meso-Aryl substituted rubyrin and its higher homologues: Structural characterization and chemical properties

Article abstract of DOI:10.1002/chem.200701909

meso-Aryl substituted rubyrin ([26]hexaphyrin(1.1.0.1.1.0)) 2 and a series of rubyrin-type large expanded porphyrins were obtained from a facile one-pot oxidative coupling reaction of meso-pentafluorophenyl substituted tripyrrane 1. The structures of two of the resulting products were determined by single-crystal X-ray diffraction analysis. Whereas [52]dodecaphyrin-(1.1.0.1.1. 0.1.1.0.1.1.0) 4 takes a symmetric helical conformation, the larger species, [62]pentadecaphyrin-(1.1.0.1.1.0.1.1.0.1.1.0.1.1.0) 5, adopts a nonsymmetric distorted conformation in the solid state that contains an intramolecular helical structure. The ability of rubyrin 2 to act as an anion receptor in its diprotonated form (2.2H⁺) was demonstrated in methanolic solutions. Oxidation of 2 with MnO₂ gave [24]rubyrin 6, a species that displays antiaromatic characteristics. [26]Rubyrin 2 and [24]rubyrin 6 both underwent metallation when reacted with Zn-(OAc)₂ to give the corresponding biszinc(II) complexes 7 and 8 quantitatively without engendering a change in the oxidation state of the ligands. As a result, complexes 7 and 8 exhibit aromatic and antiaromatic character, respectively. NICS calculation on these compounds also supported aromaticity of 2 and 7, and antiaromaticity of 6 and 8.

Full text of DOI:10.1002/chem.200701909

DOI: 10.1002/chem.200701909
meso-Aryl Substituted Rubyrin and Its Higher Homologues:
Structural Characterization and Chemical Properties
Soji Shimizu,[a] Won-Seob Cho,[b] Jonathan L. Sessler,*[b] Hiroshi Shinokubo,[a] and
Atsuhiro Osuka*[a]
Abstract: meso-Aryl substituted rubyr-
in ([26]hexaphyrin(1.1.0.1.1.0)) 2 and a
series of rubyrin-type large expanded
porphyrins were obtained from a facile
one-pot oxidative coupling reaction of
meso-pentafluorophenyl substituted tri-
pyrrane 1. The structures of two of the
resulting products were determined by
single-crystal X-ray diffraction analysis.
nonsymmetric distorted conformation
in the solid state that contains an intra-
molecular helical structure. The ability
of rubyrin 2 to act as an anion receptor
in its diprotonated form (2·2H+) was
demonstrated in methanolic solutions.
in 2 and [24]rubyrin 6 both underwent
metallation when reacted with Zn-
AHCTRE(NUG OAc)2 to give the corresponding bis-
zinc(II) complexes 7 and 8 quantita-
tively without engendering a change in
the oxidation state of the ligands. As a
result, complexes 7 and 8 exhibit aro-
matic and antiaromatic character, re-
spectively. NICS calculation on these
compounds also supported aromaticity
of 2 and 7, and antiaromaticity of 6
and 8.
Oxidation of
2
with MnO2 gave
[24]rubyrin 6, a species that displays
antiaromatic characteristics. [26]Rubyr-
Whereas
[52]dodecaphyrin-
(1.1.0.1.1.0.1.1.0.1.1.0) 4 takes a sym-
metric helical conformation, the larger
Keywords:
aromaticity
anion recognition
conjugation
·
·
·
species,
[62]pentadecaphyrin-
macrocyclic ligands · porphyrinoids
(1.1.0.1.1.0.1.1.0.1.1.0.1.1.0) 5, adopts a
Introduction
study of three-dimensional micro-fabrication, optical data
storage, and optical limiting effects.[7,8]
Expanded porphyrins[1] are a relatively new class of func-
tional molecules that have attracted considerable attention
recently as selective anion receptors,[2] ligands for transition
and lanthanoid ions,[3,4] photodynamic therapy photosensitiz-
ers,[5] and magnetic resonance imaging contrast agents.[6] Ex-
panded porphyrins have also proved useful in the study of
aromatic effects in large heterannulenes,[1] while, more re-
cently, their large two-photon absorption cross sections,
have made certain expanded porphyrins attractive for the
Many of the unique properties of fully conjugated ex-
panded porphyrins, including whether they display aromatic
features or display nonlinear optical effects, depend directly
on the size of the macrocyclic ring.[1,7–18] Accordingly, one
important research direction in expanded porphyrin chemis-
try has involved the preparation of ever larger systems. Un-
fortunately, the synthesis of such compounds remains chal-
lenging. Indeed, until recently, expanded porphyrins larger
than octaphyrins were exceedingly rare.[15,19–21] Over the
course of the last decade, the Kyoto group has developed a
series of meso-aryl and meso-trifluoromethyl substituted ex-
panded porphyrins, including a number that have demon-
strated unique electronic properties and chemical reactivi-
ty.[17,21] Unfortunately, the synthetic strategies developed ini-
tially, involving the direct condensation of mono-pyrrole
precursors with electron deficient aldehydes, are plagued by
an inevitable drop in yield as the size of the macrocyclic
products increases. Therefore, increasingly we have explored
the use of preformed multi-pyrrole fragments. In the course
of these investigations, specifically those targeting the syn-
thesis of meso-pentafluorophenyl substituted rubyrin 2[22,23]
and [38]nonaphyrin(1.1.0.1.1.0.1.1.0) 3,[24] we noticed that
[a] Dr. S. Shimizu, Prof. Dr. H. Shinokubo, Prof. Dr. A. Osuka
Department of Chemistry, Graduate School of Science
Kyoto University, Sakyo-ku, Kyoto 606-8502 (Japan)
Fax : (+81)75-753-3970
E-mail: osuka@kuchem.kyoto-u.ac.jp
[b] Dr. W.-S. Cho, Prof. Dr. J. L. Sessler
Department of Chemistry and Biochemistry
1 University Station-A5300, The University of Texas at Austin
Austin, Texas 78712-0165 (USA)
Fax : (+1)512-471-7550
E-mail: sessler@mail.utexas.edu
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
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FULL PAPER
oxidative coupling reaction involving tripyrrane 1 gave
higher homologues in addition to 2 and 3. We have now
studied this condensation process in greater detail and wish
to report here the successful isolation and characterization
of [52]dodecaphyrin(1.1.0.1.1.0.1.1.0.1.1.0) 4 and [62]penta-
decaphyrin(1.1.0.1.1.0.1.1.0.1.1.0.1.1.0) 5. Pentadecaphyrin 5
is, to the best of our knowledge, the second largest expand-
ed porphyrins structurally characterized to date.
oxidized to its antiaromatic derivative, [24]rubyrin 6, by
treatment with MnO2. As such, system 2 stands a further
rare example of a switchable aromatic–antiaromatic mole-
cule.
Results and Discussion
In addition to reporting the isolation of expanded por-
phyrins 4 and 5, we present further studies involving meso-
pentafluorophenyl substituted rubyrin 2. These involve anal-
yses of its anion binding behavior of the diprotonated form
and oxidation/metallation studies of the neutral macrocycle,
respectively. The first of these studies was motivated by the
finding in initial work that diprotonated rubyrin, 2·2H+, ex-
hibited two different conformations depending on the
choice of counter anions.[23] Such an observation, which is
thought to reflect the greater flexibility of meso-substituted
Synthesis of meso-pentafluorophenyl substituted rubyrin
and its higher homologues: In the course of our studies on
expanded porphyrins, we noted the benefit of the electron-
deficient pentafluorophenyl group for the formation of
meso-aryl substituted expanded porphyrins from the acid
catalyzed reaction between aromatic aldehydes and simple
pyrroles.[17] Although the role of this and related deactivat-
ing groups remains to be elucidated fully, one plausible ex-
planation is that such substituents stabilize the porphyrino-
gen precursors, presumed to be formed during the course of
reaction, against possible scrambling under acidic conditions.
In light of such considerations, we examined the oxidative
coupling reaction of meso-pentafluorophenyl substituted tri-
pyrrane 1 under conditions similar to those used in the syn-
thesis of core-modified rubyrins by Chandrashekar et al.
(TFA, CH2Cl2, followed by chloranil).[22] After work up, the
resulting reaction mixture was separated over a neutral alu-
mina column to give two fractions. The second violet frac-
tion proved to be rubyrin ([26]hexaphyrin(1.1.0.1.1.0)) 2, a
product that was further purified by chromatography over
silica gel to give a pure sample in 24% yield.[23] The first
fraction was subject to further separation using size exclu-
sion chromatography; this provided the previously charac-
terized species [38]nonaphyrin(1.1.0.1.1.0.1.1.0) 3 in 9%
yield,[23] as well as the new products [52]dodecaphyrin-
À
rubyrins around the pyrrole pyrrole linkage than the corre-
sponding meso-free, b-alkyl functionalized derivatives,[22a]
thus provided a specific incentive to examine the anion
binding properties of 2·2H+ in detail. Surprisingly, this is
something that has not been in the case of meso-functional-
ized (as opposed to b-alkyl substituted) expanded poprhyr-
ins.
The second set of studies was motivated by recent find-
ings that certain hexapyrrolic expanded porphyrins undergo
conversion from the corresponding aromatic to antiaromatic
forms (or vice versa) upon metallation.[7b,25,26] Such “switch-
ing”, which gives rise to easy-to-assess changes in electronic
and optical properties, is noteworthy given the normal diffi-
culties associated with isolating planar structures with
Hückel-type antiaromatic conjugation.[25,26] It was thus of in-
terest to see if rubyrin 2, a 26 p-electron species, could be
oxidized to the corresponding 24 p-electron antiaromatic
species and whether prior or concurrent metallation was re-
quired to effect the transformation. As detailed below,
meso-pentafluorophenyl substituted rubyrin 2 can indeed be
(1.1.0.1.1.0.1.1.0.1.1.0)
4
and
[62]pentadecaphyrin-
(1.1.0.1.1.0.1.1.0.1.1.0.1.1.0) 5 in isolated yields of 2.7 and
1.5%, respectively (Scheme 1). The isolated yields of 4 and
5, while modest, proved fully reproducible.
Scheme 1. Synthesis of rubyrin and its higher homologues.
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A. Osuka, J. L. Sessler et al.
Spectroscopic and structural characterization of [52]dodeca-
phyrin(1.1.0.1.1.0.1.1.0.1.1.0) and [62]pentadecaphyrin-
(1.1.0.1.1.0.1.1.0.1.1.0.1.1.0): High-resolution electrospray-
ionization time-of-flight (HR-ESI-TOF) mass spectrometry
revealed an ion peak at m/z 2207.2089, a finding consistent
with 4 being a dodecapyrrolic macrocycle (calcd for a parent
[MÀÀH] peak, C104N12F40H31, 2207.2161). The absorption
spectrum showed rather featureless broad bands at 327, 412,
1
657, and 784 nm (Figure 1). The H NMR spectrum of 4 was
characterized by a very simple pattern of signals at d 7.18,
7.05, 6.78, 6.71, 6.35, and 6.19 ppm, ascribable to the b-CH
protons, and resonances at d 14.40, 8.66, and 7.23 ppm due
to the NH protons, as would be anticipated for a highly sym-
metric conformation (see Supporting Information). This as-
signment was also supported by the clean nature of the
19F NMR spectrum recorded for 4.
Figure 2. X-ray crystal structure of 4, top view (top) and side view
(below). The thermal ellipsoids are scaled to the 50% probability level.
The pentafluorophenyl substituents at the meso-positions are omitted for
clarity in the side view.
mained broad upon cooling at À208C, displaying signals in a
range of 4–8 ppm. The 19F NMR spectrum of 5 at À208C in
CDCl3 was still broad, but was sufficiently well resolved to
allow for assignment (see Supporting Information). In par-
ticular, the integrations are consistent with the number of
fluorine atoms at the various positions (e.g., twenty fluorine
atoms for the ortho and meta positions and ten atoms for
the para substituents). These spectral features can be under-
stood in terms of compound 5 being non-rigid in solution
and subject to considerable conformational flexibility under
Figure 1. UV/Vis absorption spectra of rubyrin and its higher homologues
recorded in CH2Cl2.
Single crystals of 4 were obtained via the vapor diffusion
of hexane into a CH2Cl2 solution. X-ray diffraction analysis
of 4 revealed a near D2h symmetric twisted structure consist-
ing of two inward-orienting tripyrrane units (pyrroles L, A,
and B, and F, G, and H) and two tripyrrane units with two
inverted pyrrole rings (pyrroles C, D, and E, and I, J, and K)
(Figure 2). Intramolecular hydrogen bonding interactions
were observed between adjacent sets of amine-like pyrrole
NH protons and neighboring unprotonated imine-like nitro-
gen atoms (N-H···N distances and angles; 2.10 Š and 123.88
(pyrroles A and B), 2.21 Š and 122.98 (pyrroles F and G),
2.10 Š and 123.88 (pyrrole G and H), and 2.21 Š and 122.18
(pyrroles L and A)). Presumably, these multiple hydrogen-
bonding interactions serve to maintain the conformation
seen in the solid state and help account for the rigid struc-
ture observed in solution, as inferred from the NMR spec-
tral data alluded to above.
1
conditions of the H and 19F NMR spectroscopic analyses.
Macrocycle 5 was characterized more fully by single crys-
tal X-ray diffraction analysis; the resulting structure re-
vealed the system to be nonsymmetric with a helically
wound moiety (Figure 3). Intramolecular hydrogen bonding
networks between the amine NH and the imine N of pyrrole
rings in 5 presumably serve to stabilize this helical confor-
mation (N-H···N distances and angles; 2.18 Š and 122.28
(pyrroles A and B), 2.15 Š and 122.08 (pyrroles B and C),
2.17 Š and 120.48 (pyrroles G and H), 2.15 Š and 122.58
(pyrroles H and I), 2.29 Š and 122.18 (pyrroles J and K),
2.08 Š and 124.78 (pyrroles K and L), and 2.01 Š and 127.38
(pyrroles N and O)). As can be seen from inspection of
Figure 3, the thermal ellipsoids of the smaller helical moiety
from the pyrrole A to the pyrrole D are larger than the
others. This proved true even though the data set was re-
corded at low temperature (À1838C). Moreover, several
sets of crystals of 5 were subject to structural analyses. In all
cases, the thermal ellipsoids were always comparatively
large at this moiety, and in some instances this part proved
highly disordered. These findings are thought to reflect the
The HR-ESI-TOF mass spectrum of 5 revealed a parent
ion peak at m/z 2756.2530 ([MÀÀH]; calcd for C130N15F50H36
2756.2485) corresponding to the proposed structure. The ab-
sorption spectrum exhibited relatively sharp bands at 379
and 598 nm, along with a broad absorption tail that reaches
1
into the near-IR region (Figure 1). The H NMR spectrum
of 5 was too broad to assign at room temperature and re-
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Rubyrin and Higher Homologues
FULL PAPER
a conformation of type I, the
conformation of the diprotonat-
ed species was found to depend
on the choice of acid; a type II
conformation is preferred upon
protonation with trifluoroacetic
acid, whereas a type III confor-
mation is preferred upon proto-
nation with HCl. These struc-
tural changes were fully charac-
terized by NMR and X-ray
analyses. Due to changes in the
conjugation pathway, these two
protonated forms exhibit differ-
ent
Soret-like
absorptions,
namely at 555 nm for type II
and at 518 nm for type III (cf.
Supporting Information).
We have now extended our
protonation studies to include
H2SO4 and H3PO4. Both of
these acids give rise to proton-
ated forms of 2 (i.e., 2·2H+)
characterized by split Soret-like
bands at 520 and 550 nm that,
on the basis of the above find-
ings, are interpreted in terms of
the presence of both type II
and III conformations. To the
extent such an interpretation is
correct, it indicates that the
choice of counter anion can
have a large effect on the con-
formation of 2·2H+. This, in
turn, prompted us to investigate
the anion recognition behavior
of 2·2H+ in greater detail. To-
wards this end, hydriodic acid
Figure 3. X-ray crystal structure of 5, top view (top) and side view (below). The thermal ellipsoids are scaled
to the 50% probability level. The pentafluorophenyl substituents at the meso-positions are omitted for clarity
in the side view.
conformational flexibility observed in solution, and thus
help account for the broad signals observed in the solution
was chosen as a proton source with the expectation that the
counter iodide anion would be weakly bound to 2·2H+ and
hence easily exchanged by other anions.[27] Thus, hydriodic
acid was added to a about 4.0”10À3 mm solution of rubyrin
2 while the changes in the UV/Vis absorption spectrum
were recorded. After the addition of ꢀ25 molar equivalents,
no further spectral changes were seen, leading to the infer-
ence that full conversion to the diprotonated form 2·2H+
1
phase H and 19F NMR spectra.
Anion recognition behavior of protonated rubyrin: As pre-
viously reported, rubyrin 2 can take three different confor-
mations (type I, II, and III) depending upon its protonation
state (see below).[23] Whereas the free base rubyrin 2 adopts
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A. Osuka, J. L. Sessler et al.
had been effected (see Supporting Information). The final
spectrum obtained in this way is characterized by a Soret-
like band at about 550 nm, from which it is inferred that
2·2H+ exists in a type II conformation. The methanolic solu-
tion of 2·2H+ produced in this way was then subject to titra-
tion with either chloride (0–2.0 equiv) or sulfate (0–
4.3 equiv) anions (studied as their corresponding tetrabuty-
lammonium and tetramethylammonium salts, respectively)
and again the spectral changes were recorded (cf. Figure 4).
anion exchange of iodide anion by sulfate anion, which
causes rigidification of a type II conformation. In both titra-
tion experiments, standard curve fits matched well with a
1:1 binding profile (cf. Supporting Information), which was
also supported by Job-plot analyses. Given this, apparent
binding constants on the order of 104 mÀ1 and 105 mÀ1 for
chloride anion and sulfate anion, respectively, could be esti-
mated from the absorption changes at 513 nm and at
552 nm, respectively. However, it is important to appreciate
that these values represent formal exchange constants (unit-
less) and can be taken as apparent affinity constants only to
the extent the limiting assumption that the initial iodide
anion is not bound is valid. Further, as with many analyses
of this type, the actual values were found to be rather sensi-
tive to the extent to which the anion salt in question, the
methanol solvent, and the rubyrin itself were rigorously
dried.
Oxidation of [26]rubyrin to antiaromatic [24]rubyrin: An at-
tractive feature of nitrogen-containing porphyrins and ex-
panded porphyrins is the ability to adopt two-electron oxi-
dation and reduction reactions through facile release and
uptake of two hydrogen atoms on the nitrogen atoms; in
principle, this allows for the existence of neutral forms for
multiple oxidation states.[14,17,21,25,28] A two-electron redox
process is expected to induce a sharp Hückel type aromatic-
to-antiaromatic switch when the macrocyclic framework is
maintained, as recently demonstrated for bis-goldACTHRE(UNG III) hexa-
phyrin(1.1.1.1.1.1) complexes[25b,c] and a uranyl hexaphyr-
in(0.0.0.0.0.0) complex.[26c] However, we are unaware of any
fully characterized system of expanded porphyrins where
this has been demonstrated in the absence of metallation
except for meso-pentafluorophenyl substituted N-fused
[24]pentaphyrin[25a] and tetrathiaoctaphyrins.[14] However
their paratropic ring current effect were found to be subtle
presumably because of their highly distorted structures. We
thus keen to determine if such a conversion could be carried
out in the case of rubyrin 2 ([26]hexaphyrin(1.1.0.1.1.0)).
This species was thus treated with DDQ. Although this did
indeed provide a new spot on the thin layer chromatogram,
we could not purify the resulting putative new compound
through silica gel or alumina column. While not fully estab-
lished, the problem appeared to be that under both sets of
Figure 4. UV/Vis absorption spectral changes of protonated rubyrin 2
(2·2H+) (4.0”10À3 mm methanolic solution as produced by pretreatment
with 25 equiv of hydriodic acid) observed upon the addition of a) tetra-
butylammonium chloride (0–2.0 equiv of ClÀ) and b) tetramethylammo-
nium sulfate (0–4.3 equiv of SO42À).
Addition of chloride anion resulted in a decrease in the ab-
sorbance intensity at 552 nm and an increase in that at
513 nm. On the basis of the prior analyses (see above), such
a result is considered consistent with a change in conforma-
tion from type II to type III as the result of chloride-for-
iodide anion exchange (Figure 4a). In contrast, the addition
of sulfate anion led to a decrease in the intensity of the
shoulder-like absorption at 513 nm and an increase in the in-
tensity of the absorbance at 552 nm (Figure 4b). This result,
which stands in contrast to the formation of two protonated
conformers upon simple protonation of 2 with sulfuric acid,
is of particular interest, since it indicates that the pre-organi-
zation of type II conformation has a large influence on the
anion recognition properties. In fact, we interpret the titra-
tion experiment (Figure 4b) with sulfate anion in terms of
chromatographic conditions facile reduction back to
2
occurs. Therefore, 2 was subject to oxidation with MnO2.
The use of this inorganic oxidant simplified purification, in
that after the reaction, the residual inorganic solids were
simply removed by filtration to give the oxidized form of 2,
the [24]rubyrin 6, in almost quantitative yield. Compound 6
can be easily reduced back to 2 upon treatment with
NaBH4.
The formally antiaromatic species 6 was fully character-
ized by spectroscopic means. Consistent with the proposed
1
structure, the H NMR spectrum of 6 recorded in CDCl3 so-
lution revealed resonances for the inner b-CH protons as a
pair of doublet (J=4.8 Hz) at d 12.57 and 11.80 ppm, re-
spectively. Signals ascribable to the outer b-CH protons
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Rubyrin and Higher Homologues
FULL PAPER
were seen at d 5.82, 5.66, 5.41, and 5.18 ppm; these appeared
as two pairs of mutually coupled signals (J=4.6 Hz)
(Figure 5). An extremely down-field shifted signal at d
species differ from one another only in terms of the number
of p-electrons within their respective conjugation pathways
and the charges of the fully deprotonated forms as a direct
consequence of the differing
number
of
NH
protons.
Zinc(II) was chosen for these
studies because it is diamagnet-
ic and would permit the pro-
posed metal insertion reactions
to be followed by NMR spec-
troscopy.
Initially, metallation of 2 with
ZnACHTRE(UNG OAc)2 was attempted in a
mixture of CH2Cl2 and metha-
nol with zinc acetate. The met-
allation proceeded smoothly
with the color of the solution
Figure 5. 1H NMR spectrum of 6 recorded in CDCl3.
21.77 ppm was assigned to the inner NH protons. HH-
COSY spectrum of 6 revealed correlation between the
peaks at d 12.57 and 11.80 ppm, 5.82 and 5.66 ppm, and 5.41
and 5.18 ppm, respectively. Further structural information
was provided by ROESY measurement, which reveals prox-
imity of spatially neighboring protons (see Supporting Infor-
mation). In the ROESY spectrum of 6, the correlation be-
tween the inner NH proton and the outward orienting pyr-
role proton at 12.57 ppm was observed. In addition, in-depth
inspection of the 1H NMR spectrum of 6 reveals double-
doublet splitting of the peak at 5.18 ppm, which is accounted
for the long range proton-proton coupling between NH and
the outer b-CH. Overall, these findings are consistent with
the oxidized species 6 possessing a rectangular conformation
similar to that of 2 in solution, as well as displaying a dis-
tinctive paratropic ring current, which presumably arises
from its 24 p-electron conjugation pathway.[25,26] The UV/Vis
spectrum of 6 recorded in dichloromethane solution exhibit-
ed a blue-shift in the less intense Soret-like band (relative to
2) but no appreciable low-energy Q-band like band. Such
spectral features mirror those seen for the antiaromatic gold
complexes of the [28]hexaphyrins (Figure 6).[25b] Unfortu-
nately, while compound 6 proved stable under typical labo-
ratory conditions, all efforts to obtain single crystals of this
species that would be suitable for X-ray analysis were sty-
mied due to its poor solubility and lack of long-term stabili-
ty.
Figure 6. UV/Vis absorption spectra of rubyrin 2, [24]rubyrin 6, and their
respective bis-zinc complexes 7 and 8 as recorded in CH2Cl2.
changing from violet to bright red, a transformation that
was ascribed to quantitative conversion to the bis-zinc com-
plex 7 (see below). After the metallation was complete, the
solvent was removed using a rotary evaporator. The residue
was then dissolved in a small amount of CH2Cl2, upon which
hexane was layered to induce precipitation of the inorganic
salts. These latter were then removed by filtration. A pre-
liminary single crystal X-ray diffraction analysis was per-
formed; the resulting structure revealed a bis-zinc complex
possessing a type III conformation, in which the two cations
are bound by two dipyrromethene subunits, while the two
remaining pyrrolic units do not interact with the zinc ions
(see Supporting Information). The two zinc ions are bridged
by an acetate anion as well as by an oxygen atom of a hy-
droxy group. Consistent with the formulation of 7 as a com-
Zinc metallations of 2 and 6: Given the fact that aromatic
and antiaromatic forms had been characterized previously
for several other hexapyrrolic
1
plex of [26]rubyrin, the H NMR spectrum of 7 clearly indi-
expanded
porphyrins
(see
above), it was of interest to us
to see if metallation of the two
rubyrins 2 and 6 would lead to
changes in oxidation state. Such
a study is made more compel-
ling by the fact that these two
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A. Osuka, J. L. Sessler et al.
cated a strong diatropic ring current. This was evidenced by
signals for those protons held within the center of the ring,
namely those for the acetate, hydroxy, and the uncomplexed
pyrrolic NH group, at d À2.87, À8.97, and À5.14 ppm, re-
spectively, being positioned in a shielded region. Conversely,
the signals ascribable to the outer b-CH protons were ob-
served in a deshielded region at d 10.88, 10.37, 9.57, 9.52,
9.34, and 9.04 ppm.
The bis-zinc complex of the oxidized [24]rubyrin 6 was
synthesized and purified in a similar way. The absorption
spectrum of the resulting bis-zinc complex, 8, is character-
ized by an ill-defined broad absorption without any Q-like
absorption (Figure 6). Moreover, its 1H NMR spectrum
shows three peaks due to the b-CH protons at d 6.16, 5.22,
and 5.15 ppm, as well a peak at 4.02 ppm assigned to the
methyl groups of the two bound acetate anions. Thus, even
though the species were produced by independent synthesis,
the chemical shifts of the methyl group of the bridging ace-
tate was “shifted” from À2.87 ppm in 7 to 4.02 ppm in 8, a
finding that is fully consistent with the large proposed differ-
ences in the electronics of these two systems.
spectrum of the bis-zinc complex of this antiaromatic
[24]rubyrin exhibits a slight up-field shift for the outer b-CH
proton resonances and a down-field shift of the inner ace-
tate protons due to the presumed presence of a paratropic
ring current effect. Taken in concert, such findings provide
support for the notion that both rubyrins 2 and 6 undergo
zinc(II) metallation without a change in their respective oxi-
dation states; thus the number of p-electrons in the bis-
zinc(II) complexes 7 and 8 are the same as those present in
2 and 6, respectively. On this basis, they are characterized
aromatic and antiaromatic, respectively.
Quantitative analyses of aromaticity and antiaromaticity of
compounds 2, 6, 7, and 8: To evaluate aromaticity and anti-
aromaticity of rubyrins 2, 6, 7, and 8, we have carried out
theoretical calculations by the DFT (B3LYP/6-31G*
(LANL2DZ for Zn)) method[29,30] on the model compounds
where pentafluorophenyl substituents of the real systems
were replaced with protons for simplicity. Nucleus-inde-
pendent chemical shifts (NICS)[31] were obtained as the
qualitative measure for aromaticity and antiaromaticity on
optimized structures. The NICS values were calculated on
the global ring center of macrocycles, as well as centers of
each pyrrole unit and formal ring structure between two ad-
jacent pyrroles, as shown in Figure 8. Clearly, [26]rubyrin 2
is strongly aromatic, for which the NICS value was calculat-
ed to be À18.2 ppm at the center of the molecule. On the
other hand, [24]rubyrin 6 exhibits a moderately positive
NICS value (+6.5 ppm) at the center. Moreover, NICS
values at the points 3, 5, 7, 9, 11, and 13 are substantially
large positive (11.3–16.1 ppm, Table 1). On the basis of
these results, we conclude that [24]rubyrin 6 has substantial
antiaromaticity, which is also experimentally indicated by
The structure of 8 was elucidated by single crystal X-ray
analysis, and was found to possess a type III conformation.
Thus, the two zinc ions coordinated by this antiaromatic ru-
byrin were found to be bridged by an acetate group, as well
as by a single oxygen atom arising from the other acetate
counter anion (Figure 7). Although the non-identical nature
of the acetate bridging seen in the solid state is slightly dif-
ferent from what might be inferred from the symmetrical
1
structure one would infer from the H NMR spectrum, the
difference can easily be accounted for in terms of “normal”
variations, such as crystal packing or, more likely, fast ex-
1
change on the H NMR time scale. In any event, apart such
1
small differences, it is important to appreciate that the struc-
ture of 8 is consistent with its formulation as a 24p-electron
expanded porphyrin. For instance, the side view of this com-
plex reveals the highly distorted nature of the structure.
the H NMR and absorption spectra.
For bis-zinc complexes of rubyrins, NICS values of 7 and
8 at the ring centers are À25.4 and +9.2 ppm, respectively,
suggesting 7 to be aromatic and 8 to be antiaromatic. This
indication is also in good agreement with experimental data.
1
Nonetheless, in spite of its nonplanar nature, the H NMR
Figure 7. X-ray crystal structure of 8, top view (left) and side view (right). The thermal ellipsoids are scaled to the 50% probability level. The pentafluor-
ophenyl substituents at the meso-positions are omitted for clarity in the side view.
2674
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 2668 – 2678

Rubyrin and Higher Homologues
FULL PAPER
as an effective anion receptor
for the chloride and sulfate
anions, as revealed by anion ex-
change experiments monitored
by
UV/Vis
spectroscopy.
[26]Rubyrin 2 exhibits a dia-
tropic ring current, as would be
expected given its 26 p-electron
aromatic nature. In contrast, its
two-electron oxidized form,
[24]rubyrin 6, prepared by the
oxidation of 2 with MnO2, dis-
plays a paratropic ring current
consistent with an antiaromatic
character. Both 2 and 6 are
metallated readily with zinc(II)
ions to provide the correspond-
ing bis-zinc complexes 7 and 8
in quantitative yields. Both sets
of complexes adopt type III
conformations with all the pyr-
role rings pointing inward. They
both retain the fundamental
conjugation pathways present
in the initial non-metallated
species (i.e., ligands 2 and 6).
They thus display diatropic and
Figure 8. Optimized structures of models for compounds 2, 6, 7, and 8. Points for the NICS calculations are
also shown.
paratropic ring currents, respectively, which are also sup-
ported by quantitative analyses of the optimized structures
by using NICS values. The unique properties displayed by
these rubyrins make both of them and their larger conge-
ners, including 3, 4 and 5, attractive systems with which to
study the effects of both cation coordination and anion bind-
ing on conformational motion and aromatic–antiaromatic
switching. Further studies of these and related systems are
thus ongoing in our laboratories.
Table 1. NICS values [ppm] of the selected points as depicted in
Figure 8.
Point
2
6
7
8
1
2
3
4
5
6
7
8
À18.2
À12.4
À19.5
À13.4
À17.1
À3.5
6.5
6.5
À25.4
À10.8
À22.6
À10.9
À20.4
À17.4
À20.8
À10.8
À22.6
À10.9
À20.4
À17.4
À20.8
9.2
À1.6
23.4
À3.7
23.0
À1.5
18.8
À2.4
23.3
À4.6
23.5
À2.2
19.0
11.3
À5.9
16.1
À5.2
12.1
6.5
11.3
À5.9
16.1
À5.2
12.1
9.7
À18.1
À12.4
À18.8
À13.4
À17.1
À3.5
9
10
11
12
13
14
15
Experimental Section
À18.1
À15.4
À15.4
General procedure: All reagents and solvents were of commercial re-
agent grade and were used without further purification except where
noted. Dry CH2Cl2 was obtained by distilling over CaH2. 1H and
19F NMR spectra were recorded on a JEOL ECA-600 spectrometer (op-
erating as 600.17 MHzfor 1H and 564.73 MHzfor 19F) using the residual
solvent as the internal reference for 1H (d=7.260 ppm for CDCl3) and
hexafluorobenzene as an external reference for 19F (d=À162.9 ppm).
Spectroscopic grade CH2Cl2 was used as the solvent for all spectroscopic
studies, except those involving anion binding, which were carried out in
methanol. UV-visible absorption spectra were recorded on a Shimadzu
UV-3100 spectrometer. Mass spectra were recorded on a JEOL HX-110
spectrometer using the positive-FAB ionization method with an acceler-
ating voltage 10 kV and a 3-nitrobenzylalcohol matrix, or on a Shimadzu/
9.7
Again, NICS values of 8 at centers of formal ring structures
between two adjacent pyrroles (point 3, 5, 7, 9, 11, and 13)
are significantly large positive (18.8–23.5 ppm).
Conclusion
KRATOS KOMPACT MALDI
4 spectrometer using the positive-
The coupling reaction of tripyrrane 1 with TFA and chlora-
nil provide large expanded porphyrins 4 and 5 in addition to
2 and 3. [62]Pentadecaphyrin(1.1.0.1.1.0.1.1.0.1.1.0.1.1.0) 5 is
the second largest expanded porphyrins structurally charac-
terized so far. The protonated form of rubyrin rubyrin 2 acts
MALDI ionization method. ESI-TOF-MS spectra were recorded on a
BRUKER microTOF, using either the positive and negative-ion modes;
acetonitrile was used as the solvent. Preparative separations were per-
formed by silica gel flash column chromatography (Merck Kieselgel 60H
Art. 7736), silica gel gravity column chromatography (Wako gel C-400),
Chem. Eur. J. 2008, 14, 2668 – 2678
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2675

A. Osuka, J. L. Sessler et al.
or recycling preparative GPC-HPLC (JAI LC-908 with preparative
JAIGEL-2H, 2.5H, and 3H columns).
b-CH), 7.05 (brs, 4H; b-CH), 6.78 (d, J=4.6 Hz, 4H; b-CH), 6.71 (d, J=
4.0 Hz, 4H; b-CH), 6.35 (brs, 4H; b-CH), 6.19 ppm (s, 4H; b-CH);
19F NMR (565 MHz, CDCl3, 298 K): d=À136.3 (d, J=24.1 Hz, 4F; o-F),
À136.4 (d, J=20.7 Hz, 4F; o-F), À139.5 (dd, J=23.3 Hz, J=7.7 Hz; 4F;
o-F), À140.0 (brs, 4F; o-F), À153.4 (t, J=20.7 Hz, 4F; p-F) À153.6 (q,
J=20.7 Hz, 4F; p-F), À158.2 (m, 4F; m-F), À160.2 (brs, 4F; m-F),
À161.8 (m, 4F; m-F), À162.0 ppm (m, 4F; m-F); UV/Vis (CH2Cl2): lmax
(e)=327 (69000), 412 (70000), 657 (136000), 784 nm (54000 mÀ1 cmÀ1);
HR-ESI-TOF-MS: m/z (%): calcd for C104N12F40H31: 2207.2161; found:
2207.2089 (100) [MÀÀH].
Crystallographic data collection and structure refinement: Data collec-
tion for compound 8 was carried out at low temperature (À1538C) on a
Rigaku RAXIS-RAPID with graphite monochromated MoKa radiation
(l=0.71069 Š), whereas data for 4 and 5 was collected at À1838C on a
Bruker SMART APEX with graphite monochromated MoKa radiation
(l=0.71069 Š). Details of the crystallographic data are listed in Table 2.
Table 2. Crystallographic details for 4, 5, and 8.
meso-Pentafluorophenyl
substituted
[62]pentadecaphyr-
in(1.1.0.1.1.0.1.1.0.1.1.0.1.1.0) 5: 19F NMR (565 MHz, CDCl3, 253 K): d=
À134.5 (brs, 1F; o-F), À136.0–À137.19 (broad peaks, 12F; o-F), À138.0
(brs, 1F; o-F), À139.2 (brs, 1F; o-F), À139.3 (d, J=21.5 Hz, 1F; o-F),
À139.5 (d, J=22.6 Hz, 1F; o-F), À140.9 (brs, 1F; o-F), À141.3 (brs, 1F;
o-F), À141.5 (brs, 1F; o-F), À151.8 (brs, 1F; p-F), À152.1 (brs, 2F; p-F),
À152.5 (brs, 1F; p-F), À152.5–À153.1 (broad peaks, 3F; p-F), À154.0
(brs, 1F; p-F), À154.2 (brs, 1F; p-F), À154.8 (brs, 1F; p-F), À159.6 (brs,
1F; m-F), À160.6 (m, 2F; m-F), À160.8 (brs, 1F; m-F), À161.3–À161.4
(broad peaks, 3F; m-F), À161.6 (brs, 2F; m-F), À162.2 (m, 1F; m-F),
À162.5 (m, 1F; m-F), À162.6 (m, 1F; m-F), À163.0–À163.1 (m, 2F; m-
F), À163.4 (brs, 1F; m-F), À164.1 (brs, 1F; m-F), À164.4–À164.5 (m, 3F;
m-F), À165.4 ppm (brs, 1F; m-F); UV/Vis (CH2Cl2): lmax (e)=379
(97000), 598 nm (120000 mÀ1 cmÀ1); HR-ESI-TOF-MS: m/z (%): calcd
for C130N15F50H36: 2756.2485; found: 2756.2530 (100) [MÀÀH].
[24]Rubyrin 6: An excess amount of MnO2 (ca. 10 equiv) was added to a
dichloromethane solution of 2. This addition caused the color of the solu-
tion to turn from violet to dark orange. After confirmation the oxidation
was complete (as inferred from TLC analysis), the insoluble material
(presumed to be inorganic salts) was removed by filtration. Since the sol-
ubility of 6 is poor once dried, the initial filtrate obtained as the result of
this operation was used for all analyses and reactions. 1H NMR
(600 MHz, CDCl3, 298 K): d=21.77 (brs, 2H; NH), 12.57 (d, J=4.8 Hz,
2H; b-CH), 11.80 (d, J=4.8 Hz, 2H; b-CH), 5.82 (d, J=4.6 Hz, 2H; b-
CH), 5.66 (d, J=4.6 Hz, 2H; b-CH), 5.41 (d, J=4.6 Hz, 2H; b-CH),
5.18 ppm (d, J=4.6 Hz, 2H; b-CH); 19F NMR (565 MHz, CDCl3, 298 K):
d=À135.7 (brs, 4F; o-F), À138.1 (d, J=15.7 Hz, 4F; o-F), À151.6 (t, J=
21.6 Hz, 2F; p-F), À152.1 (t, J=19.1 Hz, 2F; p-F), À160.5 (m, 4F; m-F),
À162.2 ppm (m, 4F; m-F); UV/Vis (CH2Cl2): lmax (e)=439 (50100),
506 nm (39900 mÀ1 cmÀ1); HR-ESI-TOF-MS: m/z (%): calcd for
C52N6F20H14Na1, 1125.0853; found: 1125.2715 (100) [M++Na].
4
5
8
empirical formula
Mr
crystal system
space group
a [Š]
b [Š]
c [Š]
a [8]
b [8]
C
104N12F40H36Cl12 C137N15F50H53
C56N6F20H18O4Zn2
1349.50
monoclinic
P21/a (14)
13.851(3)
23.024(5)
17.798(4)
90
2710.91
triclinic
P1 (2)
2858.94
monoclinic
P21/c (14)
36.091(3)
24.210(2)
15.1930(14)
90
99.043(2)
90
13110(2)
4
¯
9.7166(15)
15.142(2)
18.476(3)
84.703(2)
75.046(2)
89.022(2)
2614.9(7)
1
106.429(7)
90
g [8]
V [Š3]
Z
5444(2)
4
1.646
1 [gcmÀ3
]
1.722
1.448
m [mmÀ1
(000)
]
0.448 (MoKa
1344
)
0.137 (MoKa
5720
0.30x0.30x0.10 0.25x0.25x0.10
)
1.003 (MoKa
2672
)
FACHTREUNG
crystal size [mm3] 0.80x0.40x0.20
2qmax [8]
T [K]
diffractometer
total reflections
unique reflections 9162
reflection used
parameters
absorption
correction
R1
50.0
90(2)
50.0
90(2)
55.0
123(2)
Raxis Rapid
52391
12390
12390
795
Smart Apex
24893
Smart Apex
66886
22974
22974
1822
9162
822
empirical
empirical
none
0.0958
0.2985
1.048
0.1031
0.2357
1.129
0.0781
0.2248
1.050
wR2
GOF
Bis-zinc complex of [26]rubyrin 7: To a dichloromethane solution of 2
containing a small amount of methanol, was added a mixture of zinc ace-
tate (ca. 10 equiv) and sodium acetate (ca. 10 equiv). Stirring the result-
ing solution caused the color to change from violet to bright red. The so-
lution was filtered off and all the solvent was removed under reduced
pressure to give a crude solid. This solid was redissolved in dichlorome-
thane, layered with hexane and allowed to stand overnight. The solution
was filtered and the filtrate was concentrated to dryness to give 7.
1H NMR (600 MHz, CDCl3, 298 K): d=10.88 (brs, 2H; b-CH), 10.37
(brs, 2H; b-CH), 9.57 (d, J=4.1 Hz, 2H; b-CH), 9.52 (brs, 2H; b-CH),
9.34 (d, J=4.1 Hz, 2H; b-CH), 9.04 (brs, 2H; b-CH), À2.87 (s, 3H;
CH3COO), À5.14 (s, 2H; NH), À8.97 ppm (brs, 1H; OH); 19F NMR
(565 MHz, CDCl3, 298 K): d=À134.67 (d, J=20.7 Hz, 2F; o-F), À136.96
(d, J=20.7 Hz, 2F; o-F), À137.91 (d, J=27.3 Hz, 2F; o-F), À138.55 (d,
J=20.7 Hz, 2F; o-F), À152.25 (t, J=20.7 Hz, 2F; p-F), À152.58 (t, J=
20.7 Hz, 2F; p-F), À161.73 (t, J=27.6 Hz, 2F; m-F), À161.98–
À162.24 ppm (m, 6F; m-F); UV/Vis (CH2Cl2): lmax (e)=521 (301000),
850 nm (37000 mÀ1 cmÀ1); MALDI-TOF-MS: m/z (%): calcd for
C52N6F20H14Zn2: 1229.9544; found: 1229.15 (100) [M+ÀOAcÀOH].
The structures were solved by direct methods (Sir 97[32] or SHELXS-
97[33]) using the full-matrix least square technique (SHELXL-97).[33] Sol-
vent molecules contained in the lattice of 8 were severely disordered and
could not be resolved. The program SQUEEZE[34a] in PLATON[34b] was
used to remove the solvent electron density.
CCDC 658457 (4), 658458 (5) and 658459 (8) 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.
Synthesis of rubyrin 2 and higher homologues: TFA (30.6 mL, 0.4 mmol)
was added to a solution of 1 (450 mg, 0.81 mmol) in CH2Cl2 (90 mL). The
resulting solution was stirred for 90 min at room temperature under a ni-
trogen atmosphere. At this point, chloranil (594 mg, 2.63 mmol) was
added and the mixture was heated at reflux for a further 90 min. The re-
action was quenched by the addition of aqueous NaHCO3, and the or-
ganic layer was washed once with water, and dried over anhydrous
Na2SO4. After removal of solvent, the crude product was purified over a
neutral alumina column using a mixture of CH2Cl2/hexane as an eluent.
After elution of deeply colored fractions, a purple fraction was eluted
with CH2Cl2, which gave 2 (107 mg, 24%). The first eluted fraction from
this alumina column was further purified by gel-permeation chromatogra-
phy to give 3, 4, and 5 in yields of 9.0, 2.7, and 1.5%, respectively.
Bis-zinc complex of [24]rubyrin 8: This complex was prepared in a fash-
ion identical to that used to prepare 7. 1H NMR (600 MHz, CDCl3,
298 K): d=6.16 (d, J=4.8 Hz, 4H; b-CH), 5.22 (d, J=4.9 Hz, 4H; b-
CH), 5.15 (s, 4H; b-CH), 4.02 ppm (s, 6H; CH3COO); 19F NMR
(565 MHz, CDCl3, 298 K): d=À138.3 (d, J=20.9 Hz, 4F; o-F), À140.7
(d, J=19.9 Hz, 4F; o-F), À152.38 (t, J=20.7 Hz, 4F; p-F), À160.9 ppm
(m, 8F; m-F); UV/Vis (CH2Cl2): lmax (e)=317 (18000), 423 (48000), 555
meso-Pentafluorophenyl
substituted
[52]dodecaphyr-
in(1.1.0.1.1.0.1.1.0.1.1.0) 4: 1H NMR (600 MHz, CDCl3, 298 K): d=14.40
(brs, 2H; NH), 8.66 (brs, 4H; NH), 7.23 (brs, 2H; NH), 7.18 (brs, 4H;
2676
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 2668 – 2678

Rubyrin and Higher Homologues
FULL PAPER
(68000), 639 nm (11000 mÀ1 cmÀ1); HR-ESI-FT-ICR-MS: m/z (%): calcd
for C54N6F20H15O2Zn2, 1286.9520; found: 1286.9504 (100) [M+ÀOAc].
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This work was partly supported by Grant-in-Aids (No. 18850003 to S.S.
and 19205006 to A.O.) from Japan Society for the Promotion of Science
(JSPS) and by the U.S. National Institutes of Health (grant no. GM
588907 to J.L.S.). The authors would like to thank Dr. Elisa Tomat for
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