Effects of Mixed Valency in an Fe-Based Framework: Coexistence of Slow Magnetic Relaxation, Semiconductivity, and Redox Activity

A 2-D coordination framework, (NEt4)2[Fe2(fan)3] (1·5­(acetone); H2fan = 3,6-difluoro-2,5-dihydroxy-1,4-benzoquinone), was synthesized and structurally characterized. The compound is structurally analogous to a formerly elucidated framework, (NEt4)2[Fe2(can)3] (H2can = 3,6-dichloro-2,5-dihydroxy-1,4-benzoquinone), and adopts a 2-D (6,3) topology with the symmetrical stacking of [Fe2(fan)3]2– sheets that are held in position by the NEt4 + cations between the sheets. The investigation of the dc and ac magnetic properties of 1·5­(acetone) revealed ferromagnetic ordering behavior and slow magnetization relaxation, as evinced from ac susceptibility measurements. Furthermore, the exposure of 1·5­(acetone) to air led to the formation of a heptahydrate 1·7H 2 O which displayed distinct magnetic properties. The study of the redox state and extent of delocalization in 1·5­(acetone) was undertaken via crystallography, in combination with Mössbauer and vis–NIR spectroscopy, to reveal the mixed-valence and delocalized nature of the as-synthesized material. As a result, the conductivity studies conducted on a pressed pellet showed a relatively high conductivity of 1.8 × 10–2 S cm–1 (300 K). In order to compare structurally related anilate-based structures, a relationship among the redox state, spectroscopic properties, and electronic properties was elucidated in this work. A preliminary investigation of 1·5­(acetone) as a candidate anode material in lithium ion batteries revealed a high reversible capacity of 676.6 mAh g–1 and high capacity retention.