Two new Co coordination polymers with multifunctional 5-amino-2,4,6-tribromoisophthalic acid and flexible isomeric bis(imidazole) ligands: preparation, crystal structure and characterization.

Two new Co coordination polymers (CPs), namely, catena-poly[[[(5-amino-2,4,6-tribromobenzene-1,3-dicarboxylato-κO)aquacobalt(II)]-bis[μ-1,3-bis(imidazol-1-ylmethyl)benzene-κN:N']] 4.75-hydrate], {[Co(CHBrNO)(CHN)(HO)]·4.75HO}, (1), and poly[(μ-5-amino-2,4,6-tribromobenzene-1,3-dicarboxylato-κO:O)[μ-1,2-bis(imidazol-1-ylmethyl)benzene-κN:N']cobalt(II)], [Co(CHBrNO)(CHN)], (2), have been synthesized successfully by the assembly of multifunctional 5-amino-2,4,6-tribromoisophthalic acid (HATBIP) and Co ions in the presence of the flexible isomeric bis(imidazole) ligands 1,3-bis(imidazol-1-ylmethyl)benzene (mbix) and 1,2-bis(imidazol-1-ylmethyl)benzene (obix). The isomeric mbix and obix ligands have a big influence on the structures of CPs (1) and (2). CP (1) is composed of chains of nanometre-sized elliptical rings, in which the Co atom exhibits a distorted octahedral coordination geometry and ATBIP acts as a monodentate ligand. Two adjacent chains are interlinked by π-π stacking interactions and hydrogen bonds, resulting in a supramolecular double chain. Hydrogen-bonded R(16) rings extend adjacent supramolecular double chains into a two-dimensional supramolecular layer. Halogen bonding and a hydrogen-bonded R(8) ring further link the two-dimensional supramolecular layers, leading to the formation of a three-dimensional supramolecular network. The Co ion in CP (2) is tetracoordinated, exhibiting a distorted tetrahedral configuration. The ATBIP ligand exhibits a bis(monodentate) coordination bridging mode, linking adjacent Co ions into zigzag chains, which are further bridged by the auxiliary bridging obix ligand, resulting in a two-dimensional (4,4) topological network. Interlayer hydrogen and halogen-halogen bonding further extend the two-dimensional layers into a three-dimensional supramolecular network. A detailed analysis of the solid-state UV-Vis-NIR diffuse-reflectance spectra of (1) and (2) indicates that a wide optical band gap exists in both (1) and (2). CP (1) exhibits an irreversible dehydration-rehydration behaviour.