5.1 Platinum-Pyridyl-Carboxylate Multicomponent
Assembly
Although the use of multiple pyridyl-based ligands demands a creative use of steric
bulk, size selectivity, or other method to prevent a statistical mixture of coordination combinations, it is possible to select two different donors that differ enough
such that there is an inherent selectivity for heteroligation. This strategy has been
explored experimentally with the discovery that combinations of pyridyl and
carboxylate donors favor mixed Pt-O, N coordination motifs when placed in
solution with platinum acceptors [74–77].
Simple [4+2+2] assemblies are the multicomponent analogue of [4+4] square
reactions. Whereas the use of two different lengths of linear pyridyl ligands
would give a mixture of products, the linear carboxylate and pyridyl ligands
funnel exclusively to heteroligated coordination environments to deliver
rectangles as the sole reaction product [78]. This strategy was described qualitatively as based on “charge separation” by which the anionic carboxylate ligands
and neutral pyridyl ligands are paired up so as to reduce the amount of
electrostatic repulsion between like ligands with the same charge. That said,
the preference for heteroligation is probably due to a number of effects, with
varying contributions depending on the specific system being considered. These
factors include a potential alleviation of ring strain, orbital-related phenomena
such as cis influences (akin to the better known trans influence in which multiple
bonds to the same sigma-type d
2
x À y
2 orbital reduce subsequent bond enthalpy),
and electrostatic effects. Although the magnitude of these contributions and the
exploration of other factors remains an ongoing effort at the forefront of multicomponent assembly, in practice a number of different SCCs have already taken
advantage of this heteroligation motif. Similar factors may also play a role in
mixed pyridyl/imidazole systems, in which some preference for heteroligation
has also been observed [79].
Because multicomponent assembly provides a way to include multiple ligands in
a single SCC scaffold, it becomes possible to access increasingly more complex
structures by using functionalized ligands. For instance, a four-component assembly, in which ten molecular building blocks from four unique species fuse into a
single discrete species, is possible. In this example, the dipyridyl donor used is built
upon a bis(pyridinium)ethane core, which can act as a guest for crown ethers,
forming [3]catenane species [80].
5.2 Prismatic Metallacages
One structure type that can take advantage of multicomponent assembly is that
of the prismatic SCCs [81]. Rather than using a molecular clip to bridge
polygonal faces, the requisite 90
angles can be encoded using traditional ditopic
Coordination-Driven Supramolecular Macromolecules via the Directional. . .
241
Assembly
Although the use of multiple pyridyl-based ligands demands a creative use of steric
bulk, size selectivity, or other method to prevent a statistical mixture of coordination combinations, it is possible to select two different donors that differ enough
such that there is an inherent selectivity for heteroligation. This strategy has been
explored experimentally with the discovery that combinations of pyridyl and
carboxylate donors favor mixed Pt-O, N coordination motifs when placed in
solution with platinum acceptors [74–77].
Simple [4+2+2] assemblies are the multicomponent analogue of [4+4] square
reactions. Whereas the use of two different lengths of linear pyridyl ligands
would give a mixture of products, the linear carboxylate and pyridyl ligands
funnel exclusively to heteroligated coordination environments to deliver
rectangles as the sole reaction product [78]. This strategy was described qualitatively as based on “charge separation” by which the anionic carboxylate ligands
and neutral pyridyl ligands are paired up so as to reduce the amount of
electrostatic repulsion between like ligands with the same charge. That said,
the preference for heteroligation is probably due to a number of effects, with
varying contributions depending on the specific system being considered. These
factors include a potential alleviation of ring strain, orbital-related phenomena
such as cis influences (akin to the better known trans influence in which multiple
bonds to the same sigma-type d
2
x À y
2 orbital reduce subsequent bond enthalpy),
and electrostatic effects. Although the magnitude of these contributions and the
exploration of other factors remains an ongoing effort at the forefront of multicomponent assembly, in practice a number of different SCCs have already taken
advantage of this heteroligation motif. Similar factors may also play a role in
mixed pyridyl/imidazole systems, in which some preference for heteroligation
has also been observed [79].
Because multicomponent assembly provides a way to include multiple ligands in
a single SCC scaffold, it becomes possible to access increasingly more complex
structures by using functionalized ligands. For instance, a four-component assembly, in which ten molecular building blocks from four unique species fuse into a
single discrete species, is possible. In this example, the dipyridyl donor used is built
upon a bis(pyridinium)ethane core, which can act as a guest for crown ethers,
forming [3]catenane species [80].
5.2 Prismatic Metallacages
One structure type that can take advantage of multicomponent assembly is that
of the prismatic SCCs [81]. Rather than using a molecular clip to bridge
polygonal faces, the requisite 90
angles can be encoded using traditional ditopic
Coordination-Driven Supramolecular Macromolecules via the Directional. . .
241