Mixed amido/imido/guanidinato complexes of niobium: potential precursors for MOCVD of niobium nitride thin films

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Abstract
Novel mixed amido/imido/guanidinato complexes of niobium are reported. The complexes were synthesized by insertion of two equivalents of di-isopropylcarbodiimide (i-Pr-cdi) or bis-cyclohexylcarbodiimide (Cy-cdi) respectively, into the niobium–amido bonds of [Nb(NR2)3(N-t-Bu)] (1, R = Me; 2, R = Et) starting out from [NbCl3(N-t-Bu)(py)2] and the respective LiNR2 reagent (py = pyridine). Four representative examples of these mixed ligand amido/imido/guanidinato compounds were synthesized and were characterized by 1H-NMR, 13C-NMR, 15N-NMR, CHN-analysis, mass spectrometry and infra-red spectroscopy. The molecular structures of [Nb(NR2){η2-(i-Pr-N)2C(NR2)}2(N-t-Bu)] (3, R = Me; 4, R = Et) in the solid state were determined by single-crystal X-ray diffraction studies and are discussed together with the molecular structure of the starting compound [Nb(NMe2)3(N-t-Bu)] (1). The thermal properties of the new compounds depend on the substitution at the guanidinato ligand. Complexes of i-Pr-cdi are significantly more volatile than complexes of Cy-cdi as revealed by thermogravimetric analysis. Preliminary experiments using 3 as a single-molecule source for metal–organic chemical vapour deposition (MOCVD) in the absence of ammonia indicate the formation of the stoichiometric, and surprisingly carbon-free, cubic niobium nitride phase.

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