Ketoiminate versus diketonate ligand to improve lithium precursor properties: Structural consideration of two analogues

Abstract

An approach is proposed to design new lithium MOCVD precursors by minor ligand modification. It has been shown that β-ketoiminate ligand can significantly improve the properties of volatile lithium complexes compared to β-diketonate analogues. For this purpose, the structure and properties of two volatile lithium complexes with similar bulky ligands are compared: for β-diketonate Li(tmhd) (1) (tmhd = 2,2,6,6-tetramethyl-heptane-3,5-dionate or dipivaloylmethanate, tBu-CO-CH-CO-tBu, where tBu = C(CH3)3) and β-ketoiminate Li(tmha) (2) (tmha = 2,2,6,6-tetramethyl-heptane-3-imino-5-onate, tBu-CO-CH-C(NH)-tBu). Compound 2 is new, and is formally derived from 1 by substitution of oxygen donor atom by NH-group. The compounds were prepared by the same technique and were studied by X-ray diffraction and IR and Raman spectroscopy. Molecular structure of these complexes is similar, built of tetrameric aggregates [LiL]4 with cubane-like Li4O4 core. Both 1 and 2 crystallize in C2/c space group with Z = 4 and Z’ = 0.5 but with different packing motif. According to DSC data, only 2 experiences a crystal-to-crystal phase transition at 384 K, which manifests itself as a shift of molecular rows along the a axis, while geometry of [Li(tmha)]4 and crystal space group remain unchanged. Analysis of intermolecular contacts using Hirshfeld surface analysis showed that [Li(tmha)]4 molecules are less densely packed compared to [Li(tmhd)]4. This explains the greater volatility of 2 compared to 1, due to the lower value of the lattice energy.