Conformational Flexibility of Hybrid [3]- and [4]-Rotaxanes
Selena J. Lockyer, Selina Nawaz, Adam Brookfield, Alistair J. Fielding, Íñigo J. Vitórica‐Yrezábal, Grigore A. Timco, Neil A. Burton, Alice M. Bowen, Richard E. P. Winpenny, Eric J. L. McInnes
Abstract
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.