5 Multicomponent Self-Assembly
The SCCs discussed above all use two components, a donor and an acceptor, in the
formation of identical metal–ligand bonds. The use of only two types of
components greatly simplifies the self-assembly process. However, given the
favorable energetics associated with forming regular, unstrained metallacycles,
the combination of carefully selected precursors exceeding only two components
can still result in well-defined products. For instance, the concept of selforganization and size-selective self-assembly has been explored by Stang and
coworkers, defining conditions by which desired structural outcomes can be
favored over statistical product mixtures [6].
It is also possible to develop self-assembly reactions whereby the combination of
more than two types of precursors does not select for singular, two-component
assemblies, but rather favors the sole formation of one type of discrete SCC
containing all three building blocks. Such reactions have been referred to as
“multicomponent assembly.” Although somewhat of a misnomer given that even
simple two-component assemblies technically involve multiple components, this
moniker is intended to reflect those mixtures with three or more different building
blocks.
In order for such multicomponent assemblies to occur with efficiency, there
must be a thermodynamic preference for heteroligated coordination environments
about the metal centers being used [67]. In some sense, the Re-based prismatic
SCCs that form molecular clips during the self-assembly process can be thought of
as multicomponent assemblies owning to the preference of the Re centers to acquire
one bridging ligand, often a chelator, along with a polypyridyl donor. This process
has been most systematically explored with platinum-based acceptors and pyridyl
and carboxylate donors.
A second method is to use spatial control, wherein homoleptic coordination is
hindered by the size and shape of the ligands used. In such cases, it is possible to
form discrete SCCs with multiple ligands at each metal node, as elegantly
demonstrated by Schmittel and coworkers using phenanthroline-based donors
[68–70]. If substituted pyridyl ligands are combined with unfunctionalized
analogues in mixture with acceptors containing cis-oriented substitutionally labile
sites, heteroligation will occur due to the steric constraints associated with
coordinating two of the bulkier pyridyl ligands to the same metal node, as
demonstrated by Fujita and coworkers [71, 72].
It should be noted that, in some cases, exploiting the kinetics of a system can
also afford control over multicomponent assembly. For instance, Lusby, Barran,
and coworkers demonstrated the formation of selected isomeric SCCs by varying
the sequence of addition of building blocks in a Pt-based system with
metal–ligand bonds inert enough to avoid rapid funneling to a single thermodynamic product [73]
240
T.R. Cook and P.J. Stang
The SCCs discussed above all use two components, a donor and an acceptor, in the
formation of identical metal–ligand bonds. The use of only two types of
components greatly simplifies the self-assembly process. However, given the
favorable energetics associated with forming regular, unstrained metallacycles,
the combination of carefully selected precursors exceeding only two components
can still result in well-defined products. For instance, the concept of selforganization and size-selective self-assembly has been explored by Stang and
coworkers, defining conditions by which desired structural outcomes can be
favored over statistical product mixtures [6].
It is also possible to develop self-assembly reactions whereby the combination of
more than two types of precursors does not select for singular, two-component
assemblies, but rather favors the sole formation of one type of discrete SCC
containing all three building blocks. Such reactions have been referred to as
“multicomponent assembly.” Although somewhat of a misnomer given that even
simple two-component assemblies technically involve multiple components, this
moniker is intended to reflect those mixtures with three or more different building
blocks.
In order for such multicomponent assemblies to occur with efficiency, there
must be a thermodynamic preference for heteroligated coordination environments
about the metal centers being used [67]. In some sense, the Re-based prismatic
SCCs that form molecular clips during the self-assembly process can be thought of
as multicomponent assemblies owning to the preference of the Re centers to acquire
one bridging ligand, often a chelator, along with a polypyridyl donor. This process
has been most systematically explored with platinum-based acceptors and pyridyl
and carboxylate donors.
A second method is to use spatial control, wherein homoleptic coordination is
hindered by the size and shape of the ligands used. In such cases, it is possible to
form discrete SCCs with multiple ligands at each metal node, as elegantly
demonstrated by Schmittel and coworkers using phenanthroline-based donors
[68–70]. If substituted pyridyl ligands are combined with unfunctionalized
analogues in mixture with acceptors containing cis-oriented substitutionally labile
sites, heteroligation will occur due to the steric constraints associated with
coordinating two of the bulkier pyridyl ligands to the same metal node, as
demonstrated by Fujita and coworkers [71, 72].
It should be noted that, in some cases, exploiting the kinetics of a system can
also afford control over multicomponent assembly. For instance, Lusby, Barran,
and coworkers demonstrated the formation of selected isomeric SCCs by varying
the sequence of addition of building blocks in a Pt-based system with
metal–ligand bonds inert enough to avoid rapid funneling to a single thermodynamic product [73]
240
T.R. Cook and P.J. Stang