Cationic cyclopentadienyliron complexes

Cationic polyferrocenes containing pendent cyclopentadienyliron moieties as well as azo dyes in their backbone, 210, have been prepared.261 These polymers displayed excellent solubility in polar organic solvents and exhibited max around 419 nm. Electrochemical studies showed that these polymers underwent two redox couples corresponding to the two different iron centers. The oxidation of the ferrocene occurred at Ey2 = 0.89 V and the reduction of the cationic cyclopentadienyliron complex occurred atEV2 = —1.42 V. Photolytic cleavage of the cationic cyclopentadienyliron moieties produced the neutral polyferrocene analogs. 

The synthesis of cationic organoiron containing polymethacrylates and polystyrenes has been reported. The radical polymerization of methacrylate and styrene monomers containing cationic cyclopentadienyliron complexes produced polymers with cationic cyclopentadienyliron complexes in their side chains (Scheme 2.57).265,266 Monomer 216 and polymer 217 were both redox active and underwent reversible reduction processes. An organic analog of the cationic organoiron polymer was obtained by photolytic cleavage of the cationic cyclopentadienyliron... 

Interest in organometallic maaomolecules has grown exponentially ever since Arimoto and Haven first polymerized vinylferrocene in 1955 [1]. Organometallic polymers are known to possess unique optical, magnetic, and thermal properties which allow for potential applications as chemical sensors, electrocatalysts, modified electrodes, and photo-active molecular devices [2-7]. Organoiron polymers are one of the most prevalent classes of organometallic polymers, with many reports on their synthesis and properties published over the past 50 years [8-11]. Of the many varieties of organoiron species, ferrocene and cationic cyclopentadienyliron complexes are most commonly incorporated into polymers. 

The electrochemical properties of the trimetallic monomers were examined using cyclic voltammetry. It has been established that cationic cyclopentadienyliron complexes undergo reduction processes, while ferrocene and functionalized ferro-cenes undergo oxidation processes. Figure 25 shows the cyclic voltammogram of complex 49. The 1/2 value obtained for the oxidation of the neutral iron center was 0.994 y and the 1/2 value obtained for the reduction of the two terminal cationic iron centers was -1.12 V... 

Polymers containing pendent cationic cyclopentadienyliron complexes, as well as ferrocene and azo dyes in the backbone 71 have also been reported. These brightly... 

Arenes, on complexation with Cr, Fe, Mn, and so forth, acquire strongly electrophilic character such complexes in reactions with nucleophiles behave as electrophilic nitroarenes.71 Synthesis of aromatic nitriles via the temporary complexation of nitroarenes to the cationic cyclopentadienyliron moiety, cyanide addition, and oxidative demetalation with DDQ has been reported (Eq. 9.43).72... 

Cationic cyclopentadienyliron dicarbonyl (Fp) alkene complexes are generally reactive towards a wide variety of nucleophiles, including nitrogen nucleophiles, but they too generate stable chemical step (usually oxidation). This makes catalysis impossible and severely limits application of this methodology to organic synthesis (equation 21 ).36 However, in contrast to palladium, iron is relatively inexpensive and stoichiometric procedures... 

In contrast to many stable transition metal-alkyne complexes, cationic cyclopentadienyliron-alkyne complexes are reactive toward a range of nucleophiles. However, since most of the nucleophiles studied were carbanions, discussion of this chemistry is deferred to Volume 4, Chapter 3.2, Section 3.2.3. 

Stereochemical investigations of the above mentioned reactions of cyclopentadienyliron complexes have been helpful in understanding the mechanism of [2+1] cyclopropanation of alkenes with cationic cyclopentadienyliron carbene complexes. Based on the results of these investigations, the cyclopropanation is believed to occur due to electrophilic attack by the carbene center of metallacarbenoid 5 at the less substituted position of the alkene to produce cationic complex 6 which undergoes back-side ring closure to alford 7 or front-side ring closure to afford 8 depending on cation stability, lifetime and rotation of the fS-y bond in 6. ... 

At low concentrations (0.025 M) the photolysis of these polymers resulted in the decoloration due to the degradation of the azo dye.260 After the cationic cyclopentadienyliron moieties are cleaved from the polymer, they form complexes with the azo chromophore, which then leads to photodegradation of the azo chromophore. 

Cationic star-shaped molecules containing cyclopentadienyliron complexes, where the organo-metallic cations are evenly distributed throughout the molecule branches, 250, have been reported (Scheme 2.67).295 296 291 Electrochemical studies of these materials indicated that the iron centers underwent reversible reductions. For the hexametallic star-shaped molecule (n = 1), two redox potentials were observed at KU2 = I -20 and —1.30 V. 

Star-shaped molecules containing cationic arene complexes of iron and ruthenium have been reported by Astruc and co-workers.298 Utilizing the activating nature of the cyclopentadienyliron moiety on the complexed arene, 260 was converted into 262 via bromobenzylation. The photolysis of 262 gave 263, which was subsequently reacted with 264 to give the hexametallic complex 265. Further nucleophilic aromatic substitution reactions with phenol 266 gave the allyl-substituted complex 267 (Scheme 2.70). 

A number of research groups have reported the preparation of a large number of star-shaped molecules and dendrimers containing ferrocenyl or arene cyclopentadienyliron complexes at the core or the peripheries.300-320 A number of these dendrimers were prepared via cyclopentadienyliron-mediated per-alkylation, -benzylation and -allylation reactions of cationic tri-, tetra- and hexamethylbenzene complexes. These dendrimers were multifunctional materials and have been used in the synthesis of branched organic and organometallic polymers. 

Star and dendrimer core molecules were prepared by the peralkylation or allylation of cyclopentadienyliron complexes containing methyl-substituted arenes.298,301,302,304-311,333 The preparation of water-soluble metallodendrimers containing six cationic cyclopentadienyliron moieties, 281, has also been reported.301 Dendrimer 281 was tested for potential use as a redox catalyst for the cationic reduction of nitrates and nitrites to ammonia. 

The synthesis of a water-soluhle metallodendrimer containing six cationic cyclopentadienyliron moieties was also reported (249). This dendrimer was examined as a redox catalyst for the cathodic reduction of nitrates and nitrites to ammonia. Star-shaped polyaromatic ether complexes of cyclopentadienyliron were recently reported hy Ahd-El-Aziz and co-workers (250). These complexes contained up to 15 cationic cyclopentadienyliron moieties pendent to aromatic rings in the star branches (111). Electrochemical analysis of these star polymers showed that the iron centers underwent reversible reduction processes. 

Star polymers and dendrimers have been synthesized by Astruc using cyclopentadienyliron-mediated peralkylation, benzylation, and allylation reactions of cationic tri-, tetra-, and /iexa-methylbenzene cyclopentadienyliron com-plexes. " " These star and dendritic polymers contained cationic cyclopentadienyliron moieties at the core and/or the periphery. The cathodic reduction of nitrates and nitrites to ammonia has been achieved using a water-soluble dendrimer containing six cationic cyclopentadienyliron moieties as a redox catalyst. " The octametallic star (41) was prepared by deprotonation of permethylated iron complexes. ... 

Polymerization of Olefinic Monomers Functionalized with Cationic Cyclopentadienyliron Arene Complexes... 

Astruc and coworkers have reported the synthesis of highly branched polymers coordinated to cyclopentadienyliron and pentamethylcyclopentadienyl-ruthe-niiun cations." The catalytic and sensing ability of star polymers and dendrimers has also been reviewed. Multifunctional core molecules suitable for the synthesis of star and dendritic materials were synthesized by peralkylation or aUylation of methyl-substituted arene complexes of cyclopentadienyliron." The benzylic protons on these complexed arenes are acidic, which permits their facile alkylation. These branched polymers contained cationic cyclopentadienyliron moieties at the core and/or the periphery. The synthesis of a water-soluble metallodendrimers... 

Two main classes of T -dienyliron complexes are known, namely the cationic tricarbonyliron complexes and neutral cyclopentadienyliron compounds. The cyclopentadienyl (Cp) ligand is relatively inert to a broad variety of reaction conditions. It is often introduced as a ligand to tune the properties of the iron complex, as a chiral auxiliary,or in material science in organoiron pol5miers. However, it is only rarely transformed itself into a more elaborate organic product. For this reason, the chemistry of the cyclopentadienyl ligand will not be discussed in more detail in this chapter. Tricarbonyl( n -dienylium)iron complexes, on the other hand, represent versatile electrophilic building blocks for the attachment of 1,3-butadiene, cyclohexadiene, or aromatic moieties to nucleophilic molecules. 

Reduction of arene-Tr-cyclopentadienyliron(II) cations, which are iso-electronic with the di-ir-cyclopentadienylcobalt cation, with lithium aluminium hydride yields the neutral arenecyclopentadiene-iron complexes (100) ... 

The requirement for AlCh adds some obvious limitations, but a modest number of substituted arenes have been successfully coordinated to the [FeCp]+ unit. Photoinduced arene exchange has been described43 and is particularly effective with the -r -chlorobenzene-r -cyclopentadienyliron cation without the need for Lewis acids.42 More basic (electron rich) arenes readily replace chlorobenzene, leading to FeCp cation complexes of O-phenylethyl-p-toluenesulfonate and variously substituted thiophenes.42 The charged complexes are not purified by conventional organic techniques, such as chromatography, but re-crystallization is possible. The complexes are very air and heat stable again, methods of removal of the arene from the Fe are few. The simplest is pyrolysis at >200 C.46... 

The occurrence of stable carbene complexes of Groups VI and VII is well documented (87, 96, 202), but the 77-cyclopentadienyliron carbene carbonyl cations (103, 151) appear unstable. 

If cyclopentadienyliron dicarbonyl halides are allowed to react with themselves in the presence of Lewis acids, cations are formed in which the new substituent is the cyclopentadienyliron dicarbonyl halide itself, for example, [CjH6Fe(CO)i-X-(CO)2-FeC8H8]+ (X = Cl, Br, I). All three cations can be prepared best by treatment of the corresponding halides with boron trifluoride diethyl etherate all are isolated as tetrafluoro-borates.18 The bromine complex can also be obtained by a more complicated procedure by the reaction between C8H8Fe-(CO)2Br and AlBr3 in liquid sulfur dioxide 16 the iodine cation can be isolated from a melt of cyclopentadienyliron dicarbonyl iodide and aluminum chloride.17 In the latter two cases the hexafluorophosphate salts can be obtained. These binuclear cations are of special interest, because they are cleaved by electron donors,16-17 e.g., aniline, pyridine, benzonitrile, acetonitrile, acrylonitrile, with the formation of the corresponding [C8H8Fe(CO)2L]+ cations and the parent halide. Equations for preparation of the tetrafluoroborate are ... 

Cycloaddition reactions of 18-electron transition metal ti -allyl complexes with unsaturated electrophiles to form five-membered rings have been extensively investigated. These transformations constituted a family of metal-assisted cycloaddition reactions in which the metal functions as an electron-donor center. These are typically two-step processes that involve the initial formation of a dipolar metal r) -alkene intermediate (2) and subsequent internal cyclization (equation 2). The most extensively investigated application of this methodology has been with dicarbonyl-ii -cyclopentadienyliron (Fp) complexes from the laboratory of Rosenblum. These (ri -allyl)Fp complexes are available either by metallation of allyl halides or tosylates with a Fp anion, or by deprotonation of (alkene)Fp cations. ... 

Cationic cyclopentadienyl iron fragments have been generated by photolysis of [(C5H5)Fe(C6H6)] [PF6] and shown to form transient cationic triple-decker sandwich complexes with [(t-Bu3C3P2)Fe(r-Bu2P3)]. Similar photolytic removal of cyclopentadienyliron moieties has been utilized as a key step in the synthesis of ether and thioether building blocks for polymer synthesis. ... 

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