X-ray Crystallographic and NMR Spectroscopic Study of (η2-Alkene)(μ-alkyne)pentacarbonyldicobalt Complexes: Arrested Pauson−Khand Reaction Intermediates

The hexacarbonyldicobalt complexes of a range of 5-alkynyl-5H-dibenzo[a,d]cycloheptenes, 11a−e, readily undergo loss of a carbonyl ligand with concomitant formation of pentacarbonyldicobalt clusters, 12a−e, in which the vacant coordination site on cobalt is now occupied by the C(10)−C(11) double bond of the central seven-membered ring. These (η2-alkene)(μ-alkyne)pentacarbonyldicobalt complexes provide structural models of the first step of the proposed mechanism of the Pauson−Khand process for the formation of cyclopentenones via the coupling of an alkene, an alkyne, and a source of carbon monoxide. X-ray crystallographic data reveal that the distance between the alkene carbons and the nearest alkyne carbon is approximately 2.85 Å, slightly shorter than the theoretically predicted value of ∼2.95 Å. VT NMR data for the interconversion of 11b and 12b yielded activation energies for the forward and reverse processes, 29 and 15 kcal mol−1, respectively, and the enthalpy change for the endothermic process (14 kcal mol−1); these match very well the earlier predictions from DFT calculations. Although the ethynyl-hexacarbonyldicobalt complex, 11f, does not suffer facile elimination of CO, it does participate in an intermolecular PKR with norbornadiene. Likewise, attempts to extend the reach of the alkynyl moiety through incorporation of an additional methylene group yield only an intermolecular PKR product. It is suggested that the pentacarbonyl complexes, 12a−e, do not continue along the PKR pathway because of the unfavorable relative orientation of the cobalt-coordinated alkyne and alkene.