Structural, Thermal, and Magnetic Study of Solvation Processes in Spin-Crossover [Fe(bpp)₂][Cr(L)(ox)₂]₂·<i>n</i>H₂O Complexes

The influence of lattice water in the magnetic properties of spin-crossover [Fe(bpp)2]X2·nH2O salts [bpp = 2,6-bis(pyrazol-3-yl)pyridine] is well-documented. In most cases, it stabilizes the low-spin state compared to the anhydrous compound. In other cases, it is rather the contrary. Unraveling this mystery implies the study of the microscopic changes that accompany the loss of water. This might be difficult from an experimental point of view. Our strategy is to focus on some salts that undergo a nonreversible dehydration−hydration process without loss of crystallinity. By comparison of the structural and magnetic properties of original and rehydrated samples, several rules concerning the role of water at the microscopic level can be deduced. This paper reports on the crystal structure, thermal studies, and magnetic properties of [Fe(bpp)2][Cr(bpy)(ox)2]2·2H2O (1), [Fe(bpp)2][Cr(phen)(ox)2]2·0.5H2O·0.5MeOH (2), and [Fe(bpp)2][Cr(phen)(ox)2]2·5.5H2O·2.5MeOH (3). Salt 1 contains both high-spin (HS) and low-spin (LS) Fe2+ cations in a 1:1 ratio. Dehydration yields the anhydrous spin-crossover compound with T1/2↓ = 353 K and T1/2↑ = 369 K. Rehydration affords the dihydrate [Fe(bpp)2][Cr(bpy)(ox)2]2·2H2O (1r) with 100% HS Fe2+ sites. Salt 2 also contains both HS and LS Fe2+ cations in a 1:1 ratio. Dehydration yields the anhydrous spin-crossover compound with T1/2↓ = 343 K and T1/2↑ = 348 K. Rehydration affords [Fe(bpp)2][Cr(phen)(ox)2]2·0.5H2O (2r) with 72% Fe2+ sites in the LS configuration. The structural, magnetic, and thermal properties of these rehydrated compounds 1r and 2r are also discussed. Finally, 1 has been dehydrated and resolvated with MeOH to give [Fe(bpp)2][Cr(bpy)(ox)2]2·MeOH (1s) with 33% HS Fe2+ sites. The influence of the guest solvent in the Fe2+ spin state can anticipate the future applications of these compounds in solvent sensing.晶格水对自旋交叉[Fe(bpp)₂]X₂·nH₂O盐（bpp = 2,6-双(吡唑-3-基)吡啶）磁性能的影响已有充分研究。多数情况下，相较于无水化合物，晶格水可稳定低自旋态；但在部分体系中则恰好相反。揭开这一谜团需要探究伴随失水过程的微观结构变化，而这在实验层面往往颇具挑战。 我们的研究策略聚焦于一类可发生不可逆脱水-水合过程且不会丧失结晶性的盐类。通过对比原始样品与再水合样品的结构与磁性能，可推导出关于水分子微观作用的若干规律。本文报道了[Fe(bpp)₂][Cr(bpy)(ox)₂]₂·2H₂O（1）、[Fe(bpp)₂][Cr(phen)(ox)₂]₂·0.5H₂O·0.5MeOH（2）以及[Fe(bpp)₂][Cr(phen)(ox)₂]₂·5.5H₂O·2.5MeOH（3）的晶体结构、热分析及磁性能。 盐1中Fe²+阳离子以1:1的比例同时存在高自旋（HS）与低自旋（LS）构型。脱水后得到无水自旋交叉化合物，其降温相变临界温度T₁/₂↓=353 K，升温相变临界温度T₁/₂↑=369 K。再水合后得到二水合物[Fe(bpp)₂][Cr(bpy)(ox)₂]₂·2H₂O（1r），其中Fe²+位点均为高自旋态。 盐2同样以1:1比例共存HS和LS构型的Fe²+阳离子，脱水后得到无水自旋交叉化合物，T₁/₂↓=343 K，T₁/₂↑=348 K。再水合后得到[Fe(bpp)₂][Cr(phen)(ox)₂]₂·0.5H₂O（2r），其中72%的Fe²+位点处于低自旋构型。本文还讨论了再水合产物1r与2r的结构、磁性能与热性质。 最后，我们将盐1脱水后用甲醇（MeOH）溶剂再溶剂化，得到[Fe(bpp)₂][Cr(bpy)(ox)₂]₂·MeOH（1s），其中33%的Fe²+位点为高自旋态。客体溶剂对Fe²+自旋态的影响，可为这类化合物在溶剂传感领域的未来应用提供前瞻性参考。