Amorphous Porous Polyurethanes as Macromolecular Ligands for Palladium Catalysts

Abstract

Porous organic polymers (POPs) having urethane bonds were prepared through polyaddition of 4,4′-bis(hydroxylmethyl)-2,2′-bipyridyl with 1,3,5-triisocyanato-benzene and 1,3,5-tris(4-isocyanatophenyl)benzene, resulting in POP1 and POP2, respectively. The resulting POPs were complexed with Pd(OAc)2 to form POP-bound pre-catalysts, i.e., Pd(OAc)2@POP1 and Pd(OAc)2@POP2. Pd(II)@POP1 showed higher catalytic activities in Suzuki-Miyaura cross-coupling reactions compared with 2,2′-bipyridyl-Pd(OAc)2, as the reference “monomeric” catalyst. The higher catalytic activities of Pd(OAc)2@POP1 were interpreted in terms of the rigid framework of POP1. Within the POP1 framework, Pd may be more efficiently protected from agglomeration, which eventually leads to inactive palladium black formation. Unlike covalent organic frameworks (COFs) with crystalline structure, the present POPs are amorphous but capable, as polymeric ligands, of leading to active catalytic species.