Quantum Mechanistic Studies of the Oxidation of Ethylene by Rhenium Oxo Complexes

Emmanuel Adu Fosu,Albert Aniagyei,Michael Kojo Ainooson,Collins Obuah,Louis Hamenu,Krishna K Govender,Marcelino Maneiro

doi:10.1155/2021/7931956

Abstract

Transition-metal-mediated oxygen transfer reactions are of importance in both industry and academia; thus, a series of rhenium oxo complexes of the type ReO3L (L = O−, Cl−, F−, OH−, Br−, I−) and their effects as oxidation catalysts on ethylene have been studied. The activation and reaction energies for the addition pathways involving multiple spin states (singlet and triplet) have been computed. In all cases, structures on the singlet potential energy surfaces showed higher stability compared to their counterparts on the triplet potential energy surfaces (PESs). Frontier Molecular Orbital calculations show electrons flow from the HOMO of ethylene to the LUMO of rhenium for all complexes studied except ReO4− where the reverse case occurs. In the reaction between ReO3L (L = O−, Cl−, F−, OH−, Br−, and I−) and ethylene, the concerted [3 + 2] addition pathway on the singlet PES leading to the formation of dioxylate intermediate is favored over the [2 + 2] addition pathway leading to the formation of a metallaoxetane intermediate and subsequent rearrangement to the dioxylate. The activation and the reaction energies for the formation of the dioxylate on the singlet PES for the ligands studied followed the order O− > OH− > I− > F− > Br− > Cl− and O− > OH− > F− > I− > Br− > Cl−, respectively. Furthermore, the activation and the reaction energies for the formation of the metallaoxetane intermediate increase in the order O− > OH− > I− > Br− > Cl− > F− and O− > Br− > I− > Cl− > OH− > F−, respectively. The subsequent rearrangement of the metallaoxetane intermediate to the dioxylate is only feasible in the case of ReO4−. Of all the complexes studied, the best dioxylating catalyst is ReO3Cl (singlet surface) and the best epoxidation catalyst is ReO3F (singlet surface).

Highlights

One of the primary goals in chemical research is to develop novel catalytic reactions that increase the selectivity and efficiency of chemical processes [1, 2]
Experimental and theoretical works over the past year Journal of Chemistry have shown the stability of epoxide formation via the catalyzed oxidation pathways by early transition metals such titanium, vanadium, and chromium [7,8,9] whereas oxo complexes such as ruthenium tetroxide [9], osmium tetroxide [4], and permanganate [10] tend to prefer cisdihydroxylate olefinic substrates [11]
Suggestions about the catalytic oxidation of the ethylene by OsO4 complex was that the addition mechanistic pathway is energetically favorable for the formation of dioxylate via a [3 + 2] insertion of the O Os O moiety across the olefinic bond which forms a dioxylate intermediate which has been experimentally characterized with its hydrolysis forming diols [6]

Summary

Introduction

One of the primary goals in chemical research is to develop novel catalytic reactions that increase the selectivity and efficiency of chemical processes [1, 2]. Aniagyei et al [16] concluded that the catalytic oxidation of ethylene by the ReO3L complex (L = OCH3, O−, CH3, NPH3 Cl−, and Cp) shows both kinetic and thermodynamic favorability on the [3 + 2] pathway leading to the formation of a metallacycle over the [2 + 2] mechanistic pathway which forms an oxetane intermediate before rearranging into a dioxylate intermediate on the singlet PES. It is evident from the profile diagrams (Figure S2) that an epoxide is formed through a rearrangement of the oxetane when the oxidation of ethylene is catalyzed by ReO3Cl. In all cases, there were no triplet transition states observed.

Results

Conclusion