Polymorphism of Alkali Bis(fluorosulfonyl)amides (M[N(SO<sub>2</sub>F)<sub>2</sub>], M = Na, K, and Cs)
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The polymorphic behavior of Na, K, and Cs salts of the bis(fluorosulfonyl)amide anion N(SO2F)2– has been investigated by means of differential scanning calorimetry (DSC), single-crystal and powder X-ray diffraction, and Raman spectroscopy. All of the polymorphs observed in the present work (three for Na[N(SO2F)2], two for K[N(SO2F)2], and two for Cs[N(SO2F)2]) are stable enough for analyses at room temperature. With increasing temperature, form II of Na[N(SO2F)2] undergoes a solid–solid phase transition to form I, whereas another form (form III) crystallizes from the melt upon cooling. The anions in forms I and II of Na[N(SO2F)2] have trans and cis conformations, respectively, at 113 K, while cis–trans disorder is observed for the anion in form I at 298 K. Form I of K[N(SO2F)2], with a melting point of 375 K, is the stable form at room temperature, whereas solidification from the molten state during DSC gives rise to form II with a melting point of 336 K. Both forms I and II of K[N(SO2F)2] have anions in the cis conformation. The difference between the two potassium polymorphs arises from their crystal packing modes. In the case of Cs[N(SO2F)2], form I melts at 387 K, whereas form II undergoes a solid–solid transition to form I at 330 K. The anion of form I in Cs[N(SO2F)2] has an oxygen/fluorine disorder that exhibits an oxygen/fluorine eclipsed conformation, even at 113 K. The powder X-ray diffraction pattern of form II matches that of the previously known Cs[N(SO2F)2] structure of the trans conformer. Vibrational frequencies observed with Raman spectroscopy do not necessarily show the same trend as those calculated for the energy-minimized cis or trans conformers in the gas phase due to packing effects.
