Nanocrystalline Lanthanide Nitride Materials Synthesised by Thermal Treatment of Amido and Ammine Metallocenes: X‐ray Studies and DFT Calculations

Abstract

The decomposition process of ammine lanthanide metallocenes was studied by X‐ray diffractometry, spectroscopy and theoretical investigations. A series of ammine‐tris(η⁵‐cyclopentadienyl)lanthanide(III) complexes 1‐Ln (Lanthanide (Ln)=Sm, Gd, Dy, Ho, Er, Yb) was synthesised by the reaction of [Cp₃Ln] complexes (Cp=cyclopentadienyl) with liquid ammonia at −78 °C and structurally characterised by X‐ray diffraction methods, mass spectrometry and vibrational (IR, Raman) spectroscopy. Furthermore, amido‐bis(η⁵‐cyclopentadienyl)lanthanide(III) complexes 2‐Ln (Ln=Dy, Ho, Er, Yb) were synthesised by heating the respective ammine adduct 1‐Ln in an inert gas atmosphere at temperatures of between 240 and 290 °C. X‐ray diffraction studies, mass spectrometry and vibrational (IR, Raman) spectroscopy were carried out for several 2‐Ln species and proved the formation of dimeric μ₂‐bridged compounds. Species 1‐Ln are highly reactive coordination compounds and showed different behaviour regarding the decomposition to 2‐Ln. The reaction of 1‐Ln and 2‐Ln with inorganic bases yielded lanthanide nitride LnN powders with an estimated crystallite size of between 40 and 90 nm at unprecedented low temperatures of 240 to 300 °C. Temperature‐dependent X‐ray powder diffraction and transmission electron microscopy (TEM) investigations were performed and showed that the decomposition reaction yielded nanocrystalline material. Structural optimisations were carried out for 1‐Ln and 2‐Ln by employing density functional (DFT) calculations. A good agreement was found between the observed and calculated structural parameters. Also, Gibbs free energies were calculated for 1‐Ln, 2‐Ln and the pyrolysis reaction to the nitride material, and were found to fit well with the expected ranges.