Counterion-Dependent Valence Tautomerization of Ferrocenyl-Conjugated Pyrylium Salts

1-Ferrocenylethynylanthraquinone (1-FcAq), which is a donor (D)−acceptor (A) conjugated compound consisting of a ferrocene (Fc) acting as a donor, an anthraquinone (Aq) acting as an acceptor, and an ethynyl linker, undergoes a cyclocondensation reaction with strong organic acid, and forms 2-ferrocenyloxodihydrodibenzochromenylium salts ([1-FcPyl]+X− where X = TFSI, TfO, PF6, and BF4). [1-FcPyl]+ were also characterized as conjugated donor−acceptor compounds, and electrochemical properties, UV−vis absorption spectra, single-crystal X-ray analysis, and TD-DFT calculations have indicated that the LUMO level of [1-FcPyl]+ is lower than that of 1-FcAq because of the much larger π-conjugated system in [1-FcPyl]+. Variable-temperature Mössbauer spectroscopy (12−300 K) showed that Fe(II) was dominant for the TFSI−, PF6−, and BF4− salts of [1-FcPyl]+; although the Fe(III) species was also observed at all temperature ranges, the molar ratio of Fe(III) species increased at higher temperatures in the TFSI− and PF6− salts. This finding indicates that valence tautomerization (VT) between 1-FcPyl+ and 1-Fc+Pyl occurs in the solid state of the TFSI− and the PF6− salts, but not in the BF4− salt. Variable-temperature (3.5−310 K) IR spectroscopy showed that the frequencies of the skeletal vibration of the ferrocene moiety decreased with increasing temperature in the TFSI− and PF6− salts, indicating the development of a ferrocenium-like character. The precision of the bond lengths of the [1-FcPyl]+ moiety (0.003−0.004 Å) determined by single-crystal X-ray analysis (113 and 273 K) is not sufficient to demonstrate the effect of the counterion on VT. The dihedral angle between the ferrocene and the pyrylium moieties in the BF4− salt (11.25(15)°) is larger than that in the TFSI− (6.63(12)°) and PF6− (9.55(15)°) salts. Furthermore, the planarity of the acceptor moiety (estimated from the dihedral angle between Ph1 and Ph2) is lower in the BF4− salt compared with that of other salts. These increased dihedral angles might cause a weaker D−A interaction and a destabilization of the acceptor moiety (i.e., raising a LUMO level), leading to lower stability of the Fe(III) (1-Fc+Pyl) species. Variable-temperature X-ray powder diffraction (VT XRPD, 100−300 K) revealed that the temperature dependence of the Fe−P distance in the PF6− salt was smaller than that of the Fe−B distance in the BF4− salt. Our interpretation of this phenomenon is that the molar ratio of the Fe(III) species is increased in the PF6− salt, and that the Coulombic force between the ferrocene moiety and PF6− anion increases, preventing an increase in the Fe−P distance. This indicates that the electrostatic interaction between the [1-FcPyl]+ moiety and the counteranion may affect the occurrence of VT.