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Deciphering the Influence of Meridional versus Facial Isomers in Spin Crossover Complexes
Chemistry - A European Journal ( IF 3.9 ) Pub Date : 2018-10-18 , DOI: 10.1002/chem.201804161
Timothée Lathion 1 , Laure Guénée 2 , Céline Besnard 2 , Azzedine Bousseksou 3 , Claude Piguet 1
Affiliation  

Chelate coordination of non‐symmetrical didentate pyrazine‐benzimidazole (L1) or pyridine‐benzimidazole (L2) N‐donor ligands around divalent iron in acetonitrile produces stable homoleptic triple‐helical spin crossover [Fe(Lk)3]2+ complexes existing as mixtures of meridional (C1‐symmetry) and facial (C3‐symmetry) isomers in slow exchange on the NMR timescale. The speciation deviates from the expected statistical ratio mer/fac=3:1, a trend assigned to the thermodynamic trans‐influence, combined with solvation effects. Consequently, the observed spin state FeIIlow‐spin↔FeIIhigh‐spin equilibria occurring in [Fe(Lk)3]2+ refer to mixtures of complexes in solution, an issue usually not considered in this field, but which limits rational structure‐properties correlations. Taking advantage of the selective and quantitative formation of isostructural facial isomers in non‐constrained related spin crossover d‐f helicates (HHH)‐[LnFe(Lk)3]5+ (Ln is a trivalent lanthanide, Lk=L5, L6), we propose a novel strategy for assigning pertinent thermodynamic driving forces to each spin crossover triple‐helical isomer. The different enthalpic contributions to the spin state equilibrium found in mer‐[Fe(Lk)3]2+ and fac‐[Fe(Lk)3]2+ reflect the Fe−N bond strengths dictated by the trans‐influence, whereas a concomitant solvent‐based entropic contribution reinforces the latter effect and results in systematic shifts of the spin crossover transitions toward higher temperature in the facial isomers.

中文翻译:

了解自旋交叉复合物中子午线与脸部异构体的影响

乙腈中二价铁周围非对称二齿酸吡嗪-苯并咪唑(L1)或吡啶-苯并咪唑(L2N-供体配体的螯合产生稳定的均相三螺旋自旋交换[Fe(L k3 ] 2+配合物在NMR时标上缓慢交换的子午(C 1对称)和面部(C 3对称)异构体的混合物。形态偏离预期的统计比mer / fac = 3:1,该趋势分配给热力学反演影响力与溶剂化作用相结合。因此,所观察到的自旋状态的Fe II低自旋↔Fe II高自旋中出现平衡的[Fe(大号ķ3 ] 2+是指在溶液中的复合物的混合物,一个问题通常不是这方面考虑,但其限制合理的结构特性相关性。利用非限制性相关自旋交联d-螺旋(HHH)-[LnFe(L k3 ] 5+(Ln是三价镧系元素,L k = L5)中同构面部异构体的选择性和定量形成的优势L6),我们提出了一种新颖的策略,为每个自旋交叉的三螺旋异构体分配相关的热力学驱动力。自旋状态平衡不同焓贡献在发现聚体-的[Fe(大号ķ3 ] 2+FAC - [Fe(上大号ķ3 ] 2+反映由所规定的Fe-N系列的粘结强度反式-influence,反之,基于溶剂的熵贡献则增强了后者的效果,并导致自旋交叉转变系统地转变为面部异构体中更高的温度。
更新日期:2018-10-18
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