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

Abstract

1112-1118The reaction of [OsX2 L2] [X = Cl or Br; L = 2-(m-tolylazo)pyridine (tapy), 1; L = quinolin-8-olate (oxin), 2] with tertiary phosphine (PR3 = PPh3 or PPh2Me) bases in boiling aqueous alcohol affords complexes of the types [OsX(PR3)L2]z + (L = tapy, Z = 1, 3; L = oxin, Z = 0, 5) and [Os(PR3)2 L2]n+ (L =tapy,n = 2, 4;L = oxin, n = 0, 6), which when charged isolated as their perchlorates. The osimum(IV) counterparts [OsX(PR3)-(oxin2)1+ (7) and osimum(III) species [Os(PR3)2 (oxin)2]1+ (8) of [OsX(PR3)(oxin)2] (5) and [Os(PR3)(oxin)2] (6), respectively are also stable enough for isolation by oxidation with cerium(IV). The new complexes have been characterized by spectroscopic, magnetic, electrochemical and other physicochemical results. Using 1H NMR results (δMe of tapy/PPh2Me) in selected cases the stereochemistry of OsL2 fragment in these mixed L-PR3 complexes has been proposed. The diamagnetic osmium(II) complexes display a number of spin -allowed and spin -forbidden MLCT transitions whereas LMCT bands are seen in paramagnetic osmium (III) and osmium(IV) compounds. All the complexes are electroactive and exhibit osmium (III) - osmium (II) response in MeCN solutions; additionally, oxin complexes show osmium(IV)osmium(III) couple within accessible potential window. The stability of metal oxidation levels is discussed in terms of the electrochemical results and correlated to the π-acceptor strengths of these ligands