Synthesis, structure, spectra and redox chemistry of iron(III) complexes of tridentate pyridyl and benzimidazolyl ligands

Abstract

A series of high-spin octahedral 1 : 2 iron(I1r) complexes of Schiff bases derived from salicylaldehyde and aromatic amines and the 1 : 1 and 1 : 2 complexes of bis(pyridin-2-yl)-aza and bis(benzimidazol-2-yl)-aza and -thioether ligands have been isolated. The crystal structure of trichloro[bis(pyridin-2-ylmethyl)amine]iron(r11) has been determined. It contains two crystallographically independent molecules in the asymmetric unit cell. In each molecule iron(rI1) possesses a rhombically distorted octahedral co-ordination, constituted by all the three nitrogens of the facial ligand and three chloride ions. The effects of stereochemical and/or donor atom variations on the UV/VIS and EPR spectra and Fe"'-Fe" redox potentials and hence on the Lewis acidity of the iron(m) centre are discussed. The phenolate-to-iron(m) charge-transfer band energy of [FeL' ,]Cl [HL' = N-(pyridin-2-ylmethyl)salicylideneamine] is higher and its Fe"'-Fe" redox potential more negative than those of the corresponding 1 : 1 complex.

EPR and semi-empirical studies as tools to assign the geometric structures of FeIII isomer models for transferrins

“…43 This effect has also been observed in several other complexes with the same number of phenolate groups and a variety of other pendant arms with different Lewis basicity. [17][18][19]41 The electronic spectrum of 1 was also recorded in the solid state (KBr pellets, diffuse reflectance) and revealed the same behavior observed in acetonitrile (λ max at 538 and 318 nm), indicating no changes in the coordination sphere of 1 when in solution, in agreement with the ESI-MS mass spectral data.Electrochemistry and spectroelectrochemistry. The redox behavior of 1, A and B was investigated by cyclic voltammetry and, as observed using all other techniques discussed above, it is quite similar.…”

“…This value is cathodically shifted (-0.16 V versus NHE) when compared to that of [Fe(bbpen)] + (-0.42 V versus NHE), 16 reflecting the decrease in the Lewis acidity on the Fe III center resulting from the ligand basicity increase. 17,18 Regarding the electrochemical properties of transferrins, the redox potential of -0.52 V versus NHE is in close proximity to that observed for [Fe(bbimen)] + (-0.58 V versus NHE), which indicates that it is a good model for the redox potential of transferrins.…”

“…This value is cathodically shifted (-0.16 V versus NHE) when compared to that of [Fe(bbpen)] + , 16 reflecting the decrease in the Lewis acidity on the Fe III center resulting from the ligand basicity increase. 17,18 Regarding the electrochemical properties of transferrins, the redox potential of -0.52 V versus NHE is in close proximity to that observed for [Fe(bbimen)] + (-0.58 V versus NHE), which indicates that it is a good model for the redox potential of transferrins.In order to investigate the spectral changes during the Fe III → Fe II redox process, spectroelectrochemical measurements were carried out under the same experimental conditions as those employed in the CV studies. A decrease in the LMCT phenolate → Fe III band at 542 nm, with a simultaneous increase in a new band at 420 nm was observed ( Figure 5).…”

“…The bands at lower energy (321 and 542 nm) can be ascribed to LMCT transitions from the p π orbital on the phenolate oxygen to the half-filled d π* (t 2g ) and d σ* (e g ) on Fe III . 11,[15][16][17][18][19][20]41 Comparing the UV-Visible data of 1 with those reported 16,20 for the analogous [Fe(bbpen)] + , it is observed that there is a hypsochromic shift of the p π → d π* LMCT band from 574 to 542 nm when pyridine groups in H 2 bbpen are replaced by 1-methylimidazole groups in H 2 bbimen. These results are interpreted as a consequence of the greater basicity of the 1-methylimidazole groups (pKa 1 ≅ 2.06) 42 compared to the pyridine groups (pKa 1 < 1.3).…”

“…8, 2006 tyrosinate proteins. [15][16][17][18][19][20] Very recently, we reported 16 the synthesis and full characterization of a series of Fe III complexes, [Fe(bbpen-X)] + , using hexadentate ligands (H 2 bbpen-X) containing pyridine and phenol pendant arms. In this paper, we show that the reaction of the analogous H 2 bbimen (Scheme 1) with Fe(ClO 4 ) 3 .…”