(Deoxycholic acid)2:Ferrocene: A Phase Transition Determined by the Dynamic Behavior of the Included Guest Molecules

Abstract

The deoxycholic acid inclusion complex (DCA)2:ferrocene (1) undergoes a “gradual” phase transition above ambient temperature, “completed” by ∼360 K. The phase transition is characterized by using single-crystal X-ray diffraction and 13C CP/MAS NMR spectroscopy. 13C CP/MAS NMR spectra of 1 suggest two DCA molecules in the crystallographic unit cell, which on heating become increasingly similar, until beyond ∼345 K only one molecular type is detected. The crystal structure has been determined at 360, 294, 200, and 100 K. Single-crystal diffraction data show a similar phase change at around 350 K. At T ≤ 294 K the structure is solved in P212121, and at 360 K, the a axis is halved in a high-temperature form with symmetry P22121. In the temperature region 320−340 K, the single-crystal X-ray diffraction structure cannot be solved adequately. The atomic displacement ellipsoids of the ferrocene molecule at the various temperatures are consistent with a wobble about the molecular 5-fold axis proposed previously from 2H NMR studies. The transition, although most clearly observed in the positions of the deoxycholic acid molecules, is largely dominated by the dynamic behavior of the included guest ferrocene molecules. The mechanistic nature of and the similarities and differences in the way this transition is detected by NMR and XRD techniques are discussed.