pH-controlled assembly of hybrid architectures based on Anderson-type polyoxometalates and silver coordination units

Abstract

Three new architectures based on Anderson-type polyoxometalates, (3-H₂pya)[(3-Hpya)₂Ag][AgAlMo₆H₆O₂₄]·3H₂O 1, HNa₂[(3-pya)(3-Hpya)Ag]₂[AlMo₆H₆O₂₄]·8H₂O 2 and [(3-Hpya)₂Ag][(H₂O)₂Ag]₂[AlMo₆H₆O₂₄]·2H₂O 3 (3-Hpya = 3-(3-pyridyl)acrylic acid), have been synthesized at the different pH values and characterized by elemental analysis, IR spectroscopy, TG analysis, powder X-ray diffraction and single crystal X-ray diffraction. Compound 1 was obtained at lower pH value (2.50), and represents a 3D host–guest compound containing the Ag-3-Hpya coordination complex guest and the 3D [AgAlMo₆H₆O₂₄]²⁻ host. The host framework exhibits a 4-connected diamond topology, and is constructed from [AlMo₆H₆O₂₄]³⁻ clusters connected by Ag⁺ cations. When the pH value was increased slightly, compound 2 was obtained with a 1D chain structure built up of Anderson polyoxoanions, Ag-3-Hpya coordination complexes and binuclear sodium clusters. By further increasing the pH value (3.50), compound 3 was isolated as a 2D network in which [AlMo₆H₆O₂₄]³⁻ clusters are linked together by Ag⁺ cations and Ag-3-Hpya coordination complexes. Their structural differences reveal that the pH value of the reaction system is the key factor influencing the structure and topology of three compounds. The UV-visible-NIR diffuse reflectivity spectra of 1–3 show that they can be regarded as a wide gap semiconductor. Furthermore, the pyrolysis of 1–3 produces three nanocomposites 1′–3′ composed of silver microparticles dispersed in the metal oxides. The photocatalytic properties of 1′–3′ have been investigated.