Polymorphism of Alkali Bis(fluorosulfonyl)amides (M[N(SO₂F)₂], M = Na, K, and Cs)

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