An Iron(III) Iodosylbenzene Complex: A Masked Non-Heme FeVO

Influence of Equatorial Co-Ligands on the Reactivity of LFeIIIOIPh

Previous biomimetic studies clearly proved that equatorial ligands significantly influence the redox potential and thus the stability/reactivity of biologically important oxoiron intermediates; however, no such studies were performed on FeIIIOIPh species. In this study, the influence of substituted pyridine co-ligands on the reactivity of iron(III)-iodosylbenzene adduct has been investigated in sulfoxidation and epoxidation reactions. Selective oxidation of thioanisole, cis-cyclooctene, and cis- and trans-stilbene in the presence of a catalytic amount of [FeII(PBI)3](OTf)2 with PhI(OAc)2 provide products in good to excellent yields through an FeIIIOIPh intermediate depending on the co-ligand (4R-Py) used. Several mechanistic studies were performed to gain more insight into the mechanism of oxygen atom transfer (OAT) reactions to support the reactive intermediate and investigate the effect of the equatorial co-ligands. Based on competitive experiments, including a linear free-energy relationship between the relative reaction rates (logkrel) and the σp (4R-Py) parameters, strong evidence has been observed for the electrophilic character of the reactive species. The presence of the [(PBI)2(4R-Py)FeIIIOIPh]3+ intermediates and the effect of the co-ligands was also supported by UV-visible measurements, including the color change from red to green and the hypsochromic shifts in the presence of co-ligands. This is another indication that the title iron(III)-iodosylbenzene adduct is able to oxygenate sulfides and alkenes before it is transformed into the oxoiron form by cleavage of the O-I bond.

Structure and Reactivity of a Seven-Coordinate Ruthenium Iodosylbenzene Complex

Seven-coordinate (CN7) ruthenium-oxo species have attracted much attention as highly reactive intermediates in both organic and water oxidation. Apart from metal-oxo, other metal-oxidant adducts, such as metal-iodosylarenes, have also recently emerged as active oxidants. We reported herein the first example of a CN7 Ru-iodosylbenzene complex, [Ru^IV(bdpm)(pic)₂(O)I(Cl)Ph]⁺ (H₂bdpm = [2,2'-bipyridine]-6,6'-diylbis(diphenylmethanol); pic = 4-picoline). The X-ray crystal structure of this complex shows that it adopts a distorted pentagonal bipyramidal geometry with Ru-O(I) and O-I distances of 2.0451(39) and 1.9946(40) Å, respectively. This complex is highly reactive, and it readily undergoes O-atom transfer (OAT) and C-H bond activation reactions with various organic substrates. This work should provide insights for the development of new highly reactive oxidizing agents based on CN7 geometry.

Oxidation of Aldehydes into Carboxylic Acids by a Mononuclear Manganese(III) Iodosylbenzene Complex through Electrophilic C-H Bond Activation

The oxidation of aldehyde is one of the fundamental reactions in the biological system. Various synthetic procedures and catalysts have been developed to convert aldehydes into corresponding carboxylic acids efficiently under ambient conditions. In this work, we report the oxidation of aldehydes by a mononuclear manganese(III) iodosylbenzene complex, [Mn^III(TBDAP)(OIPh)(OH)]²⁺ (1), with kinetic and mechanistic studies in detail. The reaction of 1 with aldehydes resulted in the formation of corresponding carboxylic acids via a pre-equilibrium state. Hammett plot and reaction rates of 1 with 1°-, 2°-, and 3°-aldehydes revealed the electrophilicity of 1 in the aldehyde oxidation. A kinetic isotope effect experiment and reactivity of 1 toward cyclohexanecarboxaldehyde (CCA) analogues indicate that the reaction of 1 with aldehyde occurs through the rate-determining C-H bond activation at the formyl group. The reaction rate of 1 with CCA is correlated to the bond dissociation energy of the formyl group plotting a linear correlation with other aliphatic C-H bonds. Density functional theory calculations found that 1 electrostatically interacts with CCA at the pre-equilibrium state in which the C-H bond activation of the formyl group is performed as the most feasible pathway. Surprisingly, the rate-determining step is characterized as hydride transfer from CCA to 1, affording an (oxo)methylium intermediate. At the fundamental level, it is revealed that the hydride transfer is composed of H atom abstraction followed by a fast electron transfer. Catalytic reactions of aldehydes by 1 are also presented with a broad substrate scope. This novel mechanistic study gives better insights into the metal oxygen chemistry and would be prominently valuable for development of transition metal catalysts.

Unmasking the Iron-Oxo Bond of the [(Ligand)Fe-OIAr]2+/+ Complexes

ArIO (ArI = 2-(^tBuSO₂)C₆H₄I) is an oxidant used to oxidize Fe^II species to their Fe^IV-oxo state, enabling hydrogen-atom transfer (HAT) and oxygen-atom transfer (OAT) reactions at low energy barriers. ArIO, as a ligand, generates masked Feⁿ═O species of the type Fe^(n-2)-OIAr. Herein, we used gas-phase ion-molecule reactions and DFT calculations to explore the properties of masked iron-oxo species and to understand their unmasking mechanisms. The theory shows that the I-O bond cleavage in [(TPA)Fe^IVO(ArIO)]²⁺ (1²⁺, TPA = tris(2-pyridylmethyl)amine)) is highly endothermic; therefore, it can be achieved only in collision-induced dissociation of 1²⁺ leading to the unmasked iron(VI) dioxo complex. The reduction of 1²⁺ by HAT leads to [(TPA)Fe^IIIOH(ArIO)]²⁺ with a reduced energy demand for the I-O bond cleavage but is, however, still endothermic. The exothermic unmasking of the Fe═O bond is predicted after one-electron reduction of 1²⁺ or after OAT reactivity. The latter leads to the 4e^- oxidation of unsaturated hydrocarbons: The initial OAT from [(TPA)Fe^IVO(ArIO)]²⁺ leads to the epoxidation of an alkene and triggers the unmasking of the second Fe═O bond still within one collisional complex. The second oxidation step starts with HAT from a C-H bond and follows with the rebound of the C-radical and the OH group. The process starting with the one-electron reduction could be studied with [(TQA)Fe^IVO(ArIO)]²⁺ (2²⁺, TQA = tris(2-quinolylmethyl)amine)) because it has a sufficient electron affinity for electron transfer with alkenes. Accordingly, the reaction of 2²⁺ with 2-carene leads to [(TQA)Fe^IIIO(ArIO)]²⁺ that exothermically eliminates ArI and unmasks the reactive Fe^V-dioxo species.

Intercepting a transient non-hemic pyridine N-oxide Fe(III) species involved in OAT reactions

In the context of bioinspired OAT catalysis, we developed a tetradentate dipyrrinpyridine ligand, a hybrid of hemic and non-hemic models. The catalytic activity of the iron(III) derivative was investigated in the presence of iodosylbenzene. Unexpectedly, MS, EPR, Mössbauer, UV-visible and FTIR spectroscopic signatures supported by DFT calculations provide convincing evidence for the involvement of a relevant Fe^III-O-N_Py active intermediate.

Cooperative Co-Activation of Water and Hypochlorite by a Non-Heme Diiron(III) Complex

Aqueous solutions of the iron(III) complex of N,N,N'-tris(2-pyridylmethyl)ethylenediamine-N'-acetate (tpena) react with hypochlorite (ClO^-) to produce the reactive high-valent [Fe^IV(O)(tpena)]⁺. Under catalytic conditions, in bicarbonate-buffered media (pH 8) with a set ionic strength (10 mM NaCl), kinetic analysis shows that two equivalents of [Fe^IV(O)(tpena)]⁺ per one ClO^- are produced, with benign chloride ions the only byproduct. An unprecedented supramolecular activation of ClO^- by {(HCO₃)⊂[(tpena)Fe^III(μ-O)Fe^III(Htpena)]}²⁺ is proposed. This mode of activation has great advantage for use in the catalytic oxidation of C-H bonds in water since: (i) the catalyst scaffold is protected from oxidative degradation and (ii) undesirable radical side reactions which produce toxic chlorinated compounds are circumvented by this novel coactivation of water and ClO^-. The unique activation mechanism by the Fe-tpena system makes possible the destruction of organic contaminants as an add-on technology to water disinfection by chlorination, demonstrated here through (i) the catalytic oxidation of micropollutant metaldehyde, and (ii) mineralization of the model substrate formate. The resting-state speciation at pH 3, 5, 7, and 9, as well as the catalytically active iron speciation are characterized with Mössbauer and EPR spectroscopy and supported by DFT calculations. Our study provides fundamentally new insights into the design and activation mode of iron-based catalysts relevant to applications in water remediation.

Hydride-Transfer Reaction to a Mononuclear Manganese(III) Iodosylarene Complex

Metal iodosylarene species have received interest because of their potential oxidative power as a catalyst. We present the first example of hydride-transfer reactions to a mononuclear manganese(III) iodosylbenzene complex, [Mn^III(TBDAP)(OIPh)(OH)]²⁺ (1; TBDAP = N,N-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane), with dihydronicotinamide adenine dinucleotide (NADH) analogues. Kinetic studies show that hydride-transfer from the NADH analogues to 1 occurs via a proton-coupled electron transfer, followed by a rapid electron transfer.

Engineering the Oxidative Potency of Non-Heme Iron(IV) Oxo Complexes in Water for C-H Oxidation by a cis Donor and Variation of the Second Coordination Sphere

A series of iron(IV) oxo complexes, which differ in the donor (CH₂py or CH₂COO^-) cis to the oxo group, three with hemilabile pendant donor/second coordination sphere base/acid arms (pyH/py or ROH), have been prepared in water at pH 2 and 7. The ν_Fe═O values of 832 ± 2 cm^-1 indicate similar Fe^IV═O bond strengths; however, different reactivities toward C-H substrates in water are observed. HAT occurs at rates that differ by 1 order of magnitude with nonclassical KIEs (k_H/k_D = 30-66) consistent with hydrogen atom tunneling. Higher KIEs correlate with faster reaction rates as well as a greater thermodynamic stability of the iron(III) resting states. A doubling in rate from pH 7 to pH 2 for substrate C-H oxidation by the most potent complex, that with a cis-carboxylate donor, [Fe^IVO(Htpena)]²⁺, is observed. Supramolecular assistance by the first and second coordination spheres in activating the substrate is proposed. The lifetime of this complex in the absence of a C-H substrate is the shortest (at pH 2, 3 h vs up to 1.3 days for the most stable complex), implying that slow water oxidation is a competing background reaction. The iron(IV)═O complex bearing an alcohol moiety in the second coordination sphere displays significantly shorter lifetimes due to a competing selective intramolecular oxidation of the ligand.

Photosynthesis of a Dihydroimidazopyridine Chelate Shines Light on the Reactions of a Photoactivated Iron(III) Complex with O2

The high-spin (S = ⁵/₂) meridional diastereoisomer of [Fe^III(tpena)]²⁺ (tpena = N,N,N'-tris(2-pyridylmethyl)ethylendiamine-N'-acetate), mer-[Fe(tpena)]²⁺, undergoes photolytic CO₂ release to produce an iron(II) intermediate of a radical dihydroimidazopyridine ligand (L^•). The structure of this unprecedented transient iron(II)(L^•) complex is supported by UV-vis and Mössbauer spectroscopies, DFT calculations, as well as the X-ray structural characterization of an μ-oxo iron(III) complex of the oxidized derivative of L^•, namely, [Fe^III₂O(Cl)₂(L⁺)₂](ClO₄)₄(MeCN)₂ (L⁺ = 2-(2-(bis(pyridin-2-ylmethyl)amino)ethyl)-2,3-dihydro-1H-imidazo[1,5-a]pyridin-4-ium). [Fe^III₂O(Cl)₂(L⁺)₂]⁴⁺ is obtained only in the absence of O₂. Under aerobic conditions, O₂ will intercept the iron(II)(L^•) complex to form a putative Fe(III)-alkylperoxide complex which cascades to an iron(II) complex of SBPy3 (SBPy3 = N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-aldimine). Thus, through different oxidative pathways, the unknown ligand L⁺ or SBPy3 forms by loss of a one-carbon-atom or a two-carbon-atom unit, respectively, from the glycyl arm of tpena. Acceleration of the photodecarboxylation step is achieved by addition of thiocyanate because of transient formation of a more photoreactive NCS^- adduct of [Fe(tpena)]²⁺. This has allowed for kinetic observation of the reaction of [Fe^II(L^•)]²⁺ with O₂ which is, unexpectedly, promoted also by light. We propose that this corresponds to the energy needed for the conversion of the ring-closed radical ligand L^• to a ring-opened tautomer to allow for O₂ insertion between the C and Fe atoms of the iron(II) complex.

Unprecedented Reactivities of Highly Reactive Manganese(III)-Iodosylarene Porphyrins in Oxidation Reactions

We report that Mn(III)-iodosylarene porphyrins, [Mn^III(Porp)(^sArIO)]⁺, are capable of activating the C-H bonds of hydrocarbons, including unactivated alkanes such as cyclohexane, with unprecedented reactivities, such as a low kinetic isotope effect, a saturation behavior of reaction rates, and no electronic effect of porphyrin ligands on the reactivities of [Mn^III(Porp)(^sArIO)]⁺. In oxygen atom transfer (OAT) reactions, the sulfoxidation of para-X-substituted thioanisoles by [Mn^III(Porp)(^sArIO)]⁺ affords a very unusual behavior in the Hammett plot with the saturation behavior of reaction rates and no electronic effect of porphyrin ligands on reactivities. The reactivities and mechanisms of [Mn^III(Porp)(^sArIO)]⁺ are then compared with those of the corresponding Mn^IV(Porp)(O) complex. The present study reports the first example of highly reactive Mn(III)-iodosylarene porphyrins with unprecedented reactivities in C-H bond activation and OAT reactions.

Theoretical Study on the Structural-Function Relationship of Manganese(III)-Iodosylarene Adducts

Metal-iodosylarene complexes have been recently viewed as a second oxidant alongside of the well-known high-valent metal-oxo species. Extensive efforts have been exerted to unveil the structure-function relationship of various metal-iodosylarene complexes. In the present manuscript, density functional theoretical calculations were employed to investigate such relationship of a specific manganese-iodosylbenzene complex [Mn^III(TBDAP)(PhIO)(OH)]²⁺ (1). Our results fit the experimental observations and revealed new mechanistic findings. 1 acts as a stepwise 1e+1e oxidant in sulfoxidation reactions. Surprisingly, C-H bond activation of 9,10-dihydroanthracene (DHA) by 1 proceeds via a novel ionic hydride transfer/proton transfer (HT/PT) mechanism. As a comparison to 1, the electrophilicity of an iodosylbenzene monomer PhIO was investigated. PhIO performs concerted 2e-oxidations both in sulfoxidation and C-H activation. Hydroxylation of DHA by PhIO was found to proceed via a novel ionic and concerted proton-transfer/hydroxyl-rebound mechanism involving 2e-oxidation to form a transient carbonium species.

Mechanistic dichotomies in redox reactions of mononuclear metal–oxygen intermediates

This review article focuses on various mechanistic dichotomies in redox reactions of metal–oxygen intermediates with the emphasis on understanding and controlling their redox reactivity from experimental and theoretical points of view.

Iodosylbenzene Coordination Chemistry Relevant to Metal-Organic Framework Catalysis

Hypervalent iodine compounds formally feature expanded valence shells at iodine. These reagents are broadly used in synthetic chemistry due to the ability to participate in well-defined oxidation-reduction processes and because the ligand-exchange chemistry intrinsic to the hypervalent center allows hypervalent iodine compounds to be applied to a broad array of oxidative substrate functionalization reactions. We recently developed methods to generate these compounds from O₂ that are predicated on diverting reactive intermediates of aldehyde autoxidation toward the oxidation of aryl iodides. Coupling the aerobic oxidation of aryl iodides with catalysts that effect C-H bond oxidation would provide a strategy to achieve aerobic C-H oxidation chemistry. In this Forum Article, we discuss the aspects of hypervalent iodine chemistry and bonding that render this class of reagents attractive lynchpins for aerobic oxidation chemistry. We then discuss the oxidation processes relevant to the aerobic preparation of 2-(tert-butylsulfonyl)iodosylbenzene, which is a popular hypervalent iodine reagent for use with porous metal-organic framework (MOF)-based catalysts because it displays significantly enhanced solubility as compared with unsubstituted iodosylbenzene. We demonstrate that popular synthetic methods to this reagent often provide material that displays unpredictable disproportionation behavior due to the presence of trace impurities. We provide a revised synthetic route that avoids impurities common in the reported methods and provides access to material that displays predictable stability. Finally, we describe the coordination chemistry of hypervalent iodine compounds with metal clusters relevant to MOF chemistry and discuss the potential implications of this coordination chemistry to catalysis in MOF scaffolds.

What factors tune the chemical equilibrium between metal-iodosylarene oxidants and high-valent metal-oxo ones?

Metal-iodosylarene complexes (1) and high-valent metal-oxo complexes (2) are two key reactive intermediates in oxygenation reactions.

Photoinduced O2-Dependent Stepwise Oxidative Deglycination of a Nonheme Iron(III) Complex

The iron(III) complex [Fe(tpena)]²⁺ (tpena = N, N, N'-tris(2-pyridylmethyl)ethylendiamine- N'-acetate) undergoes irreversible O₂-dependent N-demethylcarboxylation to afford [Fe^II(SBPy3)(MeCN)]²⁺ (SBPy3 = N, N-bis(2-pyridylmethyl)amine- N-ethyl-2-pyridine-2-aldimine), when irradiated with near-UV light. The loss of a mass equivalent to the glycyl group in a process involving consecutive C-C and C-N cleavages is documented by the measurement of the sequential production of CO₂ and formaldehyde, respectively. Time-resolved UV-vis absorption, Mössbauer, EPR, and Raman spectroscopy have allowed the spectroscopic characterization of two iron-based intermediates along the pathway. The first of these, proposed to be a low-spin iron(II)-radical ligand complex, reacts with O₂ in the rate-determining step to produce a putative alkylperoxide complex. DFT calculations suggest that this evolves into an Fe(IV)-oxo species, which can abstract a hydrogen atom from a cis methylene group of the ligand to give the second spectroscopically identified intermediate, a high-spin iron(III)-hydroxide of the product oxidized ligand, [Fe^III(OH)(SBPy₃)]²⁺. Reduction and exchange of the cohydroxo/water ligand produces the crystallographically characterized products [Fe^II(SBPy3)(X)]^2+/3+, X = MeCN, [Zn(tpena)]⁺.

Isolable iodosylarene and iodoxyarene adducts of Co and their O-atom transfer and C-H activation reactivity

We report an unusual series of discrete iodosyl- and iodoxyarene adducts of Co(ii) including detailed studies of their O-transfer reactivity and mechanism.

We report an unusual series of discrete iodosyl- and iodoxyarene adducts of Co. The formation of these adducts was confirmed by a suite of techniques including single crystal X-ray diffraction. The reactivity of these adducts with O-atom acceptors and an H-atom donor has been investigated with particular focus on elucidating mechanistic details. Detailed kinetic analysis allows for discrimination between proposed oxo and adduct mediated mechanisms. In particular, these reactions have been interrogated by competition experiments with isotopically labelled mixtures which shows that all of the studied adducts display a large KIE. These studies suggest different mechanisms may be relevant depending on subtle substituent changes in the adduct complexes. Reactivity data are consistent with the involvement of a transient oxo complex in one case, while the two other systems appear to react with substrates directly as iodosyl- or iodoxyarene adducts. These results support that reactivity typically ascribed to metal-oxo complexes, such as O-atom transfer and C–H activation, can also be mediated by discrete transition metal iodosyl- or iodoxyarene adducts that are frequent intermediates in the generation of oxo complexes. The influence of additional Lewis acids such as Sc³⁺ on the reactivity of these systems has also been investigated.

Electrochemically Generated cis-Carboxylato-Coordinated Iron(IV) Oxo Acid-Base Congeners as Promiscuous Oxidants of Water Pollutants

The nonheme iron(IV) oxo complex [Fe^IV(O)(tpenaH)]²⁺ and its conjugate base [Fe^IV(O)(tpena)]⁺ [tpena^- = N,N,N'-tris(2-pyridylmethyl)ethylenediamine-N'-acetate] have been prepared electrochemically in water by bulk electrolysis of solutions prepared from [Fe^III₂(μ-O)(tpenaH)₂](ClO₄)₄ at potentials over 1.3 V (vs NHE) using inexpensive and commercially available carbon-based electrodes. Once generated, these iron(IV) oxo complexes persist at room temperature for minutes to half an hour over a wide range of pH values. They are capable of rapidly decomposing aliphatic and aromatic alcohols, alkanes, formic acid, phenols, and the xanthene dye rhodamine B. The oxidation of formic acid to carbon dioxide demonstrates the capacity for total mineralization of organic compounds. A radical hydrogen-atom-abstraction mechanism is proposed with a reactivity profile for the series that is reminiscent of oxidations by the hydroxyl radical. Facile regeneration of [Fe^IV(O)(tpenaH)]²⁺/ [Fe^IV(O)(tpena)]⁺ and catalytic turnover in the oxidation of cyclohexanol under continuous electrolysis demonstrates the potential of the application of [Fe^III(tpena)]²⁺ as an electrocatalyst. The promiscuity of the electrochemically generated iron(IV) oxo complexes, in terms of the broad range of substrates examined, represents an important step toward the goal of cost-effective electrocatalytic water purification.

Accelerated Oxygen Atom Transfer and C-H Bond Oxygenation by Remote Redox Changes in Fe3 Mn-Iodosobenzene Adducts

We report the synthesis, characterization, and reactivity of [LFe₃ (PhPz)₃ OMn(^s PhIO)][OTf]_x (3: x=2; 4: x=3), where 4 is one of very few examples of iodosobenzene-metal adducts characterized by X-ray crystallography. Access to these rare heterometallic clusters enabled differentiation of the metal centers involved in oxygen atom transfer (Mn) or redox modulation (Fe). Specifically, ⁵⁷ Fe Mössbauer and X-ray absorption spectroscopy provided unique insights into how changes in oxidation state (Fe^III₂ Fe^II Mn^II vs. Fe^III₃ Mn^II ) influence oxygen atom transfer in tetranuclear Fe₃ Mn clusters. In particular, a one-electron redox change at a distal metal site leads to a change in oxygen atom transfer reactivity by ca. two orders of magnitude.

Metal-Assisted Oxo Atom Addition to an Fe(III) Thiolate

Cysteinate oxygenation is intimately tied to the function of both cysteine dioxygenases (CDOs) and nitrile hydratases (NHases), and yet the mechanisms by which sulfurs are oxidized by these enzymes are unknown, in part because intermediates have yet to be observed. Herein, we report a five-coordinate bis-thiolate ligated Fe(III) complex, [Fe^III(S₂^Me2N₃(Pr,Pr))]⁺ (2), that reacts with oxo atom donors (PhIO, IBX-ester, and H₂O₂) to afford a rare example of a singly oxygenated sulfenate, [Fe^III(η²-S^Me2O)(S^Me2)N₃(Pr,Pr)]⁺ (5), resembling both a proposed intermediate in the CDO catalytic cycle and the essential NHase Fe-S(O)^Cys114 proposed to be intimately involved in nitrile hydrolysis. Comparison of the reactivity of 2 with that of a more electron-rich, crystallographically characterized derivative, [Fe^IIIS₂^Me2N^MeN₂^amide(Pr,Pr)]^- (8), shows that oxo atom donor reactivity correlates with the metal ion's ability to bind exogenous ligands. Density functional theory calculations suggest that the mechanism of S-oxygenation does not proceed via direct attack at the thiolate sulfurs; the average spin-density on the thiolate sulfurs is approximately the same for 2 and 8, and Mulliken charges on the sulfurs of 8 are roughly twice those of 2, implying that 8 should be more susceptible to sulfur oxidation. Carboxamide-ligated 8 is shown to be unreactive towards oxo atom donors, in contrast to imine-ligated 2. Azide (N₃^-) is shown to inhibit sulfur oxidation with 2, and a green intermediate is observed, which then slowly converts to sulfenate-ligated 5. This suggests that the mechanism of sulfur oxidation involves initial coordination of the oxo atom donor to the metal ion. Whether the green intermediate is an oxo atom donor adduct, Fe-O═I-Ph, or an Fe(V)═O remains to be determined.

Iodosylbenzene and iodylbenzene adducts of cerium(IV) complexes bearing chelating oxygen ligands

Reactions of [Ce(IV)(LOEt)2Cl2] (LOEt(-) = [Co(η(5)-C5H5){P(O)(OEt)2}3](-)) and [Ce(μ-O){N(Pr(i)2PO)2}4Cl2] with PhIO afford the λ3-iodane complexes [Ce(IV)(LOEt)2{OI(Cl)Ph}2] and [Ce{N(Pr(i)2PO)2}3{OI(Cl)Ph}], respectively, whereas that between [Ce(IV)(LOEt)2Cl2] and PhIO2 or excess PhIO yields the λ5-iodane adduct [Ce(IV)(LOEt)2{OI(O)ClPh}2]. The crystal structures of the Ce(IV)λ3- and λ5-iodane complexes have been determined and their oxo transfer reactivities have been investigated.

Advances in Synthetic Applications of Hypervalent Iodine Compounds

The preparation, structure, and chemistry of hypervalent iodine compounds are reviewed with emphasis on their synthetic application. Compounds of iodine possess reactivity similar to that of transition metals, but have the advantage of environmental sustainability and efficient utilization of natural resources. These compounds are widely used in organic synthesis as selective oxidants and environmentally friendly reagents. Synthetic uses of hypervalent iodine reagents in halogenation reactions, various oxidations, rearrangements, aminations, C-C bond-forming reactions, and transition metal-catalyzed reactions are summarized and discussed. Recent discovery of hypervalent catalytic systems and recyclable reagents, and the development of new enantioselective reactions using chiral hypervalent iodine compounds represent a particularly important achievement in the field of hypervalent iodine chemistry. One of the goals of this Review is to attract the attention of the scientific community as to the benefits of using hypervalent iodine compounds as an environmentally sustainable alternative to heavy metals.

A coordinatively flexible hexadentate ligand gives structurally isomeric complexesM2(L)X3(M= Cu, Zn;X= Br, Cl)

Polypyridyl multidentate ligands based on ethylenediamine backbones are important metal-binding agents with applications in biomimetics and homogeneous catalysis. The seemingly hexadentate tpena ligand [systematic name:N,N,N′-tris(pyridin-2-ylmethyl)ethylenediamine-N′-acetate] reacts with zinc chloride and zinc bromide to form trichlorido[μ-N,N,N′-tris(pyridin-2-ylmethyl)ethylenediamine-N′-acetato]dizinc(II), [Zn2(C22H24N5O2)Cl3], and tribromido[μ-N,N,N′-tris(pyridin-2-ylmethyl)ethylenediamine-N′-acetato]dizinc(II), [Zn2Br3(C22H24N5O2)]. One ZnIIion shows the anticipated N5O coordination in an irregular six-coordinate site and is linked by ananticarboxylate bridge to a tetrahedral ZnX3(X= Cl or Br) unit. In contrast, the CuIIions in aquatribromido[μ-N,N,N′-tris(pyridin-2-ylmethyl)ethylenediamine-N′-acetato]dicopper(II)–tribromido[μ-N,N,N′-tris(pyridin-2-ylmethyl)ethylenediamine-N′-acetato]dicopper(II)–water (1/1/6.5) [Cu2Br3(C22H24N5O2)][Cu2Br3(C22H24N5O2)(H2O)]·6.5H2O, occupy two tpena-chelated sites, one a trigonal bipyramidal N3Cl2site and the other a square-planar N2OCl site. In all three cases, electrospray ionization mass spectra were dominated by a misleading ion assignable to [M(tpena)]+(M= Zn2+and Cu2+).

Reduction of hypervalent iodine by coordination to iron(iii) and the crystal structures of PhIO and PhIO2

Iodine L₃-edge XANES spectra using reference compounds with formal iodine oxidation states spanning −1 to +7 show that iodine in an Fe(iii) coordinated iodosylbenzene (PhIO) is reduced compared to parent hypervalent PhIO which has been structurally characterized.

Mononuclear Nonheme Iron(III)-Iodosylarene and High-Valent Iron-Oxo Complexes in Olefin Epoxidation Reactions

High-spin iron(III)-iodosylarene complexes are highly reactive in the epoxidation of olefins, in which epoxides are formed as the major products with high stereospecificity and enantioselectivity. The reactivity of the iron(III)-iodosylarene intermediates is much greater than that of the corresponding iron(IV)-oxo complex in these reactions. The iron(III)-iodosylarene species-not high-valent iron(IV)-oxo and iron(V)-oxo species-are also shown to be the active oxidants in catalytic olefin epoxidation reactions. The present results are discussed in light of the long-standing controversy on the one oxidant versus multiple oxidants hypothesis in oxidation reactions.

Properties and reactivities of nonheme iron(IV)-oxo versus iron(V)-oxo: long-range electron transfer versus hydrogen atom abstraction

Recent work of Nam and co-workers [J. Yoon, S. A. Wilson, Y. K. Jang, M. S. Seo, K. Nehru, B. Hedman, K. O. Hodgson, E. Bill, E. I. Solomon and W. Nam, Angew. Chem., Int. Ed., 2009, 48, 1257] on a biomimetic iron complex implicated a mixture of iron(IV)-oxo and iron(V)-oxo intermediates but the latter could not be spectroscopically characterized, hence its involvement was postulated. To gain insight into the relative activity of these iron(IV)-oxo versus iron(V)-oxo intermediates, we have performed an extensive density functional theory (DFT) study on the chemical properties of the chemical system of Nam et al., namely [Fe(O)(BQEN)(NCCH3)](2+/3+) with BQEN = N,N'-dimethyl-N,N'-bis(8-quinolyl)ethane-1,2-diamine and their reactivity in hydrogen atom abstraction from ethylbenzene. We show that the perceived iron(V)-oxo species actually is an iron(IV)-oxo ligand cation radical, similar to cytochrome P450 compound I. Moreover, this intermediate has an extremely large electron affinity and therefore can abstract electrons from substrates readily. In our particular system, this means that prior to the hydrogen atom abstraction, an electron is abstracted to form an iron(IV)-oxo species, which subsequently abstracts a hydrogen atom from the substrate. Thus, our calculations show for the first time how some nonheme iron complexes react by long-range electron transfer and others directly via hydrogen atom abstraction. We have rationalized our results with detailed thermochemical cycles that explain the observed reactivity patterns.