A comparison of the coordination preference of Cd, Zn, Cu(II) with flexible homophthalic acid and rigid bipyridine ligands

The mild hydrothermal reaction of Cd, Zn, or Cu(II) acetate salts with the deprotonated flexible linker homophthalic acid (H₂hmph) and the rigid molecule 4,4′-bipyridine (bpy) produced the following complexes, featured structurally by the assembly of various metal carboxylate subunits cohered further by the bpy ancillary ligand: {[Cd₂(hmph)₂(bpy)(H₂O)₂]·H₂O}_n (1) manifests cadmium carboxylate zigzag chain motifs, containing the alternation of a hmph-bridged Cd₂(hmph)₂ dimer and an edge-shared Cd₂N₄O₁₀ dimer, connected further by the bpy ligands to form a layered structure with a large 50-membered ring; [Zn(hmph)(bpy)]_n (2) processes hmph-bridged Cd₂(hmph)₂ dimer motifs inter-linked further by the bpy ligands to generate mutually embedded layered structures with square (4,4) grids; and {[Cu(hmph)(bpy)]·H₂O}_n (3) consists of copper carboxylate helix motifs, featuring hmph-bridged CuN₂O₄ octahedra cross-linked further by the bpy ligands into a twofold-interpenetrating 3D chiral framework with a homochiral topology of a quartz dual net. This well-defined synthetic system was proposed to highlight the fact that organic linkers display markedly different coordination preferences at specific metal ions. In addition, the fluorescence properties of complexes 1 and 2 were rationalized in terms of the local ligand environments in the crystal structures, and magnetic properties of complex 3 are also given.