A series of novel Anderson-type polyoxometalate-based MnII complexes constructed from pyridyl-derivatives: assembly, structures, electrochemical and photocatalytic properties

Abstract

Three novel Anderson-type polyoxometalates (POMs)-based metal–organic complexes (MOCs), namely, [Mn₂(4-pdtz)₂[CrMo₆(OH)₅O₁₉](H₂O)₄] (1), {Mn(3-dpye)_0.5[CrMo₆(OH)₆O₁₈](H₂O)}·(3-H₂dpye)_0.5 (2), [Mn₂(3-H₂dpye)(TeMo₆O₂₄)(H₂O)₆]·4H₂O (3) (4-pdtz = 4-pyridino-bistriazol, 3-dpye = N,N′-bis(3-pyridinecarboxamide)-1,2-ethane), were 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 [CrMo₆(OH)₅O₁₉]⁴⁻ (CrMo₆) polyoxoanions bridge the Mn^II ions to generate a 1D Mn–CrMo₆ inorganic chain, on which the 4-pdtz ligands hang. In 2, the adjacent CrMo₆ polyoxoanions are linked by Mn^II ions to form a 2D inorganic layer, which is further linked by 3-dpye forming a 3D metal–organic framework (POMOF). Interestingly, the large voids of the 3D POMOF accommodate free protonated 3-H₂dpye molecules. In complex 3, the adjacent TeMo₆O₂₄⁶⁻ (TeMo₆) polyoxoanions are linked by Mn^II ions to form a 1D inorganic chain, the protonated 3-H₂dpye act as bidentate ligands connecting two Mn^II centers with the oxygen atoms from amide groups, forming a steady 2D metal organic layer. The effects of various coordination behaviors of the pyridyl-based ligands and central metal ions on the structures of the title complexes have been discussed. The title complexes exhibit excellent electrocatalytic activity towards the reduction of bromate and hydrogen peroxide. In addition, the redox potentials of complex 3 is highly sensitive to pH and may be used as a kind of potential pH sensor. The photocatalytic properties of the title complexes under UV and visible irradiation have been investigated in detail.