It has been suggested that using π basic mixed-valence metals with low-spin d5/6 electronic configurations that are octahedrally coordinated to π acidic bridging ligands could provide high electronic interaction and delocalization throughout a coordination compound. One reported phase with an intriguing electronic structure is Fe(tri)2 (tri– = 1,2,3-triazolate) with octahedral FeII ions bridged by π acidic azolate ligands. Stoichiometric oxidants such thianthrenium tetrafluoroborate were utilized to oxidize FeII centers and afford [FeII(1-x)FeIIIx(tri)2](BF4)x (x = 0.1, 0.25, and 0.5) frameworks. Our preliminary results suggest that the initial conductivity of Fe(tri)2, &lt; 10–9 Scm–1, can be increased up to 0.3 Scm–1 through a careful stoichiometric oxidation of FeII centers. Mössbauer and UV-vis-NIR spectra suggest that this unusually large increase in the conductivity can be attributed to the intervalence charge transfer between low-spin FeII and low-spin FeIII centers. This mixed-valence FeII/III framework displays the highest electrical conductivity for any three dimensionally conducting metal-organic frameworks. In addition, Cr triazolate framework was synthesized by a solvothermal reaction of CrII salt and 1,2,3-triazole. Elemental analysis, N2 adsorption isotherm, and UV-vis-NIR spectrum suggest a spontaneous oxidation of some CrII centers and yields a mixed-valence CrII/III framework, [CrII0.73CrIII0.27(tri)2](BF4)0.27. This new, mixed-valence CrII/III framework is permanently porous with the BET surface area of 130 m2g–1 and displays the electrical conductivity of 6.7 × 10–3 Scm–1. Magnetic susceptibility measurements indicate a spontaneous magnetization with the ferrimagnetic ordering temperature of 250 K, which is the highest for a porous magnet. These results highlight the advantage of transition-metal azolate system as a promising scaffold for achieving long-range electronic conduction and high temperature magnetic ordering in porous MOFs.

Electronic conductivity and magnetic ordering in mixed-valence M(1,2,3-triazolate)2 (M = CrII/III, FeII/III) metal-organic frameworks

253rd National Meeting (2017)

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