Controlling Switching in Bistable [2]Catenanes by Combining Donor–Acceptor and Radical–Radical Interactions

Two redox-active bistable [2]­catenanes composed of macrocyclic
polyethers of different sizes incorporating both electron-rich 1,5-dioxynaphthalene
(DNP) and electron-deficient 4,4′-bipyridinium (BIPY2+) units, interlocked mechanically with the tetracationic cyclophane
cyclobis­(paraquat-p-phenylene) (CBPQT4+), were obtained by donor–acceptor template-directed syntheses
in a threading-followed-by-cyclization protocol employing Cu­(I)-catalyzed
azide–alkyne 1,3-dipolar cycloadditions in the final mechanical-bond
forming steps. These bistable [2]­catenanes exemplify a design strategy
for achieving redox-active switching between two translational isomers,
which are driven (i) by donor–acceptor interactions between
the CBPQT4+ ring and DNP, or (ii) radical–radical
interactions between CBPQT2(•+) and BIPY•+, respectively. The switching processes, as well as the nature of
the donor–acceptor interactions in the ground states and the
radical–radical interactions in the reduced states, were investigated
by single-crystal X-ray crystallography, dynamic 1H NMR
spectroscopy, cyclic voltammetry, UV/vis spectroelectrochemistry,
and electron paramagnetic resonance (EPR) spectroscopy. The crystal
structure of one of the [2]­catenanes in its trisradical tricationic
redox state provides direct evidence for the radical–radical
interactions which drive the switching processes for these types of
mechanically interlocked molecules (MIMs). Variable-temperature 1H NMR spectroscopy reveals a degenerate rotational motion
of the BIPY2+ units in the CBPQT4+ ring for
both of the two [2]­catenanes, that is governed by a free energy barrier
of 14.4 kcal mol–1 for the larger catenane and 17.0
kcal mol–1 for the smaller one. Cyclic voltammetry
provides evidence for the reversibility of the switching processes
which occurs following a three-electron reduction of the three BIPY2+ units to their radical cationic forms. UV/vis spectroscopy
confirms that the processes driving the switching are (i) of the donor–acceptor
type, by the observation of a 530 nm charge-transfer band in the ground
state, and (ii) of the radical–radical ilk in the switched
state as indicated by an intense visible absorption (ca. 530 nm) and
near-infrared (ca. 1100 nm) bands. EPR spectroscopic data reveal that,
in the switched state, the interacting BIPY•+ radical
cations are in a fast exchange regime. In general, the findings lay
the foundations for future investigations where this radical–radical
recognition motif is harnessed in bistable redox-active MIMs in order
to achieve close to homogeneous populations of co-conformations in
both the ground and switched states.