Atropisomerization in cis-[Pd(2-C6BrF4)2L2] (L = thioether):: A dual mechanism involving ligand-dissociative and nondissociative competitive pathways

Interconversion of the syn and anti rotational isomers of cis-[Pd(2-C6BrF4)(2)(tht)(2)] (1) (tht = tetrahydrothiophene) takes place very fast in CDCl3 solution. The process has been studied by F-19 NMR magnetization transfer experiments. The first-order syn-to-anti atropisomerization rate constant k(ab) decreases with the addition of tht until it reaches an asymptotic value (1.419 +/- 0.012 s(-1) at 299.1 K). This behavior reveals a dual mechanism involving two competitive pathways: the rotation of the aryl group in the four-coordinate complex 1 and the rotation in a three-coordinate species formed by tht dissociation from 1. The latter is a clear-cut example of a mechanism starting with a neutral ligand dissociation in an organopalladium(II) complex. The activation parameters associated to each of these two pathways are Delta H double dagger = 83 +/- 3 kJ mol(-1) and Delta S double dagger = 37 +/- 10 J K-1 mol(-1), for the nondissociative path, and Delta H double dagger = 77 +/- 3 kJ mol(-1) and Delta S double dagger = 28 +/- 10 J K-1 mol(-1) for the dissociative contribution. Similar energy is required (at 293 K) for the aryl rotation directly in the four-coordinate complex 1 (Delta G double dagger = 72 +/- 4 kJ mol(-1)) or via tht dissociation (Delta G double dagger = 70 +/- 4 kJ mol(-1)); hence, the two pathways make noticeable contributions to the atropisomerization process.