The bis(μ‐oxo)dicopper(III) species [CuIII2(μ‐O)2(m‐XYLMeAN)]2+ ( 1 ) promotes the electrophilic ortho‐hydroxylation–defluorination of 2‐fluorophenolates to give the corresponding catechols, a reaction that is not accomplishable with a (η2:η2‐O2)dicopper(II) complex. Isotopic labeling studies show that the incoming oxygen atom originates from the bis(μ‐oxo) unit. Ortho‐hydroxylation–defluorination occurs selectively in intramolecular competition with other ortho‐substituents such as chlorine or bromine. 相似文献
Mononuclear MnIII–peroxo and dinuclear bis(μ‐oxo)MnIII2 complexes that bear a common macrocyclic ligand were synthesized by controlling the concentration of the starting MnII complex in the reaction of H2O2 (i.e., a MnIII–peroxo complex at a low concentration (≤1 mM ) and a bis(μ‐oxo)MnIII2 complex at a high concentration (≥30 mM )). These intermediates were successfully characterized by various physicochemical methods such as UV–visible spectroscopy, ESI‐MS, resonance Raman, and X‐ray analysis. The structural and spectroscopic characterization combined with density functional theory (DFT) calculations demonstrated unambiguously that the peroxo ligand is bound in a side‐on fashion in the MnIII–peroxo complex and the Mn2O2 diamond core is in the bis(μ‐oxo)MnIII2 complex. The reactivity of these intermediates was investigated in electrophilic and nucleophilic reactions, in which only the MnIII–peroxo complex showed a nucleophilic reactivity in the deformylation of aldehydes. 相似文献
The present study focuses on the formation and reactivity of hydroperoxo–iron(III) porphyrin complexes formed in the [FeIII(tpfpp)X]/H2O2/HOO? system (TPFPP=5,10,15,20‐tetrakis(pentafluorophenyl)‐21H,23H‐porphyrin; X=Cl? or CF3SO3?) in acetonitrile under basic conditions at ?15 °C. Depending on the selected reaction conditions and the active form of the catalyst, the formation of high‐spin [FeIII(tpfpp)(OOH)] and low‐spin [FeIII(tpfpp)(OH)(OOH)] could be observed with the application of a low‐temperature rapid‐scan UV/Vis spectroscopic technique. Axial ligation and the spin state of the iron(III) center control the mode of O? O bond cleavage in the corresponding hydroperoxo porphyrin species. A mechanistic changeover from homo‐ to heterolytic O? O bond cleavage is observed for high‐ [FeIII(tpfpp)(OOH)] and low‐spin [FeIII(tpfpp)(OH)(OOH)] complexes, respectively. In contrast to other iron(III) hydroperoxo complexes with electron‐rich porphyrin ligands, electron‐deficient [FeIII(tpfpp)(OH)(OOH)] was stable under relatively mild conditions and could therefore be investigated directly in the oxygenation reactions of selected organic substrates. The very low reactivity of [FeIII(tpfpp)(OH)(OOH)] towards organic substrates implied that the ferric hydroperoxo intermediate must be a very sluggish oxidant compared with the iron(IV)–oxo porphyrin π‐cation radical intermediate in the catalytic oxygenation reactions of cytochrome P450. 相似文献
Ferric–hydroperoxo complexes have been identified as intermediates in the catalytic cycle of biological oxidants, but their role as key oxidants is still a matter of debate. Among the numerous synthetic low‐spin FeIII(OOH) complexes characterized to date, [(L52)Fe(OOH)]2+ is the only one that has been isolated in the solid state at low temperature, which has provided a unique opportunity for inspecting its oxidizing properties under single‐turnover conditions. In this report we show that [(L52)Fe(OOH)]2+ decays in the presence of aromatic substrates, such as anisole and benzene in acetonitrile, with first‐order kinetics. In addition, the phenol products are formed from the aromatic substrates with similar first‐order rate constants. Combining the kinetic data obtained at different temperatures and under different single‐turnover experimental conditions with experiments performed under catalytic conditions by using the substrate [1,3,5‐D3]benzene, which showed normal kinetic isotope effects (KIE>1) and a notable hydride shift (NIH shift), has allowed us to clarify the role played by FeIII(OOH) in aromatic oxidation. Several lines of experimental evidence in support of the previously postulated mechanism for the formation of two caged FeIV(O) and OH . species from the FeIII(OOH) complex have been obtained for the first time. After homolytic O? O cleavage, a caged pair of oxidants [FeIVO+HO . ] is generated that act in unison to hydroxylate the aromatic ring: HO . attacks the ring to give a hydroxycyclohexadienyl radical, which is further oxidized by FeIVO to give a cationic intermediate that gives rise to a NIH shift upon ketonization before the final re‐aromatization step. Spin‐trapping experiments in the presence of 5,5‐dimethyl‐1‐pyrroline N‐oxide and GC‐MS analyses of the intermediate products further support the proposed mechanism. 相似文献
A bis(μ‐oxido)dinickel(III) complex was synthesized and characterized by single crystal X‐ray diffraction, resonance Raman, and ESI‐mass measurements. Magnetic susceptibility measurements by SQUID and EPR spectroscopy reveal that the complex has a triplet ground state, which is unprecedented for high‐valent metal (M) complexes with [M2(μ‐O)2] diamond core. DFT studies indicate ferromagnetic coupling of the nickel(III) centers. The complex exhibits hydrogen abstraction reactivity and oxygenation reactivity toward external substrates. 相似文献
The synthesis and crystal structure elucidation of a novel dinuclear heteroleptic copper(II) complex has led to an alternative mechanism in the formation of covalent hydrates. During further studies on the synthesis and properties of [Cu2(ophen)2] ( 1 ), a dinuclear complex of copper(I) with 1 H‐[1,10]‐phenanthrolin‐2‐one (Hophen), two intermediates/alternative products 2 and 3 were isolated. The dinuclear, antiferromagnetic complex [Cu2(ophen)2(phen)2] ? (NO3)2 ? 9H2O ( 3 , phen=1,10‐phenanthroline) contains two five‐coordinate copper(II) ions, both with trigonal‐bipyramidal coordination, which are bridged together through deprotonated hydroxyl groups with a Cu? Cu non‐bonding distance of 3.100 Å. Complex [Cu(phen)2(H2O)] ? (NO3)2 ( 2 ) is a polymorph of a previously reported material. The occurrence of 2 and 3 has led us to propose a variation to the Gillard mechanism for the formation of covalent hydrates in bidentate N‐heterocycles in which the attacking nucleophile may be the deprotonated form of 2 , [Cu(phen)2(OH)]?, rather than free OH?. 相似文献
The mechanism and enantioselectivity of the asymmetric Baeyer–Villiger oxidation reaction between 4‐phenylcyclohexanone and m‐chloroperoxobenzoic acid ( m ‐CPBA ) catalyzed by ScIII–N,N′‐dioxide complexes were investigated theoretically. The calculations indicated that the first step, corresponding to the addition of m ‐CPBA to the carbonyl group of 4‐phenylcyclohexanone, is the rate‐determining step (RDS) for all the pathways studied. The activation barrier of the RDS for the uncatalyzed reaction was predicted to be 189.8 kJ mol?1. The combination of an ScIII–N,N′‐dioxide complex and the m ‐CBA molecule can construct a bifunctional catalyst in which the Lewis acidic ScIII center activates the carbonyl group of 4‐phenylcyclohexanone while m ‐CBA transfers a proton, which lowers the activation barrier of the addition step (RDS) to 86.7 kJ mol?1. The repulsion between the m‐chlorophenyl group of m ‐CPBA and the 2,4,6‐iPr3C6H2 group of the N,N′‐dioxide ligand, as well as the steric hindrance between the phenyl group of 4‐phenylcyclohexanone and the amino acid skeleton of the N,N′‐dioxide ligand, play important roles in the control of the enantioselectivity. 相似文献
The synthesis of α‐substituted carbonyl compounds is of great importance due to their ubiquity in both natural and man‐made biologically active compounds. The field of hypervalent iodine chemistry has been a great contributor to access these molecules. For example, the α‐oxidation of carbonyl compounds has been one of the most investigated iodine(III)‐mediated stereoselective transformations. Yet, it is also the transformation that has met the most challenge in terms of achieving high stereoselectivities. The different mechanistic pathways of the iodine(III)‐mediated α‐tosyloxylation of ketones have been investigated. The calculations suggest an unprecedented iodine(III)‐promoted enolization process. Indications that iodonium intermediates could serve as proficient Lewis acids are reported. This concept could have broad impact and foster new developments in the field of hypervalent iodine chemistry. 相似文献
The kinetics and mechanisms of the copper(II)‐catalyzed GSH (glutathione) oxidation are examined in the light of its biological importance and in the use of blood and/or saliva samples for GSH monitoring. The rates of the free thiol consumption were measured spectrophotometrically by reaction with DTNB (5,5′‐dithiobis‐(2‐nitrobenzoic acid)), showing that GSH is not auto‐oxidized by oxygen in the absence of a catalyst. In the presence of Cu2+, reactions with two timescales were observed. The first step (short timescale) involves the fast formation of a copper–glutathione complex by the cysteine thiol. The second step (longer timescale) is the overall oxidation of GSH to GSSG (glutathione disulfide) catalyzed by copper(II). When the initial concentrations of GSH are at least threefold in excess of Cu2+, the rate law is deduced to be ?d[thiol]/dt=k[copper–glutathione complex][O2]0.5[H2O2]?0.5. The 0.5th reaction order with respect to O2 reveals a pre‐equilibrium prior to the rate‐determining step of the GSSG formation. In contrast to [Cu2+] and [O2], the rate of the reactions decreases with increasing concentrations of GSH. This inverse relationship is proposed to be a result of the competing formation of an inactive form of the copper–glutathione complex (binding to glutamic and/or glycine moieties). 相似文献
The right mix does the trick : Elusive {Ni0(η6‐arene)} moieties can be dramatically stabilized by the N‐heterocyclic silylene ligand 1 , which has a zwitterionic mesomeric structure. The σ, π‐acid–base synergism between nickel and 1 explains the unexpectedly high stability of the new silylene complexes 2 , which enables arene exchange studies at a Ni0 center. Addition of B(C6F5)3 to 2 affords the zwitterionic silylene complex 3 (see scheme, R=2,6‐iPr2C6H3).
Mononuclear metal–dioxygen species are key intermediates that are frequently observed in the catalytic cycles of dioxygen activation by metalloenzymes and their biomimetic compounds. In this work, a side‐on cobalt(III)–peroxo complex bearing a macrocyclic N‐tetramethylated cyclam (TMC) ligand, [CoIII(15‐TMC)(O2)]+, was synthesized and characterized with various spectroscopic methods. Upon protonation, this cobalt(III)–peroxo complex was cleanly converted into an end‐on cobalt(III)–hydroperoxo complex, [CoIII(15‐TMC)(OOH)]2+. The cobalt(III)–hydroperoxo complex was further converted to [CoIII(15‐TMC‐CH2‐O)]2+ by hydroxylation of a methyl group of the 15‐TMC ligand. Kinetic studies and 18O‐labeling experiments proposed that the aliphatic hydroxylation occurred via a CoIV–oxo (or CoIII–oxyl) species, which was formed by O? O bond homolysis of the cobalt(III)–hydroperoxo complex. In conclusion, we have shown the synthesis, structural and spectroscopic characterization, and reactivities of mononuclear cobalt complexes with peroxo, hydroperoxo, and oxo ligands. 相似文献
Recently, it was shown that μ‐oxo‐μ‐peroxodiiron(III) is converted to high‐spin μ‐oxodioxodiiron(IV) through O?O bond scission. Herein, the formation and high reactivity of the anti‐dioxo form of high‐spin μ‐oxodioxodiiron(IV) as the active oxidant are demonstrated on the basis of resonance Raman and electronic‐absorption spectral changes, detailed kinetic studies, DFT calculations, activation parameters, kinetic isotope effects (KIE), and catalytic oxidation of alkanes. Decay of μ‐oxodioxodiiron(IV) was greatly accelerated on addition of substrate. The reactivity order of substrates is toluene<ethylbenzene≈cumene<trans‐β‐methylstyrene. The rate constants increased proportionally to the substrate concentration at low substrate concentration. At high substrate concentration, however, the rate constants converge to the same value regardless of the kind of substrate. This is explained by a two‐step mechanism in which anti‐μ‐oxodioxodiiron(IV) is formed by syn‐to‐anti transformation of the syn‐dioxo form and reacts with substrates as the oxidant. The anti‐dioxo form is 620 times more reactive in the C?H bond cleavage of ethylbenzene than the most reactive diiron system reported so far. The KIE for the reaction with toluene/[D8]toluene is 95 at ?30 °C, which the largest in diiron systems reported so far. The present diiron complex efficiently catalyzes the oxidation of various alkanes with H2O2. 相似文献
The kinetics of the metal exchange reactions between open‐chain Gd(DTPA)2? and Gd(DTPA‐BMA), macrocyclic Gd(DOTA)? and Gd(HP‐DO3A) complexes, and Cu2+ ions were investigated in the presence of endogenous citrate, phosphate, carbonate and histidinate ligands in the pH range 6–8 in NaCl (0.15 M ) at 25 °C. The rates of the exchange reactions of Gd(DTPA)2? and Gd(DTPA‐BMA) are independent of the Cu2+ concentration in the presence of citrate and the reactions occur via the dissociation of Gd3+ complexes catalyzed by the citrate ions. The HCO3?/CO32? and H2PO4? ions also catalyze the dissociation of complexes. The rates of the dissociation of Gd(DTPA‐BMA), catalyzed by the endogenous ligands, are about two orders of magnitude higher than those of the Gd(DTPA)2?. In fact near to physiological conditions the bicarbonate and carbonate ions show the largest catalytic effect, that significantly increase the dissociation rate of Gd(DTPA‐BMA) and make the higher pH values (when the carbonate ion concentration is higher) a risk‐factor for the dissociation of complexes in body fluids. The exchange reactions of Gd(DOTA)? and Gd(HP‐DO3A) with Cu2+ occur through the proton assisted dissociation of complexes in the pH range 3.5–5 and the endogenous ligands do not affect the dissociation rates of complexes. More insights into the interaction scheme between Gd(DTPA‐BMA) and Gd(DTPA)2? and endogenous ligands have been obtained by acquiring the 13C NMR spectra of the corresponding diamagnetic Y(III)‐complexes, indicating the increase of the rates of the intramolecular rearrangements in the presence of carbonate and citrate ions. The herein reported results may have implications in the understanding of the etiology of nephrogenic systemic fibrosis, a rare disease that has been associated to the administration of Gd‐containing agents to patients with impaired renal function. 相似文献
Hydrothermal reactions of metal nitrates and ligand bis(5‐(pyridine‐2‐yl)‐1,2,4‐triazol‐3‐yl)methane (H2L1) gave three cluster compounds, {Cr2}, {Zn12} and {Fe8}. Notably, methylene group of H2L1 was in situ oxidized either to hydroxymethylated (L2‐O)3− in the metallo‐ring {Zn12} or to a rigid carbonylated (L3=O)2− in the screw‐type {Fe8}. In light of comparative experimental results, NO3− was deduced to be of a catalytic role in the ligand oxidation. Metal ion could be regarded as an “induced” tool for clusters generation in self‐assembly process. 相似文献
The catalytic activity of the N‐tailed (“biuret”) TAML (tetraamido macrocyclic ligand) activators [Fe{4‐XC6H3‐1,2‐( N COCMe2 N CO)2NR}Cl]2? ( 3 ; N atoms in boldface are coordinated to the central iron atom; the same nomenclature is used in for compounds 1 and 2 below), [X, R=H, Me ( a ); NO2, Me ( b ); H, Ph ( c )] in the oxidative bleaching of Orange II dye by H2O2 in aqueous solution is mechanistically compared with the previously investigated activator [Fe{4‐XC6H3‐1,2‐( N COCMe2 N CO)2CMe2}OH2]? ( 1 ) and the more aggressive analogue [Fe(Me2C{CON(1,2‐C6H3‐4‐X) N CO}2)OH2]? ( 2 ). Catalysis by 3 of the reaction between H2O2 and Orange II (S) occurs according to the rate law found generally for TAML activators (v=kIkII[FeIII][S][H2O2]/(kI[H2O2]+kII[S]) and the rate constants kI and kII at pH 7 both decrease within the series 3 b > 3 a > 3 c . The pH dependency of kI and kII was investigated for 3 a . As with all TAML activators studied to‐date, bell‐shaped profiles were found for both rate constants. For kI, the maximal activity was found at pH 10.7 marking it as having similar reactivity to 1 a . For kII, the broad bell pH profile exhibits a maximum at pH about 10.5. The condition kI?kII holds across the entire pH range studied. Activator 3 b exhibits pronounced activity in neutral to slightly basic aqueous solutions making it worthy of consideration on a technical performance basis for water treatment. The rate constants ki for suicidal inactivation of the active forms of complexes 3 a – c were calculated using the general formula ln([S0]/[S∞])=(kII/ki)[FeIII]; here [FeIII], [S0], and [S∞] are the total catalyst concentration and substrate concentration at time zero and infinity, respectively. The synthesis and X‐ray characterization of 3 c are also described. 相似文献