Fluxional interconversion of divalent palladium complexes having NSNSN ligands between flexible SNS and rigid NNN-coordinated structures.

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.