Cation–anion packing and molecular motion in (m-fluoroanilinium)(dibenzo[18]crown-6)[Ni(mnt)2]−(CH3CN)0.25 crystals

Abstract

The hydrogen-bonding (m-fluoroanilinium)(dibenzo[18]crown-6) supramolecule was complexed with the monovalent [Ni(mnt)₂]⁻ (mnt⁻² = maleonitriledithiolate) anion and the crystal structure, thermal stability, and dielectric properties were investigated with respect to the motional freedom of m-fluoroanilinium (m-FAni⁺) in (m-FAni⁺)(dibenzo[18]crown-6)[Ni(mnt)₂]⁻·(CH₃CN)_0.25 (1). The π-planar [Ni(mnt)₂]⁻ anions were stacked along the a-axis and the orientational disorder of fluorine atoms in the m-FAni⁺ cation was observed by X-ray crystal structural analysis at 100 K. Thermogravimetry and differential scanning calorimetry analyses and dielectric constant measurements showed an anomaly at 360 K during the first heating process, where the CH₃CN molecule was released from the crystal while maintaining the block-shape of the single crystal. The cation–anion packing was compared with that of ferroelectric (m-FAni⁺)(dibenzo[18]crown-6)[Ni(dmit)₂]⁻ (2) and ferromagnetic (m-FAni⁺)(dibenzo[18]crown-6)[CrMn(oxalate)₃]·(CH₃CN)(CH₃OH) (3). The orientational disorder of the F-sites of m-FAni⁺ in crystal 1 was associated with the presence of the CH₃CN molecule, which was different from the two-fold flip-flop motion of m-FAni⁺ in ferroelectric crystal 2. Potential energy calculations revealed asymmetric double minimum-type potential energy with energy barriers of 180 and 900 kJ mol⁻¹. The cation–anion packing of crystal 1 was loose compared with that of crystals 2 and 3; therefore, the thermal stability of crystal 1 was lower than that of crystals 2 and 3. The two-dimensional anionic layer in crystals 2 and 3 enhanced the thermal stability of the crystals, whereas the one-dimensional π-stacking structure of anions in 1 decreased its thermal stability.