Acta Cryst. (2008). B64, 738-749 [ doi:10.1107/S0108768108034046 ]
Abstract: The structures of five more phases of the structural family of compounds [M(H2O)2(15-crown-5)](NO3)2 have been determined. All of these phases are stable at room temperature or above, but transform to other phases if cooled slowly. All five phase transitions take place without significant damage to the single crystal. The M = Co phase, which has a three-dimensional hydrogen-bonding pattern, is a disordered version of the structure known at room temperature; Z' changes from 2 to ½. The other four structures have a two-dimensional hydrogen-bonding pattern, are all modulated variants of the same basic cell (or subcell) and are at least mostly ordered. For M = Mg and Zn the structure found somewhat above room temperature is the Z' = 8 variant found previously for M = Fe. For M = Cu and Ni the Z' = 2 phase found is the same as seen previously for one of the room-temperature polymorphs with M = Mn. There are now two phases and one transition known for each M studied that has a two-dimensional hydrogen-bonding pattern. The changes in the modulation patterns during these transitions are Z' 3 ![[left right arrow]](/logos/entities/harr_rmgif.gif)
8 (Mg, Fe, Zn), Z' 3 2 (Mn, Ni) and Z' 5 2 (Cu). In all two-dimensional hydrogen-bonded crystals the alternation pattern of conformational enantiomers along the modulation direction becomes more perfect above the phase transition. All stable [M(H2O)2(15-crown-5)](NO3)2 phases whose existence at accessible temperatures is indicated by differential scanning calorimetry (DSC) have now been characterized.
Keywords: phase transitions; hydrogen bonding.
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