Spectroscopic and structural characterization of copper(II) and palladium(II) complexes of a lichen substance usnic acid and its derivatives. Possible forms of environmental metals retained in lichens

Abstract

The metal binding properties of a phenolic lichen substance usnic acid (UA) and its acetyl and enamine derivatives 9-O-acetylusnic acid (MAUA), 7,9-di-O-acetylusnic acid (DAUA), Δ2,11-enaminousnic acid (EUA), and N-substituted Δ2,11-enaminousnic acids have been studied by synthetic and spectroscopic methods, and the structures of copper(II) and palladium(II) complexes have been established by the X-ray diffraction method. Cu(II) reacted with UA and DAUA to give the binary complexes Cu(UA)2·H2O and Cu(DAUA)2, respectively, and Cu(bpy) (bpy=2,2′-bipyridine) formed ternary complexes with UA and DAUA. Pd(II) also reacted with UA, DAUA, EUA, and N-substituted Δ2,11-enaminousnic acids to give the corresponding binary complexes. All the isolated complexes are insoluble in water and soluble in most organic solvents. They exhibited very strong absorption and circular dichroism spectral peaks in the UV region. The 1H-NMR spectrum in CDCl3 of the Pd(II) complex of N-phenyl-Δ2,11-enaminousnic acid (PEUA), Pd(PEUA)2·C6H6, showed that the C4-proton signal suffered a large upfield shift (0.86 ppm) due to the ring current effect of the N-phenyl moiety. X-Ray crystal structure analysis has been performed for Cu(bpy)(UA)(ClO4)·CH3OH, Pd(MEUA)2·C6H6, and Pd(PEUA)2·C6H6. Cu(bpy)(UA)(ClO4)·CH3OH has a square-pyramidal structure with the two nitrogen atoms of bpy and the two oxygen atoms of the mono-deprotonated B ring of UA in the equatorial positions, while Pd(II) binds with two molecules of MEUA or PEUA in the trans configuration through the nitrogen and oxygen atoms with deprotonation. The N-phenyl ring of PEUA in Pd(PEUA)2·C6H6 was revealed to be located close to the C4 proton as indicated by 1H-NMR. Isolation of Cu2(bpy)2(UA)(NO3)2·2H2O suggests that UA has two metal binding sites that can form polymeric complexes. The present results substantiate the metal binding ability and the structures of the complexes of usnic acid and other substances from lichens as biomonitors of environmental metal ions.