Tricarbonyl cyclooctatetraene iron

The three reported preparations of tricarbonyI(cyclo-octatetraene)iron involve the interaction of cycloocta-tetraene with iron pentacarbonyl. Although the yields arc good, considerable amounts of CgHg[Fe(CO)3]2 and CgHsFe2(CO)7 are often obtained as well. The reaction betw cen cyclooctatetraene and triiron dodecacarbonyl takes place more rapidly and gives a product of better purity. 

Caution. This preparation should be performed in a hood inasmuch as small amounts of iron pentacarbonyl are occasionally formed in the reaction. 

Tricarbonyl(cyclooctatetraene)iron forms red-brown air-stable crystals which melt at 94°. It is insoluble in water but dissolves readily in organic solvents. It is easily sublimed. X-ray studies indicate that the metal is bound to two double bonds only, and the unsaturation is also shown by the protonation in concentrated sulfuric or other strong acid solutions to give the tricarbonylbicyclo[5.1.0]-octadieniumiron ion, C8H9Fe(CO)3 , and by the formation of a Diels-Alder adduct with tetracyanoethylene.  

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Diiron enneacarbonyl, synthesis 46 Triiron dodecacarbonyl, synthesis 46 Tricarbonyl(cyclooctatetraene)iron, synthesis 47 Iron carbonyl complexes of triphenylphosphine, triphenyl-arsine, and triphenylstibine, synthesis 48 cis-Dinitrobis(ethylenediamine)cobalt(III) nitrite and nitrate, synthesis 60... 

This method can be employed to the formation of acyclic and cyclic Ti -dienyl-iumiron complexes. T) -Cycloheptadienyliumiron and n -qrclooctadienyliumiron complex salts are readily available using this procedure starting from T -cycloheptatriene- or (T -cyclooctatriene)iron complexes, respectively. Protonation of tricarbonyl(cyclooctatetraene)iron by noncoordinating acids is known to afford cyclopropane annulated Ti -cycloheptadienyliumiron complexes. This procedure has been exploited for the synthesis of c -2-(2 -carboxycyclopropyl)glycines. ... 

Figure 27-12 Mossbauer spectrum of cyclooctatetraene iron tricarbonyl in octane at 78°K. Notice that a spread of Doppler velocities of about 3 mm sec-1 is enough to give the full spectrum. (Courtesy of Professor R. C. D. Breslow and the Journal of the American Chemical Society.)...

A Mossbauer spectrum that has helped to corroborate the structure of cyclooctatetraene iron tricarbonyl is shown in Figure 27-12. The separation of the two absorption peaks in Figure 27-12 corresponds to a sample Doppler velocity of 1.23 mm sec-1. This Doppler effect means that there is the very small energy difference of 1.4 X 10 6 kcal mole 1 in the two transitions shown. 

Fig. 7.9 The molecular structures of butadiene and cyclooctatetraene iron tricarbonyl determined by X-ray structure analysis (carbon and hydrogen atoms of the olefmic ligands are omitted for clarity)...

Two Fe(CO)3 complexes of dimeric cyclooctatetraenes (29 and 30) are formed by irradiation of cyclooctatetraene iron tricarbonyl in the presence of free cyclooctatetraene, posing a formidable problem with respect to the mechanism of this cycloaddition reaction 244,403,407,4io). 

Fig. 2. The structure of cyclooctatetraene-iron tricarbonyl, after Dickens and Lipscomb. The C—C bond distances of the diene-Fe(CO)s unit are each 1.42 A.

The anomalous case of the cyclooctatetraene-iron tricarbonyl complex. 

The preparation of cyclooctatetraene-iron tricarbonyl (XLI) was reported independently in 1959 by three separate research groups (18,19, 20,44, 66). 

Reactions of diene-Fe(CO)3 complexes with other molecules capable of forming ligands with metals in low oxidation states can result in either displacement of the diene ligand or of one of the carbonyl groups. For example, the Fe(CO)3 complexes of with triphenylphosphine with liberation of the hydrocarbon ligands 34,19). Cyclooctatetraene-iron tricarbonyl gives cyclooctatetraene when treated with P 3 but with As 3 or Sb 3 the... 

The iron-catalyzed isomerization of allylic alcohols can be combined with addition of aldehydes in a one-step procedure to give the corresponding aldol products (Scheme 4-293). Efficient iron catalysts for this transformation are pentacarbonyliron [Fe(CO)5], (tricarbonyl)(benzylideneaceton)iron [(dba)Fe(CO)3], or (tri-carbonyl)(cyclooctatetraene)iron [(cot)Fe(CO)3]. ... 

Recently, in a study of a large number of organo-iron compounds 120), it has been found possible to treat the isomer shift as being due to a sum of partial isomer shifts 8,. from individual ligands which for 7r-cyclopentadienyl derivatives could be correlated with proton NMR chemical shifts in the same compounds. The Mossbauer spectra of two cases of theoretical interest are worthy of mention. First, the spectra of both the mono- and binuclear iron tricarbonyl derivatives of cyclooctatetraene 115) show similar 8 and A values. The small 8 values are consistent with zero oxidation state for the iron atom and essentially complete covalent bonding between the iron and the tt electrons of the ring, so that, at least for the mononuclear... 

Another aspect of the chemistry of M(CO) fragments that the computed molecular orbital diagram could help to explain was the structure of the cyclooctatetraene complexes of iron and chromium tricarbonyl.10 Chromium tricarbonyl was shown to have three relatively low-lying vacant orbitals, with the right spatial characteristics to be able to accept electron donation from three of the double bonds of cyclooctatetraene, and accordingly adopts the r geometry shown in Figure 10.4. In iron... 

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