New mechanistic insights into intramolecular aromatic ligand hydroxylation and benzyl alcohol oxidation initiated by the well-defined (µ-peroxo)diiron(iii) complex.

A (µ-peroxo)diiron(iii) complex [Fe2(LPh4)(O2)(Ph3CCO2)]2+ (1-O2) with a dinucleating ligand (LPh4), generated from the reaction of a carboxylate bridged diiron(ii) complex [Fe2(LPh4)(Ph3CCO2)]2+ (1) with dioxygen in CH2Cl2, provides a diiron(iv)-oxo species as an active oxidant which is involved in either aromatic ligand hydroxylation or benzyl alcohol oxidation.

New mechanistic insight into intramolecular arene hydroxylation initiated by (µ-1,2-peroxo)diiron(III) complexes with dinucleating ligands.

(µ-1,2-Peroxo)diiron(iii) complexes (-R) with dinucleating ligands (R-L) generated from the reaction of bis(µ-hydroxo)diiron(ii) complexes [Fe2(R-L)(OH)2](2+) (-R) with dioxygen in acetone at -20 °C provide a diiron-centred electrophilic oxidant, presumably diiron(iv)-oxo species, which is involved in aromatic ligand hydroxylation.

Observation of a Cu(II)(2) (µ-1,2-peroxo)/Cu(III)(2) (µ-oxo)(2) equilibrium and its implications for copper-dioxygen reactivity.

Synthesis of small-molecule Cu2 O2 adducts has provided insight into the related biological systems and their reactivity patterns including the interconversion of the Cu(II) 2 (µ-η(2) :η(2) -peroxo) and Cu(III) 2 (µ-oxo)2 isomers. In this study, absorption spectroscopy, kinetics, and resonance Raman data show that the oxygenated product of [(BQPA)Cu(I) ](+) initially yields an "end-on peroxo" species, that subsequently converts to the thermodynamically more stable "bis-µ-oxo" isomer (Keq =3.2 at -90 °C). Calibration of density functional theory calculations to these experimental data suggest that the electrophilic reactivity previously ascribed to end-on peroxo species is in fact a result of an accessible bis-µ-oxo isomer, an electrophilic Cu2 O2 isomer in contrast to the nucleophilic reactivity of binuclear Cu(II) end-on peroxo species. This study is the first report of the interconversion of an end-on peroxo to bis-µ-oxo species in transition metal-dioxygen chemistry.

Aliphatic C-H bond activation initiated by a (mu-eta2:eta2-peroxo)dicopper(II) complex in comparison with cumylperoxyl radical.

A (mu-eta(2):eta(2)-peroxo)dicopper(II) complex, [Cu(2)(H-L)(O(2))](2+) (1-O(2)), supported by the dinucleating ligand 1,3-bis[bis(6-methyl-2-pyridylmethyl)aminomethyl]benzene (H-L) is capable of initiating C-H bond activation of a variety of external aliphatic substrates (SH(n)): 10-methyl-9,10-dihydroacridine (AcrH(2)), 1,4-cyclohexadiene (1,4-CHD), 9,10-dihydroanthracene (9,10-DHA), fluorene, tetralin, toluene, and tetrahydrofuran (THF), which have C-H bond dissociation energies (BDEs) ranging from approximately 75 kcal mol(-1) for 1,4-CHD to approximately 92 kcal mol(-1) for THF. Oxidation of SH(n) afforded a variety of oxidation products, such as dehydrogenation products (SH((n-2))), hydroxylated and further-oxidized products (SH((n-1))OH and SH((n-2))=O), dimers formed by coupling between substrates (H((n-1))S-SH((n-1))) and between substrate and H-L (H-L-SH((n-1))). Kinetic studies of the oxidation of the substrates initiated by 1-O(2) in acetone at -70 degrees C revealed that there is a linear correlation between the logarithms of the rate constants for oxidation of the C-H bonds of the substrates and their BDEs, except for THF. The combination of this correlation and the relatively large deuterium kinetic isotope effects (KIEs), k(2)(H)/k(2)(D) (13 for 9,10-DHA, approximately > 29 for toluene, and approximately 34 for THF at -70 degrees C and approximately 9 for AcrH(2) at -94 degrees C) indicates that H-atom transfer (HAT) from SH(n) (SD(n)) is the rate-determining step. Kinetic studies of the oxidation of SH(n) by cumylperoxyl radical showed a correlation similar to that observed for 1-O(2), indicating that the reactivity of 1-O(2) is similar to that of cumylperoxyl radical. Thus, 1-O(2) is capable of initiating a wide range of oxidation reactions, including oxidation of aliphatic C-H bonds having BDEs from approximately 75 to approximately 92 kcal mol(-1), hydroxylation of the m-xylyl linker of H-L, and epoxidation of styrene (Matsumoto, T.; et al. J. Am. Chem. Soc. 2006, 128, 3874).

Oxidation reactivity of bis(mu-oxo) dinickel(III) complexes: arene hydroxylation of the supporting ligand.

In the nick(el) of time: Bis(mu-oxo) dinickel(III) complexes 2 (see scheme), generated in the reaction of 1 with H(2)O(2), are capable of hydroxylating the xylyl linker of the supporting ligand to give 3. Kinetic studies reveal that hydroxylation proceeds by electrophilic aromatic substitution. The lower reactivity than the corresponding mu-eta(2):eta(2)-peroxo dicopper(II) complexes can be attributed to unfavorable entropy effects.

Reactions of copper(II)-H2O2 adducts supported by tridentate bis(2-pyridylmethyl)amine ligands: sensitivity to solvent and variations in ligand substitution.

The copper(II) complexes 1(H) and 1(Ar(X)), supported by the N,N-di(2-pyridylmethyl)benzylamine tridentate ligand (L(H)) or its derivatives having m-substituted phenyl group at the 6-position of pyridine donor groups (L(Ar(X))), have been prepared, and their reactivity toward H2O2 has been examined in detail at low temperature. Both copper(II) complexes exhibited a novel reactivity in acetone, giving 2-hydroxy-2-hydroperoxypropane (HHPP) adducts 2(H) and 2(Ar(X)), respectively. From 2(Ar(X)), an efficient aromatic ligand hydroxylation took place to give phenolate-copper(II) complexes 4(Ar(X)). Detailed spectroscopic and kinetic analyses have revealed that the reaction proceeds via an electrophilic aromatic substitution mechanism involving copper(II)-carbocation intermediates 3(Ar(X)). Theoretical studies at the density functional theory (DFT) level have strongly implicated conjugate acid/base catalysis in the O-O bond cleavage and C-O bond formation steps that take the peroxo intermediate 2(Ar(X)) to the carbocation intermediate 3(Ar(X)). In contrast to the 2(Ar(X)) cases, the HHPP-adduct 2(H) reacted to give a copper(II)-acetate complex [Cu(II)(L(H))(OAc)](ClO4) (6(H)), in which one of the oxygen atoms of the acetate co-ligand originated from H2O2. In this case, a mechanism involving a Baeyer-Villiger type 1,2-methyl shift from the HHPP-adduct and subsequent ester hydrolysis has been proposed on the basis of DFT calculations; conjugate acid/base catalysis is implicated in the 1,2-methyl shift process as well. In propionitrile, both 1(H) and 1(Ar(X)) afforded simple copper(II)-hydroperoxo complexes LCu(II)-OOH in the reaction with H2O2, demonstrating the significant solvent effect on the reaction between copper(II) complexes and H2O2.

Ligand effects on dioxygen activation by copper and nickel complexes: reactivity and intermediates.

Copper and nickel complexes having various active-oxygen species M n -O 2 ( n = 1 or 2), such as trans-(micro-1,2-peroxo)Cu (II) 2, bis(micro-oxo)M (III) 2, bis(micro-superoxo)Ni (II) 2, and ligand-based alkylperoxo-M (II) n , can be produced by a series of tetradentate tripodal ligands (TMPA analogues) containing sterically demanding 6-methyl substituent(s) on the pyridyl group(s), where TMPA = tris(2-pyridylmethyl)amine. Roles of the methyl substituent(s) for the formation of the active-oxygen species and their oxidation reactivities are reported.

Sequential reaction intermediates in aliphatic C-H bond functionalization initiated by a bis(mu-oxo)dinickel(III) complex.

The reaction of [Ni2(OH)2(Me2-tpa)2]2+ (1) (Me2-tpa = bis(6-methyl-2-pyridylmethyl)(2-pyridylmethyl)amine) with H2O2 causes oxidation of a methylene group on the Me2-tpa ligand to give an N-dealkylated ligand and oxidation of a methyl group to afford a ligand-based carboxylate and an alkoxide as the final oxidation products. A series of sequential reaction intermediates produced in the oxidation pathways, a bis(mu-oxo)dinickel(III) ([Ni2(O)2(Me2-tpa)2]2+ (2)), a bis(mu-superoxo)dinickel(II) ([Ni2(O2)2(Me2-tpa)2]2+ (3)), a (mu-hydroxo)(mu-alkylperoxo)dinickel(II) ([Ni2(OH)(Me2-tpa)(Me-tpa-CH2OO)]2+ (4)), and a bis(mu-alkylperoxo)dinickel(II) ([Ni2(Me-tpa-CH2OO)2]2+ (5)), was isolated and characterized by various physicochemical measurements including X-ray crystallography, and their oxidation pathways were investigated. Reaction of 1 with H2O2 in methanol at -40 degrees C generates 2, which is extremely reactive with H2O2, producing 3. Complex 2 was isolated only from disproportionation of the superoxo ligands in 3 in the absence of H2O2 at -40 degrees C. Thermal decomposition of 2 under N2 generated an N-dealkylated ligand Me-dpa ((6-methyl-2-pyridylmethyl)(2-pyridylmethyl)amine) and a ligand-coupling dimer (Me-tpa-CH2)2. The formation of (Me-tpa-CH2)2 suggests that a ligand-based radical Me-tpa-CH2* is generated as a reaction intermediate, probably produced by H-atom abstraction by the oxo group. An isotope-labeling experiment revealed that intramolecular coupling occurs for the formation of the coupling dimer. The results indicate that the rebound of oxygen to Me-tpa-CH2* is slower than that observed for various high-valence bis(mu-oxo)dimetal complexes. In contrast, the decomposition of 2 and 3 in the presence of O2 gave carboxylate and alkoxide ligands, respectively (Me-tpa-COO- and Me-tpa-CH2O-), instead of (Me-tpa-CH2)2, indicating that the reaction of Me-tpa-CH2* with O2 is faster than the coupling of Me-tpa-CH2* to generate ligand-based peroxyl radical Me-tpa-CH2OO*. Although there is a possibility that the Me-tpa-CH2OO* species could undergo various reactions, one of the possible reactive intermediates, 4, was isolated from the decomposition of 3 under O2 at -20 degrees C. The alkylperoxo ligands in 4 and 5 can be converted to a ligand-based aldehyde by either homolysis or heterolysis of the O-O bond, and disproportionation of the aldehyde gives a carboxylate and an alkoxide via the Cannizzaro reaction.

Intramolecular arene hydroxylation versus intermolecular olefin epoxidation by (mu-eta2:eta2-peroxo)dicopper(II) complex supported by dinucleating ligand.

A discrete (mu-eta2:eta2-peroxo)Cu(II)2 complex, [Cu2(O2)(H-L)]2+, is capable of performing not only intramolecular hydroxylation of a m-xylyl linker of a dinucleating ligand but also intermolecular epoxidation of styrene via electrophilic reaction to the C=C bond and hydroxylation of THF by H-atom abstraction.

Structure and dioxygen-reactivity of copper(I) complexes supported by bis(6-methylpyridin-2-ylmethyl)amine tridentate ligands.

The structure and dioxygen-reactivity of copper(I) complexes R supported by N,N-bis(6-methylpyridin-2-ylmethyl)amine tridentate ligands L2R[R (N-alkyl substituent)=-CH2Ph (Bn), -CH2CH2Ph (Phe) and -CH2CHPh2(PhePh)] have been examined and compared with those of copper(I) complex (Phe) of N,N-bis[2-(pyridin-2-yl)ethyl]amine tridentate ligand L1(Phe) and copper(I) complex (Phe) of N,N-bis(pyridin-2-ylmethyl)amine tridentate ligand L3(Phe). Copper(I) complexes (Phe) and (PhePh) exhibited a distorted trigonal pyramidal structure involving a d-pi interaction with an eta1-binding mode between the metal ion and one of the ortho-carbon atoms of the phenyl group of the N-alkyl substituent [-CH2CH2Ph (Phe) and -CH2CHPh2(PhePh)]. The strength of the d-pi interaction in (Phe) and (PhePh) was weaker than that of the d-pi interaction with an eta2-binding mode in (Phe) but stronger than that of the eta1 d-pi interaction in (Phe). Existence of a weak d-pi interaction in (Bn) in solution was also explored, but its binding mode was not clear. Redox potentials of the copper(I) complexes (E1/2) were also affected by the supporting ligand; the order of E1/2 was Phe>R>Phe. Thus, the order of electron-donor ability of the ligand is L1Phe

Synthesis and reactivity of a (mu-1,1-hydroperoxo)(mu-hydroxo)dicopper(II) complex: ligand hydroxylation by a bridging hydroperoxo ligand.

A new tetradentate tripodal ligand (L3) containing sterically bulky imidazolyl groups was synthesized, where L3 is tris(1-methyl-2-phenyl-4-imidazolylmethyl)amine. Reaction of a bis(mu-hydroxo)dicopper(II) complex, [Cu2(L3)2(OH)2]2+ (1), with H2O2 in acetonitrile at -40 degrees C generated a (mu-1,1-hydroperoxo)dicopper(II) complex [Cu2(L3)2(OOH)(OH)]2+ (2), which was characterized by various physicochemical measurements including X-ray crystallography. The crystal structure of 2 revealed that the complex cation has a Cu2(mu-1,1-OOH)(mu-OH) core and each copper has a square pyramidal structure having an N3O2 donor set with a weak ligation of a tertiary amine nitrogen in the apex. Consequently, one pendant arm of L3 in 2 is free from coordination, which produces a hydrophobic cavity around the Cu2(mu-1,1-OOH)(mu-OH) core. The hydrophobic cavity is preserved by hydrogen bondings between the hydroperoxide and the imidazole nitrogen of an uncoordinated pendant arm in one side and the hydroxide and the imidazole nitrogen of an uncoordinated pendant arm in the other side. The hydrophobic cavity significantly suppresses the H/D and 16O/18O exchange reactions in 2 compared to that in 1 and stabilizes the Cu2(mu-1,1-OOH)(mu-OH) core against decomposition. Decomposition of 2 in acetonitrile at 0 degrees C proceeded mainly via disproportionation of the hydroperoxo ligand and reduction of 2 to [Cu(L3)]+ by hydroperoxo ligand. In contrast, decomposition of a solid sample of 2 at 60 degrees C gave a complex having a hydroxylated ligand [Cu2(L3)(L3-OH)(OH)2]2+ (2-(L3-OH)) as a main product, where L3-OH is an oxidized ligand in which one of the methylene groups of the pendant arms is hydroxylated. ESI-TOF/MS measurement showed that complex 2-(L3-OH) is stable in acetonitrile at -40 degrees C, whereas warming 2-(L3-OH) at room temperature resulted in the N-dealkylation from L3-OH to give an N-dealkylated ligand, bis(1-methyl-2-phenyl-4-imidazolylmethyl)amine (L2) in approximately 80% yield based on 2, and 1-methyl-2-phenyl-4-formylimidazole (Phim-CHO). Isotope labeling experiments confirmed that the oxygen atom in both L3-OH and Phim-CHO come from OOH. This aliphatic hydroxylation performed by 2 is in marked contrast to the arene hydroxylation reported for some (mu-1,1-hydroperoxo)dicopper(II) complexes with a xylyl linker.

Reversible O-O bond cleavage and formation of a peroxo moiety of a peroxocarbonate ligand mediated by an iron(III) complex.

A mononuclear iron(III) complex containing a peroxocarbonate ligand, [Fe(qn)2(O2C(O)O)]- (qn = quinaldinate), underwent the reversible O-O bond cleavage and reformation of the peroxo group via the formation of FeIV=O or FeV=O species, which was confirmed by the resonance Raman and ESI-TOF/MS measurements.

Structural and spectroscopic characterization of (mu-hydroxo or mu-oxo)(mu-peroxo)diiron(III) complexes: models for peroxo intermediates of non-heme diiron proteins.

(mu-Hydroxo or oxo)(mu-1,2-peroxo)diiron(III) complexes having a tetradentate tripodal ligand (L) containing a carboxylate sidearm [Fe2(mu-OH or mu-O)(mu-O2)(L)2]n+ were synthesized as models for peroxo-intermediates of non-heme diiron proteins and characterized by various physicochemical measurements including X-ray analysis, which provide fundamental structural and spectroscopic insights into the peroxodiiron(III) complexes.