Synthesis and Thermoelectric Properties of Nickel Tetrathiolate Metallopolymers

Sulfur–nickel (S–Ni) complex polymers are characterized by a planar tetra-coordinated structure in which four sulfur atoms surround a central nickel atom and exhibit stable electrical and thermal properties even in the absence of external doping. In this study, a new synthetic strategy incorporating cyanoethyl groups is employed to improve the solubility of the ligand precursors. Utilizing this approach, the following three novel S–Ni complex polymers are successfully synthesized: NiTTFtt, with tetrathiafulvalene (TTF) as the donor unit in the backbone, NiDMTtt, featuring a thiophene-inserted TTF backbone, and NiDMT2tt, integrating thienothiophene into the TTF backbone. As a result, the NiDMTtt and NiDMT2tt each exhibit bent-shaped configurations at the alkene part. To further tune the polymer composition, two alkali metal counterions (Na+ and K+) are introduced, along with partial incorporation of Ni2+ ions, thus resulting in the formation of six distinct S–Ni complex polymers. These materials are processed into thermoelectric pellets for performance evaluation. Among them, the NiTTFtt exhibits the highest electrical conductivity of 575.90 S cm–1, while the NiDMTtt and NiDMT2tt display lower electrical conductivities but higher Seebeck coefficients of up to 37.09 μV K–1. Further, the effects of doping with alkali metal ions are systematically investigated, thereby revealing a new strategy for enhancing the thermoelectric properties of S–Ni complex polymers. This work provides valuable insights into the molecular design and functional tuning of metallopolymers for high-performance thermoelectric applications.