Study of spin crossover in an iron(II) tris(diimine) system tuned by counter anions

Investigations of spin-crossover in an iron(II) tris(diimine) system, based on the ligands L-1 (4,5-dimethy2-(pyridine-2-yl)imidazole) and L-2 (1-acetic ethyl-2-(pyridine-2-yl)-imidazole), have been presented in this paper. The methyl groups at adjacent sites to the donor atoms in the ligand L1 generated the crossover situation for complexes Fe(L-1)(3)(ClO4-)(2) (1), Fe(L-1)(3)(BF4-)(2) (2), Fe(L-1)(3)(PF6-)(2) (3) and Fe(L-1)(3)(SbF6-)(2) (4), while the acetic ethyl group at the non-coordinating NH site of the ligand L2 resulted in an increase in the ligand field and generated a low-spin (LS) state for the complex Fe(L-2)(3)(ClO4-)(2) (5). Complexes 1 and 5 were structurally characterized by single-crystal X-ray diffraction at different spin-states, while the phase purity of complexes 2-4 was confirmed by PXRD. Raman, IR and UV-Vis absorption spectra and magnetic properties of 1-4 were obtained, which revealed the influence of different counter anions on the spin transition temperatures. The blue-shift of IR and Raman spectra, red-shift and decrease in molar absorptivity of UV-Vis absorption spectra all follow the sequence 1 < 3 < 2 < 4. (C) 2016 Elsevier Ltd. All rights reserved.