Iron-silica complexes

There are very few examples of asymmetric synthesis using optically pure ions as chiral-inducing agents for the control of the configuration at the metal center. Chiral anions for such an apphcation have recently been reviewed by Lacour [19]. For example, the chiral enantiomerically pure Trisphat anion was successfully used for the stereoselective synthesis of tris-diimine-Fe(ll) complex, made configurationally stable because of the presence of a tetradentate bis(l,10-phenanthroline) ligand (Fig. 9) [29]. Excellent diastereoselectivity (>20 1) was demonstrated as a consequence of the preferred homochiral association of the anion and the iron(ll) complex and evidence for a thermodynamic control of the selectivity was obtained. The two diastereoisomers can be efficiently separated by ion-pair chromatography on silica gel plates with excellent yields. 

Previous studies by Sorokin with iron phthalocyanine catalysts made use of oxone in the oxidation of 2,3,6-trimethylphenol [134]. Here, 4 equiv. KHSO5 were necessary to achieve full conversion. Otherwise, a hexamethyl-biphenol is observed as minor side-product. Covalently supported iron phthalocyanine complexes also showed activity in the oxidation of phenols bearing functional groups (alcohols, double bonds, benzylic, and allylic positions) [135]. Besides, silica-supported iron phthalocyanine catalysts were reported in the synthesis of menadione [136]. 

Since iron phthalocyanine complexes can be activated and stabilized by a chlorine ring substitution (12), the activity of iron hexadecachlorophthalocyanine (7b) immobilized on silica was examined for the synthesis of 4, 5, and 6 with TBHP as oxidant. 

On the other hand, Tilley et al. have reported a synthesis of a well-defined tris(tert-butoxy)siloxy-iron(lll) complex [13] as well as respective molecular siloxide complexes of cobalt [14] and copper [15], which appear to become precursors for their grafting onto silica and application as catalysts for oxidation of alkanes, alkenes and arenes by hydrogen peroxide. 

A new strategy has been used by Tilley et al. to prepare a series of single-site catalysts that consist of iron [13] and cobalt [14] centers supported on mesoporous SBA-15 silica. The iron centers were introduced via grafting reactions of the tris(tert-butoxy)siloxy-iron(III) complex [Fe(OSi Bu3)3(THF)] with SBA-15 in dry hexane to form, finally, an immobilized iron(III) complex of the type [(=SiO) Fe 0Si(0 Bu)3 2(THF)j and to eliminate H0Si(0 Bu)3. Calcination of these species... 

Diene iron tricarbonyl complexes are prepared by thermal or photochemical reaction of conjugated dienes with iron pen-tacarbonyl in the presence of TMANO, triiron dodecacarbonyl, ()]" -benzylidenacetone)iron tricarbonyl, diiron nonacarbonyl, or diiron nonacarbonyl absorbed on silica gel in the absence of solvent. The latter method is particnlarly usefiil for the preparation of complexes from polar electron-rich dienes and heterodienes. A reductive complexation of cycloheptatrienes using iron tricarbonyl and sodium borohydride to give cyclo-heptadiene iron tricarbonyl has been developed (Scheme 126). 

Garrels et al. (1973) believe that the BIF must have been formed in restricted basins in semi-enclosed water bodies, periodically communicating with the ocean via channels or over bars. Deposition of silica occurred mainly during evaporation, but deposition of iron was complex and is explained both by oxidation (hematite facies) and by evaporation (silicate and carbonate facies) and sulfate reduction (sulfide facies). It is suggested that the spatial distribution of the sedimentary facies of the BIF will correspond to the well-known scheme of James (1954), but to explain the similarity of banding in the face of different causes of precipitation of the iron raises difficulties. 

Diiron nonacarbonyl undergoes oxidative addition of vinyl disilanes at room temperature to give oxidative addition product 28, which is isolated by chromatography on silica gel (Eq. 11) [25]. The structure of 28 was determined by H NMR spectroscopy to be (organosilyl)(r 3-l-silapropenyl)iron(II) complexes rather than simple bis(organosilyl)iron(II) complexes. 

Various systems oxidizing hydrocarbons and containing iron ions have been described which can be considered as models of non-heme mono- and dioxygenases [87] (see also Chapter X). Mononuclear iron derivatives have been used as catalysts in oxidations modeling the action of methane monooxygenase [88]. For example, a mononuclear iron carboxylate complex immobilized on a modified silica surface catalyzes oxidation of hexane in the presence of mercap-... 

In cements produced under reducing conditions bivalent iron is present in the form of pleochlorite (a complex solid solution of ferric and ferrous iron, silica, and magnesia, of... 

Figure 1. SEM images of a typical nascent PE film produced by ethylene polymerization over flat silica with anchored bis(imino)pyridyl iron(II) complex. Polymerization occurred at room temperature in toluene with 2 bar ethylene pressure for 24 h. The polymer film is broken into islands, which are connected by stress fibers, (a) and (b) Top view images, (c) side view of the polymer film.

To establish the interaetion of the impregnating solutions with the surface of silica supports, Terorde measured the amount of water required to completely remove different iron salts applied onto the surface of silicagel in a column chromatograph [41], It appeared that an elution volume of water of 5.0 ml was required to remove the iron ammonium citrate, and of 6.2 ml to remove the Mohr s salt. Iron(III) chloride called for an elution volume of 6.3 ml, and iron(III) nitrate for a volume of 7.7 ml. Apparently, the iron species in the initially impregnated solution of iron ammonim citrate does not interact strongly with the silica surface. The relatively small interaction of the iron(III) complex of citric acid with the surface of silica indicates that the presence of the citric acid affects the adsorbed layer of water molecules that remains within the partially dried impregnated support. A thicker, less mobile layer of water molecules, citric acid anions, and iron(III) ions remains than without citric acid, where a layer of about six fairly mobile water molecules is present. 

In this paper a study is presented on the preparation of a series of supported catalysts by precipitation of metal cyanide complexes in the presence of suspended supports. As supports alumina, titania, and silica, have been used. The metals studied comprise iron, cobalt, nickel, copper, manganese, palladium, and molybdenum. Both monometallic, bimetallic and even trimetallic cyanides were precipitated. The stoichiometry of the precipitated complexes was controlled by the valency of the metal ions and by using both nitroprusside and cyanide complexes. Electron microscopy was used to evaluate the distribution of the deposited complex cyanides on the supports. 57Fe-M6ssbauer spectra were measured on the dried precipitated complexes to gain information on the chemical composition of the iron containing complexes. 

As there now exists a large body of laboratory studies on each of the variable systems, for example the effect of die lime/silica ratio in the slag on the desulphurization of liquid iron, the most appropriate phase compositions can be foreseen to some extent from these laboratory studies when attempting to optimize the complex indusuial process. The factorial uials are not therefore a shot in the dark , but should be designed to take into account die laboratory information. Any qualitative difference between die results of a factorial uial, and the expectations predicted from physico-chemical analysis might suggest the presence of a variable which is important, but which was not included in the nials. 

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