Charge Delocalization and Bulk Electronic Conductivity in the Mixed-Valence Metal-Organic Framework Fe(1,2,3-triazolate) 2 (BF 4 ) x

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Abstract

Metal-organic frameworks are of interest for use in a variety of electrochemical and electronic applications, although a detailed understanding of their charge transport behavior, which is of critical importance for enhancing electronic conductivities, remains limited. Herein, we report isolation of the mixed-valence framework materials, Fe(tri) 2 (BF 4 ) x (tri - = 1,2,3-triazolate; x = 0.09, 0.22, and 0.33), obtained from the stoichiometric chemical oxidation of the poorly conductive iron(II) framework Fe(tri) 2 , and find that the conductivity increases dramatically with iron oxidation level. Notably, the most oxidized variant, Fe(tri) 2 (BF 4 ) 0.33 , displays a room-temperature conductivity of 0.3(1) S/cm, which represents an increase of 8 orders of magnitude from that of the parent material and is one of the highest conductivity values reported among three-dimensional metal-organic frameworks. Detailed characterization of Fe(tri) 2 and the Fe(tri) 2 (BF 4 ) x materials via powder X-ray diffraction, Mössbauer spectroscopy, and IR and UV-vis-NIR diffuse reflectance spectroscopies reveals that the high conductivity arises from intervalence charge transfer between mixed-valence low-spin Fe II/III centers. Further, Mössbauer spectroscopy indicates the presence of a valence-delocalized Fe II/III species in Fe(tri) 2 (BF 4 ) x at 290 K, one of the first such observations for a metal-organic framework. The electronic structure of valence-pure Fe(tri) 2 and the charge transport mechanism and electronic structure of mixed-valence Fe(tri) 2 (BF 4 ) x frameworks are discussed in detail.

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Park, J. G., Aubrey, M. L., Oktawiec, J., Chakarawet, K., Darago, L. E., Grandjean, F., … Long, J. R. (2018). Charge Delocalization and Bulk Electronic Conductivity in the Mixed-Valence Metal-Organic Framework Fe(1,2,3-triazolate) 2 (BF 4 ) x. Journal of the American Chemical Society, 140(27), 8526–8534. https://doi.org/10.1021/jacs.8b03696

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