Xanthene-modified and hangman iron corroles

Iron corroles modified with a xanthene scaffold are delivered from easily available starting materials in abbreviated reaction times. These new iron corroles have been spectroscopically examined with particular emphasis on defining the oxidation state of the metal center. Investigation of their electronic structure using (57)Fe Mo?ssbauer spectroscopy in conjunction with density functional theory (DFT) calculations reveals the non-innocence of the corrole ligand. Although these iron corroles contain a formal Fe(IV) center, the deprotonated corrole macrocycle ligand is one electron oxidized. The electronic ground state of these complexes is best described as an intermediate spin S = 3/2 Fe(III) site strongly antiferromagnetically coupled to the S = 1/2 of the monoradical dianion corrole [Fe(III)Cl-corrole(+?)]. We show here that iron corroles as well as xanthene-modified and hangman xanthene iron corroles are redox active and catalyze the disproportionation of hydrogen peroxide via the catalase reaction, and that this activity scales with the oxidation potential. The meso position of corrole macrocycle is susceptible toward nucleophilic attack during catalase turnover. The reactivity of peroxide within the hangman cleft reported here adds to the emerging theme that corroles are good at catalyzing two-electron activation of the oxygen-oxygen bond in a variety of substrates.     

Extended X-ray absorption fine structure and nuclear resonance vibrational spectroscopy reveal that NifB-co, a FeMo-co precursor, comprises a 6Fe core with an interstitial light atom

NifB-co, an Fe-S cluster produced by the enzyme NifB, is an intermediate on the biosynthetic pathway to the iron molybdenum cofactor (FeMo-co) of nitrogenase. We have used Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy together with (57)Fe nuclear resonance vibrational spectroscopy (NRVS) to probe the structure of NifB-co while bound to the NifX protein from Azotobacter vinelandii. The spectra have been interpreted in part by comparison with data for the completed FeMo-co attached to the NafY carrier protein: the NafY:FeMo-co complex. EXAFS analysis of the NifX:NifB-co complex yields an average Fe-S distance of 2.26 A and average Fe-Fe distances of 2.66 and 3.74 A. Search profile analyses reveal the presence of a single Fe-X (X = C, N, or O) interaction at 2.04 A, compared to a 2.00 A Fe-X interaction found in the NafY:FeMo-co EXAFS. This suggests that the interstitial light atom (X) proposed to be present in FeMo-co has already inserted at the NifB-co stage of biosynthesis. The NRVS exhibits strong bands from Fe-S stretching modes peaking around 270, 315, 385, and 408 cm(-1). Additional intensity at approximately 185-200 cm(-1) is interpreted as a set of cluster "breathing" modes similar to those seen for the FeMo-cofactor. The strength and location of these modes also suggest that the FeMo-co interstitial light atom seen in the crystal structure is already in place in NifB-co. Both the EXAFS and NRVS data for NifX:NifB-co are best simulated using a Fe 6S 9X trigonal prism structure analogous to the 6Fe core of FeMo-co, although a 7Fe structure made by capping one trigonal 3S terminus with Fe cannot be ruled out. The results are consistent with the conclusion that the interstitial light atom is already present at an early stage in FeMo-co biosynthesis prior to the incorporation of Mo and R-homocitrate.     

Molecular structure of an Fe(IV) species:

The molecular structure of the formal iron(IV) porphyrinate derivative, [[Fe(TTP)]2N]SbCl6 (TTP = tetratolylporphyrinate), is reported. The structural parameters are compared to the previously reported species [Fe(TPP)]2N, in which the iron oxidation state is +3.5. Both the equatorial and axial bond distances in [[Fe(TTP)]2N]SbCl6 are slightly shortened and consistent with an increased formal charge on iron. The value for the axial Fe-N distance is 1.6280(7) A, and the average value of the equatorial Fe-Np distances is 1.979(5) A. The M?ssbauer isomer shift decreases upon oxidation, again consistent with an increase in formal charge. Values for the isomer shift at room temperature are -0.13 mm/s for [[Fe(TTP)]2N]SbCl6 and 0.04 mm/s for [Fe(TTP)]2N. Crystal data for [[Fe(TTP)]2N]SbCl6 are as follows: orthorhombic, space group Fddd, Z = 8, a = 23.689(2) A, b = 31.056(3) A, c = 22.7788(18) A.     

Multielectron redox chemistry of iron porphyrinogens

Iron octamethylporphyrinogens were prepared and structurally characterized in three different oxidation states in the absence of axial ligands and with sodium or tetrafluoroborate as the only counterions. Under these conditions, the iron- and ligand-based redox chemistry of iron porphyrinogens can be defined. The iron center is easily oxidized by a single electron (E(1/2) = -0.57 V vs NHE in CH(3)CN) when confined within the fully reduced macrocycle. The porphyrinogen ligand also undergoes oxidation but in a single four-electron step (E(p) = +0.77 V vs NHE in CH(3)CN); one of the ligand-based electrons is intercepted for the reduction of Fe(III) to Fe(II) to result in an overall three-electron oxidation process. The oxidation equivalents in the macrocycle are stored in C(alpha)-C(alpha) bonds of spirocyclopropane rings, formed between adjacent pyrroles. EPR, magnetic and Mossbauer measurements, and DFT computations of the redox states of the iron porphyrinogens reveal that the reduced ligand gives rise to iron in intermediate spin states, whereas the fully oxidized ligand possesses a weaker sigma-donor framework, giving rise to high-spin iron. Taken together, the results reported herein establish a metal-macrocycle cooperativity that engenders a multielectron chemistry for iron porphyrinogens that is unavailable to heme cofactors.     

Synthesis and characterization of pyridine amide cation radical complexes of iron: stabilization due to coordination with low-spin iron(III) center

We reported the synthesis and characterization of peptide complexes of low-spin iron(III) [Fe(bpb)(py)2][ClO4] (1) and Na[Fe(bpb)(CN)2] (2) [H2bpb = 1,2-bis(pyridine-2-carboxamido)benzene; py = pyridine], where iron is coordinated to four nitrogen donors in the equatorial plane with two amide nitrogen anions and two pyridine nitrogen donors (Ray, M.; Mukherjee, R.; Richardson, J. F.; Buchanan, R. M. J. Chem. Soc., Dalton Trans. 1993, 2451). Chemical oxidation of 2 and a new low-spin iron(III) complex Na[Fe(Me6bpb)(CN)2].H2O (4) [synthesized from a new iron(III) complex [Fe(Me6bpb)(py)2][ClO4] (3) (S = 1/2)] [H2Me6bpb = 1,2-bis(3,5-dimethylpyridine2-carboxamido)-4,5-dimethylbenzene) by (NH4)2Ce(NO3)6 afforded isolation of two novel complexes [Fe(bpb)-(CN)2] (5) and [Fe(Me6bpb)(CN)2].H2O (6). All the complexes have been characterized by physicochemical techniques. While 1-4 are brown/green, 5 and 6 are violet/bluish violet. The collective evidence from infrared, electronic, M?ssbauer, and 1H NMR spectroscopies, from temperature-dependent magnetic susceptibility data, and from cyclic voltammetric studies provides unambiguous evidence that 5 and 6 are low-spin iron(III) ligand cation radical complexes rather than iron(IV) complexes. Cyclic voltammetric studies on isolated oxidized complexes 5 and 6 display identical behavior (a metal-centered reduction and a ligand-centered oxidation) to that observed for complexes 2 and 4, respectively. The M?ssbauer data for 6 are almost identical with those of the parent compound 4, providing compelling evidence that oxidation has occurred at the ligand in a site remote from the iron atom. Strong antiferromagnetic coupling (-2J > or = 450 cm(-1)) of the S = 1/2 iron atom with the S = 1/2 ligand pi-cation radical leads to an effectively S = 0 ground state of 5 and 6. The oxidized complexes display 1H NMR spectra (in CDCl3 solution), characteristic of diamagnetic species.     

The correlation of redox potential, HOMO energy, and oxidation state in metal sulfide clusters and its application to determine the redox level of the FeMo-co active-site cluster of nitrogenase

This paper describes a procedure that permits the total charge state (i.e., oxidation state) of a complex molecule to be obtained from its redox potential data by comparison with good data (both charge state and redox potential) for reference compounds that are chemically similar. The link between the reference data and the unknown compound is made by the calculated energies of the Fermi level or highest occupied molecular orbital (HOMO). The HOMO energies are calculated by unrestricted density functional methods (DMol) for the reference compounds in their known charge states, and a graphical correlation of HOMO energy and redox potential for oxidation (corresponding to loss of an electron from the HOMO) is constructed. The measured redox potential of the unknown is then applied to the correlation to yield the HOMO energy of the unknown, against which the calculated HOMO energies for various charge states of the unknown are assessed. This method is generally applicable. Using 26 reference data, the method is used here to determine the resting redox state, [NFe6MoS9]0, of the core of the FeMo cofactor (FeMo-co, bound to the MoFe protein) which is the active site of nitrogen-fixing enzymes. The analysis also shows that if the atom at the center of FeMo-co is C rather than N, then FeMo-co must be protonated in its resting state, but if FeMo-co is N-centered, it would not be protonated in the resting state.     

Single- and double-cubane clusters in the multiple oxidation states [VFe(3)S(4)](3+,2+,1+)

A new series of cubane-type [VFe(3)S(4)](z)() clusters (z = 1+, 2+, 3+) has been prepared as possible precursor species for clusters related to those present in vanadium-containing nitrogenase. Treatment of [(HBpz(3))VFe(3)S(4)Cl(3)](2)(-) (2, z = 2+), protected from further reaction at the vanadium site by the tris(pyrazolyl)hydroborate ligand, with ferrocenium ion affords the oxidized cluster [(HBpz(3))VFe(3)S(4)Cl(3)](1)(-) (3, z = 3+). Reaction of 2 with Et(3)P results in chloride substitution to give [(HBpz(3))VFe(3)S(4)(PEt(3))(3)](1+) (4, z = 2+). Reaction of 4 with cobaltocene reduced the cluster with formation of the edge-bridged double-cubane [(HBpz(3))(2)V(2)Fe(6)S(8)(PEt(3))(4)] (5, z = 1+, 1+), which with excess chloride underwent ligand substitution to afford [(HBpz(3))(2)V(2)Fe(6)S(8)Cl(4)](4)(-) (6, z = 1+, 1+). X-ray structures of (Me(4)N)[3], [4](PF(6)), 5, and (Et(4)N)(4)[6] x 2MeCN are described. Cluster 5 is isostructural with previously reported [(Cl(4)cat)(2)(Et(3)P)(2)Mo(2)Fe(6)S(8)(PEt(3))(4)] and contains two VFe(3)S(4) cubanes connected across edges by a Fe(2)S(2) rhomb in which the bridging Fe-S distances are shorter than intracubane Fe-S distances. M?ssbauer (2-5), magnetic (2-5), and EPR (2, 4) data are reported and demonstrate an S = 3/2 ground state for 2 and 4 and a diamagnetic ground state for 3. Analysis of (57)Fe isomer shifts based on an empirical correlation between shift and oxidation state and appropriate reference shifts results in two conclusions. (i) The oxidation 2 --> 3 + e(-) results in a change in electron density localized largely or completely on the Fe(3) subcluster and associated sulfur atoms. (ii) The most appropriate charge distributions are [V(3+)Fe(3+)Fe(2+)(2)S(4)](2+) (Fe(2.33+)) for 1, 2, and 4 and [V(3+)Fe(3+)(2)Fe(2+)S(4)](3+) (Fe(2.67+)) for 3 and [V(2)Fe(6)S(8)(SEt)(9)](3+). Conclusion i applies to every MFe(3)S(4) cubane-type cluster thus far examined in different redox states at parity of cluster ligation. The formalistic charge distributions are regarded as the best current approximations to electron distributions in these delocalized species. The isomer shifts require that iron atoms are mixed-valence in each cluster.     

Silylation of iron-bound carbon monoxide affords a terminal Fe carbyne

A series of monocarbonyl iron complexes in the formal oxidation states 0, +1, and +2 are accessible when supported by a tetradentate tris(phosphino)silyl ligand (SiP(iPr)(3) = [Si(o-C(6)H(4)PiPr(2))(3)](-)). X-ray diffraction (XRD) studies of these carbonyl complexes establish little geometrical change about the iron center as a function of oxidation state. It is possible to functionalize the terminal CO ligand of the most reduced carbonyl adduct by addition of SiMe(3)(+) to afford a well-defined iron carbyne species, (SiP(iPr)(3))Fe≡C-OSiMe(3). Single-crystal XRD data of this iron carbyne derivative reveal an unusually short Fe≡C-OSiMe(3) bond distance (1.671(2) ?) and a substantially elongated C-O distance (1.278(3) ?), consistent with Fe-C carbyne character. The overall trigonal bipyramidal geometry of (SiP(iPr)(3))Fe≡C-OSiMe(3) compares well with that of the corresponding carbonyls, (SiP(iPr)(3))Fe(CO)(-), (SiP(iPr)(3))Fe(CO), and (SiP(iPr)(3))Fe(CO)(+). Details regarding the electronic structure of the carbyne complex have been explored via the collection of comparative M?ssbauer data for all of the complexes featured and also via DFT calculations. In sum, these data point to a strongly π-accepting Fischer-type carbyne ligand that confers stability to a low-valent iron(0) rather than high-valent iron(IV) center.     

On the reactivity of mononuclear iron(V)oxo complexes

Ferric tetraamido macrocyclic ligand (TAML)-based catalysts [Fe{C(6)H(4)-1,2-(NCOCMe(2)NCO)(2)CR(2)}(OH(2))]PPh(4) [1; R = Me (a), Et (b)] are oxidized by m-chloroperoxybenzoic acid at -40 °C in acetonitrile containing trace water in two steps to form Fe(V)oxo complexes (2a,b). These uniquely authenticated Fe(V)(O) species comproportionate with the Fe(III) starting materials 1a,b to give μ-oxo-(Fe(IV))(2) dimers. The comproportionation of 1a-2a is faster and that of 1b-2b is slower than the oxidation by 2a,b of sulfides (p-XC(6)H(4)SMe) to sulfoxides, highlighting a remarkable steric control of the dynamics. Sulfide oxidation follows saturation kinetics in [p-XC(6)H(4)SMe] with electron-rich substrates (X = Me, H), but changes to linear kinetics with electron-poor substrates (X = Cl, CN) as the sulfide affinity for iron decreases. As the sulfide becomes less basic, the Fe(IV)/Fe(III) ratio at the end of reaction for 2b suggests a decreasing contribution of concerted oxygen-atom transfer (Fe(V) → Fe(III)) concomitant with increasing electron transfer oxidation (Fe(V) → Fe(IV)). Fe(V) is more reactive toward PhSMe than Fe(IV) by 4 orders of magnitude, a gap even larger than that known for peroxidase Compounds I and II. The findings reinforce prior work typecasting TAML activators as faithful peroxidase mimics.     

A density functional theory study on the active center of Fe-only hydrogenase: characterization and electronic structure of the redox states

We have carried out extensive density functional theory (DFT) calculations for possible redox states of the active center in Fe-only hydrogenases. The active center is modeled by [(H(CH(3))S)(CO)(CN(-))Fe(p)(mu-DTN)(mu-CO)Fe(d)(CO)(CN(-))(L)](z)() (z is the net charge in the complex; Fe(p)= the proximal Fe, Fe(d) = the distal Fe, DTN = (-SCH(2)NHCH(2)S-), L is the ligand that bonds with the Fe(d) at the trans position to the bridging CO). Structures of possible redox states are optimized, and CO stretching frequencies are calculated. By a detailed comparison of all the calculated structures and the vibrational frequencies with the available experimental data, we find that (i) the fully oxidized, inactive state is an Fe(II)-Fe(II) state with a hydroxyl (OH(-)) group bonded at the Fe(d), (ii) the oxidized, active state is an Fe(II)-Fe(I) complex which is consistent with the assignment of Cao and Hall (J. Am. Chem. Soc. 2001, 123, 3734), and (iii) the fully reduced state is a mixture with the major component being a protonated Fe(I)-Fe(I) complex and the other component being its self-arranged form, Fe(II)-Fe(II) hydride. Our calculations also show that the exogenous CO can strongly bond with the Fe(II)-Fe(I) species, but cannot bond with the Fe(I)-Fe(I) complex. This result is consistent with experiments that CO tends to inhibit the oxidized, active state, but not the fully reduced state. The electronic structures of all the redox states have been analyzed. It is found that a frontier orbital which is a mixing state between the e(g) of Fe and the 2 pi of the bridging CO plays a key role concerning the reactivity of Fe-only hydrogenases: (i) it is unoccupied in the fully oxidized, inactive state, half-occupied in the oxidized, active state, and fully occupied in the fully reduced state; (ii) the e(g)-2 pi orbital is a bonding state, and this is the key reason for stability of the low oxidation states, such as Fe(I)-Fe(I) complexes; and (iii) in the e(g)-2 pi orbital more charge accumulates between the bridging CO and the Fe(d) than between the bridging CO and the Fe(p), and the occupation increase in this orbital will enhance the bonding between the bridging CO and the Fe(d), leading to the bridging-CO shift toward the Fe(d).     

Determination of aqueous plutonium oxidation states by solvent extraction

The reliability of two solvent extraction techniques for the determination of Pu oxidation states in solution was tested with low-ionic-strength solutions and with high-Na and high-Mg brines that contained Pu concentrations sufficient for spectrophotometric analysis. One procedure only differentiates between reduced Pu [Pu(III) and Pu(IV)] and oxidized Pu [Pu(V) and Pu(VI)], whereas the second procedure was designed to differentiate between Pu(IV), Pu(V), and Pu(VI) in solution. Both procedures successfully differentiated between oxidized and reduced Pu in both dilute solutions and brines when tested with samples that contained only the Pu(IV), Pu(V), or Pu(VI) oxidation states. However, when the second solvent extraction procedure, which differentiates between Pu(V) and Pu(VI), was employed for solutions that did not contain a strong oxidant to maintain the Pu(VI) oxidation state, significant quantities of Pu(VI) were reduced to Pu(V) during extraction, indicating that accurate quantification of Pu(V) and Pu(VI) is not possible with this procedure.Work supported by the U. S. Department of Energy under Contract DE-ACO6-76RLO 1830.     

P(+) state of nitrogenase p-cluster exhibits electronic structure of a [fe(4)s(4)](+) cluster

Mo nitrogenase consists of two component proteins: the Fe protein, which contains a [Fe(4)S(4)] cluster, and the MoFe protein, which contains two different classes of metal cluster: P-cluster ([Fe(8)S(7)]) and FeMoco ([MoFe(7)S(9)C·homocitrate]). The P-cluster is believed to mediate the electron transfer between the Fe protein and the MoFe protein via interconversions between its various oxidation states, such as the all-ferrous state (P(N)) and the one- (P(+)) and two-electron (P(2+)) oxidized states. While the structural and electronic properties of P(N) and P(2+) states have been well characterized, little is known about the electronic structure of the P(+) state. Here, a mutant strain of Azotobacter vinelandii (DJ1193) was used to facilitate the characterization of the P(+) state of P-cluster. This strain expresses a MoFe protein variant (designated ΔnifB β-188(Cys) MoFe protein) that accumulates the P(+) form of P-cluster in the resting state. Magnetic circular dichroism (MCD) spectrum of the P-cluster in the oxidized ΔnifB β-188(Cys) MoFe protein closely resembles that of the P(2+) state in the oxidized wild-type MoFe protein, except for the absence of a major charge-transfer band centered at 823 nm. Moreover, magnetization curves of ΔnifB β-188(Cys) and wild-type MoFe proteins suggest that the P(2+) species in both proteins have the same spin state. MCD spectrum of the P(+) state in the ΔnifB β-188(Cys) MoFe protein, on the other hand, is associated with a classic [Fe(4)S(4)](+) cluster, suggesting that the P-cluster could be viewed as two coupled 4Fe clusters and that it could donate either one or two electrons to FeMoco by using one or both of its 4Fe halves. Such a mode of action of P-cluster could provide energetic and kinetic advantages to nitrogenase in the complex mechanism of N(2) reduction.     

Mechanistic studies on peroxide activation by a water-soluble iron(III)-porphyrin: implications for O-O bond activation in aqueous and nonaqueous solvents

The reactions of a water-soluble iron(III)-porphyrin, [meso-tetrakis(sulfonatomesityl)porphyrinato]iron(III), [Fe(III)(tmps)] (1), with m-chloroperoxybenzoic acid (mCPBA), iodosylbenzene (PhIO), and H(2)O(2) at different pH values in aqueous methanol solutions at -35 degrees C have been studied by using stopped-flow UV/Vis spectroscopy. The nature of the porphyrin product resulting from the reactions with all three oxidants changed from the oxo-iron(IV)-porphyrin pi-cation radical [Fe(IV)(tmps(*+))(O)] (1(++)) at pH<5.5 to the oxo-iron(IV)-porphyrin [Fe(IV)(tmps)(O)] (1(+)) at pH>7.5, whereas a mixture of both species was formed in the intermediate pH range of 5.5-7.5. The observed reactivity pattern correlates with the E degrees' versus pH profile reported for 1, which reflects pH-dependent changes in the relative positions of E degrees'(Fe(IV)/Fe(III) ) and E degrees'(P(*+)/P) for metal- and porphyrin-centered oxidation, respectively. On this basis, the pH-dependent redox equilibria involving 1(++) and 1(+) are suggested to determine the nature of the final products that result from the oxidation of 1 at a given pH. The conclusions reached are extended to water-insoluble iron(III)-porphyrins on the basis of literature data concerning the electrochemical and catalytic properties of [Fe(III)(P)(X)] species in nonaqueous solvents. Implications for mechanistic studies on [Fe(P)]-catalyzed oxidation reactions are briefly addressed.     

57Fe ENDOR spectroscopy and 'electron inventory' analysis of the nitrogenase E4 intermediate suggest the metal-ion core of FeMo-cofactor cycles through only one redox couple

N(2) binds to the active-site metal cluster in the nitrogenase MoFe protein, the FeMo-cofactor ([7Fe-9S-Mo-homocitrate-X]; FeMo-co) only after the MoFe protein has accumulated three or four electrons/protons (E(3) or E(4) states), with the E(4) state being optimally activated. Here we study the FeMo-co (57)Fe atoms of E(4) trapped with the α-70(Val→Ile) MoFe protein variant through use of advanced ENDOR methods: 'random-hop' Davies pulsed 35 GHz ENDOR; difference triple resonance; the recently developed Pulse-Endor-SaTuration and REcovery (PESTRE) protocol for determining hyperfine-coupling signs; and Raw-DATA (RD)-PESTRE, a PESTRE variant that gives a continuous sign readout over a selected radiofrequency range. These methods have allowed experimental determination of the signed isotropic (57)Fe hyperfine couplings for five of the seven iron sites of the reductively activated E(4) FeMo-co, and given the magnitude of the coupling for a sixth. When supplemented by the use of sum-rules developed to describe electron-spin coupling in FeS proteins, these (57)Fe measurements yield both the magnitude and signs of the isotropic couplings for the complete set of seven Fe sites of FeMo-co in E(4). In light of the previous findings that FeMo-co of E(4) binds two hydrides in the form of (Fe-(μ-H(-))-Fe) fragments, and that molybdenum has not become reduced, an 'electron inventory' analysis assigns the formal redox level of FeMo-co metal ions in E(4) to that of the resting state (M(N)), with the four accumulated electrons residing on the two Fe-bound hydrides. Comparisons with earlier (57)Fe ENDOR studies and electron inventory analyses of the bio-organometallic intermediate formed during the reduction of alkynes and the CO-inhibited forms of nitrogenase (hi-CO and lo-CO) inspire the conjecture that throughout the eight-electron reduction of N(2) plus 2H(+) to two NH(3) plus H(2), the inorganic core of FeMo-co cycles through only a single redox couple connecting two formal redox levels: those associated with the resting state, M(N), and with the one-electron reduced state, M(R). We further note that this conjecture might apply to other complex FeS enzymes.     

Characterization of a tricationic trigonal bipyramidal iron(IV) cyanide complex, with a very high reduction potential, and its iron(II) and iron(III) congeners

Currently, there are only a handful of synthetic S = 2 oxoiron(IV) complexes. These serve as models for the high-spin (S = 2) oxoiron(IV) species that have been postulated, and confirmed in several cases, as key intermediates in the catalytic cycles of a variety of nonheme oxygen activating enzymes. The trigonal bipyramidal complex [Fe(IV)(O)(TMG(3)tren)](2+) (1) was both the first S = 2 oxoiron(IV) model complex to be generated in high yield and the first to be crystallographically characterized. In this study, we demonstrate that the TMG(3)tren ligand is also capable of supporting a tricationic cyanoiron(IV) unit, [Fe(IV)(CN)(TMG(3)tren)](3+) (4). This complex was generated by electrolytic oxidation of the high-spin (S = 2) iron(II) complex [Fe(II)(CN)(TMG(3)tren)](+) (2), via the S = 5/2 complex [Fe(III)(CN)(TMG(3)tren)](2+) (3), the progress of which was conveniently monitored by using UV-vis spectroscopy to follow the growth of bathochromically shifting ligand-to-metal charge transfer (LMCT) bands. A combination of X-ray absorption spectroscopy (XAS), Mo?ssbauer and NMR spectroscopies was used to establish that 4 has a S = 0 iron(IV) center. Consistent with its diamagnetic iron(IV) ground state, extended X-ray absorption fine structure (EXAFS) analysis of 4 indicated a significant contraction of the iron-donor atom bond lengths, relative to those of the crystallographically characterized complexes 2 and 3. Notably, 4 has an Fe(IV/III) reduction potential of ～1.4 V vs Fc(+/o), the highest value yet observed for a monoiron complex. The relatively high stability of 4 (t(1/2) in CD(3)CN solution containing 0.1 M KPF(6) at 25 °C ≈ 15 min), as reflected by its high-yield accumulation via slow bulk electrolysis and amenability to (13)C NMR at -40 °C, highlights the ability of the sterically protecting, highly basic peralkylguanidyl donors of the TMG(3)tren ligand to support highly charged high-valent complexes.     

Theoretical studies of biological nitrogen fixation. I. Density functional modeling of the Mo-site of the FeMo-cofactor

The Mo-site and its ligand environment of the FeMo-cofactor (FeMo-co) were studied using the hybrid density functional method B3LYP. The structure and stability of the model complex (S-ligand)3(N-ligand)Mo[(S)-OCH(CH3)C(O)O-] along with its various protonated and reduced/oxidized forms were calculated. Several hypotheses were tested: (i) ligand environment of the Mo-site, (ii) monodentate vs bidentate coordination of the Mo-bound homocitrate ligand, (iii) substrate coordination to the Mo center, and (iv) Mo-His interaction. It was found that the decoordination of one of the homocitrate (lactate in the model) "legs", the bidentate-->monodentate rearrangement, does not occur spontaneously upon either single/double protonation or one-electron reduction. However, it could occur only upon substrate coordination to the Mo-center of the single-protonated forms of the complex. It was shown that one-electron reduction, single-protonation, and substrate coordination facilitate the bidentate<-->monodentate rearrangement of the homocitrate (lactate) ligand of FeMo-co. It was demonstrated that the smallest acceptable model of His ligand in FeMo-co is methylimidazolate (MeIm-). Our studies suggest that the epsilon-N of the FeMo-co-bound His residue is not protonated, and as a consequence the cluster is tightly bound to the protein matrix via a strong Mo-N delta bond.     

A density functional study of O-O bond cleavage for a biomimetic non-heme iron complex demonstrating an Fe(v)-intermediate

Density functional theory using the B3LYP hybrid functional has been employed to investigate the reactivity of Fe(TPA) complexes (TPA = tris(2-pyridylmethyl)amine), which are known to catalyze stereospecific hydrocarbon oxidation when H(2)O(2) is used as oxidant. The reaction pathway leading to O-O bond heterolysis in the active catalytic species Fe(III)(TPA)-OOH has been explored, and it is shown that a high-valent iron-oxo intermediate is formed, where an Fe(V) oxidation state is attained, in agreement with previous suggestions based on experiments. In contrast to the analogous intermediate [(Por.)Fe(IV)=O](+1) in P450, the TPA ligand is not oxidized, and the electrons are extracted almost exclusively from the mononuclear iron center. The corresponding homolytic O-O bond cleavage, yielding the two oxidants Fe(IV)=O and the OH. radical, has also been considered, and it is shown that this pathway is inaccessible in the hydrocarbon oxidation reaction with Fe(TPA) and hydrogen peroxide. Investigations have also been performed for the O-O cleavage in the Fe(III)(TPA)-alkylperoxide species. In this case, the barrier for O-O homolysis is found to be slightly lower, leading to loss of stereospecificity and supporting the experimental conclusion that this is the preferred pathway for alkylperoxide oxidants. The difference between hydroperoxide and alkylperoxide as oxidant derives from the higher O-O bond strength for hydrogen peroxide (by 8.0 kcal/mol).     

One-electron oxidized product of difluoroiron(III) porphyrin: is it iron(IV) porphyrin or iron(III) porphyrin π-cation radical?

The electronic structure of [Fe(TMP)F(2)], which is formally a one-electron oxidation equivalent above [Fe(III)(TMP)F(2)](-), has been examined in solution by (1)H NMR, UV-Vis, and M?ssbauer spectroscopy. In CD(2)Cl(2)-CD(3)OD solution at 193 K, the pyrrole-H and m-H signals appeared at 128.2 and 116.7 ppm, respectively. The UV-Vis spectrum showed broad absorption bands at 560-680 nm. The M?ssbauer spectrum taken in frozen toluene-methanol solution exhibited a very broad single line from which the IS and QS values were determined by computer simulation to be 0.50 and 0.14 mm s(-1), respectively. On the basis of these results, it was concluded that the one-electron oxidized product of [Fe(TMP)F(2)](-) should be formulated as the iron(III) radical cation [Fe(III)(TMP˙)F(2)], not as iron(IV) porphyrin [Fe(IV)(TMP)F(2)] as previously suggested.     

Electronic configuration of five-coordinate high-spin pyrazole-ligated iron(II) porphyrinates

Pyrazole, a neutral nitrogen ligand and an isomer of imidazole, has been used as a fifth ligand to prepare two new species, [Fe(TPP)(Hdmpz)] and [Fe(Tp-OCH(3)PP)(Hdmpz)] (Hdmpz = 3,5-dimethylpyrazole), the first structurally characterized examples of five-coordinate iron(II) porphyrinates with a nonimidazole neutral ligand. Both complexes are characterized by X-ray crystallography, and structures show common features for five-coordinate iron(II) species, such as an expanded porphyrinato core, large equatorial Fe-N(p) bond distances, and a significant out-of-plane displacement of the iron(II) atom. The Fe-N(pyrazole) and Fe-N(p) bond distances are similar to those in imidazole-ligated species. These suggest that the coordination abilities to iron(II) for imidazole and pyrazole are very similar even though pyrazole is less basic than imidazole. Mo?ssbauer studies reveal that [Fe(TPP)(Hdmpz)] has the same behavior as those of imidazole-ligated species, such as negative quadrupole splitting values and relative large asymmetry parameters. Both the structures and the Mo?ssbauer spectra suggest pyrazole-ligated five-coordinate iron(II) porphyrinates have the same electronic configuration as imidazole-ligated species.