Halides iron complexes

Table 9-3. log Ki values for iron(iii) and lead(ii) halide complexes. 

Iron hydride complexes can be synthesized by many routes. Some typical methods are listed in Scheme 2. Protonation of an anionic iron complex or substitution of hydride for one electron donor ligands, such as halides, affords hydride complexes. NaBH4 and L1A1H4 are generally used as the hydride source for the latter transformation. Oxidative addition of H2 and E-H to a low valent and unsaturated iron complex gives a hydride complex. Furthermore, p-hydride abstraction from an alkyl iron complex affords a hydride complex with olefin coordination. The last two reactions are frequently involved in catalytic cycles. 

Typically, the synthesis of the precursor iron(II) and cobalt(II) halide complexes proceeds by treatment of an anhydrous or hydrated divalent iron or cobalt halide (e.g. MX2 jcH20 or MX2 X = Cl or Br) with 1 (and also 2-4) in... 

Our work on the bifunctional activation of CO insertion was prompted by the thought that strong molecular Lewis acids should be more effective and more general than simple cations. It already had been observed that molecular Lewis acids would promote a molecular Fischer-Tropsch type reaction (5), and that iron diene complexes can be converted to polycyclic ketones by the action of aluminum halides, equation 7,(18), but information on the course of these reactions was sketchy. 

Recently, the iron-promoted Barbier-type addition of alkyl halides to aromatic aldehydes has been reported (Equation (26)).326 According to the proposed mechanism, the initial step is the formation of an alkyl radical, which can be reduced to the corresponding carbanion. This carbanion nucleophile can react, while coordinated to the iron pentacarbonyl complex, with the corresponding aldehyde. This stoichiometric method is limited with respect to substrate scope and yield. The same authors have also developed the Reformatsky-type addition of cr-halosub-stituted carbonitriles to aldehydes and ketones in the presence of iron pentacarbonyl.3... 

Iron(II) formate dihydrate, 14 537 Iron(II) fumarate, 14 537 Iron gelbs, 19 399, 400 Irondl) gluconate dihydrate, 14 541 Iron group carbides, 4 690-692 Iron halides, 14 537-540 Iron hydroxide, water exchange rates and activation parameters of hexaaqua complexes, 7 589t Iron(II) hydroxide, 14 542 Iron(III) hydroxide, 14 542 Iron hydroxides, 14 541—542 Iron(II) iodide, 14 540 Iron(III) iodide, 14 540 Iron/iron alloy plating, 9 813—814. See also Fe entries... 

Selected examples of the conversion of benzyl halides into arylacetic acids using iron carbonyl complexes... 

Carbonyl halide complexes of platinum(IV) are less common. The reaction of [Pt(CO)2]s with chloride ion in an aqueous solution of iron(HI) ions gives Pt(CO)H2Cl2, which adds chloride to give Pt(CO)H2ClJ.313,314 Addition of chlorine to a thionyl chloride solution of Pt(CO)2Cl2 at room temperature results in the rapid formation of Pt(CO)Cl (equation 106), which shows a carbonyl stretch at 2191 cm-1. The yellow-orange compound is stable toward Cl- but reacts with water to form C02.315... 

In other variations ketones are produced. The acyliron monoanion may be alkylated again with another alkyl halide to form a transient acyl-alkyl iron intermediate, which rapidly decomposes into ketone and the polynuclear iron carbonyl complex. This reaction is limited, however, because only very reactive alkylating agents such as methyl, allyl, and benzyl halides will react with the weakly nucleophilic acyliron monoanions ... 

Halides. All of the anhydrous and hydrated binary halides of iron(II) and iron(III) are known with the exception of the hydrated iodide of iron(III). A large number of complex iron halides have been prepared and characterized (6). 

Iron halides react with halide salts to afford anionic halide complexes. Because iron(III) is a hard acid, the complexes that it forms are most stable with F and decrease in both coordination number and stability with heavier halides. No stable I complexes are known. [FeF5(H20)]2 is the predominant iron fluoride species in aqueous solution. The [FeF6]3 ion can be prepared in fused salts. Whereas six-coordinate [FeClJ3 is known, four-coordinate complexes are favored for chloride. Salts of tetrahedral [FeClJ can be isolated if large cations such as tetraphenylarsonium or tetraalkylammonium are used. [FeBrJ is known but is thermally unstable and disproportionates to iron(II) and bromine. Complex anions of iron(II) halides are less common. [FeClJ2 has been obtained from FeCl2 by reaction with alkali metal chlorides in the melt or with tetraethylammonium chloride in deoxygenated ethanol. 

Addition of nucleophiles to a carbon monoxide ligand of pentacarbonyliron provides anionic acyliron intermediates which can be trapped by electrophiles (H+ or R—X) to furnish aldehydes or ketones [18]. However, carbonyl insertion into alkyl halides using iron carbonyl complexes is more efficiently achieved with disodium tetracarbonylferrate (Collman s reagent) and provides unsymmetrical ketones (Scheme 1.2) [19, 20]. Collman s reagent is extremely sensitive towards air and moisture, but offers a great synthetic potential as carbonyl transfer reagent. It can be prepared by an in situ procedure starting from Fe(CO)5 and Na-naphthalene [20]. 

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