragment from
the expected product. Such unusual reactivity, indeed, clearly shows limitations of the BOPHY
platform. Indeed, BODIPYs and aza-BODIPYs, in which the deprotonated pyrrolic nitrogen atoms
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have strong -donor properties, hydrazine-type nitrogen atoms in BOPHYs are less efficient -
donors. As shown below on the basis of DFT calculations, the addition of the strong electron-
withdrawing cyano group at the carbon atom directly connected to the hydrazine bridge in BOPHY
core dramatically reduces its basicity and triggers extrusion of the BF2 group located next to it.
Scheme 3. The synthesis of compounds 12, 13, and 14.
1
All new BOPHYs and their intermediates were characterized by H, 13C, and 2D NMR
spectroscopy, UV-vis spectroscopy, and high-resolution mass spectrometry (Supporting
Information). In addition, the structures of compounds 7 and 13 were elucidated by the X-ray
crystallography (Figures 1 and 3).
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C(4)
C(2)
O(1)
C(11)
C(10)
C(3)
C(1)
N(2)
N(1)
O(2)
C(14)
N(2)ʹ
B(1)
C(16)
F(1)
C(15)
C(17)
F(2)
Fe(1)
Figure 3. ORTEP of compound 13 with ellipsoids shown at 50% probability level. Hydrogen
atoms, certain symmetry-generated atom labels, side-chain atom labels, and a disordered solvent
molecule (CHCl3) found in the lattice omitted for clarity. Selected bond distances (Å) and angles
(°): B(1)-F(1) 1.383(3), B(1)-F(2) 1.372(3), B(1)-N(1) 1.521(4), B(1)-N(2)' 1.601(4), C(1)-N(2)
1.317(3), C(1)-C(2) 1.383(4), C(2)-N(1) 1.387(3), C(2)-C(3) 1.405(4), C(3)-C(10) 1.389(4),
C(10)-C(14) 1.414(4), C(14)-N(1) 1.359(3), C(14)-C(15) 1.445(3), C(15)-C(16) 1.332(4); F(2)-
B(1)-F(1) 110.4(2), F(2)-B(1)-N(1) 113.3(2), F(1)-B(1)-N(1) 111.8(2), F(2)-B(1)-N(2)' 106.6(2),
F(1)-B(1)-N(2)' 107.7(2), N(1)-B(1)-N(2)' 106.7(2).
Six-membered chelate rings of one and two BF2 moieties, respectively, are observed,
consistent with the structures of the parent BOPHY and Me4BOPHY derivatives,[10] with the
inversion center seen in the binuclear BOPHY 13 (Figure 3) absent in the singly-BF2 substituted
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compound 7 (Figure 1).[10] In compound 7 the two planes formed by the two halves of the molecule
[Plane 1: N(1), C(3), C(4), C(5), C(6); Plane 2: N(2), N(3), C(17), C(18), C(19), C(20), C(21),
O(3), O(4)] of the molecule are canted at an 18.5° angle, hinged approximately at the B(1)-C(1)-
C(2)-N(4) vector but with B(1), C(1) and the cyano substituent only in plane with the atoms in
Plane 1. This implies that the chromophore in 7 is conjugated only throughout one half of the
molecule; indeed, both hydrazine moieties retain alternating double/single-bond character [C(17)-
N(2) 1.3178(15), C(17)-C(18) 1.3956(17); C(2)-N(3) 1.3131(15), C(2)-C(3) 1.4212(16) Å],
similar to what was observed with the parent BOPHY.[10] While the phenyl substituents are twisted
out of the plane of each pyrrole, one of the ester side-chains [O(1), O(2)] is co-planar with the
chromophore. The other ester [O(3), O(4)] is twisted at a ~ 20° angle.
In comparison, the 16 atoms in the core of BOPHY 13 [B(1), C(1), N(1), N(2), C(2), C(3), C(10),
C(14) and symmetry-generated equivalents] are essentially coplanar, with the vinylferrocene
substituents [C(15), C(16), C(17) and one cyclopentadienyl ring] also residing within the
chromophore plane (Figure 3). This is consistent with the rigidly planar solid-state structures of
the parent BOPHY and Me4BOPHY, where only the fluorine atoms deviating from the plane of
the tetracyclic chromophore.[10] In BOPHY 13, both phenyl and ester substituents are twisted
perpendicular to the molecular core, implying a break in conjugation at these positions.
For both compounds 7 and 13, the pyrrole N-B distances are shorter than the hydrazine N-
B distances, consistent with anionic character at the pyrrolic nitrogen [7: B(1)-N(1) 1.5290(17) cf.
B(1)-N(2) 1.5859(17); 13: B(1)-N(1) 1.521(4) cf. B(1)-N(2)' 1.601(4) Å]. The presence of a
strongly electron-withdrawing cyano group at C(2) in compound 7 does not appear to affect the
C-N distance adjacent this position [7: C(2)-N(3) 1.3131(15) cf. 13: C(1)-N(2) 1.317(3) Å] but the
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C-C bond distance adjacent is statistically longer [7: C(2)-C(3) 1.4212(16) cf. 13: C(1)-C(2)
1.383(4) Å].
The UV-vis spectra of new BOPHYs are shown in Figure 2. In case of organic systems 4,
6, and 7, their UV-vis spectra are close to the simple BOPHYs.[10] Indeed, the UV-vis spectrum of
the starting BOPHY 4 is dominated by two bands at 473 and 454 nm, which are close to the parent
BOPHY (467 and 444 nm).[10] The UV-vis spectrum of unstable cyanogroup-containing BOPHY
6 is significantly red-shifted and dominated by bands at 542 and 510 nm. Similar to meso-cyano
BODIPY 10[8a] and in agreement with DFT calculations discussed below, the lowering of the
LUMO energy is primarily responsible for such a low-energy shift. Finally, extrusion of one of
the BF2 groups from BOPHY 6 leads to the high-energy shift of two most intense transitions,
which observed in 7 at 505 and 478 nm. The UV-vis spectra of the ferrocene-containing
compounds 12 and 13 are close to those observed earlier for diferrocene-BOPHY[12] and ferrocene-
BODIPYs[7a] in which the vinylferrocene fragment is attached to the -pyrrolic position of
chromophore. In particular, the UV-vis spectra of 12 and 13 are dominated by a relatively narrow
peak observed at 497 and 514 nm and broader peak observed at 606 and 656 nm for 12 and 13,
respectively (Figure 2). A clear red-shift of the narrow, predominantly * transition occurring
from parent BOPHY 4 (474 nm), to mono-Fc-BOPHY 12 (497 nm) and bis-Fc-BOPHY 13 (514
nm) reflects the increased length of the -system in 12 and 13. The influence of the electron-
withdrawing carbethoxy groups in new BOPHY 13 can be clearly seen in comparison with the
UV-vis spectrum of published earlier bis-Fc-BOPHY system[12] as both observed in the latter
compound (564 and 694 nm) are red-shifted compared to analagous bands in BOPHY 13. The
fluorescence spectra of BOPHY 4 and “half-BOPHY” 7 are shown in Figure 4 and are similar to
the fluorescence spectra of published earlier parent BOPHYs.[10]
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4
7
0.9
0.6
0.3
0.0
500
600
700
Wavelength, nm
Figure 4. Normalized emission spectra of compounds 4 and 7 in DCM.
In particular, the excitation and emission spectra are not exact mirror images of each other.
Indeed, BOPHY 4 has a fluorescent spectrum, which has a single prominent peak at 506 nm
complemented with a shoulder at 538 nm. The “half-BOPHY” compound 7 has a fluorescence
spectrum dominated by the peak at 548 nm and a shoulder at 580 nm. In both cases, large Stokes
shifts (1650 cm-1 for 4 and 1555 cm-1 for 7) are indicative of significant geometry reorganization
upon excitation of these compounds. The most striking difference between BOPHY 4 and “half-
BOPHY” 7 is in their fluorescence quantum yields. Indeed, BOPHY 4 is highly fluorescent (F =
0.98), which is close to the parent BOPHY.[10] On the other hand, the fluorescence quantum yield
of the less planar “half-BOPHY” 7 (F = 0.011) is about 100 times lower. Because of the low
stability of BOPHY 6, its fluorescence was not studied. As expected, fluorescence in the ferrocene-
containing BOPHYs 12 and 13 is quenched, which confirms the presence of an effective electron-
transfer pathway from the ferrocene-centered MOs to the photoexcited states in these compounds.
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Redox properties of compounds 4, 7, 12, and 13 were probed by electrochemical (cyclic
voltammetry, CV as well as differential pulse voltammetry, DPV, Table S1, Supporting
Information) methods and spectroelectrochemical approaches. In case of the organic system 4 no
oxidation and one quasi-reversible reduction at -1.34 V was observed in CV and DPV experiments.
At the same time, compound 7 showed two irreversible oxidation and reduction processes.
(Supporting Information). Similar irreversible single reduction process was observed within
electrochemical window for the ferrocene-containing complexes 12 and 13 (Figure 5).
0
0
1
0
1
0
-1
-2
-1
-1
-1
-1
0
-1
0
+
+
Potential, V (FcH/FcH )
Potential, V (FcH/FcH )
Figure 5. CV/DPV spectra of compounds 12 (left), 13 (right) in DCM/TBAP system.
Ferrocene-centered oxidation processes in organometallic BOPHYs 12 and 13, however,
are reversible, close in potential, and observed at ~100 mV more positive compared to the parent
ferrocene. No BOPHY-centered oxidation process was observed within electrochemical window
in complexes 12 and 13. The most interesting observation is that both ferrocene groups in BOPHY
13 oxidize at the same potential (although oxidation wave is quite broad and can be deconvoluted
into two individual oxidation processes with ~80-100 mV separation in DCM/0.05M TBAF
system, see Supporting Information), while similar oxidation processes in reported earlier bis-Fc-
BOPHY[12] are well separated (~200 mV) when studied in the same solvent/electrolyte system,
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which could be indicative of lack of the electronic coupling between ferrocene groups in BOPHY
13.
Figure 6. Spectroelectrochemical oxidation of compounds 12 (left), 13 (right) in 0.15 M
DCM/TFAB solvent system.
In order to probe potential electronic coupling between two ferrocene fragments in BOPHY 13,
we conducted spectroelectrochemical experiments on BOPHYs 12 and 13 (Figure 6 and
Supporting Information). In case of the reference BOPHY 12, during the first oxidation under
spectroelectrochemical conditions the broad MLCT band at ~600 nm disappears, while small
higher-energy shift from 491 to 485 nm was observed for the most intense * transition. In
addition, appearance of the low-intensity broad band at ~930 nm ( ~900 M-1 cm-1) was observed
during 12 → [12]+ transformation. Persistence of the most intense transition in [12]+ along with
disappearance of the broad band at ~600 nm was already been observed in vinylferrocene-
containing BODIPYs[7a] and is clearly indicative of the ferrocene group oxidation in 12. This
interpretation also correlates well with electrochemical data on BOPHY 12. The first oxidation
process in BOPHY 12 is fully reversible as reduction of [12]+ under spectroelectrochemical
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conditions lead to recovery of neutral BOPHY 12. In agreement with electrochemical data on
BOPHY 13, oxidation of this compound under spectroelectrochemical conditions is indicative of
only single transformation (Figure 6). Again, during oxidation, a broad MLCT band at ~650 nm
disappears, the most intense * transition shifts from 523 to 515 nm without loss in intensity,
and the new low-intensity band at ~930 nm ( ~700 M-1 cm-1) appears in the UV-vis-NIR spectrum
of [13]2+. Again, the UV-Vis-NIR spectrum of [13]2+ is very close to that of [12]+ and is indicative
of the simultaneous oxidation of both ferrocene groups in BOPHY 13. The lack of observation of
the inter-valence charge-transfer (IVCT) band upon careful oxidation of 13 under
spectroelectrochemical conditions is suggestive of the absence of electronic communication
between ferrocene groups in this compound. Interestingly, appearance and disappearance of a
broad IVCT band of relatively low intensity was clearly documented in the case of parent
diferrocenyl-containing BOPHY, which is along with electrochemical data indicative of electronic
coupling in this compound.[12] As this moment, it is not clear what is the main reason for the
electronic coupling observed in reported earlier diferrocenyl-containing BOPHY[12] and similar
BOPHY 13. One might speculate that the BOPHY 13 has more conformational flexibility in its
ester and phenyl groups, which can modulate the electronic structure of the BOPHY core in
solution, while the previously published diferrocene BOPHY system is conformationally rigid and
thus possesses a firm electronic structure.
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160
158
156
154
152
150
148
154
152
150
148
146
144
142
BOPHY
Substituents
CN
BOPHY
Substituents
0
10 20 30 40 50 60 70 80 90 100
% Composition
0
10 20 30 40 50 60 70 80 90 100
% Composition
150
148
146
144
142
140
138
136
BOPHY
Substituents
CN
0
10 20 30 40 50 60 70 80 90 100
% Composition
254
204
202
200
198
196
194
192
252
250
248
246
244
BOPHY
Substituents
Fc
242
BOPHY
Substituents
Fc
0
10 20 30 40 50 60 70 80 90 100
% Composition
0
10 20 30 40 50 60 70 80 90 100
% Composition
Figure 7. DFT-predicted compositions of frontier orbitals for compounds 4, 6, 7, 12, and 13.
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In order to explain the selective BF2 fragment extrusion from the target BOPHY 6 and correlate
the experimental electrochemical and spectroscopic data with electronic structure of the new
BOPHYs, we conducted DFT and TDDFT calculations on the target compounds. The molecular
orbital compositions for frontier orbitals predicted by DFT calculations is presented in Figure 7,
the DFT-predicted energy diagram is shown in Figure 8 and an important frontier orbitals are
depicted in Figure 9 and Supporting Information Figures S30 – S34.
Figure 8. DFT-predicted energy diagram for the frontier MOs in 4, 6, 7, 12, and 13.
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Figure 9. DFT-predicted frontier molecular orbitals for 4, 6, 7, 12, and 13. BOPHY-centered
predominant -orbital s HOMO-2 and HOMO-4 are shown for 12 and 13, respectively.
In general, DFT results correlate very well with the experimental data. In particular, since bridging
carbon in BOPHY core contributes into LUMO but not HOMO, introduction of the electron-
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withdrawing cyano group at this position significantly stabilizes energy of the LUMO (0.580 eV)
and similar LUMO stabilization was observed in the case of meso-cyanoBODIPY.[8a] DFT
calculations also correctly predict stepwise reduction of the HOMO-LUMO energy gap in the
order of 4 > 7 > 6, which correlates well with the observed in UV-vis spectroscopic stepwise red
shifts of the lowest energy bands in these compounds. Conjugation of one (12) or two (13)
vinylferrocene fragments to the BOPHY core leads to the appearance of the ferrocene-centered
orbitals within the HOMO energy region. Indeed, HOMO and HOMO-1 in 12 as well as HOMO
– HOMO-3 in 13 have predominant ferrocene character, while the highest energy predominantly
BOPHY core-centered orbitals are HOMO-2 (12) and HOMO-4 (13) (Figure 7). DFT-predicted
energy of the ferrocene-centered HOMO in 12 and 13 is very close to each other, which correlate
well with experimentally observed first oxidation potential in these compounds. In addition, much
lower HOMO-LUMO energy gap in 12 and 13 predicted by the DFT calculations correlate well
with the experimentally observed low-energy MLCT transitions for these compounds.
DFT calculations predict that the B-N(pyrrole) bond distance remains unaffected upon
cyanation of BOPHY 4 to BOPHY 6 (the largest predicted change is 0.002 Å), while the B-
N(hydrazine) bond distances are quite different in BOPHY 6 with the bond adjacent to the cyano
group being by 0.03 Å longer than the one opposite to the cyano group. Such elongation of the B-
N bond weakens the interaction between the BF2 fragment and hydrazine nitrogen atom and
facilitates formation of 7 from BOPHY 6. Analysis of the Mülliken charges in BOPHY 6 is also
correlate well with geometric changes in this molecule. Indeed, the cyano group bearing bridging
carbon in BOPHY 6 has a significantly larger positive charge (+0.461) compared to the same
carbon atom in BOPHY 4 (+0.109), while weakening of the B-N(hydrazine) bond in 6 results in
more negative charge at this atom (-0.349) compared to that in 4 (-0.260). Elimination of the BF2
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fragment from BOPHY 6 leads to formation of 7 in which the charge at the bridging carbon atom
is significantly reduced (+0.292) compared to that in 6 (+0.461). Thus, DFT predicts that the
cyanation of the BOPHY core at the bridging position leads to a substantial depletion of the
electron density at the carbon atom, with elongation thus weakening of the B-N(hydrazine) bond
that facilitates extrusion of the BF2 fragment from BOPHY 6 and formation of the “half-BOPHY”
compound 7.
0.3
1.0
Exp.
Exp.
0.5
0.0
0.0
0.5
TDDFT
TDDFT
0.8
0.4
0.0
0.8
0.4
0.0
0.5
0.0
0.0
300
400
500
600
300
400
500
600
700
800
Wavelength, nm
Wavelength, nm
0.20
Exp.
0.15
0.10
0.05
0.00
TDDFT
0.8
0.4
0.0
0.5
0.0
300
400
500
600
Wavelength, nm
Figure 10. Experimental (top) and TDDFT-predicted (bottom) UV-vis spectra of 4, 6, and 7.
TDDFT-predicted UV-Vis spectra of 4, 6, 7, 12, and 13 in comparison with the experimental data
are shown in Figure 1. In the case of symmetric organic BOPHY 4, the TDDFT-predicted spectrum
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is dominated by a single intense band, which has * character and dominated by the HOMO-
LUMO single-electron excitation. In agreement with experiment, TDDFT calculations predict that
the asymmetric organic systems 6 and 7 will also have one intense low-energy band formed by the
predominant HOMO-LUMO single-electron excitation. This band, however, in 6 and 7 is
complemented by several lower intensity excited states predominantly formed by the HOMO-1,
HOMO-2 → LUMO single-electron excitations, which explains the broader nature of the low-
energy band experimentally observed for compounds 6 and 7. In case of the organometallic
BOPHYs 12 and 13, the lowest-energy band has MLCT character and dominated by the HOMO
→ LUMO single-electron excitation. In addition to this band, TDDFT correctly predicts that the
most intense band in the UV-vis spectra of 12 and 13 should be formed by predominant
contribution from the HOMO-2 → LUMO (12) or HOMO-4 → LUMO (13) single-electron
excitations and thus have * character. Overall, TDDFT calculations correctly predict trends in
energies and intensities of the major transitions observed in compounds 4, 6, 7, 12, and 13 and
correlate well with the experimental data.
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Figure 11. Experimental (top) and TDDFT-predicted (bottom) UV-vis spectra of 12 and 13.
Conclusions
In this paper, we have proposed a synthetic strategy for preparation of BOPHYs with electron-
withdrawing ester groups appended at the -pyrrolic positions of BOPHY core. It has been shown
that the methyl groups in new BOPHYs are very reactive and can be involved in condensation
reactions with ferrocenecarboxaldehyde resulting in mono- and diferrocene-containing BOPHYs.
Our attempts to insert a cyano group at the bridging carbon atom of BOPHY core were
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unsuccessful, but in one case, we were able to purify and crystallographically characterize a unique
“half-BOPHY” system in which the BF2 fragment adjacent to the cyano group was extruded from
the core chromophore. DFT and TDDFT calculations on all target materials correlate well with the
experimental data and are suggestive of the dramatic reduction of the nitrogen atom basicity at the
hydrazine bridge of BOPHY upon introduction of the cyano group at the bridging carbon atom.
Such reduction of basicity is primarily responsible for a low stability of cyanated BOPHY core.
Experimental part
Reagents and Materials
Solvents were purified using standard approaches: toluene was dried over sodium metal, DCM
was dried over calcium hydride. Ferrocene carboxaldehyde was purchased from Sigma Aldrich,
pyrrole 1 [13] and BODIPY derivative 11 [7a] were prepared as described earlier.
Spectroscopy Measurements
A Varian CARY 50 UV-Vis spectrophotometer was used to collect UV-Vis data. Electrochemical
cyclic voltammetry (CV) and differential pulse voltammetry (DPV) measurements were conducted
using a CH Instruments electrochemical analyzer utilizing a three-electrode scheme with platinum
working, auxiliary and Ag/AgCl reference electrodes. DCM was used as solvents and 0.1 M
solution of tetrabutylammonium perchlorate (TBAP) and 0.05M tetrabutylammonium
terakis(pentafluorophenyl)borate (TBAF) were used as supporting electrolytes.
Spectroelectrochemical experiments on 12 and 13 were conducted in DCM/0.15M TBAF system.
Taking into consideration broad nature of the first oxidation wave in 13, its oxidation was carefully
monitored by changing potentials in 25 mV increments. In all cases, experimental redox potentials
were corrected using decamethylferrocene (Fc*H) as an internal standard. NMR spectra were
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recorded on a Bruker Avance instrument with a 300 MHz frequency for protons and 75 MHz
frequency for carbons. Chemical shifts are reported in parts per million (ppm) and referenced to
tetramethylsilane (Si(CH3)4) as an internal standard. High-resolution mass spectra of all
compounds were recorded using a Bruker micrOTOF-QIII. Varian CARY 50 UV-Vis
spectrophotometer was used to collect UV-Vis data. Fluorescence spectra were recorded on a Cary
Eclipse spectrometer. Quantum yields were determined using BOPHY molecule as a reference
[10] and calculated according to the following equation:
2
퐴 퐷 휂
푠 푠
푟
훷 = 훷
s
r
2
퐴 퐷 휂
푟 푟
푠
Where 훷 is the reported quantum yield of the reference, D is the integrated emission spectra, A
r
is the absorbance at the excitation wavelength and η is the refractive index of the solvent used.
X-ray Crystallography
Single crystals suitable for X-ray diffraction experiments of dyes 7 and 9 were prepared
through the slow evaporation of saturated isopropanol and CHCl3/MeOH solutions, respectively.
Data collection and refinement details are provided below.
Compound 7: An orange, multi-faceted crystal of suitable size (0.170 x 0.140 x 0.100 mm) and
quality was selected from a representative sample of crystals of the same habit using an optical
microscope and mounted onto a MiTiGen loop. X-ray data were obtained on a Bruker D8 QUEST
ECO CMOS diffractometer (Mo sealed X-ray tube, K = 0.71073 Å) at 150 K. All diffractometer
manipulations, including data collection, integration and scaling were carried out using the Bruker
APEX3 software suite.[14a] An absorption correction was applied using SADABS.[14a] The space
group was determined on the basis of systematic absences and intensity statistics and the structure
was solved by direct methods and refined by full-matrix least squares on F2. The structure was
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solved in the triclinic space group P-1 using XS[14b] (incorporated in SHELXTL). No obvious
missed symmetry was reported by PLATON.[14c] All non-hydrogen atoms were refined with
anisotropic thermal parameters. Hydrogen atoms were places in idealized positions and refined
using a riding model. Disordered molecules of the crystallization solvent (iPrOH) were found in
the lattice, but could not be adequately modelled and so was removed using the SQUEEZE
protocol,[14c] with the results appended to the CIF. A solvent accessible void of 240 Å3 was
calculated with 52 e- found in the void, consistent with ~1.5 molecules of iPrOH. The structure
was refined (weighted least squares refinement on F2) and the final least-squares refinement
converged to R1 = 0.0477 (I > 2σ(I), 9682 data) and wR2 = 0.1198 (F2, 39146 data, 392 parameters).
Compound 13: An dark purple-red, multi-faceted crystal of suitable size (0.150 x 0.080 x 0.030
mm) and quality was selected from a representative sample of crystals of the same habit using an
optical microscope and mounted onto a MiTiGen loop. X-ray data were obtained on a Bruker D8
QUEST ECO CMOS diffractometer (Mo sealed X-ray tube, K = 0.71073 Å) at 150 K. All
diffractometer manipulations, including data collection, integration and scaling were carried out
using the Bruker APEX3 software suite.[14a] An absorption correction was applied using
SADABS.[14a] The space group was determined on the basis of systematic absences and intensity
statistics and the structure was solved by direct methods and refined by full-matrix least squares
on F2. The structure was solved in the triclinic space group P-1 using XS[14b] (incorporated in
SHELXTL). No obvious missed symmetry was reported by PLATON.[14c] All non-hydrogen atoms
were refined with anisotropic thermal parameters. Hydrogen atoms were places in idealized
positions and refined using a riding model. One molecule of the crystallization solvent (CHCl3)
was found present in the lattice, disordered about a special position. The structure was refined
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(weighted least squares refinement on F2) and the final least-squares refinement converged to R1
= 0.0467 (I > 2σ(I), 5731 data) and wR2 = 0.1176 (F2, 31262 data, 335 parameters).
Computational Details.
All computations were preformed using the Gaussian 09 software package running under UNIX
OS.[15] The BOPHYs 4 and 13 were optimized in Ci point group, while other compounds were
optimized in C1 point group. In all calculations the TPSSh hybrid exchange-correlation
functional[16] was used along with Wachters full-electron[17] basis set for the iron atom(s) and the
6-311G(d)[18] basis set for all other atoms for all calculations. Solvation effects were modeled by
the PCM approach[19] using DCM for the solvent in all calculations. Frequencies were calculated
for all optimized geometries in order to ensure that final geometries represent a minima on the
potential energy surface. PCM-TDDFT calculations were conducted for the first 30 (organic
molecules) or 50 (organometallic compounds) excited states. Molecular orbital contributions were
compiled from single point calculations using the program QMForge.[20]
Synthesis.
5-Formyl-2-methyl-4-phenyl-1H-pyrrole-3-carboxylic acid ethyl ester (2).
The formylation of pyrrole 1 was carried out by modified to previously described procedure.[21]
POCl3 (10.1 g, 65.8 mmol, 1.1 eq) was added dropwise into DMF (14 mL, 3 eq) solution in an ice
bath, and stirred for 30 min at 0oC. Then the solution of pyrrole 1 (13.7 g, 59.8 mmol, 1 eq) in
45mL of DMF was added, and the resulting mixture was left to stir for another 15 minutes. The
resulting mixture is then heated in an oil bath at 85oC for 1.5 h. The solution was then cooled down
to room temperature, diluted with ice water (50m L), and neutralized with K2CO3. The resulting
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precipitate was filtered off and dried. The crude product was recrystallized from acetonitrile,
1
yielding 11.6 g (75%) of compound 2. H NMR (300 MHz, DMSO-d6) 12.54 (s, 1H), 9.13 (s,
1H), 7.38 (s, 5H), 4.04 (q, JH,H = 7.1 Hz, 2H), 2.49 (s, 3H), 1.00 (t, JH,H = 7.1 Hz, 3H).
Compound 3: The mixture of pyrrole aldehyde 2 (900 mg, 3.5 mmol) in 20 mL of ethanol was
heated at 60 oC until the precipitate dissolved. Then hydrazine monohydrate (105 mg, 2.1 mmol)
and a catalytic amount of glacial acetic acid were added to the reaction flask. The resulting mixture
was stirred at room temperature for 2 hours. The resulting yellow precipitate was filtered off,
washed with a small amount of ethanol, and air dried. Yield 680 mg (75%). 1H NMR (300 MHz,
CDCl3): 9.09 (s, 2H), 8.11 (s, 2H), 7.37-7.30 (m, 10H), 4.11 (q, JH,H = 7.1 Hz, 4H), 2.58 (s, 6H),
13
2.33 (s, 6H), 1.07 (t, JH,H =7.1 Hz, 6H); C NMR (75 MHz, CDCl3):150.08, 139.63, 133.75,
133.13, 130.67, 127.61, 127.32, 124.12, 59.63, 14.24, 14.10; HRMS (APCI positive) calcd for
C30H30N4O4 [M + H]+: 511.2340, found 511.2443.
Compound 4: Schiff base 3 (3.5 g, 6.86 mmol) was dissolved in BF3 Et2O (25 mL). DIEPA (14.4
mmol, 1.85 g, 2.51 mL) was then added dropwise into the solution. The resulting mixture was
stirred for 1 h at 80 oC in an oil bath. After cooling to room temperature the solution was slowly
added into cold methanol solution (30 mL) and stirred for another 30 min. The resulting precipitate
was filtered as fluorescent orange powder to give 3.30 g (Yield 80 %) of pure product 4. 1H NMR
(300 MHz, CDCl3):  7.85 (s, 2H), 7.48-7.33 (m, 10H), 4.18 (q, JH,H = 7.1 Hz, 4H), 2.80 (s, 6H),
1.08 (t, JH,H = 7.1 Hz, 6H); 13C NMR (75 MHz, CDCl3): 163.64, 154.18, 146.81, 139.98, 131.22,
130.26, 129.29, 128.29, 122.91, 119.72, 60.47, 14.34, 13.87; HRMS (APCI positive) calcd for
C30H28N4O4B2F4 [M + H]+: 606.2316, found 606.2431.
Compound 5: Compound 4 (3.6 g, 5.94 mmol) was dissolved in dry DCM (40 mL). Then KCN
(8.9 mmol, 580 mg) and 18-crown-6 ether (11.88 mmol, 3.13 g) were added to the solution. The
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resulting mixture was stirred for 30 min until fluorescence disappeared and the solution became
colorless. Then hexane (80 mL) was added to the solution and left overnight. After 12 h the
solution was decanted and the resulting white oil was washed with hexane, and dried to give 2.99
g (Yield 80%) of pale gray crystals of compound 5. 1H NMR (300 MHz, DMSO-d6):  7.46-7.20
(m, 11H), 5.30 (s, 1H), 4.07-3.96 (m, 4H), 3.55 (s, 24H), 2.56 (s, 3H), 2.52 (s, 3H) 1.03-0.95 (m,
6H);
13C
NMR
(75
MHz,
CDCl3):

Attempted preparation of BOPHY dye 6: To the solution of 5 (1.3 g, 1.43 mmol) in THF (20
mL), bromine (5.72 mmol, 0.3 mL) was added dropwise. The resulting mixture was stirred for 20
minutes at room temperature. Then the solution was dumped into ice water (40 mL) and stirred for
30 min. Then the resulting precipitate was filtered and dried. The crude product was purified by
column chromatography on silica gel using DCM as the eluent to give 398 mg (Yield 47%) of
compound 4 and 442 mg (Yield 53%) of compound 7.
BOPHY 7: 1 H NMR (300 MHz, CDCl3): 11.09 (s, 1H), 7.71 (s, 1H), 7.49-7.30 (m, 10H), 4.17
(q, JH,H = 7.1 Hz, 2H), 4.07 (q, JH,H =7.1 Hz, 2H), 2.75 (s, 3H), 2.71 (s, 3H), 1.07 (t, JH,H = 7.1 Hz,
13
3H), 0.95 (t, JH,H =7.1 Hz, 3H); C NMR (75 MHz, CDCl3): 164.29, 163.16, 149.95, 148.64,
143.45, 137.86, 132.76, 130.98, 130.48, 129.46, 128.64, 128.31, 127.70, 122.83, 117.07, 112.86,
60.66, 59.92, 15.57, 13.93, 13.81; HRMS (APCI positive) calcd for C31H28N5O4BF2 [M + H]+:
584.2281, found 584.2448.
BOPHYs 12 and 13: The mixture of 4 (400 mg, 0.66 mmol), ferrocenecarboxaldehyde (353 mg,
1.65 mmol), piperidine (28 0mg, 3.3 mmol), and acetic acid (396 mg, 6.6 mmol) in dry toluene
(20 mL) was refluxed for 5 h. After cooling to room temperature, the solution was washed with
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