Cationic Metal-Organic Frameworks Synthesized from Cyclotetraphosphazene Linkers with Flexible Tentacles

Abstract

Metal-organic frameworks (MOFs) based on group 12 metals, namely, [Zn2(O3PyCP)Cl4] (Zn-O3PyCp), [Cd(O3PyCP)(NO3)2].H2O (Cd-O3PyCp), and [Hg(O3PyCP)Cl2] (Hg-O3PyCp), were synthesized using a novel flexible molecular building block, octakis(3-pyridyloxy)cyclotetraphosphazene (O3PyCP), and group 12 metal salts (ZnCl2, Cd-(NO3)2.6H2O, and HgCl2). The crystals of each of the cationic frameworks were structurally characterized by single-crystal X-ray diffraction, Fourier transform infrared, and thermogravimetric analysis. Because of the flexibility of the O3PyCP building block, we were able to observe three distinct cationic frameworks with group 12 metal ions. We performed the atomically detailed simulations to compute H2 adsorption isotherms of the resulting frameworks at a pressure range of 0-1 bar at 77 K. Zn-O3PyCp exhibits the highest H2 uptake capacity (58.42 cm3 STP/g).