2D Conductive Iron-Quinoid Magnets Ordering up to T_c = 105 K via Heterogenous Redox Chemistry

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Abstract

We report the magnetism and conductivity for a redox pair of iron-quinoid metal-organic frameworks (MOFs). The oxidized compound, (Me₂NH₂)₂[Fe₂L₃]·2H₂O·6DMF (LH₂ = 2,5-dichloro-3,6-dihydroxo-1,4-benzoquinone) was previously shown to magnetically order below 80 K in its solvated form, with the ordering temperature decreasing to 26 K upon desolvation. Here, we demonstrate this compound to exhibit electrical conductivity values up to σ = 1.4(7) × 10-2 S/cm (E_a = 0.26(1) cm-1) and 1.0(3) × 10-3 S/cm (E_a = 0.19(1) cm-1) in its solvated and desolvated forms, respectively. Upon soaking in a DMF solution of Cp₂Co, the compound undergoes a single-crystal-to-single-crystal one-electron reduction to give (Cp₂Co)_1.43(Me₂NH₂)_1.57[Fe₂L₃]·4.9DMF. Structural and spectroscopic analysis confirms this reduction to be ligand-based, and as such the trianionic framework is formulated as [FeIII₂(L3-•)₃]3-. Magnetic measurements for this reduced compound reveal the presence of dominant intralayer metal-organic radical coupling to give a magnetically ordered phase below T_c = 105 K, one of the highest reported ordering temperatures for a MOF. This high ordering temperature is significantly increased relative to the oxidized compound, and stems from the overall increase in coupling strength afforded by an additional organic radical. In line with the high critical temperature, the new MOF exhibits magnetic hysteresis up to 100 K, as revealed by variable-field measurements. Finally, this compound is electrically conductive, with values up to σ = 5.1(3) × 10-4 S/cm with E_a = 0.34(1) eV. Taken together, these results demonstrate the unique ability of metal-quinoid MOFs to simultaneously exhibit both high magnetic ordering temperatures and high electrical conductivity.

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