Silver complexes with olefins

The use of silver fluoroborate as a catalyst or reagent often depends on the precipitation of a silver haUde. Thus the silver ion abstracts a CU from a rhodium chloride complex, ((CgH )2As)2(CO)RhCl, yielding the cationic rhodium fluoroborate [30935-54-7] hydrogenation catalyst (99). The complexing tendency of olefins for AgBF has led to the development of chemisorption methods for ethylene separation (100,101). Copper(I) fluoroborate [14708-11-3] also forms complexes with olefins hydrocarbon separations are effected by similar means (102). 

Olefin Complexes. Silver ion forms complexes with olefins and many aromatic compounds. As a general rule, the stabihty of olefin complexes decreases as alkyl groups are substituted for the hydrogen bonded to the ethylene carbon atoms (19). 

Finally, binuclear lanthanide(III)-silver(I) shift reagents are noteworthy. These form complexes with olefins, aromatic rings, halogenated saturated hydrocarbons, and phosphines. Due to the lack of polar groups, these functionalities do not give significant LIS with common mononuclear LSR. Applications of this binuclear technique have been reviewed261 for example, the Z- and E-isomers of 2-octene can be differentiated. 

Also in the 1930s, detailed studies about the thermodynamic stability of adducts of silver(I) with olefins were carried out by Howard Lucas and coworkers, who determined the equilibrium constants between the hydrated Ag+ ion and the corresponding cationic olefin silver(I) complex in dilute aqueous solutions of silver nitrate [25]. In the context of this work, Saul Winstein and Lucas made an initial attempt to describe the interaction between Ag+ and an olefin by quantum mechanics [26]. Assisted by Linus Pauling, they explained the existence of olefin silver(I) compounds in terms of resonance stabilization between the mesomeric forms shown in Fig. 7.4. Following this idea, Kenneth Pitzer proposed a side-on coordination of Ag+ to the olefin in 1945 and explained the stability of the corresponding 1 1 adducts as due to an argentated double bond , in analogy to his concept of the protonated double bond [27]. He postulated that the unoccupied s-orbital of silver(l) allowed the formation of a bond with the olefin, similar to the s-orbital of the proton. 

The inability of normal shift reagents to complex with olefinic double bonds has been overcome by the use of a mixture of silver heptafluorobutyrate and europium or praeseodymium heptafluoro-octanedionate, the silver interacting with the olefin and the lanthanide with the carboxylate group. (5) The shifts observed are, however, rather small, as expected in view of the distance of the lanthanide from the hydrogens. 

A non-dependence of the thermodynamic equilibrium constant on the solvent for two different types of diols was found 34>, which indicated that Ag+ as well as undissociated AgN03 formed complexes with olefins, comparable with mercury salt-olefin complexes 35>. Further formation constant investigations 36> by gas chromatography of silver complexes of cyclo-olefins had shown that methyl substitution at the double bond markedly reduced the stability and... 

In several papers silver-containing complexes were used as active components of selective stationary phases. Cook and Givand [60], for example, used silver ion complexes with different compounds as stationary phases. The best separation was obtained with the silver complex with pyridine. Complexes with 2,6-dimethylpyridine, quinoline, isoquinoline, 2,2-bipyrryl, etc., did not interact with olefins, which can probably be explained by steric factors [60]. 

The preparation and properties of copper(i) and silver trifluoromethanesulphon-ates, and their complexes with olefins and aromatic compounds have been reported extensively. ... 

The fact that silver ions form complexes with olefins has led to at least one patent for separation of unsaturated from saturated hydrocarbons (13). Very recently, however, Amoco was forced by economics to shelve plans to replace distillation columns in olefin plants with membrane-based separators involving hollow fibers of cellulose acetate saturated with solutions of silver nitrate (37). 

Copper, silver, and gold form complexes with olefins of the following composition [CuX(olefin)] (X = C1, Br olefin = ethylene, propylene, butenes, COT, NBD, etc. dienes in these complexes are monodentate ligands), [Cu2X2(COD)2],... 

Mono- and diglycerides can be separated from triglycerides by column chromatography on silica gel (Hirsch and Ahrens 1958), florisil (Carroll 1961), and polymerized soy bean oil according to Hirsch (1963). Further resolution of triglycerides is possible on columns of silica gel impregnated with silver nitrate (de Vries 1962) and thin-layer plates prepared in the same way (Barrett et al. 1962, 1963). Silver ions complex with olefinic double bonds with the cis-forms... 

A number of different silver salts have been used for complexation with olefins, e.g. AgNOs, AgBp4, etc. The system that has been studied in great detail is AgNOs in water. The fallowing complexation reactions have been su ested (Herberhold, 1974) ... 

The overwhelming majority of studies in the field of membrane gas separation are aimed at the development of efficient steady-state processes. This rational. approach is directly related to the important advantage of the membrane gas separations, i.e. low process maintenance. However, for many mixtures the efficient separation is not easy to achieve under the steady-state conditions. One of the challenging problems of this kind is the separation of mixtures of light paraffins and olefins, in particular of ethane and ethylene. Thus far the most intensively explored way to achieve high separation selectivities for such mixture is to load the membranes with substances capable of forming the molecular complexes with olefins (e.g., silver compounds). These complexes are the selective carriers of the olefin molecules through the membranes. 

Anhydrous silver hexafluorophosphate [26042-63-7] AgPF, as well as other silver fluorosalts, is unusual in that it is soluble in ben2ene, toluene, and xylene and forms 1 2 molecular crystalline complexes with these solvents (91). Olefins form complexes with AgPF and this characteristic has been used in the separation of olefins from paraffins (92). AgPF also is used as a catalyst. Lithium hexafluorophosphate [21324-40-3] LiPF, as well as KPF and other PF g salts, is used as electrolytes in lithium anode batteries (qv). 

Scheme 61 Epoxidation of electron deficient olefins with silver complexes.

The first metal-olefin complex was reported in 1827 by Zeise, but, until a few years ago, only palladium(II), platinum(Il), copper(I), silver(I), and mercury(II) were known to form such complexes (67, 188) and the nature of the bonding was not satisfactorily explained until 1951. However, recent work has shown that complexes of unsaturated hydrocarbons with metals of the vanadium, chromium, manganese, iron, and cobalt subgroups can be prepared when the metals are stabilized in a low-valent state by ligands such as carbon monoxide and the cyclopentadienyl anion. The wide variety of hydrocarbons which form complexes includes olefins, conjugated and nonconjugated polyolefins, cyclic polyolefins, and acetylenes. 

Anhydrous silver-olefin complexes are readily dissociable, low-melting, and variable in composition 92a, 176, 183). Cyclic olefins and polyolefins form stable complexes with silver nitrate or perchlorate, but again the Stoichiometry of the complexes varies considerably, sometimes depending on the conditions of preparation. The following types have been isolated [Ag(un)2]X (un = e.g., cyclohexene, a- and /3-pinene) ISO), [Ag(diene)]X diene = e.g., dicyclopentadiene 220), cyclo-octa-1,5-diene 50, 130), bi-cyclop, 2,1 ]hepta-2,5-diene 207), and cyclo-octa-1,3,5-triene 52), and [Ag2(diene)]X2 (diene = e.g., cyclo-octa-1,3- and -1,4-diene 180), bi-cyclo[2,2,l]hepta-2,5-diene 1) and tricyclo[4,2,2,0]-decatriene 10)). Cyclo-octatetraene (cot) forms three adducts with silver nitrate 52), viz., [Ag(cot)]NOs, [Ag(cot)2]N03, and [Ag3(cot)2](N03)3. On heating, the first two lose cyclo-octatetraene and all three decompose at the same temperature. From the stoichiometry of the above complexes it appears that the... 

The ease with which olefins form complexes with metals naturally led to investigation of acetylenes as ligands but until recent years only a few ill-defined, unstable acetylene complexes of copper and silver were known. Now complexes of acetylenes with metals of the chromium, manganese, iron, cobalt, nickel, and copper subgroups are known. These complexes fall naturally into two classes—those in which the structure of the acetylene is essentially retained and those in which the acetylene is changed into another ligand during complex formation. Complexes of the first class are discussed here and the second class is discussed in Section VI. 

Complexes in which the acceptor is a metal ion and the donor an olefin or an aromatic ring (n donors do not give EDA complexes with metal ions but form covalent bonds instead).41 Many metal ions form complexes, which are often stable solids, with olefins, dienes (usually conjugated, but not always), alkynes, and aromatic rings. The generally accepted picture of the bonding in these complexes,44 first proposed by Dewar,45 can be illustrated for the complex in which silver ion is bonded to an olefin. There are two bonds between the metal ion and the olefin. One is a a bond formed by overlap of the filled -it orbital of the olefin with the empty 5s orbital of the silver ion, and the other a -it bond... 

Complexes of Olefines and. Silver Ion.—Much work has been done on the interaction of the silver ion, Ag+, with unsaturated and aromatic hydrocarbons. (Mercuric ion and some other metal ions also react with carbon-carbon double bonde.) The structure proposed by Win-gtein and Lucas11 is probably essentially correct. Let us consider a silver ion and ethylene. The system is an electron-deficient one there are 12 valence electrons and 13 valence orbitals (including one orbital for the silver ion). We may write three structures for the complex ... 

Olefins - [FEEDSTOCKS - COALCHEMICALS] (Vol 10) - [FEEDSTOCKS-PETROCHEMICALS] (VollO) - [HYDROCARBONS - SURVEY] (Vol 13) -m automobile exhaust [EXHAUSTCONTROL, AUTOMOTIVE] (Vol 9) -catalyst for stereospeafic polymerization [TITANIUMCOMPOUNDS - INORGANIC] (Vol 24) -esters from [ESTERIFICATION] (Vol 9) -hydroxylation using H202 [HYDROGEN PEROXIDE] (Vol 13) -luminometer ratings [AVIATION AND OTHER GAS TURBINE FUELS] (Vol 3) -osmium oxidations of [PLATINUM-GROUP METALS, COMPOUNDS] (Vol 19) -polymerization [SULFONIC ACIDS] (Vol 23) -reaction with EDA [DIAMINES AND HIGHER AMINES ALIPHATIC] (Vol 8) -silver complexes of [SILVER COMPOUNDS] (Vol 22)... 

Trans olefins form weaker ir-complexes with silver ions than do cis olefins hence cis-trans isomers can be separated using silver ion chromatography. Initially silver ions were used in conjunction with TLC, with silver nitrate being incorporated in the silica gel layer. In recent years the technique has been adapted to HPLC. 

Silver ions react readily with olefins, forming a silver-olefin complex according to the reaction ... 

After successful application of the silver catalyst shown in olefin aziridination (Section 6.1.1), He and coworkers showed that intramolecular amidation was possible with both hydrocarbon-tethered carbamates and sulfamate esters.24 They found that only the Bu3tpy silver complex could catalyze efficient intramolecular amidation, while other pyridine ligands gave either dramatically lower yields or complicated product mixtures. In an interesting control study, both copper and gold were also tested in this reaction. Both the copper and gold Bu tpy complexes can mediate olefin aziridination, but only silver can catalyze intramolecular C-H amidation, indicating that the silver catalyst forms a more reactive metal nitrene intermediate. 

A paper dating from 1976 investigated the reaction of olefins or alcohols with cationic copper and silver complexes of carbon monoxide 49 The remarkable outcome of these kinetic measurements is that there is no difference between the catalytic behavior of Cu(CO) + and Ag(CO)2 +. 

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