A series of multi-dimensional metal–organic frameworks with trans-4,4′-azo-1,2,4-triazole: polymorphism, guest induced single-crystal-to-single-crystal transformation and solvatochromism

Abstract

In this work, a flexible multi-dentate trans-4,4′-azo-1,2,4-triazole (atrz) ligand has been employed, and a series of novel coordination polymers, namely [Fe(μ-atrz)(NCS)₂(H₂O)₂]_n (1), [Fe(μ-atrz)(NCS)₂(H₂O)₂]_n (2), {[Fe(μ-atrz)(μ-Cl)(H₂O)₂]·Cl·H₂O}_n (3), {[Fe(μ-atrz)(atrz)₂(H₂O)₂]·2Br·4H₂O}_n (4), {[Fe(μ-atrz)(atrz)₂(H₂O)₂]·(BF₄)₂·2H₂O}_n (5), {[Cu(μ-atrz)₃]·(BF₄)₂·2H₂O}_n (6), {[Cu(μ-atrz)₃]·(BF₄)₂}_n (7), {[Cu₂(μ₄-atrz)(μ₂-OH)₂(H₂O)₄]·SO₄·3.75H₂O}_n (8), {[Zn(μ-atrz)(NCS)₂]·0.5H₂O}_n (9) and {[Zn(μ-atrz)(NCS)₂]}_n (10) have been isolated. 1 and 2 present one-dimensional (1D) polymorphic iron(II)–triazole frameworks, in which bicycle-pedal motion of the azo moieties of atrz can be observed. Further, when different iron(II) salts are used, 1D iron(II)–triazole frameworks 3–5 with different 1D bridging motifs can be isolated. 6 has a 3D nano-porous Cu^II framework with dimensions of 11.709(1) Å × 12.201(1) Å. Interestingly, 6 exhibits guest induced single-crystal-to-single-crystal (SC-SC) structural transformation to obtain another 3D nano-porous Cu^II framework 7 with dimensions of 11.621(6) Å × 16.674(1) Å. 8 has a 2D cationic Cu^II framework constructed with infinite (CuO)_n chains and unique μ₄-bridging atrz ligands. Anion exchange experiment indicates an anion-induced structural transformation from the 3D Cu^II framework 6 to the 2D Cu^II framework 8 confirmed by PXRD patterns. Further, when Zn(NCS)₂ is used in the reaction system, 9 can be isolated which exhibits guest induced SC-SC transformation to obtain 10 accompanied by a color change from red (9) to pale pink (10). Variable temperature magnetic susceptibility measurements (2–300 K) reveal anti-ferromagnetic interactions between central iron(II) ions for 1–5. The polymorphic iron(II) frameworks, guest induced single-crystal-to-single-crystal transformation and solvatochromism also reveal great potential for the construction of flexible frameworks employing 4,4′-azo-1,2,4-triazole as basic building blocks.