A molecular placeholder strategy gives access to a family of transition-metal functionalized vanadium oxide clusters

Systematic access to metal-functionalized polyoxometalates has thus far been limited to lacunary tungsten oxide and molybdenum oxide clusters. The first controlled, stepwise bottom-up assembly route to metal-functionalized molecular vanadium oxides is now presented. A di-vacant vanadate cluster with two metal binding sites, (DMA)₂[V₁₂O₃₂Cl]³⁻ (DMA=dimethylammonium) is formed spontaneously in solution and characterized by single-crystal X-ray diffraction, ESI mass spectrometry, ⁵¹V NMR spectroscopy, and elemental analyses. In the cluster, the metal binding sites are selectively blocked by hydrogen-bonded DMA placeholder cations. Reaction of the cluster with transition metals TM (Fe³⁺, Co²⁺, Cu²⁺, Zn²⁺) gives access to mono-functionalized vanadate clusters (DMA)[{TM(L)}V₁₂O₃₂Cl]ⁿ⁻ (L=ligand). Metal binding is accomplished by significant distortions of the vanadium oxide framework reminiscent of a pincer movement. Cluster stability under technologically relevant conditions in the solid-state and solution is demonstrated.