Meso-hydroxysubporphyrins: A cyclic trimeric assembly and a stable meso-Oxy radical

Article abstract of DOI:10.1002/anie.201501592

Treatment of meso-chlorosubporphyrin with potassium hydroxide in DMSO followed by aqueous work up and recrystallization gave a cyclic trimer consisting of meso-hydroxysubporphyrin units linked between the central boron atoms and meso-hydroxy groups. Solutions of this trimer are nonfluorescent, but become fluorescent when exposed to acid or base, since hydrolytic cleavage of the axial B-O bonds generates the meso-hydroxysubporphyrin monomer or its oxyanion. Ring cleavage of the trimer was also effected by reaction with phenylmagnesium bromide to produce meso-hydroxy-B-phenyl subporphyrin, which can be quantitatively oxidized with PbO₂ to furnish a subporphyrin meso-oxy radical as a remarkably stable species as a result of spin delocalization over almost the entire molecule.

Full text of DOI:10.1002/anie.201501592

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201501592
German Edition: DOI: 10.1002/ange.201501592
Porphyrinoids
meso-Hydroxysubporphyrins: ACyclic Trimeric Assembly and a Stable
meso-Oxy Radical**
Daiki Shimizu, Juwon Oh, Ko Furukawa,* Dongho Kim,* and Atsuhiro Osuka*
Abstract: Treatment of meso-chlorosubporphyrin with potas-
sium hydroxide in DMSO followed by aqueous work up and
recrystallization gave a cyclic trimer consisting of meso-
hydroxysubporphyrin units linked between the central boron
atoms and meso-hydroxy groups. Solutions of this trimer are
nonfluorescent, but become fluorescent when exposed to acid
graphic analysis when stabilized by coordinating solvents
À
(typically pyridine), but forms a C(15) C(15’)-linked dimer
when such coordinating solvents are not present.[2b,c] The
stability of this porphyrin meso-oxy radical was improved
somewhat by the rational installation of a bulky tert-butyl
group at the 15-position, but even with such steric protection,
30% degradation occurred after two weeks in solution.[3]
Recently, [26]hexaphyrins have emerged as effective por-
phyrinic platforms for radical stabilization because of their
large conjugated electronic networks that favorably accom-
modate spin delocalization, as seen for their fairly stable
meso-oxy radicals.[4]
À
or base, since hydrolytic cleavage of the axial B O bonds
generates the meso-hydroxysubporphyrin monomer or its
oxyanion. Ring cleavage of the trimer was also effected by
reaction with phenylmagnesium bromide to produce meso-
hydroxy-B-phenyl subporphyrin, which can be quantitatively
oxidized with PbO2 to furnish a subporphyrin meso-oxy
radical as a remarkably stable species as a result of spin
delocalization over almost the entire molecule.
Since the first synthesis of tribenzosubporphines in
2006,[5a] the potential of subporphyrins, ring-contracted cous-
ins of the porphyrin, has been increasingly recognized as
a result of their attractive chemical, optical, and electro-
chemical properties.[5] In most cases so far, subporphyrins
have been explored as functional pigments and their optical
properties have been effectively controlled by taking advant-
age of the large influence that the meso substituents exert on
the electronic network of subporphyrin. Herein we report the
synthesis and characterization of meso-hydroxysubporphyr-
ins, which have the ability to form a self-assembled trimer
through an axial ligand exchange reaction with the meso-
hydroxy group, and can also stabilize a meso-oxy radical
species to such a remarkable extent that it can be manipulated
like a usual closed-shell organic molecule in the presence of
air and moisture.
T
he exploration of stable radicals[1a] has been one of the
central topics in organic radical chemistry in light of their
diverse applications such as in organic synthesis,[1b] polymer
chemistry,[1c] magnetic materials,[1d] organic batteries,[1e] quan-
tum information science,[1f] and bioimaging.[1g] Stabilization of
organic radicals is usually achieved through effective steric
protection and/or extensive spin delocalization.
Oxophlorin and meso-hydroxyporphyrin, a tautomeric
pair that plays an important role in the catabolism of heme
moieties, have been known to be easily and reversibly
oxidized to the corresponding radical, a porphyrin meso-oxy
radical.[2] This radical has been demonstrated to be stable
enough for electrochemical investigations and crystallo-
meso-Chlorosubporphyrin 2 (Scheme 1) was prepared by
chlorination of meso-free subporphyrin 1 with N-chlorosuc-
cinimide in 87% yield.[6] Subporphyrin 1 can now be prepared
[*] D. Shimizu, Prof. Dr. A. Osuka
Department of Chemistry, Graduate School of Science
Kyoto University
Sakyo-ku, Kyoto 606-8502 (Japan)
E-mail: osuka@kuchem.kyoto-u.ac.jp
J. Oh, Prof. Dr. D. Kim
Spectroscopy Laboratory for Functional p-Electronic Systems and
Department of Chemistry, Yonsei University
Seoul 120-749 (Korea)
E-mail: dongho@yonsei.ac.kr
Prof. Dr. K. Furukawa
Centre for Instrumental Analysis, Niigata University
Nishi-ku, Niigata, 950-2181 (Japan)
E-mail: kou-f@chem.sc.niigata-u.ac.jp
Scheme 1. Subporphyrins studied herein.
[**] The work at Kyoto was supported by the JSPS KAKENHI Grant
Numbers No. 25220802 and 25620031. The work at Yonsei was
supported by the Global Research Laboratory
in an improved yield of 17% by modification of an existing
procedure that originally gave 1 in 9.7% yield from 5,15-
diphenyltripyrrane (for details see the Supporting Informa-
tion).[7] By following recently developed SNAr methods,[6]
subporphyrin 2 was treated with an excess of potassium
hydroxide in dimethyl sulfoxide (DMSO) at 508C for 12 h.
(2013K1A1A2A02050183) through the National Research Founda-
tion of Korea (NRF) funded by the Ministry of Science, ICT
(Information and Communication Technologies), and Future Plan-
ning.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201501592.
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Surprisingly, the reaction furnished cyclic trimer 3 as a yellow
solid in 94% yield after usual aqueous work up and
recrystallization. During the work up procedure, the organic
layer initially displayed fluorescence, but the fluorescence
was visibly diminished following removal of the solvent under
reduced pressure, thus indicating that the cyclic trimer forms
preferentially at high concentration. High-resolution atmos-
pheric-pressure-chemical-ionization time-of-flight (HR-
APCI-TOF) mass spectrometry exhibited a signal for the
parent cation of 3 at m/z = 1228.4213 (calcd 1228.4191 for
[3]+; [C81H4811B3N9O3]+). The H NMR spectrum of 3 shows
1
a sharp and simple spectrum that is consistent with its
symmetric structure. A doublet observed at d = 6.60 ppm,
which is related to the pyrrolic b-protons (Ha, see Scheme 1),
is considerably high-field shifted compared with previously
observed values for pyrrolic b-proton resonances of mono-
meric subporphyrins (ca. d = 7.7–8.3 ppm). This effect is most
likely due to the shielding effects of the diatropic ring currents
residing on adjacent subporphyrin units.
X-Ray crystallographic analysis has revealed that the
structure of 3 is indeed a self-assembled molecular triangle, in
which the meso-hydroxy groups are bonded to the central
boron atoms of adjacent subporphyrins (Figure 1). In 3, the
Figure 2. UV/Vis absorption (solid) and emission (dashed) spectra of
3 (in CH2Cl2), 4, and 5 (in CH2Cl2/MeOH with an excess of acid or
base for 4 and 5).
separation distance of 7 Š is adequate for through-space
excitonic interaction. Interestingly, spectroscopic solutions of
trimer 3 were practically nonfluorescent (Ff < 10À3) in aprotic
solvents such as CH2Cl2, CHCl3, acetone, toluene, acetoni-
trile, and DMF (changing the polarity of the medium has no
effect), but became fluorescent upon the addition of protic
solvents such as methanol. The dropwise addition of trifluoro-
acetic acid (TFA) or potassium tert-butoxide to a solution of
cyclic trimer 3 in CH2Cl2/MeOH = 1:1 (Figure 2) results in the
absorption profile undergoing a distinct spectral transforma-
tion. In the case of the dropwise addition of TFA, the gradual
disappearance of the initial spectral profile containing a split
Soret-like band at 349 nm coincides with the gradual appear-
ance of a simple nonsplit Soret-like band at 366 nm. In the
case of the dropwise addition of a dilute solution of KOtBu,
the initial spectral profile of the cyclic trimer is gradually
replaced with a profile containing a Soret-like band at 364 nm
which has a pronounced shoulder. These spectral trans-
formations have been ascribed to the gradual turnover of
subporphyrin monomer 4 or the corresponding anion 5,
respectively. Namely, methanol serves to monomerize 3
through an axial ligand exchange reaction, with the rate of
the process being enhanced under acidic or basic conditions.
This has been confirmed by 1H NMR experiments of 3
conducted in CDCl3/CD3OD (see Figure S3-6 in the Support-
ing Information).
Femtosecond transient absorption (TA) measurements
were carried out to probe the excited-state dynamics of 3
(Figure 3a). The TA spectra of trimer 3 showed a character-
istic broad excited-state absorption (ESA) signal at wave-
lengths longer than 515 nm.[13] However, the TA spectra of 3
rapidly decayed with time components of 2 and 14 ps. Such
a fast TA decay feature of 3 was in sharp contrast to that of
monomer 4, which exhibited typical TA spectral features of
subporphyrins and a lifetime of 2.3 ns (see Figure S6-4 in the
Supporting Information). Although the detailed mechanism
of the accelerated decay in 3 remains unclear, we suspect that
the cyclic system assumes a structural nature and bond
character reminiscent of oxosubphlorin, which may facilitate
the accelerated decay. The transient absorption anisotropy
measurement of 3 revealed a depolarization with a time
component of 0.7 ps (see Figure S6-5 in the Supporting
Information), which allowed us to estimate an exciton
Figure 1. X-ray crystal structures of 3: a) side view and b) top view.
Thermal ellipsoids are set at the 50% probability level. All hydrogen
atoms and solvent molecules are omitted for clarity.
bowl depths[8] of the constituent subporphyrin units are 1.267,
1.354, and 1.356 Š, which are in line with values previously
reported for subporphyrins. Compared with the reference
À
subporphyrin 6, which shows a B O bond distance of 1.439 Š
and a C O bond distance of 1.367 Š,[9] subporphyrin trimer 3
À
À
shows distinctly longer B O bonds (1.480(4), 1.481(4), and
1.489(4) Š) and shorter C O bonds (1.339(4), 1.347(4), and
À
1.351(4) Š), thus indicating a slight meso-oxosubphlorin bond
character. This type of cyclic trimer was reported for
b-hydroxymetalloporphyrins,[10] while meso-hydroxyporphyr-
ins gave dimeric complexes.[11] The angle between the axial
ligand on boron and the meso substituent (i.e. ]O-B-C) in
subporphyrin is approximately 1208, which is ideal for the
formation of a cyclic trimer.[12]
The UV/Vis absorption spectrum of 3 (Figure 2) displays
a split Soret-like band with maxima at 349 and 371 nm and
a Q-like band at 490 nm. The split Soret-like band suggests
that strong exciton coupling exists between the neighboring
subporphyrin units, since the average interchromophoric
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subporphyrin oxy radical 8 is exceptionally stable toward air
and moisture, thereby allowing its manipulation like normal
closed-shell organic molecules. Radical 8 did not show any
change in the solid state and was stable in solutions under
open air at room temperature for up to one month or longer.
Fortunately, we obtained single crystals of 8 and con-
firmed its structure. As shown in Figure 4, 8 is a relatively rare
Figure 4. X-ray crystal structures of 8: a) side view, b) top view. Two
independent molecules are shown. Thermal ellipsoids are set at the
50% probability level. All hydrogen atoms and solvent molecules are
omitted for clarity.
Figure 3. Transient absorption spectra and decay profiles of 3 (a) and
8 (b) in toluene. TA decay profiles of 3 and 8 were obtained by
monitoring at 490 and 750 nm, respectively.
bowl-shaped curved radical[17] with two independent mole-
cules paired with their convex surfaces aligned in a comple-
À
mentary off-set head to head fashion. The C O bond lengths
are 1.224(4) and 1.240(4) Š, being considerably shorter than
hopping time of 2.1 ps on the basis of regular polygon model
for cyclic molecular arrays [see Eq. (1)].[14] These significantly
different excited-state dynamics of trimer 3 and monomer 4
rationalize the sharp contrast in their fluorescence features,
and suggest a potential for subporphyrin-based sensoring
systems.
that of meso-ethoxysubporphyrin 6 (1.367 Š), thus indicating
À
a substantial double-bond character of the C O bond. This
partial double-bond character is further supported by the
characteristic carbonyl-like IR absorption band at 1591 cmÀ1
(calcd: 1660 cmÀ1, see Figure S6-3 in the Supporting Infor-
mation). The bowl depths were 1.322 and 1.337 Š, which are
in the range of those of usual subporphyrins. The pairing of
the two molecules in the unit cell of 8 is aided by mutual CH/p
interactions between the b-hydrogen atom and axial phenyl
group, which results in partial p stacking with a shortest
separation between the b-carbon atoms of 3.853 Š.
thop
h
ꢀ ꢁi
tdep
¼
ð1Þ
2p
N
4 1 À cos2
In the next step, we examined the reactions of 3 with
Subporphyrin 8 is NMR silent as a result of its radical
character and exhibits an ill-defined absorption spectrum
without a sharp Soret-like band or Q-like band. Instead,
a weak and broad band is observed around 1200 nm that is
characteristic of radical porphyrinoid species (Figure 5a). On
the basis of the absorption edge at around 1400 nm, the
excitation energy of the S1 state has been estimated to be
approximately 0.89 eV. Oxy radical 8 was quantitatively
reduced with hydrazine monohydrate to give meso-oxy
phenylmagnesium bromide as an alternative approach for
effecting ring cleavage.[15] Namely, a solution of 3 in 1,4-
dioxane was treated with phenylmagnesium bromide in THF
to give meso-hydroxy-B-phenylsubporphyrin 7 in 54% yield.
It is noteworthy that the reaction of 3 with phenylmagnesium
bromide in THF without dioxane gave B-phenyl-meso-
triphenylsubporphyrin exclusively, thus indicating that the
Grignard reagent attacked both the central boron and
carbonyl-like meso-hydroxy carbon atoms. Therefore, the
addition of dioxane is important in modulating the reactivity
of phenylmagnesium bromide toward 3. Intriguingly, subpor-
phyrin 7 was gradually oxidized in solution under air to give
subporphyrin meso-oxy radical 8. The 1H NMR spectrum of 7
in CDCl3 was silent, probably because of a small presence of
the radical species 8, but became sharp upon addition of one
drop of hydrazine monohydrate to the solution (see the
Supporting Information).[16] Oxy radical 8 was quantitatively
obtained by oxidation of 7 with PbO2. HR-APCI-TOF mass
spectrometry indicated a signal of the parent cation of 8 at m/
z = 486.1776 (calcd 486.1778 for [8]+; [C33H2111BN3O]+).
Despite a smaller porphyrin-like 14p electronic network,
1
anion 9 (conjugate base of 7), as confirmed by H NMR and
UV/Vis measurements (see the Supporting Information). The
radical character of 8 is also evident from its excited-state
dynamics. The TA spectra of 8 decayed rapidly with a time
constant of 1.1 ps (Figure 3b). Since radical species show
ultrafast relaxation process by the greater density of states,[4]
such an extremely short excited-state lifetime of 8 is
consistent with its radical character.
The electronic-spin structure of 8 was examined by ESR
spectroscopy in a degassed 2-methyltetrahydrofuran solution
at room temperature. The ESR spectrum of 8 showed a signal
(g = 2.0039) with hyperfine couplings, which has been simu-
lated as shown in Figure 5b. Simulation of the spectra shows
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by temperature-dependent magnetic susceptibility measure-
ments (SQUID). The Bleaney–Bowers singlet-triplet model
was employed to reproduce the observed cT plot with fitted
parameters f1 = 0.979, f2 = 0.0311, and J1/kB = À64.2 K [see
Figure 5d and Eq. (2)]. This analysis indicated a relatively
strong antiferromagnetic interaction, which can be inter-
preted in terms of intermolecular p-p interactions that are
feasible because of its spin-delocalizing nature.
NAg2m2B
kB
1
NAg2m2B
þ f2
2kB
ꢀ
ꢁ
cT ¼ f1
ð2Þ
2J1
BT
3 þ exp À k
In summary, meso-hydroxysubporphyrin was efficiently
synthesized from meso-chlorosubporphyrin by an SNAr
reaction with KOH. The resulting meso-hydroxysubpor-
phyrin underwent self-assembly to give a subporphyrin-
based cyclic trimer linked at the axial sites of the central
boron atoms and the meso-hydroxy groups. Solutions of this
trimer are nonfluorescent, but become fluorescent when
monomerized to meso-hydroxysubporphyrin upon treatment
with methanol. The reaction of the cyclic trimer with phenyl-
magnesium bromide gave a meso-hydroxy-B-phenylsubpor-
phyrin, which was quantitatively oxidized with PbO2 to give
a subporphyrin meso-oxy radical. This radical is remarkably
stable under ambient conditions, thus allowing its manipu-
lation like a normal closed-shell organic molecule. The
stability of this radical has been ascribed to effective
delocalization of the spin over almost the entirety of the
subporphyrin electronic network. Finding applications for this
very stable subporphyrin meso-oxy radical in materials
chemistry is a topic worthy of further investigation.
Figure 5. a) UV/Vis absorption spectrum of 8, b) the ESR spectrum
observed at room temperature and corresponding simulated spectra,
c) spin density distribution calculated at the UB3LYP/6-311G(d) level
(isovalue: 0.001; the molecular geometry was optimized from the X-ray
crystal structure), and d) temperature-dependent magnetic susceptibil-
ity in the solid state [dots: observed, solid: calculated; see also
Eq. (2)].
the spin is coupled with the b-protons and even with the
protons on the meso-phenyl groups (see Figure S4-1 in the
Supporting Information). Density functional theory (DFT)
calculations[18] of 8 at the UB3LYP/6-311G(d) level also
revealed extensive delocalization of spin density over the
subporphyrin 14p electronic system and even over the meso-
phenyl substituents (Figure 5c). This efficient delocalization
should play a crucial role for the stabilization of the radical in
addition to the steric effect of meso- and axial-phenyl groups,
which probably prevent dimerization at the meso-carbon
atoms. Time-dependent DFT (TD-DFT) calculation of 8 at
UB3LYP/6-311G(d) indicates the lowest energy absorption
band is assigned to the HOMO-SOMO excitation (at
1239 nm, oscillator strength: f = 0.005) and the successive
absorption band is ascribed to SOMO-LUMO excitation (at
624 nm, f = 0.068), which is consistent with the observed
spectrum (see Figure S10-3 in the Supporting Information).
The electrochemical properties of 8 were evaluated by
cyclic voltammetry (CV) in CH2Cl2 containing 0.1m
nBu4NPF6 as a supporting electrolyte versus the ferrocene/
ferrocenium cation (see Figure S9 in the Supporting Infor-
mation). Fairly reversible one-electron oxidation and reduc-
tion waves were observed at 0.34 Vand À0.51 V, respectively,
in line with its stable radical character possessing a SOMO
orbital. The excitation energy gap to the S1 state can be
estimated to be 0.85 eV on the basis of these values. The
magnetic character of 8 in the solid state was also examined
Keywords: boron · porphyrinoids · radicals · self-assembly ·
subporphyrin
How to cite: Angew. Chem. Int. Ed. 2015, 54, 6613–6617
Angew. Chem. 2015, 127, 6713–6717
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Published online: April 9, 2015
´
b) A. L. Balch, L. Latos-Graz˙ynski, B. C. Noll, M. M. Olmstead,
Angew. Chem. Int. Ed. 2015, 54, 6613 –6617
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
6617