Natural cellulose-based Cu(II) complex: An eco-friendly nanocatalyst for Ullmann condensations at room temperature

Abstract

A bio-degradable and environmentally friendly heterogeneous Cu(II) complex have been synthesized from a jute nanocellulose-based poly(amidoxime) ligand for Ullmann condensation and synthesis of flexible liquid crystals. The bio-nanocellulose surface was modified via graft co-polymerization with acrylonitrile, and the poly(acrylonitrile) was converted into a bidentate poly(amidoxime) ligand via amidoximation. Further, this ligand was treated with an aqueous solution of CuSO4 resulted in strong complex formation (NCL-Cu(II)@PA) between the amidoxime ligands and Cu(II) species. This Cu(II) complex was successfully characterized and applied to the C–O bond formation via Ullmann condensation using ultra low loading (5–50 mol ppm of Cu) of metal species. A variety of alkyl/benzyl halides and 1-phenyl-2-bromoethanone were selectively forwarded the Ullmann condensation to afford the corresponding ethers quantitatively at room temperature. The NCL-Cu(II)@PA provided excellent catalytic performance in the condensation reaction with unrivalled TON 180000 and TOF 9000 h−1. The NCL-Cu(II)@PA was easily recovered from the reaction vessel and reused for up to seven cycles without a noteworthy drop in the respective ether. Furthermore, some azobenzene core flexible liquid crystal derivatives were also synthesized using the nanocellulose-based catalyst NCL-Cu(II)@PA. According to the STM results, the lengths of the bright aromatic disc-core and the azobenzene units were 0.77 ± 0.1 nm and 1.0 ± 0.2 nm.