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.

(Trimethylsilyl)indene exists as a pair of enantiomers which interconvert by means of Me3Si migrations. The mechanism involves successive [1,5]-suprafacial sigmatropic shifts via the corresponding isoindene, which can be trapped with tetracyanoethylene as the Diels−Alder adduct, 7. The X-ray crystal structure of 7 reveals that the TCNE approaches from the opposite face to the Me3Si substituent. Tris(indenyl)methylsilane, 5, also undergoes a series of [1,5]-silatropic shifts which interconvert the RRR, RRS, RSS, and SSS isomers, where the R and S labels refer to the absolute configuration of C(1) in each indenyl ring. By use of 1H−1H COSY, 1H−13C and 1H−29Si shift-correlated NMR spectra, the proton and carbon-13 nuclei in all four different indenyl ring environments can be unequivocally assigned. The molecular dynamics of 5 have been elucidated by means of 1D-selective inversion experiments together with 2D-EXSY data, and the exchange pathways between indenyl sites can be conveniently mapped onto a hypercube. Moreover, 5 yields a triple Diels−Alder adduct, 12, with TCNE; the Si-CH3 1H NMR resonance in 12 is found at −1.8 ppm suggesting that this methyl lies inside a cavity formed by the three phenyls.

The favored pathways for the haptotropic shifts from (η6-cpp)ML n to (η5-cpp)ML n , where cppH is 4H-cyclopenta[def]phenanthrene and ML n = Fe(C5H5) or Mn(CO)3, have been investigated by means of extended Hückel molecular orbital calculations, and energy hypersurfaces for these processes have been obtained. These data suggest the intermediacy of an exocyclic (η3-cpp)ML n species, stabilized by the presence of a naphthalene-type 10π aromatic system. In an attempt to generate (η3-cpp)Fe(CO)(C5H5), the corresponding (η1-cpp)Fe(CO)2(C5H5) complex was prepared and allowed to decompose; the major products were [(C5H5)Fe(CO)2]2 and the cpp trimer, C15H8(C15H9)2, 20, that was shown to adopt a rigid geometry with C 2 symmetry. Treatment of (η5-cpp)Mn(CO)3 with triethylphosphine yields (η1-cpp)Mn(CO)3(PEt3)2, 23, which exhibits hindered rotation about the C(4)−Mn bond with a barrier of 16.5 kcal mol-1. The molecule [(η6-cppH)Fe(C5H5)]PF6, 2b, and also the manganese complex 23 have been characterized by X-ray crystallography. The relevance of these (and other literature data) to the mechanisms of haptotropic shifts are discussed.

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