Singly N-confused [26]hexaphyrin: A binucleating porphyrinoid ligand for mixed metals in different oxidation states

Article abstract of DOI:10.1002/anie.201006784

(Chemical Equation Presented) Mix and match: The title porphyrinoid has been synthesized and shows unique coordination chemistry by incorporating two different metals with different oxidation states (see figure). X-ray crystallography was used to confirm

Full text of DOI:10.1002/anie.201006784

Communications
DOI: 10.1002/anie.201006784
Porphyrinoids
Singly N-Confused [26]Hexaphyrin: A Binucleating Porphyrinoid
Ligand for Mixed Metals in Different Oxidation States**
Sabapathi Gokulnath, Keisuke Yamaguchi, Motoki Toganoh, Shigeki Mori, Hidemitsu Uno, and
Hiroyuki Furuta*
The primary goal in the search for binucleating ligands relies
on creating new bimetallic species with two metal ions in close
proximity—this feature leads to intriguing magnetochemistry,
optical properties, catalytic activity, and functional models for
metalloenzymes.[1] Binucleating ligands can be acyclic or
macrocyclic in nature and depending on the donor atoms
present they can be symmetric or unsymmetric.[2] Numerous
nonconjugated binucleating ligands have been prepared since
the first discovery of classical Schiff-base-type “Robson
ligands” and many of these ligands can provide either
homodinuclear or heterodinuclear complexes.[3] However,
our current interest focuses on utilizing the porphyrinoids as
conjugated p-donating ligands by taking advantage of con-
formational and geometrical factors.[4]
various divalent metal ions as a result of the two rigid NNNO
cavities.[9] However, all those previously reported hexa-
phyrin(1.1.1.1.1.1) systems tend to provide a symmetric
coordinating environment with NNCC or NNNO cavities
for bis-metal in-plane complexes in which both the metal ions
Porphyrins are well-known p-conjugated macrocycles that
can chelate a variety of metals by the interior four nitrogen
atoms (i.e. NNNN cavity). Owing to their rigidity and limited
cavity size, porphyrins usually capture metals in a 1:1 fashion
to afford mononuclear metal complexes.[5] For the multi-
metal coordination in a cavity, expanded porphyrins having
larger cavities than porphyrins are promising.[6] In fact, bis-
metal coordination has been achieved with hexaphyrin
systems. In these systems the ligand could take a rectangular
conformation with multi-donor sites inside the cavities similar
to porphyrin pockets.[7] For example, regular meso-aryl
[26]hexaphyrin(1.1.1.1.1.1) was shown to afford bis-metal
complexes with trivalent metal such as CuIII, AgIII, and AuIII
by forming four metal–carbon bonds inside the twin NNCC
cavities.[8] In contrast, doubly N-confused hexaphyrin (N2CH;
Scheme 1), which has two confused pyrrole rings in the
framework, was shown to serve as a binucleating ligand for
Scheme 1. Conceptual representations of bis-metal complexes of vari-
ous hexaphyrins.
[*] Dr. S. Gokulnath, K. Yamaguchi, Dr. M. Toganoh, Prof. Dr. H. Furuta
Department of Chemistry and Biochemistry
Graduate School of Engineering, Kyushu University
Fukuoka 819-0395 (Japan)
are in the same oxidation states (+ 2 or + 3).[8,9] Thus, for
coordination of two different metal ions in variable oxidation
states, an unsymmetrical binucleating environment would be
necessary in the macrocycles. This environment could be
attained by incorporating one confused pyrrole unit into the
hexaphyrin system, namely a singly N-confused hexaphyrin
(N1CH). We found that such a modification can offer a unique
coordinating environment with NNNC and NNCO cavities to
chelate metal ions in two different oxidation states. In
general, our approach opens up new possibilities in the
coordination chemistry for mixed-metal porphyrinoids.
Herein, we report the syntheses as well as structural and
redox properties of singly N-confused meso-aryl hexaphyrins
3 and 4, mono-AuIII complex 5, and bis-(AuIII,PtII) complex 6.
The 26p and 28p hexaphyrin ligands, 3 and 4, were prepared
in 23% and 11% yields, respectively, by an acid-catalyzed
Fax: (+81)92-802-2865
E-mail: hfuruta@cstf.kyushu-u.ac.jp
Dr. S. Mori
Integrated Center for Sciences, Ehime University
Matsuyama 790-8577 (Japan)
Prof. Dr. H. Uno
Graduate School of Science and Engineering, Ehime University
Matsuyama 790-8577 (Japan)
[**] This work was supported by the Grant-in-Aid for Scientific Research
(21108518) and the Global COE Program Science for Future
Molecular Systems from the Ministry of Education, Culture, Sports,
Science and Technology of Japan.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201006784.
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[3+3] condensation of N-confused tripyrrane dicarbinol 1 and
tripyrrane 2 (Scheme 2).[10] Interestingly, the conformational
flexibility of 3 is evident at room temperature and results in
X-ray-quality crystals of 3 in the neutral form were unsuc-
cessful, but fortunately crystals suitable for X-ray analysis
were obtained from slow diffusion of n-hexane into a solution
of 3 in CH2Cl2/trifluoroacetic acid (TFA; 5:1). The X-ray
structure revealed the monoprotonated structure of 3 with a
twisted hexapyrrolic frame-
1
very broad H NMR signals in CDCl3, and such conforma-
tional dynamics was frozen at low temperature.[11a] The
work (Figure 2a).[11b] Inter-
estingly, effective hydrogen-
bonding interactions are
found for pyrrole ring
B···TFA···ring E, thus forcing
the pyrrole subunits to be
tilted or fully inverted to
maintain a twisted conforma-
tion.[11a]
First, a monometalation
reaction was performed on
the aromatic [26]hexaphyrin
3 by treatment with AuCl·S-
(CH3)2 in
a mixture of
CH2Cl2 and methanol (5:1)
for 1 hour at room temper-
ature. Purification by using a
short column of silica gel
afforded mono-AuIII com-
plex 5 in 74% yield. The X-
ray structure shows a rectan-
Scheme 2. Preparation of N-confused hexaphyrin (3 and 4) and metal complexes (5 and 6). Reaction
conditions. a) BF3·Et2O (0.5 equiv), 1 h, RT; b) DDQ (3.5 equiv), 14 h, RT; c) AuCl·SMe2 (4 equiv), 1 h, RT;
d) [PtCl2(PhCN)2] (4 equiv), 6 h, RT. DDQ=2,3-dichloro-5,6-dicyano-1,4-benzoquinone.
1H NMR spectrum at À608C exhibits well resolved signals
and reveals a strong diatropic ring current arising from its 26p
aromatic circuit. A signal at d = 9.46 ppm for the b proton of
the N-confused pyrrole rings and six sets of multiplet signals
appear between d = 8.35–9.25 ppm for the remaining outer
b protons. In addition, three signals appear at d = 1.22, 0.61,
and 0.06 ppm for inner NH protons and a pair of multiplet
signals around d = 0.84 and À0.72 ppm correspond to the
inner b protons of ring B. These data unambiguously indicate
the inversion of the regular pyrrole ring away from the
macrocyclic cavity, while the confused pyrrole ring points
towards the macrocyclic cavity—this is in sharp contrast to the
structure of N2CH with two confused pyrrole rings that are
inverted.[9] In contrast, the reduced isomer 4 with a 28p
conjugated circuit exhibits a well resolved spectrum at room
temperature featuring three signals at d = 6.91, 10.41, and
12.03 ppm resulting from NH protons, nine different signals in
the range d = 5.43–7.54 ppm ascribed to the pyrrolic b pro-
tons, and a signal at d = 6.74 ppm for the b proton of the
confused pyrrole rings, therefore suggesting that the structure
of 4 is nonsymmetric and nonaromatic.
The UV/Vis absorption spectrum of 3 displays an intense
Soret-like band at 570 nm, which is a characteristic band for
hexaphyrin systems, and Q-bands between 700 and 980 nm, in
line with its aromaticity. On the other hand, 4 features two
broad bands in the region from 400 to 600 nm arising from its
28p nonaromatic conjugated circuit (Figure 1).[12] Notably,
[26]hexaphyrin 3 and [28]hexaphyrin 4 undergo reversible
redox interconversion with the aid of simple redox reagents
such as DDQ and NaBH4.[13] Several attempts to obtain the
Figure 1. UV/Vis/NIR absorption spectra of 3 (blue), 4 (red), 5
(green), and 6 (black) in CH2Cl2.
gular and a fairly planar framework with a mean plane
deviation of 0.697 ꢀ from the plane defined by 36 atoms on
pyrrole rings and methine carbon atoms (excluding the
oxygen atom; Figure 2b).[11b] The Au atom is located within
the NNNC cavity in a square-planar fashion, in which three
À
À
Au N and one Au C bond lengths are 2.033(6), 2.028(6),
2.073(4), and 2.027(5) ꢀ, respectively. This square-planar
arrangement is similar to that observed for a previously
reported N-confused porphyrin to AuIII complex.[14] The NH
proton (at N4) forms two intramolecular hydrogen bonds with
À
neighboring atoms (N4-H···N3 and N4 H···O1 with distances
of 2.245(5) ꢀ for N4···N3 and 2.404(4) ꢀ for N4···O1). The
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Communications
Figure 2. ORTEP drawings of a) monoprotonated 3, b) 5, and c) 6; top views (upper) and side views (lower). A TFA counter anion in 3 and meso-
aryl groups in side views were omitted for clarity. Hydrogen-bonding interactions in 5 are indicated by broken lines. Thermal ellipsoids are drawn
at the 30% probability level.
bond length between the carbon and oxygen atoms are in the
5 features an intense Soret-like band at 619 nm and Q-like
bands at 785, 868, 919, and 1046 nm, whereas 6 exhibits ill-
defined bands between 403 and 774 nm (Figure 1). Interest-
ingly, the absorption at approximately 1400 nm in 6 reaches
deep into the near infrared region, and is significantly red-
shifted relative to 5.
The electrochemical characteristics of 5 and 6 were
studied by cyclic voltammetry in CH2Cl2 using tetrabutylam-
monium perchlorate as an electrolyte (Figure 3). The mono-
AuIII complex 5 undergoes two reversible one-electron
oxidations at 0.07 and 0.41 V, and one irreversible (state a)
and one reversible reduction at À1.12 and À0.84 V, respec-
tively. Meanwhile, complex 6 displays two reversible one-
electron oxidations at 0.49 and 0.85 Vand two reversible one-
electron reductions at À0.28 and À0.77 V. The difference in
the first oxidation (Eox1) and first reduction (Ered1) is found to
be 0.77 V, and therefore indicates a narrowing of the HOMO–
LUMO gap with respect to 5 (0.91 V) that is consistent with
the observed red-shift in the absorption spectra. In the
absorption spectra, only a slight red shift is observed for
[15]
À
range of a C O double bond, that is, 1.215(8) ꢀ.
A diatropic ring current in 5 are evident from its 1H NMR
spectrum, in which a sharp singlet at d = 9.56 ppm is observed
for the b proton of the confused pyrrole rings, as well as a
multiplet at d = 8.52 ppm and three doublets at d = 8.31, 8.23,
and 8.14 ppm are observed for the remaining outer b protons.
The presence of inner NH and b protons is confirmed by the
singlet signals in the upfield region at d = 1.78 and 0.21 ppm,
respectively.
Next, a solution of 5 in o-Cl2C6H4 was heated at reflux
with [PtCl2(PhCN)2] in the presence of NaOAc under
anaerobic conditions, and provided bis-(AuIII,PtII) complex 6
in 86% yield. The structure of 6 was determined by X-ray
analysis and is quite similar to that of mono-AuIII complex 5
with a mean plane deviation of 0.648 ꢀ (Figure 2c).[11b] As
expected, the PtII ion is coordinated to the NNCO cavity in a
square-planar geometry and the basic skeleton keeps the
À
original structure as that of 5. In particular, two Pt N bonds
À
with pyrrolic nitrogen atoms and a Pt O bond with an oxygen
atom of the confused pyrrole ring were formed. The bond
À
À
À
lengths between Pt O, Pt C, and the two Pt N bonds are
2.047(8), 2.002(1), 2.056(8), and 2.028(9) ꢀ, respectively.
Interestingly, lone pair electrons on the oxygen atom partici-
pated in PtII coordination and resulted in the alteration of the
À
C O bond length with a distance of 1.292(2) ꢀ, which is
longer than was observed for 5. This bond length is between
[16]
À
À
that of a C O single bond and a C O double bond.
Importantly, this structure leads to the assignment of a
AuIII···PtII system without involvement of axial or bridging
ligands. Also, the distance between two metal centers is long
(4.362 ꢀ), hence no direct metal–metal interactions are found
in the solid state.[11a]
From the well-resolved 1H NMR spectrum, complex 6 was
further assigned as being diamagnetic. Specifically, complex 6
showed a multiplet signal at d = 9.10 ppm for the outer
b proton and six set of multiplet signals between d = 9.52–
8.51 ppm for the remaining outer b protons, thus revealing a
diatropic ring current. The electronic absorption spectrum of
Figure 3. Cyclic voltammogram traces of 5 (top) and 6 (bottom) in
anhydrous CH2Cl2 containing 0.1m of TBAClO4. Scan rate=0.1VsÀ1
.
State a denotes an irreversible process. TBA=tetra-n-butylammonium.
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mono-AuIII complex 5 (3: 1028 nm, 5: 1048 nm), and subse-
quent PtII metalation results in the significant red-shift (6:
1400 nm; see inset of Figure 1). Taken together, the first AuIII
metalation gives only small perturbation on the HOMO–
LUMO energy gap, while the second PtII metalation causes
significant change. The HOMO–LUMO energy gaps as well
as observed absorption spectra are in good agreement with
theoretical estimation.[11a]
The unique electrochemical features of 6 could be
rationalized by a metal-assisted endocyclic extension of the
porphyrin p-system. The HOMO and LUMO of 5 are quite
similar to those of 3. The contribution from metal d-orbitals is
rather small in 5, whereas a marked difference is observed for
6. Thus, a contribution from the metal d-orbitals is observed
both in the HOMO and LUMO of 6. More importantly,
phyrin p-system. These results indicate large electronic
changes in [26]hexaphyrin can be caused by the incorporation
of two dissimilar metals in different oxidation states. Fur-
thermore, tuning the properties of these functional systems
may be useful for various applications such as molecular
sensors, magneto devices, and organic functional dyes.[19]
Further extension of this unique coordination chemistry and
study of their photophysical properties by incorporating
various 3d metal ions into this versatile porphyrinoid ligand
is in progress.
Received: October 28, 2010
Revised: December 15, 2010
Published online: January 30, 2011
Keywords: aromaticity · conformation · gold · platinum ·
=
contribution from the C C p-orbital at the inverted normal
.
porphyrinoids
pyrrole ring is clearly observed in the HOMO (Figure 4).
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=
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