Syntheses, structures and properties of three cluster-based coordination polymers: influence of the metal ions on the ligand coordination mode and crystal chirality

Three cluster-based coordination polymers, namely [Zn3(bpy)3(hip)2] · 5H2O (1), [Co3(bpy)3(hip)2] · 5H2O (2) and [Cd3(bpy)3(hip)2] (3) (bpy=2,2′-bipyridine, hip=4-hydroxyl-isophthalate) were synthesized and structurally characterized. X-ray single-crystal structural analyses revealed that both 1 and 2 crystallize in the chiral space group P21, while 3 crystallizes in the centric space group Pccn. Compounds 1 and 2 are isomorphous and both have (4,4) topological layered structures constructed from trinuclear metal clusters. Compound 3 also shows layered structure of (4,4) topology constructed from trinulear Cd(II) cores. The layers are stacked in a staggered ⋯ABAB⋯ fashion in 1 and 2 but in an overlapped ⋯AAA⋯ fashion in 3. There are two types of coordination modes of hip ligand in 1 and 2 but only one in 3. The structural difference between 1 (or 2) and 3 may be attributed to the difference of metal ion nature such as the ionic radius and coordination preference, resulting in the different orientation fashions of the auxiliary bpy ligands, stacking fashions of the layers, as well as chirality of the crystals. The chiral structures of 1 and 2 were also confirmed by measurements of powder second-harmonic-generation (SHG) measurements, which show that 1 and 2 have SHG intensity of 0.50 and 0.02 relative to that of urea, respectively.