Formation of cyclic trimers and tetramers of zinc(II) pyrazolylporphyrins by mutual coordination

Article abstract of DOI:10.1039/b006371l

The self-assembly of zinc(II) porphyrins bearing a pyrazole substituent are reported. ¹H NMR, UV/Vis and VPO measurements indicate the formation of a cyclic trimer of zinc(II) pyrazol-4-ylporphyrin and a tetramer of zinc(II) 3,5-dimethypyrazol-4-ylporphyrin by mutual coordination.

Full text of DOI:10.1039/b006371l

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Formation of cyclic trimers and tetramers of zinc(II)
pyrazolylporphyrins by mutual coordination
Chusaku Ikeda,* Noriko Nagahara, Naoki Yoshioka and Hidenari Inoue
Department of Applied Chemistry, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Y okohama
223-8522, Japan. E-mail: c05351=educ.cc.keio.ac.jp
Received (in Montpellier, France) 2nd August 2000, Accepted 8th September 2000
First published as an Advance Article on the web 25th October 2000
The self-assembly of zinc(II) porphyrins bearing a pyrazole substituent are reported. 1H NMR, UV/Vis and VPO
measurements indicate the formation of a cyclic trimer of zinc(II) pyrazol-4-ylporphyrin and a tetramer of zinc(II)
3,5-dimethypyrazol-4-ylporphyrin by mutual coordination.
In recent years, numerous attempts have been made to design
porphyrin assemblies with a particular geometry because they
have the potential capability to act as artiÐcial photosynthetic
systems and optical molecular devices.1 Self-assembling stra-
tegies based on reversible, non-covalent interactions such as
metal coordination2 and hydrogen bonding3 have been widely
used to construct porphyrin assemblies since their sponta-
neous assembly can lead to large-scale ordered arrays, which
can hardly be achieved by covalent bonding only. In particu-
lar, the metal coordination approach to porphyrin architec-
ture in which a peripheral ligand substituent coordinates to
the porphyrin central metal ion has been extensively studied
using pyridyl, hydroxyl, imidazolyl and aminophenyl metallo-
porphyrins because the assembled structure can be easily con-
trolled by the combination of metal ion species and donor
ligands.4h7 Recently, we have reported the synthesis of novel
pyrazolylporphyrins and their self-assembly by intermolecular
hydrogen bonding between peripheral pyrazole groups.8 Here
we sought to extend our investigation and focused on the
coordination properties of pyrazolylporphyrins because pyra-
zoles are known to be excellent ligands and to form intermo-
lecular hydrogen bonds. The spectroscopic characterization of
self-assembled zinc(II) pyrazolylporphyrins has revealed the
formation of mutually coordinated cyclic trimers and tetra-
mers in solution, dependent on the substituent on the pyra-
zolyl ring.
showed good solubility in most non-polar solvents such as
chloroform and toluene. The ability of Zn1 and Zn2 to associ-
ate in solution was examined by UV/Vis absorption, 1H
NMR spectroscopy and vapor pressure osmometry (VPO).
Both zinc(II) porphyrins exhibited concentration-dependent
1H NMR and UV/Vis spectra in chloroform and toluene but
not in solvents with coordinating properties such as pyridine,
suggesting the formation of coordination assemblies. Hydro-
gen bonding between pyrazole units was negligible in Zn1 and
Zn2 because the stability constant for axial coordination of
pyrazole and 3,5-dimethylpyrazole is much larger than that
for hydrogen bonding between pyrazoles.9
Self-assembly of Zn1
Fig. 1 shows a typical 1H NMR spectrum of Zn1 in
CD Cl ,10 together with that of the free-base porphyrin H 1.
2
2
2
Although the spectral pattern of H 1 is characteristic of
2
monomeric porphyrin, the spectrum of Zn1 is completely dif-
ferent from that of H 1. The spectral assignment based on
2
variable temperature measurements and COSY experiments
revealed that the pyrazole ring protons and pyrrole-b
protons experience large upÐeld shifts. The degree of these
1
upÐeld shifts, *d,11 for pyrazole-H
and pyrrole-b is 3.7
3, 5
1
and 1.7 ppm, respectively, in 10 mM CD Cl at 295 K; these
2
2
values became smaller upon dilution or increasing tem-
perature. The upÐeld shifts observed for the pyrazole ring
protons indicate that the pyrazole nitrogens coordinate to the
central zinc(II) ions and come into the shielding region of the
coordinated porphyrin ring to a†ord the self-assembly of Zn1
by coordination. The proton signals assignable to the oxy-
phenyl ring were split into two peaks with an integrated ratio
2 : 1, showing that the aryl protons of the 10 and 20 positions
are not equivalent to those of the 15 position in the assembly.
At ambient temperature, the signals of the pyrazole ring
protons were nearly coalesced around 4.5 ppm but no
coalescence of pyrazole protons was observed in the mixture
of pyrazole and zinc(II) tetraphenylporphyirn (ZnTPP) or in
that of H 1 and ZnTPP. This observation indicates that the
2
exchange lifetime for self-assembly of Zn1 becomes longer
than that for simple axial coordination, probably due to coo-
perative formation of the assembly (vide infra).
The UV/Vis spectra of Zn1 recorded in CDCl 12 also
exhibited a concentration dependence. In dilute solution
below 0.01 mM, Zn1 showed an intense B band and two weak
Q bands, which are typical of the zinc(II) porphyrin13 and
whose absorbance obeyed the BeerÈLambert law, indicating
that Zn1 exists as a monomer in these dilute conditions.
Results and discussion
3
The monopyrazolylporphyrins H 1 and H 2 were synthesized
2
2
via precursor porphyrins having a p-methoxybenzyl substit-
uent as a protecting group on the pyrazole NH group. The
desired zinc(II) complexes Zn1 and Zn2 were obtained by the
conventional insertion of zinc(II) ions. Both Zn1 and Zn2
DOI: 10.1039/b006371l
New J. Chem., 2000, 24, 897È902
897
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2000

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Fig. 1 Typical 1H NMR spectra of H 1 (6 mM, top) and Zn1 (10 mM, bottom) in CD Cl at 295 K. Pyrrole-b protons of Zn1 are overlapped
2
2
2
1
with aryl protons. See insert for the labeling of pyrrole-b protons. Asterisks denote the residual solvent peaks.
Fig. 2 Absorption spectra of Zn1 recorded at 0.004 mM in CDCl at 295 K (È È È), 2 mM at 295 K (É É É) and 2 mM at 273 K (ÈÈ). Quartz cells
3
of 10 and 0.025 mm path length were used for the measurements.
However, the B band became broader upon increasing con-
centration to 2 mM and was split into two bands at 422 and
430 nm at 273 K (Fig. 2).14 Compared with the NMR spectral
data, these spectral changes may be ascribed to the formation
of a self-assembly by coordination. It is well known that the B
band of zinc(II) porphyrins is red-shifted upon coordination of
an axial ligand such as pyridine or pyrazole13b but it does not
split in that process. Therefore, the spectral change observed
in the B band of Zn1 may be attributed to the excitonic
interaction15 between the large transition dipoles of the con-
stituent porphyrin chromophores in the assembly. The Q(1,0)
band was also red-shifted from 552 to 564 nm upon increasing
the concentration but did not split, which is very similar to
the spectral change caused by coordination of axial ligands to
zinc(II) porphyrins.
In order to conÐrm the nature of the self-assembly struc-
ture, a curve-Ðtting analysis was carried out for the change of
observed molar absorption coefficient at 552 nm where mono-
Fig. 3 Curve Ðtting analysis of the concentration dependence of the
meric Zn1 shows its Q(1,0) band. The dilution curves observed
molar absorption coefficient of Zn1 at 552 nm recorded in 0.1 and 1.0
at 295 K in CDCl were Ðtted to the model for a cooperative
mm path length quartz cells in CDCl : (L) experimental data, (È É È)
3
3
n-merization process (association number n \ 2, 3, 4) by
dimer model, (ÈÈ) trimer model, (É É É) tetramer model.
898
New J. Chem., 2000, 24, 897È902

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SaundersÈHyne analysis (Fig. 3).16 The dilution curve Ðts well
with the optimized trimer model throughout the concentra-
tion range, but the best-Ðt dimer or tetramer models deviate
substantially from the experimental data. This analysis indi-
cates that Zn1 forms a trimer and the equilibrium in the self-
assembly process is nearly complete above ca. 10 mM.
In this context, a series of VPO measurements were carried
out between 5 and 20 mM using the same solvent as that for
the UV/Vis spectra (CDCl , 25 ¡C, benzil and TPP as
3
standards). The average molecular weight obtained was
3260 ^ 110, corresponding to a trimer of Zn1 (FW \ 3161 g
mol~1). The combined VPO, UV/Vis and 1H NMR spectral
results demonstrate that Zn1 forms a self-assembled trimer by
coordination of the pyrazole nitrogen to the central zinc(II)
Fig. 5 1H NMR spectra of Zn2 (10 mM) in CDCl at 295 K. A
3
broad peak around [2 ppm was assigned to pyrazole-Me .
3
ca. 10 mM. The splitting of the B band of Zn1 appeared only
below [7 ¡C at 2 mM, but for Zn2 it occurred even at
ambient temperature at the same concentration. These di†er-
ences between Zn1 and Zn2 are most likely caused by the
stronger basicity of 3,5-dimethylpyrazole compared to pyra-
zole. Furthermore, an interesting di†erence between these
porphyrins was observed in the SaundersÈHyne analysis of
the concentration dependence of the observed molar coeffi-
ion. The binding constant for the trimer, K , was estimated to
3
be 3.5 ^ 0.5 ] 107 M~2 from the Ðtting. If we assume a linear
trimer (oligomer), the binding constant for chain extension
(K \ K \ K
) can be estimated to be 5.5 ] 103 M~1
1,2
2,3
n,n`1
from K . However, this value is apparently larger than the
3
binding constant of 630 M~1 for the axial coordination of
pyrazole to zinc(II) tetratolylporphyrin,17 suggesting the co-
operative formation of a mutually coordinated cyclic trimer.
Moreover, the possibility of the open polymeric structure was
ruled out because the dilution curve did not Ðt the non-
cooperative linear polymerization isotherm.7a These results
indicate that Zn1 self-assembles into a mutually coordinated
cyclic trimer structure at higher concentrations owing to three
coordination bonds between central zinc(II) ions and pyrazolyl
nitrogen atoms (Fig. 4).
cient of the Q band (e at 552 nm, in CDCl ). The analysis for
3
Zn2 gave a best Ðt to the tetramer model, and not to a dimer,
trimer or non-cooperative polymer model (Fig. 6), in contrast
to the trimer model found for Zn1. These results for Zn2 also
agree well with those from the VPO measurements recorded
under the same conditions as for Zn1. The average molecular
weight of 4570 ^ 340 obtained corresponds to the molecular
weight of the tetramer of Zn2 (FW \ 4324 g mol~1). The
curve Ðtting to the tetramer model gave a binding constant
K \ 3.0 ^ 1.0 ] 1013 M~3 (295 K, in CDCl ) which is too
Self-assembly of Zn2
4
3
The 1H NMR and UV/Vis spectra of Zn2, possessing a 3,5-
dimethylpyrazole ring, also showed concentration and/or tem-
perature dependencies, but the spectral behavior was quite dif-
ferent from that of Zn1. The most striking feature was
observed in 1H NMR spectra of Zn2. Though the upÐeld
shifts of proton signals due to the pyrrole-b and pyrazole ring
were observed as in Zn1, all the signals of Zn2 were broad
above ca. 1 mM in CDCl , CD Cl , or toluene-d (Fig. 5)
large to correspond to the formation of a liner tetramer.19 As
a result, the cooperative formation of a cyclic tetramer struc-
ture of Zn2 by mutual coordination (Fig. 7) is strongly sug-
gested by the 1H NMR, UV/Vis spectra and VPO
measurements.
Consideration of assembled structures
3
2
2
5
The fact that the association number of Zn1 (n \ 3) is di†erent
from that of Zn2 (n \ 4) was somewhat unexpected because
both Zn1 and Zn2 have a similar pyrazol-4-yl group while the
two methyl groups on the pyrazole ring seemed to a†ect only
and no sharp spectra could be obtained, even upon lowering
the temperature to 220 K. These spectral features suggest the
formation of coordination polymers or oligomers that have a
Ñexible conformation,18 although a chemical exchange process
may be an alternative explanation.
the pK value, which inÑuences the binding affinities only.
a
To discuss why di†erent assemblies are formed for Zn1 and
At higher concentrations, Zn2 showed a split B band and a
Zn2 and why Zn1 forms a trimer and Zn2 forms a tetramer
rather than other oligomers, we calculated the geometry of an
axially coordinated pyrazole ligand to zinc(II) tetra-
red shift of the Q band, similar to Zn1. The j
of the split B
max
band and the red-shifted Q band of Zn2 were quite similar to
those of Zn1, though these spectral changes occurred at lower
concentrations. The red shift of the Q band of Zn2 was com-
plete above ca. 1 mM, while that of Zn1 was complete above
Fig. 6 Curve Ðtting analysis of the concentration dependence of the
molar absorption coefficient of Zn2 at 552 nm in CDCl : (…) experi-
3
Fig. 4 Proposed structure of the self-assembled cyclic trimer of Zn1.
The meso p-octyloxyphenyl group is omitted for clarity.
mental data, (È È È) dimer model, (É É É) trimer model, (ÈÈ) tetramer
model.
New J. Chem., 2000, 24, 897È902
899

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tration dependence of their UV/Vis spectra demonstrated the
formation of the cyclic trimer and tetramer by mutual coordi-
nation. These results reveal that pyrazoles can be utilized as
versatile building blocks in coordination porphyrin architec-
ture and the structure can be easily controlled by the substit-
uent at the 3,5-positions of the pyrazole ring. We are currently
studying the porphyrin architecture using both metal coordi-
nation and hydrogen bonding in a cooperative fashion. The
results will be published elsewhere.
Experimental
Instrumental
1H NMR spectra were recorded on a Jeol 300 MHz LABDA-
300 spectrometer, and chemical shifts are reported relative to
internal TMS at 0.0 ppm. VPO measurements were carried
out with a Knauer K-7000 vapor pressure osmometer. UV/Vis
spectra were obtained with a Jasco V-570 spectrometer.
Fig. 7 Proposed structure of the self-assembled cyclic tetramer of
Zn2. The meso p-octyloxyphenyl groups are omitted for clarity.
CDCl stabilized with silver foil was used to prevent demetal-
3
lation of zinc(II) porphyrins.
phenylporphryin as a model system by geometry optimization
using the MOPAC program.20 The calculated results of the
model system are that the coordination angle a is 59¡ for an
unsubstituted pyrazole ligand and 65¡ for 3,5-dimethyl-
pyrazole, where a is deÐned as the angle between the porphy-
rin mean plane and the CH bond at position 3 in an axially
coordinated pyrazole ring (Fig. 8). These results clearly
showed that a is di†erent for these two ligands, probably due
to the steric repulsion of the methyl groups. Furthermore, if
the angle a is assumed to be the angle between porphyrin
mean planes in the self-assembled pyrazolylporphyrins, a
trimer structure seems reasonable for self-assembled Zn1 since
the internal angle in a triangle is 60¡. The formation of a tetra-
mer of Zn2 rather than a trimer might also be explained by
the value of the angle a, 65¡, which is not the preferred one for
the trimer. Of course, some extent of deviation in the coordi-
nation angle may occur to form a thermodynamically stable
structure. However, space Ðlling modeling indicated that the
core of the cyclic trimer structure of Zn1 is very small and
severe steric repulsion in the Zn2 assembly could occur
between methyl groups belonging to neighboring porphyrins
in the trimer. From this consideration, a cyclic tetramer struc-
ture can be regarded as the most preferable structure for Zn2
as the smallest cyclic structure which is stable in solution from
the thermodynamic point of view.21 Considering the coordi-
nation angle a of the 3,5-dimetylpyrazole ring, the cyclic tetra-
mer structure of Zn2 must not be square and it seems to form
a twisted parallelogram in a ““ÑexibleÏÏ geometry, which agrees
well with the NMR spectral results.
Synthesis
The two pyrazolecarboxaldehydes used in this study were syn-
thesized using reported procedures.22
5-(1-p-Methoxybenzylpyrazol-4-yl)-10,15,20-tris(p-octyl-
oxyphenyl)porphyrin
(pMB-H 1).
1-p-Methoxybenzyl-
2
pyrazole-4-carboxaldehyde (2.6 g, 12.3 mmol) and p-
octyloxybenzaldehyde (8 ml, 33 mmol) were added to 310 mL
reÑuxing propionic acid. Freshly distilled pyrrole (3.1 mL, 45
mmol) was then added and the mixture was reÑuxed for 10 h.
The propionic acid was removed by distillation under reduced
pressure to give a dark residue. This was taken up into the
minimum amount of chloroform and transferred to a 500 mL
round bottomed Ñask, ca. 100 g of alumina (grade I, Merck)
was added, and the solvent was removed via rotary evapo-
ration. The resulting dry powder was placed on the top of a
dry silica column and the porphyrins were eluted with toluene
and then with tolueneÈethyl acetate (10 : 1). The second band
was collected and crystallized from dichloromethaneÈ
methanol to give the title compound (0.7 g). Yield: 8%. 1H
NMR (CDCl ): d [2.72 (s, 2H, inner NH), 0.93 (t, 9H, CH
,
3
3h
J \ 6.6), 1.30È1.51 (m, 24H, CH
), 1.64 (quint, 6H, J \ 6.8,
2dhg
CH ), 1.99 (quint, 6H, J \ 6.8, CH ), 3.88 (s, 3H, OMe), 4.26
2c 2b
(t, 6H, J \ 6.6, CH ), 5.56 (s, 2H, NCH ), 7.02 (d, 2H, J \ 8.5
2a
2
ArH, p-methoxyphenyl), 7.28 (d, 6H, J \ 7.3 ArH , p-
m
octyloxyphenyl), 7.37 (d, 2H, J \ 8.5 ArH, p-methoxyphenyl),
8.10 (d, 6H, J \ 7.3 ArH, p-octyloxyphenyl), 8.14, 8.35 (each s,
pyrazole-H ), 8.85È8.88 (m, 6H, pyrrole-H ), 9.04 (d, 2H,
3,5
b
J \ 4.6 Hz pyrrole-H ). Anal. calc. for C
C 78.39, H 7.66, N 7.51. Found: C 78.39, H 7.61, N 7.40%.
Mp \ 154È155 ¡C.
H
N O É 0.5H O:
b
73 84
6
4
2
Conclusion
Novel zinc(II) pyrazolylporphyrins were synthesized and their
self-assembled structures were studied. 1H NMR spectra,
VPO measurements and curve-Ðtting analyses for the concen-
5-(1-p-Methoxybenzyl- 3,5-dimethylpyrazol- 4-yl)- 10,15,20-
tris(p-octyloxyphenyl)porphyrin (pMB-H 2). This compound
2
was synthesized in the same manner as pMB-H 1using 1-p-
2
methoxybenzyl-3,5-dimethylpyrazole-4-carboxaldehyde,
p-
octyloxybenzaldehyde and pyrrole. Yield: 5%. 1H NMR
(CDCl ): d [2.74 (s, 2H, inner NH), 0.94 (t, 9H, J \ 6.6
3
CH ), 1.25È1.50 (m, 24H, CH
), 1.64 (quint, 6H, J \ 6.8,
3h
2dhg
CH ), 1.99 (quint, 6H, J \ 6.8, CH ), 2.09, 2.22 (each s, 6H,
2c
2b
pyrazole-Me ), 3.87 (s, 3H, OMe), 4.26 (t, 6H, J \ 6.6, CH ),
3,5
2a
5.56 (s, 2H, NCH ), 7.02 (d, 2H, J \ 8.5 ArH, p-
2
methoxyphenyl), 7.27 (d, 6H, J \ 7.3 ArH , p-octyloxyphenyl),
m
7.36 (d, 2H, J\8.5 ArH, p-methoxyphenyl), 8.10 (d, 6H,
J \ 7.3 ArH , p-octyloxyphenyl), 8.97 (s, 4H, pyrrole-H ), 8.98
o
b
(d, 2H, J \ 4.6 pyrrole-H ), 9.00 (d, 2H, J \ 4.6 pyrrole-H ).
b
b
Anal. calc. for C
H
N O É 3CH OH: C 75.94, H 8.17, N
Fig. 8 Schematic view of the axially coordinated pyrazole ligand.
The porphyrin plane is shown as a thick line.
75 88
6
4
3
6.81. Found: C 75.78, H 7.56, N 6.65%. Mp \ 100È103 ¡C.
900
New J. Chem., 2000, 24, 897È902

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2
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764.
5-(Pyrazol-4-yl)- 10,15,20-tris(p- octyloxyphenyl)porphyrin
(H 1). In 60 mL of triÑuoroacetic acid, 1.1 g of pMB-H 1 (0.9
2
2
mmol) was dissolved and the mixture was reÑuxed for 3 days.
The mixture was neutralized with a saturated solution of
NaHCO . The porphyrin was extracted with chloroform and
3
washed with water. The organic layer was dried with anhy-
drous Na SO and the solvent was removed via rotary evapo-
2
4
ration. The crude porphyrin was loaded onto a SiO column
2
and the desired porphyrin was eluted with tolueneÈethyl
3
acetate (3 : 1). Crystallization from dichloromethaneÈmethanol
gave the title compound (530 mg). Yield: 56%. 1H NMR
(CDCl ): d [2.74 (s, 2H, inner-NH), 0.94 (t, 9H, J \ 6.8
3
CH ), 1.30È1.50 (m, 24H, CH
), 1.63 (quint, 8H, J \ 6.8
3h
2dhg
CH ), 1.98 (quint, 8H, J \ 6.8 CH ), 4.23 (t, 6H, J \ 6.3
2c
2b
CH ), 7.26 (d, 6H, J \ 8.6 ArH ), 8.09 (d, 6H, J \ 8.6 ArH ),
2a
8.37 (s, 2H, pyrazole-H ), 8.86 (s, 4H, pyrrole-H ), 8.89 (d,
3,5
m
o
b
2H, J \ 4.9 pyrrole-H ), 9.03 (d, 2H, J \ 4.9 Hz pyrrole-H ).
b
b
Anal. calc. for C
H
N O : C 78.91, H 7.74, N 8.46. Found:
65 76
6 3
4
C 78.87, H 7.53, N 8.39%. Mp [ 300 ¡C.
5-(3,5-Dimethylpyrazol-4-yl)-10,15,20-tris(p-octyloxyphenyl)-
porphyrin (H 2). This compound was synthesized in the same
2
manner as H 1 using pMB-H 2 as the starting material.
2
2
Yield: 65%. 1H NMR (CDCl ): d [2.74 (s, 2H, inner NH),
3
0.94 (t, 9H, J \ 6.6, CH ), 1.30È1.55 (m, 24H, CH
), 1.63
3h
2c
2dhf
(quint, 6H, J \ 6.8, CH ), 1.98 (quint, 6H, J \ 6.8, CH ),
2b
2.19 (s, 6H, pyrazole-Me ), 4.24 (t, 6H, J \ 6.5, CH ), 7.27
3,5 2a
(d, 6H, J \ 8.6, ArH ), 8.10 (d, 6H, J \ 8.6, ArH ), 8.84 (d, 2H,
m
o
J \ 4.6, pyrrole-H ), 8.86 (s, 4H, pyrrole-H ), 8.89 (d, 2H,
b
b
J \ 4.6, Hz pyrrole-H ). Anal. calc. for C
H N O É H O: C
b
67 80
6
3
2
77.72, H 7.98, N 8.11. Found: C 77.47, H 7.85, N 7.73%.
Mp \ 140È142 ¡C.
[5-(Pyrazol-4-yl)-10,15,20-tris(p-octyloxyphenyl)porphy-
rinato] zinc(II) (Zn1). To a stirred solution of 0.19 g (0.19
mmol) of H 1 in 100 mL of CH Cl was added 0.85 g (3.9
5
(a) J. Wojacznski and L. L. Grazynski, Inorg. Chem., 1995, 34,
1044; (b) J. Wojacznski and L. L. Grazynski, Inorg. Chem., 1995,
34, 1054; (c) J. Wojacznski and L. L. Grazynski, Inorg. Chem.,
1996, 35, 4812; (d) J. Wojacznski, L. L. Grazynski, M. M. Olm-
stead and A. L. Balch, Inorg. Chem., 1997, 36, 4548.
(a) Y. Kobuke and H. Miyaji, J. Am. Chem. Soc., 1994, 116, 4111;
(b) Y. Kobuke and H. Miyaji, Bull. Chem. Soc. Jpn., 1996, 69,
3563.
2
2 2
mmol) of Zn(OAc) in 50 ml MeOH. The mixture was stirred
2
and reÑuxed for 10 h. The solvent was diluted with CH Cl
and washed with water. The organic phase was separated,
2
2
6
7
then dried over Na SO . The solvent was removed in vacuo
2
4
and recrystallization from CH Cl ÈMeOH gave the title com-
(a) M. Gardner, A. J. Guerin, C. A. Hunter, U. Michelsen and C.
Rotger, New J. Chem., 1999, 23, 309; (b) H. J. Kim, N. Bampos
and J. K. M. Sanders, J. Am. Chem. Soc., 1999, 121, 8120; (c) S. L.
Darling, E. Stulz, N. Feeder, N. Bampos and J. K. M. Sanders,
New J. Chem., 2000, 24, 261; (d) L. Giribabu, T. A. Rao and B. G.
Maiya, Inorg. Chem., 1999, 38, 4971.
2
2
pound (0.19 g). Yield: 95%. 1H NMR (CD Cl , 10 mM, 295
2
2
K): d 0.89 (t, 3H, J \ 6.8, CH ), 0.97 (t, 6H, J \ 6.8, CH ),
3h
3h
1.26È1.54 (m, 24H, CH
), 1.70 (m, 6H, CH ), 1.91 (quint,
2dhf
2c
2H, J \ 6.8, CH ), 2.06 (quint, 4H, J \ 6.8, CH ), 4.14 (t,
2b
2b
2H, J \ 6.4, CH ), 4.31 (t, 4H, J \ 6.4, CH ), 4.52 (br, 2H,
2a 2a
pyrazole-H ), 7.18 (d, 4H, overlapping of pyrrole-H with
3, 5
ArH ), 7.30 (d, 4H, J \ 8.3, ArH ), 8.06 (d, 2H, J \ 8.3,
8
9
C. Ikeda, N. Nagahara, E. Motegi, N. Yoshioka and H. Inoue,
Chem. Commun., 1999, 1759.
b
The binding constants for the axial coordination of pyrazole and
3,5-dimethylpyrazole were determined to be 6.3 ] 102 and
3.9 ] 103 M~1, respectively, from UV/Vis titration in toluene at
295 K. These values are much larger than that for hydrogen
bonding between pyrazoles (20È40 M~1). For the determination
of binding constants for hydrogen bonding, see ref. 8.
m
o
m
ArH ), 8.14 (d, 4H, J \ 8.3, ArH ), 8.64 (d, 2H, J \ 4.7,
o
pyrrole-H ), 8.91, 8.94 (each d, 2H ] 2, J \ 4.7 Hz, pyrrole-
b
H ). Anal. calc. for C
H
N O Zn: C 74.09, H 7.17, N 7.98.
b
65 75
6
3
Found: C 73.80, H 7.07, N 7.85% Mp \ 145È150 ¡C.
10 Almost the same spectra were obtained in CDCl or toluene-d .
3 5
We used CD Cl here to avoid the overlapping of the residual
[5-(3,5-Dimethylpyrazol-4-yl)-10,15,20-tris(p-octyloxy-
2
2
solvent peak with aryl proton peaks around 7.2 ppm.
11 *d is deÐned as *d \ d(metal free porphyrin) [ d(zinc(II)
complex).
phenyl)porphyrinato] zinc(II) (Zn2). This compound was syn-
thesized as for Zn1 using H 2. Yield: 98%. 1H NMR
2
(pyridine-d ): d 0.88 (t, 9H, J \ 6.6, CH ), 1.20È1.40 (m, 24H,
12 For UV/Vis measurements, CDCl stabilized with silver foil was
5
3h
3
CH ), 1.57 (br, 6H, CH ), 1.93 (quint, 6H, J \ 6.8, CH ),
2dhf
2.51 (s, 6H, pyrazole-Me ), 4.22 (t, 6H, J \ 6.5, CH ), 7.46
3,5 2a
(m, 6H, ArH ), 8.36 (m, 6H, ArH ), 9.30 (m, 6H, pyrrole-H ),
used as solvent to prevent Zn1 from demetallating.
2c
2b
13 (a) D. Dolphin, T he Porphyrins, Academic Press, New York,
1979, vol. 3, ch. 1; (b) C. H. Kirksey, P. Hambright and C. B.
Storm, Inorg. Chem., 1969, 8, 2141.
14 Complete formation of the assembly at room temperature
requires a higher concentration. The highest concentration for
observation of the B band was 2 mM in a 0.025 mm path length
cell.
m
o
b
9.41 (d, 2H, J \ 4.6 Hz, pyrrole-H ). Anal. calc. for
b
C
H
N O Zn É MeOH: C 73.39, H 7.43, N 7.55. Found: C
67 78
6 3
73.72, H 7.09, N 7.39%. Mp [ 300 ¡C.
15 M. Kasha, H. R. Rawls and M. A. El-bayoumi, Pure. Appl.
References and notes
Chem., 1965, 11, 371.
16 When considering the equilibrium between monomer and n-mer,
1
(a) M. R. Wasielewski, Chem. Rev., 1992, 92, 435; (b) T. Hayashi
and H. Ogoshi, Chem. Soc. Rev., 1997, 26, 355; (c) M. D. Ward,
Chem. Soc. Rev., 1997, 26, 365; (d) A. Harriman and J. P.
Sauvage, Chem. Soc. Rev., 1996, 25, 41.
the observed molar absorption coefficient (e ) can be expressed
obs
as e \ MC be ] nKCnbe N/C
where b is the path length of
obs
1
0
1
n
total
the cell, K is the equilibrium constant between monomer and
n-mer, C is the concentration of monomer and C
is the total
1
total
New J. Chem., 2000, 24, 897È902
901

View Article Online
concentration, e and e are the molar absorption coefficient for
19 If we assume the formation of a linear tetramer, the binding con-
0
n
monomer and n-mer, respectively. The unknown parameters e ,
stant for chain extension (K \ K \ K ) is estimated to be
0
1,2
2,3
3,4
e , and K for each n value were evaluated from arbitrarily chosen
3.1 ] 104 M~1, which is much larger than the binding constant
of 4.4 ] 103 M~1 for the axial coordination of 3,5-dimethyl-
pyrazole to zinc(II) tetratolylporphyirn in toluene at 295 K.
20 J. J. P. Stewart, MOPAC97, Fujitsu Ltd, Tokyo, Japan, 1998.
21 S. C. Zimmerman and B. F. Duerr, J. Org. Chem., 1992, 57, 2215.
22 A. Werner, A. SanchezÈMigallon, A. Fruchier, J. Elguero, C.
FernandezÈCastano and C. FocesÈFoces, T etrahedron, 1995, 51,
4779.
n
values of C in a trial and error manner. For the detailed pro-
1
cedure, see: (a) M. Saunders and J. B. Hyne, J. Chem. Phys., 1958,
29, 1319; (b) K. A. Conner, Binding Constants, Wiley, New York,
1987, ch. 5.
17 This value was obtained in toluene at 295 K by the UV/Vis titra-
tion method.
18 For the porphyrin oligomer that showed broad NMR signals, see
ref. 2a and 7b.
902
New J. Chem., 2000, 24, 897È902