Osmium-(V), -(IV) and -(III) complexes with tetradentate dianionic chelating ligands

Osmium-(III) and -(IV) complexes of the ligands salen [H2salen = ethylenebis(salicylidineimine)], bpb [H2bpb = 1,2-bis(pyridine-2-carboxamido)benzene] and bpc [H2bpc = 4,5-dichloro-1,2-bis(pyridine-2-carboxamido)benzene] were synthesised by reducing the corresponding trans-dioxoosmium(VI) complexes with PPh3. The structure of [OsIV(salen)(OPri)2] has been determined by X-ray crystallography: monoclinic, space group C2/c(no. 15), a= 15.367(2), b= 7.847(1), c= 17.999(2)Å, β= 95.36(1)° and Z= 4. Electrochemical studies of trans-dialkoxyoosmium(IV) complexes in acetonitrile showed a reversible oxidation wave assignable to the oxidation of OsIV to OsV. Some trans-dialkoxyoosmium(V) complexes were characterised by UV/VIS spectroscopy. The osmium(III) complexes of salen, bpb and bpc were found to catalyse the oxidation of alkenes by iodosylbenzene.