Synthesis, characterisation and coordination chemistry of novel chiral N,N-dialkyl-N′-menthyloxycarbonylthioureas. Crystal and molecular structures of N,N-diethyl-N′-(−)-(3R)-menthyloxycarbonylthiourea and cis-(S,S)-[Pt(L)Cl(DMSO)] [where HL = N-(+)-(3R)-menthyloxycarbonyl-N′-morpholinothiourea or N-benzoyl-N′,N′-diethylthiourea]

Abstract

The novel ligands N,N-diethyl-N′-(−)-(3R)-menthyloxycarbonylthiourea (HL³), N,N-diethyl-N′-(+)-(3R)-menthyloxycarbonylthiourea (HL⁴), N-(−)-(3R)-menthyloxycarbonyl-N′-morpholinothiourea (HL⁵) and N-(+)-(3R)-menthyloxycarbonyl-N′-morpholinothiourea (HL⁶) have been prepared and characterised. The molecular structure of HL³ has been confirmed by X-ray crystallography. The reaction of these N,N-disubstituted-N′-menthyloxycarbonylthiourea ligands with cis-[PtCl₂(DMSO)₂] in the presence of sodium acetate yields geometric isomers of the resultant [Pt(L)Cl(DMSO)] complexes, that is the DMSO is sulfur bonded to the platinum in either a cis-(S,S) or trans-(S,S) arrangement with respect to the sulfur donor atom of the chelated ligand. This is in contrast to the complexation reaction of cis-[PtCl₂(DMSO)₂] with N-benzoyl-N′,N′-diethylthiourea (HL¹) or N-benzoyl-N′-morpholinothiourea (HL²) which yields only one [Pt(L)Cl(DMSO)] complex in which the DMSO is in a cis-(S,S) arrangement with respect to the sulfur donor atom of the chelated ligand. The molecular structures of cis-(S,S)-[Pt(L)Cl(DMSO)], where L = (L¹)⁻ or (L⁶)⁻, have been determined by X-ray crystallography. The difference in the coordination chemistry of the acylthiourea and alkoxycarbonylthiourea ligands has been examined further by treating the [Pt(L)Cl(DMSO)] complexes with PPh₃ to give the corresponding mono- and bis-(phosphine) complexes, [Pt(L)Cl(PPh₃)] and [Pt(L)(PPh₃)₂]⁺. The ³¹P NMR studies of these complexes reveal that the alkoxycarbonylthiourea ligands bind less strongly than the acylthiourea ligands, which is consistent with the crystallographic studies. The weaker binding properties of the alkoxycarbonylthiourea ligands might be a possible explanation for the observed geometric isomerisation of the complexes and that the mechanism could involve a chelate ring opening step.