New series of Zn^II/Cd^II mixed ligand coordination polymers: Toward the design of metallogels

A new series of mixed ligand based coordination polymers (CPs) has been synthesized by reacting a new ligand, namely, hpdia (5-(4-hydroxyphenylazo)isophthalic acid); various bispyridyl coligands (2,2′-bipyridine (2,2′-bpy), 4,4′-bipyridine (4,4′-bpy), and 1,2-di(4-pyridyl)ethylene (bpe)); and Zn(NO₃)₂/Cd(NO₃)₂ under solvothermal conditions. The molecular structure of hpdia was established by analyzing the single crystal X-ray diffraction (SXRD) data of the sodium salt of hpdia which turned out to be an alkali coordination polymer with a formula [(H₂O)₄Na₂(μ-hpdia)4]_∞ (CP1). SXRD analyses of the other transition metal compounds, thus synthesized, revealed that they were all coordination polymers (except DCC3—a discrete coordination complex) having formulas [{(H₂O)Zn(μ-hpdia)(2,2′-bpy)}·H₂O]_∞ (CP2), [(H₂O)Zn₂(μ-hpdia)₂(2,2′-bpy)₂]₂ (DCC3), [{(H₂O)Cd(μ-hpdia)(2,2′-bpy)}·H₂O]_∞ (CP4), [{(H₂O)Zn(μ-hpdia)(4,4′-bpy)}·H₂O]_∞ (CP5), [Zn_0.5(hpdia)(μ-4,4′-bpy)_0.5]_∞ (CP6), [{(H₂O)₂Cd₂(μ-hpdia)₂(μ-4,4′-bpy)}·2H₂O]_∞ (CP7), and [{Zn₂(μ-hpdia)₂(μ-bpe)}·H₂O]_∞ (CP8). The majority of the CPs were lattice occluded molecular solids. Remarkably, the reactants of all of the coordination polymers and also DCC3 showed the ability to produce metallogels which were characterized by rheology and scanning electron microscopy (SEM). The results supported the structural consideration based on which these metallogelators were designed