Fabrication of new metal phosphonates from tritopic trisphosphonic acid containing methyl groups and auxiliary ligands: syntheses, structures and gas adsorption properties

Abstract

Hydrothermal reactions of benzene-1,3,5-triyltris(methylene)triphosphonic acid (H6L1) or (2,4,6-trimethylbenzene-1,3,5-triyl)tris(methylene)triphosphonic acid (H6L2) with metal sulfates, 4,4′-bipyridyl (4,4′-bipy), 2,2′-bipyridyl (2,2′-bipy), 1,10-phenanthroline (1,10-phen) and 1,2-di(4-pyridyl)ethylene (dpe), afforded eight new transition metal trisphosphonates, namely, [Cu2(H3L1)2(H2O)2][H2dpe][H2O]4 (1), [Ni1.5(H3L2)(H2O)2][H2O] (2), [Ni(H4L2)(1,10-phen)(H2O)2] (3), [Ni(H4L2)(2,2′-bipy)(H2O)2] (4), [Cu(H4L2)(4,4′-bipy)] (5), [Cu(H4L2)(1,10-phen)][H2O] (6), [Cu(H4L2)(2,2′-bipy)(H2O)][H2O]0.13 (7), and [Mn1.5(H3L2)(H2O)2][H2O]1.1 (8). The single-crystal X-ray diffraction measurements indicate that compounds 2 and 8 are isostructural and feature 3D framework structures with (5,6,8)-net topology; 1, 5 and 6 exhibit 2D layered structures; whereas 3, 4 and 7 have 0D dimer structures. The trisphosphonate ligands (H6L1 and H6L2) in these eight compounds adopt either cis–cis–cis or cis–trans–trans conformation with or without the existence of an auxiliary N-containing ligand. The attachment of methyl groups to the benzene core can enhance the crystallinity and enrich the structure diversity. The introduction of 2,2′-bipy and 1,10-phen usually result in low dimensionality whereas 4,4′-bipy can extend the structure to a higher dimensionality. The activated sample of compound 5 has a permanent 1D nanoporous structure and shows high selective adsorption of CO2 over CH4.