The Absolute Configuration of Palladium(II) and Ruthenium(II) Pseudochiral Centers in either in Chiral or Achiral Environments

The meso-dithioxamide H-((R)-1-(1-phenyl)ethyl)-NSC-CSN-((S)-1-(1-phenyl)ethyl)-H (H2-mesoDTO) bonds
[(η6-p-cymene)chlorido-ruthenium(II)]+ or [(η3-allyl)-palladium(II)]+ fragment and provides the Cs symmetrical
complexes [(η6-p-cymene)ClRu(H-mesoDTO κ-S,S Ru)] (1) and [(η3-allyl)palladium(H-mesoDTO κ-S,S Pd)] (2). These complexes are pseudochiral, and each of them exists as a mixture of two symmetrical meso forms. The improper symmetry of [(η3-allyl)palladium(H-mesoDTO κ-S,S Pd)] has been broken in two different ways: (i) by changing the symmetrical allyl moiety with a η3-crotyl frame or (ii) by substituting the residual amidic hydrogen in the dithiooxamidate ligand with a M(PR3)Cl+ fragment (M= Pd or Pt and PR3 = triorganophosphine). As a consequence, a chiral plane is added to the pseudochiral palladium center, and two pairs of enantiomers are formed in each case.
Furthermore, [(η6-p-cymene)chlorido-ruthenium(II)]+ and [(η3-allyl)-palladium(II)]+fragments have been joined by means of the binucleating meso-dithiooxamidate ligand in a κ-S,S Ru κ-N,N Pd coordination mode. The resulting Cs-symmetrical complex [(η6-p-cymene)ClRu(μ-mesoDTO κ-S,S Ru κ-N,N Pd)Pd(η3-allyl)] (8) possesses two pseudochiral metal centers, and it is therefore a mixture of four isomeric meso forms. All of these isomers in a chloroform solution interconvert in that both palladium and ruthenium invert their configurations. A mechanism of epimerization for both palladium and ruthenium is proposed. The absolute configurations of pseudochiral palladium in [(η3-allyl)(c)-Pd(μ-((R)-1-(1-phenyl)ethyl)-NSC-CSN-((S)-1-(1-phenyl)ethyl) κ-N,N (c)-Pd κ-S,S (A,C)-Pd)(A,C)-Pd(trinpropyl-phosphine)Cl] (6) and of pseudochiral palladium and ruthenium in [(η3-allyl)(c)-Pd(μ-((R)-1-(1-phenyl)ethyl)-NSC-CSN-((S)-1-(1-phenyl)ethyl) κ-N,N (c)-Pd κ-S,S(s)-Ru)(s)-Ru(η6-isopropyltoluene)Cl] (8) are provided. A suitable stereochemical notation is proposed for bimetallic complexes containing pseudochiral centers in either a chiral or an achiral environment.