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

Abstract

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.