Assembly of a tetranuclear host with a tris(benzene-o-dithiolato) ligand

Article abstract of DOI:10.1002/chem.200901443

The preparation of the first dinuclear triple-stranded helicates based on bis(benzene-o-dithiolato) and mixed benzene-o-dithiolato/catecholato ligands is analyzed. The analogous ligand reacts with Ti⁴⁺ to yield a dinuclear triple-stranded helic

Full text of DOI:10.1002/chem.200901443

COMMUNICATION
DOI: 10.1002/chem.200901443
Assembly of a Tetranuclear Host with a Tris(benzene-o-dithiolato) Ligand
Birgit Birkmann,[a] Roland Frçhlich,[b] and F. Ekkehardt Hahn*[a]
Metallosupramolecular chemistry combines versatile poly-
dentate organic ligands and transition metals in the design
and synthesis of sophisticated assemblies of noncovalently
attached components.[1] Various supramolecular architec-
tures, such as helicates,[1e,2] boxes,[3] grids,[4] squares,[5] molec-
ular containers,[6] and others,[7] have been obtained from
metal-directed spontaneous self-assembly reactions. Among
these, metallohelicates[1e,2] have gathered special interest
due to the presence of the helical structure in nature. The
DNA motif has been transferred to metallosupramolecular
dinuclear double- and triple-stranded helical complexes by
using ligands with amine or catecholato[2,8] donor groups.
Recently, we prepared the first dinuclear triple-stranded hel-
icates based on bis(benzene-o-dithiolato)[9] and mixed ben-
zene-o-dithiolato/catecholato ligands.[10]
clusters of type [M4(A)4]mÀ. We have transferred this design
principle to tris(benzene-o-dithiol) ligands. Ligand H6-B,
however, is flexible enough to react with Ti4+ to yield the
Raymond et al. introduced a 1,5-naphthalene-bridged di-
catechol ligand H4-L,[6d,11] which was designed to prevent the
formation of a dinuclear triple-standed helicate [M2(L)3]nÀ
and thus reacted with metal ions to yield tetranuclear tetra-
hedral clusters of type [M4(L)6]2nÀ in which the dicatecholato
ligands bridge the vertices of the tetrahedron.[6d,11] We
showed that the analogous bis(benzene-o-dithiol) ligand,
against our expectations, reacts with Ti4+ to yield a dinu-
clear triple-standed helicate.[12] Since linear bis(benzene-o-
dithiol) ligands appear unsuitable for the generation of a
tetrahedral tetranuclear cluster, we turned to tripodal tris-
(benzene-o-dithiol) ligands. Raymond et al.[6c,11] and Al-
brecht et al.[13] have shown that tripodal tricatechol ligands
like H6-A are capable of forming tetranuclear tetrahedral
mononuclear siderophor analogue[14] complex anion
[Ti(B)]2À.[15] Reduction of the lengths of the ligand arms by
one methylene group each leads to the essentially planar
ligand H6-1, which like H6-A is incapable of forming mono-
nuclear chelate complexes. We report here on the prepara-
tion of ligand H6-1, its reaction with Ti4+, and on the unusu-
al structural properties of the tetranuclear pseudo-tetrahe-
dral cluster [Ti4(1)4]8À obtained in this reaction (Scheme 1).
The synthesis of the C3-symmetric ligand H6-1 was ach-
ieved by following
a previously published procedure
(Scheme 1). 1,3,5-Triaminobenzene (2),[6c] obtained by re-
duction of 3,5-dinitroaniline with Raney-Ni, was reacted
with 2,3-di(isopropylmercapto)benzoic acid chloride (3)[16]
to produce the S-alkylated ligand precursor 4. Elimination
of the isopropyl groups with sodium/naphthalene provided
ligand H6-1 in a yield of 82% (see Supporting Informa-
tion).[16]
[a] Dr. B. Birkmann, Prof. Dr. F. E. Hahn
Institut fꢀr Anorganische und Analytische Chemie
Westfꢁlische Wilhelms-Universitꢁt Mꢀnster
Corrrensstrasse 36, 48149 Mꢀnster (Germany)
Fax : (+49)251-833-3108
E-mail: fehahn@uni-muenster.de
[b] Dr. R. Frçhlich
Reaction of ligand H6-1 with [TiACHTNUTRGNEUNG(OPr)4] in methanol, in
Organisch-Chemisches Institut
Westfꢁlische Wilhelms-Universitꢁt Mꢀnster
Corrensstrasse 40, 48149 Mꢀnster (Germany)
the presence of Li2CO3/K2CO3, led to the formation of a
dark red solution (lmax =542 nm), indicating the formation
of the {TiS6}2À chromophore (Scheme 1).[17] Complex
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901443.
LixK
8ÀxACHTUNGERTN[NUNG Ti4(1)4] was not isolated. Instead the alkali metal
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Figure 1. 1H NMR spectrum of [Et4N]
8ACHTNUGTRENUNG[Ti4(1)4] at ambient temperature.
characteristic set of signals for the proton resonances of the
benzene-o-dithiolato donor unit, two doublet of doublets at
d=7.01 and 6.96 ppm, respectively, and one triplet at d=
6.65 ppm, reflecting the AMX spin system.
In a second attempt, ligand H6-1 was treated with [Ti-
AHCTUNGTRENNUNG
Scheme 1. Synthesis of ligand H6-1 and of the tetranuclear complex
[Et4N]8A[Ti4(1)6].
C
Scheme 2. Synthesis of LiKACTHNGUTREN[NUG Me4N]6ACHTUGNRTEN[NUNG Ti4(1)4].
cations were exchanged for tetraethylammonium cations.
The use of the organic cations led to a red precipitate and
allowed for the isolation of the complex as a red solid. Since
a ligand:metal ratio of 1:1 was used and ligand H6-1 is not
capable of forming mononuclear complexes, formation of
a dark red precipitate. The ESI(negative ions) mass spec-
trum showed peaks for the anions [[Me4N]3A
[Ti4(1)4]]5À and
[[Me4N]5A
[Ti4(1)4]]3À at m/z=580.2, and 1016.7 amu with the
correct isotope distribution, respectively (see Supporting In-
formation). Recrystallization of this precipitate from DMF/
CTHUNGTRENNUNG
CTHUNGTRENNUNG
the tetranuclear complex [Et4N]
The small number of resonances in the H NMR spectrum
of [Et4N]8A[Ti4(1)4] (in [D7]DMF/[D3]acetonitrile, Figure 1)
[Ti4(1)4] can be assumed.
CH3CN yielded red crystals of LiK
that were suitable for an X-ray diffraction analysis.
Complex LiK[Me4N]6A[Ti4(1)4]·6DMF crystallizes in the
ACHTUNTGRNENGU[Me4N]6AHCTUNGTRENN[GUN Ti4(1)4]·6DMF
1
G
N
CHTUNGTRENNUNG
indicates the presence of only one highly symmetric species
in solution. In case of encapsulation of some tetraethylam-
monium cations within the cavity of complex ion [Ti4(1)4]8À,
two sets of proton resonances with a highfield shift for those
of the encapsulated cations would be expected.[6d] However,
only one set of sharp resonances (d=1.14 (t) and 3.18 ppm
(q)) was observed for all tetraethylammonium cations at
ambient temperature.
monoclinic space group C2/c with the octaanion residing on
a crystallographic twofold axis (Figure 2). The asymmetric
1
unit contains = of the octaanion, three Me4N+ ions, and
2
three DMF molecules. The potassium cation resides on the
twofold axis passing through the center of the octaanion.
One half of a positive charge per asymmetric unit could not
be located. We assume that each asymmetric unit either con-
1
tains = of a disordered lithium cation or that a lithium
2
All signals corresponding to the ligand framework in com-
pound [Et4N]8ACHTUNGTRENNUNG[Ti4(1)4] are only slightly shifted relative to
those observed for the free ligand. The resonance for the
amide protons was detected as a singlet at d=10.17 ppm
and is slightly shifted highfield compared to the value re-
corded for the free ligand (Dd=0.51 ppm). This observation
cation is also located on the twofold axis. We have previous-
ly described similar problems in locating the light lithium
cation in X-ray diffraction studies on dinuclear triple-strand-
ed helicates.[10c] Each titanium atom is coordinated by six
sulfur atoms in a strongly distorted octahedral fashion with
calculated twist angles of F=33.08 (Ti1) and 39.68 (Ti2).
Two titanium atoms adopt the D configuration (Ti1, Ti1*)
and the other two the L configuration (Ti2, Ti2*), which is a
À
excludes the formation of strong intramolecular N H···S hy-
drogen bonds in solution.[9a,c] The spectrum also reveals the
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Host–Guest Chemistry
COMMUNICATION
Figure 2. Molecular structure of the octaanion [Ti4(1)4]8À in LiK
ACTHNUGTRNEUGN[Me4N]6-
ACHTUNGTRENNUNG[Ti4(1)4]·6DMF. Hydrogen atoms, counterions and solvent molecules
have been omitted and ellipsoids for phenyl carbon atoms have been
drawn at an arbitrary small scale for clarity.
rare feature for supramolecular tetrahedral clusters of type
[M4(L)4]nÀ that normally possess four homochiral metal cen-
ters.
Long and variable N···S separations in the range of 3.309
À
to 4.127 ꢃ confirm the absence of intramolecular N H···S
hydrogen bonds in the solid state as was already concluded
Figure 3. Molecular structure of the [Ti4(1)4]8À ion with the encapsulated
cations and schematic representation of the C3-symmetric ligands in the
from the 1H NMR spectrum for the situation in solution.
anions [Ti4(A)4]8À and [Ti4(1)4]8À
.
À
The Ti S bond lengths fall in the range previously reported
for titanium complexes with three amide-substituted ben-
zene-o-dithiolato ligands.[9,10] The C2S2Ti heterocycle is bent
zimidazolium salts can be converted into benzimidazolin-2-
ylidenes upon C2 deprotonation.[19]
Reaction of ligand H6-1 with [TiACTHUNGTERNN(UG OPr)4] and Li2CO3/
À
along the S S vector (range of dihedral angles between the
C6H3S2 and S2Ti planes 2.7–15.28) as has been observed ear-
lier for related complexes.[9,10,12,17]
K2CO3 in methanol followed by addition of four equivalents
The [Ti4(1)4]8À ion contains not only a potassium cation,
but also an additional four Me4N+ ions (Figure 3, top). This
behavior is in remarkable contrast to the situation described
for complex ion [Ti4(A)4]8À with the tricatecholato ligand,[6c]
for which no encapsulation of any cations has been observed
due to the limited space within the cluster anion. The trica-
techolato ligand in [Ti4(A)4]8À is essentially planar with
of N,Nꢄ-dimethylbenzimidazolium bromide gave a red pre-
cipitate. The H NMR spectrum of this solid in [D7]DMF/
CD2Cl2 (Figure 4, black line) showed the signals for the
[Ti4(1)4]8À ion (vide supra) and four broad resonances for
the benzimidazolium cation. Compared to the 1H NMR
1
À
strong intramolecular N H···O hydrogen bonds that cause
the coplanar orientation of the phenylene backbone with
the catecholato donor groups (Figure 3, bottom left).[6c]
Ligand 16À in [Ti4(1)4]8À is not planar and no N H···S hydro-
À
gen bonds have been observed. The benzene-o-dithiolato
donor groups are oriented essentially perpendicular to the
central phenylene group (range of dihedral angles between
the C6H3 and C6H3S2 planes 73.5–89.98, see ligand bridging
Ti1, Ti1* and Ti2* in Figure 3, top). This ligand conforma-
tion (Figure 3, bottom right) together with the nonplanar
C2S2Ti heterocycles generates a much larger and much more
open cavity in [Ti4(1)4]8À than was found for [Ti4(A)4]8À,
which in turn allows the encapsulation of the five cations
(one K+ and four NMe4+).
Molecular containers like complex anion [Ga4(L)6]12À
have found applications as molecular flasks in both catalysis
and reaction control.[18] Encouraged by the encapsulation of
tetramethylammonium cations observed with [Ti4(1)4]8À, we
tried to encapsulate the larger benzimidazolium cation. Ben-
Figure 4. 1H NMR spectra of benzimidazolium bromide in the absence
(red) and in the presence (black) of the tetranuclear host [Ti4(1)4]8À at
ambient temperature.
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spectrum of benzimidazolium bromide, measured in the ab-
sence of complex anion [Ti4(1)4]8À (Figure 4, red line), all
resonances for the benzimidazolium cations are shifted high-
field in the presence of [Ti4(1)4]8À. Both, the highfield shift
of the resonances of the benzimidazolium cation and the ob-
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benzimidazolium cations between the inside and the outside
of the octanuclear [Ti4(1)4]8À octaanion.[20] Formation of a
contact ion pair by p–p interactions of the benzimidazolium
cation with aromatic rings at the outside of the cluster anion
are not likely due to the substitution pattern of both the
benzimidazolium cation and the aromatic rings of ligand 16À.
We have prepared the tris(benzene-o-dithiol) ligand H6-1,
which reacts with Ti4+ to give the tetranuclear cluster anion
[Ti4(1)4]8À. This cluster anion exhibits some unusual proper-
ties when compared to the analogous derivative with a trica-
techolato ligand. The [Ti4(1)4]8À ion contains simultaneously
metal centers with the D and the L configuration and the
tris(benzene-o-dithiolato) ligand is due to a lack of intramo-
lecular hydrogen bonds not planar, but contains C6H3S2
donor groups which are rotated by about 908 relative to the
central phenylene linker. This leads to an enlarged and
much more open cavity of the cluster anion, which has been
shown to be capable of hosting different cations. As we
have already described for dinuclear triple-stranded heli-
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benzene-o-dithiolato donor groups also leads to a significant
change in the coordination chemistry of phenylene-bridged
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Acknowledgements
This work was supported by the Deutsche Forschungsgemeinschaft
(IRTG 1444) and the Fonds der Chemischen Industrie. We thank Dr.
Alexander Hepp for measuring the NMR spectra and Dr. Heinrich Luft-
mann for measuring the mass spectra.
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Keywords: coordination chemistry · host–guest systems ·
metallosupramolecular architectures
· self-assembly · S
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Received: May 29, 2009
Published online: August 11, 2009
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