Mixed-Ligand Approach to Design of Heterometallic Single-Source Precursors with Discrete Molecular Structure

Heterometallic single-source precursors for the Pb/Fe = 1:1 oxide materials, PbFe­(β-dik)4 (β-dik = hexafluoroacetylacetonate (hfac, 1), acetylacetonate (acac, 2), and trifluoroacetylacetonate (tfac, 4)), have been isolated by three different solid-state synthetic methods. The crystal structures of heterometallic diketonates 1, 2, and 4 were found to contain polymeric chains built on alternating [Fe­(β-dik)2] and [Pb­(β-dik)2] units that are held together by bridging M–O interactions. Heterometallic precursors are highly volatile, but soluble only in coordinating solvents, in which they dissociate into solvated homometallic fragments. In order to design the heterometallic precursor with a proper metal/metal ratio and with a discrete molecular structure, we used a combination of two different diketonate ligands. Heteroleptic complex Pb2Fe2­(hfac)6­(acac)2 (5) has been obtained by optimized stoichiometric reaction of an addition of homo-Fe­(acac)2 to heterometallic Pb2Fe­(hfac)6 (3) diketonate that can be run in solution on a high scale. The combination of two ligands with electron-withdrawing and electron-donating groups allows changing the connectivity pattern within the heterometallic assembly and yields the precursor with a discrete tetranuclear structure. In accord with its molecular structure, heteroleptic complex 5 is soluble even in noncoordinating solvents and was found to retain its heterometallic structure in solution. Thermal decomposition of heterometallic precursors in air at 750 °C resulted in the target Pb2­Fe2O5 oxide, a prospective multiferroic material. Prolonging the annealing time or increasing the decomposition temperature leads to another phase-pure lead–iron oxide Pb­Fe12O19 that is a representative of the important family of magnetic hexaferrites.