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 Zn-II or Mn-II 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)center dot 2(MeOH)](n), (M = Zn-II or Mn-II), in which acetate anions serve as bridges. However, when replacing Zn-II or Mn-II acetate with Zn-II or Cd-II 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 = Zn-II or Cd-II) 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 pi center dot center dot center dot pi 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 Zn-II and Cd-II complexes were fluorescent with medium intensity in the solid state.