Cyclooctatetraene complexes with cobalt

Cyclooctatetraene also forms interesting complexes with cobalt, rhodium, nickel, and other transition metals, but these will not be elaborated on here. It should also be mentioned that other eight-mem-bered ring systems, such as 1,5-cyclooctadiene, 1,3,5- and 1,3,6-cyclo-octatrienes, etc., form a variety of metal, t complexes. The most recent survey of cyclooctatetraene and related metal, t complexes is the review by Fischer and Werner (100) as well as earlier reviews by these authors (99) f and Bennett (11). 

Cyclooctatetraene reacts with Co(CO)2(C5H5) to produce (CgHg)-Co(C5H5) 158, 268, 431) and (CsH, )[60(05115)]2 227). Preparation of the complexes by reaction of cyclooctatetraene with cobaltous acetyl-acetonate and ethoxydiethylaluminum has also been reported 611). Spectral data indicate that in the mononuclear complex the olefin is coordinated as a 1,5-diene with the tub conformation 158, 226, 227, 268). X-Ray data 220) show structure (169) for the binuclear complex. 

Other structurally characterized metal carbonyl complexes with bi- and trifurcation at a carbonyl oxygen atom include, for example, the cycloheptatrienyl and cyclooctatetraene cobalt clusters Co4 CO)6ir -C ii9) r -C-J ii ), 92, Co4(CO)e-(t/ -CsHsXlti-CsHg), 93, and Co4(CO)8(Ai3-C8H8), 94 [153]. 

Upon treatment with suitable cobalt complexes, methylbutynol cyclizes to a 1,2,4-substituted benzene. Nickel complexes give the 1,3,5-isomer (196), sometimes accompanied by linear polymer (25) or a mixture of tetrasubstituted cyclooctatetraenes (26). 

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. 

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). 

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). 

---