Mechanistic Insights on the Formation of a Carbodiimide Ion from Urea in La 2 O 2 NCN 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 La 2 O 2 NCN by employing the "proanion" strategy without using NH 3 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 in situ 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 CO 2 gas, resulting in the formation of an amorphous phase rich in NCN 2- . Following further heating to 600 °C, La 2 O 2 NCN crystallizes, enhancing its crystallinity as the temperature increases. These findings elucidate the formation mechanism of La 2 O 2 NCN, introduce the "proanion method" for the alternative synthesis of metal (oxy)carbodiimides, and expand their potential for applications as functional materials.