Unique interaction between directly linked laminated π planes in the benzonorrole dimer

Article abstract of DOI:10.1002/anie.201203712

Direct link: Two directly linked benzonorrole dimers were prepared and characterized, and both have short interplane distances less than 3.5 A. While a strong π-π interaction was recognized in the oxidized form (left), only a negligible π-π interaction was observed in the reduced form (right) in spite of its shorter π-π distance. Copyright

Full text of DOI:10.1002/anie.201203712

Angewandte
Chemie
DOI: 10.1002/anie.201203712
Porphyrinoids
Unique Interaction between Directly Linked Laminated p Planes in the
Benzonorrole Dimer**
Motoki Toganoh, Yasunori Kawabe, Hidemitsu Uno, and Hiroyuki Furuta*
Interactions between two p planes are an important subject
for study because they are ubiquitous and often play
important roles in functional molecules.[1] In particular, the
interaction between porphyrins and related macrocycles have
gathered much attention because of its crucial role in
biochemistry as well as materials science.[2] To study the
interaction between porphyrinoids, the connection of two
p planes would be one of the most straightforward strategies.
In the dimers having a direct covalent linkage, two p planes
are usually arranged in a side-by-side manner to form tapelike
structures.[3] In contrast, the laminated face-to-face dimers
having a direct covalent linkage are rarely reported, plausibly
because of the lack of an efficient synthetic methodology
(Figure 1). Alternatively, the arrangement of p planes in
a directly linked, face-to-face p dimer having shorter p–p
distances.
In the directly linked face-to-face p dimer, we think that
the interaction between the two p planes can be discussed
based on molecular orbitals. Thus, in analogy to the typical
interaction between the two p orbitals in ethylene (Fig-
ure 2a), bonding and antibonding interactions between two
p planes are considered for the face-to-face structure. When
Figure 2. Bonding and antibonding interactions between two orbitals:
a) p orbitals, b) non-interacting face-to-face p planes, and c) strongly
interacting face-to-face p planes.
almost no orbital interaction exists, bonding and antibonding
orbitals will be degenerate, and have almost the same energies
as those of the original orbitals (Figure 2b). In contrast, when
strong orbital interactions exist, large energy splitting will be
observed (Figure 2c).
For the construction of the directly linked, face-to-face
p planes, a member of carbacorrole is utilized, since it has
a reactive carbon atom inside the macrocycle and would be
appropriate for this purpose. We have recently developed
a unique strategy to prepare carbacorroles, namely the N-
linked strategy. The N-linked strategy rests on the formation
Figure 1. Directly linked p planes.
a face-to-face manner can be achieved readily with the help of
a rigid bridge or metal coordination, even though the
distances between two p planes tend to be long (over
3.7 ꢀ).[4,5] Herein we have successfully constructed directly
linked, face-to-face p planes having short p–p distances (less
than 3.5 ꢀ) using a corrole isomer, and a unique interaction
between the two p planes was observed. To the best of our
knowledge, this is the first example in porphyrin chemistry of
À
of a N C bond for macrocyclization (Figure 3). The first
example was found during the synthetic study of the N-
confused corrole, as the N-linked corrole (named norrole)
was isolated accidentally as a by-product.[6] Subsequently, the
C-fused norrole was synthesized, and thus suggested the
generality of the N-linked strategy as well as the high
reactivity of the inner carbon atom.[7] Additionally, Lash
and co-workers reported a novel porphyrin isomer possessing
[*] Dr. M. Toganoh, Y. Kawabe, Prof. Dr. H. Furuta
Department of Chemistry and Biochemistry
Graduate School of Engineering
Kyushu University, Fukuoka 819-0395 (Japan)
E-mail: hfuruta@cstf.kyushu-u.ac.jp
Prof. Dr. H. Furuta
International Research Center for Molecular Systems
Kyushu University, Fukuoka 819-0395 (Japan)
Prof. Dr. H. Uno
Department of Molecular Science, Integrated Center for Science
Ehime University, Matsuyama 790-8577 (Japan)
[**] The present work was supported by the Grant-in-Aid for Scientific
Research (22350020 and 23108715) from the Ministry of Education,
Culture, Sports, Science and Technology of Japan.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201203712.
Figure 3. N-Confused corrole and norrole derivatives.
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a similar N linkage.[8] Although high reactivity of the N-linked
bipyrrole moiety motivated us to use norrole derivatives, the
reported synthetic route only gave norrole in a tiny amount
and thus an alternative method for the synthesis of norrole
derivatives was essential. A simple remedy would be utiliza-
tion of a substituted pyrrole to block the other reactive site or
the formation of an N-confused corrole. Herein an indole was
adopted as a substituted pyrrole, such that the target molecule
would be benzonorrole.
Synthesis of a benzonorrole precursor, N-confused ben-
zobilane, could be achieved by a strategy similar to that for
the synthesis of N-confused bilane (Scheme 1).[6,9] Starting
Figure 4. Top and side views of the X-ray structures of 8 (a,c) and 9
(b,d). The meso-C6F5 groups are omitted for clarity in the side views.
Thermal ellipsoids are shwon at the 30% probability level.[16]
a 35.18 angle to the tripyrrane plane, an angle which is
significantly larger than that of 8 (3.18).
The uniqueness of benzonorrole among porphyrinoids
became evident when it was further oxidized by 2,3-dichloro-
5,6-dicyano-1,4-benozoquinone (DDQ), thus forming the
unprecedented directly linked dimers. Upon treatment of 9
with DDQ in CH3CN at reflux, cis-BNdimer and trans-
BNdimer were obtained in 75 and 20% yields, respectively
(Scheme 2a). Both structures were confirmed by X-ray
crystallographic analyses. Importantly, the carbon–carbon
Scheme 1. Preparation of benzonorrole. TFA=trifluoroacetic acid,
THF=tetrahydrofuran, TIPS=triisopropylsilyl.
from indole, formylation and subsequent protection with
a triisopropylsilyl group gave 2 in 76% yield (2 steps).[10] The
reaction of 2 with C6F5MgBr gave carbinol 3, and subsequent
acid condensation with pyrrole afforded the N-confused
benzodipyrromethane 4 quantitatively. Then 4 was acylated
with S-2-pyridyl pentafluorobenzothioate to give the mono-
acyl derivative 5 in 67% yield. Reduction of 5 and subsequent
acid condensation with dipyrromethane gave the N-protected
N-confused benzobilane 6 (65% yield, 2 steps). Finally,
Scheme 2. a) DDQ oxidation of 9. b) Interconversion between cis-
BNdimer and BNdimerH2. The photos show the color in CH2Cl2
solution.
deprotection of
6
with tetrabutylammonium fluoride
bond lengths between the two benzonorrole units are
1.357(5) and 1.336(3) ꢀ for cis-BNdimer and trans-BNdimer,
respectively, thus suggesting that those bonds are double
bonds. Furthermore, reduction of cis-BNdimer with
TsNHNH2 gave BNdimerH2 quantitatively, and the color of
solution changed significantly (Scheme 2b). In the X-ray
structure the bond length between the two benzonorrole units
is 1.476(3) ꢀ, which is typical for a single bond between two
sp2-carbon atoms.[13] Upon DDQ oxidation, BNdimerH2
(TBAF) gave the N-confused benzobilane 7 quantitatively.
Oxidative cyclization of the N-confused benzobilane 7
was satisfactorily achieved by the aid of copper metal. Upon
treatment of 7 with p-chloranil and then with Cu(OAc)2·H2O,
the CuIII/benzonorrole 8 was obtained in 68% yield. Deme-
tallation of 8 by Zn/HCl gave the free-base benzonorrole 9 in
94% yield.[11] The total yield from indole to benzonorrole is
21% (10 steps), which is considerably high among corrole
isomers.[12] The structures of 8 and 9 were elucidated by X-ray
crystallographic analysis (Figure 4). Complex 8 has a nearly
planar conformation. In contrast, the indole ring of 9 sits at
reformed
cis-BNdimer
quantitatively.
Surprisingly,
TsNHNH2 reduction of trans-BNdimer did not proceed and
never gave the expected reduced product.
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Structural parameters for the X-ray structures of BNdi-
merH2, cis-BNdimer, and trans-BNdimer are summarized in
Figure 5. In BNdimerH2, the two benzonorrole planes are
close (3.14 ꢀ) and almost parallel (6.58) to each other, such
Figure 6. Absorption spectra of BNdimerH2, cis-BNdimer, and 9 in
CH2Cl2.
Figure 5. Structural parameters of BNdimerH2, cis-BNdimer, and trans-
BNdimer on the basis of their X-ray structures.
Table 1: Oxidation and reduction potentials of BNdimerH2, cis-BNdimer,
and 9.[a]
Compound
Eox
Ered
DE (dDE)
that a through-space p–p interaction can be expected. The
bond length [1.476(3) ꢀ] and angle (55.18) between the two
indole planes is appropriate for 2,2’-biindolyl.[14] Similarly, the
two benzonorrole planes are close (3.46 ꢀ) and almost
parallel (5.48) to each other in cis-BNdimer. A sharp contrast
is observed at the 2,2’-biindolyl moiety. The bond between the
two indole rings is a double bond [1.357(5) ꢀ], which is
similar to that of indigo (1.34 ꢀ),[15] and the angle between
two indole planes becomes small (10.08). Accordingly, the
difference between BNdimerH2 and cis-BNdimer in the
interactions between the two p planes could be derived not
from the relative position of two benzonorrole planes but
from the connection mode between the indole rings. The
structural parameters for trans-BNdimer are similar to those
of cis-BNdimer. Since no reduced product corresponding to
trans-BNdimer was obtained yet, the following discussion is
based on the comparison between BNdimerH2 and cis-
BNdimer (see the Supporting Information for properties of
trans-BNdimer).[16]
Absorption spectra of BNdimerH2, cis-BNdimer, and 9 in
CH2Cl2 are shown in Figure 6. In the benzonorrole monomer
9, absorption maxima are observed at l = 611 and 645 nm,
which are slightly shorter than those of the parent norrole
(l = 618 and 659 nm).[6] The absorption maximum of BNdi-
merH2 (l = 633 nm) is comparable to that of 9, thus suggest-
ing the weak interaction between two benzonorrole units in
BNdimerH2. Contrastingly, a remarkable red shift is observed
in cis-BNdimer and its absorption maximum reaches up to l =
845 nm. Such a large red shift would indicate that the
two benzonorrole p orbitals are strongly interacting
with each other in cis-BNdimer.
BNdimerH2
cis-BNdimer
9
0.24[b]
0.47[b]
0.31[b]
À1.42[b]
À0.98[c]
À1.46[c]
1.66 (À0.11)
1.45 (À0.32)
1.77 (0.00)
[a] in eV (vs. Fc/Fc+, Pt electrode, 100 mVsÀ1 scan rate).[b] Determined
by differential pulse voltammetry. [c] Determined by cyclic voltammetry.
0.11 eV from that of 9, thus suggesting weak interactions
between the two p planes. Meanwhile a large decrease by
0.32 eV was observed for cis-BNdimer (1.45 eV), thus imply-
ing a strong interaction between the two p planes. The
electrochemical properties are in good agreement with the
photophysical properties.
Finally, density functional theory (DFT) calculations were
performed on the model compounds of benzonorrole (9),
BNdimerH2, and cis-BNdimer, wherein the meso-C6F5 groups
were replaced by hydrogen atoms. The Kohn–Sham orbitals
are shown in Figure 7 and the orbital energies are summarized
in Figure 8. For clarity, half of the structures are shown for
BNdimerH2 and cis-BNdimer in Figure 7a. Roughly speak-
ing, the p-orbital shapes of BNdimerH2 and cis-BNdimer are
similar to those of the monomer, thus suggesting molecular
orbitals of benzonorrole dimers can be constructed simply
from a linear combination of those of the monomer as
postulated in Figure 2. In the case of BNdimerH2, both
HOMO and LUMO are almost degenerate (Figure 8a). This
degeneracy means that the p orbitals of the two monomer
units do not interact with each other in spite of the short p–p
Electrochemical measurements also revealed the
unique electronic structure of cis-BNdimer. The
oxidation and reduction potentials of BNdimerH2,
cis-BNdimer, and 9 in a 0.1m Bu4NPF6 solution of
CH2Cl2 are listed in Table 1. The values of Eox-Ered
(DE) for BNdimerH2 and cis-BNdimer can be
regarded as an approximate measure of the interaction
between two benzonorrole p planes. The DE of 9 is
1.77 eV, which can be taken as a standard value in the
case of no interaction between the two p planes. The
Figure 7. Molecular orbitals of a) benzonorrole, BNdimerH2, and cis-BNdimer
DE of BNdimerH2 (1.66 eV) decreased only by (top views), and b) cis-BNdimer (side views).
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Keywords: density functional calculations ·
.
dimerization · indoles · p–p interactions ·
porphyrinoids
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Figure 8. Interactions between the two benzonorrole p orbitals in a) BNdimerH2 and
b) cis-BNdimer.
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HOMOÀ1 could be due to the larger angle of the indole ring
relative to the benzonorrole plane (56.68). In contrast, the
degeneracy of the HOMO and LUMO is completely lost in
cis-BNdimer (Figure 8b), even with a longer p–p distance
(3.36 ꢀ). As illustrated in Figure 7b, HOMOÀ2 and HOMO
could be regarded as an in-phase and out-of-phase combina-
tion, respectively, of the benzonorrole p orbital. As a result of
the splitting of the energy level, even another orbital
(HOMOÀ1) exists between them. The same could be said
for the LUMO and LUMO+1 (see Figure S13 in the
Supporting Information). Accordingly, a much narrower
HOMO–LUMO gap of 1.88 eV is observed in cis-BNdimer,
and it conforms to the experimental results. The marked
contrast in the orbital interactions between BNdimerH2 and
cis-BNdimer should originate not in the p–p distance but in
the bonding mode between two p planes. The difference may
also affect the aromaticity of the dimers and, thus partial
support for this is found in the weakening of the ring current
1
in cis-BNdimer as inferred from the H NMR chemical-shift
calculations (Figure S12 in the Supporting Information).
In summary, we have synthesized benzonorrole dimers, in
which two benzonorrole units are directly linked to form face-
to-face p dimers with short p–p distances. Although no
splitting of the orbital energy levels was observed in
BNdimerH2, it was clearly observed in cis-BNdimer. Consid-
ering similar relative positions of the two benzonorrole units,
this contrast was surprising and highlights an important
feature of the p–p interaction in the directly linked system.
Thus, the bonding mode between the two p planes are more
important than the p–p distance. Meanwhile, covalent
functionalization as well as the coordination chemistry of
benzonorrole would be an interesting subject, as it is the first
norrole derivative that is readily available. Further studies on
the benzonorrole chemistry as well as the p–p interactions in
the directly linked system will be reported in the near future.
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[16] CCDC 879598 (8), and CCDC 879599 (9), CCDC 886783 (cis-
BNdimer), CCDC 879601 (trans-BNdimer), and CCDC 879602
(BNdimerH2) contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
Received: May 14, 2012
Revised: June 18, 2012
Published online: && &&, &&&&
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Communications
Porphyrinoids
M. Toganoh, Y. Kawabe, H. Uno,
H. Furuta*
&&&& — &&&&
Unique Interaction between Directly
Linked Laminated p Planes in the
Benzonorrole Dimer
Direct link: Two directly linked benzo-
norrole dimers were prepared and char-
acterized, and both have short interplane
distances less than 3.5 ꢀ. While a strong
p–p interaction was recognized in the
oxidized form (left), only a negligible p–p
interaction was observed in the reduced
form (right) in spite of its shorter p–p
distance.
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
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www.angewandte.org
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These are not the final page numbers!