5.3 Supramolecule-to-Supramolecule Transformation
Another extension of multicomponent assembly is supramolecule-tosupramolecule transformation. Such transformations can be triggered by external
means, such as light, solvent, or chemical signals [84–89]. For Pt-based SCCs, this
process exploits the same preference for heteroligation as is used in multicomponent assembly. In short, when a homoligated Pt-pyridyl SCC is mixed with a
Pt-carboxylate SCC, both initial structures become kinetic intermediates in the
context of forming a new, heteroligated structure. As such, upon their combination
in solution, the reversibility of Pt-ligand coordination sunders both original species
and the system ultimately arrives at a new multicomponent assembly [78]. This is
most simply illustrated by mixing three [4+4] Pt-pyridyl squares with four [3+3]
Pt-carboxylate triangles. Since this delivers the Pt acceptor in a 4:2:2 ratio with the
linear pyridyl and linear carboxylate donors, the stoichiometry is set to afford a
[4+2+2] rectangle quantitatively (Fig. 9).
This transformation process works equally well to deliver multicomponent
prismatic SCCs. Both initial SCCs must contain 90
acceptors and a single type
of donor. Since the final prisms typically contain a polypyridyl donor at each end,
the first initial SCC must be composed of exclusively these two building blocks. It
is possible to form discrete SCCs using both tritopic and tetratopic planar donors
with 90
acceptors. When a tripyridyl donor is mixed with a 90
acceptor in a 6:4
ratio, a truncated tetrahedron is obtained. This establishes the formation of a
trigonal prism as the result of the supramolecule-to-supramolecule transformation
in the form of a [6+3+2] self-assembly. This means that for every truncated
tetrahedron, the potential for two trigonal prisms exists relative to the number of
pyridyl donors. However, there is a shortage of platinum acceptor, which must be
corrected by the addition of a Pt-carboxylate triangle. Serendipitously, the combination of a [6+4] truncated tetrahedron and two [3+3] triangles gives a total of two
pyridyl donors, 12 platinum acceptors, and six carboxylate donors, which is the
exact ratio needed to form two trigonal prisms.
Likewise, the self-assembly between certain tetragonal donors and 90
acceptors
results in the formation of an open box trigonal prism, where three donors form the
square faces along the width of the prism, joined at each vertex by a sum total of six
platinum acceptors. This trigonal prism can be transformed into its multicomponent
counterpart upon the addition of the same carboxylate triangle. Since only three
pyridyl donors are found in each open box prism, each pair of prisms affords
enough donors for three transformed SCCs. This demands four equivalents of
triangle in order to achieve the necessary [8+4+2] prism stoichiometry.
A second type of transformation does not involve the mixture of two discrete
homoligated SCCs to form a third heteroligated multicomponent structure, but
rather takes a single homoligated SCC and transforms it upon the addition of an
exogenous small molecule. One strategy to achieve this, developed by Stang and
coworkers, is to adjust the angularity of a precursor after it has already been used to
Coordination-Driven Supramolecular Macromolecules via the Directional. . .
243
Another extension of multicomponent assembly is supramolecule-tosupramolecule transformation. Such transformations can be triggered by external
means, such as light, solvent, or chemical signals [84–89]. For Pt-based SCCs, this
process exploits the same preference for heteroligation as is used in multicomponent assembly. In short, when a homoligated Pt-pyridyl SCC is mixed with a
Pt-carboxylate SCC, both initial structures become kinetic intermediates in the
context of forming a new, heteroligated structure. As such, upon their combination
in solution, the reversibility of Pt-ligand coordination sunders both original species
and the system ultimately arrives at a new multicomponent assembly [78]. This is
most simply illustrated by mixing three [4+4] Pt-pyridyl squares with four [3+3]
Pt-carboxylate triangles. Since this delivers the Pt acceptor in a 4:2:2 ratio with the
linear pyridyl and linear carboxylate donors, the stoichiometry is set to afford a
[4+2+2] rectangle quantitatively (Fig. 9).
This transformation process works equally well to deliver multicomponent
prismatic SCCs. Both initial SCCs must contain 90
acceptors and a single type
of donor. Since the final prisms typically contain a polypyridyl donor at each end,
the first initial SCC must be composed of exclusively these two building blocks. It
is possible to form discrete SCCs using both tritopic and tetratopic planar donors
with 90
acceptors. When a tripyridyl donor is mixed with a 90
acceptor in a 6:4
ratio, a truncated tetrahedron is obtained. This establishes the formation of a
trigonal prism as the result of the supramolecule-to-supramolecule transformation
in the form of a [6+3+2] self-assembly. This means that for every truncated
tetrahedron, the potential for two trigonal prisms exists relative to the number of
pyridyl donors. However, there is a shortage of platinum acceptor, which must be
corrected by the addition of a Pt-carboxylate triangle. Serendipitously, the combination of a [6+4] truncated tetrahedron and two [3+3] triangles gives a total of two
pyridyl donors, 12 platinum acceptors, and six carboxylate donors, which is the
exact ratio needed to form two trigonal prisms.
Likewise, the self-assembly between certain tetragonal donors and 90
acceptors
results in the formation of an open box trigonal prism, where three donors form the
square faces along the width of the prism, joined at each vertex by a sum total of six
platinum acceptors. This trigonal prism can be transformed into its multicomponent
counterpart upon the addition of the same carboxylate triangle. Since only three
pyridyl donors are found in each open box prism, each pair of prisms affords
enough donors for three transformed SCCs. This demands four equivalents of
triangle in order to achieve the necessary [8+4+2] prism stoichiometry.
A second type of transformation does not involve the mixture of two discrete
homoligated SCCs to form a third heteroligated multicomponent structure, but
rather takes a single homoligated SCC and transforms it upon the addition of an
exogenous small molecule. One strategy to achieve this, developed by Stang and
coworkers, is to adjust the angularity of a precursor after it has already been used to
Coordination-Driven Supramolecular Macromolecules via the Directional. . .
243