Mi Sook Seo

Axial Ligand Effects on the Geometric and Electronic Structures of Nonheme Oxoiron(IV) Complexes

A series of complexes [Fe(IV)(O)(TMC)(X)](+) (where X = OH(-), CF3CO2(-), N3(-), NCS(-), NCO(-), and CN(-)) were obtained by treatment of the well-characterized nonheme oxoiron(IV) complex [Fe(IV)(O)(TMC)(NCMe)](2+) (TMC = tetramethylcyclam) with the appropriate NR4X salts. Because of the topology of the TMC macrocycle, the [Fe(IV)(O)(TMC)(X)](+) series represents an extensive collection of S = 1 oxoiron(IV) complexes that only differ with respect to the ligand trans to the oxo unit. Electronic absorption, Fe K-edge X-ray absorption, resonance Raman, and Mossbauer data collected for these complexes conclusively demonstrate that the characteristic spectroscopic features of the S = 1 Fe(IV)=O unit, namely, (i) the near-IR absorption properties, (ii) X-ray absorption pre-edge intensities, and (iii) quadrupole splitting parameters, are strongly dependent on the identity of the trans ligand. However, on the basis of extended X-ray absorption fine structure data, most [Fe(IV)(O)(TMC)(X)](+) species have Fe=O bond lengths similar to that of [Fe(IV)(O)(TMC)(NCMe)](2+) (1.66 +/- 0.02 A). The mechanisms by which the trans ligands perturb the Fe(IV)=O unit were probed using density functional theory (DFT) computations, yielding geometric and electronic structures in good agreement with our experimental data. These calculations revealed that the trans ligands modulate the energies of the Fe=O sigma- and pi-antibonding molecular orbitals, causing the observed spectroscopic changes. Time-dependent DFT methods were used to aid in the assignment of the intense near-UV absorption bands found for the oxoiron(IV) complexes with trans N3(-), NCS(-), and NCO(-) ligands as X(-)-to-Fe(IV)=O charge-transfer transitions, thereby rationalizing the resonance enhancement of the nu(Fe=O) mode upon excitation of these chromophores.

A Highly Reactive Mononuclear Non-Heme Manganese(IV)–Oxo Complex That Can Activate the Strong C–H Bonds of Alkanes

A mononuclear non-heme manganese(IV)-oxo complex has been synthesized and characterized using various spectroscopic methods. The Mn(IV)-oxo complex shows high reactivity in oxidation reactions, such as C-H bond activation, oxidations of olefins, alcohols, sulfides, and aromatic compounds, and N-dealkylation. In C-H bond activation, the Mn(IV)-oxo complex can activate C-H bonds as strong as those in cyclohexane. It is proposed that C-H bond activation by the non-heme Mn(IV)-oxo complex does not occur via an oxygen-rebound mechanism. The electrophilic character of the non-heme Mn(IV)-oxo complex is demonstrated by a large negative ρ value of -4.4 in the oxidation of para-substituted thioanisoles.

A Mononuclear Non-Heme Manganese(IV)−Oxo Complex Binding Redox-Inactive Metal Ions

Redox-inactive metal ions play pivotal roles in regulating the reactivities of high-valent metal-oxo species in a variety of enzymatic and chemical reactions. A mononuclear non-heme Mn(IV)-oxo complex bearing a pentadentate N5 ligand has been synthesized and used in the synthesis of a Mn(IV)-oxo complex binding scandium ions. The Mn(IV)-oxo complexes were characterized with various spectroscopic methods. The reactivities of the Mn(IV)-oxo complex are markedly influenced by binding of Sc(3+) ions in oxidation reactions, such as a ~2200-fold increase in the rate of oxidation of thioanisole (i.e., oxygen atom transfer) but a ~180-fold decrease in the rate of C-H bond activation of 1,4-cyclohexadiene (i.e., hydrogen atom transfer). The present results provide the first example of a non-heme Mn(IV)-oxo complex binding redox-inactive metal ions that shows a contrasting effect of the redox-inactive metal ions on the reactivities of metal-oxo species in the oxygen atom transfer and hydrogen atom transfer reactions.

A mononuclear nonheme iron(IV)-oxo complex which is more reactive than cytochrome P450 model compound I

A highly reactive mononuclear nonheme iron(IV)-oxo complex with a low-spin (S = 1) triplet ground state in both C–H bond activation and oxo transfer reactions is reported; this nonheme iron(IV)-oxo complex is more reactive than an iron(IV)-oxo porphyrin π-cation radical (i.e., a model of cytochrome P450 compound I) and is the most reactive species in kinetic studies among nonheme iron(IV)-oxo complexes reported so far. DFT calculations support the experimental results with extremely low activation barriers in the C–H bond activation of cyclohexane and 1,4-cyclohexadiene. The DFT calculations reveal that the S = 1 state is set up to easily lead to the highly reactive S = 2 high-spin iron(IV)-oxo species.

Experiment and Theory Reveal the Fundamental Difference between Two‐State and Single‐State Reactivity Patterns in Nonheme FeIVO versus RuIVO Oxidants

Recent developments in the emerging field of nonheme iron chemistry have provided chemists with a number of synthetic mononuclear nonheme iron(IV) oxo complexes, which have been implicated as the key reactive intermediates in enzymatic and biomimetic oxidation processes. A notable example is the recently synthesized iron(IV) oxo complex bearing a nonheme macrocyclic ligand [Fe(O)(tmc)(NCCH3)] 2+ (1-NCCH3; tmc = 1,4,8,11-tetramethyl-1,4,8,11tetraazacyclotetradecane; Figure 1a). Characterization of the iron oxo species by spectroscopic techniques and X-ray crystallography and studies of their reactivity patterns in various oxidation reactions have created a great opportunity for understanding the chemical and physical properties of these complexes. 3] Substitution of the acetonitrile ligand of 1-NCCH3 with a variety of anionic axial ligands (e.g., X = CF3CO2 , N3 , or SR ) made it possible to demonstrate that the reactivity of [Fe(O)(tmc)(X)] (1-X) is significantly affected by the nature of the axial ligands in a manner that depends on the type of reaction. 5] Thus, while electron-donating axial ligands diminished the oxidative reactivity of 1-X in oxotransfer reactions (towards PPh3, for which the reactivity order is 1-NCCH3> 1-CF3CO2> 1-N3> 1-SR), they enhanced the reactivity of 1-X in H-abstraction reactions (from phenol O H and alkyl aromatic C H bonds), that is, a reactivity order of 1’-SR> 1-N3> 1-CF3CO2> 1-NCCH3. [5] Theoretical studies proposed that the puzzling reactivity trends arose from the fact that these nonheme iron(IV) oxo reagents have two closely lying spin states, a ground state with a triplet spin state (T) and a low-lying quintet spin state (Q), as shown schematically in Figure 1b. 7] Thus, the triplet state has a high energy barrier, while the quintet state has a much lower barrier that crosses through the larger triplet barrier. Therefore, the H-abstraction reactions proceed on the two energy surfaces, and the resulting blended reactivity is behind the unusual reactivity patterns. Without the two-state blend, the reactivity on any one of the spin-state surfaces was shown to follow the electrophilicity of 1-X. How can one test this two-state reactivity (TSR) concept for the dichotomic reactivity pattern in oxo-transfer and H-abstraction processes by [Fe(O)(tmc)(X)]? The most straightforward way to interrogate the TSR hypothesis is to design a family of metal oxo complexes as closely analogous to [Fe(O)(tmc)(X)] as possible, while replacing only the iron ion with a different metal ion so that the quintet state becomes energetically inaccessible. Theory predicts that such probe complexes will not exhibit dichotomic reactivity trends in oxo-transfer and H-abstraction reactions. To test this prediction, we focused on Ru oxo complexes due to their propensity to prefer low-spin states to high-spin states. Accordingly, we synthesized ruthenium(IV) oxo analogues bearing different axial ligands [Ru(O)(tmc)(X)] (2-X; Figure 1a) and examined their reactivity in oxo-transfer and H-abstraction reactions. Herein we report experimental and theoretical studies on the effects of axial ligands of the Ru oxo complexes in oxo-transfer and H-abstraction reactions. Figure 1. a) The structures of [Fe(O)(tmc)(X)] (1-X) and [Ru(O)(tmc)(X)] (2-X); b) the TSR scenario in H-abstraction reactions by [Fe(O)(tmc)(X)].

Reversible O−O Bond Cleavage and Formation between Mn(IV)-Peroxo and Mn(V)-Oxo Corroles

Mn(IV)-peroxo and Mn(V)-oxo corroles were synthesized and characterized with various spectroscopic techniques. The intermediates were directly used in O-O bond cleavage and formation reactions. Upon addition of proton to the Mn(IV)-peroxo corrole, the formation of the Mn(V)-oxo corrole was observed. Interestingly, addition of base to the Mn(V)-oxo corrole afforded the formation of the Mn(IV)-peroxo corrole. Thus, we were able to report the first example of reversible O-O bond cleavage and formation reactions using in situ generated Mn(IV)-peroxo and Mn(V)-oxo corroles.

Water as an Oxygen Source: Synthesis, Characterization, and Reactivity Studies of a Mononuclear Nonheme Manganese(IV) Oxo Complex†

High-valent manganese oxo species have been invoked as key intermediates in the oxidation of organic substrates by heme and nonheme manganese catalysts, and in water oxidation by the oxygen-evolving complex (OEC) in photosystem II (PS II). To elucidate the chemical and physical properties of high-valent manganese oxo intermediates, a number of heme and nonheme Mn and Mn oxo complexes have been synthesized, and characterized by using various spectroscopic methods and X-ray crystallography. The reactivities of these species have also been investigated in oxidation reactions, such as C H bond activation, olefin epoxidation, halogenation, and hydrideand electron-transfer reactions. In PS II, the oxidation of water by the OEC induces the generation of high-valent Mn oxo species by a protoncoupled electron transfer (PCET) mechanism. The oxygen atom in the Mn oxo intermediate is derived from water. Biomimetic studies have established the formation of ruthenium oxo complexes in water oxidation in the presence of a strong oxidant, such as Ce or [Ru(bpy)3] 3+ (bpy = 2,2 bipyridine). Very recently, we have generated mononuclear nonheme Fe oxo complexes using water as an oxygen source and Ce as an one-electron oxidant. Since it has been proposed that high-valent manganese oxo species are generated by the oxidation of water at the OEC of PS II, we attempted to generate high-valent manganese oxo species in a similar fashion. Herein, we report the generation of a mononuclear nonheme Mn oxo complex using water as an oxygen source and Ce as an one-electron oxidant. The spectroscopic characterization and DFT-optimized structure of the intermediate are also reported. We also report the reactivity of the nonheme Mn oxo complex in oxygenation reactions. Addition of cerium(IV) ammonium nitrate (CAN; 8 mm) to a reaction solution containing [Mn(BQCN)](CF3SO3)2 (1; 2 mm ; BQCN = N,N’-dimethyl-N,N’-bis(8-quinolyl)cyclohexanediamine; see the crystal structure of 1 in Figure 1 and

Redox-inactive metal ions modulate the reactivity and oxygen release of mononuclear non-haem iron( III )–peroxo complexes

Redox-inactive metal ions that function as Lewis acids play pivotal roles in modulating the reactivity of oxygen-containing metal complexes and metalloenzymes, such as the oxygen-evolving complex in photosystem II and its small-molecule mimics. Here we report the synthesis and characterization of non-haem iron(III)–peroxo complexes that bind redox-inactive metal ions, (TMC)FeIII–(μ,η2:η2-O2)–Mn+ (Mn+ = Sr2+, Ca2+, Zn2+, Lu3+, Y3+ and Sc3+; TMC, 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). We demonstrate that the Ca2+ and Sr2+ complexes showed similar electrochemical properties and reactivities in one-electron oxidation or reduction reactions. However, the properties and reactivities of complexes formed with stronger Lewis acidities were found to be markedly different. Complexes that contain Ca2+ or Sr2+ ions were oxidized by an electron acceptor to release O2, whereas the release of O2 did not occur for complexes that bind stronger Lewis acids. We discuss these results in the light of the functional role of the Ca2+ ion in the oxidation of water to dioxygen by the oxygen-evolving complex. Non-haem iron(III)-peroxo complexes that bind redox-inactive metal ions are synthesized to investigate the role of the Ca2+ ion in the oxidation of water to dioxygen in photosystem II. The electrochemical properties and reactions of these compounds with an electron donor and an acceptor are found to be markedly dependent on the Lewis acidity of redox-inactive metal ions.

Sulfur versus Iron Oxidation in An Iron-Thiolate Model Complex

In the absence of base, the reaction of [Fe(II)(TMCS)]PF6 (1, TMCS = 1-(2-mercaptoethyl)-4,8,11-trimethyl-1,4,8,11-tetraazacyclotetradecane) with peracid in methanol at -20 °C did not yield the oxoiron(IV) complex (2, [Fe(IV)(O)(TMCS)]PF6), as previously observed in the presence of strong base (KO(t)Bu). Instead, the addition of 1 equiv of peracid resulted in 50% consumption of 1. The addition of a second equivalent of peracid resulted in the complete consumption of 1 and the formation of a new species 3, as monitored by UV-vis, ESI-MS, and Mössbauer spectroscopies. ESI-MS showed 3 to be formulated as [Fe(II)(TMCS) + 2O](+), while EXAFS analysis suggested that 3 was an O-bound iron(II)-sulfinate complex (Fe-O = 1.95 Å, Fe-S = 3.26 Å). The addition of a third equivalent of peracid resulted in the formation of yet another compound, 4, which showed electronic absorption properties typical of an oxoiron(IV) species. Mössbauer spectroscopy confirmed 4 to be a novel iron(IV) compound, different from 2, and EXAFS (Fe═O = 1.64 Å) and resonance Raman (ν(Fe═O) = 831 cm(-1)) showed that indeed an oxoiron(IV) unit had been generated in 4. Furthermore, both infrared and Raman spectroscopy gave indications that 4 contains a metal-bound sulfinate moiety (ν(s)(SO2) ≈ 1000 cm (-1), ν(as)(SO2) ≈ 1150 cm (-1)). Investigations into the reactivity of 1 and 2 toward H(+) and oxygen atom transfer reagents have led to a mechanism for sulfur oxidation in which 2 could form even in the absence of base but is rapidly protonated to yield an oxoiron(IV) species with an uncoordinated thiol moiety that acts as both oxidant and substrate in the conversion of 2 to 3.

Highly reactive nonheme iron(III) iodosylarene complexes in alkane hydroxylation and sulfoxidation reactions.

High-spin iron(III) iodosylarene complexes bearing an N-methylated cyclam ligand are synthesized and characterized using various spectroscopic methods. The nonheme high-spin iron(III) iodosylarene intermediates are highly reactive oxidants capable of activating strong C-H bonds of alkanes; the reactivity of the iron(III) iodosylarene intermediates is much greater than that of the corresponding iron(IV) oxo complex. The electrophilic character of the iron(III) iodosylarene complexes is demonstrated in sulfoxidation reactions.