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Title: Studies on activation of Nitrido complexes of osmium and manganese towards alkane oxidation and N...N coupling
Other Titles: E han dan luo he wu dui wan jing yang hua zuo yong yu meng han dan luo he wu de ou lian zuo yong zhi cui hua yan jiu
Authors: Yiu, Shek Man (姚碩珉)
Department: Dept. of Biology and Chemistry
Degree: Doctor of Philosophy
Issue Date: 2005
Publisher: City University of Hong Kong
Subjects: Alkanes -- Oxidation
Manganese compounds
Osmium compounds
Notes: CityU Call Number: QD181.O7 Y58 2005
Includes bibliographical references (leaves 174-188)
Thesis (Ph.D.)--City University of Hong Kong, 2005
xix, 188 leaves : ill. ; 30 cm.
Type: Thesis
Abstract: This work is divided into two parts. Part I is concerned with the stoichiometric and catalytic oxidation of alkanes using a bimetallic osmium/iron system. Part II describes the kinetics and mechanism of N…N coupling reaction of (salen)manganese(V) imido complexes. In part I, it was found that by simply mixing the tetrahedral nitridotrioxoosmate (VIII) ion, [OsVIII(N)(O)3]¯, with a few equivalents of FeCl3 in CH2Cl2/HOAc, a very active and efficient system for the stoichiometric oxidation of alkanes is generated. The active intermediate is proposed to be a binuclear μ-nitrido species: [OsVIII(N)(O)3]¯ + FeCl3 → [(O)3Os≡N-FeCl3]¯ An intermediate (A) is formed between [OsVIII(N)(O)3]¯ and FeCl3, which can be detected by UV-Vis spectroscopy. The formation of A (kfor) follows pseudo-first-order kinetics. The rate constants (kfor) are independent of the concentration of cyclohexane (kfor = 104 – 111 s¯¹ at 25.0 ℃). The decomposition of the active intermediate also obeys first-order kinetics in the presence of excess cyclohexane; the rate constant (kdec) increases with [cyclohexane], and it reaches saturation at high [cyclohexane]. These results are consistent with the reaction scheme shown in Equations 1-3. [OsVIII(N)(O)3]- + FeCl3 → A (kfor) (1) A + RH → [A, RH](K) (2) [A, RH] → products (k) (3) The observed rate constant for the decay of the intermediate (kdec), is related to the K and k, by the following equation: kdec = kK[RH]/ 1 + K[RH] (4) A non-linear least-squares fit of the data to Equation 4 gives k = (7.38 ± 0.37) x 10-2 s-1, K = (1.3 ± 0.1) M-1 for c-C6H12 at 25.0 ℃. The oxidation of alkanes by [OsVIII(N)(O)3]-/FeCl3 can be made catalytic by using 2,6-dichloropyridine N-oxide (Cl2pyO), tert-butyl hydroperoxide (TBHP) or hydrogen peroxide (H2O2) as the terminal oxidant. In part II, it was found that treatment of (salen)manganese(V) nitrido complexes with trifluoroacetic anhydride (TFAA) or trifluoroacetic acid (TFA) results in a facile N…N coupling reaction to produce dinitrogen. Evidence suggests that the active species for the coupling reaction is an imidomanganese(V) trifluoroacetate species for TFA and an acylimidomanganese(V) trifluoroacetate species for TFAA. It is proposed that coupling of these species results in the initial formation of a μ-diazene complex of manganese(III), [(L)MnIII(N2R2)MnIII(L)]2+, which then decomposes to give dinitrogen.
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