ESR and magnetic studies of octahedral [Fe(III)(Cl)(pcd)(H2O)(DMSO)] (pcd = pyridine-2,6-dicarboxylato) compound showing Fe(III) species with different spin states in solution

By direct synthesis has been obtained the octahedral complex [Fe(III)(Cl)(pdc)(H2O)(DMSO)] 1 (pdc = pyridyn-2,6-dicarboxylato), which shows a supramolecular structure and a clear and large molecular distortion. UV–Vis spectrum of 1 shows at 741 nm, 836 nm (ε = 100 M−1 cm−1) and 970 nm (ε = 1600 M−1 cm−1) typical iron d–d transitions. FIR spectrum of 1 shows vibrations υFeO, υFeN and υFeCl. NMR 1H spectroscopy is typical of paramagnetic compounds. The susceptibility data fit to a Hamiltonian, Hex = −2J S1 · S2, using two models, give close exchange interaction constant values of: J = −1.803 cm−1, g = 1.810 and J = −1.102 cm−1, g = 1.832. X-band ESR spectra of powder sample of 1 at 300 K/77 K show broad signals at H ≈ 56.70 mT/64.53 mT, characteristic of ferromagnetic absorptions. X-band spectra of 1 at 77 K in methanol solution at different concentrations show typical signals of Fe(III) species with high spin, S = 5/2, quantum mixed spin state (qms), 5/2, 3/2, which are present in ESR spectra of peroxidase enzymes, and a low spin S = 1/2. 1 has been used as catalyst in the guaiacol test, characteristic of peroxidase enzymes, giving positive by visual observation and by UV–Vis spectroscopy. By ERS spectroscopy has been observed the characteristic π-cationic-radical ferryl, enzyme covalently modified, when the peroxidase enzyme is active. The presence of Fe(III) species with qms observed by ESR spectra of 1, and its catalytic activity, both characteristic of peroxidase enzymes, are a significant conclusion of the work.

► We have proved the correlation between Fe(III)-spin-state and peroxidase activity.
► ESR spectra of solution samples show species with different iron(III) spin state.
► By ESR spectrometry it has been identified quantum mixes S = 3/2, S = 5/2 of Fe(III).
► Structural analyses is consistent with Fe(III) antiferromagnetic exchange interaction.