The wide availability of planar, polytopic ligands makes finding suitable “face”
components a facile task. These panel-like tectons need then be linked by building
blocks that will occupy the remaining edges of a given prism. For two-component
assembly, the archetypal design strategy is to use so-called “molecular clip”
precursors comprising two metal centers, each with a single substitutionally labile
site held parallel to one another. This provides a 0
directionality of the coordination
vectors, with a displacement that will determine the ultimate length of the prism.
An example of such a design uses tripyridyl 120
ligands, thus enforcing a
trigonal prismatic structure. These tritopic ligands may be fused with diplatinum
molecular clips supported by anthracene backbones [55]. The resulting [3+2]
assembly establishes a general [n+2] self-assembly process, where n is the number
of sides of the polygon found at the ends of the prism (i.e., n ¼ 3, trigonal prism;
n ¼ 4, tetragonal prism, etc.). The dimensions of a prism can be tuned in all
directions either by changing the size of the molecular clip, thus adjusting the
length of a prism, or by extending or contracting the extent to which the binding
sites of the polygonal panel building block point into space, thereby enhancing or
attenuating the width of the structure [56, 57]. Complementary prisms can also be
made by generating 0
clip-like donors, such as functionalizing an anthracene
backbone with two pyridyl groups instead of the organoplatinum moieties of the
prisms described above [58]. Due to the slight (~11
) splay associated with the
coordination vectors of the pyridyl groups, these clips are well suited to interact
with trimetallic acceptors with angularity between 108
and 112
. As such, the
resulting prisms are slightly puckered relative to their idealized geometric
analogues, which are rigidly planar at the faces.
When a tetratopic panel is used in place of the pioneering tritopic examples, the
self-assembly process requires but a simple change to the stoichiometry of clip
required. Once adjusted, the quantitative formation of tetragonal prisms via [4+2]
assembly can take place [59].
Of course, this process is by no means limited to platinum-based prisms. Areneruthenium molecular clips are more than suitable for self-assembly reactions and
are often compatible with the same polypyridyl ligands used with Pt-based molecular clips. As such, the combination of oxalate-bridged and other chelate-bridged
diruthenium clips with tritopic donors furnishes analogous trigonal prismatic SCCs
[60]. Likewise, alkoxide-bridged rhenium centers can serve as molecular clips to
deliver hexanuclear trigonal prisms under solvothermal conditions [61]. The generation of alkoxide-bridged clips during self-assembly has also been used to form
tetragonal prisms [62]. For Re-based prisms, some systems offer two routes to
formation: either the molecular clip precursor can be generated in an independent
step and then used for self-assembly [63], or it can be formed in a single-pot
reaction, whereby 11 unique components come together to form a given trigonal
prism [64]
A second prismatic structure achievable by coordination-driven self-assembly
are the so-called “open boxes.” Rather than positioning a polytopic ligand at the
ends of the prism, this design uses square-like panels to complete the sides of the
construct, forming, for instance, trigonal prisms [65] or hexagonal prisms [66].
Coordination-Driven Supramolecular Macromolecules via the Directional. . .
239
components a facile task. These panel-like tectons need then be linked by building
blocks that will occupy the remaining edges of a given prism. For two-component
assembly, the archetypal design strategy is to use so-called “molecular clip”
precursors comprising two metal centers, each with a single substitutionally labile
site held parallel to one another. This provides a 0
directionality of the coordination
vectors, with a displacement that will determine the ultimate length of the prism.
An example of such a design uses tripyridyl 120
ligands, thus enforcing a
trigonal prismatic structure. These tritopic ligands may be fused with diplatinum
molecular clips supported by anthracene backbones [55]. The resulting [3+2]
assembly establishes a general [n+2] self-assembly process, where n is the number
of sides of the polygon found at the ends of the prism (i.e., n ¼ 3, trigonal prism;
n ¼ 4, tetragonal prism, etc.). The dimensions of a prism can be tuned in all
directions either by changing the size of the molecular clip, thus adjusting the
length of a prism, or by extending or contracting the extent to which the binding
sites of the polygonal panel building block point into space, thereby enhancing or
attenuating the width of the structure [56, 57]. Complementary prisms can also be
made by generating 0
clip-like donors, such as functionalizing an anthracene
backbone with two pyridyl groups instead of the organoplatinum moieties of the
prisms described above [58]. Due to the slight (~11
) splay associated with the
coordination vectors of the pyridyl groups, these clips are well suited to interact
with trimetallic acceptors with angularity between 108
and 112
. As such, the
resulting prisms are slightly puckered relative to their idealized geometric
analogues, which are rigidly planar at the faces.
When a tetratopic panel is used in place of the pioneering tritopic examples, the
self-assembly process requires but a simple change to the stoichiometry of clip
required. Once adjusted, the quantitative formation of tetragonal prisms via [4+2]
assembly can take place [59].
Of course, this process is by no means limited to platinum-based prisms. Areneruthenium molecular clips are more than suitable for self-assembly reactions and
are often compatible with the same polypyridyl ligands used with Pt-based molecular clips. As such, the combination of oxalate-bridged and other chelate-bridged
diruthenium clips with tritopic donors furnishes analogous trigonal prismatic SCCs
[60]. Likewise, alkoxide-bridged rhenium centers can serve as molecular clips to
deliver hexanuclear trigonal prisms under solvothermal conditions [61]. The generation of alkoxide-bridged clips during self-assembly has also been used to form
tetragonal prisms [62]. For Re-based prisms, some systems offer two routes to
formation: either the molecular clip precursor can be generated in an independent
step and then used for self-assembly [63], or it can be formed in a single-pot
reaction, whereby 11 unique components come together to form a given trigonal
prism [64]
A second prismatic structure achievable by coordination-driven self-assembly
are the so-called “open boxes.” Rather than positioning a polytopic ligand at the
ends of the prism, this design uses square-like panels to complete the sides of the
construct, forming, for instance, trigonal prisms [65] or hexagonal prisms [66].
Coordination-Driven Supramolecular Macromolecules via the Directional. . .
239