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

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.

Distinct Oxygenation Difference between Manganese(IV) Hydroxo and Oxo Moieties: Electron Transfer versus Concerted Oxygen Transfer

“…Fujii and co-workers investigated the physical chemistry property differences among [ . [6] To the best of our knowledge, there is no reported example that addresses the oxygenation difference among reactive metal-hydroxo and -oxo moieties, even though metal-oxo moieties mediated oxygenations have been widely investigated. [7] Herein, we discuss the oxygenation of PPh 3 with the [Mn 6 ) 2 complex to address this issue.…”

“…[6] To the best of our knowledge, there is no reported example that addresses the oxygenation difference among reactive metal-hydroxo and -oxo moieties, even though metal-oxo moieties mediated oxygenations have been widely investigated. [7] Herein, we discuss the oxygenation of PPh 3 with the [Mn 6 ) 2 complex to address this issue. Although PPh 3 oxidation is not a particularly interesting or useful reaction in itself, it provides a platform to understand the oxygenation difference between the Mn IV ÀOH and Mn IV =O reactive groups (neither the Mn IV À OH nor the Mn IV =O group in this manganese(IV) complex is capable of epoxidizing olefins by itself [8] ).…”

“…These results further suggest that this manganese catalyst can perform versatile oxidation reactions, depending on specific substrates and reaction conditions. [5,8] The manganese(IV) complex, [Mn 6 ) 2 , was synthesized according to the literature. [9] As reported earlier, its first pK a value (6.86) is related to the removal of the first proton to…”

“…[6,9] Accordingly, two different manganese(IV) species could be identified by UV/ Vis spectroscopy under specific pH conditions in acetone/ water (ratio 4:1; Figure 1). At pH 4.0, the absorption of the manganese(IV) species at l max = 555 nm corresponds to [Mn ).…”

“…To distinguish the reactivity differences of M n + ÀOH and M n + =O moieties in oxidation reactions, one of us together with Busch investigated the hydrogen abstraction of Mn IV À OH and Mn IV =O moieties using a manganese(IV) complex having a dihydroxide ligand, [Mn IV 6 ) 2 (Me 2 EBC: 4,11-dimethyl-1,4,8,11-tetraazabicycloA C H T U N G T R E N N U N G [6.6.2]hexadecane). [5] It was found that the Mn 2 ] can react at a rate that is more than 40 times faster than the Mn…”