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

Abstract

The reaction of [OsX₂ L₂] [X = Cl or Br; L = 2-(<i>m-</i>tolylazo)pyridine (tapy), <b>1</b>; <b>L</b> = quinolin-8-olate (oxin), 2] with<i> </i>tertiary phosphine (PR₃ = PPh₃ or PPh₂Me) bases in boiling<i> </i>aqueous alcohol affords complexes of the types [OsX(PR₃)L₂]^{z +}

(L = tapy, Z = 1, <b>3</b>; L = oxin, Z = 0, <b>5</b>) and [Os(PR₃)₂ L₂]ⁿ⁺ (L =tapy,n = 2, <b>4</b>;L = oxin, n = 0, 6), which when charged isolated as their perchlorates. The osimum(IV) counterparts [OsX(PR₃)-(oxin₂)¹⁺ (<b>7</b>) and osimum(III) species [Os(PR₃)₂ (oxin)₂]¹⁺ (<b>8</b>) of [OsX(PR₃)(oxin)₂] (5) and [Os(PR₃)(oxin)₂] (<b>6</b>), 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 (δ_Me 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.