The first X-ray structurally characterized M3L2 cage-like complex with tetrahedral metal centres and its encapsulation of a neutral guest molecule

Article abstract of DOI:10.1039/b000259n

The first X-ray structurally characterized M₃L₂ cage-like complex with tetrahedral metal centres [Zn₃(tib)₂-(OAc)₆]·xH₂O [tib = 1,3,5- tris(imidazol-1-ylmethyl)benzene, x ~ 4] obtained by reaction of zinc(II) acetate with the tripodal ligand tib exhibits guest inclusion properties of neutral molecule such as synthetic camphor in aqueous solution.

Full text of DOI:10.1039/b000259n

The first X-ray structurally characterized M3L2 cage-like complex with
tetrahedral metal centres and its encapsulation of a neutral guest molecule
Hong-Ke Liu,a Wei-Yin Sun,*a De-Jian Ma,a Kai-Bei Yub and Wen-Xia Tanga
a Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing
210093, China. E-mail: sunwy@netra.nju.edu.cn
b Analysis Center, Chengdu Branch of Chinese Academy of Science, Chengdu 610041, China
Received (in Cambridge, UK) 4th January 2000, Accepted 28th February 2000
The first X-ray structurally characterized M3L2 cage-like
complex with tetrahedral metal centres [Zn3(tib)2-
sulfoxide under strong alkali (KOH) conditions.† Its trinuclear
zinc(II) complex [Zn3(tib)2(OAc)6]·xH2O (x ≈ 4), was synthe-
sized by self-assembly of the tib ligand with zinc acetate
dihydrate in ethanol solution.‡ An X-ray crystal structural
analysis provides direct evidence that the complex is an M3L2
cage.§ As shown in Fig. 1, two tib ligands that are both in cis,
cis, cis-conformations adopt a face-to-face orientation and are
joined together by three zinc atoms to generate an individual
three-dimensional cage. Highly disordered water molecules
may present in the cage as suggested by elemental analysis.‡
Each zinc atom is four-coordinate with two oxygen atoms from
two acetate anions and two nitrogen atoms of imidazole from
two different tib ligands. The coordination geometry of the zinc
atoms is distorted tetrahedral with coordination angles ranging
from 101.5(4) to 123.3(7)°. The distance between the zinc
atoms are 9.10 Å and the two benzene ring planes are strictly
parallel with each other with a separation of 9.49 Å.
(OAc)6]·xH2O [tib
= 1,3,5-tris(imidazol-1-ylmethyl)ben-
zene, x ≈ 4] obtained by reaction of zinc(II) acetate with the
tripodal ligand tib exhibits guest inclusion properties of
neutral molecule such as synthetic camphor in aqueous
solution.
Metal-directed assembly has been proved to be a useful
methodology in supramolecular chemistry.1 Frameworks with
specific topologies such as honeycomb grids and cages have
been obtained by assembly of suitable metal ions with rationally
designed tripodal ligands such as L1, L2 and L3.2–4 Among
It is clear from the X-ray structural analysis that the title
complex has a large cavity inside the cage and therefore is
these, cage-like complexes have attracted much attention owing
to their interesting properties and possible applications, e.g.
molecular and chiral recognition.5 The groups of Fujita, Steel,
Robson, Stang and Raymond have performed outstanding work
in the construction of cage-like complexes.3–8 Most recently, an
M12L8 type cage was obtained by assembly of a tri-bidentate
ligand with Cu(II),8 and larger cages with four organic moieties
and eight-coordinate copper(III) centres have been reported by
Beer’s group.9 However up to now, to our knowledge, no M3L2
cages have been isolated and characterized by X-ray structural
analysis, although a guest-induced Pd3L2 cage-like complex [L
= 1,3,5-tris(4-pyridylmethyl)benzene] and analogous cages
have been reported.6 Moreover, in reported M6L4 cage-like
complexes, the metal ions are essentially square planar, e.g.
Pd(II), Pt(II) or octahedral e.g. Ga(III), Fe(III),3,4,6,7,10 with no
case of the metal ion being tetrahedral being reported. Herein
we report the assembly and X-ray crystal structure of an M3L2
cage-like complex formed between a novel tripodal ligand,
1,3,5-tris(imidazol-1-ylmethyl)benzene (tib), and zinc ion,
which has tetrahedral configuration. The complex shows the
interesting property of guest inclusion in aqueous solution.
The tib ligand was prepared from 1,3,5-tris(bromome-
thyl)benzene and imidazole in a molar ratio of 1:3 in dimethyl
Fig. 1 Perspective view (a) and space-filling representation (b) of the X-ray
crystal structure of [Zn3(tib)2(OAc)6]. Selected bond lengths (Å) and angles
(º): Zn–O1 1.84(2), Zn–N1 2.022(7); O1–Zn–O1A 123.3(7), O1A–Zn–
N1A 110.3(4), O1–Zn–N1A 101.5(4), O1A–Zn–N1 101.5(4), O1–Zn–N1
110.3(4), N1A–Zn–N1 109.6(4).
DOI: 10.1039/b000259n
Chem. Commun., 2000, 591–592
This journal is © The Royal Society of Chemistry 2000
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We are grateful for funding from the National Nature Science
Foundation of China for financial support of this work.
Notes and references
† The tib ligand was prepared from 1,3,5-tris(bromomethyl)benzene and
imidazole in a molar ratio of 1:3 in dimethyl sulfoxide under strong alkali
1
conditions and was isolated in 30% yield. H NMR (D2O): d 5.08 (6H, s,
CH2), 6.93 (3H, s, 4Him), 6.95 (3H, s, 5Him), 6.96 (3H, s, Hbz), 7.63 (3H, s,
2Him). Him refers to imidazole protons and Hbz refers to benzene-ring
protons.
‡ Experimental: a solution of tib (32 mg, 0.1 mmol) in ethanol (10 ml) was
added to a ethanol (10 ml) solution of Zn(OAc)2·2H2O (33 mg, 0.15 mmol)
at room temperature. The mixture was filtered after stirring for ca. 10 min
and the filtrate was allowed to stand at ambient temperature for several days.
Colorless crystals were collected in 56% yield. Single prismatic crystals
suitable for X-ray diffraction analysis were obtained by slow evaporation of
the filtrate. (Found: C, 45.94; H, 4.82; N, 13.24. C48H62N12O16Zn3
{[Zn3(tib)2](OAc)6·(H2O)4} requires C, 45.78; H, 4.96; N, 13.34%. 1H
NMR (D2O, 298 K): d 1.80 (18H, s, CH3), 5.18 (12H, s, CH2), 6.67 (6H, s,
4Him), 7.07 (6H, s, 5Him), 7.09 (6H, s, Hbz), 7.81 (6H, s, 2Him).
§ Crystal data for [Zn3(tib)2](OAc)6: Mw = 1187.14, rhombohedral, space
group R32, a = 17.661(2), b = 17.661(2), c = 20.722(4) Å, g = 120°, U
= 5597.5(14) Å3, Z = 3, Dc = 1.057 g cm23, m = 1.007 mm21, F(000) =
1836, T = 294(2) K. A single crystal with dimensions of 0.56 3 0.54 3
0.24 mm was mounted and data collection were performed on a Siemens-P4
four-circle diffractometer by the w-scan technique using graphite-mono-
chromated Mo-Ka radiation (l = 0.71073 Å). 2385 reflections were
collected of which 2081 are independent (Rint = 0.0231). The structure was
solved by direct method with SHELXS-86 and refined by full-matrix least-
squares calculations on F2 with SHELXL-93. The final weighting scheme
Fig. 2 1H NMR spectra in D2O at 293 K: (a) camphor (3.0 mM) +
[Zn3(tib)2(OAc)6] (3.0 mM); (b) downfield region for [Zn3(tib)2(OAc)6]
(3.0 mM); (c) camphor (3.0 mM). c refers to complexed species; h to host
species and g to guest species.
expected to have the ability to encapsulate guest molecules.6
Thus we observed remarkable ability of the complex to
encapsulate neutral molecules in its aqueous solution. It has
been reported that 1H NMR spectroscopy is a powerful method
for investigation of host–guest complexation.6,11 Fig. 2(a)
shows the 1H NMR spectrum of the complexation of the
[Zn3(tib)2(OAc)6] with camphor in deuterated aqueous solution.
Two kinds of signal were observed for both the host and guest
species when the guest camphor was added to a D2O solution of
[Zn3(tib)2(OAc)6]. One set remains unshifted compared with
the corresponding signals of the host [Fig. 2(b)] and of the guest
[Fig. 2(c)], i.e. the unshifted signals originate from un-
complexed species. The other set of upfield shifted signals are
from the guest camphor molecules, while for the signals of the
host molecule, both up- and down-field shifts were observed.
These up- and down-field shifted signals for the host and guest
species arise from complexed species.11 The integration ratio of
the host and guest signals in the 1H NMR spectrum confirmed
the 1:1 stoichiometry for the complexation between the cage
complex and camphor and indicated that ca. 60% of the
camphor was complexed [Fig. 2(a)]. The results indicate that
the camphor molecule, which has a diameter of ca. 6 Å, is large
enough so that species inside and outside of the cavity of the
complex can be distinguished on the NMR time scale. The
broadening of the signals as observed in Fig. 2(a) is due to slow
exchange between the free and complexed species, since the
signals become much broader when the temperature is raised.
The kinetic process of the complexation of the cage with
camphor was investigated by two-dimensional exchange spec-
troscopy (2D EXSY).¶ The rate constants for the forward
(complexation) and reverse (decomplexation) process are 3.9 3
103 M21 s21 and 32.9 s21, respectively, giving an equilibrium
constant of 117.6 M21. For chain-like molecules such as
ethanol, ethyl acetate, diethyl ether or n-butyl alcohol, no
complexation was observed.
2
was w = 1/[s2(Fo2) + (0.1209P)2 + 0.0000P] where P = (Fo2 + 2Fc )/3; R1
= 0.0684 and wR2 = 0.1641 [I > 2s(I)], max. min. residual density:
+0.434, 20.282 e Å23. CCDC 182/1563. See http://www.rsc.org.suppdata/
cc/b0/b000259n for crystallographic files in .cif format.
¶ 1H NMR data were obtained from a Bruker AM 500 spectrometer. 2D
EXSY spectral measurements were carried out using the phase-sensitive
NOESY pulse sequence with a mixing time of 20 ms: R. R. Ernst, G.
Bodenhausen and A. Wokau, Principles of Nuclear Magnetic Resonance in
One and Two Dimensions, Oxford University Press, Oxford, 1983; G.
Bodenhausen, H. Kogler and R. R. Ernst, J. Magn. Reson., 1984, 58,
370.
1 See, for example: Comprehensive Supramolecular Chemistry, ed. J. L.
Atwood, J. E. D. Davies, D. D. MacNicol, F. Vogtle and J. M. Lehn.
Pergamon, Oxford, 1996, vol. 9.
2 G. B. Gardner, D. Venkataraman, J. S. Moore and S. Lee, Nature, 1995,
374, 792.
3 M. Fujita, D. Oguro, M. Miyazawa, H. Oka, K. Yamaguchl and K.
Ogura, Nature, 1995, 378, 469.
4 C. M. Hartshorn and P. J. Steel, Chem. Commun., 1997, 541.
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10 For example: T. N. Parac, D. L. Caulder and K. N. Raymond, J. Am.
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In conclusion, the present study shows that an M3L2 cage is
assembled from five small component molecules by the rational
design of a suitable tripodal ligand and metal ions, and provides
example of supramolecules with interesting properties such as
guest inclusion.
Communication b000259n
592
Chem. Commun., 2000, 591–592