Mixed-valent, heteroleptic homometallic diketonates as templates for the design of volatile heterometallic precursors

Abstract

A novel series of mixed-valent, heteroleptic transition metal diketonates that can be utilized as prospective single-source precursors for the low-temperature preparation of oxide materials are reported. The first mixed-valent iron β-diketonates with different Fe^III/Fe^II ratios have been synthesized by applying the mixed-ligand approach. Based on nearly quantitative reaction yields and analysis of iron–oxygen bonds, these compounds were formulated as [Fe^III(acac)₃][Fe^II(hfac)₂] (1) and [Fe^II(hfac)₂][Fe^III(acac)₃][Fe^II(hfac)₂] (2). In the above heteroleptic complexes, the Lewis acidic, coordinatively unsaturated Fe^II centers chelated by two hfac (hexafluoroacetylacetonate) ligands with electron-withdrawing substituents maintain bridging interactions with oxygen atoms of electron-donating acac (acetylacetonate) groups that chelate the neighboring Fe^III atoms. Switching the ligands on Fe^III and Fe^II atoms in starting reagents resulted in the instant ligand exchange between iron centers and in yet another polynuclear homometallic diketonate [Fe^II(hfac)₂][Fe^III(acac)₂(hfac)][Fe^II(hfac)₂] (3) that adheres to the same bonding pattern as in complexes 1 and 2. The proposed synthetic methodology has been extended to design heterometallic diketonates with different M : M′ ratios. Homometallic parent molecules have been used as templates to obtain heterometallic mixed-valent [Fe^III(acac)₃][Mn^II(hfac)₂] (4) and [Ni^II(hfac)₂][Fe^III(acac)₃][Ni^II(hfac)₂] (5) complexes. The combination of two different diketonate ligands with electron-donating and electron-withdrawing substituents was found to be crucial for maintaining the above mixed-valent heterometallic assemblies. Theoretical investigation of two possible “isomers”, [Fe^III(acac)₃][Mn^II(hfac)₂] (4) and [Mn^III(acac)₃][Fe^II(hfac)₂] (4′) provided an additional support for the metal site assignment giving a preference of 9.78 kcal mol⁻¹ for the molecule 4. Heterometallic complexes obtained in the course of this study have been found to act as effective single-source precursors for the synthesis of mixed-transition metal oxide materials M_xM^′_2−xO₃ and M_xM^′_1−xO. The title highly volatile precursors can be used for the low-temperature preparation of both amorphous and crystalline heterometallic oxides in the form of thin films or nanosized particles that are known to operate as efficient catalysts in oxygen evolution reaction.