Deprotonation of low-spin mononuclear iron(III)–hydroperoxide complexes give transient blue species assigned to high-spin iron(III)–peroxide complexes

Abstract

Purple iron(III)–hydroperoxide complex ions [Fe(Rtpen)(η¹-OOH)]²⁺ can be reversibly deprotonated to give transient blue species showing spectroscopic properties consistent with iron(III)–peroxide complexes; a novel ferryl species is produced in MS/MS experiments with the iron(III) hydroperoxide ion.

References

1. L. Que and R. Y. N. Ho, Chem. Rev., 1996, 96, 2607.
2. J. W. Sam, X.-J. Tang and J. Peisach, J. Am. Chem. Soc., 1994, 116, 5250; R. J. Guajardo, J. D. Tan and P. K. Mascharak, Inorg. Chem., 1994, 33, 2838.
3. I. Bernal, I. M. Jensen, K. B. Jensen, C. J. McKenzie, H. Toftlund and J. P. Tuchagues, J. Chem. Soc., Dalton Trans., 1995, 3667.
4. K. B. Jensen, Ph.D. Thesis, Odense University, Denmark, 1997.
5. C. Kim, K. Chen, J. Kim and L. Que, Jr., J. Am. Chem. Soc., 1997, 119, 5964; M. Lubben, A. Meetsma, E. C. Wilkinson, B. Feringa and L. Que, Jr., Angew. Chem., Int. Ed. Engl., 1995, 34, 2048; Y. Zang, T. E. Elgren, Y. Dong and L. Que, Jr., J. Am. Chem. Soc., 1993, 115, 811.
6. S. Ahmad, J. D. McCallum, A. K. Shiemke, E. H. Appelman, T. M. Loehr and J. Sanders-Loehr, Inorg. Chem., 1988, 27, 2230.
7. F. Neese and E. I. Solomon, J. Am. Chem. Soc., 1998, 120, 12 829.
8. Purple species, λ_max/nm: [Fe(metpen)(OOH)]²⁺, 536; [Fe(ettpen)(OOH)]²⁺, 536: blue species, λ_max/nm: [Fe(metpen)(OO)]²⁺, 748; [Fe(ettpen)(OO)]²⁺, 747.
9. The related compounds [Fe(tpen)](ClO₄)₃[tpen =N,N,N′,N′-tetrakis(2-pyridylmethyl)ethane-1,2-diamine and Na[Fe(edta)](edta^4–= ethylendiaminetetraacetate) were used to calibrate the low-spin iron(III) and high-spin iron(III) signals, respectively. Possible differences in the rates of relaxation for the various iron compounds were eliminated by calibrating at several probe temperatures. Thus an estimation of the percentage conversion in the reactions with hydrogen peroxide was made. 100% of the iron(II) species [Fe(bztpen)Cl]⁺ can be converted to the low-spin iron(III) species [Fe(bztpen)X]²⁺ and [Fe(bztpen)OOH]²⁺(relative amounts depending on the H₂O₂ excess); 25% of the low-spin [Fe(bztpen)OOH]²⁺ can be converted to the high-spinsignal assigned to [Fe(bztpen)OO]⁺. There are two factors which might be responsible for the apparent incomplete conversion: (i) All the reactions are in equilibrium (as depicted in Scheme 1) and the iron(II) species are favoured. (ii) Decomposition of all or some of the iron(III) species depicted in Scheme 1 and formation of EPR silent oxo-bridged dimers. In support of the second explanation peaks assignable to [(bztpen)ClFe(µ-O)FeCl(bztpen)]²⁺(m/z 522) are indeed observed in ESI mass spectra of the purple and blue solutions.
10. N. Kitajima, K. Fujisawa, C. Fujimoto, Y. Moro-oka, S. Hashimoto, T. Kitagawa, K. Toriumi, K. Tatsumi and A. Nakamara, J. Am. Chem. Soc., 1992, 114, 1277.