Metal ions induced various polymolybdate-based metal–organic complexes with a pyridyl-amide-carboxylate ligand: Synthesis, structures and selective separation of cationic dyes

Abstract Three polymolybdate-based metal–organic complexes (MOCs) constructed from a pyridyl-amide-carboxylate ligand, 6-(pyridine-3-ylcarbamoyl)picolinic acid (HPPA), namely, H 2 [Cd 2 (PPA) 2 (H 2 O) 2 ](β-Mo 8 O 26 )·2CH 3 OH·9H 2 O ( 1 ), H 2 [Co 4 (PPA) 2 (H 2 O) 12 (Mo 10 O 34 )]·6H 2 O ( 2 ), [Ni 4 (PPA) 4 (H 2 O) 6 (β-Mo 8 O 26 )]·8H 2 O ( 3 ), have been successfully synthesized under solvothermal (methanol–water mixed solvent) conditions. Characterized by single-crystal X-ray diffraction analyses, 1 exhibits a 3D supramolecule framework, which is built from binuclear [Cd 2 (PPA) 2 (H 2 O) 2 ] 2+ subunits and β-[Mo 8 O 26 ] 4− anions through the non-covalent interactions; In 2 , Co II was used and the [Mo 10 O 34 ] 8− anion was in situ constructed, which were linked by [Co 2 (PPA)(H 2 O) 6 ] 3+ subunits to yield a 1D double chain. The 1D chains were further linked by H-bond to afford a 3D supramolecular framework; 3 is also a 3D supramolecular structure based on 1D [Ni 2 (PPA) 2 (H 2 O) 2 (β-Mo 8 O 26 )] 2− anionic chains and the free [Ni 2 (PPA) 2 (H 2 O) 4 ] 2+ units with the H-bond interaction. The adsorption behaviors for organic dyes of the title complexes were investigated. All the title complexes show efficient adsorption capacity for cationic dyes methylene blue (MB) and gentian violet (GV), especially complex 3 can selectively separate MB and GV from the mixture of MB&MO/GV&MO, respectively. In addition, the electrochemical properties of the title complexes also have been reported.