Heterometallic molecular precursors for a lithium–iron oxide material: synthesis, solid state structure, solution and gas-phase behaviour, and thermal decomposition

Abstract

Three heterometallic single-source precursors with a Li : Fe = 1 : 1 ratio for a LiFeO oxide material are reported. Heterometallic compounds LiFeL (L = tbaoac (1), ptac (2), and acac(3)) have been obtained on a large scale, in nearly quantitative yields by one-step reactions that employ readily available reagents. The heterometallic precursor LiFe(acac) (3) with small, symmetric substituents on the ligand (acac = pentane-2,4-dionate), maintains a 1D polymeric structure in the solid state that limits its volatility and prevents solubility in non-coordinating solvents. The application of the unsymmetrical ligands, tbaoac (tert-butyl acetoacetate) and ptac (1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedionate), that exhibit different bridging properties at the two ends of the ligand, allowed us to change the connectivity pattern within the heterometallic assembly. The latter was demonstrated by structural characterization of heterometallic complexes LiFe(tbaoac) (1) and LiFe(ptac) (2) that consist of discrete heterocyclic tetranuclear molecules LiFeL. The compounds are highly volatile and exhibit a congruent sublimation character. DART mass spectrometric investigation revealed the presence of heterometallic molecules in the gas phase. The positive mode spectra are dominated by the presence of [M - L] peaks (M = LiFeL). In accord with their discrete molecular structure, complexes 1 and 2 are highly soluble in nearly all common solvents. In order to test the retention of the heterometallic structure in solution, the diamagnetic analog of 1, LiMg(tbaoac) (4), has been isolated. Its tetranuclear molecular structure was found to be isomorphous to that of the iron counterpart. H andLi NMR spectroscopy unambiguously confirmed the presence of heterometallic molecules in solutions of non-coordinating solvents. The heterometallic precursor 1 was shown to exhibit clean thermal decomposition in air that results in phase-pure α-modification of layered oxide LiFeO, the prospective cathode material for lithium ion batteries.

Three to tango requires a site-specific substitution: heterotrimetallic molecular precursors for high-voltage rechargeable batteries

“…However, for successful application of mass spectrometry it is important that heterometallic molecules exhibit a certain degree of structural integrity in maintaining heterometallic ions at experimental conditions and do not undergo excessive fragmentation/recombination processes in the mass spectrometer. To this end, the Direct Analysis in Real Time (DART) mass spectrometry has already demonstrated3,49,54,55 great promise in retention of heterometallic assemblies due to its soft ionization characteristics, allowing for the effective analysis of the “gas-phase structure”, as well as for identifying the chemical compositions based on the detected ionic species and their isotope distribution patterns.…”

“…Therefore, there is a great need for alternative synthetic approaches that allow one to avoid the above problems. One of the ways to overcome the difficulties in preparation of quaternary oxide materials is the use of molecular precursors that contain all necessary metals in the proper ratio and decompose at lower temperatures to yield phase-pure nanosize target materials 49–51. The intimate mixing of different metals within hetero tri metallic precursors ensures the homogeneity of metal distribution in the target products as well as facilitates a rapid interaction between the constituent elements of the molecule upon thermolysis to circumvent the formation of unwanted intermediates 52,53…”

A three body problem: a genuine heterotrimetallic molecule vs. a mixture of two parent heterobimetallic molecules

“…Magnesium was found to support the tetranuclear type of structure Li 2 Mg 2 L 6 with a number of unsymmetric ligands, all isomorphous to those of 3d transition metals. 47 Divalent Mg can be neither reduced nor oxidized in the presence of divalent Ni and Co. Its atomic number and monoisotopic mass (23.9850) are far from those of the 3d transition metals of interest. Finally, the Li 2 Mg 2 (tbaoac) 6 molecule has been thoroughly characterized 47 in solutions of non-coordinating solvents by multinuclear NMR spectroscopy.…”

“…DART (Direct Analysis in Real Time) mass spectrometry has already been established as a powerful tool for investigating the gas phase behaviour of volatile heterometallic compounds 42 , 43 , 47 as well as for confirmation of transition metal oxidation states. 49 In this work, we extended the application scope of the technique for analysis of hetero tri metallic compounds primarily based on their isotope distribution patterns.…”

“…We proposed to utilize hetero bi metallic analogue Li 2 M 2 L 6 , which is a tetranuclear cyclic molecule ( Scheme 1 ) with two transition metal sites. Molecules of that type were shown to be readily prepared on a large scale and to be capable of accommodating a number of divalent metals that prefer an octahedral environment, including even main group metals like Mg. 41 , 47 Construction of such cyclic molecules requires the utilization of unsymmetric ligands (diketonates and beta-ketoesters) that feature different (small/bulky) substituents at the two ligand ends. The ligand oxygen located under small (CH 3 , CF 3 ) substituents acts as a chelating–bridging, while the other one, under bulky (O t Bu, t Bu) groups, is purely chelating.…”