Ferrocenyl naphthalenes: substituent- and substitution pattern-depending charge transfer studies

The synthesis of a series of ferrocenyl-functionalized naphthalenes of type 2-Fc-C₁₀H₇ (3a), 1-Fc-2-R-C₁₀H₆ (3b, R = OMe; 3c, R = Me; 3d, R = H; 3e, R = CH(O)), 1,1'-(C₁₀H₇)₂Fc' (4), 1-Br-4-Fc-C₁₀H₆ (6a), 1-Br-5-Fc-C₁₀H₆ (6b), 1-Br-8-Fc-C₁₀H₆ (6c), 2-Br-6-Fc-C₁₀H₆ (6d), 1,4-Fc₂-C₁₀H₆ (7a), 1,5-Fc₂-C₁₀H₆ (7b), 1,8-Fc₂-C₁₀H₆ (7c) and 2,6-Fc₂-C₁₀H₆ (7d) (Fc = Fe(η⁵-C₅H₄)(η⁵-C₅H₅), Fc' = Fe(η⁵-C₅H₄)₂) is reported. They are accessible either by the Suzuki-Miyaura or Negishi C,C cross-coupling reaction of FcB(OH)₂ (1a) or FcZnCl (1b) with the appropriate bromo-naphthalenes 2a-e and 5a-d, respectively. The molecular structures of 3a-c, 3e, 4, 6b-d and 7a-d in the solid state were determined by single-crystal X-ray diffraction analysis. They show inter- (3b,c,e, 6b,d, 7a) and intramolecular (7c) π-interactions in the form of T-shaped or parallel displaced π arrangements (3c,e, 6b), whereby 3e displays a columnar stacking of the condensed aromatic unit with plane distances of 3.485(5) to 3.525(5) Å. The (spectro)electrochemical behaviour of 3-4 and 6-7 in dichloromethane in the presence of the weakly coordinating anion [B(C₆F₅)₄]^- is discussed, showing reversible redox events in the range of -140-150 mV vs. FcH/FcH⁺. The electrochemical response of 3a-e and 4 depends on the electron-withdrawing and -donating groups present. The redox processes of mono Fc-substituted 6a-d are affected by the naphthalene substitution pattern, which also influences the redox separations ΔE of Fc₂-naphthalenes 7a-d, confirming a significant effect of the different electron transfer pathways through the aromatic core. The UV/vis/NIR spectra of mixed-valent [7a,b,d]⁺ show broad and weak absorptions in the NIR region, allowing a classification as weakly coupled class II systems according to Robin and Day.