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)ethyny l)lithium gives, after hydrolysis, the analogous alkynol; subsequent additi on of dicobalt carbonyl and then fluoroboric acid yields the corresponding fluorenyl, indenyl, or cyclopentadienyl cation stabilized by complexation t o a tricarbonylcobalt moiety. Variable-temperature NMR data on these cluste r cations, and on their bis(diphenylphosphino)methane derivatives, reveal t hat 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 specie s [((fluorenyl)=C=CSiMe3)FeCo(CO)(6)] (22) and [((2,3-diphenylindenyl)=C=CS iMe3)FeCo(CO)(6)] (27). In these structural models for the cationic complex es, the Fe-C(9) distance in 22 is 2.626(11) Angstrom, while in the indenyl system 27 the Fe-C(1) distance is 2.347(7) Angstrom, again indicating that the 8 pi indenyl cation interacts more strongly with the metal center than does the 12 pi fluorenyl cation.