Charge Delocalization and Bulk Electronic Conductivity in the Mixed-Valence Metal–Organic Framework Fe(1,2,3-triazolate)₂(BF₄)_<i>x</i>

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(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 Fe^II/III centers. Further, Mössbauer spectroscopy indicates the presence of a valence-delocalized Fe^II/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.