Copper Fixation to Guaiacyl Lignin Units by Nitrogen Donor Ligands

Abstract

Several new copper(II) complexes with guaiacyl lignin models vanillin (HL1) [Cu(L1)2(nia)2] (1) (nia = nicotinamide) or vanillic acid (HL2) [Cu(L2)2(nia)2] (2) [Cu2(μ-L2)4(nia)2] (3), and [Cu(L2)2(Hetam)2] (4) (Hetam = ethanolamine) were isolated and characterized. The molecular structure of complex 1 reveals bidentate vanillin (HL1) coordination via the methoxy and the deprotonated hydroxy groups. On the other hand, the vanillic acid (HL2) complexes 2 - 4 show a deprotonated carboxylate group with chelating coordination mode in 2, bridging in 3 and monodentate coordination in 4. The mononuclear complexes 1, 2 and 4 show a distorted trans octahedral coordination sphere with pairs of monodentate and chelating ligands. A replacement of the monodentate nicotinamide ligand in 2 with the bidentate ethanolamine ligand in 4 changes the coordination mode of the vanillic acid anion from bidentate (complex 2) to monodentate (complex 4). This shift inside the coordination sphere reveals different O-Cu-O Jahn-Teller axes by the vanillic acid anion in 2 and ethanolamine in 4. Empty channels are present in the crystal structure of the dinuclear complex 3, stabilized by hydrogen bonds and π-π stacking.