fac-Specific syntheses of homochiral [Fe(NN′)3]2+ complexes (NN′ = pyridine keto-hydrazone); origins of the stereoselectivity
作者:Cynthia Paul Sebli、Suzanne E. Howson、Guy J. Clarkson、Peter Scott
DOI:10.1039/c000815j
日期:——
The 2-pyridinehydrazones (from condensation of pyridine-2-carbaldehyde and hydrazines) have previously been noted to have poor ligating ability as a result of a sterically demanding planar conformation. Destabilisation of this conformation is achieved through simple use of the ketohydrazones, and as a result the diamagnetic chiral tris-bidentate diimine complexes fac-[FeL3]2+ are readily isolated. In the solid state, inter-ligand ÏâÏ stacking and complex/counter-ion H-bonding are apparent, and these features persist in solution according to dynamic NMR spectra, which also indicate extremely high stereoselectivity for the fac isomers (>200â:â1). The compounds crystallise as conglomerates, and time-resolved CD spectra of non-racemic samples indicate a high degree of persistence of chirality (racemisation t1/2ca 77 min). Variations of solvent and counter-ion indicate that H-bonding is unimportant in determining the structure of the cation. The fac-selectivity arises in the induction of a chiral conformation in the coordinated ligand, and the fact that such non-planar ligands can only be accommodated about the Fe(II) centre if they all have the same absolute configuration. Adding a hydrazine N-methyl group increases the steric demand further, while retaining the novel non-planar conformation, and as a result paramagnetic chiral bis-bidentate complexes such as [FeL72(CH3CN)2]2+ are readily available.
2 吡啶肼(由吡啶-2-甲醛和肼缩合而成)由于具有立体要求较高的平面构象,因此连接能力较差。通过简单地使用酮酰肼就可以破坏这种构象的稳定性,因此很容易分离出二磁性手性三公头二亚胺配合物 fac-[FeL3]2+。在固态下,配体间的ÏâÏ堆叠和络合物/反离子间的氢键作用十分明显,根据动态核磁共振光谱,这些特征在溶液中也会持续存在,这也表明 fac 异构体具有极高的立体选择性(>200â:â1)。化合物结晶为团聚体,非外消旋样品的时间分辨 CD 光谱表明手性具有高度的持久性(外消旋化 t1/2ca 77 分钟)。溶剂和反离子的变化表明,氢键在决定阳离子结构方面并不重要。面选择性是由于配位配体的手性构象的诱导作用,而这种非平面配体只有在绝对构型相同的情况下才能被容纳在 Fe(II) 中心周围。添加肼 N-甲基会进一步增加立体需求,同时保留新颖的非平面构象,因此顺磁手性双同位配合物(如 [FeL72(CH3CN)2]2+)很容易获得。