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The Coupling of N‑Heterocyclic Carbenes to Terminal Alkynes at Half Sandwich Cobalt NHC Complexes

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journal contribution
posted on 10.06.2019, 00:00 by Katharina Lubitz, Udo Radius
Studies on the reactivity of complexes of the type [(η5-C5H5)­Co­(NHC)­(olefin)] with internal and terminal alkynes are presented. The half sandwich complexes [(η5-C5H5)­Co­(iPr2Im)­(η2-C2H4)] (1, iPr2Im = 1,3-di-isopropyl-imidazolin-2-ylidene) and [(η5-C5H5)­Co­(Dipp2Im)­(η2-C2H3SiMe3)] (2, Dipp2Im = 1,3-bis­(2,6-di-isopropyl-phenyl)­imidazolin-2-ylidene) are not good catalysts for the [2 + 2 + 2] cyclotrimerization of diphenylacetylene and phenylacetylene. Stoichiometric reactions of 1 and 2 with diphenylacetylene, phenylacetylene, and p-tolylacetylene have shown that the behavior of internal and terminal alkynes toward complexes [(η5-C5H5)­Co­(NHC)­(olefin)] is different. The reaction of 1 and 2 with diphenylacetylene leads to the formation of [(η5-C5H5)­Co­(R12Im)­(η2-C2Ph2)] (R1 = iPr 3, Dipp 4). Complex 4 reacts at elevated temperatures with another equivalent of diphenylacetylene to afford [(η5-C5H5)­Co­(η4-C4Ph4)] (5). Complexes 1 and 2 react with phenylacetylene and p-tolylacetylene with coupling of the NHC and the alkyne in the coordination sphere of the cobalt atom. Depending on the NHC ligand used, the complexes [(η5-C5H5)­Co­(C­{R}C­{H}­{Dipp2Im})] (R = Ph 6, Tol 7) and [(η5-C5H5)­Co­(η4-{(RER-NHC)­C4H2R2}] (R = Ph 8, Tol 9) are formed. In course of these reactions the NHCs migrate from the metal atom to the alkyne α carbon atom. For the NHC iPr2Im a second equivalent of the alkyne reacts with 1 with ring opening of the NHC.