Ruthenium(II) complexes of a tripodal phosphite ligand: Synthesis, characterization, and applications in catalytic dehydrogenation

The scaffold tris(8-quinolinyl)phosphite, (P(Oquin)₃), can bind to metal centers as a bi- or a tridentate tripodal ligand, depending on the synthetic conditions and characteristics of the metal atom. Here the study of the coordinating properties of such chelate was extended to the synthesis and characterization of well-defined Ru(II)-halide complexes. The compound [κ³(N,P,N){P(Oquin)₃}RuCl₂(PPh₃)] was obtained by simple ligand exchange on the precursor [(PPh₃)₃RuCl₂]. Two isomers were observed by means of ³¹P{¹H} NMR spectroscopy, and were partially separated by recrystallization in refluxing solvents, thus allowing their structure elucidation. X-ray diffraction analysis revealed that the two crystalline isomers have the formula [κ³(N,P,N){P(Oquin)₃}RuCl₂(PPh₃)] with the phosphite ligand coordinated to the Ru(II) center either in a meridional or in a facial disposition. The mixture [κ³(N,P,N){P(Oquin)₃}RuCl₂(PPh₃)] undergoes phosphine dissociation at temperatures over 100 °C, leading to the formation of [κ⁴(P,N₃){P(Oquin)₃}RuCl₂]. This compound displays a tetradentate P(Oquin)₃ chelate and is a suitable catalyst for the dehydrogenation of formic acid and the dehydrocoupling of methylphenylsilane.