- C3-Symmetric Trinuclear Molybdenum Cluster Sulfides: Configurational Stability, Supramolecular Stereocontrol, and Absolute Configuration Assignment
Richard Frantz, Eva Guillamón, Jérôme Lacour, Rosa Llusar, Victor Polo, Cristian Vicent
Inorg. Chem. 2007, 46, 10717-10723
The chiral C3-symmetric [Mo3S4Cl3(dppe)3]+ cluster [dppe = 1,2-bis(diphenylphosphinoethane),P or M enantiomers] with incomplete cuboidal structure is shown to be configurationally stable at room temperature and configurationally labile at elevated temperature using enantiopure Δ- or Λ-TRISPHAT [(tris(tetrachlorobenzenediolato)phosphate(V)] anions both as chiral NMR solvating and asymmetry-inducing reagents. It is evidenced that the enantiomers of this trinuclear cluster cation can equilibrate at higher temperature (typically 72 °C), and in the presence of the hexacoordinated phosphate anion, a moderate level of stereocontrol (1.2:1) can be achieved. It results in a diastereomeric enrichment of the solution in favor of the heterochiral ion pairs, e.g.,M+ Δ- or P+ Λ-. At higher temperature, a partial racemization of the TRISPHAT anion is also observed, and precipitation at room temperature of [rac-Mo3S4Cl3(dppe)3][rac-TRISPHAT] salts from the diastereomeric enriched solution improves the diastereomeric purity of the mother liquor to a 4:1 ratio. A low-energy pathway for the inverconversion between the [P-Mo3S4Cl3(dppe)3]+ and [M-Mo3S4Cl3(dppe)3]+ enantiomers has been found using combined quantum mechanics and molecular mechanics methodologies. This pathway involves two intermediates with three transition state regions, which result from the partial decoordination of the diphosphane coordinated at each metal center. Such decoordination creates a vacant position on the metal, producing a Lewis acidic site that presumably catalyzes the TRISPHAT epimerization.
open archive unige:6914