Characterizing X-Ray and Solution State Conformations for a Model Qubit System: {Cr7Ni} Ring Rotaxanes on a Mixed Metal Triangle

The synthesis of a series of [4]rotaxanes, each consisting of three [2]rotaxanes joined via a central {CrNi₂} triangular linker, is reported. The resultant four [4]rotaxanes were characterized by single crystal X-ray diffraction and electron paramagnetic resonance (EPR) spectroscopy. Orientation-selective 4-pulse double electron–electron resonance (DEER) measurements between the three {Cr₇Ni} rings incorporated in each [4]rotaxane reveal that each system is conformationally fluxional in solution, with the most abundant conformations found to differ significantly from the crystal structure geometry for each compound. The degree of similarity between conformations is evaluated using a novel application of the earth mover’s distance analysis.

A set of four macromolecular assemblies is synthesized, each containing four paramagnetic groups. Their crystal structures are determined by X-ray Diffraction and the paramagnetic groups are initially probed by continuous-wave electron paramagnetic resonance (EPR). With orientationally selective pulsed EPR, we determine the most common molecular geometries adopted by each system in solution, and the deviation thereof from the corresponding crystal structure is evaluated for each system with a novel application of the earth mover’s distance.