5.4 Post-Self-Assembly Functionalization
Although functionalization can be introduced using the transformations described
above, more traditional methods can also be used, wherein an intact SCC
participates in an established organic transformation by virtue of selecting
precursors with reactive functional groups. Using these precursors in a multicomponent assembly provides a way to functionalize structures that would be difficult
to achieve by other means. Carboxylate donors built on phenyl backbones can often
include a second functionality attached. These ditopic donors would be limited to
2D metallacycles for homoligated SCCs; however, the development of multicomponent assembly permits their use to form functionalized prisms. In this way, Stang
and coworkers developed a suite of tetragonal prisms with a range of pendant
functionalities, including amines, ferrocenes, alkoxides, etc [90]. Although the
caveat exists that these functionalities must not interfere with the metal–ligand
coordination that is at the heart of the self-assembly process, in practice even
moderately Lewis-basic moieties may be used.
When amine- or maleimide-functionalized carboxylate donors are used, the
resulting SCCs are well suited to interact with isocyanates and maleic anhydride
(in the case of the amine variant) or undergo Diels–Alder reactions (for the
maleimide variant), providing a means to attach functionalities directly to the
edges of a prismatic SCC. When redox-active groups containing ferrocene are
used, the resulting functionalized prism is amenable to electrochemical characterization to confirm that a quantitative coupling takes place without affecting the core
of the SCC [91].
This technique does not require multicomponent assembly, and can be
demonstrated on simpler platforms as a proof-of-concept. One example is postself-assembly click chemistry [92]. By designing a parent cyclooctynefunctionalized metallacycle, Stang and coworkers demonstrated self-assembly
functionalization with a variety of azide-bearing small molecules, from simple
benzylazide to a significantly more complex biotin azide substrate. In all cases,
[3+2] Huisgen cycloadditions took place under mild conditions (Fig. 10) [93].
6 Summary, Conclusion, and Outlook
The development of the directional bonding approach to coordination-driven selfassembly has given rise to a vast molecular library of building blocks and supramolecular coordination complexes. From the design principles first established
using simple self-assembly reactions to obtain square metallacycles, increasing
complex polygons and polyhedra have been constructed, demanding creative new
approaches to encode angularity and deconstruct target geometries, but without
requiring significant synthetic redesign. Whereas natural systems deftly manipulate
ensemble of weak non-directional interactions, chemists can mimic this approach
using metal–ligand bonding, which simplifies the process by allowing a higher
degree of control.
Coordination-Driven Supramolecular Macromolecules via the Directional. . .
245
Although functionalization can be introduced using the transformations described
above, more traditional methods can also be used, wherein an intact SCC
participates in an established organic transformation by virtue of selecting
precursors with reactive functional groups. Using these precursors in a multicomponent assembly provides a way to functionalize structures that would be difficult
to achieve by other means. Carboxylate donors built on phenyl backbones can often
include a second functionality attached. These ditopic donors would be limited to
2D metallacycles for homoligated SCCs; however, the development of multicomponent assembly permits their use to form functionalized prisms. In this way, Stang
and coworkers developed a suite of tetragonal prisms with a range of pendant
functionalities, including amines, ferrocenes, alkoxides, etc [90]. Although the
caveat exists that these functionalities must not interfere with the metal–ligand
coordination that is at the heart of the self-assembly process, in practice even
moderately Lewis-basic moieties may be used.
When amine- or maleimide-functionalized carboxylate donors are used, the
resulting SCCs are well suited to interact with isocyanates and maleic anhydride
(in the case of the amine variant) or undergo Diels–Alder reactions (for the
maleimide variant), providing a means to attach functionalities directly to the
edges of a prismatic SCC. When redox-active groups containing ferrocene are
used, the resulting functionalized prism is amenable to electrochemical characterization to confirm that a quantitative coupling takes place without affecting the core
of the SCC [91].
This technique does not require multicomponent assembly, and can be
demonstrated on simpler platforms as a proof-of-concept. One example is postself-assembly click chemistry [92]. By designing a parent cyclooctynefunctionalized metallacycle, Stang and coworkers demonstrated self-assembly
functionalization with a variety of azide-bearing small molecules, from simple
benzylazide to a significantly more complex biotin azide substrate. In all cases,
[3+2] Huisgen cycloadditions took place under mild conditions (Fig. 10) [93].
6 Summary, Conclusion, and Outlook
The development of the directional bonding approach to coordination-driven selfassembly has given rise to a vast molecular library of building blocks and supramolecular coordination complexes. From the design principles first established
using simple self-assembly reactions to obtain square metallacycles, increasing
complex polygons and polyhedra have been constructed, demanding creative new
approaches to encode angularity and deconstruct target geometries, but without
requiring significant synthetic redesign. Whereas natural systems deftly manipulate
ensemble of weak non-directional interactions, chemists can mimic this approach
using metal–ligand bonding, which simplifies the process by allowing a higher
degree of control.
Coordination-Driven Supramolecular Macromolecules via the Directional. . .
245