Synthesis, crystal structures, EPR and DFT studies of first row transition metal complexes of lignin model compound ethylvanillin

The transition metal complexes of ethylvanillin [Mn(L)2(H2O)2] (1), [Co(L)2(H2O)2] (2), [Ni(L)2(H2O)2] (3), and [Zn(L)2(H2O)2] (4) have been synthesized. The geometric and electronic structure of 1, 2, 3 and 4 has been solved by X-ray diffraction analysis and EPR spectroscopy. They are octahedral complexes with a cis arrangement of the two water molecules. All the structures were calculated by DFT methods at the level of theory B3P86/6-311g. The EPR spectra of 1 are isotropic both at 298 and 77 K with g values at 2.034 ± 0.010 and 2.040 ± 0.010, respectively indicating Mn(II) ion (3d5) with a S = 5/2 spin. 1 shows a hyperfine structure with six strong absorptions, corresponding to the |−1/2, m> → |1/2, m> ‘allowed’ transitions (ΔM = ±1, Δm = 0), and five pairs of ‘forbidden’ absorptions (ΔM = ±1, Δm = ±1), between the Δm = 0 hyperfine transitions in an organic solvent such as DMF, DMSO and CH3CN. EPR spectroscopy and DFT calculations suggest that in the temperature range 77–298 K 2 presents a high-spin S = 3/2 state, whereas the low-spin state S = 1/2 begins to be populated at temperatures higher than 77 K (liquid nitrogen temperature). In 3, the weak signal due to a small amount of an octahedral Ni(III) complex (NiL3), is characterized by a rhombic spectrum. DFT simulations on 4 indicate that the octahedral structure with a cis arrangement of the two water ligands is more stable than the octahedral one with a trans arrangement and the tetrahedral geometry.