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.