Stable π Radical from a Contracted Doubly N-Confused Hexaphyrin by Double Palladium Metalation

Article abstract of DOI:10.1002/anie.201502285

A contracted doubly N-confused dioxohexaphyrin derivative served as a dinucleating metal ligand for unsymmetrical coordination. The complexation of two palladium(II) cations led to the formation of π-radical species that were persistent in atmospheric air

Full text of DOI:10.1002/anie.201502285

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201502285
German Edition: DOI: 10.1002/ange.201502285
Expanded Porphyrins
Stable p Radical from a Contracted Doubly N-Confused Hexaphyrin
by Double Palladium Metalation**
Yutaka Hisamune, Keiichi Nishimura, Koji Isakari, Masatoshi Ishida, Shigeki Mori,
Satoru Karasawa, Tatsuhisa Kato, Sangsu Lee, Dongho Kim,* and Hiroyuki Furuta*
Abstract: A contracted doubly N-confused dioxohexaphyrin
derivative served as a dinucleating metal ligand for unsym-
metrical coordination. The complexation of two palladium(II)
cations led to the formation of p-radical species that were
persistent in atmospheric air in the presence of moisture.
Effective delocalization of an unpaired electron over the
hexaphyrin backbone could contribute to the distinct chemical
stability.
unique “NNCC” coordination cavities to afford various
dinucleating organometallic complexes with a variety of
conformation owing to their inherent topological flexibility.[4]
In contrast, the synthetic analogue of hexaphyrin with two N-
confused pyrrole rings, namely, the doubly N-confused
dioxohexaphyrin 2, displays more versatility but with
a defined conformation in metal complexation.[5] The rela-
tively rigid “NNNO” donor environments of 2 facilitated the
coordination of various divalent metals (e.g., zinc(II), cop-
per(II)) with predictable conformations (Scheme 1).
E
xpanded porphyrinoids have attracted great interest owing
to their unique optical properties as well as redox non-
innocence.[1] For example, the meso-aryl-substituted hexa-
phyrin 1, which consists of six pyrrole rings, exhibits
absorption/emission in the near-infrared (NIR) region and
is a representative derivative capable of redox switching
between a Hückel aromatic 26p species and an antiaromatic
(or Mçbius aromatic) 28p congener.[2] Such redox-rich
p systems can stabilize the open-shell radical species by the
use of their giant p-conjugated networks in some cases.[3]
From the viewpoint of the coordination chemistry of
expanded porphyrinoids, hexaphyrin 1 can provide two
Upon the structural modification of 1 through an “N-
confusion” approach, two metal cations with different oxida-
tion states (e.g., a combination of gold(III) and platinum(II))
were coordinated site selectively in a singly N-confused
oxohexaphyrin 3.[6] To further extend the coordination
chemistry of expanded porphyrinoids, we examined the
skeletal modification of 2, which was expected to produce
an unsymmetrical coordination environment, for example, by
the loss of a meso methine carbon atom within the macro-
cycle. The contraction of the hexaphyrin skeleton would lead
to an unsymmetrical dinucleating coordination environment
with different cavity sizes for metal cations. Herein, we report
the synthesis of a novel contracted version of doubly N-
confused dioxo[26]hexaphyrin with an embedded bipyrrole
unit (i.e., 5,10,15,20,25-pentakis(pentafluorophenyl)-substi-
tuted doubly N-confused dioxo[26]hexaphyrin(1.1.1.1.1.0),
4). The dioxo[26]hexaphyrin 4 coordinated two palladium(II)
atoms to produce an unprecedented stable p-radical complex
[*] Y. Hisamune, K. Nishimura, K. Isakari, Dr. M. Ishida,
Prof. Dr. H. Furuta
Department of Chemistry and Biochemistry, Graduate School of
Engineering, and Education Center for Global Leaders in Molecular
Systems for Devices, Kyushu University
Fukuoka 819-0395 (Japan)
E-mail: hfuruta@cstf.kyushu-u.ac.jp
under ambient conditions rather than a mixed-valent (PdII/III
species.
)
Dr. S. Mori
Integrated Center for Science, Ehime University
Matsuyama 790-8577 (Japan)
The contracted hexaphyrin analogue 4 was synthesized in
a stepwise manner (Scheme 2). Regioselective monoacylation
of the N-confused tripyrrane derivative 5 gave compound 6 in
40% yield.[7] After the reduction of derivative 6 with NaBH4,
subsequent condensation with an excess amount of 5 afforded
the doubly N-confused hexapyrrane precursor 7 in 61% yield.
A [6+0]-type intramolecular oxidative cyclization with 2,3-
dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as the oxi-
Prof. Dr. S. Karasawa
Graduate School of Pharmaceutical Sciences, Kyushu University
Fukuoka 812-8582 (Japan)
Prof. Dr. T. Kato
Institute for Liberal Arts and Sciences, Kyoto University
Kyoto 606-8501 (Japan)
S. Lee, Prof. Dr. D. Kim
dant
yielded
the
doubly
N-confused
[24]hexa-
Spectroscopy Laboratory for Functional p-Electronic Systems and
Department of Chemistry, Yonsei University
Seoul 120-749 (Korea)
phyrin(1.1.1.1.1.0) 8 in 18% yield. Finally, the hexaphyrin 8
underwent reductive oxygenation with sodium dithionite in
aqueous THF to give the desired doubly N-confused dioxo-
[26]hexaphyrin 4.
The structure of 4 was characterized by high-resolution
(HR) mass spectrometry and NMR spectroscopy (see the
Supporting Information). The parent-ion peak of 4 observed
at m/z 1314.0917 is consistent with the proposed structure
([M]+ calcd for C59H14F25N6O2: 1314.0857). The 1H NMR
spectrum of 4 in CDCl3 showed the presence of three sets of
E-mail: dongho@yonsei.ac.kr
[**] We acknowledge financial support through Grants-in-Aid (25248039
to H.F. and 26810024 to M.I.) from the JSPS and by the Global
Research Laboratory (2013K1A1A2A02050183 to D.K.) of the
National Research Foundation of Korea (NRF), funded by the
Ministry of Science, ICT (Information and Communication Tech-
nologies) and Future Planning.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201502285.
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The chemical-shift difference
À
between interior N H atoms and
exterior b-H atoms, DdCH–NH, was
estimated to be 17.0 ppm, which
suggests a stronger aromaticity as
a result of the diatropicity of the
26p-conjugated circuit. The nega-
tive nucleus-independent chemi-
cal-shift (NICS) value calculated
at the global center of the macro-
cycle (d = À13.3 ppm) supports
distinct aromaticity (see Fig-
ure S11 in the Supporting Infor-
mation).
The UV/Vis/NIR absorption
spectrum of 4 shows characteristic
aromatic features: an intense
Soret band split at lmax = 568 and
606 nm and well-resolved Q-like
bands at 726, 792, 877, and
1000 nm (Figure 1). Fluorescence
emission of 4 was observed at
Scheme 1. Structural modifications of hexaphyrin skeletons for metal complexation.
lmax = 1010 nm with
a
small
Stokes shift (99 cmÀ1), again sup-
porting the aromatic nature of 4
(see Figure S12).
Upon palladium complexa-
tion, the electronic properties of
aromatic hexaphyrin 4 were dra-
matically altered. Metalation was
performed by mixing a solution of
4
in CH2Cl2/MeOH with Pd-
(OAc)2 (10 equiv) at room tem-
perature. Silica-gel column-chro-
matographic separation then gave
a pure green fraction of the bis-
palladium complex 4-Pd2 in 75%
yield. The UV/Vis/NIR absorp-
tion spectrum of the product
exhibited a less intense Soret-like
band at lmax = 637 nm (e = 5.9 ”
104 mÀ1 cmÀ1) and broad Q-like
bands reaching into the NIR
region (Figure 1).[8] The complex
exhibited a parent-ion peak at m/
z = 1520.8564 ([M]+ calcd for
C59H10F25N6O2Pd2: 1520.8529) in
the HR mass spectrum, thus indi-
cating the formation of a bispalla-
dium complex, 4-Pd2.
Scheme 2. Synthesis of the contracted doubly N-confused dioxohexaphyrin bispalladium(II) complex 4-
Pd2. TFA=trifluoroacetic acid.
The structure of bispalladium
complex 4-Pd2 was determined
unambiguously by X-ray crystallo-
interior NH hydrogen atoms with high-field resonances (i.e.,
d = À6.23, À2.58, À1.11 ppm), which were assigned by a D2O-
exchange experiment. The peripheral b-pyrrolic hydrogen
atoms appeared at relatively low field around 9.3–10.8 ppm.
Comparison with the spectrum of the hexaphyrin 2 led us to
anticipate the rectangular-type conformation in solution.[5]
graphic analysis (Figure 2; see also Figure S13).[9] Two palla-
dium cations reside within the macrocyclic NNNO cavities
with square-planar geometry. The overall core structure of 4-
Pd2 adopts a coplanar conformation. Both inner carbonyl
groups are oriented toward the same side of the macrocyclic
plane. The carbonyl oxygen atom located on the side of the
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Figure 1. UV/Vis/NIR spectra of 4 and 4-Pd2 in CH2Cl2.
Figure 3. a) EPR spectrum of 4-Pd2 in toluene at 300 K. b) Spin-denÀs1ity
map calculated at the UB3LYP/LANL2DZ and 6-31G** level. c) cmol
T plot of solid 4-Pd2, as determined by SQUID. Inset: cmol T–T plot of
solid 4-Pd2.
–
result indicates the formation of an organic p-radicaloid
species upon palladium complexation of the contracted
[26]hexaphyrin 4.
To examine the above rationale further, we determined
the temperature-dependent magnetic susceptibility of solid 4-
Pd2 by the use of a superconducting quantum interference
device (SQUID; Figure 3c). The cmol T value of 4-Pd2
recorded at room temperature is nearly identical to the
theoretical index of 0.375 emuKmolÀ1 expected for the
Figure 2. X-ray crystal structure of 4-Pd2 with thermal ellipsoids at
50% probability: a) top view, b) side view, and c,d) stacked dimeric
structures. Solvent molecules and meso aryl groups are omitted for
clarity in (b–d).
doublet species.[10] The cmol value decreased gradually as
À1
À1
smaller coordination sphere deviates significantly from the
mean plane of the macrocycle (mean deviation value defined
by 35 atoms: 0.36 Š).
the temperature was lowered. Fitting analysis of the cmol
value in the range from 300 to 60 K by the use of the Curie–
Weiss equation gave rise to a negative Weiss constant, q =
À42.9 K, which suggests the existence of an intermolecular
antiferromagnetic interaction (see Figure S15). The crystal-
packing structure of 4-Pd2 indeed revealed the formation of
a weak p-stacking dimer with slipped parallel orientations
(Figure 2d). The two interacting macrocyclic planes in the
dimer have an intermolecular short atomic contact of 3.266 Š
(see Figure S16), which is less than the sum of the van der
Waals radii of two carbon atoms (3.4 Š).[11] This arrangement
may arise from SOMO–SOMO overlap, despite the presence
of the bulky meso pentafluorophenyl rings of 4-Pd2 (Fig-
ure 2c).
In an effort to determine the redox properties of 4-Pd2, we
carried out cyclic voltammetry in CH2Cl2 containing 0.1m n-
tetrabutylammonium hexafluorophosphate as the electrolyte.
A reversible one-electron oxidation wave at Eox = 0.29 V
(versus ferrocene/ferrocenium) and two reversible one-elec-
tron reduction waves at Ered = À0.18 and À1.38 V were
observed (Figure 4). These waves were assigned by spectro-
The bispalladium complex 4-Pd2 demonstrated a unique
paramagnetic character. The 1H NMR spectrum of 4-Pd2
exhibited no resonance peaks in the usual diamagnetic
region (À10– + 15 ppm). Instead, an active electron para-
magnetic resonance (EPR) signal at a g-tensor of 2.0021 was
observed in toluene at 300 K (Figure 3a). The shape of the
EPR signal of solid 4-Pd2 remained unchanged at a lower
temperature (ca. 4 K; see Figure S14). Quantification of the
amount of unpaired spin and the presence of no signals
corresponding to DmS =Æ 2 in the half-field region of 4-Pd2
suggested a typical organic monoradical (S = 1/2) nature. In
fact, unrestricted DFT (UB3LYP) calculations of the struc-
ture of 4-Pd2 reproduced the coplanar structure with an open-
shell electronic character. The spin-density map of 4-Pd2
demonstrates that the unpaired electron is delocalized over
the whole p framework of the hexaphyrin ligand, whereas
marginal spin densities exist on both palladium centers
(Mulliken spin density: 0.013951, 0.008796; Figure 3b). This
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of 4-Pd2 was estimated to be 2.6 ppm, which supports this
conclusion (see Figure S22). In fact, this neutral radical
species 4-Pd2 reveals significant chemical stability toward
dioxygen as well as moisture (see Figure S23). The rigid and
larger p-conjugated hexaphyrin scaffold could enable stabi-
lization of the unpaired electron through delocalization over
the macrocyclic circuit and kinetic blocking by the meso aryl
groups.[3]
The third-order nonlinear optical (NLO) response of 4-
Pd2 is also of interest in terms of the open-shell electronic
structure, which gives rise to underlying static and dynamic
polarizability.[15] The two-photon-absorption (TPA) cross-
section value (s(2)) of 4-Pd2 was determined by an open-
aperture Z-scan method in the wavelength region ranging
from 1500 to 2200 nm, in which one-photon absorption can be
ignored (see Figure S24). The radical complex 4-Pd2 exhibited
a maximum TPA cross-section value of 2100 GM upon
photoexcitation at 1500 nm, whereas the closed-shell bispal-
ladium(II) complex 2-Pd2 exhibited a maximum value of
900 GM at the same photoexcitation wavelength. The larger
TPA cross-section value of 4-Pd2 might be attributed to its
distinct spin-delocalized p-radical nature.
In conclusion, we have synthesized and characterized
a novel skeletally contracted doubly N-confused hexaphyrin
derivative 4, which acts as a ligand for unsymmetrical metal
coordination, that is, in the trianionic and dianionic NNNO
cavities inside the macrocycle.[16] The bispalladium(II) com-
plex 4-Pd2 showed a highly stable p-radical character
identified by various spectroscopic and magnetometric
means as well as theoretical calculations. The durability of
the 25p neutral hexaphyrin radical 4-Pd2 under ambient
conditions is notable and of scientific importance for the
further extension of unsymmetrical-hexaphyrin-based coor-
dination chemistry.
Figure 4. Cyclic voltammogram of 4-Pd2 in CH2Cl2 containing 0.1m
nBu4NPF6. Scan rate: 100 mVsÀ1. The cationic, neutral, and anionic
states are assigned as states I–III, respectively.
electrochemical analysis of 4-Pd2, whereby absorption spectra
were obtained during electrochemical reaction in a thin-
layered cell (see Figure S17). The original absorption spec-
trum of neutral 4-Pd2 (i.e., state II) was changed upon
oxidation at 0.5 V. The spectrum of the product (i.e., state I)
after the oxidation of 4-Pd2 exhibited a broad spectral feature
with intense bands in the UV region, which could be
interpreted according to a typical 4n p-electronic structure.[12]
In contrast, a different pattern of spectral change was
observed upon reduction at À0.5 V (state III): A Soret-like
but broad band at lmax = 677 nm and relatively intense Q-
bands with vibronic structures were observed in the NIR
region. This spectral feature was reproduced by the chemical
reduction of 4-Pd2 with decamethylferrocene (1 equiv; see
Figure S18). Intriguingly, the absorption spectrum of the one-
electron-reduced anionic product derived from 4-Pd2 resem-
bles that of the bispalladium(II) doubly N-confused dioxo-
[26]hexaphyrin 2-Pd2, thus suggesting the regeneration of
aromaticity in anionic 4-Pd2.[13] The remarkably narrow
HOMO–LUMO energy gap of 0.47 V could be rationalized
by a significant anodic shift of the first reduction potential
relative to the potentials of the free base 4; first oxidation and
reduction potentials of Eox = 0.29 V and Ered = À1.37 V were
found (see Figure S19 and Table S2). This result is consistent
with the theoretical calculations. The marked stabilization of
the b-spin LUMO of 4-Pd2 contributes to the narrow energy
gap (see Figure S20).
Keywords: expanded porphyrins · N-confusion · palladium ·
stable radicals · two-photon absorption
How to cite: Angew. Chem. Int. Ed. 2015, 54, 7323–7327
Angew. Chem. 2015, 127, 7431–7435
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radical 4-Pd2, bispalladium complexation by the hexaphyrin
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[7] A neglible amount of the product formed by monoacylation at
the opposite a position was detected. The regioselectivity of the
acylation of 3 is discussed in the Supporting Information.
[8] A lowest-energy transition at 1818 nm (0.68 eV, f = 0.0002) was
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transition by a time-dependent (TD) DFT calculation according
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Received: March 11, 2015
Published online: May 7, 2015
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