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.

Synthesis, Characterization and Anticancer Studies of Rh(I), Rh(III), Pd(II) and Pt(II) Complexes Bearing A Dithiooxamide Ligand

“…Six mononuclear complexes of Rh(III) ( 1 ), Rh(I) ( 2 ), Pd(II) ( 3‐4 ) and Pt(II) ( 5 ‐ 6 ) were prepared using a (μ‐chloro)‐bridged metal complex ( 1 ‐ 4 ) or PtCl 2 (dmso) 2 ( 5 ‐ 6 ) as starting materials. Complex [( η 3 ‐allyl)Pd(H‐isopropyl 2 DTO κ ‐ S,S Pd)] ( 3 ), has been previously reported by us . Complexes IP1‐IP4 (ion pair intermediates of compounds 1–4 ) and 1–4 , [L n M(H‐isopropyl 2 DTO κ‐S,S M)] (L n M=(C 5 Me 5 )RhCl, ( 1 ); (COD)Rh, ( 2 ); ( η 3 ‐allyl)Pd, ( 3 ); and (tri n propyl‐phosphine)PdCl, ( 4 )) respectively, were obtained by the following process:…”

“…H 2 ‐isopropyl 2 DTO is a bidentate sulfur ligand which, by reacting with chloro bridges, can cleave them and form mononuclear complexes . Structurally, this secondary dithiooxamide is a chelating binucleating ligand which can also form binuclear complexes depending on the stoichiometric ratios and the nature of the chlorido‐bridged dimers .…”

Allylpalladium Dimers with Metals Connected by Binucleating Dithiooxamidates in Two Different Coordination Modes: Solution Behavior and Solid-State Structure

“…[(η 3 -Allyl)Pd(HR 2 -DTO k-S,S Pd)] was prepared following a reported procedure. 11 Secondary DTOs, H 2 R 2 DTO [R = methyl, ethyl, isopropyl, benzyl, isoamyl, 1-(S)-(1-phenyl)ethyl, and meso/rac-(1-phenyl)ethyl], were prepared according to literature methods. 12 The meso-DTO H-(R)-1-(1-phenyl)ethyl)NSCÀCSN-(S)-1-(1-phenyl)ethyl)-H was obtained by fractional crystallization from ethanol and an equimolar mixture of meso-and rac-(1-phenyl)ethyldithiooxamide obtained from rac-(1-phenyl)ethylamine.…”

“…10 Moreover, it has been observed that the steric hindrance of the ancillary ligands in a given metal fragment can also favor the formation of one of the two possible linkage isomers of the k-N,N 0 MLn k-S,S 0 M 0 L 0 n bimetallic complexes. 11 Finally, it has been shown that the bulkiness of nitrogen DTO substituents can direct the synthesis of homobimetallic complexes in an "exo" k-N,S coordination mode toward one of the two possible conformers. 8 The present paper explores the capability of a secondary DTO to act as a binucleating ligand when the ancillary ligands in the binucleated metal frames have modest steric crowding.…”

Mechanisms in solution

“…In a chloroform solution all of these isomers interconvert in that both palladium and ruthenium invert their configurations. A mechanism of epimerization for both palladium and ruthenium is proposed 43. The phosphinito-bridged Pt(I) complex [(PHCy 2 )Pt(m-PCy 2 ){k 2 P,O-m-P(O)Cy 2 }-Pt(PHCy 2 )](Pt-Pt) reversibly adds H 2 under ambient conditions, giving cis-[(H)(PHCy 2 )-Pt 1 (m-PCy 2 )(m-H)Pt 2 (PHCy 2 ){kPP(O)Cy 2 }](Pt-Pt), which slowly isomerizes spontaneously into the corresponding more stable isomer trans-[(PHCy 2 )(H)Pt(m-PCy 2 )(m-H)-Pt(PHCy 2 ){kP-P(O)Cy 2 }](Pt-Pt).…”