Formation of arenes and of tetracarbonyl(hexatrienediyl) dicobalt ("flyover") complexes from Co2(CO)8

The cobalt carbonyl mediated cyclotrimerization of alkynes to yield both arenes and "flyover" complexes (which contain a C6Co2 ring) has been reexamined with particular regard to the intermediacy of cobaltacyclopentadienes. Since the latter can now be obtained in controlled fashion from (alkyne)Co-2(CO)(6) and can react with a third alkyne under mild conditions by using trimethylamine-N-oxide as promoter, the incorporation of three different alkynes into both arenes and flyovers has become possible. Cobaltacyclopentadienes, like their precursors, catalyze alkyne cyclotrimerization, but flyovers do not do so under comparable conditions and hence are not on the main reaction path to arenes. In some circumstances cleavage of cobaltacyclopentadienes must occur to account for the observed arene products. Crystal structures of two flyover complexes are reported.