Investigations on the Spin States of Two Mononuclear Iron(II) Complexes Based on N-Donor Tridentate Schiff Base Ligands Derived from Pyridine-2,6-Dicarboxaldehyde

Iron(II)-Schiff base complexes are a well-studied class of spin-crossover (SCO) active species due to their ability to interconvert between a paramagnetic high spin-state (HS, S = 2, ${}^5$T${}_2$) and a diamagnetic low spin-state (LS, S = 0, ${}^1$A${}_1$) by external stimuli under an appropriate ligand field. We have synthesized two mononuclear FeII complexes, viz., [Fe(L${}^1$)${}_2$](ClO${}_4$)${}_2$.CH${}_3$OH (1) and [Fe(L${}^2$)${}_2$](ClO${}_4$)${}_2$.2CH${}_3$CN (2), from two N${}_6$–coordinating tridentate Schiff bases derived from 2,6-bis[(benzylimino)methyl]pyridine. The complexes have been characterized by elemental analysis, electrospray ionization–mass spectrometry (ESI-MS), Fourier-transform infrared spectroscopy (FTIR), solution state nuclear magnetic resonance spectroscopy, ${}^1$H and ${}_{13}$C NMR (both theoretically and experimentally), single-crystal diffraction and magnetic susceptibility studies. The structural, spectroscopic and magnetic investigations revealed that 1 and 2 are with Fe–N${}_6$ distorted octahedral coordination geometry and remain locked in LS state throughout the measured temperature range from 5–350 K.