Changing the bridging connectivity pattern within a heterometallic assembly: design of single-source precursors with discrete molecular structures

Abstract

Three heterometallic single-source precursors with a proper Li : Co ratio for the lithium ion battery cathode material LiCoO₂ are reported. The heterometallic compounds LiCoL₃ (L = acac (1), tbaoac (2) or dhd (3)) were obtained on a large scale, in nearly quantitative yield using a one-step reaction that employs readily available starting reagents. The first of these precursors, heterometallic diketonate LiCo(acac)₃ (1) (acac = pentane-2,4-dionate), was found to have a polymeric structure in the solid state that limits its volatility and prevents solubility in non-coordinating solvents. In order to design precursors with discrete molecular structures, we used asymmetric ligands, tert-butyl acetoacetate (tbaoac) and 2,2-dimethyl-hexanedionate (dhd), that exhibit different bridging properties at the two ends of the ligand and thus allow one to effectively change the bridging connectivity pattern within the heterometallic assembly. As a result, the heterometallic complexes LiCo(tbaoac)₃ (2) and LiCo(dhd)₃ (3), which consist of discrete tetranuclear molecules of Li₂Co₂L₆, were successfully isolated and characterized. Out of the three heterometallic compounds reported in this work, LiCo(tbaoac)₃ (2) appears to meet the application demands for an ideal single-source precursor. In accord with its discrete molecular structure, the complex is soluble in nearly all common solvents and retains its heterometallic structure in solution. Thus, it can be effectively used for the preparation of thin films by direct liquid injection techniques. The compound is highly volatile and exhibits a congruent sublimation with heterometallic fragments present in the gas phase. In addition, it shows the lowest decomposition temperature leading to phase-pure LiCoO₂ and can be utilized for the preparation of nanoparticles of this cathode material. Importantly, single-source precursor 2 does not impose any measurable ligand extra cost on the formation of the target cathode material.