A metallosupramolecular tetrahedron with a huge internal cavity

Article abstract of DOI:10.1039/b309495b

A huge molecular tetrahedral complex forms quantitatively by self-assembly from four ligands L-H₆ and four titanium-(IV) ions; in the solid state it encapsulates four {K(DMF)₃}⁺ units in its interior.

Full text of DOI:10.1039/b309495b

A metallosupramolecular tetrahedron with a huge internal cavity†
Markus Albrecht,*a Ingo Janser,a Sebastian Meyer,a Patrick Weisb and Roland Fröhlichc
a Institut für Organische Chemie der RWTH-Aachen, Professor-Pirlet-Straße 1, D-52074 Aachen, Germany.
E-mail: markus.albrecht@oc.rwth-aachen.de; Fax: +49 241 80 92385; Tel: +49 241 80 94678
b Institut für Physikalische Chemie der Universität Karlsruhe, Fritz-Haber-Weg, D-76128 Karlsruhe,
Germany
c
Organisch-Chemisches Institut der Universität Münster, Corrensstraße 40, D-48149 Münster, Germany
Received (in Cambridge, UK) 8th August 2003, Accepted 22nd October 2003
First published as an Advance Article on the web 3rd November 2003
A huge molecular tetrahedral complex forms quantitatively
by self-assembly from four ligands L-H6 and four titanium-
On the other hand, a series of organic ligands with three
pyridine units, which possess a C3-axis, are used to obtain huge
metal complexes with big cavities and different geome-
tries.20–22 Hereby not only guests are encapsulated in the
interior23 but in addition highly reactive intermediates can be
stabilized24 or chemical reactions can be promoted.25
Our aim was to obtain a tetrahedral M4(ligand)4 complex, but
to extend the size of the ligand compared to those, which are
described in the literature,15–18 so that a large cavity is formed
which has the ability to bind either one big or several small
guest molecules. Therefore we prepared ligand L–H6 (Fig. 1) by
reduction of the corresponding tri-nitro derivative26 followed by
condensation with 2,3-dihydroxybenzaldehyde.27 Compound
L–H6 was thus obtained in 74% over two steps.
A coordination study of the derivative L–H6 with TiO(acac)2
and alkali metal carbonate (molar ratio: 1 : 1 : 1) was performed
in DMF at 80 °C for 5 h, to guarantee the thermodynamically
driven quantitative formation of the tetranuclear complexes
M8[Ti4L4] (M = Li, Na, K).
All three salts M8[Ti4L4] show very similar simple NMR
spectra in CD3OD e.g. for the potassium salt we detected signals
at d = 9.08 (s), 7.34 (d, J = 7.9 Hz), 7.28 (d, J = 7.4 Hz), 7.04
(d, J = 7.9 Hz), 6.57 (t, J = 7.4 Hz), and 6.50 (d, J = 7.4 Hz).
The simplicity of the spectra shows the high symmetry of the
obtained coordination compounds. The composition of the
complexes to be M8[Ti4L4] is deduced from FT-ICR MS (in
methanol). Only peaks which can be assigned to the tetranuclear
coordination compounds are found. For the representative
potassium complex K8[Ti4L4] signals are observed which all
show the expected isotopic pattern. The peaks with only the
(
IV) ions; in the solid state it encapsulates four {K(DMF)3}+
units in its interior.
Container-molecules1 possess the potential to bind guests,2 to
stabilise reactive intermediates3 or to promote chemical reac-
tions in their interior.4 Usually the preparation of the containers
has to be performed in costly multistep syntheses.5
An easy way to obtain container-molecules is the self-
assembly of molecular building blocks into highly sophisticated
supramolecular aggregates, e.g. by hydrogen bonding inter-
actions.6 Metal coordination also represents an attractive way to
obtain cage molecules by simple self-assembly processes from
organic donor molecules and appropriate metal ions. Such metal
containing container-molecules possess the advantages of good
accessibility and relative high stability.7,8
A simple geometric body which forms an internal cavity is
the tetrahedron.9 Different approaches can be envisaged to form
molecular tetrahedra in metal-directed self-assembly pro-
cesses.8 On the one hand, linear ligands with two metal binding
sites can be located on the edges of the tetrahedron and a
M4(ligand)6 complex is obtained. Starting with the pioneering
work of Saalfrank,10 many such complexes have been de-
scribed,11,12 the inclusion of guest species has been studied and
reactive species could be stabilized inside of the cavity.13
On the other hand, ligands, which possess three metal-
binding sites and an idealized C3-axis, can connect the metal
ions by spanning the faces (Fig. 1).14 Three examples are
already described in the literature, where tetrahedral coordina-
tion compounds M4(ligand)4 were obtained and structurally
characterized.15–18 It should be mentioned, that related ligands
also can form [M6(ligand)6] complexes.11,18
major isotopes are observed at m/z = 1444.5 {K3H3[Ti4L4]}22
1425.5 {K2H4[Ti4L4]}22, 1406.5 {KH5[Ti4L4]}22, 962.7
{K3H2[Ti4L4]}32 {K2H3[Ti4L4]}32
950.0 937.3
{KH4[Ti4L4]}32 925.0 {H5[Ti4L4]}32
722.0 {K3H
[Ti4L4]}42, 712.2 {K2H2[Ti4L4]}42, 702.8 {KH3[Ti4L4]}42
and 693.5 {H4[Ti4L4]}42
,
,
,
Due to the small size of the previously described ligands, no
inclusion of guests could be observed in the interior of the
tetrahedral complexes.15–18 However, o,oA,oB-aminotrisbenzoic
acid forms a related tetrahedral aggregate by hydrogen bonding
and encapsulates one molecule of ethanol.19
,
,
,
.
Crystallization of the complex salts M8[Ti4L4] from DMF/
ether affords single crystals as needles with a length of up to one
cm. Due to the high content of solvent molecules in the crystal
and the connected instability, we were able to obtain only the X-
ray structure of the potassium salt.‡
K8[Ti4L4]·23DMF crystallizes in the monoclinic space group
C2/c. The solid state structure of the octaanion [Ti4L4]82 is
represented in Fig. 2a. Four titanium(IV) ions are located on the
corners of a slightly distorted tetrahedron with all four complex
units having the same configuration. The triangular faces are
blocked by four of the ligands L which each coordinate to three
of the metal centers. A huge internal cavity is formed. Four of
the potassium counter cations are located in the interior of the
cavity, each binding to the three internal oxygen atoms of the
four titanium tris(catecholate) units.28 Additionally each po-
tassium cation binds three DMF molecules (Fig. 2b). In the
center of [{(DMF)3K}47{Ti4L4}]42 is still some space left
which is filled with solvent molecules. Those molecules are
severely disordered and can not be resolved by X-ray crys-
tallography. However, it can be estimated that about three DMF
molecules can fit into the center of the cavity.
Fig. 1 Representation of the design principle to obtain a molecular
tetrahedron from C3-symmetric ligands and the preparation of L–H6 and
M8[Ti4L4].
† Dedicated to Professor Bernt Krebs on the occasion of his 65th
birthday.
2854
CHEM. COMMUN., 2003, 2854–2855
This journal is © The Royal Society of Chemistry 2003

Notes and references
‡ Crystal
data
for
[K(C3H7NO)3]4[C156H96N16O24Ti4]·K4(C3-
H7NO)10(C3H7NO)2/2, M = 4764.10, monoclinic, space group C2/c (No.
15), a = 35.628(1), b = 32.922(1), c = 26.300(1) Å, b = 123.79(1)°, V =
25637.6(14) Å3, Dc = 1.234 g cm23, m = 3.26 cm21, Z = 4, l = 0.7103
Å, T = 198 K, 44276 reflections collected (±h, ±k, ±l), [(sinq)/l] = 0.54
Å21, 16324 independent (Rint = 0.084) and 9559 observed reflections [I 4
2 s(I)], 1291 refined parameters, R = 0.129, wR2 = 0.324. CCDC 210953.
See http://www.rsc.org/suppdata/cc/b3/b309495b/ for crystallographic data
in .cif or other electronic format.
1 D. J. Cram and J. M. Cram, Container molecules and their guests, RSC,
Cambridge, 1994.
2 A. Jasat and J. C. Sherman, Chem. Rev., 1999, 99, 931; R. C. Helgeson,
K. Paek, C. B. Knobler, E. F. Maverick and D. J. Cram, J. Am. Chem.
Soc., 1996, 118, 5590; J. C. Sherman and D. J. Cram, J. Am. Chem. Soc.,
1989, 111, 4527.
3 B. R. Beno, C. Sheu, K. N. Houk, R. Warmuth and D. J. Cram, Chem.
Commun., 1998, 301.
4 R. Warmuth, Eur. J. Org. Chem., 2001, 423; R. Warmuth, Chem.
Commun., 1998, 59.
5 E. g.: E. Graf and J.-M. Lehn, J. Am. Chem. Soc., 1975, 97, 5022; F. P.
Schmidtchen and G. Müller, J. Chem. Soc. Chem. Commun., 1984,
1115.
6 L. R. MacGillivray and J. L. Atwood, Nature, 1997, 389, 469; R. M.
Grotzfeld, N. Branda and J. Rebek, Science, 1996, 271, 487.
7 For an example see: P. Klüfers and J. Schuhmacher, Angew. Chem.,
1994, 106, 1925; Angew. Chem. Int. Ed., 1994, 33, p. 1863.
8 D. Caulder and K. N. Raymond, J. Chem. Soc., Dalton Trans., 1999,
1185.
9 L. R. MacGillivray and J. L. Atwood, Angew. Chem., 1999, 111, 1243;
Angew. Chem. Int. Ed., 1999, 38, p. 1018.
10 R. W. Saalfrank, A. Stark, K. Peters and H. G. Von Schnering, Angew.
Chem., 1988, 100, 878; Angew. Chem. Int. Ed., 1988, 27, p. 851; R. W.
Saalfrank and I. Bernt, Curr. Opin. Solid State & Materials Science,
1998, 3, 407.
11 D. W. Johnson and K. N. Raymond, Inorg. Chem., 2001, 40, 5157.
12 T. N. Parac, D. Caulder and K. N. Raymond, J. Am. Chem. Soc., 1998,
120, 8003.
13 M. Ziegler, J. L. Brumaghim and K. N. Raymond, Angew. Chem., 2000,
112, 4285; Angew. Chem. Int. Ed., 2000, 39, p. 4119.
14 D. W. Johnson, J. Xu, R. W. Saalfrank and K. N. Raymond, Angew.
Chem., 1999, 111, 3058; , Angew. Chem. Int. Ed., 1999, 38, p. 2882; S.
Hiraoka, T. Yi, M. Shiro and M. Shionoya, J. Am. Chem. Soc., 2002,
124, 14510; M. Albrecht, Angew. Chem., 1999, 111, 3671; Angew.
Chem. Int. Ed., 1999, 38, p. 3463.
Fig. 2 Representations of the solid state structure of (a) the octaanion
[Ti4L4]82, (b) the coordination at an encapsulated {K(DMF)3}-unit, and (c)
the four central nitrogen atoms of ligands L and the four titanium(IV) ions.
Those eight atoms define the cavity size of the supramolecular tetrahedron
(yellow: titanium, blue: potassium, grey: carbon, green: nitrogen, red:
oxygen; hydrogen atoms are omitted for clarity).
15 A. J. Amoroso, J. C. Jefferey, P. L. Jones, J. A. McCleverty, P. Thornton
and M. D. Ward, Angew. Chem., 1995, 107, 1577; Angew. Chem. Int.
Ed., 1995, 34, p. 1443.
Fig. 2c represents the titanium atoms and the central nitrogen
atoms of the ligand, which all together define the size of the
cavity. Hereby the titanium(IV) ions (Ti…Ti = 16.7–17.1 Å) as
well as nitrogen atoms (N…N = 9.7–10.0 Å) form two
tetrahedra with the smaller one fitted into the bigger one.
Titanium–nitrogen distances within the ligands are 10.0 to 10.2
Å, while the Ti–N distances through the cavity are in the region
of 16.3 Å.
The concept to use ligands with C3-symmetry to obtain
tetranuclear [M4(ligand)4] complexes was already introduced
by others.15,17 However, we describe the first example of a
tetrahedral tetranuclear coordination compound which binds
guest species in its interior. The cavity of the complex possesses
an extraordinary size and is able to encapsulate four
{K(DMF)3}-units and three additional DMF molecules. There-
fore this huge coordination compound should be well suited for
the investigation of guest encapsulation and guest exchange in
solution. The big pores of the tetrahedron hereby should not be
a problem as was shown by the work of Fujita.20 Corresponding
studies are momentarily going on in our laboratories.
16 D. Caulder, C. Brückner, R. E. Powers, S. König, T. N. Parac, J. A.
Leary and K. N. Raymond, J. Am. Chem. Soc., 2001, 123, 8923.
17 C. Brückner, R. E. Powers and K. N. Raymond, Angew. Chem., 1998,
110, 1937; Angew. Chem. Int. Ed., 1998, 37, p. 1837.
18 R. W. Saalfrank, H. Glaser, B. Demleitner, F. Hampel, M. M.
Chowdhury, V. Schünemann, A. X. Trautwein, G. B. M. Vaughan , R.
Yeh, A. V. Davis and K. N. Raymond, Chem. Eur. J., 2002, 8, 493.
19 J. E. Field, M. Y. Combariza, R. W. Vachet and D. Venkataraman,
Chem. Commun., 2002, 2260.
20 Review: M. Fujita, Chem. Soc. Rev., 1998, 27, 417.
21 B. Olenyuk, A. Fechtenkötter and P. J. Stang, J. Chem. Soc., Dalton
Trans., 1998, 1707.
22 Review: S. Leininger, B. Olenyuk and P. J. Stang, Chem. Rev., 2000,
100, 853.
23 M. Fujita, K. Umemoto, M. Yoshizawa, N. Fujita, T. Kusukawa and K.
Biradha, Chem. Commun., 2001, 509.
24 M. Yoshizawa, T. Kusukawa, M. Fujita and K. Yamaguchi, J. Am.
Chem. Soc., 2000, 122, 6311.
25 H. Ito, T. Kusukawa and M. Fujita, Chem. Lett., 2000, 598; M.
Yoshizawa, Y. Takeyama, T. Okano and M. Fujita, J. Am. Chem. Soc.,
2003, 125, 3243.
26 O. Temme, T. Dickner, S. Laschat, R. Fröhlich, S. Kotila and K.
Bergander, Eur. J. Org. Chem., 1998, 651.
This work was supported by the Fonds der Chemischen
Industrie and the Deutsche Forschungsgemeinschaft (SPP
1118). We thank Professor Dr M. Kappes and the Nano-
technology Institute, Forschungszentrum Karlsruhe for facili-
tating the ESI-MS measurements.
27 M. Albrecht, S. Kamptmann and R. Fröhlich, Polyhedron, 2003, 22,
643.
28 M. Albrecht, H. Röttele and P. Burger, Chem. Eur. J., 1996, 2, 1264.
CHEM. COMMUN., 2003, 2854–2855
2855