GoldIII-IridiumIII hybrid complexes of hexaphyrin(1.1.1.1.1.1)

Article abstract of DOI:10.1002/asia.201300320

Au revoIR: Hexaphyrin Au^III-Ir^III complexes were prepared from the reaction of a mono-Au^III [26]hexaphyrin complex with [IrCl(cod)]₂ followed by treatment with pyridine. N-Fusion reaction of this complex gave a syn-doubly N-fused hexaphyrin as the major product, which displayed antiaromatic properties such as strong paratropic ring current, a small HOMO-LUMO gap and a low-energy forbidden absorption. The oxidation of this fused product with MnO₂ afforded an oxidatively cleaved product.

Full text of DOI:10.1002/asia.201300320

COMMUNICATION
DOI: 10.1002/asia.201300320
GoldIII–IridiumIII Hybrid Complexes of Hexaphyrin(1.1.1.1.1.1)
Koji Naoda, Hirotaka Mori, and Atsuhiro Osuka*[a]
In the last two decades, expanded porphyrins that possess
larger pyrrolic macrocyclic rings in comparison to porphyr-
ins have received increasing attention due to their ability to
accomplish metalation with various metal ions and realize
versatile electronic states.[1] One such compound reported
here is [26]hexaphyrin 1[2] that gives a range of metal com-
plexes such as aromatic, antiaromatic,[3] Mçbius aromatic,[4]
Mçbius antiaromatic,[5] and stable radical species,[6] depend-
ing upon the coordinated metal, the number of p-electrons
participating in conjugation, and the molecular topology.
[26]Hexaphyrin 1 has a planar rectangular shape harnessing
26p electrons and therefore exhibits a strong diatropic ring
current, while its reduced congener, [28]hexaphyrin 2, exists
as a dynamic mixture of conformers in solution with twisted
Mçbius aromatic species being the predominant conformers
(Scheme 1).[7] As seen for bis-AuIII and mono-AuIII com-
plexes 3 and 5, AuIII-metalation helps to rigidify the planar
conformations, thus facilitating the formation of Hꢀckel an-
tiaromatic species occupying 28p electronic states such as
4.[3] Mono-AuIII hexaphyrin complex 5, which can be pre-
pared by selective mono-metalation with NaACTHNUGTRENUNG[AuCl4] and
Ag3PO4,[8] has a pre-organized CCNN cavity and is a useful
precursor for the preparation of hybrid bis-metal complexes
such as 6a (AuIII–AgIII), 6b (AuIII–CuIII), and 6c (AuIII–
RhIII; see Scheme 1). Stimulated by the intriguing properties
and reactivities of IrIII-porphyrnoids,[9,10] we examined the
iridium metalation of 5.
Scheme 1. Redox interconversions of free-base and bis-AuIII complexes
of hexaphyrins and formation of hybrid bis-metal complexes. py=pyri-
dine.
Initially, mono-AuIII hexaphyrin 5 was reacted with [IrCl-
A
complex 7 was obtained in 62% yield after purification by
silica gel column chromatography (Scheme 2). The high-res-
olution electrospray-ionization time-of-flight (HR-ESI-
TOF) mass spectrum of 7 indicated the parent ion peak at
m/z=2003.0560 ([M+H]+) (calcd for C76H19N8F30AuIr: m/
z=2003.0550). The 1H NMR spectrum of 7 displays four
doublet resonances corresponding to the outer-b protons in
the range of 9.18–9.66 ppm and three upfield-shifted signals
refluxing pyridine, by following a previously reported proce-
dure used to prepare 6c.[3d] Ir-metalated products were ob-
tained in low yields with poor reproducibility. In the next
step, we employed the metalation procedure used for the
IrIII complex of 5,10,15-tris(pentafluorophenyl)corrole.[11b]
solution of 5 in THF was refluxed in the presence of [IrCl-
ACHTUNGTRENNUNG(cod)]2 and NaOAc for 3 hours under nitrogen atmosphere,
to which pyridine was added, and the resulting solution was
stirred for an additional 1 hour. After being passed through
a short basic alumina column, [26]hexaphyrin AuIII–IrIII
[a] K. Naoda, H. Mori, 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
Scheme 2. Ir metalation to a mono-AuIII hexaphyrin 5. py=pyridine.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201300320.
Conditions: a) [IrClACTHNGUTRENNU(G cod)]2 (10 equiv) and NaOAc (10 equiv), then pyri-
dine (excess) in THF. b) NaBH4 (20 equiv) in CH2Cl2/MeOH (10:1).
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Atsuhiro Osuka) et al.
due to the axial pyridyl protons at 0.82, 4.19, and 5.38 ppm.
Such resonances indicate a strong diatropic ring current
present in compound 7. Its crystal structure was determined
by X-ray diffraction analysis (Figure 1a).[12] The IrIII ion is
Figure 2. UV/Vis/NIR absorption spectra of 7, 9, and 12 in CH2Cl2. Peaks
marked with an asterisk are originate from the solvent.
tense Soret-like band at 556 nm and broad Q-like bands at
706, 1150, and 1352 nm, which are characteristic of aromatic
porphyrinoids (Figure 2). Nucleus-independent chemical
shift (NICS) values[13] of 7 have been calculated to be large
negative (see the Supporting Information), also confirming
its aromatic nature.
Reduction of 7 with NaBH4 proceeded quantitatively to
III
give [28]hexaphyrin AuIII Ir complex 8. The HR-ESI-TOF
À
mass spectrum of 8 indicated the parent ion peak at m/z=
2004.0641 ([M]+) (calcd for C76H20N8F30AuIr: m/z=
1
2004.0628). In the H NMR spectrum, the outer pyrrolic NH
protons are shifted to high field at À7.98 ppm, the outer-
b proton resonances are in the range of À1.83–À3.02 ppm,
and the axial pyridyl protons are inversely shifted down-
field; a-protons at 29.39, b-protons at 14.66, and a g-proton
at 13.47 ppm, thus providing evidence of a remarkably
strong paratropic ring current. The absorption spectrum of 8
shows an ill-defined Soret band and no Q-bands. Beside
these features, weak absorption bands are observed at 1163,
1500, and 2000 nm, probably related to a transition to
a dark state (see the Supporting Information).[14] [28]Hexa-
phyrin 8 is stable enough for analysis by NMR and UV/Vis/
NIR spectroscopies as well as cyclic voltammetry measure-
ments, but is gradually oxidized to 7 in solution in open air.
One tactic to lock a [28]hexaphyrin framework is to
impose an intramolecular N-fusion reaction.[15] Thus, we ex-
amined the feasibility of N-fusion reactions by heating a so-
lution of 7 in 1,2-dichlorobenzene at 1508C. These condi-
tions actually afforded the doubly N-fused hexaphyrin syn-
isomer 9 in 58% yield as a dark violet solid. A rearranged
doubly N-fused hexaphyrin anti-isomer 11 was also obtained
Figure 1. X-ray crystal structures of (a) 7, (b) 9, and (c) 12 and (a) select-
ed bond lengths of 7. Left: top view; Right: side view. The thermal ellip-
soids are scaled to (a) 50% probability and (b,c) 30% probability. Sol-
vent molecules and pentafluorophenyl groups in the side view are omit-
ted for clarity.
bound within the NNCC pocket in an octahedral manner
with two axially coordinated pyridines completing the coor-
dination sphere. To the best of our knowledge, IrIII azulipor-
phyrin is the only reported example of IrIII carbaporphyri-
[10d]
À
À
À
noids bearing a Ir C bond.
N(1) Au and N(2) Au bonds
1
are 2.099 and 2.105 ꢂ, respectively, which are significantly
as a minor product. The H NMR spectrum of hexaphyrin 9
À
longer than the corresponding trans C Au bonds; 1.997 ꢂ
is similar to that of 8, indicating a strong paratropic ring cur-
rent by displaying remarkably upfield-shifted outer-b proton
resonances in the range of 1.55–2.43 ppm and a-, b-, and g-
protons of the axial pyridine at 19.68 ppm, 11.45 ppm, and
11.10 ppm, respectively. The lack of the two fluorine atoms
À
À
for C(12) Au and 2.021 for C(28) Au. A similar feature is
observed for the IrIII site; 2.096 and 2.217 ꢂ for N(4) Ir and
N(5) Ir, and 2.010 and 2.014 ꢂ for C(27) Ir and C(13) Ir
(Figure 1a). The absorption spectrum of 7 features an in-
À
À
À
À
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Atsuhiro Osuka) et al.
at the ortho position of the pentafluorophenyl groups was
also confirmed by 19F NMR analysis. The UV/Vis/NIR spec-
trum shows ill-defined Soret-like bands and a low energy
band in the range of 1000–2500 nm related to the transition
to a dark state, which is characteristic of antiaromatic por-
phyrinoids (Figure 2).[14] The structure of 9 was revealed by
X-ray diffraction analysis to be a syn-doubly N-fused prod-
uct at the Au-site (Figure 1b). The observed preferential N-
fusion reaction at the Au-site may be rationalized in terms
of electrophilicity, with the AuIII-complex having a more
electrophilic nature when compared to the IrIII-complex.
Quite large positive NICS values of 9 were calculated inside
the macrocycle (ca. +39 to +45 ppm).[16] This computational
study also supported a strong antiaromatic nature for 9. For-
tunately, the structure of the minor product 11 has been re-
vealed by X-ray diffraction analysis (see the Supporting In-
formation and Scheme 3). Interestingly, one of the fused
thogonal geometry of the linkage. The two carbonyl groups
are arranged in a roughly antiparallel fashion with a distance
of about 3 ꢂ. The H NMR spectrum of 12 displays eight
1
resonances related to the pyrrolic outer-b proton in the
range of 5.32–7.35 ppm, while UV/Vis/NIR spectra exhibit
ill-defined Soret-like bands at 438 nm and 625 nm and
a broad absorbance in the NIR region in the range of 900–
1400 nm (Figure 2). Collectively, complex 12 can be assigned
as a nonaromatic species; however, it is interesting to point
out that there is a considerable through-space interaction
between the two carbonyl groups in its LUMO and
LUMO+2 (see the Supporting Information).
The electrochemical properties of 7, 8, 9, and 12 were
studied by cyclic voltammetry (Figure 3 and the Supporting
Information). The hexaphyrin 7 exhibits three reversible ox-
idations at Eox =0.520, 0.916, and 1.316 eV, and three rever-
sible reductions at Ered =À0.796, À1.256, and À1.884 eV,
Scheme 3. N-Fusion reaction of 7 and oxidative cleavage and rearrange-
ment reaction. Conditions: a) 1,2-dichlorobenzene, 1508C. b) MnO2, air,
CH2Cl2, room temperature.
Figure 3. Cyclic voltammograms of the hexaphyrin 7 and 9 in anhydrous
CH2Cl2 with 0.1m Bu4NPF6 as a supporting electrolyte, and Ag/AgClO4
as a reference electrode. Fc/Fc+ was used as an external reference.
pyrroles underwent a rearrangement to form a six-mem-
bered lactam ring. Adventitious water is the most likely
source of oxygen. It is conceivable that 11 is formed via an
oxidative rearrangement of the putative anti-N-fusion prod-
uct 10.
Finally, the syn-doubly N-fused hexaphyrin 9 was oxidized
with MnO2, thereby giving rise to a pronounced color
change from dark violet to clear green. After the standard
work-up procedures, product 12 was isolated in about 50%
yield. X-ray diffraction analysis revealed the structure of 12,
in which the meso-carbon and pyrrolic a-carbon in one of
the N-fused moieties is oxidatively cleaved (Figure 1c). The
covalently linked macrocyclic structure of 12 is still retained
through the 1,2-tetrafluorophenylene linkage, but the mac-
rocyclic conjugation is interrupted due to the practically or-
thus indicating its electron-rich nature as compared with 3
(Eox1 =0.899 and Ered1 =À0.309) and 5 (Eox1 =0.945 and
Ered1 =À0.409). The more electron-deficient nature of the
AuIII metalation site may explain why the regioselective N-
fusion reaction occurs there. Complexes 8 and 9 show oxida-
tion waves at very low potentials, À0.036 and À0.436 eV for
8 and 0.228 and À0.124 eV for 9. The very low oxidation po-
tential values of 8 may help to explain its chemical instabili-
ty. Consistent with the assignment as antiaromatic species,
the electrochemical HOMO–LUMO gaps of 8 and 9 are
both quite small, 0.608 and 0.744 eV, respectively.
III
In summary, the AuIII Ir hybrid complex of [26]hexa-
À
phyrin 7 was prepared by the reaction of mono-AuIII
[26]hexaphyrin complex 5 with [IrClACTHNUTRGEN(UNG cod)]2. Complex 7 was
quantitatively reduced to the corresponding [28]hexaphyrin
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Atsuhiro Osuka) et al.
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complex syn-isomer 9, preferentially as a locked antiaromat-
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electrochemical studies of these AuIII Ir hybrid complexes
À
helped to reveal their rather electron-rich properties as com-
III
pared with AuIII Au complexes. Attempts to utilize these
À
hybrid complexes in catalytic applications are now in prog-
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Acknowledgements
This work was supported by Grants-in-Aid (no. 22245006 (A) and
20108001 “pi-Space”) from MEXT.
Keywords: aromaticity · conjugation · iridium · metalation ·
N-fusion reactions
[12] 7: C86.50H30F30N8AuIr, Mr =2140.35, monoclinic, space group C 2/
c
(no. 15), a=37.4313(7), b=20.4320(4), c=23.0628(4) ꢂ, b=
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=
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2s(I)),
Rw =0.2788
(all
data),
GOF=1.032.
12:
ꢀ
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via www.ccdc.cam.ac.uk/data_request/cif.
´
´
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Received: March 11, 2013
Published online: && &&, 0000
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COMMUNICATION
Au revoIR: Hexaphyrin AuIII–IrIII
complexes were prepared from the
reaction of a mono-AuIII [26]hexa-
Hybrid Complexes
Koji Naoda, Hirotaka Mori,
Atsuhiro Osuka*
phyrin complex with [IrCl
C
&&&& — &&&&
lowed by treatment with pyridine. N-
Fusion reaction of this complex gave
a syn-doubly N-fused hexaphyrin as
the major product, which displayed
antiaromatic properties such as strong
paratropic ring current, a small
GoldIII–IridiumIII Hybrid Complexes of
Hexaphyrin(1.1.1.1.1.1)
HOMO–LUMO gap and a low-energy
forbidden absorption. The oxidation of
this fused product with MnO2 afforded
an oxidatively cleaved product.
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These are not the final page numbers! ÞÞ