Spin Crossover in Iron(III) Quinolylsalicylaldiminates: The Curious Case of [Fe(qsal-F)(2)](Anion)

A family of iron(III) spin crossover complexes with different counteranions, [Fe(qsal-F)(2)]A (qsal-F = 4-fluoro-2-[(8-quinolylimino)methyl]phenolate; A = PF61, OTf(2), NO(3), ClO(4), BF5, or NCS6) have been prepared. All compounds are isostructural and crystallize (triclinic P1- space group) with two independent iron(III) centers (Fe1 and Fe2) in the asymmetric unit. No solvent molecules are found in the crystal lattice, allowing us to directly probe the relative influence of anion variation on the spin crossover characteristics. The crystal packing is governed by three types of pp interactions (type A, type B, and type C), which form undulating 1D chains. Additional interactions (pF, CH...O/F, and P4AE) connect the neighboring chains to form a complex supramolecular network. Hirshfeld surface analysis supports these findings. The anions are located between the cationic [Fe(qsal-F)](+) chains; hence, similar interchain distances (d) are observed irrespective of the anion. However, the interplane distances (dplane) are influenced by the crystal packing and increase proportionally with the anion size. Magnetic studies reveal that smaller anions tend to stabilize the low-spin state (NO33, ClO(4), and BF(5)), while larger anions (PF(1) and OTf(2)) exhibit lower transition temperatures (Tfor 1 = 200 K and T for 2 = 190 K) and gradual spin crossovers. The anomaly is 6, where, despite having the smallest anion, it exhibits the lowest transition temperature with magnetic hysteresis in the first step (T1/2 down arrow = 170 K and T1/2 down arrow = 157 K). This suggests the size, shape, and supramolecular connectivity of the anion all influence the magnetic properties.