Capped subporphyrins

Article abstract of DOI:10.1002/chem.200900879

Capped subporphyrins 12-16 with C₃ molecular symmetry were synthesized from 5,10,15-tri(3-aminophenyl)-substituted subporphyrin 8 and tripodal trialdehydes 2-6 by Lindsey's entropically favored macrocyclization. Xray diffraction analysis has re

Full text of DOI:10.1002/chem.200900879

FULL PAPER
DOI: 10.1002/chem.200900879
Capped Subporphyrins
Yasuhide Inokuma and Atsuhiro Osuka*[a]
Abstract: Capped subporphyrins 12–16
with C3 molecular symmetry were syn-
thesized from 5,10,15-tri(3-aminophen-
yl)-substituted subporphyrin 8 and tri-
podal trialdehydes 2–6 by Lindseyꢀs en-
tropically favored macrocyclization. X-
ray diffraction analysis has revealed
that the concave surface of the subpor-
phyrin core is selectively capped with a
13 exhibit tight structures, in which the
cap and subporphyrin core are found
much closer with average interplanar
separations of 3.56 and 3.15 ꢁ, respec-
tively. Variable-temperature 1H NMR
measurements revealed that subpor-
phyrins 12, 13, and 16 undergo spiral
Of these, subporphyrin 13, with a 1,3,5-
tri(alkoxycarbonyl)benzene cap strap-
ped by three-atom arms, exhibits a con-
siderably slow spiral interconversion
with a large enthalpy change of DHꢀ =
76.4 kJmolÀ1 and a characteristic red-
shift of the Soret-like band and en-
interconversions between
P
and
M
hancement of the QACTHNGUETRNNU(G 0,0) band. These
forms depending on the arm length
and the electronic nature of the cap.
properties are ascribed to considerable
through-space charge-transfer interac-
tions between the electron-deficient
cap and the subporphyrin core and the
1,3,5-substituted
benzene
moiety.
Capped subporphyrins 15 and 16, with
a five-atom arm and thus large inner
cavities, exhibit solvent incorporation
behavior in their crystal structures. On
the other hand, subporphyrins 12 and
Keywords: cage compounds · por-
phyrinoids · spiral interconversions ·
substituent effects
À
multiple CH p interactions.
Introduction
the meso positions of subporphyrin to tune the electronic
properties of the subporphyrins.[4–7] For example, elongation
of the p-electronic network by substitution of oligo(p-phe-
nyleneethynylene) arms resulted in the intensification and
redshift of the Soret-like band and the enhancements in
both the fluorescence quantum yield and the two-photon ab-
sorption (TPA) cross section.[5] Substitution with the 4-(N,N-
dialkylamino)phenyl group caused strong intramolecular
charge-transfer interactions, which gave substantial pertur-
bation on the absorption and fluorescence spectra of subpor-
phyrin.[6] Considerable through-bond interactions between
two subporphyrin cores were observed for the 4,4’-bipheny-
lene-bridged dimer in the ground state, but not for the 3,3’-
homologue.[7] On the other hand, chemical modulations at
the b positions provide large electronic impacts on the sub-
porphyrin macrocycle. Hence, hydrogenation of one b=b
double bond of subporphyrin yielded meso-aryl subchlorin,
which displays a blueshifted and broadened Soret-like band
and redshifted and intensified Q-like bands.[8] A hexabromi-
nation–Stille coupling strategy provided hexaethynylated
subporphyrins in high yields, which exhibited large TPA
cross sections up to 2800 GM.[9] Importantly, most of these
attractive properties are very difficult or almost impossible
for porphyrins in which peripheral substituents are arranged
in an almost orthogonal manner.
The chemistry of subporphyrin, which is a genuine contract-
ed porphyrin with a 14p-aromatic macrocycle, began with
our discovery of tribenzosubporphines in 2006.[1] The follow-
ing years have witnessed the exploration of an increasing
number of subporphyrins and their related compounds with
rich optical, electrochemical, and structural properties.[2–9]
Among them, meso-aryl-substituted subporphyrins[4] have
been found to be particularly interesting in light of their
highly tunable optical and electrochemical properties by
large meso-aryl substituents. These features mainly arise
from free rotation of the 2,6-unsubstituted aryl substituents
at meso positions, which allow effective conjugative interac-
tions between the meso-aryl groups and the subporphyrin.
Similarly, various functional groups have been introduced to
[a] Dr. Y. Inokuma, Prof. Dr. A. Osuka
Department of Chemistry, Graduate School of Science
Kyoto University, Sakyo-ku, Kyoto 6068502 (Japan)
Fax : (+81)75-753-3970
E-mail: osuka@kuchem.kyoto-u.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900879.
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Subporphyrins adopt a bowl-shaped triangular structure,
which is common for subphthalocyanines.[10] Recently, such
bowl-shaped p-conjugated systems have attracted increasing
attention in light of curved aromatic systems, unique recog-
nition abilities, and partial structural components of ful-
lerenes and carbon nanotubes.[11] Whereas the representative
bowl-shaped aromatic hydrocarbon systems, such as suma-
nenes and corranulenes, are known to undergo bowl-to-bowl
flipping,[12] none such dynamic motions have been observed
for subporphyrins or subphthalocyanines, which suggests
their structural robustness. We thought that this structural
feature of subporphyrins should be advantageous for fabri-
cation at the concave face, whereas such a face-selective fab-
rication has been quite rare both for subporphyrins and sub-
phthalocyanines. Rare examples include 1) a chirally dis-
criminating assembly of subphthalocyanines, which was re-
ported to provide a dimeric cage with linkages at the con-
cave face[13] and 2) a boron axial-ligand exchange reaction
used to construct complementary face-to-face subporphyrin
dimers with linkages at the convex face.[14]
Herein, we report the synthesis and characterization of
capped subporphyrins in which a 1,3,5-trisubstituted ben-
zene cap is connected to a subporphyrin core at the concave
side. Whereas there are many examples of capped porphyr-
ins in the literature,[14–16] there are no such precedents for
subporphyrin. It can be expected for capped subporphyrins
that 1) intramolecular through-space p–p interactions be-
tween the concave face and the 1,3,5-trisubstituted cap is
possible, 2) a guest molecule of a suitable size and shape can
be encapsulated into a cavity created by the subporphyrin
core and cap, and 3) such a subporphyrin cap restricts free
rotation of the meso-aryl substituents, hence influencing the
electronic interaction of the subpoprhyrin core with the
meso-aryl substituents. During
linkage.[16,17] First, we attempted the direct method for the
synthesis of capped subporphyrins. To preserve C3 molecular
symmetry of the subporphyrin core, 1,3,5-trisubstituted ben-
zene moiety was employed as a cap model. Scheme 1 shows
tripodal aryl aldehydes 1–6 used for the direct synthesis. The
arm length is defined by the number of atoms between the
central benzene moiety and the peripheral benzaldehyde
unit; this is also indicated in Scheme 1.
An equimolar mixture of pyridine-tri-N-pyrrolylborane
(7) and a tripodal aryl aldehyde was treated with trifluoro-
acetic acid (TFA) in 1,2-dichlorobenzene at 08C. After the
acid catalyst was quenched with pyridine, the solution was
heated at reflux under aerobic conditions. However, this
standard reaction for meso-aryl-substituted subporphyrins
did not yield capped subporphyrin. Therefore, we examined
the stepwise approach. One of the most efficient stepwise
synthetic protocols of capped porphyrin was developed by
Lindsey and Mauzerall, who employed entropically con-
trolled macrocyclization between the a,a,a,a-atropisomer of
meso-tetrakis(2-aminophenyl)porphyrin and
a tetraalde-
hyde.[17] As such, the stereocontrolled a,a,a,a-atropisomer
was required for the synthesis of capped porphyrins because
of restricted rotation of the meso-aryl substituents in por-
phyrins. It was thought, however, that such a care would not
be obligatory in the application of this strategy to subpor-
phyrins owing to free rotation of the meso-aryl substituents.
We thus examined a similar macrocyclization reaction for
meso-tri(3-aminophenyl)-substituted subporphyrin 8 and tri-
podal aldehydes 1–6.
Subporphyrin 8 was first synthesized from meso-tri(3-ni-
trophenyl)-substituted subporphyrin
9 by means of an
SnCl2·2H2O reduction in a mixture of EtOH and hydrochlo-
ric acid. Although the reduction of three nitro groups of 9
this work, we confirmed all of
these expectations and addi-
tionally found that spiral inter-
conversions of capped subpor-
phyrins depend upon the length
of bridges that connect the sub-
porphyrin core and cap and
upon the electronic nature of
the cap.
Results and Discussion
Synthesis: There are two syn-
thetic approaches to capped
porphyrins: One is the direct
synthesis of a porphyrin macro-
cycle from pyrrole and a cova-
lently linked tetraaldehyde.[15]
The other is a stepwise synthe-
sis that consists of the initial
porphyrin synthesis and subse-
quent capping with quadruple Scheme 1. Structures of tripodal aryl aldehydes 1–6. The arm lengths are also indicated.
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Subporphyrins
FULL PAPER
proceeded quantitatively, the very poor yield of 9 (less than
1%) posed a serious obstacle for further investigation. As
an alternative route, meso-tris(3-bromophenyl)-substituted
subporphyrin 10 was prepared from 7 and 3-bromobenzalde-
hyde in 4.9% yield under the standard conditions.[4c] Subpor-
phyrin 10 was coupled with benzamide in the presence of
sharp singlet at d=8.13 ppm due to the six b protons and a
single set of signals due to the 3-aminophenyl groups (a sin-
glet at d=7.41 ppm, two doublets at d=7.40 and 6.92 ppm,
and a triplet at d=7.45 ppm), indicating free rotation of
meso-3-aminophenyl substituents in solution.
The stepwise capping reaction was first examined for 8
and trialdehyde 1. These two components were condensed
under various reaction conditions in the presence of an acid
catalyst. However, the 1H NMR spectrum of the reaction
products showed complicated signal patterns, and no evi-
dence was obtained for the expected capped subporphyrin.
Although two intense cation peaks at m/z 911.3 and 929.1,
which can be assigned to borenium cations of the doubly
linked dyad [8+1ÀOMeÀ2H2O]+ and the singly linked
[Pd2ACHTUNGTRENNUNG(dba)3], Cs2CO3, and Xantphos under the reaction con-
ditions reported by Yin and Buchwald to furnish meso-
tris(3-benzamidophenyl)-substituted subporphyrin 11 in
82% yield.[18] The amido groups in 11 were readily hydro-
lyzed with NaOH in a mixture of EtOH/THF/water to give
subporphyrin 8 in 92% yield. Hence, the overall yield of 8
from 7 was 3.7% yield over 3 steps (Scheme 2). The
1H NMR spectrum of subporphyrin 8 in CDCl3 exhibits a
dyad
[8+1ÀOMe-H2O]+
(C63H44BN6O=911.37
and
C63H46BN6O2 =929.38), respec-
tively, were observed in the
MALDI-TOF mass spectrum of
the reaction mixture, signals
due to the triply linked Schiff-
base cage could not be record-
ed. These results indicated that
the arm length of 1 was too
short to link three meso-aryl
substituents of 8. Next, the cap-
ping reaction of 8 and 2 was ex-
amined. Compound 2 (1 equiv)
in a small amount of CH2Cl2
was added to a solution of 8 in
a
mixed solvent of CH2Cl2/
iPrOH (v/v=4:1) in the pres-
ence of TFA (5 equiv) and the
solution was stirred at room
temperature for 1 day. MALDI-
TOF mass analysis of the reac-
tion solution exhibited an in-
tense cation signal at m/z 941.5,
which corresponded to [8+
2ÀOMeÀ3H2O]+ (i.e., [12-
SchiffÀOR]+; C63H42BN6O3 =
941.34). Without purification,
this Schiff base cage was re-
duced with NaBH3CN to give
capped subporphyrin 12 in
80% yield (Scheme 3). Under
the similar reaction conditions,
tripodal aldehydes 3, 4, 5, and 6
also afforded corresponding
capped subporphyrins 13, 14,
15, and 16 in 59, 58, 85, and
70%
yields,
respectively
(Scheme 4).
It is noteworthy that capping
reactions at high concentrations
of
8 and tripodal aldehydes
Scheme 2. Synthetic routes to meso-tris(3aminophenyl) substituted subporphyrin 8. dba=dibenzylideneacetone. (ꢀ0.5 mm) resulted in the for-
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A. Osuka and Y. Inokuma
Scheme 3. Synthesis of capped subporphyrin 12.
mation of orange precipitates of polymeric materials and
the high yields of capped subporphyrins 12–16 were accom-
plished only in homogeneous solutions under dilute condi-
tions (60 mm) without isolation of the Schiff bases. High-res-
olution electrospray ionization (HR-ESI) mass spectra of
capped subporphyrins 12–16 exhibited intense borenium
cation signals at m/z 947.3884, 1031.3736, 1037.4205,
1079.4680, and 1121.4045, which correspond with the calcu-
Three meso-aryl rings of the subporphyrin tilt in the same
direction, thus forming a helical structure. In the unit cell,
two pairs of P and M spiral forms were found to construct a
racemic crystal. Interestingly, the dihedral angles between
the meso-aryl ring and the mean plane defined by six b
carbon atoms are 52.1, 62.8, and 68.68, which are distinctly
larger than those of 2,6-unsubstituted aryl rings on subpor-
phyrins, which are generally found to be in the range of 40–
558. This is explained in terms of the capping effect, which
does not allow free rotation of the meso-aryl substituents,
but rather forces them to adopt larger dihedral angles
(Figure 1).
A single crystal of 13 suitable for X-ray analysis was
grown from a mixture of DMSO/MeOH. The crystal struc-
ture of 13 also shows a spiral conformation similar to 12.
The tilting angles of the meso-aryl substituents are observed
in the range of 43.1–54.68 with respect to the six b carbon
mean plane. The average distance between the b carbon
lated
values of [MÀOMe]+ =947.3886, 1031.3733,
1037.4203, 1079.4673, and 1121.4051, respectively.
X-ray crystallographic analysis: The solid-state structures
were unambiguously determined by X-ray diffraction analy-
sis for capped subporphyrins 12, 13, 15, and 16. The bowl-
shaped subporphyrin core of 12 was capped with the 1,3,5-
triphenoxymethylbenzene moiety at the concave side. The
average distance from the mean plane of the aryl ring of the
cap to the b carbon atom of the subporphyrin is 3.56 ꢁ.
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Subporphyrins
FULL PAPER
Scheme 4. Synthesis of capped subporphyrins 13–16.
mean plane and the aryl ring in the cap is only 3.15 ꢁ,
which is the shortest in the series, although the arm length
of 13 (3 atoms) is longer than 12 (2 atoms). Such a remarka-
bly short interplanar distance suggests considerable through-
space p–p interactions between the subporphyrin core and
the cap. Although there are many examples of convex-to-
plane- or concave-to-convex-type p–p interactions in the
solid state seen in fullerene–porphyrin cocrystals,[19] and
belt- and bowl-shaped hydrocarbons,[20] such concave-to-
plane-type p–p interactions are rather rare.[21] This capped
subporphyrin model might be a good platform to evaluate
suitable interplanar separation or orientation of p-
Capped subporphyrin 15 was recrystallized from a mix-
ture of CHCl3/MeOH to afford single crystals of good quali-
ty (Figure 2A). The crystal structure of 15 also shows a con-
cave-capped spiral structure. Two chloroform molecules
used as the solvent were found close to 15 in the crystal lat-
tice. Interestingly, one is placed at the center of the inner
cavity of 15, whereas the other is located between two
meso-appended arm units. The chloroform molecule inside
the cavity is orientationally disordered and the major orien-
tation (62% occupancy) is shown in Figure 2A. In the other
form, the chloroform molecule is arranged upside down
with an occupancy of 38%. Owing to the long arm (5
atoms) and the chloroform clathration, the inner cavity of
15ꢁCHCl3 is enlarged to be 6.7 ꢁ in height.
ACHTUNGTRENNUNG(concave)–pACHTUNGTRENNUNG(plane) interactions. Furthermore, in the helical
structure of 13, each of the three 1,2-phenylene rings in the
À
bridging arms is located close to a b-C H position. The dis-
A cubic crystal of 16 was obtained from a mixed solvent
system of CH2Cl2/mesitylene/MeOH (Figure 2B). In the
crystal structure, the axial methoxo ligand of 16 was hydro-
lyzed into the hydroxo form (i.e., 16-OH) by adventitious
water. Capped subporphyrin 16-OH also exhibits a concave-
tances from the mean plane of the 1,2-phenylene ring to the
nearest b hydrogen atom are in the range of 2.59–2.75 ꢁ
and those to the nearest b carbon atom is in the range of
À
3.41–3.52 ꢁ, indicating intramolecular CH p interactions.
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A. Osuka and Y. Inokuma
tion of excess amounts of mesi-
tylene in CDCl3 at room tem-
perature. However, the proton
signals due to 16 did not under-
go any specific shift, even in the
presence of
a large excess
amount of mesitylene. We also
attempted 1H NMR, UV/Vis
absorption, and fluorescence ti-
tration analyses with 15 and 16
in the presence of C3 symmetric
guests, for example, 1,3,5-trime-
thoxybenzene, 1,3,5-tribromo-
benzene, chloroform, and tri-
phenylphosphane, but, none of
them exhibited a particular af-
finity towards the cavities.
These results suggested that the
affinity and selectivity of these
guest molecules toward the
inner cavities of 15 and 16 in
solution were poor. Neverthe-
less, the solid-state structures of
15ꢁCHCl3 and 16-OHꢁmesity-
lene indeed demonstrated the
encapsulating ability of capped
subporphyrins,
encouraging
their use as a host for a guest
with C3-molecular symmetry. In
addition, we have recently dis-
closed characteristic structural
changes for the meso-[4-(N,N-
Figure 1. X-ray crystal structures of A) 12 and B) 13.
dibenzylaminophenyl)]-substi-
tuted subporphyrins, in which
capped, helical structure with pseudo-C3 molecular symme-
slight but distinctive benzoquinone-type bond length alter-
nations have been observed for meso-attached benzene
rings.[6] However, such remarkable bond length discrimina-
tions are not confirmed for capped subporphyrins 12, 13,
15ꢁCHCl3, and 16-OHꢁmesitylene, which suggests little in-
fluence of 3-aminophenyl groups on the electronic proper-
ties of the subporphyrin core in the ground state.
try[22] along the B O bond axis. In this crystal structure, one
À
mesitylene molecule is precisely packed in the cavity of 16-
OH, and three mean planes of the aryl ring of the cap, mesi-
tylene, and six b carbon atoms of the subporphyrin are ar-
ranged in a parallel manner. The interplanar distances are
3.46 ꢁ between the aryl ring of the cap and mesitylene and
3.16 ꢁ between the six b carbon mean plane and mesitylene,
suggesting favorable p–p stacking between the three aro-
matic segments. Interestingly, the interplanar distance be-
tween the mesitylene and the subporphyrin core in 16-OH is
quite similar to that between the aryl ring of the cap and
the subporphyrin core in 13, which might suggest a suitable
interplanar separation of around 3.15 ꢁ for the favorable p-
1
1H NMR spectroscopy: The H NMR spectrum of 12 shows
a C3-symmetric signal pattern featuring two singlets at d=
5.77 and 7.52 ppm due to the aryl protons of the cap (Ha,
designated in Scheme 3) and the b-pyrrolic protons at room
temperature, both of which are significantly high-field shift-
ed compared with those of the fragment molecules 2 and 8.
On the basis of the crystal structure, the former high-field
shift can be ascribed to the diatropic ring-current effect of
the 14p-aromatic subporphyrin core and the latter is under-
standable in terms of the local ring current of the 1,2-phen-
ylene moiety of the bridging arm. Similar spectral patterns
were observed for 14, 15, and 16 in CDCl3, in which the pyr-
ACHTUNGTRENNUNG(concave)–pACHTUNGTRENNUNG(plane) stacking of capped subporphyrins. The
inner cavity height of 16-OHꢁmesitylene is thus 6.6 ꢁ,
which is similar to that of 15ꢁCHCl3. The dihedral angles of
the meso-aryl rings of 15ꢁCHCl3 are in the range of 45.4–
60.28 and those of 16-OHꢁmesitylene are 51.38 each with
respect to the six b carbon mean plane, which are slightly
smaller than those of 12. To evaluate the affinity of mesity-
lene for the cavity of capped subporphyrin 16 in solution, a
1H NMR titration experiment was carried out by the addi-
rolic
b protons were observed at d=7.78, 7.28, and
7.27 ppm, respectively, as a sharp singlet. The aryl protons
(Ha) of the cap are observed at d=5.54, 5.51, and 8.40 ppm
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Subporphyrins
FULL PAPER
Figure 2. X-ray crystal structures of capped subporphyirn A) 15ꢁCHCl3 and B) 16-OHꢁmesitylene. Only the major orientation of disordered chloroform
molecule inside the cavity is shown for (A).
for 14, 15, and 16, respectively, with high-field shifts com-
pared with those of 4, 5, and 6 observed at d=6.19, 6.09,
and 8.87 ppm. These data indicated the influences of the di-
atropic ring currents, similar to the case of 12. Such shield-
ing effects for the cap proton Ha decreased in the order of
12@14>15>16 (Dd=1.75, 0.65, 0.58, and 0.47 ppm, respec-
tively), probably reflecting the average distance between the
cap and the subporphyrin core. With the solid-state struc-
pairs of split two peaks at d=7.17 and 6.00, 4.73 and 4.70,
and 5.48 and 5.03 ppm, respectively. In particular, the signals
due to Hk displayed a clear geminal coupling feature with
J
gem =10.1 Hz (Figure 3A). The large difference in chemical
shifts of the two b-proton signals indicated their magnetical-
ly nonequivalent environments, hence supporting a C3-sym-
metrical helical structure of 13 in solution, similar to that
found in the crystal structure. The high-field shifted signal at
d=6.00 ppm has been assigned to the b protons close to the
1,2-phenylene rings in the bridge (magenta in the top view
of Figure 1B). Similarly, separated signals due to Hf and Hk
can be explained in terms of a spiral conformation of 13.
The NOESY spectrum of 13 in [D6]DMSO at room temper-
ature also indicates a helical structure, featuring through-
space proton correlations between the cap Ha and meso-aryl
He protons, and the b proton at d=6.00 ppm and Hh or Hi.
In particular, the strong correlation between Ha and He is in-
dicative of close stacking between the aryl ring of the cap
1
tures and the observed single sets of H NMR spectroscopy
signals, the capping reaction has been concluded to occur
with complete concave-face selectivity for 12, 15, and 16.
1
The similar H NMR spectroscopy data of 14 strongly sug-
gested the same concave-capped structure. The concave-cap-
ping selectivity can be readily ascribed to considerable steric
effects exerted by the axially coordinated boron ligand at
the convex face. Generally the solubilities of capped subpor-
phyrins are much lower than 8. Particularly, the solubility of
13 is very poor, which does not allow us to detect a clear
1H NMR spectrum in CDCl3.
1
and the subporphyrin core in solution. These H NMR spec-
tral features of 13 are different from those of 12, 14, 15, and
16, since the b- and benzylic protons in 12, 14, 15, and 16
are recorded as sharp and coalesced signals in CDCl3 at
room temperature (see Figure S1 in the Supporting Informa-
tion), despite their spiral crystal structures. Since subpor-
phyrins 12, 14, 15, and 16 exhibit essentially the same
1H NMR spectral pattern in [D6]DMSO, showing sharp b-
proton signals at d=7.32, 7.47, 7.29, and 7.17 ppm, respec-
tively, and coalesced benzyl protons due to Hf and Hk, the
solvent effect would be marginal. It can be considered that
Spiral interconversion: VT-NMR spectroscopy: In the mean-
time, we found that 13 was reasonably soluble in DMSO
1
and then examined a H NMR spectrum in [D6]DMSO. At
1
room temperature, the H NMR spectrum of 13 exhibited a
C3-symmetric signal pattern, featuring a singlet due to the
aryl cap Ha proton at d=6.56 ppm, which undergoes the
largest high-field shift in the series (Dd=2.14 ppm) com-
pared with trialdehyde 3. Interestingly, signals due to the b
and benzyl protons Hf and Hk (see Scheme 4) resonate as
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A. Osuka and Y. Inokuma
tons was observed at d=À0.03
to 0.08 ppm in the same signal
intensity almost independently
of the temperature. Thus, the
observed dynamic motion of 13
is neither bowl-to-bowl inver-
sion nor an axial-ligand ex-
change reaction, but intercon-
version of the helical structures
with
temperature-dependent
exchange rates. The exchange
rates of 13 were estimated to be
k=14, 90, 510, 2050, 9000, and
40000 sÀ1 at 25, 40, 60, 80, 100,
and 1208C, respectively, by sim-
ulating the NMR spectroscopy
signal patterns for the benzyl
proton Hk.
In the cases of 12 and 16,
similar spectral changes were
observed in CD2Cl2 upon de-
creasing
the
temperature
(Figure 4 and the Supporting
Information). The capped sub-
porphyrin 12 exhibited sharp
signals due to the b-, benzyl-Hf
and Hk protons at d=7.48, 4.41,
and 3.89 ppm, respectively, at
258C in CD2Cl2. These peaks
were separated into three pairs
of two broad signals at À758C
as shown in Figure 4A. The six
b protons of 12 resonated at
d=7.73 and 7.12 ppm at
Figure 3. a) 600 MHz VT-1H NMR spectra of 13 in [D6]DMSO (*: solvent peaks), b) simulated 1H NMR spec-
tra for Hf, and c) interconversion between the P and M spiral forms.
rotational freedom of the meso-aryl groups is considerably
suppressed in 12–16 due to the cap that connects to three
meso-aryl substituents. Thus, the unique 1H NMR spectral
features of 13 may be assigned to a small exchange rate of
two spiral conformations, that is, P and M spiral (Figure 3C)
with respect to the 1H NMR timescale, whereas such ex-
À908C, each signal corresponded to three protons. The
1H NMR spectroscopy simulations were performed for Hf
and Hk in the range from À90 to À158C, which allowed the
estimation of the temperature-dependent exchange rate con-
stants as k=100 to 35000 sÀ1 (Figure 4B). Signals due to the
b-, Hf, and ethylene chain protons of 16 also exhibited sig-
nificant broadening at À708C in CD2Cl2. Finally at À908C,
the signals of the b protons were divided into two parts at
d=7.25 and 6.81 ppm, which were well reproduced by the
simulated spectra with k=80 sÀ1 (Figure S3 in the Support-
ing Information).
The exchange rate constants of 12, 13, and 16 were ana-
lyzed by Eyring plots, in which these capped subporphyrins
displayed good linear correlations (Figure 5). The activation
parameters of 13 in DMSO were calculated to be DHꢀ =
76.4 kJmolÀ1 and DSꢀ =34.5 JmolÀ1 KÀ1, hence, the energy
barrier of the interconversion between the P and M spiral
forms was estimated to be DGꢀ298 =66.1 kJmolÀ1 at 258C.
Table 1 summarizes the activation parameters thus calcu-
lated for 12, 13, and 16. Interestingly, a negative value of
À45.7 JmolÀ1 KÀ1 was obtained for the entropy change of 12,
while the other two subporphyrins exhibited positive DSꢀ
values. This can be simply explained in terms of the short
1
change rates are likely to be larger than the H NMR time-
scale for the other capped subporphyrins 12, 14, 15, and 16.
To verify this hypothesis, variable-temperature (VT)
1H NMR spectra of 12, 13, and 16 were examined. Upon in-
creasing the temperature, the split signals due to the b-, Hf,
and Hk protons of 13 indeed showed significant peak broad-
ening and finally coalesced at 1208C into sharp singlets at
d=6.71 and 5.28 ppm due to the b- and Hf protons, and a
doublet (JACHTUNGTRENNUNG
(CH2,NH)=5.5 Hz) at 4.75 ppm due to Hk
(Figure 3). The other proton signals, such as Ha–He, and Hg–
Hj, did not exhibit any characteristic shift or coalescence be-
havior and the total molecular symmetry of C3 was pre-
served in a range from 25 to 1408C. The temperature-depen-
dent spectral changes of the b-, Hf, and Hk protons are re-
versible and the original spectrum of 13 was completely re-
covered upon cooling to room temperature. During the
measurement, a sharp singlet due to the axial methoxo pro-
6870
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Chem. Eur. J. 2009, 15, 6863 – 6876

Subporphyrins
FULL PAPER
ture also indicate its structural
tightness. The largest DHꢀ and
the energy barrier DGꢀ were
298
obtained for 13. Considering
the crystal structure, through-
space interactions between the
aryl ring of the cap and the sub-
porphyrin core along with char-
acteristic intramolecular CH–p
interactions presumably play an
important role in stabilizing the
spiral conformation. It is worth
noting that the DHꢀ value of
subporphyrin 16 (50.4 kJmolÀ1)
is considerably larger than that
of 12 (28.9 kJmolÀ1). Although
the resulting DGꢀ of 16 is the
298
smallest in the series due to its
positive large entropy factor;
this result might suggest favora-
ble interactions between the
1,3,5-tri(alkoxycarbonyl)ben-
zene moiety and subporphyrin
1
Figure 4. a) 600 MHz VT-1H NMR spectra of 12 in CD2Cl2 (*: solvent peaks), and b) simulated H NMR spec-
tra for benzylic proton Hf and Hk.
Table 1. Structural data and activation parameters[a] of spiral motion of
12, 13, and 16.
12
13
16
arm length[b]
aryl cap
2 atoms
3 atoms
5 atoms
tilting angles[c] 52.1
43.1
51.3
[8]
62.8
68.6
28.9
À45.7
46.0
54.6
76.4
34.5
51.3
51.3
50.4
67.9
DHꢀ [kJmolÀ1
DSꢀ
]
[JmolÀ1 KÀ1
]
DGꢀ
43.5
66.1
16
30.2
Figure 5. Eyring plots for capped subporphyrins 12, 13, and 16.
298
[kJmolÀ1
]
k298 [sÀ1
]
2.2ꢃ105
3.2ꢃ107
[d]
[a] Activation parameters were calculated by simulating the VT-1H NMR
spectra of 12 and 16 in CD2Cl2 and 13 in [D6]DMSO. [b] Number of
linker atoms between the aryl group of the cap and the 1,2-phenylene
ring. [c] Dihedral angles between the meso-aryl ring and the mean plane
of six b carbon atoms in the crystal structure. The tilting angle of 16 was
calculated from the structure of 16-OHꢁmesitylene. [d] Exchanging rate
constant between P and M spiral forms at 298 K. These values were cal-
culated based on the Eyring equation.
arm length. In the transition state, it is expected that the
meso-aryl substituents are arranged in an almost perpendic-
ular manner with respect to the subporphyrin mean plane,
and the distance from the aryl ring of the cap to the subpor-
phyrin core is longer than that of the spiral forms. In case of
12, such conformational change results in the formation of a
tight transition structure in which the free motion of the
linker unit is restricted and three meso-aryl rings are forced
to synchronize the tilting behavior due to the short strap. In
contrast, capped subporphyrins 13 and 16, bearing longer
arms, are expected to obtain the degree of freedom in the
transition structures rather than their spiral forms. Such in-
fluences of the arm length on the structure can be seen in
their dihedral angles of meso-aryl rings in the solid state.
Significantly large dihedral angles of 12 in the crystal struc-
core rather than 1,3,5-trialkoxymethylbenzene analogue in
the capped system. In short, the dynamic spiral interconver-
sion can be controlled entropically by arm length and en-
thalpically by intramolecular interactions between the sub-
porphyrin core and the aryl ring of the cap. Such an adjusta-
ble feature of dynamic spiral interconversion is quite impor-
tant both for biological movements[23] and for constructing
Chem. Eur. J. 2009, 15, 6863 – 6876
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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6871

A. Osuka and Y. Inokuma
artificial nanomachines.[24] In this view, capped subporphyr-
ins are also a promising platform owing to their highly tuna-
ble properties.
sorption spectra of the capped subporphyrins 15 and 16,
with five atom arms, much broader and redshifted Soret-like
bands were observed at 382 and 380 nm, respectively. The
fluorescence spectra of 12 and 15 displayed rather distin-
guishable vibronic structures at 507 and 532 nm, and 517
and 544 nm, respectively, whereas 14 and 16 have ill-defined
spectral shapes as well as non-capped subporphyrin 8. These
minor differences in the absorption and fluorescence spectra
can be attributed to the conformational properties, such as
structural rigidity, restriction of the free rotation of meso-
aryl rings, and solvent accessibility into the cavity. The most
significant perturbation was found in the absorption spec-
trum of 13, in which a sharp Soret-like band was recorded at
387 nm with the largest bathochromic shift in the series. Be-
Optical properties: The UV/Vis absorption spectra of sub-
porphyrins 8 and 12–16 recorded in CH2Cl2 are shown in
Figure 6A. Recently, we have reported largely perturbed op-
sides, the QACTHNUGRTNEUNG(0,0) band of 13 also exhibits a characteristic
redshift to 497 nm with concurrent enhancement. Interest-
ingly, the fluorescence spectrum of 13 shows a fine vibronic
structure with band peaks at 515 and 546 nm. The Stokes
shift of 13 thus calculated is 703 cmÀ1, which is the smallest
in the series, indicating its rigid structure in the excited
state. The fluorescence quantum yields of capped subpor-
phyrins are comparable to that of the mother subporphyrin
8 (Table 2). However, those of 1,3,5-tri(alkoxycarbonyl)ben-
zene-capped subporphyrins 13 and 16 are slightly but dis-
tinctively enhanced compared with 8.
Table 2. Optical properties of subporphyrins 8 and 12–16.
Absorption
Soret [nm]
Fluorescence
QAHCTUNTGERN(GUNN 0,0) [nm] lmax [nm] FF [%]
Stokes shift [cmÀ1
]
8
377
489
487
497
487
493
492
525
507
515
515
517
534
12.2
13.2
16.6
13.3
15.6
16.7
1402
810
703
1116
942
1598
12 377
13 387
14 377
15 382
16 380
Figure 6. A) UV/Vis absorption spectra and B) fluorescence spectra of 8
and 12–16 in CH2Cl2. Fluorescence spectra were recorded by excitation
at the absorption maxima (377–382 nm).
tical properties of meso-(4-(N,N-dialkylamino)phenyl)-sub-
stituted subporphyrins[6] which exhibited redshifted absorp-
tion spectra and intensified fluorescence compared with
meso-triphenyl-substituted subporphyrin (17). Contrary to
these 4-aminophenyl-substituted analogues, 3-aminophenyl-
substituted subporphyrin 8 shows only modest changes in
the absorption and fluorescence spectra. This difference can
be attributed to small conjugative interaction of 3-amino-
phenyl substituent. Since the capping reaction can be con-
sidered merely as an N-alkyla-
DFT calculations: DFT calculations were performed for 8,
12, and 13 at the B3LYP/6-31G(d) level by using the Gaussi-
an 03 package (Figure 7).[25] The optimized structures of 12
and 13 were calculated based on their crystal structures
(data shown in Table 3 and the structure of 8 was generated
from the crystal structure of meso-triphenylsubporphyrin 17.
Subporphyrin 8 has four frontier orbitals similar to 17.
Capped subporphyrin 12 also provides a similar four orbital
set, which is indicative of a small direct influence of the cap
on the electronic properties of the subporphyrin core in the
ground state. Unlike 8 and 12, four vacant orbitals were
found for capped subporphyrin 13 in a similar energy range,
along with a couple of degenerated HOMOs. The orbital co-
efficients in the LUMO and LUMO+1 of 13 are localized
at the aryl ring of the cap (Figure 7). LUMO+2 and
LUMO+3 correspond to the unoccupied e orbitals due to
the subporphyrin core. Namely, LUMO levels of the elec-
tron-deficient 1,3,5-tri(alkoxycarbonyl)benzene moiety are
slightly lower than those of meso-(3-aminophenyl)subpor-
phyrin. For comparison, frontier orbital energy levels of the
tion reaction at the meso-substi-
tuted 3-aminophenyl groups, the
direct p-electronic impact of the
cap to the subporphyrin core
should be trivial. In fact, the
spectral shapes of capped sub-
porphyrins 12 and 14 are analo-
gous to that of 8, although slight
broadening of the Soret-like
band was observed. In the ab-
6872
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Chem. Eur. J. 2009, 15, 6863 – 6876

Subporphyrins
FULL PAPER
Figure 7. Molecular orbital (MO) diagrams of 8, 12, and 13. MOs of LUMO and LUMO+1 for 13 are localized on the aryl rings of the cap, therefore,
the orbital diagrams are drawn as bottom views.
cap units of 12–16, that is, 1,3,5-triphenoxymethylbenzene
(18), 1,3,5-trimethoxybenzene (19), and trimethylbenzene-
1,3,5-tricarboxylate (20), were calculated at the B3LYP/6-
31G(d) level (Figure S4 in the Supporting Information). The
energy levels of the HOMOs for 18–20 are considerably
lower than that of subporphyrin 8. Also, the LUMO of 20 is
coincidentally similar in energy to that of 8 because of three
electron-withdrawing ester groups. Although any significant
orbital overlap or hybridization between the cap unit and
the subporphyrin core were not found in the calculated
MOs,[26] such low-lying LUMOs of the cap would be advan-
tageous for through-space p–p interactions with the concave
surface of the subporphyrin core, as observed in the absorp-
tion spectrum of 13. In a previous report, similar absorption
spectral changes have been observed for a capped porphyrin
with an electron-deficient cap.[27] Judging from the experi-
mental and theoretical results, weak incipient charge-trans-
fer interactions can also be expected for capped subporphyr-
in 13. In addition, similar cap-dependent unoccupied orbitals
were found in the LUMO and LUMO+1 of subporphyrin
16. The observed absorption spectrum of 16, however, did
not exhibit characteristic spectral changes compared with
13. It is more likely that the spectral shape of 16 is rather
similar to that of 15 despite the difference in the aryl ring of
the cap. This situation suggests that structural restraints and
dynamic motions in solution based on the arm length are
particularly important to induce efficient through-space in-
teractions between the cap and the core as well as its p-elec-
tronic characters.
Conclusions
Capped subporphyrins 12–16 were synthesized in good
yields from 8 and tripodal trialdehydes 2–6. The complete
concave-face selectivity of these capping reactions was un-
1
ambiguously confirmed by H NMR spectroscopy and X-ray
crystallographic analysis. Capped subporphyrins 15 and 16,
Chem. Eur. J. 2009, 15, 6863 – 6876
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6873

A. Osuka and Y. Inokuma
Table 3. Crystallographic data for compounds 12, 13, 15ꢁCHCl3, and 16-OHꢁmesitylene.
using positive-ion mode. Preparative
separations were performed by silica
gel gravity column chromatography
(Wako gel C-300) or size-exclusion gel
permeation chromatography (GPC)
(Bio-Rad Bio-Beads S-X1, packed
with THF in a 8ꢃ50 or 4ꢃ90 cm gravi-
ty column). Crystallographic data were
12
13
15ꢁCHCl3
16-OHꢁmesitylene
C69H55B1N6O10·C9H12
formula
C64H51B1N6O4·
2CH2Cl2
0.20ꢃ0.20ꢃ0.10
1148.77
monoclinic
P21/n
16.799(5)
22.051(7)
17.270(5)
90
C67H51B1N6O7·
C2H6S1O1,C1H4O1
0.40ꢃ0.30ꢃ0.20
1173.12
monoclinic
C2/c
30.0417(11)
17.2535(19)
29.0055(18)
90
C70H63B1N6O7·CHCl3,
0.8CHCl3, CH3OH
0.65ꢃ0.30ꢃ0.20
1356.44
orthorhombic
Pbca
16.7601(15)
20.6452(18)
38.996(3)
90
crystal size [mm]
formula weight
crystal system
space group
a [pm]
b [pm]
c [pm]
a [8]
0.35ꢃ0.35ꢃ0.20
1259.19
cubic
Pa-3
collected on
a
Rigaku R-AXIS
23.300(3)
23.300(3)
23.300(3)
90
90
90
12649(3)
8
123(2)
1.322
3707
1654
RAPID diffractometer with graphite
monochromated MoKa radiation (l=
0.71075 ꢁ) at À1508C (for 12, 13, and
16-OHꢁmesitylene) or on a Bruker
SMART APEX with graphite mono-
chromated MoKa radiation (l=
b [8]
118.974(10)
90
5597(3)
4
108.808(3)
90
14231.5(19)
8
90
90
13493(2)
8
g [8]
V [nm3]
Z
0.71073 ꢁ)
15ꢁCHCl3).
Methoxo(5,10,15-tri(3-
at
À1838C
(for
T [K]
123(2)
1.363
123(2)
1.095
90(2)
1.335
1calcd [mgmÀ3
]
nitrophenyl)subporphyrinato)boron-
AHCTUNGTRENGN(UN III) (9): This compound was synthe-
total reflns
9689
16052
11880
unique reflns
5895
0.269
8003
7957
0.101
m [mmÀ1
]
0.088
sized from pyridine tri-N-pyrrolylbor-
ane (2.00 g, 6.94 mmol) and 3-nitro-
benzaldehyde (3.10 g, 20.8 mmol) in
the presence of TFA (0.75 mL) ac-
cording to the general procedure for
the synthesis of meso-triarylsubpor-
phyrin. The product was purified by
silica gel column chromatography
(eluent: CH2Cl2/AcOEt=95:5). Subse-
quent recrystallization from CH2Cl2/
MeOH gave 9 as an orange powder
(41 mg, 0.9%). 1H NMR (600 MHz,
R1 [I>2s(I)]
wR2 (all data)
GOF
0.0753
0.1278
1.022
720912
0.0859
0.1564
0.958
0.0981
0.3063
1.035
720914
0.0995
0.1920
1.041
CCDC[a] number
720913
720915
[a] CCDC-720912, 720913, 720914, and 720915 contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
with an arm length of five atoms, have large inner cavities
CDCl3): d=8.85 (s, 3H; meso-Ar-o-H), 8.50 (d, J=7.4 Hz, 3H; meso-Ar-
o or p-H), 8.43 (d, J=7.4 Hz, 3H; meso-Ar-o or p-H), 8.14 (s, 6H; b-H),
7.92 (t, J=7.4 Hz, 3H; meso-Ar-m-H), 0.82 ppm (s, 3H; axial-OMe);
11B NMR (193 MHz, CDCl3): d=À15.4 ppm (s, 1B); 13C NMR (150 MHz,
CDCl3): d=148.6, 141.4, 138.8, 138.5, 129.9, 127.4, 123.1, 122.7, 118.6,
46.8 ppm (axial-OMe); UV/Vis (in CH2Cl2): l (e)=373 (141000), 460 nm
(14000mÀ1 cmÀ1); fluorescence (in CH2Cl2, lex =373 nm): lmax =512 nm,
that are enough to encapsulate solvent molecules, such as
mesitylene and chloroform. On the other hand, capped sub-
porphyrins 12 and 13 exhibit rather tight structures, in which
the cap units are closely stacked to the subporphyrin core.
In particular, the intramolecular p–p interactions and multi-
ple CH–p interactions of 13 result in a drastic decrease in
the exchange rate between the P and M spiral forms in solu-
tion compared with 12 and 16. Besides, the through-space
intramolecular interactions of 13 lead to the remarkable per-
turbations in the absorption spectrum. MO calculations also
suggested favorable orbital interactions between the subpor-
phyrin core and the cap through the low-lying LUMOs of
the 1,3,5-tri(alkoxycarbonyl)benzene moiety. Further fabri-
cation of capped subporphyrins are actively in progress in
our laboratory.
FF =8ꢃ10À3
;
HR-ESIMS-TOF (positive mode): m/z calcd for
C33H18N6B1O6: 605.1381 [MÀOMe]+; found: 605.1396.
Methoxo(5,10,15-tri(3-bromophenyl)subporphyrinato)boron
(III)
(10):
This compound was synthesized from pyridine tri-N-pyrrolylborane
(2.00 g, 6.94 mmol) and 3-bromobenzaldehyde (2.45 mL, 21.0 mmol) in
the presence of TFA (1.03 mL) according to the general procedure. The
product was purified by silica gel column chromatography (eluent:
CH2Cl2/hexane/ether=1:2:1). Subsequent recrystallization from CH2Cl2/
MeOH gave 10 as an orange powder (252 mg, 4.9%). 1H NMR
(600 MHz, CDCl3): d=8.19 (s, 3H; meso-Ar-o-H), 8.13 (s, 6H; b-H), 7.99
(d, J=8.1 Hz, 3H; meso-Ar-o-H), 7.78 (d, J=7.8 Hz, 3H; meso-Ar-p-H),
7.58 (t, J=7.8 Hz, 3H; meso-Ar-m-H), 0.81 ppm (s, 3H; axial-OMe);
11B NMR (193 MHz, CDCl3): d=À15.3 ppm (s, 1B); 13C NMR (150 MHz,
CDCl3): d=141.3, 139.2, 135.9, 131.9, 131.2, 130.3, 122.9, 122.5, 119.3,
46.9 ppm (axial-OMe); UV/Vis (in CH2Cl2): l (e)=373 (158000), 460 nm
(13000mÀ1 cmÀ1); fluorescence (in CH2Cl2, lex =373 nm): lmax =517 nm,
FF =0.139; HR-ESIMS-TOF (positive mode): m/z calcd for
C34H21N3B1Br3ONa: 759.9207 [M+Na]+; found: 759.9213.
Experimental Section
General: All reagents and solvents were of commercial reagent grade
1
and were used without further purification. H, 11B, and 13C NMR spectra
Methoxo(5,10,15-tri(3-benzamidophenyl)subporphyrinato)boron
(11): Subporphyin 10 (140 mg, 190 mmol), benzamide (83 mg, 84 mmol),
[Pd2A(dba)3] (4.9 mg, 5.3 mmol), Xantphos (9.2 mg, 15.9 mmol), Cs2CO3
ACHTUNGTRENNUNG(III)
were recorded on a JEOL delta-600 spectrometer, and chemical shifts
were reported on the delta scale in ppm relative to internal standards
CHCl3 (d=7.26 ppm for 1H, 77.16 ppm for 13C), CHDCl2 (d=5.30 ppm
CTHUNGTRENNUNG
1
1
(260 mg, 800 mmol), and degassed 1,4-dioxane (6 mL) were added to a
20 mL Schlenk tube under an N2 atmosphere. The resulting mixture was
degassed through three freeze–thaw cycles and then heated to 908C for
1 day. After cooling the solution the reaction mixture was passed through
a short Celite pad and the solvent was evaporated. The crude product ob-
tained was purified by GPC column chromatography (4ꢃ90 cm) and the
axial ligand of the separated product was completely converted to the
methoxo form by several dissolve–evaporate cycles in a mixture of
for H), DMSO (d=2.50 ppm for H), and an external standard BF3·OEt2
in CDCl3 (d=0.00 ppm for 11B)). Spectroscopic grade solvents were used
for all spectroscopic studies without further purification. UV/Vis absorp-
tion spectra were recorded on a Shimadzu UV-3100 spectrometer. Fluo-
rescence spectra were recorded on a Hamamatsu Photonics C9920-02
spectrometer and absolute fluorescence quantum yields were measured
by a photon-counting method using an integration sphere. ESI-TOF mass
spectra were recorded on a Bruker Daltonics micro TOF LC instrument
6874
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Chem. Eur. J. 2009, 15, 6863 – 6876

Subporphyrins
FULL PAPER
CH2Cl2/MeOH (1:1). Recrystallization of the residue from CH2Cl2/
hexane gave subporphyrin 11 (133 mg, 82%). 1H NMR (600 MHz,
[D6]DMSO): d=10.63 (s, 3H; NH) 8.62 (s, 3H; meso-Ar-o-H), 8.37 (s,
6H; b-H), 8.12 (d, J=8.2 Hz, 3H; meso-Ar-o or p-H), 8.04 (d, J=7.4 Hz,
6H; Ph-o-H), 7.09 (d, J=8.2 Hz, 3H; meso-Ar-o or p-H), 7.78 (t, J=
7.8 Hz, 3H; meso-Ar-m-H), 7.62 (t, J=7.4 Hz, 3H; Ph-p-H), 7.56 (t, J=
7.3 Hz, 6H; Ph-m-H), 0.63 ppm (s, 3H; axial-OMe); 11B NMR (193 MHz,
CDCl3): d=À15.5 ppm (brs, 1B); 13C NMR (150 MHz, CDCl3): d=140.8,
138.8, 137.7, 134.6, 131.6, 129.2, 128.3, 127.3, 125.1, 123.3, 122.4, 120.1,
119.9, 46.6 ppm (axial-OMe); UV/Vis (in CH2Cl2): l (e)=376 (164000),
J=5.5 Hz, 6H; benzyl-Hf), 3.90 (s, 6H; benzyl-Hk), 0.56 ppm (s, 3H;
axial-OMe); 11B NMR (193 MHz, CDCl3): d=À15.9 ppm (s, 1B); HR-
ESIMS-TOF (positive mode): m/z calcd for C63H48N6B1O3: 947.3886
[MÀOMe]+; found: 947.3884; UV/Vis (in CH2Cl2): l (e) 377 (144000),
461 (16000), 487 nm (11000mÀ1 cmÀ1); fluorescence (in CH2Cl2, lex
377 nm): lmax =507 nm, FF =0.132.
=
Capped subporphyrin 13: Yield: 59%; 1H NMR (600 MHz, [D6]DMSO
at 258C): d=7.75 (d, J=7.7 Hz, 3H; Hg or Hj), 7.66–7.62 (t, t, and d, 9H;
Hh, Hi, and Hg or Hj), 7.50 (t, J=7.8 Hz, 3H; Hc), 7.19 (d, J=7.3 Hz, 3H;
Hb), 7.17 (brs, 3H; b-H), 7.13 (d and brt, 6H; Hd and NH), 6.75 (s, 3H;
He), 6.56 (s, 3H; cap-Ha), 6.00 (brs, 3H; b-H), 5.48 (d, Jgem =10.1 Hz,
3H; benzyl-Hk), 5.03 (d, Jgem =10.1 Hz, 3H; benzyl-Hk), 4.73 (brd, 3H;
benzyl-Hf), 4.70 (brd, 3H; benzyl-Hf), À0.03 ppm (s, 3H; axial-OMe);
1H NMR (600 MHz, [D6]DMSO at 1208C): d=7.79 (d, J=7.4 Hz, 3H;
Hg or Hj), 7.65 (d, J=7.4 Hz, 3H; Hg or Hj), 7.62 (t, J=7.3 Hz, 3H; Hh
or Hi), 7.58 (t, J=7.3 Hz, 3H; Hh or Hi), 7.50 (t, J=7.3 Hz, 3H; Hc), 7.24
(d, J=7.3 Hz, 3H; Hb), 7.14 (d, J=8.2 Hz, 3H; Hd), 6.84 (s, 3H; He),
6.74 (brt, 3H; NH), 6.71 (brs, 6H; b-H), 6.67 (s, 3H; cap-Ha), 5.28 (s,
6H; benzyl-Hk), 4.76 (d, J=5.5 Hz, 6H; benzyl-Hf), 0.08 ppm (s, 3H;
462 nm (12000mÀ1 cmÀ1); fluorescence (in CH2Cl2, lex =373 nm); lmax
=
520 nm, FF =0.178; HR-ESIMS-TOF (positive mode): m/z: calcd for
C55H39N6B1O4Na: 881.3027 [M+Na]+; found: 881.3022.
Methoxo(5,10,15-tri(3-aminophenyl)subporphyrinato)boronACHTNUTGRNEUNG(III) (8)
Reduction of nitro groups: Subporphyrin 9 (10–50 mg) was dissolved in a
minimal amount of chloroform, the solution was diluted with ethanol
(15 mL) and 1m aqueous HCl (15 mL) and SnCl2·2H2O (20 equiv) were
added. The resulting mixture was vigorously stirred at 708C for 7 h. (The
reaction was monitored by TLC. In case the reaction was sluggish, anoth-
er 5–10 mL of 1m aqueous HCl was added to the solution.) After cooling,
1m aqueous NaOH was added to make the solution basic and the prod-
uct was repeatedly extracted with CH2Cl2 until the aqueous layer became
colorless. The combined organic layer was washed with brine, dried over
anhydrous Na2SO4, and the solvent was evaporated. The solid obtained
was dissolved in methanol (ca. 20 mL) and the solution was heated at
reflux for 30 min. After evaporating the solvent, the axially methoxo-co-
ordinated product was recrystallized from CH2Cl2/hexane to give subpor-
phyrin 8 quantitatively as an orange solid.
axial-OMe); UV/Vis (in CH2Cl2 at 258C):
l (e) 387 (136000), 471
(13000), 497 nm (17000mÀ1 cmÀ1); fluorescence (in CH2Cl2, lex =387 nm):
lmax =515 nm, FF =0.166; HR-ESIMS-TOF (positive mode): m/z calcd
for C66H48N6BO6 and C67H51N6BO7Na: 1031.3733 [MÀOMe]+, 1085.3810
[M+Na]+; found: 1031.3736, 1085.3785.
Capped subporphyrin 14: Yield: 58%; 1H NMR (600 MHz, CDCl3 at
258C): d=7.78 (s, 6H; b-H), 7.77 (d, J=7.3 Hz, 3H; Hb), 7.55 (d, J=
7.4 Hz, 3H; Hg), 7.53 (t, J=7.4 Hz, 3H; Hc), 7.28 (d, J=7.4 Hz, 3H; Hi),
7.10 (t, J=7.4 Hz, 3H; Hh), 6.93 (d, J=8.1 Hz, 3H; Hd), 6.76 (d, J=
8.4 Hz, 3H; Hj), 6.64 (s, 3H; He), 5.54 (s, 3H; cap-Ha), 4.23 (brs, 6H;
benzyl-Hf), 4.20 (brs, 3H; NH), 3.99 (brt, 6H; ethylene), 3.72 (brt, 6H;
ethylene), 0.75 ppm (s, 3H; axial-OMe); 11B NMR (193 MHz, CDCl3):
d=À15.3 ppm (s, 1B); UV/Vis (in CH2Cl2): l (e)=377 (132000), 461 nm
(18000mÀ1 cmÀ1); fluorescence (in CH2Cl2, lex =377 nm): lmax =515 nm,
FF =0.133; HR-ESIMS-TOF (positive mode) m/z calcd for
C66H54N6B1O6: 1037.4203 [MÀOMe]+; found: 1037.4205.
Hydrolysis of amido groups: Subporphyrin 11 (133 mg, 155 mmol) and
NaOH (5.0 g) were dissolved in
a mixture of EtOH (30 mL)/THF
(3 mL)/water (10 mL). The resulting solution was heated at reflux over-
night. After cooling the system, dilute hydrochloric acid (ꢂ10%) was
added to make the solution weakly basic. The product was extracted with
CH2Cl2 until the colored aqueous layer became colorless. The combined
organic layer was washed with brine, dried over Na2SO4, and the solvent
was evaporated. The axial ligand of the crude product was converted into
the methoxo form through three dissolve–evaporate cycles in a mixture
of CH2Cl2/MeOH (1:1). Finally, the crude product was recrystallized
Capped subporphyrin 15: Yield: 85%; 1H NMR (600 MHz, CDCl3 at
258C): d=7.64 (d, J=7.3 Hz, 3H; Hb), 7.60 (d, J=7.6 Hz, 3H; Hg), 7.53
(t, J=8.3 Hz, 3H; Hc), 7.36 (t, J=6.8 Hz, 3H; Hi), 7.28 (s, 6H; b-H), 7.20
(t, J=7.3 Hz, 3H; Hh), 7.00 (d, J=8.3 Hz, 3H; Hd), 6.83 (d, J=7.8 Hz,
3H; Hj), 6.54 (s, 3H; He), 5.51 (s, 3H; cap-Ha) 4.53 (brs, 3H; NH), 4.31
(brs, 6H; benzyl-Hf), 3.93 (t, J=5.0 Hz, 6H; -O-CH2-CH2-), 3.66 (t, J=
5.0 Hz, 6H; -O-CH2-CH2-), 1.99 (quintet, J=5.0 Hz, 6H; -O-CH2-CH2-),
0.57 ppm (s, 3H; axial-OMe); 11B NMR (193 MHz, CDCl3): d=
from CH2Cl2/hexane to give
8
(92 mg, 92%). 1H NMR (600 MHz,
CDCl3): d=8.13 (s, 6H; b-H), 7.45 (t, J=7.6 Hz, 3H; meso-Ar-m-H),
7.41 (s, 3H; meso-Ar-o-H), 7.40 (d, J=6.4 Hz, 3H; meso-Ar-o-H), 6.92
(d, J=8.7 Hz, 3H; meso-Ar-p-H), 3.92 (brs, 6H; NH2), 0.81 ppm (s, 3H;
axial-OMe); 11B NMR (193 MHz, CDCl3): d=À15.3 ppm (s, 1B);
13C NMR (150 MHz, CDCl3): d=146.6, 140.9, 138.4, 129.5, 123.9, 122.2,
120.7, 120.0, 114.6, 46.8 ppm (axial-OMe); UV/Vis (in CH2Cl2): l (e)=
À15.8 ppm (s, 1B); UV/Vis (in CH2Cl2):
l (e)=382 (124000), 467
(13000), 493 nm (14000mÀ1 cmÀ1); fluorescence (in CH2Cl2, lex =382 nm);
lmax =517 nm, FF =0.156; HR-ESIMS-TOF (positive mode): m/z calcd
for C69H60N6B1O6: 1079.4673 [MÀOMe]+; found: 1079.4680.
377 (170000), 489 nm (12000mÀ1 cmÀ1); fluorescence (in CH2Cl2, lex
=
377 nm): lmax =522 nm, FF =0.09; HR-ESIMS-TOF (positive mode): m/z
Capped subporphyrin 16: Yield: 70%; 1H NMR (600 MHz, CDCl3 at
258C): d=8.40 (s, 3H; cap-Ha), 7.63 (d, J=7.3 Hz, 3H; Hb), 7.56 (d, J=
7.8 Hz, 3H; Hg), 7.53 (t, J=7.3 Hz, 3H; Hc), 7.38 (t, J=6.8 Hz, 3H; Hi),
7.27 (s, 6H; b-H), 7.18 (t, J=7.4 Hz, 3H; Hh), 6.99 (d, J=7.3 Hz, 3H;
Hd), 6.82 (d, J=7.8 Hz, 3H; Hj), 6.48 (s, 3H; He), 4.54 (brs, 9H; NH and
-OCH2-), 4.31 (brd, 6H; benzyl-Hf), 4.08 (brs, 6H; -OCH2-), 0.56 ppm (s,
3H; axial-OMe); 11B NMR (193 MHz, CDCl3): d=À15.8 ppm (s, 1B);
calcd for C33H24N6B1: 515.2156 [M-OMe]+; found: 515.2159.
General procedure for preparing capped subporphyrins: Trifluoroacetic
acid (10 mL) and a tripodal aldehyde (1.0 equiv, 27.5 mmol) were added to
a solution of subporphyrin 8 (15.0 mg, 27.5 mmol) in a mixture of CH2Cl2
(360 mL) and iPrOH (90 mL) and the solution was stirred at room tem-
perature for 1 day. After the addition of NaBH3CN (150 mg), the solu-
tion was stirred additional 1 day. The resulting solution was concentrated
to ca. 20 mL by evaporation and then poured into water (50 mL). The
product was extracted with CH2Cl2 and the organic layer was washed
with water, dried with Na2SO4, and the solvent was evaporated to dry-
ness. The crude product was purified by gravity GPC column chromatog-
raphy (4ꢃ90 cm, eluent: THF). The axial ligand of the obtained product
was completely converted into the methoxo form through dissolve–evap-
UV/Vis (in CH2Cl2)
l (e) 380 (108000), 466 (11000), 492 nm
(12000mÀ1 cmÀ1); fluorescence (in CH2Cl2, lex =380 nm); lmax =534 nm,
FF =0.167; HR-ESIMS-TOF (positive mode): m/z calcd for
C69H54N6B1O9: 1121.4051 [MÀOMe]+; found: 1121.4045.
orate cycles in
a mixture of CH2Cl2/MeOH. Recrystallization from
CH2Cl2/MeOH (for 12, 13, 14, and 16) or CH2Cl2/hexane (for 15) gave
the corresponding capped subporphyrin as an orange powder.
Capped subporphyrin 12: Yield: 80%; 1H NMR (600 MHz, CDCl3 at
258C): d=7.66 (d, J=7.3 Hz, 3H; Hb), 7.62 (d, J=6.8 Hz, 3H; Hg), 7.52
(t, J=7.6 Hz, 3H; Hc), 7.52 (s, 6H; b-H), 7.33 (t, J=7.3 Hz, 3H; Hi), 7.14
(s, 3H; He), 7.11 (t, J=6.9 Hz, 3H; Hh), 6.98 (d, J=8.2 Hz, 3H; Hd), 6.80
(d, J=8.3 Hz, 3H; Hj), 5.77 (s, 3H; cap-Ha), 4.95 (brt, 3H; NH), 4.43 (d,
Acknowledgements
This work was supported by a Grant-in-Aid (A) (No. 19205006) for Sci-
entific Research from MEXT. Y.I. thanks the JSPS Research Fellowship
for Younger Scientists.
Chem. Eur. J. 2009, 15, 6863 – 6876
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6875

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Received: April 3, 2009
Published online: June 5, 2009
6876
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Chem. Eur. J. 2009, 15, 6863 – 6876