A three body problem: a genuine heterometallic molecule a mixture of two parent heterometallic molecules.

This work raises a fundamental question about the "real" structure of molecular compounds containing three different metals: whether they consist of genuine heterometallic species or of a mixture of parent heterometallic species. Heterotrimetallic complex LiCoNi(tbaoac) (, tbaoac = -butyl acetoacetate) has been designed based on the model tetranuclear structure featuring two transition metal sites in order to be utilized as a molecular precursor for the low-temperature preparation of the LiCoNiO battery cathode material. An investigation of the structure of appeared to be very challenging, since the Co and Ni atoms have very similar atomic numbers, monoisotopic masses, and radii as well as the same oxidation state and coordination number/environment. Using a statistical analysis of heavily overlaid isotope distribution patterns of the [LiMM'L] (M/M' = Co, Ni, and CoNi) ions in DART mass spectra, it was concluded that the reaction product contains both heterotrimetallic and bimetallic species. A structural analogue approach has been applied to obtain LiMMg(tbaoac) (M = Co () and Ni ()) complexes that contain lighter, diamagnetic magnesium in the place of one of the 3d transition metals. X-ray crystallography, mass spectrometry, and NMR spectroscopy unambiguously confirmed the presence of three types of molecules in the reaction mixture that reaches an equilibrium, LiML + LiMgL ↔ 2LiMMgL, upon prolonged reflux in solution. The equilibrium mixture was shown to have a nearly statistical distribution of the three molecules, and this is fully supported by the results of theoretical calculations revealing that the stabilization energies of heterometallic assemblies fall exactly in between those for the parent heterometallic species. The LiCoNiO quaternary oxide has been obtained in its phase-pure form by thermal decomposition of heterometallic precursor at temperatures as low as 450 °C. Its chemical composition, structure, morphology, and transition metal distribution have been studied by X-ray and electron diffraction techniques and compositional energy-dispersive X-ray mapping with nanometer resolution. The work clearly illustrates the advantages of heterometallic single-source precursors over the corresponding multi-source precursors.