Anion Modified Spin Crossover in [Fe(qsal-4-F)]+ Complexes with a 4-Position Substituted Qsal Ligand

Four iron(III) complexes, [Fe(qsal-4-F)2]Y·sol (Hqsal-4-F = 4-fluoro-N-(8-quinolyl)salicylaldimine; Y = NO3–, sol = 0.91MeOH·0.57H2O (1NO3); Y = PF6– (2PF6); Y = BF4– (3BF4); Y = OTf–, sol =1.5MeOH (4OTf)), with a new 4-position substituted qsal type ligand Hqsal-4-F have been synthesized and structurally and magnetically characterized. Complexes 1NO3–3BF4 consist of 1D chains formed by the [Fe(qsal-4-F)2]+ cations connected by π–π and C–H···O interactions, which are further linked by more weak interactions to form 2D layers and 3D networks. On the other hand, complex 4OTf has a structure of nearly isolated 1D column where the [Fe(qsal-4-F)2]+ cations are connected by π–π, C–H···π, and C–F···π interactions. Magnetic studies revealed the occurrence of two-step symmetry-breaking SCO in 1NO3 and two-step gradual SCO in 2PF6. Complex 3BF4 undergoes a gradual SCO, whereas 4OTf remains almost high-spin. The smaller anions tend to stabilize the low-spin state, while larger anions tend to stabilize the high-spin state. In addition, the intermediate spin state of 1NO3 could be thermally trapped by quenching from the high temperature, thereby kinetically suppressing the spin transition to the full low-spin state. This work represents a good example that the position of the substituent and the anions plays critical roles in the preparation of SCO materials with tunable properties.