Spontaneous formation of a chiral supramolecular superhelix in the crystalline state using a single-stranded tetranuclear metallohelicate

Article abstract of DOI:10.1039/b810426c

An unprecedented one-handed coiled-coil structure was formed in the crystal of a Zn₃La tetranuclear metallohelicate, in which the handedness of both the helical component and the helical array was well controlled by the chiral auxiliary of the flexible acyclic ligand. The Royal Society of Chemistry.

Full text of DOI:10.1039/b810426c

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Spontaneous formation of a chiral supramolecular superhelix in the
crystalline state using a single-stranded tetranuclear metallohelicatew
Shigehisa Akine, Takashi Matsumoto and Tatsuya Nabeshima*
Received (in Cambridge, UK) 19th June 2008, Accepted 24th July 2008
First published as an Advance Article on the web 29th August 2008
DOI: 10.1039/b810426c
An unprecedented one-handed coiled-coil structure was formed
in the crystal of a Zn3La tetranuclear metallohelicate, in which
the handedness of both the helical component and the helical
array was well controlled by the chiral auxiliary of the flexible
acyclic ligand.
The superhelix is a hierarchical structure in which a helix is
coiled to produce a more ordered helix. The structure plays an
important role in biopolymers such as supercoiled DNAs1 and
the a-helical coiled-coil2 structure of proteins. There are also
artificial coiled-coil structures, and some synthetic helical
polymers form a higher-ordered spiral liquid crystal, which
is observable by an electron microscope.3 In such cases, the
higher-ordered structure is maintained by many simultaneous
weak interactions among the chains.
Fig. 1 Supramolecular superhelix formed by crystallization of a
chiral metallohelicate.
Recently, a non-covalent strategy has been frequently
employed to make artificial molecular helices, including a wide
range of metallohelicates with double and triple helical struc-
tures.4 Functional metallohelicates were also used to invert the
handedness in response to external stimuli.5 However, an
artificial coiled-coil structure based on the supramolecular
self-assembly of discrete helical subunits has been rarely
explored, mainly due to the difficulty of controlling the
handedness of both the higher-ordered helix and the helical
subunits.6 In this communication, we report an unprecedented
hierarchical supramolecular superhelix, in which an array of
helical metal complexes with a chiral auxilliary forms the
coiled-coil structure in the crystal (Fig. 1). The chiral auxiliary,
introduced into the acyclic oligooxime ligand, effectively
regulates the handedness of the single-stranded tetranuclear
metallohelicate. The chiral moiety also contributes to the
formation of a one-handed helical array of the helical subunits
in the crystalline state.
in order to obtain the one-handed helix (Scheme 1).9,10 Salen-
metal analogues with such a chiral auxiliary have been utilized
as an excellent enantioselective catalyst for organic reac-
tions.11 This chiral salen moiety is placed in the middle of
the acyclic oligooxime framework, taking into consideration
the reactivity of the imine and oxime CQN bonds; the former
suffers from recombination of the CQN bonds while the latter
does not.12 If the salen moiety is introduced into other
positions, the recombination of the imine CQN bonds may
produce a mixture of several compounds with different chain
lengths. The target ligand H6L2 was prepared by the reaction
of a commercially available chiral diamine with 2 equiv. of the
terminal unit (ESIw).
The complexation of the chiral ligand H6L2 with zinc(II)
acetate produced a mixture of soluble zinc(II) complexes,
which were not characterized. However, when 1 equiv. of
We used a tetranuclear single-stranded metallohelicate
[L1Zn3La]3+ as the basic framework of the helical subunit.
The heteronuclear complex can be spontaneously formed by
the complexation of the acyclic oligooxime ligand (H6L1) with
two kinds of metal ions.7,8 We introduced a chiral salen unit
having a trans-1,2-diphenylethylene group into the framework
Graduate School of Pure and Applied Sciences, University of
Tsukuba, Tsukuba, Ibaraki 305-8571, Japan.
E-mail: nabesima@chem.tsukuba.ac.jp; Fax: +81-29-853-4507;
Tel: +81-29-853-4507
w Electronic supplementary information (ESI) available: Experimental
synthetic procedure and characterizations of H6L. CCDC reference
number 692370. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b810426c
Scheme 1 Single-stranded tetranuclear metallohelicate bearing a
chiral auxiliary.
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Fig. 2 Part of the 1H NMR spectrum of [L2Zn3La]3+ at 400 MHz in
CDCl3–CD3OD (1 : 1). The two diastereomers are indicated by J
and K.
lanthanum(III) acetate was present in the mixture, simple and
sharp signals were observed in the 1H NMR spectrum. The
results strongly indicate the formation of a single-stranded
metallohelicate, similar to the achiral analogue [L1Zn3La]3+ 7
.
The formation of the heterotetranuclear complex [L2Zn3La]3+
was confirmed by the ESI mass spectrum.
We have already reported that the helix inversion of the
achiral helical complex [L1Zn3La]3+ is very slow on the NMR
timescale, even at 353 K in CDCl3–CD3OD (1 : 1).7 If the helix
inversion of the chiral complex [L2Zn3La]3+ occurs at a
similar rate, the right- and left-handed forms must be sepa-
rately observed in the NMR spectra because they are a
diastereomeric pair. Indeed, the 1H NMR spectrum of
[L2Zn3La]3+ showed two sets of signals (Fig. 2), for example,
two singlets for the methoxy protons were observed for the
major and minor isomers at 3.76 and 3.79 ppm, respectively.
The ratio was 71 : 29, indicating that either of the right- or
left-handed diastereomers is preferentially formed. The
energy difference between the two forms is estimated to be
2.2 kJ molÀ1 at 298 K.
Fig. 3 (a) Change in CD spectra of [L2Zn3La]3+ recorded every
10 min after dissolving in chloroform–methanol (1 : 1). (b) Change
in the CD intensity monitored at 350 nm (J). The solid line indicates
1
the exponential decay (t = 43.7 min) fitted to the observed data.
2
isodichroic points were observed. This decay obeyed first-
order kinetics and the half-life was calculated to be 43.7 min
by a nonlinear least-squares analysis (Fig. 3b). The decrease of
the intensity can be attributed to the inversion between the
right- and left-handed forms of the helical complex. Since it
took several days to prepare the single crystals, the minor
isomer in the mother liquor probably isomerized to the major
one keeping the equilibrium.
From a solution of the isomeric mixture, single crystals of
the metallohelicate [L2Zn3La]3+ were obtained. The 1H NMR
spectrum of the crystals, which was recorded immediately after
dissolving, showed only one set of signals. The observed set
was found to correspond to the major isomer of the two
diastereomers mentioned above. We obtained the pure one-
handed isomer by crystallization. However, after the solution
was allowed to stand for 4 h at room temperature, we again
observed the two sets of signals corresponding to the two
isomers in a ratio of 71 : 29, which is exactly the same as that
observed for the mixture of H6L2, zinc(II), and lanthanum(III).
Obviously, the isomerization took place to afford the equili-
brated mixture.
An X-ray crystallographic analysis was carried out using the
single crystals of [L2Zn3La(OAc)3(H2O)(MeOH)]Á3CHCl3Á
MeOH (Fig. 4a).13,14 The complex crystallizes in the tetra-
gonal, space group P43, and the unit cell contains four
molecules of the left-handed (M) isomer of the complex.
Consequently, the absolute configuration of the major isomer
observed in solution corresponds to the left-handed (M)
isomer. The three zinc atoms sit in the two types of N2O2
sites (two salamo and one salen sites), which surround the
central lanthanum atom (La1) forming Zn–O–La m-phenoxo
bridges. The ligand moiety (L2)6À forms a single helix and the
winding angle of the helix is 4191.15 The structural feature of
[L2Zn3La]3+ is similar to that of the achiral analogue
[L1Zn3La]3+ (winding angle: 4211).7 Thus, the chiral auxiliary
(chiral salen unit) can be incorporated into the helical frame-
work without any significant distortion of the molecular
structure.
This equilibration process can also be monitored by the
change in the CD signals (Fig. 3a). The CD spectrum of
[L2Zn3La]3+ recorded immediately after dissolving in
chloroform–methanol (1 : 1) showed negative peaks at
283 and 350 nm and a positive one at 312 nm. The CD
intensity at 350 nm gradually decreased to 37% of the initial
intensity after the solution was allowed to stand at 22 1C for
4 h. During the isomerization, the maximum wavelength and
peak profile did not significantly change and several
It is worthwhile to point out that the space group P43 has a
4-fold left-handed screw axis along the crystallographic c axis.
As a result, a helical array of the helical complex is formed, in
which the helix axis of each tetranuclear complex [L2Zn3La]3+
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developing novel superhelices with bio-inspired functions
and a self-replication system based on the helical metal
complexes.
Notes and references
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and G. Hopfgartner, Chem. Rev., 1997, 97, 2005–2062.
5 T. Nabeshima, A. Hashiguchi, T. Saiki and S. Akine, Angew.
Chem., Int. Ed., 2002, 41, 481–484.
6 For examples of chirality control of single-stranded helical com-
plexes, see: (a) T. Mizutani, S. Yagi, A. Honmaru and H. Ogoshi,
J. Am. Chem. Soc., 1996, 118, 5318–5319; (b) T. Mizutani, S. Yagi,
T. Morinaga, T. Nomura, T. Takagishi, S. Kitagawa and H.
Ogoshi, J. Am. Chem. Soc., 1999, 121, 754–759.
7 S. Akine, T. Taniguchi, T. Matsumoto and T. Nabeshima, Chem.
Commun., 2006, 4961–4963.
8 For shorter analogues of [L1Zn3M]n+, see: (a) S. Akine, T.
Taniguchi and T. Nabeshima, Angew. Chem., Int. Ed., 2002, 41,
4670–4673; (b) S. Akine, T. Taniguchi, T. Saiki and T. Nabeshima,
J. Am. Chem. Soc., 2005, 127, 540–541; (c) S. Akine, T.
Matsumoto, T. Taniguchi and T. Nabeshima, Inorg. Chem.,
2005, 44, 3270–3274; (d) S. Akine, T. Taniguchi and T. Nabeshima,
Tetrahedron Lett., 2006, 47, 8419–8422; (e) S. Akine, T. Taniguchi
and T. Nabeshima, J. Am. Chem. Soc., 2006, 128, 15765–15774.
9 For metallofoldamers bearing a chiral salen moiety, see: (a)
Z. Dong, R. J. Karpowicz, Jr, S. Bai, G. P. A. Yap and J. M.
Fox, J. Am. Chem. Soc., 2006, 128, 14242–14243; (b) Z. Dong, G.
P. A. Yap and J. M. Fox, J. Am. Chem. Soc., 2007, 129,
11850–11853.
Fig. 4 (a) Molecular structure of [L2Zn3La(OAc)3(H2O)(MeOH)]
with thermal ellipsoids plotted at the 50% probability level. (b) Space
filling representation of the crystal structure showing a helical array
along the crystallographic c axis.
10 For single-stranded helical complexes of salen-containing
polymers, see: (a) Y. Dai, T. J. Katz and D. A. Nichols, Angew.
Chem., Int. Ed. Engl., 1996, 35, 2109–2111; (b) H.-C. Zhang, W.-S.
Huang and L. Pu, J. Org. Chem., 2001, 66, 481–487; (c) Y.
Furusho, T. Maeda, T. Takeuchi, N. Makino and T. Takata,
Chem. Lett., 2001, 1020–1021.
11 For reviews, see: (a) E. N. Jacobsen, in Catalytic Asymmetric
Synthesis, ed. I. Ojima, VCH, New York, 1993, pp. 159–202; (b)
T. Katsuki, Coord. Chem. Rev., 1995, 140, 189–214; (c) E. N.
Jacobsen, Acc. Chem. Res., 2000, 33, 421–431.
is tilted from the main axis (c axis) by ca. 801. This makes a
coiled-coil structure—a supramolecular superhelix—in the
crystal lattice (Fig. 4b). To the best of our knowledge, this is
the first example of a coiled-coil structure formed by the
folding and assembly of flexible acyclic molecules. The helical
arrangement probably results from the weak intermolecular
interactions such as the C–HÁ Á ÁO and C–HÁ Á Áp ones operating
between the helical subunits. The intermolecular interactions
may contribute to the exclusive formation of the left-handed
isomer during the crystallization process.
12 (a) S. Akine, T. Taniguchi and T. Nabeshima, Chem. Lett., 2001,
682–683; (b) S. Akine, T. Taniguchi, W. Dong, S. Masubuchi and
T. Nabeshima, J. Org. Chem., 2005, 70, 1704–1711.
13 Crystallographic analysis of [L2Zn3La(OAc)3(H2O)(MeOH)]Á
3CHCl3ÁMeOH (C61H64Cl9LaN6O21Zn3 = 1871.25): tetragonal,
P43, a = 18.2391(19) A, c = 22.795(4) A, V = 7583.0(17) A3, Z =
4, m(MoKa) = 1.879 mmÀ1, rcalcd = 1.639 g cmÀ3, F(000) = 3760,
71 388 reflections measured (Rigaku R-AXIS Rapid), 17 275
unique (Rint = 0.0303), T = 120 K, R1 = 0.0386 (I 4 2s(I)),
wR2 = 0.1011 (all data), Flack w = À0.009(8).
In the formation process of the chiral superhelical structure
in this study, the M/M intermolecular interaction was more
favourable than the P/P or M/P one. Such a system is
important from the viewpoint of a self-sorting system for
optical purity amplification. The finely designed small subunits
lead to the well-controlled secondary structure (helical con-
formation) and the higher-ordered structure (superhelix), as
seen in most of the functional biomolecules. We are now
14 G. M. Sheldrick, SHELXL-97, Program for refinement of crystal
structures, University of Gottingen, Germany, 1997.
¨
15 Defined as the sum of seven O–M–O angles.
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4606 | Chem. Commun., 2008, 4604–4606