Construction of Metal−Organic Frameworks (M = Cd(II), Co(II), Zn(II), and Cu(II)) Based on Semirigid Oxadiazole Bridging Ligands by Solution and Hydrothermal Reactions

Two new symmetric and unsymmetrical semirigid organic ligands 2,5-bis(4-(1H-1,2,4-triazol-1-ylmethyl)phenyl)-1,3,4-oxadiazole (L1) and 4-(1H-1,2,4-trizol-1-ylmethyl)benzoic acid (L2) have been prepared. L2 was generated from L1 by a
hydrothermal reaction via the hydrolysis of the central oxadiazole ring of L1. The solution and hydrothermal reactions of L1 with
various inorganic M(II) (M = Cd, Co, Zn, and Cu) ions have been investigated. Compounds [Cd(L1)2](NO3)2 (1), [Cu(L1)2](NO3)2
(2), and [Cu(L1)]Cl2·2(H2O) (3) were obtained in solution by a combination of L1 with corresponding metal salts, while compounds
Zn(L2)2 (4), Co(L2)2 (5), and [Co(L2)2]·H2O (6) were synthesized by a hydrothermal technique with in situ generated unsymmetrical
L2 ligand. In addition, compounds 4−6 could also be prepared by direct combination of L2 with corresponding metal salts under
the same hydrothermal conditions. All six new coordination polymers were fully characterized by infrared spectroscopy, elemental
analysis, single-crystal X-ray diffraction, and thermogravimetric analysis. Compounds 1−3 are isostructural and feature a one-dimensional chain motif composed of an M2L2-macroring as the based building block. Compounds 4 and 5 form a novel chiral and
acentric non-interpenetrating two-dimensional network consisting of right- and left-handed helical ML chains, respectively. Compound
6 adopts a unique non-interpenetrating three-dimensional framework, in which the inorganic ···Co−O−Co−O··· chains are connected
to each other through unsymmetric L2 ligands. In the solid state, L1−L2 and their complexes 1−6 are luminescent. The Kurtz
powder measurements show that both 4 and 5 are nonlinear optical (NLO) active and have potential applications as NLO-active
materials.