Tuning the spin-crossover temperature of polynuclear iron(II)–triazole complexes in solution by water and preparation of thermochromic fibers

The spin-crossover temperatures of alkyl-substituted Fe²⁺–triazole complexes with the counter ions 4-dodecylbenzenesulfonate (DBS⁻) and 2-naphthalenesulfonate (2ns⁻) are strongly influenced by water in the dissolved state, whereby the low-spin state is stabilized by water. This effect was found to be more pronounced in complexes with the counter ion DBS⁻ compared to 2ns⁻. It appears that non-coordinating water molecules interact with the counter ion and thus alter the ligand-field splitting. Notably, the spin-crossover temperature depends only on the Fe²⁺/H₂O ratio and not on the concentration of the complex in solution. As a consequence, the spin-crossover temperature can be controlled by the Fe²⁺/H₂O ratio (for DBS⁻ complexes within a temperature range of 7–47 °C), and on the other hand small amounts of water (in the ppm range) can be detected in apolar solvents. Furthermore, fibers of blends of the complex [Fe(C₁₈trz)₃](DBS)₂ with ultra-high molecular weight polyethylene (UHMWPE) could be prepared. In these fibers, spin-crossover is maintained leading to pronounced thermochromism as in the bulk material, indicating essentially no interaction between UHMWPE and Fe²⁺–triazole complex.