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Biochemistry Biochemistry Z 0250 Dioxygen Activation at Mononuclear Nonheme Iron Active Sites: Enzymes, Models, and Intermediates -[517 refs.]. -(COSTAS, M.; MEHN, M. P.; JENSEN, M. P.; QUE*, L. J.; Chem. Rev. (Washington, D. C.) 104 (2004) 2, 939-986; Dep. Chem., Univ. Minn., Minneapolis, MN 55455, USA; Eng.) -Lindner 21-291

Mononuclear iron(II) alpha-keto carboxylate and carboxylate compounds of the sterically hindered tridentate face-capping ligand Tp(Ph2) (Tp(Ph2) = hydrotris(3,5-diphenylpyrazol-1-yl)borate) were prepared as models for the active sites of nonheme iron oxygenases. The structures of an aliphatic alpha-keto carboxylate complex, [Fe(II)(Tp(Ph2))(O(2)CC(O)CH(3))], and the carboxylate complexes [Fe(II)(Tp(Ph2))(OBz)] and [Fe(II)(Tp(Ph2))(OAc)(3,5-Ph(2)pzH)] were determined by single-crystal X-ray diffraction, all of which have five-coordinate iron centers. Both the alpha-keto carboxylate and the carboxylate compounds react with dioxygen resulting in the hydroxylation of a single ortho phenyl position of the Tp(Ph2) ligand. The oxygenation products were characterized spectroscopically, and the structure of the octahedral iron(III) phenolate product [Fe(III)(Tp(Ph2))(OAc)(3,5-Ph(2)pzH)] was established by X-ray diffraction. The reaction of the alpha-keto carboxylate model compounds with oxygen to produce the phenolate product occurs with concomitant oxidative decarboxylation of the alpha-keto acid. Isotope labeling studies show that (18)O(2) ends up in the Tp(Ph2) phenolate oxygen and the carboxylate derived from the alpha-keto acid. The isotope incorporation mirrors the dioxygenase nature of the enzymatic systems. Parallel studies on the carboxylate complexes demonstrate that the oxygen in the hydroxylated ligand is also derived from molecular oxygen. The oxygenation of the benzoylformate complex is demonstrated to be first order in metal complex and dioxygen, with activation parameters DeltaH++ = 25 +/- 2 kJ mol(-1) and DeltaS++ = -179 +/- 6 J mol(-1) K(-1). The rate of appearance of the iron(III) phenolate product is sensitive to the nature of the substituent on the benzoylformate ligand, exhibiting a Hammett rho value of +1.3 indicative of a nucleophilic mechanism. The proposed reaction mechanism involves dioxygen binding to produce an iron(III) superoxide species, nucleophilic attack of the superoxide at the alpha-keto functionality, and oxidative decarboxylation of the adduct to afford the oxidizing species that attacks the Tp(Ph2) phenyl ring. Interestingly, the alpha-keto carboxylate complexes react 2 orders of magnitude faster than the carboxylate complexes, thus emphasizing the key role that the alpha-keto functionality plays in oxygen activation by alpha-keto acid-dependent iron enzymes.

Many nonheme iron-dependent enzymes activate dioxygen to catalyze hydroxylations of arene substrates. Key features of this chemistry have been developed from complexes of a family of tetradentate tripodal ligands obtained by modification of tris(2-pyridylmethyl)amine (TPA) with single alpha-arene substituents. These included the following: -C(6)H(5) (i.e., 6-PhTPA), L(1); -o-C(6)H(4)D, o-d(1)-L(1); -C(6)D(5), d(5)-L(1); -m-C(6)H(4)NO(2), L(2); -m-C(6)H(4)CF(3), L(3); -m-C(6)H(4)Cl, L(4); -m-C(6)H(4)CH(3), L(5); -m-C(6)H(4)OCH(3), L(6); -p-C(6)H(4)OCH(3), L(7). Additionally, the corresponding ligand with one alpha-phenyl and two alpha-methyl substituents (6,6-Me(2)-6-PhTPA, L(8)) was also synthesized. Complexes of the formulas [(L(1))Fe(II)(NCCH(3))(2)](ClO(4))(2), [(L(n)())Fe(II)(OTf)(2)] (n = 1-7, OTf = (-)O(3)SCF(3)), and [(L(8))Fe(II)(OTf)(2)](2) were obtained and characterized by (1)H NMR and UV-visible spectroscopies and by X-ray diffraction in the cases of [(L(1))Fe(II)(NCCH(3))(2)](ClO(4))(2), [(L(6))Fe(II)(OTf)(2)], and [(L(8))Fe(II)(OTf)(2)](2). The complexes react with tert-butyl hydroperoxide ((t)()BuOOH) in CH(3)CN solutions to give iron(III) complexes of ortho-hydroxylated ligands. The product complex derived from L(1) was identified as the solvated monomeric complex [(L(1)O(-))Fe(III)](2+) in equilibrium with its oxo-bridged dimer [(L(1)O(-))(2)Fe(III)(2)(mu(2)-O)](2+), which was characterized by X-ray crystallography as the BPh(4)(-) salt. The L(8) product was also an oxo-bridged dimer, [(L(8)O(-))(2)Fe(III)(2)(mu(2)-O)](2+). Transient intermediates were observed at low temperature by UV-visible spectroscopy, and these were characterized as iron(III) alkylperoxo complexes by resonance Raman and EPR spectroscopies for L(1) and L(8). [(L(1))Fe(II)(OTf)(2)] gave rise to a mixture of high-spin (S = 5/2) and low-spin (S = 1/2) Fe(III)-OOR isomers in acetonitrile, whereas both [(L(1))Fe(OTf)(2)] in CH(2)Cl(2) and [(L(8))Fe(OTf)(2)](2) in acetonitrile afforded only high-spin intermediates. The L(1) and L(8) intermediates both decomposed to form respective phenolate complexes, but their reaction times differed by 3 orders of magnitude. In the case of L(1), (18)O isotope labeling indicated that the phenolate oxygen is derived from the terminal peroxide oxygen via a species that can undergo partial exchange with exogenous water. The iron(III) alkylperoxo intermediate is proposed to undergo homolytic O-O bond cleavage to yield an oxoiron(IV) species as an unobserved reactive intermediate in the hydroxylation of the pendant alpha-aryl substituents. The putative homolytic chemistry was confirmed by using 2-methyl-1-phenyl-2-propyl hydroperoxide (MPPH) as a probe, and the products obtained in the presence and in the absence of air were consistent with formation of alkoxy radical (RO(*)). Moreover, when one ortho position was labeled with deuterium, no selectivity was observed between hydroxylation of the deuterated and normal isotopomeric ortho sites, but a significant 1,2-deuterium shift ("NIH shift") occurred....

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dinitrogen activation ͉ high-valent iron ͉ nitrogenase ͉ high-valent iron nitride ͉ spectroscopy

A new family of low-coordinate nickel imides supported by 1,2-bis(di-tert-butylphosphino)ethane was synthesized. The oxidation of nickel(II) complexes led to the formation of both aryl- and alkyl-substituted nickel(III) imides, and examples of both types have been isolated and fully characterized. The aryl substituent that proved most useful in stabilizing the Ni(III)-imide moiety was the bulky 2,6-dimesitylphenyl. The two nickel(III)-imide compounds showed different variable-temperature magnetic properties, but analogous EPR spectra at low temperatures. In order to account for this discrepancy, a low-spin/high-spin equilibrium was proposed to take place for the alkyl-substituted imide nickel(III) complex. This proposal was supported by DFT calculations. DFT calculations also indicated that the unpaired electron is mostly localized on the imide nitrogen for the nickel(III) complexes. The results of reactions carried out in the presence of hydrogen donors supported the findings from DFT calculations that the adamantyl substituent was a significantly more reactive hydrogen-atom abstractor. Interestingly, the steric properties of the 2,6-dimesitylphenyl substituent are important not only in protecting the Ni=N core but also in favoring one rotamer of the resulting nickel(III) imide, by locking in the phenyl ring in a perpendicular orientation with respect to the NiPP plane.

With the goal of gaining insight into the structures of peroxo intermediates observed for oxygen activating nonheme diiron enzymes, a series of metastable synthetic diiron(III)-peroxo complexes with [FeIII2(µ-O)(µ-1,2-O2)] cores has been characterized by X-ray absorption and resonance Raman spectroscopy. EXAFS analysis shows that this basic core structure gives rise to an Fe-Fe distance of ~3.15 Å; the distance is decreased by 0.1 Å upon introduction of an additional carboxylate bridge. In corresponding resonance Raman studies, vibrations arising from both the Fe-O-Fe and the Fe-O-O-Fe units can be observed. A change in the Fe-Fe distance affects the ν(O-O) mode, as well as the νsym(Fe-O-Fe) and the νasym(Fe-O-Fe) modes. Indeed a linear correlation can be discerned between the ν(O-O) frequency of a complex and its Fe-Fe distance among the subset of complexes with [FeIII2(µ-OR)(µ-1,2-O2)] cores (R = H, alkyl, aryl, or no substituent). These experimental studies are complemented by a normal coordinate analysis and DFT calculations.