Paramagnetic ruthenium(III) ortho-metallatedcomplexes. Synthesis, spectroscopic and redoxproperties

Abstract

The reaction of (CS)Cl(PPh₃)₂Ru^II (μ-Cl)₂ Ru^II (PPh₃)₂Cl(CS), A with the phenolic Schiff base ligands o-(OH)C₆H₄C(H)⊗N-C₆H₄(R), (R=p-OMe, Me, H, Cl, NO₂; H₂L¹-H₂L₅) in methanol under aerobic conditions afforded the complexes Ru^III(HL)₂(PPh₃) Cl, 1 and Ru^III(L)(PPh₃)(CH₃OH)Cl, 2. In complexes 1 both the ligands (HL^-) are bound to the metal center at the deprotonated phenolic oxygen and azomethine nitrogen and in the complexes 2 the L^2- is in tridentate C,N,O mode where cyclometallation takes place from the ortho carbon atom of the amine fragment of H₂L. During the reaction the metal ion is oxidized from the starting Ru^II in A to Ru^III in the products 1 and 2. The complexes (1 and 2) are nonconducting and behave as one-electron paramagnets. Complexes display rhombic EPR spectra that have been analyzed to furnish values of axial (ν ) and rhombic (V) distortion parameters as well as energies of the two expected ligand field transitions (ν ₁ and ν ₂) within the t₂ shell. One of the transitions (ν ₂) has been observed in the predicted region. The complexes exhibit moderately strong ligand-to-metal charge-transfer transition in the visible region and intraligand transitions in the UV region. The complexes are electroactive and show ruthenium (IV)-ruthenium(III) (E_½ , 0.75-0.88 V vs. Ag/AgCl) and ruthenium(III)-ruthenium(II) (E_½ , -0.42 to -0.59 V) couples. The E_½ values vary linearly with the Hammett constant of the substituents R. The role of coordination of phenolato function in stabilizing the unusual paramagnetic ruthenium(III) oxidation state in the complexes 2 is noted.