A meso-spiro[cyclopentadiene-isoporphyrin] from a phenylethynyl porphyrin platinum(II) pincer complex

Article abstract of DOI:10.1002/anie.201109091

A good catch: The oxidation of a phenylethynyl Pt^II pincer complex with iodine led to formal reductive elimination but without the liberation of Pt^II, which was caught by the 2-pyridyl substituents on the Ni^II porphyrin. The unique reactivity of the Pt^II pincer complex led to the formation of a meso-spiro[cyclopentadiene- isoporphyrin]. Copyright

Full text of DOI:10.1002/anie.201109091

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DOI: 10.1002/anie.201109091
Porphyrinoids
A meso-Spiro[Cyclopentadiene-Isoporphyrin] from a Phenylethynyl
Porphyrin Platinum(II) Pincer Complex**
Shoma Anabuki, Hiroshi Shinokubo,* Naoki Aratani,* and Atsuhiro Osuka*
Pincer-type organometallic complexes have received much
attention as non-linear optical (NLO) materials, light emitting
dyes, and highly active and stable catalysts in a number of
organic transformations.[1] The tridentate ligand strongly
binds a metal to prevent ligand dissociation, thus achieving
high thermal stability. We have explored porphyrin pincer
complexes bearing palladium or platinum metal bound to the
tridentate ligand consisting of a porphyrin meso carbon and
two 2-pyridyl groups substituted at the adjacent b posi-
tions.[2–4] These porphyrin pincer complexes exhibit catalytic
reactivity in Heck reactions,[2a,b] redox-responsive pivotal
conformational switching,[4a] and large two-photon absorption
cross-sections.[2c] Porphyrin pincer complexes may serve as
precursors for peripheral functionalization of porphyrin, but
such a possibility has been scarcely tested to date. As a rare
case, we have reported that the oxidation of the phenyl-
platinum(II) pincer complex 1 with iodine induced a facile
meso-phenylation by reductive elimination (Scheme 1, (1)).[5]
Herein, we report quite different chemical behaviors of
(phenylethynyl)platinum(II) pincer complex 4 upon treat-
ment with iodine. Interestingly, in this case, reductive
elimination occurs to allow a carbon–carbon bond formation
but without liberation of platinum(II) metal, which is left
tightly bound by the two 2-pyridyl groups. Furthermore,
a
meso-spiro[cyclopentadiene-isoporphyrin] was formed
unexpectedly as a doubly phenylethynylated product. Iso-
porphyrins are porphyrin tautomers that carry an interrupted
Scheme 1. Synthesis and transformations of porphyrin pincer com-
plexes 1–6,9. Ar=3,5-di-tert-butylphenyl.
[*] S. Anabuki, Dr. N. Aratani, Prof. Dr. A. Osuka
Department of Chemistry
Graduate School of Science, Kyoto University
Sakyo-ku, Kyoto 606-8502 (Japan)
E-mail: aratani@kuchem.kyoto-u.ac.jp
osuka@kuchem.kyoto-u.ac.jp
macrocyclic conjugation owing to the presence of an sp3-
hybridized meso carbon. The existence of this tautomer was
first suggested by Woodward in 1961,[6] whose prediction was
first confirmed by Dolphin et al. by their synthesis.[7] Iso-
porphyrins have been considered to be key intermediates in
the heme oxidation catalyzed by heme oxygenase,[8] but
synthetic investigations of isoporphyrins have been rather
limited.[9]
Pincer complex 3 was prepared according to our reported
procedure,[5] and was converted into phenylethynyl pincer
complex 4 by a ligand-exchange reaction (Scheme 2). This
complex was sensitive to hydrolytic cleavage on a silica gel
column and was thus isolated by recrystallization from
Dr. N. Aratani
PRESTO, Japan Science and Technology Agency (Japan)
Prof. Dr. H. Shinokubo
Department of Applied Chemistry
Graduate School of Engineering, Nagoya University
Chikusa-ku, Nagoya 464-8603 (Japan)
E-mail: hshino@apchem.nagoya-u.ac.jp
[**] This work was partly supported by Grants-in-Aid for Scientific
Research (Nos. 19205006 (A) and 20108001 “pi-Space”) and for
Young Scientists (A) from MEXT, and by JST PRESTO program.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201109091.
a
mixture of CH2Cl2 and methanol in 72% yield
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(Scheme 1, (2)). The high-resolution electrospray ionization
time-of-flight (HR-ESI-TOF) mass and 1H NMR spectra of 4
are consistent with the structure, which has been confirmed by
X-ray diffraction analysis (Figure 1a).[10] The porphyrin ring
of 4 is distorted to a saddle shape owing to the peripheral
metalation, which is similar to the porphyrin palladium(II)
pincer complexes.[2a]
To induce the reductive elimination of 4, oxidation of the
PtII center was attempted by treatment with iodine. Initially,
complex 4 was reacted with iodine under the reaction
conditions used for the meso-phenylation of 1.[5] Aqueous
work up produced a complicated mixture from which
a product was isolated in a trace amount. Fortunately, good-
quality crystals were obtained, allowing the structural deter-
mination by X-ray diffraction analysis as meso-phenacylated
porphyrin 5. In this complex, the platinum(II) ion is bound to
the a position of the phenacyl group, the two nitrogen atoms
of the 2-pyridyl groups, and iodide to give a square-planar
coordination (Figure 1b).[10] Consistent with the structure, the
parent ion peak of 5 was observed at m/z = 1524.4638 (calcd
for C80H83N6ONiPtI = 1524.4697 [M]+) in the HR-ESI-TOF
1
mass spectrum, and the H NMR spectrum of 5 at À608C
exhibits a singlet signal at 6.44 ppm for Ha, a set of three
signals for the phenyl group, and sets of non-equivalent
signals for both the porphyrinic b protons and pyridyl protons
(Supporting Information). On the basis of the consideration
that 5 should be a hydrolyzed product, we attempted to
improve the yield of 5 by the addition of water or hydroxide
ion. After extensive optimization, the complex 5 was obtained
in 39% yield by refluxing a mixture of 4 in chlorobenzene and
THF in the presence of I2 and aqueous KOH at 1008C for 12 h
(Scheme 1, (3)).
In a next step, we examined the reaction of 4 with I2 under
rigorously anhydrous conditions. Treatment of 4 with I2 in
anhydrous CHCl3 at 08C for 12 h furnished meso-alkenylated
porphyrin 6 in 48% yield after recrystallization from aceto-
nitrile (Scheme 1, (4)). The parent ion peak of 6 was observed
at m/z = 1634.3634 (calcd for C80H82N6NiPtI2 = 1634.3715
[M]+) in the HR-ESI-TOF mass spectrum, indicating an
addition of I2. The 1H NMR spectrum of 6 now exhibits higher
symmetry: a set of porphyrin signals were observed at 8.69–
8.61 ppm and the phenyl protons at 7.18–6.46 ppm. The X-ray
diffraction analysis revealed the structure of 6 to be a meso-
alkenyl porphyrin (Figure 1c).[10] Compound 6 is sensitive to
water and could be easily hydrolyzed by aqueous KOH to
afford 5 quantitatively.
The formation of 5 and 6 can be understood in terms of
iodination of 4 to form PtIV pincer porphyrin 7[11] and
subsequent reductive elimination to yield meso-ethynyl
porphyrin 8 (Scheme 2). The acetylene moiety in 8 is
activated by the interaction with the captured PtII ion, so
that nucleophilic attack on the ethynyl group in 8 by
hydroxide or iodide ion is facilitated to form 5 or 6
(Scheme 2).
Furthermore, during the analysis of the reaction of 4 with
I2 under anhydrous conditions, we noticed the formation of
a very polar byproduct in addition to 6, which was charac-
terized as meso-spiro[cyclopentadiene-isoporphyrin] 9. This
unique product was obtained in 22% yield (on the basis of the
Figure 1. X-ray crystal structures of a) 4, b) 5, c) 6, and d,e) 9.
Ni green, Pt yellow, N blue, O red, I purple. tert-Butyl groups, hydrogen
atoms, disordered parts, and solvent molecules are omitted for clarity.
Ellipsoids are set at 50% (4) and 30% probability (5, 6, and 9).
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of 7 to afford pincer complex iodide 10 and
iodoethynylbenzene. Subsequent insertion of
iodoethynylbenzene to 4 to form 1,3-enyny-
lated pincer complex 11, which is converted
into 9 via I2-assisted intramolecular cycliza-
tion.[12] To confirm this mechanism, the reac-
tion of 4 with iodine in the presence of 5 equiv
iodoethynylbenzene was attempted, which
indeed led to the formation of 9 in 68% yield.
As a separate test reaction, we examined
the reaction of meso-[phenylethynylbis(tri-
phenylphosphino)platinum(II)]
porphyrin
13[13] with iodine (Scheme 3). Interestingly,
this reaction did not produce the reductive
elimination product but instead afforded
iodinated PtII complex 14 in 85% yield,
suggesting that the unique reactivity of the
complex 4 arises from the tightly bound
pincer structure.
The UV/Vis absorption spectra of 4–6 and
9 are shown in Figure 2. The complexes 5 and
6 exhibit broader Soret bands and red-shifted
Q bands relative to those of 4. The cationic
isoporphyrin 9 displays a significantly red-
shifted Q band reaching to infrared region up
to 1200 nm, which is characteristic of isopro-
phyrins but much more red-shifted than those
of other isoporphyrin metal complexes (typ-
ically around 800–900 nm).[9]
In summary, the oxidation of phenyl-
Scheme 2. Plausible mechanisms for the formation of 5, 6, and 9.
ethynyl PtII pincer complex 4 with iodine led
amount of 4) and 44% yield (on the
basis of the phenylethynyl moiety)
under the conditions shown in
Scheme 1, (5). The parent ion peak of
9 was observed at m/z = 1735.4126
(calcd for C88H87N6NiPtI2 = 1735.4106
[MÀI3]+) in its HR-ESI-TOF mass
spectrum, thus indicating the presence
Scheme 3. Formation of 14 from the reaction of 13 with iodine.
of an additional phenylethynyl unit.
1
The H NMR spectrum of 9 exhibited
less-deshielded signals for the porphyr-
inic b protons, thus indicating a loss of a diatropic ring
current. Single crystals of 9 suitable for X-ray diffraction
analysis were grown by slow vapor diffusion of methanol into
its chloroform solution. The X-ray diffraction study revealed
a newly formed cyclopentadiene moiety that is connected at
the meso position in a spiro manner with a dihedral angle of
73.28 relative to the porphyrin mean plane, thus disrupting
a porphyrin conjugated aromatic circuit to form an isopor-
phyrin skeleton (Figure 1d,e).[10] The diamagnetic character
of 9 indicates a low-spin NiII ion in the isoporphyrin ligand. As
À
an isoporphyrin is a monoanionic ligand, I3 is found as
a counterion to balance the charge for the resulting NiII
isoporphyrin cation. It is worth noting that 9 is the first
example of nickel(II) isoporphyrin.[9l]
A plausible reaction mechanism for the formation of 9 is
shown in Scheme 2, which involves the reductive elimination
Figure 2. UV/Vis absorption spectra of 4–6 and 9 in CH2Cl2.
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[9] For details about synthetic isoporphyrins, see; a) A. Harriman,
to formal reductive elimination, but without the liberation of
PtII, which was caught by the 2-pyridyl pincer substituents.
Importantly, the remaining PtII ion activates the ethynyl
moiety by p coordination to assist the facile formations of 5
and 6. We also found the reaction conditions under which
meso-spiro[cyclopentadiene-isoporphyrin] 9 was formed in
good yield by an additional coordination–insertion of iodoe-
thynylbenzene to 4 followed by I2-assisted intramolecular
cyclization. Further exploration of unique reactions of
porphyrin pincer complexes for porphyrin peripheral modi-
fications is actively in progress in our laboratories.
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Received: December 23, 2011
Published online: February 15, 2012
Keywords: isoporphyrins · pincer ligands · platinum ·
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porphyrinoids · reductive elimination
[10] Crystal data for 4: C80H82N6NiPt·5CHCl3, Mr = 1973.12, triclinic,
¯
space group P1 (no. 2), a = 14.5824(12), b = 17.7665(15), c =
17.7761(15) ꢀ, a = 91.675(2), b = 102.2010(10), g =
98.3630(10)8, V= 4444.9(6) ꢀ3, Z = 2, T= 90(2) K, Dcalcd
1.474 gcmÀ3
R1 = 0.0503 (I > 2s(I)), Rw = 0.1384 (all data),
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=
,
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93(2) K,
D , R1 = 0.0858 (I > 2s(I)), Rw =
calcd = 1.045 gcmÀ3
0.2397 (all data), GOF = 0.833; 6: C80H82N6NiPtI2, Mr =
¯
1635.12, triclinic, space group P1 (no. 2), a = 11.7306(2), b =
14.8418(4), c = 26.9882(5) ꢀ, a = 99.1516(10), b = 98.6274(14),
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1.294 gcmÀ3
, R1 = 0.0969 (I > 2s(I)), Rw = 0.2611 (all data),
GOF = 1.007; 9: C88H87N6Ni1Pt1I5·2.73CHCl3·0.27I2, Mr =
¯
2511.35, triclinic, space group P1 (no. 2), a = 15.2725(3), b =
17.3994(3), c = 19.6955(4) ꢀ, a = 88.3329(7), b = 69.6501(7),
g = 74.1108(7)8, V= 4706.89(16) ꢀ3, Z = 2, T= 93(2) K, Dcalcd
=
1.772 gcmÀ3, R1 = 0.0973 (I > 2s(I)), Rw = 0.2465 (all data),
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