High-valent iron

High-valent iron commonly denotes compounds and intermediates in which iron is found in a formal oxidation state > 3 that show a number of bonds > 6 with a coordination number ≤ 6. The term is rather uncommon for hepta-coordinate compounds of iron.[1] It has to be distinguished from the terms hypervalent and hypercoordinate, as high-valent iron compounds neither necessarily violate the 18-electron rule nor necessarily show coordination numbers > 6. 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.

ferrate(VI) ion, [FeO4]2−

Oxoiron compounds[2]

Oxoferryl species are commonly proposed as intermediates in catalytic cycles, especially biological systems in which O2 activation is required. Diatomic oxygen has a high reduction potential (E0 = 1.23 V), but the first step required to harness this potential is a thermodynamically unfavorable one electron reduction E0 = -0.16 V. This reduction occurs in nature by the formation of a superoxide complex in which a reduced metal is oxidized by O2. The product of this reaction is a peroxide radical that is more readily reactive. The abundance of these species in nature and the chemistry that is available to them are the reasons why the study of these compounds is important. A widely applicable method for the generation of high-valent oxoferryl species is the oxidation with iodosobenzene:

symbolic oxidation of an iron compound using iodosobenzene; L denotes the supporting ligand

Fe(IV)O
thiolate-ligated oxoiron(IV)

Several syntheses of oxoiron(IV) species have been reported.[3] 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). Thiolate-ligated oxoiron(IV) is formed by the oxidation of a precursor, [FeII(TMCS)](PF6) (TMCS = 1-mercaptoethyl-4,8,11-trimethyl-1,4,8,11-tetraza cyclotetradecane), and 3-5 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.[4]

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.[5]

High-valent iron bispidine complexes can oxidize cyclohexane to cyclohexanol and cyclohexanone in 35% yield with an alcohol to ketone ratio up to 4.[6]

Fe(V)O

FeVTAML(=O), TAML = tetra-amido macrocyclic ligand, is formed by the reaction of [FeIII(TAML)(H2O)](PPh4) with 2-5 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.[7]

Fe(VI)O
Main article: Ferrate(VI)

Ferrate(VI) is an inorganic anion of chemical formula [FeO4]2−. It is photosensitive and contributes a pale violet colour to its compounds and solutions. It is one of the strongest water-stable oxidising species known. Although it is classified as a weak base, concentrated solutions of ferrate(VI) are only stable at high pH.

Electronic structure

The electronic structure of porphyrin oxoiron compounds has been reviewed.[8]

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.

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 Mossbauer, EXAFS, IR, and DFT calculations. Unlike the ferrate(VI) ion, compound 5 is diamagnetic.[19]

μ-Nitrido compounds and oxidation catalysis[20]

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

Electronic structure

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

References
  1. Craig et al. Dalton Trans., 2010, 39, 4874-4881 doi:10.1039/B927032A
  2. Que et al.; Journal of Inorganic Biochemistry Volume 100, Issue 4, April 2006, Pages 421-433;doi:10.1016/j.jinorgbio.2006.01.014
  3. Yee, Gereon M.; Tolman, William B. (2015). "Chapter 5, Section 2.2.4 Fe(IV)-Oxo Intermediates". In Peter M.H. Kroneck and Martha E. Sosa Torres (ed.). 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.
  4. Bukowski, M. R., Koehntop, K. D., Stubna, A., Bominaar E. L., Halfen, J. A., Munck, E., Nam, W., Que, L., Science, 310, 1000-1002, 2005; doi:10.1126/science.111909
  5. Rohde, J.-U., In, J.-H., Lim, M. H., Brennessel, W. W., Bikowski, M. R., Stubna, A., Munck, E., Name, W., Que, L., Science, 299, 1037-1039, 2003; doi:10.1126/science.299.5609.1037
  6. Comba, P. et al.; Inorg. Chem., 2009, 48 (21), pp 10389–10396; doi:10.1021/ic901702s
  7. Oliveira, F. T., Chanda, A., Banerjee, D., Shan, X., Mondal, S., Que, L., Bominaar, E. L., Munck, E., Collins, T. J., Science, 315, 835-838, 2007; doi:10.1126/science.1133417
  8. Fujii, H.; Coordination Chemistry Reviews Volume 226, Issues 1-2, March 2002, Pages 51-60; doi:10.1016/S0010-8545(01)00441-6
  9. Berry, J.F.; Comments on Inorganic Chemistry, 30: 28–66, 2009; doi:10.1080/02603590902768875
  10. Peters, J.C., Mehn, M.P.; Journal of Inorganic Biochemistry Volume 100, Issue 4, April 2006, Pages 634-643; doi:10.1016/j.jinorgbio.2006.01.023
  11. Tyler, D. R., Crossland, J. E.; Coordination Chemistry Reviews 254 (2010) 1883–1894; doi:10.1016/j.ccr.2010.01.005
  12. Nakamoto, K.; Coordination Chemistry Reviews Volume 226, Issues 1-2, March 2002, Pages 153-165; doi:10.1016/S0010-8545(01)00425-8
  13. Nakamoto, K.; Journal of Molecular Structure Volumes 408-409, 1 June 1997, Pages 11-16; doi:10.1016/S0022-2860(96)09670-6
  14. Peters, Jonas C.; Que, Lawrence, Jr. et al.; Inorg. Chem., 2007, 46 (14), pp 5720–5726; doi:10.1021/ic700818q
  15. Smith et al.; Angewandte Chemie International Edition Volume 48, Issue 17, pages 3158–3160, April 14, 2009; doi:10.1002/anie.200900381
  16. Meyer et al.; Angewandte Chemie International Edition Volume 47, Issue 14, pages 2681–2684, March 25, 2008, April 14, 2009; doi:10.1002/anie.200800600
  17. Wagner, W.D.; Nakamoto, K.; J. Am. Chem. Soc., 1988, 110 (12), pp 4044–4045; doi:10.1021/ja00220a057
  18. Wagner, W.D.; Nakamoto, K.; J. Am. Chem. Soc., 1989, 111 (5), pp 1590–1598; doi:10.1021/ja00187a010
  19. Berry, J. F., Bill, E., Bothe, E., George, S. D., Miener, B., Neese, F., Wieghardt, K., Science, 312, 1937-1941, 2006; doi:10.1126/science.1128506
  20. Review: Que, L., Tolman, W.B.; Nature 455, 333-340 (18 September 2008); doi:10.1038/nature07371
  21. Sorokin, A.B.; Kudrik, E.V.; Bouchu, D.; Chem. Commun., 2008, 2562-2564; doi:10.1039/B804405H
  22. Dey, A.; Ghosh, A.; J. Am. Chem. Soc., 2002, 124 (13), pp 3206–3207; doi:10.1021/ja012402s

See also

Further reading
  • Solomon et al.; Angewandte Chemie International Edition Volume 47, Issue 47, pages 9071–9074, November 10, 2008; doi:10.1002/anie.200803740
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