BIMETALLIC ALKOXIDES OF NIOBIUM

The formation of the I-III group metal alkoxoniobates has been studied using the physicochemical analysis method, namely the investigation of solubility isotherms for the M(OR)(n)-Nb(OR)(5)-ROH systems (M = Li, Ba, Sc, La, R = 2Et, Pr-i). The crystallization of the following complexes has been observed: LiNb(OEt)(6) (I), [LiNbO(OEt)(4)(EtOH)](2) (II), [BaNbO(OEt)(5)(EtOH)(1,5)](4) (III), BaNb2(OPri)(12)((PrOH)-O-i)(2) (IV) and LaNb2(OPri)(13) (V). It has been found that the bimetallic isopropoxide of Sc and Mb does not exist in the solid state. The oxo complexes Il and III are formed on storage (or more rapidly on refluxing) of the solutions of non-ore compounds in alcohols in the presence of dry air and are, in fact, the products of these compounds oxidation by oxygen. The structure of II is built of dimeric molecules with the metal-oxygen core analogous to that of Ti-4(OMe)(16). The molecule of III is constructed around the [Ba4O4] cubane-like unit, whose oxygen vertices are occupied by the oxo atoms of NbO(OEt)(5) octahedra. The structures of the molecules of IV, V and recently studied Nb(OPri)(5) are based on the common motif featuring the M(2)O(2) four-membered cycles in edge sharing octahedra. All the observed [NbMNb] units are nonlinear, the molecule of V being asymmetric, for the central [LaO6] octahedron is sharing simultaneously an edge with one [NbO6] octahedron and a face with the other. The conditions leading to the formation of oxo groups and the influence of the latter on the complexation process have been discussed. The literature data on the existence of 3d transition metals alkoxoniobates have not been confirmed. It has been found that interaction of Nb and Co or Ni isopropoxides takes place only due to preliminary micro-hydrolysis or pyrolysis and leads to formation of oxoalkoxoniobates.