Title

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

Abstract

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.

Department(s)

Chemistry

Comments

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

Citation

File Type

text

Language(s)

English

Rights

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

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