[Mn(tmc)(O₂)]⁺: A Side‐On Peroxido Manganese(III) Complex Bearing a Non‐Heme Ligand

Abstract

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 (

Reactivity of a cobalt(III)-peroxo complex in oxidative nucleophilic reactions

“…S3). It is worth noting that 2 does not react with triphenylposphine, thioanisole, and cyclooctene, as observed in the reactions of other metal-peroxo complexes [26,33,34]. This observation, combined with the result of Hammett study (vide infra), indicates that 2 has a nucleophilic character, as we have reported in the deformylation of aldehydes by [M III (TMC)(O 2 )] + (M = Fe for 2 Fe and Mn for 2 Mn ) [23,26].…”

“…S3). Product analysis of the final reaction mixture with GC/GC-MS and HPLC revealed the formation of cyclohexene ($70% based on 2) and acetophenone ($95% based on 2) in the reactions of CCA and 2-PPA, respectively, with the concurrent formation of formate [23,26]. Under the identical conditions, only trace amounts of products were formed in the absence of the cobalt complex.…”

“…It has been known that metal-peroxo species react with aldehydes to afford the corresponding deformylated products [6,7,23,26]. Kinetic studies of 2 with cyclohexanecarboxaldehyde (CCA) in CH 3 CN at 0°C show a first-order decay profile (Fig.…”

“…The nucleophilic character of cobalt(III)-peroxo species has also been demonstrated briefly in the reaction of benzoyl chloride [29]. As our ongoing [26] were prepared according to the literature methods.…”

“…, and a macrocyclic tetradentate TMC ligand, [Mn III (TMC)(O 2 )] + (2 Mn )[26]. The latter manganese(III)-peroxo intermediate has shown reactivities in aldehyde deformylation reactions, where cyclohexanecarboxaldehyde was deformylated to give cyclohexene as a product…”