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

[Image: see text] The synthesis, structures, and properties of [4]- and [3]-rotaxane complexes are reported where [2]-rotaxanes, formed from heterometallic {Cr(7)Ni} rings, are bound to a fluoride-centered {CrNi(2)} triangle. The compounds have been characterized by single-crystal X-ray diffraction and have the formulas [CrNi(2)(F)(O(2)C(t)Bu)(6)]{(BH)[Cr(7)NiF(8)(O(2)C(t)Bu)(16)]}(3) (3) and [CrNi(2)(F)(O(2)C(t)Bu)(6)(THF)]{(BH)[Cr(7)NiF(8)(O(2)C(t)Bu)(16)]}(2) (4), where B = py-CH(2)CH(2)NHCH(2)C(6)H(4)SCH(3). 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.