Cationic D4R zinc phosphate–anionic polyoxometalate hybrids: synthesis, spectra, structure and catalytic studies

Abstract

A facile synthetic strategy for the generation of a new class of inorganic–organic–inorganic hybrids is reported. Replacement of labile DMSO ligands from the preformed Double-Four-Ring (D4R) zinc phosphate cluster [Zn(dipp){(CH₃)₂SO}]₄ by the addition of 4-aminopyridine (4-ampyr) results in the isolation of another discrete D4R cluster [Zn(dipp)(4-ampyr)]₄ (1), which forms a hydrogen-bonded framework in solid-state. If 1-methyl-4,4′-bipyridinium salts are employed instead, tetra-cationic D4R cluster [Zn(dipp)(L)]₄[PF₆]₄ (2) and [Zn(dipp)(L)]₄[ClO₄]₄ (3) (L = 1-methyl-4,4′-bipyridinium, dipp = 2,6-di-iso-propylphenylphosphate) are isolated. Compound 2 was reacted with three different polyoxometalates, [TBA]₄[Mo₈O₂₆] (POM-1), [TBA]₄[PMo₁₁VO₄₀] (POM-2), and [TBA]₄[SiMo₁₂O₄₀] (POM-3) to obtain amorphous hybrids, [Zn(dipp)(L)]₄[Mo₈O₂₆] (4), [Zn(dipp)(L)]₄[PMo₁₁VO₄₀] (5), and [Zn(dipp)(L)]₄[SiMo₁₂O₄₀] (6), respectively. All the hybrid materials have been characterized by analytical and spectroscopic studies. The molecular structure of 1 has also been determined by single-crystal XRD measurements. N₂ gas sorption analyses show moderate BET surface area and also establish the mesoporous nature of the hybrids 4–6. The catalytic potential of hybrids 4, 5 and 6 has been explored towards epoxidation of cyclohexene.