Tautomerism in novel oxocorrologens

Article abstract of DOI:10.1002/chem.200701428

A novel corrole-type macrocycle, oxocorrologen (2), substituted with hemiquinone groups, has been synthesized. It was found to undergo multiple tautomerism of its exchangeable protons between electronegative atom sites at the macrocyclic core (nitrogen atoms) and periphery (phenol oxygen atoms). Alkylation at one macrocyclic nitrogen atom with a 4-nitrobenzyl group gave 3, which can exist in only two tautomeric forms depending on the solvent. Tautomerism has beenstudied by means of ¹H NMR spectroscopy in a variety of solvents and solvent mixtures. Tautomer structure assignments have been supported by DFT calculations of the relative energies of the tautomers. X-ray crystallography of the N-nitrobenzyl derivative has revealed that intramolecular hydrogen bonding may be responsible for stabilizing the observed tautomers. The solvent dependence of the tautomerism of 2 and 3 confers solvatochromism. Electrochemical measurements on 2 and 3 in their respective quinone forms have revealed irreversible processes, but indicate that they are both electron-deficient with a small HOMOLUMO gap and first reduction potentials close to those of fullerene electron acceptors.

Full text of DOI:10.1002/chem.200701428

DOI: 10.1002/chem.200701428
Tautomerism in Novel Oxocorrologens
Yongshu Xie,[a, d] Jonathan P. Hill,*[a] Amy Lea Schumacher,[b] Paul A. Karr,[b]
Francis DꢀSouza,*[b] Christopher E. Anson,[c] Annie K. Powell,[c] and Katsuhiko Ariga[a]
1
Abstract: A novel corrole-type macro-
cycle, oxocorrologen (2), substituted
with hemiquinone groups, has been
synthesized. It was found to undergo
multiple tautomerism of its exchangea-
ble protons between electronegative
atom sites at the macrocyclic core (ni-
trogen atoms) and periphery (phenol
oxygen atoms). Alkylation at one mac-
rocyclic nitrogen atom with a 4-nitro-
benzyl group gave 3, which can exist in
only two tautomeric forms depending
on the solvent. Tautomerism has been
studied by means of H NMR spectros-
copy in a variety of solvents and sol-
vent mixtures. Tautomer structure as-
signments have been supported by
DFT calculations of the relative ener-
gies of the tautomers. X-ray crystallog-
raphy of the N-nitrobenzyl derivative
has revealed that intramolecular hydro-
gen bonding may be responsible for
stabilizing the observed tautomers. The
solvent dependence of the tautomerism
of 2 and 3 confers solvatochromism.
Electrochemical measurements on
2
and 3 in their respective quinone forms
have revealed irreversible processes,
but indicate that they are both elec-
tron-deficient with a small HOMO–
LUMO gap and first reduction poten-
tials close to those of fullerene electron
acceptors.
Keywords: corroles · N-alkylation ·
porphyrinoids · quinones · tauto-
merism
Introduction
the respective molecule, with the isomers referred to as tau-
tomers. Probably the most common example of the phenom-
enon is prototropic keto–enol tautomerism, in which an ex-
changeable proton can be bonded at oxygen or another elec-
tronegative atom within the same molecule yielding two dis-
tinct structures.[2] Despite its conceptual simplicity, this fea-
ture is of paramount importance in biochemical systems[3,4]
and also from the point of view of chemical reactivity.[5] The
existence of an individual molecule in multiple stable states
is also important with regard to potential applications, espe-
cially if each state has discrete electrochemical and/or spec-
troscopic characteristics. This would permit the construction
of molecular multi-throw switches and other devices with
potential for molecular level information processing.[6,7] Tau-
tomeric variation of molecular structure should be of use
since it is a facile process that can be influenced by an exter-
nal stimulus such as incident electromagnetic radiation.[8,9]
In the realm of tetrapyrrole chemistry, there is no more fa-
miliar example of tautomerism than that of the porphyrin
pyrrolic amine groups at the core of the macrocycle[10] or at
the periphery in N-confused porphyrins.[11] Moreover, com-
pounds containing the corrole framework are currently a
subject of intensive investigation because of their exception-
al properties related to advanced applications[12–14] and be-
cause of recent advances in synthetic procedures.[15] Here,
we report multiple prototropic tautomerism[16] in a novel ox-
Isomerism of a molecule occurring by an easy conversion
between different forms as a result of the movement of one
or more hydrogen atoms is known as prototropic tautomer-
ism[1] and is intimately related to the structural identity of
[a] Dr. Y. Xie, Dr. J. P. Hill, Dr. K. Ariga
WPI Center for Materials Nanoarchitectonics
and Supermolecules Group
National Institute for Materials Science
Namiki 1-1, Tsukuba, Ibaraki 305-0044 (Japan)
Fax : (+81)29-860-4832
E-mail: Jonathan.Hill@nims.go.jp
[b] Dr. A. L. Schumacher, Prof. P. A. Karr, Prof. F. DꢀSouza
Department of Chemistry, Wichita State University
1845 Freemount, Wichita, Kansas 67260-0051 (USA)
Fax : (+1)316-978-3431
E-mail: Francis.DSouza@wichita.edu
[c] Dr. C. E. Anson, Prof. A. K. Powell
Institute for Inorganic Chemistry
University of Karlsruhe, Geb. 30.45
Engesserstrasse 15, 76128 Karlsruhe (Germany)
[d] Dr. Y. Xie
Current address: Key Laboratory for Advanced Materials
and Institute of Fine Chemicals
East China University of Science and Technology
Meilong Rd. 130, Shanghai 200237 (PR China)
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
ocorrologen-type molecule.[17] We also report the controlling
effect on this tautomerism of alkylation at one of the central
nitrogen atoms.
Results and Discussion
To address the problem of constructing a molecule possess-
ing multiple stable states, we considered that the asymmetry
of the corrole macrocycle would be of importance when
combined with the tautomerism of oxoporphyrinogen 1[18,19]
(see Figure S1) and we sought to advance this idea by the
design of 2, the corrole analogue of 1. If the molecular sym-
metry of 1 is broken by deliberate removal of one pyrrole–
pyrrole bridging group, then compound 2 is obtained. Al-
though oxoporphyrinogen compounds have been known for
some time, the corresponding oxocorrologen compounds
have not been investigated. We speculated that the quinoi-
dal substituents might also stabilize an oxocorrologen, and
this was proven by our synthesis of compound 2. The mole-
cule thus obtained possesses four different electronegative
atom sites (as opposed to two in 1 or corrole), and as a
result this molecule can exist as several possible tautomers
(see Scheme 1). These tautomers can be classified according
to the state of protonation at the macrocyclic nitrogen
atoms. Thus, type 1 possesses four exchangeable NH pro-
tons, type 2 has three NH groups, type 3 has two NH groups,
and type 4 contains one exchangeable NH proton. The syn-
thesis of 2 was accomplished by a known method.[15]
The tautomerization of 2 can be controlled to some
1
extent by using different solvents, as shown by the H NMR
spectra of 2 in various solvents in Figure 1. In polar solvents
such as [D6]DMSO (Figure 1a)i), b)i)), a pure tautomer is
present and it can be assigned the structure 2a since all ni-
trogen atoms are protonated and all oxygen atoms are non-
protonated (type 1). The spectrum features four resonances
due to pyrrolic protons (in [D6]DMSO two are overlapped)
and three due to the alkene protons of the meso-substituents
since these groups are not free to rotate. The broadness of
this spectrum is most likely due to a rapid exchange of sol-
vent hydrogen-bonded at the amino groups. Tautomer 2a is
analogous to the porphyrinogen form of compound 1.[18] In
polar media, the porphyrinogen form 2a is present because
of hydrogen bonding to the solvent molecules; indeed, oxo-
porphyrinogens have a known propensity to bind solvents
through this interaction.[20] The resonances due to the NH
protons, the meso-substituent alkene protons, and the pyr-
rolic b-protons have the expected relative integrals of 4:6:8
(see the Supporting Information). Dissolution in other sol-
vents results in tautomeric mixtures. Tautomers 2b (type 2)
and 2c (type 3) appear when 2 is dissolved in less polar sol-
vents ([D8]toluene: Figure 1a)ii), b)ii); CDCl3: Figur-
e 1a)iii), b)iii); 40% (v/v) CD2Cl2 in [D14]hexane: Figur-
e 1a)iv), b)iv)). Both tautomers possess phenolic protons
and the type 3 structure of 2c is assigned on the basis of the
presence of two phenolic and two NH proton resonances in
Scheme 1. Structures of compounds 1 and 2 and the possible tautomers
of 2 classified according to the positions of the exchangeable protons.
Note that 2d is antiaromatic.
NH peaks indicates an intramolecular hydrogen-bonding in-
teraction.[21] It is apparent that while a three-way tautomer-
1
its H NMR spectrum. The substantial downfield shift of the
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J. P. Hill, F. D’Souza et al.
of tautomers 2b and 2c in CD2Cl2/[D14]hexane or CDCl3 are
E
R
T
U
H
C
A
G
also reasonably well resolved, d) when the pyrrole N is pro-
tonated, that is, NH, resonances due to protons attached to
the pyrrole b-positions are broader than those of protons on
pyrrole groups with imine-type nitrogen atoms. First, 2D-
COSY spectra were used to determine resonances due to
pyrrole protons attached to the same pyrrole nucleus, and to
determine which meso-substituent resonances were related.
Subsequently,
a combination of qualitative steady-state
NOE experiments and 2D-NOESY spectra was used to col-
lect information on the relative positions of the pyrrole
units and the meso-substituents. As a starting point for the
assignments, we located the resonances due to the aromatic
protons of the phenolic substituents since these appear as
simple singlets and should stimulate NOE responses with
two pyrrole groups. Irradiation of the protons of the alkene
or aromatic units of the meso-substituents, in conjunction
with the 2D-COSY spectra, proved sufficient to assign the
relative positions of the pyrrole groups and meso-substitu-
ents. Either correlations due to long-distance coupling be-
tween the pyrrole NH and the corresponding b-H or line
broadening in the 1D proton NMR spectra revealed the po-
sitions of the NH groups. Protons adjacent to the corrole
pyrrole–pyrrole bond were assigned on the basis of their
lack of NOE response upon irradiation of any of the meso-
substituent protons.
Tautomer 2a is predominant (>95 mol%) in [D8]THF so-
lution, and COSY and NOESY spectra were used to assign
all of its resonances (Figure 2). [D8]THF was used as solvent
because of the poor solubility of 2 in [D6]DMSO (in which
only tautomer 2a is present). Signals between d=7.00 and
6.75 ppm are due to the protons of the pyrrole groups, while
those between d=8.00 and 7.60 ppm are due to the protons
at the meso-substituent (in this case, alkene-H). Correlations
between these peaks in the 2D-COSY and 2D-NOESY
spectra were used to confirm the structure of this tautomer
and to assign the origin of the resonances. Also, resonances
due to the tert-butyl groups could be unequivocally assigned
(see the Supporting Information), and long-range couplings
between pyrrole b-H and the corresponding pyrrolic amine
protons were evidenced. The most upfield resonance of the
pyrrole groups was assigned to the outer proton of the cor-
role-type pyrrole group since it showed no NOE correlation
with any meso-substituent. Subsequent correlations are
shown in Figure 2.
Figure 1. 1H NMR spectra of 2: a) aromatic region, b) pyrrole NH region.
Solvents: i) [D6]DMSO (2a), ii) [D8]toluene (2b: 70%; 2c: 30%),
iii) CDCl3 (2b: 31%; 2c: 69%), and iv) CD2Cl2 in [D14]hexane (40% v/v)
(2b: 19%; 2c: 81%).
ism is operative for compound 2, only the tautomer 2a is
available in a demonstrably pure form at one extreme of
solvent polarity. 2b is not isolable, being present at a maxi-
mum level of ꢀ70% in [D8]toluene, and the maximum level
of 2c is about 80% at the other accessible extreme of sol-
vent non-polarity in this system. Interestingly, the tautomer-
ism of 2 does not extend detectably to an isomer containing
three phenolic protons (i.e., a type 4 tautomer) under any
conditions. This is most probably due to the antiaromatic
character and hence the reduced stability of type 4 tauto-
mers.[22] Alternatively, the polarity-dependent trend towards
stability of the more highly phenol-substituted tautomers
suggests that a medium of extremely low polarity might be
necessary to observe this species, although decreasing solu-
bility in nonpolar solvents such as hexane may preclude
such an observation.
Since the tautomerism of 2 is sensitive to solvent polarity,
we used this fact, in conjunction with correlation NMR and
NOE NMR spectroscopies, to determine the identities of
the tautomers present among the possible tautomeric struc-
tures. Initially, the NH/OH ratio of the tautomers could be
deduced from the 1D proton NMR spectra since meso-sub-
stituents such as phenol groups should be rotatable and this
is reflected in the presence of a singlet resonance due to the
phenyl protons of these substituents. The relative peak areas
of the resonances in spectra of mixtures of tautomers can be
used to assign resonances to particular tautomers. Overlap-
ping peaks can also be detected from the integrated intensi-
ties. Peaks due to phenol resonances are also a good indica-
tor of the tautomer type since types 1, 2, and 3 contain no,
one, and two exchangeable phenol protons, respectively. In
the present case, it is fortuitous that a) tautomer 2a is avail-
able in pure or nearly pure form, b) resonances in the spec-
tra of mixtures of tautomers 2b and 2c in [D8]toluene are
very well resolved, c) resonances in the spectra of mixtures
For tautomer 2b, the highest molar proportion of 70% is
found in [D8]toluene. The proton NMR spectrum in
[D8]toluene could be partly assigned using 2D-COSY, but
2D-NOESY spectra were less informative because of the
peaks due to residual toluene, which appeared at around
d=7.0 ppm. The structure of the oxocorrologen framework
of 2b was assigned with the aid of difference NOE spectros-
copy, the relevant NOE signals being indicated in Figure 3.
The resulting structure immediately discounts possible tau-
tomer 2b(ii). The positions of the central protons were as-
signed on the basis of the broadening of the b-pyrrolic reso-
nances of the relevant pyrrolic groups, which is caused by
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Oxocorrologen Tautomerism
FULL PAPER
Figure 3. a) 2D-COSY (1H,1H) and b) i)–ix) steady-state NOE enhance-
ments obtained by irradiation at the indicated resonances of tautomer 2b
(downward arrows). Peaks and correlations due to 2c are indicated. The
strongly negative peak at the irradiation position has been removed for
clarity. c) Assignments of peaks and elicited NOEs.
Figure 2. a) 2D-COSY (1H,1H) and b) 2D-NOESY (1H,1H) for tautomer
2a. Minor peaks due to 2b are labeled. c) Assignment of the resonances
of 2a, with an indication of NOE correlations (arrows).
NOE signals expected from their free rotation, irradiation
of either of the alkene protons on the quinone group unex-
pectedly results in an NOE enhancement for two pyrrolic
protons as well as a negative peak for the remaining alkene
proton on the same substituent. Additionally, resonances
due to quinone alkene protons are usually broad and sharp-
en at decreased temperatures, suggesting some dynamic pro-
cess. To explain these phenomena, we consulted a molecular
model of 2c. In the model (Figure 5), the phenol groups
show the expected non-coplanarity with the oxocorrole mac-
rocycle, but the quinone group also displays a marked and
unexpected non-coplanarity. Furthermore, there appear to
be two possible extremes of this arrangement, although
long-range coupling with the respective NH protons.[23]
There is one other interesting feature of the spectra of 2b.
In solutions in chlorinated solvents or tetrahydrofuran, all
three amine protons are observable. However, in toluene
the broadest of these peaks cannot be seen and two reso-
nances (due to the b-H of one pyrrole group) are additional-
ly shifted to higher field by around 0.5 ppm.
For tautomer 2c in CDCl3 (Figure 4), NOE correlations
between two meso-phenol substituents and resonances from
a single pyrrole group already discount tautomers 2c
A
and 2c(iv). While the phenol substituents both give the two
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J. P. Hill, F. D’Souza et al.
Figure 5. Model of the twisting about the lone quinonoid group in tauto-
mer 2c, which results in broadening of the resonance due to its alkenyl
protons and permits an NOE with both pyrrole resonances adjacent to
this substituent. Note that the conformers are chiral. tert-Butyl and other
meso-substituents have been removed for clarity.
of observing a similar double NOE response. For 2c, the po-
sitions of the NH protons could again be assigned based on
the broadening of the corresponding b-pyrrole-H resonan-
ces.
To further observe the continuous change in the composi-
1
tion of the tautomeric mixture, a H NMR titration of 2 in
CD2Cl2 with [D8]THF was conducted and the result is
shown in Figure 6. As the proportion of [D8]THF was in-
creased, tautomers 2b and 2c gradually disappeared, which
was accompanied by a concurrent increase in the proportion
of 2a. At the starting point of the titration (100% CD2Cl2),
2c accounted for around 80 mol% of the 2 present. The
changes in the distribution of the tautomers during the titra-
tion are shown in Figure 6b. The existence of the discrete
tautomers 2a, b, and c can, in fact, be attributed to competi-
tive intramolecular/intermolecular hydrogen bonding.[21]
The 1H NMR spectra clearly demonstrate that both 2b
and c contain NH protons for which the chemical shifts
(13.5<d<18 ppm) indicate the occurrence of hydrogen
bonding, but these chemical shifts are not influenced by the
presence of a hydrogen-bond acceptor, such as a polar sol-
vent. For 2c, this indicates two non-identical intramolecular
hydrogen bonds, while 2b possesses one imine group, which
can be involved in an intramolecular hydrogen bond, leaving
the two amine groups available for intermolecular hydrogen
bonds. However, the titration indicates that for 2b only one
of these NH protons (that of ring c; see Figure 6a) actually
interacts strongly with a hydrogen-bond acceptor (i.e. THF).
This assignment can be made on the basis of the increased
flexibility about ring c relative to that about ring a (see Fig-
ure 2a), and is consistent with the findings of an X-ray crys-
tal structure determination for an N-alkyl derivative of 3
(see below).
Figure 4. a) 2D-COSY (1H,1H) and b) i)–v) steady-state NOE enhance-
ments obtained by irradiation at the indicated resonances of tautomer 2c
(downward arrows). Peaks and correlations due to 2b are indicated.
c) Assignments of peaks and elicited NOEs. Spectrum b) i) was used to
differentiate between protons j and k.
these extremes are chemically identical. At room tempera-
ture, there should be an equilibrium between these two
states leading to a broadening of the resonance due to the
alkene proton of the quinone meso-substituent. One further
indication of this non-coplanar arrangement is that there
exists the possibility of an NOE enhancement of the signals
of both pyrrole b-protons adjacent to this meso-substituent.
The negative signal might then be due to mixing of the
NOEs, although its intensity could suggest another mecha-
nism. It should be noted that the quinone groups in the
computed structure of 2b are also non-coplanar, but the in-
creased flexibility of the macrocycle compared to doubly in-
tramolecularly hydrogen-bonded 2c reduces the likelihood
1
In addition to the H NMR spectroscopic studies, we per-
formed a DFT computational analysis of the tautomers at
the B3LYP/6-31G(d) level using the Gaussian program (see
Figure 7). The energies of the tautomers relative to the por-
phyrinogen form 2a proved to be consistent with the tauto-
meric assignment derived from the NMR data. Thus, tauto-
mer 2b is more stable than the other possible tautomers by
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Oxocorrologen Tautomerism
FULL PAPER
cludes intramolecular H-bonds
and imposes a porphyrinogen-
like alternating puckered con-
formation of the pyrrole
groups. 2b has a single intramo-
lecular H-bond, which reduces
the flexibility of the macrocycle
and induces some non-coplanar-
ity of the quinone groups. The
double intramolecular H-bond-
ing in 2c results in near-planari-
ty of the macrocycle, forcing
the remaining quinone group
into a more non-coplanar con-
formation. The latter is reflect-
ed in the unusual NOE re-
sponse for 2c about the solitary
quinone moiety. Notably, the
possible tautomer 2c(i) does
not contain intramolecular H-
bonds, perhaps because of
steric effects, and this is reflect-
ed in its relatively poor stabili-
ty.
In seeking a means of further
controlling the tautomerism
and to assist in the assignment
of the relevant tautomer struc-
tures, N-alkylation of 2 was at-
tempted using various alkylat-
ing agents. This resulted in
complex mixtures of products,
in contrast to the highly regio-
selective N-alkylation observed
in the case of oxoporphyrino-
gen 1.[24,25] However, one prod-
1
Figure 6. a) 1H NMR titration of 2 in CD2Cl2 with [D8]THF. b) Percentages of 2a, b, and c during H NMR ti-
tration of 2 in CD2Cl2 with [D8]THF determined from the ratio of peak areas.
at least 7.43 kJmolÀ1. For 2c, this value falls to 2.96 kJmolÀ1,
although only two of the tautomers are stabilized, that is,
one with the two phenol groups at the 5,10-positions and an-
other with the phenol groups at the 5,15-positions. Taken to-
gether, the computed stability of the 5,10-tautomer relative
to its 5,15-isomer and the presence of two phenol resonan-
uct, 3, was obtained in unusually high yield and could be iso-
lated in pure form following the alkylation of 2 using 4-ni-
trobenzyl bromide; its crystal structure is shown in Figure 8.
Presumably, alkylation occurs preferentially at the porphy-
rin-type pyrrolic nitrogen because of the ease with which
these groups achieve a porphyrinogen conformation relative
to the a–a-linked corrole-type pyrrole groups. In the crystal
structure of 3b, exchangeable protons were located and
their positions were refined, and bond length and bond
angle analyses confirmed these locations. For the quinone
1
ces in the H NMR spectra support our assignment of the
structure of 2c. However, further computational analysis re-
garding the distribution of 2c and its 5,15-isomer revealed
an approximately 80:20 ratio of the two tautomers (see the
Supporting Information), although this was difficult to con-
À
oxygens O1 and O3, the C O bond lengths are 1.2294(3)
1
firm by H NMR because of the proximity of the resonances
and 1.2400(3) Š, which correspond to C=O double bonds.
of the phenolic protons. Additionally, the energy of the
type 4 tautomer, bearing three phenol groups, relative to 2a
(+59.39 kJmolÀ1) was indicative of a substantial loss of sta-
bility, which helps to explain its absence from our observa-
tions of this system. The computed structures of the tauto-
mers and their relative energies depend on the existence of
intramolecular hydrogen-bonding interactions and how
these influence the planarity of the tetrapyrrole. Thus, 2a
has all four central nitrogen atoms protonated, which pre-
The corresponding bond length for the phenolic C37-O2
À À
bond is 1.3719(3) Š. The larger C N C bonding angles for
N1, N2, and N4 relative to N3 indicate the NH character of
these atoms. Furthermore, this crystal structure confirms the
intramolecular hydrogen bonding between the proton at N2
and nitrogen N3. There is an intramolecular p–p stacking in-
teraction between the phenyl group of the N-alkyl substitu-
ent and the meso-substituent at C5, with a centroid–centroid
distance of 3.876(1) Š (see Figure 8). The molecular packing
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J. P. Hill, F. D’Souza et al.
Figure 8. ORTEP representation of the X-ray structure of 3b (tert-butyl
groups and H-atoms bonded to C have been omitted for clarity) and its
solvent-dependent tautomerism. Selected bond lengths and bond angles
À
À
À
(Š or 8): C23 O1 1.2294(3), C37 O2 1.3719(3), C51 O3 1.2400(3); C1-
N1-C4 109.20(3), C6-N2-C9 111.21(3), C11-N3-C14 107.28(2), C15-N4-
C18 112.66(3); N3···H2 1.9464(5); N3···H2-N2 132.16(3). Centroid (C20-
C21-C22-C23-C24-C25)–centroid (C63-C64-C65-C66-C67-C68) distance:
3.876(1) Š.
is strongly influenced by intermolecular noncovalent interac-
À
tions. First, a C H···O interaction exists between the nitro
group of the benzyl N-substituent and the methylene group
of the corresponding substituent in a neighboring molecule
(see Figure 9a). Second, the phenolic OH group is engaged
in hydrogen bonding with a quinone oxygen atom of a 3,5-
di-tert-butyl-4-oxocyclohexadienyl group, also in an adjacent
molecule (see Figure 9b).
The most notable property of this molecule is that it still
has the ability to tautomerize between forms 3a and b
Figure 7. DFT computational analysis of the tautomers of
2
at the
À
Figure 9. a) Intermolecular C H···O hydrogen bond involving the nitro
B3LYP/6-31G(d) level. Energy difference (DE in kJmolÀ1) of each possi-
ble tautomer is given relative to tautomer 2a. Tautomers 2b and 2c are
favored, supporting the assignment made on the basis of NMR spectros-
copy.
À
group in the X-ray crystal structure of 3b. C H···O bond length: 2.48 Š,
angle: 153.998. b) Hydrogen-bonding interaction between the phenolic
O H and a quinone group in an adjacent molecule of 3b. O H···O bond
length: 2.39(2) Š, angle 128.9(17)8.
À
À
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Oxocorrologen Tautomerism
FULL PAPER
under control by solvent polarity, a feature that does not
persist in the N-alkyl derivatives of 1. Figure 10 shows the
proton NMR spectra of the pure tautomers of 3. The num-
Figure 11. Electronic absorption spectra of 2 and 3 in the solvents indicat-
ed.
Further to the multiple tautomerism exhibited by 2 and
its modulation by N-alkylation, the electrochemical behavior
was investigated and compared with that of the structurally
similar derivatives of 1.[24c,d] Cyclic voltammograms of com-
pounds 1, 2, and 3 in DMSO are shown in Figure 12. DMSO
Figure 10. 1H NMR spectra of 3 in a) CD2Cl2 (3b), b) [D6]DMSO/CD2Cl2
(80% v/v) (3a) (*residual solvent).
bers of amine and phenol protons confirm the tautomeric
assignments. In contrast to the mixture of 2b and c in
CD2Cl2, pure 3b could be observed by NMR spectroscopy
and its spectrum is consistent with the crystal structure. Fur-
thermore, the crystal structure confirms intramolecular hy-
drogen bonding in 3a, as was inferred for 2b and c from
Figure 12. Cyclic voltammograms of i) 2, ii) 3, and iii) 1 in DMSO contain-
ing 0.1m (TBA)ClO4. Scan rate=100 mVsÀ1
.
was chosen since in this solvent all of these compounds exist
in the oxo (quinone) tautomeric form, that is, only one type
of species was expected to be present. The redox processes
were found to be irreversible, perhaps due to the associated
electron/proton coupled reactions. Of interest are the values
of the reduction potential and the HOMO–LUMO gap of 2
in comparison to those of 1. Compounds 2 and 1 possess
three and four hemi-quinonoid entities, respectively, and so
reduction of 2 is expected to be more difficult than that of
1. However, reductions of both 2 and 3 were found to be
facile and were located at Epc =À1.17 V vs Fc/Fc+, as com-
pared to a value of Epc =À1.32 V vs Fc/Fc+ for 1 in DMSO.
This suggests increased electron deficiency of 2 as compared
to 1. An estimation of the HOMO–LUMO gap (the poten-
tial difference between the first oxidation and first reduc-
tion) also yielded a smaller value for 2 (ꢀ1.42 V) as com-
pared to that for 1 (>1.53 V). In fact, oxidation of 1 could
1
their H NMR spectra. The intramolecular hydrogen bond-
ing is a key feature of this system since it confers both sol-
vent dependency and thermal stability on the tautomers and
permits us to control the tautomerism by varying the polari-
ty of the medium under ambient conditions rather than by
varying the temperature.
The electronic absorption spectrum of the oxocorrologen
2 is dependent on both the nature of the solvent and the
state of N-alkylation of the macrocycle. For solutions of 2 in
dichloromethane, the absorption maximum appears at
450 nm and is accompanied by a broad absorption at 700 nm
(see Figure 11). Interestingly, titration of a solution of 2 in
dichloromethane with DMSO or methanol permits observa-
tion of the electronic spectrum of 2a, in which all nitrogen
atoms bear protons.
Chem. Eur. J. 2007, 13, 9824 –9833
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www.chemeurj.org
9831

J. P. Hill, F. D’Souza et al.
filtration, washed with MeOH, and chromatographed on silica gel eluting
with dichloromethane. The second (dark-green) band eluting after the
porphyrin by-product was collected and further purified by column chro-
matography (silica; CH2Cl2/hexane 2:3) to afford 2 as a dark-green mi-
crocrystalline solid (135 mg, 1%). 1H NMR (CD2Cl2 containing 10%
[D6]DMSO, 300 MHz, 298 K): d=11.75 (br, 2H; NH), 10.94 (br, 2H;
NH), 7.92 (s, 2H; cyclohexadienyl-H), 7.60 (s, 2H; cyclohexadienyl-H),
not be recorded within the anodic potential window of
DMSO. The absorption spectra of 2 and 3 reveal red-shifted
bands compared to the spectrum of 1 in DMSO, which may
be attributed to the smaller HOMO–LUMO gap (see the
Supporting Information). The facile reduction and smaller
HOMO–LUMO gap for 2 as compared to 1 are noteworthy
observations. The first reduction potentials of 2 and 3 are
comparable to those of the widely used benzoquinone and
fullerene electron acceptors,[26] suggesting that oxocorroles
are promising candidates for use as components of novel
donor–acceptor systems.
7.56 (s, 2H; cyclohexadienyl-H), 6.81 (d, 3J
A
H), 6.71 (m, 4H; pyrrolic H), 6.63 (d, 3J
AHCTREUNG
1.35, 1.29, 1.27 ppm (3”s, 54H; tBu); UV/Vis (CH2Cl2): lmax (e)=391
(36100), 462 (64200), 676 nm (34500 molÀ1 dmÀ3 cmÀ1); MALDI-MS (di-
thranol): m/z: 910 [M+].
Compound 3: Compound 2 (18.2 mg, 0.02 mmol), 4-nitrobenzyl bromide
(21.6 mg, 0.1 mmol), and K2CO3 (0.3 g, 2.2 mmol) were added to anhy-
drous ethanol (5 mL). The resulting mixture was refluxed under dry air
for 7 h. The solvent was then removed under reduced pressure, the resi-
due was dissolved in dichloromethane, and the resulting solution was
washed with water and dried over Na2SO4. The crude product was puri-
fied by column chromatography on silica gel eluting with 50% dichloro-
methane in hexane to afford 3 as a dark-green solid (8.5 mg, 34%). 1H
NMR (CD2Cl2, 300 MHz, 298 K): d=13.90 (br, 1H; NH), 9.02 (br, 1H;
NH), 7.97 (s, 1H), 7.86 (d, 3J=8.5 Hz, 2H), 7.76 (s, 1H), 7.38–7.41 (m,
Conclusion
In summary, we have observed an unusual example of com-
plex multiple tautomerism in a novel oxocorrole derivative.
Alkylation of this compound at one macrocyclic nitrogen
atom modulates the tautomerism and permits switching be-
tween two pure isomers of the N-alkyl oxocorrole by simple
variation of solvent polarity. Isolation of the tautomers of 2
was more challenging, but high polarity yields a porphyrino-
gen form with other tautomers accessible at varying levels
by changing the solvent polarity. Electrochemical studies
have revealed electron deficiency and a smaller HOMO–
LUMO gap for oxocorroles 2 and 3 as compared to oxopor-
phyrinogen 1. We are currently seeking to control the tauto-
merism more finely by using a variety of stimuli. Further-
more, the construction of novel donor–acceptor systems
through N-alkyl derivatization[24e] of oxocorroles to observe
the effect of this tautomerization on potential intramolecu-
lar electron- and energy-transfer processes is currently un-
derway. We believe that this system is an important initial
step towards the development of a molecular switching
manifold based on poly-tautomerism.
3
3H), 7.08 (d, J=4.1 Hz, 1H), 6.96–6.99 (m, 3H), 6.89–6.93 (m, 2H), 6.85
(d, 3J
N
N
A
ACHTREUNG
A
ACHTREUNG
15.4 Hz, 1H; benzylic H), 1.07, 1.06, 1.02, 1.01, 0.93, 0.90 ppm (6”s, 54H;
tBu); UV/Vis (CH2Cl2): lmax (e)=361 (16400), 479 (86400), 694 nm
(26900 molÀ1 dmÀ3 cmÀ1); MALDI-MS (dithranol): m/z: 1044 [M+].
X-ray crystallography: Crystals suitable for X-ray analysis were grown
from a solution of 3 in dichloromethane/octane. 3b·2C8H18: C84H113N5O5,
F
W =1272.79 gmolÀ1, green block 0.42”0.36”0.29 mm3, monoclinic, P21/
c, a=20.7896(7), b=14.6515(5), c=25.7342(9) Š, b=112.105(1)8, V=
7262.4(4) Š3, F(000)=2768, 1calcd =1.146 gmolÀ1, m(MoKa)=0.071 mmÀ1
N
G
,
T=100 K, 40778 data measured on a Bruker SMART Apex diffractome-
ter, of which 13982 were unique (Rint =0.0332); 712 parameters refined
2
against Fo (all data), final wR2 =0.1356, S=0.987, R1 (9803 data with I>
À3
2s(I))=0.0503, largest final difference peak/hole=+0.62/À0.30 eŠ
.
Structure solution by direct methods and full-matrix least-squares refine-
ment against F2 (all data) using SHELXTL.[27] The coordinates of the
NH and OH H-atoms were refined; H atoms bound to C were placed in
calculated positions. The crystal structure contains two octane molecules
of crystallization per corrole molecule; these were found to be severely
disordered and were handled using the SQUEEZE option in
PLATON.[28] CCDC 641061 (3a) contains the supplementary crystallo-
graphic 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.
Experimental Section
General: Solvents and reagents used in this study were obtained from Al-
drich Chemical Co., Tokyo Kasei Chemical Co., or Wako Chemical Co.
Solvents for NMR spectroscopic measurements were obtained from
Cambridge Isotope Laboratories Inc. Electronic absorption spectra were
measured using a Shimadzu UV-3600 UV/Vis/NIR spectrophotometer.
1
All H NMR spectra were obtained using a JEOL AL300BX spectrome-
Acknowledgements
ter. Mass spectra were measured using a Shimadzu-Kratos Axima CFR+
MALDI-TOF mass spectrometer with dithranol as matrix. X-ray crystal-
lography was carried out on a Bruker SMART Apex diffractometer
equipped with a CCD area detector using MoKa radiation. Electrochemis-
try was performed using a three-electrode system. A platinum button
electrode was used as the working electrode. A platinum wire served as
the counter electrode and an Ag/AgCl electrode was used as the refer-
ence. Solutions were purged with argon prior to the electrochemical
measurements. All experiments were carried out at 23Æ18C.
Compound 2: Pyrrole (4.4 mL, 0.064 mol) was added to a refluxing mix-
ture of propionic acid (300 mL) and 3,5-di-tert-butyl-4-hydroxybenzalde-
hyde (15 g, 0.064 mol) and reflux was continued for 3 h under air. The
black solution was then concentrated to half of its original volume. The
mixture was allowed to stand open to air at room temperature for 5 d,
and then MeOH (200 mL) was added. The precipitate was collected by
This work was supported by a Grant-in-Aid for Science Research in a
Priority Area “Super-Hierarchical Structures” from the MEXT, Japan,
the National Science Foundation (Grant 0453464 to F.D.), the donors of
the Petroleum Research Fund administered by the American Chemical
Society, and the Deutsche Forschungsgemeinschaft.
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Chem. Eur. J. 2007, 13, 9824 –9833

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Received: September 11, 2007
Published online: October 19, 2007
Chem. Eur. J. 2007, 13, 9824 –9833
ꢁ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9833