Magneto-Structural Correlation of Five-Coordinate Trigonal Bipyramidal High Spin Cobalt(II) Complexes.

Here, we combined magnetometry, multi-frequency electronic paramagnetic resonance, and wave function based ab initio calculations to investigate magnetic properties of two high spin Co(II) complexes Co(BDPRP) (BDPRP=2,6-bis((2-(S)-di(4-R)phenylhydroxylmethyl-1-pyrrolidi-nyl)methyl)pyridine, R=H for 8; R=tBu for 9). Complexes 8 and 9 featuring effective D3h symmetry were found to possess D=24.0 and 32.0 cm-1, respectively, in their S=3/2 ground states of 1 e ' ' d x z / y z 4 1 e ' d x y / x 2 - y 2 2 1 a 1 ' d z 2 1 ${{\left(1{{\rm e}}^{{\rm { {^\prime}}}{\rm { {^\prime}}}}\right({d}_{xz/yz}\left)\right)}^{4}{\left(1{{\rm e}}^{{\rm { {^\prime}}}}\right({d}_{{xy/{x}^{2}-y}^{2}}\left)\right)}^{2}{\left(1{{\rm a}}_{1}^{{\rm { {^\prime}}}}\right({d}_{{z}^{2}}\left)\right)}^{1}}$ . Ligand field analyses revealed that the low-lying d-d excited states make either positive or vanishing contributions to D. Hence, total positive D values were measured for 8 and 9, as well as related D3h high spin Co(II) complexes. In contrast, negative D values are usually observed for C3v congeners. In-depth analyses suggested that lowering symmetry from D3h to C3v induces orbital mixing between 1 e d x z / y z ${1{\rm e}\left({d}_{xz/yz}\right)}$ and 2 e d x y / x 2 - y 2 ${2{\rm e}\left({d}_{{xy/{x}^{2}-y}^{2}}\right)}$ and admixes excited state 4 A 2 1 e → 2 e ${{}^{4}{{\rm A}}_{2}\left(1e\to 2e\right)}$ into the ground state. Both factors turn the total D value progressively negative with the increasing distance (δ) of the Co(II) center out of the equatorial plane. Therefore, δ determines the sign and magnitude of final D values of five-coordinate trigonal bipyramidal S=3/2 Co(II) complexes as measured for a series of such species with varying δ.

A Mechanistic Spectrum of O-H Bond Cleavage Observed for Reactions of Phenols with a Manganese Superoxo Complex.

Reaction of a rare and well-characterized MnIII-superoxo species, Mn(BDPBrP)(O2⋅) (1, H2BDPBrP=2,6-bis((2-(S)-di(4-bromo)phenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine), with 4-dimethylaminophenol at -80 °C proceeds via concerted proton electron transfer (CPET) to produce a MnIII-hydroperoxo complex, Mn(BDPBrP)(OOH) (2), alongside 4-dimethylaminophenoxy radical; whereas, upon treatment with 4-nitrophenol, complex 1 undergoes a proton transfer process to afford a MnIV-hydroperoxo complex, [Mn(BDPBrP)(OOH)]+ (3). Intriguingly, the reactions of 1 with 4-chlorophenol and 4-methoxyphenol follow two routes of CPET and sequential proton and electron transfer to furnish complex 2 in the end. UV-vis and EPR spectroscopic studies coupled with DFT calculations provided support for this wide mechanistic spectrum of activating various phenol O-H bonds by a single MnIII-superoxo complex, 1.

Insights into dioxygen binding on metal centers: an ab initio multireference electronic structure analysis.

Why does binding of dioxygen (O2) to metal centers, the initial step of O2 storage, transportation, and activation, almost inevitably induce metal-to-O2 single-electron transfer and generate superoxo (O2-˙) species, instead of genuine O02 adducts? To address this question, this study describes highly correlated wavefunction-based ab initio calculations using CASSCF/NEVPT2 (CASSCF = complete active space self-consistent field, and NEVPT2 = N-electron valence state second-order perturbation theory) approaches to explore the electronic-structure evolution of O2 association on Fe(II)(BDPP) (H2BDPP = 2,6-bis((2-(S)-diphenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) and Co(II)(BDPP) to produce S = 3 Fe(III)(BDPP)(O2-˙) (1) and Co(III)(BDPP)(O2-˙) (2). CASSCF/NEVPT2 calculations suggest that the processes furnishing 1 and 2 feature an avoided crossing resulting from interactions of two diabatic curves, of which one is characterized as Co(II) and Fe(II) centers interacting with a triplet O2 ligand and the other as Co(III) and Fe(III) centers bound to a superoxo ligand. In both cases, the avoided crossing induces a one-electron transfer from the divalent metal center to the incoming O2 ligand and leads to formation of trivalent metal-O2-˙ complexes. To facilitate the interpretation of complicated multireference wavefunctions, we formulated two-fragment spin eigenfunctions utilizing Clebsch-Gordan coefficients (CGCs) to rationalize computed spin populations on the metal centers and the O2 ligand and compared these results with usual valence bonding (VB) analyses. It turns out that both methods give the same results and are complementary to each other. Finally, the limitation of DFT approaches in describing complex electronic structures involving metal-ligand magnetic couplings is delineated.

A Manganese(IV)-Hydroperoxo Intermediate Generated by Protonation of the Corresponding Manganese(III)-Superoxo Complex.

Earlier work revealed that metal-superoxo species primarily function as radicals and/or electrophiles. Herein, we present ambiphilicity of a MnIII-superoxo complex revealed by its proton- and metal-coupled electron-transfer processes. Specifically, a MnIV-hydroperoxo intermediate, [Mn(BDPBrP)(OOH)]+ (1, H2BDPBrP = 2,6-bis((2-(S)-di(4-bromo)phenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) was generated by treatment of a MnIII-superoxo complex, Mn(BDPBrP)(O2•) (2) with trifluoroacetic acid at -120 °C. Detailed insights into the electronic structure of 1 are obtained using resonance Raman and multi-frequency electron paramagnetic resonance spectroscopies coupled with density functional theory calculations. Similarly, the reaction of 2 with scandium(III) triflate was shown to give a Mn(IV)/Sc(III) bridging peroxo species, [Mn(BDPBrP)(OO)Sc(OTf)n](3-n)+ (4). Furthermore, it is found that deprotonation of 1 quantitatively regenerates 2, and that one-electron oxidation of the corresponding MnIII-hydroperoxo species, Mn(BDPBrP)(OOH) (3), also yields 1.

Dinitrosyliron Complex [(PMDTA)Fe(NO)2]: Intermediate for Nitric Oxide Monooxygenation Activity in Nonheme Iron Complex.

Despite a comprehensive study on the biosynthesis and function of nitric oxide, biological metabolism of nitric oxide, especially when its concentration exceeds the cytotoxic level, remains elusive. Oxidation of nitric oxide by O2 in aqueous solution has been known to yield NO2-. On the other hand, a biomimetic study on the metal-mediated conversion of NO to NO2-/NO3- via O2 reactivity disclosed a conceivable pathway for aerobic metabolism of NO. During the NO-to-NO3- conversion, transient formation of metal-bound peroxynitrite and subsequent release of •NO2 via O-O bond cleavage were evidenced by nitration of tyrosine residue or 2,4-di-tert-butylphenol (DTBP). However, the synthetic/catalytic/enzymatic cycle for conversion of nitric oxide into a nitrite pool is not reported. In this study, sequential reaction of the ferrous complex [(PMDTA)Fe(κ2-O,O'-NO2)(κ1-O-NO2)] (3; PMDTA = pentamethyldiethylenetriamine) with NO(g), KC8, and O2 established a synthetic cycle, complex 3 → {Fe(NO)2}9 DNIC [(PMDTA)Fe(NO)2][NO2] (4) → {Fe(NO)2}10 DNIC [(PMDTA)Fe(NO)2] (1) → [(PMDTA)(NO)Fe(κ2-O,N-ONOO)] (2) → complex 3, for the transformation of nitric oxide into nitrite. In contrast to the reported reactivity of metal-bound peroxynitrite toward nitration of DTBP, peroxynitrite-bound MNIC 2 lacks phenol nitration reactivity toward DTBP. Presumably, the [(PMDTA)Fe] core in {Fe(NO)}8 MNIC 2 provides a mononuclear template for intramolecular interaction between Fe-bound peroxynitrite and Fe-bound NO-, yielding Fe-bound nitrite stabilized in the form of complex 3. This [(PMDTA)Fe]-core-mediated concerted peroxynitrite homolytic O-O bond cleavage and combination of the O atom with Fe-bound NO- reveals a novel and effective pathway for NO-to-NO2- transformation. Regarding the reported assembly of the dinitrosyliron unit (DNIU) [Fe(NO)2] in the biological system, this synthetic cycle highlights DNIU as a potential intermediate for nitric oxide monooxygenation activity in a nonheme iron system.

Hydrogen Bond-Enabled Heterolytic and Homolytic Peroxide Activation within Nonheme Copper(II)-Alkylperoxo Complexes.

To explore the reactivity of copper-alkylperoxo species enabled by the heterolytic peroxide activation, room-temperature stable mononuclear nonheme copper(II)-alkylperoxo complexes bearing a N-(2-ethoxyethanol)-bis(2-picolyl)amine ligand (HN3O2), [CuII(OOR)(HN3O2)]+ (R = cumyl or tBu), were synthesized and spectroscopically characterized. A combined experimental and computational investigation on the reactivity and reaction mechanisms in the phosphorus oxidation, C-H bond activation, and aldehyde deformylation reactions by the copper(II)-alkylperoxo complexes has been conducted. DFT-optimized structures suggested that a hydrogen bonding interaction exists between the ethoxyethanol backbone of the HN3O2 ligand and either the proximal or distal oxygen atom of the alkylperoxide moiety, and this interaction consequently results in the enhanced stability of the copper(II)-alkylperoxo species. In the phosphorus oxidation reaction, both experimental and computational results indicated that a phosphine-triggered heterolytic O-O bond cleavage occurred to yield phosphine oxide and alcohol products. DFT calculations suggested that (i) the H-bonding between the ethoxyethanol backbone and distal oxygen of the alkylperoxide moiety and (ii) the phosphine binding to the proximal oxygen of the alkylperoxide moiety engendered the heterolytic peroxide activation. In the C-H bond activation reactions, temperature-dependent reactivity of the copper(II)-alkylperoxo complexes was observed, and a relatively strong activation energy of 95 kcal mol-1 was required to promote the homolytic peroxide activation. A rate-limiting hydrogen atom abstraction reaction of xanthene by the putative copper(II)-oxyl radical resulted in the formation of the dimeric copper product and the substrate radical that further underwent autocatalytic oxidation reactions to form an oxygen incorporated product. Finally, amphoteric reactivity of copper(II)-alkylperoxo complexes has been assessed by conducting kinetic studies and product analysis of the aldehyde deformylation reaction.

Ambient Stable Cyanomethylcopper(III) Complex: a Strong Cu-Csp3 Bond Supported by a PS3-Tripodal Chelator.

An organocuprate(III) complex, [Cu(TMSPS3)(CH2CN)]- (2), was identified along with a sequential derivative, [Cu(TMSPS3)(CN)]- (3), and an intermediate, [Cu(TMSPS3)(HN═PPh3)] (4), formed in a relative transformation. Apical ligands among these complexes all strongly associate with a robust trigonal copper(III) platform. The nature of the ligand binding was spectroscopically and computationally investigated through a series of copper(III) complexes. The bonding along the principal C3 axis is adaptable, and σ interaction dominates the axial ligand coordination, where the cyanomethyl group exhibits the strongest bonding. Complex 2 is a scarce example of a thermostable aliphatic organocuprate(III) compound, which sheds some light on the organocopper(III) chemistry widely involved in many copper-mediated catalyses.

Mononuclear Manganese(III) Superoxo Complexes: Synthesis, Characterization, and Reactivity.

Metal-superoxo species are typically proposed as key intermediates in the catalytic cycle of dioxygen activation by metalloenzymes involving different transition metal cofactors. In this regard, while a series of Fe-, Co-, and Ni-superoxo complexes have been reported to date, well-defined Mn-superoxo complexes remain rather rare. Herein, we report two mononuclear MnIII-superoxo species, Mn(BDPP)(O2•-) (2, H2BDPP = 2,6-bis((2-(S)-diphenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) and Mn(BDPBrP)(O2•-) (2', H2BDPBrP = 2,6-bis((2-(S)-di(4-bromo)phenylhydroxyl-methyl-1-pyrrolidinyl)methyl)pyridine), synthesized by bubbling O2 into solutions of their MnII precursors, Mn(BDPP) (1) and Mn(BDPBrP) (1'), at -80 °C. A combined spectroscopic (resonance Raman and electron paramagnetic resonance (EPR) spectroscopy) and computational study evidence that both complexes contain a high-spin MnIII center (SMn = 2) antiferromagnetically coupled to a superoxo radical ligand (SOO• = 1/2), yielding an overall S = 3/2 ground state. Complexes 2 and 2' were shown to be capable of abstracting a H atom from 2,2,6,6-tetramethyl-1-hydroxypiperidine (TEMPO-H) to form MnIII-hydroperoxo species, Mn(BDPP)(OOH) (5) and Mn(BDPBrP)(OOH) (5'). Complexes 5 and 5' can be independently prepared by the reactions of the isolated MnIII-aqua complexes, [Mn(BDPP)(H2O)]OTf (6) and [Mn(BDPBrP)(H2O)]OTf (6'), with H2O2 in the presence of NEt3. The parallel-mode EPR measurements established a high-spin S = 2 ground state for 5 and 5'.

Conversion of a Fleeting Open-Shell Iron Nitride into an Iron Nitrosyl.

Terminal metal nitrides have been proposed as key intermediates in a series of pivotal chemical transformations. However, exploring the chemical activity of transient tetragonal iron(V) nitrides is largely impeded by their facile dimerization in fluid solutions. Herein, in situ EPR and Mössbauer investigations are presented of unprecedented oxygenation of a paramagnetic iron(V) nitrido intermediate, [FeV N(cyclam-ac)]+ (2, cyclam-ac- =1,4,8,11-tetraazacyclotetradecane-1-acetate anion), yielding an iron nitrosyl complex, [Fe(NO)(cyclam-ac)]+ (3). Further theoretical studies suggest that during the reaction a closed-shell singlet O atom is transferred to 2. Consequently, the N-O bond formation does not follow a radical coupling mechanism proposed for the N-N bond formation but is accomplished by three mutual electron-transfer pathways between 2 and the O atom donor, thanks to the ambiphilic nature of 2.

Synthesis, Structural, Spectral, and Electrochemical Studies of Selenabenziporphyrin and Its Pd(II) Complex.

A new nonaromatic selenabenziporphyrin was synthesized by (3 + 1) condensation of m-benzitripyrrane and 2,5-bis[( p-tolyl)hydroxymethyl] selenophene under mild trifluoroacetic acid-catalyzed reaction conditions. The selenabenziporphyrin was characterized by high-resolution mass spectrometry, one- and two-dimensional NMR spectroscopy, and X-ray crystallography. The crystal structure revealed that the macrocycle was planar with moderately tilted m-phenylene ring and that the phenylene ring completely blocks the macrocyclic π-delocalization. The selenabenziporphyrin exhibits one broad absorption band at 645 nm along with one sharp band at 415 nm, and electrochemical studies revealed that the macrocycle was electron-deficient. The selenabenziporphyrin readily forms organometallic Pd(II) complex when treated with PdCl2 in CH3CN/CHCl3 at reflux followed by recrystallization. The X-ray structure revealed that the Pd(II) ion was coordinated with two pyrrole "N"s, selenophene "Se", and m-phenylene ring "C" in square-planar fashion, and the complex retained its nonaromatic nature. The Pd(II) complex exhibits ill-defined absorption bands, and it was more electron-deficient than free-base selenabenziporphyrin macrocycle. Time-dependent density functional theory studies supported the experimental observations.

Synthesis of Quaternary-Carbon-Containing ß2,2 -Amino Acids by the RhI -Catalyzed Enantioselective Arylation of α-Substituted ß-Nitroacrylates.

An enantioselective RhI -catalyzed conjugate addition reaction of α-substituted ß-nitroacrylates with various arylboronic acids by using chiral RhI diene catalysts is described for the first time. The addition reaction proceeds under mild conditions in a range of common organic solvents and additives, and it affords the corresponding quaternary-carbon-containing α,α-disubstituted ß-nitropropionate products in up to 63 % yield and 99 % ee. Reaction of either (E)- or (Z)-ß-nitroacrylates provided the same enantiomer of the product, and a range of esters and aryl groups were tolerated. To demonstrate the utility of the method, ethyl (R)-1,1-methyl-1-phenyl-3-nitropropionate, prepared herein, was converted to the non-proteinogenic ß2,2 -amino acid, (R)-2-(aminomethyl)-2-phenylpropanoic acid, and to the ß2,2 -lactam, (R)-3-methyl-3-phenylazetidin-2-one. In addition, a tripeptide, which comprised l-phenylalanine, l-alanine, and ß2,2 -amino acid 7, was also synthesized.

Calixsmaragdyrin: A Versatile Ligand for Coordination Complexes.

The Ru(II) and BF2 complexes of calixsmaragdyrin were prepared under simple reaction conditions and characterized by HR-MS, 1D and 2D NMR spectroscopy, optical spectroscopy, and electrochemistry, and the structure of the Ru(II) complex of calixsmaragdyrin was elucidated by X-ray crystallography. The crystal structure of the Ru(II) complex revealed that the Ru(II) ion is hexacoordinate with the three pyrrole nitrogen ligands from the tripyrrin unit of the calixsmaragdyrin macrocycle, and the remaining coordination sites of Ru(II) ion were occupied by two carbonyl groups and one hydroxyl (-OH) group. The calixsmaragdyrin macrocycle in the Ru(II) complex was distorted with a dome-like structure. In the BF2 complex of calixsmaragdyrin, the BF2 unit was bound to two pyrrolic nitrogens of the dipyrrin moiety of calixsmaragdyrin as deduced by detailed 1- and 2-dimensional NMR spectroscopy studies. The Ru(II) complex displayed a strong Soret-like absorption band at 449 nm with the absence of Q-bands, whereas the BF2 complex showed a Soret-like band at 475 nm with two well-defined Q-bands at 787 and 883 nm, respectively. Quantum mechanical DFT calculations yielded relaxed equilibrium structures that were similar to the X-ray crystal structures, and the related charge density distributions indicated that the d orbital of the Ru(II) ion was contributing to the HOMO and LUMO states. In addition, TD-DFT calculations successfully reproduced the large bathochromic shifts, oscillator strengths, and electronic transitions that were observed in the experimental absorption spectra of all three complexes. Both the Ru(II) and the BF2 complexes of calixsmaragdyrin were stable under redox conditions.

Construction of Novel Cyclic Tetrads by Axial Coordination of Thiaporphyrins to Tin(IV) Porphyrin.

We report the formation of new cyclic porphyrin tetrads 1 and 2, which were obtained from the reaction between dihydroxytin(IV) porphyrin and cis-dihydroxy-21-thiaporphyrin/21,23-dithiaporphyrin. The unique oxophilicity of tin(IV) porphyrin was the driving force for the formation of these tetrads. Moreover, these novel tetrads represent the first examples of cyclic porphyrins containing tin(IV) that are constructed exclusively on the basis of the "Sn-O" interaction without any other complementary, noncompetitive mode of interactions. The molecular structures of the cyclic tetrads have been investigated by matrix-assisted laser desorption ionization time-of-flight mass spectrometry, NMR spectroscopy, quantum-mechanical calculations, and, in one case, single-crystal X-ray crystallography. The X-ray structure revealed that the two cis-dihydroxy-N2S2 porphyrins were coordinated at the axial positions of two tin(IV) porphyrins, leading to the symmetric cyclic tetrad structure. The optical properties of tetrads were studied, and these compounds were stable under redox conditions. Preliminary photophysical studies carried out on the tetrads indicated efficient energy transfer from tin(IV) porphyrin to the thiaporphyrin unit, which highlights their potential applications in energy and electron transfer in the future.

A Structurally Characterized Nonheme Cobalt-Hydroperoxo Complex Derived from Its Superoxo Intermediate via Hydrogen Atom Abstraction.

Bubbling O2 into a THF solution of CoII(BDPP) (1) at -90 °C generates an O2 adduct, Co(BDPP)(O2) (3). The resonance Raman and EPR investigations reveal that 3 contains a low spin cobalt(III) ion bound to a superoxo ligand. Significantly, at -90 °C, 3 can react with 2,2,6,6-tetramethyl-1-hydroxypiperidine (TEMPOH) to form a structurally characterized cobalt(III)-hydroperoxo complex, CoIII(BDPP)(OOH) (4) and TEMPO•. Our findings show that cobalt(III)-superoxo species are capable of performing hydrogen atom abstraction processes. Such a stepwise O2-activating process helps to rationalize cobalt-catalyzed aerobic oxidations and sheds light on the possible mechanism of action for Co-bleomycin.

Ring Opening of a meso-Triaryl 25-Oxasmaragdyrin Macrocycle by m-Chloroperoxybenzoic Acid.

Smaragdyrin, a class of expanded porphyrin macrocycles, upon treatment with meta-chloroperoxybenzoic acid (mCPBA) underwent oxidative ring opening to form an unprecedented linear pentaheterocyclic compound. The linear pentaheterocyclic compound was freely soluble in common organic solvents and characterized in detail by HRMS, 1D and 2D NMR spectroscopy, and X-ray crystallography. Our preliminary studies indicated that the linear pentaheterocyclic compound can specifically sense anions such as H2 PO4- and CN- ions, which was corroborated by absorption and fluorescence studies.

Rhenium(I) Tricarbonyl Complexes of meso-Tetraaryl-21,23-diheteroporphyrins.

The dithia/diselena meso-tetraarylporphyrins have a lesser tendency to form metal complexes because of the larger size of the heteroatom(s), which shrinks the cavity size, and the heteroatoms also have poor coordinating ability to bind metal ions. The first example of a rhenium(I) tricarbonyl complex of 21,23-diselenaporphyrin was synthesized by treating 5,10,15,20-tetra-p-tolyl-21,23-diselenaporphyrin with Re(CO)5Cl in chlorobenzene at reflux temperature and its structural properties were compared with our earlier reported rhenium(I) complex of tetraaryl-21,23-dithiaporphyrin. The crystal structures of rhenium(I) complexes of diheteroporphyrins revealed that the Re(I) ion binds to both the Se/S atoms and one of the N atoms of the porphyrin core along with three terminal carbonyl groups in an octahedral fashion. The rhenium(I) complexes of 21,23-diheteroporphyrins are stabilized by a large counterion, the trichloro-bridged dirhenium(I) ion. We also present a detailed account of the spectral and redox properties of rhenium(I) tricarbonyl complexes of 21,23-diheteroporphyrins.

Spectroscopic and Theoretical Study of Spin-Dependent Electron Transfer in an Iron(III) Superoxo Complex.

It was shown previously (J. Am. Chem. Soc. 2014, 136, 10846) that bubbling of O2 into a solution of Fe(II)(BDPP) (H2BDPP = 2,6-bis[[(S)-2-(diphenylhydroxymethyl)-1-pyrrolidinyl]methyl]pyridine) in tetrahydrofuran at -80 °C generates a high-spin (SFe = (5)/2) iron(III) superoxo adduct, 1. Mössbauer studies revealed that 1 is an exchange-coupled system, [Formula: see text], where SR = (1)/2 is the spin of the superoxo radical, of which the spectra were not well enough resolved to determine whether the coupling was ferromagnetic (S = 3 ground state) or antiferromagnetic (S = 2). The glass-forming 2-methyltetrahydrofuran solvent yields highly resolved Mössbauer spectra from which the following data have been extracted: (i) the ground state of 1 has S = 3 (J < 0); (ii) |J| > 15 cm(-1); (iii) the zero-field-splitting parameters are D = -1.1 cm(-1) and E/D = 0.02; (iv) the major component of the electric-field-gradient tensor is tilted ≈7° relative to the easy axis of magnetization determined by the MS = ±3 and ±2 doublets. The excited-state MS = ±2 doublet yields a narrow parallel-mode electron paramagnetic resonance signal at g = 8.03, which was used to probe the magnetic hyperfine splitting of (17)O-enriched O2. A theoretical model that considers spin-dependent electron transfer for the cases where the doubly occupied π* orbital of the superoxo ligand is either "in" or "out" of the plane defined by the bent Fe-OO moiety correctly predicts that 1 has an S = 3 ground state, in contrast to the density functional theory calculations for 1, which give a ground state with both the wrong spin and orbital configuration. This failure has been traced to a basis set superposition error in the interactions between the superoxo moiety and the adjacent five-membered rings of the BDPP ligand and signals a fundamental problem in the quantum chemistry of O2 activation.

A Stable Seven-Membered Heterocycle, Containing B, C, N, O, and P Atoms, inside a Smaragdyrin Macrocycle.

Unprecedented examples of smaragdyrin macrocycles containing seven membered heterocyclic rings were synthesized under simple reaction conditions in high yields. The heterocycle formed inside smaragdyrin macrocycle is rare example of heterocycle containing five different atoms, such as B, C, N, O, and P atoms. The mixed B(III) and P(V) complexes of smaragdyrin macrocycles showed new structural, spectral, and electrochemical properties.

Ambient Stable Trigonal Bipyramidal Copper(III) Complexes Equipped with an Exchangeable Axial Ligand.

A stable trigonal bipyramidal copper(III) complex, [PPN][Cu((TMS)PS3)Cl] (1, wherein PPN represents bis(triphenylphosphine)iminium), was synthesized from CuCl2/PPNCl via intramolecular copper(II) disproportionation. Under ambient conditions, the axial chloride of 1 is exchangeable in solution thus making 1 serve as an intermediate to prepare trigonal bipyramidal copper(III) derivatives, e.g., [PPN][Cu((TMS)PS3)(N3)] (2) and [Cu((TMS)PS3)(DABCO)] (3). Diamagnetic complexes 1-3 were fully characterized by X-ray crystallography, NMR, UV-vis, and Cu K-edge absorption spectroscopy. A series of UV-vis titrations were performed to investigate the relative ligand affinity toward the [Cu((TMS)PS3)] moiety, verifying the 1:1 binding equilibrium between various ligands. Compared to known copper(III) compounds, Cu K-edge absorptions of 1-3 possess lower pre-edge energy and higher shakedown transition energy, which, respectively, attribute to the electron donation from (TMS)PS3(3-) ligand and their trigonal ligand field.

Characterization of a paramagnetic mononuclear nonheme iron-superoxo complex.

O2 bubbling into a THF solution of Fe(II)(BDPP) (1) at -80 °C generates a reversible bright yellow adduct 2. Characterization by resonance Raman and Mössbauer spectroscopy provides complementary insights into the nature of 2. The former shows a resonance-enhanced vibration at 1125 cm(-1), which can be assigned to the ν(O-O) of a bound superoxide, while the latter reveals the presence of a high-spin iron(III) center that is exchange-coupled to the superoxo ligand, like the Fe(III)-O2(-) pair found for the O2 adduct of 4-nitrocatechol-bound homoprotocatechuate 2,3-dioxygenase. Lastly, 2 oxidizes dihydroanthracene to anthracene, supporting the notion that Fe(III)-O2(-) species can carry out H atom abstraction from a C-H bond to initiate the 4-electron oxidation of substrates proposed for some nonheme iron enzymes.