Transient Intermediates from Mn(salen) with Sterically Hindered Mesityl Groups: Interconversion between MnIV-Phenolate and MnIII-Phenoxyl Radicals as an Origin for Unique Reactivity

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Abstract

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, OMnIV(salen), HO-Mn IV(salen), and H2O-MnIII(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 MnIII(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 OMnIV(salen) (1.58 A); the electronic configuration of H2O-MnIII(salen (+*)) is MnIII-phenoxyl radical, while those of OMnIV(salen) and HO-MnIV(salen) are MnIV-phenolate. Among OMnIV(salen), HO-MnIV(salen), and H2O-MnIII(salen (+*)), only the OMnIV(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 OMnIV(salen) state.

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