Highly symmetric organic ligand-capped Lindqvist structures derived from 3d-elements

Abstract

Interaction of M(acac)₂ with Ti(OEt)₄ associated with hydrolysis or thermolysis provides with high yields, through the decomposition of the initially formed M₂Ti₂(acac)₄(OEt)₈, M = Ni (1), Co (2), poorly soluble Lindqvist-type products with the formula M₅TiO(acac)₆(OEt)₆, M = Ni (3), Co (4), Mg (5). The compounds 3–5 are isostructural and display a perfect statistical disorder between the OEt groups and 1/2 of the β-diketonate, acac, ligand. The metal atoms constitute a regular octahedron, centered by the oxo-oxygen atom. There is only one symmetrically independent metal site, which imposes it to contain 1/6 of titanium and 5/6 of the divalent metal atom. The oxygen atoms on the surface of the octahedron are doubly bridging and are located above the twelve edges of the octahedron, as is usual for the Lindqvist-type structures. The coordination of the metal atoms is completed by a single terminal oxygen atom. The complexes 3–5 are volatile with decomposition, releasing only homometallic products into the gas phase in the case of 3 and 4, but with a considerable contribution of molecular evaporation in the case of 5. The intermediates 1 and 2 belong to the tetramolybdate, M₄O₁₆, structure type. Highly soluble 1 and 2 can be considered as attractive molecular precursors for the incorporation of 3 and 4 into porous matrices for the preparation of combustion catalysts. Application of the same synthetic approach to n-propoxide complexes offers in a Co–Ti system a derivative with a different composition and resolved metal and ligand disorder, Co₄Ti₂O(acac)₄(OⁿPr)₁₀ (6), which, however, still follows the Lindquist type and displays high (tetragonal) symmetry for both the molecule and the crystal structure.