Dynamic covalent
chemistry (DCvC) is a powerful means
by which
to rapidly prepare complex structures from simple molecular building
blocks. Effective DCvC behavior is contingent upon the reversibility
of covalent bond formation. Stabilized radical species, therefore,
have been effectively used for these applications. In earlier work
we demonstrated that properly substituted 1-arylurazolyl radicals
showed promise as oxygen-insensitive heterocyclic N-centered radicals
with a propensity for reversible bond formation. In this work we have
synthesized several tethered bis(urazolyl) diradicals, varying by
the type and length of connectivity between the urazole rings, and
tested them for DCvC behavior. We have found that when the two aryl
rings to which the urazolyl radical sites are attached are tethered
by a chain of five or more carbons, equilibrium mixtures of monomeric
and dimeric species are formed by N–N bond formation between
two radical sites. DCvC behavior is observed that is sensitive to
changes in temperature, concentration, and (to a lesser extent) solvent.
In general, the dimer species is favored at lower temperatures and
higher concentrations.