D3h-Symmetric Porphyrin-Based Rigid Macrocyclic Ligands for Multicofacial Multinuclear Complexes in a One-Nanometer-Sized Cavity

Article abstract of DOI:10.1002/chem.201501854

The one-step synthesis of D_3h-symmetric cyclic porphyrin trimers 1 composed of three 2,2′-[4,4′-bis(methoxycarbonyl)]bipyridyl moieties and three porphyrinatozinc moieties was achieved from a nickel-mediated reductive coupling of meso-5,15-bis(6-chloro-4-methoxycarbonylpyrid-2-yl)porphyrinatozinc. Although cyclic trimers 1 were obtained as a mixture that included other cyclic and acyclic porphyrin oligomers, an extremely specific separation was observed only for cyclic trimers 1 when using columns of silica gel modified with pyrenylethyl, cyanopropyl, and other groups. Structural analysis of cyclic trimers 1 was carried out by means of NMR spectroscopy and X-ray crystallography. Treatment of an η³-allylpalladium complex with a cyclic trimer gave a tris(palladium) complex containing three η³-allylpalladium groups inside the space, which indicated that the bipyridyl moieties inside the ring could work as bidentate metalloligands. Separation anxiety: Chromatography on a cyanopropyl-modified column supplied a cyclic porphyrin trimer efficiently from a mixture of acyclic and cyclic oligomers (see scheme; cod=1,5-cyclooctadiene, bpy=2,2′-bipyridyl). Treatment of an η³-allylpalladium complex with the cyclic trimer gives a tris(palladium) complex, which indicates that the cyclic trimer is a versatile macrocyclic ligand for multicofacial multimetallic complexes.

Full text of DOI:10.1002/chem.201501854

DOI: 10.1002/chem.201501854
Full Paper
&
Separation Methods |Hot Paper|
D -Symmetric Porphyrin-Based Rigid Macrocyclic Ligands for
3
h
Multicofacial Multinuclear Complexes in a One-Nanometer-Sized
Cavity
Yohei Ohkoda, Akane Asaishi, Tomoya Namiki, Tomoaki Hashimoto, Midori Yamada,
Koichiro Shirai, Yuta Katagami, Tomoaki Sugaya, Makoto Tadokoro,* and Akiharu Satake*
[
a]
Abstract: The one-step synthesis of D -symmetric cyclic
silica gel modified with pyrenylethyl, cyanopropyl, and other
groups. Structural analysis of cyclic trimers 1 was carried out
by means of NMR spectroscopy and X-ray crystallography.
3h
porphyrin trimers 1 composed of three 2,2’-[4,4’-bis(meth-
oxycarbonyl)]bipyridyl moieties and three porphyrinatozinc
moieties was achieved from a nickel-mediated reductive
coupling of meso-5,15-bis(6-chloro-4-methoxycarbonylpyrid-
3
Treatment of an h -allylpalladium complex with a cyclic
3
trimer gave a tris(palladium) complex containing three h -al-
2
-yl)porphyrinatozinc. Although cyclic trimers 1 were ob-
lylpalladium groups inside the space, which indicated that
the bipyridyl moieties inside the ring could work as biden-
tate metalloligands.
tained as a mixture that included other cyclic and acyclic
porphyrin oligomers, an extremely specific separation was
observed only for cyclic trimers 1 when using columns of
Introduction
such a reaction field. Macrocyclic porphyrin oligomers are can-
didates that can serve as multinuclear coordination sites within
[
8]
Metal-catalyzed reactions and molecular transformations in
their rigid skeletons. Macrocyclic porphyrin oligomers are in-
teresting materials for photochemical and electronic organic
[
1]
nanometer-sized cavities are attracting attention in organic
synthesis as biomimetic artificial enzymes and metalloenzymes,
in which selective reactions are expected to proceed under
[
9]
[10]
devices, as well as in the field of molecular recognition, se-
[
1a,10b]
lective reactions,
and in ion-transport materials because
[
2]
[2a,3]
[11]
ambient conditions. Bi- and multimetallic
catalysts and
of their nanometer-sized rigid pores. Synthetic strategies for
the production of covalently linked cyclic porphyrin oligomers
involve either macrocyclization of appropriately long acyclic
[
4]
[5]
their related di- and multinuclear metal complexes are also
attracting attention because of their high potential for selec-
tive transformations and multiredox reactions to activate and
transform unreactive and inert molecules and chemical bonds.
Therefore, their combined study, that is, the construction of
multimetallic systems with nanometer-sized cavities, is a chal-
lenging target for the next generation of biomimetic and envi-
ronmentally benign catalysts in organic synthesis.
[
10e,12]
porphyrin oligomers prepared by multistep synthesis,
or
a one-step synthesis from single or multiple porphyrin units by
[
13]
coupling reactions in the presence or absence of templates.
In both of these synthetic approaches, separation of the target
cyclic porphyrin oligomer from a mixture that contains other
cyclic oligomers and acyclic oligomers is a laborious step that
can lead to reduced yield. Gel permeation chromatography
(GPC) is a common method used to isolate macrocyclic com-
pounds from mixtures. In general, cyclic compounds tend to
be eluted later from the GPC column than the corresponding
acyclic compounds because the hydrodynamic volumes of
cyclic compounds are smaller than those of linear com-
[
6]
[7]
Macrocyclic and pseudomacrocyclic multidentate ligands,
in which all of the coordination sites are directed toward the
center of the macrocycle, are potential candidates for cofacial
dinuclear and multicofacial multinuclear complexes in which
the metal ions work to manipulate a guest molecule coopera-
tively. Appropriate spaces to accept desired molecules in mac-
rocycles with their rigid skeletons are required to construct
[
12b,c,14]
pounds.
However, the difference is generally small.
Therefore, a long column length and/or repeated chroma-
tographic runs are often necessary to isolate the target cyclic
oligomer from the mixture.
[
a] Y. Ohkoda, A. Asaishi, T. Namiki, T. Hashimoto, M. Yamada, K. Shirai,
Y. Katagami, Dr. T. Sugaya, Prof. M. Tadokoro, Prof. A. Satake
Graduate School of Chemical Sciences and Technology
Tokyo University of Science, 1-3 Kagurazaka
Shinjuku-ku, Tokyo 162-8601 (Japan)
We have designed new D -symmetric macrocyclic ligands
3
h
1
for, at most, hexanuclear multicofacial metal complexes
(
Figure 1). The ligands of 1 are composed of three 2,2’-bipyrid-
E-mail: tadokoro@rs.kagu.tus.ac.jp
yl moieties and three porphyrinato moieties and, in total, six
metal ions can be accepted by the three bidentate ligands and
the three tetradentate ligands. To increase the solubility of
satake@rs.kagu.tus.ac.jp
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201501854.
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multinuclear complexes, and also for further functionalization
outside the macrocyclic ligands, we planned to introduce six
carboxylic ester groups on the 4,4’-positions of each 2,2’-bipyr-
idyl moiety. Because an orthogonal conformation between
each porphyrin moiety and the 2,2’-bipyridyl group is predict-
ed, the three porphyrin groups are expected to arrange them-
selves to produce a three-dimensional prism-shaped inner
space. On the meso positions of the three porphyrins, a sub-
mixture, specific retention phenomena were observed by chro-
matographic analysis with various modified columns of silica
gel. Herein, we report on this unusual separation behavior, as
well as the efficient isolation of C-3mer 1, and that the C-3mer
has the potential to form versatile macrocyclic ligands for mul-
ticofacial multinuclear complexes.
stituent group (denoted by R in Figure 1) can be introduced Results and Discussion
during the synthesis of the porphyrin parts. This can be used
Synthesis and isolation of the C-3mer
to control the size of the mouth to the inner space, as well as
the size of the inner space. In this study, mesityl (Mes) and
ethyl substituents were examined as a bulky aryl group and
a small alkyl group, respectively, denoted by 1a and 1b in
Figure 1.
Porphyrins 2a and 2b were synthesized as follows. The corre-
sponding Mes- or ethyl-substituted dipyrromethanes (3a and
3b) and 6-chloro-4-methoxycarbonylpyridyl-2-paraldehyde (4)
were condensed in the presence of trifluoroacetic acid, fol-
lowed by oxidation with p-chloranil to give the free-base por-
phyrins 5a and 5b in 31 and 25% yield, respectively. Incorpo-
ration of zinc(II) ions into the free-base porphyrins gave 2a
and 2b quantitatively (Scheme S1 in the Supporting Informa-
tion). Referring to our previous conditions for the synthesis of
We planned to achieve the one-step synthesis of 1 by
a nickel-mediated reductive coupling reaction of bis[2-(6-chlo-
ropyridyl)]porphyrinatozinc 2 (Scheme 1). The reaction led to
the formation of a mixture composed of the target cyclic
trimer (C-3mer) and other cyclic and acyclic porphyrin oligo-
mers. During the isolation of the C-3mer from the complex
[
15]
bis(porphyrins) connected through 2,2’-bpy, nickel-mediated
Figure 1. The design of macrocyclic ligand 1 (C-3mer) composed of three 4,4’-methoxycarbonyl-2,2’-bipyridine and three porphyrin groups: a) top view,
b) side view. The three 2,2’-bipyridine parts are expected to give metal complexes inside the ring, as shown with gray spheres in a). The six carboxylic acid de-
rivatives outside the ring are expected to increase solubility in various solvent systems. A triangular prism with a triangular space (ca. 1 nm each side) is ex-
pected in the orthogonal conformation, as shown in b).
Scheme 1. Synthetic scheme for the preparation of cyclic trimers (C-3mer) 1a and 1b; COD=1,5-cyclooctadiene; 2,2’-bpy=2,2’-bipyridine.
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reductive coupling of 2 was performed. Under these condi-
tions, zinc porphyrins and carboxylic ester moieties in the sub-
strates did not interfere with the reaction. A 1:1 mixture of
[
Ni(cod) ] (7–20 equiv) and 2,2’-bpy (7–20 equiv) was treated
2
with porphyrinatozinc 2 in anhydrous THF or anhydrous DMF
under an argon atmosphere at 508C for 12 h. When ethyl-sub-
stituted 2b was used as the starting material, the coupling re-
0
action proceeded in THF, and seven equivalents of the Ni –bpy
system were enough to consume most of the starting material.
However, in the case of the Mes-substituted complex 2a, the
conditions discussed above were insufficient, and treatment
0
with 20 equivalents of the Ni –bpy system in the more active
[
16]
solvent DMF gave better results with regard to the reprodu-
cible consumption of starting materials. After the reactions
were completed, the mixture was washed with an aqueous
ammonium solution to remove excess of nickel species, and
the organic layer was extracted with CHCl . The organic layer
3
was analyzed by means of a GPC system fitted with three poly-
styrene-based GPC columns (two TSKgel G2500HHR (exclusion
limit 20000 Da, 7.8 mm inner diameter (I.D.)”30 cm) and
a TSKgel G2000HHR (exclusion limit 10000 Da, 7.8 mm I.D.”
3
0 cm)) connected in series to a photodiode array detector.
The elution profiles from the GPC column monitored at l=
56 and 566 nm, respectively, are depicted as solid lines in
5
Figure 2. Only porphyrin derivatives can be observed at this
detection frequency. The plain Gaussian curves shown in
Figure 2, which were obtained by deconvolution analysis by
using OriginPro8 software, revealed that more than 10 compo-
nents containing porphyrin oligomers were present in the
crude samples. The ratio of each component varied in each ex-
periment, even under the same conditions, and the represent-
ed profiles are most typical. In the sample analyzed in Fig-
ure 2a and b, the component eluting at 22.5 and 25.6 mL, re-
spectively (shaded areas, assigned as C-3mer, see below), were
obtained as the dominant species. The crude mixture was fur-
ther separated by using a preparative GPC column packed
with polystyrene-based gel (Biorad S-X1 (exclusion limit
Figure 2. Bold lines in a) and b): typical GPC chromatograms of reaction mix-
tures obtained by nickel-mediated coupling of 1a (R=Mes) and 1b
(R=ethyl) (column: TOSOH TSKgel G2500HHR (300 mm (length))”2 (exclu-
sion limit, 20000 Da)+TOSOH TSKgel G2000HHR (300 mm (length))”1 (ex-
À1
clusion limit, 10000 Da), eluent: pyridine, flow rate: 1.0–1.2 mLmin ), moni-
tored at: a) l=556, and b) 566 nm). EV: elution volume. Plain lines in a) and
b): deconvolution analysis of the chromatograms by Gaussian curve fitting
by means of OriginPro8 software. The shaded peaks indicate C-3mers: a) 1a,
and b) 1b.
1
4000 Da), 3 cm diameter”1 m length) with a mixture of tolu-
ene and pyridine (85:15 v/v) as the eluent under atmospheric
pressure. MALDI-TOF MS analysis of the fractions showed mass
fragments that were approximately two to five times larger
than those of starting monomers 2, which suggested that the
reductive coupling reaction produced di- to pentameric por-
phyrin oligomers. Some estimated structures are shown in
Scheme 1. The terminal moieties of acyclic porphyrin oligo-
mers, indicated as X in A-nmer in Scheme 1, were assigned as
H, Cl, or OH, but some unidentified signals were also observed
in the MALDI-TOF MS analyses.
method could be used to isolate the C-3mer 1b, other chro-
matographic techniques involving modified silica gel were sur-
veyed to find a more efficient method. Four analytes, com-
pound 2b and three fractions containing mainly acyclic dimer
(ethyl A-2mer), acyclic trimer (ethyl A-3mer), and cyclic trimer
(ethyl C-3mer), were prepared to compare the elution behavior
in various modified silica-based columns. Eight HPLC columns
(COSMOSILꢁ series), C18-MS-II, Cholester, PYE, pNAP, NPE, PBB-
R, NPE, and CN-MS (4.6 mm I.D.”15 cm), supplied by Nacalai
Tesque were tested. The chemical structures of the functional
groups on these columns are shown in Figure 3. Thus, octadec-
yl, cholesterol ether, pyrenylethyl, naphthylethyl, pentabromo-
benzyl ether, nitrophenylethyl, and cyanopropyl moieties were
attached to silica gel (5 mm particle, 120 Š pore). GPC and
MALDI-TOF MS analyses of the four analytes used herein are
shown in Figures S1–S4 in the Supporting Information. A mix-
ture of toluene and pyridine was used as the eluent. The polar-
ity parameters of the solvents were 2.3 (toluene) and 5.3 (pyri-
The following detailed analysis describes the case of using
ethyl-substituted 2b as a starting material for the coupling re-
action. In the GPC chromatographic separation, compounds
corresponding to the trimer were detected in two fractions,
with the later eluting compound assigned as the target C-
3
mer 1b based on high-resolution MALDI-TOF MS analysis.
However, in addition to the target C-3mer 1b, the fraction con-
tained other mono- and dimeric porphyrin derivatives; thus
further purification was required. Although a recycling GPC
[
17]
dine); therefore, the combination of these solvents worked
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Figure 3. Chemical structures of functional groups bonded to silica gel in
modified silica gel columns.
well to elute all of the analytes without adsorption in the sta-
tionary phase.
Chromatograms of the analytes obtained by column chro-
matography with CN-MS as the stationary phase are shown in
Figure 4a–d; as a reference, a chromatogram of chloroform
Figure 4. Chromatograms of: a) 2b, b) C-3mer, c) A-2mer, d) A-3mer, and
e) chloroform with analytical HPLC column CN-MS (4.6 mm (ID), 15 cm
(length)). A mixture of toluene and pyridine (15:85 v/v) was used as the
eluent.
(
10 mL) is also illustrated in Figure 4e . The peak at 3.88 min in
each chromatogram in Figure 4a–d is a shock peak of chloro-
form used for dilution of the analytes. Peaks of compound 2b,
A-2mer, and A-3mer eluted earlier than the shock peak, which
suggested that almost no retention occurred under these con-
ditions, with the A-3mer eluting earliest. In contrast, the C-
as t . Retention factors, k, are listed in Table 1. The retention
0
factors of C-3mer in C18-MS-II and Cholester were 0.04 and
0.02, respectively, which were similar to those of porphyrinato-
zinc 2b and C-3mer 1b and suggested that no interaction was
observed between C-3mer 1b and the stationary phase in
C18-MS-II and Cholester. The retention factors of C-3mer 1b in
PYE, pNAP, and PBB-R were in the range 0.88–1.68, whereas
that of 2b remained below 0.1, which indicated that a specific
interaction between C-3mer 1b and the stationary phase was
observed.
3
mer eluted much later (13.4 min) than those peaks, and im-
purities contained in this analyte eluted earlier (3–11 min) than
the C-3mer. These results indicated the specific interaction of
C-3mer with silica gel modified by cyanopropyl groups.
If we assume that the A-3mer was not retained on the CN-
MS column at all, then the retention time of this compound
could be defined as t . Retention factors, k=(tÀt )/t , can be
0
0
0
calculated based on t , as shown for Run 1 in Table 1. Here, k ,
Larger retention factors for C-3mer (ca. 3) were observed
with NPE and CN-MS, and the retention factors of k2b remained
0
2b
kCHCl3 and kC-3mer 1b are the retention factors of compounds 2b,
CHCl , and C-3mer 1b, respectively. If a retention factor (k) is
3
larger than one, the retention time of the analyte is more than
two times longer than that of A-3mer. The large difference in
the retention times of A-3mer and C-3mer is noteworthy.
Other chromatograms obtained when using C18-MS-II, Choles-
ter, PYE, pNAP, PBB-R, and NPE columns are shown in Fig-
ures S5–S7 in the Supporting Information. In the case of C18-
MS-II and Cholester, C-3mer 1b was not retained as well as 2b,
A-2mer, and A-3mer (Figure S5 in the Supporting Information).
In contrast, specific retention of C-3mer 1b was observed in
PYE, pNAP, NPE, and PBB-R columns (Figures S6 and S7 in the
Supporting Information). In all of the chromatograms, A-3mer
eluted first and the retention times of A-3mer can be defined
Table 1. Retention factors (k) of compounds on modified silica gel HPLC
columns.
[a]
Run
Column
CN-MS
Retention factors
k
2b
k
CHCl3
k
C3mer 1b
1
2
3
4
5
6
7
0.08
0.08
0.08
0.08
0.08
0.10
0.09
0.14
0.25
0.24
0.12
0.10
0.10
0.14
2.95
0.04
0.02
0.88
0.94
1.68
3.03
C
18-MS-II
Cholester
pNAP
PBB-R
PYE
NPE
0 0 0
[a] k=(tÀt )/t ; t : retention time of A-3mer.
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below 0.1. For comparison, TLC analysis was performed with
an unmodified SiO plate with a mixture of toluene/pyridine
2
(
15:85) as the eluent; in this case, all of the analytes (2b, A-
2
1
mer, A-3mer, and C-3mer) eluted to the top of the plate (R =
f
), which indicated that there was almost no interaction of the
analytes with the silanol residues in this solvent system. The
specific separation mechanism of C-3mer 1b on some modi-
fied silica gel columns is discussed below. After the survey, cya-
nopropyl-modified silica gel chromatography was selected for
use in the more efficient separation of cyclic trimers from the
crude reaction mixtures. In Figure 5a and b, chromatograms of
the reaction mixtures of the C-3mers synthesized from 2a and
2
b obtained with a shorter CN-MS column (I.D.=4.6 mm”
5
0 mm) are shown. The reaction mixtures are the same sam-
ples that were analyzed by using the GPC systems shown in
Figure 2a and b, respectively. The peaks occurring at 2.8 and
6
mL in Figure 5a and b, respectively, correspond to the cyclic
trimers 1a and 1b, respectively. These were eluted much later
than most of the other oligomers, even from the crude reac-
tion mixtures. The tailing behavior observed in the ethyl-sub-
stituted cyclic trimer 1b suggests a stronger interaction with
cyanopropyl moieties than that with the Mes-substituted cyclic
trimer 1a. The peaks that eluted later than the cyclic trimers,
at 4.7 (Figure 5a) and 21 mL (Figure 5b), were assigned to
a Mes-substituted monohydrolyzed cyclic trimer (Figures S8
and S9 in the Supporting Information) and an ethyl-substituted
cyclic tetramer (Figure S10 in the Supporting Information), re-
spectively, from MALDI-TOF MS analysis. Finally, preparative
chromatographic separation of both Mes- and ethyl-substitut-
ed cyclic trimers 1a and 1b from the corresponding crude re-
action mixtures was carried out on silica gel modified with cya-
nopropyl groups (Cyanogel) purchased from the Yamazen Cor-
poration for flash chromatography. These were purified by Cya-
nogel chromatography with a mixture of pyridine and toluene
Figure 5. Chromatograms of reaction mixtures obtained by nickel-mediated
coupling of: a) 1a, and b) 1b by using a Nacalai Tesque 5CN-MS HPLC
column ((4.6 mm (ID)”50 mm (length)), eluent: toluene/pyridine=15:85
À1
(v/v), flow rate: 0.5 mLmin , monitored at l=556 nm. Peaks at 2.7 mL in a),
and 6 mL in b) correspond to 1a and 1b, respectively. Inset in b): magnifica-
tion of the chromatogram from 10 to 30 mL elution volume. The same sam-
ples were used in Figures 2a and 5a, and Figures 2b and 5b, respectively.
(
85:15) as the eluent. This purification method was much
spectroscopy techniques: HMQC and HMBC (Figures S11–S14
1
easier than that for the GPC method. Any cyanopropyl-modi-
fied silica gel was reused several times to purify different crude
mixtures. After purification by Cyanogel column chromatogra-
phy, the ethyl-substituted C-3mer 1b was washed with a 0.3m
aqueous solution of citric acid and then with deionized water
in the Supporting Information). Similar to the H NMR spec-
trum of 1a, the spectrum of 1b also showed simple signals,
which indicated that 1b had a D -symmetric structure (Fig-
3h
ure S16 in the Supporting Information). In the case of the
1
H NMR spectrum of 1b, coordinated pyridine molecules were
in CHCl . The concentrated organic layer was further purified
observed at d=4.3, 3.8, and 2.2 ppm as broad signals. The
3
by using an unmodified SiO2 column chromatography with
source of the pyridine molecules was the eluent used in the
1
CHCl and ethyl acetate (5:1) as eluents to give 1b in a 9.0 %
Cyanogel column chromatography. Because the H NMR spec-
3
yield. Mes-substituted C-3mer 1a was further purified by re-
crystallization from 2-butanone and methanol, after Cyanogel
column purification, to give 1a in 13.1 % yield.
trum of 1b changed slightly, depending on the amount of pyr-
idine molecules in CDCl , NMR spectroscopy structural analysis
3
of 1b was performed in [D ]pyridine. All 1D and 2D NMR spec-
5
troscopy data and assignments of 1b in [D ]pyridine are
5
shown in Figures S18–S23 in the Supporting Information. In
the case of 1a, no residual pyridine was observed in the NMR
spectra.
NMR spectroscopy structural analysis of the C-3mers
NMR spectroscopy structural analysis of cyclic trimers 1a and
1
1
b was carried out. The H NMR spectra of 1a and the corre-
sponding monomeric zinc porphyrin 2a in CDCl are shown in
Figure 6a and b, respectively.
3
X-ray crystallography of C-3mer 1a
Similar to the spectrum of 2a, the spectrum of the C-3mer
An ORTEP model and the molecular structure of C-3mer 1a are
shown in Figure 8 and Figure S24 in the Supporting Informa-
tion, respectively. A deformed triangular prism shape was ob-
served, in which all six methyl ester groups were directed
1
1
a showed only a single set of signals, which indicated that
a had a D -symmetric structure. All of the signals of 1a
3h
could be assigned, as shown in Figure 7, by using 2D NMR
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1
Figure 6. H NMR spectra (300 MHz, in CDCl
3
) of: a) 1a, and b) 2a. Assignment of signals a–j in spectrum a) correspond to a–j in Figure 7.
The angle of the porphyrin
plane tilted away from the per-
pendicular of the mean plane
containing the three 2,2’-bpy
parts is 318 (Figure 9). This result
suggests that, in solution, the
porphyrin moiety in 1a can
rotate
approximately
Æ308
along the axis through the two
meso positions connected to the
two pyridyl parts.
Mechanisms of specific separa-
tions of C-3mer on modified
silica gel columns
From the chemical structures of
the modified columns shown in
Figure 3 and the retention fac-
tors of C-3mer 1b on the col-
umns listed in Table 1, the col-
umns can be classified into three
groups. Two columns, C18-MS-II
and Cholester, have nonaromatic
1
13
Figure 7. Assignments of H (300 MHz) and C NMR (75 MHz) signals of cyclic trimer 1a in CDCl
3
.
toward the outside, whereas the nitrogen atoms on the bipyr-
idyl groups were directed toward the inside.
acyclic and cyclic residues, and they have almost no retention
capability for C-3mer 1b (<0.04). These columns are classified
as group A. Three columns, PYE, pNAP, and PBB-R, have non-
polar aromatic residues, and their retention capability for C-
The distance between the two zinc ions was 9.64–9.70 Š
(
Table 2), which indicated that C-3mer 1a had a one-nanome-
ter-sized space inside. Although the D symmetry of 1a was
3
h
1
observed in the H NMR data, a C -symmetric structure was ob-
s
[
a]
served in the crystal structure because of the incline in one of
the three porphyrin groups in 1a, which allowed it to interact
with a bipyridyl moiety in a neighboring cyclic porphyrin 1a.
The bipyridyl (C(17)ÀC(21)) and Mes (C(67)ÀC(69)) moieties on
the symmetric plane passing through the Zn(2), C(66), C(70),
and C(71) atoms are disordered, as shown in Figure S24 in the
Supporting Information.
Table 2. Zinc center distances [Š] for 1a.
Bond
Distance
Zn(1)ÀZn(2)
9.6404(12)
9.7046(16)
Zn(1)ÀZn(1)#1
[
a] Symmetry transformations used to generate equivalent atoms: #1
x,Ày+1/2,z.
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moment must be larger than the 2,2’-bpy groups on ethyl A-
2
mer and ethyl A-3mer, which can assume different conforma-
tions by rotating along the CÀC bond between two adjacent
pyridyl moieties. Pyridine is considered to be a polar solvent
that cancels out the relatively weak dipole–dipole interactions
between the nitrobenzene moieties and ethyl A-2mer and
ethyl A-3mer, and only strong dipole–dipole interactions with
the NPE column for C-3mer 1b were observed, even in polar
pyridine. However, on PYE, chloroform acted in an entirely dif-
ferent manner. Thus, when chloroform was used as an eluent
on a PYE column (group B), both ethyl A-3mer and C-3mer 1b
were eluted immediately with slight tailing. The t values of
R
the peak maxima were earlier than that of 2b (Figure S27 in
the Supporting Information). The specific retention of C-3mer
1
b on a PYE column was only observed when pyridine was
used as an eluent, as shown in Figure S6a in the Supporting In-
formation. This observation suggests that chloroform suppress-
es any interaction between the pyrenyl groups on the PYE
column and C-3mer 1b, whereas pyridine helps interactions of
the PYE column and C-3mer 1b. To obtain information on the
interaction of chloroform with C-3mer 1b, pyridine-free C-
Figure 8. ORTEP drawing of 1a. Ellipsoids are drawn at the 50 % probability
level; hydrogen atoms, coordinated methanol molecules, and disordered
moieties were omitted for clarity. Symmetry transformations used to gener-
ate equivalent atoms: #1 x,Ày+1/2,z.
3
mer 1b was prepared by washing with a solution of citric
acid several times until any residual pyridine was removed
completely.
1
The H NMR spectrum of pyridine-free C-3mer 1b in CDCl3
exhibited very broad signals, which indicated that conforma-
tional isomers existed and that the exchange rate among the
isomers was slow on the NMR spectroscopy timescale (Fig-
ure S28 in the Supporting Information). The broad signals
became sharp with D3h symmetry in the presence of trace
Figure 9. Molecular packing of 1a observed by X-ray single-crystal analysis;
hydrogen atoms are omitted for clarity. A light-colored molecule is an adja-
cent one (1a): a) top view, b) side view, c) front view. The methyl group ob-
served in the center of the C-3mer in a) is disordered at the two carbon po-
sitions.
amounts of pyridine or when only [D ]pyridine was used (Fig-
5
ures S16 and S18 in the Supporting Information).
These NMR spectroscopy observations can be explained by
the chloroform molecules being held tightly within the C-3mer
with several conformational arrangements and the exchange
rate among the conformers being slow. A coordinating solvent,
such as pyridine, probably increases the rate of conformational
exchange by rapid coordination exchange between the ligand
and zinc ion and, during this process, the chloroform mole-
cules held within the C-3mer are ejected from the inside.
To represent the chromatographic conditions of a PYE
3
mer 1b is moderate (0.88–1.68). These columns are classified
as group B. The last two columns, NPE and CN-MS, have polar
residues, and they retained C-3mer 1b the most strongly (ca.
3
.0). These columns are classified as group C. Because of the
difference in chemical structures between groups B and C, the
mechanism of their specific separation was considered to be
different.
1
column with pyridine, a H NMR spectroscopy titration experi-
To discuss these mechanisms, the effect of the solvent
eluent was examined. When chloroform was used as an eluent
on the NPE column (group C), acyclic ethyl A-3mer and C-3mer
ment of C-3mer 1b in the presence of N-methylimidazole (Im)
with pyrene was carried out (Figure S29 in the Supporting In-
formation). We used Im instead of pyridine because the associ-
ation constant of zinc tetraphenylporphyrin is larger (logK=
1
b were adsorbed on the column and were not eluted within
[
18]
a retention time (t ) of 30 min, whereas monomeric 2b was
4.66) than that of pyridine (logK=3.52) , and monitoring the
R
1
eluted at t =4 min immediately (Figure S25 in the Supporting
H NMR signal of the N-methyl part was applicable.
R
Information). Increasing the polarity of the eluent by using
a toluene/pyridine system on the NPE column caused immedi-
ate elution of ethyl A-2mer and ethyl A-3mer, followed by a sig-
nificantly delayed elution of C-3mer 1b (Figure S26 in the Sup-
porting Information). These observations are explained by
dipole–dipole interactions between nitrobenzene moieties on
the NPE column and the functional groups on ethyl A-2mer,
ethyl A-3mer, and C-3mer 1b. Because of the restricted confor-
mation of the 2,2’-bpy groups of the C-3mer, the partial dipole
A mixture of C-3mer 1b (3.6 mm) and Im (ca. 20 equiv) in
(CDCl ) was prepared. Under these conditions, all zinc moie-
2
2
ties had to have been coordinated with Im from both the
inside and outside of the C-3mer to give five-coordinated Im–
zinc porphyrin complexes (B in Figure S29 in the Supporting
1
Information). The H NMR spectrum obtained is shown in Fig-
ure S30 in the Supporting Information, and a broad signal as-
signed to the N-methyl group was observed at d=3.25 ppm.
The chemical shift of the signal maximum of the broad signal
Chem. Eur. J. 2015, 21, 11745 – 11756
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is reasonable because an average signal caused by exchange
of one Im molecule coordinated from the inside of the C-3mer,
two Im molecules coordinated from the outside of the C-3mer,
and about 17 free Im molecules. The chemical shifts of Im mol-
ecules coordinated with the outside and inside of the C-3mer
were estimated to be d=2.21 and À1.3 ppm, respectively,
from the complex of monomeric C-3mer 1b and Im
columns. The inclusion of nonpolar p aromatic residues on the
columns into the C-3mer, and dipole–dipole interactions of
polar residues on the columns with the partial dipole moiety
of the C-3mer are the most probable mechanisms for group B
and C columns, respectively. Used as an eluent, pyridine has
a dual role as a coordination ligand and as a polar solvent for
each mechanism.
(
0.25 equiv) in (CDCl ) , and from variable-temperature NMR
2 2
spectroscopy experiments of C-3mer with Im (0.5 equiv) at
À408C. At room temperature, these coordinating Im molecules
exchanged rapidly with free Im molecules, the N-methyl group
of which was observed at d=3.64 ppm, to give a broad signal
at d=3.25 ppm as an averaged signal. The addition of pyrene
to the mixture caused a downfield shift of the signal, which
almost stopped at d=3.32 ppm when about 20 equivalents of
pyrene had been added to the mixture (Figure S31 in the Sup-
porting Information). The resulting chemical shift is reasonable
if three molecules are coordinated on the outside and no Im
molecules exist on the inside.
Complexation of the bipyridyl parts in the C-3mer 1a
Complexation of the bipyridyl parts in cyclic trimer 1a was ex-
II
amined for various metal ions with compounds such as Ni Cl
2
I
and Cu I, which are employed in fluorescence spectroscopy.
The addition of transition-metal ions caused fluorescence
quenching from cyclic trimer 1a, which suggested a heavy-
metals effect after complexation of the metal ions with the bi-
pyridyl parts of 1a. For a more quantitative analysis, titration
3
of 1a with an h -allylpalladium tetrafluoroborate complex was
3
carried out. A solution of h -allylpalladium tetrafluoroborate
The change in chemical shift of the pyrene molecules added
during the titration also caused a downfield shift of the signals
from the initial value upon addition of 0.9 equivalents of
pyrene, and the signal eventually approached the value of free
pyrene (Figure S32 in the Supporting Information). These ob-
servations indicate that the pyrene molecules pushed out the
Im molecules from inside the C-3mer, and the pyrene mole-
cules then became included inside the C-3mer as a host–guest
complex. Based on molecular modeling, only one pyrene mole-
cule can exist in the center of a cavity of C-3mer, and there-
fore, the observed change in chemical shift suggests that
pyrene molecules in the system are exchanged.
complex in CHCl was added portionwise (0.5 equiv each) to
3
a solution of cyclic trimer 1a (6 equiv) in CHCl . After stirring
3
for 15 min after each addition, each aliquot was diluted 1000
times to give a solution of about 0.38 mm in chloroform. UV/Vis
and fluorescence spectra of the diluted samples were recorded
(Figure 10).
In the UV/Vis spectra, the maximum of the Soret band shift-
ed toward the blue, from l=425.5 to 422.5 nm, whereas the
maxima of the Q bands shifted toward the red, from l=560
and 606 to 564 and 630 nm, with significant broadening. At
the same time, a successive absorption or scattering in the
wavelength range from l=320 to 700 nm was observed. In
the fluorescence spectra, addition of two equivalents of a palla-
dium complex resulted in almost complete quenching of the
Similarly, an NMR spectroscopy titration experiment of C-
3
mer 1b with nitrobenzene was also carried out. Although the
3
phenomenon of ejecting Im molecules from inside C-3mer was
observed (Figure S33 in the Supporting Information), no inclu-
sion of nitrobenzene in the C-3mer was observed (Figure S34
in the Supporting Information). This result also suggests that
the mechanism of specific chromatographic separation of C-
fluorescence from 1a. The mixture of 1a with an h -allylpalladi-
um tetrafluoroborate complex (6 equiv) was analyzed by using
1
ESI-TOF MS and H NMR spectroscopy in CDCl . From the ESI
3
+
mass spectrum, the [1a+(allylpalladium)3]3 species and its
derivatives were observed as the dominant ones (Figure S35 in
the Supporting Information). By comparing the NMR spectrum
3
mer on group C columns is different from that on group B
3
Figure 10. a) UV/Vis spectral changes to 1a upon addition of an h -allylpalladium tetrafluoroborate complex in CHCl
3
. (Normalized at the peak maxima of
their Soret bands lꢀ425 nm.) Inset: the expanded spectra from l=370 to 470 nm. b) Fluorescence spectral changes of the same titration of a) excited at
l=556 nm. (Their intensities were normalized by being divided with the absorbance at the excited wavelength.)
Chem. Eur. J. 2015, 21, 11745 – 11756
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1
3
3
Figure 11. H NMR spectra (500 MHz, in CDCl
3
) of: a) 1a, b) a mixture of 1a with h -allylpalladium tetrafluoroborate complex (6 equiv), and c) h -allylpalladium
tetrafluoroborate complex.
3
Demetalation of zinc ions from C-3mer 1a and incorporation
of iron(III) ions
of a mixture of 1a and the h -allylpalladium tetrafluoroborate
complex (Figure 11 b) with reference spectra (Figure 11 a and
3
c), characteristic h -allylpalladium species were observed at d=
1
.8, À0.7, and À1.4 ppm, and were considered to be located
To demonstrate that the three tetradentate porphyrinato parts
in cyclic trimer 1a also worked as versatile ligands for multico-
facial multimetalloporphyrin complexes, demetalation of the
zinc ions from 1a and incorporation of iron(III) ions was carried
out. Iron porphyrins are well known as oxidation catalysts and
within the cyclic porphyrin.
Assignment of the allylpalladium species was performed by
1
1
1
using the coupling constants in the H NMR, H– HCOSY,
NOESY, and HMQC spectra, as shown in Figure S36 in the Sup-
porting Information. Both NMR spectroscopy and ESI-MS data
[
19]
as a model for cytochrome P450. Treatment of 1a with tri-
clearly show the formation of the [1a–(allylPd) ] complex
fluoroacetic acid (TFA) and concentrated H SO in dry CHCl at
3
2
4
3
(
Scheme 2). These complexation experiments indicate that the
08C, followed by neutralization with NaHCO gave the free-
3
three bipyridyl parts can work as bidentate bis(nitrogen) li-
gands.
base porphyrin 6a. The structure of 6a was confirmed by
means of NMR spectroscopy (Figures S37 and S38 in the Sup-
porting Information) and high-resolution MALDI-TOF MS analy-
sis (Figures S39 and S40 in the Supporting Information). The
D -symmetric structure was observed in the NMR spectrum.
3
h
Treatment of [Fe(CO) ] and iodine with 6a in toluene, followed
5
by ligand exchange with a 1m solution of HCl, gave corre-
sponding iron(III) C-3mer 7a (Scheme 3). Structural analysis
was performed by UV/Vis spectroscopy and MALDI-TOF MS, as
shown in Figures S41 and S42 in the Supporting Information.
This observation suggests that the C-3mers 1 are precursors
for cyclic tris(metallo)porphyrin derivatives.
Conclusion
The one-step synthesis of D -symmetric cyclic porphyrin tri-
3
h
mers 1 was achieved from a nickel-mediated reductive cou-
pling of bis(chloropyridyl)porphyrinatozinc 2. Although cyclic
trimers 1 were obtained as a mixture that contained other
cyclic and acyclic porphyrin oligomers, cyclic trimers 1 could
3
Scheme 2. Complexation of an h -allylpalladium complex with C-3mer 1a.
Chem. Eur. J. 2015, 21, 11745 – 11756
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a cooperative effect to form a host–guest complex is unique.
The two different separation mechanisms for C-3mer observed
for group B and C columns will be applicable for the isolation
of other macrocyclic compounds, and a combination of meth-
ods will allow new strategies to be pursued to prepare various
macrocycles.
Experimental Section
Synthesis of 1a
[
Ni(cod) ] (174 mg, 0.63 mmol) and 2,2’-bpy (101 mg, 0.65 mmol)
2
were placed in a 200 mL of Schlenk flask under a flow of Ar. Dry
DMF (60 mL) was added to the mixture, and the mixture was
stirred for 5 min at 258C to give a dark-purple solution. Porphyrina-
tozinc 2a (30.7 mg, 0.032 mmol) and dry DMF (29 mL) were added
to the mixture and stirred under argon for 16 h at 508C. CHCl3
(
20 mL) was added to the mixture, which was washed with an
aqueous solution of citric acid (0.3m, 80 mL) four times. Subse-
quently, the organic layer was washed with an aqueous solution of
NH OH (25 %, 20 mL) three times. The organic layer was filtered
4
through a membrane filter (Omnipore, 0.1 mm), and the filtrate was
concentrated to give a purple solid (33.3 mg). The crude mixture
was purified by Cyanogel (Yamazen YFLC Gel, 40 mm dp, 60 Š
pore) column chromatography with a mixture of toluene and pyri-
dine (15:85 v/v) as the eluent to give 1a (5.5 mg). This sample was
recrystallized from 2-butanone and methanol to afford pure 1a
Scheme 3. Demetalation of 1a (R=Mes) into free-base C-3mer 6a and incor-
poration of iron(III) ion into 6a to give iron(III) C-3mer 7a.
1
13
(
3.7 mg, 13.1 %). All of the H and C NMR signals in CDCl were
3
assigned as shown in Figure 7. The 1D and 2D NMR data are
shown in Figure 6 and Figures S11–S14 in the Supporting Informa-
tion. MALDI-TOF-MS (dithranol): m/z calcd for [C156H120N O Zn +
18
12
3
be isolated efficiently from the mixtures by using cyanopropyl-
modified silica gel column chromatography. Structural analysis
of cyclic trimers 1 was carried out by using NMR spectroscopy
and X-ray crystallography. As a result, in solution, the cyclic
trimers were found to assume an apparent D -symmetric mac-
+
+
Na] [M+Na] : 2651.7100; found: 2651.7112 (Figure S15 in the
Supporting Information); UV/Vis (pyridine): l
(e)=432.2 (7.0”
max
À1
5
4
4
À1
1
0 ), 564.2 (4.3”10 ), 610.2 nm (1.3”10 m cm ).
3h
Allylpalladium complex 1a–(allylPd)3
rocyclic structure, in which three bipyridyl parts were directed
toward the center and six carboxylic ester groups existed out-
3
A stock solution of h -allylpalladium tetrafluoroborate complex was
3
3
prepared from h -allylpalladium chloride dimer (3.9 mg, 11 mmol)
side the ring. Treatment of an h -allylpalladium complex with
and silver tetrafluoroborate (7.3 mg, 37 mmol) in chloroform
cyclic trimer 1a gave tris(palladium) complex 1a–(allylPd)3,
which indicated that the bipyridyl moieties inside the ring
could work as bidentate metalloligands. The zinc porphyrin
parts also worked as tetradentate metallo ligands of an iron
porphyrin complex after acidic demetalation of the zinc ions.
These demonstrations show that cyclic trimers 1 are versatile
macrocyclic ligands for multicofacial multimetallic complexes.
Unique catalytic activity for multimetallic sites within the re-
stricted approximately one-nanometer-sized space is expected.
From the effect of the solvents chloroform and pyridine on
PYE columns and the results from NMR spectroscopy titration
experiments of C-3mer 1b with pyrene, the following interest-
ing relationship between chloroform, pyridine, and pyrene in
C-3mer was revealed. Although pyrene molecules alone could
not push out chloroform molecules held in C-3mer, this
became possible with the assistance of a coordinating solvent,
such as pyridine. The coordination of pyridine with zinc ions in
the C-3mer overcame the interaction of chloroform molecules
inside the C-3mer, and the exchange rate of the coordination
bond between the inner and outer moieties was rapid. Then,
pyrene molecules could be inserted inside the C-3mer. Such
(
(
0
1.0 mL) at 08C. This stock solution was added portionwise
0.5 equiv each up to 6 equiv) to a solution of 1a (1.0 mg,
.38 mmol) in degassed CHCl (1.0 mL). After 15 min of stirring for
3
each addition, each aliquot was diluted 1000 times to give a solu-
tion of approximately 0.38 mm in chloroform. UV/Vis and fluores-
cence spectra of the diluted sample were collected (see text and
Figure 10). A mixture of 1a and h -allylpalladium tetrafluoroborate
complex (6 equiv) was analyzed by ESI-MS and H NMR spectrosco-
3
1
py (see text and Figure 11 b and Figure S35 in the Supporting Infor-
mation).
Demetalation of zinc porphyrin 1a to free-base porphyrin
6
a
TFA (2.0 mL) was added dropwise to a solution of 1a (5.0 mg) in
dry chloroform (2.0 mL) at 08C to give a green solution. Subse-
quently, concentrated H SO (0.2 mL) was added dropwise to give
a pale-brown suspension. The reaction mixture was added to a sa-
turated solution of NaHCO to maintain pH 8–9. The organic layer
was extracted with chloroform, dried over anhydrous Na SO , and
concentrated under reduced pressure. The residue was purified by
flash column chromatography on a short SiO column to give free-
base C-3mer 6a (4.0 mg) as a brown solid. H NMR (300 MHz,
2
4
3
2
4
2
1
Chem. Eur. J. 2015, 21, 11745 – 11756
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CDCl ): d=9.09 (d, 2H, J=1.4 Hz), 8.87 (d, 2H, J=1.4 Hz), 8.57 (d,
125, 7354–7359; u) J. Hong, K. E. Djernes, I. Lee, R. J. Hooley, F. Zaera,
ACS Catal. 2013, 3, 2154–2157; v) J. Liu, L. Chen, H. Cui, J. Zhang, L.
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Toste, K. N. Raymond, R. G. Bergman, J. Am. Chem. Soc. 2014, 136,
3
4
2
H, J=4.6 Hz), 8.36 (d, 4H, J=4.6 Hz), 7.07 (brs, 2H), 6.85 (brs,
H), 4.13 (s, 6H), 2.55 (s, 6H), 1.73 (s, 6H), 1.01 (s, 6H), À3.17 ppm
1
3
(
(
(
s, 2H); C NMR (75 MHz, CDCl ): d=166.1 (C=O), 162.9 (C), 156.5
C), 139 (broad), 138.3 (C), 137.4 (C), 136.9 (C), 131 (broad), 128.5
CH), 127.6 (CH), 121.1 (CH), 118.3 (C), 116.9 (C), 53.2 (CH3),
3
14409–14412.
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1.5 ppm (CH3); MALDI-TOF-MS (dithranol): m/z calcd for
+
+
[
C156H126N O +H] , [C
H
N O +Na] : 2443.9875, 2465.9695;
18
12
156 126 18 12
+
+
found: 2443.9812 [M+H] , 2465.9697 [M+Na] ; UV/Vis (CHCl ):
3
lmax =419.9, 516.5, 591.4, 647.5 nm.
Synthesis of iron(III) porphyrin 7a
[
Excess I2 (76.5 mg) was added to a solution of 6a (6.5 mg,
2
.7 mmol) in toluene (10.0 mL) under Ar. To the mixture, an excess
of [Fe(CO) ] (40 mL) was added. After 5 min of stirring at RT, the
5
mixture was heated at 608C for 1 h under an Ar atmosphere. At
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8C, a 1m solution of HCl was added to the mixture, and the mix-
115, 28–126.
ture was stirred for 5 min. The organic layer was washed with dis-
tilled water and brine, dried over anhydrous Na SO , and concen-
trated under reduced pressure to give 7a (6.7 mg). This sample
was analyzed by MALDI-TOF MS and UV/Vis spectroscopy, as
shown in Figures S42 and S43 and Figure S41 in the Supporting In-
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3
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4
20.2 (0.435), 515.0 (0.077), 580.0 (0.026), 670.0 nm (0.014).
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[
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A.S. acknowledges financial support by KAKENHI (24510140)
from the Japan Society for the Promotion of Science (JSPS).
Keywords: column chromatography
·
macrocycles
·
multinuclear complex · porphyrinoids · synthesis design
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Received: May 12, 2015
Published online on July 14, 2015
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim