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

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posted on 2009-11-20, 00:00 authored by Braulio Rodriguez-Molina, Ma. Eugenia Ochoa, Norberto Farfán, Rosa Santillan, Miguel A. García-Garibay
We describe in this paper the synthesis, crystallization, and solid-state 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-d2 and 3-d30, 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 13C CPMAS NMR revealed insufficient chemical shift dispersion for a dynamic characterization of the rotation of compounds 2 and 3 (including 3-d30), the use of quadrupolar echo 2H NMR methods with 2-d2 revealed a rotational site exchange with a barrier of ca. 8.5 kcal/mol (35.5 kJ mol−1) for the pyridine group in 2.