A series of two-dimensional microporous triazole-functionalized metal–organic frameworks with the new multi-dentate ligand 1-(4-aminobenzyl)-1,2,4-triazole: single-crystal-to-single-crystal transformation, structural diversity and luminescent sensing

Abstract

In this work a new flexible multi-dentate 1-(4-aminobenzyl)-1,2,4-triazole (abtz) ligand has been designed and synthesized, and a series of two-dimensional (2D) triazole-functionalized metal–organic frameworks, namely {[Cd(abtz)₂(H₂O)₂]·(ClO₄)₂·H₂O}_n (1), {[Cd(abtz)₂(H₂O)₂]·(SiF₆)}_n (2), {[Cd(abtz)₂(H₂O)₂]·(ClO₄)_1.64·(HSO₄)_0.36·1.36H₂O}_n (3), {[Cd(abtz)₂(NCS)]·(ClO₄)}_n (4) and {[Ag₂(μ₂-abtz) (μ₃-abtz)(ClO₄)]·(ClO₄)}_n (5) have been isolated. Firstly, when abtz and corresponding cadmium(II) salts are employed, new rectangular-grid 2D coordination frameworks containing free ClO₄⁻ or SiF₆²⁻ anions, {[Cd(abtz)₂(H₂O)₂]·(ClO₄)₂·H₂O}_n (1) and{[Cd(abtz)₂(H₂O)₂]·(SiF₆)}_n (2), can be isolated. Furthermore, in 1 weakly H-bonded ClO₄⁻ anions present within the voids can be partly exchanged by HSO₄⁻ anions to generate the anion exchanged crystalline product {[Cd(abtz)₂(H₂O)₂]·(ClO₄)_1.64·(HSO₄)_0.36·1.36H₂O}_n (3) in single-crystal-to-single-crystal (SC–SC) manner. On the other hand, 1 can be used as a precursor to react with NH₄NCS to obtain the 2D bilayer framework {[Cd(abtz)₂(NCS)]·(ClO₄)}_n (4) under hydrothermal conditions. This also presents a novel example of structural transformation with bond rupture/formation from a mono-layer framework 1 to a 2D bi-layer framework 4. Furthermore, when AgClO₄ is used to replace Cd(ClO₄)₂ in the reaction system, another 2D layer framework {[Ag₂(μ₂-abtz)(μ₃-abtz) (ClO₄)]·(ClO₄)}_n (5) with unique (2,4,5) connected topology can be isolated. The solid state luminescence properties and PXRD patterns of 1–5 have been investigated. Additionally, luminescence measurements illustrate that complexes 4 and 5 represent novel examples as luminescent probes of new sensing materials. 4 shows highly sensitive response to acetone and Fe³⁺. Besides, 5 is the first report of multidimensional polymers based on triazole derivatives with luminescent response to small biological molecules such as vitamin C in water solutions.