Mono- and dicationic divalent palladium complexes having 2,6-bis(N-heteroarylsulfanylmethyl)pyridine ligands (NSNSN ligands) were synthesized and characterized. The NSNSN ligands were fixed in a rac-SNS tridentate coordination mode in the solid state, while the equilibria among meso-SNS, rac-SNS, and NNN isomers were observed in solution. The equilibrium between the SNS and NNN isomers could be modulated by temperature, as well as by the steric and electronic factors of the NSNSN and monodentate ligands. Lowering the temperature tended to make NNN isomers more predominant compared with SNS isomers. On the other hand, the steric demand between the ligands in the complexes shifted the equilibrium from NNN to SNS isomers. Introduction of pyridyl groups instead of pyrimidyl groups as N-heteroarenes also shifted the equilibrium to SNS isomers. DFT calculation indicated rapid ring inversion of the metallacycle moieties and relatively slow S-inversion in the SNS isomers, a result that was in good agreement with the experimentally observed dynamic behaviors. Both the experimental and theoretical results revealed that the SNS isomers had flexible structures in solution, whereas the NNN isomers were rigid and less dynamic. The mechanistic pathways for interconversion between SNS and NNN isomers were also calculated. Such calculations indicated that a pathway featuring a relatively unstable, distorted ax-SNN intermediate was plausible. The intermediate had an N-heteroarene on the coordinated sulfur atom at an axial position.
合成并表征了具有 2,6-双(N-杂芳基
硫甲基)
吡啶配体(NSNSN
配体)的单二价和二价
钯配合物。在固态下,NSNSN
配体固定为 rac-SNS 三位配位模式,而在溶液中则观察到了介-SNS、rac-SNS 和 NNNN 异构体之间的平衡。SNS 和 NNN 异构体之间的平衡可受温度以及 NSNSN 和单齿
配体的立体和电子因素的影响。与 SNS 异构体相比,温度越低,NNN 异构体越占优势。另一方面,配合物中
配体之间的立体需求使平衡从 NNN 异构体转向 SNS 异构体。引入
吡啶基基团而不是
嘧啶基团作为 N-异构体也会使平衡转向 SNS 异构体。DFT 计算表明,在 SNS 异构体中,
金属环分子的环反转速度很快,而 S 反转速度相对较慢,这一结果与实验观察到的动态行为十分吻合。实验和理论结果都表明,SNS 异构体在溶液中具有柔性结构,而 NNN 异构体则是刚性的,动态性较弱。此外,还计算了 SNS 和 NNN 异构体之间相互转化的机理途径。计算结果表明,以相对不稳定的扭曲 ax-SNN 中间体为特征的途径是可行的。该中间体在轴向位置的配位
硫原子上有一个 N-杂
环戊烯。