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.     

Reaction of (mu-oxo)diiron(III) core with CO2 in N-methylimidazole: formation of mono(mu-carboxylato)(mu-oxo)diiron(III) complexes with N-methylimidazole as ligands

Several iron(III) complexes with N-methylimidazole (N-MeIm) as the ligand have been synthesized by using N-MeIm as the solvent. Under anaerobic conditions, [Fe(N-MeIm)(6)](ClO(4))(3) (1) reacts with stoichiometric amounts of water in N-MeIm to afford the (mu-oxo)diiron(III) complex, [Fe(2)(mu-O)(N-MeIm)(10)](ClO(4))(4) (3). Exposure of a solution of 3 in N-MeIm to stoichiometric and excess CO(2) gives rise to the (mu-oxo)(mu-carboxylato)diiron(III) species [Fe(2)(mu-O)(mu-HCO(2))(N-MeIm)(8)](ClO(4))(3) (4) and the methyl carbonate complex [Fe(2)(mu-O)(mu-CH(3)OCO(2))(N-MeIm)(8)](ClO(4))(3) (5), respectively. Formation of the formato-bridged complex 4 upon fixation of CO(2) by 3 in N-MeIm is unprecedentated. Methyl transfer from N-MeIm to a bicarbonato-bridged (mu-oxo)diiron(III) intermediate appears to give rise to 5. Complex 3 is a good starting material for the synthesis of (mu-oxo)mono(mu-carboxylato)diiron(III) species [Fe(2)(mu-O)(mu-RCO(2))(N-MeIm)(8)](ClO(4))(3) (where R = H (4), CH(3) (6), or C(6)H(5) (7)); addition of the respective carboxylate ligand in stoichiometric amount to a solution of 3 in N-MeIm affords these complexes in high yields. Attempts to add a third bridge to complexes 4, 6, and 7 to form the (mu-oxo)bis(mu-carboxylato)diiron(III) species result in the isolation of the previously known triiron(III) mu-eta(3)-oxo clusters [[Fe(mu-RCO(2))(2)(N-MeIm)](3)O](ClO(4)) (8). The structures of 3, 4, 6, and 7 allow one, for the first time, to inspect the various features of the [Fe(2)(mu-O)(mu-RCO(2))](3+) moiety with no strain from the ligand framework.     

Reactivity of a (mu-oxo)(mu-hydroxo)diiron(III) diamond core with water, urea, substituted ureas, and acetamide

A series of iron(III) complexes of the tetradentate ligand BPMEN (N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)ethane-1,2-diamine) were prepared and structurally characterized. Complex [Fe(2)(mu-O)(mu-OH)(BPMEN)(2)](ClO(4))(3) (1) contains a (mu-oxo)(mu-hydroxo)diiron(III) diamond core. Complex [Fe(BPMEN)(urea)(OEt)](ClO(4))(2) (2) is a rare example of a mononuclear non-heme iron(III) alkoxide complex. Complexes [Fe(2)(mu-O)(mu-OC(NH(2))NH)(BPMEN)(2)](ClO(4))(3) (3) and [Fe(2)(mu-O)(mu-OC(NHMe)NH)(BPMEN)(2)](ClO(4))(3) (4) feature N,O-bridging deprotonated urea ligands. The kinetics and equilibrium of the reactions of 1 with ligands L (L = water, urea, 1-methylurea, 1,1-dimethylurea, 1,3-dimethylurea, 1,1,3,3-tetramethylurea, and acetamide) in acetonitrile solutions were studied by stopped-flow UV-vis spectrophotometry, NMR, and mass spectrometry. All these ligands react with 1 in a rapid equilibrium, opening the four-membered Fe(III)(mu-O)(mu-OH)Fe(III) core and forming intermediates with a (HO)Fe(III)(mu-O)Fe(III)(L) core. The entropy and enthalpy for urea binding through oxygen are DeltaH degrees = -25 kJ mol(-1) and DeltaS degrees = -53.4 J mol(-1) K(-1) with an equilibrium constant of K(1) = 37 L mol(-1) at 25 degrees C. Addition of methyl groups on one of the urea nitrogen did not affect this reaction, but the addition of methyl groups on both nitrogens considerably decreased the value of K(1). An opening of the hydroxo bridge in the diamond core complex [Fe(2)(mu-O)(mu-OH)(BPMEN)(2)] is a rapid associative process, with activation enthalpy of about 60 kJ mol(-1) and activation entropies ranging from -25 to -43 J mol(-1) K(-1). For the incoming ligands with the -CONH(2) functionality (urea, 1-methylurea, 1,1-dimethylurea, and acetamide), a second, slow step occurs, leading to the formation of stable N,O-coordinated amidate diiron(III) species such as 3 and 4. The rate of this ring-closure reaction is controlled by the steric bulk of the incoming ligand and by the acidity of the amide group.     

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.     

Structural and spectroscopic studies of valence-delocalized diiron(II,III) complexes dupported by carboxylate-only bridging ligands

The synthesis, molecular structures, and spectroscopic properties of a series of valence-delocalized diiron(II,III) complexes are described. One-electron oxidation of diiron(II) tetracarboxylate complexes afforded the compounds [Fe(2)(mu-O(2)CAr(Tol))(4)L(2)]X, where L = 4-(t)BuC(5)H(4)N (1b), C(5)H(5)N (2b), and THF (3b); X = PF(6)(-) (1b and 3b) and OTf(-) (2b). In 1b-3b, four mu-1,3 carboxylate ligands span relatively short Fe...Fe distances of 2.6633(11)-2.713(3) A. Intense (epsilon = 2700-3200 M(-1) cm(-1)) intervalence charge transfer bands were observed at 620-670 nm. EPR spectroscopy confirmed the S = (9)/(2) ground spin state of 1b-3b, the valence-delocalized nature of which was probed by X-ray absorption spectroscopy. The electron delocalization between paramagnetic metal centers is described by double exchange, which, for the first time, is observed in diiron clusters having no single-atom bridging ligand(s).     

End-on and side-on peroxo derivatives of non-heme iron complexes with pentadentate ligands: models for putative intermediates in biological iron/dioxygen chemistry

Mononuclear iron(III) species with end-on and side-on peroxide have been proposed or identified in the catalytic cycles of the antitumor drug bleomycin and a variety of enzymes, such as cytochrome P450 and Rieske dioxygenases. Only recently have biomimetic analogues of such reactive species been generated and characterized at low temperatures. We report the synthesis and characterization of a series of iron(II) complexes with pentadentate N5 ligands that react with H(2)O(2) to generate transient low-spin Fe(III)-OOH intermediates. These intermediates have low-spin iron(III) centers exhibiting hydroperoxo-to-iron(III) charge-transfer bands in the 500-600-nm region. Their resonance Raman frequencies, nu(O)(-)(O), near 800 cm(-)(1) are significantly lower than those observed for high-spin counterparts. The hydroperoxo-to-iron(III) charge-transfer transition blue-shifts and the nu(O)(-)(O) of the Fe-OOH unit decreases as the N5 ligand becomes more electron donating. Thus, increasing electron density at the low-spin Fe(III) center weakens the O-O bond, in accord with conclusions drawn from published DFT calculations. The parent [(N4Py)Fe(III)(eta(1)-OOH)](2+) (1a) ion in this series (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) can be converted to its conjugate base, which is demonstrated to be a high-spin iron(III) complex with a side-on peroxo ligand, [(N4Py)Fe(III)(eta(2)-O(2))](+) (1b). A detailed analysis of 1a and 1b by EPR and M?ssbauer spectroscopy provides insights into their electronic properties. The orientation of the observed (57)Fe A-tensor of 1a can be explained with the frequently employed Griffith model provided the rhombic component of the ligand field, determined by the disposition of the hydroperoxo ligand, is 45 degrees rotated relative to the octahedral field. EXAFS studies of 1a and 1b reveal the first metrical details of the iron-peroxo units in this family of complexes: [(N4Py)Fe(III)(eta(1)-OOH)](2+) has an Fe-O bond of 1.76 A, while [(N4Py)Fe(III)(eta(2)-O(2))](+) has two Fe-O bonds of 1.93 A, values which are in very good agreement with results obtained from DFT calculations.     

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).     

Synthesis and spectroscopy of micro-oxo (O(2)(-))-bridged heme/non-heme diiron complexes: models for the active site of nitric oxide reductase

In this paper, we describe the synthesis and study of a series of heme/non-heme Fe-O-Fe' complexes supported by a porphyrin and the tripodal nitrogen ligand TMPA [TMPA = tris(2-pyridylmethyl)amine]. The complete synthesis of [((6)L)Fe-O-Fe(X)](+) (1) (X = OMe(-) or Cl(-), 69:31 ratio), where (6)L is the dianion of 5-(o-O-[(N,N-bis(2-pyridylmethyl)-2-(6-methoxyl)pyridinemethanamine)phenyl]-10,15,20-tris(2,6-difluorophenyl)porphine, is reported. The crystal structure for 1.PF(6) reveals an intramolecular heme/non-heme diferric complex bridged by an Fe-O-Fe' moiety; 90 degree angle (Fe-O-Fe') = 166.7(3) degrees, and d(Fe.Fe') = 3.556 A. Crystal data for C(70)H(57)ClF(12)Fe(2)N(8)O(3)P (1.PF(6)): triclinic, Ponemacr;, a = 13.185(3) A, b = 14.590 (3) A, c = 16.885(4) A, alpha = 104.219(4) degrees, beta = 91.572(4) degrees, gamma = 107.907(4) degrees, V = 2977.3(11) A(3), Z = 2, T = 150(2) K. Complex 1 (where X = Cl(-)) is further characterized by UV-vis (lambda(max) = 328, 416 (Soret), 569 nm), (1)H NMR (delta 27-24 [TMPA -CH(2)-], 16.1 [pyrrole-H], 15.2-10.5 [PY-3H, PY-5H], 7.9-7.2 [m- and p-phenyl-H], 6.9-5.8 [PY-4H] ppm), resonance Raman (nu(as)(Fe-O-Fe') 844 cm(-)(1)), and M?ssbauer (delta(Fe) = 0.47, 0.41 mm/s; deltaE(A) = 1.59, 0.55 mm/s; 80 K) spectroscopies, MALDI-TOF mass spectrometry (m/z 1202), and SQUID susceptometry (J = - 114.82 cm(-)(1), S = 0). We have also synthesized a series of 3-, 4-, and 5-methyl-substituted as well as selectively deuterated TMPA(Fe') complexes and condensed these with the hydroxo complex (F(8))FeOH or (F(8)-d(8))FeOH to yield "untethered" Fe-O-Fe' analogues. Along with selective deuteration of the methylene hydrogens in TMPA, complete (1)H NMR spectroscopic assignments for 1 have been accomplished. The magnetic properties of several of the untethered complexes and a comparison to those of 1 are also presented. Complex 1 and related species represent good structural and spectroscopic models for the heme/non-heme diiron active site in the enzyme nitric oxide reductase.     

Structural and spectroscopic characterization of copper(II) tolfenamate complexes

The synthesis and characterization of four new solid complexes, Cu(tolf)₂L₂ (tolf = tolfenamate, L = 2-pyridylmethanol (2-pyme), 3-pyridylmethanol (3-pyme), nicotinamide (na)) and Cu(tolf)₂(dena)₂(H₂O)₂ (dena = N,N-diethylnicotinamide) is reported. The composition and stereochemistry as well as the mode for ligand coordination have been determined by elemental analysis, IR, electronic and EPR spectra. The carboxyl group of the tolfenamate anion coordinates to the Cu(II) atom as an unidentate or as a chelating ligand. The EPR spectra of the powdered solids are consistent with spin S = ½. The crystal structure of Cu(tolf)₂(dena)₂(H₂O)₂ has been determined at 293 K. The Cu(II) atom has a tetragonal–bipyramidal arrangement (CuO₄N₂). The spectroscopic data indicate that each copper(II) atom in Cu(tolf)₂L₂ has a tetragonal–bipyramidal environment built up by bidentate unsymmetrically coordinate tolfenamates and unidentate N-donor atom ligands.     

Catalytic oxyfunctionalization of saturated hydrocarbons by non-heme oxo-bridged diiron(III) complexes: role of acetic acid on oxidation reaction

Oxo-bridged diiron(III) complexes [Fe₂O(L¹)₂(H₂O)₂](ClO₄)₄ (1) and [Fe₂O(L²)₂(H₂O)₂](ClO₄)₄ (2), where L¹ and L² are tetradentate N-donor N,N′-bis(2-pyridylmethyl)-1,2-cyclohexanediamine and N,N′-bis(2-pyridylmethyl)ethane-1,2-diamine respectively, have been isolated as synthetic models of non-heme iron oxygenases and characterized by physicochemical and spectroscopic methods. Both the complexes have been studied as catalysts for the oxyfunctionalization of saturated hydrocarbons using green hydrogen peroxide (H₂O₂) as oxidant under mild conditions. The selectivity (A/K) and regioselectivity (3°/2°) in oxidative C–H functionalization of alkanes suggests the involvement of metal-based intermediate in the oxygenation reaction. The catalytic efficiency is found to be strongly dependent on the presence of acetic acid. Remarkable increase in conversion and selectivity favoring the formation of alcohols in the oxidation of cyclohexane and cyclooctane and exclusive hydroxylation of adamantane with drastic enhancement of regioselectivity has been achieved by the addition of acetic acid in the presence of H₂O₂.

     

Transient absorption studies of the Pacman effect in spring-loaded diiron(III) mu-oxo bisporphyrins

Picosecond transient absorption spectroscopy of diiron(III) mu-oxo bisporphyrins appended to xanthene, (DPX)Fe2O and (DPXM)Fe2O, and dibenzofuran (DPD)Fe2O have been investigated in order to decipher the effect of a spring-loaded cleft on their photophysics and attendant oxidation photocatalysis. The tension of the cofacial pocket is systematically tuned with the bridge span and meso-substitution opposite to the bridge; the distances of the relaxed cofacial pockets and clamped Fe-O-Fe pockets are known from X-ray crystallography (Deltad(M-M)(relaxed-clamped)=4.271 A (DPD), 2.424 A (DPXM), 0.208 A (DPX)). The photophysical and chemical properties of these cofacial platforms are compared to the unbridged diiron(III) mu-oxo analogue, (Etio)2Fe2O. Photon absorption by the diiron(III) mu-oxo chromophore prompts Fe-O-Fe photocleavage to release the spring and present a PFeIVO/PFeII pair (P=porphyrin subunit); net photooxidation is observed when oxygen atom transfer to substrate occurs before the spring can reclamp to form the mu-oxo species. The inherent lifetimes of the PFeIVO/PFeII pairs for the four compounds are surprisingly similar (tau[(DPD)Fe2O]=1.36(3) ns, tau[(DPX)Fe2O]=1.26(5) ns, tau[(DPXM)Fe2O]=1.27(9) ns, and tau[(Etio)2Fe2O]=0.97(3) ns), considering the structural differences arising from tensely clamped (DPD and DPXM), relaxed (DPX), and unbridged (Etio) cofacial architectures. However, the rates of net oxygen atom transfer for (DPD)Fe2O and (Etio)2Fe2O are found to be 4 orders of magnitude greater than that of (DPX)Fe2O and 2 orders of magnitude greater than that of (DPXM)Fe2O. These results show that the spring action of the cleft, known as the Pacman effect, does little to impede reclamping to form the mu-oxo species but rather is manifest to opening the cofacial cleft to allow substrate access to the photogenerated oxidant. Consistent with this finding, photooxidation efficiencies decrease as the steric demand of substrates increase.     

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).     

Synthesis and characterization of two binuclear iron(III) complexes of aminoethanol derivatives of aminophenol as models for non-heme iron enzymes active sites

Two aminoethanol derivatives of aminophenol ligands were synthesized and characterized by IR and ¹H NMR spectroscopies. The binuclear iron(III) complexes of these ligands have been prepared and characterized by IR, ¹H NMR and UV-Vis spectroscopic techniques, cyclic voltammetry, single crystal X-ray diffraction and magnetic susceptibility studies. X-ray analysis revealed binuclear complexes, Fe₂(L₂), in which Fe(III) centers are surrounded by two phenolate and hydroxyl oxygen atoms, and amine nitrogens of the ligands. The metal active sites of both complexes are held together by the two above mentioned hydroxyl bridges. Variable temperature magnetic susceptibility indicates antiferromagnetic coupling between the iron centers of both complexes. This exchange coupling is stronger for Fe₂(L^ae)₂, such that it shows a room temperature strong coupling between the two iron centers. The investigated complexes undergo irreversible electrochemical oxidation and reduction.     

Modeling dioxygen-activating centers in non-heme diiron enzymes: carboxylate shifts in diiron(II) complexes supported by sterically hindered carboxylate ligands

General synthetic routes are described for a series of diiron(II) complexes supported by sterically demanding carboxylate ligands 2,6-di(p-tolyl)benzoate (Ar(Tol)CO(2)(-)) and 2,6-di(4-fluorophenyl)benzoate (Ar(4-FPh)CO(2)(-)). The interlocking nature of the m-terphenyl units in self-assembled [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)L(2)] (L = C(5)H(5)N (4); 1-MeIm (5)) promotes the formation of coordination geometries analogous to those of the non-heme diiron cores in the enzymes RNR-R2 and Delta 9D. Magnetic susceptibility and M?ssbauer studies of 4 and 5 revealed properties consistent with weak antiferromagnetic coupling between the high-spin iron(II) centers. Structural studies of several derivatives obtained by ligand substitution reactions demonstrated that the [Fe(2)(O(2)CAr')(4)L(2)] (Ar' = Ar(Tol); Ar(4-FPh)) module is geometrically flexible. Details of ligand migration within the tetracarboxylate diiron core, facilitated by carboxylate shifts, were probed by solution variable-temperature (19)F NMR spectroscopic studies of [Fe(2)(mu-O(2)CAr(4-FPh))(2)-(O(2)CAr(4-FPh))(2)(THF)(2)] (8) and [Fe(2)(mu-O(2)CAr(4-FPh))(4)(4-(t)BuC(5)H(4)N)(2)] (12). Dynamic motion in the primary coordination sphere controls the positioning of open sites and regulates the access of exogenous ligands, processes that also occur in non-heme diiron enzymes during catalysis.     

Structural and spectroscopic characterization of metastable thiolate-ligated manganese(III)-alkylperoxo species

Metastable Mn-peroxo species are proposed to form as key intermediates in biological oxidation reactions involving O(2) and C-H bond activation. The majority of these have yet to be spectroscopically characterized, and their inherent instability, in most cases, precludes structural characterization. Cysteinate-ligated metal-peroxos have been shown to form as reactive intermediates in both heme and nonheme iron enzymes. Herein we report the only examples of isolable Mn(III)-alkylperoxo species, and the first two examples of structurally characterized synthetic thiolate-ligated metal-peroxos. Spectroscopic data, including electronic absorption and IR spectra, and ESI mass spectra for (16)O vs (18)O-labeled metastable Mn(III)-OOR (R = (t)Bu, Cm) are discussed, as well as preliminary reactivity.     

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.     

Dioxygen activation at non-heme diiron centers: characterization of intermediates in a mutant form of toluene/o-xylene monooxygenase hydroxylase

We report the generation and characterization of an intermediate in a mutant form of the toluene/o-xylene monooxygenase hydroxylase component from Pseudomonas stutzeri OX1. The reaction of chemically reduced I100W variant in the presence of the coupling protein, ToMOD, with dioxygen was monitored by stopped-flow UV/visible spectroscopy. Rapid-freeze quench (RFQ) samples were also generated for EPR and M?ssbauer spectroscopy. A transient species is observed in the UV/visible spectrum with an absorption maximum at 500 nm. EPR and M?ssbauer spectra of RFQ samples identified this species as a diiron(III,IV) cluster spin-coupled to a neutral W radical. A diamagnetic precursor to the mixed-valent diiron(III,IV) was also observed at an earlier time point, with M?ssbauer parameters typical of high-spin FeIII. We have tentatively assigned this antiferromagnetically coupled diiron(III) intermediate as a peroxo-bridged cluster, and this complex has also been observed in preliminary studies of the wild-type hydroxylase.