The spontaneous but controlled generation of functional architectures by self-assembly has emerged as a major development in the field of supramolecular chemistry, aiming at the design of self-organising systems of increasing complexity [1,2]. Of special interest among the great variety of possible superstructures are those containing metal ions arranged in a grid-type fashion. As multisite species, they present intriguing features that make them potential components of nanoscale information storage devices.

It is crucial to obtain an atomic level crystal structure in order to understand the stability criteria and self-assembly mechanisms of such compounds. However, high molecular weight species tend to produce poor and unstable van-der-Waals crystals not amenable to structure solution and refinement by laboratory methods. It is only possible to obtain tractable data on such crystals using a high-intensity synchrotron X-ray source.


Fig. 34: Space-filling representation of the crystal structure of the [4x4]PbII16 grid complex.


Among the most complicated of the systems studied so far are the [4x4]PbII16 grid (Figure 34) and [4#4]PbII12 double-cross (Figure 35) architectures. These systems contain arrays of lead ions in a well-defined arrangement, which is accessible in a single operational step via self-assembly of respectively 24 and 16 components. This self-organisation is driven by the formation of respectively 96 and 48 coordinative bonds between the tetratopic terpyridine ligand and the lead(II) ions.


Fig. 35: Space-filling representation of the crystal structure of the [4#4]PbII12 double-cross complex.


X-ray diffraction measurements were carried out at beamline ID11 using a wavelength of 0.32826 Å. The best crystals which could be produced were very poor (> 2º mosaic!), small (some tens of µm3) and unstable in air (lifetimes outside the mother liquor <1 hour). Structure solution and refinement required accumulation of data from several crystals and extensive data treatment.

The [4x4]PbII16 grid and the [4#4]PbII12 double-cross result from the appropriate design of the ligand which is programmed for efficient self-assembly under the principle of "maximum coordination site occupation". The coordination array is further stabilized by - interactions, which stack subsets, and by internal bridge-type coordination of the lead cations by the triflate counterions, which decrease the total charge of the complex cation and thus the coulombic repulsion between the PbII cations.

The overall total volumes of the [4x4]PbII16 grid and of the [4#4]PbII12 double-cross are ~ 8.6 nm3 and 6.9 nm3 respectively, placing these entities within the nanostructural domain.

In terms of programmed self-assembly, the formation of the different metallo-architectures underlines the fact that, despite the role of the coordination interactions and of maximal site occupation, other factors, such as stacking, preferential pyridine coordination, and binding of anions and solvent molecules may interfere and influence the nature of the favoured output species. Atomic level crystal structures provide crucial information for understanding such self-assembly processes.

[1] J.-M. Lehn, Supramolecular Chemistry-Concepts and Perspectives, VCH, Weinheim, chap 9 (1995).
[2] M. Barboiu, G.B.M. Vaughan, N. Kyritsakas, J.-M. Lehn, Chem. Eur J., 9, 763 (2003).

Principal Publication and Authors:
M. Barboiu (a,d), G.B.M. Vaughan (b), R. Graff (c),
J.-M Lehn (d), J. Am. Chem. Soc. 125, 10257 (2003).
(a) Institut Européen des Membranes, Montpellier (France)
(b) ESRF
(c) Service Commun de RMN, Strasbourg (France)
(d) ISIS, Laboratoire de Chimie Supramoléculaire, Strasbourg (France)