Cooperative effect of bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylate and selective bipyridine ligands on coordination network assembly and metallocycle generation

Three new coordination polymers [Co2(ceta)(bpy)1.5(H2O)3]·4H2O (1), [Ni2(ceta)(bpe)2(H2O)3]·11H2O (2) and [Ni2(ceta)(bpp)2(H2O)2]·2H2O (3) (ceta = bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylate, bpy = 4,4′-bipyridine, bpe = 1,2-bis(4-pyridyl)ethylene, bpp = 1,3-bis(4-pyridyl)propane) have been synthesized by hydrothermal reaction and their crystal structures determined. In 1, ceta ligands act as tetradentate bridges bound to Co atoms, resulting in the formation of (4,4) sublayer based on Co4(ceta)4 metallocycles. The adjacent layers are pillared by bpy triple-bridges forming a 3D condensed structure with pcu topology. 2 has a novel porous 3D framework based on Ni6(ceta)6 metallocycles, linked by tetradentate ceta ligands and triple helical [Ni(bpe)]n chains. 3 features a unique 2D molecular network composed of [Ni2(ceta)]n chains and contains small empty spaces inside the [Ni2(bpp)2] 22-membered metallocycles. The structural diversities indicate that structural features of the ceta ligand, as well as the different lengths and conformations of the bipyridine ligands play crucial roles in modulating the structures of the coordination polymers. A discussion of the crystal structures, as well as the coordination behavior of the tetracarboxylate ligands with different bipyridine ligands is provided.

Graphical abstractThe combination of a tetradentate bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylate ligand and three types of bipyridine ligands afford three mixed-ligand frameworks, where the acidic ligand of changeable coordination modes develops various secondary building units, such as metallocycles and chains, which are interlinked by the auxiliary bipyridine ligands with increased length and different conformations, displaying interesting structural diversity.Figure optionsDownload full-size imageDownload as PowerPoint slide