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.