Nonheme iron-oxo and -superoxo reactivities: O2 binding and spin inversion probability matter

DFT calculated barriers for C-H activation of 1,4-cyclohexadiene by nonheme iron(IV)-oxo and iron(III)-superoxo species show that the experimental trends can be explained if the spin inversion probability of the TMC iron(IV)-oxo is assumed to be poor. Also, the TMC iron(III)-superoxo reaction proceeds with an endothermic O(2)-binding energy followed by an intrinsically reactive quintet state.     

Regioselectivity of aliphatic versus aromatic hydroxylation by a nonheme iron(II)-superoxo complex

Many enzymes in nature utilize molecular oxygen on an iron center for the catalysis of substrate hydroxylation. In recent years, great progress has been made in understanding the function and properties of iron(IV)-oxo complexes; however, little is known about the reactivity of iron(II)-superoxo intermediates in substrate activation. It has been proposed recently that iron(II)-superoxo intermediates take part as hydrogen abstraction species in the catalytic cycles of nonheme iron enzymes. To gain insight into oxygen atom transfer reactions by the nonheme iron(II)-superoxo species, we performed a density functional theory study on the aliphatic and aromatic hydroxylation reactions using a biomimetic model complex. The calculations show that nonheme iron(II)-superoxo complexes can be considered as effective oxidants in hydrogen atom abstraction reactions, for which we find a low barrier of 14.7 kcal mol(-1) on the sextet spin state surface. On the other hand, electrophilic reactions, such as aromatic hydroxylation, encounter much higher (>20 kcal mol(-1)) barrier heights and therefore are unlikely to proceed. A thermodynamic analysis puts our barrier heights into a larger context of previous studies using nonheme iron(IV)-oxo oxidants and predicts the activity of enzymatic iron(II)-superoxo intermediates.     

Identification of an "end-on" nickel-superoxo adduct, [Ni(tmc)(O2)]+

An "end-on" Ni2+-superoxo adduct has been prepared via two independent synthetic routes and its structure ascertained by spectroscopic and computational methods. The new structure type in nickel coordination chemistry is supported by resonance Raman and EPR spectroscopic features, the former displaying a high frequency nu (O-O) mode (1131 cm-1) consistent with significant superoxo character. The Ni2+-superoxo adduct oxidizes PPh3 to OPPh3 in quantitative yield.     

Structure of a new trinuclear nickel(II) complex with a salen-type bisoxime ligand

A new trinuclear Ni(II) complex {[NiL(DMF)]₂(OAc)₂Ni}·2DMF (H₂L = 4,4′-dichloro-2,2′-[(1,3-propylene)dioxybis(nitrilomethylidyne)]diphenol) is synthesized and characterized by elemental analyses, IR spectra, UV-Vis spectra and X-ray crystallography. The results show that the Ni(II) complex consists of three Ni(II) ions, two tetradentate (μ-L)^2? units, two coordinated μ-acetate ions, two coordinated DMF molecules, and two crystallization DMF molecules. All hexacoordinated Ni(II) ions of the complex have a slightly distorted octahedral geometry. The crystal packing of the Ni(II) complex reveals a notable feature of this structure: the formation of an infinite supramolecular 2D layered structure by virtue of intermolecular C-H…O hydrogen bonding.     

Steric effect on construction of extended architectures of Ni(II) complexes directed by intermolecular C-H...F and C-H...O interactions

Five new Ni(II) complexes with pyridine carboxamide ligands have been synthesized and the crystal structures of three of the complexes were determined. Strong distortion effects of 6-methyl substitution were observed in the complexes with 6-methyl-substituted pyridyl bpb ligands. The C-H...F and C-H...O hydrogen bond interactions build extended architectures in the crystals studied. This result suggests that the steric effect of 6-methyl substitution plays an important role in the distortion of the structure, and the 6-methyl substitution can facilitate hydrogen bond interactions between methyl hydrogen atoms and O(carbonyl) or F atoms. Twelve Ni(II) complexes, including seven complexes reported previously, show reversible redox behavior, implying that the reduced Ni(I) state of each complex is stable in the time scale of CV measurement. The steric effect of R₁ substituent and the electronic effects of X₁ and X₂ groups were found to be the main factors contributing to the shift of the redox potential of the Ni(II) complexes     

Catalytic mechanism of dopamine beta-monooxygenase mediated by Cu(III)-oxo

Mechanisms of dopamine hydroxylation by the Cu(II)-superoxo species and the Cu(III)-oxo species of dopamine beta-monooxygenase (DBM) are discussed using QM/MM calculations for a whole-enzyme model of 4700 atoms. A calculated activation barrier for the hydrogen-atom abstraction by the Cu(II)-superoxo species is 23.1 kcal/mol, while that of the Cu(III)-oxo, which can be viewed as Cu(II)-O*, is 5.4 kcal/mol. Energies of the optimized radical intermediate in the superoxo- and oxo-mediated pathways are 18.4 and -14.2 kcal/mol, relative to the corresponding reactant complexes, respectively. These results demonstrate that the Cu(III)-oxo species can better mediate dopamine hydroxylation in the protein environment of DBM. The side chains of three amino acid residues (His415, His417, and Met490) coordinate to the Cu(B) atom, one of the copper sites in the catalytic core that plays a role for the catalytic function. The hydrogen-bonding network between dopamine and the three amino acid residues (Glu268, Glu369, and Tyr494) plays an essential role in substrate binding and the stereospecific hydroxylation of dopamine to norepinephrine. The dopamine hydroxylation by the Cu(III)-oxo species is a downhill and lower-barrier process toward the product direction with the aid of the protein environment of DBM. This enzyme is likely to use the high reactivity of the Cu(III)-oxo species to activate the benzylic C-H bond of dopamine; the enzymatic reaction can be explained by the so-called oxygen rebound mechanism.     

Synthesis and crystal structure of the nickel(II) complex with 2-diphenylphosphinobenzaldehyde 2-quinolinylhydrazone

2-Diphenylphosphinobenzaldehyde 2-quinolinylhydrazone (HL) and its nickel(II) complex [Ni(L)(CH₃COO)] were synthesized. The structure of the complex was determined by X-ray diffraction. The nickel ion in the molecule of the complex has a square PNN’O donor environment. Owing to C-H...O intermolecular hydrogen bonds, centrosymmetric dimers are formed in the crystal.     

Oxygen activation by the noncoupled binuclear copper site in peptidylglycine alpha-hydroxylating monooxygenase. Reaction mechanism and role of the noncoupled nature of the active site

Reaction thermodynamics and potential energy surfaces are calculated using density functional methods to investigate possible reactive Cu/O(2) species for H-atom abstraction in peptidylglycine alpha-hydroxylating monooxygenase (PHM), which has a noncoupled binuclear Cu active site. Two possible mononuclear Cu/O(2) species have been evaluated, the 2-electron reduced Cu(II)(M)-OOH intermediate and the 1-electron reduced side-on Cu(II)(M)-superoxo intermediate, which could form with comparable thermodynamics at the catalytic Cu(M) site. The substrate H-atom abstraction reaction by the Cu(II)(M)-OOH intermediate is found to be thermodynamically accessible due to the contribution of the methionine ligand, but with a high activation barrier ( approximately 37 kcal/mol, at a 3.0-A active site/substrate distance), arguing against the Cu(II)(M)-OOH species as the reactive Cu/O(2) intermediate in PHM. In contrast, H-atom abstraction from substrate by the side-on Cu(II)(M)-superoxo intermediate is a nearly isoenergetic process with a low reaction barrier at a comparable active site/substrate distance ( approximately 14 kcal/mol), suggesting that side-on Cu(II)(M)-superoxo is the reactive species in PHM. The differential reactivities of the Cu(II)(M)-OOH and Cu(II)(M)-superoxo species correlate to their different frontier molecular orbitals involved in the H-atom abstraction reaction. After the H-atom abstraction, a reasonable pathway for substrate hydroxylation involves a "water-assisted" direct OH transfer to the substrate radical, which generates a high-energy Cu(II)(M)-oxyl species. This provides the necessary driving force for intramolecular electron transfer from the Cu(H) site to complete the reaction in PHM. The differential reactivity pattern between the Cu(II)(M)-OOH and Cu(II)(M)-superoxo intermediates provides insight into the role of the noncoupled nature of PHM and dopamine beta-monooxygenase active sites, as compared to the coupled binuclear Cu active sites in hemocyanin, tyrosinase, and catechol oxidase, in O(2) activation.     

A chromium(III)-superoxo complex in oxygen atom transfer reactions as a chemical model of cysteine dioxygenase

Metal-superoxo species are believed to play key roles in oxygenation reactions by metalloenzymes. One example is cysteine dioxygenase (CDO) that catalyzes the oxidation of cysteine with O(2), and an iron(III)-superoxo species is proposed as an intermediate that effects the sulfoxidation reaction. We now report the first biomimetic example showing that a chromium(III)-superoxo complex bearing a macrocyclic TMC ligand, [Cr(III)(O(2))(TMC)(Cl)](+), is an active oxidant in oxygen atom transfer (OAT) reactions, such as the oxidation of phosphine and sulfides. The electrophilic character of the Cr(III)-superoxo complex is demonstrated unambiguously in the sulfoxidation of para-substituted thioanisoles. A Cr(IV)-oxo complex, [Cr(IV)(O)(TMC)(Cl)](+), formed in the OAT reactions by the chromium(III)-superoxo complex, is characterized by X-ray crystallography and various spectroscopic methods. The present results support the proposed oxidant and mechanism in CDO, such as an iron(III)-superoxo species is an active oxidant that attacks the sulfur atom of the cysteine ligand by the terminal oxygen atom of the superoxo group, followed by the formation of a sulfoxide and an iron(IV)-oxo species via an O-O bond cleavage.     

Nickel(0)-catalyzed isomerization of an aryne complex: formation of a dinuclear Ni(I) complex via C-H rather than C-F bond activation

The reaction of the aryne complex (PEt3)2Ni(eta2-C6H2-4,5-F2) with a catalytic amount of Ni(PEt3)2 results in a dinuclear Ni(I) complex from the coupling of the isomer (PEt3)2Ni(eta2-C6H2-3,4-F2), obtained via rearrangement of the aromatic C-H bonds, which demonstrates that Ni(PEt3)2 is kinetically capable of C-H bond activation, even in the presence of C-F bonds. The intermediate [(PEt3)2Ni]2(mu-eta2:eta2-C6H2-4,5-F2) was isolated and crystallographically characterized; the mu-eta2:eta2-bonding mode observed is unprecedented in aryne chemistry.     

Conversion of methane to methanol at the mononuclear and dinuclear copper sites of particulate methane monooxygenase (pMMO): a DFT and QM/MM study

Methane hydroxylation at the mononuclear and dinuclear copper sites of pMMO is discussed using quantum mechanical and QM/MM calculations. Possible mechanisms are proposed with respect to the formation of reactive copper-oxo and how they activate methane. Dioxygen is incorporated into the Cu(I) species to give a Cu(II)-superoxo species, followed by an H-atom transfer from a tyrosine residue near the monocopper active site. A resultant Cu(II)-hydroperoxo species is next transformed into a Cu(III)-oxo species and a water molecule by the abstraction of an H-atom from another tyrosine residue. This process is accessible in energy under physiological conditions. Dioxygen is also incorporated into the dicopper site to form a (mu-eta(2):eta(2)-peroxo)dicopper species, which is then transformed into a bis(mu-oxo)dicopper species. The formation of this species is more favorable in energy than that of the monocopper-oxo species. The reactivity of the Cu(III)-oxo species is sufficient for the conversion of methane to methanol if it is formed in the protein environment. Since the sigma orbital localized in the Cu-O bond region is singly occupied in the triplet state, this orbital plays a role in the homolytic cleavage of a C-H bond of methane. The reactivity of the bis(mu-oxo)dicopper species is also sufficient for the conversion of methane to methanol. The mixed-valent bis(mu-oxo)Cu(II)Cu(III) species is reactive to methane because the amplitude of the sigma singly occupied MO localized on the bridging oxo moieties plays an essential role in C-H activation.     

Glyoxalase I-type hemithioacetal isomerization reactivity of a mononuclear Ni(II) deprotonated amide complex

The synthesis, characterization, and hemithioacetal isomerization reactivity of a mononuclear Ni(II) deprotonated amide complex, [(bppppa-)Ni]ClO4.CH3OH (1, bppppa- = monoanion of N,N-bis-[(6-phenyl-2-pyridyl)methyl]-N-[(6-pivaloylamido-2-pyridyl)methyl]amine), are reported. Complex 1 was characterized by X-ray crystallography, 1H NMR, UV-vis, FTIR, and elemental analysis. Treatment of 1 with an equimolar amount of the hemithioacetal PhC(O)CH(OH)SCD3 in dry acetonitrile results in the production of the thioester PhCH(OH)C(O)SCD3 in approximately 60% yield. This reaction is conveniently monitored via 2H NMR spectroscopy. A protonated analogue of 1, [(bppppa)Ni](ClO4)2 (2), is unreactive with the hemithioacetal, thus indicating the requirement of the anionic chelate ligand in 1 for hemithioacetal isomerization reactivity. Complex 1 is unreactive with the thioester product, PhCH(OH)C(O)SCD3, which indicates that the pKa value for the PhCH(OH)C(O)SCD3 proton of the thioester must be significantly higher than the pKa value of the C-H proton of the hemithioacetal (PhC(O)CH(OH)SCD3). Complex 1 is the first well-characterized Ni(II) coordination complex to exhibit reactivity relevant to Ni(II)-containing E. coli glyoxalase I. Treatment of NiBr2.2H2O with PhC(O)CH(OH)SCD3 in the presence of 1-methylpyrrolidine also yields thioester product, albeit the reaction is slower and involves the formation of multiple -SCD3 labeled species, as detected by 2H NMR spectroscopy. The results of this study provide the first insight into hemithioacetal isomerization promoted by a synthetic Ni(II) coordination complex versus a simple Ni(II) ion.     

Synthesis, spectroscopic, thermal, crystal characterization and biological activity of [Ni(phen)3][Ni(dipic)2]2.17H2O (H2dipic:pyridine-2,6-dicarboxylic acid, phen: 1,10-phenanthroline)

The new [Ni(phen)₃][Ni(dipic)₂]₂.17H₂O (1) (phen = 1,10-phenanthroline, dipic = dipicolinate)) has been prepared and characterized by elemental analysis, IR, UV-Vis, magnetic measurement and single crystal X-ray diffraction. The complex consists of two tris(1,10-phenanthroline)nickel(II) cations, two bis(dipicolinato)nickelate(II) anions and seventeen uncoordinated water molecules. The Ni(II) complex crystallizes in the triclinic space group P-1. The complex consisting of cation has distorted octahedral coordination by three bidentate phen ligands. In the complex anion, each dipic ligand simultaneously exhibits tridentate coordination modes through N atom of pyridine ring and oxygen atoms of the carboxylate groups. The crystal packing of 1 is a composite of intermolecular hydrogen bonding, π-π and C-H?π interactions. The complex has also been investigated in terms of biological activity and it showed high activity against S. aureus from Gram positive bacteria and C. albicans from yeast tested.     

Kinetics, thermodynamics, and effect of BPh3 on competitive C-C and C-H bond activation reactions in the interconversion of allyl cyanide by [Ni(dippe)

Reaction of [(dippe)Ni(micro-H)](2) with allyl cyanide at low temperature quantitatively generates the eta(2)-olefin complex (dippe)Ni(CH(2)=CHCH(2)CN) (1). At ambient temperature or above, the olefin complex is converted to a mixture of C-CN cleavage product (dippe)Ni(eta(3)-allyl)(CN) (3) and the olefin-isomerization products (dippe)Ni(eta(2)-crotonitrile) (cis- and trans-2), which form via C-H activation. The latter are the exclusive products at longer reaction times, indicating that C-CN cleavage is reversible and the crotononitrile complexes 2 are more thermodynamically stable than eta(3)-allyl species 3. The kinetics of this reaction have been followed as a function of temperature, and rate constants have been extracted by modeling of the reaction. The rate constants for C-CN bond formation (the reverse of C-CN cleavage) show a stronger temperature dependence than those for C-CN and C-H activation, making the observed distribution of C-H versus C-CN cleavage products strongly temperature-dependent. The activation parameters for the C-CN formation step are also quite distinct from those of the C-CN and C-H cleavage steps (larger DeltaH(++) and positive DeltaS(++)). Addition of the Lewis acid BPh(3) to 1 at low temperature yields exclusively the C-CN activation product (dippe)Ni(eta(3)-allyl)(CNBPh(3)) (4). Independently prepared (dippe)Ni(crotononitrile-BPh(3)) (cis- and trans-7) does not interconvert with 4, indicating that 4 is the kinetic product of the BPh(3)-mediated reaction. On standing in solution at ambient temperature, 4 decomposes slowly to complex 5, with structure [(dippe)Ni(eta(3)-allyl)(N triple bond C-BPh(3)), while addition of a second equivalent of BPh(3) immediately produces [(dippe)Ni(eta(3)-allyl)](+)[Ph(3)BC triple bond NBPh(3)](-) (6). Comparison of the barriers to pi-sigma allyl interconversion (determined via dynamic (1)H NMR spectroscopy) for all of the eta(3)-allyl complexes reveals that axial cyanide ligands facilitate pi-sigma interconversion by moving into the P(2)Ni square plane when the allyl group is sigma-bound.     

Synthesis and Crystal Structure of (1-Naphthyl){[3- (2,6-dibromo-4-methylphenylimino)methyl]-5-chloro- 2-hydroxy-benzaldehyde}(triphenylphosphine) Nickel(II) Triphenylphosphine

1INTRODUCTION Over the past decades,olefin oligomerization and polymerization based on late transition metal catalysts have been the most exciting developments in the area of organometallic chemistry and polymer science[1～3].In this broad context,neutral nickel(II)complexes have attracted much attention for their less sensitivity to protonic solvents and polar mon-mers.The most typical one is the SHOP-type cata-lyst[4～6],which contains an anionic[P,O]chelate ring and shows high activ…     

Synthesis of an anthracenyl bridged porphyrin–corrole bismacrocycle. Physicochemical and electrochemical characterisation of the biscobalt μ-superoxo derivative

Dicobalt or heterobimetallic cofacial bisporphyrins are up till now amongst the very few molecular electrocatalysts able to promote the direct reduction of dioxygen to water via a four-electron process in acidic medium. Numerous studies have been devoted to elucidate the key steps of this catalytic reaction and an important result has revealed an unexpected high dioxygen affinity for a mixed valence Co(II)/Co(III) cofacial porphyrin, the key intermediate complex being a μ-superoxo derivative. At the same time, the great importance assumed by ‘Pacman’ porphyrins and the recent developments in corrole chemistry have provided the stimulation to synthesise porphyrin–corrole dyads which might also transport and/or activate dioxygen. In the present paper, we report the stepwise synthesis and characterisation of a cofacial porphyrin–corrole bearing an anthracenyl bridge, (PCA)H₅ where PCA is the pentaanion of 1-(13,17-diethyl-2,3,7,8,12,18-hexamethylporphyrin–5-yl)-8-(7,8,12,13-tetramethyl-2,3,17,18-tetraphenylcorrol-10-yl) anthracene. The synthesis and characterisation of the μ-superoxo Co(III)/Co(III) complex 〚(PCA)Co₂Im₂〛(μ-O₂) is also described.     

Vibrational-electronic interaction in the infrared circular dichroism spectra of transition-metal complexes

The circular dichroism spectra of the quasi-tetrahedral complexes of transition-metal ions with (?)-spartein, [M(l-sp)Cl₂], over the C-H stretching vibration range, show an optical activity which is weak in the Zn(II) case, but enhanced in the Co(II) and Ni(II) analogues through the coupling of the sharp vibrational transition with underlying broad d-d electronic excitation.     

Mechanistic analysis of iridium(III) catalyzed direct C-H arylations: a DFT study

In a recent experimental work the Ir complex [Ir(cod)(py)(PCy(3))](PF(6)) (that is, Crabtree's catalyst) has been shown to catalyze the C-H arylation of electron-rich heteroarenes with iodoarenes using Ag(2)CO(3) as base. For this process, an electrophilic metalation mechanism, (S(E)Ar) has been proposed as operative mechanism rather than the concerted metalation-deprotonation (CMD) mechanism, widely implicated in Pd-catalyzed arylation reactions. Herein we have investigated the C-H activation step for several (hetero)arenes catalyzed by a Ir(III) catalyst and compared the data obtained with the results for the Pd(II)-catalyzed C-H bond activation. The calculations demonstrate that, similar to Pd(II)-catalyzed reactions, the Ir(III)-catalyzed direct C-H arylation occurs through the CMD pathway which accounts for the experimentally observed regioselectivity. The transition states for Ir(III)-catalyzed direct C-H arylation feature stronger metal-C((arene)) interactions than those for Pd(II)-catalyzed C-H arylation. The calculations also demonstrate that ligands with low trans effect may decrease the activation barrier of the C-H bond cleavage.     

Structural, spectroscopic, and magnetic study of bis(9,10-dihydro-9-oxo-10-acridineacetate)bis(imidazole)bis(methanol) nickel(II)

The mixed ligand complex [Ni(CMA)2(im)2(MeOH)2] (where CMA = 9,10-dihydro-9-oxo-10-acridineacetate ion, im = imidazole) was prepared, and its crystal and molecular structure were determined. The nickel ions are hexa-coordinated by four oxygen atoms of the carboxylate and hydroxyl groups and by two imidazole nitrogen atoms, to form a distorted octahedral arrangement. The structure consists of a one-dimensional network of the complex molecules connected by strong intermolecular hydrogen bonds. The weak intermolecular C-H...X hydrogen bonds and stacking interactions make up the 2-D structure. Very strong intramolecular hydrogen bonds significantly affect the geometry and vibrational characteristics of the carboxylate group. The UV-vis-NIR electronic spectrum was deconvoluted into Gaussian components. Electronic bands of the Ni(II) ion were assigned to suitable spin-allowed transitions in the D4h symmetry environment. The single ion zero-field splitting (ZFS) parameters for the S = 1 state of Ni(II), as well as the g components, have been determined by high-field and high-frequency EPR (HF-HFEPR) spectroscopy over the frequency range of 52-432 GHz and with the magnetic fields up to 14.5 T: D = 5.77(1) cm-1, E = 1.636(2) cm-1, gx = 2.29(1), gy = 2.18(1), and gz = 2.13(1). These values allowed us to simulate the powder magnetic susceptibility and field-dependent magnetization of the complex.