Cyclooctatetraene, complexes with

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. 

Cyclooctatetraene Complexes with Structures Determined by X-Ray Analysis... 

Cyclization with various nickel complex catalysts gives up to 97% selectivity to a mixture of cyclooctatetraene derivatives, with only 3% of benzene derivatives. The principal isomer is the symmetrical l,3,5,7-cyclooctatetraene-l,3,5,7-tetramethanol (29). 

Among the compounds that form complexes with silver and other metals are benzene (represented as in 9) and cyclooctatetraene. When the metal involved has a coordination number >1, more than one donor molecule participates. In many cases, this extra electron density comes from CO groups, which in these eomplexes are called carbonyl groups. Thus, benzene-chromium tricarbonyl (10) is a stable compound. Three arrows are shown, since all three aromatic bonding orbitals contribute some electron density to the metal. Metallocenes (p. 53) may be considered a special case of this type of complex, although the bonding in metallocenes is much stronger. 

Cyclooctatetraene was reduced electrochemically to cyclooctatetraenyl dianion. In DMF the product is mostly (92%) 1,3,5-cyclooctatriene at —1.2 V. If the potential is lowered the main product is 1,3,6-cyclooctatriene. Previous experiments, in which the anion radical was found to be disproportionated, were explained on the basis of reactions of the cyclooctatetraene dianion with alkali metal ions to form tightly bound complexes, or with water to form cyclooctatrienes. The first electron transfer to cyclooctatetraene is slow and proceeds via a transition state which resembles planar cyclooctatetraene102. 

Another example concerns the initial electronic reduction of a-nitrostilbene (Todres et al. 1982, 1985, Todres and Tsvetkova 1987, Kraiya et al. 2004). The reduction develops according to direction a in Scheme 2.9 if the mercury cathode as well as cyclooctatetraene dianion are electron sources and according to direction b if the same stilbene enters the charge-transfer complexes with bis(pyridine)-tungsten tetra(carbonyl) or uranocene. For direction b, the charge-transfer bands in the electronic spectra are fixed. So the mentioned data reveal a great difference in electrochemical and chemical reduction processes a and b as they are marked in Scheme 2.9. 

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... 

Cyclooctatetraene (continued) with iron, 12 267-273 with nickel, 12 307, 308 with palladium, 12 315 with platinum, 12 319 with rhodium, 12 304 with ruthenium, 12 280 with silver, 12 340, 342, 343, 346 complexes of, 4 81 iron complexes of, 4 89 Cyclooctatriene 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)-... 

In certain cases cyclization of cyclooctatetraene occurs with Ru3(CO)j2, and pentalene complexes [62] are formed (209, 210), which are similar but not structurally related to the complexes [65] resulting from azulene and Ru3(CO)j2 (96, 700). 

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... 

Neutral ligands can also bond in a variety of ways. Cyclooctatetraene can act as an alkene (Tj2), a diene (T)4), a triene (T]6), or a tetraene (r 8), and the reactivity of the ligand changes accordingly. These are all ft complexes with the metal above or below the black portion of the ring and with the thick bond to the metal at right angles to the alkene plane. 

The products obtained in the reaction of the cyclooctatetraene hafnium complex with phosphaalkynes are temperature-controlled. If 131b is treated with t-BuC=P at 0°C, three molar equivalents of the phosphaalkyne are... 

Tris(cyclopentadienyl)lanthanide complexes with steri-cally more crowded Cp ligands such as C5Me4R (R = Me, Et, Tr, and SiMe3) are not assessable by simple metathesis between lanthanide trihalides and the respective alkali metal salt of the bulky Cp ligand. For instance, Cp 3Sm, obtainable from Cp 2Sm and cyclooctatetraene, reacts with THF with ring-opening forming Cp 2Sm[0(CH2)4Cp ](THF) (equation 14). 

The complexation with the dianion of cyclooctatetraene leads to the formation of the mixed or half-sandwich complexes see Half-sandwich Complexes) as shown in Scheme The coordination of the planar... 

One possible approach to the preparation of an all-hydrocarbon tripledecker compound, with the necessary 34-electron configuration, is to attempt to oxidize or reduce a near-miss complex (42), that is, one containing the correct number of ligands and metals but with an excess or deficiency of electrons. Two such attempts, involving complexes with bridging cyclooctatetraene (cot) ligands, have been reported. 

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

The bicyclic adducts 6 and 7 were obtained from cyclooctatetraene and 3//-l,2,4-triazole-3,5(4//)-diones by preliminary complexation with tricarbonyl iron6 7. Only one diastereomer was obtained, but it was not possible to ascertain if the metal was in the endo or exo position relative to the N-N bridge. Decomplexation was accomplished with oxidizing agents7 8. 

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