rtl-M-MOFs (M = Cu, Zn) with a T-shaped bifunctional pyrazole-isophthalate ligand showing flexibility and S-shaped Type F-IV sorption isotherms with high saturation uptakes for M = Cu

Abstract

Two new rtl-MOFs rtl-[Cu(HIsa-az-dmpz)] and rtl-[Zn(HIsa-az-dmpz)] have been synthesized by using the new bifunctional ligand 5-(4-(3,5-dimethyl-1H-pyrazolyl)azo)isophthalic acid (H₃Isa-az-dmpz). Both frameworks are potentially porous structures with DMF molecules included in the channels of the as synthesized materials. The flexible MOF rtl-[Cu(HIsa-az-dmpz)] undergoes a reversible phase change into a closed form upon activation. Consequently, rtl-[Cu(HIsa-az-dmpz)] shows S-shaped Type F-IV adsorption profiles or a gate-opening effect at cryogenic temperatures with high saturation uptakes of 360 cm³ g⁻¹ for N₂ at 77 K and 310 cm³ g⁻¹ for CO₂ at 195 K. These profiles together with the reversibility could be reproduced upon repeated measurements on the same materials. The gravimetric high-pressure CO₂ adsorption shows a gate-opening at ∼10 bar with an uptake of 332 mg g⁻¹. rtl-[Zn(HIsa-az-dmpz)] undergoes an irreversible transformation into a non-porous phase upon activation.