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 (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.