Ruthenium-mediated reduction of oximes to imines. Synthesis, characterization and redox properties of imine complexes of ruthenium

Abstract

Reaction of three oxime ligands (oximes of salicylaldehyde (HL¹–O), 2-hydroxyacetophenone (HL²–O) and 2-hydroxynaphthylaldehyde (HL³–O); where H stands for the phenolic proton and O for the oxime oxygen) with [Ru(PPh₃)₃Cl₂] in a 1∶1 molar ratio brings about reduction of the oximes to imines and affords complexes of the form [Ru(PPh₃)₂(L)Cl₂], where L stands for the deprotonated imine ligand which is coordinated as a N,O-donor forming a six-membered chelate ring. The structure of the [Ru(PPh₃)₂(L²)Cl₂] complex has been solved by X-ray crystallography. The coordination sphere around ruthenium is composed of NOP₂Cl₂ with the two PPh₃ ligands in mutually trans and the two chlorides in mutually cis positions. The [Ru(PPh₃)₂(L)Cl₂] complexes are one-electron paramagnetic (low-spin d⁵, S = 1/2) and show rhombic EPR spectra in 1∶1 dichloromethane–toluene solution at 77 K. In dichloromethane solution the [Ru(PPh₃)₂(L)Cl₂] complexes show several intense LMCT transitions in the visible region, together with a weak ligand field transition near 1700 nm. Cyclic voltammetry on the [Ru(PPh₃)₂(L)Cl₂] complexes shows a ruthenium(III)–ruthenium(II) reduction near −0.4 V vs. SCE and a ruthenium(III)–ruthenium(IV) oxidation in the range 0.88–1.15 V vs. SCE.