Programmed single step self-assembly of a [2 x 2] grid architecture built on metallic centers of different coordination geometries

Article abstract of DOI:10.1039/b402854f

A novel type of 2 x 2 grid type architecture incorporating two ions of different geometries results from the single step directed self-assembly of a bischelating ligand combining bridged bidentate and tridentate complexation subunits, designed so as to lead to the ion selective and toposelective introduction of two Zn(II) and two Cu(I) cations at well-defined, diagonally located positions of, respectively, octahedral and tetrahedral coordination geometry.

Full text of DOI:10.1039/b402854f

Programmed single step self-assembly of a [2 3 2] grid architecture built
on metallic centers of different coordination geometries
Anne Petitjean†,a Nathalie Kyritsakasb and Jean-Marie Lehn*a
a Laboratoire de Chimie Supramoléculaire, ISIS ULP, 8 allée Gaspard Monge, BP 70028, F-67083
Strasbourg Cedex, France. E-mail: lehn@isis.strasbg.fr
b Service Commun de Rayons X, 4 rue Blaise Pascal, F-67070 Strasbourg Cedex, France
Received (in Cambridge, UK) 26th February 2004, Accepted 2nd April 2004
First published as an Advance Article on the web 23rd April 2004
A novel type of 2 3 2 grid type architecture incorporating two
ions of different geometries results from the single step directed
self-assembly of a bischelating ligand combining bridged
bidentate and tridentate complexation subunits, designed so as
to lead to the ion selective and toposelective introduction of two
Treating ligand LH with half an equivalent of each zinc triflate
and copper tetrakisacetonitrile tetrafluoroborate in a CDCl3–
CD3OD–CD3CN (2:1:1) mixture in the presence of one equivalent
of tetramethylammonium hydroxide, gave a deep purple solution of
1
the complex [Zn2Cu2L4]2+. Its H NMR spectrum is displayed in
Zn(II) and two Cu(
I) cations at well-defined, diagonally located
Fig. 1(c), together with the spectra of the free anionic ligand and
that of the red ZnL2 complex.12 The peaks were assigned on the
basis of their COSY and ROESY 2D proton NMR spectra.
In the free ligand, the preferential transoid conformation of the
pyridine-pyrimidine type unit is responsible for the high chemical
shift of the signal of the pyrimidine proton 1, which directly faces
two nitrogen atoms. It is displaced by 1 ppm on introduction of half
an equivalent of ZnII, mainly due to conformational change.
Electro-spray mass spectrometry shows that two ligands assemble
positions of, respectively, octahedral and tetrahedral coordina-
tion geometry.
Self-assembly has proven to be a highly valuable process in the
construction of elaborate architectures starting from simple build-
ing blocks.1 The information encoded in molecular scaffolds may
be read by algorithms based on hydrogen bonding,1,2 donor/
acceptor1b interactions and metal ion coordination,1 in a supramo-
lecular processing manner.3
Metal coordination chemistry has allowed to direct the self-
assembly towards the generation of discrete well-defined metal-
losupramolecular architectures, yielding a variety of outputs such
as linear and circular helicates,1,4 racks,5 cages6 and grids7 based on
the preferred coordination geometry of the metal center and the
structure of the ligands.
The regularity of the networks expressed in grid-type archi-
tectures7 makes them attractive objects for nanotechnology and
information storage, since they yield ordered 2D array of ion dots.1a
Homometallic grids can be constructed on the basis of tetrahedral
(CuI, AgI)7a–c or octahedral (ZnII, CoII, FeII, etc.)7d–g coordination
geometry with ligands containing respectively bidentate or tri-
dentate complexation subunits. The resulting homometallic com-
plexes display a variety of interesting electrochemical7f and
magnetic7g properties.
The construction of heterometallic [2 3 2] grids presenting
different precisely located metal cations has been achieved by
stepwise strategies.8 Along the way towards self-organization by
selection,9 as seen in the generation of mixed ligand grids7c and the
selective assembly of ligand components in a dynamic library of
ligands,10 it is particularly valuable to devise procedures allowing
the spontaneous assembly of mixed metal grids by selection from a
mixture of metal ions and ligand(s) under the control of the
processing of suitable binding information through specific coor-
dination algorithms.
Scheme 1 Ligand LH and its complexes ZnL2 and [Zn2Cu2L4]2+
.
We now report the ion selective and toposelective self-assembly
of mixed metal [2 3 2] grid architectures combining two cations of
octahedral and two cations of tetrahedral coordination geometry
with four identical ligands, presenting two different coordination
subunits, in a single step from a mixture of ligands and cations.
Such a process bears relation to the self-assembly of heterotopic
double helicates.11
Ligand LH (Scheme 1)12 was designed so as to offer a bidentate
unit of a,aA-bipyridine type as a privileged binding site for
tetrahedral coordination, and a tridentate unit including a 8-hydrox-
yquinoline group, destined to accommodate octahedral coordina-
tion geometry. The latter further provides tunability through its
acido-basic activity. The bidentate and tridentate sites share a
bridging pyrimidine unit participating in both complexations.
Fig. 1 1H NMR spectra of L2 in CDCl3–CD3OD 1.0:1.0 (a), ZnL2 in
CDCl3–CD3OD 1.0:1.0 (b) and [Zn2Cu2L4]2+ in CDCl3–CD3OD–CD3CN
2.0:1.0:1.0 (c). The signals corresponding to protons a and b (Scheme 1) are
highlighted in grey.
† Present address: Department of Chemistry M/C 127-72, Caltech, 1200 E.
California Blvd, Pasadena CA 91125, USA.
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T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

with a zinc cation to form a 2:1 complex ZnL2. Single crystal
diffraction confirmed that the zinc cation recognizes the hard
tridentate site in ligand L2 and that it coordinates in a distorted
octahedral fashion to the deprotonated hydroxyquinoline of two
ligands.12 Solution studies agree with this formulation. Upon
complexation in a 2:1 ligand–metal mode, the signals of protons a
and b are both shifted to lower values by approximately 1.2 ppm, as
a result of both cation complexation and shielding by the phenyl
ring.
controlled introduction of two pairs of cations along the diagonals
imposes a directionality to these systems, which may thereafter be
translated at a supramolecular level in the 1D and 2D organization
of grids into arrays.13 In addition, preliminary mass spectrometry
data show that zinc(II) can be replaced by copper(II) while
preserving the tetrameric assembly. Hence, a single cation in two
different (and possibly interconvertible) oxidation states may be
used to assemble these architectures, which may open the door to
electro-controlled self-assembled grids.
In the case of the 1:0.5:0.5 ligand–zinc–copper mixture, the
resulting deep purple complex displays a further decrease in shift of
~ 0.4 ppm for b and ~ 0.9 ppm for a, relative to ZnL2 (1.6 and 2.2
ppm respectively relative to the free anionic ligand), suggesting that
stacking is enhanced upon copper complexation.
The present results achieve the single step, programmed self-
assembly of heteronuclear arrays of metal ions of well-defined
geometry with ion selectivity and toposelectivity, a given metal ion
being selected for binding at a precisely determined location. These
features bear relation to the potential interest of such arrays for local
information storage and addressing as well as to processes of self-
organization by selection.9
The values of the chemical shifts closely resemble those obtained
with pyrimidine bridged [2 3 2] grids based on octahedral
centers,7e for which, however, the signals of both protons a and b
are split, due to hindered rotation of the phenyl group within the
grid architecture, which renders them inequivalent. In the present
case, the two ortho a and meta b protons are equivalent on the NMR
time scale. The signal for the b protons is actually broadened [Fig.
1(c)], indicative of intermediate rotation rate around the phenyl–
pyrimidine bond. The mass spectral data agree with a 4:2:2 ligand–
zinc–copper composition (peaks at 1018.2 and 2187 respectively
for the 2+ species and the 1+ species with a triflate anion).
The grid type structure of the [Zn2Cu2L4]2+ complex was
confirmed by single crystal diffraction (Fig. 2).‡ In contrast to the
fully octahedral site homonuclear grids,7d,e this mixed tetrahedral/
octahedral center [2 3 2] grid deviates from a strictly square
geometry; it is diamond shaped with an angle of about 100°
between the planes of the ligand at each octahedral Zn(II) site. The
Cu–Zn distances (edges) range from 6.5 to 6.7 Å. These features
correlate with a large difference in the length of the two diagonals,
the Zn–Zn distance being much shorter (8.5 Å) than the Cu–Cu
distance (10 Å). The phenyl groups are held within stacking
between two parallel ligands, as in homonuclear octahedrally based
[2 3 2] grids.7e Their orientation relative to the pyrimidine differs
though, since the torsion angle is only 50° compared to the usual
90°. Two enantiomeric grids are present in the unit cell.
Notes and references
‡ Crystal data for [Zn2Cu2L4]2+: C232H200Cu4N32O8Zn4·3BF4·F3CSO-
3·5CH3OH·2H2O, M = 4685.76, purple prism, triclinic, a = 17.8009(3), b
= 18.2756(3), c = 21.3141(4) Å, V = 5887.1(2) Å3, a = 17.80009(3), b
= 66.993(5), g = 67.425(5)°, space group P1, Z = 1, m = 0.842 mm21
,
¯
35 951 data measurements, 11 153 data measurements with I > 3s(I), R =
0.105, Rw = 0.137. Anions (BF4 and triflate) are disordered in the crystal.
See http://www.rsc.org/suppdata/cc/b4/b402854f/ for crystallographic data
in CIF or other electronic format.
1 (a) J.-M. Lehn, Supramolecular Chemistry – Concepts and Per-
spectives, VCH, Weinheim, 1995, ch. 9; (b) D. Philp and J. F. Stoddart,
Angew. Chem., Int. Ed. Engl., 1996, 35, 1154.
2 D. C. Sherrington and K. A. Taskinen, Chem. Soc. Rev., 2001, 30, 83;
M. J. Krische and J.-M. Lehn, Struct. Bonding, 2000, 96, 3.
3 J.-M. Lehn, Chem. Eur. J., 2000, 6, 2097.
4 C. Piguet, G. Bernardinelli and G. Hopfgartner, Chem. Rev., 1997, 97,
2005.
5 G. Hanan, C. Arana, J.-M. Lehn, G. Baum and D. Fenske, Chem. Eur.
J., 1996, 2, 1292.
6 (a) P. N. W. Baxter, in Comprehensive Supramolecular Chemistry, Vol.
9, ed. J. L. Atwood, J. E. D. Davies, D. D. MacNicol, F. Vögtle, J.-M.
Lehn, Pergamon, Oxford, 1996, p. 165; (b) M. Fujita, Chem. Soc. Rev.,
1998, 27, 417.
The complex cation [Zn2Cu2L4]2+ is the first [2 3 2] grid self-
assembled from two cations of different coordination geometries.
The generation of a well-defined species results from the proper
reading of the electronic information (namely the coordination
number and the hard/soft nature) of the binding sites by the cation
effectors. The high coordination number hard zinc(II) cation
recognizes the anionic tridentate unit while soft tetracoordinate
7 (a) M.-T. Youinou, N. Rahmouni, J. Fischer and J. A. Osborn, Angew.
Chem., Int. Ed. Engl., 1992, 31, 733; (b) P. N. W. Baxter, J.-M. Lehn,
J. Fischer and M.-T. Youinou, Angew. Chem., Int. Ed. Engl., 1994, 33,
2284; (c) G. Hanan, D. Volkmer, U. S. Schubert, J.-M. Lehn, G. Baum
and D. Fenske, Angew. Chem., Int. Ed. Engl., 1997, 36, 1842; (d) J.
Rojo, F. J. Romero-Salguero, J.-M. Lehn, G. Baum and D. Fenske, Eur.
J. Inorg. Chem., 1999, 1421; (e) M. Ruben, E. Breuning, M. Barboiu, J.-
P. Gisselbrecht and J.-M. Lehn, Chem. Eur. J., 2003, 9, 291; (f) M.
Ruben, E. Breuning, J.-M. Lehn, V. Ksenofontov, F. Renz, P. Guetlich
and G. B. M. Vaughan, Chem. Eur. J., 2003, 9, 4422.
copper( ) prefers to bind to the bipyridine type portion. The
I
8 (a) D. M. Bassani, J.-M. Lehn, K. Fromm and D. Fenske, Angew. Chem.,
Int. Ed., 1998, 37, 2364; (b) L. Uppadine and J.-M. Lehn, Angew.
Chem., Int. Ed., 2004, 43, 240.
9 J.-M. Lehn, Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 4763.
10 J. R. Nitschke and J.-M. Lehn, Proc. Natl. Acad. Sci. U. S. A., 2003, 100,
11 970.
11 V. Smith and J.-M. Lehn, Chem. Commun., 1996, 2733; J.-C. G. Bünzli
and C. Piguet, Chem. Rev., 2002, 102, 1897.
12 The synthesis and properties of ligand LH and of its complexes will be
described in detail in a forthcoming paper: A. Petitjean, N. Kyritsakas
and J.-M. Lehn, manuscript in preparation. All compounds had
properties in agreement with their structure.
13 For grid arrays organized in two dimensions by hydrogen bonding
recognition groups see: E. Breuning, U. Ziener, J.-M. Lehn, E. Wegelius
and K. Rissanen, Eur. J. Inorg. Chem., 2001, 1515.
Fig. 2 Molecular structure of [Zn2Cu2L4]2+: (a) side view; (b) top view. The
n-butyl chains are represented in light grey; solvent molecules and triflate
anions are omitted for clarity. Stick and ball representation were used for the
ligands and cations respectively.
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