Hetero<i>tri</i>metallic Precursor with 2:2:1 Metal Ratio Requiring at Least a Pentanuclear Molecular Assembly

Abstract

This work represents an important step in the quest to make hetero<i>multi</i>metallic molecules featuring specific metal types and complicated metal ratios. The rational design, synthesis, and characterization of a complex hetero<i>tri</i>metallic single-source molecular precursor for the next generation sodium-ion battery cathode material, Na₂Mn₂FeO₆, is described. A unique pentametallic platform [Mn^II(ptac)₃-Na-Mn^III(acac)₃-Na-Mn^II(ptac)₃] (<b>1</b>) was derived from the known polymeric structure of [NaMn^II(acac)₃]_∞, through a series of elaborate design procedures, such as mixed-ligand, unsymmetric ligand, and mixed-valent approaches. Importantly, the application of those techniques results in a molecule with distinctively different transition metal positions in terms of ligand environment and oxidation states. An isovalent substitution of Fe^III for the central Mn^III ion forms the target hetero<i>tri</i>metallic precursor [Mn^II(ptac)₃-Na-Fe^III(acac)₃-Na-Mn^II(ptac)₃] (<b>3</b>) with an appropriate metal ratio of Na:Mn:Fe = 2:2:1. The arrangement of metal ions and ligands in this pentametallic assembly was confirmed by single crystal X-ray investigation. The unambiguous assignment of the positions and oxidation states of the Periodic Table neighbors Fe and Mn in <b>3</b> has been achieved by a combination of investigative techniques that include synchrotron resonant diffraction, X-ray multiwavelength anomalous diffraction, X-ray fluorescence spectroscopy, Mössbauer spectroscopy, and gas-phase DART mass spectrometry. The hetero<i>tri</i>metallic single-source precursor <b>3</b> was shown to exhibit a clean decomposition pattern yielding the phase-pure P2-Na₂Mn₂FeO₆ quaternary oxide with high uniformity of metal ion distribution as confirmed by electron microscopy.