A mechanistic study of the reaction between a diiron(II) complex [FeII(2)(mu-OH)2(6-Me3-TPA)2](2+) and O2 to form a diiron(III) peroxo complex

A kinetic study of the reaction between a diiron(II) complex [Fe(II)(2)(mu-OH)(2)(6-Me(3)-TPA)(2)](2+) 1, where 6-Me(3)-TPA = tris(6-methyl-2-pyridylmethyl)amine, and dioxygen is presented. A diiron(III) peroxo complex [Fe(III)(2)(mu-O)(mu-O(2))(6-Me(3)-TPA)(2)](2+) 2 forms quantitatively in dichloromethane at temperatures from -80 to -40 degrees C. The reaction is first order in [Fe(II)(2)] and [O(2)], with the activation parameters DeltaH(double dagger) = 17 +/- 2 kJ mol(-1) and DeltaS(double dagger) = -175 +/- 20 J mol(-1) K(-1). The reaction rate is not significantly influenced by the addition of H(2)O or D(2)O. The reaction proceeds faster in more polar solvents (acetone and acetonitrile), but the yield of 2 is not quantitative in these solvents. Complex 1 reacts with NO at a rate about 10(3) faster than with O(2). The mechanistic analysis suggests an associative rate-limiting step for the oxygenation of 1, similar to that for stearoyl-ACP Delta(9)-desaturase, but distinct from the probable dissociative pathway of methane monoxygenase. An eta(1)-superoxo Fe(II)Fe(III) species is a likely steady-state intermediate during the oxygenation of complex 1.     

Reactions of the peroxo intermediate of soluble methane monooxygenase hydroxylase with ethers

Soluble methane monooxygenase (sMMO) isolated from Methylococcus capsulatus (Bath) utilizes a carboxylate-bridged diiron center and dioxygen to catalyze the conversion of methane to methanol. Previous studies revealed that a di(mu-oxo)diiron(IV) intermediate termed Q is responsible for the catalytic activity with hydrocarbons. In addition, the peroxodiiron(III) intermediate (H(peroxo)) that precedes Q formation in the catalytic cycle has been demonstrated to react with propylene, but its reactivity has not been extensively investigated. Given the burgeoning interest in the existence of multiple oxidants in metalloenzymes, a more exhaustive study of the reactivity of H(peroxo) was undertaken. The kinetics of single turnover reactions of the two intermediates with ethyl vinyl ether and diethyl ether were monitored by single- and double-mixing stopped-flow optical spectroscopy. For both substrates, the rate constants for reaction with H(peroxo) are greater than those for Q. An analytical model for explaining the transient kinetics is described and used successfully to fit the observed data. Activation parameters were determined through temperature-dependent studies, and the kinetic isotope effects for the reactions with diethyl ether were measured. The rate constants indicate that H(peroxo) is a more electrophilic oxidant than Q. We propose that H(peroxo) reacts via two-electron transfer mechanisms, and that Q reacts by single-electron transfer steps.     

Iron(II) complexes with amide-containing macrocycles as non-heme porphyrin analogues

Iron(II) complexes of macrocyclic pentadendate ligands 3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),14,16-triene-2,13-dione (H2pydioneN5) and 16-chloro-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),14,16-triene-2,13-dione (H2pyCldioneN5) were synthesized and fully characterized. Complexes with one or two deprotonated amide groups of H2pydione were both isolated. In the former case the metal ion has a distorted octahedral coordination sphere; in the latter case the complex adopts a pentagonal-bipyramidal geometry. NMR experiments show that the protonation state of the ligand is preserved in a dimethyl sulfoxide (DMSO) solution. The complexes maintain a high-spin state even at low temperatures. Detailed kinetic studies of oxygenation of the iron(II) complexes showed that the deprotonation state of the complex has a profound effect on the reactivity with dioxygen. Oxygenation of the dideprotonated complex of iron(II), Fe(pydioneN5), in aprotic solvents proceeds via a path that is analogous to that of iron(II) porphyrins: via iron(III) superoxo and diiron(III) peroxo species, as evidenced by the spectral changes during the reaction, which is second-order in the concentration of the iron(II) complex, and with an inverse dependence of the reaction rate on the concentration of dioxygen. The final products of oxygenation are crystallographically characterized iron(III) mu-oxo dimers. We have also found that the presence of 1-methylimidazole stabilizes the diiron peroxo intermediate. The reaction of Fe(pydioneN5) with dioxygen in methanol is distinctly different under the same conditions. The reaction is first-order in both iron(II) complex and dioxygen, and no intermediate is spectroscopically observed. Similar behavior was observed for the monodeprotonated complex Fe(HpydioneN5)(Cl). The presence of an accessible proton either from the solvent (reactions in methanol) or from the complex itself (in Fe(HpydioneN5)(Cl)) proves sufficient to alter the oxygenation pathway in these macrocyclic systems, which is reminiscent of the properties of iron(II) porphyrin complexes. The new amidopyridine macrocycles can be considered as new members of the "expanded porphyrin analogue" family. The expansion of the cavity provides control over the spin state and availability of protons. These macrocyclic systems also allow for easy synthetic modifications, paving the way to new, versatile metal complexes.     

Dioxygen binding to complexes with Fe(II)2(mu-OH)2 cores: steric control of activation barriers and O2-adduct formation

A series of complexes with [Fe(II)(2)(mu-OH)(2)] cores has been synthesized with N3 and N4 ligands and structurally characterized to serve as models for nonheme diiron(II) sites in enzymes that bind and activate O(2). These complexes react with O(2) in solution via bimolecular rate-limiting steps that differ in rate by 10(3)-fold, depending on ligand denticity and steric hindrance near the diiron center. Low-temperature trapping of a (mu-oxo)(mu-1,2-peroxo)diiron(III) intermediate after O(2) binding requires sufficient steric hindrance around the diiron center and the loss of a proton (presumably that of a hydroxo bridge or a yet unobserved hydroperoxo intermediate). The relative stability of these and other (mu-1,2-peroxo)diiron(III) intermediates suggests that these species may not be on the direct pathway for dioxygen activation.     

Mechanistic studies on the formation and reactivity of dioxygen adducts of diiron complexes supported by sterically hindered carboxylates

Dioxygen activation by enzymes such as methane monooxygenase, ribonucleotide reductase, and fatty acid desaturases occurs at a nonheme diiron active site supported by two histidines and four carboxylates, typically involving a (peroxo)diiron(III,III) intermediate in an early step of the catalytic cycle. Biomimetic tetracarboxylatodiiron(II,II) complexes with the familiar "paddlewheel" topology comprising sterically bulky o-dixylylbenzoate ligands with pyridine, 1-methylimidazole, or THF at apical sites readily react with O(2) to afford thermally labile peroxo intermediates that can be trapped and characterized spectroscopically at low temperatures (193 K). Cryogenic stopped-flow kinetic analysis of O(2) adduct formation carried out for the three complexes reveals that dioxygen binds to the diiron(II,II) center with concentration dependences and activation parameters indicative of a direct associative pathway. The pyridine and 1-methylimidazole intermediates decay by self-decomposition. However, the THF intermediate decays much faster by oxygen transfer to added PPh(3), the kinetics of which has been studied with double mixing experiments in a cryogenic stopped-flow apparatus. The results show that the decay of the THF intermediate is kinetically controlled by the dissociation of a THF ligand, a conclusion supported by the observation of saturation kinetic behavior with respect to PPh(3), inhibition by added THF, and invariant saturation rate constants for the oxidation of various phosphines. It is proposed that the proximity of the reducing substrate to the peroxide ligand on the diiron coordination sphere facilitates the oxygen-atom transfer. This unique investigation of the reaction of an O(2) adduct of a biomimetic tetracarboxylatodiiron(II,II) complex provides a synthetic precedent for understanding the electrophilic reactivity of like adducts in the active sties of nonheme diiron enzymes.     

Toward functional carboxylate-bridged diiron protein mimics: achieving structural stability and conformational flexibility using a macrocylic ligand framework

A dinucleating macrocycle, H(2)PIM, containing phenoxylimine metal-binding units has been prepared. Reaction of H(2)PIM with [Fe(2)(Mes)(4)] (Mes = 2,4,6-trimethylphenyl) and sterically hindered carboxylic acids, Ph(3)CCO(2)H or Ar(Tol)CO(2)H (2,6-bis(p-tolyl)benzoic acid), afforded complexes [Fe(2)(PIM)(Ph(3)CCO(2))(2)] (1) and [Fe(2)(PIM)(Ar(Tol)CO(2))(2)] (2), respectively. X-ray diffraction studies revealed that these diiron(II) complexes closely mimic the active site structures of the hydroxylase components of bacterial multicomponent monooxygenases (BMMs), particularly the syn disposition of the nitrogen donor atoms and the bridging μ-η(1)η(2) and μ-η(1)η(1) modes of the carboxylate ligands at the diiron(II) centers. Cyclic voltammograms of 1 and 2 displayed quasi-reversible redox couples at +16 and +108 mV vs ferrocene/ferrocenium, respectively. Treatment of 2 with silver perchlorate afforded a silver(I)/iron(III) heterodimetallic complex, [Fe(2)(μ-OH)(2)(ClO(4))(2)(PIM)(Ar(Tol)CO(2))Ag] (3), which was structurally and spectroscopically characterized. Complexes 1 and 2 both react rapidly with dioxygen. Oxygenation of 1 afforded a (μ-hydroxo)diiron(III) complex [Fe(2)(μ-OH)(PIM)(Ph(3)CCO(2))(3)] (4), a hexa(μ-hydroxo)tetrairon(III) complex [Fe(4)(μ-OH)(6)(PIM)(2)(Ph(3)CCO(2))(2)] (5), and an unidentified iron(III) species. Oxygenation of 2 exclusively formed di(carboxylato)diiron(III) compounds, a testimony to the role of the macrocylic ligand in preserving the dinuclear iron center under oxidizing conditions. X-ray crystallographic and (57)Fe M?ssbauer spectroscopic investigations indicated that 2 reacts with dioxygen to give a mixture of (μ-oxo)diiron(III) [Fe(2)(μ-O)(PIM)(Ar(Tol)CO(2))(2)] (6) and di(μ-hydroxo)diiron(III) [Fe(2)(μ-OH)(2)(PIM)(Ar(Tol)CO(2))(2)] (7) units in the same crystal lattice. Compounds 6 and 7 spontaneously convert to a tetrairon(III) complex, [Fe(4)(μ-OH)(6)(PIM)(2)(Ar(Tol)CO(2))(2)] (8), when treated with excess H(2)O.     

Synthesis and spectroscopic studies of non-heme diiron(III) species with a terminal hydroperoxide ligand: models for hemerythrin

Two compounds, [Fe2(mu-OH)(mu-Ph4DBA)(TMEDA)2(OTf)] (4) and [Fe2(mu-OH)(mu-Ph4DBA)(DPE)2(OTf)] (7), where Ph4DBA(2-) is the dinucleating bis(carboxylate) ligand dibenzofuran-4,6-bis(diphenylacetate), have been prepared as synthetic models for the dioxygen-binding non-heme diiron protein hemerythrin (Hr). X-ray crystallography reveals that, in the solid state, these compounds contain the asymmetric coordination environment found at the diiron center in the reduced form of the protein, deoxyHr. M?ssbauer spectra of the models (4, delta = 1.21(2), DeltaE(Q) = 2.87(2) mm s(-1); 7, delta(av) = 1.23(1), DeltaE(Qav) = 2.79(1) mm s(-1)) and deoxyHr (delta = 1.19, DeltaE(Q) = 2.81 mm s(-1)) are also in good agreement. Oxygenation of the diiron(II) complexes dissolved in CH2Cl2 containing 3 equiv of N-MeIm (4) or neat EtCN (7) at -78 degrees C affords a red-orange solution with optical bands at 336 nm (7300 M(-1) cm(-1)) and 470 nm (2600 M(-1) cm(-1)) for 4 and at 334 nm (6400 M(-1) cm(-1)) and 484 nm (2350 M(-1) cm(-1)) for 7. These spectra are remarkably similar to that of oxyHr, 330 nm (6800 M(-1) cm(-1)) and 500 nm (2200 M(-1) cm(-1)). The electron paramagnetic resonance (EPR) spectrum of the cryoreduced, mixed-valence dioxygen adduct of 7 displays properties consistent with a (mu-oxo)diiron(II,III) core. An investigation of 7 and its dioxygen-bound adduct by extended X-ray absorption fine structure (EXAFS) spectroscopy indicates that the oxidized species contains a (mu-oxo)diiron(III) core with iron-ligand distances in agreement with those expected for oxide, carboxylate, and amine/hydroperoxide donor atoms. The analogous cobalt complex [Co2(mu-OH)(mu-Ph4DBA)(TMEDA)2(OTf)] (6) was synthesized and structurally characterized, but it was unreactive toward dioxygen.     

Synthetic analogue of the [Fe(2)(mu-OH)(2)(mu-O(2)CR)](3+) core of soluble methane monooxygenase hydroxylase via synthesis and dioxygen reactivity of carboxylate-bridged diiron(II) complexes

We describe the synthesis and dioxygen reactivity of diiron(II) tetracarboxylate complexes [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(N,N-Me(2)en)(2)] (2) and [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(N,N-Bn(2)en)(2)] (6), where Ar(Tol)CO(2)(-) = 2,6-di(p-tolyl)benzoate. These complexes were prepared as models for the diiron(II) center in the hydroxylase component of soluble methane monooxygenase (MMOH). Compound 6 reacts with dioxygen to afford PhCHO in approximately 60(5)% yield, following oxidative N-dealkylation of the pendant benzyl group on the diamine ligand. The diiron(III) complex [Fe(2)(mu-OH)(2)(mu-O(2)CAr(Tol))(O(2)CAr(Tol))(3)(N-Bnen)(N,N-Bn(2)en)] (8) was isolated from the reaction mixture. The 4.2 K M?ssbauer spectrum of 8 displays a single quadrupole doublet with parameters delta = 0.48(2) mm s(-1) and Delta E(Q) = 0.61(2) mm s(-1). The [Fe(2)(mu-OH)(2)(mu-O(2)CR)](3+) core structure in 8 matches that of the fully oxidized form of MMOH. The conversion of 6 to 8 closely parallels the chemistry of MMOH in which an O(2)-derived oxygen atom is inserted into the C-H bond of methane. Several reaction pathways are considered to account for this novel chemical transformation, and these are compared with mechanistic frameworks previously developed for related cytochrome P450 and copper(I) dioxygen chemistry.     

Mechanistic Studies of the Formation and Decay of Diiron(III) Peroxo Complexes in the Reaction of Diiron(II) Precursors with Dioxygen

Mechanistic studies of the reactions of three analogous alkoxo-bridged diiron(II) complexes with O(2) have been carried out. The compounds, which differ primarily in the steric accessibility of dioxygen to the diiron(II) center, form metastable &mgr;-peroxo intermediates when studied at low temperature. At ambient temperatures, these intermediates decay to form (&mgr;-oxo)polyiron(III) products. The effect of ligand steric constraints on the O(2) reactivity was investigated. When access to the diiron center was unimpeded, the reaction was first-order with respect to both [Fe(II)(2)] and [O(2)] and the activation parameters for O(2) addition were similar to those for O(2) reacting with the dioxygen transport protein hemerythrin. When the binding site was occluded, however, reduced order with respect to [O(2)] was observed and a two-step mechanism was required to explain the kinetic results. Decay of all three peroxide intermediates involves a bimolecular event, implying the formation of tetranuclear species in the transition state.     

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. In corresponding resonance Raman studies, vibrations arising from both the Fe-O-Fe and the Fe-O-O-Fe units can be observed. Importantly a linear correlation can be discerned between the nu(O-O) frequency of a complex and its Fe-Fe distance among the subset of complexes with [Fe(III)(2)(mu-OR)(mu-1,2-O(2))] cores (R = H, alkyl, aryl, or no substituent). These experimental studies are complemented by a normal coordinate analysis and DFT calculations.     

Synthesis, characterization, and dioxygen reactivity of tetracarboxylate-bridged Diiron(II) complexes with coordinated substrates

The synthesis and characterization of [Fe(2)(micro-O(2)CAr(Tol))(4)L(2)] complexes, where L is benzylamine or 4-methoxybenzylamine (BA(p)()(-)(OMe)), are described. The reaction of the latter diiron(II) complex with dioxygen at -78 degrees C affords a metastable mixed-valent Fe(II)Fe(III) green intermediate. When O(2) is introduced at ambient temperature, N-dealkyation occurs to yield anisaldehyde, eliminating N-oxidation as a viable pathway for the reaction. Use of [Fe(2)(micro-O(2)CAr(T)(omicron)(l))(4)(alpha-d(1)-BA(p)()(-)(OMe))(2)] allowed a deuterium kinetic isotope of approximately 3 to be determined.     

Modeling features of the non-heme diiron cores in O2-activating enzymes through the synthesis, characterization, and oxidation of 1,8-naphthyridine-based complexes

Multidentate naphthyridine-based ligands were used to prepare a series of diiron(II) complexes. The compound [Fe(2)(BPMAN)(mu-O(2)CPh)(2)](OTf)(2) (1), where BPMAN = 2,7-bis[bis(2-pyridylmethyl)aminomethyl]-1,8-naphthyridine, exhibits two reversible oxidation waves with E(1/2) values at +310 and +733 mV vs Cp(2)Fe(+)/Cp(2)Fe, as revealed by cyclic voltammetry. Reaction with O(2) or H(2)O(2) affords a product with optical and M?ssbauer properties that are characteristic of a (mu-oxo)diiron(III) species. The complexes [Fe(2)(BPMAN)(mu-OH)(mu-O(2)CAr(Tol))](OTf)(2) (2) and [Fe(2)(BPMAN)(mu-OMe)(mu-O(2)CAr(Tol))](OTf)(2) (3) were synthesized, where Ar(Tol)CO(2)(-) is the sterically hindered ligand 2,6-di(p-tolyl)benzoate. Compound 2 has a reversible redox wave at +11 mV, and both 2 and 3 react with O(2), via a mixed-valent Fe(II)Fe(III) intermediate, to give final products that are also consistent with (mu-oxo)diiron(III) species. The paddle-wheel compound [Fe(2)(BBAN)(mu-O(2)CAr(Tol))(3)](OTf) (4), where BBAN = 2,7-bis(N,N-dibenzylaminomethyl)-1,8-naphthyridine, reacts with dioxygen to yield benzaldehyde via oxidative N-dealkylation of a benzyl group on BBAN, an internal substrate. In the presence of bis(4-methylbenzyl)amine, the reaction also produces p-tolualdehyde, revealing oxidation of an external substrate. A structurally related compound, [Fe(2)(BEAN)(mu-O(2)CAr(Tol))(3)](OTf) (5), where BEAN = 2,7-bis(N,N-diethylaminomethyl)-1,8-naphthyridine, does not undergo N-dealkylation, nor does it facilitate the oxidation of bis(4-methylbenzyl)amine. The contrast in reactivity of 4 and 5 is attributed to a difference in accessibility of the substrate to the diiron centers of the two compounds. The M?ssbauer spectroscopic properties of the diiron(II) complexes were also investigated.     

Reactions of meso-hydroxyhemes with carbon monoxide and reducing agents in search of the elusive species responsible for the g = 2.006 resonance of carbon monoxide-treated heme oxygenase. Isolation of diamagnetic iron(II) complexes of octaethyl-meso-hydroxyporphyrin

To examine possible models for the g = 2.006 resonance seen when the hydroxylated heme-heme oxygenase complex in the Fe(III) state is treated with CO, the reactivities of CO and reducing agents with (py)(2)Fe(III)(OEPO) and [Fe(III)(OEPO)](2) (OEPO is the trianion of octaethyl-meso-hydroxyporphyrin) have been examined. A pyridine solution of (py)(2)Fe(III)(OEPO) reacts in a matter of minutes with zinc amalgam (or with hydrazine) under an atmosphere of dioxygen-free dinitrogen to produce bright-red (py)(2)Fe(II)(OEPOH).2py.0.33H(2)O, which has been isolated in crystalline form. The (1)H NMR spectrum of (py)(2)Fe(II)(OEPOH) in a pyridine-d(5) solution is indicative of the presence of a diamagnetic compound, and no EPR resonance was observed for this compound. Treatment of a solution of (py)(2)Fe(II)(OEPOH) in pyridine-d(5) with carbon monoxide produces spectral changes after a 30 s exposure that are indicative of the formation of diamagnetic (OC)(py)Fe(II)(OEPOH). Treatment of a green pyridine solution of (py)(2)Fe(III)(OEPO) with carbon monoxide reveals a slow color change to deep red over a 16 h period. Although a resonance at g = 2.006 was observed in the EPR spectrum of the sample during the reaction, the isolated product is EPR silent. The spectroscopic features of the final solution are identical to those of a solution formed by treating (py)(2)Fe(II)(OEPOH) with carbon monoxide. Addition of hydrazine to solutions of (OC)(py)Fe(II)(OEPOH) produces red, diamagnetic (OC)(N(2)H(4))Fe(II)(OEPOH).py in crystalline form. The X-ray crystal structures of (py)(2)Fe(II)(OEPOH).2py.0.33H(2)O and (OC)(N(2)H(4))Fe(II)(OEPOH).py have been determined. Solutions of diamagnetic (OC)(N(2)H(4))Fe(II)(OEPOH).py and (OC)(py)Fe(II)(OEPOH) are extremely air sensitive and are immediately converted in a pyridine solution into paramagnetic (py)(2)Fe(III)(OEPO) in the presence of dioxygen.     

Functional mimic of dioxygen-activating centers in non-heme diiron enzymes: mechanistic implications of paramagnetic intermediates in the reactions between diiron(II) complexes and dioxygen

Two tetracarboxylate diiron(II) complexes, [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(C(5)H(5)N)(2)] (1a) and [Fe(2)(mu-O(2)CAr(Tol))(4)(4-(t)BuC(5)H(4)N)(2)] (2a), where Ar(Tol)CO(2)(-) = 2,6-di(p-tolyl)benzoate, react with O(2) in CH(2)Cl(2) at -78 degrees C to afford dark green intermediates 1b (lambda(max) congruent with 660 nm; epsilon = 1600 M(-1) cm(-1)) and 2b (lambda(max) congruent with 670 nm; epsilon = 1700 M(-1) cm(-1)), respectively. Upon warming to room temperature, the solutions turn yellow, ultimately converting to isolable diiron(III) compounds [Fe(2)(mu-OH)(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)L(2)] (L = C(5)H(5)N (1c), 4-(t)BuC(5)H(4)N (2c)). EPR and M?ssbauer spectroscopic studies revealed the presence of equimolar amounts of valence-delocalized Fe(II)Fe(III) and valence-trapped Fe(III)Fe(IV) species as major components of solution 2b. The spectroscopic and reactivity properties of the Fe(III)Fe(IV) species are similar to those of the intermediate X in the RNR-R2 catalytic cycle. EPR kinetic studies revealed that the processes leading to the formation of these two distinctive paramagnetic components are coupled to one another. A mechanism for this reaction is proposed and compared with those of other synthetic and biological systems, in which electron transfer occurs from a low-valent starting material to putative high-valent dioxygen adduct(s).     

Water affects the stereochemistry and dioxygen reactivity of carboxylate-rich diiron(II) models for the diiron centers in dioxygen-dependent non-heme enzymes

Carboxylate-bridged high-spin diiron(II) complexes with distinctive electronic transitions were prepared by using 4-cyanopyridine (4-NCC(5)H(4)N) ligands to shift the charge-transfer bands to the visible region of the absorption spectrum. This property facilitated quantitation of water-dependent equilibria in the carboxylate-rich diiron(II) complex, [Fe(2)(mu-O(2)CAr(Tol))(4)(4-NCC(5)H(4)N)(2)] (1), where (-)O(2)CAr(Tol) is 2,6-di-(p-tolyl)benzoate. Addition of water to 1 reversibly shifts two of the bridging carboxylate ligands to chelating terminal coordination positions, converting the structure from a paddlewheel to a windmill geometry and generating [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(4-NCC(5)H(4)N)(2)(H(2)O)(2)] (3). This process is temperature dependent in solution, rendering the system thermochromic. Quantitative treatment of the temperature-dependent spectroscopic changes over the temperature range from 188 to 298 K in CH(2)Cl(2) afforded thermodynamic parameters for the interconversion of 1 and 3. Stopped flow kinetic studies revealed that water reacts with the diiron(II) center ca. 1000 time faster than dioxygen and that the water-containing diiron(II) complex reacts with dioxygen ca. 10 times faster than anhydrous analogue 1. Addition of {H(OEt(2))(2)}{B}, where B(-) is tetrakis(3,5-di(trifluoromethyl)phenyl)borate, to 1 converts it to [Fe(2)(mu-O(2)CAr(Tol))(3)(4-NCC(5)H(4)N)(2)](B) (5), which was also structurally characterized. Mossbauer spectroscopic investigations of solid samples of 1, 3, and 5, in conjunction with several literature values for high-spin iron(II) complexes in an oxygen-rich coordination environment, establish a correlation between isomer shift, coordination number, and N/O composition. The products of oxygenating 1 in CH(2)Cl(2) were identified crystallographically to be [Fe(2)(mu-OH)(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(4-NCC(5)H(4)N)(2)].2(HO(2)CAr(Tol)) (6) and [Fe(6)(mu-O)(2)(mu-OH)(4)(mu-O(2)CAr(Tol))(6)(4-NCC(5)H(4)N)(4)Cl(2)] (7).     

Stopped-flow Fourier transform infrared spectroscopy of nitromethane oxidation by the diiron(IV) intermediate of methane monooxygenase

The hydroxylase component (MMOH) of soluble methane monooxygenase from Methylococcus capsulatus (Bath) was reduced to the diiron(II) form and then allowed to react with dioxygen to generate the diiron(IV) intermediate Q in the first phase of a double-mixing stopped-flow experiment. CD3NO2 was then introduced in the second phase of the experiment, which was carried out in D2O at 25 degrees C. The kinetics of the reaction of the substrate with Q were monitored by stopped-flow Fourier transform infrared spectroscopy, observing the disappearance of the asymmetric NO2 bending vibration at 1548 cm-1. The data were fit to a single-exponential function, which yielded a kobs of 0.45 +/- 0.07 s-1. This result is in quantitative agreement with a kobs of 0.39 +/- 0.01 s-1 obtained by observing the disappearance of Q by double-mixing stopped-flow optical spectroscopy at its absorption maximum of 420 nm. These results provide for the first time direct monitoring of the hydroxylation of a methane-derived substrate in the MMOH reaction pathway and demonstrate that Q decay occurs concomitantly with substrate consumption.     

Structural, EPR, and Mössbauer characterization of (μ-alkoxo)(μ-carboxylato)diiron(II,III) model complexes for the active sites of mixed-valent diiron enzymes

To obtain structural and spectroscopic models for the diiron(II,III) centers in the active sites of diiron enzymes, the (μ-alkoxo)(μ-carboxylato)diiron(II,III) complexes [Fe(II)Fe(III)(N-Et-HPTB)(O(2)CPh)(NCCH(3))(2)](ClO(4))(3) (1) and [Fe(II)Fe(III)(N-Et-HPTB)(O(2)CPh)(Cl)(HOCH(3))](ClO(4))(2) (2) (N-Et-HPTB = N,N,N',N'-tetrakis(2-(1-ethyl-benzimidazolylmethyl))-2-hydroxy-1,3-diaminopropane) have been prepared and characterized by X-ray crystallography, UV-visible absorption, EPR, and M?ssbauer spectroscopies. Fe1-Fe2 separations are 3.60 and 3.63 ?, and Fe1-O1-Fe2 bond angles are 128.0° and 129.4° for 1 and 2, respectively. M?ssbauer and EPR studies of 1 show that the Fe(III) (S(A) = 5/2) and Fe(II) (S(B) = 2) sites are antiferromagnetically coupled to yield a ground state with S = 1/2 (g= 1.75, 1.88, 1.96); M?ssbauer analysis of solid 1 yields J = 22.5 ± 2 cm(-1) for the exchange coupling constant (H = JS(A)·S(B) convention). In addition to the S = 1/2 ground-state spectrum of 1, the EPR signal for the S = 3/2 excited state of the spin ladder can also be observed, the first time such a signal has been detected for an antiferromagnetically coupled diiron(II,III) complex. The anisotropy of the (57)Fe magnetic hyperfine interactions at the Fe(III) site is larger than normally observed in mononuclear complexes and arises from admixing S > 1/2 excited states into the S = 1/2 ground state by zero-field splittings at the two Fe sites. Analysis of the "D/J" mixing has allowed us to extract the zero-field splitting parameters, local g values, and magnetic hyperfine structural parameters for the individual Fe sites. The methodology developed and followed in this analysis is presented in detail. The spin Hamiltonian parameters of 1 are related to the molecular structure with the help of DFT calculations. Contrary to what was assumed in previous studies, our analysis demonstrates that the deviations of the g values from the free electron value (g = 2) for the antiferromagnetically coupled diiron(II,III) core in complex 1 are predominantly determined by the anisotropy of the effective g values of the ferrous ion and only to a lesser extent by the admixture of excited states into ground-state ZFS terms (D/J mixing). The results for 1 are discussed in the context of the data available for diiron(II,III) clusters in proteins and synthetic diiron(II,III) complexes.     

Copper-dioxygen adducts and the side-on peroxo dicopper(II)/bis(mu-oxo) dicopper(III) equilibrium: Significant ligand electronic effects

The variation of ligand para substituents on pyridyl donor groups of tridentate amine copper(I) complexes was carried out in order to probe electronic effects on the equilibrium between mu-eta2:eta2-(side-on)-peroxo [Cu(II)2(O2(2-))]2+ and bis(mu-oxo) [Cu(III)2(O(2-))2] species formed upon reaction with O2. [Cu(I)(R-PYAN)(MeCN)n]B(C6F5)4 (R-PYAN = N-[2-(4-R-pyridin-2-yl)-ethyl]-N,N',N'-trimethyl-propane-1,3-diamine, R = NMe2, OMe, H, and Cl) (1R) vary over a narrow range in their Cu(II)/Cu(I) redox potentials (E(1/2) vs Fe(cp)2(+/0) = -0.40 V for 1(NMe2), -0.38 V for 1(OMe), -0.33 V for 1H, and -0.32 V for 1Cl) and in C-O stretching frequencies of their carbonyl adducts, 1R-CO: nu(C-O) = 2080, 2086, 2088, and 2090 cm(-1) for R = NMe2, OMe, H, and Cl, respectively. However, within this range of electronic properties for 1R, dioxygen reactivity is significantly affected. The reaction of 1Cl or 1H with O2 at -78 degrees C in CH2Cl2 gives UV-vis and resonance Raman spectra indicative of a mu-eta2:eta2-(side-on)-peroxo dicopper(II) adduct (2R). Compound 1(OMe) reacts with O2, yielding equilibrium mixtures of side-on peroxo (2(OMe)) and bis(mu-oxo) (3(OMe)) species. Oxygenation of 1(NMe2) leads to the sole generation of the bis(mu-oxo) dicopper(III) complex (3(NMe2)). A solvent effect was also observed; in acetone or THF, increased ratios of bis(mu-oxo) relative to side-on peroxo complex are observed. Thus, the equilibrium between a dicopper side-on peroxo and bis(mu-oxo) species can be tuned by ligand design-specifically, more electron donating ligands favor the formation of the latter isomer, and the peroxo/bis(mu-oxo) equilibrium can be shifted from one extreme to the other within the same ligand system. Observations concerning the reactivity of the dioxygen adducts 2H and 3(NMe2) toward external substrates are also presented.     

XAS characterization of end-on and side-on peroxoiron(III) complexes of the neutral pentadentate N-donor ligand N-methyl-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine

Peroxo intermediates are implicated in the catalytic cycles of iron enzymes involved in dioxygen metabolism. X-ray absorption spectroscopy has been used to gain insight into the iron coordination environments of the low-spin complex [Fe(III)(Me-TPEN)(eta(1)-OOH)](2+)(1) and the high-spin complex [Fe(III)(Me-TPEN)(eta(2)-O(2))](+)(2)(the neutral pentadentate N-donor ligand Me-TPEN =N-methyl-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine) and obtain metrical parameters unavailable from X-ray crystallography. The complexes exhibit relatively large pre-edge peak areas of approximately 15 units, indicative of iron centers with significant distortions from centrosymmetry. These distortions result from the binding of peroxide, either end-on hydroperoxo for 1 (r(Fe-O)= 1.81A) or side-on peroxo for 2 (r(Fe-O)= 1.99 A). The XAS analyses of 1 strongly support a six-coordinate low-spin iron(III) center coordinated to five nitrogen atoms from Me-TPEN and one oxygen atom from an end-on hydroperoxide ligand. However, the XAS analyses of 2 are not conclusive: Me-TPEN can act either as a pentadentate ligand to form a seven-coordinate peroxo complex, which has precedence in the DFT geometry optimization of [Fe(III)(N4Py)(eta(2)-O(2))](+)(the neutral pentadentate N-donor ligand N4Py =N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), or as a tetradentate ligand with a dangling pyridylmethyl arm to form a six-coordinate peroxo complex, which is precedented by the crystal structure of [Fe(2)(III)(Me-TPEN)(2)(Cl)(2)(mu-O)](2+).     

Facile ring opening of iron(III) and iron(II) complexes of meso-amino-octaethylporphyrin by dioxygen

Pyridine solutions of ClFe(III)(meso-NH(2)-OEP) undergo oxidative ring opening when exposed to dioxygen. The high-spin iron(III) complex, ClFe(III)(meso-NH(2)-OEP), has been isolated and characterized by X-ray crystallography. In the solid state, it has a five-coordinate structure typical for high-spin (S = 5/2) iron(III) complex. In chloroform-d solution, ClFe(III)(meso-NH(2)-OEP) displays an (1)H NMR spectrum characteristic of a high-spin, five-coordinate complex and is unreactive toward dioxygen. However, in pyridine-d(5) solution a temperature-dependent equilibrium exists between the high-spin (S = 5/2), six-coordinate complex, [(py)ClFe(III)(meso-NH(2)-OEP)], and the six-coordinate, low spin (S = 1/2 with the less common (d(xz)d(yz))(4)(d(xy))(1) ground state)) complex, [(py)(2)Fe(III)(meso-NH(2)-OEP)](+). Such pyridine solutions are air-sensitive, and the remarkable degradation has been monitored by (1)H NMR spectroscopy. These studies reveal a stepwise conversion of ClFe(III)(meso-NH(2)-OEP) into an open-chain tetrapyrrole complex in which the original amino group and the attached meso carbon atom have been converted into a nitrile group. Additional oxidation at an adjacent meso carbon occurs to produce a ligand that binds iron by three pyrrole nitrogen atoms and the oxygen atom introduced at a meso carbon. This open-chain tetrapyrrole complex itself is sensitive to attack by dioxygen and is converted into a tripyrrole complex that is stable to further oxidation and has been isolated. The process of oxidation of the Fe(III) complex, ClFe(III)(meso-NH(2)-OEP), is compared with that of the iron(II) complex, (py)(2)Fe(II)(meso-NH(2)-OEP); both converge to form identical products.