Pseudopolymorphism based on 1D metallacyclic chains constructed from an angular zwitterionic ditopic diacid organic linker

The reaction of Cd(NO3)2·4H2O and 4-carboxy-1-(4-carboxybenzyl)pyridinium bromide (H2LBr) under different reaction conditions generates two pseudopolymorphs, {[Cd(L)2]·2H2O}⊃C3H6O (1α) and {[Cd(L)2]·2H2O}⊃3H2O (1β). Crystal structure analysis reveals that complexes 1α and 1β feature an essentially identical 1D metallacyclic chain structure, and their structural differences mainly stem from different packing modes of the identical 2D sheets, consisting of identical 1D metallacyclic chains linked via π⋯π interactions. Corresponding to the different packing modes, complexes 1α and 1β have distinctly different space groups: the acentric orthorhombic space group Fdd2 for 1α and the centric monoclinic space group I2/c for 1β. For acentric 1α crystals, modest powder second-harmonic-generation (SHG) efficiency is observed at room temperature.