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

Abstract

We report the magnetism and conductivity for a redox pair of iron-quinoid metal-organic frameworks (MOFs). The oxidized compound, (MeNH)[FeL]·2HO·6DMF (LH = 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 S/cm (E = 0.26(1) cm) and 1.0(3) × 10 S/cm (E = 0.19(1) cm) in its solvated and desolvated forms, respectively. Upon soaking in a DMF solution of CpCo, the compound undergoes a single-crystal-to-single-crystal one-electron reduction to give (CpCo)(MeNH)[FeL]·4.9DMF. Structural and spectroscopic analysis confirms this reduction to be ligand-based, and as such the trianionic framework is formulated as [Fe(L)]. Magnetic measurements for this reduced compound reveal the presence of dominant intralayer metal-organic radical coupling to give a magnetically ordered phase below T = 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 S/cm with E = 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.

Is iron unique in promoting electrical conductivity in MOFs?

“…40,41 This has been shown already with two MOFs based on 1,2,4,5-tetrahydroxybenzene and its derivatives, where the ligands coexist in the semiquinone and quinone states, which gives rise to high electrical conductivity (10 –3 to 10 –1 S cm –1 ). 42,43 Therefore, redox-active metal ions and organic ligands are desirable when designing electrically conductive MOFs.…”

Thermally Induced Valence Tautomeric Transition in a Two‐Dimensional Fe‐Tetraoxolene Honeycomb Network

“…For this reason, the combination of Fe II /Cl 2 An 2− , where Cl 2 An 2− is 2,5‐dichloro‐3,6‐dihydroxo‐1,4‐benzoqunonate, is intriguing for the construction of magnetic materials. Harris and co‐workers recently reported a high T c magnet ( T c =105 K) in a honeycomb layered system . Notably, the same group quite recently reported an interesting Fe II /Cl 2 An 2− chain compound, which exhibited TDET with charge distributions of [Fe II ‐Cl 2 An 2− ] at T > T 1/2 and [Fe III ‐Cl 2 An .3− ] at T < T 1/2 , where T 1/2 =213 K .…”

“…The oxidation states of the Cl 2 An n − ligand could be identified by the characteristic bond lengths of the four C−O bonds, since the average C−O bond length increases as the negative charge of Cl 2 An n − increases in going from Cl 2 An 2− to Cl 2 An .3− . Based on several examples of metal‐Cl 2 An n − complexes reported previously, the mean C−O bond length can be determined to be in the range of 1.244–1.270 Å for Cl 2 An 2− and 1.291–1.312 Å for Cl 2 An .3− (Table S3; see the references given therein).…”

Dielectric Tunability, Expanding the Function of Metal‐Organic Frameworks

“…Metal‐organic frameworks (MOFs), known by their structural diversity and controllability, have attracted great many attention due to their potential application in gas absorption and separation, ionic exchange and identification, conductive, catalytic, magnetic, and even ferroelectric materials. However, the application of MOFs in dielectric tunability under electric‐field has not been reported yet so far, despite that dielectric tunability under electric‐field has potential applications in electrically tunable microwave devices, such as voltage‐controlled oscillators, band pass filters, phase shifters, and wireless communications .…”

“…The relative tunability (defined as [e 0 (0) -e 0 (E) ]/e 0 (0) Â 100% ¼ Δe 0 /e 0 (0) Â 100%) values for 1 are up to 35% (at 410 K) for E?c and 21% (at 380 K) for E||c, while these for 2 are 14% for E?c and 11.5% for E||c. Investigation on the mechanism of the dielectric tunability in 1 and 2 reveals that the activation energy for the electron hopping between two adjacent metal ions and the magnetic exchange interaction play a key role in the dielectric tunability of these materials.Metal-organic frameworks (MOFs), known by their structural diversity and controllability, have attracted great many attention due to their potential application in gas absorption and separation, [1] ionic exchange and identification, [2] conductive, [3] catalytic, [4] magnetic, [5] and even ferroelectric [6] materials. However, the application of MOFs in dielectric tunability under electric-field has not been reported yet so far, despite that dielectric tunability under electric-field has potential applications in electrically tunable microwave devices, such as voltage-controlled oscillators, band pass filters, phase shifters, and wireless communications.…”