TetrazineBox: A Structurally Transformative Toolbox

Synthetic
macrocycles capable of undergoing allosteric regulation
by responding to versatile external stimuli are the subject of increasing
attention in supramolecular science. Herein, we report a structurally
transformative tetracationic cyclophane containing two 3,6-bis­(4-pyridyl)-l,2,4,5-tetrazine
(4-bptz) units, which are linked together by two p-xylylene bridges. The cyclophane, which possesses modular redox
states and structural post-modifications, can undergo two reversibly
consecutive two-electron reductions, affording first its bisradical
dicationic counterpart, and then subsequently the fully reduced species.
Furthermore, one single-parent cyclophane can afford effectively three
other new analogs through box-to-box cascade transformations, taking
advantage of either reductions or an inverse electron-demand Diels–Alder
(IEDDA) reaction. While all four new tetracationic cyclophanes adopt
rigid and symmetric box-like conformations, their geometries in relation
to size, shape, electronic properties, and binding affinities toward
polycyclic aromatic hydrocarbons can be readily regulated. This structurally
transformative tetracationic cyclophane performs a variety of new
tasks as a result of structural post-modifications, thus serving as
a toolbox for probing the radical properties and generating rapidly
a range of structurally diverse cyclophanes by efficient divergent
syntheses. This research lays a solid foundation for the introduction
of the structurally transformative tetracationic cyclophane into the
realm of mechanically interlocked molecules and will provide a toolbox
to construct and operate intelligent molecular machines.