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

Reaction of three phenolate ligands, viz. salicylaldehyde (HL1), 2-hydroxyacetophenone (HL2) and 2-hydroxynaphthylaldehyde (HL3), (abbreviated in general as HL, where H stands for the phenolic proton) with [Ru(PPh3)(3)Cl-2] in 1:1 mole ratio gives complexes of the type [Ru(PPh3)(3)(L)Cl-2]. The structure of the [Ru(PPh3)(2)(L-2)Cl-2] complex has been solved by X-ray crystallography. The coordination sphere around ruthenium is O2P2Cl2 with a cis-trans-cis geometry, respectively. The [Ru(PPh3)(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(PPh3)(2)(L)Cl-2] complexes show several intense LMCT transitions in the visible region. Reaction between the phenolic ligands and [Ru(PPh3)(2)Cl-2] in 2:1 mole ratio in the presence of a base affords the [Ru(PPh3)(2)(L)(2)] complexes in two isomeric forms. H-1 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. H-1 NMR spectra of the other isomer shows that it has C-2 symmetry. The structure of the isomer of the [Ru(PPh3)(2)(L-1)(2)] complex has been solved by X-ray crystallography. The coordination sphere around ruthenium is O2P with a cis-trans-cis disposition of the carbonylic oxygens, phenolate oxygens and phosphorus atoms, respectively. The [Ru(PPh3)(2)(L)(2)] complexes are diamagnetic (low-spin d(6), S = 0) and show intense MLCT transitions in the visible region. Cyclic voltammetry on the [Ru(PPh3)(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(PPh3)(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. (C) 2000 Elsevier Science Ltd. All rights reserved.