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We have recently reported a copper(II)-superoxide complex supported by an N3-tridentate ligand (LN3), which exhibits a similar structure and reactivity to those of a putative reactive intermediate involved in the catalytic reactions of copper monooxygenases such as peptidylglycine α-hydroxylating monooxygenase (PHM) and dopamine β-monooxygenase (DβM). In this study, we have synthesised and characterised copper complexes supported by a related sulphur-containing ligand (LN2S) to get insight into the notable electronic effect of the sulphur donor atom in the reaction with cumene hydroperoxide, inducing efficient heterolytic O-O bond cleavage. [ABSTRACT FROM AUTHOR]

Copper(ii) complexes 1aand 1b, supported by tridentate ligand bpa [bis(2-pyridylmethyl)amine] and tetradentate ligand tpa [tris(2-pyridylmethyl)amine], respectively, react with cumene hydroperoxide (CmOOH) in the presence of triethylamine in CH3CN to provide the corresponding copper(ii) cumylperoxo complexes 2aand 2b, the formation of which has been confirmed by resonance Raman and ESI-MS analyses using 18O-labeled CmOOH. UV-vis and ESR spectra as well as DFT calculations indicate that 2ahas a 5-coordinate square-pyramidal structure involving CmOO−at an equatorial position and one solvent molecule at an axial position at low temperature (−90 °C), whereas a 4-coordinate square-planar structure that has lost the axial solvent ligand is predominant at higher temperatures (above 0 °C). Complex 2b, on the other hand, has a typical trigonal bipyramidal structure with the tripodal tetradentate tpa ligand, where the cumylperoxo ligand occupies an axial position. Both cumylperoxo copper(ii) complexes 2aand 2bare fairly stable at ambient temperature, but decompose at a higher temperature (60 °C) in CH3CN. Detailed product analyses and DFT studies indicate that the self-decomposition involves O–O bond homolytic cleavage of the peroxo moiety; concomitant hydrogen-atom abstraction from the solvent is partially involved. In the presence of 1,4-cyclohexadiene (CHD), the cumylperoxo complexes react smoothly at 30 °C to give benzene as one product. Detailed product analyses and DFT studies indicate that reaction with CHD involves concerted O–O bond homolytic cleavage and hydrogen-atom abstraction from the substrate, with the oxygen atom directly bonded to the copper(ii) ion (proximal oxygen) involved in the C–H bond activation step. [ABSTRACT FROM AUTHOR]

Electron transfer oxidation reaction of bis(dithiolene)monooxomolybdenum(iv) (MoIVOLx) complexes is studied as a model of oxidative-half reaction of arsenite oxidase molybdenum enzymes. The reactions are revealed to involve proton-coupled electron transfer. Electrochemical oxidation of MoIVOLxyields the corresponding bis(dithiolene)dioxomolybdenum(vi) complexes in basic solution, where the conversion of MoIVOLdmedsupported by a smaller electron donating dithiolene ligand (1,2-dicarbomethoxyethylene-1,2-dithiolate, Ldmed) to MoVIO2Ldmedis faster than that of MoIVOLbdtwith a larger electron donating dithiolene ligand (1,2-benzenedithiolate, Lbdt) under the same conditions. Titration experiments for the electrochemical oxidation reveal that the reaction involves two-electron oxidation and two equivalents of OH−consumption per MoIVOLx. In the conversion process of MoIVOLxto MoVIO2Lx, the five-coordinate bis(dithiolene)monooxomolybdenum(v) complex (MoVOLx) being a one-electron oxidized species of MoIVOLxis suggested to react with OH−. MoVOLxreacts with OH−in CH3CN or C2H5CN in a 2 : 2 ratio to give one equivalent MoIVOLxand one equivalent MoVIO2Lx, which is confirmed by the UV–vis and IR spectroscopies. The low temperature stopped-flow analysis allows investigations of the mechanism for the reaction of MoVOLxwith OH−. The kinetic study for the reaction of MoVOLdmedwith OH−suggests that MoVOLdmedreacts with OH−to give a six-coordinate oxo-hydroxo-molybdenum(v) species, MoVO(OH), and, then, the resulting species undergoes successive deprotonation by another OH−and oxidation by a remaining MoVOLdmedto yield the final products MoIVOLdmedand MoVIO2Ldmedcomplexes in a 1 : 1 ratio. In this case, the MoVO2species are involved as an intermediate in the reaction. On the other hand, in the reaction of MoVOLbdtwith OH−, coordination of OH−to the MoVcentre to give a six-coordinate MoVO(OH)Lbdtspecies becomes the rate limiting step and other intermediates are not suggested. On the basis of these results, the ligand effects of the dithiolene ligands on the reactivity of the bis(dithiolene)molybdenum complexes are discussed. [ABSTRACT FROM AUTHOR]

A series of (μ-η2:η2-disulfido)dinickel(II) complexes 2nhave been synthesized by the reaction of Na2S2and nickel(II) complexes 1nsupported by tris[(pyridin-2-yl)methyl]amine (TPA; 10) [(6-methylpyridin-2-yl)methyl]bis[(pyridin-2-yl)methyl]amine (Me1TPA; 11), bis[(6-methylpyridin-2-yl)methyl][(pyridin-2-yl)methyl]amine (Me2TPA; 12) and tris[(6-methylpyridin-2-yl)methyl]amine (Me3TPA; 13), respectively, and characterised by UV-vis and resonance Raman spectroscopy. X-ray crystallographic analyses on 22and 23supported by Me2TPA and Me3TPA, respectively, have revealed that the Ni2S2core is largely bent (∼30°) along the S–S axis, being in sharp contrast to the planar Ni2S2core structure of the (μ-η2:η2-disulfido)dinickel(II) complexes reported so far. The UV-vis spectra of 20and 21supported by TPA and Me1TPA, respectively, exhibiting an intense absorption band at ∼360 nm together with a shoulder around 400 nm and a weak and broad absorption band around 450–600 nm, are very close to those of 22and 23, suggesting that 20and 21also exhibit a similar distorted Ni2S2core structure. Resonance Raman spectra of 2nshowed a characteristic S–S stretching vibration mode at ∼450 cm−1with an isotope shift Δν(32S–34S) = 10–15 cm−1. The reaction of 2nwith (p-Me-C6H4)3P gave (p-Me-C6H4)3PS quantitatively based on 2n. Hammett analysis on the sulfur atom transfer process from 2nto the phosphine derivatives [(p-X-C6H4)P; X = OMe, Me, H and Cl] has indicated that the reaction involves an electrophilic ionic mechanism. Moreover, the order of reactivity of 2ntoward PPh3has been found as 20> 23> 21> 22. On the basis of these results, the ligand effects of MenTPA on the structure and reactivity of the nickel(II) complexes have been discussed. [ABSTRACT FROM AUTHOR]

Reaction of β-diketiminate copper(ii) complexes and Na2S2resulted in formation of (μ-η2:η2-disulfido)dicopper(ii) complexes (adduct formation) or β-diketiminate copper(i) complexes (reduction of copper(ii)) depending on the substituents of the supporting ligands. In the case of sterically less demanding ligands, adduct formation occurred to provide the (μ-η2:η2-disulfido)dicopper(ii) complexes, whereas reduction of copper(ii) took place to give the corresponding copper(i) complexes with sterically more demanding β-diketiminate ligands. Spectroscopic examinations of the reactions at low temperature using UV-vis and ESR as well as kinetic analysis have suggested that a 1 : 1 adduct LCuII–S–SNa with an end-on binding mode is initially formed as a common intermediate, from which different reaction pathways exist depending on the steric environment of the metal-coordination sphere provided by the ligands. Thus, with the sterically less demanding ligands, rearrangement of the disulfide adduct from end-on to side-on followed by self-dimerisation occurs to give the (μ-η2:η2-disulfido)dicopper(ii) complexes, whereas such an intramolecular rearrangement of the disulfide co-ligand does not take place with the sterically more demanding ligands. In this case, homolytic cleavage of the CuII–S bond occurs to give the reduced copper(i) product. The steric effects of the supporting ligands have been discussed on the basis of detailed analysis of the crystal structures of the copper(ii) starting materials. [ABSTRACT FROM AUTHOR]