Paramagnetic ruthenium(III) ortho-metallated complexes. Synthesis, spectroscopic and redox properties

Abstract

The reaction of (CS)Cl(PPh 3) 2Ru II (μ-Cl) 2 Ru II (PPh 3) 2Cl(CS), A with the phenolic Schiff base ligands o-(OH)C 6H 4C(H)N-C 6H 4(R), (R= p-OMe, Me, H, Cl, NO 2; H 2L 1–H 2L 5) in methanol under aerobic conditions afforded the complexes Ru III(HL) 2(PPh 3)Cl, 1 and Ru III(L)(PPh 3)(CH 3OH)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 H 2L. 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 (▵) and rhombic (V) distortion parameters as well as energies of the two expected ligand field transitions ( ν 1 and ν 2) within the t 2 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 1/2, 0.75–0.88 V vs. Ag/AgCl) and ruthenium(III)–ruthenium(II) ( E 1/2, −0.42 to −0.59 V) couples. The E 1/2 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.