Ruthenium(II) bipyridine complexes with modified phenolic Schiff base ligands. Synthesis, spectroscopic characterization and Redox properties

Abstract

A group of stable new ruthenium(II) mixed-ligand tris-chelated complexes of the type [Ru(bpy) 2L]ClO 4 ( 1–6), (bpy = 2,2′-bipyridine; L = deprotonated form of the HL ligands, o-(HO)C 6H 3(R)C(R′)NCH 2C 6H 5 or o-(HO)C 6H 3(R)C(R′)NNHC 6H 5; where R = H, p-NO 2 and R′ = H, CH 3) have been synthesized and characterized. The complexes are essentially diamagnetic and behave as 1 : 1 electrolytes in acetonitrile solution. They display two metal-to-ligand-charge-transfer (MLCT) transitions near 500 and 400 nm respectively and intra ligand π-π ∗ transitions in the UV-region. In acetonitrile solution the complexes exhibit weak emission from the lowest energy MLCT band at room-temperature. The quantum yields of the complexes are found to be in the range 0.0004–0.01. In acetonitrile solution the complexes show quasi-reversible ruthenium(II)-ruthenium(III) oxidation couples in the range 0.33 a ̊ 0.70 V and irreversible ruthenium(III)-ruthenium(IV) oxidations in the range 1.53 a ̊ 1.95 V vs SCE . Two successive reversible bipyridine reductions are observed for each complex in the ranges −1.4 a ̊ −1.62 V and −1.59 a ̊ −1.85 V vs SCE respectively. The presence of trivalent ruthenium in the oxidized solution for one complex 5 is evidenced by the rhombic EPR spectrum with g values, g 1 = 2.389, g 2 = 2.081 and g 3 = 1.810. The EPR spectrum of the coulometrically oxidized species, 5 + has been analyzed to furnish values of axial ( Δ = 4745 cm −1) and rhombic (V = 3692 cm −1) distortion parameters as well as the energies of the two expected ligand field transitions ( ν 1 = 3071 cm −1 and ν 2 = 6819 cm −1) within the t 2 shell. One of the ligand field transitions has been observed experimentally at 6578 cm −1 by near-IR spectrum which is close to the computed ν 2 value.