Comparison between a Diarylferrocenylmethylium Ion and Its Isolobal Cobalt Species: Similarities and Differences

Abstract

Reaction of {(1,2,8,9-η)-tricyclo[7.5.0.01,7]tetradeca-1,8-diene}{η5-(methoxycarbonyl)cyclopentadienyl}cobalt (9) with either 2 mol of arylmagnesium bromide or aryllithium and subsequent hydrolysis yields the alcohols {(1,2,8,9-η)tricyclo[7.5.0.01,7]tetradeca-1,8-diene}{η5-(diarylhydroxymethyl)cyclopentadienyl}cobalt (13a−c). The treatment of the yellow colored alcohols 13a−c with HBF4 in ether affords the tetrafluoroborates of the violet colored carbocations 14a−c. Investigations of single crystals of 14b by means of the X-ray technique show a displacement of the C1−Cexo bond by α = 10.8° toward the metal and a pronounced bond alternation in the fulvene moiety. The bending of C1−Cexo is considerably larger than in the corresponding alcohol 13b (α = 2.5°). The interaction between the exo-carbon and the metal in 14b is further substantiated by comparison of the 13C-NMR chemical shift of 14b with 13b (Δδ = −65 ppm). The comparison of the data obtained for 14b with those of diphenylferrocenylcarbenium ion 1b suggests a stronger bonding between metal and fulvene moiety in 1b as compared to 14b. These results are substantiated by the results of ab initio calculations on 1b and 4a which show a smaller positive charge at the exo-carbon in 4a than in 1b and a stronger bond index between the exo-carbon and the metal in 1b as compared to 4a.