Revealing the diastereomeric nature of pincer terdentate nitrogen ligands 2,6-bis(arylaminomethyl)pyridine through coordination to palladium
作者:Ana Arnáiz、José V. Cuevas、Gabriel García-Herbosa、Arancha Carbayo、Juan A. Casares、Enrique Gutierrez-Puebla
DOI:10.1039/b201319c
日期:——
Palladium complexes of formula [PdLCl]X (X = Cl 1, BF42) containing the pincer terdentate ligands L [L = 2,6-(ArNHCH2)2C5H3N (Ar = 4-MeC6H41a and 2a, 4-MeOC6H41b and 2b); L = 2,6-(4-MeC6H4NMeCH2)2C5H3N 1c and 2c] have been synthesised as cis–trans diastereomeric mixtures. The chiral nature of both amine arms of the pincer ligands accounts for the mixtures of isomers. All the complexes show dynamic behavior assigned to conversion
between diastereomers. A ΔG‡273 of 68.9 kJ mol−1 for 2a and a ΔG‡273 of 65.8 kJ mol−1 for 2c were found through complete DNMR line-shape analysis of the –CH2– signals in the 1H spectra. The cis–trans conversion requires the inversion of the pyramidal nitrogen of one of the coordinated amine arms. Two putative pathways have been proposed for such inversion: a) dissociation of one of the amine nitrogen atoms followed by inversion of the nitrogen atom and reformation of the nitrogen–palladium bond and b) deprotonation of one of the N–H bond leading to an amido intermediate which can reprotonate on the other side. Experimental evidence is given for both mechanisms. The structural characterization of 1a
is described.
含有钳状三齿配体 L [L = 2,6-(ArNHCH2)2C5H3N (Ar = 4-MeC6H41a 和 2a, 4-MeOC6H41b 和 2b); L = 2,6-(4-MeC6H4NMeCH2)2C5H3N 1c 和 2c] 的式 [PdLCl]X (X = Cl 1, BF42) 钯配合物已合成为顺反非对映异构体混合物。钳形配体的两个胺臂的手性是产生异构体混合物的原因。所有复合物都显示出非对映异构体之间转换的动态行为。通过对 1H 光谱中的 -CH2- 信号进行完整的 DNMR 线形分析,发现 2a 的 ΔG‡273 为 68.9 kJ mol-1,2c 的 ΔG‡273 为 65.8 kJ mol-1。顺反转化要求其中一个配位胺臂的金字塔氮发生反转。这种反转有两种可能的途径:a)其中一个胺氮原子解离,然后氮原子反转,氮-钯键重构;b)其中一个 N-H 键去质子化,产生一个氨基中间体,该中间体可以在另一侧再质子化。实验证明了这两种机制。介绍了 1a 的结构特征。