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 AgReO 4 in dichloromethane and the ferrocenium(1+) salts were isolated, after recrystallization from acetone, as dark-blue or dark-green crystals of [1,2-BUT + ][ReO 4 − ] and [1,3-BUT + ][ReO 4 − ]. 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 Fe-C distances in both cations span a greater range and are, on average, longer than in the corresponding neutral ferrocenes. UV-vis spectra revealed that λ max for yellow 1,2-BUT (442 nm) is lower than for orange 1,3-BUT (466 nm) and that λ max for blue 1,2-BUT + (674 nm) is smaller than for green 1,3-BUT + (682 nm). In addition, E 1/2 (+/0) values determined by cyclic voltammetry in dichloromethane, reported here for the first time, show that 1,3-BUT (−0.24 V vs. Fe(Cp) 2 +/0) is 50 mV easier to oxidize than 1,2-BUT (−0.19 V vs. Fe(Cp) 2 +/0). A comparison of the structure of [1,2-BUT + ][ReO 4 − ] with that of the literature compound [1,3-DEC + ][ReO 4 − ] shows that the shortest Fe ... Re distances in the two salts are ca. 5.8 Å, 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-2nonyl)ferrocenium(1+)). This surprising result is significant as far as differences in anionextraction equilibria reported previously.