Adaptation toward Restricted Conformational Dynamics: From the Series of Neutral Molecular Rotors

A semirigid ligand, 1,4-bis(2-nonylbenzimidazol-1-ylmethyl)benzene (Nbenzbix), containing a long alkyl chain substituted stator and a rotating unit was designed and synthesized. Three dinuclear metallacycles [(Re(CO)3)(μ-L)(μ-Nbenzbix)(Re(CO)3)] (1, H2-L = H2-dhnq = 6,11-dihydroxy-5,12-naphthacenedione; 2, H2-L = H2-dhaq = 1,4-dihydroxy-9,10-anthraquinone; and 3, H2-L = H2-CA = 2,5-dichloro-3,6-dihydroxy-p-benzoquinone) were synthesized from Re2(CO)10, Nbenzbix, and H2-L units. Compounds 1–3 and the ligand Nbenzbix were characterized by elemental analysis, FT-IR, and 1H NMR spectroscopy. Compound 1 was further characterized by a single-crystal X-ray diffraction analysis. The dynamic properties of 1–3 in solution were studied using variable-temperature 1H NMR spectroscopy, and the findings indicate that the p-phenylene unit in the metallacycle rotates in solution. To prevent the stator units from undergoing conformational changes due to the syn/anti arrangement of the benzimidazolyl units, a long-chain alkyl group was introduced at the 2-position of the benzimidazolyl unit. Molecular modeling calculations indicate that the energy barrier for the p-phenylene rotating unit in the metallacycle would be very low. Hence, these neutral metallacycles can be regarded as surface-mounted altitudinal rotors in which the bischelating unit is related to the surface, the nonylbenzimidazolyl units are related to the stators, and the p-phenylene is related to the rotating unit.