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The carboxylate adducts of OsO4 ([OsO4(X)]−, X− = AcO− and BzO−) were isolated and their structures, physicochemical properties and reactivity were explored. The C(sp3)−H oxygenation mechanism by the carboxylate adducts was discussed based on their kinetics and DFT studies.

A tin(II) complex coordinated by a sterically demanding o-phenylenediamido ligand is synthesized. The ligand is redox-active to reach a tin(II) complex with the diiminobenzosemiquinone radial anion in the oxidation by AgPF6. The tin(II) complex reacts with a series of nosylazides (x-NO2C6H4-SO2-N3; x = o, m, or p) at -30 °C to yield the corresponding nitrene radical bound tin(II) complexes. The nitrene radical complexes exhibit C(sp3)-H activation and amination reactivity.

The isolation, characterization, and dioxygen reactivity of monomeric [(TPA)M

The redox state of the metallomonooxygenases is finely tuned by imposing specific coordination environments on the metal center to reduce the activation energy for the generation of active-oxygen species and subsequent substrate oxygenation reactions. In this study, copper(II) complexes supported by a series of linear tetradentate ligands consisting of a rigid 6-, 7-, or 8-membered cyclic diamine with two pyridylmethyl (-CH2Py) side arms (L6Pym2, L7Pym2, and L8Pym2) are employed to examine the effects of the coordination environment on the reactivity of their acylperoxide adduct complexes. The UV-vis and electron paramagnetic resonance spectroscopic data indicate that the ligand-field splitting between the dx2-y2 and dz2 orbitals of the starting copper(II) complexes increase with an increase of the ring size of the diamine moiety (L6Pym2 → L7Pym2 → L8Pym2). In the reaction of these copper(II) complexes with m-chloroperbenzoic acid (m-CPBA), the L6Pym2 complex gives a stable m-CPBA adduct complex, whereas the L7Pym2 and L8Pym2 complexes are immediately converted to the corresponding m-chlorobenzoic acid (m-CBA) adducts, indicating that the reactivity of the copper(II) acylperoxide complexes largely depends on the coordination environment induced by the supporting ligands. Density functional theory (DFT) calculations on the m-CPBA adduct complexes show that the ligand-field-splitting energy increases with an increase of the ring size of the diamine moiety, as in the case of the starting copper(II) complexes, which enhances the reactivity of the m-CPBA adduct complexes. The reasons for such different reactivities of the m-CPBA adduct complexes are evaluated by using DFT calculations.

A cyclometalated osmium(III) complex, [OsIIICl(phpy)(tpy)]+ (OsIII(phpy); phpy = 2-C6H4-2′-C5H4N-κC,N) is synthesized from OsIIICl3(tpy) (tpy = 2,2′;6′,2″-terpyridine) and 2-phenylpyridine (H-phpy)...

Hydroxylation of cyclohexane with m-chloroperbenzoic acid was examined in the presence of an iron(III) complex supported by a trianionic planar tetradentate ligand. The present reaction system shows a high turnover number of 2750 with a high product selectivity of alcohol (93%). The turnover frequency was 0.51 s-1, and the second-order rate constant (k) for the C-H bond activation of cyclohexane was 1.08 M-1 s-1, which is one of the highest values among the iron complexes in the oxidation of cyclohexane so far reported. The present catalytic system can be adapted to the hydroxylation of substrates having only primary C-H bonds such as 2,2,3,3-tetramethylbutane as well as gaseous alkanes such as butane, propane, and ethane. The involvement of an iron(III) acyl peroxido complex as the reactive species was suggested by spectroscopic measurements of the reaction solution.

C(sp2)–H iodination of m-xylene promoted by a rhodium(III) complex supported by a redox-active ligand, (nBu4N)[RhIIIL3−(MeCN)Cl] (1Cl), is reported. The iodination reaction involves an electrophili...

The dinuclear copper enzyme, tyrosinase, activates O2 to form a (μ-η2 :η2 -peroxido)dicopper(II) species, which hydroxylates phenols to catechols. However, the exact mechanism of phenolase reaction in the catalytic site of tyrosinase is still under debate. We herein report the near atomic resolution X-ray crystal structures of the active tyrosinases with substrate l-tyrosine. At their catalytic sites, CuA moved toward l-tyrosine (CuA1 → CuA2), whose phenol oxygen directly coordinates to CuA2, involving the movement of CuB (CuB1 → CuB2). The crystal structures and spectroscopic analyses of the dioxygen-bound tyrosinases demonstrated that the peroxide ligand rotated, spontaneously weakening its O-O bond. Thus, the copper migration induced by the substrate-binding is accompanied by rearrangement of the bound peroxide species so as to provide one of the peroxide oxygen atoms with access to the phenol substrate's ϵ carbon atom.

Cupin superfamily proteins (TM1459) work as a macromolecular ligand framework with a double-stranded β-barrel structure ligating to a Cu ion through histidine side chains. Variegating the first coordination sphere of TM1459 revealed that H52A and H54A/H58A mutants effectively catalyzed the diastereo- and enantioselective Michael addition reaction of nitroalkanes to an α,β-unsaturated ketone. Moreover, calculated substrate docking signified C106N and F104W single-point mutations, which inverted the diastereoselectivity of H52A and further improved the stereoselectivity of H54A/H58A, respectively.

The rhodium (III) complexes supported by a di-deprotonated o-phenylenediamido ligand, ([Cp*RhIII(C6H4N2tBu2Ph22−)] ([Cp*RhIII(L2−)], 1), and one-electron and two-electron oxidized ligands, [Cp*RhII...

DFT calculations are carried out to investigate the geometric effects of the supporting ligands in the relative energies of the (μ–η2:η2-peroxido)CuIICuII complex 1 and the bis(μ-oxido)CuIIICuIII complex 2. The N3-tridentate ligand bearing acyclic propane diamine framework La preferentially provided 1, whereas the N3-tridentate ligand with cyclic diamine framework such as 1,4-diazacycloheptane Lb gave 2 after the oxygenation of the corresponding CuI complexes as reported previously [S. Itoh, et al., Inorg. Chem., 2014, 53, 8786–8794]. Calculations at the B3LYP*-D3 level of theory can reasonably explain the experimental results in relative energies, structures and harmonic frequencies of 1 and 2. Perturbational effects of the diamine chelates of La and Lb especially on the equilibrium of 1 and 2 are investigated in detail. In the range from 2.30 A to 3.40 A of the N–N distance in the diamine moiety, 1 is more stable than 2 by 8.4 kcal mol−1 at the distance of 3.40 A. Calculated potential energies indicate that the decrease in the N–N distance is associated with a decrease in energy of 2, leading that 2 can be most stabilized at the N–N distance of 2.60 A. Furthermore, molecular orbitals analyses are performed to explain that the energy gaps between the σ* orbital of the O–O bond and the dx2−y2 orbitals of the CuII ions of 1 get small as the diamine moiety is shrunk, leading to facilitate the O–O bond cleavage from 1 to 2.

The reaction of osmium tetroxide (OsO

A tin(II) complex coordinated by a sterically demanding

Mechanistic studies are performed on the alkane hydroxylation with m -CPBA ( m -chloroperbenzoic acid) catalyzed by nickel(II) complexes, Ni II (L). In the oxidation of cycloalkanes, Ni II (TPA) acts as an efficient catalyst with a high yield and a high alcohol selectivity. In the oxidation of adamantane, the tertiary carbon is predominantly oxidized. The reaction rate shows first-order dependence on [substrate] and [Ni II (L)] but is independent on [ m CPBA]; v obs = k 2 [substrate][ Ni II (L)]. The reaction exhibited a relatively large kinetic deuterium isotope effect ( KIE ) of 6.7, demonstrating that the hydrogen atom abstraction is involved in the rate-limiting step of the catalytic cycle. Furthermore, Ni II (L) supported by related tetradentate ligands exhibit apparently different catalytic activity, suggesting contribution of the Ni II (L) in the catalytic cycle. Based on the kinetic analysis and the significant effects of O 2 and CCl 4 on the product distribution pattern, possible contributions of (L)Ni II -O• and the acyloxyl radical as the reactive oxidants are discussed.

A tetrahedral CuII alkylperoxido complex [CuII (TMG3 tach)(OOCm)]+ (1OOCm ) (TMG3 tach={2,2',2''-[(1s,3s,5s)-cyclohexane-1,3,5-triyl]tris-(1,1,3,3-tetramethyl guanidine)}, OOCm=cumyl peroxide) is prepared and characterized by UV/Vis, cold-spray ionization mass spectroscopy (CSI-MS), resonance Raman, and EPR spectroscopic methods. Product analysis of the self-decomposition reaction of 1OOCm in acetonitrile (MeCN) indicates that the reaction involves O-O bond homolytic cleavage of the peroxide moiety with concomitant C-H bond activation of the solvent molecule. When an external substrate such as 1,4-cyclohexadiene (CHD) is added, the O-O bond homolysis leads to C-H activation of the substrate. Furthermore, the reaction of 1OOCm with 2,6-di-tert-butylphenol derivatives produces the corresponding phenoxyl radical species (ArO. ) together with a CuI complex through a concerted proton-electron transfer (CPET) mechanism. Details of the reaction mechanisms are explored by DFT calculations.

Oxido-iron(IV) porphyrin π-radical cation species are involved in a variety of heme-containing enzymes and have characteristic oxidation states consisting of a high-valent iron center and a π-conjugated macrocyclic ligand. However, the short lifetime of the complex has hampered detailed reactivity studies. Reported herein is a remarkable increase in the lifetime (80 s at 10 °C) of FeIV (TMP+. )(O)(Cl) (2; TMP=5,10,15,20-tetramesitylporphyrin dianion), produced by the oxidation of FeIII (TMP)(Cl) (1) by ozone in α,α,α-trifluorotoluene (TFT). The lifetime is 720 times longer compared to that of the currently most stable species reported to date. The increase in the lifetime improves the reaction efficiency of 2 toward inert alkane substrates, and allowed observation of the reaction of 2 with a primary C-H bond (BDEC-H =ca. 100 kcal mol-1 ) directly. Activation parameters for cyclohexane hydroxylation were also obtained.

Much recent attention has been focused on the structure and reactivity of transition-metal superoxide complexes, among which mononuclear copper(II)-superoxide complexes are recognized as key reactive intermediates in many biological and abiological dioxygen-activation processes. So far, several types of copper(II)-superoxide complexes have been developed and their electrophilic reactivity has been explored in C–H and O–H bond activation reactions. Here we demonstrate that a mononuclear copper(II)-(end-on)superoxide complex supported by a N-[(2-pyridyl)methyl]-1,5-diazacyclooctane tridentate ligand can induce catalytic C–C bond formation reaction between carbonyl compounds (substrate) and the solvent molecule (acetone), giving β-hydroxy-ketones (aldol). Kinetic and spectroscopic studies at low temperature as well as DFT calculation studies support a nucleophilic reactivity of the superoxide species toward the carbonyl compounds, providing new insights into the reactivity of transition-metal superoxide species not only in biological oxidation reactions but also in synthetic organic chemistry. Copper(II)-superoxide complexes are known to behave as electrophiles but there are few examples of nucleophilic reactivity by such species. Here a mononuclear copper(II)-superoxide species is shown to promote simple aldol reactions, and this reactivity is found to be sensitive to minor changes to the ligand structure.

Oxido-alcoholato- and oxido-thiolato-molybdenum(VI) complexes bearing two ene-1,2-dithiolate ligands (cyclohexene-1,2-dithiolate) are prepared as synthetic models of molybdenum(VI) reaction centers of dimethyl sulfoxide reductase family of molybdenum enzymes. These complexes are prepared by oxygen atom transfer from tertiary amine N-oxide (trimethylamine N-oxide and N,N-dimethylaniline N-oxide) to the five-coordinate alcoholato- and thiolato-molybdenum(IV) complexes, and are characterized by UV–vis, cold-spray-ionization mass, resonance Raman, and 1H NMR spectroscopies. The oxygen atom transfer reactions are studied kinetically at a low temperature (−40 °C) to demonstrate that the reactivity of the thiolato-molybdenum(IV) complex is higher than that of alcoholato-molybdenum(IV) complex by about 7 times, and that the oxygen atom transfer reactivity increases with increasing the electron withdrawing ability of the p-substituent of N,N-dimethylaniline N-oxide derivatives. Mechanistic details are discussed based on the reactivity studies.

Hydroxylation of cyclohexane with

This mini-review article summarizes the O2-reactivity of copper(I) complexes supported by a series of 2-(2-pyridyl)ethylamine (Pye) based ligands, which was developed by Karlin and his coworkers for biomimetic copper-oxygen chemistry. Oxygenation reaction of the copper(I) complexes supported by simple bis[2-(2-pyridyl)ethyl]amine tridentate ligands (R′Pye2R) mainly provided (μ-η2:η2-peroxido)dicopper(II) complex 5, but the reaction rates were significantly altered by the ligand substituents R and R′. The peroxide complex 5 induced aliphatic hydroxylation at the benzylic position of the ligand sidearm, when R = –CH2CH2Ph. Mechanistic studies on the aliphatic ligand hydroxylation revealed that the real reactive intermediate was a bis(μ-oxido)dicopper(III) speceis 6 generated in situ by the O–O bond homolysis of 5. On the other hand, peroxide complex 5 induced direct hydroxylation of phenolate derivatives to the corresponding catechols via electrophilic aromatic substitution mechanism, which can be regarded as a model reaction of tyrosinase. When the reaction of copper(I) complexes and O2 was conducted by using 2-(2-pyridyl)ethylamine didentate ligands (R″Pye1R,R′), not only the (μ-η2:η2-peroxido)dicopper(II) complex 5, but also the bis(μ-oxido)dicopper(III) speceis 6 and a mixed-valent bis(μ3-oxido)tricopper(II,II,III) complex 7 were generated, the product type of which was also controlled by the ligand substituents (R, R′, and R″). Reactivities of the generated complexes supported by the didentate ligands R″Pye1R,R′ were also explored. Notably, a tridentate ligand consisting with 2-(2-pyridyl)ethylamine (Pye1) and 8-membered cyclic diamine (RL8Pye) gave a mononuclear copper(II) endo-on superoxide complex 1, when the supported copper(I) complex and O2 was reacted at a low temperature. Such a unique ligand effect of the RL8Pye system were examined using a series of related tridentate ligands. Furthermore, asymmetric (μ-η1:η2-peroxido)dicopper(II) complex 8 was successfully obtained by using a newly designed penta-pyridine ligand (Pye2PyPye2).

Cupin superfamily proteins (TM1459) work as a macromolecular ligand framework with a double-stranded beta-barrel structure ligating to a Cu ion through histidine side chains. Variegating the first coordination sphere of TM1459 revealed that H52A and H54A/H58A mutants effectively catalyzed the diastereo- and enantio-selective Michael addition reaction of nitroalkanes to an α,β-unsaturated ketone. Moreover, in silico substrate docking signified C106N and F104W single-point mutations, which inverted the diastereoselectivity of H52A and further improved the stereoselectivity of H54A/H58A, respectively.

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.

Six-coordinate rhodium(III) complexes coordinated by a planar trianionic ligand (L3-) are synthesized. One of the axial positions is occupied by chloride (Cl-), bromide (Br-), or iodide (I-), and another axial position is coordinated by a solvent molecule such as acetonitrile (AN), water (H2O), tetrahydrofuran (THF), or pyridine (PY) to complete an octahedral rhodium(III) center; [RhIII(L3-)(X)(Y)]- (1X/Y; X = Cl-, Br-, or I-, Y = AN, H2O, THF, or PY). Coordination of the AN, H2O, and THF ligands to the metal center is rather weak, so that these solvent molecules are easily replaced by PY to give [RhIII(L3-)(Cl)(PY)]-. In the electrochemical measurements, all complexes have two reversible redox couples based on the ligand-centered oxidation L3- to L•2- and to L-, as reflected by the very similar redox potentials regardless of the different axial ligands. The rhodium(III) complexes catalyze C-H bond amination of xanthene with tosyl azide (TsN3). Because the yields of the aminated product are nearly the same among the complexes, replacement of the axial solvent ligands with TsN3 readily occurs to give a nitrene-radical-bound rhodium(III) complex, [RhIII(L•2-)(N•Ts)(X)]-, as an active oxidant, which is generated by one-electron transfer from the trianionic L3- to the nitrene nitrogen atom. Generation of such a diradical intermediate was substantiated by the direct reaction of 1Cl/AN with TsN3 in the absence of the substrate (xanthene). In this case, a rhodium(III) iminosemiquinone complex, 2, was generated by the intramolecular reaction between the nitrene-radical moiety and the radical moiety of the ligand L•2-.

To discuss the relationship between the structure and the oxygen reduction reaction (ORR) activity of pyridylmethylamine copper complexes containing carboxy groups, CuII complexes of a N,N-bis(2-pyridylmethyl)phenylmethylamine tridentate ligand (L1) or its carboxy derivatives (2-{[bis(2-pyridylmethyl)amino]methyl}benzoic acid: L2, 3-{[bis(2-pyridylmethyl)amino]methyl}benzoic acid: L3, or 4-{[bis(2-pyridylmethyl)amino]methyl}benzoic acid: L4) were synthesized and characterized. The X-ray crystal structures of mononuclear CuII complexes ligated by L2–L4 were obtained as dichloride complexes, and a crystallographic analysis revealed that all of them have a square pyramidal geometry. The ESR spectroscopy showed that these complexes also had a square pyramidal geometry in a neutral aqueous solution. Cyclic voltammetry analysis in a neutral aqueous solution suggested that only the CuII complex of L2 shows intramolecular interaction between the carboxy group and copper ion in the aqueous solution. The introduction of a carboxy group dramatically increased the amount of the CuII complexes adsorbed on carbon black, which can slightly increase the ORR activity.

Copper(II) complexes supported by N 3 -tridentate ligands, consisting of a rigid cyclic diamine (8-membered cyclic-diamine; L8 or 7-membered cyclic-diamine; L7 ) and a 2-(2-pyridyl)ethyl (-CH 2 CH 2 Py) group, were synthesized and structurally characterized. Reaction of the copper(II) complexes and cumene hydroperoxide (CmOOH) in the presence of triethylamine in CH 3 CN gave the corresponding cumylperoxide complexes L8 Cu II OOCm and L7 Cu II OOCm. The UV–vis and EPR spectra suggested that L8 Cu II OOCm takes a tetrahedrally distorted structure, whereas L7 Cu II OOCm has a planar geometry in solution. Resonance Raman spectra of these alkylperoxide complexes indicated that the O-O stretching vibration energy of L8 Cu II OOCm ( ν O–O = 878 cm − 1 ) is somewhat lower than that of L7 Cu II OOCm ( ν O–O = 881 cm − 1 ). Such a difference in O-O bond strength is reflected to the reactivity difference of these two alkylperoxide complexes. Namely, the reactivity L8 Cu II OOCm toward CHD (1,4-cyclohexadiene) as well as solvent molecule (CH 3 CN) is higher than that of L7 Cu II OOCm due to the weaker O-O bond of the former complex as compared to that of the latter complex. Geometric effects on the reactivity induced by the supporting ligands are discussed.

A new tridentate N3 ligand (TMG3tach) consisting of cis,cis-1,3,5-triaminocyclohexane (tach) and three N,N,N′,N′-tetramethylguanidino (TMG) groups has been developed to prepare copper complexes with a tetrahedral geometry and a labile coordination site. Treatment of the ligand with CuIIX2 (X = Cl and Br) gave copper(II)-halide complexes, [CuII(TMG3tach)Cl]+ (2Cl) and [CuII(TMG3tach)Br]+ (2Br), the structures of which have been determined by X-ray crystallographic analysis. The complexes exhibit a four-coordinate structure with C3v symmetry, where the labile halide ligand (X) occupies a position on the trigonal axis. 2Br was converted to a methoxido-copper(II) complex [CuII(TMG3tach)(OMe)](OTf) (2OMe), also having a similar four-coordinate geometry, by treating it with an equimolar amount of tetrabutylammonium hydroxide in methanol. The methoxido-complex 2OMe was further converted to the corresponding phenolato-copper(II) (2OAr) and thiophenolato-copper(II) (2SAr) complexes by ligand exchange reactions wit...

Thermally stable TM1459 cupin superfamily protein from Thermotoga maritima was repurposed as an osmium (Os) peroxygenase by metal-substitution strategy employing the metal-binding promiscuity. This novel artificial metalloenzyme bears a datively bound Os ion supported by the 4-histidine motif. The well-defined Os center is responsible for not only the catalytic activity but also the thermodynamic stability of the protein folding, leading to the robust biocatalyst (Tm ≈ 120 °C). The spectroscopic analysis and atomic resolution X-ray crystal structures of Os-bound TM1459 revealed two types of donor sets to Os center with octahedral coordination geometry. One includes trans-dioxide, OH, and mer-three histidine imidazoles (O3N3 donor set), whereas another one has four histidine imidazoles plus OH and water molecule in a cis position (O2N4 donor set). The Os-bound TM1459 having the latter donor set (O2N4 donor set) was evaluated as a peroxygenase, which was able to catalyze cis-dihydroxylation of several alken...

Werner type six-coordinate rhodium(iii) complexes coordinated by a planar trianionic ligand and two axial aniline ligands are synthesised. The trianionic ligand behaves as a redox-active ligand to form a ligand radical species upon one-electron oxidation of the complex. The rhodium(iii) complexes catalyse C-H amination of external substrates such as xanthene with tosylazide as the nitrene source. DFT-calculation and kinetic deuterium isotope effects indicate that a di-radical rhodium(iii) complex formed by one-electron transfer from the redox-active ligand to the nitrene group works as a reactive intermediate to induce aliphatic C-H activation.

To unveil the activation of dioxygen on the copper centre (Cu2O2core) of tyrosinase, we performed X-ray crystallograpy with active-form tyrosinase at near atomic resolution. This study provided a novel insight into the catalytic mechanism of the tyrosinase, including the rearrangement of copper-oxygen species as well as the intramolecular migration of copper ion induced by substrate-binding.