摘要:

AbstractMetal chelates are suitable electrocatalysts for the reduction of dioxygen. The selectivity of the reduction process (H2O2or H2O as end product) depends on which catalyst is used. Comparable compounds, such as cobalt and iron phthalocyanine (CoPc and FePc), show different selectivities. CoPc catalyzes the dioxygen reduction pathway with H2O2as the end product, whereas FePc gives H2O. The interaction between the catalyst and dioxygen is crucial for the selectivity of the reduction process. In this review, the most important models for the adsorption of dioxygen on metal chelates are discussed.Most of these models predict that dioxygen is adsorbed in a so‐called “end‐on (bent)” configuration on to the metal phthalocyanines. The models also predict that, for the dioxygen adduct of CoPc, the oxygen fragment is more negatively charged than for the adduct of FePc, and that this may be the reason for the observed difference in selectivity. Alternatively, dioxygen can bind in between two metal phthalocyanine molecules. For these dioxygen‐bridged dimers, as can be derived from adsorption (bonding) models, the cleavage of the oxygen‐oxygen bond occurs more easily in the case of the FePc dioxygen dimer than in the corresponding

ISSN:0165-0513    

DOI:10.1002/recl.19901090202

出版商:WILEY‐VCH Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractTransfer sulfonations of anisole using various polymethylbenzenesulfonic acids (PoMBR's) catalyzed by trifluoromethanesulfonic acid at 25 °C are described. The observed reactivity order for transfer sulfonation increases in the order 2,4‐DiMBS, 2,6‐DiMBS, 2,3,4,5‐TeMBS (all non‐reactive)<2,3,5,6‐TeMBS ≈︁ 2,4,6‐TrMBS<2,3,4,6‐TeMBS

ISSN:0165-0513    

DOI:10.1002/recl.19901090203

出版商:WILEY‐VCH Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractThe reactivity of pure isolated copper(I) 2‐halobenzoates has been studied at 20 and 80 °C either with or without oxidizing agents (O2and CuBr2). Furthermore, these complexes have been reacted at 80°C either alone or in combination with sodium acetylacetonate,i.e.under “Hurtley reaction conditions”. Corresponding reactivity of the mixed (2‐halobenzoato)(mesityl)copper(I) clusters, wherein the mesityl fragment can be considered as an intramolecularly bonded nucleophile, has also been studied.The organic products of these reactions and their distribution, measured by combined gas chromatography‐mass spectrometry (GC–MS) and gas–liquid chromatography (GLC), for both types of reactions showed remarkable similarities. The principal products of these reactions,e.g.biaryls, methyl 2‐(1‐acetyl‐2‐oxopropyl)benzoate and 2‐mesitylbenzoate derivatives, are formed by a halogen substitution reaction. Compared to the chlorobenzoate derivatives, the more reactive bromo species generate more by‐products. The presence of a coordinating solvent, such as acetylacetone, proved to be necessary in order to obtain a good yield from the condensation reaction between the copper(I) 2‐halobenzoates and sodium acetylacetonate. A detailed mechanism involving dinuclear copper(I) intermediates is prop

ISSN:0165-0513    

DOI:10.1002/recl.19901090204

出版商:WILEY‐VCH Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractIt can be shown that alkaline hydrolysis of four‐coordinated phosphorus (PIV) compounds is substantially accelerated if the trigonal‐bipyramidal (TBP) five‐coordinated phosphorus (PV) intermediate in the reaction can exhibit conformational transmission. With the compounds5a‐c, it was shown that this acceleration is an entropy effect. Pseudorotation in the PV‐TBP intermediate is facilitated if conformational transmission occurs, and therefore the entropy loss during build‐up of the PV‐TBP is reduced. This evidently results in a lower free energy of activation for the reaction. For the phosphate triester systems6aand6b, it was also found that the occurrence of conformational transmission speeds up the hydrolysis reaction, although the effect is less pronounced than for compounds5a‐c. The cyclopentylmethoxy group (6a) and the tetrahydrofurfuryloxy group (6b) show slight preferences for equatorial and axial positioning in the transient PV‐TBP, respectively. This study shows for the first time that reaction rate and product selectivity of hydrolyses of phosphate esters can be determined in part by the conformational tr

ISSN:0165-0513    

DOI:10.1002/recl.19901090205

出版商:WILEY‐VCH Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractThe Bond‐Order Conservation principle is shown to be rigorously true within the Valence‐Bond approximation excluding ionogenic terms. After a short discussion ofShustorovich's adaptation of this method to chemisorption on metals, a new improved expression for the activation energy of dissociation is derived, based on a transition‐state model derived from Extended Hückel calculations.It appears to be essential to account for the weakening of the surface‐surface metal bonds.New expressions for atomic and molecular adsorption energies are derived.A major result of this work is a significant reduction in the activation energy for dissociation due to loss of surface metal‐metal atom bonding upon che

ISSN:0165-0513    

DOI:10.1002/recl.19901090206

出版商:WILEY‐VCH Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractThe spontaneous hydrolysis of 2,4‐dinitrophenyl phosphate dianion (2,4‐DNPP) is speeded by cationic micelles of cetyltrimethylammonium salts [CTAX, X = Cl, Br, (SO4)0.5] and of bolaform(22)X2[Me3N+(CH2)nN+Me32X−,n= 22, X = Br, (SO4)0.5]. The shorter‐chain bolaform surfactants (n= 16, 12, X = Br) are less effective. Twin‐chain surfactants of didodecyl and dicetyldimethylammonium salts (C12H25)2NMe2X, X = Br, Cl, (SO4)0.5and (C16H33)2NMe2X, X = Cl, (SO4)0.5and pyridinium‐ion surfactants C16H33NC5H5Cl and its 4‐(dimethylamino) derivative, also speed hydrolysis. The first‐order rate constants increase to limiting values (k′M) when substrate is fully bound and these values are not very sensitive to the nature of the surfactant and are generally greater than in water by factors of 20–110. However, the extent of binding of 2,4‐DNPP is sensitive to the nature of the surfactant. Tertiary amines,e.g., pyridine and DABCO speed the reaction in CTACl by generating an interme

ISSN:0165-0513    

DOI:10.1002/recl.19901090207

出版商:WILEY‐VCH Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractThe synthesis and characterization of the copper tetrafluoroborate compound containing the bidentate N,S ligand 5‐methyl‐4‐(ethylthiomethyl)imidazole (memi), [Cu(memi)2(BF4)2], is described. The activities of this compound and of [Cu(memi)2(H2O)2](NO3)2as oxidation catalysts have been determined in the reaction of 2,6‐dimethylphenol with molecular dioxygen to give polyphenylene oxide (PPO) and diphenoquinone (DPQ). For optimum catalysis, an additional base is required in order to dehydronate the phenol. Triethylamine proved to be most useful as a base. Initial reaction rates encountered are between 1 × 10−5and 6 × 10−5M O2/s andca.50% of the phenol was converted after a reaction time of one hour. Less than 10% phenol was converted into DPQ. Both compounds have comparable reactivities.A single crystal of [Cu(memi)2(BF4)2] was used in an X‐ray structure determination: monoclinic space groupC2/c, a= 13.844(4) Å,b= 11.251(1) Å,c= 15.753(4) Å, β = 111.39 (2)°, Z = 4 andT= 293 K. The structure was solved using heavy‐atom techniques and refined by least‐squares methods to a residualRvalue of 0.050. The copper ion is in a centrosymmetric distorted octahedral environment with all chemically identical donor atoms intranspositions. The tetrafluoroborate anions are coordinated to the copper ion with relatively short distances,i.e.2.55 Å; Cu‐N and Cu‐S

ISSN:0165-0513    

DOI:10.1002/recl.19901090208

出版商:WILEY‐VCH Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractPropene metathesis has been studied at 295 K and 1 atm in He or in H2with MoO3−x/Al2O3catalysts which were reduced at 673 K. Some catalysts were modified by exposing them to diazomethane vapor prior to testing. The diazomethane was synthesized shortly before use; it was stored by dissolving its vapor in di‐n‐butyl phthalate. Metathesis of the unmodified catalysts revealed an induction period during which the activity increased; this period is markedly shorter in H2than in He. In He, metathesis is the sole reaction, in H2it is accompanied by a prevailing hydrogenolysis. No induction period is visible for catalysts modified with diazomethane in accordance with the assumption that surface carbenes are reaction intermediates. The activity is, however, lowered by this treatment, as catalyst deactivation is enhanced by the pretreatment with diazome

ISSN:0165-0513    

DOI:10.1002/recl.19901090209

出版商:WILEY‐VCH Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractThe chemisorption of H2and of CO and the catalysis of neopentane conversion have been studied on two groups of Y‐zeolite‐supported Pd samples. One group was prepared under conditions which result in the formation of “grape‐shaped” Pd particles filling adjacent supercages. In the second group, pretreatment conditions were chosen which yield isolated Pd particles of the size of the supercages. Both groups of Pd/NaY samples differ in adsorption of H2and CO; the COads/Hadsratios are in agreement with the different particle morphologies. The selectivity of neopentane conversion does not detectable depend on Pd morphology; however, the activation energy for reaction is significantly higher for the samples of the fi

ISSN:0165-0513    

DOI:10.1002/recl.19901090210

出版商:WILEY‐VCH Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractHighly active heterogeneous rhodium hydrogenation catalysts ‐ rhodium on support ‐ have been prepared by pyrolysis of commercially available Rh4(CO)12, highly dispersed on a suitable support,e.g.SrTiO3, TiO2‐anatase, TiO2‐rutile, BaTiO3or SrZrO3. The inactive materials initially obtained were activated by exposure to air at room temperature. The activities of these catalysts were determined in the liquid‐phase hydrogenation of aromatic compounds at atmospheric pressure and at low temperatures (max. 70 °C). They appear to be at least twice as active as catalysts prepared from impregnated RhCl3or [RhCl(CO)2]2on SrTiO3or a commercially available catalyst (5% Rh on Al2O3).The gas‐phase hydrogenation of benzene at room temperature takes place exothermally. Very high activities were also found in the liquid‐phase hydrogenation of alkenes and alkynes, whereas low activities were shown in the hydrogenation of nitriles. Almost no activity was observed in the liquid‐phase hydrogenation of nitro compounds and ketones.Catalysts, prepared from Rh6(CO)16, showed lower activities as compared with those prepared from Rh4(CO)12. Hardly any activity was found using Ir4(CO)12as starting carbonyl cluster and no activity was observed when Ru3(CO)12, Co4(CO)12, Os3(CO),2or F

ISSN:0165-0513    

DOI:10.1002/recl.19901090211

出版商:WILEY‐VCH Verlag    

年代:1990

数据来源: WILEY