pH-dependent two novel 3D polynuclear cobalt(ii) cluster-based metal–organic frameworks constructed from a tri-pyridyl-bis-amide and a polycarboxylate: assembly, structures and properties

Abstract

The hydrothermal reaction of CoCl₂ with N,N′-bis(pyridine-3-yl)pyridine-3,5-dicarboxamide (L) and 1,3,5-benzenetricarboxylic acid (H₃BTC) mixed ligands affords two novel polynuclear cobalt cluster-based metal–organic frameworks (MOFs), [Co₃(L)₂(BTC)₂(H₂O)₂]·6H₂O (1) and {[Co₂(L)(BTC)(OH)]·2.5H₂O}₂ (2). Single crystal X-ray diffraction analysis reveals that the Co^II ions in complex 1 are connected by BTC anions with a four-coordinated mode to form a two dimensional (2D) undulating layer based on dinuclear [Co₂(COO)₄] units, which is extended into a 3D (3, 4, 4, 5)-connected MOF by the L ligand with μ₃-bridging coordination mode. In complex 2, the BTC ligand displays a six-coordinated mode to connect Co^II ions into a 2D corrugated layer derived from tetranuclear [Co₄(μ₃-OH)₂(COO)₆] motifs, and the layers are further extended into a 3D (3, 8)-connected tfz-d MOF via μ₂-bridging L ligands. The title complexes exhibit a (4²·6³·8⁵)₂(4²·6³·8)₂(6·8²)₂(6²·8⁴) topology for 1 and a (4³)₂(4⁶·6¹⁸·8⁴) topology for 2. The pH value, as well as the various coordination modes of the tri-pyridyl-bis-amide L and BTC ligands, play a crucial role in the construction of the diverse topological structures of 1 and 2. Complex 1 exhibits excellent photocatalytic activity for dye degradation under UV light. In addition, the electrochemical behaviour and magnetic and fluorescent properties of the two complexes have also been investigated.