Variation of structures of coordination polymers of Ca(II), Sr(II), and Ba(II) with a tripodal ligand: synthesis, structural, and gas adsorption studies

Abstract

The podand ligand tris-{2-(4-carboxyphenoxy)ethyl}amine (ptaH) readily reacts with Ca(II), Sr(II), and Ba(II) nitrates in a water-THF mixture at room temperature to afford {[Ca(ptaH)(HO)]·6HO}, 1, {[Sr(ptaH)(HO)]·6HO}, 2, and {[Ba(ptaH)(HO)]·HO}, 3. The crystallographic studies performed on these systems revealed that their structures are based on infinite chains of interlinked metallocycles which give rise to open frameworks with the pores being filled with hydration water molecules. The XRPD experiments performed after solvent removal upon heating are consistent with framework stability for 1 and a structural change for 2 and 3. The solid-gas adsorption measurements performed on 1 agree with the typical behavior of an ultramicroporous material in which the N molecules are not able to diffuse through its porous framework at 77 K; however appreciable amounts of CO are readily incorporated at 293 K. By contrast, compounds 2 and 3 do not adsorb either N molecules at 77 K or CO at 293 K, which is indicative of framework collapse upon solvent removal.