Zinc(II) carboxylates with imidazole and 2-methylimidazole: unprecedented cyclic dimer and polynuclear coordination polymers based on bridging phthalate ions

Abstract

Novel zinc(II) coordination polymer [Zn(Pht)(Im) 2] n ( 1), unique cyclic dimer [Zn(Pht)(Im)(H 2O)] 2 ( 2) and coordination polymer [Zn(Pht)(2-MeIm)] n ( 3) (where Pht 2−=dianion of o-phthalic acid, Im=imidazole, 2-MeIm=2-methylimidazole) have been synthesized and their structures characterized by X-ray crystallography. Both carboxylate groups of the phthalate dianion coordinate in a monodentate mode bridging two Zn(II) ions with formation of 1D zigzag chain in 1. Two Im ligands play a double role: they coordinate the metal atom and join the coordination polymers via NH⋯O hydrogen bonds into a 3D-network. Complex 2 represents the first example of a metallocycle in which two Zn(II) atoms are bridged by two phthalate ligands forming a 14-membered ring. The Zn(II) atom is coordinated by two O atoms of Pht 2− carboxylate groups, N atom of the Im and O atom of the water molecule with formation of tetrahedral NO 3 metal-binding site. The molecules of 2 form a 3D-network through NH⋯O and OH⋯O hydrogen bonds between carboxylate groups of Pht 2−, Im and water molecules. Compound 3 reveals the characteristic structural features of both compounds 1 and 2. One pair of zinc atoms is linked by 1,6-bridge of two phthalate ligands, while the other is held together through syn– syn bridging carboxylate groups in a 1,3-fashion from the same phthalates. As a result, infinite chains with alternation of 14-membered and eight-membered cycles are formed. Additional hydrogen bonds link the chains to form a 2D-network. IR and 1H NMR spectra of 1– 3 are also reported.