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The molecular and electronic structure of octahedral tris(phenolato)iron(III) complexes and their phenoxyl radical analogues: A Mossbauer and resonance Raman spectroscopic study
Citation key ISI:000082488300013
Author Snodin, M D and Ould-Moussa, L and Wallmann, U and Lecomte, S and Bachler, V and Bill, E and Hummel, H and Weyhermuller, T and Hildebrandt, P and Wieghardt, K
Pages 2554-2565
Year 1999
ISSN 0947-6539
Journal Chem. Eur. J
Volume 5
Number 9
Month SEP
Abstract Hexadentate macrocyclic ligands containing a 1,4,7-triazacyclononane backbone and three N-bound pendent-arm phenolates form extremely stable neutral complexes with (FeCl3)-Cl-III,. The octahedral complexes [(FeL)-L-III] undergo three reversible one-electron oxidation processes to yield the mono- and dications, [FeL](+) and [FeL](2+), which are stable in solution for hours. whereas the trications, [FeL](3+), are only stable in solution on the time scale of a cyclic voltammetric experiment. These oxidations are shown to be ligand- rather than metal-centered. Three coordinated phenoxyl radicals are formed successively as shown conclusively by Mossbauer spectroscopy, The neutral, mono-, di-, and tricationic species each contain an octahedral, high-spin ferric ion (S-Fe=(5)/(2)), which is intramolecularly antiferromagnetically coupled to the spin (S = (1)/(2)) of the bound phenoxyl ligands to yield an S-t =2 ground state for the monocation and an S-t=(3)/(2) ground state for the dications as shown by EPR spectroscopy. The vibrations of the coordinated phenolate are observed by resonance Raman (RR) spectroscopy by excitation in resonance with the phenolate-to-iron charge-transfer (CT) transition above 500 nm or, alternatively, of the coordinated phenoxyl by excitation in resonance with the intraligand pi–>pi* transition at about 410nm. Use of O-18 isotopomers selectively labeled at the phenolic oxygen allowed the identification of the C-O stretching and Fe-O stretching and bending modes. It is shown that the substitution pattern of phenolates and phenoxyls in their respective ol ortho and par para positions and the charge of the complexes have a pronounced influence on the vibrational modes observed.
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