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In order to elucidate the interactions of copper with wood, three mononuclear copper( II) coordination compounds with a vanillinate anion, cis-[Cu(C 8 H 7 O 3 ) 2 (H 2 O) 2 ] (1), trans- [Cu(C 8 H 7 O 3 ) 2 (H 2 O) 2 ]·2H 2 O (2), and trans-[Cu(C 8 H 7 O 3 ) 2 (H 2 O) 2 ] (3), have been characterized. X-ray structure analysis of the cis isomer 1 reveals two bidentate vanillinate ions coordinated via methoxy (Cu−O1 2.260(2) Å ) and deprotonated hydroxy oxygen atoms (Cu−O2 1.909(2) Å ), and two water molecules (Cu−O1w 2.087(2) Å ) in the octahedral CuO 6  chromophore. Two axes O1−Cu−O1w’ in the octahedron have the same length, while the third axis O 2 −Cu−O 2 ’ is shorter. This is in agreement with the room temperature EPR spectrum of 1, showing two signals (g 12  2.302, g3 2.005), but interestingly, three signals (g 1  2.393, g 2  2.214, g 3  2.010) in the 115 K spectrum were found. The same coordination atoms were found also in the trans isomer 2 (Cu−O2 1.950(2), Cu−O1w 1.994(2), Cu−O1 2.334(2) Å ), however here, two axes of almost equal length are short (O2−Cu−O2’ O1w−Cu−O1w’), while the third axis is longer (O1−Cu−O1’). On the other hand, three (rhombic) signals (g 1  2.289, g 2  2.163, g 3  2.086) in the room temperature EPR spectrum of 2 suggest three different axes in the coordination octahedron. In the EPR spectrum, of the second trans complex 3, a slightly rhombically distorted elongated axial spectrum is found. The 115 K EPR spectra of the two trans complexes 2 and 3 do not differ significantly from the features observed at room temperature. These results indicate that there is not always a straightforward correlation between the results of XRD structure analysis and EPR spectroscopy. Nevertheless, both methods can act also complementarily and give a deeper insight into the nature of copper(II) chromophores.

zeitschrift für naturforschung b

Fixation of Copper(II) Ions in Aqueous Solution to Lignin Model Compound Vanillin in an Absence of the Nitrogen Donor Ligands; Structural and EPR Correlation