Chemistry:High-valent iron

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Short description: Iron in an oxidation state higher than III
Ferrate(VI) ion, [FeO4]2−

High-valent iron commonly denotes compounds and intermediates in which iron is found in a formal oxidation state greater than +3 that show a number of bonds above 6 with a coordination number no more than 6.[according to whom?] The ferrate(VI) ion [FeO4]2− was the first structure in this class synthesized. The synthetic compounds discussed below contain highly oxidized iron in general, as the concepts are closely related.

Oxoiron compounds

Oxoferryl species are common examples of high-valent iron complexes. Such compounds are prepared by oxidation of ferrous complexes with iodosobenzene:[1][2]

(mac)FeL
2 + OIPh → (mac)Fe=O(L) + IPh + L
(mac = tetradentate macrocyclic ligand)

Fe(IV)O

Thiolate-ligated oxoiron(IV)

Several syntheses of oxoiron(IV) species have been reported. The simplest are mixed-metal oxides of the form MFeO3, with M being Ba, Ca, or Sr. However, those compounds do not have discrete iron anions.[3]

Isolated oxoiron(IV) species are known with more complicated ligands. These compounds model biological complexes such as cytochrome P450, NO synthase, and isopenicillin N synthase. Two such reported compounds are thiolate-ligated oxoiron(IV) and cyclam-acetate oxoiron(IV).[4]

Thiolate-ligated oxoiron(IV) is formed by the oxidation of a precursor, [FeII(TMCS)](PF6) (where TMCS is 1-mercaptoethyl-4,8,11-trimethyl-1,4,8,11-tetraazacyclotetradecane), and 3 to 5 equivalents of H2O2 at −60 °C in methanol. The iron(IV) compound is deep blue in color and shows intense absorption features at 460 nm, 570 nm, 850 nm, and 1050 nm. This species FeIV(=O)(TMCS)+ is stable at −60 °C, but decomposition is reported as temperature increases. Compound 2 was identified by Mössbauer spectroscopy, high resolution electrospray ionization mass spectrometry (ESI-MS), X-ray absorption spectroscopy, extended X-ray absorption fine structure (EXAFS), ultraviolet–visible spectroscopy (UV-vis), Fourier-transform infrared spectroscopy (FT-IR), and results were compared to density functional theory (DFT) calculations.[5]

Tetramethylcyclam-supported oxoiron(IV)

Tetramethylcyclam oxoiron(IV) is formed by the reaction of FeII(TMC)(OTf)2, TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane; OTf = CF3SO3, with iodosylbenzene (PhIO) in CH3CN at −40 °C. A second method for formation of cyclam oxoiron(IV) is reported as the reaction of FeII(TMC)(OTf)2 with 3 equivalents of H2O2 for 3 hours. This species is pale green in color and has an absorption maximum at 820 nm. It is reported to be stable for at least 1 month at −40 °C. It has been characterized by Mössbauer spectroscopy, ESI-MS, EXAFS, UV-vis, Raman spectroscopy, and FT-IR.[6]

High-valent iron bispidine complexes can oxidize cyclohexane to cyclohexanol and cyclohexanone in 35% yield with an alcohol-to-ketone ratio up to 4.[7]

Fe(V)O

FeVTAML(=O), where TAML is a tetra-amido macrocyclic ligand, is formed by the reaction of [FeIII(TAML)(H2O)](PPh4) with 2 to 5 equivalents of meta-chloroperbenzoic acid at −60 °C in n-butyronitrile. This deep green compound (two λmax at 445 and 630 nm respectively) is stable at 77 K. The stabilization of Fe(V) is attributed to the strong π-donor capacity of deprotonated amide nitrogens.[8]

Fe(VI)O

Main article: Chemistry:Ferrate(VI)

Ferrate(VI) is found in the inorganic anion [FeO4]2−. It has been isolated as the potassium salt, potassium ferrate. It is a strong water-stable oxidizing agent. Its solutions are stable at high pH.

Nitridoiron and imidoiron compounds

Generation of an nitridoiron(VI) complex

Nitridoiron[9] and imidoiron[10] compounds are closely related to iron-dinitrogen chemistry.[11] The biological significance of nitridoiron(V) porphyrins has been reviewed.[12][13] A widely applicable method to generate high-valent nitridoiron species is the thermal or photochemical oxidative elimination of molecular nitrogen from an azide complex.

(L)Fen
N
3 → (L)Fen+2
N + N
2
Symbolic oxidative elimination of nitrogen yields a nitridoiron complex; L denotes the supporting ligand.

Fe(IV)N

Several structurally characterized nitridoiron(IV) compounds exist.[14][15][16]

Fe(V)N

The first nitridoiron(V) compound was synthesised and characterized by Wagner and Nakamoto (1988, 1989) using photolysis and Raman spectroscopy at low temperatures.[17][18]

Fe(VI)N

A second FeVI species apart from the ferrate(VI) ion, [(Me3cy-ac)FeN](PF6)2, has been reported. This species, is formed by oxidation followed by photolysis to yield the Fe(VI) species. Characterization of the Fe(VI) complex was done by Mössbauer, EXAFS, IR, and DFT calculations. Unlike the ferrate(VI) ion, compound 5 is diamagnetic.[19]

μ-Nitrido compounds and oxidation catalysis

Bridged μ-nitrido diiron phthalocyanine compounds such as iron(II) phthalocyanine catalyze the oxidation of methane to methanol, formaldehyde, and formic acid using hydrogen peroxide as sacrificial oxidant.[20][21]

Electronic structure

Nitridoiron(IV) and nitridoiron(V) species were first explored theoretically in 2002.[22]

See also

References

  1. ↑ Que, L. Jr.; Shan, X. (April 2006). "High-valent nonheme iron–oxo species in biomimetic oxidations". Journal of Inorganic Biochemistry 100 (4): 421–433. doi:10.1016/j.jinorgbio.2006.01.014. 
  2. ↑ Fujii, H. (March 2002). "Electronic structure and reactivity of high-valent oxo iron porphyrins". Coordination Chemistry Reviews 226 (1–2): 51–60. doi:10.1016/S0010-8545(01)00441-6. 
  3. ↑ Housecroft, C. E.; Sharpe, A. G. (2018). Inorganic Chemistry (5th ed.). Prentice-Hall. p. 769. ISBN 978-0273742753. 
  4. ↑ Yee, G. M.; Tolman, W. B. (2015). "Chapter 5, Section 2.2.4 Fe(IV)-Oxo Intermediates". in Kroneck, P. M. H.; Sosa Torres, M. E.. Sustaining Life on Planet Earth: Metalloenzymes Mastering Dioxygen and Other Chewy Gases. Metal Ions in Life Sciences. 15. Springer. pp. 145–146. doi:10.1007/978-3-319-12415-5_5. ISBN 978-3-319-12414-8. 
  5. ↑ Bukowski, M. R.; Koehntop, K. D.; Stubna, A.; Bominaar, E. L.; Halfen, J. A.; Munck, E.; Nam, W.; Que, L. (2005). "A thiolate-ligated nonheme oxoiron(IV) complex relevant to cytochrome P450". Science 310: 1000–1002. doi:10.1126/science.1119092. 
  6. ↑ Rohde, J.-U.; In, J.-H.; Lim, M. H.; Brennessel, W. W.; Bikowski, M. R.; Stubna, A.; Munck, E.; Nam, W. et al. (2003). "Crystallographic and spectroscopic characterization of a nonheme Fe(IV)=O Complex". Science 299: 1037–1039. doi:10.1126/science.299.5609.1037. 
  7. ↑ Comba, P.; Maurer, M.; Vadivelu, P. (2009). "Oxidation of cyclohexane by high-valent iron bispidine complexes: tetradentate versus pentadentate ligands". Inorganic Chemistry 48 (21): 10389–10396. doi:10.1021/ic901702s. 
  8. ↑ Oliveira, F. T.; Chanda, A.; Banerjee, D.; Shan, X.; Mondal, S.; Que, L.; Bominaar, E. L.; Munck, E. et al. (2007). "Chemical and spectroscopic evidence for an FeV-oxo complex". Science 315: 835–838. doi:10.1126/science.1133417. 
  9. ↑ Berry, J. F. (2009). "Terminal nitrido and imido complexes of the late transition metals". Comments on Inorganic Chemistry 30: 28–66. doi:10.1080/02603590902768875. 
  10. ↑ Peters, J. C.; Mehn, M. P. (April 2006). "Mid- to high-valent imido and nitrido complexes of iron". Journal of Inorganic Biochemistry 100 (4): 634–643. doi:10.1016/j.jinorgbio.2006.01.023. 
  11. ↑ Tyler, D. R.; Crossland, J. E. (2010). "Iron–dinitrogen coordination chemistry: Dinitrogen activation and reactivity". Coordination Chemistry Reviews 254: 1883–1894. doi:10.1016/j.ccr.2010.01.005. 
  12. ↑ Nakamoto, K. (March 2002). "Resonance Raman spectra and biological significance of high-valent iron(IV,V) porphyrins". Coordination Chemistry Reviews 226 (1–2): 153–165. doi:10.1016/S0010-8545(01)00425-8. 
  13. ↑ Nakamoto, K. (June 1997). "Structure, spectra and biological significance of high-valent iron(IV,V) porphyrins". Journal of Molecular Structure 408–409: 11–16. doi:10.1016/S0022-2860(96)09670-6. 
  14. ↑ Rohde, J.-U.; Betley, T. A.; Jackson, T. A.; Saouma, C. T.; Peters, J. C.; Que, L. Jr. (2007). "XAS characterization of a nitridoiron(IV) complex with a very short Fe−N bond". Inorganic Chemistry 46 (14): 5720–5726. doi:10.1021/ic700818q. 
  15. ↑ Scepaniak, J. J.; Young, J. A.; Bontchev, R. P.; Smith, J. M. (April 2009). "Formation of ammonia from an iron nitrido complex". Angewandte Chemie International Edition 48 (17): 3158–3160. doi:10.1002/anie.200900381. 
  16. ↑ Vogel, C.; Heinemann, F. W.; Sutter, J.; Anthon, C.; Meyer, K. (March 2008). "An iron nitride complex". Angewandte Chemie International Edition 47 (14): 2681–2684. doi:10.1002/anie.200800600. 
  17. ↑ Wagner, W. D.; Nakamoto, K. (1988). "Formation of nitridoiron(V) porphyrins detected by resonance Raman spectroscopy". Journal of the American Chemical Society 110 (12): 4044–4045. doi:10.1021/ja00220a057. 
  18. ↑ Wagner, W. D.; Nakamoto, K. (1989). "Resonance Raman spectra of nitridoiron(V) porphyrin intermediates produced by laser photolysis". Journal of the American Chemical Society 111 (5): 1590–1598. doi:10.1021/ja00187a010. 
  19. ↑ Berry, J. F.; Bill, E.; Bothe, E.; George, S. D.; Miener, B.; Neese, F.; Wieghardt, K. (2006). "An octahedral coordination complex of iron(VI)". Science 312: 1937–1941. doi:10.1126/science.1128506. 
  20. ↑ Sorokin, A. B.; Kudrik, E. V.; Bouchu, D. (2008). "Bio-inspired oxidation of methane in water catalyzed by N-bridged diiron phthalocyanine complex". Chemical Communications 44 (22): 2562–2564. doi:10.1039/B804405H. 
  21. ↑ Que, L.; Tolman, W. B. (18 September 2008). "Biologically inspired oxidation catalysis". Nature 455: 333–340. doi:10.1038/nature07371. 
  22. ↑ Dey, A.; Ghosh, A. (2002). "“True” iron(V) and iron(VI) porphyrins: a first theoretical exploration". Journal of the American Chemical Society 124 (13): 3206–3207. doi:10.1021/ja012402s. 

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