C–H Bond Activation of Simple Alkenes with (C 5 H 5 )CO Fragments: Preparation, Molecular Structure, and Dynamical Behaviour of Tri‐ and Tetranuclear Cyclopentadienylcobalt Cluster complexes with μ‐Cycloalkyne Ligands

Vicinal C–H bonds of the cycloalkenes C n H 2n (n = 5–8) were activated upon treatment under mild conditions with reactive sources of the (C 5 H 5 )CO fragment. With [C 5 H 5 )CO(C 2 H 4 ) 2 ] ( 2a ), the trinuclear cluster complexes [H 2 ((C 5 H 5 )CO) 3 (μ 3 ‐C 2 (CH 2 ) n‐2 )] ( 5a : n = 5; 5c : n = 5d : n = 8) were formed. Using the more reactive [(C 5 H 5 ) 2 Co]/K, 5a – 5d were obtained, along with small amounts of the tetranuclear [{(C 5 H 5 )CO} 4 r{μ 4 ‐C 2 (CH 2 ) n‐2 }] ( 8a–c : n = 5–7). X‐ray structure analyses of 5c, d and 8b, c were performed. A μ‐‖ (orμ 3 ‐η 1 :η 2 :η 1 ) coordination of the cycloalkyne ligands was found in the trinuclear cluster complexes. The tetranuclear 8b, c had the cycloalkynes quadruply bridging (μ 4 ‐η 1 :η 2 :η 2 :η 1 _ a butterfly arrangement of the metal atoms. The acetylenic carbon–carbon bonds are considerably lengthened upon coordination [1.392(4), 1.398(3) å in 5c d ; 1.464(8), 1.499(5) Å in 8b, c ]. Three dynamic processes – alkyne “walk” on top of the CO 3 triangle, μ 2 ‐hydride migration, and μ 2 /μ 3 ‐hydride exchange – are operational in the trinuclear 5a–d. Dynamic NMR data for 5a are consistent with a “windshield‐wiper” mechanism for the alkyne migration. Hydride exchange is associated with a higher energy barrier, and is probably and independent dynamic process.