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 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.