Co-ordination chemistry of higher oxidation states. Part 23. Synthesis and properties of tetrahalogenoiridium(IV) complexes, [IrL2X4][X = Cl or Br; L = pyridine, PR3, AsR3, SbR3, SR2, or SeR2]. Crystal and molecular structure of trans-[Ir(AsEt3)2Br4

Abstract

The iridium(III) anions trans-[IrL2Cl4]–[L = pyridine (py), PEt3, PEt2Ph, PEtPh2, AsEt3, AsMe2Ph, SMe2, or SeMe2] have been prepared and oxidised with chlorine to purple iridium(IV)trans-[IrL2Cl4]. Dark green trans-[IrL2Br4](L = py, PEt3, PMe2Ph, AsEt3, AsMe2Ph, or SMe2) were obtained similarly from trans-[IrL2Br4]– and Br2 or HNO3. The [IrL2Cl4](L = PPh3 AsPh3, or SbPh3) are made directly from IrCl3·nH2O + 2L, followed by chlorination of the intermediate [(IrL2Cl3)n] produced; cis-[IrL2Cl4](L = py or SbMe3) and trans-[IrL2X4]–(L = TeMe2, X = Cl; L = SeMe2, X = Br) are also described. The iridium(IV) complexes have been characterised by analysis, i.r., and u.v.–visible spectroscopy and for the complex [Ir(AsEt3)2Br4] by X-ray diffraction. Crystals of trans-[Ir(AsEt3)2Br4] are monoclinic, space group P21/n with a= 8.442(1), b= 13.933(2), c= 9.617(1)A, β= 103.85(1)°, Z= 2, and R= 0.041 for 1 826 reflections [F > 3σ(F)]. Discrete centrosymmetric molecules with Ir–Br 2.455(1), 2.463(1) and Ir–As 2.489(1)A are present. Assignments are proposed for the u.v.–visible spectra of trans-[IrL2X4] in terms mainly of πX, σX, σL→ Ir (t2g) ligand-to-metal charge-transfer transitions. For most couples, [IrL2X4]–[lrL2X4]–, the electron-transfer reaction is close to reversible and formal potentials, estimated from the cyclic voltammograms, correlate well with spectroscopic data. The voltammograms for four anions (X = Cl, L = TeMe2 and X = Br, L = SbPh3, SeMe2, or dimethyl sulphoxide) show that the corresponding iridium(IV) complexes have half-lives of only a few seconds and this explains the failure to isolate them from chemical syntheses. Cyclic voltammograms for [RhL2Cl4]–(L = PEt3, SMe2, SeMe2, or py) show that oxidation occurs at more positive potentials, but the rhodium(IV) complexes are unstable. Neutral iridium(III) complexes [IrL3X3] are not oxidised by X2 or HNO3, and possible reasons for this and the crucial role of the [IrL2X4]– intermediates in the preparation of [IrL2X4] are discussed.