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

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.