Nature of the stabilization of a carbenium ion adjacent to two organometallic groups. [Mo2Cp2(CO)4(.mu.-FcCHC:C(CH2)2CH3]+, BF4-: x-ray structure and NMR dynamic investigation

[Co2 (CO)6 (Alkynyl)] Complexes of Dibenzosuberyl and Dibenzosuberenyl Carbocations: Dibenzotropylium or Dibenzoheptafulvene?

The reaction of dibenzo[a,d]cycloheptan-5-one (dibenzosuberone) and dibenzo[a,d]cyclohept-10-en-5-one (dibenzosuberenone) with aryl- or trimethylsilylacetylides led to the formation of the corresponding alkynyldibenzosuberols and alkynyldibenzosuberenols. Treatment with dicobalt octacarbonyl and then with bis(diphenylphosphino)methane (dppm) furnished the corresponding [Co₂ (CO)₄ (dppm)(alkynol)] clusters 25 and 29. Upon protonation with HBF₄ at 203 K to generate the relevant cobalt-stabilised cations, the dibenzosuberyl system 30 exhibited fluxionality such that the cation migrated between cobalt centres. Variable-temperature ³¹ P NMR spectroscopy revealed a barrier of approximately 12.5 kcal mol^-1 . In contrast, in the supposedly aromatic [Co₂ (CO)₄ (dppm)(dibenzosuberenyl)]⁺ cation (31), which would be expected to have less need of cobalt stabilisation, the barrier was too high to be measured experimentally, but is certainly in excess of 16 kcal mol^-1 . These data were rationalised by DFT calculations on the structures and energies of the relevant ground states and transition states, which suggested that the nonplanar alkynyldibenzosuberenyl moiety in 31 is better regarded as a neutral dibenzoheptafulvene coordinated to a cationic alkynyl-dicobalt cluster. The question of the bonding of both aromatic and antiaromatic cations to alkyne-dicobalt clusters is considered, and it is proposed that their stabilities, when complexed, parallel the inversion of (4n+2) π and 4n π systems seen under photochemical conditions.

Bonding, Hyperfine Interactions, and Lattice Dynamics of Cationic and Neutral Ferrocenyl-Substituted Allylic and Cumulenic Compounds

The synthesis and spectroscopic characterization (by mass, UV-visible, IR, NMR and (57)Fe Mössbauer spectroscopy) of three new compounds related to seven recently published ferrocenyl-substituted allylic and cumulenic compounds are reported and have made it possible to effect a systematic evaluation of the parameters extractable from temperature-dependent gamma ray resonance studies. For the neutral species, the hyperfine parameters (IS and QS) of the sigma-bonded ferrocenyl groups are insensitive to the structural details of the allylic or cumulenic carbon framework. The temperature dependence of the recoil-free fraction is dominated by the Cp ring-iron atom interaction, and is not diagnostic of the detailed molecular architecture. In the case of the carbocationic complexes, two distinct iron sites are resolvable from the Mössbauer data. The detailed temperature-dependent behavior of the hyperfine parameters and recoil-free fraction for 10 complexes with either equivalent termini, linked by similar, but different types of bridges (neutral 1-5), or with inequivalent moieties linked by related or identical connectors (cationic 6-10), has been elucidated over the temperature range 90 </= T </= 300 K. An X-ray crystallographic study of (R,S)-ferroceno[2,3-a]inden-1-one proved the racemic nature of this progenitor of compound 10: monoclinic, P2(1)/c, a = 9.29.3(2) pm, b = 1083.7(2) pm, c = 1345.7(3) pm, beta = 108.41(3), V = 1.2859(5) nm(3), Z = 4, R(F(o)) = 0.0574, R(w)() (F(o)(2)) = 0.1394.