Permanent porosity and role of sulfonate groups in coordination networks constructed from a new polyfunctional phosphonato-sulfonate linker molecule

Abstract

The new linker molecule (H₂O₃PCH₂)₂N-CH₂C₆H₄SO₃H, (4-{[bis(phosphonomethyl)amino]methyl}benzene-sulfonic acid, H₅L), bearing both phosphonic and sulfonic acid groups, was employed for the synthesis of new coordination polymers (CPs). Four new CPs of composition [Mg(H₃L)(H₂O)₂]·H₂O (1), [Mg₂(HL)(H₂O)₆]·2H₂O (2), [Ba(H₃L)(H₂O)]·H₂O (3) and [Pb₂(HL)]·H₂O (4), were discovered using high-throughput methods and all structures were determined by single-crystal X-ray diffraction (SCXRD). With increasing ionic radius of the metal ion, an increase in coordination number from CN = 6 (Mg²⁺) to CN = 9 (Ba²⁺) and an increase in the dimensionality of the network from 1D to 3D is observed. This is reflected in the composition of the IBU and the number of metal ions that are connected by each linker molecule, i.e. from three in 1 to ten in 4. The connection of the IBUs leads to 1D and 2D structures in 1 and 2 with non-coordinating sulfonate groups, while 3 and 4 crystallise in MOF-type structures and coordination of the sulfonate groups is observed. The compounds exhibit thermal stabilities between 200 (2) and 345 °C (4) as proven by variable temperature powder X-ray diffraction (VT-PXRD) measurements. Title compound 4 contains micropores of 4 × 2 Å and reversible H₂O uptake of 50 mg g⁻¹ was demonstrated by vapour sorption measurements, making it the first porous metal phosphonatosulfonate. Detailed characterisation, i.e. CHNS and TG analysis as well as NMR and IR spectroscopy measurements confirm the phase purity of the title compounds.