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Open AccessArticle

A Charge-Transfer Salt Based on Ferrocene/Ferrocenium Pairs and Keggin-Type Polyoxometalates

1
Departamento de Química Inorgánica, Facultad de Ciencia y Tecnología, Universidad del País Vasco UPV/EHU, P.O. Box 644, 48080 Bilbao, Spain
2
Centro de Física de Materiales CFM (CSIC–UPV/EHU), Paseo Manuel Lardizabal 5, 20018 Donostia-San Sebastián, Spain
3
Departamento de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco UPV/EHU, P. O. Box 644, 48080 Bilbao, Spain
4
BCMaterials, Basque Center for Materials, Applications and Nanostructures, UPV/EHU Science Park, 48940 Leioa, Spain
*
Author to whom correspondence should be addressed.
Molecules 2018, 23(12), 3150; https://doi.org/10.3390/molecules23123150
Received: 13 November 2018 / Revised: 27 November 2018 / Accepted: 29 November 2018 / Published: 30 November 2018
(This article belongs to the Special Issue Polyoxometalates and Polyoxometalate-Based Systems)
A new hybrid inorganic-organometallic salt has been obtained from the reaction of the Keggin-type silicotungstate anion with ferrocene in a water/methanol mixture as a result of the partial oxidation of ferrocene molecules to ferrocenium cations. Single-crystal X-ray diffraction analysis reveals the presence of four ferrocenium (FeIII) cations and one ferrocene (FeII) molecule per plenary Keggin anion in the crystal structure of [FeIII (Cp)2]4[SiW12O40]·[FeII(Cp)2]·2CH3OH (1). Compound 1 thus constitutes the first example in the literature in which ferrocenium and ferrocene species coexist in the structure of a polyoxometalate-based salt. The two crystallographically independent ferrocenium species in the asymmetric unit of 1 exhibit different configurations: One displays an eclipsed conformation with ideal D5h symmetry, whereas the conformation in the other one is staggered D5d. The crystal packing of 1 can be best described as an organometallic sub-lattice of ferrocenium and ferrocene species linked by a network of π-π interactions that generates rectangular cavities of about 14 × 10 Å in which strings of Keggin anions and methanol molecules are hosted, further connected to each other via weak OPOM···CMeOH-OMeOH···OPOM type hydrogen bonds. The charge-transfer nature of the salt has been studied by solid-state diffuse reflectance UV-Vis spectroscopy and the presence of magnetically isolated FeIII/FeII centres has been confirmed by Mössbauer spectroscopy. A topological study carried out on all of the pristine ferrocenyl species deposited in the Cambridge Structural Database (CSD) has allowed two main conclusions to be drawn: (1) these species tend to adopt extreme conformations (either eclipsed or staggered) with less than a 15% of examples showing intermediate states and (2) the oxidation state of the iron centres can be unequivocally assigned on the basis of a close inspection of the Fe···Cp distances, which allows ferrocene neutral molecules and ferrocenium cations to be easily distinguished. View Full-Text
Keywords: polyoxometalates; ferrocene; single-crystal x-ray diffraction; mössbauer spectroscopy polyoxometalates; ferrocene; single-crystal x-ray diffraction; mössbauer spectroscopy
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MDPI and ACS Style

Artetxe, B.; Iturrospe, A.; Vitoria, P.; Ruiz-Bilbao, E.; S. Garitaonandia, J.; Gutiérrez-Zorrilla, J.M. A Charge-Transfer Salt Based on Ferrocene/Ferrocenium Pairs and Keggin-Type Polyoxometalates. Molecules 2018, 23, 3150.

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