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.