Tetrathiafulvalene-2,3,6,7-tetrathiolate (TTFtt^n–, n = 4, 3, and 2) is a sulfur-based, redox-active ligand. The related dinulcear complexes and coordination polymers exhibit various properties such as high electrical conductivity, near-infrared fluorescence, and unusual spin transitions. However, most reported TTFtt compounds consist of diamagnetic metals, and the effect of unpaired metal spins in this ligand scaffold remains relatively unexplored. In Part One of this presentation, we study the exchange coupling (J) between TTFtt^3– radical and paramagnetic metals for the first time using a dinuclear Mn(II) complex. Magnetometry measurements show that the antiferromagnetic coupling in this complex is weaker than that in other reported Mn(II)–radical compounds. Experimental and computational results suggest that the small J might stem from poor overlap between Mn- and TTFtt-based magnetic orbitals, as well as insignificant spin density on the coordinating S-atoms. These factors counteract any expected increase in J from the high sulfur-metal covalency, demonstrating that several competing effects should be considered in the design of molecule-based magnetic compounds. In Part Two of this presentation, we study the electronic and magnetic properties of a series of TTFtt coordination polymers (CPs) based on Ni and Fe. With metal(II) as starting materials, charge transfer occurs between Fe(II) and the TTFtt^2- ligand, engendering a semiconductive Fe(III) product. By contrast, the oxidation state of Ni(II) remains unchanged in the reported CP which exhibits metallic transport. With mixed-metal approach, the conductivity of resulting CPs can be tuned across two orders of magnitude. Moreover, Fe-containing compounds show a spin-glass-like transition accompanied by an increase in conductivity at low temperature. Analogous to metallic alloy systems, Fe(III) serves as a magnetic impurity when introduced into the Ni(II) CP, leading to the negative magnetoresistance. This work provides an additional strategy to control the TTFtt oxidation state, and demonstrates that paramagnetic metals could offer more functionalities to conductive TTFtt materials.

Chen-Yu Lien

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