Synthesis and structural study of polynuclear lithium oxoethoxo tantalates

LiTa 9 O 1 3 (OEt) 2 0 .2EtOH (I) is a side product of the electrochemical synthesis of Ta(OEt) 5 carried out using LiCl as a supporting electrolyte. This product crystallizes in a minor yield upon the storage of crude liquid Ta(OEt) 5 . Treatment of I with a hot ethanol solution of LiOEt yields Li 8 Ta 8 O 1 4 (OEt) 2 0 .12EtOH (II). The structures of I and II have been determined using X-ray crystallography. Molecule I, LiTa 9 H(μ 3 -O) 4 (μ-O) 9 (μ-OEt) 6 (OEt) 1 5 (EtOH), is located on a threefold axis that passes through the lithium atom, which coordinates one disordered solvation ethanol molecule and three μ 3 -oxo groups belonging to the [Ta 6 O 6 (OR) 1 5 ] ring fragment made of six octahedra. The next layer is formed by the [Ta 3 O 7 (OR) 6 ] fragment made of three octahedra linked to the previous fragment through six μ-oxo ligands. In molecule II, Li 8 Ta 8 H 8 (μ 4 -O) 2 (μ 3 -O) 8 (μ-O) 4 (μ 3 -OEt) 2 (μ-OEt) 1 0 (OEt) 1 6 .4EtOH, one can distinguish two groups of four [TaO 6 ] octahedra that form [Ta 4 ] tetrahedra sharing two vertices represented by μ 4 -oxo groups. Four Li atoms are located between the [Ta 4 ] tetrahedra thus forming a "interlayer" comprising two distorted [LiO 4 ] tetrahedra and two distorted [LiO 5 ] tetragonal pyramids. The other four tetrahedrally coordinated Li atoms are located on the periphery of the molecule and are linked through [O(OR)] edges with Ta atoms. Each Ta atom coordinates three terminal OR groups and one μ-OR group. Four solvation ethanol molecules are hydrogen-bonded with the terminal OR groups at Ta atoms.