Synthesis and characterization of new platinum(II) and platinum(IV) triphyrin complexes

Article abstract of DOI:10.1021/ic302624h

Metalation of 6,13,20,21-tetrakis(4-methylphenyl)-22H-tribenzo[14] triphyrin(2.1.1) with PtCl₂ gave a platinum(II) complex having a square-planar coordination structure with two pyrrolic nitrogen atoms and two chloride ions, with a saddle-shaped macrocycle. This platinum(II) complex was easily oxidized by air to an octahedral platinum(IV) complex coordinated by three pyrrolic nitrogen atoms as a tridentate monoanionic cyclic ligand and three chloride ions. When platinum(II) triphyrin was crystallized in air, an oxygen atom was incorporated between two α-carbon atoms of the pyrroles as an oxygen bridge to intercept the 14π aromatic system.

Full text of DOI:10.1021/ic302624h

Communication
pubs.acs.org/IC
Synthesis and Characterization of New Platinum(II) and Platinum(IV)
Triphyrin Complexes
Zhaoli Xue,†,‡ Daiki Kuzuhara,† Shinya Ikeda,† Tetsuo Okujima,§ Shigeki Mori,⊥ Hidemitsu Uno,§
and Hiroko Yamada*,†,∥
†Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma 630-0192, Japan
§Department of Chemistry and Biology, Graduate School of Science and Engineering, Ehime University, Matsuyama 790-8577, Japan
⊥Department of Molecular Science, Integrated Center for Sciences, Ehime University, Matsuyama 790-8577, Japan
∥CREST, JST, Chiyoda-ku, Tokyo 102-0076, Japan
S
* Supporting Information
We have described the formation of octahedral metal
complexes of [14]triphyrin(2.1.1) with MnI, ReI, and RuII ions
ABSTRACT: Metalation of 6,13,20,21-tetrakis(4-methyl-
phenyl)-22H-tribenzo[14]triphyrin(2.1.1) with PtCl2 gave
so far.4b,5 The triangle macrocycle is a good tridentate
a platinum(II) complex having a square-planar coordina-
tion structure with two pyrrolic nitrogen atoms and two
chloride ions, with a saddle-shaped macrocycle. This
platinum(II) complex was easily oxidized by air to an
octahedral platinum(IV) complex coordinated by three
pyrrolic nitrogen atoms as a tridentate monoanionic cyclic
ligand and three chloride ions. When platinum(II)
triphyrin was crystallized in air, an oxygen atom was
incorporated between two α-carbon atoms of the pyrroles
as an oxygen bridge to intercept the 14π aromatic system.
monoanionic ligand for octahedral coordination. Considering
the tridentate structure of [14]triphyrin(2.1.1), it is challenging
to make the square-planar platinum(II) complex. In the present
study, we will report the successful metalation of 1 with PtCl2 in
toluene to give PtIITriP (2). The platinum(II) complex was easily
oxidized to PtIVTriP (3) in the presence of oxygen in solution
(Scheme 1) or was bridged by an oxygen atom between α-carbon
atoms of neighboring pyrroles (PtIIOTriP, 2-O) during
crystallization.
Scheme 1. Synthesis of 2, 2-O, and 3
ontracted porphyrins, a new branch of the porphyrin
Cfamily, have attracted considerable attention in recent years
because of their potential applications in a variety of high-
technology fields.1 With the exception of subpyriporphyrin and
some [18]triphyrins,2 ring-contracted porphyrinoids(1.1.1)
commonly contain three pyrrole or isoindoline moieties and
exist only as boron compounds.3 Moreover, removal of the
central boron atom of these porphyrinoids(1.1.1) has not been
achieved. In 2008, using a modified Lindsey condensation
method, we first reported a facile synthesis of meso-aryl-
substituted free-base [14]triphyrin(2.1.1) (TriP, 1).4 Since
then, meso-free and β-free triphyrins have been prepared by
McMurry coupling or [2 + 1] condensation reactions.5,6 These
novel complexes have made metalation of contracted porphyrins
possible, which has attracted increasing attention in tuning the
optical and electric properties.4b,5
Among metalloporphyrins, platinum-based complexes have
received special attention in light of chromophores for a variety
of applications, such as organic field-effect transistors,7,8 organic
light-emitting diodes,9−11 and photodynamic therapy
(PDT).12,13 In general, platinum(II) and platinum(IV) porphyr-
ins were prepared using platinum(II) and platinum(IV) reagents,
respectively.14 Platinum(II) porphyrins tend to give a square-
planar structure, and platinum(IV) porphyrins give an octahedral
structure with two more anionic ligands. The direct oxidation of a
platinum(II) complex to a platinum(IV) complex was reported
only by an electrochemical procedure.15
The synthesis of 1 was already reported.4a 1 was treated with
PtCl2 following the conventional procedure generally used for
the synthesis of platinum(II) porphyrins.9 A toluene solution of 1
in a Schlenk flask was refluxed for 12 h in the presence of 10 equiv
of PtCl2 under a nitrogen atmosphere. Subsequent separation
over silica gel column chromatography using CHCl3 as the eluent
provided 3 as a green fraction (Rf = 0.71) as a trace amount and
then 2 as a yellow-green fraction (Rf = 0.12) in 59% yield. Next
the same reaction was performed in air, and then 2 and 3 were
obtained in 47% and 11% yield, respectively. High-resolution
electrospray ionization mass spectrometry (ESI-MS) analysis of
2 and 3 gave parent ion peaks that match well with the calculated
ones (Figures S1 and S2 in the Supporting Information, SI). 1H
Received: November 30, 2012
Published: February 4, 2013
© 2013 American Chemical Society
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Inorganic Chemistry
Communication
Figure 1. ORTEP diagrams of 2, 2-O, and 3. Top views: (a) 2, (c) 2-O, and (e) 3. Side views: (b) 2, (d) 2-O, and (f) 3. Thermal ellipsoids are drawn at
50% probability with the phenyl groups omitted. Solvent molecules and hydrogen atoms, except N−H in part a, are also omitted for clarity.
NMR spectra of 2 and 3 are shown in Figure S3 in the SI along
with that of 1. In line with the symmetric structure, 2 shows a
simple spectrum of the aromatic protons and two singlet peaks at
2.69 and 2.41 ppm for the methyl protons of the tolyl groups.
The 1H NMR spectrum of 3 is rather broad at room temperature
(Figure S3c in the SI), but the peaks at 8.45, 7.82, 7.48, and 7.21
ppm become increasingly sharper upon a decrease in temper-
ature to 213 K, indicating that the broad spectrum is due to
conformational flexibility (Figure S3e in the SI). Broadening of
the meso-aryl groups was also observed for rhenium(I) and
ruthenium(II) complexes of tetraaryltribenzotriphyrins.4b,5,16
molecule 2-O was positioned in a mirror plane. In 2-O, an
oxygen atom is incorporated between the C(2) and C(2′) atoms
to form a 2,5-dihydrofuran ring, with a C(2)−O bond length of
1.47(1) Å. The whole molecular π-conjugation system of 2-O is
interrupted with sp3 C(2) atoms. The bond length of C(1)−
C(1′) [1.331(1) Å] was close to that of a normal CC double
bond and significantly shorter than that of 2 [1.386(7) Å]. The
bond lengths connected to the C(2) atom [1.53 (1) Å for C(1)−
C(2); 1.52(1) Å for C(2)−C(3); 1.49(1) Å for C(2)−N(1)] in
2-O are obviously longer than the corresponding bond lengths of
2, 1.475(19), 1.384(19), and 1.352(6) Å, respectively. The newly
formed dihydrofuran moiety showed chair shape, bent by 33.18°
at the C(1)−C(1′) axis (Figure S6b in the SI). The mutual angle
between two pyrrole rings coordinated to a platinum ion was
109.04°, and the C(2)−C(2′) distance was only 2.32 Å,
indicating a deeper saddle shape compared with 2. The angle
between the noncoordinated pyrrole ring and the NNClCl plane
of 2-O was 46.53°, lifting the platinum plane 2.86° more than 2.
Single crystals of 3 were obtained from a toluene/n-hexane
solution. X-ray structure analysis (Figures 1e,f and S6c and
Tables S1−S6 in the SI) revealed a bowl-shaped conformation
for 3, similar to that of rhenium(I) complex.4b The distance
between the PtIV ion and the NNN plane was 1.275 Å and was
shorter than the corresponding distance in the rhenium(I)
complex by 0.201 Å, and platinum(IV) complex was flatter than
that of the rhenium(I) complex. The bond length of C(1)−
C(16) [1.43(1) Å] was longer than those of 2 and 2-O, and the
bond lengths of the macrocyclic ring suggested clear 14π
conjugation (Table S2 in the SI).
Absorption spectra of 2 and 3 along with free-base 1 are shown
in Figure 2. Platinum complexes show broad Soret bands at the
region around 470 nm and Q bands in the region from 600 to 700
nm, respectively, which are considerably red-shifted and
broadened compared with those of 1. The Soret band of 3 is
more red-shifted and the Q band is more blue-shifted than those
of 2. Unfortunately, the absorption spectrum of 2-O was not
obtained because the spectrum of 2-O in solution was the same
as that of 2. 2-O will be unstable in solution.
The electrochemical properties of 2 and 3 were examined by
cyclic and differential pulse voltammetry (Table S7 and Figures
S7 and S8 in the SI) in CH2Cl2 containing 0.1 M Bu4NPF6 at
room temperature. Complex 2 gave four irreversible waves with
1
The transformation from 2 to 3 was confirmed by H NMR
investigation in CDCl3. When the change of the 1H NMR spectra
of 2 in air was measured (Figure S4 in the SI), the two methyl
peaks of the platinum(IV) complex at 2.76 and 2.49 ppm started
to be observed at 2 h and the two methyl peaks of the
platinum(II) complex at 2.69 and 2.41 ppm disappeared
completely after 96 h. On the other hand, when the air in the
NMR tube was changed to a strict argon atmosphere using a
freeze−thaw method, this transformation did not occur after 96 h
(Figure S5 in the SI). These results suggest that 2 is oxidized to 3
by oxygen gas. 3 was also obtained as the sole product in 41%
yield by the reaction of 1 with (Bu4N)2PtCl6, as a platinum(IV)
source, in a mixture of toluene and acetic acid.
Crystallization of 2 was performed with a N2-bubbled solvent
(CH2Cl2/CH3OH) under a nitrogen atmosphere in a glovebox.
The crystal structure of 2 is shown in Figures 1a,b and S6a in the
SI and summarized in Tables S1−S6 in the SI. The PtII ion was
coordinated to two pyrrolic nitrogen atoms and two chloride ions
with the following bond lengthes: 2.021(4) Å for Pt−N(1) and
2.314(1) Å for Pt−Cl in a cis configuration to form a rigid square-
planar conformation. The triphyrin macrocycle shows a saddle
shape with Cs symmetry. The mutual angle between two pyrrole
rings coordinated to a PtII ion is 119.36° with a C(2)−C(2)′
distance of 3.131(7) Å. The angle between the noncoordinated
pyrrole ring and the NNClCl plane was 43.69° (Figure S6a ni the
SI).
When 2 was crystallized from a CH2Cl2/EtOH solution in air,
green plate-shaped crystals were obtained. It was a new PtIITriP
complex, 2-O, with an oxygen adduct between two pyrrole rings,
as shown in Figures 1c,d and S6b and Tables S1−S6 in the SI.
The space group of the crystal for 2-O was P21/m, and the
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Inorganic Chemistry
Communication
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ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray crystallographic data in CIF format, synthetic details, NMR
spectra, cyclic voltammograms, and crystallographic data. This
material is available free of charge via the Internet at http://pubs.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: hyamada@ms.naist.jp. Fax:+81-743-72-6042.
■
Present Address
‡Department of Applied Chemistry, Jiangsu University,
Zhenjiang 212013, People’s Republic of China.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank to Mika Yamamura and Yuriko Nishikawa for
ESI-MS measurements. This work was supported by a JSPS
Postdoctoral Fellowship for Foreign Researchers (to Z.X.) and
partly supported by a Grant-in-Aid (Grant 24655034 to H.Y. and
D.K.) and the Green Photonics Project in NAIST sponsored by
the Ministry of Education, Culture, Sports, Science and
Technology, MEXT, Japan.
DEDICATION
Dedicated to the memory of Dr. Christian G. Claessens.
■
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