Atropisomerization in cis-[Pd(2-C6BrF4)(2)L-2] (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 valu e (1.419 +/- 0.012 s(-1) at 299.1 K). This behavior reveals a dual mechanis m involving two competitive pathways: the rotation of the aryl group in the four-coordinate complex 1 and the rotation in a three-coordinate species f ormed by tht dissociation from 1. The latter is a clear-cut example of a me chanism starting with a neutral ligand dissociation in an organopalladium(I I) complex. The activation parameters associated to each of these two pathw ays are Delta H double dagger = 83 +/- 3 kJ mol(-1) and Delta S double dagg er = 37 +/- 10 J K-1 mol(-1), for the nondissociative path, and Delta H dou ble 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 (De lta 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 noticea ble contributions to the atropisomerization process.