Synthesis and Reactions of Alkyl Cobalt Complexes Containing a Quadridentate Nitrogen Donating Ligand PDF Download
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Languages : en
Pages : 274
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Author:
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Category :
Languages : en
Pages : 274
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Author: S. Y. Lee
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Category :
Languages : en
Pages :
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Author: Kihanduwage N. Gunawardhana
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Category : Charge exchange
Languages : en
Pages : 213
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The chemical or electrochemical reduction of the trifluoroacetyl complex CF3COCo(CO)3PPh3 involves a single electron transfer yielding trifluoromethyl radical and an anionic cobalt carbonyl complex. The mechanism is proposed to involve electron transfer followed by initial dissociation of either a carbonyl or phosphine ligand from the 19-electron [CF3COCo(CO)3PPh3]- anion. The resulting 17-electron intermediate undergoes subsequent one-electron reductive elimination of trifluoromethyl radical by homolytic cleavage of the carbon-carbon bond of the trifluoroacetyl group. The CF3. radical can be trapped by either benzophenone anion, forming the anion of [alpha]-(trifluoromethyl)benzhydrol, or Bu3SnH, yielding CF3H. The final organometallic product is an 18-electron anion, either [Co(CO)4]- or [Co(CO)3(PPh3)]-, depending upon which ligand is initially lost. The chemical or electrochemical reduction CF3Co(CO)3PPh3 is a two-electron process involving heterolytic cobalt-carbon bond cleavage to yield trifluoromethyl anion and cobalt carbonyl anions. The trifluoromethyl anion rapidly decomposes to fluoride and difluorocarbene. This carbene may dimerize to form C2F4. The unstable fluoro carbene can also be trapped by cyclohexene. The mechanism proposed for the reduction of C6F5Co(CO)3PPh3 involves a homolytic cobalt-carbon bond cleavage to form C6F5. radical. The resultant C6F5. radical abstracts hydrogen or deuterium from the solvent or trace amounts of water to produce C6F5H or C6F5D. With an excess of reducing agent this C6F5. radical can be further reduced to C6F5- anion before forming pentafluorobenzene by protonation. The inorganic fragment, the 18-electron [Co(CO)3PPh3]- anion, may participate in a ligand exchange reaction to form [Co(CO)4]-. In addition, interesting reactivity was observed between C6F5Co(CO)3PPh3 and tin hydrides, deuterides and chlorides without any reducing agents. We have demonstrated that ligand replacement reactions can be used for the synthesis of new cobalt-NHC complexes with fluorinated alkyl, acyl and aryl ligands. In addition, the X-ray crystal structure of CF3COCo(CO)3PPh3 was obtained to compare the bond lengths and bond angles with other related compounds. An unusual Co-C(acyl) bond length was observed for CF3COCo(CO)3PPh3. Considering the bond lengths of other alkyl and acyl complexes, it can generally be argued that the position of the alkyl/acyl equilibrium varies with the Co-C(alkyl/acyl) bond length.
Author: Deborah Marina Tonei
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Category : Alkylation
Languages : en
Pages : 440
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Author: Noel John Farrier
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Category : Cobalt compounds
Languages : en
Pages : 392
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Author: Omar Khalid Al-Duaij
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Category :
Languages : en
Pages :
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Category :
Languages : en
Pages :
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Category : Dissertations, Academic
Languages : en
Pages : 628
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Author: Isabell Sarah Regina Karmel
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Category :
Languages : en
Pages : 219
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Author: Yunzhou Chen
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Category : Alkanes
Languages : en
Pages : 0
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Direct use of dioxygen (O2) in functionalizing organic molecules is highly desirable. In nature, enzymes perform alkane oxidation efficiently at ambient conditions. The transition metals involved in the active sites of enzymes play vital roles in binding with O2 and transferring electrons and protons during metabolism. Many metal-oxygen species, such as hydro(alkyl) peroxo complexes, are invoked as reactive intermediates in these biological processes. Given the complexity of enzymes, studying the reactivity of these enzymes with simple synthetic coordination compounds is one of the strategies. This thesis is mainly concerned with the oxidation of alkanes and alkenes catalyzed by tailor-made cobalt(III) alkylperoxo complexes at ambient conditions. In the first part, we report the design and synthesis of the highly electrophilic cobalt(III) alkylperoxo complex, [CoIII(qpy)(OOtBu)(NCCH3)]2+ (2), supported by a planar tetradentate quaterpyridine ligand (qpy = 2,2′:6′,2′′:6′′,2′′′-quaterpyridine). This complex activates C(sp3)–H bonds of a variety of organic molecules at ambient conditions and yields a series of alkylperoxo complexes with the general formula [CoIII(qpy)(OOR)(NCCH3)]2+ [RH = Et2O (3), THF (4), tBuOMe (5), ethylbenzene (6), toluene (7), cyclopentene (8), and 3-hexyne (9)], which have been well characterized by various spectroscopic techniques including NMR, ESI-MS, UV-vis, FT-IR, and CHN elemental analysis. The structures of these complexes have also been characterized by X-ray crystallography. In the second part, the mechanism for the alkane oxidation catalyzed by [CoIII(qpy)(OOR)(NCCH3)]2+ was extensively studied at room temperature and one atmospheric pressure. NMR study reveals the reaction stoichiometry. ESI-MS study indicates exogeneous O2 is crucial with the support of 18O-labeled experiments. Kinetics study by UV-vis and a significant kinetic isotopic effect resulted for the oxidation of ethylbenzene by 2 suggest a rate-limiting hydrogen-atom abstraction from organic substrates (R′H) by [CoIII(qpy)OOR]2+ via the proximal oxygen atom of the peroxo group (i.e., [CoIII(qpy)OOR]2+ + R′H → [CoII(qpy)]2+ + R′• + ROOH). The resulting alkyl radical R′• bound with O2 to form alkyl peroxyl radical R′OO•, which was rapidly scavenged by the [CoII(qpy)]2+ to give another alkylperoxo complex [CoIII(qpy)OOR′]2+. The proposed mechanism in the peroxidation of organic molecules b y alkyl(hydro)peroxo complexes is unprecedented. In the third part, we examine the catalytic properties of [CoIII(qpy)(OOR)(NCCH3)]2+ in aerobic oxidation of various substrates. Using ethylbenzene, cumene, cyclopentene, and cyclohexene as the substrates, [CoIII(qpy)(OOR)(NCCH3)]2+ are found to be active and robust catalysts to produce the corresponding hydroperoxides, alcohols, and ketones catalytically. A turnover of >3000 is achieved in the oxidation of cyclohexene for 7 d. In the fourth part, the reactivities of [CoIII(qpy)OOR]2+ with alkenes were explored. Alkenes with weak C–H bonds (e.g., 1,4-cyclohexadiene and cycloalkenes) resulted in C–H functionalization. In case there are no weak C–Hs in the alkenes (e.g., styrene), [CoIII(qpy)OOR]2+ catalyzes the polymerization of styrenes in O2 to produce polyalkylperoxo species. The [CoIII(qpy)(OOCH(OOtBu)CH2Ph)(NCCH3)]2+ ( bisalkylperoxo 11), has complex, been isolated and characterized by ESI-MS, NMR, and X-ray crystallography. In summary, this work demonstrates the highly electrophilic character of Co(III) alkylperoxo complexes supported by the qpy ligand. Under ambient conditions, these complexes are suitable catalysts to perform aerobic peroxidation of a variety of alkanes and alkenes
