Synthesis, Characterization, and Rotational Dynamics of Crystalline Molecular Compasses with N-Heterocyclic Rotators

We describe in this paper the synthesis, crystallization, and solid-suite NMR dynamics of molecules intended to emulate the structure and function of macroscopic compasses. The desired structures consist of polar pyridine (2) and pyridazine (3) groups as well as their corresponding N-oxides (2O and 3O), each linked axially to two bulky triphenyl methyl groups by 1,4-triple bonds. The structures are such that the central polar heterocycles may rotate about the dialkyne axle while being sterically shielded by the two trityl groups. In addition to the synthesis of samples With natural isotopic abundance, we describe the preparation of 2-d(2) and 3-d(30), one labeled with deuteria. in the pyridine rotator and the other fully deuterated in the two trityl groups in the stator. Crystal structures of 2 and 3 revealed packing motifs analogous to those previously reported for samples prepared with substituted phenylene rotators. While solid-state NMR measurements by C-13 CPMAS NMR revealed insufficient chemical shift dispersion fora dynamic characterization of the rotation of compounds 2 and 3 (including 3-d(30)), the use of quadrupolar echo H-2 NMR methods with 2-d(2) revealed it rotational site exchange with a barrier of ca. 8.5 kcal/mol (35.5 kJ mol(-1)) for the pyridine group in 2.