3.5
Mechanically Interlocked Molecules Containing CBPQT
4+
Ring
Mechanically interlocked molecules (MIMs) [14a] have been considered as a type of
nonclassic molecules. On the one hand, different from those normal or “classic”
molecules whose atoms are all connected covalently with the molecular moiety,
MIMs contain multiple components, between which covalent bonds are absent.
When some noncovalent interactions occur between the interlocked molecular
components, these MIMs might exist in some specific co-conformations. The preference of these co-conformations could be switched by using some external stimuli
to tune these intercomponent intramolecular noncovalent interactions including
either weakening the primary one or strengthening the secondary one, which results
in mechanical movement of these molecular components with respect to each other.
This behavior affords MIMs the ability to develop smart materials whose physical
properties could be reversibly controlled. On the other hand, these molecular
components are mechanically interlocked with each other, in reminiscence of the
many interlocked rings in a necklace or metal chain. Without destroying at least one
covalent bond, the architecture of a MIM would remain intact. This feature distinguishes MIMs from those supramolecular complexes, whose molecular components
can undergo reversible association/dissociation.
The often studied MIMs include rotaxanes and catenanes, both of which contain a
macrocyclic component that encircles either a dumbbell-shaped or another ring
component, respectively. When a linear molecule is encircled by a ring, this system
is called a pseudorotaxane, which is a type of supramolecular complexes. Pseudorotaxanes are often used as the precursors in the synthesis of rotaxanes and
catenanes, when the terminal groups of the former undergo reactions with larger
and bulky molecules, or each other, respectively. We are going to discuss it in more
detail in the coming section.
3.5.1 Catenanes Containing CBPQT
4+ Ring
The often used approach to obtain catenanes is the template-directed synthesis.
Some noncovalent interactions are employed to drive a macrocycle to encircle a
thread-shaped molecule, forming a so-called pseudorotaxane. The supramolecular
driving forces are of importance, given that they lead to enthalpy release to compensate the entropy loss during the association of the supramolecular complex.
When the two groups undergo reaction with each other or another molecule
containing two reacting units simultaneously, which has been called “clipping,” a
catenane is generated.
The formation of the donor-acceptor catenanes containing CBPQT
4+ ring relies
on the supramolecular interactions between a π-electron-rich ring and a CBPQT
4+
ring. The driving forces include donor-acceptor interactions, hydrogen bonding,
C–H•••π, as well as hydrophobic effect in aqueous solutions, as we mentioned
before. The π-electron-rich ring thus often contains π-electron-rich unit such as
3 Host-Guest Chemistry of a Tetracationic Cyclophane, Namely, Cyclobis. . .
61
Mechanically Interlocked Molecules Containing CBPQT
4+
Ring
Mechanically interlocked molecules (MIMs) [14a] have been considered as a type of
nonclassic molecules. On the one hand, different from those normal or “classic”
molecules whose atoms are all connected covalently with the molecular moiety,
MIMs contain multiple components, between which covalent bonds are absent.
When some noncovalent interactions occur between the interlocked molecular
components, these MIMs might exist in some specific co-conformations. The preference of these co-conformations could be switched by using some external stimuli
to tune these intercomponent intramolecular noncovalent interactions including
either weakening the primary one or strengthening the secondary one, which results
in mechanical movement of these molecular components with respect to each other.
This behavior affords MIMs the ability to develop smart materials whose physical
properties could be reversibly controlled. On the other hand, these molecular
components are mechanically interlocked with each other, in reminiscence of the
many interlocked rings in a necklace or metal chain. Without destroying at least one
covalent bond, the architecture of a MIM would remain intact. This feature distinguishes MIMs from those supramolecular complexes, whose molecular components
can undergo reversible association/dissociation.
The often studied MIMs include rotaxanes and catenanes, both of which contain a
macrocyclic component that encircles either a dumbbell-shaped or another ring
component, respectively. When a linear molecule is encircled by a ring, this system
is called a pseudorotaxane, which is a type of supramolecular complexes. Pseudorotaxanes are often used as the precursors in the synthesis of rotaxanes and
catenanes, when the terminal groups of the former undergo reactions with larger
and bulky molecules, or each other, respectively. We are going to discuss it in more
detail in the coming section.
3.5.1 Catenanes Containing CBPQT
4+ Ring
The often used approach to obtain catenanes is the template-directed synthesis.
Some noncovalent interactions are employed to drive a macrocycle to encircle a
thread-shaped molecule, forming a so-called pseudorotaxane. The supramolecular
driving forces are of importance, given that they lead to enthalpy release to compensate the entropy loss during the association of the supramolecular complex.
When the two groups undergo reaction with each other or another molecule
containing two reacting units simultaneously, which has been called “clipping,” a
catenane is generated.
The formation of the donor-acceptor catenanes containing CBPQT
4+ ring relies
on the supramolecular interactions between a π-electron-rich ring and a CBPQT
4+
ring. The driving forces include donor-acceptor interactions, hydrogen bonding,
C–H•••π, as well as hydrophobic effect in aqueous solutions, as we mentioned
before. The π-electron-rich ring thus often contains π-electron-rich unit such as
3 Host-Guest Chemistry of a Tetracationic Cyclophane, Namely, Cyclobis. . .
61