Cyclooctatetraene, complexes formed from

A very similar study has been performed on the complex formed from silver nitrate and cyclooctatetraene 12,13). This polymeric complex is obtained by reaction of aqueous silver nitrate with cyclooctatetraene dissolved in petroleum ether. Like the benzene-silver perchlorate described above, this complex decomposes readily and hence was examined in an atmosphere of the ligand. Complete bond distance and angle data are available for its structure as shown in Table V.l 13). The molecular structure of the 1 1 silver nitrate-cyclo-octatetraene dimer has been determined by X-ray crystallographic analysis 15). 

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) ). 

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). 

Cuprous chloride forms crystalline complexes with numerous unsaturated hydrocarbons, of which one has already been mentioned. In several of these Cu(i) forms two bonds to Cl and a third to a multiple bond of the hydrocarbon, the three bonds from Cu being coplanar. In the 1,5-cyclooctatetraene complex, CuCl. the Cu and Cl atoms form an infinite chain (Fig. 25.5(a)), while in... 

It is also not clear why the diolefin 1,5-cyclooctadiene forms a weaker bond to silver than cyclooctene. The effects of strain differences within the olefins and/or within the complexes is not apparent30, but in any case suggests, as noted by the original authors, a negligible interaction between the metal and the second double bond. However, the cyclooctatetraene complex is asserted to be polymeric in solution and so currently precludes obtaining useful information derived from these complexation studies of this species. 

Additional improvements in preparations of polyacetylene came from several developments. One is the use of metathesis polymerization of cyclooctatetraene, catalyzed by a titanium alkylidene complex. The product has improved conductivity, though it is still intractable and unstable. By attaching substituents it is possible to form soluble and more stable materials that can be deposited from solution on various substrates. Substitution, however, lowers the conductivity. This is attributed to steric factors introduced by the substituents that force the double bonds in the polymeric chains to twist out of coplanarity." Recently, a new family of substituted polyacetylenes was described. These polymers form from ethynylpyridines as well as from ethynyldipyridines. The polymerization reaction takes place spontaneously by a quatemization process ... 

Nickel plays a role in the Reppe polymeriza tion of acetylene where nickel salts act as catalysts to form cyclooctatetraene (62) the reduction of nickel haUdes by sodium cyclopentadienide to form nickelocene [1271 -28-9] (63) the synthesis of cyclododecatrienenickel [39330-67-1] (64) and formation from elemental nickel powder and other reagents of nickel(0) complexes that serve as catalysts for oligomerization and hydrocyanation reactions (65). 

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. 

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). 

Of the cyclic olefins, norbornadiene replaces two CO groups from one Co to yield a labile complex 159, 160, 235), cyclooctatetraene replaces the axial CO ligands from all three cobalt atoms 53) and is itself replaced by other Lewis bases 330), and cyclopentadiene forms the unusual complex [95] with Co3(CO)gCMe 159,160). A few catalytic reactions were observed with methinyltricobalt enneacarbonyls including the dimerization of norbornadiene 160, 235) and the polymerization of functional olefins 312) with different Co3(CO)9CX. 

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). 

One of the first and perhaps most interesting examples of a metal-assisted, symmetry-forbidden reaction was Reppe s synthesis of cyclo-octatetraene from acetylene 34). in a careful study of this system, Schrauzer proposed a concerted mechanism in which the four a bonds of the cyclo-octatetraene are essentially formed simultaneously 35). He proposed an octahedral complex (54) with four acetylene hgands fitted to adjacent ligand coordination positions, spatially defining the incipient cyclooctatetraene. 

Stable olefin-Ni(O) complexes are formed also with 1,5-cycloocta-diene (COD) and cyclooctatetraene (COT), by displacement of cyclo-dodecatriene from (Ci2Hig)Ni (608) or by reduction of nickel acetyl-acetonate (73). The COD complex has also been produced (419) by treating anhydrous NiClg with an excess of iso-C3H7MgBr and COD in ether under UV irradiation. 

Acrylonitrile and related compounds displace all the carbonyl groups from nickel carbonyl to form [(RCH CHCN)2Ni], in which the nitrile bonds through the olefinic double bond 222, 418). The bis(acrylonitrile) complex catalyzes many reactions, including the conversion of acrylonitrile and acetylene to heptatrienenitrile and the polymerization of acetylene to cyclooctatetraene 418). Cobalt carbonyl gave a brown-red amorphous material with acrylonitrile, which had i cn absorptions typical of uncoordinated nitrile groups, but interestingly, the presence of C=N groups was also indicated 419). In acidic methanol, cobalt carbonyl converts a,j8-unsaturated nitriles to saturated aldehydes 459). 

As an example of a more complex structure, the bicyclopropenyl derivative 11 was formed in good yield. Finally, 8-chlorobicyclo[5.1.0]octa-2,4-diene reacted with potassium tert-butoxide to give cyclooctatetraene from the e.TO-monochloride. ... 

A thermally unstable complex of rhodium(I), [ir-CgH8RhCl]2, prepared from ethanolic rhodium(III) chloride and cyclooctatetraene, shows two proton resonance lines at 5.8 r and 4.3 r in carbon disulfide, and probably also contains the tub form of the olefin (13). 

A mixture of cycloocta-l,3,5-triene and cycloocta-l,3,6-triene formed by partial reduction of cyclooctatetraene is the usual starting point for the study of cyclooctatriene complexes, and obviously complexes may be derived from both isomers. Also, the 1,3,5-isomer is in thermal equilibrium with its valence tautomer, bicyclo[4.2.0]octa-2,4-diene (XXVII), at 100° C (30), from which complexes may also be formed. Although silver nitrate... 

---