Synthesis and biradicaloid character of doubly linked corrole dimers

Article abstract of DOI:10.1021/ja062654z

Directly linked corrole dimer 1, which has a planar cyclooctatetraene in its core, has been synthesized from 2-borylcorrole in excellent yield. 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) oxidation of 1 provides its oxidized form 4, which has two NH p

Full text of DOI:10.1021/ja062654z

Published on Web 09/01/2006
Synthesis and Biradicaloid Character of Doubly Linked Corrole Dimers
Satoru Hiroto,† Ko Furukawa,‡ Hiroshi Shinokubo,*,† and Atsuhiro Osuka*,†
Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Sakyo-ku, Kyoto 606-8502, Japan, and
CREST & PRESTO, Japan Science and Technology Agency (JST), and Institute for Molecular Science, Myodaiji,
Okazaki 444-8585, Japan
Received April 17, 2006; Revised Manuscript Received August 17, 2006; E-mail: hshino@kuchem.kyoto-u.ac.jp; osuka@kuchem.kyoto-u.ac.jp
Scheme 1a
Cyclic π-conjugated systems have attracted considerable interest
in terms of their applications as molecular materials having
conductive or nonlinear optical properties, as building blocks for
supramolecular structures, and for complexation with metals or ions.
Our group has developed meso-meso, â-â, â-â, triply linked
porphyrin oligomers, which exhibit extremely low-energy electronic
absorption bands that reach into the infrared region because of the
extensive full π-conjugation.1 Recently, this strategy has been
extended to a two-dimensional porphyrin sheet that bears a planar
cyclooctatetraene (COT) core, which exhibits paratropic magnetic
effects.2 This direct fusion strategy has been now applied to corrole,
a porphyrin-like 18-π aromatic macrocycle except for one direct
pyrrole-pyrrole linkage, which displays a unique ability to stabilize
unusually high valence state transition metals.3,4 Some nickel(II)
and copper(II) corroles have been reported to show radical charac-
ter.5 We report the synthesis and anomalous properties of the doubly
linked corrole dimer 1 and its metal complexes that hold a formal
COT moiety in the center of the molecule. Notably, oxidized coun-
terparts of 1 and its zinc(II) complex exhibit biradicaloid character.
The synthesis of doubly linked corrole dimer 1 began with
2-borylcorrole 2 (Scheme 1).6 Palladium-catalyzed oxidative cou-
pling of 2 using chloroacetone as an oxidant afforded the 2,2′-
linked corrole dimer 3 in excellent yield.7 Further oxidation of 3
with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in toluene
provided one product, 4, as an air- and moisture-stable solid in
moderate yield. The electronic absorption spectrum of 4 showed a
broad spectrum reaching the near-infrared region. Reduction of 4
with NaBH4 resulted in a solution color change from brown to green
and gave 1 quantatively. Its parent mass ion peak was observed at
m/z ) 1587.1096 (calcd for (C74H17N8F30)- ) 1587.1103 [(M -
H)-]) in its high-resolution electrospray ionization time-of-flight
(HR ESI-TOF) mass spectrum. The 1H NMR spectrum of 1
elucidated its symmetrical structure, exhibiting one singlet peak
(8.91 ppm) and two doublet peaks (8.66 and 8.48 ppm) in the
deshielded region. The electronic absorption spectrum of 1 exhibits
three main bands at 396, 472, and 721 nm, which are characteristic
of corroles but slightly red-shifted compared to those of monomers
(Figure 1).
a Reaction conditions: (a) PdCl2(dppb), chloroacetone, THF, H2O at 70
°C, 6 h, 82%; (b) DDQ, toluene, at 50 °C, 1.5 h; (c) NaBH4, THF, MeOH,
room temperature, 30 min, 67% from 3.
Figure 1. Electronic absorption spectra of 1 (black solid line), 4 (red solid
line), 5 (black dashed line), and 6 (red dashed line). All spectra were
recorded in CH2Cl2 solution.
Scheme 2
DDQ oxidation of 1 yielded 4 quantitatively, indicating that 4
is an oxidized form of 1 (Scheme 2). To gain knowledge about
these compounds, we performed the metalation of 1 and 4. When
1 was subjected to the conditions for cobalt metalation of corrole
monomers, the bis-cobalt(III) complex 5 was obtained in good yield.
On the other hand, 4 was zincated with Zn(OAc)2/2H2O in refluxing
CHCl3 to afford the bis-zinc complex 6 as a stable brown solid.
Since corroles usually serve as ligands for trivalent metals, it is
quite rare that a corrole can accommodate divalent metal ions such
as the zinc ion.8 These complexes showed electronic absorption
spectra similar to those of the corresponding free bases 1 and 4,
respectively, indicating preservation of their electronic and geo-
metric structures upon metalation. Complex 6 showed its parent
mass ion peak at m/z ) 1714.5249 (calcd for (C74H12F30N8Zn2)-
) 1714.5272 ([M]-)) and exhibited very broad (virtually no signal)
1H NMR spectra in CD2Cl2 and THF-d8, even at -90 °C. The
structures of the two complexes were finally confirmed by X-ray
single-crystal analysis. Curiously, both complexes exhibited quite
planar structures, including the COT cores (Figure 2). These
† Kyoto University and JST.
‡ Institute of Molecular Science.
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J. AM. CHEM. SOC. 2006, 128, 12380-12381
10.1021/ja062654z CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
calculated to be -178 cm-1. Calculation at the CASSCF(2,2)/
631TZVP//UBLYP/631TZVP level gave an admixture of 43.5%
of the doubly excited configuration 1ΦH,HfL,L into the singlet ground
state. From this result, the singlet biradical character of 6 is
estimated to be approximately 87%.
In summary, we have synthesized and characterized doubly
linked corrole dimers. The oxidized corrole dimer 6 shows unusual
biradical character, which has been indicated by UV-vis absorption
spectra, 1H NMR spectra, magnetic measurements, and DFT
calculations. X-ray analysis of 6 has revealed a planar COT core.
Although several stable singlet biradicals have been reported, this
is the first report of biradicaloids based on porphyrinoids, to the
best of our knowledge. The high stability of 6 in the air at ambient
temperature is notable, and its application in molecular devices and
material science is of worthy of further investigation.
Figure 2. X-ray crystal structures of 5‚(PPh3)2 and 6‚(dioxane)2. Top views
of 5‚(PPh3)2 (a) and 6‚(dioxane)2 (b), and side views of 5‚(PPh3)2 (c) and
6‚(dioxane)2 (d). The thermal ellipsoids are scaled to the 50% probability
level. meso-Substituents are omitted for clarity.
Acknowledgment. We thank Prof. Shigeki Kato (Kyoto Uni-
versity) for his advice on calculations. Helpful discussions with
Profs. Yasushi Morita, Takashi Kubo (Osaka University), Kunio
Awaga (Nagoya University) are also acknowledged. We also thank
Dr. Kenji Yoza (Bruker AXS), Prof. Norihiro Tokitoh, and Prof.
Takahiro Sasamori (Kyoto University) for X-ray analyses.
Supporting Information Available: General procedures, spectral
data for compounds, and CIF files. This material is available free of
charge via the Internet at http://pubs.acs.org.
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Figure 3. Temperature-dependent SQUID magnetic susceptibility of 6.
structures suggest that the ligand of 6, free base 4, has two NH
protons in each of the corrole cores. This suggestion is also
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1586.1007 for 4 (calcd for (C74H17F30N8)- ) 1586.1024 ([M]-)).
The ESR spectrum of 6 was measured at room temperature, and
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(Figure 3), which revealed that the øpT value was temperature
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equation.10 The result is well consistent with ESR measurement of
6 (Supporting Information). Cyclic voltammetry of 6 also supported
its biradical structure, exhibiting two reversible oxidation potentials
at 0.20 and 0.38 V (vs ferrocene/ferrocenium couple (Fc/Fc+)) and
two reversible and one irreversible reduction potentials at -0.19,
-0.34, and -1.9 V, respectively. The gap between the first
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(11) 631TZVP denotes a basis set consisting of TZVP for Zn and 6-31G(d)
for the rest.
oxidation and reduction potentials (∆E ) E1 - E1Red) was only
Ox
0.40 V, suggesting nonbonding character of the frontier orbital of
6.9a
The observed biradical character was supported by theoretical
calculations at the BLYP/631TZVP level.11 Symmetry-breaking of
the UDFT solution lowered the energy 5.1 kJ/mol from the
nonradical state. In addition, the UDFT calculations confirmed that
the S ) 0 state lies below the S ) 1 state, and the JS-T value was
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