STABLE AND CRYSTALLINE ALLYLIUM AND ALLENYLIUM SALTS WITH FERROCENYL SUBSTITUENTS

Allylic and allenylic carbenium ions, which have been reported in the literature to be labile or not isolable at all, have no inherent insta bility. Progressive substitution with ferrocenyl groups affords allyli c 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 met allocenes is illustrated in cyclopentadienyl( 1,3-diferrocenyl-1-ylium pentalenyl)iron tetrafluoroborate, a compound which survives exposure to sunlight in aqueous solution for days. In addition to full characte rization for these allylium salts by the usual analytical and spectros copic methods, X-ray analysis of cyclopentadienyl(1,3-diferrocenyl- 1- yliumpentalenyl)iron tetrafluoroborate showed (a) deviations of the re gular conformation of the ferrocenyl substituents in the 1,3-positions that are similar to those observed in other simple metallocenyl-stabi lized carbenium ions, indicative of electronic stabilization by intram olecular 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 dissymmetri c structure is most likely the result of crystal forces, and (c) the a nnelated ferrocene of this formal pentalene system is undistorted, rul ing out (together with results from solution NMR measurements) any sig nificant conjugation; therefore, this cation should be envisaged as an allylium system and not as an antiaromatic cyclopentadienyl cation. I n the case of allenylium <-> propargylium ions, attachment of two ferr ocenes results in labile or intermediate propargylium ions, which can be trapped with triethylamine to afford ammonium allenes, formally Lew is base complexed allenylium salts, with an interesting bent allene un it as is evidenced by single-crystal structure analysis. Introduction of a third ferrocene into the allenylium <-> propargylium system affor ds 1,3,3-triferrocenylallen-1-ylium tetrafluoroborate, the first stabl e and fully characterized allenylium compound, with only minor resonan ce contributions from the propargylium structure, proven most clearly by the observation of an extremely intense and acceptor-shifted cumule nic stretching vibration of 2151 cm(-1) in the IR spectrum.