Keywordsbioinorganic chemistry iron-oxo; nonheme iron complexes; high-valent compounds; enzyme models High-valent oxoferryl intermediates have been proposed as the active oxidants in the catalytic cycles of a wide range of mononuclear non-heme oxygen activating enzymes.[1] These high-valent species have now been spectroscopically characterized for four enzymes and were found in all instances to contain high-spin (S = 2) iron(IV) centers.[2] Contemporaneously, the first examples of the existing family of synthetic nonheme oxoiron(IV) complexes were characterized, [3][4][5] which are exclusively octahedral and in all but one case exhibit the S = 1, rather than S = 2, spin-state. Given that DFT suggests higher reactivity for an S = 2 oxoiron(IV) unit, [6,7] it is perhaps not surprising that there is a scarcity of such complexes. Indeed, the only example to date is [Fe IV (O)(H 2 O) 5 ] 2+ (1),

A series of iron(II) bis(triflate) complexes containing tripodal tetradentate nitrogen ligands with pyridine and dimethylamine donors of the type [N(CH(2)Pyr)(3-n)()(CH(2)CH(2)NMe(2))(n)] [n = 0 (tpa, 1), n = 1 (iso-bpmen, 3), n = 2 (Me(4)-benpa, 4), n = 3 (Me(6)-tren, 5)] and the linear tetradentate ligand [(CH(2)Pyr)MeN(CH(2)CH(2))NMe(CH(2)Pyr), (bpmen, 2)] has been prepared. The preferred coordination geometry of these complexes in the solid state and in CH(2)Cl(2) solution changes from six- to five-coordinate in the order from 1 to 5. In acetonitrile, the triflate ligands of all complexes are readily displaced by acetonitrile ligands. The complex [Fe(1)(CH(3)CN)(2)](2+) is essentially low spin at room temperature, whereas ligands with fewer pyridine donors increase the preference for high-spin Fe(II). Both the number of pyridine donors and the spin state of the metal center strongly affect the intensity of a characteristic MLCT band around 400 nm. The catalytic properties of the complexes for the oxidation of alkanes have been evaluated, using cyclohexane as the substrate. Complexes containing ligands 1-3 are more active and selective catalysts, possibly operating via a metal-based oxidation mechanism, whereas complexes containing ligands 4 and 5 give rise to Fenton-type chemistry.

[FeIV(O)(TMG3tren)]2+ (1; TMG3tren = 1,1,1-tris{2-[N2-(1,1,3,3-tetramethylguanidino)]ethyl}amine) is a unique example of an isolable synthetic S = 2 oxoiron(IV) complex, which serves as a model for the high-valent oxoiron(IV) intermediates observed in nonheme iron enzymes. Congruent with DFT calculations predicting a more reactive S = 2 oxoiron(IV) center, 1 has a lifetime significantly shorter than related S = 1 oxoiron(IV) complexes. Self-decay of 1 exhibits strictly first-order kinetic behavior and is unaffected by solvent deuteration, suggesting an intramolecular process. This hypothesis was supported by ESI-MS analysis of the iron products and a significant retardation of self-decay upon use of a perdeuteromethyl TMG3tren isotopomer, d36-1 (KIE = 24 at 25°C). The greatly enhanced thermal stability of d36-1 allowed growth of diffraction quality crystals for which a high-resolution crystal structure was obtained. This structure showed an Fe=O unit (r = 1.661(2) Å) in the intended trigonal bipyramidal geometry enforced by the sterically bulky tetramethylguanidinyl donors of the tetradentate tripodal TMG3tren ligand. The close proximity of the methyl substituents to the oxoiron unit yielded three symmetrically oriented short C-D···O non-bonded contacts (2.38 – 2.49 Å), an arrangement that facilitated self-decay by rate-determining intramolecular hydrogen atom abstraction and subsequent formation of a ligand-hydroxylated iron(III) product. EPR and Mössbauer quantification of the various iron products, referenced against those obtained from reaction of 1 with 1,4-cyclohexadiene, allowed formulation of a detailed mechanism for the self-decay process. The solution of this first crystal structure of a high-spin (S = 2) oxoiron(IV) center represents a fundamental step on the path towards a full understanding of these pivotal biological intermediates.

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