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

A group of stable new ruthenium(II) mixed-ligand tris-chelated complexes of the type [Ru (bpy)(2)L]ClO4 (bpy = 2,2'-bipyridine; L = deprotonated form of the HL ligands, o-(HO)-C6H3(R)C(R')=N-CH2-C6H5 or o-(HO)-C6H3(R)C(R')=N-NH-C6H5; where R = H, p-NO2 and R' = H, CH,) 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 pi-pi* 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 --> 0.70 V and irreversible ruthenium (III)-ruthenium(IV) oxidations in the range 1.53 --> 1.95 V vs SCE. Two successive reversible bipyridine reductions are observed for each complex in the ranges -1.4 --> -1.62 V and -1.59 --> -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 (Delta = 4745 cm(-1)) and rhombic (V = 3692 cm(-1)) distortion parameters as well as the energies of the two expected ligand held transitions (nu(1) = 3071 cm(-1) and nu(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 nu(2) value. (C) 1998 Elsevier Science Ltd. All rights reserved.