Iron bipyridine complexes

There is a brief but eomprehensively referenced section on iron in a review of homoleptie 2,2-bipyridyl eomplexes " iron-terpy (3 pages) and iron-quaterpy (1/2 page) complexes have also been briefly reviewed." Absorption spectra and photochemistry of apprcmriate iron diimine complexes are ineluded in a text on polypyridyl and porphyrin complexes." A review of the applieation of ehiral 2,2 -bipyridines, 1,10-phenanthrolines, and 2,2 6, 2"-terpyridines in homogeneous eatalysis eontains a little material on iron complexes of such ligands." ... 

On the other hand, Takacs and coworkers added organometallic reducing agents to the reaction mixture and promoted the formation of low-valent iron(O) bipyridine complexes. The mechanism of the low-valent iron-catalyzed Alder-ene reaction involves coordination of the two starting materials within the ligand sphere of the iron, which makes the Woodward-Hoffmann rules for such reactions obsolete [11]. Thereby, the scope of the reactions was broadened so that alkenes and 1,3-dienes could also be used as educts in a formal [4 + 4]-cycloisomerization (Scheme 9.3) [12]. Intriguingly, the diastereoselectivity of the cydopentane products can be influenced by either the application of the 2Z-isomer 3 or the 2 E-isomer 4. Especially the E-isomers 4 gave almost exclusive cis selectivity [13]. 

There has been considerable interest in the chemistry and electronic structures of cobalt and iron complexes of a s-l,2-disub-stituted ethene-1,2-dithiol1 2 and their Lewis base adducts.3-5 The complexes, and their Lewis base adducts which may contain pyridine, phosphines, NO, dipyridyl (2,2/-bipyridine), etc., are capable of undergoing reversible one-electron transfer reactions, thereby generating a series of complex ions differing from each other by only one unit of electric charge. 

The widespread occurrence of iron ores, coupled with the relative ease of extraction of the metal, has led to its extensive use as a constructional material with the result that the analysis of steels by both classic wet and instrumental methods has been pursued with vigour over many years.3 Iron complexes are themselves widely used as the basis of convenient analytical methods for the detection and estimation of iron down to parts per million. Familiar tests for iron(III) in aqueous solution include the formation of Prussian blue as a result of reaction with [Fe(CN)6]4, and the formation of the intensely red-coloured [Fe(H20)5SCN]2+ on reaction with thiocyanate ion.4 Iron(II) forms particularly stable red tris chelates with a,a -diimines such as 1,10-phenanthroline or 2,2 -bipyridine that have been used extensively in spectrophotometric determinations of iron and in the estimation of various anions.5 In gravimetric estimations, iron(III) can be precipitated as the insoluble 8-hydroxyquinoline or a-nitroso-jS-naphthol complex which is then ignited to Fe203.6 In many situations the levels of free [Fe(H20)6]3+ may be controlled through complex formation by addition of edta. 

This triad of elements have played a crucial role in the development of the chemistry of 2,2 -bipyridine. The characteristic red color of [Fe(bpy)3l + was first observed by Blau in his pioneering studies on 2,2 -bipyridine (73-75), and iron complexes of bpy have continued to be of interest in the past century. The complexes of iron, ruthenium, and osmium probably account for about a third of all literature references to 2,2 -bipyridine complexes. This in part represents the facile synthesis of the complexes, their high stability, and extensive redox chemistry. The recent interest in the use of these compounds as photocatalysts has led to an explosive interest in the literature. Recent reviews have concerned themselves generally or partially with the chemistry of iron (342, 552, 688, 814) and ruthenium (800, 803-806, 814) complexes of 2,2 -bipyridine, so these complexes are not discussed further here. In particular, the reader is referred to excellent recent reviews of the photochemical applications of these compounds (41, 43, 44, 176, 194, 443, 624, 625, 877, 954). 

NjCsoHsj, 3,11,15,192226-Hexaazatricy-clo( 11.7.7. F loctacosa-1,5,7,9(28), 12,14,19,212 nonaene, 3,11-dibenzyl-1420-dimethyl-2,12-diphenyl-, iron complex, 27 280 nickel complex, 27 277 N8Cl20jRuC3 H3j 6H20, Ruthenium(II), tris(2,2 -bipyridine)-, dichloride, hexa-hydrate, 28 338... 

The photoconversion efficiency for iron complexes [FeLs] and [FeL2(X)2], (L = 4,4 -dicarboxylic acid-2,2 -bipyridine and 5,5 -dicarboxylic acid-2,2 -bipyridine) has been reported [63]. The [FeL2(X)2] type complexes show two broad MLCT bands in the visible region. In both cases, the photoconversion efficiency of the high-energy band was reported in the range of 40%. Nonetheless, there is virtually no photocurrent contribution... 

Reactivity of dinuclear iron complexes with simple bidentate ligands such as 2,2 -bipyridine and 1,10-phenanthroline for oxidation of alkanes, toluene, dimethyl sulfide, trans-stilbene and adamantane was studied in CHjCN under Ar in the presence of oxidants (TBHP, CHP or H2O2). The effect of the number of the bridging ligand was found remarkable monobridged Fe20(L)4(X)2(C104)4 >... 

For the approach detailed above to be successful, the metal binding domain must form robust complexes, yet retain synthetic flexibility. Although substituted phenanthroline and bipyridine easily form self-assembled receptors (12), the substitution stability of terpyridine (Fig. 2) makes this ligand more suited for the creation of libraries. The choice of metal also permits some flexibility in library creation. Iron complexes form easily through the addition of ammonium iron(II) sulfate hexahydrate, but can be cleaved by the addition of chelating guests (11,13), Cobalt complexes are also easily formed using cobalt(II)... 

Oxygenation of catechols with molecular oxygen by bipyridine(pyridine)iron(III) and related iron complexes... 

Scheme 8. Proposed mechanism of the intra- and extra-diol oxygenations of DTBCH2 catalyzed by a (bipyridine)(pyridine)iron complex [30, 99].

Allylic ethers may react with 2,3-disubstituted 1,3-butadienes in an iron-catalyzed process in two different modes depending on the ligand (Scheme 4-312). A [4+4] ene reaction is favored when a bipyridine(diene)iron complex is used as catalyst to form the corresponding vinyl ethers. If l,2-bis(diphenylphosphano)ethane is used as ligand, a 1,4-hydrovinylation of the diene is observed. The active Fe(0) catalysts are synthesized from Fe(ll) or Fe(III) salts by reduction with Grignard or trialkylaluminum reagents. ... 

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