MIXED-LIGAND CHELATES OF OSMIUM(II/III) 8-QUINOLINOLATES - SYNTHESIS, CHARACTERIZATION AND REDOX BEHAVIOR

The synthesis and characterization of six mixed-ligand complexes of type [OsQL2]Z+ (4) [L = 2,2'-bipyridine (bpy; L1, 2) and 2-(m-tolylazo)pyridine (tap; L2, 3); Q = deprotonated forms of 8-quinolinol (HQ1) and 2-methyl-8-quinolinol (HQ2); Z = 1 for osmium(II) and Z = 2 for osmium(III)] as perchlorates are described. For L = bpy only, osmium(III) chelates were obtained and the route for interconversion of osmium(III) and osmium(II) species was found. The chelates exhibit osmium(IV)-osmium(III) and osmium(III)-osmium(II) couples in the range 0.1-1.6 V vs SCE. The redox stability of the metal centre in these complexes and in OsQ3 is correlated on the basis of the pi-acceptor properties of these ligands. Ligand reductions are observable in successive one-electron transfer steps in the range -0.3 to -2.3 V. The non-innocent N,N ligands provide one redox orbital for each alpha-diimine/azoimine group. Complete/nearly complete redox series have been identified.