Halide Effects on the Stability of Osmium Indenylidene Complexes: Isolation, Characterization, and Reactivities

[(η6-p-cymene)­OsI2]2 was found
to be a good catalyst for the cyclopropanation
reaction of isobutyl vinyl ether with 3-tert-butyldiazoindene.
In contrast, the complexes [(η6-p-cymene)­OsX2]2 (X = Cl, Br) are poorer catalysts
for the reaction. The interesting halide effect can be related to
the tendency of osmium indenylidene complexes [(η6-p-cymene)­OsX2(indenylidene)] (6) (derived from the reactions of [(η6-p-cymene)­OsX2] (5) with diazoindene,
X = Cl, Br, I) to undergo migratory insertion reactions to give η5-haloindenyl complexes. While the chloro indenylidene complex 6Cl readily rearranges to the corresponding η5-chloroindenyl complex 8Cl, the iodo indenylidene complex 6I can be stored in solutions for weeks without appreciable
decomposition. The thermal stability of the bromo indenylidene 6Br is between those of Cl- and I-substituted indenylidenes.
Computational studies have been carried out to understand the experimental
observations. The indenylidene complexes described here represent
the first well-characterized mononuclear indenylidene complexes synthesized
from diazoindene precursors.

[(η⁶-对伞花烃)二碘合锇(Ⅱ)]₂被证实是乙烯基异丁基醚（isobutyl vinyl ether）与3-叔丁基重氮茚（3-tert-butyldiazoindene）的环丙烷化反应（cyclopropanation）的高效催化剂。与之形成对比的是，配合物[(η⁶-对伞花烃)OsX₂]₂（X=Cl、Br）对该反应的催化性能较差。这一值得关注的卤离子效应（halide effect），可与锇茚亚基配合物（osmium indenylidene complexes）[(η⁶-对伞花烃)OsX₂(茚亚基)]（6，由[(η⁶-对伞花烃)OsX₂]（5）与重氮茚反应得到，X=Cl、Br、I）发生迁移插入反应（migratory insertion）生成η⁵-卤代茚基配合物（η⁵-haloindenyl complexes）的倾向相关。其中，氯代茚亚基配合物6Cl可快速重排为对应的η⁵-氯代茚基配合物8Cl，而碘代茚亚基配合物6I可在溶液中稳定存放数周而无显著分解。溴代茚亚基配合物6Br的热稳定性（thermal stability）介于氯代与碘代取代茚亚基配合物之间。为阐明该实验观测结果，研究人员已开展了计算研究（computational studies）。本文所报道的茚亚基配合物，是首例由重氮茚前驱体（diazoindene precursors）合成且得到充分表征的单核茚亚基配合物（mononuclear indenylidene complexes）。