Protonation of a peroxodiiron(III) complex and conversion to a diiron(III/IV) intermediate: implications for proton-assisted O-O bond cleavage in nonheme diiron enzymes.

Oxygenation of a diiron(II) complex, [Fe(II)(2)(μ-OH)(2)(BnBQA)(2)(NCMe)(2)](2+) [2, where BnBQA is N-benzyl-N,N-bis(2-quinolinylmethyl)amine], results in the formation of a metastable peroxodiferric intermediate, 3. The treatment of 3 with strong acid affords its conjugate acid, 4, in which the (μ-oxo)(μ-1,2-peroxo)diiron(III) core of 3 is protonated at the oxo bridge. The core structures of 3 and 4 are characterized in detail by UV-vis, Mössbauer, resonance Raman, and X-ray absorption spectroscopies.

Synthesis and characterization of three-coordinate Ni(III)-imide complexes.

A new family of low-coordinate nickel imides supported by 1,2-bis(di-tert-butylphosphino)ethane was synthesized. 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.

Facile Routes to Manganese(II) Triflate Complexes.

Manganese(II) chloride reacts with trimethylsilyl triflate (TMS(OTf) where OTf = (-)OSO(2)CF(3)) in a 1:1 mixture of acetonitrile and tetrahydrofuran, and after recrystallization affords the linear coordination polymer [Mn(II)(CH(3)CN)(2)(OTf)(2)](n). Each distorted octahedral manganese(II) center in the polymeric chain has trans-acetonitriles and the remaining equatorial coordination positions are occupied by the bridging triflate anions. Dissolving [Mn(II)(CH(3)CN)(2)(OTf)(2)](n) in equal volumes of acetonitrile and pyridine followed by recrystallization with diethyl ether yields trans-[Mn(II)(C(5)H(5)N)(4)(OTf)(2)].

Synthesis and characterization of sterically encumbered β-ketoiminate complexes of iron(II) and zinc(II).

The synthesis, structure, and spectroscopic signatures of a series of four-coordinate iron(II) complexes of β-ketoiminates and their zinc(II) analogues are presented. An unusual five-coordinate iron(II) triflate with three oxygen bound protonated β-ketoimines is also synthesized and structurally characterized. Single-crystal X-ray crystallographic analysis reveals that the deprotonated bis(chelate)metal complexes are four-coordinate with various degrees of distortion depending on the degree of steric bulk and the electronics of the metal center.

Synthesis and characterization of iron(II) quinaldate complexes.

Treatment of iron(II) chloride or iron(II) bromide with 2 equiv of sodium quinaldate (qn = quinaldate or C(10)H(6)NO(2)(-)) yields the coordinatively unsaturated mononuclear iron(II) quinaldate complexes Na[Fe(II)(qn)(2)Cl].DMF and Na[Fe(II)(qn)(2)Br].DMF (DMF = N,N-dimethylformamide), respectively.

Spectroscopic and computational studies of (mu-oxo)(mu-1,2-peroxo)diiron(III) complexes of relevance to nonheme diiron oxygenase intermediates.

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 [Fe(III)(2)(mu-O)(mu-1,2-O(2))] cores has been characterized by X-ray absorption and resonance Raman spectroscopies, EXAFS analysis shows that this basic core structure gives rise to an Fe-Fe distance of approximately 3.15 A; the distance is decreased by 0.1 A upon introduction of an additional carboxylate bridge.

On the feasibility of N2 fixation via a single-site FeI/FeIV cycle: Spectroscopic studies of FeI(N2)FeI, FeIV[triple bond]N, and related species.

The electronic properties of an unusually redox-rich iron system, [PhBP(R)3]Fe-Nx (where [PhBP(R)3] is [PhB(CH2PR2)3]-), are explored by Mössbauer, EPR, magnetization, and density-functional methods to gain a detailed picture regarding their oxidation states and electronic structures. The complexes of primary interest in this article are the two terminal iron(IV) nitride species, [PhBP(iPr)3]Fe[triple bond]N (3a) and [PhBP(CH2Cy)3]Fe[triple bond]N (3b), and the formally diiron(I) bridged-Fe(mu-N2)Fe species, {[PhBP(iPr)3]Fe}2(mu-N2) (4). Complex 4 is chemically related to 3a via a spontaneous nitride coupling reaction.

Vibrational spectroscopy and analysis of pseudo-tetrahedral complexes with metal imido bonds.

A number of assignments have been previously posited for the metal-nitrogen stretch (nu(M-NR)), the N-R stretch (nu(MN-R)), and possible ligand deformation modes associated with terminally bound imides. Here we examine mononuclear iron(III) and cobalt(III) imido complexes of the monoanionic tridentate ligand [PhBP3] ([PhBP3] = [PhB(CH2PPh2)3]-) to clarify the vibrational features for these trivalent metal imides. We report the structures of [PhBP3]FeNtBu and [PhBP3]CoNtBu.

Mid- to high-valent imido and nitrido complexes of iron.

This short review summarizes recent advances in the synthesis and reactivity of iron imides and nitrides. Both monometallic and multimetallic assemblies featuring these moieties are discussed. Recent synthetic approaches have led to the availability of new mid- to high-valent iron imides and nitrides, allowing us to begin assembling models to describe the factors influencing their relative stabilities and reactivity patterns.

High-spin and low-spin iron(II) complexes with facially-coordinated borohydride ligands.

Rare examples of monometallic high-spin and low-spin L3Fe(H3BH) complexes have been characterized, where the two L3 ligands are [TpPh2] and [PhBP3] ([TpPh2] = [HB(3,5-Ph2pz)3]- and [PhBP3] = [PhB(CH2PPh2)3]-). The structures are reported wherein the borohydride ligand is facially coordinated to the iron center in each complex. Density functional methods have been employed to explain the bonding in these unusual iron(II) centers.

Heterolytic H2 activation mediated by low-coordinate L3Fe-(mu-N)-FeL3 complexes to generate Fe(mu-NH)(mu-H)Fe species.

The diiron mu-nitride complexes, {L3FeII(mu-N)FeIIL3}- and L3FeIII(mu-N)FeIIL3, heterolytically activate hydrogen (1 atm) at ambient temperature in solution (L3 = [PhB(CH2PPh2)3]-). These transformations lead to structurally unique {L3FeII(mu-NH)(mu-H)FeIIL3}- and L3FeIII(mu-NH)(mu-H)FeIIL3 products. X-ray data establish a marked reduction in the Fe-Fe distance upon H2 uptake, and spectroscopic data establish both FeIIFeII species to be diamagnetic, whereas the FeIIIFeII species, L3FeIII(mu-N)FeIIL3 and L3FeIII(mu-NH)(mu-H)FeIIL3, populate doublet ground states with thermally accessible higher spin states.

Oxygenase activity in the self-hydroxylation of (s)-2-hydroxypropylphosphonic acid epoxidase involved in fosfomycin biosynthesis.

The last step of the biosynthesis of fosfomycin is the conversion of (S)-2-hydroxypropylphosphonic acid (HPP) to fosfomycin by HPP epoxidase (HppE), which is a mononuclear non-heme iron-dependent enzyme. The apo-HppE from Streptomyces wedmorensis is colorless, but turns green with broad absorption bands at 430 and 680 nm after reconstitution with ferrous ion under aerobic conditions. Resonance Raman studies showed that this green chromophore arises from a bidentate iron(III)-catecholate (DOPA) complex, and the most likely site of modification is at Tyr105 on the basis of site-specific mutagenesis results.

Oxygen activation by nonheme iron(II) complexes: alpha-keto carboxylate versus carboxylate.

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.

Biomimetic aryl hydroxylation derived from alkyl hydroperoxide at a nonheme iron center. Evidence for an Fe(IV)=O oxidant.

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.