Synthesis, crystal structures, and luminescent properties of Cd(ii) coordination polymers assembled from asymmetric semi-rigid V-shaped multicarboxylate ligands

Abstract

A series of seven Cd(II) coordination polymers, namely [Cd₃(L¹)₂(phen)₃]_n·(H₂O)_2n (1), [Cd₃(L¹)₂(2,2′-bpy)₃(H₂O)₂]_n·(H₂O)_3n (2), [Cd₃(L¹)₂(4,4′-bpy)₂]_n·(H₂O)_4n (3), [Cd₃(L²)₂(phen)₂]_n (4), [Cd₃(L²)₂(2,2′-bpy)₂]_n (5), [Cd₃(L³)₂(phen)₂(H₂O)]_n·(H₂O)_2n (6), and [Cd₃(L⁴)₂(phen)₂(H₂O)₂]_n·(H₂O)_3n (7), have been prepared on the basis of four asymmetric semi-rigid V-shaped multicarboxylate ligands including 4-(2-carboxyphenoxy)phthalic acid (H₃L¹), 3-(2-carboxyphenoxy)phthalic acid (H₃L²), 3-(4-carboxyphenoxy)phthalic acid (H₃L³), and 4-(4-carboxyphenoxy)phthalic acid (H₃L⁴) with the help of 1,10-phenanthroline (phen), 2,2′-bipyridine (2,2′-bpy), or 4,4′-bipyridine (4,4′-bpy) as secondary ligand. Single-crystal X-ray diffraction analysis reveals that compounds 1, 4, 5, and 6 exhibit one-dimensional (1D) double chain structures constructed from trinuclear Cd(II) clusters, which further forms 2D supramolecular architecture via intermolecular π–π interaction. Complex 2 possesses a 3D supramolecular network assembled from 2D layered architecture composed of dinucelar cadmium subunits and isolated Cd(II) ions via intermolecular π–π interaction. Compound 3 shows a 3D framework structure which is generated from 2D layered motif comprised of trinuclear cadmium subunits pillared by 4,4′-bpy ligands, while the 3D framework of 7 is constructed from two different directional ribbon chains consisting of dinuclear cadmium clusters and isolated Cd(II) ions as node. The different molecular structure for compounds 1–3 formed from the same L¹ ligand reveals the spacer effect of secondary ligands on tuning the structure of coordination polymers, while the structural difference among 1, 4, 6, and 7 built from different asymmetric semi-rigid V-shaped multicarboxylate ligand (L¹, L², L³, or L⁴) but employing the same secondary ligand of phen reveals the important role of the positions of carboxylic substituents at the asymmetric multicarboxylate ligand in the formation of coordination compounds. Photophysical properties over the series of complexes 1–7 have been systematically and comparatively investigated.