Exceptional sensitivity to the synthetic approach and halogen substituent for Zn(II) coordination assemblies with 5-halonicotinic acids

Seven Zn(II) coordination complexes with 5-halonicotinic acids (HL-X, X = F, Cl, or Br) have been synthesized with different synthetic approaches, including layer diffusion or stirring method in an ambient environment and solvothermal synthesis at 100 °C. Assembly of HL-F with Zn(II) under different conditions will yield the same 2D network of [Zn(L-F)2]n (1). Interestingly, three distinct complexes, a 3D framework {[Zn2(L-Cl)4(H2O)](H2O)6}n (2) and two 2D pseudo-polymorphic isomers {[Zn(L-Cl)2](H2O)1.5}n (3) and {[Zn2(L-Cl)4](H2O)}n (4) can be obtained by reacting HL-Cl with Zn(II) under layer diffusion, stirring, and solvothermal conditions, respectively. Furthermore, replacing the -Cl substituent with -Br on the HL-X ligand will also afford three diverse coordination assemblies of 3D {[Zn2(L-Br)4(H2O)](CH3OH)2.5}n (5), mononuclear [Zn(HL-Br)2(H2O)4][L-Br]2 (6), and 2D {[Zn(L-Br)2](H2O)1.15}n (7) depending on the synthetic pathways. Beyond the significant influence of the synthetic approach, which will lead to the formation of various crystalline products, the halogen substitution effect of HL-X ligands on the coordination motifs has also been demonstrated. In addition, thermal stability and fluorescence for these crystalline materials will be presented.