Anion Control of the Self-Assembly of One-Dimensional Molecular Ladders vs Three-Dimensional Cross-like Arrays Based on a Bidentate Schiff Base Ligand

The reaction of a conjugated bispyridyl-based Schiff base ligand L (L = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene)
with transition metal ions afforded two different structure types of coordination polymers depending on the choice of counteranions.
The combination of ZnII or MnII acetate and L in solution yielded two novel infinite one-dimensional (1D) molecular ladder complexes
1 and 2 with a molecular composition of [M2L2(OAc)4·2(MeOH)]n, (M = ZnII or MnII), in which acetate anions serve as bridges.
However, when replacing ZnII or MnII acetate with ZnII or CdII nitrate in the same reaction condition, two isomorphous 1D linear
coordination polymers 3 and 4 with a molecular composition of [ML(NO3)2(H2O)2]n, (M = ZnII or CdII) were obtained. Interestingly,
the 1D polymeric chains of 3 and 4 could be further assembled to a three-dimensional (3D) microporous cross-like array with
layer-to-layer alternate arrangement resulting from interchain π···π stacking and hydrogen-bonding interactions with the presence
of coordinated nitrate anions and water molecules. There are remarkably distinct coordination and molecular packing modes in the
two types of complexes regardless of the nature of the metal ions used. These results indicate that the nature of the anions, which
play different roles in forming two types of crystals, is the key factor directing the structural topologies. IR spectra and thermal
behaviors of all four complexes have been characterized, and photoluminescent studies revealed that the ZnII and CdII complexes
were fluorescent with medium intensity in the solid state.