Molecular Crystals with Moving Parts: Synthesis, Characterization, and Crystal Packing of Molecular Gyroscopes with Methyl-Substituted Triptycyl Frames

We report a highly convergent synthesis for the preparation of molecular gyroscopes consisting of para-phenylene rotors linked by triple bonds to methyl-substituted triptycenes acting as pivots and encapsulating frames. The desired 1,4-bis[2-(2,3,6,7,12,13-hexamethyl-10-alkyl-9-triptycyl)-ethynyl]benzenes were prepared from 2,3-dimethyl-1,3-butadiene using Diels-Alder cycloadditions and Pd(0)-catalyzed coupling as the key reactions. The main challenge in the synthesis came about in the preparation of 9-alkynyl-triptycenes by Diels-Alder reaction of benzynes and 9-alkynyl-2,3,6,7-tetramethylanthracenes. These reactions occurred with chemical yields and regioselectivities that were strongly influenced by steric and electronic effects of substituents at C10 of the anthracene core. Anthracenes with methyl, propyl, and phenyl substituents were utilized to complete the synthesis of their corresponding molecular gyroscopes, and their solid-state structures were determined by single-crystal X-ray diffraction analysis. Examination of these results indicated that, as expected, the bulky triptycyl groups encourage crystallization motifs that create more free volume around the phenylene rotor, as needed to facilitate fast gyroscopic motion in the solid state.