Assembly and photocatalysis of two novel 3D Anderson-type polyoxometalate-based metal–organic frameworks constructed from isomeric bis(pyridylformyl)piperazine ligands

Abstract

Two novel Anderson-type polyoxometalates (POMs)-based metal–organic frameworks (MOFs), namely, H{Cu₂(μ₂-OH)₂L¹[CrMo₆(OH)₆O₁₈]}·4H₂O (1), {Cu₂L²[CrMo^VI₅Mo^V(OH)₆O₁₈](H₂O)₄}·4H₂O (2) (L¹ = N,N′-bis(3-pyridinecarboxamide)-piperazine, L² = N,N′-bis(4-pyridinecarboxamide)-piperazine), are hydrothermally synthesized and structurally characterized by single-crystal X-ray diffraction, IR spectra, powder X-ray diffraction (PXRD) and thermogravimetric analyses (TGA). In complex 1, the hexadentate [CrMo₆(OH)₆O₁₈]³⁻ polyoxoanion bridges the Cu^II ions to generate a 2D Cu–POM inorganic layer, which is further extended by the μ₂-bridging L¹ ligands (via ligation of pyridyl nitrogen atoms) to form a 3D MOF with a 4,6-connected {4⁴·6¹⁰·8}{4⁴·6²} topology. Complex 2 is also a 3D POM-based MOF exhibiting a {4²·8⁴} topology, which is constructed from the quadridentate [CrMo^VI₅Mo^V(OH)₆O₁₈]⁴⁻ polyoxoanions and μ₄-bridging L² ligands (via ligation of pyridyl nitrogen and carbonyl oxygen atoms). The different coordination modes of POM polyanions and the isomeric bis(pyridylformyl)piperazine ligands play key roles in the construction of the title complexes. In addition, the photocatalytic activities of the title complexes on the degradation of methylene blue (MB) under UV, visible light and sunlight irradiation have been investigated in detail.