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Please use this identifier to cite or link to this item: http://hdl.handle.net/2031/8817

Title: Kinetics and mechanisms of the reaction of a (salen)ruthenium(VI) nitrido complex and ferrate(VI) with some organic substrates
Other Titles: Han xi fu jian de dan hua liao pei he wu he gao tie suan yan yu yi xie you ji di wu de fan ying dong li xue yu ji li yan jiu
含希夫碱的氮化釕配合物和高鐵酸鹽與一些有機底物的反應動力學與機理研究
Authors: Xie, Jianhui (謝建暉)
Department: Department of Biology and Chemistry
Degree: Doctor of Philosophy
Issue Date: 2015
Publisher: City University of Hong Kong
Subjects: Ruthenium compounds.
Chemical kinetics.
Phenols -- Oxidation.
Notes: CityU Call Number: QD181.R9 X53 2015
x, 210 pages : illustrations ; 30 cm
Thesis (Ph.D.)--City University of Hong Kong, 2015.
Includes bibliographical references (pages 197-210)
Type: Thesis
Abstract: The study of high-valent metal nitrido and oxo complexes has attracted much attention in recent decades. This thesis mainly consists of two parts, the first part focuses on the kinetics and mechanisms of the reaction of a ruthenium(VI) nitrido complex, ([Ru(salchda)(N)(CH3OH)]+(RuN, salchda = N ,N'-bis(salicylidene)-o-cyclohexyldiamine) with anilines, alkynes, phenols and hydroquinones. The second part is concerned with the kinetics and mechanism of the oxidation of phenols with ferrate(VI). The reaction of RuN with aniline was investigated in 1,2-dichloroethane. A diazonium complex [Ru(L)(N2Ph)(NH2Ph)]+ and a ruthenium(III) bis(aniline) complex [Ru(L)(NH2Ph)2]+ are isolated from the red and green solution, respectively. Benzene and N2 were also detected. The kinetics of this reaction were studied under pseudo-first-order conditions in 1,2-dichloroethane. Three steps were observed. The first step is first order in [RuN] and second order in [aniline], with the third-order rate constant, k3 = (2.60±0.05) x 10 M-2 s-1. The second step is first order in both [RuN] and [aniline], with the second-order rate constant, k2 = (5.76±0.16) x 10M-1 s-1. The third step is independent of both [RuN] and [aniline], with the first-order rate constant k = (3.98±0.08) x 10-3 s-1. No kinetic isotopic effect was observed for the first two steps when aniline-d7 was used. The substituent effect was studied for the first step. A linear Hammett correlation was found between log(kobsX/kobsH)and σp with the reaction constant ρ = -(6.57±0.35), suggesting nucleophilic attack of PhNH2 at the nitrido ligand of RuN. The proposed mechanism involves an initial equilibrium coordination of aniline, which activates RuN towards nucleophilic attack by the second aniline, and this is followed by proton and electron transfer to generate a diazonium intermediate. This intermediate finally decomposes to give benzene and N2. The kinetics of the reaction of RuN with phenylacetylenes and aliphatic alkynes have been studied in methanol, acetonitrile and 1,2-dichloroethane. The first step follows the rate law - d[RuN]/dt = k2[RuN][phenylacetylene] and the second-order rate constants for phenylacetylene k2 = (8.99±0.24) x 10-4 M-1 s-1 in acetonitrile and k2 = (3.04±0.16) x 10-4 M-1 s-l in 1,2-dichloroethane, at 298.0 K. The reaction can be accelerated in the presence of pyridine in 1,2-dichloroethane and the equilibrium constant between RuN and pyridine is found to be (16.4±1.3) M-1 at 298.0 K in 1,2-dichloroethane. No kinetic isotopic effect was observed when phenylacetylene-d was used as substrate. A linear Hammett correlation was found between log(k2X/k2H) and σ+ with the reaction constant ρ = -(1.1±0.1), suggesting that the generation of a vinyl radical is involved in the rate-limiting step. The mechanism may be electrophilic attack of RuN at one of the carbon of triple bond to form a vinyl radical followed by rebound to N to form a very unstable aziro intermediate which undergoes ring opening by CH3OH, H2O or pyridine to generate the corresponding imime complexes. The kinetics of the reaction of RuN with phenol were studied in methanol in the presence of pyridine. With excess phenol, the reaction follows pseudo-first-order kinetics. The pseudo-first-order rate constant is independent of [RuN] and exhibits saturation behavior on [phenol]. The ESI/MS spectra indicate that phenol is attached to RuN with the loss of 1 m/z unit. In the proposed mechanism, RuN interacts with phenol to form an adduct which then loses an H atom. The reaction of RuN with hydroquinone was studied in acetonitrile and 1,2-dichloroethane. In acetonitrile, RuN seems to undergo ligand-induced N...N coupling reaction. When the reaction was carried out in the presence of pyridine in 1,2-dichloroethane, the reaction shows saturation kinetics on [hydroquinone]. Only [Ru(salchda)(py)2]+ was detected in the ESI/MS. With 0.1 M hydroquinone, a linear relation was obtained between log(pseudo-first-order rate constants) and O-H BDE (Bond Dissociation Enthalpies) of hydroquinones. RuN is proposed to oxidize hydroquinone via an H-atom abstraction mechanism. The oxidation of phenol by ferrate(VI) has been studied in aqueous solutions. Only one step is observed at 505nm due to decay of ferrate. Two steps are found at 398 nm, which is consistent with the formation and decomposition of intermediate 4,4'-biphenoquione. With excess phenol, the consumption of ferrate follows pseudo-first-order kinetics. The pseudo-first-order rate constant is independent of [RuN], and depends linearly on [phenol] with the second-order rate constant k2 = (1.50±0.02) x 10 2 M-1 s-1 at pH=7.23, T = 298.0 K and I = 0.3 M. The activation enthalpy, ∆H≠ and activation entropy, ∆S≠, were found to be (7.9±1.0) kcal mol-1 and -(22±3) cal mol-1 K-1, respectively. The acidity effects on k2 are studied over the range of pH 6.22 - 9.30 at 298.0 K and I = 0.3 M. The second-order rate constants of the reaction between HFeO4- and PhOH, FeO4 2- and PhOH, FeO4 2- and PhO- are found to be (2.11±0.59) x 10 2 M-1 s-1, (9.44±0.61) x 10 M-1 s-1 and (5.00±0.53) x 10 2 M-1 s-1, respectively. The kinetics were also carried out in D2O at T = 298.0 K and I = 0.3 M. At pH 7.00, 7.23 7.36 and 8.00, the solvent kinetic isotope effects, k(H2O)/k(D2O) were found to be 3.02, 2.62, 2.19 and 1.79, repectively. A linear correlation between log(k2) and O-H BDE of phenols bearing electron-donating groups is obtained. For the mechanism below pH 9.0, HFeO4- oxidizes phenol via HAT and FeO4 2- oxidizes phenol via PCET (Proton-Coupled Electron Transfer). The oxidation of phenols with electron-donating groups proceeds via HAT, while phenols with electron-withdrawing groups are oxidized through PCET.
Online Catalog Link: http://lib.cityu.edu.hk/record=b4963871
Appears in Collections:BCH - Doctor of Philosophy

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