A combined theoretical and experimental approach to determine the right choice of co-ligand to impart spin crossover in Fe(ii) complexes based on 1,3,4-oxadiazole ligands

Abstract

We present the synthesis of two new novel tetradentate ligands based on 1,3,4-oxadiazole, 2-(2-pyridyl)-5-[N,N-bis(2-pyridylmethyl)aminomethyl]-1,3,4-oxadiazole (L^{TetraPy–ODA}) and 2-(2-phenyl)-5-[N,N-bis(2-pyridylmethyl)aminomethyl]-1,3,4-oxadiazole (L^{TetraPh–ODA}). The ligands were used to prepare six mononuclear complexes [Fe^II(L^{TetraPy–ODA})(NCE)] (C1–C3) and [Fe^II(L^{TetraPh–ODA})(NCE)] (C4–C6) where E = S, Se or BH₃. In addition, the ligand L^{TetraPy–ODA} was employed in the synthesis of a new di-nuclear complex [Fe^II₂(L^TetraPh)](ClO₄)₄·1 CH₃NO₂·1.5 H₂O (C7). Characterization of all complexes was carried out using single-crystal X-ray crystallography, elemental analysis, and infrared spectroscopy. Magnetic susceptibility measurements, performed in the temperature range of 2–300 K using a SQUID magnetometer, revealed spin crossover behaviour exclusively in the mononuclear complexes C3 and C6, in which two monodentate NCBH₃⁻ co-ligands coordinate. The presence of the lattice solvent was found to be crucial to the spin transition property, with complex C3 exhibiting a switching temperature (T_1/2) of approximately 165 K and C6 approximately 194 K. The other four mononuclear complexes C1, C2, C4, C5, as well as the dinuclear complex C7 are locked in the high spin state over the measured temperature range. Density Functional Theory (DFT) calculations were performed on complexes C1–C6 to rationalise the observed magnetic behaviour, demonstrating the significant effect of the NCS⁻, NCSe⁻ and NCBH₃⁻ co-ligands ligands on the spin-crossover behaviour of the [Fe^II(L)(NCE)] complexes.