Metal self-recognition: A pathway to control the formation of dihelicates and mesocates

Article abstract of DOI:10.1039/c2dt31770b

We have studied the factors that affect the formation of different metallosupramolecular architectures by metal direct self-assembly. A synthetic route has been developed to obtain mesocates or dihelicates selectively. For this purpose a series of five bi

Full text of DOI:10.1039/c2dt31770b

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PAPER
Metal self-recognition: a pathway to control the formation of dihelicates and
mesocates†
Miguel Martínez-Calvo,a María J. Romero,a Rosa Pedrido,*a,b Ana M. González-Noya,*a,b
Guillermo Zaragozac and Manuel R. Bermejoa
Received 2nd August 2012, Accepted 6th September 2012
DOI: 10.1039/c2dt31770b
We have studied the factors that affect the formation of different metallosupramolecular architectures by
metal direct self-assembly. A synthetic route has been developed to obtain mesocates or dihelicates
selectively. For this purpose a series of five bisthiosemicarbazone ligands derived from 1,3-
diacetylbenzene were designed and synthesised. Namely H2LMe, H2LEt, H2LPh, H2LPhN and H2LPhOMe
with different substituents on the 4-N terminal position of the thiosemicarbazone strands. The Co(II),
Zn(II) and Cd(II) complexes of these ligands were prepared by an electrochemical procedure.
Crystallographic studies revealed the formation of mesocates for Co(II) and Zn(II) metal ions whereas the
Cd(II) complexes gave a bishelical supramolecular structure. Therefore these bisthiosemicarbazone ligands
enable the selective isolation of mesocate or dihelicate complexes by using a specific metal ion.
the binding sites must be disposed so that they could bridge two
Introduction
or more metal ions, otherwise simple mononuclear complexes
may result.8,9 If this requirement is fulfilled, homochiral (Δ and
In the past two decades a great deal of attention has been paid to
the self-assembly of metallosupramolecular architectures.1,2
Λ) helicates are formed as a racemic mixture if the achiral
ligands are twisted around the metal ions.10 The side-by-side
Supramolecular aggregates of different shapes and functionalities
have been obtained by using well-designed organic ligands in
combination with appropriate metal ions or metal complex
fragments.3–6 Among the wide set of multinuclear supramolecu-
lar systems, helicates are probably the most widely studied spe-
cies1d,7 and are used as simple models to study the elemental
principles that direct the self-assembly process. The study of
helicates has shown that the specificity of the supramolecular
self-assembly depends on the symmetry of the molecular com-
ponents: i.e. the stereoelectronic preference of the metal ions and
the arrangement of the binding sites in the precursor ligand.
A judicious choice of the bridging ligands and the metal ions
is therefore crucial for the deliberate formation of helicates. With
regard to the ligands, a minimum requirement seems to be that
coordination of the ligands, however, results in achiral meso-
helicates or mesocates, in which metal centres exhibit opposite
chirality (Δ and Λ).
Helicates are being intensively studied because of their
involvement in new areas of research such as anion sensors,11
luminescence, magnetism, chirality, molecular machines,12,13
guest recognition and DNA binding.14 In contrast, mesocates
have received significantly less attention, despite the great poten-
tial applications of these achiral structures, which include mole-
cular wires and molecular switches to name but two examples in
information storage and processing nanotechnology.15
The selective formation of either the helical or meso form has
been a topic of debate since the first mesocate was reported by
Albrecht in 1995.16 The formation of mesocates instead of heli-
cates appears to be controlled by a variety of factors, including
the particular ligand architecture, the variation of the metal and
the incorporation of a guest molecule.
In a first approach, Albrecht et al. proposed the well-known
odd–even rule, which states that the length of the alkyl spacer
between two bidentate chelating units of the bridging ligand can
determine whether the resulting complex will have a helical
(even) or a meso-helical (odd) structure.17–20 However, Dolphin
and co-workers demonstrated that this rule is not universal,
because ligands with a single methylene spacer can form both
diastereoisomeric species from the same reaction.21 The same
group also investigated the interconversion between helicates
and mesocates by changing reaction conditions such as solvent
aDepartamento de Química Inorgánica, Centro Singular de
Investigación en Química Biolóxica e Materiais Moleculares (CIQUS),
Universidade de Santiago de Compostela, Santiago de Compostela,
Galicia, E-15782, Spain. E-mail: manuel.bermejo@usc.es
bDepartamento de Química Inorgánica, Facultade de Ciencias,
Universidade de Santiago de Compostela, Lugo, Galicia, E-27002,
Spain. E-mail: rosa.pedrido@usc.es, ana.gonzalez.noya@usc.es
cUnidade de Difracción 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
†Electronic supplementary information (ESI) available: Synthetic
schemes, NMR spectra, crystal packing figures, crystal data, main bond
distances and angles and hydrogen bond parameters. CCDC
893868–893874. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2dt31770b
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and temperature.22 It was found that bond rotation in the ligand
allows the transition between the two conformers under the reac-
tion conditions and this results in both structures.
Our aim was to elucidate the factors that determine the selective
formation of helicates and mesocates. These metal ions can give
rise to tetrahedral or octahedral kernels by coordination with two
or three ligand strands. Electrochemical interaction of these
ligands with cobalt, zinc and cadmium plates was carried out
and the subsequent formation of helical or meso-helical metallic
arrangements was analysed.
An additional hypothesis was proposed by Stack and co-
workers, who stated that a chiral ligand backbone should force
the formation of a helicate and disfavour the formation of a
meso-complex.23 Raymond’s group contradicted both hypo-
theses with an example of both helical and meso complexes
assembled by the same chiral ligand. This study went one step
further in demostrating that the helicate-to-mesocate interconver-
sion could be driven by the formation of a solvent host–guest
complex.24
Results and discussion
Ligands
The tetradentate ligands H2LR (R = Me, Et, Ph, PhN, PhOMe)
are composed of two flexible bidentate [NS] domains separated
by a rigid aromatic spacer (Scheme 1), thus fulfilling the require-
ments generally established to assemble helical or meso-helical
structures.1 These ligands were synthesised by the reaction of
1,3-diacetylbenzene and the corresponding thiosemicarbazide
precursor, as detailed in the Experimental section. Recrystallisa-
tion of the ligand H2LMe from methanol yielded good quality
crystals that were studied by crystallography. Our interest in ana-
lysing the structure of this ligand lay in the possibility of predict-
ing the conformational changes required in the ligand to make
coordination to two metal centres possible and thus afford
helical or meso-helical complexes.
More recently, Wu et al. showed that the encapsulation of
anions may be responsible for the anion-dependent formation of
helicates versus mesocates.25
All of the aforementioned precedents demonstrate that the
existing studies concerning the selective discrimination between
mesocate/helicate is mainly focused on factors that are directly
related to the ligand design.26–28 To date, only a few partial
studies have shown that different metal ions give rise to different
supramolecular structures based on their coordination require-
ments, as these can have a clear preference for tetrahedral [Cu(I),
Ag(I)] or octahedral kernels [Fe(II), Mn(II)].1 In contrast, a simple
methodology based on metal ion recognition that would allow
the selective isolation of helicates and mesocates has not yet
been systematically investigated.
Thiosemicarbazone ligands are well-known skeletons both in
terms of their coordination chemistry and their wide range of
biological applications.29 Our research group has pioneered the
use of these types of ligands as precursors of different supra-
molecular arrays. For example, we have reported mesocates, heli-
cates and cluster thiosemicarbazone-derived helicates with
unique architectures and promising new functionalities.6b,30,31
We report here the first strategy to achieve synthetic control
over the isolation of dinuclear helical or meso-helical complexes
by metal-ion recognition. For this purpose we designed a series
of five tetradentate bisthiosemicarbazone ligands H2LR with
different terminal substituents (R = Me, Et, Ph, PhOMe, PhNO2)
(Scheme 1). As the metal ions we chose Co(II), Zn(II) and Cd(II).
The crystal structure of H2LMe consists of discrete molecules
with the two thiosemicarbazone arms adopting an anti-arrange-
ment and an E conformation in relation to the two imine bonds
(Fig. 1a, Table S1†). As expected, this conformation is mainly
determined by the existence of both intra- and intermolecular
hydrogen bonds (Fig. 1b, Table S2†). The imine nitrogen atom
and the NH thioamide group of each ligand arm are involved in
the intramolecular hydrogen bonds.
Non-classical intramolecular interactions also exist between
the aromatic protons H6 and H10 and the imine nitrogen atoms
N3 and N4, respectively. Furthermore, the establishment of inter-
molecular hydrogen bonds between the NH hydrazide group of
one ligand arm and the sulfur atom of another neighbouring mole-
cule gives rise to zig-zag chains within the crystal cell (Fig. 1b).
Scheme 1 Bisthiosemicarbazone ligands H2LR (R = Me, Et, Ph, PhN, PhOMe).
13396 | Dalton Trans., 2012, 41, 13395–13404
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Fig. 2 ORTEP representation of the mesocate [Co2(LMe)2] 1.
Fig. 1 (a) ORTEP representation of H2LMe and (b) zig-zag chains
In order to complete our systematic investigation we per-
formed the electrochemical oxidation of a cobalt plate in a con-
ducting acetonitrile solution of the ligands H2LMe, H2LPh and
H2LPhN. This procedure led to the isolation of the Co(II) com-
assembled in the [H2LMe] crystal cell.
As far as the H2LMe arrangement is concerned, it must be
noted that the donor atoms in one of the bidentate domains (N3/
S1) are oriented in opposite directions. For this reason the
coordination of these two donor atoms to the same metal ion
would require a conformational rotation in order to make the
imine nitrogen and the thioamide sulfur atoms point towards the
same metal centre.32,33 Furthermore, bearing in mind that the
helicate is the minimum energy complex34 and the optimal con-
formation taken by the free ligand H2LMe (Fig. 1), the helicate
structure should be theoretically more favoured for assemblies of
the type [M2L2].
plexes
[Co2(LMe)2]·H2O,
[Co2(LPh)2]·2H2O
and
[Co2(LPhN)2]·4H2O. The ESI mass spectra of these complexes
R
contained peaks due to [Co2L2 + H]+. This evidence, together
with the analytical data and the conductivity values, led us to
propose a neutral [Co2L2] stoichiometry for these compounds.
The magnetic moments of the Co(II) complexes at room tempera-
ture are close to that expected for magnetically diluted CoII ions
in a tetrahedral environment (4.2–4.3 B.M.). These complexes
were also characterized by IR spectroscopy.
X-ray quality crystals of [Co2(LMe)2] 1 (Fig. 2, Table S4†)
were grown by slow evaporation of the mother liquors from the
electrochemical synthesis of compound 5. In this complex two
dianionic strands [LMe 2−
are coordinated to two cobalt atoms
]
Helicate or mesocate assembly
through an [N2S2] donor system that consists of a nitrogen imine
atom and a thioamide sulfur atom from one of the ligand arms.
This coordination mode gives rise to a side-by-side coordination,
thus generating a meso-helical structure.
The second step in our study was the preparation of the com-
plexes of the ligands H2LR (R = Me, Et, Ph, PhN, PhOMe) with
different divalent metal ions, such as Co(II), Zn(II) and Cd(II). It
must be noted that Ni(II) and Cu(II) were not included in our
study because there is evidence that these two metals induce cat-
alytic hydroxylation of the central arene of the ligand strands,31
thus precluding the formation of helical or meso-helical
arrangements.
The investigation was initially carried out with Co(II), which
has a d7 electronic configuration. The coordinative preferences
of Co(II) must be considered with care since it is well known that
the coordination geometry of the metal depends on the low/high
spin state of the metal, which in turn depends on the ligand field
strength. Regarding the preferred geometries, cobalt(II) is known
to have a four coordinate (tetrahedral) and six coordinate (octa-
hedral) stereochemistry.
The coordination geometry around the metal ion is distorted
tetrahedral, as evidenced by the values of the bond angles
between the atoms and the ligand donor system (see Table S5†).
The bond distances Co–N and Co–S are in the order of those
found in the literature for Co(II) complexes with thiosemi-
carbazone ligands.37 The intradinuclear Co–Co distances for
complexes with R terminal substituents methyl, ethyl and meth-
oxyphenyl are 7.0188(13) Å, 6.7802(20) Å35 and 6.8934(6)36 Å,
respectively. Therefore, it appears that the introduction of bulky
groups in position 4 of the N terminal branches of the thiosemi-
carbazone ligands leads to a slight decrease in the intermetallic
distances.
Deprotonation of the NH hydrazide groups and coordination
to the metal ion through the imine nitrogen and the thioamide
sulfur atoms leads to charge delocalization along the ligand. Fur-
thermore, coordination to the cobalt centre gives rise to a confor-
mational change in the thiosemicarbazone arms with respect to
the imine bonds. Thus, the configuration changes from E/E in
the free ligand to E/Z in the cobalt mesocate [Co2(LMe)2] 1.
The crystal lattice of complex 1 reveals the formation of
hydrogen bonds between thioamide groups and imine nitrogen
atoms of a neighbouring complex molecule (Fig. S6, Table S6†).
The Co(II) complexes studied are of additional interest
because they are amongst the few examples of cobalt mesocates
Recently, we published the unique structures of the mesocates
Et 35
[Co2L2
]
and [Co2L2PhOMe],36 which arise from the electro-
chemical interaction of a cobalt plate with the ligands H2LEt and
H2LPhOMe, respectively. Both compounds were previously
reported as they were of additional interest beyond their meso-
cate structure. For example, [Co2L2Et] crystallised as an un-
expected mixture of linkage and conformational isomers,
whereas [Co2L2PhOMe] featured a novel type of supramolecular
grid structure, a “grid-of-mesocates”, in which the grid nodes are
simple meso-helical units. These two mesocate compounds are
included in this work for comparative purposes.
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described in the literature.15a,38 More specifically, the only
examples reported to date of Co(II) mesocates with bisthiosemi-
carbazone helicands are compound 1 and the previously reported
observation that may be attributed to the existence of isomers in
solution. However, the NMR spectrum of the methoxyphenyl
compound contains a single set of resonances. This spectro-
scopic evidence, together with the poor quality of the nitro
complex spectrum, does not provide any valuable information
about the structure of these complexes in solution.
[Co2L2 ]
Et 35 and [Co2L2PhOMe].36
The formation of meso-helicates in all these cases indicates
that an increase in the size of the 4-N terminal substituents of the
thiosemicarbazone ligands does not affect the type of structure
obtained, but it does affect the size and shape of these
mesocates.
Slow evaporation of the acetonitrile mother liquors obtained
in the synthesis of [Zn2(LEt)2]2
afforded a different solvent
30a
free complex 2. Recrystallisation of the phenyl and nitrophenyl
compounds from chloroform and acetone, respectively, afforded
crystals suitable for study by X-ray diffraction. The structures of
the complexes were found to be [Zn2(LPh)2]·CH3Cl 3 and
[Zn2(LPhN)2]2·5(CH3)2CO 4. Slow evaporation from the
methoxyphenyl mother liquors yielded crystals corresponding to
[Zn2(LPhOMe)2] 5 (Table S7†).
The dinuclear zinc complexes 2, 3, 4 and 5 exhibit a box-like
achiral meso-helical structure, because the two ligands are side-
by-side rather than twisted around the zinc(II) ions (Fig. 3–5).
The [LR]2− bridging ligands act in a fully deprotonated
It was decided to assess a different dn metal ion with the aim
of assessing whether the electronic configuration has any effect
on the structure of the assembled complex. Having ruled out
Ni(II) and Cu(II) (vide supra), we chose the d10 ion Zn(II), which
has a softer acid character compared to Co(II). In addition the
ligand-field stabilization energy gain cannot be accomplished by
the Zn(II) closed shell d10 ions. The arrangement of the ligands
in the Zn(II) compounds is the result of an equilibrium between
the attractive electrostatic metal–ligand forces and the repulsion
of the bound residues and this equilibrium may lead to the for-
mation of both octahedral and tetrahedral arrangements.
It should be noted that the structures of the mesocate com-
plexes [Zn2(LMe)2]2 and [Zn2(LEt)2]2·0.5(C3H6O) were published
previously as part of our investigations carried out to establish a
successful route to obtain cluster helicates selectively.30a The
compounds are included here for comparative purposes.
Oxidation of a zinc plate in the presence of the ligands H2LR
(R = Ph, PhN, PhOMe) in a conducting acetonitrile solution
yielded the complexes [Zn2(LPh)2]·2H2O, [Zn2(LPhN)2]·3H2O
and [Zn2(LPhOMe)2]·2H2O, respectively. The characterization
data for these complexes (ESI, elemental analysis and IR spec-
troscopy) allow us to propose [Zn2LR2] stoichiometries for these
1
compounds. The H NMR spectra of the zinc complexes do not
contain a signal for the hydrazide NH proton, thus confirming
the bisdeprotonation of the ligand. In addition, the methyl and
phenyl signals show splitting of the terminal NH signal, an
Fig. 4 ORTEP representation of the mesocates [Zn2(LPh)2]2·CHCl3
and [Zn2(LPhOMe)2]2·CHCl3 5.
3
Fig. 3 ORTEP representation of the linkage isomers (a) (top) and (b)
(bottom) of mesocate [Zn2(LEt)2]2·2.
Fig. 5 ORTEP representation of the conformational isomers of meso-
cate [Zn2(LPhN)2]2·5(CH3)2CO 4.
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bisbidentate fashion and coordinate the zinc(II) ions through the
imine nitrogen and the thioamide sulfur atoms. The zinc(II) ions
have a distorted tetrahedral four-coordination environment, as
evidenced by the bond angles (Tables S8–S10†). The bond dis-
tances (Tables S8–S10†) between the metal ion and the kernel
donors are in the same order as the values found for Zn(II) com-
plexes derived from thiosemicarbazone ligands.30,39 The inter-
metallic Zn–Zn distances for our complexes with R terminal
substituents methyl, ethyl, phenyl, nitrophenyl and methoxyphe-
nyl are 7.1588(11),30a 6.9146(11)30a and 7.0290(8)/7.4407(56),
6.9611(5), 6.8941(12)/6.8030(11) and 5.9262(5) Å, respectively.
As in the case of the Co(II) mesocates, functionalisation of the
thiosemicarbazone branches with more bulky groups leads to a
slight decrease in the metal–metal distances.
(Fig. 7 and S9†). These interactions give rise to an ordered
crystal lattice with the acetone molecules trapped inside.
The conformational isomers of 4 arise from the different
spatial orientation adopted by one of the terminal nitro phenyl
chains in each ligand strand (Fig. 6). The different arrangement
of these conformers is clearly shown by the values of the S–Zn–S
bond angles (see Table S6,† Fig. 6), as well as by the different
chelate ring angles (84.60° in isomer a, 77.20° in isomer b). The
Zn–Zn intermetallic distance is very similar in the two confor-
mers (vide supra).
The crystal cell of the complexes 3 and 5 reveals the establish-
ment of hydrogen bonds between the sulfur atoms and the NH
thioamide groups of a neighbouring complex (Table S7†), a situ-
ation that gives rise to the growth of chains in the crystal lattice.
In addition, in complex 3 π–π stacking interactions occur
between the aromatic rings of the spacer (centroid–centroid dis-
tance 3.778(2) Å, Fig. S7†).
As mentioned before for the Co(II) mesocates, the Zn(II) ana-
logues also have a Z/E configuration of the thiosemicarbazone
branches with respect to the imine bonds, thus confirming the
need for conformational rotation to achieve meso-helical
arrangements. The meso-helical structures achieved with Zn(II)
ions also demonstrate that a change from Co(II) to a similar-sized
metal ion like Zn(II), which lacks ligand-field stabilization
energy, does not influence the arrangement of the bisthiosemicar-
bazone ligands around the two metal ions. Besides, a change in
the size and nature of the 4-N terminal substituents of the thio-
semicarbazone ligands does not have any effect on the type of
structure obtained (mesocates) but it does influence the compac-
ticity of the meso-helical molecules.
The next step in our investigation was to study another metal-
related factor that may be involved in the assembly process, i.e.
the size of the metal ion. Thus, keeping the d10 closed-shell
nature, we chose the larger metal ion Cd(II). Cd(II) can be also
found in tetrahedral and octahedral arrangements and a preferred
coordination kernel cannot be defined.
Mesocate-type structures have been found on several
occasions for zinc complexes with [N2O2] ligands26b,40 and
[N4]41 donors derived from pyridine, pyrazole-phenol or chiral
imines, with tetracoordinated metal centres in all cases.
However, there are very few examples of zinc mesocates derived
from thiosemicarbazone ligands.42
It is worth noting that the asymmetric unit of [Zn2(LEt)2]2·2
contains two linkage isomers (Fig. 3), whereas in the case of
mesocate [Zn2(LPhN)2]2·5(CH3)2CO 4 the crystal cell exhibits
two conformational isomers (Fig. 5).
The formation of linkage isomers in 2 could be attributed to
the fact that [NS] thiosemicarbazone domains can coordinate
either the imine or the hydrazide nitrogen atoms and one can
therefore consider that these domains can behave in an [(Nim/
Nhy)S] ambidentate manner (Scheme 2). Conformers [Nim/S],
a, exhibit behaviour normally observed in thiosemicarbazonate
complexes, giving rise to five-membered chelate rings. However,
in the linkage isomer [Nhy/S], b, both thiosemicarbazone
strands employ different nitrogen atoms for coordination: one of
them uses the imine nitrogen atom in a similar manner to the
[Nim/S] isomer, but the second domain is bound to the zinc
atoms by the hydrazide nitrogen atom instead. This alternative
coordination mode is less frequently observed in the coordi-
nation chemistry of thiosemicarbazones. The Zn–Zn bond dis-
tance is higher in the b isomer [7.4407(56) Å] in comparison to
the a isomer [7.0290(8) Å].
The presence of the two conformers in the unit cell has been
found previously30a,35 and could be attributed to the need for
geometrical optimisation of the hydrogen bond contacts between
the conformers. Thus, conformational isomers of complex 4 are
connected by hydrogen bonds between the NH thioamide groups
and the thioamide sulfur atoms of the neighbouring conformer,
as well as with oxygen atoms from solvent molecules. Moreover,
there are π–π stacking interactions between the π-cloud of the
NO2 groups and the spacer aromatic rings of a neighbouring con-
former, with N–centroid distances of 3.582(7) and 3.625(8) Å
Fig. 6 Overlapped coordination kernels for the conformational isomers
of mesocate 4.
Scheme 2 Ambidentate behaviour of the hydrazide N–N group in a
thiosemicarbazone binding domain.
Fig. 7 Non-covalent interactions between conformational isomers of
mesocate [Zn2(LPhN)2]2·5(CH3)2CO 4.
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Recently we reported the cadmium helicate [Cd2(LPh)2]2,
which assembles into chains of enantiomers in the solid state,43
and [Cd2(LMe)2]2·DMSO, which forms a novel supramolecular
topology that we named “grid-of-helicates” with the DMSO
molecule acting as a superglue agent.44 In order to provide
further support for these results we performed the electrochemi-
cal oxidation of a cadmium plate in the presence of acetonitrile
solutions of ligands H2LR (R = Et, PhN, PhOMe). These synth-
eses gave rise to the complexes [Cd2(LEt)2]·H2O,
[Cd2(LPhN)2]·3H2O and [Cd2(LPhOMe)2], respectively. The ESI
mass spectra of these complexes are consistent with the for-
mation of dinuclear species since peaks due to [M2L2 + H]+
were identified. The elemental analysis data and the conductivity
values confirm the neutral and dinuclear nature of all these com-
plexes. The 1H NMR spectra of the cadmium complexes are also
indicative of the dianionic character of the ligand in the com-
plexes, but do not provide further information about their struc-
ture in DMSO solutions, as was the case for the previously
discussed zinc derivatives.
The Cd(II) complexes were also characterized by means of
113Cd NMR studies (Fig. S5†). Thus, the 113Cd NMR spectra
exhibit signals at 559.0 and 542.4 ppm for ethyl and methoxy-
phenyl complexes, respectively, and these values are close to
those found for Cd(II) complexes with [N2S2] coordination
environments in DMSO solutions. The positions of these signals
indicate that the tetracoordinated kernel found in the solid state
(vide infra) is retained in solution.45 In the case of the nitrophe-
nyl complex, the signal appears at 454.4 ppm (Fig. S1†) and this
is indicative of the coordination of a DMSO solvent molecule to
the Cd(II) ions in solution, thus making the compound an unsatu-
rated helicate.
bond distances Cd–N and Cd–S in 6 are in the order of those
found in the literature for Cd(II) complexes derived from thio-
semicarbazone ligands.37b,43,46 The intrahelicate Cd–Cd dis-
tances are 5.2911(3), 5.2492(3) and 5.0178(4) Å for complexes with
terminal substituents methyl, phenyl and nitrophenyl, respecti-
vely. As expected, helicates have more compact structures than
mesocates. Furthermore, in a similar way to the Co(II) and Zn(II)
mesocates, functionalisation of the thiosemicarbazone branches
with more bulky groups leads to a slight decrease in the metal–
metal distances.
As demonstrated previously for the analogue [Cd2(LPh)2]2,
helicate 6 exhibits π–π stacking interactions between the nitro-
phenyl terminal groups of neighbouring molecules, thus giving
rise to parallel infinite chains of helicate diastereoisomers. This
packing mode differs from that found in the previously reported
[Cd2(LPh)2]2, in which dihelical enantiomer pairs assembled into
an infinite linear chain.43 Both cases are clear examples of hier-
archical self-assembly because discrete dihelicate complexes
aggregate in a second supramolecular approach (Fig. 9).11a,47
This result also confirms that the presence of an aromatic ring at
each of the four ends of the dihelicate favours aggregation into
1D chains through π-stacking interactions, whereas functionalisa-
tion of the supramolecule with alkyl groups leads to the assem-
bly of hydrogen-bonded 1D chains or unsaturated grid-of-
helicates instead.
The X-ray diffraction studies carried out on the Co(II) and Zn(II)
mesocates 1–5 and the Cd(II) dihelicate 6 showed significant
differences in terms of the conformation of the ligand arms.
Thus, the configuration adopted by the ligand with respect to the
imine bonds when they are bound to Co(II) and Zn(II) ions in
meso-helical structures is E/Z (or pseudo-C), whereas for the
Cd(II) helicates an E/E configuration (also called pseudo-S) is
observed (Fig. 10). This fact implies that dihelicate formation
requires fewer conformational rotations and is therefore the ther-
modynamically most stable metal structure. Moreover, the Cd(II)
helicates prove that an increase in the degree of hindrance in the
ligand by functionalisation of the terminal NH thioamide group
does not affect the type of supramolecular structure assembled
but it does affect the crystal packing mode (1D chains or grid-of-
helicates).
Recrystallisation of the nitrophenyl complex from acetone
yielded crystals suitable for X-ray diffraction studies. Analysis of
the structural data revealed a helical structure of formula
[Cd2(LPhN)2]·4(CH3)2SO 6 (Table S7†) (Fig. 7). The helicate is
constructed by two bideprotonated ligand units (LPhN 2−
)
arranged in a helical mode around two cadmium(II) metal ions
(Fig. 8). The two enantiomers (P and M) alternate through the
crystal cell. The dihelicate 6 is additionally solvated by four
DMSO molecules.
The cadmium atoms adopt a distorted tetrahedral kernel by
coordination with the imine nitrogen and the thioamide sulfur
atoms belonging to two intertwined dianionic ligand threads.
The existence of a distorted tetrahedral environment is confirmed
by the values of the metal-donor set bond angles (Table S13†).
This distortion is higher than that exhibited by the meso-helical
arrangements in the cases of cobalt(II) and zinc(II) ions. The
Fig. 9 Aggregation of 6 by π-stacking interactions into infinite chains
Fig. 8 ORTEP diagram of the dihelicate 6.
of diastereoisomers.
13400 | Dalton Trans., 2012, 41, 13395–13404
This journal is © The Royal Society of Chemistry 2012

View Article Online
commercially available and were used without further purifi-
cation. Metals (Aldrich) were used as ca. 2 × 2 cm2 plates.
Physical measurements
Elemental analyses (C, H and N) were performed on a Carlo
Erba EA 1108 analyser. 1H NMR spectra were recorded on
Bruker DPX-250 and Varian Inova 500 spectrometers using
DMSO-d6 as a solvent. 113Cd NMR spectra were recorded on a
Bruker AMX-500 spectrometer using DMSO-d6 as a solvent.
Chemical shifts are expressed relative to tetramethylsilane (1H
NMR) and 0.1 M Cd(ClO4)2
ences. Infrared spectra were measured on KBr pellets on a Bio-
Rad FTS 135 spectrophotometer in the range 4000–600 cm−1
(
113Cd NMR) as external refer-
.
Fig. 10 Representation of the naked ligand [LPhN 2−
and (b) helicate 6.
]
in (a) mesocate 4
Electrospray ionization (ESI) mass spectra were registered on an
API4000 Applied Biosystems mass spectrometer. Matrix
Assisted Laser Desorption Ionisation Time of Flight (MALDI-
TOF) mass spectra were registered on a Bruker Autoflex spec-
trometer using DCTB as the matrix. Molar conductivity values
were obtained at 25 °C from a 10−3 M solution in acetone on a
Crison micro CM 2200. Room temperature magnetic suscepti-
bility was measured using a Digital Measurement system
MSB-MKI, calibrated using mercury tetrakis(isothiocyanato)-
cobalt(II).
Bearing in mind that the increase in radius of first-row tran-
sition metal cations through the series is not large [72 pm for
Co(II) and 74 pm for Zn(II) in four-coordinated tetrahedral
environments], a similar behaviour is expected for the ligand
when it coordinates to Co(II) and Zn(II) ions. The formation of
meso-helical structures for Co(II) and Zn(II) ions but the for-
mation of helicates for Cd(II) confirms this prediction.
In the cases of Zn(II) mesocates and Cd(II) helicates, both
metal ions have the same d10 electronic configuration and this
does not provide crystal stabilization – although they do differ in
their ionic radius [92 pm for Cd(II) in a four-coordinated tetrahe-
dral environment]. This size increase may explain the fact that
the ligand strands are helically arranged around the two
cadmium atoms in order to satisfy their tetrahedral coordinative
requirements.
Ligand synthesis
H2LMe. The synthesis and characterization of the ligand bis(4-
N-methylthiosemicarbazone)-1,3-diacetylbenzene were pre-
viously reported by us.30a Recrystallisation from methanol of the
isolated solid yielded crystals of [H2LMe] and these were studied
by X-ray diffraction.
All of the results obtained for the series of complexes con-
structed with the H2LR thiosemicarbazone ligands and the
selected metal ions Co(II), Zn(II) and Cd(II) by means of electro-
chemical synthesis are summarised above. The data obtained
clearly indicate that we can selectively control the isolation of
helicate or mesocate compounds by metal self-assembly
recognition.
H2LEt. The synthesis and characterization of the ligand bis(4-
N-ethylthiosemicarbazone)-1,3-diacetylbenzene, H2LEt, were
previously reported by our group.30a
H2LPh. The ligand bis(4-N-phenylthiosemicarbazone)-1,3-di-
acetylbenzene, H2LPh, was prepared by condensation of 1,3-dia-
cetylbenzene (0.73 g, 4.5 mmol) with 4-N-phenyl-3-
thiosemicarbazide (1.11 g, 9 mmol) in ethanol (125 mL). The
solution was heated under reflux for 10 h and concentrated with
a Dean–Stark trap to ca. 20 mL (Scheme S1†). The pale yellow
precipitate was collected by filtration. The resulting solid was
finally washed with diethyl ether (3 × 10 mL) and dried
in vacuo. Yield: 1.23 g, 82%. Anal. C26H28N6S2O2 requires: C,
60.0; H, 5.0; N, 16.1; S, 12.3. Found: C, 59.9; H, 5.1; N, 16.0;
S, 12.1. MS ESI (m/z) 519.2 [H2L − H]−; IR (KBr, cm−1):
ν(NH) 3307 (w), 3242 (w), ν(CvN + C–N) 1595 (w), 1544 (s),
1521 (s), 1477 (m), ν(CvS) 1111 (m), 800 (w), ν(N–N) 1030
(m); 1H NMR δH(DMSO-d6, ppm): 10.53 (s, 2H), 10.01 (s, 2H),
8.38 (s, 1H), 8.03 (d, 2H, J = 7.7 Hz), 7.44 (t, 1H, J = 7.7 Hz),
7.34 (d, 4H, J = 8.8 Hz), 6.90 (d, J = 8.8 Hz), 3.77 (s, 6H), 2.42
(s, 6H).
Conclusions
The coordination of [NS] bisbidentate ligands to different metal
ions has allowed us to obtain mesocate or dihelicate complexes
by direct metal-ion recognition: meso-helical compounds are
formed in the cases of Co(II) and Zn(II) whereas Cd(II) ions give
rise to double helicates. The size of the metal ion seems to be
the key factor that determines the conformational change of the
ligand upon metal coordination and therefore the formation of
mesocates or helicates.
Experimental
Materials
H2LPhN. The ligand bis(4-N-(4-nitrophenyl)thiosemicarba-
zone)-1,3-diacetylbenzene, H2LPhN, was prepared by conden-
sation of 1,3-diacetylbenzene (0.73 g, 4.5 mmol) with 4-N-(4-
nitrophenyl)-3-thiosemicarbazide (1.28 g, 9 mmol) in ethanol
All solvents, 1,3-diacetylbenzene, 4-methoxyphenyl-isothiocya-
nate, hydrazine monohydrate, 4-N-R-3-thiosemicarbazides (R =
Me, Ph, PhNO2) and tetraethylammonium perchlorate are
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 13395–13404 | 13401

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(125 mL). The solution was heated under reflux for 10 h and
concentrated with a Dean–Stark trap to ca. 20 mL (Scheme 1).
The yellow precipitate was collected by filtration. The resulting
solid was washed with diethyl ether (3 × 10 mL) and dried
in vacuo. Yield: 1.21 g, 81%. Anal. C24H22N8O4S2 requires: C,
52.4; H, 4.0; N, 20.4; S, 11.9%. Found: C, 51.8; H, 4.1; N, 20.5;
S, 11.8; MS ESI (m/z): 549.1 [H2L − H]−; IR (KBr, cm−1):
ν(NH) 3283 (w), 3198 (w), 3263 (m), ν(CvN + C–N)
1597 (w), 1546 (s), 1477 (m), ν(CvS) 1113 (m), 852 (m),
ν(N–N) 1059 (w), ν(NO2) 1504 (m), 1334 (s). 1H NMR
δH(DMSO-d6, ppm): 11.05 (s, 2H), 10.43 (s, 2H), 8.40 (s, 1H),
8.15 (d, 4H, J = 9.0 Hz), 8.06 (d, 2H, J = 7.8 Hz), 7.98 (d, 4H,
J = 9.0 Hz), 7.50 (t, 1H, J = 7.8 Hz), 2.46 (s, 6H).
quantities and with great care), was electrolyzed for 62 min
using a current of 10 mA. The resulting green solid was filtered
off, washed with diethyl ether and dried in vacuo.
[Co2(LMe)2]·H2O. Yield:
0.13
g,
95%.
Anal.
Co2C28H38N12S4O requires: C, 49.4; H, 4.1; N, 18.2; S, 13.9.
Found: C, 48.9; H, 4.2; N, 18.1; S, 14.0. MS ESI (m/z) 394.2
[ML + H]+, 730.0 [ML2 + H]+, 787.0 [M2L2 + H]+; IR (KBr,
cm−1): ν(OH) 3426 (w), ν(NH) 3369 (w), ν(CvN + C–N) 1589
(w), 1547 (w), 1491 (s), ν(CvS) 1097 (w), 797 (w), ν(N–N)
1049 (w); ΛM (μS cm2) = 17.5; μ (B.M.) = 4.2. Slow evaporation
of the mother liquors gave crystals of complex [Co2(LMe)2] 1
that were suitable for X-ray diffraction studies.
The synthesis and structure of [Co2(LEt)2]35 and [Co2(L-
PhOMe)2]36 were previously reported by us.
Precursor 4-N-(4-methoxyphenyl)-3-thiosemicarbazide
H2PhOMe. Hydrazine monohydrate (1.48 g, 30 mmol) was
slowly added to a solution of 4-methoxyphenyl-isothiocyanate
(2.47 g, 15 mmol) in 20 mL of absolute ethanol with stirring at
0 °C (Scheme S2†). After 1 h the white solid was filtered off,
washed with diethyl ether and dried under vacuum. Yield: 2.55 g
(85%). Anal. C8H11N3SO requires: C, 48.7; H, 5.6; N, 21.3; S,
16.2. Found: C, 48.6; H, 5.6; N, 21.3; S, 16.2. MS ESI (m/z)
196.1 [H2PhOMe − H]−; IR (KBr, cm−1): ν(NH) 3321 (m),
3275 (w), 3167 (m), ν(CvN + C–N) 1529 (vs), 1512 (s), 1491
[Co2(LPh)2]·2H2O. Yield:
0.11
g,
78%.
Anal.
Co2C48H48N12S4O2 requires: C, 53.8; H, 4.5; N, 15.7; S, 12.0.
Found: C, 53.9; H, 4.2; N, 16.2; S, 12.0. MS ESI (m/z) 518.0
[ML + H]+, 979.3 [ML2 + H]+, 1035.1 [M2L2 + H]+; IR (KBr,
cm−1): ν(OH) 3402 (w), ν(NH) 3305 (w), ν(CvN + C–N) 1599
(m), 1530 (s), 1494 (s), ν(CvS) 1113 (w), 752 (w), ν(N–N)
1055 (w); ΛM (μS cm2) = 12.3; μ (B.M.) = 4.2.
[Co2(LPhN)2]·4H2O. Yield: 0.10 g, 82%. Anal. Co2C48H50-
N16S4O13 requires: C, 42.4; H, 4.1; N, 16.5; S, 9.4. Found: C,
42.3; H, 3.8; N, 16.4; S, 8.8. MS ESI 609.1 [ML + H]+, MAL-
DI-TOF (m/z) 608.1 [ML + H]+, 668.6 [M2L + H]+, 1215.1
[M2L2]+; IR (KBr, cm−1): ν(OH) 3417 (w), ν(NH) 3303 (w),
ν(CvN + C–N) 1597 (m), 1546 (m), 1497 (s), 1479 (s), ν(NO2)
1329 (s), ν(CvS) 1113 (m), 851 (w), ν(N–N) 1057 (w); ΛM (μS
cm2) = 10.1; μ (B.M.) = 4.3.
1
(m), ν(CvS) 1105 (w), 833 (w), ν(N–N) 1034 (m); H NMR
δH(DMSO-d6, ppm): 9.51 (s, 1H), 8.97 (s, 1H), 7.42 (s, 2H, J =
8.4 Hz), 6.84 (d, 2H, J = 8.4 Hz), 4.72 (s, 2H), 3.73 (s, 3H).
H2LPhOMe. The thiosemicarbazone ligand H2LPhOMe was
obtained by condensation of the previously synthesised 4-N-(4-
methoxyphenyl)-3-thiosemicarbazide (1.12 g, 9 mmol) with 1,3-
diacetylbenzene (0.73 g, 4.5 mmol) (Scheme S1†). For this reac-
tion both reactants were ground in an agate mortar until a homo-
geneous mixture was obtained. The mixture was introduced into
a vial and was sonicated for 6 h. The resulting yellow oil was tri-
turated with 20 mL of absolute ethanol to give a white solid,
which was filtered off, washed with diethyl ether and dried
under vacuum. Yield: 1.27 g, 85%. Anal. C26H28N6S2O2
requires: C, 60.0; H, 5.0; N, 16.1; S, 12.3. Found: C, 59.9; H,
5.1; N, 16.0; S, 12.1. MS ESI (m/z) 519.2 [H2L − H]−; IR (KBr,
cm−1): ν(NH) 3307 (w), 3242 (w), ν(CvN + C–N) 1595 (w),
1544 (s), 1521 (s), 1477 (m), ν(CvS) 1111 (m), 800 (w),
The crystal structures of [Zn2(LMe)2] and [Zn2(LEt)2]·0.5
(C3H6O) were published previously.30a Recrystallisation of the
[Zn2(LEt)2] acetonitrile mother liquors afforded crystals corre-
sponding to the different solvent free structure 2.
[Zn2(LPh)2]·2H2O. Yield:
0.09
g,
69%.
Anal.
Zn2C48H44N12S4O2 requires: C, 53.4; H, 4.1; N, 15.6; S, 11.9.
Found: C, 53.2; H, 4.1; N, 15.2; S, 11.6. MS ESI (m/z) 520.2
1
[ML + H]+, 587.3 [M2L + H]+, 1041.3 [M2L2 + H]+; H NMR
δH(DMSO-d6, ppm): 9.45 (s, 1H), 9.01 (s, 1H), 8.57 (s, 1H),
7.75 (d, 4H, J = 7.7 Hz), 7.61 (t, 1H, J = 6.6 Hz), 7.38 (d, 2H, J
= 8.4 Hz), 7.31 (t, 1H, J = 8.4 Hz), 6.98 (m, 4H, J = 7.7,
6.6 Hz), 6.86 (t, 1H, J = 6.6 Hz), 2.78 (s, 3H), 2.34 (s, 3H); IR
(KBr, cm−1): ν(OH) 3395 (w), ν(NH) 3312 (w), ν(CvN + C–N)
1597 (w), 1529 (s), 1493 (s), ν(CvS) 1120 (m), 753 (w),
ν(N–N) 1032 (w); ΛM (μS cm2) = 11.7. Recrystallisation of
[Zn2(LPh)2]·2H2O from CHCl3 led to the isolation of yellow
crystals of [Zn2(LPh)2]·2CHCl3 3.
1
ν(N–N) 1030 (m); H NMR δH(DMSO-d6, ppm): 10.53 (s, 2H),
10.01 (s, 2H), 8.38 (s, 1H), 8.03 (d, 2H, J = 7.7 Hz), 7.44 (t, 1H,
J = 7.7 Hz), 7.34 (d, 4H, J = 8.8 Hz), 6.90 (d, J = 8.8 Hz), 3.77
(s, 6H), 2.42 (s, 6H).
Synthesis of the complexes. The metal complexes were
obtained using an electrochemical procedure.48 As an example
the synthesis of [Co2(LMe)2]·H2O is described below.
[Zn2(L2PhN)2]·3H2O. Yield: 0.09 g, 73%. Anal. Zn2C48H46-
N16S4O11 requires: C, 43.1; H, 3.9; N, 16.8; S, 9.6. Found: C,
43.3; H, 3.5; N, 17.0; S, 9.9. MS ESI (m/z) 636.9 [ML + Na]+,
680 [M2L + H]+; MS MALDI-TOF (m/z) 613.1 [ML]+; 1H
NMR δH(DMSO-d6, ppm): 10.06 (s, 1H), 9.95 (s, 2H), 8.18 (m,
4H + 4H), 8.05 (m, 1H), 8.03 (m, 1H), 7.87 (m, 1H), 2.34 (s,
6H); IR (KBr, cm−1): ν(OH) 3402 (sh), ν(NH) 3364 (w), 3290
(w), ν(CvN + C–N) 1597 (m), 1547 (m), 1495 (s), 1475 (s),
ν(NO2) 1329 (s), ν(CvS) 1113 (m), 851 (w), ν(N–N) 1067 (w);
ΛM (μS cm2) = 9.3. Recrystallisation of [Zn2(LPhN)2]·3H2O from
[Co2(LMe)2]·H2O. An acetonitrile solution of the ligand con-
taining about 10 mg of tetraethylammonium perchlorate as the
supporting electrolyte was electrolyzed using a platinum wire as
the cathode and a metal plate as the anode. The cell can be sum-
marised as: Pt(−)|H2LR + MeCN|Co(+). A solution (0.1 g,
0.19 mmol) of the ligand in acetonitrile (80 mL), containing
10 mg of tetraethylammonium perchlorate (CAUTION: although
problems were not encountered in this work, all perchlorate com-
pounds are potentially explosive and should be handled in small
13402 | Dalton Trans., 2012, 41, 13395–13404
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X-ray crystallography
acetone led to the isolation crystals of the complex
[Zn2(LPhN)2]2·5(CH3)2CO 4 and these were characterized by
X-ray diffraction.
Data for H2LMe, 2, 3, 4, 5 and 6 were collected on a Bruker
APPEX 2 diffractometer and data for 1 were collected on a
Bruker SMART 1000 diffractometer, all using graphite-mono-
chromated Mo-Kα radiation (λ = 0.71073 Å) from a fine-focus
sealed tube source (at 100 K). The computations and reduction
were carried out with APEX2. In all cases an empirical absorp-
tion correction was applied using SADABS.49 All of the struc-
tures were solved by SIR-9750 except for 3, which was solved by
SIR200451 and all were refined by full-matrix least-squares tech-
niques against F2 using SHELXL-97.52 Positional and anisotro-
pic atomic displacement parameters were refined for all
heteroatoms. The hydrogen atom positions were included in the
model by electronic density and refined isotropically [Uiso(H) =
1.2Ueq(atom)] or were geometrically calculated and refined using
a riding model (isotropic thermal parameters 1.2–1.5 times those
of their carrier atoms). Criteria for a satisfactory complete analy-
sis were the ratios of “rms” shift to standard deviation of less
than 0.001 and no significant features in the final difference
[Zn2(LPhOMe)2]·2H2O. Yield: 0.10 g, 76%. Anal. Zn2C52H56-
N12S4O6 requires: C, 51.9; H, 4.7; N, 14.0; S, 10.7. Found: C,
51.6; H, 4.5; N, 14.3; S, 10.4. MS ESI (m/z) 583.1 [ML + H]+,
650.3 [M2L + H]+, 1103.3 [ML2 + H]+; MS MALDI-TOF (m/z)
583.2 [ML]+, 1169.4 [M2L2 + H]+; 1H NMR δH(DMSO-d6,
ppm): 8.95 (s, 2H), 8.41 (s, 1H), 7.68 (d, 2H, J = 7.7 Hz), 7.59
(d, 4H, J = 8.8 Hz), 7.41 (m, 1H, J = 7.7 Hz), 6.82 (d, 4H, J =
8.8 Hz), 3.76 (s, 6H), 2.26 (s, 6H); IR (KBr, cm−1): ν(OH)
3436 (w), ν(NH) 3397 (m), ν(CvN + C–N) 1593 (w), 1509 (s),
1474 (s), ν(CvS) 1100 (w), 797 (w), ν(N–N) 1034 (m); ΛM
(μS cm2) = 5.7. Slow evaporation of the mother liquors of
[Zn2(LPhOMe)2]·2H2O yielded the new crystalline complex
[Zn2(LPhOMe)2] 5, which was characterized by X-ray diffraction.
The synthesis and full characterization of [Cd2(LMe)2] and
[Cd2(LPh)2] have recently been reported by us.43,44
maps. Molecular graphics were obtained with ORTEP.53
A
[Cd2(LEt)2]·H2O. Yield: 0.12 g, 82%. Anal. Cd2C32-
H46N12S4O1 requires: C, 39.7; H, 4.8; N, 17.4; S, 13.3. Found:
C, 39.7; H, 4.8; N, 17.4; S, 13.3. MS ESI (m/z) 476.9
summary of the crystal data, experimental details and refinement
results is given in Tables S1, S4, S7 and S12.† Significant bond
distances and angles are summarized in Tables S3, S5, S8, S9,
S10 and S13,† while hydrogen bond parameters are shown in
Tables S2, S6, S11 and S14.†
1
[ML + H]+, 589.4 [M2L + H]+, 948.0 [M2L2 + H]+; H NMR
δH(DMSO-d6): 8.34 (s, 1HA), 7.73 (d, 1HB, J = 7.4 Hz), 7.57 (d,
2HA, J = 7.6 Hz), 7.48 (dd, 1HB, J1 = 8.2 Hz, J2 = 6.9 Hz), 7.37
(s broad, 1HB), 7.20 (t, 1HB, J = 7.6 Hz), 6.98 (t, 1HB, J =
6.2 Hz), 6.84 (t, 1HB, J = 4.9 Hz), 6.73 (t, 2HA, J = 4.6 Hz),
3.40 (m, 4HA), 3.18 (m, 4HB), 2.43 (s, 6HA), 1.79 (s, 6HB), 1.16
(t, 6HA, J = 6.9 Hz), 1.08 (t, 6HB, J = 6.9 Hz) ppm. IR (KBr,
cm−1): ν(OH) 3420 (w), ν(NH) 3348, 3327 (w), ν(CvN + C–N)
1566 (m), 1515 (m), ν(CvS) 1091 (m), 846 (w), ν(N–N)
1057 (w); ΛM (μS cm2) = 4.0.
CCDC 893868–893874 contain the supplementary crystallo-
graphic data for this paper.
References
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[Cd2(LPhN)2]·3H2O. Yield: 0.12 g, 82%. Anal. Cd2C48H46-
N16S4O11 requires: C, 41.9; H, 3.4; N, 16.3; S, 9.3. Found:
C, 41.5; H, 3.3; N, 16.0; S, 8.9. MALDI-TOF (m/z) 663.1
[ML + H]+, 773.5 [M2L + H]+; 1H NMR δH(DMSO-d6): 9.66 (s,
2H, H2), 8.64 (s, 1H, H3), 7.94 (d, 4H, H4 + 4H, H6), 7.70 (m,
2H, H5), 7.56 (m, 1H, H7), 2.33 (s, 6H, H8); 113Cd NMR
(DMSO-d6): δ 454.4 ppm; IR (KBr, cm−1): ν(OH) 3405 (w),
ν(NH) 3367 (w), ν(CvN + C–N) 1597 (m), 1544 (m), 1495 (s),
1472 (s), ν(NO2) 1328 (s), ν(CvS) 1113 (m), 848 (w), ν(N–N)
1062 (w); ΛM (μS cm2) = 8.3. Recrystallisation from acetone
yielded crystals of [Cd2(LPhN)2]·4(CH3)2SO 6 that were suitable
for X-ray diffraction.
4 M. Yoshizawa, Y. Takeyama, T. Kusukawa and M. Fujita, Angew. Chem.,
Int. Ed., 2002, 41, 1347.
5 D. W. Johnson, J. D. Xu, R. W. Saalfrank and K. N. Raymond, Angew.
Chem., Int. Ed., 1999, 38, 2882.
[Cd2(LPhOMe)2]. Yield: 0.11 g, 78%. Anal. Cd2C52H52-
N12S4O4 requires: C, 49.5; H, 4.2; N, 13.3; S, 10.2. Found: C,
49.0; H, 4.1; N, 13.0; S, 9.9. MS ESI (m/z) 633.1 [ML + H]+,
1153.2 [ML2 + H]+; MS MALDI-TOF (m/z) 1263.1 [M2L2 + H];
1H NMR δH(DMSO-d6): 8.84 (s, 2H, H2), 8.48 (s, 1H, H3), 7.66
(d, 2H, H4, J = 7.7 Hz), 7.62 (d, 4H, H6, J = 8.8 Hz), 7.55
(t, 1H, H5, J = 7.7 Hz), 6.75 (d, 4H, H7, J = 8.8 Hz), 3.75
(s, 6H, H8), 2.28 (s, 6H, H9); 113Cd NMR (DMSO-d6):
δ 542.4 ppm; IR (KBr, cm−1): ν(NH) 3397 (w), ν(CvN + C–N)
1591 (w), 1509 (s), 1474 (s), ν(CvS) 1113 (w), 797 (w),
ν(N–N) 1034 (w); ΛM (μS cm2) = 4.1.
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