Crystallization of Calcium Vanadate Solids from Solution: A Metathetic Route

Microwave mediated metathetic solid state reaction between NaVO3 or NH4VO3 and CaCl2 followed by dissolution in aqueous solution resulted in the crystallization of four calcium vanadate hydrates: CaV2O6·4H2O (1), CaV2O6·4.25H2O (2), CaV2O6·5H2O (3), and (NH4)2[{Ca(H2O)7}2V10O28]·2H2O (4). Single crystal X-ray diffraction analysis of the three pseudopolymorphs 1−3 revealed that the crystal packing is dominated by the presence of ladder-type {V2O6} chains made of edge-shared VO5 square pyramids interconnected via calcium hydrate chains. The way the system accomplishes the variation in water of hydration through interesting H-bonding interactions between calcium hydrates and the vanadate chains is an example of smart crystal engineering; 3 shows the occurrence of an unusual tetrameric H-bonded water cluster. In contrast, 4 is a decavanadate cluster based solid. The paper proposes a retrosynthetic approach to understand the crystal packing of the vanadate solids through intuitive mechanisms that provide insights into the aggregations occurring in solution and supramolecular interactions observed in the solid state.