Counter ions in tuning the extended arrangements of pyrazolone-thiosemicarbazone-based Fe(III) complexes

Herein, we report the syntheses, crystal structures, and magnetic properties of the title compounds [Fe(III)L₂]Cl·CH₃OH, 1 and [Fe(III)L₂]NH(C₂H₅)₃, 2 formulated from pyrazolone-based thiosemicarbazone ligand (L) at room temperature. Structurally, the Fe(III) spin centre was chelated under N₂O₂S₂ coordination environment and then completed by charge-neutralizing counter ions for charge balance, i.e., the anion chloride (Cl⁻) for 1 and the cation triethylammonium (NH(C₂H₅)₃⁺) for 2. Interestingly, the counter ions guided extended arrangement facilitated by halogen bonding of the Cl⁻ and hydrogen bonding of the NH(C₂H₅)₃⁺ were mainly responsible for obtaining the diamond- and hexagonal-shaped spin centre arrangements for 1 and 2, respectively. Magnetic examination revealed that both compounds are high-spin (HS) at all the measurement temperature ranges, and the experimental bond distances (at 300 K) were also in good agreement for the HS state. Strong ligand field chemical modification on the ligand that stabilizes the low spin (LS) state is suggested to realize the spin crossover (SCO) phenomenon. Moreover, further biological and catalytic investigation of the reported new compounds is suggested, as thiosemicarbazone-containing HS metal complexes are good candidates for these applications.