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

Abstract

This study affords mechanistic insights into the formation mechanism of carbodiimide ions (NCN2–) 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 NCN2–. 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 NCN2– anions by releasing CO2 gas, resulting in the formation of an amorphous phase rich in NCN2–. 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.