Cationic iridium complexes with C2-symmetry binaphthalene-core disulfide ligands: Synthesis and catalytic activity in the hydrogenation of alkenes

Mononuclear cationic Ir(I)-cyclooctadiene complexes containing three different C2-symmetrical binaphthalene-templated sulfide ligands, featuring alkyl groups of increasing steric demand onto the donor centres, have been prepared and characterized. Variable temperature NMR spectra provide evidence that, regardless of the bulk of the alkyl substituent on the sulfur, the chelate coordination of the ligands proceeds in all cases with complete stereoselectivity at the newly generated S-stereocentres affording just one stereoisomer. This species features a seven-membered chelate ring in a frozen conformation where the diequatorial S-alkyl substituents are disposed in anti-relationship and the stereogenic S-donors display the same configuration. The oxidative addition of hydrogen to these complexes proceeds smoothly affording in every case one single cis-dihydride complex whose structure in one case has been cleared by correlated NMR spectra. The cationic complexes derived from these ligands are catalysts of modest value for the hydrogenation of α,β-unsaturated acid derivatives where they produce nearly racemic products in moderate yields.