Metal-O 2 adducts, such as metal-superoxo and -peroxo species, are key intermediates often detected in the catalytic cycles of dioxygen activation by metalloenzymes and biomimetic compounds. The synthesis and spectroscopic characterization of an end-on nickel(II)-superoxo complex with a 14-membered macrocyclic ligand was reported previously. Here we report the isolation, spectroscopic characterization, and high-resolution crystal structure of a mononuclear side-on nickel(III)-peroxo complex with a 12-membered macrocyclic ligand, [Ni(12-TMC)(O 2 )] + (1) 4,7,4,7,. Different from the end-on Ni(II)-superoxo complex, the Ni(III)-peroxo complex is not reactive in electrophilic reactions, but is capable of conducting nucleophilic reactions. The Ni(III)-peroxo complex transfers the bound dioxygen to manganese(II) complexes, thus affording the corresponding nickel(II) and manganese(III)-peroxo complexes. The present results demonstrate the significance of supporting ligands in tuning the geometric and electronic structures and reactivities of metal-O 2 intermediates that have been shown to have biological as well as synthetic usefulness in biomimetic reactions.Metalloenzymes activate dioxygen to carry out a variety of biological reactions including biotransformation of naturally occurring molecules, oxidative metabolism of xenobiotics, and oxidative phosphorylation. One goal in biomimetic research is to understand the mechanistic details of dioxygen activation and oxygenation reactions and the structures of reactive intermediates occurring at the active sites of the metalloenzymes 1 . In the unified mechanism of dioxygen activation, dioxygen first binds to a reduced metal center that forms metal-superoxo and -peroxo intermediates, followed by O-O bond cleavage leading to the formation of high-valent metal-oxo species that are believed to carry out substrate oxidations 1 . Among the metal-oxygen intermediates, mononuclear metal-O 2 adducts, such * Corresponding authors: wwnam@ewha.ac.kr, edward.solomon@stanford.edu. Correspondence and requests for materials should be addressed to W.N. Author contributions: J.C., E.I.S., and W.N. conceived and designed the experiments; J.C., R.S., J.A., S.Y.K., and M.K. performed the experiments; J.C., R.S., J.A., M.K., and T.O. analyzed the data; J.C., R.S., E.I.S., and W.N. co-wrote the paper. 25 . We now report for the first time the synthesis, spectroscopic and electronic properties, and crystal structure of a mononuclear side-on (η 2 ) nickel(III)-peroxo complex stabilized by a 12-membered macrocyclic ligand, [Ni(III)(12-TMC)(O 2 )] + (1) (12-TMC = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane). The reactivities of the Ni(III)-peroxo complex in electrophilic and nucleophilic reactions and peroxo group transfer to other metal complexes have been discussed as well. Results and discussionThe starting nickel complex, [Ni(12-TMC)(CH 3 CN)] 2+ (3), was synthesized and characterized with UV-vis absorption spectroscopy, electrospray ionization mass spectrometry (ESI MS), and X...

The chemistry of metal-O 2 complexes has attracted much interest in the biological and bioinorganic chemistry communities, as such species are generated as key intermediates in the activation of dioxygen by metalloenzymes and corresponding model compounds. [1] In biomimetic studies, a number of metal-O 2 adducts have been synthesized and characterized with various spectroscopic methods, and their reactivities in the oxidation of organic substrates have been extensively investigated.[2] For example, peroxidoiron(III) complexes with heme and non-heme ligands have been synthesized as chemical models of cytochrome P450 aromatase and Rieske dioxygenases, and their reactivities have been demonstrated in various nucleophilic reactions, such as aldehyde deformylation. [3, 4] Peroxidocopper(III) and -nickel-(III) complexes have been synthesized and characterized recently, but their reactivities have not been well established in oxidative nucleophilic reactions of organic substrates. [2d,e, 5] Peroxidomanganese(III) complexes are also invoked as reactive intermediates in the reactions of Mn-containing enzymes, such as manganese superoxide dismutase, catalase, and the oxygen-evolving complex of photosystem II. [6] In biomimetic studies, a number of Mn-peroxido complexes have been synthesized and characterized with a variety of spectroscopic methods including X-ray crystallography. A notable example is the first X-ray crystal structure of a side-on peroxido manganese(III) porphyrin complex ([Mn III -(tpp)(O 2 )] À ; tpp = meso-tetraphenylporphyrin), reported by Valentine and co-workers.[7] The second crystal structure of a monomeric side-on peroxido manganese(III) complex bearing a non-porphyrinic ligand was reported by Kitajima et al. [8] However, reactivities of the peroxidomanganese(III) complexes have been rarely investigated in oxidation reactions. In the present work, we synthesized a peroxidomanganese (

The dark side of the Mn: A manganese(III) complex bearing a 13-membered macrocyclic ligand (1, see picture) binds a peroxo ligand in a side-on eta(2) fashion. The reactivity of 1 is influenced by the introduction of anionic ligands trans to the peroxo group. Electronic and structural changes upon trans-ligand binding explain the increased nucleophilicity of the resulting complexes 1-X.

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