Azo-Bridged Cesium Salt of Phenolate/Triazolide as an Unprecedented Carboxylation Catalyst for 1,3-Disubtituted Ureas from CO₂ and Amines

The azo-bridged cesium salt of phenolate/triazolide (IUPAC name: cesium salt of 4-(1H-1,2,4-triazol-3-ylazo)-phenol) (2a, ABC-PT) is successfully prepared from the equimolar neutralization of corresponding precursor, 4-(1H-1,2,4-triazol-3-ylazo)-phenol with Cs₂CO₃, and is characterized by elemental analysis, thermogravimetric analysis, Fourier-transform infrared spectroscopy, and NMR. This compound is applied as a highly active catalyst for the direct synthesis of 1,3-disubstituted urea as well as polyureas with high yield and selectivity from various amines and CO₂. It is observed that the high activity of the prepared catalyst is associated with the synergistic effect of azo linkage as well as azolide and phenolate group in the structure of the compound, which act as CO₂-philic moieties due to their resonance structures. These explain the high stability against water and air, thus resulting in an unprecedented turnover frequency value of 858 h⁻¹ in the carboxylation reaction. In addition, the catalyst is found to be thermally stable and retains its original activity after recycling four times.