Structural characterization of Sr-Ti and Ba-Ti catecholate complexes: single source precursors for SrTiO₃ and BaTiO₃ binary oxides

Titanium catecholate complex H₂Ti(cat)₃ (1) is synthesized from the direct reaction of Ti(OⁱPr)₄ with catechol (H₂cat). Treatment of 1 with NEt₃ gives ionic complex (HNEt₃)₂[Ti(cat)₃] (2). Reactions of 1 with SrCO₃ or with BaCO₃ afford the corresponding catecholate complexes with approximate composition Sr[Ti(cat)₃]·3H₂O (3) and Ba[Ti(cat)₃]·3H₂O (4), of which the formula was proposed according to the previous literature report and microanalytical data. Water soluble crystalline materials [Sr(H₂O)₅]₂[Ti₂O₂(cat)₄]·6H₂O (5) and [Ba(H₂O)₄(C₃H₆O)]₂[Ti₂O₂(cat)₄]·2C₃H₆O (6) are isolated in low yields by repeated recrystallization of 3 and 4 from a mixture of water and acetone at room temperature. The single crystal X-ray diffraction studies reveal that they fail to show the discrete Ti(cat)₃²⁻ unit as suggested previously, but they contain a dimeric [Ti₂O₂(cat)₄]⁴⁻ fragment with two bridging oxo ligands and two chelating catecholate ligands associated with each Ti atom. The latter is further linked to the hydrated Sr²⁺ or the Ba²⁺ counter cations through axial oxygen atoms of the catecholate ligands and the unique bridging oxo ligand. Crystal data for 5: a = 7.8251(1), b = 11.3739(2), c = 11.4980(2) Å, α = 91.942(1), β = 100.441(1), γ = 103.061(1)° with Z = 1 in space group P1̄. For complex 6: a = 9.6450(3), b = 10.2092(3), c = 13.3098(4) Å, α = 18.192(1), β = 85.876(1), γ = 73.475(1)° with Z = 1 in space group P1̄. Conversion to the respective SrTiO₃ and BaTiO₃ phases by calcination under oxygen atmosphere is confirmed by TG analysis and X-ray powder analysis.