Boron(III) induced skeletal rearrangement of hexaphyrin(1.1.1.1.1.1) to hexaphyrin(2.1.1.0.1.1)

Article abstract of DOI:10.1002/anie.201001052

(Figure Presented) Who eight all the πs? Hexaphyrin(1.1.1.1.1.1) rearranges into figure-of-eight hexaphyrin(2.1.1.0.1.1) upon B^III complexation (see scheme). Redox reactions convert between 30π and 28π electron sys- terns requiring the B^II

Full text of DOI:10.1002/anie.201001052

Angewandte
Chemie
DOI: 10.1002/anie.201001052
Rearrangement
Boron(III) Induced Skeletal Rearrangement of Hexaphyrin(1.1.1.1.1.1)
to Hexaphyrin(2.1.1.0.1.1)**
Kohei Moriya, Shohei Saito, and Atsuhiro Osuka*
In recent years, increasing attention has been focused on
expanded porphyrins[1] that exhibit attractive properties, such
as multi-metal coordination,[2] facile redox interconversion,[3]
anion sensing,[4] large nonlinear optical properties,[5] and
Mꢀbius aromaticity.[6] In addition, skeletal rearrangements
and the resulting intriguing chemical reactivities, which are
often triggered upon metalation have been studied.[7] In 2003,
Vogel et al. found that metalation of 5,24-dioxo-octa-
phyrin(1.1.1.0.1.1.1.0) with nickel(II) caused a rearrangement
to give a bis-nickel(II) complex of spirodicorrole through the
extrusion of two carbon dioxide molecules.[8] They also
disclosed that the metalation of [34]octaphyrin(1.1.1.0.1.1.1.0)
with palladium(II) caused a rearrangement to a bis-spirodi-
porphyrin skeleton.[8]
In the last decade, we have focused on the exploration of
the chemistry of meso-aryl substituted expanded porphyr-
ins.[9] In the course of these studies, we found two thermal
Scheme 1. Splitting reactions of meso-aryl expanded porphyrins:
splitting reactions: 1) from bis-copper(II) [36]octa-
phyrin(1.1.1.1.1.1.1.1) to two CuII [18]porphyrins (Sche-
me 1a)[10] and 2) from a putative CuII–BIII complex of
[32]heptaphyrin(1.1.1.1.1.1.1) to a CuII [18]porphyrin
and a BIII [14]subporphyrin (Scheme 1b).[11] Both
reactions are apparently initiated by the transannular
p-electron interactions at the hinge positions. These
results made us envisage a similar splitting reaction of
[28]hexaphyrin(1.1.1.1.1.1) (1)[12] to produce BIII
[14]subporphyrin molecules as a result of appropriate
material balance (Scheme 1c).
a) [36]octaphyrin to two [18]porphyrins, b) [32]heptaphyrin to [14]sub-
porphyrin and [18]porphyrin, and c) [28]hexaphyrin to two [14]subpor-
phyrins.
To test this idea, we examined the reaction of
[28]hexaphyrin 1 with boron(III) reagents. Com-
pound 1 was treated with 50 equivalents of BBr3 in
the presence of NEt(iPr)2 and the resulting mixture
was heated under reflux for 1 h (Scheme 2, Proto-
col A), which yielded a complicated mixture, from
which blue complex 2 was isolated in 22% yield as the
major product. The structure of 2 was determined by
Scheme 2. BIII insertion and resulting rearrangement of 1. *=sp3-hybridized
carbon atom.
X-ray diffraction analysis to be a transannularly a,a-
bridged hexaphyrin bis-boron(III) complex.
(Figure 1).[13] The two bridged a-carbon atoms and neighbor-
ing two meso-carbon atoms are sp3-hybridized as evinced by
the 13C NMR spectrum that displays corresponding signals at
d = 41.29 and 83.06 ppm. In accord with the structure, the
1H NMR spectrum of 2 supports a C2 symmetrical molecular
structure and the absence of a macrocyclic ring current. The
11B NMR chemical shift is d = À1.91 ppm. The UV/Vis
[*] K. Moriya, S. Saito, 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
[**] This work was supported by Grants-in-Aid from MEXT (Nos.
19205006 (A) and 20108001 “pi-Space”). S.S. acknowledges JSPS
Fellowship for Young Scientists.
absorption spectrum of 2 shows broad bands at 371, 414,
and 645 nm probably arising from the tripyrromethene
moieties (Supporting Information). The structure of 2 sug-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001052.
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Figure 2. X-ray crystal structure of 4: a) top view and b) side view;
meso-C6F5 substituents in the side view are omitted for clarity.
Figure 1. X-ray crystal structure of 2: a) top view and b) side view;
meso-C6F5 substituents in the side view are omitted for clarity.
gests that the transannular interactions are mandatory during
the complexation of BIII by 1 but do not drive the molecule to
make the double meso,a-linkage that is required for the
targeted splitting reaction.[10b]
We found that the treatment of 1 with 200 equivalents of
BBr3 in the presence of NEt(iPr)2 and subsequent aqueous
work-up (Scheme 2, Protocol B) gave the reddish brown 3 as
the main product (see below), which slowly changed under
ambient conditions to purple bis-boron(III) complex 4. The
synthetic procedure was optimized by a final oxidation with
3 equivalents of MnO2 to produce 4 immediately (in 55%
yield from 1). Employing Protocol B, complex 2 was not
detected, while following Protocol A a trace amount of 4 was
obtained (Supporting Information). Quite unexpectedly, the
structure of 4 was revealed by single-crystal X-ray diffraction
analysis to be a bis-BIII complex of [28]hexaphyrin(2.1.1.0.1.1)
that has a figure-of-eight conformation containing a directly
connected bipyrrole unit and a s-trans 1,3-butadienyl linkage
at the opposite position (Figure 2).[13] Distinct bond-length
alternation along the 28p electron circuit is also apparent.
Bond lengths of the directly connected a–a carbon bond and
the central bond in the s-trans 1,3-butadienyl linkage are 1.46
and 1.45 ꢁ, respectively, which are similar to that of the
Scheme 3. Possible mechanism from 1 to 4. *=sp3-hybridized carbon
atom.
À
central C C bond of 1,3-butadiene (1.48 ꢁ). The formation of
4 requires a drastic and unprecedented skeletal rearrange-
ment, namely the transposition of an aryl-substituted meso-
position over pyrrole units. The mechanism of this rearrange-
ment is still not settled, but is considered to involve the
formation of a–a and meso–meso bonds and the subsequent
cleavage of two meso–a bonds (Scheme 3 and Supporting
Information). This route is different from the mechanism of
the anticipated splitting reaction to produce two subporphyr-
ins, which requires the formation of two meso–a bonds and
the subsequent cleavage of two meso–a bonds.[10b]
As indicated by the crystal structure, the complex 4 has
two tri-N-pyrrolyl hydroxy boron(III) hemi-macrocycles,
whose axial hydroxy ligands can be exchanged by alkoxy
ligands upon treatment with alcohol as found for the
subporphyrins (Supporting Information).[11b] In line with the
C2 symmetrical structure, the 1H and 19F NMR spectra of 4 are
simple, exhibiting signals of the peripheral b-protons at d =
8.76, 6.56, 6.23, 5.94, and 5.46 ppm in a integral ratio of
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1:1:1:1:2 with a signal at d = 4.09 ppm for the axial hydroxy
group. The 11B NMR spectrum showed a signal in a relatively
deshielded region at d = 5.51 ppm. The 1H NMR signal at d =
8.76 ppm can be assigned to the Ha b-protons located above
the twisted position of the figure-of-eight conformation
(Scheme 2).[14] Overall, these NMR data, the calculated
molecular orbital diagram, and the NICS[15] values (d = +
6.78 to + 10.93 ppm) at the points inside the macrocycle,
including the center of the macrocycle and the average
positions among two nitrogen atoms and two alpha carbons of
neighboring pyrrole units, indicated weak 28p Hꢂckel anti-
aromaticity (Supporting Information). The UV/Vis/NIR
absorption spectrum of 4 shows some broad bands, without
low-energy bands in near-IR region, which are also consistent
with its antiaromatic character (Supporting Information).[5,16]
Cyclic voltammetry measurement revealed reversible oxida-
tion potentials at 0.24 and 0.54 V (vs. Fc/Fc+; Fc =
[(C5H5)2Fe]) and reduction potentials at À0.67 and À1.07 V,
hence indicating a small electrochemical HOMO–LUMO gap
(0.91 V), which is characteristic of antiaromatic species
(Supporting Information).[16] The weakness of antiaromaticity
may be ascribed to the large dihedral angle (728) between
directly linked bipyrrole unit.
The free-base [26]hexaphyrin(2.1.1.0.1.1) (5; Scheme 5)
was obtained from 4 in 70% yield upon treatment with a large
excess of MnO2 (ca. 300–400 equiv) at room temperature. The
loss of BIII ions is induced by the change in the macrocyclic p
network from a 28p to 26p system and thus from being a
tetraanionic to a dianionic ligand. The parent ion peak of 5
was observed at m/z 1461.0933 (calcd for C66H15F30N6 [M +
H]+: 1461.0874). The 1H NMR spectrum showed a broad
signal at d = 11.41 ppm arising from NH and six signals for b-
protons in the range of d = 6.93–6.34 ppm, indicating intra-
molecular hydrogen bonds between aminic/iminic pyrrole
subunits and the non-aromaticity of the twisted 26p con-
jugated system. The DFT optimized structure of 5 indicated a
figure-of-eight macrocycle with a trans-vinylene linkage. The
calculated NICS values (d = À0.31 to À3.70 ppm) at the
corresponding points inside the macrocycle support its non-
aromaticity (Supporting Information). The UV/Vis/NIR
absorption spectrum of 5 shows rather broad bands at 345,
548, and 653 nm with relatively small absorption coefficients
(e ꢀ 67000mÀ1 cmÀ1), which is consistent with the non-aroma-
ticity.[11] To our knowledge this is the first synthesis of
[26]hexaphyrin(2.1.1.0.1.1).[19] The bis-palladium(II) complex
6 was synthesized from 5 in 22% yield upon treatment with
Pd(OAc)2 (Scheme 5). The crystal structure of 6 shows that
the two PdII ions coordinated in a square-planar manner are
bridged by the oxygen atom attached to a C2 carbon linkage
(Figure 3).[13] The oxygenated meso-carbon atom is sp3-
Importantly, the redox interconversion between 3 and 4
was confirmed as shown in Scheme 4. The complex 3, which
was formed as the initial product in Protocol B (Scheme 2),
Scheme 5. BIII removal from 4 to give [26]hexaphyrin(2.1.1.0.1.1) 5 and
its subsequent PdII metalation to give 6.
Scheme 4. Switching BIII coordination mode by the redox interconver-
sion of hexaphyrin(2.1.1.0.1.1).
has been characterized to be [30]hexaphyrin(2.1.1.0.1.1) with
BIII ions in a trigonal coordination environment. The complex
3 was slowly oxidized to 4 when left standing in the air. High-
resolution ESI-TOF mass spectroscopy showed the parent-
ion peak of 3 at m/z 1480.0861 (calcd for C66H12B2F30N6 [M]+:
1480.0856). The 1H NMR spectrum shows six doublets at d =
7.66, 7.62, 7.53, 7.24, 7.21, and 5.44 ppm without any signal for
axial ligands in line with the assigned weakly aromatic 30p
hexaphyrin structure. The density functional theory (DFT;
B3LYP/6-31G(d))[17] optimized structure of 3 indicates the
figure-of-eight conformation with a trans vinylene linkage and
the NICS value (d = À5.72 to À11.70 ppm) at the correspond-
ing positions inside the macrocycle, whose significant negative
values demonstrate macrocyclic diatropic ring current (Sup-
porting Information). The reversible interconversion between
3 and 4 is interesting, in that switching between tetrahedral
versus trigonal BIII coordination occurs upon the change in
the number of p electrons in hexaphyrin(2.1.1.0.1.1).[18]
Figure 3. X-ray crystal structure of 6: a) top view and b) side view;
meso-C6F5 substituents are omitted in the side view for clarity.
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[13] See the Supporting Information for crystallographic details.
CCDC 765595 (2), 765596 (4), 765597 (6) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[14] S. Saito, J.-Y. Shin, J. M. Lim, K. S. Kim, D. Kim, A. Osuka,
hybridized, hence disrupting the macrocyclic p conjugation.
The NMR and absorption spectra of 6 are also consistent with
the structure (Supporting Information). These results indicate
that the hexaphyrin 5 can serve as a metal-binding ligand
despite its distorted figure-of-eight structure.
In summary, the unprecedented rearrangement from
hexaphyrin(1.1.1.1.1.1) into hexaphyrin(2.1.1.0.1.1) was
found upon BIII complexation. Within
a
hexa-
phyrin(2.1.1.0.1.1) skeleton, the reversible interconversion
between trigonal versus tetrahedral BIII coordination was
demonstrated by the redox reactions between 30p versus 28p
electronic system. The free-base hexaphyrin(2.1.1.0.1.1),
obtained by the oxidative removal of the boron centers, was
shown to serve as a ligand for two PdII centers. Boron(III)-
induced skeletal rearrangements of other expanded porphyr-
ins are worthy of further investigation, and are being studied
in our laboratory.
Received: February 20, 2010
Published online: May 5, 2010
Keywords: aromaticity · boron · porphyrinoids · rearrangement
.
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