Counterion-Dependent Valence Tautomerization of Ferrocenyl-Conjugated Pyrylium Salts

Abstract

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, PF(6), and BF(4)). [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 pi-conjugated system in [1-FcPyl](+). Variable-temperature Mossbauer spectroscopy (12-300 K) showed that Fe(II) was dominant for the TFSI(-), PF(6)(-), and BF(4)(-) 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 PF(6)(-) 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 PF(6)(-) salts, but not in the BF(4)(-) 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 PF(6)(-) salts, indicating the development of a ferrocenium-like character. The precision of the bond lengths of the [1-FcPyl](+) moiety (0.003-0.004 A) 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 BF(4)(-) salt (11.25(15) degrees) is larger than that in the TFSI(-) (6.63(12) degrees) and PF(6)(-) (9.55(15) degrees) salts. Furthermore, the planarity of the acceptor moiety (estimated from the dihedral angle between Ph1 and Ph2) is lower in the BF(4)(-) 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 PF(6)(-) salt was smaller than that of the Fe-B distance in the BF(4)(-) salt. Our interpretation of this phenomenon is that the molar ratio of the Fe(III) species is increased in the PF(6)(-) salt, and that the Coulombic force between the ferrocene moiety and PF(6)(-) 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.

Oxidized Styrylruthenium–Ferrocene Conjugates: From Valence Localization to Valence Tautomerism

“…As was previously found for other valence‐tautomeric systems,[15b], [15e], [36b], , the distribution of the different VTs of the 2 ‐acac + radical cation is affected by its environment. As is illustrated in Figure , the positions and relative intensities of the ν(CO) bands change in different solvents.…”

Why is Ferrocene so Exceptional?

“…This taumerization, in which the cationic iron sandwich form is favored at high temperatures for the PF 6 – salt, was not observed for the BF 4 – counterpart, due to increased coulombic attraction between this smaller counter‐anion and the localized oxonium charge. This situation is somewhat reminiscent of that of α‐ferrocenylcarbonium (vide supra) . Along similar lines, this research group also reported remarkable ferrocene–dithiolene hybrids that display temperature‐gated proton‐coupled electron transfer or reversed intramolecular charge transfer in which the redox properties of ferrocene play a main role.…”

“…The Nishihara group discovered a counter‐anion‐dependent and thermally modulated valence tautomerization of ferrocenyl‐conjugated pyrylium salts formed by strong‐acid‐mediated cyclocondensation of 1‐(ferrocenylethynyl)anthraquinone yielding 2‐ferrocenyl‐7‐oxodihydrodibenzochromenylium salts [Equation ] …”

Double Protonation of 1,5-Bis(triarylaminoethynyl)anthraquinone To Form a Paramagnetic Pentacyclic Dipyrylium Salt

“…The IVCT band of [1,5-Am 2 Pyl 2 ] 2+ was shifted to longer wavelengths relative to [1-AmPyl] + and [1-FcPyl] + 10,11 because its pentacyclic structure possesses a more extended π-conjugated system compared with the tetracyclic compounds. The lowering of the LUMO level in [1,5-Am 2 Pyl 2 ] 2+ relative to other reported protonated compounds [10][11][12] was expected to produce more effective intramolecular charge transfer. Cyclic voltammetry of 1,5-Am 2 Aq, [1,5-Am 2 Pyl] + , and [1,5-Am 2 Pyl 2 ] 2+ in 0.1 M Bu 4 NClO 4 -CH 2 Cl 2 revealed two reversible one-electron reduction steps ascribed to the anthraquinone (Aq) and its protonated moieties (Pyl + and Pyl 2 2+ ) (Supporting Information, Figure S9).…”

“…Because the electronic structure of a molecule directly influences its physical properties, 5-7 the electronic properties can be employed to achieve switching behavior in molecular devices. 8,9 We have previously described a new class of D-A molecules comprising a 1:1 donor/acceptor formula, 1-RAq, [10][11][12] in which R () ferrrocenyl (Fc), phenyl, m-tolyl, or p-tolyl) and Aq (anthraquinone) undergo intramolecular cyclization to give pyrylium cations [1-RPyl] + . This phenomenon can be used to lower the energy of the lowest unoccupied molecular orbital Inorg.…”