Anion-driven supramolecular modulation of spin-crossover properties in mononuclear iron(III) Schiff-base complexes

A series of Fe(III) complexes [Fe(5-F-sal-N-1,4,7,10)]Y (Y = PF₆^- for 1, Y = ClO₄^- for 2, Y = I^- for 3 and Y = NO₃^- for 4) have been prepared. Single-crystal X-ray crystallographic studies show that complex 1 crystallizes in the orthorhombic Pna2₁ space group and complexes 2-4 have an isomorphous structure and crystallize in the same monoclinic space group, P2₁/n. Complexes 2-4 have two independent molecules (Fe1 and Fe2) in the unit cell. Magnetic susceptibility measurements demonstrated that complexes 1 and 3 showed a gradual one-step SCO behavior (T_1/2 for 1 = 177 K and for 3 = 227 K) without thermal hysteresis. The magnetic behavior of 2 shows an incomplete two-step SCO process at T_1/2 = 114 K and 170 K, respectively, while 4 is in a high-spin state at all measured temperatures. A careful evaluation of the supramolecular structures of these complexes revealed correlation between the supramolecular packing forces and their SCO behavior. The crystal structure of 1 consists of a three-dimensional (3D) extended network constructed from N-H⋯F and C-H⋯F hydrogen bonds, and C-H⋯π and C⋯C short contacts. In compounds 2-4, the crystal packing is governed by C⋯C, C-H⋯π and p-π interactions for the Fe1 centers and by C-H⋯π/O interactions for the Fe2 centers, which form 1D chains. Additional interactions (C-H⋯F and N-H⋯O/I) connect the neighboring chains and planes to form a complex supramolecular network. The anion⋯π interactions in 4 provide a means for preventing SCO occurring at low temperatures. This suggests that the supramolecular connectivity of the anions influences the magnetic properties.