New Insights into the Mechanism of Hydroperoxide Activation by Investigation of Dynamic Processes in the Coordination Sphere of Seven-Coordinated Molybdenum Peroxo Complexes

Abstract

Seven‐coordinated molybdenum oxobisperoxo complexes with chelate nitrogen donors like pyrazolylpyridines are catalysts for the epoxidation of olefins. An NMR spectroscopic and quantumchemical study on the fluxionality of the chelate ligand proves that during this process partial ligand dissociation takes place. This gave rise to a detailed theoretical study on the activation of CH3OOH at the model complex (NH3)2MoO(O2)2 including dissociation of one of the ammonia ligands and proton transfer from the hydroperoxide to one of the peroxo ligands.

Effect of an Ionic Liquid on the Catalytic Performance of Thiocyanatodioxomolybdenum(VI) Complexes for the Oxidation of Cyclooctene and Benzyl Alcohol

“…These seven-coordinate molybdenum peroxo complexes are active catalysts for the epoxidation of olefins with TBHP, especially when L is a neutral bidentate N,N-ligand such as a substituted pyrazolylpyridine [40,43,44]. Mechanistic studies have indicated that the oxidising agent coordinates to the Mo VI centre, undergoes a proton transfer to one of the peroxo ligands and is activated for oxygen transfer by g 2 -coordination [45]. A catalytic run carried out with the solid 2* under conditions equivalent to those used for the fresh catalyst precursor 2 (Table 2) revealed a similar performance for the oxidation of cyclooctene.…”

Olefin Epoxidation with Transition Metal η2-Peroxo Complexes: The Control of Reactivity

“…Due to the coordination of the oxidant, such as H 2 O 2 or tBuOOH, to the metal center and due to the high activation barriers for the stabilized complexes, these catalytic processes must involve different species formed in subsequent reaction steps. Thiel and co-workers recently gave experimental [13] and theoretical [17] evidence for a potential proton transfer from an alkyl hydroperoxide ligand ROOH that is coordinated to the metal center, to an η 2 -peroxo group of [MoO(η 2 -O 2 ) 2 (NH 3 )(ROOH)]. During intramolecular proton transfer, the metalladioxirane moiety M(O 2 ) is opened and species of the type [MoO(η 2 -O 2 )(η 1 -OOH)(NH 3 )(η 1 -OOR)] are formed.…”

“…[16] Note that, in the catalytic systems, the coordination of the oxidant such as hydrogen peroxide, silyl peroxides, or tert-butyl hydroperoxide to the metal center and subsequent proton transfer can yield different species which are potentially more reactive than the parent η 2 -peroxo complex. [10,13,17,21] Unless otherwise Scheme 3. Concepts for the control of reactivity of olefin oxidation with Mimoun-type peroxo complexes stated, transfer of the peroxo oxygen atom, which is located trans to the phosphane oxide, has been considered.…”

Transition Metal Peroxides. Synthesis and Role in Oxidation Reactions*Dedicated, on the occasion of his 80th birthday, to our ‘vitae ac studiorum magister in secunda et in adversa fortuna’ Emeritus Professor Giorgio Modena, Padova University, who showed us the way toward Chemistry.

“…On the basis of kinetic, spectroscopic and theoretical studies, the authors provided a mechanism, whose key steps are sketched in Scheme 12. In this context a major role appears to be played by the fluxionality of the particular ligands used 157 .…”