Thermal Splitting of Bis-Cu(II) Octaphyrin(1.1.1.1.1.1.1.1) into Two Cu(II) Porphyrins

Article abstract of DOI:10.1021/ja031935t

When heated, bis-Cu(II) octaphyrin(1.1.1.1.1.1.1.1) is quantitatively split into two Cu(II) porphyrins both in solution and film states, which is accompanied by large absorption spectral changes. Copyright

Full text of DOI:10.1021/ja031935t

Published on Web 02/24/2004
Thermal Splitting of Bis-Cu(II) Octaphyrin(1.1.1.1.1.1.1.1) into Two Cu(II)
Porphyrins
Yasuo Tanaka, Wataru Hoshino, Soji Shimizu, Katsuyuki Youfu, Naoki Aratani, Norihiko Maruyama,†
†
Shizuo Fujita, and Atsuhiro Osuka*
Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Core Research for EVolutional Science
and Technology (CREST), Japan Science and Technology Agency (JST), Kyoto 606-8502, Japan, and
International InnoVation Center, Kyoto UniVersity, Kyoto 606-8501, Japan
Received December 24, 2003; E-mail: osuka@kuchem.kyoto-u.ac.jp
Currently, increasing attention has been focused on the chemistry
of expanded porphyrins that have pyrrolic subunits with more than
four porphyrins.1 An intriguing promise of this new class of
macrocycles may be the ability to chelate two or more metal ions
with the aid of large and flexible ligands. Despite this promise,
there have been only scattered reports on the metalation of expanded
porphyrins thus far.2 Rare interesting examples include the
Chart 1. Ar ) Pentafluorophenyl
-5
3
4
metalation of amethyrin and doubly N-confused hexaphyrin that
have been shown to chelate two metal ions with notable metal-
metal interactions. More recently, Vogel et al. disclosed interesting
metalation behaviors of octaphyrins, in which Ni(II)-metalation of
a diketo octaphyrin(1.1.1.0.1.1.1.0) induced a drastic skeletal
rearrangement to a bis-Ni(II) spirodicorrole and Pd(II)-metalation
of octaphyrin(1.1.1.0.1.1.1.0) triggered a different skeletal re-
ordering in the transition state than the initial state. The reaction
rates were virtually not influenced upon the increase of solvent
-
4
-1
-4 -1
5
polarity, 2.3 × 10
s
in nonpolar toluene and 2.2 × 10
s
in
arrangement to a bis-Pd(II) bis-spiroporphyrin. In this communica-
polar DMF at 100 °C, suggesting only a small polarity change in
the transition state compared to the starting substrate 1(Cu2). More
importantly the splitting reaction has not been observed either for
1 or for 1(Cu) even for prolonged refluxing in toluene solutions,
highlighting the crucial importance of two Cu(II) ion coordination
in 1 for the splitting reaction. We have tested the metalation of 1
with a variety of metal ions with a view to find the similar splitting
reaction and have found that the formation of porphyrin derivatives
was noted in small amounts for some metals such as Pd(II), Co(II),
and Ni(II) ions, but the quantitative splitting into two porphyrins
was only observed for Cu(II) ion.
tion, we report the thermal splitting reaction of bis-Cu(II) complex
of octaphyrin(1.1.1.1.1.1.1.1) into two Cu(II) porphyrins.
Octaphyrin(1.1.1.1.1.1.1.1) 1 was prepared by our synthesis of
meso-aryl expanded porphyrins6b (see Chart 1). In the solid-state,
1
takes a figure-eight structure with two identical porphyrin-like
hemi-macrocyclic tetrapyrrolic ligands that are pertinent for metal-
ion coordination.6 A solution of 1 in toluene was stirred at 50 °C
a
in the presence of 10 equiv of Cu(OAc) and 5 equiv of NaOAc
2
and the metalation process was monitored by UV-vis absorption
spectroscopy (Figure 1). The nonaromatic [36]octaphyrin 1 exhib-
ited absorption bands at 410 and 643 nm, which changed to the
absorption bands at 412 and 668 nm after 2 h and to the absorption
bands at 351 and 522 nm after 4 h. These spectral changes have
been ascribed to sequential formation of mono-Cu(II) octaphyrin
2
X-ray diffraction analysis of 1(Cu2) reveals C symmetry of the
molecule and the presence of two copper atoms centrally bound
within the core of a severely distorted octaphyrin macrocycle
(
Figure 2).9 Two copper atoms are both coordinated with a
1(Cu) and bis-Cu(II) octaphyrin 1(Cu2), by referring to the
porphyrin-like hemi-macrocyclic ligand with Cu-N1, Cu-N2, Cu-
N3, and Cu-N4 bond distances being 2.06, 1.97, 1,98, and 2.02
respective authentic absorption spectra. To our surprise, the
absorption spectrum of this reaction mixture changed dramatically
upon prolonged stirring and after 2 weeks finally displayed an
absorption spectrum with a sharp band at 408 nm and two small
bands at 535 and 570 nm. The final spectrum apparently indicated
the formation of Cu(II) porphyrin 2 that was actually isolated in
67% yield from this reaction mixture. Elevating the reaction
temperature to 111 °C (refluxing toluene) completed the splitting
reaction within 2 h, providing 2 in 91% yield.
Copper complexes 1(Cu) and 1(Cu2) were prepared under
controlled metalation conditions in 77 and 81% yields, respectively
7
(
Supporting Information). With pure 1(Cu2) in hand, the activation
parameters of the splitting reaction were determined. The splitting
reaction of 1(Cu2) was found to be an unimolecular process with
q
q
8
∆
H ) 68.2 kJ/mol and ∆S ) -133 eu. Despite the dissociative
nature of the splitting reaction, the observed large negative
activation entropy is notable, since it suggests a greater degree of
Figure 1. UV-vis absorption spectra recorded during the metalation of 1
with Cu(OAc)2 at 50 °C in toluene.
†
International Innovation Center, Kyoto University.
3046
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J. AM. CHEM. SOC. 2004, 126, 3046-3047
10.1021/ja031935t CCC: $27.50 © 2004 American Chemical Society

C O MMU N I C A T I O N S
the film of 1(Cu2) at 200 °C, however, gave rise to clean splitting
into 2 as judged from newly appearing absorption bands, 413
(Soret), 537 (Q), and 573 (Q) nm, encouraging the use of this
process in patterned FET and thermal/optical recording/reading
devices.
As demonstrated above, the Cu(II) metalation of octaphyrin-
(1.1.1.1.1.1.1.1) gave rise to the facile splitting reaction triggered
by increased distortion in the bis-metal complex 1(Cu2), which
provided a rare example of “molecular mitosis” for expanded
porphyrins.11 Studies on the metalation of other meso-aryl expanded
porphyrins are actively in progress to explore their novel reactivities
and potentials.
Figure 2. X-ray structures of 1(Cu2). (a) Top view and (b) side view.
The thermal ellipsoids were scaled to the 50% probability level. Hydrogen
atoms and pentafluorophenyl substituents are omitted for clarity.
Acknowledgment. This work was supported by Grant-in-Aids
for Scientific Research (B) (No. 15350022) from the Ministry of
Education, Culture, Sports, Science and Technology of Japan.
Å, respectively, and the Cu-Cu distance is 5.42 Å. A tripyrrolic
unit consisting of pyrroles B, C, and D is relatively flat with the
mean-plane deviation of 0.17 Å, while the pyrrole A plane is tilted
by 63° with respect to the above plane and the Cu atom is displaced
out of this plane by 0.757 Å. Collectively, the coordinating structure
of 1(Cu2) is severely distorted, which is in sharp contrast to the
porphyrin 2 that exhibits a quite planar structure as shown by its
X-ray structure with the mean plane deviation of only 0.015 Å
Supporting Information Available: Synthetic procedures and
spectral data of 1(Cu) and 1(Cu2), absorption spectral changes of 1-
(Cu2) during heating in the film state, Arrhenius and Eyring plots,
DSC experiments. CIF files for the X-ray structural analysis of 1(Cu2)
and 2. This material is available free of charge via the Internet at http://
pubs.acs.org.
(
Supporting Information).10 These features suggest that the relief
References
of strain in 1(Cu2) may be a main driving force for this thermal
splitting reaction. We thus examined the thermal behavior of 1-
(1) (a) Jasat, A.; Dolphin, D. Chem. ReV. 1997, 97, 2267. (b) Lash, T. D.
Angew. Chem., Int. Ed. 2000, 39, 1763. (c) Furuta, H.; Maeda H.; Osuka,
A. Chem. Commun. 2003, 1795. (d) Sessler, J. L.; Seidel, D. Angew.
Chem., Int. Ed. 2003, 42, 5134. (e) Setsune, J.; Maeda, S. J. Am. Chem.
Soc. 2000, 122, 12405. (f) Chandrashekar T. K. Venkatraman, S. Acc.
Chem. Res. 2003, 36, 676.
(Cu2) by differential scanning calorimetry (DSC) at a rate of 25
°
C/min, which revealed a sharp exothermic response between 240
and 280 °C, and the porphyrin 2 was obtained in high yield from
this thermolysis experiment. Such a response was not observed for
either 1 or 1(Cu). From the DSC experiment, the formation energy
of 2 was estimated to be 135 kJ/mol, which can be considered to
arise from a stability difference between 1(Cu2) and 2. One possible
mechanism of the splitting reaction may be 2π + 2π cycloaddition
to give spirocyclobutane intermediate 3, which is split into two
molecules of 2 via a cycloreversion reaction.
(2) (a) Charriere, R.; Jenny, T. A.; Rexhausen, H.; Gossauer, A. Heterocycles
1
993, 36, 1561. (b) Dolphin, D.; Rettig, S. J.; Tang, H.; Wijesekera, T.;
Xie, L. J. Am. Chem. Soc. 1993, 115, 9301.
(
3) Weghorn, S. J.; Sessler, J. L.; Lynch, V.; Baumann, T. F.; Sibert, J. W.
Inorg. Chem. 1996, 35, 1089.
(
4) Srinivasan, A.; Ishizuka, T.; Osuka, A.; Furuta, H. J. Am. Chem. Soc.
2003, 125, 878.
(5) Vogel, E.; Michels, M.; Zanders, L.; Lex, J.; Tuzun, N. S.; Houk, K. N.
Angew. Chem., Int. Ed. 2003, 42, 2857.
(
6) (a) Shin, J.-Y.; Furuta, H.; Yoza, K.; Igarashi, S.; Osuka, A. J. Am. Chem.
Soc. 2001, 123, 7190. (b) Taniguchi, R.; Shimizu, S.; Suzuki, M.; Shin,
J.-Y.; Furuta, H.; Osuka, A. Tetrahedron Lett. 2003, 44, 2505.
(7) 1(Cu) and 1(Cu2) showed the parent ion peaks at m/z ) 2009.0444 (calcd
for C88
2
H
18
N
8
F
40Cu, 2009.0412) and m/z ) 2069.9668 (calcd for C88
H
16
N
8
F
40
-
Cu
, 2069.9651), respectively.
6
-1
(
8) The Arrhenius plot provided a preexponential factor of 2.2 × 10 s and
an activation energy of 71.1 kJ/mol.
(
9) 1(Cu2): C92H N O F40Cu , M ) 2285.25, Pbcn, with a ) 23.140(3)
21 8 10 2 w
3
Å, b ) 15.364(2) Å, c ) 24.814(3) Å, V ) 8822(1) Å , Z ) 4, µ ) 6.303
-1
3
cm , Dcalc ) 1.693 g/cm , and crystal dimensions 0.80 mm × 0.40 mm
0.20 mm. The data were collected on Bruker SMART diffractometer
at -153 °C, and the structure was solved by direct methods. It was refined
to R ) 0.077, R ) 0.096, GOF ) 1.058 for 6433 with I > 3.0 σ(I).
×
w
The present thermal splitting reaction of 1(Cu2) to 2 may be
attractive also for application to a recording device in terms of the
dramatic spectral changes as well as the perfect material balance.
We thus examined the similar splitting reaction of 1(Cu2) in a film
state. Fortunately, 1(Cu2) was found to form a good transparent
film by spin coating of its 2% toluene solution (500 rpm, 10 s and
(10) The mean plane deviation of 2 was previously reported to be 0.032 Å.
Birnbaum, E. R.; Hodge, J. A.; Grinstaff, M. W.; Schaefer, W. P.; Henling,
L.; Labinger, J. A.; Bercaw, J. E.; Gray, H. B. Inorg. Chem. 1995, 34,
3625.
(
11) Molecular mitosis reactions are known for calix[n]arenes and calix[n]-
pyrroles, but it is difficult to consider them for fully conjugated, expanded
porphyrins. (a) Gutsche, C. D. In Calixarenes, Monographs in Supramo-
lecular Chemsitry; Stoddart, J. F., Ed.; The Royal Chemical Society:
Cambridge, 1989; pp 5-58. (b) Sessler, J. L.; Anzenbacher, P.; Shriver,
J. A.; Jursikova, K.; Lynch, V. M.; Marquez, M. J. Am. Chem. Soc. 2000,
2000 rpm, 60s) onto the quartz. In the film state, 1(Cu2) exhibited
122, 12061.
broad bands at 354, 531, and 693 nm due to aggregation and was
more robust than in solution, being stable under 140 °C. Heating
JA031935T
J. AM. CHEM. SOC.
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