Synthesis, Characterization, and Stability of {Mo₁₃₂}-Type Capsules Containing Phosphorus Oxo Anion Ligands

This article deals with a set of {Mo₁₃₂}-type capsules containing phosphorus oxo anions as ligands. Two of them, with hypophosphite and phosphate ligands, respectively, were prepared and characterized by infrared (IR) spectroscopy and X-ray diffraction in Bielefeld more than two decades ago. We extended this pioneering work to phosphite and methylphosphonate ligands. All these capsules were prepared by ligand exchange in the acetate-containing {Mo₁₃₂} capsule and some of them contain residual acetate, depending on the incoming ligand and the pH of the reaction mixture. All of them were characterized by ¹H and ³¹P NMR spectroscopy in D₂O and one by single-crystal X-ray diffraction. These studies showed that side reactions occur. On the one hand, degradation of the {Mo₁₃₂} framework leads to the release of fragments that can be trapped as {MoO₃H}⁺ units by the ligands inside capsules. This was known for phosphate and is also observed for methylphosphonate. On the other hand, monitoring by NMR spectroscopy revealed that capsules slightly transform over time in solution as well as in the solid state, as evidenced by growth of characteristic signals of Strandberg-type complexes and reduced Keggin-type molybdophosphates. Decomposition, however, remains fairly low over a few months.