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The reaction of bis(thioether) complexes of (octaethylporphyrinato)ruthenium(II) with dioxygen, and the catalyzed O₂-oxidation of thioethers

University of British Columbia

This thesis describes a detailed study of the reactivity of Ru(OEP)(RR'S)₂ complexes (where OEP ≋ the dianion of 2,3,7,8,12,13,17,18-octaethylporphyrin, R ≋ methyl, ethyl or decyl, and R' ≋ methyl or ethyl) with 0₂, in various solvents. Exposure to O₂ (or air) of a benzene, toluene or methylene chloride solution containing PhCOOH and Ru(OEP)(RR’S)₂ at ambient conditions, results in the selective oxidation of the axial ligand(s) on the metalloporphyrin complex to the corresponding sulfoxide(s). For a CD₂Cl₂ solution containing ≈ 20 mM of Ru(OEP)(dms)₂ (dms ≋ dimethylsulfide) and ≈ 12 mM PhCOOH, exposed to 1 atm of O₂ at room temperature, ¹H-nmr analysis shows that most of the Ru(OEP)(dms)₂ has oxidized to Ru(OEP)(dmso)₂ over a period of 35h. Three detected intermediates are identified as Ru(OEP)(dms)(dmso)(where s indicates S-coordination of the sulfoxide), Ru(OEP)(dms)₂⁺PhC00⁻ andRu(OEP)(dms)(PhC00). To identify the products and intermediates of oxidation, the complexes in the series Ru(OEP)(RR’S)₂ , Ru(OEP)(RR'S0)₂ and Ru(OEP)(RR’S)₂ ⁺BF₄⁻, as well as Me₄N⁺Ru(OEP)(PhC00)₂, were synthesized and characterized by use of ¹H-nmr, it and uv/vis spectroscopy, cyclic voltammetry, and elemental analysis; the x-ray crystal stucture of Ru(OEP)(decMS)₂⁺BF₄⁻ was obtained. The Ru(OEP)(dmso)₂ complex exists as the bis(S-bound) isomer in the solid state, although variable temperature uv/vis studies suggest that isomerization to 0-bound species occurs in solution. The Ru(OEP)(RR'S)(RR'SO) complexes could not be isolated pure, but they were characterized in solution by ¹H-nmr, CV, and uv/vis studies. For solutions containing Ru(OEP)(Et₂SO)₂, Ru(OEP)(Et₂S)(Et₂50), Ru(OEP)(Et₂S)₂, and varying concentrations of Et₂S and Et2SO, the equilibrium and rate constants governing the relative solution concentrations of the three species were determined from stopped-flow experiments. The Ru(OEP)(RR'S)(PhCOO) complexes could not be isolated in pure form either, but solutions containing 1:1 mixtures of Ru(OEP)(dms)₂⁺BF₄⁻ and Me₄N⁺Ru(OEP)(PhC00)₂⁻ were shown by ¹H-nmr to generate Ru(OEP)(dms)(PhCOO). On the basis of detected intermediates and their properties, a "three-stage" mechanism is proposed for the O₂-oxidation of the thioether ligands of Ru(OEP)(RR’S)₂ in acidic organic media. For example, for the bis(dms) system, in the "first stage", O₂ coordinates to Ru‶(OEP)(dms) formed by dissociation of a dms ligand. This is followed by electron transfer from the metal to O₂ ; the O₂ formed is protonated by PhCOOH to yield HO₂, while Ru‷(OEP)(dms)(PhC00) is also formed. The HO₂ disproportionates to O₂ and H₂O₂ , and the latter oxidizes Et₂S to Et₂SO. In the "second stage", a Ru‷ species, probably Ru‷ (OEP)(dms)(PhCOO), is oxidized to Ru[symbol] by another Ru‷ species, probably Ru‷(OEP)(dms)₂⁺PhCOO⁻ (i.e. 2Ru‷ --> Ru[symbol] + Ru‶). During the "third stage", the Ru[symbol] species is thought to be converted to O=Ru[symbol](OEP)(dms), which then reacts with dms to produce Ru‶(OEP)(dms)(dmso). The overall process results in two moles of dms being oxidized to dmso per mole of O₂ consumed. The basic mechanism appears to be the same for the oxidation of dialkylsulfides in CH₂Cl₂, benzene or toluene, but with some differences in detail. In the presence of excess thioether, solutions of Ru(OEP)(RR’S)₂ in CH₂Cl₂,benzene or toluene, containing PhCOOH, catalyze the O₂-oxidation of thioether to sulfoxide, but light above 480 nm is required. It is believed that, under catalytic conditions, O₂ -coordination to the metal is inhibited by the presence of excess thioether, and that light is then required to provide energy for the otherwise unfavourable outer-sphere electron transfer from the metal to O₂. After the initial electron transfer, the reaction would follow the same course as in the stoichiometric oxidation. The catalytic system was studied for the case in which RR'S = Et₂S. The stoichiometry: 2Et₂S + O₂ →2Et₂SO was verified by gas chromatography and by oxygen-uptake experiments. A kinetic analysis of the gas uptake data showed that, under the experimental conditions used, the initial rate approached a maximum value for [Ru]₀ > 2 mM, [O₂ ] >0.14 M, and [PhCOOH] > 54 mM, with the limiting rate being imposed by the complete absorption of the incident light by the reaction solution. The results of a kinetic modelling analysis suggest that the photo excited state that gives rise to the observed photochemistry has a minimum lifetime of 10⁻⁸ s (in the absence of O₂ ), and that Ru(OEP)(Et₂S)(Et₂S0), which accumulates as the concentration of Et₂SO builds up, is outside of the catalytic cycle.

Thesis/Dissertation

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2008-09-25

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http://hdl.handle.net/2429/2383

Chemistry

University of British Columbia

UBCV

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