Complexes of Cyclooctatetraene

Complex (LIV) is extremely air-sensitive but is unaffected by deoxygen-ated water. This suggests that it is a tt complex rather than a compound with Ni—C r bonds (99). 

---

CyclooctatetraenylCompounds. Sandwich-type complexes of cyclooctatetraene (COT), CgH g, are well known. The chemistry of thorium—COT complexes is similar to that of its Cp analogues in steric number and electronic configurations. Thorocene [12702-09-9], COT2Th, (16), the simplest of the COT derivatives, has been prepared by the interaction of ThCl [10026-08-1] and two equivalents of K CgHg. Thorocene derivatives with alkyl-, sdyl-, and aryl-substituted COT ligands have also been described. These compounds are thermally stable, air-sensitive, and appear to have substantial ionic character. 

B. Dickens, and W. N. Lipscomb, Molecular and Valence Structures of Complexes of Cyclooctatetraene with Iron Carbonyl, J. Chem. Phys. 37, 2084-2093 (1962). 

Metal, TT complexes of cyclooctatetraene were first described by Reppe ei al. (211) in 1948. Silver and copper complexes were obtained during the synthesis of this hydrocarbon from acetylene. These complexes of cyclooctatetraene have since been investigated in considerable detail by several groups of investigators (50, 114, l j l )- An x-ray analysis of the complex [(CgHg)Ag]+N03 (XXIV) indicates that the silver ion is coordinated to two nonadjacent bonds of cyclooctatetraene but is situated at differences from each. [(CgHg)Ag]+ units are, furthermore, joined together by weak bonds. 

The 7T complexes of cyclooctatetraene, a nonaromatic but formally conjugated ring system, with many of the transition metals have been described (220). I he first such complex of the group VIB metals was reported by Winstein el al. (620) who prepared (C3H3)Mo(CO)3 (29), in good yield by reaction of the olefin with (diglyme)Mo(CO)3 in hexane... 

The complex CgHgMo(CO)3 also displays this phenomenon (379a) NMR of this in toluene solution shows a sharp singlet at 70° C, which becomes broadened (half-width 25 cps) at room temperature and freezes to a complex pattern at — 30°C. This is therefore the first known complex of cyclooctatetraene whose proton magnetic resonance shows observable change from rapid to arrested valency tautomerism. 

Compound (XLI) does not undergo a Diels-Alder reaction with maleic anhydride but recently a 1 1 adduct with tetracyanoethylene has been obtained (30, 61). Schrauzer and Eichler (61) have found that two isomeric products, presumed to be Fe(CO)3 complexes of cyclooctatetraene dimers, are obtained when (XLI) is irradiated in the presence of cyclooctatetraene. As mentioned in Section III, A, 2, these isomers, upon irradiation with Fe(CO)5, eliminate benzene to form the same binuclear complex CioHtoFe2 (CO)g. The structures of these complexes have not been rigorously established. 

Two known cobalt complexes of cyclooctatetraene are of the type CgHg(CoC5H5) , with = 1 or 2. In the mononuclear complex COT acts as a 1,5-diene with the eight-membered hydrocarbon in the tub configuration 153,154), while the binuclear complex probably has a central CgHg ligand with a plane tetragonal structure 147,154). 

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). 

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 (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... 

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... 

This inversion barrier is much lower than that observed for derivatives of cyclooctatetraene (see Section X,B), and recognition that the fluorinated ring in complexes 119 may more closely resemble a cyclooctatriene than a cyclooctatetraene provides one rationale for the difference in its dynamic behavior compared to its rj1 -heptafluorocyclooctatetraeny 1 relatives. 

One example of outer-sphere electron transfer is the reaction between dipotassium cyclooctatetraene (K2C8H8) and the cobalt complex of bis(salicylidenediamine) (ConSalen) (Levitin et al. 1971), Scheme 1-110 ... 

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... 

Because of the inherent complexity of the cyclooctatetraene system,these results can be explained without invoking adsorbed intermediates 102 but the possibility remains that adsorption plays an important role. By analogy with electrophilic additions to cyclooctatetraene 1 °3, an adsorbed cyclooctatetraene molecule would be attacked preferentially from the fold of the tub conformation with formation of encfo-8-acetoxyhomotropylium ion (11). Attack by acetate ion on 11 would give the c/s-isomer, whereas the trans-isomer might be formed from exo-8-acetoxyhomotropylium ion 12 (Eq. (33) ). 

---