Robust 1D open rack-like architecture in coordination polymers of Anderson POMs [{Na4(H2O)14}{Cu(gly)}2][TeMo6O24] and [{Cu(en)2}3{TeW6O24}]: synthesis, characterization and heterogeneous catalyticepoxidation of olefines

Abstract

Two novel organic-inorganic hybrid tungsto- and molybdo- telurates having formula [{Na₄(H₂O)₁₄}{Cu(gly)}₂][TeMo₆O₂₄] (1){gly = glycine} and [{Cu(en)₂}₃{TeW₆O₂₄}]·6H₂O {en = ethyline-diamine} (2) based on Anderson type heteropolyoxometalates (POMs) have been synthesized and characterized by X-ray crystallography. Common structural feature of both 1 and 2 is the presence of a unique 1D open rack-like architecture, where the disc shaped Anderson POMs act as steps and cationic Cu-organic complexes act as handles of the rack. In 1 the independent structural unit is a 1D coordination polymer with the above mentioned rack type architecture, while in 2, these independent rack-like architectures are further extended to a 2D coordination polymer. Heterogeneous catalysis for the epoxidation of cyclohexene and styrene by complexes 1 and 2 showed very good catalytic efficiency resulting epoxides of ∼60% yield, with dialcohol formed by the hydrolysis of epoxides, as the other major product (∼28%). Cyclic voltammetric studies of [{Na₄(H₂O)₁₄}{Cu(gly)}₂][TeMo₆O₂₄] (1) in aqueous KCl solution indicates that the redox changes occur only on the copper centers and supported by carrying out parallel experiments on the precursors like ([Cu(gly)₂]²⁺ and [TeMo₆O₂₄]⁶⁻, under the identical experimental conditions. The E_1/2 = 0.662, −0.142 and −0.332 V(vs. SCE) correspond to Cu(III) → Cu(II), Cu(II) → Cu(I) and Cu(I) → Cu(0) reductions, respectively. Thermal analyses reveal identical phase transition reactions with an exothermic peak in the DTA curve at 380 °C for 1 and an endothermic peak appears at comparatively higher temperature (408 °C) for 2 manifesting the higher stability of tungstane based POM over the molybdenum ones. EPR as well as magnetic moment results indicate that both the complexes 1 and 2 are paramagnetic with one unpaired electron per copper(II) ion.