Mixed-valent, heteroleptic homometallic diketonates as templates for the design of volatile heterometallic precursorsстатья
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Аннотация: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 b-diketonates with different FeIII/FeII 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 [FeIII(acac)3][FeII(hfac)2] (1) and [FeII(hfac)2][FeIII(acac)3][FeII(hfac)2]
(2). In the above heteroleptic complexes, the Lewis acidic, coordinatively unsaturated FeII 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 FeIII atoms. Switching the ligands on FeIII and FeII atoms in starting reagents
resulted in the instant ligand exchange between iron centers and in yet another polynuclear
homometallic diketonate [FeII(hfac)2][FeIII(acac)2(hfac)][FeII(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 : M0 ratios. Homometallic parent molecules have been used
as templates to obtain heterometallic mixed-valent [FeIII(acac)3][MnII(hfac)2] (4) and [NiII(hfac)2]-
[FeIII(acac)3][NiII(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”,
[FeIII(acac)3][MnII(hfac)2] (4) and [MnIII(acac)3][FeII(hfac)2] (40) provided an additional support for the metal
site assignment giving a preference of 9.78 kcal mol1 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 MxM0
2xO3 and MxM0
1xO. 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.
Introduction
One of the greatest technological challenges facing our global
well-being is the development of renewable energy sources.
Over the past thirty years, a great deal of research has been
focused on the development of new catalysts for the oxygen
evolution reaction