Catalytic C-H bond amination from high-spin iron imido complexes

Dipyrromethene ligand scaffolds were synthesized bearing large aryl (2,4,6-Ph(3)C(6)H(2), abbreviated Ar) or alkyl ((t)Bu, adamantyl) flanking groups to afford three new disubstituted ligands ((R)L, 1,9-R(2)-5-mesityldipyrromethene, R=aryl, alkyl). While high-spin (S=2), four-coordinate iron complexes of the type ((R)L)FeCl(solv) were obtained with the alkyl-substituted ligand varieties (for R=(t)Bu, Ad and solv=THF, OEt(2)), use of the sterically encumbered aryl-substituted ligand precluded binding of solvent and cleanly afforded a high-spin (S=2), three-coordinate complex of the type ((Ar)L)FeCl. Reaction of ((Ad)L)FeCl(OEt(2)) with alkyl azides resulted in the catalytic amination of C-H bonds or olefin aziridination at room temperature. Using a 5% catalyst loading, 12 turnovers were obtained for the amination of toluene as a substrate, while greater than 85% of alkyl azide was converted to the corresponding aziridine employing styrene as a substrate. A primary kinetic isotope effect of 12.8(5) was obtained for the reaction of ((Ad)L)FeCl(OEt(2)) with adamantyl azide in an equimolar toluene/toluene-d(8) mixture, consistent with the amination proceeding through a hydrogen atom abstraction, radical rebound type mechanism. Reaction of p-(t)BuC(6)H(4)N(3) with ((Ar)L)FeCl permitted isolation of a high-spin (S=2) iron complex featuring a terminal imido ligand, ((Ar)L)FeCl(N(p-(t)BuC(6)H(4))), as determined by (1)H NMR, X-ray crystallography, and (57)Fe Mo?ssbauer spectroscopy. The measured Fe-N(imide) bond distance (1.768(2) ?) is the longest reported for Fe(imido) complexes in any geometry or spin state, and the disruption of the bond metrics within the imido aryl substituent suggests delocalization of a radical throughout the aryl ring. Zero-field (57)Fe Mo?ssbauer parameters obtained for ((Ar)L)FeCl(N(p-(t)BuC(6)H(4))) suggest a Fe(III) formulation and are nearly identical with those observed for a structurally similar, high-spin Fe(III) complex bearing the same dipyrromethene framework. Theoretical analyses of ((Ar)L)FeCl(N(p-(t)BuC(6)H(4))) suggest a formulation for this reactive species to be a high-spin Fe(III) center antiferromagnetically coupled to an imido-based radical (J = -673 cm(-1)). The terminal imido complex was effective for delivering the nitrene moiety to both C-H bond substrates (42% yield) as well as styrene (76% yield). Furthermore, a primary kinetic isotope effect of 24(3) was obtained for the reaction of ((Ar)L)FeCl(N(p-(t)BuC(6)H(4))) with an equimolar toluene/toluene-d(8) mixture, consistent with the values obtained in the catalytic reaction. This commonality suggests the isolated high-spin Fe(III) imido radical is a viable intermediate in the catalytic reaction pathway. Given the breadth of iron imido complexes spanning several oxidation states (Fe(II)-Fe(V)) and several spin states (S=0→(3)/(2)), we propose the unusual electronic structure of the described high-spin iron imido complexes contributes to the observed catalytic reactivity.     

Substrate-triggered activation of a synthetic [Fe2(μ-O)2] diamond core for C-H bond cleavage

An [Fe(IV)(2)(μ-O)(2)] diamond core structure has been postulated for intermediate Q of soluble methane monooxygenase (sMMO-Q), the oxidant responsible for cleaving the strong C-H bond of methane and its hydroxylation. By extension, analogous species may be involved in the mechanisms of related diiron hydroxylases and desaturases. Because of the paucity of well-defined synthetic examples, there are few, if any, mechanistic studies on the oxidation of hydrocarbon substrates by complexes with high-valent [Fe(2)(μ-O)(2)] cores. We report here that water or alcohol substrates can activate synthetic [Fe(III)Fe(IV)(μ-O)(2)] complexes supported by tetradentate tris(pyridyl-2-methyl)amine ligands (1 and 2) by several orders of magnitude for C-H bond oxidation. On the basis of detailed kinetic studies, it is postulated that the activation results from Lewis base attack on the [Fe(III)Fe(IV)(μ-O)(2)] core, resulting in the formation of a more reactive species with a [X-Fe(III)-O-Fe(IV)═O] ring-opened structure (1-X, 2-X, X = OH(-) or OR(-)). Treatment of 2 with methoxide at -80 °C forms the 2-methoxide adduct in high yield, which is characterized by an S = 1/2 EPR signal indicative of an antiferromagnetically coupled [S = 5/2 Fe(III)/S = 2 Fe(IV)] pair. Even at this low temperature, the complex undergoes facile intramolecular C-H bond cleavage to generate formaldehyde, showing that the terminal high-spin Fe(IV)═O unit is capable of oxidizing a C-H bond as strong as 96 kcal mol(-1). This intramolecular oxidation of the methoxide ligand can in fact be competitive with intermolecular oxidation of triphenylmethane, which has a much weaker C-H bond (D(C-H) 81 kcal mol(-1)). The activation of the [Fe(III)Fe(IV)(μ-O)(2)] core is dramatically illustrated by the oxidation of 9,10-dihydroanthracene by 2-methoxide, which has a second-order rate constant that is 3.6 × 10(7)-fold larger than that for the parent diamond core complex 2. These observations provide strong support for the DFT-based notion that an S = 2 Fe(IV)═O unit is much more reactive at H-atom abstraction than its S = 1 counterpart and suggest that core isomerization could be a viable strategy for the [Fe(IV)(2)(μ-O)(2)] diamond core of sMMO-Q to selectively attack the strong C-H bond of methane in the presence of weaker C-H bonds of amino acid residues that define the diiron active site pocket.     

Selectivity and mechanism of hydrogen atom transfer by an isolable imidoiron(III) complex

In the literature, iron-oxo complexes have been isolated and their hydrogen atom transfer (HAT) reactions have been studied in detail. Iron-imido complexes have been isolated more recently, and the community needs experimental evaluations of the mechanism of HAT from late-metal imido species. We report a mechanistic study of HAT by an isolable iron(III) imido complex, L(Me)FeNAd (L(Me) = bulky β-diketiminate ligand, 2,4-bis(2,6-diisopropylphenylimido)pentyl; Ad = 1-adamantyl). HAT is preceded by binding of tert-butylpyridine ((t)Bupy) to form a reactive four-coordinate intermediate L(Me)Fe(NAd)((t)Bupy), as shown by equilibrium and kinetic studies. In the HAT step, very large substrate H/D kinetic isotope effects around 100 are consistent with C-H bond cleavage. The elementary HAT rate constant is increased by electron-donating groups on the pyridine additive, and by a more polar medium. When combined with the faster rate of HAT from indene versus cyclohexadiene, this trend is consistent with H(+) transfer character in the HAT transition state. The increase in HAT rate in the presence of (t)Bupy may be explained by a combination of electronic (weaker Fe=N π-bonding) and thermodynamic (more exothermic HAT) effects. Most importantly, HAT by these imido complexes has a strong dependence on the size of the hydrocarbon substrate. This selectivity comes from steric hindrance by the spectator ligands, a strategy that has promise for controlling the regioselectivity of these C-H bond activation reactions.     

Preparation of iron amido complexes via putative Fe(IV) imido intermediates

The isolation and characterization of monomeric Fe(III) amido complexes with hybrid ureate/amidate ligands is described. An aryl azide serves as the source of the amido ligand in preparing the complexes from trigonal monopyramidal Fe(II) precursors. Aryl azides more commonly react with transition metal complexes by a two-electron oxidation process to yield imido complexes, suggesting that the Fe(III) amido complexes may be formed from high valent species by hydrogen atom abstraction from an external species. The mechanistic basis for formation of the amido complexes is investigated using substrates that readily donate hydrogen atoms. Results from these experiments suggest that the Fe(III) amido complexes are generated from Fe(IV) imido intermediates that can facilitate homolytic X-H bond cleavage. The Fe(III) amido complexes are high spin (S = 5/2) with a strong absorbance band at lambdamax approximately 600 nm and extinction coefficients between 2000 and 3000 M-1 cm-1. These complexes are hygroscopic, reacting with 1 equiv of water to produce the corresponding Fe(III)-OH complexes and p-toluidine.     

DFT calculations of d0 M(NR)(CHtBu)(X)(Y) (M = Mo, W; R = CPh3, 2,6-iPr-C6H3; X and Y = CH2tBu, OtBu, OSi(OtBu)3) olefin metathesis catalysts: structural, spectroscopic and electronic properties

DFT(B3PW91) calculations have been carried out to rationalise the structural, electronic and spectroscopic properties of Mo and W imido M(NR1)(CHR2)(X)(Y) olefin metathesis catalysts by using either simplified or actual ligands of the experimental complexes. The calculated structures, energetics (preference for the syn isomer and alkylidene rotational barrier for the syn/anti interconversion), and spectroscopic properties (NMR J(C-H) coupling constants) are in good agreement with available experimental data. Additionally, the alkylidene nu(C-H) stretching frequencies, not available experimentally, have been calculated. These quasi-tetrahedral complexes have a linear imido group and a C-H alkylidene agostic interaction, which stabilizes the syn isomer. Whether looking at M(NR1)(CHR2)(X)(Y), M = Mo, W, or the isolobal Re complexes, Re(CR1)(CHR2)(X)(Y), a linear correlation is obtained between both the alkylidene nu(C-H) stretching frequencies and J(C-H) coupling constants with the calculated alkylidene C-H bond lengths. These correlations show that the strength of the alpha-C-H agostic interaction increases from alkylidyne Re to imido group 6 complexes and from Mo to W. The NBO and AIM Bader analyses show firstly that the imido and alkylidyne groups are both triply bonded to the metal, but that the triply bonded imido ligand is a weaker electron donor than the alkylidyne, hence the stronger alpha-C-H agostic interaction for group 6 imido complexes. Secondly, one of the pi bonds of the triply bonded ligand is weakened at the transition state of the alkylidene rotation: while no lone pair is formed, the metal-ligand triple bond is polarized. This is more favourable for an imido than for an alkylidyne ligand, hence the lower alkylidene rotational barrier for the former complexes. Conversely, the aryl imido is even less of an electron donor than the alkyl imido group, which in turn strengthens the alpha-C-H agostic interaction and lowers the alkylidene rotational barrier even more.     

Ligand topology effect on the reactivity of a mononuclear nonheme iron(IV)-oxo complex in oxygenation reactions

Mononuclear nonheme iron(IV)-oxo complexes with two different topologies, cis-α-[Fe(IV)(O)(BQCN)](2+) and cis-β-[Fe(IV)(O)(BQCN)](2+), were synthesized and characterized with various spectroscopic methods. The effect of ligand topology on the reactivities of nonheme iron(IV)-oxo complexes was investigated in C-H bond activation and oxygen atom-transfer reactions; cis-α-[Fe(IV)(O)(BQCN)](2+) was more reactive than cis-β-[Fe(IV)(O)(BQCN)](2+) in the oxidation reactions. The reactivity difference between the cis-α and cis-β isomers of [Fe(IV)(O)(BQCN)](2+) was rationalized with the Fe(IV/III) redox potentials of the iron(IV)-oxo complexes: the Fe(IV/III) redox potential of the cis-α isomer was 0.11 V higher than that of the cis-β isomer.     

High-valent iron and manganese complexes of corrole and porphyrin in atom transfer and dioxygen evolving catalysis

Manganese(V) imido complexes of 5,10,15-tris(pentafluorophenyl)corrole (H(3)tpfc) can be prepared by the reaction of Mn(III)(tpfc) and organic nitrene generated from either photolytic or thermal activation of organic azides. The terminal imido complexes of manganese(V) were among the first structurally characterized examples of Mn(V) terminal imido complexes in the literature. They feature a short Mn≡N triple bond and a nearly linear M[triple bond, length as m-dash]N-C angle. The ground state of (tpfc)Mn(V)(NAr) is singlet. Contrary to expectations, arylimido complexes of manganese(V) were stable to moisture and did not undergo [NR] group transfer to olefins. Manganese(V) imido corrole with an activated tosyl imido ligand was prepared from iodoimine (ArINTs) and manganese(III) corrole. The resulting complex (tpfc)Mn(NTs) is paramagnetic (S = 1), hydrolyzes to (tpfc)Mn(O) in the presence of water, abstracts hydrogen atoms from benzylic C-H bonds, and catalyzes aziridination of alkenes. Mechanistic studies on the aziridination and hydrogen atom transfer reactions are reviewed. This perspective also describes the reaction chemistry of the heme enzyme chlorite dismutase, the mechanism by which dioxygen is formed on a single-metal site, and recent advances in functional modelling of this enzyme. We also compare the reactivity of water-soluble iron versus manganese porphyrins towards the chlorite anion.     

Iron(II)-catalyzed amidation of aldehydes with iminoiodinanes at room temperature and under microwave-assisted conditions

A method for the amidation of aldehydes with PhI=NTs/PhI=NNs as the nitrogen source and an inexpensive iron(II) chloride + pyridine as the in situ formed precatalyst under mild conditions at room temperature or microwave assisted conditions is described. The reaction was operationally straightforward and accomplished in moderate to excellent product yields (20-99%) and with complete chemoselectivity with the new C-N bond forming only at the formylic C-H bond in substrates containing other reactive functional groups. By utilizing microwave irradiation, comparable product yields and short reaction times of 1 h could be accomplished. The mechanism is suggested to involve insertion of a putative iron-nitrene/imido group to the formylic C-H bond of the substrate via a H-atom abstraction/radical rebound pathway mediated by the precatalyst [Fe(py)(4)Cl(2)] generated in situ from reaction of FeCl(2) with pyridine.     

Unusual reactivity of methylene group adjacent to pyridine-2-carboxamido moiety in iron(III) and cobalt(III) complexes

The Fe(III) and Co(III) complexes of the ligand N-(2-picolyl)picolinamide (pmpH; H represents the dissociable amide hydrogen), namely, [Fe(pmp)(2)]BF(4) (1) and [Co(pmp)(2)]ClO(4) (2), have been synthesized and structurally characterized. The [bond]CH(2)[bond] moiety of pmp(-) in [M(pmp)(2)](+) (M = Fe, Co) is very reactive and is readily converted to carbonyl (C[double bond]O) group upon exposure to dioxygen. Such conversion results in [M(bpca)(2)]ClO(4) complexes (M = Fe (3), Co (5); bpcaH = bis(2-pyridylcarbonyl)amine) which have been characterized by spectroscopy and X-ray diffraction. The structure of 5 is reported here for the first time. The reactivity of the [bond]CH(2)[bond] moiety of pmp(-) has so far precluded the isolation of 1 although other metal complexes of pmp(-) have been reported years ago. The CH(2) --> C[double bond]O transformation arises from the tendency of the coordinated pmp(-) ligand to achieve further conjugation in the ligand framework and provides a better way to synthesize the metal complexes of bpcaH ligand. Reaction of 3 with NaH affords Fe(II) complex [Fe(bpca)(2)] (4) without any reduction of the ligand frame.     

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

A number of assignments have been previously posited for the metal-nitrogen stretch (nu(M-NR)), the N-R stretch (nu(MN-R)), and possible ligand deformation modes associated with terminally bound imides. Here we examine mononuclear iron(III) and cobalt(III) imido complexes of the monoanionic tridentate ligand [PhBP3] ([PhBP3] = [PhB(CH2PPh2)3]-) to clarify the vibrational features for these trivalent metal imides. We report the structures of [PhBP3]FeNtBu and [PhBP3]CoNtBu. Pseudo-tetrahedral metal imides of these types exhibit short bond lengths (ca. 1.65 A) and nearly linear angles about the M-N-C linkages, indicative of multiple bond character. Furthermore, these compounds give rise to intense, low-energy visible absorptions. Both the position and the intensity of the optical bands in the [PhBP3]MNR complexes depend on whether the substituent is an alkyl or aryl group. Excitation into the low-energy bands of [PhBP3]FeNtBu gives rise to two Raman features at 1104 and 1233 cm(-1), both of which are sensitive to 15N and 2H labeling. The isotope labeling suggests the 1104 cm(-1) mode has the greatest Fe-N stretching character, while the 1233 cm(-1) mode is affected to a lesser extent by (15)N substitution. The spectra of the deuterium-labeled imides further support this assertion. The data demonstrate that the observed peaks are not simple diatomic stretching modes but are extensively coupled to the vibrations of the ancillary organic group. Therefore, describing these complexes as simple diatomic or even triatomic oscillators is an oversimplification. Analogous studies of the corresponding cobalt(III) complex lead to a similar set of isotopically sensitive resonances at 1103 and 1238 cm(-1), corroborating the assignments made in the iron imides. Very minimal changes in the vibrational frequencies are observed upon replacement of cobalt(III) for iron(III), suggesting similar force constants for the two compounds. This is consistent with the previously proposed electronic structure model in which the added electron resides in a relatively nonbonding orbital. Replacement of the tBu group with a phenyl ring leads to a significantly more complicated resonance Raman spectrum, presumably due to coupling with the vibrations of the phenyl ring. Polarization studies demonstrate that the observed modes have A(1) symmetry. In this case, a clearer resonance enhancement of the signals is observed, supporting a charge transfer designation for the electronic transitions. A series of isotope-labeling experiments has been carried out, and the modes with the greatest metal-nitrogen stretching character have been assigned to peaks at approximately 960 and approximately 1300 cm(-1) in both the iron and cobalt [PhBP3]MNPh complexes. These results are consistent with a multiple M-N bond for these metal imides.     

Stereospecific alkane hydroxylation by non-heme iron catalysts: mechanistic evidence for an Fe(V)=O active species.

High-valent iron-oxo species have frequently been invoked in the oxidation of hydrocarbons by both heme and non-heme enzymes. Although a formally Fe(V)=O species, that is, [(Por(*))Fe(IV)=O](+), has been widely accepted as the key oxidant in stereospecific alkane hydroxylation by heme systems, it is not established that such a high-valent state can be accessed by a non-heme ligand environment. Herein we report a systematic study on alkane oxidations with H(2)O(2) catalyzed by a group of non-heme iron complexes, that is, [Fe(II)(TPA)(CH(3)CN)(2)](2+) (1, TPA = tris(2-pyridylmethyl)amine) and its alpha- and beta-substituted analogues. The reactivity patterns of this family of Fe(II)(TPA) catalysts can be modulated by the electronic and steric properties of the ligand environment, which affects the spin states of a common Fe(III)-OOH intermediate. Such an Fe(III)-peroxo species is high-spin when the TPA ligand has two or three alpha-substituents and is proposed to be directly responsible for the selective C-H bond cleavage of the alkane substrate. The thus-generated alkyl radicals, however, have relatively long lifetimes and are susceptible to radical epimerization and trapping by O(2). On the other hand, 1 and the beta-substituted Fe(II)(TPA) complexes catalyze stereospecific alkane hydroxylation by a mechanism involving both a low-spin Fe(III)-OOH intermediate and an Fe(V)=O species derived from O-O bond heterolysis. We propose that the heterolysis pathway is promoted by two factors: (a) the low-spin iron(III) center which weakens the O-O bond and (b) the binding of an adjacent water ligand that can hydrogen bond to the terminal oxygen of the hydroperoxo group and facilitate the departure of the hydroxide. Evidence for the Fe(V)=O species comes from isotope-labeling studies showing incorporation of (18)O from H(2)(18)O into the alcohol products. (18)O-incorporation occurs by H(2)(18)O binding to the low-spin Fe(III)-OOH intermediate, its conversion to a cis-H(18)O-Fe(V)=O species, and then oxo-hydroxo tautomerization. The relative contributions of the two pathways of this dual-oxidant mechanism are affected by both the electron donating ability of the TPA ligand and the strength of the C-H bond to be broken. These studies thus serve as a synthetic precedent for an Fe(V)=O species in the oxygen activation mechanisms postulated for non-heme iron enzymes such as methane monooxygenase and Rieske dioxygenases.     

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.     

Terminal cobalt(III) imido complexes supported by tris(carbene) ligands: imido insertion into the cobalt-carbene bond

Highly reactive tris-carbene Co(I) complexes [(TIMENaryl)Co]Cl react with organic azides to yield monomeric Co(III) imido complexes [(TIMENaryl)Co(NAr')](BPh4) (aryl = mes, xyl; Ar = -C6H4-CH3, -C6H4-OCH3). The cobalt-imido fragment in these complexes is electrophilic and, as a result, the imido group readily inserts into the cobalt-carbene bond, yielding bis-carbene imine cobalt complexes.     

Synthesis of U(IV) imidos from Tp*2U(CH2Ph) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) by extrusion of bibenzyl

Addition of organic azides, N(3)R (R = 2,4,6-trimethylphenyl (Mes), phenyl (Ph), 1-adamantyl (Ad)), to a solution of the uranium(III) alkyl complex, Tp*(2)U(CH(2)Ph) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) (1), results in the formation of a family of uranium(iv) imido derivatives, Tp*(2)U(NR) (2-R). Notably, these complexes were synthesized in high yields by coupling of the benzyl groups to form bibenzyl. The uranium(IV) imido derivatives, 2-Mes, 2-Ph, and 2-Ad, were all characterized by both (1)H NMR and IR spectroscopy, and 2-Mes and 2-Ad were also characterized by X-ray crystallography. In the molecular structure of 2-Mes, typical κ(3)-coordination of the Tp* ligands was observed; however in the case of 2-Ad, one pyrazole ring of a Tp* ligand has rotated away from the metal centre, forcing a κ(2)-coordination of the pyrazoles. This results in a uranium-hydrogen interaction with the Tp* B-H. Treating these imido complexes with para-tolualdehyde results in multiple bond metathesis, forming the terminal uranium(IV) oxo complex, Tp*(2)U(O), and the corresponding imine.     

Spin-state energetics and spin-crossover behavior of pseudotetrahedral cobalt(III)-imido complexes. The role of the tripodal supporting ligand

DFT calculations have underscored the importance of the tripodal supporting ligand in tuning the spin-state energetics of pseudotetrahedral transition metal imido complexes. In particular, we have focused on Co(III)-imido complexes, where our best estimate (OLYP) of the singlet-triplet splitting varies from 0.75 eV for a trisphosphine complex (1) and 0.3 eV for a tris(N-heteroyclic-carbene) complex (2) to essentially 0.0 eV for a hydrotris(pyrazolyl)borate (3) complex. The experimentally studied analogues of 1, 2, and 3 all exhibit S = 0 ground states; however, the experimental analogue of 3 exhibits spin-crossover behavior due to a low-lying S = 1 state. Interestingly, whereas all the pure functionals examined successfully predict nearly equienergetic singlet end triplet states for 3, the hybrid functionals B3LYP and O3LYP exhibit a clear (and incorrect) preference for the S = 2 state. In addition, we have also carried out an exploratory survey of Cr(III), Mn(III), and Fe(III) imido complexes with trisphosphine and hydrotris(pyrazolyl)borate (Tp) supporting ligands. Among the more interesting predictions of this study is that an FeIII(Tp)(imido) species should exhibit a high-spin S = 5/2 ground state, which would be unique for an iron-imido complex.     

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

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

Imido transfer from bis(imido)ruthenium(VI) porphyrins to hydrocarbons: effect of imido substituents, C-H bond dissociation energies, and Ru(VI/V) reduction potentials

[Ru(VI)(TMP)(NSO2R)2] (SO2R = Ms, Ts, Bs, Cs, Ns; R = p-C6H4OMe, p-C6H4Me, C6H5, p-C6H4Cl, p-C6H4NO2, respectively) and [Ru(VI)(Por)(NTs)2] (Por = 2,6-Cl2TPP, F20-TPP) were prepared by the reactions of [Ru(II)(Por)(CO)] with PhI=NSO2R in CH2Cl2. These complexes exhibit reversible Ru(VI/V) couple with E(1/2) = -0.41 to -0.12 V vs Cp2Fe(+/0) and undergo imido transfer reactions with styrenes, norbornene, cis-cyclooctene, indene, ethylbenzenes, cumene, 9,10-dihydroanthracene, xanthene, cyclohexene, toluene, and tetrahydrofuran to afford aziridines or amides in up to 85% yields. The second-order rate constants (k2) of the aziridination/amidation reactions at 298 K were determined to be (2.6 +/- 0.1) x 10(-5) to 14.4 +/- 0.6 dm3 mol(-1) s(-1), which generally increase with increasing Ru(VI/V) reduction potential of the imido complexes and decreasing C-H bond dissociation energy (BDE) of the hydrocarbons. A linear correlation was observed between log k' (k' is the k2 value divided by the number of reactive hydrogens) and BDE and between log k2 and E(1/2)(Ru(VI/V)); the linearity in the former case supports a H-atom abstraction mechanism. The amidation by [Ru(VI)(TMP)(NNs)2] reverses the thermodynamic reactivity order cumene > ethylbenzene/toluene, with k'(tertiary C-H)/k'(secondary C-H) = 0.2 and k'(tertiary C-H)/k'(primary C-H) = 0.8.     

Thermodynamics of hydrogen atom transfer to a high-valent iron imido complex

Thermodynamic investigations relevant to hydrogen atom transfer by the high-valent iron imido complex [LMesFe[triple bond]NAd]OTf have been undertaken. The complex is found to be weakly oxidizing by cyclic voltammetry (E1/2 = -0.98 V vs Cp2Fe+/Cp2Fe in MeCN). A combination of experimental and computational studies has been used to determine the acidity of LMesFe-N(H)Ad+ (pKa = 37 in MeCN), allowing the N-H BDFE (88(5) kcal/mol) to be calculated from a thermodynamic cycle. Consistent with this value, [LMesFe[triple bond]NAd]OTf reacts with 9,10-dihydroanthracene (C-H BDE = 78(1) kcal/mol) to form anthracene.     

Role of Fe(IV)-oxo intermediates in stoichiometric and catalytic oxidations mediated by iron pyridine-azamacrocycles

An iron(II) complex with a pyridine-containing 14-membered macrocyclic (PyMAC) ligand L1 (L1 = 2,7,12-trimethyl-3,7,11,17-tetra-azabicyclo[11.3.1]heptadeca-1(17),13,15-triene), 1, was prepared and characterized. Complex 1 contains low-spin iron(II) in a pseudo-octahedral geometry as determined by X-ray crystallography. Magnetic susceptibility measurements (298 K, Evans method) and M?ssbauer spectroscopy (90 K, δ = 0.50(2) mm/s, ΔE(Q) = 0.78(2) mm/s) confirmed that the low-spin configuration of Fe(II) is retained in liquid and frozen acetonitrile solutions. Cyclic voltammetry revealed a reversible one-electron oxidation/reduction of the iron center in 1, with E(1/2)(Fe(III)/Fe(II)) = 0.49 V vs Fc(+)/Fc, a value very similar to the half-wave potentials of related macrocyclic complexes. Complex 1 catalyzed the epoxidation of cyclooctene and other olefins with H(2)O(2). Low-temperature stopped-flow kinetic studies demonstrated the formation of an iron(IV)-oxo intermediate in the reaction of 1 with H(2)O(2) and concomitant partial ligand oxidation. A soluble iodine(V) oxidant, isopropyl 2-iodoxybenzoate, was found to be an excellent oxygen atom donor for generating Fe(IV)-oxo intermediates for additional spectroscopic (UV-vis in CH(3)CN: λ(max) = 705 nm, ε ≈ 240 M(-1) cm(-1); M?ssbauer: δ = 0.03(2) mm/s, ΔE(Q) = 2.00(2) mm/s) and kinetic studies. The electrophilic character of the (L1)Fe(IV)═O intermediate was established in rapid (k(2) = 26.5 M(-1) s(-1) for oxidation of PPh(3) at 0 °C), associative (ΔH(?) = 53 kJ/mol, ΔS(?) = -25 J/K mol) oxidation of substituted triarylphosphines (electron-donating substituents increased the reaction rate, with a negative value of Hammet's parameter ρ = -1.05). Similar double-mixing kinetic experiments demonstrated somewhat slower (k(2) = 0.17 M(-1) s(-1) at 0 °C), clean, second-order oxidation of cyclooctene into epoxide with preformed (L1)Fe(IV)═O that could be generated from (L1)Fe(II) and H(2)O(2) or isopropyl 2-iodoxybenzoate. Independently determined rates of ferryl(IV) formation and its subsequent reaction with cyclooctene confirmed that the Fe(IV)-oxo species, (L1)Fe(IV)═O, is a kinetically competent intermediate for cyclooctene epoxidation with H(2)O(2) at room temperature. Partial ligand oxidation of (L1)Fe(IV)═O occurs over time in oxidative media, reducing the oxidizing ability of the ferryl species; the macrocyclic nature of the ligand is retained, resulting in ferryl(IV) complexes with Schiff base PyMACs. NH-groups of the PyMAC ligand assist the oxygen atom transfer from ferryl(IV) intermediates to olefin substrates.     

Catalytic flow-injection determination of reactive iron in non-acidified water samples.

A flow-injection determination method was proposed for the speciation of reactive species of iron in non-acidified water samples. Iron was determined by the spectrophotometric monitoring of the iron-catalyzed oxidation of o-phenylenediamine with hydrogen peroxide. The reactive iron was characterized as Fe3+ and Fe(III)Li(3-in), where i = 1 or 2 for a unidentate ligand and i = 1 for a bidentate ligand (L(n-)) from the chemical equilibria of hydroxo, oxalato and fluoro Fe(III) complexes. A useful parameter was also proposed to check the reliability of the determination when suffering the adsorption of iron on the inner wall of a PTFE tube of a flow-injection system. Under the optimized FIA condition and by measuring the peak height, a linear calibration curve of iron was obtained up to 0.1 mg L(-1) with a detection limit of 0.1 microg L(-1). The proposed method was successfully applied to the speciation of reactive and unreactive iron in tap- and river-water samples.