Constructing various metal–organic frameworks by mixed pyridine–acylamide and carboxylate ligands: ring-like or helical building blocks

Abstract

A new pyridine–acylamide ligand, LLN¹,N⁴-di(pyridin-3-yl)naphthalene-1,4-dicarboxamide, has been synthesized. Based on L, four types of carboxylate ligands as co-ligands and two metal ions (Zn(II) and Cd(II)), seven new metal–organic frameworks have been synthesized and structurally characterized. Compounds 1 and 2 are composed of SBU I (a helical {Zn(L)}_n chain), while compounds 3–7 are composed of SBU II (a M₂L₂ metallocycle). In 1, with Hbtc²⁻ anions connected to the neighboring Zn(II) ions of one SBU I, an infinite 1D chain is formed, while in 2, the neighboring SBUs I are bridged by ip²⁻ anions forming a 2D structure. With SBUs II extended by oba²⁻ ligands, compound 3 shows a 2D network. Compound 4 exhibits a 1D infinite chain with SBUs II bridged by nap²⁻ ligands. Compounds 5 and 6 are isomers in which SBUs II are extended by nap²⁻ ligands to afford a 2D network. In compound 7, SBUs II are linked together by Hbtc²⁻ ligands featuring an infinite 1D chain. All of these compounds have a 3D framework governed by hydrogen bond interactions. Comparing all the compounds, it can be seen that the structural characteristics of the L ligand and organic counteranions simultaneously play an important role in their construction. Moreover, their thermostability and photoluminescence properties were explored.