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

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(acry-lonitrile) 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.