Solid State Characterization of Bridged Steroidal Molecular Rotors: Effect of the Rotator Fluorination on Their Crystallization

We describe the synthesis and crystallization of two macrocyclic steroidal compounds labeled (<i>Z</i>)-<b>1F</b>₂ and (<i>E</i>)-<b>1F</b>₂, envisioned to work as molecular rotors with 1,4-diethynyl-2,3-difluorophenylene rotators. The introduction of the fluorine atoms rendered different crystal arrays that were fully characterized by X-ray diffraction, solid state ¹³C NMR, and calorimetric experiments. In both isomers, the central molecular fragments point the <i>o</i>-fluorine atoms toward their cavity, highlighting the preferred orientation in the solid state. By analyzing the crystal array, it can be seen that while the fluorinated <i>Z</i> isomer is isomorphic to its non-fluorinated analogue, the <i>E</i> isomer shows significant structural variations in an unanticipated crystal array with two molecules per asymmetric unit (<i>Z</i>′ = 2)

Synthesis, Structure, and Local Molecular Dynamics for Crystalline Rotors Based on Hecogenin/Botogenin Steroidal Frameworks

The synthesis and solid-state characterization of a series of cyclic/acyclic molecular rotors derived from naturally occurring steroidal 12-oxosapogenins are described. The bridged molecular rotors with rigid steroidal frameworks were obtained by employing ring-closing metathesis (RCM) as a key step. The X-ray diffraction technique was employed for determination and refinement of the crystal and molecular structure of selected models giving good quality single crystals. In the case of the bridged hecogenin molecular rotor <b>11</b><i><b>E</b></i> for which poor quality crystals were obtained, an NMR crystallography approach was used for fine refinement of the structure. Solid state NMR spectroscopic techniques were applied for the study of local molecular dynamics of the featured acyclic/cyclic molecular rotors. Analysis of ¹³C principal components of chemical shift tensors and chemical shift anisotropy (CSA) as well as heteronuclear ¹H–¹³C dipolar couplings (DC) unambiguously proved that aromatic rings located in the space within the rigid steroidal framework both for cyclic and acyclic rotors are under kHz exchange regime. Experimental results were confirmed by theoretical calculations of rotation barrier on the density functional theory level. Small distinctions in the values of CSA and DC for the rotors under investigation are explained on the basis of differences in their molecular structures