Assembly and disassembly of a Zn10 high-nuclearity circular helicate

Article abstract of DOI:10.1039/c2cc30381g

A nano-scale decanuclear Zn(ii) circular helicate is synthesized without the aid of counteranions during the assembly process, and can be totally disassembled into its reactants by specific anions.

Full text of DOI:10.1039/c2cc30381g

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COMMUNICATION
Assembly and disassembly of a Zn10 high-nuclearity circular helicatew
Zih-Syuan Wu,z Jui-Ting Hsu,z Chang-Chih Hsieh and Yih-Chern Horng*
Received 17th January 2012, Accepted 9th February 2012
DOI: 10.1039/c2cc30381g
A nano-scale decanuclear Zn(II) circular helicate is synthesized
without the aid of counteranions during the assembly process, and
can be totally disassembled into its reactants by specific anions.
construct a high-nuclearity circular helicate has not been
reported before. In light of these observations, we designed
and prepared a dianionic bis-bidentate ligand to coordinate
divalent metal ions and successfully used it to obtain a neutral
decanuclear circular helicate without any entrapped counter-
anions. In addition, total disassembly of the circular helicate
by specific anions was achieved.
The self-assembly of high-nuclearity coordination compounds, in
particular, complexes exhibiting a helical array of ligand strands,
has recently drawn much attention in the field of supramolecular
chemistry.1 However, the majority of these helicates have linear
topologies with double- and triple-stranded arrangements—only
a few examples of circular helical architectures have been
reported.2 Furthermore, the underlying principles governing
the spontaneous formation of these circular helicates remain to
be thoroughly elucidated. Factors reported to influence the
formation of cyclic helicates include the size and coordination
geometry of metal ions,3 structural features of the ligands,4
intramolecular (e.g. hydrogen bonding)5 and intermolecular
(e.g. template effects of anions)6 interactions, and kinetic and
thermodynamic controls.7 However, to construct a higher-
nuclearity circular helicate, the effects of the net positive charge
(NPC) of the circular helicate portion on its formation and the
ease of isolation have to be considered. For instance, among
cationic circular helicates with published structures composed of
only divalent metal ions and neutral ligands, the maximum
reported nuclearity is five with a high NPC of 10.8,9 The only
two reported hexanuclear species were synthesized either by
using monovalent metal ions (NPC: 6)10 or through the coordi-
nation of counteranions to the divalent metal centers (NPC: 0).11
Thus far, the octonuclear circular helicate reported by Jones et al.
represents the highest nuclearity for a cyclic helicate (NPC: 3) in
the literature.12 In this case, a monoanionic bis-bidentate ligand
was used as a bridge between two adjacent Co2+ ions and a
counteranion was encapsulated in the central cavity. It seems that
the net charges of the ligand and assembled helicates play
important roles in the construction of larger cyclic circular
helicates. Although the strategy of ligand deprotonation through
electrochemical synthesis was employed by Bermejo et al. to
avoid the effects of counteranions in order to obtain cluster
helicates,13 the use of a dithiolate bis-bidentate ligand to
Recently, a 1,3-bis(methylene)phenylene spacer was employed
in the pyridylimine ligand to facilitate the formation of trinuclear
circular helicates with Ni2+ or Cu2+ ions.14 In the structures of
these helicates, the ligands adopt the most extended conformation,
with the two methylene groups pointing in opposite directions
(up and down from the phenyl ring). We forced the two
bidentate domains to orient toward the same side of the spacer
by introducing three ethyl groups on the benzene ring of the
spacer, since it is well known that the substituents attached to
the 2,4,6-positions of 1,3,5-triethylbenzene turn to orient on the
opposite side of the benzene plane relative to the ethyl groups.15
Furthermore, to minimize the positive charge associated with
high-nuclearity polymer formation when the ligand reacts with
divalent metal ions, we introduced a negative charge (thiolate)
at each binding domain. The dithiol ligand (H2L) was prepared
in a manner similar to its trithiol analogue16 in 53% yield by the
reaction of 2,4-bis(bromomethyl)-1,3,5-triethylbenzene with
2,20-aminophenyl disulfide in CH3CN followed by reduction
and protonation (Fig. 1 and ESIw). This ligand has two
bidentate chelating binding domains separated by a rigid
1,3-bis(methylene)phenylene spacer, which, together with the
expected flexibility of the ligand (the free rotation of thiolphenol
around the N–C (phenyl) bond and the stereo-configuration
around N), should prevent both aminothiophenolate units from
chelating to the same metal ion, and hence facilitate the
formation of high-nuclearity species.
With the knowledge that group 12 metals can form complexes
with various thiolate ligands,17 we investigated the complexations
of L2ꢀ with Hg2+, Cd2+ and Zn2+ ions. In a typical reaction,
1 equiv. of M2+ (M = Hg, Cd or Zn) was added to a CH3CN
solution of 1 equiv. of H2L and 2 equiv. of NEt3 with stirring to
afford a light-yellow complex as a precipitate in high yield (ESIw).
Department of Chemistry, National Changhua University of
Education, 1 Jin-De Road, Changhua 50058, Taiwan.
E-mail: ychorng@cc.ncue.edu.tw; Fax: +886 4721 1190
w Electronic supplementary information (ESI) available: Experimental
details. CCDC 856487 and 856488. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c2cc30381g
z The authors contributed equally to this work.
Fig. 1 Synthesis of H2L.
c
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The use of NaH instead of NEt3 in this gave similar results.
However, no reaction proceeded when the deprotonation step
with NEt3 was omitted. Elemental analysis results of the
precipitates were consistent with the formula (ML)n. The
1H NMR spectra of H2L (in CDCl3), Hg (in CDCl3) and Cd
(in DMSO-d6) complexes showed only half a set of protons,
indicating the chemical equivalence of the two binding domains
within one ligand (ESIw). Interestingly, the Zn complex was found
to have a complicated 1H NMR spectrum. Through 1H–1H
COSY and 1H–13C HSQC NMR experiments (ESIw), each signal
in the 1H NMR spectrum was assigned unambiguously. This
showed that only one type of ligand conformation exists with two
chemically non-equivalent binding sites. The chemical shifts of the
two sets of aromatic protons on the binding strands were spread
over 5.1 to 7.9 ppm, indicating significant shielding effects on
several protons. This shielding effect was also observed in other
polynuclear circular helicates.9,10
Diffusion of pentane into a THF/1,4-dioxane (v/v 4 : 1)
solution of the Hg complex at 4 1C afforded X-ray quality
crystals,y the solid-state structure of which revealed neutral
double-strand dinuclear mesocate Hg2L2 (ESIw). As expected,
the two binding strands of each ligand were oriented at the
same side of the spacer. Each of the Hg2+ centers coordinates
to the aminothiophenolate S donors from two adjacent ligands
with a straight linear coordination (Hg–S: 2.327(5), 2.329(5) A;
S–Hg–S = 177.4(2)1). A short Hgꢁ ꢁ ꢁO contact interaction
between Hg2+ and the O donor of 1,4-dioxane was also
revealed in the structure (Hgꢁ ꢁ ꢁO: 2.82(2) A). Unfortunately,
we were not able to obtain single crystals of the Cd complex
suitable for X-ray diffraction studies. However, a dinuclear Cd
complex is expected based on HRMS (ESI) analysis.
Recrystallization of the Zn compound from CHCl3 by
hexane diffusion at 4 1C afforded X-ray quality crystals,y the
solid-state structure of which interestingly revealed a neutral
decanuclear circular helicate (ZnL)10 (Fig. 2). This structure
represents the highest-nuclearity circular helicate reported
thus far. In the case of Zn(II), only pentanuclear circular
helicates have been reported.9
Fig. 2 (a) Crystal structure of (ZnL)10. The ligands in the same
asymmetric unit are shown in the same color. Hydrogen atoms are
omitted for clarity. (b) Schematic drawing of the decanuclear ring
showing the helical ligand arrangement.
helicate is thus solely governed by the preferred binding geometry
of the metal center, and the structural features and negative
charges of the ligand. There is absolutely no need for counter-
anions to control the ring size by dictating the assembly pathway.
The space-filling representation of the decagonal helicate
clearly indicates the nano-scale dimensions of the assembled
structure. The outside diameter of the decagonal structure is
about 2.9 nm with the thickness of the disc being about
1.2 nm, and the cavity has a diameter of B0.86 nm. Moreover,
the cyclic decameric (ZnL)10 units are packed together and
stabilized by two types of CHꢁ ꢁ ꢁp interactions (totally eight for
each unit). The molecular units stack in such a manner that
channels of approximately oval cross section extending in the
b direction are produced (ESIw).
Each Zn(II) center has a four-coordinate pseudo-tetrahedral
coordination environment bound to two bidentate aminothio-
phenolate units from two different ligands. The dihedral angle
between the ZnS2 and ZnN2 planes (62.8(1)–66.3(1)1) and the
bond distances of the Zn–S (2.260(2)–2.277(2) A) and Zn–N
(2.081(5)–2.151(4) A) for each metal center are very similar to
the values reported for mono-nuclear Zn(II) complexes with an
N2S2 coordination set.17 Due to the restricted orientation of
the two binding domains of the ligand, the distances between
adjacent Zn(II) ions (6.04–6.27 A) are much shorter than those
in the Zn pentanuclear circular helicates (over 9.0 A) reported
recently.9 The ten spacers are alternately located ‘above and
below’ the plane formed by the ten metal ions. The two
bidentate strands of each ligand, flipped differently, interact
with two metal ions from inside and outside the decagon,
respectively. The ten Zn(II) centers alternately have L- and D-
configurations, resulting in an overall meso-helicate. There are
no obvious intramolecular non-covalent interactions within
the helicate and no template molecules, whether counteranions
or solvents, were necessary to stabilize the architecture of this
cyclic species. This spontaneous formation of the high-nuclearity
Interestingly, we found that the neutral Zn(II) cyclic helicate
ꢀ
can be totally disassembled by H2PO4 ions. The reported
monomeric Zn(II) complexes with similar N2S2 donor ligands
were not luminescent in solution at RT.18 However, (ZnL)10 in
CHCl3 at RT exhibited a strong emission (at 530 nm) on
irradiating with ultraviolet light (at 265 nm). When (ZnL)10
was titrated in CHCl3 with various anions, including Clꢀ, Brꢀ,
Iꢀ, ClO4ꢀ, PF6ꢀ, NO3ꢀ, HSO4ꢀ, no change occurred in the
ꢀ
fluorescence intensity of (ZnL)10, although H2PO4 showed
c
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Chem. Commun., 2012, 48, 3436–3438 3437

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Notes and references
y Crystal data for Hg2L2ꢁ1.5C4H8O2: C52H60Hg2N4S4ꢁC4H8O2ꢁC2H4O,
M = 1402.62, monoclinic, a = 9.9374(19) A, b = 26.392(4) A, c =
12.695(2) A, a = 90.001, b = 108.149(6)1, g = 90.001, V = 3163.8(9) A3,
T = 200(2) K, space group P21/n, Z = 2, m(MoKa) = 5.020 mmꢀ1
,
9084 reflections measured, 4795 independent reflections (Rint = 0.0620).
The final R1 values were 0.0751 (I > 2s(I)). The final wR(F2) values
were 0.1810 (I > 2s(I)). The final R1 values were 0.1325 (all data). The
final wR(F2) values were 0.2104 (all data). The goodness of fit on F2 was
1.047. CCDC 856487; for (ZnL)10: C260H300N20S20Zn10, M = 5000.10,
monoclinic, a = 26.7089(14) A, b = 25.6880(11)
c = 27.0302(16) A,
A,
a = 90.001, b = 117.334(7)1, g = 90.001, V = 16474.7(15) A3, T =
110(2) K, space group P21/n, Z = 2, m(MoKa) = 0.884 mmꢀ1, 80019
reflections measured, 38154 independent reflections (Rint = 0.0809).
The final R1 values were 0.0657 (I > 2s(I)). The final wR(F2) values
were 0.1344 (I > 2s(I)). The final R1 values were 0.1730 (all data).
The final wR(F2) values were 0.1414 (all data). The goodness of fit on
F2 was 0.965. CCDC 856488.
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Fig. 3 (a) Plot of relative fluorescence intensity (I/I0) of [(ZnL)10
]
versus the mole ratio of anions added. Fluorescence (b) and absorption
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represent the spectra of H2L at 0.10 mM. The starting concentration of
(ZnL)10 for all titrations was 10 mM.
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significant quenching (Fig. 3a and b). This quenching is not
solely due to the protonation of the thiolates of the coordinated
ꢀ
L2ꢀ since even 20 equiv. of HSO4 and CH3COOH acids
have no effect on the fluꢀorescence intensity of (ZnL)10. To
better understand H2PO4 binding, titrations using absorp-
tion spectroscopies were carried out between (ZnL)10 and
different concentrations of H2PO4ꢀ. Upon the addition of
H2PO4ꢀ, the initial absorbance band of (ZnL)10 was slightly
shifted (from 300 to 313 nm) with increased intensity, and
reached a saturation point after the addition of 10 equiv.
1
(Fig. 3c). The absorption and H NMR spectra of the final
product showed characteristics nearly identical to that of the
ꢀ
simple H2L. These results indicate that H2PO4 can induce
total disassembly of the nano-scale cyclic helicate (ZnL)10 into
its reactants H2L and Zn2+. To the best of our knowledge, this
is the first example of total disassembly of synthetic circular
helicates in the literature.
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circular helicate (ZnL)10 through careful design of ligands,
including structural features and charges. The procedure does
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spontaneous formation of these high-nuclearity metal based
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ꢀ
sensing of H2PO4 anions by (ZnL)10 accompanied by total
disassembly of the helicate.
We are grateful to the National Science Council (Taiwan)
for their financial support of this work.
c
3438 Chem. Commun., 2012, 48, 3436–3438
This journal is The Royal Society of Chemistry 2012