Low-temperature enantiotropic k2 phase transition in the ionic 222-cryptand complex with LiClO4.

Crystallographic analyses at 100 and 200 K are reported for the macrobicyclic polyether 4,7,13,16,21,24-hexaoxa-1,10-diaza-bicyclo[8.8.8]hexacosane (denoted as 222-cryptand) that encapsulates a Li+ cation and then forms a complex (I) with ClO4-. Compound (I) undergoes a reversible second-order k phase transition at 253 (2) K from an almost ordered structure [space group P2(1)2(1)2(1)] at 100 K to a more disordered structure that exhibits a different unit cell [P2(1)2(1)2 (2c'=c)] above 253 (2) K. At 295 K the Li+ cation and five atoms of the perchlorate anion are each disordered over at least two positions about a crystallographic twofold axis [Chekhlov (2003). Russ. J. Coord. Chem. 29, 828-832]; as the temperature decreases the dynamic positional disorder is slowly frozen out, but is still observed for lithium even at 100 K. Based upon DFT computations, it seems that in the solid state the position of the Li+ cation in the cavity of the 222-cryptand below 253 (2) K likely corresponds to a local energy minimum; the global minimum in the gas phase corresponds to a near D3 symmetrical conformation of the 222-cryptand with the undersized Li+ cation residing in the center of its cavity.