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

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)(BF) (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), and find that the conductivity increases dramatically with iron oxidation level. Notably, the most oxidized variant, Fe(tri)(BF), 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) and the Fe(tri)(BF) 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 centers. Further, Mössbauer spectroscopy indicates the presence of a valence-delocalized Fe species in Fe(tri)(BF) at 290 K, one of the first such observations for a metal-organic framework. The electronic structure of valence-pure Fe(tri) and the charge transport mechanism and electronic structure of mixed-valence Fe(tri)(BF) frameworks are discussed in detail.

Coordination-induced reversible electrical conductivity variation in the MOF-74 analogue Fe2(DSBDC)

“…Chemically, the dopant molecules may engage with the MOF either through outer-sphere electron transfer (i.e. redox reactivity) [22][23][24][25] or through inner-sphere reactivity. The latter involves binding of guest molecules to coordinatively unsaturated metal centers to form charge transport pathways and/or to inject charge carriers.…”

Electroactive Metalorganic Frameworks

“…Parallel to the above work, Long and co‐workers reported on electrical conductivity tuning of the iron(II) MOF Fe(tri) 2 (tri − =1,2,3‐triazolate), through a post‐synthesis chemical oxidation to achieve a mixed‐valence state . In this case, the oxidation of the framework was obtained by a structural modification and the incorporation of the tetrafluoroborate anion (BF 4 − ) into the framework, yielding the crystalline and porous phases Fe(tri) 2 (BF 4 ) x ( x =0.09, 0.22, 0.33).…”

Coupled Electrical Conduction in Coordination Polymers: From Electrons/Ions to Mixed Charge Carriers

“…[53] Interestingly, Park et al synthesized Fe(tri) 2 (BF 4 ) x (x = 0.09, 0.22, and 0.33), and found that BF 4 À ions entered the frameworks via partially oxidizing the Fe II node to Fe III . [54] The extra BF 4À ions trigger ion transfer in framework as shown in Figure 5. In addition, the electronic conductivity of this CP was elevated to 0.3 S cm À 1 .…”