New cerium-loaded phosphine oxide-functionalized polyurethane foam materials – Synthesis, stability, comparison of coordination behavior and catalytic applications

Abstract

Phosphine oxides (R3P=O) are very well-known as reagents, ligands as well as catalysts in various chemical transformations. Among them, numerous molecules and polymers possessing phosphine oxide moieties have been employed as ligands in the chemistry of f-metals. In this regard, the present work describes the synthesis and characterization of three different new PO-functionalized polyurethane foams (PUFs) and their corresponding Ce-loaded PUFs (Ce-PUFs). The coordination behavior of these PUFs is evaluated with cerium salts and is compared with selected phosphine oxide molecules such as triphenylphosphine oxide (TPPO), trioctylphosphine oxide (TOPO), dibutyl phosphate (DBP), and bisphosphoramide (BAmPO). The single-crystal X-ray analysis describes that one of the Ce(III) complexes of bisphosphoramide appears as an infinite coordination polymer by intermolecularly bridged-phosphine oxide ligands. As a measure of coordination strength, the ligand-exchange reactions follow the trend as PUF > TOPO > BAmPO > TPPO > DBP and indicating the strong metal-binding capacity of PUFs more than the monomeric and dimeric phosphine oxides. According to the thermogravimetric analysis, the thermal stability of the metal complexes is observed as Ce(IV)-PUFs > [Ce(NO3)4(TOPO)4] > [Ce(NO3)4(BAmPO)2] > [Ce(NO3)4(TPPO)4] > [Ce(NO3)2(DBP)2]. Among all, the Ce-loaded foam materials displayed the highest stability and exchange the other ligands instantaneously. Further, the Ce(IV)-PUFs are found to be stable, non-leachable, and re-usable (more than 10 cycles) catalysts in high yield synthesis of biscoumarin derivatives.