The oxidative properties of a manganese(iv) hydroperoxide moiety and its relationships with the corresponding manganese(iv) oxo and hydroxo moieties

Abstract

Clear elucidation of the oxidative relationships of the active metal hydroperoxide moiety with its corresponding metal oxo and hydroxo intermediates would help the understanding of the different roles they may play in redox metalloenzymes and oxidation chemistry. Using an Mn(Me(2)EBC)Cl(2) complex, it was found that, in t-butanol-water (4 : 1) with excess H(2)O(2) at pH 1.5, the Mn(IV)-OOH moiety may exist in the catalytic solution with a mass signal of m/z = 358.1, which provides a particular chance to investigate its oxidative properties. In catalytic oxidations, the Mn(IV)-OOH moiety demonstrates a relatively poor activity in hydrogen abstraction from diphenyl methane and ethylbenzene with TOF of only 1.2 h(-1) and 1.1 h(-1) at 50 °C, whereas it can efficiently oxygenate diphenyl sulfide, methyl phenyl sulfide and benzyl phenyl sulfide with TOF of 13.8 h(-1), 15.4 h(-1) and 17.8 h(-1), respectively. In mechanistic studies using H(2)(18)O and H(2)(18)O(2), it was found that, in the Mn(IV)-OOH moiety mediated hydrogen abstraction and sulfide oxygenations, the reaction proceeds by two parallel pathways: one by direct oxygen insertion/transfer, and the other by plausible electron transfer. Together with a good understanding of the corresponding manganese(IV) oxo and hydroxo intermediates, this work provides the first chance to compare the reactivity differences and similarities of the active metal oxo, hydroxo and hydroperoxide intermediates. The available evidence reveals that the Mn(IV)-OOH moiety has a much more powerful oxidizing capability than the corresponding Mn(IV)=O and Mn(IV)-OH functional groups in both hydrogen abstraction and oxygenation.

Synthesis, Structural Studies, and Oxidation Catalysis of the Late-First-Row-Transition-Metal Complexes of a 2-Pyridylmethyl Pendant-Armed Ethylene Cross-Bridged Cyclam

“…Clearly, oxidizing Mn(PyMeEBC)Cl + to the corresponding manganese(IV) complex is not so easy as Mn(Me2EBC)Cl2 where the manganese(IV) complex can be obtained in large scale, and this may explain its relatively poor efficiency in sulfide oxygenation in which the manganese(IV) species may play significant roles. [12] In terms of utility, this selectivity for sulfoxide production may be advantageous. Organic sulfoxides are useful synthetically for the production of pharmaceuticals and other valuable chemical compounds.…”

“…[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] The manganese complex of 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane [15] (Me2EBC, Scheme 1) in particular, has a diverse and rich oxidation chemistry utilizing oxidation mechanisms ranging from hydrogen atom abstraction, electron transfer, concerted oxygen transfer, to the oxygen rebound mechanism. [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] This compound, which we propose to call "the Busch catalyst", was initially targeted as a potential oxidation catalyst because the rigid cross-bridged ligand could strongly bind the oxygen-reactive manganese ion and prevent it from being deactivated in the form of MnO2. [1] [2] [3] [4] Additional critical ligand properties are thought to be the two available cis labile coordination sites for oxidant and substrate interaction, the methyl groups sterically preventing dimerization which might deactivate the catalyst, and the saturated and all-tertiary nitrogen nature of the ligand, which minimizes the possibility of ligand oxidation and catalyst destruction.…”

Synthesis, structural studies, kinetic stability, and oxidation catalysis of the late first row transition metal complexes of 4,10-dimethyl-1,4,7,10-tetraazabicyclo[6.5.2]pentadecane

“…Complexation of Me 2 B13N4 with manganese and iron were carried out to produce novel complexes that may have interesting catalytic oxidation behavior, as do their Me 2 EBC [17][18][19][20][21][22][23][24][25][26][27][28][29] and Me 2 Bcyclen 34 analogues. Reaction of the ligand in acetonitrile with anhydrous MnCl 2 and FeCl 2 in an inert atmosphere glovebox gave good yields of the M(Me 2 B13N4)Cl 2 complexes.…”

“…Concurrent with the imaging applications of pendantarmed cross-bridged tetraazamacrocycles, Busch, with some of us, embraced the original dimethyl ligands Me 2 EBC and Me 2 Bcyclen as ideal chelates for manganese and iron for the purpose of oxidation catalysis in harsh aqueous media. [17][18][19][20][21][22][23][24][25][26][27][28][29][30] As rigidly bridged macrocycles, these small cryptands would provide complementary binding sites for these redox active metal ions, yet resist stepwise protic removal and deactivation of the potential catalysts as their oxide forms. The methyl versions of the ligands were actually preferred as they made all of the nitrogen atoms tertiary, and thus more resistant to ligand oxidation; did not chelate the metal ion, allowing two cis open sites on the metal ion for oxidant and substrate binding and activation; and even provided just enough steric bulk to prevent dimerization and inactivation of the catalysts as the oxo-bridged forms.…”

Synthesis, structural studies, and oxidation catalysis of the manganese(II), iron(II), and copper(II) complexes of a 2-pyridylmethyl pendant armed side-bridged cyclam

“…[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] The manganese complex of 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane [15] ( Me 2 EBC , Scheme 1) in particular, has a rich oxidation chemistry. [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] This compound, which we propose to call “the Busch catalyst”, was initially synthesized as an oxidation catalyst because the cross-bridged ligand could rigidly bind the oxygen-reactive manganese metal and stop it from being lost in the form of MnO 2 . [1] [2] [3] [4] Que has determined that the iron complex of Me 2 EBC is an efficient olefin epoxidation catalyst with H 2 O 2 oxidant under appropriate conditions as well.…”