Synthesis and applications of Palladium complexes Part I. The synthesis, single crystal structures and liquid crystalline phase behavior of Alkoxy substituted Tolans. Part II. Palladium-Complexes of Thioureas and phosphine sulfides

Abstract

Three series of alkoxy substituted tolans p-X-C(_6)H(_4)-C≡C-C(_6)H(_4)-p-OC(_n)H(_2n+1) [X = H (series 1), CH(_3) (series 2), OCH(_3) (series 3)] with varying chain length were synthesized by Pd/Cu-catalysed Sonogashira cross-coupling reactions of terminal alkynes and iodoarenes, crystal structures and phase behavior of three series of tolans. Twenty-eight of the tolans were structurally characterised by single-crystal X-ray diffraction. Their phase behavior was characterised by tplm; only the tolans in series 3 show liquid crystalline phases. The melting points of the tolans decrease with increasing chain length due to a higher degree of flexibility of the terminal chain. An odd-even effect is clearly observed for the clearing point of the nematic phase upon both heating and cooling, with the higher temperature for even and low temperature for odd number carbon chains for series 3.Monomeric and dimeric palladium dichloride complexes containing the monodentate thioureas, tetramethyl thiourea (tmtu) and a chiral C(_2) symmetric thiourea were synthesized. Their structures were obtained from single-crystal X-ray diffraction. The structures of the mono-palladium complexes are the trans-isomers, whereas the dimer complexes present cis-configurations, and are also the first examples of palladium complexes with sulfur-bridging thiourea ligands. NMR studies of the monomeric and dimeric complexes with tmtu reveal that, in solution, the dimeric complexes are interconverting with mono-palladium complexes, which suggests that the systems are labile. A dynamic equilibrium in solution was also observed between the rac- and meso-dipalladium complexes with the C(_2)-symmetric ligand. The energy barrier to exchange was obtained from a variable temperature NMR study. It is proposed that this equilibrium results from the monomer-dimer interconversion. S,N-bidentate ß-dimethyl and ß-monomethyl oxazoline thiourea ligands and α-isoquinoline thiourea ligands and their palladium complexes have been obtained. Single-crystal X-ray diffraction analyses allow us to distinguish between some of their atropoisomers and diastereomers. Most of the Pd complexes were shown to be monomers in the solid state, although one ligand with a cyclohexyl group formed exclusively dimers, and one formed a trimer, as well as a monomer, depending on the solvent mixture that the crystals were grown from. The ligands were found to coordinate through their S and N atoms. All of the mono-palladium complexes and most of the dimers have cis-configurations at the Pd centers; however, we also found a trans-configuration in the trimer and the dimer of one of the palladium complex of the ß-monomethyl ligands. The structures show that the ß- monomethyl oxazoline and a-isoquinoline thiourea complexes appear to be more weakly coordinated than the ß-dimethyl oxazoline ones which may be responsible for their lower stabilities in solution. The structure of the ligands (in particular the 'up' or 'down' conformation of the atropoisomers) is intimately related to their enatioselectivities in bis(methoxycarbonylation) reactions of styrene using palladium complexes. The ß-dimethyl ligands were, in general, found to be more selective in this reaction than the ß -monomethyl oxazoline ones, with palladium complexes of the latter generally found to be less stable in NMR studies. Several bis(phosphine) monosulfide and disulfide ligands and their palladium complexes have been synthesized and analyzed by single-crystal X-ray diffraction. The reaction with 1:1 molar ratios of Pd source to monosulfide ligands were found to produce monomeric palladium complexes with chelating ligands, apart from the reaction with is(diphenylphosphino)butane monosulfide, which was found to result in a dimer being formed. The reactions with 1:2 molar ratios of Pd source to monosulfide ligand gave complexes which were coordinated only through the phosphine groups, except for that with bis(diphenylphosphino)methane monosulfides which formed cafionic palladium complexes with two chelating ligands. Analogous reactions with the disulfide ligands and the Pd source [Pd(CH(_3)CN)(_4)](BF(_4))(_2) also resulted in cationic Pd complexes incorporating two chelating ligands. In some cases, in solution, the complexes showed dynamic equilibria between cis- and trans- isomers which suggests that the phosphine sulfide ligands are labile. The Pd-S-P angles in the complexes were found to be variable but, since there are none less than 90, it is unlikely that there is any ƞ(^2)-π bonding involving the P=S bond. Preliminary results show that the activities of phosphine sulfide palladium complexes in the oxidative homo-coupling of phenylacetylene are comparable to that of the commonly used pre-catalyst, PdCl(_2)(PPh(_3))(_2).