Guochuan Yin

Oxidative Reactivity Difference among the Metal Oxo and Metal Hydroxo Moieties: pH Dependent Hydrogen Abstraction by a Manganese(IV) Complex Having Two Hydroxide Ligands

Clarifying the difference in redox reactivity between the metal oxo and metal hydroxo moieties for the same redox active metal ion in identical structures and oxidation states, that is, M(n+)O and M(n+)-OH, contributes to the understanding of natures choice between them (M(n+)O or M(n+)-OH) as key active intermediates in redox enzymes and electron transfer enzymes, and provides a basis for the design of synthetic oxidation catalysts. The newly synthesized manganese(IV) complex having two hydroxide ligands, [Mn(Me(2)EBC)(2)(OH)(2)](PF(6))(2), serves as the prototypic example to address this issue, by investigating the difference in the hydrogen abstracting abilities of the Mn(IV)O and Mn(IV)-OH functional groups. Independent thermodynamic evaluations of the O-H bond dissociation energies (BDE(OH)) for the corresponding reduction products, Mn(III)-OH and Mn(III)-OH(2), reveal very similar oxidizing power for Mn(IV)O and Mn(IV)-OH (83 vs 84.3 kcal/mol). Experimental tests showed that hydrogen abstraction proceeds at reasonable rates for substrates having BDE(CH) values less than 82 kcal/mol. That is, no detectable reaction occurred with diphenyl methane (BDE(CH) = 82 kcal/mol) for both manganese(IV) species. However, kinetic measurements for hydrogen abstraction showed that at pH 13.4, the dominant species Mn(Me(2)EBC)(2)(O)(2), having only Mn(IV)O groups, reacts more than 40 times faster than the Mn(IV)-OH unit in Mn(Me(2)EBC)(2)(OH)(2)(2+), the dominant reactant at pH 4.0. The activation parameters for hydrogen abstraction from 9,10-dihydroanthracene were determined for both manganese(IV) moieties: over the temperature range 288-318 K for Mn(IV)(OH)(2)(2+), DeltaH(double dagger) = 13.1 +/- 0.7 kcal/mol, and DeltaS(double dagger) = -35.0 +/- 2.2 cal K(-1) mol(-1); and the temperature range 288-308 K for for Mn(IV)(O)(2), DeltaH(double dagger) = 12.1 +/- 1.8 kcal/mol, and DeltaS(double dagger) = -30.3 +/- 5.9 cal K(-1) mol(-1).

Oxo- and Hydroxomanganese(IV) Adducts: A Comparative Spectroscopic and Computational Study

The electronic structures of the bis(hydroxo)manganese(IV) and oxohydroxomanganese(IV) complexes [Mn(IV)(OH)(2)(Me(2)EBC)](2+) and [Mn(IV)(O)(OH)(Me(2)EBC)](+) were probed using electronic absorption, magnetic circular dichroism (MCD), and variable-temperature, variable-field MCD spectroscopies. The d-d transitions of [Mn(IV)(OH)(2)(Me(2)EBC)](2+) were assigned using a group theory analysis coupled with the results of time-dependent density functional theory computations. These assignments permit the development of an experimentally validated description for the pi and sigma interactions in this complex. A similar analysis performed for [Mn(IV)(O)(OH)(Me(2)EBC)](+) reveals that there is a significant increase in the ligand character in the Mn pi* orbitals for the Mn(IV)=O complex relative to the bis(hydroxo)manganese(IV) complex, whereas the compositions of the Mn sigma* orbitals are less affected. Because of the steric features of the Me(2)EBC ligand, we propose that H-atom transfer by these reagents proceeds via the sigma* orbitals, which, because of their similar compositions among these two compounds, leads to modest rate enhancements for the Mn(IV)=O versus Mn(IV)OH species.

The mechanic study of the Pd-catalyzed synthesis of diphenylcarbonate with heteropolyacid as a cocatalyst

The reaction to synthesize diphenyl carbonate (DPC) by an oxidative carbonylation of phenol with CO and O2 has been found to proceed through the second-order of phenol concentration. The activation energy Ea, ΔS and ΔH are 27.0 kcal mol−1, −6.43 cal mol−1 and 26.3 kcal mol−1, respectively. The kinetic and additive data obtained agree with the proposed mechanism as follows: Pd(OAc)2 reacts with an ammonium phenoxy salt to give AcOue5f8Pdue5f8OPh which then reacts with CO to form AcOue5f8Pdue5f8COOPh. This species leads to PhOue5f8Pdue5f8COOPh which undergoes reductive elimination to give DPC and Pd(0). This Pd(0) is reoxidized to Pd(II) by the help of a heteropolyacid very effectively.

Similarities and differences in properties and behavior of two H2O2-activated manganese catalysts having structures differing only by methyl and ethyl substituents

The complex [Mn(IV)(Me2EBC)(OH)2](PF6)2, in which Me2EBC is 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane, is a remarkably selective H2O2 oxidation catalyst that has been shown to be useful in removing stains from fabrics without affecting their colors. Mn(IV) is the highest oxidation state detected and the dihydroxo complex forms a peroxyhydroxy derivative that is responsible for catalytic oxidations. Study of the diethyl homolog of this catalyst has revealed surprising differences in chemical behavior. Oxidation of this new manganese complex, Mn(Et2EBC)Cl2, using aqueous H2O2, at −30°C following removal of chloride ion, yields [Mn(Et2EBC)(OH)2](PF6)2. Above 0°C, H2O2 oxidation of Mn(Et2EBC)Cl2 oxidizes the ethyl substituents. X-ray structure determinations of Et2EBC complexes with Mn(II), Mn(III), and Mn(IV) are reported. The complex [Mn(Et2EBC)(OH)2](PF6)2 displays a surprisingly mild oxidizing potential of +0.556 V for the Mn4+/Mn3+ couple; however, its hydrogen abstraction ability for selected substrates is limited by the BDECH value of 82u2009kcalu2009mol−1, the same as reported for [Mn(Me2EBC)(OH)2](PF6)2. However, unlike the methyl derivative, electrochemical results indicate a 5+/4+ couple, in addition to the expected 4+/3+ and 3+/2+ couples. The significance of these differences in behavior is discussed. Mass spectral studies have identified some products of ethyl group oxidations.

Lewis Acid Catalyzed Epoxidation of Olefins Using Hydrogen Peroxide: Growing Prominence and Expanding Range

Publisher Summary Among the many methods for synthesizing epoxides, Lewis acid catalyzed olefin epoxidation provides a special combination of capabilities including gentle and green processing and selectivity of multiple kinds. The subject has contributed both to the understanding of important basic science and a large and growing inventory of impressive oxidation reactions. Some big challenges have been met by Lewis acid catalysts, but it has yet to provide the underlying science in a major industrial process. Historically, the cost of hydrogen peroxide has been a limitation, but recent developments, especially the on-site and/or in situ generation of H 2 O 2 has been demonstrated for promising alternative processes, especially in the light olefin epoxidation industry. For commodity chemicals, costs and environmental issues are accompanied by other related challenges such as catalyst durability and productivity that thwart otherwise elegant processes. The case of the long known and arguably unique catalyst methyltrioxorhenium for light olefin oxidation is explored in this context. Attention is also directed to the growing realization that, in combination with the right ligands, late transition metal elements, including manganese and iron also catalyze olefin epoxidation reactions. Ordinarily manganese and iron perform epoxidations by the oxygen rebound mechanism of Groves, and engage in one-electron redox chemistry. To produce only epoxides in reacting with many common olefins, the activated catalyst must not abstract hydrogen atoms that would open radical routes to other products. Ligand design has produced highly selective Mn(IV) catalysts capable of converting many olefins into their oxides but limited in their ability to initiate radical processes by hydrogen abstraction.

Manganese complexes with a lengthy o -xylylene cross-bridged cyclam ligand: synthesis, characterization and catalytic hydrogen abstraction by dioxygen activation

Two ultra rigid, o-xylylene cross-bridged macrobicyclic ligands, 1,10,13,19-tetraazatricyclo[8.6.6.03,8]docosa-3,5,7-triene (H2XBC), and 13,19-dimethyl-1,10,13,19-tetraazatricyclo[8.6.6.03,8]docosa-3,5,7-triene (Me2XBC), have been synthesized and the manganese complexes have been synthesized and characterized, including an X-ray structure determination. Mn(Me2XBC)Cl2 displays a relatively high redox potential for the Mn2+/Mn3+ couple (+0.947V vs SHE, measured in CH3CN), suggesting that the manganese(III) complex may be capable of hydrogen abstraction from moderately active substrates. Direct reaction of the freshly synthesized manganese(III) complex, [Mn(Me2XBC)Cl2]PF6, with 1,4-cyclohexadiene confirmed its hydrogen abstracting ability. The manganese(II)/Me2XBC complex is activated by dioxygen in buffered basic aqueous solutions and catalyzes hydrogen abstraction from selected substrates. A possible mechanism for this manganese complex catalyzed dioxygen activation and hydrogen abstraction is proposed.