Crystallization of organically templated phosphomolybdate cluster-based solids from aqueous solution

The paper reports the synthesis and structural characterization of several organic–inorganic solids involving phosphomolybdate clusters. The Strandberg-type {P 2 Mo O 23 } and the lower-valent {P 4 Mo O 31 } cluster based solids were isolated in the presence of en (ethylenediamine) by controlling pH of the reaction medium. The lower-valent cluster invariably requires the presence of a suitable metal cation for further stabilization. A detailed investigation of the system was carried out where three different weak acids viz. oxalic acid, succinic acid and glycine were used in the entire pH range (1–12). Our results establish that the organic amine ( en ) is alone capable of reducing the molybdenum core in the absence of an organic acid at a suitable pH. Hence, pH of the reaction medium combined with suitable temperature favors the formation of lower-valent phosphomolybdate cluster. Higher pH favors the precipitation of a new sodium hydrogen phosphate. Graphical Abstract Two types of phosphomolybdate clusters, fully oxidised Strandberg-type, {P 2 Mo 5 VI O 23 } and fully reduced {P 4 Mo 6 V O 31 } are crystallized with appropriate inorganic and/or organic cations from aqueous solutions containing sodium molybdate and an organic amine (morpholine, HMTA and ethylenediamine). The paper discusses the influence of p H and temperature in dictating the nature of the cluster and the supramolecular interactions.