Self-assembly of Zn(II) porphyrin-1,2,3-triazole conjugate with alcohol glue

Article abstract of DOI:10.1246/cl.2010.252

A Zn(II) porphyrin-1,2,3-triazole conjugate (1-Zn), which can be prepared by click chemistry, afforded a unique self-assembled structure with methanol molecules as glue in the crystals and in toluene solution. The resulting assembled units form a zigzag architecture due to multipoint CH-π interactions in the solid state.

Full text of DOI:10.1246/cl.2010.252

doi:10.1246/cl.2010.252
252
Published on the web February 6, 2010
Self-assembly of Zn(II) Porphyrin-1,2,3-Triazole Conjugate with Alcohol Glue
Motoki Toganoh,1 Hiroyuki Harada,1 Yoshiya Ikawa,1,2 and Hiroyuki Furuta*1
1Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University,
744 Motooka, Nishi-ku, Fukuoka 819-0395
2PRESTO, Japan Science and Technology Agency (JST), Tokyo 102-0075
(Received December 28, 2009; CL-091152; E-mail: hfuruta@cstf.kyushu-u.ac.jp)
A Zn(II) porphyrin-1,2,3-triazole conjugate (1-Zn), which
(3-Zn) in 61% yield (2 steps). Then, deprotection of 3-Zn with
can be prepared by click chemistry, afforded a unique self-
assembled structure with methanol molecules as glue in the
crystals and in toluene solution. The resulting assembled units
form a zigzag architecture due to multipoint CH-³ interactions
in the solid state.
Bu4NF gave the terminal alkyne (4-Zn) in 99% yield and the
click reaction of 4 with an ethyl azidoacetate gave the Zn(II)
porphyrin-1,2,3-triazole conjugate (1-Zn) in 81% yield. The
free base form 1 was also prepared by demetalation of 1-Zn with
CF3CO2H in 67% yield.
1
The H NMR spectra of 1 and 1-Zn in CDCl3 (or toluene-
d8) closely resemble each other except for the signal due to the
inner NH protons at ¹2.61 ppm and the high-field shifts of the
porphyrin ¢-protons by ca. 0.1 ppm in 1, suggesting 1-Zn alone
does not form any self-recognized oligomers in solution
(Figure S1).9 For example, the signals assignable to the 1,2,3-
triazole moiety appeared at 7.93 (1) and 7.94 ppm (1-Zn). The
signals due to the ¢-phenylene moiety were observed at 7.40,
7.62 ppm for 1 and 7.41, 7.62 ppm for 1-Zn. Note that the
porphyrin derivatives possessing a coordination group at the
meso- or ¢-positions often afford the self-recognized oligomers,
where the distinct low-field shifts of some signals are observed
due to the shielding effect of a paired [18]annulenic porphyrin
macrocycle.3,10-12 The absence of oligomer formation can be
explained by steric factors. Namely, access to the center zinc
metal by the triazole nitrogen atom of another molecule is
blocked by the ¢-phenylene and CH2CO2Et groups.
Click chemistry has been widely used in construction of a
variety of conjugated molecules.1,2 Although a major role of
click chemistry is just connecting two building blocks, the
resulting bridge part, a 1,2,3-triazole unit, is sometimes used for
metal coordination. For example, a 1,2,3-triazole-appended
zinc(II) porphyrin formed a self-assembled dimer3 and con-
jugates of a biomolecule and a transition-metal complex were
readily prepared by the click reactions.4,5 In addition to these N-
coordinated complexes, the application of a 1,2,3-triazole ring to
C-coordinated complexes or N-heterocyclic carbene complexes
was also reported.6
Alternatively, a 1,2,3-triazole unit can serve as a hydrogen-
bonding acceptor or donor. Although molecular recognition or
the construction of supramolecules through hydrogen bondings
with a 1,2,3-triazole unit can be expected, such examples are
hardly ever reported. This time we have prepared a new type of
1,2,3-triazole-appended zinc(II) porphyrin and studied its assem-
bly. While direct self-recognition through metal coordination
was not observed certainly due to the steric hindrance, formation
of a 2:2 assembled structure between the zinc(II) porphyrin-
1,2,3-triazole conjugates (1-Zn) and methanol molecules was
observed unexpectedly, where the methanol molecules work as
glue through hydrogen bondings as well as metal coordination.
The zinc(II) porphyrin-1,2,3-triazole conjugate (1-Zn) was
prepared in four steps from the ¢-bromotetraphenylporphyrin
(2)7 as shown in Scheme 1. The Suzuki cross coupling reaction
of 2 with 4-[(triisopropylsilyl)ethynyl]phenylboronic acid8 and
subsequent zinc metalation gave the ¢-arylated Zn(II) porphyrin
Although 1-Zn alone did not form any oligomeric struc-
tures, it formed a 2:2 assembled structure with CH3OH, which
was demonstrated by X-ray crystallographic analysis (Figure 1).
Thus, 1-Zn was recrystallized from CH2Cl2/CH3OH to give red
plates, one of which was subjected to X-ray analysis.13 The
oxygen atom of a CH3OH molecule coordinates to the zinc(II)
metal (Zn-O: 2.149 ¡) of a porphyrin conjugate and the hydroxy
proton of the same CH3OH molecule forms hydrogen bonding
with the 1,2,3-triazole unit of another porphyrin conjugate
(O-N: 2.749 ¡). As a result, a 2:2 assembled structure was
constructed with 1-Zn and CH3OH. Furthermore, two porphyrin
conjugates interact with each other by multipoint CH-³
interactions, where the distances between the porphyrin mean
plane composed of 24 heavy atoms and the aryl protons are
2.40-2.75 ¡ on the assumption of C-H bond length being 0.93 ¡
(Figure 1b). Interestingly, synergy of these interactions resulted
in the formation of the aligned zigzag structures in a solid state
(Figure 1c).
Ph
Ph
R
Br
NH
N
N
N
N
N
N
1) Suzuki coupling
2) zinc metallation
61% (2 steps)
Zn
Ph
Ph
Ph
Ph
HN
3-Zn (R = iPr3Si)
4-Zn (R = H)
1
The variable temperature H NMR analysis of 1-Zn in the
deprotection
99%
2
Ph
Ph
Ph
presence of CH3OH implied the formation of the 2:2 assembled
structures even in solution (Figures 2 and S2). By cooling 1-Zn
and CH3OH (3.4 equiv) in toluene-d8 from 20 to ¹20 °C, high-
field shifts of the signals due to CH3OH were observed, which
would be explained by coordination of CH3OH to the center
metal of 1-Zn. The signals due to the ¢-phenylene moiety did
not shift significantly at this stage. Meanwhile, one of the two ¢-
phenylene signals showed distinct high-field shifts by further
β
N
N
N
CO2Et
N
N
click reaction
81%
M
Ph
Ph
N
N
meso
1-Zn (M = Zn(II))
1 (M = H2)
demetallation
67%
Ph
Scheme 1. Synthesis of 1-Zn.
Chem. Lett. 2010, 39, 252-253
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

253
Ph
N
(a)
(b)
Ph
N
N
Ph
N
N
Zn
N
Ph
CO2Et
N
N
N
Zn
O
N
Ph
N
N
EtO2C
Ph
N
N
Zn
N
a
b
c
de
Ph
H
Ph
N
H
CH3
H3C
O
Ph
CH3OH
O
H
O
H
H
N
Ph
CH3
O
Ph
N
N
N
Zn
Zn
N
N
a: 2.736 Å
b: 2.729 Å
c: 2.698 Å
N
d: 2.408 Å
e: 2.640 Å
N
Ph
Ph
N
N
EtO2C
Zn
N
N
N
Ph
Ph
N
EtO2C
Ph
(c)
Zn–O: 2.149 Å
O–N: 2.749 Å
(O–H: 0.913 Å)
(N–H: 1.899 Å)
Scheme 2. Formation of a 2:2 assembled structure with 1-Zn and
CH3OH.
In summary, we have synthesized a zinc(II) porphyrin-
1,2,3-triazole conjugate, in which a unique 2:2 assembled
structure with CH3OH was observed in the solid state and most
likely in toluene solution.14 This work demonstrates the ap-
plicability of a 1,2,3-triazole unit formed by a click reaction to a
hydrogen-bonding acceptor and further application of the click
chemistry in supramolecular chemistry based on hydrogen
bondings will be expected.
Figure 1. X-ray structures of 1-Zn¢CH3OH: (a) 2:2 assembled
structure by the hydrogen bondings, (b) CH-³ interactions between
porphyrin plane and aryl groups, and (c) packing diagram in the solid
state. The phenyl groups at the 5, 10, and 15 positions and the
CH2CO2Et group are omitted for clarity in (b) and (c).
The present work was supported by PRESTO from Japan
Science and Technology Agency and by a Grant-in-Aid for
Scientific Research on Innovative Areas (No. 21108518, “pi-
Space”) and the Global COE Program “Science for Future
Molecular Systems” from the MEXT, Japan.
β
-phenyl
CH3OH
CH3OH
–50 °C
–40 °C
–30 °C
–20 °C
20 °C
References and Notes
1
2
3
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8
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0
ppm
5
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equiv of CH3OH in toluene-d8.
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cooling from ¹20 to ¹50 °C. Additionally, the high-field shifts
of the hydroxy proton of the methanol became smaller or even
the low-field shifts were observed. This behavior can be
explained by the formation of the 2:2 assembled structures.
The former would be rationalized by the shielding effect
imposed by the paired porphyrin macrocycle and the latter might
be due to the hydrogen bonding of the hydroxy proton with the
1,2,3-trizaole moiety. The formation of 2:2 assembled structures
was also inferred from the severe broadening of the signals at
lower temperature. In contrast, only the large high-field shifts
7
8
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1
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13 Crystal data: 1-Zn¢CH3OH, red plate, C57H43N7O3Zn, Mr = 939.35,
monoclinic, space group C2/c (No. 15), a = 42.603(5), b =
10.2888(12), c = 21.811(3) ¡, ¢ = 105.962(2)°, V = 9192(2) ¡3,
Z = 8, T = 296(2) K, R = 0.0583 (I > 2·(I)), Rw = 0.1783 (all data),
GOF = 0.895, CCDC number 743205.
measurements were achieved on 3-Zn (Figure S3). The signals
due to the ¢-phenylene moiety of 3-Zn did not shift significantly
unlike 1-Zn. The characteristic shifts like Figure 2 were not
observed in CD2Cl2, meaning the importance of less polar
solvents in the solution phase assembly. The observation of only
one set of the signals due to CH3OH even under the presence of
excessive amounts suggested the rapid equilibrium between the
monomer and the dimer in the NMR time scale (Scheme 2).
Obviously, the initial NMR spectrum was obtained again upon
heating the chilled solution to ambient temperature.
14 Experiments with ethanol and 2-propanol in place of methanol did not
give clear evidence for formation of 2:2 assembled structures so far.
Chem. Lett. 2010, 39, 252-253
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/