Mechanical-Bond-Protected, Air-Stable Radicals

Radical templation
centered around a hetero­trisradical tricationic
inclusion complex DB•+⊂​DAPQT2(•+), assembled from an equimolar
mixture of a disubstituted 4,4′-bipyridinium radical cation
(DB•+) and an asymmetric cyclophane
bisradical dication (DAPQT2(•+)), affords
a symmetric [2]­catenane (SC·7PF6) and
an asymmetric [2]­catenane (AC·7PF6) on
reaction of the 1:1 complex with diazapyrene and bipyridine, respectively.
Both these highly charged [2]­catenanes have been isolated as air-stable
monoradicals and characterized by EPR spectroscopy. X-ray crystallography
suggests that the unpaired electrons are delocalized in each case
across two inner 4,4′-bipyridinium (BIPY2+) units
forming a mixed-valence (BIPY2)•3+ state
inside both [2]­catenanes, an observation which is in good agreement
with spin-density calculations using density functional theory. Electrochemical
studies indicate that by replacing the BIPY2+ units in
homo[2]­catenane HC•7+composed
of two mechanically interlocked cyclobis­(paraquat-p-phenylene) ringswith “zero”, one, and two
more highly conjugated diazapyrenium dication (DAP2+) units,
respectively, a consecutive series of five, six, and seven redox states
can be accessed in the resulting SC·7PF6 (0, 4+, 6+, 7+, and 8+), HC·7PF6 (0,
2+, 4+, 6+, 7+, and 8+), and AC·7PF6 (0,
1+, 2+, 4+, 6+, 7+, and 8+), respectively. These unique [2]­catenanes
present a promising prototype for the fabrication of high-density
data memories.