η5-cyclopentadienyliron(II)-[14]triphyrin(2.1.1) sandwich compounds: Synthesis, characterization, and stable redox interconversion

Article abstract of DOI:10.1002/anie.201302815

A semiferrocene complex with a [14]triphyrin(2.1.1) (TriP) ligand has been synthesized. The structure and properties are characterized by X-ray crystallographic analysis, UV/Vis spectroscopy, and variable-temperature ¹H NMR spectroscopy. The Fe^II and Fe^III complexes are electrochemically reversible. Copyright

Full text of DOI:10.1002/anie.201302815

Angewandte
Chemie
Communications
Metalloporphyrinoids
Z.-L. Xue, D. Kuzuhara, S. Ikeda,
Y. Sakakibara, K. Ohkubo, N. Aratani,
T. Okujima, H. Uno, S. Fukuzumi,
H. Yamada*
&&&& — &&&&
h5-Cyclopentadienyliron(II)–
[14]Triphyrin(2.1.1) Sandwich
Compounds: Synthesis, Characterization,
and Stable Redox Interconversion
A semiferrocene complex with
troscopy, and variable-temperature
a [14]triphyrin(2.1.1) (TriP) ligand has
been synthesized. The structure and
properties are characterized by X-ray
crystallographic analysis, UV/Vis spec-
1H NMR spectroscopy. The FeII and FeIII
complexes are electrochemically reversi-
ble.
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DOI: 10.1002/anie.201302815
Metalloporphyrinoids
h5-Cyclopentadienyliron(II)–[14]Triphyrin(2.1.1) Sandwich
Compounds: Synthesis, Characterization, and Stable Redox
Interconversion**
Zhaoli Xue, Daiki Kuzuhara, Shinya Ikeda, Yuka Sakakibara, Kei Ohkubo, Naoki Aratani,
Tetsuo Okujima, Hidemitsu Uno, Shunichi Fukuzumi, and Hiroko Yamada*
Ferrocene, an iron(II) center sandwiched by a pair of
aromatic cyclopentadienyl (Cp) ligands, is the first known
and archetypal metallocene; it was discovered in 1951.[1]
Thereafter, research into ferrocene-containing compounds
has continued apace within diverse areas, such as a redox
mediator, catalyst, electron donor, rotational hinge part, and
so on.[2] However larger macrocyclic p-conjugated systems
with monovalent anionic character has been scarcely reported
to date, which is due to the weak coordination ability of p-
extended Cp-type ligands. Especially in porprhyin families,
there are a few reports in this context:[3] 1) Cp-ScIII-porphyr-
in,[4a] Cp-ZrII-porphyrin,[4b] and Cp*-RuIV-porphycene (Cp* =
pentamethylcyclopentadienyl),[4c] although porphyrin and
porphycene are divalent ligands; and 2) b,b’-fused monoru-
thenocenylporphyrins, bisferrocenoporphyrins,[5a] metalopor-
phyrcenes,[5b] and cyclopentadienylruthenium p complexes of
subphthalocyanines,[5c] where that five-membered ring moiety
(pyrrole or cyclohexadiene moiety) acted as ligands. Only
recently, double-decker iron(II) complexes of dithiaethyne-
porphyrin[6] and N-fused porphyrin (NFP),[7] where they
behaved as macrocyclic tridentate ligands with a single
negative charge, have been reported. During the synthesis
of NFP complex, the Cp-FeII-NFP compound was detected by
mass spectroscopy; this compound has yet to be isolated. To
date, the synthesis of Cp-FeII-porphyrin sandwich compounds
remains a considerable challenge.
In 2008, we reported a facile procedure to synthesize
[14]triphyrin(2.1.1) (TriP, 1) as the first example of boron-free
ring-contracted porphyrins.[8,9] In contrast to the reported
dome-shaped boron subporphyrin complexes,[10] these novel
porphyrinoids opened up the way to a previously unexplored
region of contracted porphyrinoid coordination chemistry as
a monoanionic cyclic tridentate ligand. Owing to the flexi-
bility of the macrocycle, TriP has realized octahedral rhe-
nium(I), manganese(I), ruthenium(II), and platinum(IV)
complexes and square-planar platinum(II) complexes.[11]
Now we report herein the synthesis, characterization, and
redox behavior of novel h5-cyclopentadienyliron(II) TriP
sandwich complexes.
The metalation procedure is shown in Scheme 1. A dry
toluene solution of TriP 1 was treated with 5 equiv of
[{Fe(CO)2(Cp)}2] and refluxed for 24 h under argon. After
elimination of the solvent, the residue was dissolved in CHCl3
and the solution was filtered to remove the precipitates. The
solvent was again removed and the residue was purified by
short silica gel column chromatography using CHCl3 as an
eluent. The first eluted purple fraction was evaporated to
afford the crude product; crystallization from toluene and
pentane then gave the pure target compound in 40% yield for
2a (Ar= C6H4-p-CH3) and 45% for 2b (Ar= C6H4-p-
COOCH3). Sandwich TriP-Fe-TriP-type compounds were
not obtained.
[*] Dr. Z.-L. Xue, Dr. D. Kuzuhara, Dr. S. Ikeda, Y. Sakakibara,
Prof. N. Aratani, Prof. H. Yamada
Prof. S. Fukuzumi
Department of Bioinspired Science, Ewha Womans University
Seoul 120-750 (Korea)
Graduate School of Materials Science
Nara Institute of Science and Technology
8916-5, Takayama-cho, Ikoma 630-0192 (Japan)
E-mail: hyamada@ms.naist.jp
Prof. T. Okujima, Prof. H. Uno
Department of Chemistry and Biology
Graduate School of Science and Engineering, Ehime University
Bunkyo-cho 2–5, Matsuyama, 790–8577 (Japan)
Dr. Z.-L. Xue
Department of Applied Chemistry, Jiangsu University
Zhenjiang, 212013 (P. R. China)
[**] We are grateful to Prof. Haruyuki Nakano, Kyusyu University (Japan)
for his fruitful discussions on DFT calculations. This work was
supported by JSPS Postdoctoral Fellowship for Foreign Researchers
(to Z.-L.X.) and partly supported by Grants-in-Aid (No. 24655034 to
H.Y. and D.K., No. 20108010 to S.F. and No. 23750014 to K.O.), and
the Green Photonics Project in NAIST sponsored by the MEXT
(Japan) and NRF/MEST of Korea through the WCU (R31-2008-000-
10010-0) and GRL (2010-00353) Programs.
Prof. H. Yamada
CREST Japan Science and Technology Agency (JST)
Ikoma, 630-0192 (Japan)
Prof. N. Aratani
PRESTO Japan Science and Technology Agency (JST)
Ikoma, 630-0192 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201302815.
Prof. K. Ohkubo, Prof. S. Fukuzumi
Department of Material and Life Science, Graduate School of
Engineering
Osaka University, ALCA Japan Science and Technology Agency (JST)
Suita, Osaka 565-0871 (Japan)
2
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Scheme 1. Synthesis of Cp-FeII-TriP complexes 2a and 2b.
The structures of 2a and 2b were characterized by high-
resolution electrospray ionization time-of-flight (HR-ESI-
TOF) mass spectra and 1H, 13C, and H-H COSY NMR
spectra. HR-ESI-TOF mass spectra of 2a and 2b showed the
parent ion peaks at m/z 875.2934 (calcd for C61H45N3Fe:
875.2945 [M]+) and 1051.2512 (calcd for C65H45N3O8Fe:
1051.2557 [M]+), respectively (Supporting Information, Fig-
Figure 2. Crystal structures of a) 2a and b) 2b; top view (top) and side
view (bottom) with phenyl groups omitted. Solvent molecules and
hydrogen atoms are also omitted for clarity. Ellipsoids are set at 50%
probability. One of the four crystallographically independent molecules
in a unit cell is shown for 2a. The distances for 2a are average value
of four molecules.
1
ure S1 and S2). H NMR spectra of 1a and 2a are shown in
Figure 1 and the Supporting Information, Figures S3–S5. The
Moreover, the FeII ion is found on the top of the NNN plane
of TriP ligand. The distances between FeII ion and the four
meso-carbon plane of complexes 2a and 2b are (1.483 Æ
0.013) and 1.422 ꢀ, which are much more shorter than those
of the previously reported metallotriphyrin complexes:
1.825 ꢀ for [ReI(TriP)(CO)3], 1.678 ꢀ for [RuII-
(TriP)(CO)2Cl], and 1.631 ꢀ for [PtIV(TriP)Cl3]. The average
À
Fe N bond lengths are 1.894–1.897 ꢀ for 2a and 1.895 ꢀ for
2b, respectively, which are similar to [FeII(NFP)2].[7]
The absorption spectra of 2a and 2b have essentially
similar shapes as well as positions of the main absorptions at
374, 538, and 596 nm for 2a and 370, 545, and 592 nm for 2b
(Supporting Information, Figure S11), which are similar to
the values found in the spectrum of the [ReITriP(CO)3]
complex.[8b] The spectra of 2a and 2b are considerably blue-
shifted and broadened compared with those of free-base
TriPs, indicating strong electronic interaction between tri-
phyrin ligand and the metal d orbital. The peaks at 370 nm are
assigned to B bands, in analogy with [ReITrip(CO)3] complex.
The intense absorption peaks from 530 nm to 700 nm for 2a
and 2b can be assigned to metal-to-ligand charge transfer
(MLCT) bands on the basis of density functional theory
(DFT) calculations, in combination with weak Q-bands
(Supporting Information, Figure S12).[13] The energy level of
the highest occupied molecular orbital (HOMO) is signifi-
cantly destabilized while the lowest unoccupied molecular
orbital (LUMO) remains the same level compared to the
free-base TriP. In-depth analyses on the MOs show that both
the HOMO and LUMO are composed of d-p conjugated
orbitals, but the HOMO is composed of the top Cp ring
p orbitals while the LUMO is only composed of TriP
p orbitals. This situation is totally different as compared to
the parent [FeIICp2]. The absorption spectra of 2a and 2b
were nearly independent of solvent (Supporting Information,
Figure S13), which implied that intramolecular charge trans-
fer would not be important in the photoexcitation of such
kind of sandwich compound.
Figure 1. 1H NMR spectra of a) 1a and b) 2a in CDCl3. ? indicates
solvents.
sharp peaks observed for 2a suggest that the Fe ion is low-spin
and divalent, not trivalent. For 2a, the NH proton of 1a at
8.25 ppm disappeared and proton peaks of Cp rings at
2.84 ppm were observed at remarkably higher field owing to
the strong ring-current effect of the TriP ligand. This
phenomenon has been reported previously for the Cp-ScIII-
porphyrin.[4a] The similar trend was observed for 1b and 2b, as
shown in the Supporting Information, Figure S6–S9.
The structures of complexes 2a and 2b are unambiguously
determined by single-crystal X-ray diffraction analysis.[12] The
crystal structures of 2a and 2b are summarized in Figure 2
and the Supporting Information, Figure S10 and Table S1.
The crystal of 2a includes four independent molecules in
a unit cell. It is clear that in both 2a and 2b, iron(II) center is
sandwiched between one Cp ring and one TriP macrocyclic
ligand. The Cp ring is coordinated to FeII ion through the five
carbon atoms, with the distances of (1.694 Æ 0.012) ꢀ for 2a
and 1.694 ꢀ for 2b, which are almost the same as the distance
II
À
between Fe ion and Cp in ferrocene. The average Fe C bond
The electrochemical properties of 2a and 2b were
examined by cyclic voltammetry (CV; Figure 3; Supporting
length is 2.06–2.08 ꢀ for 2a and 2.073 ꢀ for 2b, respectively.
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Figure 3. Cyclic voltammograms of a) 2a and b) 2b in CH2Cl2 contain-
ing 0.1m nBu4NPF6, scan rate 0.1VsÀ1
.
Information, Table S2) in CH2Cl2 containing 0.1m nBu4NPF6
at room temperature. Apparently, oxidation potentials of the
FeII ion of 2a and 2b are À0.50 and À0.39 V lower than that of
ferrocene. These data are comparable to [FeIITPP] (TPP =
tetraphenylporphyrin)[14] and agree with the results of DFT
calculations (Supporting Information, Figure S12). The
second and third oxidation peaks together with the first and
second reduction peaks were assigned to the triphyrin macro-
cycle. As compared to free-base TriP, the reduction potentials
showed almost no change while the oxidation potentials
became 0.2–0.3 V higher than free-base Trip (Supporting
Information, Table S2).
Figure 4. Spectrophotometric titration of 2a with trifluoroacetic acid
The oxidized 2a (2a+) was identified by NMR, HR-ESI-
MS, and EPR measurements. When 2a was oxidized by
AgPF6, the NMR peaks were broadened in the range of 0 to
28 ppm (Supporting Information, Figure S14). The EPR
spectrum of 2a+ oxidized with p-chloranil was measured at
77 K. The largely anisotropic EPR signal was observed at g =
4.13, 2.41, and 2.04, which is identified as an FeIII complex with
the S = 5/2, 3/2 intermediate spin state (Supporting Informa-
tion, Figure S15).[15] DFT calculations also suggested that the
spin density was localized on the FeIII ion (Supporting
Information, Figure S16). The change in the absorption
spectra with the oxidation of 2a was also monitored
(Supporting Information, Figure S17a). When the applied
potential of the oxidation of 2a was kept at À0.34 V (vs. Fc/
Fc+), the peaks at 374, 540, and 589 nm decreased and at the
same time 392 and 489, 575 nm increased, which corre-
sponded to the FeIII complex in agreement with the spectrum
in the presence of AgI ion. The spectrum change was reversed
to the original spectrum by keeping the applied potential at
À1.0 V vs. Fc/Fc+ (Supporting Information, Figure S17b). The
oxidation and reduction of the Fe ion of 2a and 2b were stable
during the 20-cycle repeats (Supporting Information, Fig-
ure S18). Interestingly, the TFA titration of 2a showed
a similar UV/Vis spectral change with electrochemical
oxidation. By addition of TFA, the color of solution changed
from purple to brown and UV/Vis of 2a changed to the
similar spectrum with 2a+, which went back to the spectrum
(TFA) (a) and then with DBU (b) in CH2Cl2 at 298 K.
of 2a by addition of DBU (Figure 4). The NMR and HR-ESI-
MS spectra of 2a in the presence of TFA showed the same
spectrum to that of 2a+ (Supporting Information, Figures S14
and S19). These results indicated that 2a is easily oxidized to
2a+ by oxygen in the presence of TFA and 2a+ is remarkably
stable even under the acidic conditions.[16]
This stability allowed us to make single crystals of the
oxidized state of 2a, namely Cp-FeIII-TriP. Fortunately, the
crystallization of the oxidized species 2a with excess
CF3SO3Ag gave tiny crystals, from which we could perform
Figure 5. X-ray crystal structure of [Cp-FeIII-TriP]+[(CF3SO3)2Ag]À. Sol-
vent molecules and hydrogen atoms are omitted for clarity. Ellipsoids
are set at 20% probability.
4
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ler, D. M. Guldi, T. Torres, Angew. Chem. 2012, 124, 11499;
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the X-ray diffraction analysis (Figure 5; Supporting Informa-
tion, Table S1). The structure determined contained
[CF3SO3]2Ag]À as a counteranion. The Cp ring is coordinated
to FeIII ion through the five carbon atoms, with the distance of
1.723 ꢀ, which is longer than that of 2a, suggesting that the
additional positive charge in 2a+ is mostly localized on the
formal FeIII atom.[17]
In summary, we have successfully synthesized and char-
acterized the unique sandwich iron(II) compounds 2a and 2b
prepared from the corresponding free-base [14]triphyrin-
(2.1.1) 1a and 1b with [{Fe(CO)2(Cp)}2]. From the single-
crystal X-ray structure analysis, the central iron(II) ion is
sandwiched by one Cp ring and one triphyrin ligand. The
oxidation potential of FeII complex was lower than that of Fc
and reversible. Protonation of FeII complex with TFA gave
FeIII complex, which was reduced to FeII complex with DBU
reversibly.
´
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Received: April 5, 2013
Published online: && &&, &&&&
[11] Z. L. Xue, D. Kuzuhara, S. Ikeda, T. Okujima, S. Mori, H. Uno,
H. Yamada, Inorg. Chem. 2013, 52, 1688.
Keywords: ferrocene · metalloporphyrinoids · redox activity ·
sandwich complexes · triphyrin
.
[12] CCDC 871230 (2a), 871231 (2b), and 929554 (2a+) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[13] M. J. Frisch, et al. Gaussian09, R. C., Gaussian, Inc., Wall-
ingford CT, 2004. The full list of authors is given in Supporting
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