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.

中文翻译：

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[Fe2L3] X4系列中由溶剂和卤化物阴离子介导的立体因子共同驱动的完全，不完全和不发生自旋转变的罕见例子。

而最不拥挤的（I-）3 / I3-类似物（3）在[HS-HS]↔[LS-LS]状态下表现出完整的SCO。此外，在溴盐中观察到了有趣的两步转变，在较高温度的步骤中表现出28 K的热滞，这是迄今为止报道的Fe（II）双核三螺旋体系最大的热滞。对2的可变温度单晶X射线衍射（SCXRD）分析表明，发现该两步曲线是晶体学参数随温度以两步方式演变的结果，而不是晶体学相变的结果。迄今为止，Fe（II）双核三螺旋系统最大的热滞现象。对2的可变温度单晶X射线衍射（SCXRD）分析表明，发现该两步曲线是晶体学参数随温度以两步方式演变的结果，而不是晶体学相变的结果。迄今为止，Fe（II）双核三螺旋系统最大的热滞现象。对2的可变温度单晶X射线衍射（SCXRD）分析表明，发现该两步曲线是晶体学参数随温度以两步方式演变的结果，而不是晶体学相变的结果。