Theoretical study on selective oxidation of olefin and alcohol with Mo–peroxo amine complex using “paired interacting orbitals (PIOs)” analysis

• Published in: Journal of molecular catalysis. A, Chemical Vol. 291; no. 1; pp. 38 - 48
• Main Authors: Shiga, Akinobu, Kurusu, Yasuhiko
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 29.08.2008; Elsevier
• Subjects: Alcohol; Catalysis; Chemistry; Exact sciences and technology; General and physical chemistry; Mo–peroxo complex; Olefin; Oxidation mechanism; Paired interacting orbitals (PIOs) analysis; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; Alcohol; Paired interacting orbitals (PIOs) analysis; Olefin; Oxidation mechanism; Mo–peroxo complex; Transition element complexes; Ethylene; Theoretical study; Mo-peroxo complex; Mechanism; Hydrogen abstraction; Peroxo complex; Amine; Energy; Alkyl; Transition state; Alkanol; Epoxidation; Molybdenum complex; Oxidation; Ethylenic compound; Orbital; Electrons; Methanol
• ISSN: 1381-1169; 1873-314X
• DOI: 10.1016/j.molcata.2008.05.012

Oxidation mechanisms of ethylene and methanol on Mo(O)(OO) 2 (an amine) and the influence of amines on the oxidation are studied by using paired interacting orbital (PIO) analysis of the transition states (TS) of the ethylene epoxidation and of the methanol coordinated complexes, the methoxy complexes, and the hydrogen abstraction of the methanol oxidation. In the case of ethylene oxidation, we are able to recognize three important PIOs: PIO-1 which expresses the electron delocalization from the π-orbital of the ethylene to the σ*-orbital(O 3–O 4) of the Mo peroxo; PIO-2 which expresses the electron delocalization from the p-orbital of O 4 of the Mo peroxo to the π*-orbital of the ethylene; and PIO-3 which expresses the overlap repulsion between the ethylene and occupied (Mo–O 3–O 4) orbitals of the Mo peroxo. This overlap repulsion is enhanced by the coordination of higher alkyl amines to the Mo peroxo, because of the destabilization of these occupied (Mo–O 3–O 4) orbitals. In the case of methanol oxidation, the main components of the PIO-1 of the methanol coordinated complexes and the methoxy complexes are Mo-4d orbitals of the unoccupied orbitals of the Mo peroxo part, and the main components of the PIO-1 of the hydrogen abstraction are O 5-2p, O 6-2p and C-2p orbitals of SOMO of the Mo peroxo part. The energy of the MOs, that contain these Mo-4d, O-2p or C-2p orbitals are not influenced by the amine coordination on the Mo atom. Results suggest that ethylene epoxidation is hindered by the coordination of higher alkyl amines on Mo peroxo complexes, whereas methanol oxidation is not hindered by the coordination of the amines. Oxidation mechanisms of ethylene and methanol on Mo(O)(OO) 2 (an amine) are studied. In the case of ethylene oxidation PIO analysis reveals that the electron delocalization from the π-orbital of the ethylene to the σ*-orbital (O 3–O 4) of the Mo peroxo (the so called π-donation of ethylene), the electron delocalization from the p-orbital of O 4 of the Mo peroxo to the π*-orbital of the ethylene and the overlap repulsion between occupied orbitals of the ethylene and occupied (Mo–O 3–O 4) orbitals of the Mo peroxo. This overlap repulsion is enhanced by the coordination of higher alkyl amines to the Mo peroxo, because of the destabilization of these occupied (Mo–O 3–O 4) orbitals. So, the results suggest that ethylene epoxidation is hindered by the coordination of higher alkyl amines. On the contrary, in the case of methanol oxidation, methanol coordination to the Mo peroxo moiety, Mo–methoxy complex formation and hydrogen abstraction which are elementary reactions of methanol oxidation, are not hindered by the coordination of amines. This is one reason why a selective oxidation of alcohols is attained by addition of higher alkyl amines to the oxidation system. ▪