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

In order to elucidate the interactions of copper with wood, three mononuclear copper( II) coordination compounds with a vanillinate anion, cis-[Cu(C8H7O3)2(H2O)2] (1), trans- [Cu(C8H7O3)2(H2O)2]·2H2O (2), and trans-[Cu(C8H7O3)2(H2O)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 CuO6 chromophore. Two axes O1−Cu−O1w’ in the octahedron have the same length, while the third axis O2−Cu−O2’ is shorter. This is in agreement with the room temperature EPR spectrum of 1, showing two signals (g12 2.302, g3 2.005), but interestingly, three signals (g1 2.393, g2 2.214, g3 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 (g1 2.289, g2 2.163, g3 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.