Identification, Characterization, and Electronic Structures of Interconvertible Cobalt-Oxygen TAML Intermediates.

The reaction of Li[(TAML)CoIII]·3H2O (TAML = tetraamido macrocyclic tetraanionic ligand) with iodosylbenzene at 253 K in acetone in the presence of redox-innocent metal ions (Sc(OTf)3 and Y(OTf)3) or triflic acid affords a blue species 1, which is converted reversibly to a green species 2 upon cooling to 193 K. The electronic structures of 1 and 2 have been determined by combining advanced spectroscopic techniques (X-band electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), X-ray absorption spectroscopy/extended X-ray absorption fine structure (XAS/EXAFS), and magnetic circular dichroism (MCD)) with ab initio theoretical studies. Complex 1 is best represented as an S = 1/2 [(Sol)(TAML•+)CoIII---OH(LA)]- species (LA = Lewis/Brønsted acid and Sol = solvent), where an S = 1 Co(III) center is antiferromagnetically coupled to S = 1/2 TAML•+, which represents a one-electron oxidized TAML ligand. In contrast, complex 2, also with an S = 1/2 ground state, is found to be multiconfigurational with contributions of both the resonance forms [(H-TAML)CoIV═O(LA)]- and [(H-TAML•+)CoIII═O(LA)]-; H-TAML and H-TAML•+ represent the protonated forms of TAML and TAML•+ ligands, respectively. Thus, the interconversion of 1 and 2 is associated with a LA-associated tautomerization event, whereby H+ shifts from the terminal -OH group to TAML•+ with the concomitant formation of a terminal cobalt-oxo species possessing both singlet (SCo = 0) Co(III) and doublet (SCo = 1/2) Co(IV) characters. The reactivities of 1 and 2 at different temperatures have been investigated in oxygen atom transfer (OAT) and hydrogen atom transfer (HAT) reactions to compare the activation enthalpies and entropies of 1 and 2.

Deciphering the Mechanism of MRSA Targeting Copper(II) Complexes of NN2 Pincer-Type Ligands.

WHO lists AMR as one of the top ten global public health issues. Therefore, constant effort is needed to develop more efficient antimicrobial drugs. As a result, earth-abundant transition-metal complexes have emerged as an excellent solution. In this regard, new aminoquinoline-based copper(II) pincer complexes 1-3 were designed, synthesized, and characterized by modern spectroscopic techniques. It is worth mentioning that, at the highest concentration (1024 µg/mL) of complexes (1-3), the hemolysis was found to be <15%, implying their less toxicity. Further, the complexes effectively interfered with the growth of Gram positive MRSA and the fungus Candida albicans. Among them, complex 2 was promising (MIC = 16 µg/mL) against MRSA, which was better than the known antibacterial drug kanamycin (64 µg/mL) under identical conditions. The Alamar blue cell viability test and the MBC/MFC identified by spot assay were in accordance with MIC values. Moreover, the insilico studies explained the most probable mechanism of action as inhibition of cell wall biosynthesis and dysfunction of antibiotic sensing proteins. Similarly, the antifungal action might be due to the cell surface adhesion protein dysfunction by the complexes. Furthermore, we are expecting to draw these compounds for clinical applications.

Are Zn(II) pincer complexes efficient apoptosis inducers? a deep insight into their activity against A549 lung cancer cells.

To expand the array of chemotherapeutic drugs, earth-abundant metal complexes are found to be the future direction. In this regard, new zinc(II) complexes 1-3 of 8-aminoquinoline-based pincer ligands were synthesized, characterized and tested for their anticancer activity. The IC50 values of these complexes were estimated by an MTT assay to be 16.35-17.95 µM and 33.35-40 µM against A549 lung and MCF-7 breast cancer cells respectively. Among them, 3 was slightly better than the other complexes and, thus, subjected to detailed studies. Moreover, the ligand corresponding to 3 was less active against both the cell lines than the complex. Further, 3 showed no toxicity against normal fibroblast cell line L929, which instantly elevated the drug characteristic of our complex. An AO-EB staining assay revealed that 3 can induce apoptosis in A549, and it was quantified by flow cytometry as 22.77%. Moreover, the depolarization of the mitochondrial membrane potential determined by JC-1 staining indicated excess ROS production sites in the mitochondria, which was confirmed by carboxy-H2DCFDA staining. Interestingly, the present complexes show better activity than that of the standard drug cisplatin against A549 cells. Overall, the studies provided promising results that can be extended for clinical applications.

Deciphering the effect of amine versus imine ligands of copper(II) complexes in 2-aminophenol oxidation.

A series of amine (1-6) and imine (5',6') based copper(II) complexes with tridentate (NNO) ligand donors were synthesized and characterized using modern analytical techniques. All the complexes were subjected to 2-aminophenol (OAP) oxidation to form 2-aminophenoxazin-3-one, as a functional analogue of an enzyme, phenoxazinone synthase. In addition, a critical comparison of the reactivity using the amine-based complexes with their respective imine counterparts was achieved in both experimental as well as theoretical studies. For instance, the kinetic measurement revealed that the imine-based copper(II) complexes (kcat, 2.4 × 105-6.2 × 106 h-1) are better than amine-based (kcat, 6.3 × 104-3.9 × 105 h-1) complexes. The complex-substrate adducts [Cu(L3)(OAP)] (7) and [Cu(L3')(OAP)] (7') were characterized for both systems by mass spectrometry. Further, the DFT study was performed with amine- (3) and imine- (3') based copper(II) complexes, to compare their efficacy in the oxidation of OAP. The mechanistic investigations reveal that the key elementary step to determine the reactivity of 3 and 3' is the proton-coupled electron transfer (PCET) step occurring from the intermediates 7/7'. Further, the computed HOMO-LUMO energy gap of 7' was smaller than 7 by 0.8 eV, which indicates the facile PCET compared to that of 7. Moreover, the coupling of the OAP moiety using imine-complexes (ΔGR.E = -5.8 kcal/mol) was found to be thermodynamically more favorable than amine complexes (ΔGR.E = +3.3 kcal/mol). Overall, the theoretical findings are in good agreement with the experimental results.

Carbazole appended trans-dicationic pyridinium porphyrin finds supremacy in DNA binding/photocleavage over a non-carbazolyl analogue.

A carbazolyl appended trans-pyridyl porphyrin (1) was synthesized and its dicationic form 2 was obtained by methylation of the pyridyl group. Copper and zinc complexes of porphyrin 2 (Cu(II), 3; Zn(II), 4) were isolated and characterized by various modern spectroscopic techniques. The DNA binding properties of 2, 3, and 4 have been explored against calf thymus-DNA (CT-DNA). DNA binding was quantized using the intrinsic binding constant (Kb) that was calculated by UV-visible absorption spectroscopy, and the value Kb = 1.6 × 106 M-1 for compound 2 reveals a better interaction of 2 towards CT-DNA than those of 3 (3.1 × 105 M-1) and 4 (3.4 × 105 M-1), which follows the order 2 > 4 > 3. The fluorescence quenching efficiency and ethidium bromide quenching assay also indicated a good binding affinity of all the compounds towards CT-DNA. Furthermore, the spectroscopic data suggest that the possible mode of interaction is intercalation. The docking studies were in accordance with the experimental results. Notably, DNA cleavage studies reveal that 2 shows better damage than 3 and 4 which is in accordance with the binding affinity order 2 > 4 > 3. The observed quantum yield (2: 0.65, 3: 0.33, and 4: 0.97) and no change in DNA cleavage in the presence of NaN3 reveal the involvement of singlet oxygen. The singlet excited state lifetimes were in the range of 6.3-1.2 ns. Furthermore, these porphyrins can be investigated as interesting photosensitizers in photodynamic therapy and photochemotherapy.

A side-on Mn(III)-peroxo supported by a non-heme pentadentate N3Py2 ligand: synthesis, characterization and reactivity studies.

A mononuclear manganese(iii)-peroxo complex [MnIII(N3Py2)(O2)]+ (1a) bearing a non-heme N,N'-dimethyl-N-(2-(methyl(pyridin-2-ylmethyl)amino)ethyl)-N'-(pyridin-2-ylmethyl)ethane-1,2-diamine (N3Py2) ligand was synthesized by the reaction of [Mn(N3Py2)(H2O)](ClO4)2 (1) with hydrogen peroxide and triethylamine in CH3CN at 25 °C. The reactivity of 1a in aldehyde deformylation using 2-phenyl propionaldehyde (2-PPA) was studied and the reaction kinetics was monitored by UV-visible spectroscopy. A kinetic isotope effect (KIE) = 1.7 was obtained in the reaction of 1a with 2-PPA and α-[D1]-PPA, suggesting nucleophilic character of 1a. The activation parameters ΔH‡ and ΔS‡ were determined using the Eyring plot while Ea was obtained from the Arrhenius equation by performing the reaction between 288 and 303 K. Hammett constants (σp) of para-substituted benzaldehydes p-X-Ph-CHO (X = Cl, F, H, and Me) were linear with a slope (ρ) = 3.0. Computational study suggested that the side-on structure of 1a is more favored over the end-on structure and facilitates the reactivity of 1a.

Copper(II) complexes of tripodal ligand scaffold (N3O) as functional models for phenoxazinone synthase.

The copper(II) complexes [Cu(L)NO3] (1-9) of newer N3O ligands (L1-L9) have been synthesized and characterized. The molecular structure of 1, 4, and 7 exhibited nearly a perfect square pyramidal geometry (τ, 0.04-0.11). The Cu-OPhenolate bonds (~ 1.91 Å) are shorter than the Cu-N bonds (~ 2.06 Å) due to the stronger coordination of anionic phenolate oxygen. The Cu(II)/Cu(I) redox potentials of 1-9 appeared around -0.102 to -0.428 V versus Ag/Ag+ in water. The electronic spectra of the complexes showed the d-d transitions around 643-735 nm and axial EPR parameter (g||, 2.243-2.270; A||, 164-179 × 10-4 cm-1) that corresponds to square pyramidal geometry. The bonding parameters α2, 0.760-0.825; ß2, 0.761-0.994; γ2, 0.504-0.856 and K||, 0.698-0.954 and K⊥, 0.383-0.820 calculated from EPR spectra and energies of d-d transitions. The complexes catalyzed the conversion of substrate 2-aminophenol into 2-aminophenoxazine-3-one using molecular oxygen in the water and exhibited the yields of 41-61%. The formation of the product is accomplished by the appearance of a new absorption band at 430 nm and the rates of formation were calculated as 6.98-15.65 × 10-3 s-1 in water. The reaction follows Michaelis-Menten enzymatic reaction kinetics with turnover numbers (kcat) 9.11 × 105 h-1 for 1 and 4.66 × 105 h-1 for 9 in water. The spectral, redox and kinetic studies were performed in water to mimic the enzymatic oxidation reaction conditions.

Catalytic Four-Electron Reduction of Dioxygen by Ferrocene Derivatives with a Nonheme Iron(III) TAML Complex.

A mononuclear nonheme iron(III) complex with a tetraamido macrocyclic ligand (TAML), [(TAML)FeIII]- (1), is a selective precatalyst for four-electron reduction of dioxygen by ferrocene derivatives in the presence of acetic acid (CH3COOH) in acetone. This is the first work to show that a nonheme iron(III) complex catalyzes the four-electron reduction of O2 by one-electron reductants. An iron(V)-oxo complex, [(TAML)FeV(O)]- (2), was produced by oxygenation of 1 with O2 via the formation of triacetone triperoxide (TATP), acting as an autocatalyst that shortened the induction time for the generation of 2. Decamethylferrocene (Me10Fc) and octamethylferrocene (Me8Fc) reduced 2 to 1 by two electrons in the presence of CH3COOH to produce decamethylferrocenium cation (Me10Fc+) and octamethylferrocenium cation (Me8Fc+), respectively. Then, 1 was oxygenated by O2 to regenerate 2 via the formation of TATP. In the cases of ferrocene (Fc), bromoferrocene (BrFc) and 1,1'-dibromoferrocene (Br2Fc), initial electron transfer from ferrocene derivatives to 2 occurred; however, neither a second proton-coupled electron transfer from ferrocene derivatives to 2 nor a catalytic four-electron reduction of O2 occurred.

A High-Valent Manganese(IV)-Oxo-Cerium(IV) Complex and Its Enhanced Oxidizing Reactivity.

A mononuclear nonheme manganese(IV)-oxo complex binding the Ce4+ ion, [(dpaq)MnIV (O)]+ -Ce4+ (1-Ce4+ ), was synthesized by reacting [(dpaq)MnIII (OH)]+ (2) with cerium ammonium nitrate (CAN). 1-Ce4+ was characterized using various spectroscopic techniques, such as UV/Vis, EPR, CSI-MS, resonance Raman, XANES, and EXAFS, showing an Mn-O bond distance of 1.69 Šwith a resonance Raman band at 675 cm-1 . Electron-transfer and oxygen atom transfer reactivities of 1-Ce4+ were found to be greater than those of MnIV (O) intermediates binding redox-inactive metal ions (1-Mn+ ). This study reports the first example of a redox-active Ce4+ ion-bound MnIV -oxo complex and its spectroscopic characterization and chemical properties.

Catalytic Conversion of Atmospheric CO2 into Organic Carbonates by Nickel(II) Complexes of Diazepane-Based N4 Ligands.

Activation of CO2 and conversion into value-added products is an effective option to mitigate CO2 emission. The nickel(II) complexes [Ni(L1)](ClO4)2 1, [Ni(L2)](ClO4)2 2, and [Ni(L3)(CH3CN)2](Ph4B)2 3 of diazepane-based ligands [1,4-bis[(pyridin-2-yl-methyl)]-1,4-diazepane (L1), 1,4-bis[2-(pyridin-2-yl)ethyl]-1,4-diazepane (L2), and 4-bis[2-(quinoline-2-yl)-methyl]-1,4-diazepane (L3)] have been synthesized and structurally characterized. The complexes were employed as the catalysts for the conversion of atmospheric CO2 into organic carbonates in the absence of cocatalyst at 1 atm pressure. The single-crystal X-ray structures of 1 and 2 exhibit distorted square-planar geometry with almost identical Ni-N bond distances (1.891-1.946 Å). The geometry of the complexes rearranged into octahedral in acetonitrile, which was studied by paramagnetic 1H NMR and electronic spectra. The complexes selectively captured CO2 from the atmospheric air and readily converted epoxides into cyclic carbonates without any cocatalyst. They showed a maximum yield of 25% (TON, 500) using 1 atm air, which is drastically enhanced up to 89% (TON, 1780) using 1 atm pure CO2 gas. This is the highest catalytic efficiency reported for CO2 fixation using nickel-based catalysts to date. The CO2 fixation reaction without organic substrate showed the formation of carbonate-bridged dinuclear nickel(II) complexes. They showed characteristic absorption bands around 571-612 nm and were further confirmed by electrospray ionization mass spectrometry, IR, and single-crystal X-ray structures. The molecular structure of carbonate-bridged intermediates exhibited two Ni2+-centers with distorted square pyramidal geometries for 2a and 3a but distorted octahedral and square pyramidal geometries for 1a. The CO2 fixation reactions possibly proceeded via the formation of CO2-bound nickel species.

A Mn(iv)-peroxo complex in the reactions with proton donors.

Protons play an important role in promoting O-O or M-O bond cleavage of metal-peroxo complexes. Treatment of side-on O2-bound [PPN][MnIV(TMSPS3)(O2)] (1, PPN = bis(triphenylphosphine)iminium and TMSPS3H3 = 2,2',2''-trimercapto-3,3',3''-tris(trimethylsilyl)triphenylphosphine) with perchloric acid (HClO4) in the presence of PR3 (R = phenyl or p-tolyl) results in the formation of neutral five-coordinate MnIII(OPR3)(TMSPS3) complexes (R = phenyl, 2a; p-tolyl, 2b), which are confirmed by X-ray crystallography. Isotope labelling experiments demonstrate that the oxygen atom in the phosphine oxide product derives from the peroxo ligand of 1. Reactions of 1 with weak proton donors, such as phenylthiol, phenol, substituted phenol and methanol, are also investigated to explore the reactivity of the MnIV-peroxo complex, leading to the isolation of a series of five-coordinate [MnIII(L)(TMSPS3)]- complexes (L = phenylthiolate, phenolate or methoxide). Mechanistic aspects of the reactions of the MnIV-peroxo complex with proton donors are discussed as well.

Redox Reactivity of a Mononuclear Manganese-Oxo Complex Binding Calcium Ion and Other Redox-Inactive Metal Ions.

Mononuclear nonheme manganese(IV)-oxo complexes binding calcium ion and other redox-inactive metal ions, [(dpaq)MnIV(O)]+-M n+ (1-Mn+, M n+ = Ca2+, Mg2+, Zn2+, Lu3+, Y3+, Al3+, and Sc3+) (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino- N-quinolin-8-yl-acetamidate), were synthesized by reacting a hydroxomanganese(III) complex, [(dpaq)MnIII(OH)]+, with iodosylbenzene (PhIO) in the presence of redox-inactive metal ions (M n+). The Mn(IV)-oxo complexes were characterized using various spectroscopic techniques. In reactivity studies, we observed contrasting effects of M n+ on the reactivity of 1-M n+ in redox reactions such as electron-transfer (ET), oxygen atom transfer (OAT), and hydrogen atom transfer (HAT) reactions. In the OAT and ET reactions, the reactivity order of 1-M n+, such as 1-Sc3+ ≈ 1-Al3+ > 1-Y3+ > 1-Lu3+ > 1-Zn2+ > 1-Mg2+ > 1-Ca2+, follows the Lewis acidity of M n+ bound to the Mn-O moiety; that is, the stronger the Lewis acidity of M n+, the higher the reactivity of 1-M n+ becomes. In sharp contrast, the reactivity of 1-M n+ in the HAT reaction was reversed, giving the reactivity order 1-Ca2+ > 1-Mg2+ > 1-Zn2+ > 1-Lu3+> 1-Y3+> 1-Al3+ ≈ 1-Sc3+; that is, the higher is Lewis acidity of M n+, the lower the reactivity of 1-M n+ in the HAT reaction. The latter result implies that the Lewis acidity of M n+ bound to the Mn-O moiety can modulate the basicity of the metal-oxo moiety, thus influencing the HAT reactivity of 1-M n+; cytochrome P450 utilizes the axial thiolate ligand to increase the basicity of the iron-oxo moiety, which enhances the reactivity of compound I in C-H bond activation reactions.

A Mononuclear Non-heme Manganese(III)-Aqua Complex as a New Active Oxidant in Hydrogen Atom Transfer Reactions.

A mononuclear non-heme Mn(III)-aqua complex, [(dpaq)MnIII(OH2)]2+ (1, dpaq = 2-[bis(pyridin-2-ylmethyl)]amino- N-quinolin-8-yl-acetamidate), is capable of conducting hydrogen atom transfer (HAT) reactions much more efficiently than the corresponding Mn(III)-hydroxo complex, [(dpaq)MnIII(OH)]+ (2); the high reactivity of 1 results from the positive one-electron reduction potential of 1 ( Ered vs SCE = 1.03 V), compared to that of 2 ( Ered vs SCE = -0.1 V). The HAT mechanism of 1 varies between electron transfer followed by proton transfer and one-step concerted proton-coupled electron transfer, depending on the one-electron oxidation potentials of substrates. To the best of our knowledge, this is the first example showing that metal(III)-aqua complex can be an effective H-atom abstraction reagent.

A Manganese(V)-Oxo Tetraamido Macrocyclic Ligand (TAML) Cation Radical Complex: Synthesis, Characterization, and Reactivity Studies.

A mononuclear manganese(V)-oxo complex with tetraamido macrocyclic ligand (TAML), [MnV (O)(TAML)]- (1), is a sluggish oxidant in oxidation reactions. Herein, a mononuclear manganese(V)-oxo TAML cation radical complex, [MnV (O)(TAML+. )] (2), is reported. It was synthesized by reacting [MnIII (TAML)]- with 3.0 equivalents of [RuIII (bpy)3 ]3+ or upon addition of one-electron oxidant to 1 and then characterized thoroughly with various spectroscopic techniques along with DFT calculations. Although 1 is a sluggish oxidant, 2 is a strong oxidant capable of activating C-H bonds of hydrocarbons (i.e., hydrogen atom transfer reaction) and transferring its oxygen atom to thioanisoles and olefins (i.e., oxygen atom transfer reaction).

Enhanced Electron-Transfer Reactivity of a Long-Lived Photoexcited State of a Cobalt-Oxygen Complex.

Photodynamics and electron-transfer reactivity of an excited state derived from an earth-abundant mononuclear cobalt-oxygen complex ground state, [(TAML)CoIV(O)]2- (1; H4TAML = 3,4,8,9-tetrahydro-3,3,6,6,9,9-hexamethyl-1 H-1,4,8,11-benzotetraazo-cyclotridecane-2,5,7,10-(6 H, 11 H)tetrone), prepared by electron-transfer oxidation of Li[(TAML)CoIII]·3(H2O) (2) in a 1:1 acetonitrile/acetone solvent mixture at 5 °C, were investigated using a combination of femtosecond and nanosecond laser absorption spectroscopy. Visible light photoexcitation of 1 (λexc = 393 nm) resulted in generation of the excited state S2* (lifetime: 1.4(4) ps), detected 2 ps after laser irradiation by femtosecond laser spectroscopy. The initially formed excited state S2* converted to a lower-lying excited state, S1* (λmax = 580 nm), with rate constant kc = 7(2) × 1011 s-1 (S2* → S1*). S1* exhibited a 0.6(1) ns lifetime and converted to the initial ground state 1 with rate constant kd = 1.7(3) × 109 s-1 (S1* → 1). The same excited state dynamics was observed when 1 was generated by electron-transfer oxidation of 2 using different one-electron oxidants such as Cu(OTf)2 (OTf- = triflate anion), [Fe(bpy)3]3+ (bpy = 2,2'-bipyridine), and tris(4-bromophenyl)ammoniumyl radical cation (TBPA•+). The electron-transfer reactivity of S1* was probed by nanosecond laser photoexcitation of 1 in the presence of a series of electron donors with different one-electron oxidation potentials ( Eox vs SCE): benzene (2.35 V), toluene (2.20 V), m-xylene (2.02 V), and anisole (1.67 V). The excited state S1* engaged in electron-transfer reactions with m-xylene and anisole to generate π-dimer radical cations of m-xylene and anisole, respectively, observed by nanosecond laser transient absorption spectroscopy, whereas no reactivity was observed toward benzene and toluene. Such differential electron-transfer reactivity depending on the Eox values of electron donors allowed the estimation of the one-electron reduction potential of S1* ( Ered*) as 2.1(1) V vs SCE, which is much higher than that of the ground state ( Ered = 0.86 V vs SCE).

A mononuclear manganese(iii)-hydroperoxo complex: synthesis by activating dioxygen and reactivity in electrophilic and nucleophilic reactions.

We report the synthesis of manganese(iii)-peroxo (MnIII(O2)) and manganese(iii)-hydroperoxo (MnIII(O2H)) complexes by activating dioxygen (O2) and the amphoteric reactivity of the Mn(iii)-hydroperoxo complex in electrophilic and nucleophilic reactions.

Investigation of structural dynamics of Thrombocytopenia Cargeeg mutants of human apoptotic cytochrome c: A molecular dynamics simulation approach.

Naturally occurring mutations to cytochrome c (cyt-c) have been identified recently in patients with mild autosomal dominant thrombocytopenia (low platelet levels), which yield cyt-c mutants with enhanced apoptotic activity. However, the molecular mechanism underlying this low platelet production and enhanced apoptosis remain unclear. Therefore, an attempt is made herein for the first time to investigate the effects of mutations of glycine 41 by serine (G41S) and tyrosine 48 by histidine (Y48H) on the conformational and dynamic changes of apoptotic (Fe3+) cyt-c using all atom molecular dynamics (MD) simulations in explicit water solvent. Our 30ns MD simulations demonstrate considerable structural differences in G41S and Y48H compared to wild type (WT) cyt-c, such as increasing distances between the critical electron transfer residues results in open conformation at the heme active site, large fluctuations in ß-turns and α-helices. Additionally, although the ß-sheets remain mostly unaffected in all the three cyt-c simulations, the α-helices undergo conformational switch to ß-turns in both the mutant simulations. Importantly, this conformational switch of α-helix to ß-turn around heme active site should attributes to the loss of intraprotein H-bonds in the mutant simulations especially between NE2 (His26) and O (Pro44) in agreement with the experimental report. Further, essential dynamics analysis reveals that overall motions of WT cyt-c is mainly involved only in the first eigenvector, but in G41S and Y48H the overall motions are mainly in three and two eigenvectors respectively. Overall, the detailed atomistic level information provide a unifying description for the molecular mechanism of structural destabilization, disregulation of platelet formation and enhanced peroxidase activity of the mutant cyt-c's in the pathology of intrinsic apoptosis.

Autocatalytic dioxygen activation to produce an iron(v)-oxo complex without any reductants.

An iron(v)-oxo complex with a tetraamido macrocyclic ligand, [(TAML)FeV(O)]-, was produced by reacting [(TAML)FeIII]- with dioxygen without any electron source in acetone at 298 K. The autocatalytic mechanism of dioxygen activation for the formation of an iron(v)-oxo complex has been clarified based on the autocatalysis by radical chain initiators.

Novel nickel(ii) complexes of sterically modified linear N4 ligands: effect of ligand stereoelectronic factors and solvent of coordination on nickel(ii) spin-state and catalytic alkane hydroxylation.

A series of Ni(ii) complexes of the types [Ni(L)(CH3CN)2](BPh4)21-3, 5 and [Ni(L4)](BPh4)24, where L = N,N'-bis(2-pyrid-2-ylmethyl)-1,4-diazepane (L1), N-(6-methylpyrid-2-ylmethyl)-N'-(pyrid-2-ylmethyl)-1,4-diazepane (L2), N,N'-bis(6-methyl-2-pyridylmethyl)-1,4-diazepane (L3), N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)ethylenediamine (L5) and L4 = N,N'-bis((1-methyl-1H-imidazole-2-yl)methyl)-1,4-diazepane, have been isolated and characterized. The complex cations of 1 and 4 possess, respectively, distorted octahedral and low-spin square planar coordination geometries in which nickel(ii) is meridionally coordinated to all four nitrogen atoms of L1 and L4. DFT studies reveal that L5 with the ethylenediamine backbone coordinates in the cis-ß mode in [Ni(L5)(CH3CN)2]2+5, but in the cis-α mode in [Ni(L5)(H2O)2]2+. Also, they illustrate the role of ligand donor atom type, diazacyclo backbone and steric hindrance to coordination of pyridyl nitrogen in conferring novel coordination geometries on Ni(ii). All these complexes catalyse the oxidation of cyclohexane in the presence of m-CPBA as the oxidant up to 600 turnover numbers (TON) with relatively good alcohol selectivity (A/K, 5.6-7.2). Adamantane is oxidized to 1-adamantanol, 2-adamantanol and 2-adamantanone with high bond selectivity (3°/2°, 8.7-11.7). The incorporation of methyl substituent(s) on one (2) or both (3) of the pyridyl rings and the replacement of both the pyridylmethyl arms in 1 by imidazolylmethyl arms to give 4 decrease the catalytic efficiency. Interestingly, 5 with the cis-ß mode of coordination provides two labile cis coordination sites for oxidant binding, leading to higher total TON and product/bond selectivity.

Selective Oxygenation of Cyclohexene by Dioxygen via an Iron(V)-Oxo Complex-Autocatalyzed Reaction.

An iron complex with a tetraamido macrocyclic ligand, [(TAML)FeIII]-, was found to be an efficient and selective catalyst for allylic oxidation of cyclohexene by dioxygen (O2); cyclohex-2-enone was obtained as the major product along with cyclohexene oxide as the minor product. An iron(V)-oxo complex, [(TAML)FeV(O)]-, which was formed by activating O2 in the presence of cyclohexene, initiated the autoxidation of cyclohexene with O2 to produce cyclohexenyl hydroperoxide, which reacted with [(TAML)FeIII]- to produce [(TAML)FeV(O)]- by autocatalysis. Then, [(TAML)FeV(O)]- reacted rapidly with [(TAML)FeIII]- to produce a µ-oxo dimer, [(TAML)FeIV(O)FeIV(TAML)]2-, which was ultimately converted to [(TAML)FeV(O)]- when [(TAML)FeIII]- was not present in the reaction solution. An induction period was observed in the autocatalytic production of [(TAML)FeV(O)]-. The induction period was shortened with increasing catalytic amounts of [(TAML)FeV(O)]- and cyclohexenyl hydroperoxide, whereas the induction period was prolonged by adding catalytic amounts of a spin trapping reagent such as 5,5-dimethyl-1-pyrroline N-oxide (DMPO). The allylic oxidation of cycloalkenes was also found to depend on the allylic C-H bond dissociation energies, suggesting that the hydrogen atom abstraction from the allylic C-H bonds of cycloalkenes is the rate-determining radical chain initiation step. In this study, we have shown that an iron(III) complex with a tetraamido macrocyclic ligand is an efficient catalyst for the allylic oxidation of cyclohexene via an autocatalytic radical chain mechanism and that [(TAML)FeV(O)]- acts as a reactive intermediate for the selective oxygenation of cyclohexene with O2 to produce cyclohex-2-enone predominantly.