Synthesis and solid state characterization of molecular rotors with steroidal stators: ethisterone and norethisterone.

In this article we describe the synthesis and dynamic behavior of two new molecular rotors with 1,4-diethynylphenylene rotators axially linked to two conformationally rigid steroidal norethisterone acetate or ethisterone frames. The resulting 1,4-bis(19-nor-17alpha-ethynyltestosterone-17beta-acetate)benzene (1) and 1,4-bis(17alpha-ethynyltestosterone)benzene (2) were fully characterized in solution and in the solid state, and the rotational dynamics of the central phenylene were explored with the help of (13)C NMR with cross polarization and magic angle spinning (CPMAS), and with quadrupolar echo variable temperature (VT) (2)H NMR in the case of 1. Splitting of signals from the aromatic ring on the (13)C CPMAS NMR and a broad quadrupolar spin echo (2)H spectrum of polycrystalline samples indicated that the rotation of the central aromatic ring in these compounds was limited at ambient temperature in the solid state. Variable temperature (2)H NMR experiments at 350 K in the case of 1-d(4) suggested a 2-fold rotational exchange with upper frequency limit of ca. 10 kHz. Single crystal X-ray analysis of this compound revealed that a crowded environment around the prospective phenylene rotator is responsible of the restricted rotation in the solid state.