Published on Web 09/14/2006
Electrostatically Controlled Hierarchical Arrangement of Monocationic
Silver(I) and Dicationic Mercury(II) Ions between Disk-Shaped Template
Ligands
Shuichi Hiraoka,†,‡ Takaaki Tanaka,† and Mitsuhiko Shionoya*,†
Department of Chemistry, Graduate School of Science, The UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo
113-0033, Japan, and PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi 332-0012, Japan
Received June 25, 2006; E-mail: shionoya@chem.s.u-tokyo.ac.jp
Transition metal ions are ubiquitous constituents of functionalized
materials with unique electronic, optical, magnetic, and catalytic
properties. To date, many excellent studies have been reported on
the metal array-function relationship, which have impressed us
on the growing importance of developing novel chemical tools for
metal arrays at the atomic level.1 In particular, a precise control of
heterogeneous metal array is a prerequisite for the construction of
structurally and functionally more complex molecular architectures.
In terms of the difficulty of synthesis, the heterogeneous metal array
is much more difficult than the regular, homogeneous arrangement
of metal ions; hence one has to devise ways to control the spatial
Figure 1. Schematic representation of hierarchical arrangement of Ag+
configuration of metal centers, for instance, by making efficient
use of programmable template ligands. Moreover, metal coordina-
and Hg2+ ions between two disk-shaped hexa-monodentate ligands 1.
tion geometries, hard/soft natures of metals and ligands, and
electrostatic interactions between positively charged metal centers
should be carefully considered in the design strategy.
of 3, indicating the chemical environments of both sides of each
oxazolyl ligand in the complexes are inequivalent, which strongly
suggests the sandwich-shaped structure of M332 complexes (M )
Ag+ or Hg2+) (Figure 2b and c, respectively). The resonances for
the proton signals of oxazolyl rings, a and b of Hg332, were located
at 4.9 and 4.4 ppm, respectively, both of which shifted to downfield
greater than those of Ag332 as a result of the electron-deficient
nature of Hg2+. Thus, 1H NMR is a good indicator for differentiating
which binds to oxazolyl rings, Ag+ or Hg2+, in the isostructural
sandwich-shaped complexes. On the other hand, ligand 4 having
only three oxazolyl rings as G1 sites formed a sandwich-shaped
structure only with Ag+ to generate Ag342 (Figure 2e),4 whereas
ligand 3 with only G2 sites formed both Ag+- and Hg2+-linked
sandwich-shaped complexes. This specific formation of Ag342
should arise from minimal electrostatic repulsion between mono-
cationic Ag+ arranged in a triangle within the complex. In addition,
the 1H NMR investigation of a mixed solution containing 3, Ag+,
and Hg2+ in a 2:3:3 ratio revealed exclusive formation of Hg332.
These model studies clearly indicate that the binding specificity
for Hg2+ and Ag+ in the sandwich-shaped complexes M3L2 is
completely different. Thus, the newly designed disk-shaped hexa-
monodentate ligand 1 bearing six oxazoline rings as G1 and G2
sites was expected to establish hierarchical heterometal arrangement
of Ag+ and Hg2+ in a sandwich-shaped complex.
1H NMR study clearly demonstrated the site-specific arrangement
of monocationic Ag+ and dicationic Hg2+ between two disk-shaped
ligands 1. Upon addition of 1.5 equiv of Hg2+ to a solution of 1 in
a 1:1 mixed solvent of CD3OD and CDCl3, the proton signals of
rings G2, a and b, were observed at 5.0 and 4.6 ppm, respectively,
which are comparable to those of g and h of G2 sites in Hg332. On
the other hand, the proton signals of rings G1, c and d, showed
almost no shifts, indicating that rings G2 bind exclusively to Hg2+
(Figure 2g).5 ESI-TOF mass spectrum of the mixture displayed two
signals at m/z ) 830.4 and 1319.6, which are assignable to [Hg312‚
(OTf)3]3+ and [Hg312‚(OTf)4]2+, respectively. We therefore conclude
that three Hg2+ are put between two ligands 1 in a sandwich-shaped
We present herein a hierarchical arrangement of two kinds of
metal ions that prefer a linear two-coordinate geometry on a two-
dimensional (2D) plane using a disk-shaped hexa-monodentate
ligand 1 in which three oxazoline rings are arranged on each of
two concentric circles (Figure 1). 1H NMR and electrospray
ionization-time-of-flight (ESI-TOF) mass studies demonstrated the
quantitative formation of a sandwich-shaped discrete structure, Ag3-
Hg312, in which three monocationic Ag+ and three dicationic Hg2+
ions are site-selectively arranged on the two concentric circles from
the center in this order.
Recently, we have reported the triangular array of three Ag+
ions that prefer a linear two-coordinate geometry between two disk-
shaped ligands 2 to form a sandwich-shaped Ag322 complex
quantitatively.2 To extend this strategy to the hierarchical arrange-
ment of two kinds of metal ions on a 2D plane between two disk-
shaped ligands, we have designed a novel ligand 1 having six
oxazoline rings, three of which (rings G1, the first generation
coordination sites) are attached directly to the central benzene ring
at the 1, 3, 5 positions, and the other three of which (rings G2, the
second generation coordination sites) are arranged through a
p-phenylene linker at the 2, 4, 6 positions in a concentric pattern.
We expected that this ligand could arrange metal ions so as to
minimize electrostatic repulsion between positively charged metal
centers.
To learn first the metal binding nature of rings G2 in ligand 1,
the Ag+ and Hg2+ complexation of a model ligand 3 having only
1
three oxazolyl rings as G2 sites was examined. H NMR titration
with Ag+ and Hg2+ and ESI-TOF mass measurements revealed that
sandwich-shaped Ag332 and Hg332 complexes, respectively, are
1
quantitatively formed in solution.3 The H NMR spectra for both
complexes displayed eight resonances for eight aromatic protons
† The University of Tokyo.
‡ PRESTO, Japan Science and Technology Agency.
9
13038
J. AM. CHEM. SOC. 2006, 128, 13038-13039
10.1021/ja064194f CCC: $33.50 © 2006 American Chemical Society