Synthesis, characterisation and redox behaviour of mixed ligand complexes of osmiurn(II/III/IV)

Abstract

A group of mixed-tris chelates of osmium(II), (III) and (IV) of the general formula [OsL'L₂]^z+ [L' = N-((pyridin-2-yl )formylidene) aniline (pfan) or N-((pyridin-2-yl)benzylidene)aniline (pban); L = 2,2'-bipyridyl (bipy), 2-(III-tolylazo)pyridine (tapy) or quinolin-8-olate (oxin); z = 1-3] have been prepared, isolated as their perchlorates and characterised by physicochemical, magnetic and spectroscopic methods. The complexes exhibit several spinallowed and spin-forbidden charge-transfer transitions in the 1000-200 nm region. In MeCN solutions the chelates display osmium(IV)-osmium(III) and osmium(III)-osmium(II) couples in the range -0.4 to 1.6 V vs SCE. The stability of metal oxidation levels is discussed in terms of the electrochemical result s and correlated to the π-acceptor properties of these ligands. On the negative side of SCE, successive ligand reductions in bipy/tapy complexes are observed involving diimine (-N=C-C=N-)/ azoimine (-N=N-C=N-) group.