Conformational Flexibility of Hybrid [3]- and [4]-Rotaxanes

The
synthesis, structures, and properties of [4]- and [3]-rotaxane
complexes are reported where [2]-rotaxanes, formed from heterometallic
{Cr7Ni} rings, are bound to a fluoride-centered {CrNi2} triangle. The compounds have been characterized by single-crystal
X-ray diffraction and have the formulas [CrNi2(F)­(O2CtBu)6]­{(BH)­[Cr7NiF8(O2CtBu)16]}3 (3) and [CrNi2(F)­(O2CtBu)6(THF)]­{(BH)­[Cr7NiF8(O2CtBu)16]}2 (4), where B = py-CH2CH2NHCH2C6H4SCH3. The [4]-rotaxane 3 is an isosceles triangle of three [2]-rotaxanes bound to
the central triangle while the [3]-rotaxane 4 contains
only two [2]-rotaxanes bound to the central triangle. Studies of the
behavior of 3 and 4 in solution by small-angle
X-ray scattering and atomistic molecular dynamic simulations show
that the structure of 3 is similar to that found in the
crystal but that 4 has a different conformation to the
crystal. Continuous wave and pulsed electron paramagnetic resonance
spectroscopy was used to study the structures present and demonstrate
that in frozen solutions (at 5 K) 4 forms more extended
molecules than 3 and with a wider range of conformations.