Mechanistic Insights on the Formation of a Carbodiimide Ion from Urea in La₂O₂NCN Synthesis Based on the “Proanion” Strategy

This study affords mechanistic insights into the formation mechanism of carbodiimide ions (NCN^2-) from urea during the synthesis of La₂O₂NCN by employing the "proanion" strategy without using NH₃ gas. It is a safer, cost-effective, and environmentally friendly approach. Urea, acting as a proanion, decomposes upon heating, facilitating conversion to NCN^2-. This work meticulously examines the phase transitions and identifies intermediate species formed during the reaction using <i>in situ</i> X-ray diffraction, Fourier transform infrared spectroscopy, and thermogravimetric-differential thermal analysis-mass spectrometry. The findings present a detailed mechanism in which urea initially decomposes at 140 °C, releasing HNCO, which reacts with La(OH)₃ to immobilize NCO^- species on the surface of La(OH)₃. As the temperature reaches approximately 400 °C, these NCO^- anions transform into NCN^2- anions by releasing CO₂ gas, resulting in the formation of an amorphous phase rich in NCN^2-. Following further heating to 600 °C, La₂O₂NCN crystallizes, enhancing its crystallinity as the temperature increases. These findings elucidate the formation mechanism of La₂O₂NCN, introduce the "proanion method" for the alternative synthesis of metal (oxy)carbodiimides, and expand their potential for applications as functional materials.