Single helicates of dipalladium(II) hexapyrroles: Helicity induction and redox tuning of chiroptical properties

Article abstract of DOI:10.1002/anie.201207113

Hexapyrrole-α,ω-dialdehyde, which has eight donor atoms, afforded a dipalladium(II) single helicate (see picture; Pd pink, O red, N blue). A rapid interchange of the helical screw was slowed down by imine formation at the terminal aldehyde units with (R)-(-)-1-cyclohexylethylamine, leading to an overwhelming excess of a P-helical screw. This stable dinuclear single helicate has a redox-driven reversible change in chiroptical properties. Copyright

Full text of DOI:10.1002/anie.201207113

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Chemie
DOI: 10.1002/anie.201207113
Helical Structures
Single Helicates of Dipalladium(II) Hexapyrroles: Helicity Induction
and Redox Tuning of Chiroptical Properties**
Chaolu Eerdun, Satoshi Hisanaga, and Jun-ichiro Setsune*
Single helicates have some flexibility in their structures, in
contrast to double and triple helicates.[1] That is, their metal
sites can be exposed externally and their twist angle and
helical handedness can be changed under certain condi-
tions.[2–5] Such a dynamic structural feature of single helicates
constitutes a basis for unique molecular functions relevant to
chirality. In this context, metal complexes of open chain
oligopyrroles have been of great interest in view of their
porphyrin-like nature. A number of mononuclear complexes
Scheme 1. Synthesis of hexapyrrole 3 and the dipalladium(II) com-
of tetrapyrroles with helical structures have been reported,[6]
plexes 3Pd2, 4Pd2 (R=benzyl), 5Pd2 (R=(S)-(À)-1-phenylethyl), and
and a few examples of multinuclear tetrapyrroles are also
6Pd2 (R=(R)-(À)-1-cyclohexylethyl).
known.[7] However, control of their helical handedness is
difficult, and successful examples have been limited.[8–10]
Induction of a unidirectional helical screw and increasing
the chain length are the logical expansion in the chemistry of
oligopyrrolic helicates. Open-chain oligopyrroles with more
than five pyrrole units,[11] especially of their metal complexes,
are rare.[8c,d,10,12] A racemic mixture of dicopper(II) octapyr-
role has been reported recently as an exceptional example of
the multinuclear single helicate of long oligopyroles.[13] But,
these examples are short of structural diversity including
control of helicity. Development of longer-chain oligopyrroles
is necessary for new multinuclear single helicates. In partic-
ular, molecular design for the straightforward introduction of
a unidirectional helical screw with high stability against
racemization is very important. Herein, we describe hexa-
pyrrole-a,w-dialdehyde with eight donor atoms for metal
coordination and the terminal aldehyde groups for introduc-
tion of chirality, leading to a large excess of unidirectional
helical screw and the redox driven change of the chiroptical
properties in the dipalladium(II) single helicate.
potential bidentate ligands, various metal coordination modes
are expected for 3. When [PdCl2(MeCN)2] was reacted with 3
in a mixture of CH2Cl2/MeOH for 12 h, a dinuclear palladiu-
m(II) complex 3Pd2 was obtained in 57% yield (Scheme 1).
[16]
X-ray crystallography of 3Pd2
showed a single helicate
composed of a pair of square-planar palladium coordination
À
units with the Pd Pd distance of 3.166 ꢀ, which is shorter
than twice the van der Waals radius of palladium (3.26 ꢀ)[17]
(Figure 1). The angle between the two pyrrole mean planes in
The reaction of 6,6’-diphenyl-4,4’-dipropyl-2,2’-bis(azaful-
vene) 1[14] and 5-formyl-gem-dimethyl-2,2’-dipyrrylmethane
2[15] in CH2Cl2 was promoted by zinc p-tert-butylbenzoate to
give hexapyrrole-a,w-dialdehyde 3[16] in 73% yield after
Figure 1. X ray structures of 3Pd2 (left) and 6Pd2 (right) with an atom
numbering scheme. Propyl and phenyl groups are omitted for clarity.
À
Pd Pd distances [ꢀ]: 3.1662(5) (left) and 3.4704(5) (right).
oxidation
with
2,3-dichloro-5,6-dicyanobenzoquinone
(Scheme 1). As not only the dipyrrin unit but also the 2,2’-
bipyrrole unit and the 2-formylpyrrole unit are regarded as
the 2,2’-bipyrrole unit, the dipyrrylmethane unit, and the
dipyrrin unit is 37.48, 10.78, and 11.38, respectively. As a result,
each palladium coordination plane is helically distorted and
the twist in the central 2,2’-bipyrrole unit generates a primary
helical chirality.
[*] C. Eerdun, S. Hisanaga, Prof. J. Setsune
Department of Chemistry, Graduate School of Science
Kobe University
The variable-temperature (VT) 1H NMR spectra of 3Pd2
in CDCl3 showed that a broad singlet at 1.25 ppm owing to the
gem-dimethylmethylene bridge at 408C split into a pair of
signals at 1.7 and 0.7 ppm at À608C (Figure 2). One is similar
to the corresponding shift of 3 (1.80 ppm), while the other is
considered to reflect the magnetic circumstance inside the
cleft between two palladium coordination planes. This
observation suggests that the closed helical form of 3Pd2 is
Kobe 657-8501 (Japan)
E-mail: setsunej@kobe-u.ac.jp
[**] This work was supported by a Grant-in-Aid (No. 24550050) from the
Ministry of Education, Culture, Sports, Science and Technology
(Japan). We thank Prof. M. Tsubaki and Y. Furuie (Kobe University)
for ESR measurements.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201207113.
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1
Scheme 2. Conformational change between closed forms (P,P,P and
M,M,M), open forms (P,M,P and M,P,M), and an achiral form (M,P)
of 4Pd2 (X=N-benzyl). The models are seen along the direction of the
bipyrrole 2,2’-bond.
Figure 2. VT H NMR spectra of 3Pd2 in CDCl3. Marked signals are
due to the methyl protons of the gem-dimethylmethylene bridge.
favored in solution as well as in the crystal and the helicity
change is fast on the NMR timescale at room temperature.
The energy barrier (DG°253 ꢀ 47 kJmolÀ1) was estimated on
the basis of the coalescence temperature (À208C) and the
chemical shift difference at À608C.[18] A recent study argues
for the presence of a weak d8–d8 bonding interaction in the
dipalladium(II) complexes with a short metal–metal dis-
tance.[19] It might contribute to stabilize a closed form of 3Pd2
in comparison with other conformational isomers.
methyl singlets owing to the isomer of the least amount are
overlapped. These three isomers are in equilibrium as seen by
the change in their molar ratio from 4:10:86 at 208C to 7:13:80
at 908C in the 1H NMR in [D8]toluene (Supporting Informa-
tion, Figure S4). The stereochemical feature of the closed
form is described as (P,P,P) or (M,M,M) on the basis of three
components of the local helicity (Scheme 2). Rotation around
the bipyrrole 2,2’ bond of the (P,P,P) closed form leads to the
(P,M,P) open form that goes to the achiral form through the
flipping of the terminal formimidoylpyrrole moiety.
The oxygen atoms of the terminal formyl groups of 3Pd2
come close to the 3,3’-hydrogen atoms of the 2,2’-bipyrrole
unit when the 2,2’-bipyrrole unit takes an s-trans conforma-
tion in the course of interchanging helical chirality. Therefore,
transformation of the formyl group to a bulkier alkylimine
group would affect not only the dynamic behavior of the
conformational change but also the helical handedness if the
imine group is derived from an optically active alkylamine.
When 3Pd2 was reacted with benzylamine, (S)-(À)-1-phenyl-
ethylamine, and (R)-(À)-1-cyclohexylethylamine in CHCl3 at
reflux, the hexapyrrole-a,w-diimine dipalladium(II) com-
plexes, 4Pd2, 5Pd2, and 6Pd2, were obtained in 85%, 59%,
and 80% yield, respectively. As expected, a conformational
change of 4Pd2 was so slow that three sets of signals owing to
the conformational isomers were observed separately in the
1H NMR. The molar ratio of 8:18:74 was determined on the
basis of the singlets owing to the formimidoyl proton at 8.78,
8.52, and 7.03 ppm, respectively. A remarkable low-frequency
shift of the latter signal can be explained in terms of the
magnetic shielding effect of the closely situated Pd coordina-
tion plane in the closed form. The major isomer showed a pair
of singlets owing to the gem-dimethylmethylene bridge at 1.64
and 0.85 ppm (Supporting Information, Figures S2,S3). As
these chemical shifts are very similar to those observed for
3Pd2 at À608C, the major isomer of 4Pd2 is associated with
the closed form, which was verified by the X-ray crystallog-
As the complexes 5Pd2 and 6Pd2 have a stereogenic
center at the imine substituent, five diastereomers are
1
distinguishable. In fact, the H NMR analysis of 5Pd2 and
6Pd2 showed five and four sets of signals with the ratio of
4:1:1:1:1 and 16:1:1:1, respectively. For example, five quartets
and four quintets owing to the methine proton at the
stereogenic center of 5Pd2 and 6Pd2, respectively, are seen
(Figure 3a,b). The major isomer showed a pair of singlets
owing to the gem-dimethylmethylene bridge at 1.62 and
0.86 ppm for 5Pd2 and at 1.64 and 0.87 ppm for 6Pd2 that are
characteristic of the closed form (Supporting Information,
Figure S3). Repeated recrystallization of 6Pd2 from CH2Cl2/
[16]
raphy of 4Pd2 after recrystallization. Three small singlets
owing to the methyl protons of the gem-dimethylmethylene
bridge of the other two isomers, an open form of the C2 point
group and an achiral form of the Ci point group, were
observed at 1.85, 1.78, and 1.74 ppm with almost equal
intensity (Scheme 2). This is probably because a pair of
Figure 3. 1H NMR signals in CDCl3 due to the methine proton at the
stereogenic center of a) 5Pd2, b) 6Pd2, and c) a (P,P,P) isomer of
6Pd2.
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MeOH gave the major diastereisomer in a pure form as
evidenced by the H NMR analysis, which also showed that
racemization was negligible at ambient temperature for
a couple of days in CDCl3 (Figure 3c; Supporting Informa-
tion, Figure S5). The (P,P,P) helicity was determined by X-ray
crystallography (Figure 1).[16]
species was inferred from the selective disappearance of the
1
1H NMR signals owing to the 2,2’-bisdipyrrin part of 3Pd2
when 0.1 molar equiv of AgClO4 was added (Figure 5).
Signals for the protons far apart from the 2,2’-bisdipyrrin
part did not disappear. This observation is rationalized in
The relationship between the chiroptical property and the
helical handedness in the 2,2’-bisdipyrrin and biliverdin
derivatives has been known.[20] In accord with this, the
(P,P,P) helical form of 6Pd2 showed a positive CD signal at
699 nm, while a mirror-image CD spectrum was observed for
the enantiomer 6’Pd2 prepared from 3Pd2 and (S)-(+)-1-
cyclohexylethylamine (Figure 4). 5Pd2 which has an (S)-
stereogenic center at the imine auxiliary, showed a weak CD
Figure 5. 1H NMR spectra of 3Pd2 (8.9 mm) in CD2Cl2 (bottom) and
after adding 0.1 molar equiv of AgClO4 (59 mm) in [D8]toluene (top).
The marked signals are due to the formylpyrrole protons and the
propyl g-methyl protons.
terms of rapid electron hopping between 3Pd2 and its p cation
radical at the 2,2’-bisdipyrrin part.
One-electron oxidation of the (P,P,P) helical form of 6Pd2
occurred at by 0.07 V more negative potential than that of
3Pd2 (Supporting Information, Figure S12). The chemical
oxidation of 6Pd2 with AgClO4 completed at one molar equiv
of AgClO4. In the meanwhile, both the absorption band and
the CD Cotton effect of 6Pd2 at 699 nm shifted to 741 nm,
with isosbestic points at 428, 455, 522, 604, and 720 nm in the
UV/Vis and at 406, 556, and 723 nm in the CD (Figure 6). An
intense ESR signal at g = 2.001 with a linewidth of 6.0 G was
observed at room temperature (Supporting Information,
Figure S6). This one-electron oxidized species of 6Pd2 was
reduced cleanly by [CuI(MeCN)4]ClO4 to regenerate the
original UV/Vis and CD spectrum of 6Pd2. It is remarkable
that the helical chirality of 6Pd2 is maintained in this redox
cycle with the great change in the CD intensity, that is,
D(De) = À150 and + 117 LmolÀ1 cmÀ1, in the longer wave-
length region at 699 and 741 nm, respectively, upon oxidation
at the not so positive potential corresponding to the + 0.14 V
redox couple versus Fc/Fc+.
In conclusion, the unique dynamic structure of the single
helicate of dipalladium(II) hexapyrroles was elucidated. The
rate of the conformational change was significantly slowed
down by the conversion of the terminal formyl group into the
imine group. The closed form was generally preferred in these
helicates and the diastereoselectivity and helical handedness
were markedly influenced by the point chirality at the imine
substituent. As a matter of fact, the (P,P,P) helical isomer
formed with as high as 85% diastereoselectivity by using (R)-
(À)-1-cyclohexylethylamine and it was readily isolated in
Figure 4. UV/Vis spectra of 3Pd2, 4Pd2, 5Pd2, and 6Pd2 (bottom), and
CD spectra of 5Pd2, 6Pd2, and 6’Pd2 (top) in CH2Cl2.
signal of a negative sign at 689 nm. As 5Pd2 is an equilibrium
mixture of the five diastereomers with a 4:1:1:1:1 ratio, the
contributions of the minor diastereomers to the whole CD
spectrum would be cancelled out. Thus, the observed CD
spectrum of 5Pd2 is considered to reflect a CD profile of the
major isomer with the (M,M,M) helical conformation.
In view of the recent progress in the chiroptical molecular
switch,[21] the redox chemistry of the hexapyrrole dipalladiu-
m(II) complexes is of great interest. Cyclic voltammetry of
3Pd2 showed two redox couples at + 0.21 and + 0.60 V vs
ferrocene/ferrocenium (Fc/Fc+) standard in CH2Cl2 (Support-
ing Information, Figure S12). These values are similar to the
potentials for the ligand centered oxidation of the 2,2’-
bisdipyrrin metal complexes.[20a,22] Chemical oxidation of
3Pd2 in CH2Cl2 with AgClO4 proceeded to cause a red shift
of the major absorption band at 667 nm to 767 nm and
appearance of an intense ESR signal at g = 2.001 with
a linewidth of 5.0 G at room temperature (Supporting
Information, Figures S13,S6). This is indicative of the for-
mation of a p cation radical delocalized over the 2,2’-
bisdipyrrin chromophore. Although the hyperfine splitting
was not seen in the ESR signal, the nature of the oxidized
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Figure 6. UV/Vis (bottom) and CD (top) titration of 6Pd2 (23.6 mm) in
CH2Cl2 with AgClO4 (3.45 mm) in toluene. The dotted line is after
adding 8.7 equiv of [Cu(MeCN)4]ClO4 to the oxidized solution of 6Pd2
with AgClO4 (2.0 equiv). Inset: a plot of the change of absorption
Dabs of the 741 nm band with the added molar equiv of AgClO4.
about 50% yield by recrystallization. The combined applica-
tion of helical chirality and redox activity in the dinuclear
single helicate of hexapyrroles is quite promising in the
development of multifunctional materials.
Received: September 3, 2012
Revised: October 6, 2012
Published online: November 28, 2012
[14] J. Setsune, M. Mori, T. Okawa, S. Maeda, J. M. Lintuluoto, J.
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[16] Experimental procedures, details on the X-ray diffraction, and
analytical data of all compounds are provided in the Supporting
Information. CCDC 898174 (3), 898175 (3Pd2), 898176 (4Pd2),
and 898177 (6Pd2) 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.
Keywords: chirality · helicates · helicity · oligopyrroles ·
redox reactions
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