A new 3-D coordination polymer as a precursor for CuI-based thermoelectric composites

Two complexes, [Cu₆I₆(L1)₃]_n (I) and [Cu₄I₄(L2)₂]_n (II) (L1 = 1,4-bis(phenylthio)but-2-yne; L2 = 1,4-bis(phenylthio)butane), as precursors for thermoelectric composites were prepared using a literature procedure. During the preparation of I, an unexpected 3-D polymorph [Cu₄I₄(L1)₂]_n (1) with a triclinic space group and an infinite [CuI]_n staircase structure was obtained. This new polymorph (1) exhibited the same structure at both room temperature and 173 K. Complexes 1 and II were therefore pyrolysed to composites 2 and 3, respectively, at 400 °C under a nitrogen gas flow. Composite 3 was pale in color with a low carbon content (0.05 wt%) and easily disassembled during handling. By comparison, the high carbon containing (10.2 wt%) composite 2 can be compressed into a robust, light pellet (density 3.58 g cm⁻³), which showed a moderate to high Seebeck coefficient (543-1308 μV K⁻¹) over the temperature range 70-240 °C.