Auxiliary ligand-directed and counter anion-templated effects on coordination networks based on semirigid 2-aminodiacetic terephthalic acid ligand

Abstract

Reactions of the semirigid polycarboxylate ligand, 2-aminodiacetic terephthalic acid (H₄adtp), with Zn(II) and Cd(II) salts resulted in {[Zn₂(adtp)(H₂O)₂]·2H₂O}_n (1) and [Cd₂(adtp)(μ₂-H₂O)(H₂O)₃]_n (3), respectively. By introducing auxiliary ligand 4,4′-bipyridine (bpy), two 3D regularly arranged pillared-layered frameworks {[Zn₃(adtp)(bpy)_1.5(μ₃-OH)(H₂O)]·NO₃·3H₂O}_n (2a) and {[Cd₄(adtp)₂(bpy)(H₂O)₄]·2.5H₂O}_n (4) were obtained. Complex 1 is characterized as a two-dimensional (2D) layer structure, whereas complex 3 features a three-dimensional (3D) framework with two types of alternately arrayed channels. In complex 2a, the one-dimensional (1D) zinc chains built up of alternately interlinked tetranuclear and dinuclear units are cross-linked into 2D layers, which are further pillared by bpy into a 3D cationic framework with NO₃⁻ as counterions. As for 4, a 3D pillared-layer framework is observed and comprises carboxylic μ₂-O bridged –M–O–M– chains. The auxiliary ligand bpy exerts a structure-directing effect on the resultant 3D pillared-layer architectures in 2a and 4, wherein μ₃-OH bridged tetranuclear metal units or carboxylic μ₂-O bridged –M–O–M– chains appear. Additionally, the cationic framework in 2a is strongly dependent on counter anions and the concentration of bpy ligand. The isomorphic {[Zn₃(adtp)(bpy)_1.5(μ₃-OH)(H₂O)]·ClO₄·3H₂O}_n (2b) with ClO₄⁻ as template and counterions is also obtained. The luminescent properties of 1, 2a, 3 and 4 are investigated in detail. Compared with free H₄adtp, complex 3 exhibits a larger blue-shifted emission band. Complex 2a displays tunable photoluminescence by variation of excitation light and temperature due to multi-centers emissions.