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

Abstract

The meso-dithioxamide H-((R)-1-(1-phenyl)ethyl)-NSC-CSN-((S)-1-(1-phenyl)ethyl)-H (H(2)-mesoDTO) bonds [(η(6)-p-cymene)chlorido-ruthenium(II)](+) or [(η(3)-allyl)-palladium(II)](+) fragment and provides the C(s) symmetrical complexes [(η(6)-p-cymene)ClRu(H-mesoDTO κ-S,S Ru)] (1) and [(η(3)-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 [(η(3)-allyl)palladium(H-mesoDTO κ-S,S Pd)] has been broken in two different ways: (i) by changing the symmetrical allyl moiety with a η(3)-crotyl frame or (ii) by substituting the residual amidic hydrogen in the dithiooxamidate ligand with a M(PR(3))Cl(+) fragment (M = Pd or Pt and PR(3) = 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, [(η(6)-p-cymene)chlorido-ruthenium(II)](+) and [(η(3)-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 C(s)-symmetrical complex [(η(6)-p-cymene)ClRu(μ-mesoDTO κ-S,S Ru κ-N,N Pd)Pd(η(3)-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 [(η(3)-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(tri(n)propyl-phosphine)Cl] (6) and of pseudochiral palladium and ruthenium in [(η(3)-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(η(6)-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.