Tuning Metal Stoichiometry in Heterometal Alkoxides: First Structurally Characterised Molecular Precursor to BaZrO3

Abstract

Manipulable metal ratios are accessible in a series of heterometal Ba-Zr alkoxides by changing the size and the charge of the ligands. The synthesis and characterisation of four mixed-metal Ba-Zr species [BaZr2(OtBu)10] (1), [Ba2Zr(OtBu)8(tBuOH)(THF)2] (2), [BaZr(OtBu)6(THF)2] (3) and [BaZr(OH)(OiPr)5(iPrOH)3]2 (4), based on different Ba/Zr stoichiometries, are reported. The molecular structures of 1, 2, and 4 have been determined by single-crystal X-ray diffraction studies. 1 exhibits a bow-tie structural motif in which a trigonally distorted six-coordinate barium atom is chelated by two bipyramidal {Zr(OtBu)5}− units. 2 reveals a triangular “Ba2Zr(μ3-OtBu)2(μ2-OtBu)3” core structure formed by the fusion of three octahedra built about two Ba atoms and a Zr atom. The structure is unique in terms of the distribution of terminal ligands at each of the metal centers. One of the barium atoms possesses one –OtBu and one tBuOH as terminal ligands, while the terminal ligands on the second barium atom are two THF molecules; the zirconium center bears –OtBu groups as the terminal ligands. Hydrolysis of compound 3 with one equivalent of water, followed by alcoholysis (iPrOH) of the resulting product, gave 4. The structure of 4 shows a planar tetrametallic Ba2Zr2 frame capped by two μ3-hydroxo ligands. The coordination figure of the zirconium atoms resembles a distorted octahedron, whereas the geometry of the barium centers is best described as capped trigonal prisms. Both 3 and 4 are excellent precursors to morphologically pure BaZrO3 ceramic by the sol-gel process.