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.一系列5-炔基-5H-二苯并[a,d]环庚烯(5-alkynyl-5H-dibenzo[a,d]cycloheptenes, 11a−e)的六羰基二钴(hexacarbonyldicobalt)配合物，可轻易脱除一个羰基配体，同时生成五羰基二钴簇(pentacarbonyldicobalt clusters, 12a−e)，此时钴原子上的空配位位点被中心七元环的C(10)−C(11)双键所占据。这类(η²-烯烃)(μ-炔烃)五羰基二钴配合物，为通过烯烃、炔烃与一氧化碳源偶联合成环戊烯酮的帕松-坎德反应(Pauson−Khand reaction, PKR)所提出的反应机理的第一步提供了结构模型。X射线晶体衍射数据显示，烯烃碳原子与最近的炔烃碳原子之间的距离约为2.85 Å，略短于密度泛函理论(DFT, density functional theory)预测的~2.95 Å。针对11b与12b的互变过程的变温核磁共振(VT NMR, variable temperature nuclear magnetic resonance)数据表明，正、逆反应的活化能分别为29和15 kcal mol−1，吸热过程的焓变为14 kcal mol−1；上述结果与此前DFT计算的预测值吻合极佳。尽管乙炔基六羰基二钴配合物11f不会轻易脱除CO，但它仍可与降冰片二烯发生分子间帕松-坎德反应(PKR)。同理，通过引入额外亚甲基以延长炔基侧链的尝试，仅得到了分子间PKR产物。研究认为，五羰基配合物12a−e无法沿PKR路径继续反应，根源在于钴配位的炔烃与烯烃的相对取向不利于反应进行。