Probing the effect of heterocycle-bonding in PNX-type ruthenium pre-catalysts for reactions involving H2

Abstract

A series of ruthenium(II) complexes with three novel PNX-type pincer ligands is reported, in which X denotes a heterocyclic donor group (PNX = Ph₂PCH₂CH₂N(H)CH₂-X, X = 2-pyridyl, 2-furanyl, 2-thiophenyl or 2-pyrrolyl). The reaction of [(Ph₃P)₃RuCl₂] with one equivalent of ligand leads to the trans-dichloro complexes [(PNX)RuCl₂(PPh₃)] (1–4) for all ligands, whereas the complexation of [(Ph₃P)₃Ru(H)(Cl)(CO)] results in different types of complexes. The variation of the heterocyclic donor group is in line with different binding properties and a labilization of this group. Investigations on the catalytic activity of different types of ruthenium(II) complexes in hydrogenation and dehydrogenation reactions reveal that more labile bound heterocycle donors result in a decrease of catalytic productivity and activity.