Metal Cluster Stabilized Fluorenyl, Indenyl, and Cyclopentadienyl Antiaromatic Cations: An NMR and X-ray Crystallographic Study

Treatment of fluorenone, 2,3-diphenylindenone, tetraphenylcyclopentadienone, or 2,5-diethyl-3,4-diphenylcyclopentadienone with ((trimethylsilyl)ethynyl)lithium gives, after hydrolysis, the analogous alkynol; subsequent addition of dicobalt carbonyl and then fluoroboric acid yields the corresponding fluorenyl, indenyl, or cyclopentadienyl cation stabilized by complexation to a tricarbonylcobalt moiety. Variable-temperature NMR data on these cluster cations, and on their bis(diphenylphosphino)methane derivatives, reveal that the barrier to migration of the cationic center between cobalt cluster vertices increases in the order fluorenyl < indenyl < cyclopentadienyl and suggest that the cations with more antiaromatic character have the greatest need for charge delocalization onto the metal center. Replacement of a Co(CO)3 cationic fragment by an Fe(CO)3 unit yields the mixed-metal species [((fluorenyl)CCSiMe3)FeCo(CO)6] (22) and [((2,3-diphenylindenyl)CCSiMe3)FeCo(CO)6] (27). In these structural models for the cationic complexes, the Fe−C(9) distance in 22 is 2.626(11) Å, while in the indenyl system 27 the Fe−C(1) distance is 2.347(7) Å, again indicating that the 8π indenyl cation interacts more strongly with the metal center than does the 12π fluorenyl cation.