Dimeric iron complex

Scheme 62 Reaction scheme of dimer of dimer iron complexes as models...

There is ample experimental evidence for the presence of partial hydroxylated monomer-and dimer-iron complexes in zeolites that decompose N2O. At high temperatures N2O easily decomposes on a single-center Fe +(OH) cationic complex to form N2 and the Fe +=0 complex... 

A head-to-head dimerization of a-olefin catalyzed by a bis(imino)pyridine iron complex has been reported by Small and Marcucci [126]. This reaction delivers linear internal olefins (up to 80% linearity) from a-oleftns. The linearity of products, however, depends on the catalyst structure and the reaction conditions. 

SCHEME 34. Alternative strategy used by Constable et al. for the construction of a dimeric iron(II) complex built around a terpyridine ligand.274... 

In model studies involving Fe(n) species, three broad approaches have been used to mitigate the problem of autoxidation of the iron (Hay, 1984). These are (i) the use of low temperatures so that the rate of oxidation becomes very slow (ii) the synthesis of ligands containing steric barriers such that dimerization of the iron complex is inhibited, and (iii) immobilization of the iron complex on a solid surface such that dimerization once again will not be possible. 

In some cases where the siderophore architecture permits, the tetradentate siderophore will form a dimeric Fe2L2 complex with iron instead of the monomeric FeL form. These dimeric complexes are not discernible from their monomer counterparts... 

Although the above model was developed under non-catalytic conditions, some of the results may bear significance under natural conditions or in the presence of excess sulfite ions. Thus, the decomposition of the mono-sulfito complex was considered to be the rate-determining step in the catalytic cycle, but only estimates could be given for the rate constant in earlier studies. The comprehensive data treatment used by Lente and Fabian yielded a well established value for this parameter (106), which can then be used to improve previous kinetic models. Furthermore, the participation of reactions of the [Fe2(0H)(S03)]3+ complex was never considered in kinetic studies where excess sulfite ion was used over low iron(III) concentration in mildly acidic solution (pH 2.5-3.0). The above model predicts that in some cases the formation of the dimeric sulfito complex could make a substantial contribution to the spectral changes and omission of this species could lead to biased conclusions. Reevaluation of data sets reported earlier by including the reactions of [Fe2(0H)(S03)]3+ may resolve some of the controversies found in literature results. 

Dinuclear C B complexes (triple-decker sandwiches and dimers) Iron ... 

Carbonvlation of Benzyl Halides. Several organometallic reactions involving anionic species in an aqueous-organic two-phase reaction system have been effectively promoted by phase transfer catalysts(34). These include reactions of cobalt and iron complexes. A favorite model reaction is the carbonylation of benzyl halides using the cobalt tetracarbonyl anion catalyst. Numerous examples have appeared in the literature(35) on the preparation of phenylacetic acid using aqueous sodium hydroxide as the base and trialkylammonium salts (Equation 1). These reactions occur at low pressures of carbon monoxide and mild reaction temperatures. Early work on the carbonylation of alkyl halides required the use of sodium amalgam to generate the cobalt tetracarbonyl anion from the cobalt dimer(36). 

It is essential to characterize the reactant species in solution. One of the problems, for example, in interpreting the rate law for oxidation by Ce(IV) or Co(III) arises from the difficulties in characterizing these species in aqueous solution, particularly the extent of formation of hydroxy or polymeric species. We used the catalyzed decomposition of HjOj by an Fe(III) macrocycle as an example of the initial rate approach (Sec. 1.2.1). With certain conditions, the iron complex dimerizes and this would have to be allowed for, since it transpires that the dimer is catalytically inactive. In a different approach, the problems of limited solubility, dimerization and aging of iron(III) and (Il)-hemin in aqueous solution can be avoided by intercalating the porphyrin in a micelle. Kinetic study is then eased. 

One of the major difficulties encountered in attempts to prepare 1 1 dioxygen-iron complexes which can desorb molecular oxygen, particularly oxygenated complexes of ferroheme, is the strong driving force toward the irreversible formation of the stable p-oxo ferriheme dimer, as represented in Eq. (11). The oxygenated... 

The syntheses of iron isonitrile complexes and the reactions of these complexes are reviewed. Nucleophilic reagents polymerize iron isonitrile complexes, displace the isonitrile ligand from the complex, or are alkylated by the complexes. Nitration, sulfonation, alkylation, and bromina-tion of the aromatic rings in a benzyl isonitrile complex are very rapid and the substituent is introduced mainly in the para position. The cyano group in cyanopentakis(benzyl isonitrile)-iron(ll) bromide exhibits a weak "trans" effect-With formaldehyde in sulfuric acid, benzyl isonitrile complexes yield polymeric compositions. One such composition contains an ethane linkage, suggesting dimerization of the transitory benzyl radicals. Measurements of the conductivities of benzyl isonitrile iron complexes indicate a wide range of A f (1.26 e.v.) and o-o (1023 ohm-1 cm.—1) but no definite relationship between the reactivities of these complexes and their conductivities. 

Similarly, tricarbonyl-2,3-dimethylbutadienyl osmium (37) is a dimer wifh an analogous structure except that the 7r-bonded Os(CO)3 group is rotated through 60° relative to the iron complex. The compound cyclo-pentadienylhexakistrifluoromethylbenzene rhodium (38) is also novel in... 

Cyclohexene oxidation in the presence of the molybdenum complex, [C5Hr)Mo(CO)3]2, gave two major products at low conversion VI and VII nearly 1 1 mole ratio, Table V. The ketone, VIII, was formed in very low yield in contrast to oxidations using the iron complex. This reaction is far more selective than the oxidation of cyclohexene in the presence of Mo02(acac)2 reported by Gould and Rado (24). When a cyclohexene solution of V was exposed to [CsHsMk COJs] at 70°C, VI and VII were formed in approximately equimolar amounts (Table VI). These data show that the molybdenum complex efficiently catalyzes the epoxidation of cyclohexene by V before the allylic hydroperoxide decomposes substantially. Reaction 16 represents the predominant course of cyclohexene oxidation in the presence of cyclopentadienyltricarbonyl molybdenum dimer. 

The intermediacy of a metallacyclobutene is proposed upon reaction of the diphenylcy-clopropenone dimer spirolactone with CpCo(CO)2, ultimately yielding a >j4-vinylketene complex (equation 23 l)295a. Unlike the analogous iron complex (Section IV.B.2.a), no vinyl carbene complex was observed, and hence formation of the metallacyclobutene seems to be more likely. 

Heterometal alkoxide precursors, for ceramics, 12, 60-61 Heterometal chalcogenides, synthesis, 12, 62 Heterometal cubanes, as metal-organic precursor, 12, 39 Heterometallic alkenes, with platinum, 8, 639 Heterometallic alkynes, with platinum, models, 8, 650 Heterometallic clusters as heterogeneous catalyst precursors, 12, 767 in homogeneous catalysis, 12, 761 with Ni—M and Ni-C cr-bonded complexes, 8, 115 Heterometallic complexes with arene chromium carbonyls, 5, 259 bridged chromium isonitriles, 5, 274 with cyclopentadienyl hydride niobium moieties, 5, 72 with ruthenium—osmium, overview, 6, 1045—1116 with tungsten carbonyls, 5, 702 Heterometallic dimers, palladium complexes, 8, 210 Heterometallic iron-containing compounds cluster compounds, 6, 331 dinuclear compounds, 6, 319 overview, 6, 319-352... 

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