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

Why does binding of dioxygen (O) to metal centers, the initial step of O storage, transportation, and activation, almost inevitably induce metal-to-O single-electron transfer and generate superoxo (O˙) species, instead of genuine O02 adducts? To address this question, this study describes highly correlated wavefunction-based calculations using CASSCF/NEVPT2 (CASSCF = complete active space self-consistent field, and NEVPT2 = -electron valence state second-order perturbation theory) approaches to explore the electronic-structure evolution of O association on Fe(II)(BDPP) (HBDPP = 2,6-bis((2-()-diphenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) and Co(II)(BDPP) to produce = 3 Fe(III)(BDPP)(O˙) (1) and Co(III)(BDPP)(O˙) (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 O 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 O ligand and leads to formation of trivalent metal-O˙ complexes.

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 Mn-superoxo species, Mn(BDPP)(O⋅) (1, HBDPP=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 Mn-hydroperoxo complex, Mn(BDPP)(OOH) (2), alongside 4-dimethylaminophenoxy radical; whereas, upon treatment with 4-nitrophenol, complex 1 undergoes a proton transfer process to afford a Mn-hydroperoxo complex, [Mn(BDPP)(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 Mn-superoxo complex, 1.

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(BDPP) (BDPP=2,6-bis((2-(S)-di(4-R)phenylhydroxylmethyl-1-pyrrolidi-nyl)methyl)pyridine, R=H for 8; R=Bu for 9). Complexes 8 and 9 featuring effective D symmetry were found to possess D=24.0 and 32.

Hydrogen Atom Transfer Thermodynamics of Homologous Co(III)- and Mn(III)-Superoxo Complexes: The Effect of the Metal Spin State.

Systematic investigations on H atom transfer (HAT) thermodynamics of metal O adducts is of fundamental importance for the design of transition metal catalysts for substrate oxidation and/or oxygenation directly using O. Such work should help elucidate underlying electronic-structure features that govern the OO-H bond dissociation free energies (BDFEs) of metal-hydroperoxo species, which can be used to quantitatively appraise the HAT activity of the corresponding metal-superoxo complexes. Herein, the BDFEs of two homologous Co- and Mn-hydroperoxo complexes, and , were calculated to be 79.

Ambiphilicity of a mononuclear cobalt(III) superoxo complex.

Addition of HOTf to a mixture of CoIII(BDPP)(O2˙) (1, H2BDPP = 2,6-bis((2-(S)-diphenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) and Cp*2Fe produced H2O2 in high yield implying formation of CoIII(BDPP)(OOH) (3), and reaction of Sc(OTf)3 with the same mixture gave a peroxo-bridged CoIII/ScIII5. These findings demonstrate the ambiphilic property of CoIII-superoxo 1.

Dinitrosyliron Complex [(PMDTA)Fe(NO)]: 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 O in aqueous solution has been known to yield NO. On the other hand, a biomimetic study on the metal-mediated conversion of NO to NO/NO via O reactivity disclosed a conceivable pathway for aerobic metabolism of NO.

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 Mn-superoxo complex revealed by its proton- and metal-coupled electron-transfer processes. Specifically, a Mn-hydroperoxo intermediate, [Mn(BDPP)(OOH)] (, HBDPP = 2,6-bis((2-()-di(4-bromo)phenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) was generated by treatment of a Mn-superoxo complex, Mn(BDPP)(O) () with trifluoroacetic acid at -120 °C.

Redox reactions between chromium(VI) and hydroquinone: Alternative pathways for polymerization of organic molecules.

Chromium (VI) reduction by organic compounds is one of the major pathways to alleviate the toxicity and mobility of Cr(VI) in the environment. However, oxidative products of organic molecules receive less scientific concerns. In this study, hydroquinone (HQ) was used as a representative organic compound to determine the redox reactions with Cr(VI) and the concomitant oxidative products.

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, [Fe N(cyclam-ac)] (2, cyclam-ac =1,4,8,11-tetraazacyclotetradecane-1-acetate anion), yielding an iron nitrosyl complex, [Fe(NO)(cyclam-ac)] (3).

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 -(2-ethoxyethanol)-(2-picolyl)amine ligand (HNO), [Cu(OOR)(HNO)] (R = cumyl or Bu), 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 HNO 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.

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 Mn-superoxo species, Mn(BDPP)(O) (, HBDPP = 2,6-bis((2-()-diphenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) and Mn(BDPP)(O) (, HBDPP = 2,6-bis((2-()-di(4-bromo)phenylhydroxyl-methyl-1-pyrrolidinyl)methyl)pyridine), synthesized by bubbling O into solutions of their Mn precursors, Mn(BDPP) () and Mn(BDPP) (), at -80 °C.

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.

Nickel(iii)-mediated oxidative cascades from a thiol-bearing nickel(ii) precursor to the nickel(iv) product.

A nickel(ii) complex, Ni(HPS2) (1) that contains two pendant thiols, is rapidly aerobically oxidized in the presence of an amine to produce a diamagnetic nickel(iv) complex, Ni(PS2) (2). This process was investigated spectroscopically at a temperature of -80 °C. Absorption spectra revealed that the deprotonation of one pendant thiol of 1 triggers an oxidative cascade; EPR findings indicate that single-spin species comprised of nickel(iii) intermediates are produced in the reaction solution.

Synthesis of Tellurabenziporphyrin and Its Pd(II) Complex.

An unprecedented tellurabenziporphyrin containing C, N, and Te donor atoms was synthesized by condensing benzitripyrrane and tellurophene diol under acid catalyzed conditions. The tellurabenziporphyrin readily forms a Pd(II) complex when treated with PdCl in CHCl/CHCN. The crystal structures of tellurabenziporphyrin and its Pd(II) complex revealed that the benzene ring hinders the π-electron delocalization.

Synthetic nickel-containing superoxide dismutase attenuates para-phenylenediamine-induced bladder dysfunction in rats.

Para (p)-phenylenediamine and its toxic metabolites induce excess reactive oxygen species formation that results in bladder voiding dysfunction. We determined the effects of synthetic Ni-containing superoxide dismutase mimics and the role of oxidative stress in p-phenylenediamine-induced urinary bladder dysfunction. P-phenylenediamine (60 μg/kg/day) was intraperitoneally administered for 4 weeks to induce bladder injury in female Wistar rats.

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.

Hybrid Macrocycles of Subporphyrins and Triphyrins.

The dibenzofuran/dibenzothiophene-based nonaromatic hybrid macrocycles, exhibiting the features of both contracted macrocycles, subporphyrins and triphyrins, have been synthesized under simple reaction conditions using readily available precursors. The monoanionic new macrocyclic ligands with three donor atoms, such as two pyrrole nitrogens and one dibenzofuran oxygen or dibenzothiophene sulfur, readily form Re(I) complexes.

Calixsmaragdyrin: A Versatile Ligand for Coordination Complexes.

The Ru(II) and BF 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.

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

Bubbling O into a THF solution of Co(BDPP) (1) at -90 °C generates an O adduct, Co(BDPP)(O) (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, Co(BDPP)(OOH) (4) and TEMPO.

Synthesis of Quaternary-Carbon-Containing β -Amino Acids by the Rh -Catalyzed Enantioselective Arylation of α-Substituted β-Nitroacrylates.

An enantioselective Rh -catalyzed conjugate addition reaction of α-substituted β-nitroacrylates with various arylboronic acids by using chiral Rh 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 β-nitropropionate products in up to 63 % yield and 99 % ee. Reaction of either (E)- or (Z)-β-nitroacrylates provided the same enantiomer of the product, and a range of esters and aryl groups were tolerated.

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.

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

It was shown previously (J. Am. Chem.

Stabilization of hexa-coordinated P(v) corroles by axial silyloxy groups.

We report the stabilization of the hexa-coordination environment for P(v) corroles by using alkyl/aryl substituted silyloxy groups as axial ligands. The P(v) corroles are highly fluorescent in a hexa-coordination environment compared to in a penta-coordination environment. However, P(v) corroles generally undergo axial ligand dissociation to form a mixture of penta- and hexa-coordinated P(v) corroles in non-coordinating solvents such as toluene, CH2Cl2, CHCl3.