Mechanistic Insights on the Formation of a Carbodiimide Ion from Urea in LaONCN Synthesis Based on the "Proanion" Strategy.

This study affords mechanistic insights into the formation mechanism of carbodiimide ions (NCN) from urea during the synthesis of LaONCN by employing the "proanion" strategy without using NH gas. It is a safer, cost-effective, and environmentally friendly approach. Urea, acting as a proanion, decomposes upon heating, facilitating conversion to NCN. This work meticulously examines the phase transitions and identifies intermediate species formed during the reaction using 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) to immobilize NCO species on the surface of La(OH). As the temperature reaches approximately 400 °C, these NCO anions transform into NCN anions by releasing CO gas, resulting in the formation of an amorphous phase rich in NCN. Following further heating to 600 °C, LaONCN crystallizes, enhancing its crystallinity as the temperature increases. These findings elucidate the formation mechanism of LaONCN, introduce the "proanion method" for the alternative synthesis of metal (oxy)carbodiimides, and expand their potential for applications as functional materials.