Self-organization in coordination-driven self-assembly

Abstract

Self-assembly allows for the preparation of highly complex molecular and supramolecular systems from relatively simple starting materials. Typically, self-assembled supramolecules are constructed by combining complementary pairs of two highly symmetric molecular components, thus limiting the chances of forming unwanted side products. Combining asymmetric molecular components or multiple complementary sets of molecules in one complex mixture can produce myriad different ordered and disordered supramolecular assemblies. Alternatively, spontaneous self-organization phenomena can promote the formation of specific product(s) out of a collection of multiple possibilities. Self-organization processes are common throughout much of nature and are especially common in biological systems. Recently, researchers have studied self-organized self-assembly in purely synthetic systems. This Account describes our investigations of self-organization in the coordination-driven self-assembly of platinum(II)-based metallosupramolecules. The modularity of the coordination-driven approach to self-assembly has allowed us to systematically study a wide variety of different factors that can control the extent of supramolecular self-organization. In particular, we have evaluated the effects of the symmetry and polarity of ambidentate donor subunits, differences in geometrical parameters (e.g., the size, angularity, and dimensionality) of Pt(II)-based acceptors and organic donors, the influence of temperature and solvent, and the effects of intermolecular steric interactions and hydrophobic interactions on self-organization. Our studies have shown that the extent of self-organization in the coordination-driven self-assembly of both 2D polygons and 3D polyhedra ranges from no organization (a statistical mixture of multiple products) to amplified organization (wherein a particular product or products are favored over others) and all the way to the absolute self-organization of discrete supramolecular assemblies. In many cases, inputs such as dipolar interactions, steric interactions, and differences in the geometric parameters of subunits, used either alone or as multiple factors simultaneously, can achieve absolute self-organization of discrete supramolecules. We have also observed instances where self-organization is not absolute and varies in its deviation from statistical results. Steric interactions are particularly useful control factors for driving such amplified self-organization because they can be subtly tuned through small structural variations. Having the ability to fully understand and control the self-organization of complex mixtures into specific synthetic supramolecules can provide a better understanding of analogous processes in biological systems. Furthermore, self-organization may allow for the facile synthesis of complex multifunctional, multicomponent systems from simply mixing a collection of much simpler, judiciously designed individual molecular components.

Scheme 1

Schematic representation of the differing extents of self-organization phenomena that can occur within a complex mixture of subunits: statistical (no organization); amplified (partial organization); absolute (exclusive organization)

Scheme 2

Chemical structures of ambidentate pyridyl/carboxylate donors and their self-assembly into 2D supramolecular structures.

Scheme 3

Geometric self-organization wherein a mixture of three Pt(II)-based acceptors (3–4, 8) and linear bis-pyridyl donor 9 self-organize exclusively into supramolecular structures 10–12 with no mixed acceptor assemblies.

Scheme 4

The absolute self-organization of 3D supramolecular trigonal prism structures 16–19 as driven by differences in the size and geometry of individual subunits.

Scheme 5

Demonstration of the use of different sized molecular components as a means to drive self-organization of 2D and 3D metallosupramolecules.

Scheme 6

Simultaneous use of three different geometric parameters (angle, size, and dimensionality) to drive the self-organization of multiple discrete 2D and 3D supramolecules from within complex mixtures.

Scheme 7

Chemical structures of unsymmetrical bispyridyl donors 25–27 and schematic representation of the four different supramolecular squares (29a–d) that can be formed upon their self-assembly with Pt(II) acceptor 28.

Scheme 8

The amplified self-organization of functionalized donors into hydrophobic, amphiphilic, and hydrophilic rectangles.

Scheme 9

Representative example of the work of Nitschke and coworkers wherein metal-ligand coordination is used to collapse a dynamic library of interconverting imines into self-organized supramolecular complexes.