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), [Ni_1.5(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 [Mn_1.5(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.