A Hobson's Choice Co-Ligand in Imparting Spin Crossover in Fe(II) Complexes Based on 1,3,4-Oxadiazole Ligands

In this manuscript we report the synthesis of three methanol solvatomorphs of Fe(II) complexes with the 2-(2-phenyl)-5-[N,N-bis(2-pyridylmethyl)aminomethyl]-1,3,4-oxadiazole (L^Ph−ODA) ligand and three different NCE co-ligands where E=S, Se and BH₃ (C1–C3). The complexes obtained were characterized by single crystal X-ray crystallography and SQUID magnetometry between 2 and 300 K. Complexes C1 and C2 remain in the high spin state over the entire measured temperature window, whereas complex C3 exhibits a solvent dependent spin transition with a T_1/2 of 166 K. Upon heating the sample to 400 K and removing methanol, the complex exhibited a two-step spin transition with the first step having a T_1/2 of 215 K and a second step having a T_1/2 of 137 K.