Expanded hexapyrrolohexaazacoronenes. Near-infrared absorbing chromophores with interrupted peripheral conjugation

Article abstract of DOI:10.1021/ja508963v

A family of azacoronenes containing up to two saturated bridges at the periphery was synthesized from substituted hexapyrrolylbenzenes using a two-step condensation-aromatization procedure. The introduction of peripheral bridges provides access to nonplanar, sterically crowded systems that display complex reactivity patterns, involving stereospecific aromatization of bridges and nucleophile additions. Despite the interrupted conjugation on the periphery, the new azacoronenes have easily accessible higher oxidation levels, and a quadruply charged species was chemically generated by reaction with SbCl₅. These oxidized species show extensive π-electron conjugation and are efficient UV-vis-NIR absorbers, active up to ca. 2400 nm. Interruption of peripheral conjugation is shown to induce a tendency toward biradicaloid electron configurations in doubly oxidized species.

Full text of DOI:10.1021/ja508963v

Article
pubs.acs.org/JACS
Expanded Hexapyrrolohexaazacoronenes. Near-Infrared Absorbing
Chromophores with Interrupted Peripheral Conjugation
Elzbieta Gonka, Piotr J. Chmielewski, Tadeusz Lis, and Marcin Stępien*
́
́
̇
Wydział Chemii, Uniwersytet Wrocławski, ul. F. Joliot-Curie 14, 50-383 Wrocław, Poland
S
* Supporting Information
ABSTRACT: A family of azacoronenes containing up to two
saturated bridges at the periphery was synthesized from
substituted hexapyrrolylbenzenes using a two-step condensa-
tion−aromatization procedure. The introduction of peripheral
bridges provides access to nonplanar, sterically crowded systems
that display complex reactivity patterns, involving stereospecific
aromatization of bridges and nucleophile additions. Despite the
interrupted conjugation on the periphery, the new azacoronenes
have easily accessible higher oxidation levels, and a quadruply
charged species was chemically generated by reaction with
SbCl5. These oxidized species show extensive π-electron
conjugation and are efficient UV−vis−NIR absorbers, active
up to ca. 2400 nm. Interruption of peripheral conjugation is
shown to induce a tendency toward biradicaloid electron configurations in doubly oxidized species.
Scheme 1. Hexapyrrolohexaazacoronene (HPHAC, H0B6)
and Its Expanded Analogs Described in This Work
INTRODUCTION
■
a
Large polycyclic aromatic hydrocarbons (PAH’s) are among the
most important research targets in organic materials chemistry
because of their role as discrete graphene models (nano-
graphenes, graphene nanoribbons), characterized by precisely
defined structures and functionalizable peripheries. Seminal
advances in the field are associated with the synthetic chemistry
of hexa-peri-benzocoronene (p-HBC), which opened the path
to extremely large graphenoid molecules.1−3 This large body of
work inspired new research directions, for example, focused on
cata-fused HBC isomers and related systems.4−10 Nano-
graphene structures can be constructively modified by “doping”
the polycyclic network with heteroatoms, which, in the case of
bottom-up syntheses, can be achieved with a high degree of
structural precision. Diverse fused polycyclics containing
heteroaromatic five-membered (N,11−20 P,21 S10,22−29) and
six-membered rings (B,26,30,31 N,30−37 O,34,35 S34,35) have been
reported, showing that the inclusion of heteroatoms can
significantly alter MO energy levels, producing distinct effects
on the absorption spectra and redox chemistry of these systems.
Hexapyrrolohexaazacoronenes11,38 (HPHACs, denoted
H0B6 in Scheme 1) and the related HPHAC−HBC hybrids12
bear a structural and synthetic similarity to HBCs, because they
can be likewise prepared from star-shaped precursors, namely,
substituted derivatives of hexapyrrolylbenzene39 (H0B).
HPHAC possesses two chemically accessible oxidation levels
(radical cation and dication), which are characterized by strong
electronic absorptions extending into the near-infrared.11 At the
dication level, the HPHAC ring system can be viewed as a
nonmacrocyclic relative of cyclo[6]pyrrole,40 with which it
shares an analogous peripheral conjugation circuit. Similar
a
The ring systems are labeled as HmBn, wherein m and n denote,
respectively, the number of methylene/methine bridges and the total
number of α−α bonds and sp2 bridges (n = 0 is omitted).
properties have been described for the HPHAC−HBC hybrids,
for which the effect of juxtaposing pyrrole and benzene rings in
a single structure has been systematically explored.12
Despite their potential as precursors to large fused
heteroaromatics, the synthetic chemistry of hexapyrrolylben-
zenes has remained relatively unexplored, being currently
limited to the oxidative coupling reaction.11,38 Here we report
on the synthesis of a family of expanded HPHAC analogues
(H1B5 and H2B4, Scheme 1), which are prepared in a two
step reaction sequence that includes an acid-catalyzed bridging
step followed by oxidative fusion. We show that the peripheral
bridges are surprisingly resistant to oxidative dehydrogenation
in an unusually stereospecific manner. When dehydrogenated,
Received: September 7, 2014
© XXXX American Chemical Society
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the bridge provides a reactive site for nuclephilic addition.
Chemical and electrochemical oxidation of the expanded
azacoronenes reveals their utility as multilevel optical redox
switches active over a wide range of wavelengths.
this solvent, the rates of first and second bridging are
comparable, and consequently, a selective synthesis of the
intermediate H1Ba is not possible even with a stoichiometric
amount of the aldehyde. The selectivity of the first bridging is
however improved in dichloromethane, and H1Ba was isolated
in 43% yield from a reaction carried out in the latter solvent
(1:1 aldehyde to H0Ba ratio). In structural terms, the
condensation between H0B and aldehyde resembles the
synthesis of dipyrromethanes from monopyrroles,43 except
that in the present case, it becomes a ring-forming process and
occurs in a sterically congested environment. As a possible
consequence of these steric restrictions, the formation of the
doubly bridged species effectively proceeds in a regio- and
stereoselective manner, trans-H2Ba being the only isolable
product containing two aldehyde equivalents.
RESULTS AND DISCUSSION
■
Synthesis. The starting hexapyrrolylbenzene H0Ba was
prepared via a nucleophilic aromatic substitution reaction
between 3,4-di(4-butoxyphenyl)pyrrole19,41,42 and hexafluoro-
benzene, following an established protocol,11,39 and charac-
terized using NMR spectroscopy and mass spectrometry (see
the Supporting Information). In analogy to the previously
reported hexapyrrolylbenzene derivatives bearing electron
withdrawing groups,11 compound H0Ba was oxidized to the
corresponding HPHAC derivative H0B6a in the presence of
FeCl3. Compound H0Ba reacts with p-nitrobenzaldehyde to
yield consecutively two bridged species: H1Ba and H2Ba
(Scheme 2). Out of several common Brønsted and Lewis acids
Solution structures of H1Ba and H2Ba were unambiguously
1
confirmed by means of H and 13C NMR spectroscopy. In the
1H NMR spectrum of H1Ba (CDCl3, 330 K), the key signal of
the unique sp3 bridge is present at 5.93 ppm and is associated
with a 13C signal at 37.8 ppm via a 1JCH coupling (HSQC). The
1H NMR spectrum contains three singlets corresponding to α-
pyrrole protons, with a relative intensity of 2:4:4 (6.53, 6.47,
6.42 ppm). The latter two of these signals are significantly
broadened, indicating that the rotation of the unbridged pyrrole
groups about the C−N is partly hindered (rings B and C,
Scheme 2). At 190 K, the rotation of pyrroles and most of the
p-butoxyphenyl groups becomes slow on the NMR time scale,
resulting in a complicated spectral pattern. The p-nitrophenyl
group is still represented by only two signals, indicating that it
is oriented perpendicular to the molecular plane of symmetry
or that it rotates (or librates) with a very low energy barrier.
a
Scheme 2. Reactions of Hexapyrrolylbenzene H0Ba
1
The H NMR spectrum of trans-H2Ba (CDCl3, 300 K) is
noticeably simpler, reflecting the higher effective symmetry of
the molecule (C2h vs Cs in H1Ba). The benzylidene bridge
signal is present at 5.86 ppm, and it yields a HSQC correlation
1
with the bridge carbon at 37.5 ppm. The H NMR spectrum
contains two α-pyrrole signals of equal intensity (6.27 and 6.10
ppm), neither of which splits at low temperatures. This
observation is consistent with the equivalence of α protons in
pyrrole ring B and provides a strong indication of the C2h point
symmetry of the molecule, corresponding to the trans isomer of
H2Ba.
a
Reagents and conditions: (1) p-Nitrobenzaldehyde (1−8 equiv), BF3·
Et2O (4 equiv), CHCl3/EtOH or CH2Cl2, 1 h; (2) (i) FeCl3 (36
equiv), MeNO2, DCM, 2 h; (ii) NaCl(aq); (iii) N2H4(aq); (3) (i)
DDQ (6 equiv), DCM; (ii) H2O; (iii) N2H4(aq).
The stereochemistry of trans-H2Ba was additionally
confirmed in the solid state by means of an X-ray crystallo-
graphic analysis (Figure 1). The p-nitrophenyl substituents are
in the trans configuration inferred from the NMR data and are
located above and below the benzene core, forming an angle of
ca. 60° with the mean plane of the ring. The orientation of
pyrrole rings A relative to the benzene plane is fixed by the
presence of bridges (torsional angles of 40.2° and 47.7° for the
symmetry-independent part of the molecule), whereas the free
pyrrole ring B forms a corresponding torsion of 61.7°.
Oxidative Coupling. Oxidative coupling of the adjacent
pyrrole subunits was easily induced by reacting H1Ba with 2,3-
dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) at room tem-
perature. After reductive workup with aqueous hydrazine and
further purification steps, the expanded HPHAC analog H1B5a
was isolated in 71% yield (Scheme 2). The structure of H1B5a
differs from the generic HPHAC motif by the presence of a p-
nitrobenzylidene bridge that is inserted between one pair of
tested as potential catalysts of the above reaction, only
trifluoromethanesulfonic acid and BF3·Et2O offered sufficient
activation of the reactants at ambient conditions, with the latter
catalyst providing higher reaction yields. p-Nitrobenzaldehyde
was selected as the condensation reagent because it showed
sufficient reactivity toward H0Ba while providing good bridging
selectivity. Additionally, the downfield shifted signals of p-
nitrophenyl groups could be used diagnostically in the analysis
1
of H NMR spectra. The electron donating butoxyl groups
placed on the aryl substituents in H0Ba were found to be
essential for the appropriate activation of the pyrroles toward
electrophilic substitution, while also improving the solubility of
the products.
The choice of solvent was also found to affect the selectivity
of the bridging reaction. Highest yields of H2Ba (30%) were
obtained for condensations performed with 8 equiv of p-
nitrobenzaldehyde in chloroform containing 1% of ethanol. In
1
neighboring pyrrole rings. In the H and 13C NMR spectra
(CDCl3, 330 K), the bridge is represented by signals at 5.97
and 38.6 ppm, respectively. The effective Cs symmetry of the
B
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Figure 1. Molecular structure of trans-H2Ba, obtained from an X-ray
structural analysis. The hexapyrrolylbenzene core, benzylidene bridges,
and peripheral substituents are shown in blue, pink, and gray,
respectively. Solvent molecules and disordered groups are omitted for
clarity.
Figure 2. DFT-optimized geometries of the principal oxidation
products. The b substitution pattern used in calculations corresponds
to R = Ph, R′ = p-nitrophenyl. The conformation of H1B5b is based
on an X-ray crystal structure of H1B5a (see the Supporting
Information).
molecule is reflected by the presence of six types of
nonequivalent p-butoxyphenyl groups, all of which rotate
rapidly at 330 K.
Expanded azacoronene systems described in this work
showed a limited tendency to form diffraction quality crystals,
which could be ascribed to their unfavorable packing properties
and, occasionally, limited stability. Even when diffraction data
were obtained (as in the case of H2Ba), severe disorder of the
butoxy substituents affected the overall quality of geometrical
parameters. Structural features of the new condensed systems
were therefore probed using a robust combination of solution
NMR spectroscopy and gauge invariant atomic orbitals density
functional theory (GIAO−DFT) calculations. Computational
modeling of these systems was however complicated by their
size and conformational flexibility. To speed up the analysis
process and simplify conformational searches, p-butoxy groups
were omitted from the DFT models (b substitution pattern, R
= Ph, R′ = p-nitrophenyl), because they were unlikely to make a
significant impact on the conformational preferences of these
systems. GIAO shifts for the b-substituted structures could be
correlated with the experimental values as described in the
Supporting Information. The DFT-optimized geometry of
H1B5b, based on an X-ray crystal structure of H1B5a (Figure
S41 in the Supporting Information), is shown in Figure 2. The
H1B5 core exhibits a slight saddle-like distortion, which
partially alleviates the steric congestion of peripheral sub-
stituents. Importantly, the benzylidene bridge is tilted above the
average plane of the core atoms, with the R′ substituent
positioned above the surface of the fused ring system. This
orientation of the R′ group, which is typically preferred for
steric reasons, will be denoted endo in subsequent discussion.
Under optimized reaction conditions, the oxidation of trans-
H2Ba with DDQ (5 equiv), followed by acidic workup, yielded
a monocationic species, [H2B5a]+, which was isolated in 76%
yield in the form of a tetrafluoroborate salt (Scheme 3). The
ionic nature of [H2B5a][BF4] was reflected by its good
solubility in polar solvents such as acetonitrile, low mobility on
TLC plates, and finally, mass spectrum (m/z 2502.22). The
oxidation of trans-H2Ba is a stepwise process, and an
intermediate, denoted trans-H2B2a, was consistently identified
in partly reacted mixtures (Scheme 3) and was separated by
means of semipreparative thin-layer chromatography. The
compositions of crude reaction mixtures obtained at various
oxidation stages, indicated that the DDQ-induced dehydrogen-
ation of an sp3 bridge, leading to [H2B5a]+, occurred as the
ultimate step. On this basis, it was assumed that intermediate
α−α couplings are stereospecific, that is, the H2B2a species
retains the trans configuration of the starting material. An
additional intermediate, later identified as trans-H2B4a in an
independent synthesis (see below), was occasionally observed
in minute amounts among products of H2Ba oxidation,
providing further support for the proposed stereospecificity.
The solution structure of the [H2B5a]+ cation was
investigated using variable temperature NMR spectroscopy.
1
At 190 K, the H NMR spectrum of [H2B5a]+ is consistent
with an effective Cs molecular symmetry, with six identifiable
ABCD spin systems corresponding to p-butoxyphenyl sub-
stituents. The complete assignment of these signals was
achieved in the course of further analysis (Figure S11,
Supporting Information). The key feature of the spectrum,
however, is the presence of two nonequivalent p-nitrophenyl
substituents, which give rise, respectively, to ABCD and
AA′BB′ spin systems. This differentiation is consistent with
the ABCD substituent lying in the molecular symmetry plane
and the AA′BB′ substituent being placed orthogonally to that
plane. The latter substituent is attached to the unique sp3
bridge (δ(1H) 5.79 ppm, δ(13C) 38.4 ppm). In the HMBC
3
spectrum, ortho protons of the ABCD substituent yield J
correlations to a carbon resonating at 136.3 ppm, which was
unambigously identified as the other trigonally hybridized
bridge. The observed 13C shift indicates that the bridge does
not have a significant carbocationic character, which is expected
in view of the extensive charge delocalization in the [H2B5a]+
cation.
A DFT structural model of the monocation was developed
from the above spectroscopic data, using a simplified
substitution pattern ([H2B5b]+, R = Ph, R′ = p-nitrophenyl).
The structure shown in Figure 2 is the lowest-energy geometry
found in a restricted conformational search. If rapid libration of
the R groups is assumed, the structure is consistent with the
observed spectral symmetry and dipolar coupling pattern found
in the low-temperature ROESY spectrum of [H2B5a]+. In
C
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a
Scheme 3. Reactivity of trans-H2Ba towards Oxidants and Nucleophiles
a
In all systems, the trans and cis descriptors reflect the relative orientation of the R′ substituents.
particular, the sp2-bound p-nitrophenyl substituent is located
between the flanking R groups, although a perfect in-plane
alignment of the axes of these three substituents is apparently
not feasible sterically. Neverthless, partial interlocking of these
are indeed observed in the proton spectrum. The “5 + 1”
structure of trans-H2B2a was determined by means of a
1
1H−13C HMBC experiment, which revealed simultaneous J
3
and J coupling between one of the α-pyrrole protons and the
1
three groups leads to characteristic H shifts of the “eclipsed”
corresponding ipso-C signal (Figure S16, Supporting Informa-
tion). Such a coupling pattern is only possible for a doubly α-
unsubstituted pyrrole ring, which is only present in the “5 + 1”
ortho and meta protons (upfield of 6 ppm). The rotation of the
flanking R groups is significantly hindered and the upfield
signals can be readily observed at room temperature, providing
a useful diagnostic feature. In comparison with H1B5a, the
presence of an additional aryl group on the periphery results in
a more pronounced saddle distortion of the fused H2B5 core.
The sp3-bound p-nitrophenyl group in [H2B5a]+ adopts the
endo orientation (above the plane of the H2B5 ring). The
1
isomer. H chemical shifts of trans-H2B2a are temperature
dependent in the limit of fast substituent rotation, indicating
that the observed spectrum is averaged over several conformers.
No attempt was therefore made to correlate the experimental
shifts with GIAO data.
Reactivity of [H2B5a]+ toward Nucleophiles. In initial
experiments, the oxidative coupling of trans-H2Ba was
performed using either DDQ or FeCl3, and the reactions
were quenched by adding dilute aqueous hydrazine. This
approach invariably resulted in mixtures of products and low
reproducibility. It was subsequently found that the observed
chemistry involved initial addition of available nucleophiles
(e.g., water, chloride, hydrazine) to the sp2 bridge of the
[H2B5a]+ cation. This reactivity was explored in detail using
purified [H2B5a][BF4] and well-defined nucleophile sources
(Scheme 3). In the presence of excess methanol and anhydrous
Na2CO3, [H2B5a]+ quantitatively underwent a stereoselective
methoxide addition to yield the bis-sp3 species, trans-H2B4a-
OMe. This addition could be reversed by subsequent addition
of trifluoromethanesulfonic acid to the DCM solution of
H2B4a-OMe. Likewise, the reaction of [H2B5a][BF4] with
sodium cyanide or sodium borohydride in THF resulted in the
quantitative formation of trans-H2B4a-CN and trans-H2B4a,
respectively. The latter species was identical to that observed
spectroscopically as an intermediate in the oxidation of trans-
H2Ba to [H2B5a]+.
1
chemical shift pattern observed in the H NMR spectrum of
[H2B5a]+ could be satisfactorily correlated with the GIAO data
obtained for [H2B5b]+ as described in the Supporting
Information.
The [H2B5b]+ cation contains a fully conjugated seven-
membered ring, which is a structural feature rarely seen in
polycyclic aromatics of comparable size. The presence of this
ring induces extensive cross conjugation in the pyrrole rings,
which is reflected in the optical properties of the system. The
cation has a deep violet color in solution and exhibits a rich
absorption spectrum extending to ca. 1300 nm into the infrared
(Figure 4B, violet trace). The strongest absorptions are located
in the UV region (λmax = 279, 337 nm, ε ≈ 9.5 × 104 M−1
cm−1) and are followed in intensity by the principal NIR band
(λmax = 1081 nm, ε = 2.28 × 104 M−1 cm−1) and several bands
in the visible region (λmax = 520, 582, 667 nm, ε = (1.11−2.26)
× 104 M−1 cm−1).
1
The H NMR spectrum of trans-H2B2a is consistent with
two isomeric structures of 2-fold symmetry: the “5 + 1” isomer
with one free pyrrole ring B (effective C2 symmetry, Scheme 3)
and the “3 + 3” isomer with two singly fused rings B (Ci
symmetry). In each case, one expects six sets of p-butoxyphenyl
signals, one set of signals corresponding to the p-nitro-
benzylidene bridge, and two α-pyrrole singlets, all of which
1
The H NMR spectrum of trans-H2B4a-OMe is representa-
tive of other products of nucleophilic addition, and it shares
certain features with the spectrum of the [H2B5a]+ cation,
described above. Complete assignment of all 1H signals
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Figure 3. DFT-optimized geometries of key structures containing the H2B4 core (blue; benzylidene bridges in red). The b substitution pattern used
in calculations corresponds to R = Ph (gray), R′ = p-nitrophenyl. The methoxy group of trans-H2B4b-OMe is shown in green.
(excluding the remote parts of the butyl chains) was achieved at
200 K (Figure S13, Supporting Information). At that
temperature, the rotation of all aryl substituents was slow,
and the spectral pattern corresponded to the effective Cs
symmetry of the molecule. A distinguishing feature of the
spectrum is the signal at 2.89 ppm, which was identified as
belonging to the methoxy group on the basis of its relative
intensity and the HSQC correlation at δ(13C) 51.5 ppm. This
OMe substituent is bound to an sp3 carbon (δ(13C) 77.6 ppm),
which also bears a p-nitrophenyl group. The latter substituent
yields an ABCD spin system in the 1H spectrum, indicating that
it is locked between the two adjacent aryl groups. Those two
flanking aryls rotate slowly even at room temperature, yielding
a characteristic ABCD spin system with two upfield signals at
5.60 and 5.46 ppm (meta and ortho, respectively, 200 K). The
latter signals are diagnostically useful because they are
consistently observed for all systems containing an exo-R′
substituent. In analogy to the monocation, the other p-
nitrophenyl group, bound to the tertiary sp3 bridge (δ(1H) 5.84
ppm, δ(13C) 37.5 ppm), gave rise to an AA′BB′ spin system. A
dipolar coupling between the o-H protons of the latter
substituent and the OMe group was found in the ROESY
spectrum, providing a direct proof of the trans configuration of
H2B4a-OMe.
temperature spectrum. DFT calculations performed for
H2B4b showed that both endo−endo−trans and endo−exo−
trans structures correspond to stable energy minima of
comparable energy (Figure 3). Calculations using the standard
B3LYP functional predict the endo−endo conformer to be
more stable by 1.4 kcal/mol. For structures optimized using the
dispersion-corrected ωB97XD functional,44 the energy differ-
ence is 2.6 kcal/mol in favor of the endo−exo structure. The
observed discrepancy between these two methods may indicate
the importance of nonbonded interactions between neighbor-
ing aryl substituents in determining the conformational
preferences of the system. Importantly, the symmetry-averaged
GIAO-B3LYP shifts calculated for the endo−exo structure are
in much better agreement with the experimental values
observed in the fast exchange limit at 300 K. The B3LYP
energy difference between the endo−endo−trans and endo−
endo−cis structures of H2B4b is 11.8 kcal/mol, indicating that
the stereoselective hydride addition to [H2B5a]+ is kinetically
controlled and leads to a thermodynamically disfavored
product.
When [H2B5a][BF4] was treated with neat hydrazine
hydrate (ca. 5 equiv) in DCM-d2, the cation was quantitatively
converted into a new species, which was identified as the
hydrazine adduct H2B4a-N2H3 on the basis of low-temperature
1H NMR data. Addition of t-BuOK to the hydrazine adduct did
not alter the proton spectrum, indicating that the observed
species was indeed a neutral substituted hydrazine rather than a
1
trans-H2B4a yields a sharp H NMR spectrum at 300 K
(DCM-d2, 600 MHz), corresponding to an effective C2h
symmetry of the molecule and rapid rotation of all aryl
substituents. However, when the temperature is lowered, all
signals undergo gradual broadening, which indicates a more
complex dynamic process than the simple rotation of aryl
groups. At 170−180 K, the spectrum of trans-H2B4a was
sufficiently well-resolved to reveal the presence of two
nonequivalent benzylidene bridges (5.93 and 5.78 ppm) and
upfield shifted aryl signals characteristic of the exo arrangement
of the bridge. These spectral features were consistent with the
presence of the endo−exo conformer of trans-H2B4a, but
complete signal assignment was not possible because of
extensive signal overlap. At higher temperatures, the
benzylidene bridges move rapidly relative to the molecular
plane, leading ultimately to the fully symmetrical room-
1
hydrazinium cation. The H NMR spectrum of H2B4a-N2H3
unusually corresponds to the effective C1 symmetry even at
room temperature, though this feature is partly obscured by
dynamic broadening due to aryl rotation. At 240 K, two
broadened, scalar-coupled peaks were found at 3.56 and 3.26
ppm, which were tentatively assigned as corresponding to the
−NH− and −NH2 protons, respectively, on the basis of
exchange and ROE correlations (Figure S14, Supporting
Information). The initially formed C1-symmetric hydrazine
adduct underwent a gradual transformation into a new species,
with a spectral pattern reminiscent of trans-H2B4a-OMe
(effective Cs symmetry, upfield aryl signals at 5.72 and 5.64
ppm), and a set of resonances at 4.03 and 2.85 ppm, likewise
E
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Figure 4. Changes of UV−vis−NIR spectra observed during titrations of H1B5a (A), [H2B5a][BF4] (B), cis-H2B4a (C), and trans-H2B4a (D)
with SbCl5. Colored lines and labels indicate approximate ranges corresponding to specific oxidized forms. The NIR regions shown in insets were
recorded in separate experiments.
attributable to the NHNH2 group. It may thus be proposed that
the cis isomer of H2B4a-N2H3 is formed initially as a kinetic
addition product and undergoes subsequent equilibration with
the trans isomer.
The conversion of H2B4a-N2H3 isomers into cis-H2B4a is
proposed to proceed via their aerial oxidation to the respective
diazenes, followed by N2 elimination, analogous to that
occurring in the Wolff−Kishner reaction (Scheme 3). This
process, known to be particularly fast for tertiary RNNH
diazenes,45 is considered to proceed via (1) diazene
deprotonation, (2) abstraction of the dinitrogen molecule,
and (3) the protonation of the resulting carbanionic
The two H2B4a-N2H3 adducts are not stable in solution at
room temperature, gradually decomposing into a new species,
which was identified as the cis isomer of H2B4a. In fact, the
latter compound could be obtained on a preparative scale by
stirring a dichloromethane solution of [H2B5a][BF4] with neat
hydrazine hydrate in air. In contrast to its trans isomer, cis-
H2B4a retains the equivalence of benzylidene bridges in the 1H
NMR spectra down to at least 175 K. A GIAO calculation
performed on the optimized endo−endo conformer yielded
shifts in good agreement with the experimental values obtained
at 185 K (Figures S8 and S32, Supporting Information). The
absence of upfield shifted aryl signals, characteristic of the exo
arrangement of the R′ group (5.4−6.0 ppm for the flanking R
groups), indicates the prevalence of the endo−endo conformer
and provides indirect proof of the cis configuration (at the
B3LYP level, the endo−exo and exo−exo conformers of cis-
H2B4a are predicted to have energies higher by 28.4 and 28.0
kcal/mol, respectively). Thus, both isomers of H2B4a
preferentially adopt conformations that provide the same type
of saddle-like distortion of the aromatic core (endo−exo−trans
and endo−endo−cis, Figure 3).
intermediate.46,47 Diazenes were not detectable by H NMR
1
spectroscopy (literature NNH shifts are ca. 16 ppm48,49), in
agreement with their expected rapid decomposition. In line
with the presumed intermediacy of a diazene, the formation of
cis-H2B4a was accelerated by traces of Cu(II) ions. The trans
and cis isomers of H2B4a are thus formed according to two
distinct mechanisms (carbocationic and carbanionic, respec-
tively), which display perfect and opposite stereoselectivities.
Furthermore, the mechanism of hydrazine-mediated reduction
of [H2B5a]+ is presumably different from the analogous
reductions of the H0B6 radical cation,11 for which nucleophilic
hydrazine addition is less likely to occur.
Chemical Oxidation and Proton Transfer Reactions.
The incorporation of bridges into the peripheries of H1B5a,
cis/trans-H2B4a, and [H2B5a]+ alters the π-conjugation
topology and was expected to influence the oxidation potentials
of these systems and chromophore properties of the higher
oxidation levels. The outcome of these oxidations was difficult
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to predict because of the possible formation of interpyrrolic
methine bridges, for which analogies can be found in
oligopyrrole chemistry. The reactivity of expanded azacor-
onenes was thus systematically explored using a combination of
UV−vis spectroscopy with NMR, ESR, MS, and electro-
chemical measurements. Antimony(V) pentachloride in di-
chloromethane was chosen as the principal reagent because of
its known utility as a chemical oxidant50 and its previous
application in the chemistry of azacoronenes.11,12 While not
typically used for this purpose, SbCl5 is also capable of
methylene-to-methine oxidations.51 It should be noted,
however, that the stoichiometry of oxidations with SbCl5 is
complicated by the tendency of this compound to react with
nucleophiles (including traces of water) and by its ability to
form complex counteranions.50,51 In the experiments summar-
ized below, the oxidizing efficiency of SbCl5 was variable and
concentration-dependent. Consequently, the numbers of SbCl5
equivalents observed in different experiments could not be
directly compared and are presented solely as a measure of
titration progress. The efficiency of SbCl5 in the early stages of
some oxidations, in which it acted as a near two-electron
oxidant, may indicate the involvement of [SbCl6]− as a
secondary oxidizing species.50
Compound cis-H2B4a (ca. 0.01 mM in CH2Cl2) was titrated
with a dichloromethane solution of antimony(V) pentachloride,
and the progress of the oxidation was monitored using UV−
vis−NIR spectroscopy. From this experiment, four consecutive
oxidation events could be deduced (Figure 4C), which were
however incompletely separated and yielded only approximate
isosbestic points. All oxidized species showed non-negligible
absorption at all wavelengths from 300 to ca. 1700 nm. A
separate titration (ca. 0.1 mM cis-H2B4a in CD2Cl2) was
followed simultaneously using 1H NMR (600 MHz, 230 K) and
ESR spectroscopies. During the initial steps of the titration (0−
1
2.0 equiv), the H NMR spectrum of cis-H2B4a broadened
considerably, indicating the formation of paramagnetic species
possibly combined with chemical exchange (Figure S2,
Supporting Information). Radical species were indeed observed
in the ESR spectrum throughout the titration, and the observed
signal intensity reached a maximum at ca. 0.6−0.8 equiv of
SbCl5, followed by a smaller one at ca. 1.6 equiv, and a
minimum at ca. 2.0 equiv (Figure S21, Supporting
Information). Such changes may be thought to represent four
consecutive oxidation events, alternating between para- and
diamagnetic products. Electrospray mass spectra of the partly
oxidized cis-H2B4a enabled direct identification of the radical
cation [cis-H2B4a]•+ and the dication [cis-H2B4a]2+, which
disappeared upon further addition of SbCl5, indicating that the
more highly charged cations might not be detectable under the
ESI conditions. In the 2.0−3.0 equiv range, the room
temperature ESR signal was slowly increasing again, but the
1H NMR spectrum became remarkably well resolved, revealing
the presence of a single form with an effective symmetry
identical to that of the starting compound (C2v, Figure 5). This
The oxidative coupling of H1Ba with DDQ, described above,
produces a deep green reaction mixture prior to addition of
hydrazine, indicating that H1B5a is easily oxidized beyond its
neutral state. Reduction of the green mixture using hydrazine
hydrate diluted with D2O (1:20 volume ratio) results in a
1
H1B5a sample with undiminished intensity of the sp3-CH H
NMR signal, providing an indirect proof that the bridge is not
deprotonated during oxidation. Titration of H1B5a (ca. 4 μM
in CH2Cl2) with SbCl5 revealed three oxidation events in the
absorption spectrum (Figure 4A), the first of which was clearly
separated from the others. Unlike H1B5a, which shows weak
electronic transitions in the visible region, the oxidized forms
are green and exhibit broad overlapping absorptions that cover
the entire visible range. The first of these forms, absorbing up
to ca. 2400 nm, was identified as a radical cation [H1B5a]•+ on
the basis of a strong ESR signal (g = 2.0026) and extreme
1
broadening of the H NMR spectrum observed during the
initial titration steps (0−0.8 equiv SbCl5, ca. 3.4 mM H1B5a in
CD2Cl2). The ESR signal revealed a hyperfine splitting pattern,
which likely reflects the coupling with three nonequivalent pairs
of 14N centers, in accord with the molecular symmetry of the
radical (simulated a values 2.15, 0.95, and 0.45 G, Figure S19,
Supporting Information). We were also able to observe an
analogous coupling to six equivalent 14N nuclei for the
chemically generated [H0B6a]•+ (a = 0.63 G, Figure S18,
Supporting Information). Upon addition of ca. 1.6 equiv of
SbCl5 to H1B5a, the ESR signal almost completely disappeared
Figure 5. 1H NMR spectrum of [H2B4a]4+ generated by in situ
oxidation with SbCl5 (600 MHz, CD2Cl2, 230 K).
1
and a partly resolved H NMR spectrum was observed, which
diamagnetic species corresponded to the fourth oxidation step
observed spectrophotometrically and was accordingly identified
as the tetracation [cis-H2B4a]4+. In the 6−9 ppm range, the 1H
NMR spectrum of [cis-H2B4a]4+ (CD2Cl2, 230 K, Figure 5)
features an AA′BB′ spin system of one p-nitrophenyl group and
three ABCD spin systems corresponding to slowly rotating p-
butoxyphenyl groups. An effective singlet is found at 5.82 ppm,
which yields an HSQC correlation at 41.0 ppm, consistent with
the presence of two equivalent sp3 bridges in the structure. At
higher temperatures, the spectrum of [cis-H2B4a]4+ broadened
significantly, possibly a result of interactions with residual
paramagnetic forms. Because of increasing Coulombic
however remained too broad to enable complete assignment.
Those changes were consistent with the predominance of a
diamagnetic species, which was assumed to be the dication
[H1B5a]2+. Further titration with SbCl5 caused rapid broad-
ening of the 1H NMR spectrum, which was associated with the
development of a new, though significantly weaker, ESR signal.
These changes might be linked to a growing contribution of the
radical trication [H1B5a]•3+. Electrospray mass spectra of the
oxidized mixtures confirmed the presence of a mono and
dication species, providing an indication that the ionization
indeed occurs without deprotonation of the sp3 bridge.
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repulsions, closed-shell tetracations are rarely stable in
contiguously π-conjugated systems,52 and the successful
generation of [cis-H2B4a]4+ is in notable contrast to the
behavior of reported azacoronenes, which were not chemically
oxidized above the dication level.11,12
Scheme 4. SbCl5-Induced Oxidation of cis- and trans-
H2B4a
a
To our surprise, the response of trans-H2B4a to SbCl5
oxidation was completely different from the behavior of the
cis isomer. The initial steps of the titration (0−0.6 equiv of
1
SbCl5, 3.4 mM trans-H2B4a in CD2Cl2), monitored with H
NMR spectroscopy, revealed the concurrent formation of two
diamagnetic products. One of these species, which formed
quantitatively upon addition of ca. 0.6 equiv of SbCl5, was
1
immediately recognized as [H2B5a]+, on the basis of its H
NMR shifts, and was subsequently identified in the ESI
spectrum of the oxidized mixture. The identity of the other
product could not be determined spectroscopically because of
its transient character and broadened 1H NMR spectrum.
However, given its diamagnetic character and gradual
conversion into [H2B5a]+, the transient intermediate can be
reasonably assumed to be the corresponding dication [trans-
H2B4a]2+. A signal corresponding to this dication could be
detected using mass spectrometry (ESI) at an early stage of the
titration. No signals corresponding to radical intermediates
were detectable by ESR in the 0−0.4 equiv range, indicating
that either trans-H2B4a undergoes a two-electron oxidation
with SbCl5 or the intermadiate radical cation [trans-H2B4a]•+
rapidly disproportionates to trans-H2B4a and [trans-H2B4a]2+.
UV−vis−NIR spectroscopic data further confirmed the proton
loss occurring at the initial titration stage. The absorption
profile observed in the 0.5−1.0 equiv range was noticeably
similar to the spectrum of pure [H2B5a][BF4]. Further
oxidation steps performed on [trans-H2B4a] led to spectral
changes analogous to those observed during the titration of
[H2B5a][BF4] (Figure 4B,D). The latter compound undergoes
a two-step oxidation with SbCl5 to [H2B5a]•2+ and [H2B5a]3+,
which were both detectable in the ESI spectrum. At this stage
of titration, no narrowing of the 1H NMR spectrum was
observed, but a significant drop in the intensity of the ESR
signal was observed at ca. 2.0 and above 3.0 equiv of SbCl5. In
the electronic spectrum, the radical dication was identified in an
incompletely separated oxidation step with a discrete band at
ca. 2050 nm, whereas the ultimate trication exhibited a broad
absorption with a maximum at ca. 1310 nm.
The different behavior of cis and trans isomers of H2B4a in
the oxidation reactions is apparently caused by an unusual
stereospecificity of the deprotonation step. This behavior can
be rationalized by assuming that the proton loss from sp3
carbons in H1B5 and H2B4 derivatives is efficient only when
the bridge adopts the exo conformation (Scheme 4). Such a
bridge is only present in trans-H2B4a, enabling the
deprotonation of [trans-H2B4a]2+ to [H2B5a]+. The remain-
ing species of interest, namely, H1B5a, cis-H2B4a, and
[H2B5a]+, only contain endo bridges and the corresponding
exo conformers are too high in energy to be sufficiently
populated at room temperature. The observed stereospecificity
is thought to originate from the different alignment of the
C(sp3)−H bond with respect to the π-electron system. In the
endo conformers, the bond is held in the conjugation plane,
leading to a negligible overlap between the CH bond and the p
orbitals of adjoining sp2 carbons in the transition state (Scheme
4). In contrast, in the exo arrangement, the latter p orbitals are
roughly parallel to the CH bond, providing considerable
resonance assistance during proton abstraction. The congestion
a
Proposed transition state (TS) structures explaining the observed
stereospecificity of proton abstraction are shown in gray boxes.
of aryl substituents around the CH bond is also greater in the
cis conformer, but it is thought to contribute less to the
stereospecificity because the most likely bases participating in
the proton removal (chloride or water) are not sterically
encumbered.
The postulated deprotonation of [trans-H2B4a]2+ leading to
[H2B5a]+ is apparently kinetically controlled and is not
reversed by addition of a protic acid. When trans-H2B4a-
OMe was titrated with trifluoromethanesulfonic acid (TfOH)
in CD2Cl2, it quantitatively released [H2B5a]+. With further
addition of the acid, the monocation underwent subsequent
two-step protonation, which did not involve the interpyrrolic
methine bridge. The protons were found to bind at the pyrrole
α positions adjacent to the sp3 bridge, leading ultimately to the
tricationic species [H2B5a-H2]3+ (Scheme 5), which was
identified in the titration mixture on the basis of 1D and 2D
NMR spectra. Specifically, three tertiary sp3 sites were
identified in the HSQC map recorded at 240 K (denoted a,
b, and c, H/13C shifts of 4.76/71.2, 5.78/76.1, and 6.02/74.2
ppm, respectively). The first of these sites bears a p-nitrophenyl
1
Scheme 5. Protonation of the [H2B5a]+ Cation Using
Trifluoromethanesulfonic Acid
H
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a
Table 1. Electrochemically Determined Redox Potentials
species
H0B6a
Ered1 (ΔE)
Eox1 (ΔE)
Eox2 (ΔE)
Eox3 (ΔE)
Eox4 (ΔE)
Eox5 (ΔE)
Eox6 (ΔE)
b
−309 (60)
−244 (65)
−9 (60)
−50 (90)
186 (60)
−93 (57)
−64 (70)
76 (70)
750 (57)
602 (90)
363 (60)
300 (70)
1018 (90)
985
H1B5a
805 (70)
509 (67)
402 (100)
b
b
cis-H2B4a
632
866
b
trans-H2B4a
[H2B5a][BF4]
70 (70)
970
−741 (70)
482 (80)
a
b
In mV relative to the Fc/Fc+ internal standard. Peak-to-peak separations (mV) are given in parentheses. Anodic scan potential of an irreversible
couple.
substituent and corresponds to the original sp3 bridge in
[H2B5a]+. The 13C shift of this bridge is unusually large,
possibly as a consequence of nearby charge accumulation, but it
was correctly reproduced by a GIAO calculation (72 ppm). The
adjacency of these three sp3 centers was evident from the
events below −1.5 V were observed for bridged systems, likely
corresponding to reductions of the nitro groups on R′
substituents. Additionally, a reversible event at −741 mV was
observed for the [H2B5a]+ cation, which was assigned to a one-
electron reduction of the central π-conjugated system. The
electrochemical HOMO−LUMO gap of [H2B5a]+ (calculated
as Eox1 − Ered1) is only 0.93 V, in good agreement with the
optical gap determined from the NIR spectrum (ca. 0.95 eV at
1300 nm).
DFT Calculations and the Electronic Structure. The
introduction of benzylidene bridges into the HPHAC core has
a distinctive effect on the energies of frontier Kohn−Sham
orbitals (Figure 6; Figure S28, Supporting Information). In
3
observed vicinal couplings (3Jab = 11.2 Hz, Jac = 6.8 Hz),
providing strong evidence for the proposed protonation
pattern. The inequivalence of b and c protons is retained
even at room temperature, lending support to the assumption
that the respective sp3 sites have opposite configurations
relative to the a bridge (cis vs trans) and that the trication lacks
an effective symmetry plane. The spectrum of [H2B5a-H2]3+
was too complex to allow complete signal assignment; however,
the planarity of the other bridge was confirmed by its 13C shift
of 143.7 ppm, determined from a HMBC map. In a DFT
model, obtained in a restricted conformational search (R = Ph,
[H2B5b-H2]3+, Figure S29, Supporting Information), the
HaCCHb and HaCCHc torsions were 156° and 49°,
3
respectively. The corresponding J couplings predicted using
the Bothner-By relationship53 are 11 and 6 Hz, in good
agreement with the experimental values.
Electrochemistry. In line with the results of chemical
oxidation described above, the electrochemical analysis of the
bridged systems, carried out by means of cyclic and differential
pulse voltammetry, revealed multiple oxidation events (Table
1). In each case studied, at least three oxidations were
chemically reversible. The nonreversibility of higher oxidations
was associated with observable deposition of the analyte on the
working electrode. The bridge-free H0B6a was investigated as a
reference system and showed four oxidations, which were
cathodically shifted by 0.2−0.5 V relative to the potentials
reported earlier for its 4-(trifluoromethyl)phenyl analogue.11 In
H1B5a and cis-H2B4a, which were shown not to undergo
proton loss upon oxidation, the introduction of bridges causes a
gradual increase of the first two oxidation potentials (Eox1, Eox2),
which is interestingly accompanied by a decrease of Eox3 and
Eox4. In fact, in the case of cis-H2B4a, two additional oxidations
can be observed electrochemically, while Eox3 and Eox4 become
sufficiently low to enable four-electron chemical oxidation, as
revealed earlier by the observation of the [cis-H2B4a]4+
tetracation. Five oxidations were identified for trans-H2B4a,
at potentials remarkably different from those recorded for the
cis isomer. Both isomers contain topologically identical π-
conjugated systems, indicating that the redox dissimilarities
should result from relatively small conformational distortions of
the H2B4 moiety. Three oxidation events were observed for
the monocationic [H2B5a][BF4], which do not overlap with
any of the higher oxidations of trans-H2B4a. This observation
indicates that under the conditions of voltammetric experi-
ments, proton loss from [trans-H2B4a]2+ does not occur to a
significant extent. In contrast to H0B6a, which could not be
reduced in the accessible potential range (ca. −2.3 V), redox
Figure 6. Frontier orbital energies for the unsubstituted HmBn
azacoronenes (B3LYP/6-311++G(d,p)//6-31G(d,p)).
unsubstituted H0B6c and H1B5c (R = R′ = H), the calculated
energy of the HOMO was almost identical, and it was only
slightly lower in H2B4c. The doubly degenerate (H − 1, H −
2) level in the bridge-free HPHAC is split by symmetry
lowering in the bridged systems, with the upper level increasing
in energy by 0.69 eV on going from H0B6c to H2B4c. This
change leads to an energy difference of only 0.06 eV between
the two upper occupied levels in H2B4c. The gradual decrease
of the energy gap between HOMO and H − 1 may be
proposed to explain the progressively smaller potential
difference between the second and third oxidation measured
for H1B5a and H2B4a. In the bridged systems, the original
LUMO orbital of H0B6c is subject to gradual destabilization,
which, combined with the energy lowering of the degenerate (L
+ 1, L + 2) pair, leads to the reordering of lowest virtual levels.
The HOMO−LUMO gap increases from 2.96 eV in H0B6c to
3.29 eV in H2B4c.
The chromophore properties of expanded azacoronenes were
further probed using time-dependent density functional theory
(TD-DFT). Calculations were performed for variously oxidized
forms of H1B5b, H2B4b, and [H2B5b]+ (see the Supporting
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Information). The use of the simplified substitution pattern in
these calculations (R = Ph) precluded quantitative performance
assessment of the TD methods; however, the general features
of the absorption spectra and semiquantitative comparisons
between theory and experiment could still be made. It was
found that the Tamm−Dancoff approximation (TDA),54
employed at the B3LYP/6-31G(d,p)/PCM(CH2Cl2) level of
theory, yielded reasonably accurate though somewhat blue-
shifted spectra of closed-shell structures. The CAM-B3LYP
functional,55 which does not exhibit the self-interaction error of
B3LYP, offered qualitatively similar results, which however
suffered from a much larger blueshift, in line with a published
benchmark study.56 For neutral bridged azacoronenes H1B5b
and H2B4b, the theory correctly reproduces the ultraviolet
absorption edge in both species, although it predicts a number
of nearly forbidden transitions in the visible region. For the
oxidized forms of H1B5b, strong absorptions are predicted in
both the visible and NIR ranges of the spectrum. The lowest
energy transition of [H2B5b]+ is shown to be a nearly pure
HOMO−LUMO excitation with a predicted wavelength of 899
nm (B3LYP). Further absorptions, present in the visible region,
combine excitations from lower occupied levels to LUMO.
Lower energy absorptions are predicted for the higher
oxidation levels [H2B5b]•2+ and [H2B5b]3+, in line with the
spectral changes induced by oxidation of [H2B5a]+.
energy ordering may depend on subtle structural effects.
Experimental determination of the S−T gap could not be
performed reliably because both the cis and trans dications of
H2B4a were only generated transiently as mixtures with other
paramagnetic species. Nevertheless, the postulated [trans-
H2B4a]2+ was apparently mostly diamagnetic at 230 K, judging
by its well resolved 1H NMR spectrum. The observed decrease
of ESR signal intensity associated with the conversion of [cis-
H2B4a]•+ into [cis-H2B4a]2+ might lead to similar conclusions,
1
although no sharp H NMR spectrum could be observed for
the cis dication at 230 K. The latter observation may indicate
either a sizable triplet contribution or rapid exchange with
differently oxidized paramagnetic species (mono- and trication
radicals).
Time-dependent DFT calculations performed for the closed-
shell singlets (R) reveal strongly red-shifted bands of high
intensity (λ > 1500 nm in [cis-H2B4b]2+), which are absent in
the experimental spectra of [cis-H2B4a]2+ (Table 2, see the
Supporting Information for additional data). This red shift is
apparently not associated with the self-interaction error of
B3LYP because an even larger value is obtained with the CAM-
B3LYP functional. A more realistic result was obtained for the
open-shell singlets (U), for which the low-energy bands (above
1200 nm) are predicted to be weak, in line with the
experimental findings. In the TD-DFT spectra of the U
singlets, these weak bands are characteristically accompanied by
additional forbidden low-energy transitions. The lowest energy
transitions of significant intensity are predicted in the 770−940
nm, that is, blue-shifted relative to the position of the first broad
band in [cis-H2B4a]2+, centered at ca. 900 nm. Calculations
performed for triplet states of [cis-H2B4b]2+ and [H2B4c]2+
produced spectral patterns similar to those obtained for the
corresponding broken-symmetry singlets, differing mainly in
the relative intensities of principal bands. Overall, the TD
spectra obtained for U and T solutions provide qualitative
agreement with the experimental spectrum of [cis-H2B4a]2+. A
similarly good fit was also obtained for the tetracation [cis-
H2B4b]4+, for which the presence of NIR absorption features
at ca. 800 nm (stronger) and 1200 nm (weaker) was predicted
by the TD-DFT calculation. Unfortunately, neither the full
linear response (LR) nor TDA formalism provided adequate
results for the radical mono- and trication of H2B4b.
Biradicaloid structures have been previously observed in
hybrid azacoronenes12 and in certain macrocyclic systems.58−60
In the present series of compounds, the open-shell structure is
apparently unique to [H2B4a]2+, indicating that a double
disruption of the peripheral π-conjugation is a prerequisite for
the biradicaloid character. At the same levels of theory, broken-
symmetry wave functions were not stable for the HPHAC
dication [H0Bc]2+ nor for [H2B5c]+, which contains a single
sp3 bridge. The [H1B5]2+ dication is a borderline case, yielding
a residual open-shell contribution in the unsubstituted c
structure (⟨S2⟩ = 0.02, B3LYP) and a pure closed-shell
configuration for the b substitution pattern. The origin of the
biradicaloid character in [H2B4]2+ can be seen in the near
degeneracy of the highest occupied MOs in the neutral species
(H − 1 and HOMO, Figure 5 and Figure S28, Supporting
Information). The two orbitals are, respectively, symmetric and
antisymmetric with respect to the plane containing the sp3
bridges, having otherwise similar nodal characteristics. In the
dication, the α- and β-HOMO orbitals become spatially
disjoint, in a manner reported previously for other pyrrole-
based biradicaloids12,59 (Figure 7).
Density functional theory calculations performed for the
unsubstituted [H2B4c]2+ and the aryl-substituted [cis-
H2B4b]2+ strongly indicate that this dicationic species has
significant open-shell character (Table 2). For comparison, all
Table 2. DFT Data for the [H2B4]2+ Dication
a
b
c
property
[H2B4c]2+
[cis-H2B4b]2+
f
g
f
g
ΔE(R−U),
8.9 (21.5)
5.8 (19.2)
0.27 (0.03)
−5.5 (−19.2)
0.19 (0.70)
kcal/mol
ΔE(T−U),
kcal/mol
−0.14 (−0.11)
−9.1 (−21.7)
0.25 (0.78)
ΔE(T−R),
kcal/mol
d
⟨S2⟩
TD-B3LYP
e,f
R: 1064 (0.714)
U: 783 (0.067)
T: 828 (0.052)
R: 1506 (0.110)
U: 944 (0.114)
T: 940 (0.077)
λ/nm (f)
TD-CAM-B3LYP R: 1486 (0.576)
R: 1872 (0.451)
U: 796 (0.071)
T: 787 (0.069)
e,g
λ/nm (f)
U: 646 (0.26)
T: 665 (0.224)
a
B3LYP/6-31G(d,p) geometries optimized separately for closed-shell
singlet (R), open-shell (broken symmetry) singlet (U), and triplet (T).
b
c
DF/6-311++G(d,p)/PCM(CH2Cl2) densities. DF/6-31G(d,p)/
d
PCM(CH2Cl2) densities. Value for the U state after annihilation of
the first spin contaminant. Lowest energy electronic transitions with
e
oscillator strength f > 0.05, obtained using the Tamm−Dancoff
f
g
approximation. DF = B3LYP. DF = CAM-B3LYP.
calculations were performed using the standard B3LYP
functional and its long-range corrected variant CAM-
B3LYP,55 frequently used for the description of biradicaloid
systems.57 For both substitution patterns, closed-shell wave
functions revealed an RHF-to-UHF instability, the open-shell
solution being more stable by ca. 20 kcal/mol at the CAM-
B3LYP level. The latter functional predicts a much higher
biradicaloid character for the singlet state of both [H2B4c]2+
and [cis-H2B4b]2+ (⟨S2⟩ ≥ 0.7) than does B3LYP. Interest-
ingly, at both levels of theory, the singlet−triplet (S−T) energy
gap is close to zero suggesting that, in the actual dications of
H2B4a, both spin states may be thermally accessible and their
J
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exclusively, on pyrrole α and β carbons. The biradicaloid
[H2B4]2+ can thus be viewed as a union of two radical cations
delocalized along two disjoint peripheral conjugation paths.
This description is in good agreement with the calculated
amplitudes of α- and β-HOMO orbitals (Figure 7).
SUMMARY AND CONCLUSION
■
We have demonstrated the synthesis of azacoronene analogues
in which the peripheral conjugation is interrupted by
introduction of benzylidene bridges. Our synthetic method
employs the familiar reactivity of pyrroles toward electrophilic
reagents, which herein has been put to use in the sterically
congested setting of β-substituted hexapyrrolylbenzenes.
Oxidative dehydrogenation of the benzylidene bridges is
stereochemically controlled: it is kinetically hindered in the
prevailing endo arrangement but proceeds easily for exo-
oriented bridges. Steric effects are also at play in the
nucleophilic additions to the [H2B5a]+ cation, enabling perfect
stereocontrol in the syntheses of cis- and trans-H2B4a. Most
importantly, the disrupted peripheral conjugation is found to
provide a counterintuitive design principle for the construction
of efficient near-infrared chromophores with multiple oxidation
levels. This principle is highlighted by the extended NIR
absorption (reaching 2400 nm) in singly bridged cations and by
the generation of a contiguously π-conjugated tetracation, [cis-
H2B4a]4+, containing two peripheral bridges. These features
result from specific changes in orbital energies caused by
bridging, in particular from the gradual decrease of the energy
gap between the first and second occupied MO level. This small
gap can also be linked to the predicted open-shell character of
the doubly bridged dications. Extending the present synthetic
concept to other large heteroaromatics and other bridging units
is hoped to provide access to π-conjugated molecules with
desirable optical and electronic properties. Further work to
explore these possibilities is in progress in our laboratory.
Figure 7. HOMO orbital diagrams (0.01 au) and total SCF spin
density (0.0004 au) for the DFT-modeled singlet state of the
[H2B4c]2+ biradical (UB3LYP/6-311++G(d,p)//6-31G(d,p)).
An intuitive description of the closed/open-shell dichotomy
in the [H2B4]2+ dications is provided by the familiar concept of
Clar sextets,61,62 which is applicable to charged63,64 and
heteroaromatic65 systems. For H2B42+, four generic types of
canonical resonance structures can be distinguished (Scheme
6). Types I and II retain the unperturbed sextet in the central
Scheme 6. Symmetry-Independent Canonical Structures
Describing Closed-Shell Conjugation in [H2B4]2+ (top) and
a
the Proposed Biradicaloid Conjugation Pattern (bottom)
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures and analytical data for all new
compounds, additional NMR, ESR, and mass spectra, electro-
chemistry data, X-ray data for trans-H2Ba (CIF format), and
computational data tables, figures, and Cartesian coordinates
(PDB format). This material is available free of charge via the
Internet at http://pubs.acs.org.
a
Clar-type sextets are indicated in bold. Each dotted pathway is a
AUTHOR INFORMATION
short-hand notation for a set of localized canonical structures.
■
Corresponding Author
marcin.stepien@chem.uni.wroc.pl
benzene ring, complemented by, respectively, four and three
sextets in the pyrrole rings. On the basis of the sextet count,
type I structures might be considered preferable. These
structures may however be destabilized by the disadvantageous
placement of formal positive charges on adjacent pyrrole rings.
Interestingly, formal positive charges can only be placed on the
opposite “halves” of the H2B4 system, by introducing quinoidal
bond localization to the central benzene ring (types III and IV).
It can thus be seen that the principal canonical forms of
[H2B4]2+ are destabilized by either the spatial proximity of
positive charges or disruption of the central sextet. None of
these unfavorable effects is present in the open-shell resonance
structures V and VI, each containing five unperturbed sextets
(Scheme 6). Calculations indicate that in [H2B4]2+, charge and
spin densities (Figure 7) are distributed primarily, though not
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the National Science Center (Grant
2012/07/E/ST5/00781) is kindly acknowledged. Quantum
chemical calculations were performed in the Wrocław Center
for Networking and Supercomputing.
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