Metallocene-Terminated Allylium Salts: The Effect of End Group on Localization in Polymethines

A range of 1,3-di(metallocenyl)allylium salts [Mc(CH)3Mc‘]+[X]- {Mc, Mc‘ = ferrocenyl (Fc),
2,3,4,5,1‘,2‘,3‘,4‘-octamethylferrocen-1-yl (Fc‘ ‘), ruthenocenyl (Rc); X = BF4, PF6} was synthesized by
reaction of (2-lithiovinyl)metallocenes with formylmetallocenes, followed by treatment of the resulting alcohols
with HX. Two salts with X = BAr‘4 {Ar‘ = 3,5-(CF3)2C6H3} were synthesized by anion metathesis from the
corresponding PF6 salts. The crystal structure of [Fc‘ ‘(CH)3Fc‘ ‘]+[PF6]- contains symmetrical termethine
cations, while the same appears to be true in the disordered structure of [Fc(CH)3Fc]+[PF6]-. The formally
unsymmetrical cation in [Fc(CH)3Fc‘ ‘]+[BF4]- is only slightly unsymmetrical with little bond-length alternation
in the allylium bridge. In contrast, the crystal structures of [Rc(CH)3Rc]+[PF6]- and [Rc(CH)3Rc]+[BAr‘4]-
both contain a bond-alternated “Peierls-distorted” cation, which can be considered as a ruthenocene bridged
to a [(η6-fulvene)(η5-cyclopentadienyl)ruthenium] cation by a vinylene moiety. The strong similarity between
solid-state and solution infrared and Raman spectra of [BF4]-, [PF6]-, and [BAr‘4]- salts of [Rc(CH)3Rc]+
indicates that the C−C stretching constant in the allylium chain and, therefore, the structure, of this ion are
largely independent of the local environment, suggesting that the unsymmetrical structures observed in
the crystal structures are not simply an artifact of packing. Differences in the solvatochromism of [Rc(CH)3Rc]+ and [Fc(CH)3Fc]+ also suggest a localized structure for the former cation in solution.
Electrochemistry, UV−visible−NIR spectroscopy, and DF calculations give insight into the electronic structure
of the metallocene-terminated allylium cations. Using an analogy between polymethines and mixed-valence
compounds, the difference between the behaviors of [Fc(CH)3Fc]+ and [Rc(CH)3Rc]+ is attributed to larger
reorganization energy associated with the geometry differences between metallocene and [(η6-fulvene)(η5-cyclopentadienyl)metal] structures in the ruthenium case.