X-ray structures and electronic properties of the 1,1′,2,2′- and 1,1′,3,3′-tetra-t-butylferrocenium(1+) cations

Abstract

The compounds 1,1′,2,2′- and 1,1′,3,3′-tetra-t-butylferrocene (1,2-BUT and 1,3-BUT, respectively) were oxidized with AgReO4 in dichloromethane and the ferrocenium(1+) salts were isolated, after recrystallization from acetone, as dark-blue or dark-green crystals of [1,2-BUT+][ReO4−] and [1,3-BUT+][ReO4−]·acetone, respectively, which were suitable for X-ray diffraction. As expected from the known structures of the neutral ferrocenes, the di-t-butylcyclopentadienyl (Cp′) rings are virtually parallel in the 1,2-BUT+ cation and are tilted by 11.6° in the 1,3-BUT+ cation. The FeC distances in both cations span a greater range and are, on average, longer than in the corresponding neutral ferrocenes. UV–Vis spectra revealed that λmax for yellow 1,2-BUT (442nm) is lower than for orange 1,3-BUT (466nm) and that λmax for blue 1,2-BUT+ (674nm) is lower than for green 1,3-BUT+ (682nm). In addition, E1/2(+/0) values determined by cyclic voltammetry in dichloromethane, reported here for the first time, show that 1,3-BUT (−0.24V versus Fe(Cp)2+/0) is 50mV easier to oxidize than 1,2-BUT (−0.19V versus Fe(Cp)2+/0). A comparison of the structure of [1,2-BUT+][ReO4−] with that of the literature compound [1,3-DEC+][ReO4−] shows that the shortest Fe⋯Re distances in the two salts are ca. 5.8Å, indicating that the effective ion-pairing radii of the two cations are essentially the same in spite of the much greater overall size of the 1,3-DEC+ cation (1,3-DEC+=1,1′,3,3′-tetra(2-methyl-2-nonyl)ferrocenium(1+)). This surprising result is significant as far as differences in anion-extraction equilibria reported previously.