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.