Mechanical Bonds and Topological Effects in Radical Dimer Stabilization

While mechanical bonding stabilizes
tetrathiafulvalene (TTF) radical
dimers, the question arises: what role does topology play in catenanes
containing TTF units? Here, we report how topology, together with
mechanical bonding, in isomeric [3]- and doubly interlocked [2]­catenanes
controls the formation of TTF radical dimers within their structural
frameworks, including a ring-in-ring complex (formed between an organoplatinum
square and a {2+2} macrocyclic polyether containing two 1,5-dioxynaphthalene
(DNP) and two TTF units) that is topologically isomeric with the doubly
interlocked [2]­catenane. The separate TTF units in the two {1+1} macrocycles
(each containing also one DNP unit) of the isomeric [3]­catenane exhibit
slightly different redox properties compared with those in the {2+2}
macrocycle present in the [2]­catenane, while comparison with its topological
isomer reveals substantially different redox behavior. Although the
stabilities of the mixed-valence (TTF2)•+ dimers are similar in the two catenanes, the radical cationic (TTF•+)2 dimer in the [2]­catenane occurs only
fleetingly compared with its prominent existence in the [3]­catenane,
while both dimers are absent altogether in the ring-in-ring complex.
The electrochemical behavior of these three radically configurable
isomers demonstrates that a fundamental relationship exists between
topology and redox properties.