TUNING METAL STOICHIOMETRY IN HETEROMETAL ALKOXIDES - FIRST STRUCTURALLY CHARACTERIZED MOLECULAR PRECURSOR TO BAZRO3

Manipulable metal ratios are accessible in a series of heterometal Ba- Zr alkoxides by changing the size and the charge of the ligands. The s ynthesis 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 differ ent Ba/Zr stoichiometries, are reported. The molecular structures of 1 , 2, and 4 have been determined by single-crystal X-ray diffraction st udies. 1 exhibits a bow-tie structural motif in which a trigonally dis torted six-coordinate barium atom is chelated by two bipyramidal {Zr(O tBu)5}(-) units. 2 reveals a triangular ''Ba2Zr(mu(3)-OtBu)(2)(mu(2)-O tBu)(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. On e of the barium atoms possesses one -OtBu and one tBuOH as terminal li gands, while the terminal ligands on the second barium atom are two TH F molecules; the zirconium center bears -OtBu groups as the terminal l igands. Hydrolysis of compound 3 with one equivalent of water, followe d by alcoholysis (iPrOH) of the resulting product, gave 4. The structu re of 4 shows a planar tetrametallic Ba2Zr2 frame capped by two mu(3)- hydroxo ligands. The coordination figure of the zirconium atoms resemb les a distorted octahedron, whereas the geometry of the barium centers is best described as capped trigonal prisms. Both 3 and 4 are excelle nt precursors to morphologically pure BaZrO3 ceramic by the sol-gel pr ocess.