Origin of Decomposition in a Family of Molybdenum Precursor Compounds

Abstract

The bis­(tert-butylimido)-molybdenum­(VI) framework has been used successfully in the design of vapor-phase precursors for molybdenum-containing thin films, so understanding its thermal behavior is important for such applications. Here, we report the thermal decomposition mechanism for a series of volatile bis­(alkylimido)-dichloromolybdenum­(VI) adducts with neutral N,N′-chelating ligands, to probe the stability and decomposition pathways for these molecules. The alkyl groups explored were tert-butyl, tert-pentyl, 1-adamantyl, and a cyclic imido (from 2,5-dimethylhexane-2,5-diamine). We also report the synthesis of the new tert-octyl imido adducts, ( t OctN)2MoCl2·L (L = N,N,N′,N′-tetramethylethylenediamine or 2,2′-bipyridine), which have been fully characterized by spectroscopic techniques as well as single-crystal X-ray diffraction and thermal analysis. We found that the decomposition of all compounds follows the same general pathway, proceeding first by the dissociation of the chelating ligand to give the coordinatively unsaturated species (RN)2MoCl2. Subsequent dimerization results in either an imido bridged adduct, [(RN)­Mo­(μ-NR)­Cl2]2, or a chloride bridged adduct, [(RN)2Mo­(μ-Cl)­Cl]2, depending on the size of the R group. The dimeric species then likely undergoes an intramolecular γ-hydrogen transfer to yield a nitrido-amido adduct, (RHN)­MoNCl2, and an alkene. Ultimately, the resulting molybdenum species appears to decompose into free tert-alkylamine and Mo2N or Mo2C. The thermolysis reactions have been monitored using 1H NMR spectroscopy, and the volatile decomposition products were analyzed using gas chromatography–mass spectrometry. A key intermediate has also been detected using electron ionization high-resolution mass spectrometry. Finally, a detailed computational investigation supports the mechanism outlined above and helps explain the relative stabilities of different N,N′-chelated bis­(alkylimido)-dichloromolybdenum­(VI) adducts.

Cut-and-pasting ligands: The structure/function relationships of a thermally robust Mo(VI) precursor

“…It is well known that bis(tert-butylimido)molybdenum(VI) adducts decompose to yield isobutylene, 7,10,15,16,33,34 assuming that lower-temperature "ancillary ligand-centered" decompositions do not occur. Therefore, isobutylene was anticipated to be the primary decomposition product in the thermolysis of 1.…”

“…tert-Butylamine is a major decomposition product in the thermolysis of ( t BuN)2MoCl2 adducts. 16,33 It forms via an intramolecular γ-H transfer (of a t BuN proton) to another imido moiety, resulting in a tert-butylamido ligand and a nitrido (Mo≡N) moiety. 16 A previous report, based specifically on the thermolysis of ( t BuN)2Mo(Pz) (A), 10 proposed that γ-H activation, to yield isobutylene and a hydrogen-terminated imido (Mo=NH), occurs first.…”

“…16,33 It forms via an intramolecular γ-H transfer (of a t BuN proton) to another imido moiety, resulting in a tert-butylamido ligand and a nitrido (Mo≡N) moiety. 16 A previous report, based specifically on the thermolysis of ( t BuN)2Mo(Pz) (A), 10 proposed that γ-H activation, to yield isobutylene and a hydrogen-terminated imido (Mo=NH), occurs first. It is followed by an intramolecular proton transfer to the pyrazolate, resulting in the elimination of the neutral pyrazole ligand precursor (HPz).…”

“…This first yields isobutene and ( t BuNH)MoNCl2, the latter of which subsequently decomposes to give tert-butylamine and MoNx. 16 It has previously been shown that ( t BuN)2MoCl2•2py is a single-source precursor for the CVD of MoCxNy, with no Cl contamination. 17 Additionally, the use of chloride-containing compounds can be exploited for the deposition of chlorine-free metal films, 18 although halogens are generally avoided in precursor design as the byproducts can etch the growing film or damage a metal reactor.…”

Disturbance of Intermolecular Forces: Eutectics as a New Tool for the Preparation of Vapor-Phase Deposition Precursors

“…Alignment of the dipoles in molecules of 3 results in the head to tail arrangement shown in the Figure . Compounds 1 and have the same general structure, and we have previously shown that upon vaporization they lose their coordinative ligands, resulting in the formation of the same gas phase species, [( t BuN)Mo(μ-N t Bu)Cl2]2. 13,14 Therefore, volatilization of eutectic mixtures of these compounds (1 or 3 with 2) should result in the same surface species and uniform film composition, upon deposition.…”