The presence of mixed-valent silver in the uranyl phenylenediphosphonate framework

Abstract

In this work, a new 2D layered silver uranyl phosphonate compound Ag⁰_1.51[Ag^I₄(UO₂)₃(O₃PC₆H₄PO₃)(O₃PC₆H₄PO₃H)₂] (denoted as compound 1) was synthesized by reaction of 1,2-phenylenediphosphonic acid (H₄PDPA) with UO₂(NO₃)₂·6H₂O and AgNO₃ under mild hydrothermal conditions. The uranyl layered framework consists of two types of UO₈ and UO₇ coordination units simultaneously connected by two kinds of phenylenediphosphonate linkers. To our surprise, the presence of both silver ions and metallic silver atoms in the free space between the layers is identified by the single crystal X-ray diffraction, EDS, and XPS techniques. The ratio of Ag⁺ and Ag⁰ was estimated to be 2.81 : 1, based on the primary peak area of Ag 3d_5/2 from the XPS results, which corroborates with the ratio of 2.65 : 1 obtained from the XRD results. More importantly, the incorporation of the mixed-valent silver sites results in the quenching of the fluorescent emission of the uranyl phosphonate compound as evidenced by its single crystal fluorescence spectrum.