Chemistry of ruthenium with some phenolic ligands: synthesis, structure and redox properties

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Reaction of three phenolate ligands, viz. salicylaldehyde (HL 1), 2-hydroxyacetophenone (HL 2) and 2-hydroxynaphthylaldehyde (HL 3), (abbreviated in general as HL, where H stands for the phenolic proton) with [Ru(PPh 3) 3Cl 2] in 1:1 mole ratio gives complexes of the type [Ru(PPh 3) 2(L)Cl 2]. The structure of the [Ru(PPh 3) 2(L 2)Cl 2] complex has been solved by X-ray crystallography. The coordination sphere around ruthenium is O 2P 2Cl 2 with a cis– trans– cis geometry, respectively. The [Ru(PPh 3) 2(L)Cl 2] complexes are one-electron paramagnetic (low-spin d 5, S=1/2) and show rhombic ESR spectra in 1:1 dichloromethane–toluene solution at 77 K. In dichloromethane solution the [Ru(PPh 3) 2(L)Cl 2] complexes show several intense LMCT transitions in the visible region. Reaction between the phenolic ligands and [Ru(PPh 3) 3Cl 2] in 2:1 mole ratio in the presence of a base affords the [Ru(PPh 3) 2(L) 2] complexes in two isomeric forms. 1H NMR spectra of one isomer shows that it does not have any C 2 symmetry and has the cis– cis– cis disposition of the three sets of donor atoms. 1H NMR spectra of the other isomer shows that it has C 2 symmetry. The structure of the isomer of the [Ru(PPh 3) 2(L 1) 2] complex has been solved by X-ray crystallography. The coordination sphere around ruthenium is O 4P 2 with a cis– trans– cis disposition of the carbonylic oxygens, phenolate oxygens and phosphorus atoms, respectively. The [Ru(PPh 3) 2(L) 2] complexes are diamagnetic (low-spin d 6, S=O) and show intense MLCT transitions in the visible region. Cyclic voltammetry on the [Ru(PPh 3) 2(L)Cl 2] complexes shows a ruthenium(III)ruthenium(II) reduction near −0.3 V versus SCE and a ruthenium(III)ruthenium(IV) oxidation in the range 1.08–1.24 V versus SCE. Cyclic voltammetry on both isomers of the [Ru(PPh 3) 2(L) 2] complexes shows a ruthenium(II)ruthenium(III) oxidation within 0.09–0.41 V versus SCE, followed by a ruthenium(III)-ruthenium(IV) oxidation within 1.31–1.52 V versus SCE.