Complexes cyclooctatetraene complexes

Reduction of nitrobenzene (Grant and Streitwieser 1978, Todres et al. 1985) and 4-methoxy-nitrobenzene (Todres et al. 1985) by uranium, thorium, and lanthanum-di(cyclooctatetraene) complexes leads to azo compounds. Scheme 1.8 illustrates these reductive reactions using the di(cyclooctatetraene)-uranium complex as an example. 

Many substituted uranocenes have been made and there is a substantial body of organometallic chemistry of uranocene derivatives now known 16, 17). Some of this chemistry will be mentioned in passing but wiU not be covered in a systematic way since other reviews of the organic chemistry are available 18). The only other actinide cyclooctatetraene complex structurally characterized to date is bis[(l,3,5,7-tetramethylcyclooctatetraenyl]uranium(IV) 19), which was of interest because the presence of methyl groups allowed the planarity and relative orientation of the dianion rings to be determined. Crystal and molecular parameters for these three actinide compounds are summarized in Table 1. 

Table 1. Summary of crystal and molecular data for actinide and lanthanide cyclooctatetraene complexes...

It is interesting to compare these actinide(IV) cyclooctatetraene complexes with similar compounds of the group IVB transition elements Ti, Zr and Hf. Bis (cyclooctatetraene) complexes of aU three are known although structural data is only available for the first two. All would appear to involve both planar and non-planar COT rings and to exhibit a sHpped sandwich structure rather than the true sandwich structure of uranocene. 

Several lanthanide cyclooctatetraene complexes have been synthesized, including both the divalent metal complexes Eu(CgHg) and Yb(CgH8) 44), and the trivalent complexes [M(CgH8)2] (M =Ce, Pr, Nd, SmorTb) 35). The former were preprired by direct reaction of the metal with cyclooctatetraene in liquid ammonia. No structural data exist for these divalent compounds, but they probably involve some kind of bridging interaction since, even with coordinated solvent, one COT... 

Table 2. Comparison of M—C bond lengths in several cyclooctatetraene complexes...

A number of cyclopentadienyl-ffee amidoAn -cyclooctatetraene complexes have been reported. Sattelberger s group found that [An(cot)Cl2(thf)2] with 2Na[N(SiMe3)2] in toluene yielded the crystalline [An(cot) N(SiMe3)2 2], which for An = Th was X-ray-characterised as a four-legged piano stool , with the Th- -Centroid distance of 2.04 A, each amido ligand having one short Th- - -C contact (3.15 or 3.04 A). The compounds [U(cot)-... 

Treatment of MX5 with alkali metal naphthalenide in DME at —60 °C provides thermally unstable brown intermediates, assumed to be [M(C10H8)2] or [M(l-MeCioH7)2], which react with CO (1 atm) to give [M(CO)6]- in 30-54% yields.718 Carbonylation, using sodium in the presence of cyclooctatetraene in THF under CO (1 atm, 40 °C), has also been reported, although its efficiency is low in the case of Ta (12%).719 Cyclooctatetraene complexes are thought to form as intermediates, but the mild carbonylation conditions compared to those of [Nb(COT)3]- (50 atm, 100 °C) suggest that a compound of different stoichiometry may be involved.720... 

The TCNE adducts of a variety of other [nq4-Fe(CO)3]- complexed trienes and tetraenes have been investigated. In most cases the adducts have been oxidatively cleaved from the metal with CeIV and the free ligands recovered in good yields. Reactivity patterns generally follow those described above for cy-cloheptatriene and cyclooctatetraene complexes. Specific complexes investigated include [Fe(2,4,6-cy-clooctatrieneone)(CO)3],61 [Fe(tropone)(CO)3],62,63 [Fe(heptafulvalene)(CO)3] and various 8-substituted... 

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... 

The mass spectrum of the cyclooctatetraene complex C8H8Cr(CO)3, which has the structure (XXVIII), shows the typical stepwise loss of the... 

The mass spectrum of cyclooctatetraeneiron tricarbonyl shows stepwise loss of the three carbonyl groups from the molecular ion, followed by elimination of acetylene giving C6H6Fe+, and further breakdown gives Fe+ (119). The mass spectra of two substituted cyclooctatetraene complexes (see Table VIII) show the molecular ion and stepwise loss of the carbonyl groups (83). 

The cyclooctatetraene complexes, for example, [Nb(C8H8)3] and CH3Ta(C8H8)2, have rf, butadienelike, and rf-allyllike bonding of the rings to the metal they are nonrigid. 

Cyclooctatetraene complexes of Zr and Hf halides also undergo simple metathesis with allylmagnesium halides ... 

Cyclooctatetraene complexes of hafnium have proved to be excellent templates for the cyclotrimerization of Bu C P, whereby according to the particular reaction conditions complexes of different cyclotrimers were constructed. 

Spectroscopic investigation (227) of the cyclooctatetraene complex (CgHg)PdCl2 indicates that all double bonds in the molecule are coordinated, but the structure of the complex has not been determined. When suspended in methanol, this complex is rapidly solvolyzed to produce (1, - dichlorobis(2 - methoxy - 3,5,7 - cyclooctatrienyl)dipalladium(II) (501). Similar complexes are formed by the reaction of 1,3-cyclohexa-diene, 1,3-cycloheptadiene, or 1,3- or 1,5-cyclooctadiene with Na2PdCl4 in methanol at room temperature. The last species reacts with HCl to give (l,5-cyclooctadiene)dichloropalladium(II) (501). Pyrolysis of the complex [(CgHi20CHa)PdCl]2 obtained from 1,5-cyclooctadiene yields 1-methoxy- and 2-methoxy-l,3-cyclooctadiene (535). 

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