Identifying and quantifying the individual factors affecting the temperature and properties of the spin crossover in transition metal complexes is a challenging task, because many variables are involved. While the most decisive factor is the crystal field imparted by ligands around the active metal center, some less common actors are intramolecular steric repulsions or non-covalent interactions. A series of three Fe(II) complexes of 1,3bpp derivatives of (2-(pyrazol-1-yl)-6-(1H-pyrazol-3-yl)pyridine) have been prepared and characterized crystallographically to probe these effects: [Fe(1,3bpp)₂](ClO₄)₂ (1), [Fe(met1,3bpp)₂](ClO₄)₂ (2) and [Fe(dimet1,3bpp)₂](ClO₄)₂ (3). The ligands exhibit none, one or two methyl substituents on the pyrazol-1-yl heterocycle. These groups exert a dramatic effect on the SCO temperature in the solid state, and, most significantly, in solution (with T_SCO (3) > T_SCO (1) > T_SCO (2)). Extensive DFT calculations have unveiled the origin of these effects which lie in the intramolecular non-covalent or steric interactions rather than resulting from crystal field effects.

中文翻译：

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空间配体取代基探针对Fe（II）配合物自旋交叉的影响

识别和量化影响过渡金属络合物中自旋交联的温度和性质的各个因素是一项艰巨的任务，因为涉及许多变量。虽然最决定性的因素是活性金属中心周围的配体赋予的晶体场，但一些较不常见的作用分子是分子内空间排斥或非共价相互作用。制备了一系列的（2-（吡唑-1-基）-6-（1 H-吡唑-3-基）吡啶的1,3bpp衍生物的三种铁（II）配合物，并进行了结晶学表征以探测这些作用：[Fe（1,3bpp）₂ ]（ClO ₄）₂（1），[Fe（met1,3bpp）₂ ]（ClO ₄）₂（ 2）和[Fe（dimet1,3bpp） ₂ ]（ClO ₄） ₂（ 3）。所述配体在吡唑-1-基杂环上没有，一个或两个甲基取代基。这些基团对固态的SCO温度产生了显着影响，最显着的是在溶液中（ T _SCO（ 3）> T _SCO（ 1）> T _SCO（ 2））。广泛的DFT计算揭示了这些效应的起因，这是由于分子内非共价或空间相互作用，而不是晶场效应引起的。