1008763-40-3Relevant academic research and scientific papers
Unique properties and reactivity of high-valent manganese-Oxo versus manganese-hydroxo in the salen platform
Kurahashi, Takuya,Kikuchi, Akihiro,Shiro, Yoshitsugu,Hada, Masahiko,Fujii, Hiroshi
, p. 6664 - 6672 (2010/09/17)
To gain an understanding of oxidation reactions by MnIII(salen), a reaction of MnIII(salen) with m-chloroperoxybenzoic acid in the absence of a substrate Is investigated. UV-vis, perpendicular- and parallel-mode electron paramagnetic resonance, and X-ray absorption spectroscopy show that the resulting solution contains MnIv(salen)(O) as a major product and MnIv(salen)(OH) as a minor product. MnIv(salen)(O) readily reacts with 4-H-2,6-tert-Bu2C6H2OH (homolytic bond dissociation energy of an OH bond, BDEoH = 82.8 kcal mol-1), 4-CH3CO-2,6-tert-Bu2C6H 2OH (BDEoH = 83.1 kcal mol-1), and 4-NC-2,6-tert-Bu2C6H2OH (BDEOH = 84.2 kcal mol-1) at 203 K, following secondorder rate kinetics. MnIv(salen)(OH) reacts with 4-CH3CO-2,6-tert-Bu 2C6H2OH (BDEoH = 83.1 kcal mol -1) much more slowly under identical conditions than Mn Iv(salen)(O) and does not react with 4-NC-2,6-tert-Bu 2C6H2OH (BDEOH=84.2 kcal mol-1), suggesting that the thermodynamic hydrogen-atom-abstracting ability of Mn Iv(salen)(OH) is about 83 kcal mol-1. The rate constant for reactions of MnIv(salen)(OH) with phenols is not dependent on the concentration of phenols, suggesting that MnIv(salen)(OH) might bind phenols prior to the rate-limiting oxidation reactions. Quantum chemical calculations are carried out for MnIv(salen)(0) and Mn Iv(salen)(OH), both of which well reproduce the extended X-ray absorption fine structures as well as the electronic configurations. It is also indicated that protonation of MnIv(salen)(OH) induces a drastic electronic structural change from manganese(IV) phenolate to a manganese(III) phenoxyl radical, which is also consistent with the experimental observation.
Transient intermediates from Mn(salen) with sterically hindered mesityl groups: Interconversion between MnIV-phenolate and Mn III-phenoxyl radicals as an origin for unique reactivity
Kurahashi, Takuya,Kikuchi, Akihiro,Tosha, Takehiko,Shiro, Yoshitsugu,Kitagawa, Teizo,Fujii, Hiroshi
, p. 1674 - 1686 (2009/01/30)
In order to reveal structure-reactivity relationships for the high catalytic activity of the epoxidation catalyst Mn(salen), transient intermediates are investigated. Steric hindrance incorporated to the salen ligand enables highly selective generation of three related intermediates, O=MnIV(salen), HO-MnIV(salen), and H2O-Mn III(salen+?), each of which is thoroughly characterized using various spectroscopic techniques including UV-vis, electron paramagnetic resonance, resonance Raman, electrospray ionization mass spectrometry, 2H NMR, and X-ray absorption spectroscopy. These intermediates are all one-electron oxidized from the starting Mn III(salen) precursor but differ only in the degree of protonation. However, structural and electronic features are strikingly different: The Mn-O bond length of HO-MnIV(salen) (1.83 A) is considerably longer than that of O=MnIV(salen) (1.58 A); the electronic configuration of H2O-MnIII(salen+?) is MnIII-phenoxyl radical, while those of O=MnIV(salen) and HO-MnIV(salen) are MnIV-phenolate. Among O=Mn IV(salen), HO-MnIV(salen), and H2O-Mn III(salen+?), only the O=MnIV(salen) can transfer oxygen to phosphine and sulfide substrates, as well as abstract hydrogen from weak C-H bonds, although the oxidizing power is not enough to epoxidize olefins. The high activity of Mn(salen) is a direct consequence of the favored formation of the reactive O=MnIV(salen) state.
