Stable and crystalline allylium and allenylium salts with ferrocenyl substituents

Article abstract of DOI:10.1021/om00012a025

Allylic and allenylic carbenium ions, which have been reported in the literature to be labile or not isolable at all, have no inherent instability. Progressive substitution with ferrocenyl groups affords allylic cations, whose stability toward photochemical decay and nucleophilic collapse increases with increasing number of ferrocenyl substituents, if the allylic moiety is at least 2-fold-substituted with ferrocenes in the 1,3-positions. The unmatched cation-stabilizing capacity of metallocenes is illustrated in cyclopentadienyl(1,3-diferrocenyl-1-yliumpentalenyl)iron tetrafluoroborate, a compound which survives exposure to sunlight in aqueous solution for days. In addition to full characterization for these allylium salts by the usual analytical and spectroscopic methods, X-ray analysis of cyclopentadienyl(1,3-diferrocenyl-1-yliumpentalenyl)iron tetrafluoroborate showed (a) deviations of the regular conformation of the ferrocenyl substituents in the 1,3-positions that are similar to those observed in other simple metallocenyl-stabilized carbenium ions, indicative of electronic stabilization by intramolecular charge delocalization, (b) these two ferrocenyl substituents are structurally distorted in an unequal manner and seem to contribute unequally in this charge delocalization, but the observed dissymmetric structure is most likely the result of crystal forces, and (c) the annelated ferrocene of this formal pentalene system is undistorted, ruling out (together with results from solution NMR measurements) any significant conjugation; therefore, this cation should be envisaged as an allylium system and not as an antiaromatic cyclopentadienyl cation. In the case of allenylium ? propargylium ions, attachment of two ferrocenes results in labile or intermediate propargylium ions, which can be trapped with triethylamine to afford ammonium allenes, formally Lewis base complexed allenylium salts, with an interesting bent allene unit as is evidenced by single-crystal structure analysis. Introduction of a third ferrocene into the allenylium ? propargylium system affords 1,3,3-triferrocenylallen-1-ylium tetrafluoroborate, the first stable and fully characterized allenylium compound, with only minor resonance contributions from the propargylium structure, proven most clearly by the observation of an extremely intense and acceptor-shifted cumulenic stretching vibration of 2151 cm^-1 in the IR spectrum.