A sequentially assembled grid composed of supramolecular meso-helical nodes

Article abstract of DOI:10.1039/c1cc13582a

A unique case of a grid-of-mesocates sequentially assembled by supramolecular cobalt(ii) meso-helical units connected through hydrogen bonding is reported. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1cc13582a

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COMMUNICATION
A sequentially assembled grid composed of supramolecular meso-helical
nodesw
ab
a
Miguel Martı
nez-Calvo,a Rosa Pedrido,*ab Ana M. Gonzalez-Noya,* Marıa J. Romero,
´ ´
´
Magdalena Cwiklinska,ab Guillermo Zaragozac and Manuel R. Bermejoa
Received 16th June 2011, Accepted 18th July 2011
DOI: 10.1039/c1cc13582a
A unique case of a grid-of-mesocates sequentially assembled by
supramolecular cobalt(II) meso-helical units connected through
hydrogen bonding is reported.
supramolecular self-organization lies in the possibility of
obtaining highly extended arrays, which are potentially of
great interest for nanotechnology and molecular electronic
devices.
Among the wide variety of supramolecular species reported
to date, helicates are considered the simplest and most funda-
mental architectures.5 An appropriate choice of the bridging
ligands and the metal ions is crucial to achieve helical metallo-
structures, with the ligands twisted around the metal ions
which are located in homochiral environments (D or L). In
contrast, a side-by-side binding of the ligands around the metal
centres affords meso-helical achiral structures (the so-called
mesocates) with the metal ions exhibiting opposite chirality
(D and L).6 These meso-helical species are relatively unexplored
in comparison with the related helicates, despite their potential
applications in information storage and processing nano-
technology.7
The ability to organise functional molecules into higher
dimensional arrays with well-defined spatial relationships
between the components is one of the major goals in Supra-
molecular Chemistry. The traditional basis of this discipline
lies in the establishment of intermolecular non-covalent inter-
actions such as hydrogen bonds or p-stacking.1 Moreover, it is
well-known that simple metallo-supramolecular systems can
be assembled using metal ions and organic ligands through
coordination bonds.2 However, covalent synthesis involving
metal ions may limit the size of the metal building blocks and
thus the subsequent size and shape of the supramolecular
array. One approach to circumvent these limitations is to
use multiple recognition events in sequence. In this strategy
an initial event leads to the formation of small supramolecular
units, which can then be employed as scaffolds in a second
supramolecular assembly to achieve higher order arrays.3 In
particular, the combination of metal–ligand coordination with
hydrogen bonding could be a suitable way to achieve two-
level, sequential and hierarchical self-organisation processes,
in which the initial assembly of a coordination architecture
from given ligands and metal ions is followed by the organisa-
tion of these units into more specific arrangements through
hydrogen bonding interactions. This type of approach was
successfully applied by Lehn et al.4 to prepare a ‘grid-of-grids’,
by employing ligands conveniently designed to form double
self-complementary hydrogen bonds. The interest in sequential
The use of helical or meso-helical metallo-supramolecular
scaffolds to aggregate larger assemblies is still rare. Early work
of Hannon et al. reported a cyclic array of helicates held
together by p–p interactions,3a linear infinite chains3c or 2D
networks3d assembled from helical or meso-helical complexes
through hydrogen bonding. More recently, zig-zag aggregates8a
or chains of helicates8b have been sequentially achieved by
means of combined p-stacking and hydrogen bonding contacts.
During the last few decades there have been many studies on
the coordinative behaviour of thiosemicarbazones.9 Our group
has been a pioneer in designing a route to achieve different
functional supramolecular species by employing thiosemi-
carbazone moieties.10 Thus, careful design of the thiosemi-
carbazone ligand combined with a suitable choice of metal ion
allowed us to obtain helicates, mesocates or cluster helicates in
a selective manner. This methodology was described in a
recent work,11 in which the first example of a cobalt(II)
mesocate derived from the bisthiosemicarbazone ligand
bis(1,3-diacetylbenzene)-4-N-ethyl-3-thiosemicarbazone H2La
(Chart 1) was reported. This mesocate structure features a
simple 1D chain established by hydrogen bonds involving the
imine, the thioamide nitrogen and the sulfur atoms of the
thiosemicarbazone strand (Fig. 1 and Fig. S1).
a
´
´
´
Departamento de Quımica Inorganica, Centro de Investigacion en
´
´
Quımica Biologica y Materiales Moleculares, Universidade de
Santiago de Compostela, Santiago de Compostela, Galicia, E-15782,
Spain. E-mail: manuel.bermejo@usc.es; Tel: +34 981563100
b
´
´
Departamento de Quımica Inorganica, Facultade de Ciencias,
Universidade de Santiago de Compostela, Lugo, Galicia, E-27002,
Spain. E-mail: rosa.pedrido@usc.es, ana.gonzalez.noya@usc.es
c Unidade de Difraccio´n de Raios X, Edificio CACTUS, Universidade
de Santiago de Compostela, Campus Sur, Santiago de Compostela,
Galicia, E-15782, Spain. E-mail: g.zaragoza@usc.es
w Electronic supplementary information (ESI) available: Synthesis
and experimental data for H2Lb, selected bond distances and angles
for 1. CCDC 828824 ([Co2(Lb)2] 1). For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c1cc13582a
With this result in mind, we envisaged that it may be
possible to obtain a higher aggregation order for meso-helical
assemblies by using the sequential self-assembly approach.3a
c
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Chem. Commun., 2011, 47, 9633–9635 9633

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Therefore, tetradentate thiosemicarbazone ligands are shown
to be suitable precursors to achieve tetrahedral four-coordinated
mesocates.
Beyond the relevance of the mesocate structure itself, the
most remarkable feature of 1 lies in its crystal packing. Thus,
the establishment of hydrogen bonds between one of the
thioamide protons and the thioamide sulfur atom of a
neighbouring molecule [N6–H6Nꢀ ꢀ ꢀS2 3.415(2) A, 2 ꢂ x,
ꢂ1 ꢂ y,1 ꢂ z], together with the formation of a second
perpendicular hydrogen bond between the second thioamide
proton and the methoxyphenyl oxygen atom from another
mesocate molecule [N1–H1Nꢀ ꢀ ꢀO2, 3.222(3) A, 1 ꢂ x, ꢂy,
2 ꢂ z], gives rise to the self-assembly of a 2D supramolecular
grid structure (Fig. 3).
Chart 1 Ligands H2La and H2Lb.
Fig. 1 Representation of the 1D-chain assembled by [Co2La2] meso-
helicates.
For this purpose we designed the related ligand H2Lb
(Chart 1), which bears a terminal methoxyphenyl thioamide
group instead of ethyl. The incorporation of an additional
hydrogen bond site, conveniently arranged perpendicular with
respect to the imine–thioamide thiosemicarbazone chain,
could provide a simple pathway to increase the dimensionality
of the mesocate network.
It must be noted that the term ‘‘metallo-supramolecular
grid’’ has traditionally referred to an assembled architecture in
which ligands and metal ions form a rectangular or square
array and the ligands that form the grid nodes are crossed.
These nodes usually consist of a metal ion and polydentate
coordination sites from two or more different ligands.17 In this
context, complex 1 features a novel class of supramolecular
grid structure that might be called a ‘‘grid-of-mesocates’’, in which
the grid nodes are simple meso-helical architectures (Fig. 3).
This new type of grid-shaped solid surface constructed with
mesocates could be an interesting alternative to overcome the
synthetic difficulties involved in the generation of higher
nuclearity mesocates, which are currently being investigated
in the field of information storage devices.18
Electrochemical oxidation of a cobalt plate in a conducting
acetonitrile solution12 of the ligand H2Lb afforded a green
solid that was readily characterised. The analytical data and
the MALDI mass spectrum for this solid are consistent with
the formation of the neutral Co(II) dimeric complex [Co2(Lb)2]ꢀ
H2O (1ꢀH2O),13 which arises from the double deprotonation of
the ligand in solution. The solid was also characterised by IR,
1H NMR and magnetic measurements at room temperature.
Slow evaporation of the mother liquors allowed us to obtain
green crystals suitable for X-ray diffraction. The crystal structure
of the complex exhibits discrete dinuclear mesocate molecules
of [Co2(Lb)2] 1 (Fig. 2).z Every mesocate unit is composed
of two dianionic bridging ligands [Lb]2– that tetrahedrally
coordinate both Co(II) centres through two pairs of imine–
thiolate donors. This results in a sixteen-membered metallo-
macrocyclic ring with an internal cavity of ca. 6.523 ꢁ 3.276 A
(Fig. S2, ESIw). The intradinuclear Coꢀ ꢀ ꢀCo distance is
6.8933(6) A, whereas the shortest intermolecular separation
is 7.9131(7) A.
From the results obtained it is clear that appropriate tuning
of the bisthiosemicarbazone ligand, with a second hydrogen
bonding site placed at an angle of 901 (Fig. 4 and Fig. S3) to
the imine–thioamide skeleton, would enable not only an
increase in the assembly dimension of the array but would
Most of the mesocates published to date concern titanium,
manganese, silver, zinc and copper,14,15 with only a small
number of cobalt mesohelicates reported.16 Besides, all
of the cobalt mesocates described in the literature have a
coordination number of six and octahedral kernels, with
the only exception being the aforementioned [Co2(La)2].11
Fig. 2 ORTEP structure showing the atomic numbering scheme (a)
and stick representation (b) of meso-helical complex 1.
Fig. 3 Supramolecular grid assembled by the mesocate 1 through
hydrogen bonding interactions.
c
9634 Chem. Commun., 2011, 47, 9633–9635
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´
Fig. 4 Sequential self-assembly of a 2D grid-of-mesocates through
´
complementary hydrogen bonding.
´
also result in a unique and new square-shaped grid arrange-
ment (Fig. 4). In addition, two-level sequential self-assembly
has proven to be an appropriate strategy to gain a more in
depth knowledge of the factors involved in the formation of
new hydrogen-bonded supramolecular arrays.
´
lez-Noya, R. M. Pedrido,
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´
In summary, we have successfully obtained a new type of
extended supramolecular 2D array, the ‘‘grid-of-mesocates’’,
by using conveniently functionalised ligands and applying the
‘‘sequential self-organization strategy’’. We strongly believe
that this approach could open new perspectives for the
spontaneous, but controlled, generation of large organised
arrays with the aim of obtaining new functional nanomolecular
architectures. Further experiments aimed at increasing the ‘‘grid
of mesocates’’ dimensions are ongoing in our laboratories.
Financial support from the Xunta de Galicia
(10PXIB262132PR) and the Spanish Ministerio de Ciencia e
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10862–10864.
12 C. Oldham and D. G. Tuck, J. Chem. Educ., 1982, 59, 420–421.
13 A solution of the ligand (0.1 g, 0.19 mmol) in acetonitrile (80 cm3),
containing 10 mg of tetraethylammonium perchlorate (CAUTION:
although no problem was encountered in this work, all perchlorate
compounds are potentially explosive and should be handled with
great care), was electrolyzed for 62 minutes using a current of 10 mA.
The resulting green solid was filtered off, washed with diethyl ether
and dried in vacuo. Co2(Lb)2ꢀH2O (1ꢀH2O): Yield: 0.084 g (74%).
Anal. Co2C52H54N12S4O5 requires: C, 53.2; H, 4.6; N, 14.3; S, 10.9.
Found: C, 53.1; H, 4.6; N, 14.3; S, 10.8%. MS MALDI-TOF (m/z)
578.2 [ML], 1155.1 [M2L2]; IR (KBr, cm–1): n(OH) 3417 (w),
n(NH) 3317 (m), n(CQN + C–N) 1593 (w), 1509 (s), 1483 (m),
n(CQS) 1100 (w), 798 (w), n(N–N) 1044 (m); 1H NMR dH
(CD3CN): 140.36, 118.00, 108.63, 78.88, 62.00, 32.92, 31.16,
20.91, 19.55, 17.04, 16.00, 11.28, 10.00, 9.23, 7.62, 6.91, 6.23,
5.93, 3.16, 0.63, ꢂ16.63, ꢂ23.01, ꢂ31.10, ꢂ35.45, ꢂ48.40,
ꢂ61.50; LM (mS cm2) = 5.6; m(B. M.) = 4.1.
Innovacion and ERDF(EU) (CTQ2010-19191) is acknowledged.
´
Notes and references
z Crystal data for [Co2(Lb)2] 1: (Co2C52H52N12O4S4), Mw = 1155.20,
3
%
crystal dimensions: 0.52 ꢁ 0.16 ꢁ 0.04 mm , triclinic, P1, a =
9.1069(5), b = 10.9220(5), c = 14.6964(9) A, a = 73.752(2), b =
79.577(3), g = 65.667(2)1, V = 1275.13(12) A3, Z = 1, m = 0.874 mm–1,
F(000) = 598. Radiation l(Mo-Ka) = 0.71073 A, T = 100 K,
reflections collected/unique 42 121/5021 (Rint = 0.042), R (all data) =
0.042, wR (all data) = 0.083, GOF = 1.04, max minꢂ1 residual density
0.72/ꢂ0.36 e A–3. CCDC 828824.
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