A rare example of a complete, incomplete, and non-occurring spin transition in a [Fe2L3]X4 series driven by a combination of solvent-and halide-anion-mediated steric factors

A trend between the degree of steric congestion of the Fe(II) coordination environment and the extent of spin transition (percentage completeness) has been observed in a series of halide salts of a dinuclear triple helicate architecture with the general form [Fe₂L₃]X₄ (where X = Cl^- for 1, Br^- for 2, and (I^-)₃/I₃ ^- for 3, and L is (1E,1′E)-N,N′-(oxybis(4,1-phenylene))bis(1-(1H-imidazol-4-yl)methanimine). Crystal packing densities of adjacent helicates were found to decrease with increasing anion size. Greater steric congestion by neighboring helicates favored the [HS-HS] state of the dinuclear triple helicate architecture. As a result, the highly crowded Cl^- salt (1) did not undergo spin-crossover (SCO), the more congested Br^- salt (2) underwent an incomplete solvent-dependent transition, and the least crowded (I^-)₃/I₃ ^- analogue (3) exhibited a full SCO from the [HS-HS] ↔ [LS-LS] state. Furthermore, an interesting two-step transition was observed in the Br^- salt, exhibiting a 28 K thermal hysteresis in the higher temperature step, the largest thermal hysteresis reported to date for a Fe(II) dinuclear triple helicate system. Variable-temperature single-crystal X-ray diffraction (SCXRD) analysis of 2 demonstrated that this two-step profile was found to be the result of crystallographic parameters evolving in a two-step manner with temperature, rather than a crystallographic phase change.