Charge Delocalization and Bulk Electronic Conductivity in the Mixed-Valence Metal-Organic Framework Fe(1,2,3-Triazolate)₂(BF₄)ₓ


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(BF4)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(BF4) 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(BF4)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 FeII/III centers. Further, Mössbauer spectroscopy indicates the presence of a valence-delocalized FeII/III species in Fe(tri)2(BF4)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(BF4)x frameworks are discussed in detail.



Research Center/Lab(s)

Center for High Performance Computing Research


This research was supported by NSF award number DMR-1611525, with the exception of the measurement and analysis of the magnetic data, which were supported by the Nanoporous Materials Genome Center of the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences under Award No. DE-FG02-17ER16362.

Keywords and Phrases

Charge transfer; Conductive materials; Crystalline materials; Electric conductivity; Electronic structure; Metals; Organic polymers; Organometallics; Oxidation, Charge transport mechanisms; Electronic application; Electronic conductivity; Intervalence charge transfer; Metal organic framework; Powder X ray diffraction; Room-temperature conductivity; UV-vis-NIR diffuse reflectance spectroscopy, Iron compounds, iron; metal organic framework, Article; chemical structure; controlled study; diffuse reflectance spectroscopy; electric conductivity; infrared spectroscopy; Mossbauer spectroscopy; oxidation; stoichiometry; thermal conductivity; ultraviolet spectroscopy; X ray diffraction

International Standard Serial Number (ISSN)

0002-7863; 1520-5126

Document Type

Article - Journal

Document Version


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© 2018 American Chemical Society (ACS), All rights reserved.

Publication Date

01 Jul 2018