Synthesis, structures and properties of metal–organic frameworks prepared using a semi-rigid tricarboxylate linker

Abstract

The metal–organic frameworks formed from the reaction between cadmium(II) salts and the semi-rigid 5-((carboxymethyl)amino)isophthalic acid (H₃cmai) in water at 90 °C are dependent on the counter-ions, with [Cd₂(Hcmai)₂(H₂O)₂]·H₂O (1), [Cd(Hcmai)(H₂O)₂] (2) and [Cd₃(cmai)₂(H₂O)₄]·4H₂O (3) forming from the nitrate, chloride and the acetate salts, respectively. Compound 3 loses both lattice and coordinated water molecules on heating to 150 °C and is converted into [Cd₃(cmai)₂] (4). This reaction occurs in a single-crystal-to-single-crystal manner and was followed using variable temperature X-ray powder diffraction. The reaction of cadmium(II) nitrate with H₃cmai in a mixture of water and DMF afforded [Cd₆(cmai)₄(H₂O)_9.75(DMF)_2.25]·18H₂O·1.5DMF (5) whereas the same reaction in water and DEF in the presence of 4,4′-bipyridine led to [Cd₃(cmai)₂(H₂O)₃]·6H₂O (6). Generally, the coordination networks containing Hcmai (1–2) propagate in one or two dimensions, whereas those containing cmai (3–6) propagate in three dimensions. Use of the first row d-block salts zinc(II), copper(II) and cobalt(II) nitrate resulted in compounds [M(Hcmai)(H₂O)_x] (M = Zn, x = 2 (7), M = Cu, x = 1.2 (8), M = Co, x = 2 (9)), with 8 showing photocatalytic activity for the degradation of the dye rhodamine B. Finally, the reaction of zinc(II) acetate with H₃cmai in water and DEF yielded [Zn₂(cmai)(OH)(H₂O)₂]·3H₂O (10). Compounds 7–10 all form two-dimensional networks.