Cyclopentadienyl-iron anions

Another route to complexes with a,/i-unsaturated acyl ligands involves reaction of the dicar-bonyl(r/5-cyclopentadienyl)iron anion of 12 with a,/ -unsaturated acyl halides38. 

Treating pentacarbonyliron with dicyclopentadiene affords the dicarbonyl (ri -cyclopentadienyl)iron dimer [Cp(CO)2pe]2. Reduction of the latter with sodium amalgam provides the nucleophilic dicarbonyl(r -cyclopentadienyl)iron anion [Cp(CO)2Fe]. Allylic halides or tosylates can be reacted with this Fe(0) complex to afford ri -allyl-Fp-iron complexes (Fp = Fe(CO)2Cp, Scheme... 

Alkylation of the anion 2 with iodomethane or other haloalkanes provides alkyldicarbonyl(t/5-cyclopentadienyl)iron complexes such as 53,0 (see also Houben-Weyl, Vol. 13/9a, p 209). Migratory insertion of carbon monoxide occurs on treatment with phosphanes or phosphites9 -11 (see also Houben-Weyl, Vol. d3/9a, p257) to provide chiral iron-acyl complexes such as 6. This is the most commonly used preparation of racemic chiral iron-acyl complexes. 

Another example of a complex that obeys the 18-electron rule is ferrocene or bis(cyclopentadienyl) iron. The cyclopentadienyl anion is generated by the reaction of cydopentadiene with sodium, and ferrocene is obtained by the subsequent reaction with ferrous chloride,... 

The chemistry in this chapter originates from Pauson s discovery of bis(cyclopentadienyl)iron(II). Pauson had attempted the synthesis of the 1 Otc aromatic hydrocarbon fulvalene via an oxidative coupling/dehydro-genation of the cyclopentadienyl anion (Figure 7.24). However, Pauson... 

A98. N. N. Greenwood and T. C. Gibb, Mossbauer Spectroscopy. Chapman Hall, London, 1971. Chapter 9, pp. 221-238 Covalent iron compounds (41), treats binary carbonyls, carbonyl anions, hydride anions, substituted iron carbonyls, ferrocene and other 7r-cyclopentadienyl iron derivatives. 

The generation of the precursors for cyclopentadienyl-silanol-functionalized iron complexes involves the formation of the corresponding iron anion in a first step [6]. 2a is obtained by reductive cleavage of the methoxysilyl-cyclopentadienyl-substituted iron dimer 1 with sodium amalgam in THF (Scheme 1). This reaction is restricted to alkoxysilyl-cyclopentadienyl-fiinctionalized iron anions because of the limited access to the corresponding Si-H-functionalized iron dimers. 

Scheme 1. Synthesis of silyl-cyclopentadienyl-fimctionalized iron anions.

The study of complexes of anionic phospholyl systems continues to develop. Heating l-t-butyl-3,4-dimethylphosphole in the presence of a pinene-fused cyclopentadienyl iron carbonyl complex results in the formation of the chiral... 

Cationic olefin complexes of dicarbonyl(> -cyclopentadienyl) iron have been of wide interest in syntheses for a number of years. These complexes, generally isolated as their tetrafluoroborate or hexafluorophosphate salts, have been prepared by the reaction of Fe(f -CsHs)(CO)2Br with simple olefins in the presence of Lewis acid catalysts, by protonation of allyl ligands in Fe(t/ -CsHjXCO)2[(allyl)KC ] complexes, or by treatment of these with cationic electrophiles, by hydride abstraction from Fe(f) -CsHjXCO)2(alkyI) complexes, through reaction of epoxides with Fe(fi -CsHjXCO)2 anion followed by protonation, or by thermally induced ligand exchange between [Fe( i -CjHsXCO)2(ij -2-methyH-propene)][BF4] " or [Fe( -C,HsXCO)2(tetrahydrofuran)][BF4] and excess olefin. 

The starting materials for these complexes are dicarbonyl(cyclopentadienyl)-iron chloride (Fischer and Moser, 1970) and the dicarbonylcyclopentadienyl-iron anion [as the sodium salt (Green and Nagy. 1963)], both obtainable from tetracarbonyl (di-Tr-cyclopentadienyl)diiron which is available commercially. The reaction is carried out in tetrahydrofuran (THF). Equation (1) demonstrates the ease of manipulation of these substances in the preparation of an olefin complex from an epoxide. The tetrafluoroborate salt precipitates from ether. This reaction is also useful as a means of reducing expoxides stereospecifically to olefins with retention of configuration. The olefin is readily liberated from the complex by treatment with sodium iodide in acetone at room temperature for 15 min (Giering et ai, 1972). 

Distinction should be made between the value of the geometrical and the electrochemical (active) areas of an electrode. The meaning of geometric area is obvious. The electrochemical area should be computed on the basis of the response to a benchmark species in one of the techniques discussed in the following. Once the diffusion coefficient of the species chosen, typically one partner of a reversible redox couple, such as the hexacyanoferrate anions in water or bis(cyclopentadienyl)iron (II)—ferrocene—in organic solvent, is known, the ratio between the measured current and the expected current density constitutes a reliable estimate of the electrochemical area. The dependence of this area value on the exact nature of the electroactive species may be discarded as a first approximation, once poisoning of the electrode and the occurrence of unknown complex electrode mechanisms can be excluded. 

Alternatively, acyliron complexes can be obtained in a two-step sequence either from dicarbonyl(cyclopentadienyl)ferrates or from dicarbonyl(cyclopentadienyl)iron halides via alkyl-Fp complexes. The first method makes use of the nucleophilicity of [CpFe(CO)2] anions. Their reaction with alkyl halides provides alkyl-Fp complexes that, upon treatment with phosphanes or phosphites, undergo a migratory insertion of a carbonyl ligand into the metal-alkyl bond leading to an acyliron complex (Scheme 4-38). " ... 

A Hiickel molecular orbital calculation for the cyclopentadiene system can be carried out as illustrated in Chapter 5. As is shown in Figure 5.20, the Frost-Musulin diagram places the five molecular orbitals at energies of a + 2/3, a + 0.618/3 (2), and a — 1.618/3 (2). Because the cyclopentadienyl anion has six electrons, only the three lowest energy levels are populated and are the orbitals interacting with those on the iron. Figure 21.15 shows the orbitals of the cyclopentadienyl anion. 

In a similar manner, Jt-allyl complexes of manganese, iron, and molybdenum carbonyls have been obtained from the corresponding metal carbonyl halides [5], In the case of the reaction of dicarbonyl(r 5-cyclopentadienyl)molybdenum bromide with allyl bromide, the c-allyl derivative is obtained in 75% yield in dichloromethane, but the Jt-allyl complex is the sole product (95%), when the reaction is conducted in a watenbenzene two-phase system. Similar solvent effects are observed in the corresponding reaction of the iron compound. As with the cobalt tetracarbonyl anion, it is... 

This all explains the nature of failures of the DFT based methods in those cases, when correlations substantially come into play as in e.g. d -iron (II) ferrocene molecule. Here the errors even of advanced DFT methods become catastrophic. For example, in Ref. [86] the calculated enthalpies of dissociation of ferrocene to the free Fe + ion and two Cp anions (Cp = C5H5 - cyclopentadienyl) depending on the functional used appear to be by 3-4 eV smaller than the experimental value. The reason is transparent... 

Rhenium-acyl complexes, such as 1, are isoelectronic with the iron-acyl complexes discussed above and many reactivity patterns are common to the two groups of compounds. Treatment of complex 1 with strong bases, such as butyllithium or lithium diisopropylamide, results in abstraction of a cyclopentadienyl proton which is followed by rapid migration of the acyl ligand to the cyclopentadienyl ring to produce the metal-centered anion 384. Alkylation of 3generates a metal-alkyl species, such as 4. 

An alternative method is to use electrochemical mediators that are at a higher concentration that O2 and can therefore be shuttled back and forth between the protein and the electrode faster than the enzyme is reduced, so that the arrival of the glucose is always rate-limiting. A typical chemical that works in this way is ferrocene, which is an iron cation between two cyclopentadienyl anions, as shown in Figure 6.47. It exists in neutral and - -1 oxidation state that are readily interconvertible at metal or carbon electrodes. 

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. 

Tetracarbonyl(trifluoromethyl)iron(II) iodide reacts with C-C bond formation to give per-fluoroethene. Tetracarbonyl(perfluorohexyl)iron(II) iodide gives several products, but no per-fluorohexene resulting from /1-elimination has been found.148 However, reductive defluorination of perfluoro(methylcyclohexane) has been reported with dicarbonyl(ty5-cyclopcn-tadienyl)iron(III).212 The defluorination is accompanied by substitution of fluorine with the cyclopentadienyl anion and proton abstraction from the solvent, the latter is well known in the chemistry of fluoroaromatics with the cyclopentadienyl anion. 

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