Substitution features in the isomorphous replacement series for metal-organic compounds (NbxTa1−x)4O2(OMe)14(ReO4)2, x=0.7, 0.5, 0.3—Single-source precursors of complex oxides with organized porosity

Trimetallic oxoalkoxide complexes (Nb0.7Ta0.3)4O2(OMe)14(ReO4)2 (I), (Nb0.3Ta0.7)4O2(OMe)14(ReO4)2 (II) and (Nb0.5Ta0.5)4O2(OMe)14(ReO4)2 (III) were obtained by the interaction of rhenium heptoxide (VII) Re2O7 with niobium and tantalum alkoxides M2(OMe)10 (M=Nb, Ta) in toluene. The centrosymmetric molecules (I)–(III) can be considered as a product of condensation of two M2(OMe)9(OReO3) molecules with the formation of two oxo-bridges. The specific feature of the structure is the uneven distribution of metal atoms in the crystallographic positions, where one symmetry-independent position, connected via μ-O with a perrhenate ReO4− group, is predominantly occupied by niobium atoms, while the other one connected via alkoxide groups has a higher tantalum content. The distribution of Nb and Ta in the structure is truly even only for compound III. The niobium and tantalum content is varied to a different extent for I (less) and for II (more), which is apparently due to small differences in the sizes of these two cations, resulting in preferences for packing of different molecules in the structures. Thermal decomposition of (Nb1−xTax)4O2(OMe)14(ReO4)2 (x=0.3, 0.5, 0.7) in air leads to the formation of crystalline species of solid solutions based on tantalum and niobium oxides displaying semi-ordered pores with the size of 100−250 nm. In the dry nitrogen atmosphere, the decomposition leads to the amorphous complex oxides containing rhenium, niobium and tantalum.