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)Å, 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)Å are present. Assignments are proposed for the u.v.–visible spectra of trans-[IrL₂X₄] in terms mainly of π_X, σ_X, σ_L→ Ir (t_2g) 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.