Boron(III) Carbazosubphthalocyanines: Core-Expanded Antiaromatic Boron(III) Subphthalocyanine Analogues

Article abstract of DOI:10.1002/anie.201811420

Condensation of 1,8-diamino-3,6-dichlorocarbazole with a series of disubstituted 1,3-diiminoisoindolines, followed by treatment with BF₃?OEt₂ led to the formation of the corresponding core-expanded boron(III) subphthalocyanine analogues. These air-stable π-conjugated boron(III) carbazosubphthalocyanines possess two boron-containing seven-membered-ring units and a 16 π-electron skeleton, and represent the first examples of antiaromatic boron(III) subphthalocyanine analogues as supported by spectroscopic and theoretical studies. The molecular structure of one of these compounds was unambiguously determined by single-crystal X-ray diffraction analysis. In contrast to typical boron(III) subphthalocyanines, which adopt a cone-shaped structure, the π skeleton of this compound is almost planar.

Full text of DOI:10.1002/anie.201811420

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Accepted Article
Title: Boron(III) Carbazosubphthalocyanines: Core-Expanded
Antiaromatic Boron(III) Subphthalocyanine Analogues
Authors: Dennis K. P. Ng, Nagao Kobayashi, Joseph Y. M. Chan, and
Takahiro Kawata
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201811420
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when BCl3 and BBr3, which are common reagents for SubPc
synthesis, were used instead of BF3∙Et2O, the desired boron(III)
carbazosubphthalocyanines could not be obtained. It seems that
the axial fluoro ligand can stabilize the expanded core as in the
shifted compared with 3a. The spectrum of 3c was similar to that
of 3a. The carbazole protons of 3d resonated at d 7.03 and 6.68,
while the isoindole protons resonated at d 7.16 and 6.98. All these
signals were slightly upfield-shifted when compared with those for
carbazoles and phthalocyanines, which could be attributed to the
paratropic ring current arising from the antiaromatic core of these
compounds. The effect, however, was not as large as that of the
previously reported porphyrinoids based on the degree of upfield
shift.5-8 The relatively large upfield shift for the two isoindole
protons’ singlets for 3a, 3c, and 3d could be due to the additional
anisotropic shielding effect by the neighboring aryl rings.
Boron(III)
Carbazosubphthalocyanines:
Core-Expanded
case of the SubPc analogues containing
a peripheral seven-
Antiaromatic Boron(III) Subphthalocyanine Analogues**
membered heterocycle.17b Furthermore, when BF3∙Et2O was
added during the reaction of 1 and 2a, the one-pot reaction gave
the metal-free phthalocyanine formed by self-cyclization of 2a in
18% yield, while only 4% yield of 3a was obtained. The
experimental details and characterization data are given in the
Supporting Information.
Joseph Y. M. Chan, Takahiro Kawata, Nagao Kobayashi,* and Dennis K. P. Ng*
Abstract: Condensation of 1,8-diamino-3,6-dichlorocarbazole with a
series of disubstituted 1,3-diiminoisoindolines followed by the
treatment with BF3·OEt2 led to the formation of the corresponding
core-expanded boron(III) subphthalocyanine analogues. These air-
stable π-conjugated boron(III) carbazosubphthalocyanines possess
two boron-containing seven-membered ring units and a 16 π-electron
skeleton, which represent the first examples of antiaromatic boron(III)
subphthalocyanine analogues as supported by spectroscopic and
theoretical studies. The molecular structure of one of these
compounds was also unambiguously determined by single-crystal X-
antiaromatic azaporphyrinoids reported so far. Typically, SubPcs
contain three aza-linked isoindole units, forming a 14 π-electron
aromatic skeleton around boron center, and the molecules
a
adopt a cone-shaped structure.16 Recently, we have reported two
series of analogues in which one of the pyrrole units is replaced
with a six- or seven-membered heterocycle without alternating the
inner π-electron core (Figure 1).17 In contrast, these boron(III)
carbazosubphthalocyanines are core-expanded having a 16 π-
electron skeleton. The antiaromatic character of these
compounds has been revealed by spectroscopic and theoretical
studies as reported and discussed below.
To provide further insight about the paratropic ring current of
the core, we replaced the axial fluoro ligand of 3d with a methoxy
group by treating 3d with NaOMe in 5% MeOH in tetrahydrofuran.
The axial methoxy protons of the substituted product (compound
5) resonated as
downfield-shifted compared with that of the SubPc-OMe analogue
(at d 1.50).19 The signal also appeared at
more downfield
a singlet at d 3.73, which was significantly
a
ray diffraction analysis.
subphthalocyanines which adopt
p skeleton of this compound is almost planar.
In contrast to typical boron(III)
position than that for non-aromatic tetracoordinated B-OMe
derivatives which have the methoxy protons‘ signal typically in the
range of d 2.6-3.1.20
Scheme 1. Synthesis of boron(III) carbazosubphthalocyanines 3a-d.
a
cone-shaped structure, the
Single crystals of 3a suitable for X-ray diffraction analysis
were obtained by slow diffusion of hexane into a solution of 3a in
CHCl3. This compound crystallizes in the triclinic system with a P-
1 space group with two molecules in a unit cell. The structure also
contains two solvated CHCl3 for each molecule. Figure 2(a)
shows the top view of the molecular structure. Unlike typical
SubPcs which have a cone-shaped structure, the π skeleton of
3a is almost planar [Figure 2(b)]. The central boron atom deviates
from the mean plane defined by the three coordinating nitrogen
atoms by 0.49 Å, which is significantly smaller than the values for
typical SubPcs (0.59-0.66 Å).17b The sum of the N-B-N angles,
which is another parameter to reflect the extent of planarity, is
332⁰, which is also significantly larger than those for typical
SubPcs (308-316⁰).17b With an essentially planar structure, the
molecules stack in a head-to-tail manner with the axial fluoro
ligands pointing each other in the crystal lattice (Figure S2 in
Supporting Information). A close examination of the C-N bond
distances along the inner core did not reveal significant bond-
length alternation.
A stronger evidence of antiaromaticity came from the 11B and
19F NMR spectroscopic data. The 11B NMR spectra of 3a, 3b, 3c,
and 3d in CDCl3 showed a doublet at d 9.23 (1JBF = 52.8 Hz), 9.10
(1JBF = 56.0 Hz), 9.10 (1JBF = 54.9 Hz), and 9.13 (1JBF = 52.7 Hz),
respectively. Compared with the typical 11B chemical shifts for
SubPcs (d -11 to -16),17b,19,21 these signals were largely
downfield-shifted. Similarly, the 19F NMR signals for 3a (at d -93.7),
3b (at d -91.4), 3c (at d -91.2), and 3d (at d -91.0) were also
significantly deshifted compared with those for boron
subphthalocyanine fluoride (d -158) and our previously reported
SubPc derivatives (d -149).17b The large downfield shift of these
signals clearly indicates the presence of paratropic ring current.
The 1H NMR spectroscopic data are in accord with the result
of nucleus-independent chemical shift (NICS) calculations using
the CAM-B3LYP/6-31G(d) basis set.22 As shown in Figure 3(a),
the computed NICS values in the central core region of the density
functional theory (DFT)-optimized structure of 3a are all positive
suggesting that paratropic ring current is exceeding over diatropic
ring current, while in the outer rim region, the values become
smaller and finally change to negative. This is further confirmed
by anisotropy of the current-induced density (ACID) calculations
at the CAM-B3LYP/6-31G(d) level,23 which show counter-
clockwise paratropic ring current flow in the inner core and
clockwise diatropic ring current flow in the four fused benzo
groups [Figure 3(b)]. In contrast, clockwise diatropic ring current
flow is shown for SubPcs both in the central core and the three
benzo groups (Figure S3 in Supporting Information). The results
indicate that the effective chromophore region is that shown in
bold lines in Figure 3(c) and the outer four fused benzo groups
have little contribution in considering the theoretical absorption
spectroscopic aspect of this molecule. This can explain why these
protons are not substantially upfield-shifted as observed for other
antiaromatic porphyrin derivatives.5-8
Antiaromatic porphyrinoids with
a 4n π-electron core have
received considerable attention.1 Apart from their intrinsic interest
from theoretical point of view, these compounds exhibit intriguing
optical and electronic properties. They can serve as potential
electrode-active materials in rechargeable batteries2 and
ambipolar semiconductors for field-effect transistors.3 To date, a
number of antiaromatic porphyrins4 and their ring-expanded,5
ring-contracted (e.g. norcorroles6 and tetrahydronorcorroles7),
and core-modified (e.g. isophlorins,1c,8 oxatriphyrins,9 and
phenanthriporphyrin10
) analogues have been reported. The
aromaticity of these systems can be modulated through changing
the physical and chemical environments, and by photo-switching
the electronic states.1a Stacking of antiaromatic porphyrin
molecules, for example, would diminish the inherent
antiaromaticity and promote delocalization of the π electrons,
thereby enhancing their two-photon absorption property.11
Despite the advances in the study of these systems, antiaromatic
azaporphyrinoids are relatively scarce. To our knowledge, only a
few
such
derivatives,
including
diazaporphyrins,14
phthalocyanines,12
and
hemiporphyrazines,13
Figure 1. Molecular structures of SubPcs and their analogues.
pentabenzotriazasmaragdyrins15 have been reported so far. We
report herein the first examples of antiaromatic boron(III)
subphthalocyanine (SubPc) derivatives, namely boron(III)
carbazosubphthalocyanines, which represent the smallest
These boron(III) carbazosubphthalocyanines were prepared
in two steps as shown in Scheme 1. 1,8-Diamino-3,6-
dichlorocarbazole (1) was first treated with 2.1 equivalent of
disubstituted 1,3-diiminoisoindolines 2a-d in refluxing ethanol
overnight. The solvent was then removed, and the residue was
mixed with BF3∙Et2O and 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU) in 1-chloronaphthalene. After stirring at 150 oC for 2 h, the
corresponding boron(III) carbazosubphthalocyanines 3a-d were
formed and isolated in 24-47% yield. Attempts were made to
isolate the intermediate product of 1 and 2a, but its low solubility
in common organic solvents and high polarity hindered this
process. Based on the mass spectrometric data (Figure S1 in
Supporting Information), it is likely that the 1:2 condensed product
4 was formed. Similar condensation reactions of diamines and
1,3-diiminoisoindolines have been reported previously.18 Addition
of BF3∙Et2O led to chelation followed by cyclization. In the
absence of DBU, the yield was slightly reduced (to 25% for 3a),
indicating that it might play a role in this reaction. Interestingly,
[*]
J. Y. M. Chan, Prof. D. K. P. Ng
Department of Chemistry,
The Chinese University of Hong Kong,
Shatin, N.T., Hong Kong, China
E-mail: dkpn@cuhk.edu.hk
T. Kawata, Prof. N. Kobayashi
Department of Chemistry and Materials, Faculty of Textile Science
and Technology,
Shinshu University,
Figure 2. Molecular structure of 3a. Hydrogen atoms are omitted for clarity
and the thermal ellipsoids are drawn at the 30% probability level. (a) Top view.
(b) Side view.
The 1H NMR spectrum of 3a in CDCl3 showed two doublets at
d 7.27 and 6.91 (with a 4JHH of 2.4 Hz) for the two sets of carbazole
protons, and two singlets at d 6.69 and 6.58 for the two sets of
isoindole protons. For 3b, while the two doublets for the carbazole
Apart from 3a, we also computed the 1H NMR data for the
hypothetical boron-free analogue of 3a. The signals of the two
Ueda 386-8567, Japan
inner pyrrole protons were predicted at
d 20.47, which were
E-mail: nagaok@m.tohoku.ac.jp and/or nagaok@shinshu-u.ac.jp
This work was partially supported by a Grant-in-Aid for Scientific
Research (C), No. 18K05076, from the Japan Society for the
Promotion of Science to NK
protons (at
d 7.13 and 6.92) were essentially unshifted, the
substantially downfield-shifted compared with those of the
hypothetical boron-free SubPc analogue [at d 0.76 (convex site)
[**]
isoindole protons’ signals (at d 7.90 and 7.83) were less upfield-
:
Supporting information and the ORCID identification numbers for the
authors of this article can be found under
http://dx.doi.org/10.1002/anie
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10.1002/anie.201811420
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and 5.78 (concave site)] (Figure S4 in Supporting Information).
The NICS(0) values in the center of the boron-free analogue of 3a
and SubPc were calculated to be 8.33 and -13.5, respectively
(Figure S5 in Supporting Information). In addition, we also
calculated the chemical shifts of the OMe protons‘ signals of 5 and
SubPc-OMe. They were found to appear at d 4.21 (1 H) and 3.69
(2 H) (for 5) and at d 1.93 (1 H) and 1.06 (2 H) (for SubPc-OMe)
(Figure S6 in Supporting Information), which were in good
agreement with the experimental data. All these theoretical
results further support the antiaromatic character of the
carbazosubphthalocyanine core.
macrocycles.24,25 Corresponding to the weak absorption band in
600-900 nm, a negative MCD envelope was detected, while the
sign of MCD signal changed from positive to negative on going
from ca. 420-570 to ca. 325-420 nm. Since there are no
degenerate states in neither the ground nor excited states due to
low symmetry of the molecule, these are all Faraday B-terms
appearing closely to the absorption peaks or shoulders and arise
from mixing between closely related states linked by a magnetic
dipole transition moment.26 To reveal these spectral features, the
absorption spectrum of 3a was simulated by time-dependent
(TD)-DFT calculations at the CAM-B3LYP/6-31G(d) level [Figure
4(c)] with results for low-energy π®π* transitions summarized in
Table 1.27 Although the energies were slightly overestimated, the
experimental spectrum was nicely reproduced. The data in Table
1 suggest that, a very weak, two weak, a medium, and an intense
bands labelled respectively as S, CT, N1, and (P1+N2) bands are
in ascending energy. The S band (the HOMO®LUMO transition)
corresponds to an intrashell transition so that it carries no electric
dipole oscillator strength and is therefore very weak. The CT band
is characteristic of this system with transition occurs from an outer
benzene ring to the inner ligand. The relationship between the N
and P bands is similar to that of the Soret and Q bands in (4n+2)π
porphyrinoids so that N bands are generally weak while P bands
are stronger. In Figure 4(b), the absorptions in 600-900, 430-600,
and 390-430 nm regions correspond to S, CT+N1, and P1+N2
bands, respectively. If we know the energy of six frontier orbitals
labelled as h- (HOMO-2), h+ (HOMO-1), s- (HOMO), s+ (LUMO), l-
(LUMO+1), and l+ (LUMO+2) of the “ligand perimeter”, not only
the intensity but also the MCD sign can often be reasonably
explained.24,25
HOMO-1 and HOMO-2 may not correspond to the h+ and h-
orbitals, respectively, since they have higher coefficient in the
outer fused benzo moieties and only two nodes can be drawn for
HOMO-2. The HOMO-3 and HOMO-4 may correspond to the h+
and h- orbitals, respectively, since three nodal planes can be
drawn in the central part of their MOs (Figure S8 in Supporting
Information) and the coefficients of MOs are large even in their
central moieties. The relatively large contribution of these orbitals
to the N1 and N2 bands (Table 1) may also support this
assignment. Therefore, by assuming that the HOMO-4, HOMO-3,
HOMO, LUMO, LUMO+1, LUMO+2 correspond to the h-, h+, s-,
s+, l-, and l+ orbitals of the “ligand perimeter”, respectively, and
using the energy level values of these MOs, we can calculate the
been reported for the preparation of SubPc derivatives. The
molecular structure of one of these compounds has been
determined, which shows a planar π skeleton. Despite bond-
length alternation is not noticeably in the inner core and the
peripheral ring protons are not significantly upfield-shifted, the
positive computed NICS values, counter-clockwise ring current
flow as shown in the ACID plot in the core, calculated downfield
chemical shift values for the inner pyrrole protons of the
hypothetical boron-free analogue of 3a and the axial methoxy
protons of 5, as well as the large downshift shift of the 11B and 19
F
NMR signals strongly suggest the presence of paratropic ring
current. The antiaromatic character of these compounds has also
been inferred by UV-Vis and MCD spectroscopic studies together
relative orbital energy differences as follows: DL = 0.99, DS = 4.27, with DFT calculations. To our knowledge, these boron(III)
DH = 0.19, DLS = 4.04, DHS = 3.40, DHSL = 2(DHS-DLS) = -1.28,
DHL = DH-DL = -0.80, and SHL = DH+DL = 1.18 eV. Furthermore,
from the |DHSL| > SHL > |DHL| relationship, C2v symmetry of the
molecule, and the fact that the orbitals h- and l- do not have
carbazosubphthalocyanines represent the first examples of
antiaromatic SubPc derivatives and the smallest antiaromatic
azaporphyrinoids reported so far.
a)
identical symmetry with respect to
a plane of symmetry
perpendicular to the molecular plane, we can infer that 3a can be
classified as an orbital-shift-dominated system of S-perturbed
perimeters.24a Because DHSL is negative, the expected MCD sign
pattern for the N1 and P1 (N2) transition is a +,- in ascending
energy, which is consistent with the observed MCD signs in ca.
400-500 nm region.
Table 1. Calculated Transition Energies, Oscillator Strength f, Assignment, and
Composition for 3a
Energy/eV (nm)
2.00 (620.8)
3.23 (384.3)
f
Assign.
S
CT
Composition (weight %)
H(s-)→L(s+) (95%)
H-1→L(s+) (74%),
H-2→L(s+) (13%)
0.0031
0.0259
3.29 (367.7)
3.35 (370.2)
3.59 (345.6)
3.76 (329.4)
3.84 (322.9)
3.97 (312.2)
4.00 (309.8)
4.13 (300.6)
0.0150
0.2895
1.0200
0.0019
0.0202
0.0247
0.2140
0.2033
CT
N1
P1
N2
H-2→L(s+) (63%),
H-4(h-)→L(s+) (15%)
H-3(h+)→L(s+)(86%),
H-2→L+1(l-)(5%)
a)
450
H(s-)→L+1(l-)(45%),
H-4(h-)→L(s+)(31%)
H-4(h-)→L(s+)(42%),
H(s-)→L+1(l-)(40%)
H-16→L(s+)(78%),
H-17→L+1(l-)(10%)
H-13→L(s+)(75%),
H-16→L+1(l-)(14%)
H-13→L(s+)(54%),
H-14→L(s+)(26%)
H-14→L(s+)(55%),
H-13→L(s+)(23%)
De
399
364
385
718
780
346
411
x10
x25
416
b)
396
376
e
b)
453
708
780
The redox behavior of 3a-d and 5 was investigated using
cyclic voltammetry in CH2Cl2 (for 3a, 3d, and 5) or 1,2-
dichlorobenzene (for 3b and 3c, which had limited solubility in
CH2Cl2). All the compounds exhibited a reversible oxidation and
a reversible reduction (Figure S9 and Table S1 in Supporting
Figure 3. (a) Computed NICS values (in ppm) for the DFT-optimized structure
of 3a at various positions (2,6-dimethylphenoxy groups were omitted for clarity).
(b) Calculated ACID plot for 3a at an isovalue of 0.03. (c) The effective
chromophore region according to the NICS calculations.
c)
e
Figure 5. (a) Energy diagram and some frontier MOs of 3a. (b) Nodal patterns
of the HOMO-3, HOMO, LUMO, and LUMO+1 of 3a at an isovalue of 0.012.
Information). Compared with boron(III) subphthalocyanine
Figures 4(a) and 4(b) show the electronic absorption and
magnetic circular dichroism (MCD) spectra of 3a in CHCl3. In
contrast to the reported spectra of SubPcs, which have (4n+2)π
&'(
chloride,28 the half-wave reduction potentials ( "
) of these
#/%
compounds were substantially less negative (ca. -0.9 V vs. -1.7
V), showing that they are better oxidizing agents. The
electrochemical HOMO-LUMO gap for these compounds (1.72-
1.83 V) was in good agreement with the energy of the S-band
absorption (ca. 1.75 eV).
In summary, we have synthesized a series of novel core-
expanded SubPc analogues through condensation of diamine 1
and 1,3-diiminoisoindolines 2a-d, followed by boron-induced
complexation and cyclization. This synthetic approach has rarely
Keywords: Antiaromaticity • subphthalocyanines • boron •
Wavelength / nm
type characteristics,16 it showed
a very weak and broad
magnetic circular dichroism • ring expansion
Figure 4. Experimental (a) MCD and (b) UV-Vis spectra of 3a in CHCl3 and (c)
a simulated UV-Vis spectrum of 3a calculated at the CAM-B3LYP/6-31G(d) level.
absorption envelope in 600-900 nm and an intense peak at 416
nm with a tail extending to ca. 570 nm. The spectral features of
3b-d and 5 were similar except that an additional band at 509 nm
(for 3b) or 504 nm (for 3c) was also observed (Figure S7 in
Supporting Information), which might be due to the n®π*
transition arising from the sulfur-containing substituents. These
appear to be the spectral characteristic of 4nπ type
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A series of novel core-expanded
boron(III) subphthalocyanine
analogues, namely boron(III)
carbazosubphthalocyanines, have
been synthesized and characterized,
which represent the first examples of
antiaromatic boron(III)
subphthalocyanine derivatives and the
smallest antiaromatic
azaporphyrinoids reported so far.
Joseph Y. M. Chan, Takahiro Kawata,
Prof. Nagao Kobayashi,* and Prof.
Dennis K. P. Ng*
Page No. – Page No.
Boron(III)
Carbazosubphthalocyanines: Core-
Expanded Antiaromatic Boron(III)
Subphthalocyanine Analogues
[7]
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