Dynamic equilibrium of self-assembled cyclic trimer and tetramer of 1-methyl-5-imidazolylcobalt(III)porhyrin

Article abstract of DOI:10.1560/C64H-MNQ6-58MA-FFQ2

Dynamic equilibrium of self-assembled multi-porphyrin systems is of interest in obtaining switchable photoresponsive material, but rarely reported. 1-methyl-5-imidazolylcobalt(III)porphyrin (1Co) synthesized here assembled automatically into cyclic trimer and tetramer by intermolecular imidazolyl-cobalt(III) coordination. The trimer-tetramer equilibrium was dependent on concentration and solvent, as examined by NMR spectrometry. In CDCl₃, the tetramer formation was favored at high concentrations, as the ratio of the trimer to the tetramer was 1:2 at 14.8 mM 1Co, and shifted to 1:8 at 74 mM. Further, when the sample was concentrated from a CHCl₃ solution to dryness, the ratio increased to 1:24 on dissolution. In CD ₃OD, on the other hand, only the trimer was observed in the wide concentration range. Accordingly, both the trimeric and the tetrameric structures could be prepared selectively by the choice of concentration and solvent.

Full text of DOI:10.1560/C64H-MNQ6-58MA-FFQ2

Dynamic Equilibrium of Self-Assembled Cyclic Trimer and Tetramer of
1-Methyl-5-imidazolylcobalt(III)porphyrin
AKIRA TANAKA, AYA RYUNO, SAORI OKADA, AKIHARU SATAKE, AND YOSHIAKI KOBUKE*
Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma,
Nara 630-0192, Japan
(Received 1 February 2005)
Abstract. Dynamic equilibrium of self-assembled multi-porphyrin systems is of
interest in obtaining switchable photoresponsive material, but rarely reported.
1-methyl-5-imidazolylcobalt(III)porphyrin (1Co) synthesized here assembled auto-
matically into cyclic trimer and tetramer by intermolecular imidazolyl-cobalt(III)
coordination. The trimer–tetramer equilibrium was dependent on concentration and
solvent, as examined by NMR spectrometry. In CDCl3, the tetramer formation was
favored at high concentrations, as the ratio of the trimer to the tetramer was 1:2 at
14.8 mM 1Co, and shifted to 1:8 at 74 mM. Further, when the sample was concen-
trated from a CHCl3 solution to dryness, the ratio increased to 1:24 on dissolution. In
CD3OD, on the other hand, only the trimer was observed in the wide concentration
range. Accordingly, both the trimeric and the tetrameric structures could be prepared
selectively by the choice of concentration and solvent.
INTRODUCTION
trolled manner. We have recently reported dynamic for-
Supramolecular methodology by use of coordination mation of large cyclic multiporphyrins.9 Here, the meth-
bonds becomes more and more important for construc- odology is based on the large self-association constant
tion of large molecular systems.1 Because of their large (K = 1011 M–1 in CHCl3) for the complementary dimer
binding constants and specific coordination geometries, formation of meso-2-(1-methylimidazolyl)porphyrina-
various stable supramolecular structures can be con- tozinc.7 When two imidazolylzincporphyrins are linked
structed by appropriate choice of metal and ligand spe- through a spacer with an adjustable angle, various sizes
cies. In addition, the exchangeable nature of coordina- of macrorings were prepared and reorganized to rings of
tion bonds can be used to produce dynamic supramo- other sizes.10 In our systems, kinetically controlled prod-
lecular systems that could be applied to a dynamic com- ucts can also be isolated by the large binding constant at
binatorial library,2 molecular switch,3 or machine.4
each coordination site. The ring size preference was
Self-assembled supramolecular multiporphyrin sys- controlled by reorganization conditions, such as coordi-
tems driven by metal–ligand coordination have been nating solvents for dissociating the initial aggregates,
studied extensively5 for the last decade as mimics of contents of solvent for reorganization, rate of concentra-
light-harvesting antenna and multi-redox systems. tion change, and others.
When coordinating heteroaromatics, such as pyridine,6
If one can control the dynamic process by changing
imidazole,7 and pyrazole,8 were attached to a porphyrin, outer conditions, it becomes possible to choose or
cyclic dimer,6b,d,h,7 trimer,6c,8 tetramer,6a,c,e,g,i, 8a) and larger switch the supramolecular architectures on demand.
macrocycles6f were obtained by intermolecular coordi- Sanders and coworkers11 reported template-induced
nation. In these cases, one motif generally results in one
multiporphyrin structure in a thermodynamically con- kobuke@ms.naist.jp
*Authortowhomcorrespondenceshouldbeaddressed. E-mail:
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282
Formyl-1-methylimidazole was prepared by a modified proce-
dure of the literature.14
structural change of cyclic multiporphyrins linked by
disulfide bonds. In non-porphyrin supramolecular sys-
tems, temperature- , concentration- , and medium-de-
pendent equilibria of pyridine/palladium or platinum-
based supramolecules were reported by Fujita et al.1d,12
and Stang and coworkers.13 Here, we report solvent-
and concentration-dependent dynamic equilibrium be-
tween trimer and tetramer of 1-methyl-5-imidazolyl-
cobalt(III)porphyrins, as illustrated in Scheme 1.
2-(t-Butyldimethylsilyl)-1-methylimidazole (2)
n-BuLi (1.5 M hexane solution, 32 mL, 52 mmol) was
added dropwise to a solution of 1-methylimidazole (3.8 mL,
48 mmol) in dry THF (50 mL) at –78 ºC under nitrogen
atmosphere. The mixture was raised to –40 ºC and then cooled
again to –78 ºC. A solution of t-butyldimethylsilylchloride
(TBDMSCl, 8 g, 55 mmol) in dry THF (10 mL) was added to
the mixture at –78 ºC, allowed to raise the temperature to rt,
and stirred until the disappearance of 1-methylimidazole. Wa-
ter (100 mL) was added to the mixture, and the organic layer
was extracted with ethyl acetate (50 mL × 3). The organic
layer was dried over anhydrous Na2SO4 and evaporated under
reduced pressure. Residual TBDMSCl and 1-methylimidazole
were removed by distillation (0.1 KPa, 40 ºC) to give 2 as a
yellow solid (6.2 g, 67%). The crude material was used for the
EXPERIMENTAL
General Procedures
1H NMR spectra were recorded on a JEOL ECP-600
(600 MHz NMR) spectrometer. MALDI-TOF mass spectra
were obtained with Perseptive Biosystems Voyager DE-STR
with dithranol (Aldrich) as a matrix. ESI-mass spectra were
obtained with JEOL JMS-7000 MStation. Column chromatog-
raphy was performed with silica gel (63–210 µm, KANTO
Chemical Co.). Analytical gel permeation chromatography
1
next reaction. H NMR (270 MHz, CDCl3) δ 7.19 (1H, s,
imidazole), 6.96 (1H, s, imidazole), 3.75 (3H, s, NCH3), 0.95
(9H, s, –C(CH3)3), 0.39 (6H, s, –Si(CH3)2).
(GPC) was undertaken on a Shimadzu LC-M10 equipped with 5-Formyl-2-(t-butyldimethylsilyl)-1-methylimidazole (3)
SPD-10 AVP photodiode array detector using a TSK n-BuLi (1.5 M hexane solution, 30 mL, 48 mmol) was
G2500HHR column (TOSOH Co. polystyrene gel, 7.8 mm × added to a solution of 2 (6.2 g, 32 mmol) in dry THF (50 mL)
30 cm, exclusion limit: 2 × 104 Da) and CHCl3/MeOH (98/2) at –78 ºC under nitrogen atmosphere. The mixture was stirred
with LiBr (30 mM in MeOH) as an eluent (0.8 mL/min). 5- at rt for 3 h, and then cooled to –78 ºC. A mixture of DMF
Scheme 1. Self-assembly of 1Co and 1Zn and dynamic equilibrium between (1Co)3 and (1Co)4.
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(4 mL, 48 mmol) and dry THF (10 mL) was added to the MHz, CDCl3, 58.6 mM) δ 8.89 (2H, br, pyrrole β-H), 8.87
mixture at –78 ºC, allowed to raise the temperature to rt, and (2H, br, pyrrole β-H), 8.54 (2H, br, pyrrole β-H), 8.41–8.06
stirred until disappearance of 2. Saturated aqueous NH4Cl (6H, m, phenyl-H), 7.83–7.57 (9H, m, phenyl-H), 6.95 (2H,
solution was added to the mixture, and the organic layer was br, pyrrole β-H), 2.70 (1H, br, probably imidazole), 2.00 (3H,
washed with brine (100 mL × 2) and distilled water (100 mL × br, NCH3), 1.82 (1H, br, probably Iδ). 13C NMR assigned by
2). The organic layer was dried over anhydrous Na2SO4, and HMQC (correlating 1H NMR signal) δ 135.4 (8.36, phenyl-H),
evaporated under reduced pressure to give 3 as a yellow oil 135.2 (8.14, phenyl-H), 133.0 (8.87, pyrrole β-H ), 132.2
(6.0 g, 83%). The crude material was used for the next reac- (8.89, pyrrole b-H ), 129.5 (6.95, pyrrole β-H ), 126.7 (7.83–
1
tion. H NMR (270 MHz, CDCl3) δ 9.74 (1H, s, CHO), 7.86 7.57, phenyl-H), 31.9 (2.00, NCH3), MALDI-TOF mass
(1H, s, imidazole), 3.99 (3H, s, NCH3), 0.95 (9H, s, –C(CH3)3), (dithranol) m/z 680.70 (M+H)+. Calcd for C42H28N6Zn 680.17.
0.41 (6H, s, –Si(CH3)2).
UV-vis (λmax/nm (Abs), CHCl3, 0.15 mmol, monomer) 422.0
(0.192), 551.0 (0.008), 587.5 (0.003). Fluorescence (λEX
420 nm, λEM/nm (Intensity), CHCl3) 604.8 (61), 654.2 (64).
5-Formyl-1-methylimidazole (4)
3 (6.0 g, 26.6 mmol) was dissolved in a mixture of acetic
acid and water (2:1, 50 mL), and stirred at rt for 5 h. The
mixture was poured into water (100 mL) and adjusted to pH 8
with solid NaHCO3. Insoluble salt was filtered, and the or-
ganic layer in the filtrate was extracted with ethylether (50 mL
× 3). The organic layer was dried over anhydrous Na2SO4, and
evaporated under reduced pressure to give 4 as a brown solid
(2.0 g, 70%). The crude material was pure enough to use for
5,10,15-Triphenyl-20-(1-methyl-5-
imidazolyl)porphyrinatoCo(III)chloride (1Co)
A solution of cobalt(II)chloride (anhydrous, 40 mg,
1.82 mmol) in MeOH (5 mL) was added to a solution of 1
(22 mg, 0.036 mmol) in CHCl3 (10 mL). The mixture was
refluxed for 3 days. The mixture was diluted with CHCl3
(10 mL), and washed with 0.1 M HCl solution (5 mL) and
distilled water (5 mL × 3). The organic layer was dried over
anhydrous Na2SO4, and evaporated under reduced pressure to
give 1Co (19 mg, 78%) as a purple solid. MALDI-TOF mass
(dithranol) m/z 676.9 (M+H)+. Calcd for C42H28CoN6 675.2.
1H NMR spectra of 1Co in CDCl3, (CDCl2)2, acetone-d6,
and MeOH-d4 were obtained after dissolving 1Co sample
newly synthesized in the respective solvents (20 mM) and then
letting it stand until reaching the equilibrium. The chemical
shifts were reported by referring residual proton signals of
solvents {(CDCl2)2: 5.91; MeOH-d4: 3.30; DMF-d7: 8.01,
2.91, and 2.74; acetone-d6: 2.04 ppm} except for CDCl3
1
the next porphyrin synthesis. H NMR (270 MHz, CDCl3) δ
9.78 (1H, s, CHO), 7.79 (1H, s, imidazole-2), 7.62 (1H, s,
imidazole-4), 3.95 (3H, s, NCH3). 13C NMR (67.5 MHz,
CDCl3) δ 179.5 (CHO), 144.1 (imidazole-2C), 143.3 (imida-
zole-4C), 131.6 (imidazole-5C), 34.1 (NCH3).
5,10,15-Triphenyl-20-(1-methyl-5-imidazolyl)porphyrin (1)
Propionic acid (106 mL) was heated to 150 °C in a 300-mL
flask. 5-Formyl-1-methylimidazole 4 (0.5 g, 4.54 mmol),
benzaldehyde (1 mL, 9.08 mmol), and pyrrole (0.83 mL,
12.1 mmol) were added to the flask, successively. The mixture
was heated at 150 °C for 6 h. Propionic acid was evaporated
under reduced pressure. The residue was neutralized with
saturated aqueous NaHCO3 solution, and the organic layer was
extracted with CHCl3. The organic layer was washed with dis-
tilled water (50 mL), dried over anhydrous Na2SO4, and evapo-
rated under reduced pressure. The residue was purified by
column chromatography (SiO2, CHCl3, then CHCl3/acetone =
1
(TMS: 0 ppm). Aggregate A (trimer): H NMR (600 MHz,
CDCl3, 14.8 mM) δ 9.10 (2H, d, J = 4.8 Hz, pyrrole β-H), 8.98
(2H, d, J = 4.8 Hz, pyrrole β-H), 8.65 (2H, d, J = 4.8 Hz,
pyrrole β-H), 7.95–7.64 (15H, m, phenyl-H), 6.48 (2H, d, J =
4.8 Hz, pyrrole β-H), 1.62 (3H, s, NCH3), 1.11–1.10 (1H, m,
imidazole), –0.09 – –0.10 (1H, m, imidazole). Aggregate B
(tetramer): 1H NMR (600 MHz, CDCl3, 14.8 mM) δ 8.93 (2H,
d, J =4.8 Hz, pyrrole β-H), 8.85 (2H, d, J = 4.8 Hz, pyrrole β-
H), 8.67 (2H, d, J = 4.8 Hz, pyrrole β-H), 7.90–7.58 (15H, br,
phenyl-H), 6.93 (2H, d, J = 4.8 Hz, pyrrole β-H), 1.39 (1H, d,
J = 1.2 Hz, imidazole), 0.76 (1H, d, J = 1.2 Hz, imidazole),
0.62 (3H, s, NCH3). 13C NMR assigned by HMQC (correlating
1H NMR signal), trimer: δ 135.3 (9.10), 135.4 (8.65), 134.6
(8.98), 133 (1.10), 131.2 (6.48), 128 (–0.1), 32 (1.62), tet-
ramer: δ 135.4 (8.93), 135.4 (8.67), 134.6 (8.85), 134 (0.76),
132.2 (6.93), 124 (1.39), 32 (0.62).
1
5/1) to give porphyrin 1 (110 mg, 6%). H NMR (270 MHz,
CDCl3)δ 8.95 (2H, d, J = 4.8 Hz, pyrrole-Hβ), 8.91 (2H, d, J =
4.8 Hz, pyrrole-Hb), 8.89 (4H, s, pyrrole-Hb), 8.31–8.16 (6H,
m, phenyl-H), 8.02 (1H, d, J = 1.6 Hz, imidazole), 7.95 (1H, d,
J = 1.6 Hz, imidazole), 7.85–7.71 (9H, m, phenyl-H), 3.47
(3H, s, NCH3), –2.71 (2H, s, pyrrole-NH). MALDI-TOF mass,
(dithranol) m/z 618.28 (M+H)+, Calcd for C42H30N6 618.25.
UV-vis (λmax/nm (Abs). CHCl3) 419 (0.100), 514 (0.004), 588
(0.001), 591 (0.001), 649 (0.001). Fluorescence (λEX 420 nm,
λEM/nm (Intensity), CHCl3) 653.0 (188), 715.2 (99).
1
Aggregate A (trimer): H NMR (600 MHz, (CDCl2)2) δ
5,10,15-Triphenyl-20-(1-methyl-5-
imidazolyl)porphyrinatoZn(II) (1Zn)
A saturated solution of zinc acetate in MeOH (3 mL) was (15H, m, phenyl-H), 6.39 (2H, d, J = 5.1 Hz, pyrrole β-H),
added to a solution of 1 (22 mg, 0.036 mmol) in CHCl3 (5 mL). 1.54 (3H, s, NCH3), 1.13 (1H, s, imidazole), –0.20 (1H, s,
9.05 (2H, d, J = 5.1 Hz, pyrrole β-H), 8.91 (2H, d, J = 5.1 Hz,
pyrrole β-H), 8.60 (2H, d, J = 5.1 Hz, pyrrole β-H), 7.92–7.45
1
After stirring for 3 h at rt, the mixture was diluted with CHCl3 imidazole). Aggregate B (tetramer): H NMR (600 MHz,
(10 mL), and washed with saturated aqueous NaHCO3 solu- (CDCl2)2) δ 8.90 (2H, d, J = 5.1 Hz, pyrrole β-H), 8.84 (2H, d,
tion (10 mL) and water (10 mL). The organic layer was dried J = 5.1 Hz, pyrrole β-H), 8.65 (2H, d, J = 5.1 Hz, pyrrole β-H),
over anhydrous Na2SO4, and evaporated under reduced pres- 7.92–7.45 (15H, br, phenyl-H), 6.85 (2H, d, J = 5.1 Hz,
sure to give 1Zn (20 mg, 83%) as a purple solid. 1H NMR (600 pyrrole β-H), 1.41 (1H, s, imidazole), 0.72 (1H, s, imidazole),
Tanaka et al. / Equilibrium of Cyclic Multi-Porphyrins

284
0.60 (3H, s, NCH3). Aggregate A (trimer): 1H NMR (600 MHz,
MeOH-d4) δ 9.33 (2H, d, J = 5.1 Hz, pyrrole β-H), 9.25 (2H, d,
J = 5.1 Hz, pyrrole β-H), 8.92 (2H, d, J = 5.1 Hz, pyrrole β-H),
8.04–7.69 (15H, m, phenyl-H), 6.68 (2H, d, J = 5.1 Hz, pyrrole
β-H), 1.73 (3H, s, NCH3), 1.63 (1H, s, imidazole), –0.41 (1H,
RESULTS
Synthesis of 1Zn and 1Co
5-Formyl-1-methylimidazole (4) was prepared ac-
cording to the reported procedure.14 Free base porphyrin
1 was synthesized by condensation of aldehyde 4, ben-
zaldehyde, and pyrrole in propionic acid.15 Zincporphyrin
1Zn and cobalt(III)porphyrin 1Co were prepared in a
CHCl3–MeOH solution by treatment with zinc acetate
and cobalt(II) chloride, respectively. Structures of both
complexes were confirmed by 1H NMR, MALDI-TOF
1
s, imidazole). Aggregate A (trimer): H NMR (600 MHz,
DMF-d7) δ 9.07 (2H, d, J = 5.1 Hz, pyrrole β-H), 8.99 (2H, d,
J = 5.1 Hz, pyrrole β-H), 8.63 (2H, d, J = 5.1 Hz, pyrrole β-H),
8.04–7.64 (15H, m, phenyl-H), 6.69 (2H, d, J = 5.1 Hz, pyrrole
β-H), 1.96–1.95 (1H, m, imidazole), 1.74 (3H, s, NCH3),
0.047–0.038 (1H, m, imidazole). Aggregate A (trimer): 1H NMR
(600 MHz, acetone-d6) δ 9.04 (2H, d, J = 5.1 Hz, pyrrole β-H),
8.96 (2H, d, J = 5.1 Hz, pyrrole β-H), 8.60 (2H, d, J =
5.1 Hz, pyrrole β-H), 7.99–7.58 (15H, m, phenyl-H), 6.63 (2H,
d, J = 5.1 Hz, pyrrole β-H), 1.74 (1H, d, J = 1.4 Hz, imidazole),
1.63 (3H, s, NCH3), 0.031 (1H, d, J = 1.4 Hz, imidazole).
1
mass, and UV-vis spectra. Sharp H NMR signals and
silent ESR signal indicated the absence of paramagnetic
Co(II)porphyrin species.
Self-Assembly of 1Zn
1
When H NMR spectra of free base 1 and 1Zn
(29.3 mM) in CDCl3 were compared, large upper-field
shifts of β1 and N-Me signals were observed in 1Zn. β-
Protons, phenyl, and N-Me groups of 1Zn were as-
signed by the HMQC spectrum. Thus, β-protons at 8.91,
8.58, and 6.98 were correlated to the typical 13C signals
of pyrrole-β (130 ppm), and N-methyl protons at
2.02 ppm were correlated to that of N-methyl carbon of
imidazolylzincporphyrin (31.9 ppm).16 Two broad sig-
nals observed at 2.70 and 1.82 ppm may correspond to
Im1 and Im2 peaks, but they could not be identified from
the HMQC spectrum due to the peak broadening. Even
so, the large upper-field shift of the N-methyl moiety
certainly indicates that the imidazole moiety coordi-
nates to another zinc porphyrin. Since UV-vis spectra
were concentration-independent over 2×10–4 M, self-
organization of 1Zn should be completed in the NMR
SELECTIVE PREPARATION OF AGGREGATES
A AND B OF 1Co
A sample containing aggregate A as a main compo-
nent was obtained by stirring 1Co sample containing A
and B in MeOH for 10 min, and then evaporating the
solution under reduced pressure. The A/B content was
evaluated by 1H NMR integrations of the peaks, 6.47 for
aggregate A and 6.93 for B in CDCl3 (Fig. 2A). 1Co
newly synthesized in CHCl3 and MeOH (2:1) (see Ex-
perimental) also contains aggregate A mainly (Fig. 1A). A
sample containing aggregates A and B was obtained
when 1Co was dissolved in CDCl3 (10–70 mM). The
percentage of B increased on a time scale of a few hours
and days. The relative content was evaluated by NMR
peak integration. The prepared mixtures were applied
for ESI-mass spectrometry and GPC analysis. When
1Co (15 mg) was dissolved in CHCl3 (10 mL), and the
solution was evaporated slowly in a period of a week
under normal pressure, aggregate B became dominant
(Fig. 1D).
1
concentration (10–2 M). The broadening of H NMR
signals of 1Zn compared with those of freebase 1 indi-
cates that structural exchange between different su-
pramolecular structures and/or ligand–metal dissocia-
tion occur moderately in the NMR time scale. Since
MALDI-TOF and ESI-TOF mass spectrometries gave
only peaks corresponding to the monomer of 1Zn, the
number of aggregation could not be determined for 1Zn.
NMR MEASUREMENTS
Treatment of 1Co with Ag3PO4
Samples containing only A (Fig. 2A) and a mixture
of A and B (Fig. 2E) were dissolved in CDCl3 (11 mM).
Solid Ag3PO4 (13 mg) was added to the solutions, and
the mixture was sonicated for 1 min. After inorganic
salts were precipitated (about 10 min), 1H NMR spectra
were recorded. These cycles of sonication and standing
were repeated.
Self-Assembly of 1Co
1H NMR spectra of 1Co in CDCl3 (14.8 mM) are
shown in Fig. 1. The top one (Fig. 1A) was recorded
immediately after dissolving the sample freshly pre-
pared in CHCl3/MeOH (2:1). Four sharp doublets
(marked with filled circle) observed at 9.10, 8.98, 8.65,
and 6.48 ppm were assigned unequivocally as β4 (or β3),
β3 (or β4), β2, and β1 by H–H COSY and HMQC spectra.
ESI-Mass Measurements
ESI-mass spectra were recorded on a JEOL JMS- Coordinating imidazolyl protons were also observed as
7000 MStation using MeOH (HPLC grade) as a solvent sharp singlet signals at 1.62 (N-Me), 1.11 (Im1), and
(flow rate: 0.2 mL/min). The samples prepared in CHCl3 –0.10 (Im2), respectively. These chemical shift values
(or CDCl3) were also applied to the apparatus with indicate that the imidazolyl part is strongly shielded by
methanol as a solvent.
porphyrin ring current. Careful observation of 1A
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285
Fig. 1. NMR spectra of Co(III)porphyrin 1Co in CDCl3 (14.8 mM). (A): immediately after dissolving the sample of 1Co newly
prepared in CHCl3/MeOH (2:1), (B) after 2 days, (C) after 7 days, (D) re-dissolved sample via concentration procedure (see
text). Filled circle and open triangle indicate the aggregates A and B, respectively. Marks * and # indicate water and impurity,
respectively.
Fig. 2. NMR spectrum before and after treatment of aggregate A (A–D) and a mixture of A and B (E–H) by addition of Ag3PO4
in CDCl3 (11 mM). (A, E) before addition, (B, F) after addition and sonication for 1 min, (C, G) further sonication of B and F for
1 min, (D, H) further sonication of C and G for 1 min. Filled circle: aggregate A, open circle: modified aggregate A′, open
triangle: aggregate B.
Tanaka et al. / Equilibrium of Cyclic Multi-Porphyrins

286
reveals another tiny set of signals at 8.93, 8.85, 8.67, ences of β, and N-Me peaks are greater, and significant
6.93, 1.39, 0.76, and 0.63 ppm (marked with open tri- structural alternations must be involved. These results
angle) with a 1:8 ratio compared with the major signals. indicate that aggregate A′, similar to the aggregate A
Intensities of the latter set of signals increased during type structure, is stable even in CDCl3.
the time course of standing (Fig. 1B: after 2 days;
In order to determine the aggregation numbers in A
Fig. 1C: after 7 days), with sacrifice of the former set of and B, ESI-mass spectra were measured. A sample con-
signals. This behavior indicated that two sets of signals taining A and B in a ratio of 1.4:1 (from NMR peak
belonged to different species of aggregates. The signal integration) indicated the existence of trimer and tet-
intensity change almost reached an equilibrium after ramer along with a small amount of pentamer (Fig. 3A).
7 days, and approximately a 1:2 mixture resulted Here, m/z peaks 3520, 2809, 2099, 1743, 1388, and
(Fig. 1C). For convenience, the compound observed to 1032 could be assigned to [(1Co)5–Cl–]+, [(1Co)4–Cl–]+,
be dominant at the initial stage is defined as aggregate [(1Co)3–Cl–]+, [(1Co)5–2Cl–]2+, [(1Co)4–2Cl–]2+, and
A, and at the latter stage as B. The signals of B were [(1Co)3–2Cl–]2+, respectively. On the other hand, a
assigned as indicated in Fig. 1D based on H–H COSY sample containing only aggregate A prepared in MeOH-
and HMQC spectra. Imidazolyl signals of B were d4 showed trimer peaks exclusively, m/z 2088 ([(1Co)3–
shielded by porphyrin ring current, but the degrees for 3Cl–+2MeO–]+) and 1028 ([(1Co)3–3Cl–+MeO–]2+)
individual peaks were considerably different from those (Fig. 3D). In this sample, chloride anions in the trimer
of A. The difference will be discussed in the discussion were replaced by methoxide. From these two spectra,
section. Since both coordination aggregates A and B aggregates A and B were assigned as trimer and
showed only one set of signals for aggregated imida- tetramer, respectively. Figure 4 shows analytical gel
zolylporphyrins, each imidazolylporphyrin unit in the permeation chromatograms (GPC) of samples contain-
aggregates must locate in the identical environment. ing A and B aggregates in ratios 5:1, 1.4:1, and 1:20
Interestingly, conversion to aggregate B is pushed fur- from NMR peak integration. The relative integrated
ther by increasing the concentration. A more concen- peak areas and retention times strongly support the
trated sample in CDCl3 (74 mM) gave a 1:8.2 mixture, assignment. Although a small peak 3520 corresponding
1
showing a higher composition of aggregate B. When the to pentamer was observed in Fig. 3A, H NMR and
sample was evaporated slowly over a period of a week analytical GPC indicated only two sets of the aggre-
under normal pressure and the residual solid was dis- gates. We considered that the larger aggregate was
solved again in CDCl3, a 1:24 mixture was obtained formed from trimer and tetramer under ESI-mass ion-
(Fig. 1D). The aggregate ratio also varied depending on ization conditions.
the solvents. Percentages of aggregate A judged from
UV-vis spectra of the trimer (>95%, 3.9 mM as 1Co)
peak integration of β1 after equilibrium (20 mM as 1Co) and tetramer (>95%, 4.2 mM as 1Co) in CHCl3 are
were 28% in CDCl3, 31% in (CDCl2)2, 89% in acetone- shown in Fig. 5. Soret bands of both the trimer and the
d6, 100% in MeOH-d4, and 100% in DMF-d6.17 Aggre- tetramer were broader (half-band widths: 66 and
gate A apparently becomes dominant in polar (or coordi- 67 nm, respectively) than those of meso-tetraphenylpor-
nating) solvents. In MeOH-d4, only aggregate A was phyrinatocobalt(III) (half-band width: 58 nm)18 due to
observed in the wide concentration range 20–127 mM.
electronic interaction among the porphyrins. However,
In CDCl3, chloride anions in the aggregates are con- no remarkable difference could be observed in the
sidered to be tightly connected to cobalt(III) ions to whole area between the trimer and the tetramer.
reduce the charge character. In order to examine the
effect of counterions, the chloride anions were elimi-
DISCUSSION
nated by treatment with a Ag+ salt and substituted by the
anion of that Ag+ salt. When Ag3PO4 was added to the Structure of Trimer and Tetramer
solution of aggregate A in CDCl3, pyrrole-β protons,
Molecular modeling using molecular mechanic cal-
including β1, disappeared and new peaks appeared at culation19 was carried out for the trimer and the tetramer
slightly shifted positions (from Fig. 2A to 2D). Since the of 1Co (Fig. 6). Only one pseudo C3-symmetric struc-
shifted values were very small, aggregate A may main- ture (I) was generated as the trimer, whereas pseudo
tain the fundamental structure denoted here by aggre- C4-symmetric (II) and asymmetric (III) ones were pos-
gate A′. On the other hand, when Ag3PO4 was added to a sible as tetramers. Since the asymmetric tetramer III is
mixture of aggregates A and B in CDCl3, the peaks inconsistent with the simple NMR spectrum shown in
assigned to aggregate B almost disappeared (from 2E to Fig. 1D, the asymmetric III was excluded as a candidate.
2H), and the same peaks of Fig. 2D became dominant. In the symmetric trimer I and the tetramer II, all the
Contrary to the former case, the chemical shift differ- N-methyl groups must be located at the same side of the
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Fig. 3. ESI-mass spectra of (A) 1.4:1 mixture of A:B prepared in chloroform, (D) aggregate A prepared in methanol. (B), (C), and
(E): expanded spectra of (A) and (D).
Fig. 5. UV-vis spectra of trimer (1Co)3 (3.9 × 10–6 M, solid
line) and tetramer (1Co)4 (4.2 × 10–6 M, dashed line) in CHCl3.
Fig. 4. GPC chromatogram of A:B mixture, (A) 5:1, (B) 1.4:1,
and (C) 1:20 from NMR peak integration. Column: TSK
G2500HHR (TOSOH Co. polystyrene gel, 7.8 mm × 30 cm,
exclusion limit: 2 × 104 Da), eluent: CHCl3/MeOH (98/2) with
aggregates A and B are listed along with N-methylim-
idazole coordinated to tetraphenylporphyrinato-
cobalt(III) (TPPCo/Im). These chemical shift values
LiBr (30 mM), flow rate: 0.8 mL/min, monitored at 440 nm.
were relative to the imidazolyl protons in free base 1 as
porphyrin plane to which chloride ions are attached. the non-coordinating imidazole in 1Co and free N-
These symmetric structures provide cavities at the methylimidazole. The values of TPPCo/Im can stand as
center. In order to elucidate the two structures I and II, standards for the shielding effect by a single Co(III)por-
the magnitudes of upper-field shifts of the imidazolyl phyrin. The most characteristic signal was observed at
protons on coordination were compared. In Table 1, Im2 in aggregate A. The incremental upper-field shift
chemical shifts of coordinating imidazole protons in relative to Im2 in TPPCo/Im, (–6.48) – (–8.0) = 1.52 ppm,
Tanaka et al. / Equilibrium of Cyclic Multi-Porphyrins

288
Fig. 6. Molecular models of (I) trimer, (II) symmetric tetramer, and (III) asymmetric tetramer.
Table 1. Degrees of upper-field shifts (∆δ, ppm) of the imida-
zolyl protons in TPPCo/Im, aggregates A and B referred to that
in free base 1
Equilibrium of Trimer and Tetramer
As shown in Fig. 1, the initial mixture of trimer and
tetramer assemblies (14.8 mM of 1Co) in a ratio of 8:1
underwent a shift of the ratio to 1:2 in a relatively slow
process of 7 days. When a higher concentration was
applied, the ratio was shifted further to 1:8. Redissolv-
ing the sample concentrated from a CHCl3 solution to
dryness drove the ratio to 1:24. These results suggest
that chloroform stabilizes preferentially the tetramer
assembly and its ratio to the trimer is concentration-
dependent. Both the trimer and the tetramer are conceiv-
able from molecular modeling, and subtle factors of
TPPCo/Ima
aggregate A
aggregate B
(trimer)
(tetramer)
Im1
Im2
N-Me
–6.66
–6.48
–1.67
–6.87
–8.00
–1.81
–7.21
–6.51
–2.81
aref 20.
indicates an additional shielding by the neighboring molecular interactions may be operative to favor the
porphyrin. Since shift values of Im1 and N-Me in aggre- tetramer. Since imidazoyl ligand coordinates from in-
gate A are similar to those of TPPCo/Im, they are con- side the ring, the chloride must be located at the outside.
–
+
sidered to be shielded by only the coordinating The closer interaction of Cl(δ ) and NCH3(δ ) on coordi-
porphyrin. These observations are consistent with the nation may contribute to the tetramer stabilization.
molecular model of the trimer I. The Im2 protons are
Concentration-dependent equilibrium of two com-
located inside the cavity formed by three Co(III)por- pounds can be rationalized on the basis of the Le
phyrins and are susceptible to upper-field shift by its Châtelier principle. If one compound is favored
own and another remote porphyrin. In aggregate B, Im2 enthalpically and the other entropically, the equilibrium
is separated far from the neighboring porphyrins and is shifted by pressure, concentration, and temparature to
appears as normal. However, Im1 and N-Me are more cancel the overall change. Such phenomena and model
upper-field shifted than those in aggregate A. Because studies21 in solution are reported sometimes for su-
the N-methyl groups should be located outside the cav- pramolecular motifs. In complexation of cis-protected
ity, the shielding effect by the neighboring porphyrins square planar Pd(II) or Pt(II) metal ion to bispyridine
should be marginal. In the case of the tetramer, imi- ligand, concentration-dependent equilibrium between
dazolylporphyrins are significantly warped to coordi- square and triangle structures is observed.22 The square
nate to the next porphyrin perpendicularly. Therefore, structure is enthalpically favored due to the ditopic 90°
Im1 and N-Me are considered to be more shielded by the acceptor of Pd(II) and Pt(II) ions, whereas the triangle
coordinating porphyrin and its own porphyrin, respec- one is entropically favored due to the increase of total
tively. Distances of Im1–Co(III) ion, Im2–Co(III) ion, molecular numbers. In cases of relatively flexible
and N-Me-Co(III) ion in trimer (I) and tetramer (II) were bispyridine ligands, equilibrium of dimer and trimer
simulated by molecular mechanic calculation19 (Fig. 7). were observed.13, 23 Formation of [2]catenane of macro-
As expected, Im1 and N-Me in the tetramer come closer cyclic Pd–bipyridine complex is also concentration-
to the coordinating and its own porphyrins, respectively, dependent.12a,c In this case, [2]catenane formation is
compared with those in the trimer.
enthalpically favored in water due to hydrophobic inter-
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289
Fig. 7. Distances of Im1–Co(III) ion, Im2–Co(III) ion, and N-Me–Co(III) ion in trimer (I-a) and tetramer (II-a), A and B,
respectively. Angles of Co1-x-Co2, x-Co2-Cmeso-a, and x-Co2-Cmeso-b in trimer (I-b) and tetramer (II-b), C and D, respectively.
x = Centroid of imidazole.
action of ligand moieties, whereas non-catenane macro- MeOH or DMF could easily convert tetramer to the
cycle is entropically favored. In our experiments, the trimer assembly. In nonpolar solvents such as CDCl3 or
thermodynamic parameters of our system could not be (CDCl2)2, the monocationic Co(III)porphyrin unit
determined by temparature-dependent experiments be- should carry Cl– as an intimate counteranion, so that the
cause of the slow exchange rate at low temperature. But charge interaction in the assembly may be minimal to
a similar relationship would be operative in the trimer/ maintain the tetramer structure. When the solvent is
tetramer equilibrium of 1Co.
replaced by polar or coordinating solvent, the cationic
In principle, an entropically favored component is charges will be exposed along the porphyrin ring by
difficult to obtain as a concentrated sample.24 In the charge-stabilizing solvation. However, this will operate
present system, polar or donating solvents such as charge repulsions in the ring framework to reduce the
Tanaka et al. / Equilibrium of Cyclic Multi-Porphyrins

290
York, 2000; pp 347–368. (e) Harvey, P.D. The Porphy-
rin Handbook. Vol. 18; Kadish, K.M.; Smith, K.M.;
Guilard, R., Eds.; Elsevier Science: New York, 2003;
pp 63–250. (f) Satake, A.; Kobuke, Y. Tetrahedron
2005, 61, 13–41.
assembly number from four to three. When the counter-
anion Cl– is replaced by PO43– anion in CDCl3, the
intimate charge neutralization becomes difficult and a
similar structural transformation from tetramer to trimer
will take place in CDCl3.
(6) (a) Fleischer, E.B.; Shachter, A.M. Inorg. Chem. 1991,
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CONCLUSION
We discovered a new concentration- and solvent-depen-
dent equilibrium system of self-assembled multi-por-
phyrins. Both trimeric and tetrameric supramolecular
structures can be prepared selectively by the choice of
concentration and solvent. 1-Methyl-5-imidazolyl-
cobalt(III)porphyrin is classified as a new motif for
dynamic supramolecular architectures.
Acknowledgments. This work was supported by a Grant-in-
Aid for Scientific Research (A) and Scientific Research on
Priority Areas (No. 15036248), Reaction Control of Dynamic
Complexes, from the Ministry of Education, Culture, Sports,
Science and Technology, Japan and JSPS (Japan Society for
the Promotion of Science). A.S. acknowledges the financial
assistance of a Grant-in-Aid for Young Scientists (B) in
Grants-in-Aid for Scientific Research from JSPS.
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Tanaka et al. / Equilibrium of Cyclic Multi-Porphyrins