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]ClO₄ (1-6), (bpy = 2,2'-bipyridine; L = deprotonated form of the HL ligands,o-(HO)---C₆H₃(R)C(R')=N---CH₂---C₆H₅or o-(HO)---C₆H₃(R)C(R')=N---NH---C₆H₅; where R = H, p-NO₂ and R' = H, CH₃) 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 å 0.70 V and irreversible ruthenium(III)-ruthenium(IV) oxidations in the range 1.53 å 1.95 V vs SCE. Two successive reversible bipyridine reductions are observed for each complex in the ranges -1.4 å -1.62 V and -1.59 å -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, g₁ = 2.389,g₂ = 2.081 and g₃ = 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 (ν₁ = 3071 cm^-1 andν₂ = 6819 cm^-1) within the t₂shell. One of the ligand field transitions has been observed experimentally at 6578 cm^-1 by near-IR spectrum which is close to the computed ν₂ value.