Co-ordination chemistry of higher oxidation states. Part 27. Synthesis, properties, and electrochemistry of some cis-tetrahalogenoiridium(IV) complexes: crystal structure of [Ir(phen)Cl4]

Abstract

The iridium(IV) complexes cis-[Ir(L–L)Cl₄][L–L = MeSCH₂CH₂SMe, MeSeCH₂CH₂SeMe, 2,2′-bipyridyl (bipy), or 1,10-phenanthroline (phen)] have been prepared by Cl₂ or HNO₃ oxidation of the corresponding Ir^III anions [Ir(L–L)Cl₄]^–. Dark green cis-[Ir(L–L)Br₄](L–L = bipy or phen) and cis-[Ir(py)₂X₄](X = Cl or Br) are also described. Iridium(III) complexes [Ir{RTe(CH₂)₃TeR}Cl₄]^–(R = Me or Ph), [{Ir[Me₂Sb(CH₂)₃SbMe₂]Cl₃}_n], trans-[Ir(L–L)₂Cl₂]⁺[L–L = Ph₂PCH₂CH₂PPh₂(dppe), Me₂PCH₂CH₂PMe₂(dmpe), or o-C₆H₄(AsMe₂)₂], cis-[Ir(phen)₂Cl₂]⁺, and some rhodium(III) analogues, [Rh(L–L)Cl₄]^–(L–L = MeSCH₂CH₂SMe, MeSeCH₂CH₂SeMe, or bipy) and trans-[Rh(dppe)₂Cl₂][Rh(dppe)Cl₄], are described, but cannot be oxidised to the M^IV state. Complexes have been characterised by i.r., u.v.–visible, n.m.r. (³¹P, ⁷⁷Se, and ¹²⁵Te) spectroscopy, and, for the complex [Ir(phen)Cl₄], by X-ray diffraction. Crystals of [Ir(phen)Cl₄] are monoclinic, space group P2₁/c with a= 7.410(8), b= 19.256(4), c= 10.477(4)Å, β= 109.39(6)°, Z= 4, and R= 0.034 for 2 052 reflections. The iridium is six-co-ordinate, Ir–N = 2.042(9), Ir–Cl = 2.298(3)–2.311(3)Å. The u.v.–visible spectra of the iridium(IV) complexes are interpreted in terms of ligand-to-metal charge-transfer transitions and compared with the analogous trans-[IrL₂X₄]. Electrochemical studies show that for most of the [Ir(L–L)X₄]^––[Ir(L–L)X₄] couples the electron transfer reaction is close to reversible, with formal potentials for the oxidation occurring at more positive values than for the trans-[IrL₂X₄]^––[IrL₂X₄] analogues. The rhodium(III) anions show completely irreversible oxidation reactions.