Hydrogenperoxo-[(bztpen)Fe(OOH)]2+ and Its Deprotonation Product Peroxo-[(bztpen)Fe(O2)]+, Studied by EPR and Mössbauer Spectroscopy − Implications for the Electronic Structures of Peroxo Model Complexes

The purple hydrogenperoxo species [Fe(bztpen)(OOH)]2+ (1) can be generated in methanol solution by treatment of [Fe(bztpen)Cl]3+ with a large excess of hydrogen peroxide [A. Hazell, C. J. McKenzie, L. P. Nielsen, S. Schindler, M. Weitzer, J. Chem. Soc., Dalton Trans.2002, 310−317]. Addition of 30 equivalents of triethylamine to a solution of [Fe(bztpen)(OOH)]2+ gives the [Fe(bztpen)(O2)]+ (2) species, in which it has been proposed that the peroxo ligand is coordinated in the η2 mode. We have synthesized both 100% 57Fe-enriched non-heme FeIII peroxo species and studied them by EPR and Mossbauer spectroscopy. Species 1 displays a typical S = 1/2 low-spin ferric EPR spectrum, the g values of which have been analysed in terms of the Griffith model [J. S. Griffith, Proc. R. Soc. London, A1956, 234, 23−36]. We have thus obtained insight into the electronic structure of 1. The EPR spectrum of 2 is characteristic of a high-spin FeIII species in a nearly axial ligand field. We have determined all Mossbauer parameters for both complexes by a numerical treatment of applied field Mossbauer data. Species 1 exhibits an isomer shift δ/Fe = 0.16 mm/s and a quadrupole splitting ΔEQ = −2.08 mm/s (T = 4.2 K). In the case of 2, we obtain an isomer shift δ/Fe = 0.63 mm/s and a quadrupole splitting ΔEQ = 1.12 mm/s (T = 4.2 K). These values are close to those published very recently by Bill et al. for [Fe(trispicMeen)(OOH)]2+ (3) and [Fe(trispicMeen)(η2−OO)]+ (4) species, where bztpen and trispicMeen ligands differ by a noncoordinating group [A. J. Simaan, F. Banse, J.-J. Girerd, K. Wieghardt, E. Bill, Inorg. Chem.2001, 40, 6538−6540]. We have also used the extension of the Griffith model developed by Lang et al. for Mossbauer spectroscopy [W. T. Oosterhuis, G. Lang, Phys. Rev. 1969, 178, 439−456] to evaluate the 57Fe hyperfine tensor of 1 and reproduce the experimental data. On comparison with the Mossbauer data available for η2-peroxo FeIII complexes, our Mossbauer study of 2 agrees well with a high-spin FeIII side-on peroxo complex. Very characteristic δ/Fe and ΔEQ ranges for hydrogenperoxo and side-on peroxo FeIII species are now available, which should aid in the detection of comparable species formed in biological systems. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)