Put a Ring on It: Improving the Thermal Stability of Molybdenum Imides Through Ligand Rigidification

Volatile bis(tert-butylimido)-dichloromolybdenum(VI) compounds containing N,N’-chelating ligands, (tBuN)2MoCl2·L, have previously been used as single-source precursors for the chemical vapor deposition of high-purity Mo2N thin films. The first step in the thermolysis of these compounds is the partial dissociation of the chelating ligand to yield (tBuN)2MoCl2, which further decomposes by eliminating isobutylene and tBuNH2. The rate determining step in this process is the formation of a pentacoordinate intermediate where the previously bidentate ligand adopts a κ1-coordination. Here we show that rigidification of the ligand backbone, by incorporating various heterocycles, led to an overall increase in thermal stability (21−38 °C) of these complexes by preventing the formation of the κ1-intermediate. Formation of the κ1-intermediates is highlighted by high level calculations and is supported by experimental activation barriers. Finally, a model for the κ1-bipyridine adduct was isolated and characterized using 2-phenylpyridine. This careful control of the thermal stabilities of these compounds can lead to new vapor-phase deposition precursors for the preparation of Mo2N films.