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

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(mu-atrz)(NCS)(2)(H2O)(2)](n) (1), [Fe(mu-atrz)(NCS)(2)(H2O)(2)](n) (2), {[Fe(mu-atrz)(mu-Cl)(H2O)(2)]center dot Cl center dot H2O}(n) (3), {[Fe(mu-atrz)(atrz)(2)(H2O)(2)]center dot 2Br center dot 4H(2)O}(n) (4), {[Fe(mu-atrz)(atrz)(2)(H2O)(2)]center dot(BF4)(2)center dot 2H(2)O}(n) (5), {[Cu(mu-atrz)(3)]center dot(BF4)(2)center dot 2H(2)O}(n) (6), {[Cu(mu-atrz)(3)]center dot(BF4)(2)}(n) (7), {[Cu-2(mu(4)-atrz)(mu(2)-OH)(2)(H2O)(4)] center dot SO4 center dot 3.75H(2)O}(n) (8), {[Zn(mu-atrz)(NCS)(2)]center dot 0.5H(2)O}(n) (9) and {[Zn(mu-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) angstrom x 12.201(1) angstrom. Interestingly, 6 exhibits guest induced single-crystal-to-single-crystal (SC-SC) structural transformation to obtain another 3D nano-porous CuII framework 7 with dimensions of 11.621(6) angstrom x 16.674(1) angstrom. 8 has a 2D cationic Cu-II framework constructed with infinite (CuO)(n) chains and unique mu(4)-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.