Résultat de la recherche
Vanadate ion-catalyzed oxidation of methane with hydrogen peroxide in an aqueous solution
It was shown that, unlike methane oxidation with the reagent “hydrogen peroxide-vanadate anion-pyrazine-2-carboxylic acid (PCA)” in acetonitrile, the performance of the process in an aqueous solution is accompanied by the intense parallel degradation of the cocatalyst. Therefore, relatively high yields of methane oxidation products (largely, formic acid) cannot be attained unless a rather high PCA concentration is used. Admixtures of a strong acid (sulfuric, trifluoroacetic, or perchloric) increase the yield of the products. It was found that perchloric acid can also be used as a cocatalyst instead of PCA.
The kinetics and mechanism of oxidation of isopropanol with the hydrogen peroxide-vanadate ion-pyrazine-2-carboxylic acid system
The vanadate anion in the presence of pyrazine-2-carboxylic acid (PCA) was found to effectively catalyze the oxidation of isopropanol to acetone with hydrogen peroxide. The electronic spectra of solutions and the kinetics of oxidation were studied. The conclusion was drawn that the rate-determining stage of the reaction was the decomposition of the vanadium(V) diperoxo complex with PCA, and the particle that induced the oxidation of isopropanol was the hydroxyl radical. Supposedly, the HO• radical detached a hydrogen atom from isopropanol, and the Me2 C• (OH) radical formed reacted with HOO• to produce acetone and hydrogen peroxide. The electronic spectra of solutions in isopropanol and acetonitrile and the dependences of the initial rates of isopropanol oxidation without a solvent and cyclohexane oxidation in acetonitrile on the initial concentration of hydrogen peroxide were compared. The conclusion was drawn that hydroxyl radicals appeared in the oxidation of alkanes in acetonitrile in the decomposition of the vanadium diperoxo complex rather than the monoperoxo derivative, as was suggested by us earlier.
Dinuclear manganese complexes containing 1,4-dimethyl-1,4,7-triazacyclononane ligands as well as carboxylato and oxo bridges
The reaction of 1,4-dimethyl-1,4,7-triazacyclononane (L-Me2) with MnCl2 • 4H2O in acetonitrile gives, in the presence of sodium formate, hydrogen peroxide, triethylamine and KPF6, the dinuclear Mn(III)–Mn(IV) complex cation [(L-Me2)2Mn2 (O) 2 (OOCH)]2+ (1) which crystallises as the hexafluorophosphate salt.The analogous reaction with sodium benzoate, however, yields the dinuclear Mn(III)–Mn(III) complex cation [(L-Me2)2Mn2 (O)(OOCC6H5)2]2+ (2), isolated also as the hexafluorophosphate salt.In the case of sodium acetate, both cations, the Mn(III)–Mn(IV) complex [(L-Me2)2Mn2 (O) 2 (OOCCH3)]2+ (3) and the known Mn(III)–Mn(III) complex [(L-Me2)2Mn2 (O)(OOCCH3)2]2+ (4) are available, depending upon the molar ratio.The single-crystal X-ray structure analyses show for the green crystals of [1][PF6]1.5 [Cl]0.5 • 1.5 H2O and [3][PF6]2 • (CH3)2CO, a Mn–Mn distance of 2.620(2) and 2.628(4) Å, respectively, while for the red-violet crystal of [4][PF6]2, a Mn–Mn distance of 3.1416(8) Å is observed.All four compounds show catalytic activity for the oxidation of isopropanol with hydrogen peroxide in water and in acetonitrile to give acetone in the presence of oxalic or ascorbic acid as co-catalysts.
Supramolecular cluster catalysis : facts and problems
By checking the chemistry underlying the concept of "supramolecular cluster catalysis" we identified two major errors in our publications related to this topic, which are essentially due to contamination problems. (1) The conversion of the "closed" cluster cation [H3Ru3(C6H6)(C6Me6)2(O)]+ (1) into the "open" cluster cation [H2Ru3(C6H6)(C6Me6)2 (O)(OH)]+ (2), which we had ascribed to a reaction with water in the presence of ethylbenzene is simply an oxidation reaction which occurs in the presence of air. (2) The higher catalytic activity observed with ethylbenzene, which we had erroneously attributed to the "open" cluster cation [H2Ru3(C6H6)(C6Me6)2(O)(OH)]+ (2), was due to the formation of RuO2• nH2O, caused by a hydroperoxide contamination present in ethylbenzene.
Dinuclear Manganese Complexes Containing Chiral 1,4,7-Triazacyclononane-Derived Ligands and Their Catalytic Potential for the Oxidation of Olefins, Alkanes, and Alcohols
Five new 1,4,7-triazacyclononane-derived compds., sodium 3-(4,7-dimethyl-1,4,7-triazacyclononan-1-yl)propionate (Na[LMe2R']) as well as enantiopure (S)-1-(2-methylbutyl)-4,7-dimethyl-1,4,7-triazacyclononane (S-LMe2R''), (S,S)-trans-2,5,8-trimethyl-2,5,8-triazabicyclo[7.4.01,9]tridecane ((S,S)-LBMe3), (S)-1-(2-hydroxypropyl)-4,7-dimethyl-1,4,7-triazacyclononane (S-LMe2R), and (R)-1-(2-hydroxypropyl)-4,7-dimethyl-1,4,7-triazacyclononane (R-LMe2R), were synthesized. Reaction of MnCl2 with the chiral macrocycles S-LMe2R and R-LMe2R in aq. EtOH gives, upon oxidn. with H2O2, the brown dinuclear Mn(III)-Mn(IV) complexes which are enantiomers, [Mn2(S-LMe2R)2(?-O)2]3+ (S,S-1) and [Mn2(R-LMe2R)2(?-O)2]3+ (R,R-1). The single-crystal x-ray structure analyses of [S,S-1][PF6]3·0.5Me2CO and [R,R-1][PF6]3·0.5Me2CO show both enantiomers to contain Mn(III) and Mn(IV) centers, each of which being coordinated to three N atoms of a triazacyclononane ligand and each of which being bridged by two oxo and by two chiral hydroxypropyl pendent arms of the macrocycle. The enantiomeric complexes S,S-1 and R,R-1 catalyze the oxidn. of olefins, alkanes, and alcs. with H2O2. In the epoxidn. of indene the enantiomeric excess values attain 13%. The bond selectivities of the oxidn. of linear and branched alkanes suggest the crucial step in this process to be the attack of a sterically hindered high-valent Mn-oxo species on the C-H bond. [on SciFinder(R)]
Regioselective alkane oxygenation with H2O2 catalyzed by titanosilicalite TS-1
Titanosilicalite TS-1 catalyses oxidation of light (methane, ethane, propane and n-butane) and normal higher (hexane, heptane, octane and nonane) alkanes to give the corresponding isomeric alcohols and ketones. The oxidation of higher alkanes proceeds in many cases with a unique regioselectivity. Thus, in the reaction with n-heptane the CH2 groups in position 3 exhibited a reactivity 2.5 times higher than those of the other methylene groups. This selectivity can be enhanced if hexan-3-ol is added to the reaction mixture, the 3-CH2/2-CH2 ratio becoming 10. It is assumed that the unusual selectivity in the oxidation of n-heptane (and other higher alkanes) is due to steric hindrance in the catalyst cavity. As a result, the catalytically active species situated on the catalyst walls can only easily react with certain methylenes of the alkane, which is adsorbed in the cavity taking U-shape (hairpin) conformations.
Dinuclear iron, ruthenium and cobalt complexes containing 1,4-dimethyl-1,4,7-triazacyclononane ligands as well as carboxylato and oxo or hydroxo bridges
The reaction of 1,4-dimethyl-1,4,7-triazacyclononane (L–Me2) with FeSO4 • 7H2O in aqueous ethanol gives, in the presence of sodium carboxylates, hydrogen peroxide, sodium hydroxide and KPF6, the dinuclear Fe(III)–Fe(III) complex cations [(L–Me2)2Fe2(O)(OOCR) 2]2+ (R = H: 1, R = CH3: 2, R = C6H5: 3), which crystallise as the hexafluorophosphate salts. The corresponding reaction with RuCl3 • nH2O does not work, however, the analogous Ru(III)–Ru(III) complex [(L–Me2)2Ru2 (O)(OOCCH3)2]2+ (5) can be synthesised by reacting Ru(dmso)4Cl2 with L–Me2, HCl and air in refluxing ethanol, followed by addition of sodium acetate, the mononuclear intermediate (L–Me2)RuCl3 • H2O (4) being also isolated and characterised. The reaction of L–Me2, sodium acetate, hydrogen peroxide and triethylamine with CoCl2 • 6H2O in acetonitrile yields, however, the hydroxo-bridged Co(III)–Co(III) complex [(L–Me2)2Co2 (OH)(OOCCH3)2]3+ (6). The molecular structures of 2, 5 and 6, solved by single-crystal X-ray structure analyses of the hexafluorophosphate salts, reveal for the orange crystals of [2][PF6]2 a Fe–Fe distance of 3.104(1) Å, for the purple crystals of [5][PF6]2 a Ru–Ru distance of 3.230(1) Å, and for the violet crystals of [6][PF6]3 • (CH3)2CO a Co–Co distance of 3.358(1) Å. All six complexes show catalytic activity for the oxidation of isopropanol with hydrogen peroxide in water to give acetone in the presence of ascorbic acid as co-catalyst.
Synthesis, Molecular Structure, and Catalytic Potential of the Tetrairon Complex [Fe4(N3O2-L)4(?-O)2]4+ (L = 1-Carboxymethyl-4,7-dimethyl-1,4,7-triazacyclononane)
The reaction of Fe sulfate with 1-carboxymethyl-4,7-dimethyl-1,4,7-triazacyclononane (L) and H2O2 in aq. EtOH gives a brown dinuclear complex considered to be [Fe2(N3O-L)2(?-O)(?-OOCCH3)]+ (1), which converts upon standing in MeCN soln. into the green tetranuclear complex [Fe4(N3O2-L)4(?-O)2]4+ (2). A single-crystal x-ray structure anal. of [2][PF6]4·5MeCN reveals 2 to contain four Fe(III) centers, each of which is coordinated to three N atoms of a triazacyclononane ligand and is bridged by one oxo and two carboxylato bridges, a structural feature known from the active center of methane monooxygenase. Accordingly, 2 was found to catalyze the oxidative functionalization of methane with H2O2 in aq. soln. to give MeOH, Me hydroperoxide, and formic acid; the total turnover nos. attain 24 catalytic cycles within 4 h. To gain more insight into the catalytic process, the catalytic potential of 2 was also studied for the oxidn. of higher alkanes, cycloalkanes, and iso-PrOH in MeCN, as well as in aq. soln. The bond selectivities of the oxidn. of linear and branched alkanes suggest a ferroxy radical pathway. [on SciFinder(R)]
Oxidation of alkanes, alkenes and alcohols catalyzed by transition metal complexes
Supramolecular cluster catalysis: facts and problems
By checking the chem. underlying the concept of supramol. cluster catalysis the authors identified two major errors in their publications related to this topic, which are essentially due to contamination problems. (1) The conversion of the closed cluster cation [H3Ru3(C6H6)(C6Me6)2(O)]+ (1) into the open cluster cation [H2Ru3(C6H6)(C6Me6)2(O)(OH)]+ (2), which the authors had ascribed to a reaction with H2O in the presence of ethylbenzene is simply an oxidn. reaction which occurs in the presence of air. (2) The higher catalytic activity obsd. with ethylbenzene, which the authors had erroneously attributed to the open cluster cation 2, was due to the formation of RuO2·nH2O, caused by a hydroperoxide contamination present in ethylbenzene. [on SciFinder(R)]