Porphyrin-Based Air-Stable Helical Radicals

Article abstract of DOI:10.1002/chem.201705291

Stable helical radicals are promising multi-functional molecules in light of intriguing magnetic and chiroptical properties. Attempts were made to extend diphenylmethyl-fused Ni^II porphyrin radical to helical system as the first air-stable orga

Full text of DOI:10.1002/chem.201705291

A Journal of
Accepted Article
Title: Porphyrin-based Air-stable Helical Radicals
Authors: Atsuhiro Osuka, Kenichi Kato, Ko Furukawa, and Tadashi
Mori
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Porphyrin-based Air-stable Helical Radicals
Kenichi Kato,[a] Ko Furukawa,*[b] Tadashi Mori,[c] and Atsuhiro Osuka*[a]
Abstract: Stable helical radicals are promising multi-functional
molecules in light of intriguing magnetic and chiroptical properties.
Attempts were made to extend diphenylmethyl-fused NiII porphyrin
radical to helical system as the first air-stable organic neutral helical
radicals. Intramolecular Pd-catalyzed twofold C–H arylation of
methyl- or methoxy-introduced meso-diphenylmethyl NiII porphyrins
gave a mixture of the target and rearranged radicals. Oxidative
fusion reaction of meso-(bis(1-naphthyl)methyl) NiII porphyrins
provided doubly fused NiII porphyrin radicals. One of the helical
radicals was separated into enantiomers that showed mirror-image
CD spectra up to 1300 nm. The helical dinaphthylmethyl-fused NiII
porphyrin radical displayed solid-state magnetic property mostly
arising from monomeric radicals, different from the parent
linked porphyrin skeleton.
diphenylmethyl-fused
NiII
porphyrin
radical
that
showed
antiferromagnetic coupling due to π-stacked pairing.
Stable organic radicals are promising functional compounds
in light of characteristic electrochemical and magnetic properties
originating from unpaired electrons.[1] Incorporation of a stable
radical into a chiral framework is attractive because unique
attributes arising from spin and chirality are realized in a single
molecule.[2] As chiral stable radicals, neutral nitroxide radicals
with a chiral substituent have been studied.[3,4] These radicals
are characterized by a localized spin at the nitroxide and in
some cases the magnetic moments of the radicals have been
successfully arranged to show chiral molecular magnets with the
aid of chiral substituents.[4] In contrast, radicals extensively
delocalized over helical π-networks[5] have been quite rare, while
they are promising because of their intense circular dichroism
(CD) responses. To the best of our knowledge, only three
examples (1, 2, and 3)[6] were reported as stable neutral radicals
delocalized over helically chiral π-frameworks, which are
however somewhat unstable toward oxygen,[7] hampering further
studies on their magnetic properties.
Chart 1. Stable organic radicals with helically delocalized spin densities. Ar =
3,5-di-tert-butylphenyl.
Firstly, introduction of methyl or methoxy substituents at the
2,2'-positions in 4 was examined in order to induce helical
structures for the fused diphenylmethyl segment. Following our
previous procedure,[8] 2,18,20-trichloro-5,10,15-tris(3,5-di-tert-
butylphenyl) NiII porphyrin 6[9a] was reacted with 2,2'-substituted
diphenylmethyllithiums to produce the adducts 7Me and 7OMe
in 53% and 42% yields respectively (Scheme 1). Conversion of
these compounds into the corresponding helical radicals rac-
8Me-A and rac-8OMe-A were attempted via twofold palladium-
catalyzed intramolecular C–H arylations and spontaneous
hydrogen atom loss from the central carbon.[8] Both products
were fairly stable and isolated by silica gel column
chromatography in 48% and 79% yields respectively. Naturally,
1
these radicals are H NMR silent and ESR active, displaying a
signal at around g = 2.0036. Oxidation of these radicals with
tris(4-bromophenyl)ammoniumyl hexachloroantimonate (magic
blue) gave the corresponding close-shell cations that showed
more complicated 1H NMR spectra than expected for the cations
of rac-8Me-A or rac-8OMe-A. Characteristically, three singlet
peaks for the methyl or methoxy protons were observed,
indicating that both cations and hence their parent radicals were
Recently, we reported that diphenylmethyl-fused porphyrin
radical 4 is a very stable radical under ambient conditions owing
to effective spin-delocalization over the whole π-electronic
network.[8] Since 4 has a [4]helicene moiety as a sub-structure,
this molecular system may be promising as a platform to realize
air-stable chiral radicals whose spin is distributed over the
helicene architecture. Herein, we report the synthesis and
characterization of air-stable helical radicals based on the triply
mixtures of
a C2-symmetric and C1-symmetric products.
Examination of the integral values revealed the ratios of these
isomers to be 32/68 for rac-8Me-A/rac-8Me-B and 22/78 for
rac-8OMe-A/rac-8OMe-B, respectively. In line with these results,
we fortunately obtained two different crystals of rac-8Me-A and
rac-8Me-B and confirmed that rac-8Me-A is the target
compound and rac-8Me-B is a rearranged product (Figures
1a,b). Unfortunately, both mixtures were hard to separate by
silica gel column chromatography.
[a]
K. Kato, Prof. Dr. A. Osuka
Department of Chemistry, Graduate School of Science
Kyoto University
Sakyo-ku Kyoto, 606-8502 (Japan)
Fax: (+81) 75-753-3970
E-mail: osuka@kuchem.kyoto-u.ac.jp
Prof. Dr. K. Furukawa
[b]
Center for Instrumental Analysis
Niigata University
In the next step, we employed a bis(1-naphthyl)methane unit
as a fused segment, since it will ensure a helical structure and
additionally holds a large π-conjugated system that may be
favorable for oxidative fusion reaction. If so, we may escape the
palladium-catalyzed intramolecular C–H arylations for the fusion
step, which might induce undesirable rearrangement. SNAr
Nishi-ku, Niigata 950-2181 (Japan)
Prof. Dr. T. Mori
Department of Applied Chemistry, Graduate School of Engineering
Osaka University
[b]
Suita, Osaka 565-871 (Japan)
Supporting information for this article is given via a link at the end of
the document.
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Scheme 1. Synthesis of helical diarylmethyl-fused porphyrins. Reagents and conditions: a) (2-R-C6H4)2CHLi (2.0 equiv), THF, –98 °C to RT, 2 h; b) Pd(OAc)2 (20
mol%), PCy3•HBF4 (40 mol%), K2CO3 (5.0 equiv), toluene, reflux, overnight; c) bis(1-naphthyl)methyllithium (2.0 equiv), THF, 0 °C to RT, 1 h; d) DDQ (10 equiv),
Sc(OTf)3 (10 equiv), toluene, 110 °C, 1 h. Ar = 3,5-di-tert-butylphenyl. Selected regions of 1H NMR spectra of rac-8Me-A/rac-8Me-B, rac-8OMe-A/rac-8OMe-B,
rac-5Ar, and rac-5Mes in the presence of an excess amount of (4-Br-C6H4)3NSbCl6 in CDCl3 at 25 °C.
dicyano-p-benzoquinone (DDQ) was then attempted, which
afforded only a singly fused product after 1 h reaction at room
temperature. No doubly fused compound was detected even for
longer reaction time, and the oxidative coupling at 50 °C resulted
in severe decomposition. Use of a combination of DDQ and
Sc(OTf)3 at 110 °C produced a small amount of the doubly-fused
product. After further optimization of the reaction conditions and
workup procedure, the target compound rac-5Ar was isolated in
5.7% yield. rac-5Ar showed the HR-APCI-TOF-MS peak at m/z
= 1191.5782 (calculated for the parent ion [C83H81N458Ni]–:
1191.5809), silent 1H NMR spectrum except a broad peak due to
the tert-butyl groups, and ESR signal at g = 2.0035, suggesting
the generation of radical species with the triply linked fused
porphyrin skeleton. 10,20-Dimesityl-15-p-tolyl-substituted radical
rac-5Mes was also synthesized following the same fusion
reaction in an improved yield of 19%. Again, these helical
radicals are stable under ambient conditions and can be handled
like usual close-shell organic molecules. The corresponding
cations of these naphthalene-based radicals generated with
magic blue showed C2-symmetric 1H NMR spectra,[10] supporting
non-rearranged structures.
X-Ray crystallographic analysis revealed the structure of
rac-5Ar to possess a fully conjugated π-framework involving a
[6]helicene unit (Figures 1c,d). The molecule exists as a
monomer without any π-stacking interactions in the crystal,
though several CH–π interactions with co-crystallizing benzenes
Figure 1. X-Ray crystal structures of a) rac-8Me-A, b) rac-8Me-B, and c,d)
and other molecules were observed, different from the
rac-5Ar. Thermal ellipsoids are scaled to 50% probability. Solvent molecules,
antiparallel π-stacked dimer of diphenylmethyl-fused radical 4.[8]
tert-butyl groups, and all hydrogen atoms are omitted for clarity. e) Spin
density distribution plot of 5' calculated at the UB3LYP/6-31G(d) level, where
The sum of three angles around the central carbon is 360.0(6)°,
meso-aryl groups were replaced with phenyl groups (isovalue: 0.001).[11]
indicating perfect sp2-hybridization. The bond lengths around the
central carbon are 1.428(3) Å for C(central)–C(meso) and
1.446(4) and 1.464(4) Å for C(central)–C(1-naphthyl). These
reaction of 5-chloro-10,15,20-tris(3,5-di-tert-butylphenyl) NiII
structural data imply that the radical was stabilized by full
porphyrin 9Ar[9b] with bis(1-naphthyl)methyllithium provided
meso-(bis(1-naphthyl)methyl)porphyrin 10Ar in 92% yield.
Oxidative fusion of 10Ar with FeCl3 and 2,3-dichloro-5,6-
This feature has been supported by the calculated spin
delocalization over the whole fused π-system with more effective
contribution from the porphyrin segment than naphthyl groups.
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distribution map of the model compound 5', which shows larger
densities over the porphyrin macrocycle as compared with those
over the naphthalene moieties (Figure 1e).
As shown in Figure 2a, the UV/vis/NIR absorption spectra of
rac-5Ar and rac-5Mes are similar to that of 4, exhibiting a
relatively weak Soret-like band (around 490 nm), Q-like bands
(560–600 nm), and several broad absorption in the range of 650
to 1100 nm, and weak bands reaching to 1650 nm. These
spectra are characteristic of porphyrin radicals.[12] The redox
behaviors of rac-5Ar and rac-5Mes were studied by cyclic
voltammetry and differential pulse voltammetry in CH2Cl2
(Figures 2b,c). Both radicals displayed highly reversible
oxidation and reduction waves with narrow electrochemical
gaps. The redox potentials of each radical showed small positive
shifts by 30 to 60 mV as compared with those of diphenylmethyl-
fused porphyrin 4.[8]
Figure 3. a) Observed (circle) and simulated (line) χT values of rac-5Mes.
Applied field: 0.5 T. b) UV/vis/NIR absorption spectrum of rac-5Mes (above)
and CD spectra of separated fractions of 5Mes (below) in THF.
The helical radicals rac-5Ar and rac-5Mes are quite stable
under ambient conditions, which allowed us to examine the
optical resolution by HPLC using a chiral column. Although rac-
5Ar was inseparable under several conditions, rac-5Mes was
successfully resolved into a pair of enantiomers.[13] The CD
spectra of the separated fractions as well as the UV/vis/NIR
absorption spectrum of rac-5Mes in THF are represented in
Figure 3b. The first fraction showed a positive Cotton effect at
491 and 1299 nm and a negative one at 316, 413, 668, and
1018 nm. The CD spectrum of the second fraction was the
mirror image of that of the first. Owing to large absorption
coefficient, the intensity of Soret-like band (491 nm, |Δε| =
1.4×102 M–1 cm–1, |gCD| = |Δε/ε| = 0.99×10–3) was comparable to
that of the helicene-based signal (316 nm, |Δε| = 98 M–1 cm–1,
|gCD| = 3.8×10–3), though dissymmetry factor |gCD| was small.
Notably, longer wavelength absorptions exhibited moderate
intensity (668 nm, |Δε| = 19 M–1 cm–1, |gCD| = 2.5×10–3), and
weak but clear CD bands were observed even in the region over
1000 nm, indicating that frontier orbitals of porphyrin were
effectively desymmetrized by the incorporation of the [6]helicene
segment. The UV/vis/NIR and CD spectra were appropriately
reproduced by the TD-DFT calculations at the UB3LYP/6-
31G+(d) level with small blue shifts,[11] and the second fraction
Figure 2. a) UV/vis/NIR absorption spectra of 4, rac-5Ar, and rac-5Mes in
CH2Cl2. Cyclic voltammograms of b) rac-5Ar and c) rac-5Mes in CH2Cl2
containing 0.1 M nBu4NPF6 as a supporting electrolyte. Scan rate: 0.05 V/s;
working electrode: Pt; counter electrode: Pt wire; reference electrode:
Ag/AgNO3. Ferrocene/ferrocenium cation was used as external reference.
Potentials marked with * were determined by differential pulse voltammetry.
The magnetic susceptibility of rac-5Mes was also examined
by temperature dependent SQUID measurement for
a
polycrystalline sample (Figure 3a). The observed χT values were
well reproduced with the Curie-Weiss model with fitted
parameters (C, θp) = (0.323 emu K mol–1, –2.7 K). This result
indicates that each magnetic moment exists almost
independently with rather weak antiferromagnetic intermolecular
interactions in the solid state, which is consistent with the crystal
packing structure of rac-5Ar.
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2002, 41, 586; Angew. Chem. 2002, 114, 606; c) C. Train, M. Gruselle,
M. Verdaguer, Chem. Soc. Rev. 2011, 40, 3297.
turned out to be the (P)-isomer and the first (M)-isomer (Figure
S10-3 in SI).
In summary, diarylmethyl-fused NiII porphyrin radicals have been
extended to helical system as the first air-stable organic neutral
helical radicals. Intramolecular Pd-catalyzed twofold C–H arylation of
7Me and 7OMe gave the target helical radicals rac-8Me-A and rac-
8OMe-A but produced the rearranged products rac-8Me-B and rac-
8OMe-B as major products as well. On the other hand, the oxidative
fusion reaction of 10Ar and 10Mes provided the corresponding
doubly fused NiII porphyrin radicals rac-5Ar and rac-5Mes. The
helical radical rac-5Mes was separated into enantiomers that
showed mirror-image CD spectra up to 1300 nm. Different from the
antiferromagnetically π-stacked pairing of 4, the helical radical rac-
5Mes displayed solid-state magnetic property mostly arising from
monomeric radicals. Further studies on porphyrin-based stable
radicals are actively pursued in our laboratories.
[5]
[6]
Reviews on helicene: a) R. H. Martin, Angew. Chem. Int. Ed. 1974, 13,
649; Angew. Chem. 1974, 86, 727; b) Y. Shen, C.-F. Chen, Chem. Rev.
2012, 112, 1463.
a) A. Ueda, H. Wasa, S. Suzuki, K. Okada, K. Sato, T. Takui, Y. Morita,
Angew. Chem. Int. Ed. 2012, 51, 6691; Angew. Chem. 2012, 124,
6795; b) Y. Wang, H. Zhang, M. Pink, A. Olankitwanit, S. Rajca, A.
Rajca, J. Am. Chem. Soc. 2016, 138, 7298; c) P. Ravat, P. Ribar, M.
Rickhaus, D. Häussinger, M. Neuburger, M. Juríček, J. Org. Chem.
2016, 81, 12303. Stable diradicaloids based on [5]helicene backbones
were also reported: d) J. Liu, P. Ravat, M. Wagner, M. Baumgarten, X.
Feng, K. Müllen, Angew. Chem. Int. Ed. 2015, 54, 12442; Angew.
Chem. 2015, 127, 12619; e) P. Ravat, T. Šolomek, M. Rickhaus, D.
Häussinger, M. Neuburger, M. Baumgarten, M. Juríček, Angew. Chem.
Int. Ed. 2016, 55, 1183; Angew. Chem. 2016, 128, 1198.
[7]
These radicals were reported to be stable and characterized in solution
state under strictly oxygen-free conditions, and radical 1 was reported
to survive in the solid state in the air at –30 °C for a few weeks.
K. Kato, W. Cha. J. Oh, K. Furukawa, H. Yorimitsu, D. Kim, A. Osuka,
Angew. Chem. Int. Ed. 2016, 55, 8711; Angew. Chem. 2016, 128, 8853.
a) N. Fukui, H. Yorimitsu, A. Osuka, Angew. Chem. Int. Ed. 2015, 54,
6311; Angew. Chem. 2015, 127, 6409; b) L.-M. Jin, J.-J. Yin, L. Chen,
C.-C. Guo, Q.-Y. Chen, Synlett 2005, 2893.
[8]
[9]
Acknowledgements
The work at Kyoto was supported by Grant-in-aid for
Scientific Research (S): 25220802 and JSPS Fellows:
17J09048. K.K. appreciates JSPS’s Research Fellowship for
Young Scientists. The authors acknowledge Prof. Dr. Hideki
Yorimitsu (Kyoto University) for HRMS and JASCO Corporation
for CD measurements.
[10] The cations of rac-5Ar and rac-5Mes exhibited significantly up-field
shifted porphyrinic β-proton peaks due to the contributions of expanded
20π-antiaromatic circuits (ref. 8).
[11] a) Gaussian09, revision E.01, M. J. Frisch et al. Gaussian, Inc.
Wallingford CT, 2009; b) A. D. Becke, J. Chem. Phys. 1993, 98, 1372;
c) C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785.
[12] Stable porphyrinoid radicals: a) T. Koide, G. Kashiwazaki, M. Suzuki, K.
Furukawa, M.-C. Yoon, S. Cho, D. Kim, A. Osuka, Angew. Chem. Int.
Ed. 2008, 47, 9661; Angew. Chem. 2008, 120, 9807; b) M. Ishida, J.-Y.
Shin, J.-M. Lim, B. S. Lee, M.-C. Yoon, T. Koide, J. L. Sessler, A.
Osuka, D. Kim, J. Am. Chem. Soc. 2011, 133, 15533; c) D. Shimizu, J.
Oh, K. Furukawa, D. Kim, A. Osuka, Angew. Chem. Int. Ed. 2015, 54,
6613; Angew. Chem. 2015, 127, 6713; d) P. Schweyen, K. Brandhorst,
R. Wicht, B. Wolfram, M. Bröring, Angew. Chem. Int. Ed. 2015, 54,
8213; Angew. Chem. 2015, 127, 8331; e) D. Shimizu, J. Oh, K.
Furukawa, D. Kim, A. Osuka, J. Am. Chem. Soc. 2015, 137, 15584; f) L.
J. Esdaile, L. Rintoul, M. S. Goh, K. Merahi, N. Parizel, R. M. Wellard,
S. Choua, D. P. Arnold, Chem. Eur. J. 2016, 22, 3430; g) D. Shimizu, K.
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Keywords: radical • porphyrinoid • helical structures • circular
dichroism • fused-ring systems
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[13] Conditions for the optical resolution of rac-5Mes: SUMICHIRAL OA-
3100 column; φ = 10 mm; flow rate = 1.0 mL/min; 15 °C; eluent =
CH2Cl2/n-hexane (20/80).
a) H. Kumagai, K. Inoue, Angew. Chem. Int. Ed. 1999, 38, 1601;
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Diarylmethyl-fused NiII porphyrin
radicals have been extended to helical
system as the first air-stable organic
neutral helical radicals. Di(1-
K. Kato, K. Furukawa,* T. Mori, and A.
Osuka*
Page No. – Page No.
naphthyl)methyl-fused NiII porphyrin
radicals thus synthesized were
separated into enantiomers that
showed mirror-image CD spectra up
to 1300 nm, indicating the effective
desymmetrization of the frontier
molecular orbitals.
Porphyrin-based Air-stable Helical
Radicals
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