Porous zinc and cobalt 2-nitroimidazolate frameworks with six-membered ring windows and a layered cobalt 2-nitroimidazolate polymorph

Abstract

Polymorphs of Zn(2-nIm)₂ (compound 1) and Co(2-nIm)₂ (compounds 2 and 3) (2-nIm = 2-nitroimidazole) have been prepared by two routes: solvothermal synthesis and recrystallisation of ZIF-65(Zn/Co). Compounds 1 and 2 are isostructural, with a tetrahedrally-connected framework topology related to, but different from, that of tridymite (lonsdaleite). Single crystal X-ray diffraction analysis showed that in compound 1 (Pccn, Z = 8; a = 8.462(8) Å, b = 14.549(15) Å, c = 18.799(18) Å, V = 2314(4) ų) there is rotational disorder for two of the three crystallographically-distinct linker types, which has been investigated computationally and by solid-state NMR spectroscopy. Detailed adsorption studies on a sample of 1 prepared by recrystallisation show 1.1 mmol g⁻¹ uptake of CO₂ at 0.1 bar (25 °C) with high affinity for CO₂ over CH₄ and N₂ (adsorption enthalpies of 39.5, 26.0 and 18.5 kJ mol⁻¹, respectively). A cobalt analogue (compound 2) with improved water stability (but lower porosity) has also been prepared. Changing the conditions of synthesis and recrystallisation gives rise to a cobalt 2-nitroimidazolate (Co(2-nIm)₂, compound 3), which has a layered structure (I4₁/amd, a = 6.025(18) Å, c = 26.95(8) Å, V = 978.3(5) ų) containing sheets of tetrahedrally-connected Co²⁺ cations composed of four membered rings, without porosity.