Search for Electron Delocalization from [Fe(CN)₆]³ˉ to the Dication of Viologen in (DNP)₃[Fe(CN)₆]₂·10H₂O


K3Fe(CN)6 reacts with the viologen 1,1′-bis(2,4-dinitrophenyl)-4,4′-bipyridinium dication, (DNP)2+, to form a supramolecular complex, (DNP)3[Fe(CN)6]2·10H2O (1). The crystal structure of 1 reveals that there are two [Fe(CN)6]3- anions within an organic framework of three (DNP)2+ cations with the shortest Fe(III)···Fe(III) distances of ca. 9.8 Å, distances that minimize extensive long-range magnetic exchange coupling interactions between the [Fe(CN)6]3- anions, and, thus, 1 is paramagnetic above ca. 17 K and exhibits weak ferromagnetic coupling between 17 and 3 K and antiferromagnetic coupling between 3 and 1.8 K. The long Fe(III)···Fe(III) distances permit slow spin-spin and slow spin-lattice paramagnetic relaxation, relative to the iron-57 Larmor precession frequency, as is evidenced by the Mössbauer spectra measured between 3 and 60 K; between 85 and 295 K, rapid paramagnetic relaxation is observed. Both the slow spin-spin and slow spin-lattice relaxation are mediated by the organic, π-conjugated viologen cations. The Fe-C distances, the Mössbauer isomer shifts, the temperature dependence of the magnetic susceptibility, and the 3 K magnetization results all indicate the presence of low-spin Fe(III) ions in the [Fe(CN)6]3- anions in 1. There is no unequivocal indication of the presence of any formal electron delocalization or transfer from the [Fe(CN)6]3- anion to the (DNP)2+ cations in the results obtained from X-ray crystallography, magnetic measurements, and Mössbauer spectra. Because of enhancement of the spin-orbit coupling by the heavy-atom or-ion effect, the Fe(III) ions in the [Fe(CN)6]3- anions interact with the (DNP)2+ cations, causing them to fluoresce with increasing intensity upon cooling from 90 to 25 K when excited at 300 nm. The resulting luminescence of the viologen (DNP)2+ cation induced by the [Fe(CN)6]3- anions indicates the presence of significant mixing of the molecular orbitals derived from the [Fe(CN)6]3- anions and the molecular orbitals associated with the (DNP)2+ cations to yield bonding supramolecular orbitals in 1, a mixing that is also observed between 50 and 3 K in the temperature dependence of the isomer shift of 1.




This work was supported by the DFG, Center for Functional Nanostructures, and the Swiss National Science Foundation (Contract 200020-125175).

Keywords and Phrases

Removal; Ethylenediaminetetraacetic acids; Solid phase

International Standard Serial Number (ISSN)

0020-1669; 1520-510X

Document Type

Article - Journal

Document Version


File Type





© 2017 American Chemical Society (ACS), All rights reserved.