Combining Cobalt‐Assisted Alkyne Cyclotrimerization and Ring Formation through C–H Bond Activation: A “One‐Pot” Approach to Complex Multimetallic Structures

Abstract

Multiferrocenyl‐substituted benzenes could be obtained in a “one‐pot” reaction of 1,4‐diferrocenylbutadiyne with substoichiometric amounts of [CoCp(CO)2] (Cp = cyclopentadienyl) by combining cobalt‐assisted formal [2+2+2] cycloaddition with C–H bond activation at a single catalyst. A mechanism for the reaction is discussed. The newly synthesized hexaferrocenyl species belongs to the rather scarcely examined family of multimetallic aromatic compounds. It is one of the few examples in which the ferrocenyl units are either single bonded or 1,2‐substituted to give rise to planar chirality at three of the ferrocenyl units. Electrochemical studies showed that up to six consecutive ferrocenyl‐based redox events are observed. Within in situ spectro‐electrochemical measurements of both compounds it is shown that two intervalence charge‐transfer pathways are possible. At different excitation energies either a charge transfer through the peripheric ethylene bridging unit or through the central benzene unit occurs.

Multiferrocenyl Cobalt‐Based Sandwich Compounds

“…The formation of compounds 5a , b including cobalt‐assisted cyclotrimerization and subsequent ring‐closing C–H activation reactions has been discussed previously . Within these studies, we were able to isolate and characterize a third isomer (compound 5c ) (Figure ) (see the Experimental Section).…”

“…Reaction of FcC≡C–C≡CFc [Fc = Fe(η 5 ‐C 5 H 4 )(η 5 ‐C 5 H 5 )] ( 1 ) with Co(η 5 ‐C 5 H 5 )(CO) 2 ( 2 ) affording 3 – 7 . For 5a , b see ref …”

“…Compounds 3 – 7 are accessible by a “one‐pot” synthesis procedure including the following three reactions: (i) cyclodimerization of 1 affording cobalt cyclobutadiene cyclopentadienyl species 3a , b and 4a , b , (ii) cyclotrimerization and C–H bond activation yielding 5a , b and 5c , and (iii) cyclodimerization with consecutive CO insertion producing cobalt cyclopentadienone cyclopentadienyl compounds 6a , b , c and 7 (Figure ).…”

“…Within these studies, we were able to isolate and characterize a third isomer (compound 5c ) (Figure ) (see the Experimental Section). The formation of this compound is based on the mechanism reported for 5a , b with the generation of a symmetric type A molecule in situ (for a more detailed discussion see ref ).…”

“…Formation of 5a – c from 1 and 2 via intermediates A and B . For a detailed mechanism for the formation of 5a , b see ref …”