Synthesis, Stereochemistry and Ligating Properties of Tetraalkyl-Substituted Ethanedithioamides − Evidence for the Lowering of High-Energy Rotation Barriers under the trans Influence of Pt-σ-Bonded Aryl Groups

Abstract

The structural and coordinating properties of five tetrasubstituted tetraalkyldithioxamides (R4DTO) have been studied: tetraethyldithioxamide (Et4DTO, 1), tetrabenzyldithioxamide (Bz4DTO, 2), (E,E)-N,N′-dibenzyl-N,N′-diethyldithioxamide [(E,E)-Bz2Et2DTO, 3], (E,Z)-N,N′-dibenzyl-N,N′-diethyldithioxamide [(E,Z)-Bz2Et2DTO, 4], (Z,Z)-N,N′-dibenzyl-N,N′-diethyldithioxamide [(Z,Z)-Bz2Et2DTO, 5]. The solid structure of 4, determined by single-crystal X-ray diffraction, is characterised by two planar thioamide groups positioned almost orthogonally to each other, connected by a C−C single bond. Each of these dithioxamides showed high-energy barriers to rotation around both the C−N and the central C−C bond. This latter is a chiral torsion, so that R4DTO species exist as pairs of enantiomers. Compounds 3, 4 and 5 slowly isomerised in solution to give equimolar mixtures of the three isomers; the activation enthalpy of the process was 30.0 kcal mol−1. The ligands 1−5 reacted with [Pt(Me2SO)2X2] (X = Cl, Br, Me, Ph) to give chiral chelate complexes [(R4DTO-κ-S,S′)PtX2], but 3 (and also 4 and 5) provided a mixture of three isomeric platinum chelates in which the S,S′-coordinated ligands showed the (E,E), (E,Z) and (Z,Z) conformations. Furthermore, the presence of small amounts of the platinum complexes cis-[Pt(Me2SO)2Ph2] or [(Et4DTO-κ-S,S′)PtPh2] caused the fast isomerisation of large quantities of a given isomerically pure ligand. We have explained the collapse of the R4DTO rotational barriers by assuming that a release of electrons from the metal atom to incoming ligands in the transition state twists these into a partially reduced, dithiolenic form. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)