Double-Hinged Di-1,2,4-Triazoles for Synthesis of Breathing MOFs

Employing semi-rigid di-1,2,4-triazoles as ligands has led to the formation of a plethora of Metal-Organic Framework (MOF) architectures. The ability for the ligands to exhibit multiple conformations through a “hinge” effect allows for the formation of a multitude of MOF topologies and dimensionalities. Employment of a di-triazole containing a central trans-butene moiety was initially studied and led to the formation of two three-dimensional copper MOFs. The frameworks can be synthesized independently, but a reaction occurs in water wherein the kinetic product is used as a reagent to synthesize the topologically distinct thermodynamic product.

Additional testing of reaction conditions with the butene-containing di-triazole led to the formation of a three-dimensional breathing framework by utilizing a mixed anion system. The framework structure flexes reversibly upon removal or addition of water to form semi-hydrated and dehydrated MOFs. Single crystal X-ray analysis demonstrated the 2-butene subunit of the ligand rotates between positions, causing changes in the solvent accessible volume. This double hinge within the ligand is a built-in breathing mechanism and suggests a general synthesis for breathing MOFs.

Replacing the central butene moiety with a xylene moiety resulted in the di-triazole adopting a syn conformation between copper chains, forming two-dimensional MOFs that resemble fused 1D metal-organic nanotubes (MONTs). The 2D sheet layers can expand or contract, or, more remarkably, the phenyl ring can rotate between positions as a function of solvation. The transformations were followed by powder X-ray diffraction and solid state NMR. Additionally, frameworks which contain extended naphthyl and biphenyl linkers have been synthesized and characterized.

The syn conformation adopted by the di-triazoles was further exploited for the formation of a series of 1D MONTs. The di-triazole ligands bridge rigid metal chains while appropriate anion choice provides a “capping” of the metal fragments, leading to nanotube formation instead of 2D sheets. The pore size of the MONTs can be adjusted by changing the central portion of the double-hinged ligand, allowing for a general synthesis of MONTs. Adsorption studies of MONTs revealed selective uptake of carbon dioxide and methane with copper MONTs exhibiting the highest uptake.