The quest for mixed-metal oxide precursors based on bismuth: synthesis and molecular structure of BiTi2(µ3-O)(µ-OPri)4(OPri)5 and [Bi2(µ-OPri)2(OPri)2(acac)2]∞ (acac = acetylacetonate)†

Abstract

Various routes to mixed-metal Bi–Ti species were investigated. Various heterometallic alkoxides could be isolated by using hydrolysis reactions. The mixed-metal alkoxide BiTi₂(µ₃-O)(µ-OPrⁱ)₄(OPrⁱ)₅ was obtained by reaction between titanium isopropoxide and a bismuth oxoisopropoxide formed in situ by controlled microhydrolysis and subsequent alcoholysis of Bi(OBu^t)₃. It was characterised by X-ray crystallography, ¹H NMR and Fourier-transform IR spectroscopy. The structure is based on an isosceles triangular framework with a central triply bridging oxo ligand. Bismuth is four-co-ordinate with a stereochemically active lone pair. The titanium atoms are six-co-ordinate with a severely distorted octahedral environment. Synthesis and characterisation of the bismuth–titanium ethoxides BiTi₂O(OEt)₉ and Bi₄Ti₃O₄(OEt)₁₆ were achieved, as well as their evaluation as oxide precursors. The latter leads, after complete hydrolysis and thermal treatment at 450 °C, to the pure crystalline Bi₄Ti₃O₁₂ perovskite phase. The synthesis and structural characterisation of [Bi₂(µ-OPrⁱ)₂(OPrⁱ)₂(η²-acac)₂]_∞ (acac = acetylacetonate) are also reported. The basic structural unit is a dimer in which the five-co-ordinated metals are linked by dissymmetrical Bi–OR bridges. Each β-diketonate is chelating one metal. Chains of dimers run along the c axis via terminal semibridging isopropoxide ligands, ensuring six-co-ordination for the bismuth atoms.