Reaction of [FeL4X2][FeX4](L = OPPh3 or OAsPh3, X = Cl or Br) with sulphur dioxide to yield some rare examples of iron(III) complexes of sulphur dioxide, [FeL4{OS(O)X}2][FeX4]. The oxidation of sulphur dioxide by the iron(III) complexes and the isolation of the sulphuric acid derivatives (PPh3O)(PPh3OH)(HSO4) and (AsPh3OH)(HSO4). Crystal structure of the latter

Abstract

The complexes [FeL4X2][FeX4](L = OPPh3 or OAsPh3, X = Cl or Br), first prepared by Cotton and Gibson, have been further characterised and their reactions with sulphur dioxide investigated; these reactions have been found to be dependent on the nature of the ligand, the halide, and the reaction medium. In the solid state only the iron(III) complexes containing OAsPh3 ligands react with sulphur dioxide to form [Fe(OAsPh3)4{OS(O)X}2][FeX4](X = Cl or Br). However, in toluene slurries [FeL4{OS(O)X}2][FeX4](L = OPPh3, X = Br; L = OAsPh3, X = Cl or Br) can be isolated, but [Fe(OPPh3)4Cl2][FeCl4] is recovered unchanged. Nonetheless, filtrates from all four reactions, when exposed to moist air yield the sulphuric acid derivatives (PPh3O)(PPh3OH)(HSO4) and (AsPh3OH)(HSO4); the formation of sulphuric acid can be indefinitely delayed by maintaining dry anaerobic conditions. The compound (AsPh3OH)(HSO4) has been characterised by single-crystal X-ray analysis which shows hydrogen-bonded chains of HSO4– anions running parallel to the c axis; each hydrogensulphate ion is then hydrogen bonded to a [AsPh3OH]+ cation. The As–O bond distance observed, 1.727(5)Å, is substantially longer than that previously reported for Ph3AsO, 1.645 Å.